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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 7143

  Storage Maintenance (storm) WG         Mallikarjun Chadalapaka
  Internet Draft                                       Microsoft
  draft-ietf-storm-iscsi-cons-09.txt
  Intended status: Proposed Standard               Julian Satran
  Expires: December 2013                          Infinidat Ltd.
  Obsoletes: RFC3720, RFC3980, RFC4850, RFC5048
  Updates: RFC3721                                   Kalman Meth
                                                             IBM

                                                     David Black
                                                             EMC




               iSCSI Protocol (Consolidated)




Status of this Memo

  This Internet-Draft is submitted to IETF in full conformance with
  the provisions of BCP 78 and BCP 79.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups. Note that
  other groups may also distribute working documents as Internet-
  Drafts.

  Internet-Drafts are draft documents valid for a maximum of six
  months and may be updated, replaced, or obsoleted by other
  documents at any time. It is inappropriate to use Internet-Drafts
  as reference material or to cite them other than as "work in
  progress."

  The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/ietf/1id-abstracts.txt




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  The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html

  This Internet-Draft will expire on December 31, 2013.


Copyright Notice

  Copyright (c) 2013 IETF Trust and the persons identified as the
  document authors. All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document. Please review these documents
  carefully, as they describe your rights and restrictions with
  respect to this document. Code Components extracted from this
  document must include Simplified BSD License text as described in
  Section 4.e of the Trust Legal Provisions and are provided without
  warranty as described in the Simplified BSD License.


Abstract

  This document describes a transport protocol for SCSI that works
  on top of TCP. The iSCSI protocol aims to be fully compliant with
  the standardized SCSI Architecture Model (SAM-2). RFC 3720
  defined the original iSCSI protocol. RFC 3721 discusses iSCSI
  Naming examples and discovery techniques. Subsequently, RFC 3980
  added an additional naming format to iSCSI protocol. RFC 4850
  followed up by adding a new public extension key to iSCSI. RFC
  5048 offered a number of clarifications and a few improvements and
  corrections to the original iSCSI protocol.


  This document obsoletes RFCs 3720, 3980, 4850 and 5048 by
  consolidating them into a single document and making additional
  updates to the consolidated specification. This document also
  updates RFC 3721. The text in this document thus supersedes the
  text in all the noted RFCs wherever there is a difference in
  semantics.




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  Note: This version of the draft does not yet incorporate planned
  resolutions to some Last Call comments regarding Kerberos and
  IPsec-related security considerations.




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1. Introduction.................................................... 15

2. Acronyms, Definitions and Document Summary...................... 16
 2.1.   Acronyms ................................................   16
 2.2.   Definitions .............................................   18
 2.3.   Summary of Changes ......................................   25
 2.4.   Conventions .............................................   26
3. UML Conventions................................................. 27
 3.1.   UML Conventions Overview ................................   27
 3.2.   Multiplicity Notion .....................................   27
 3.3.   Class Diagram Conventions ...............................   28
 3.4.   Class Diagram Notation for Associations .................   29
 3.5.   Class Diagram Notation for Aggregations .................   30
 3.6.   Class Diagram Notation for Generalizations ..............   30
4. Overview........................................................ 32
 4.1. SCSI Concepts ...........................................     32
 4.2. iSCSI Concepts and Functional Overview ..................     33
   4.2.1. Layers and Sessions .................................     34
   4.2.2. Ordering and iSCSI Numbering ........................     35
     4.2.2.1. Command Numbering and Acknowledging ..............    35
     4.2.2.2. Response/Status Numbering and Acknowledging ......    39
     4.2.2.3. Response Ordering ................................    40
       4.2.2.3.1. Need for Response Ordering ...................    40
       4.2.2.3.2. Response Ordering Model Description ..........    40
       4.2.2.3.3. iSCSI Semantics with the Interface Model .....    41
       4.2.2.3.4. Current List of Fenced Response Use Cases ....    42
     4.2.2.4. Data Sequencing ..................................    43
   4.2.3. iSCSI Task Management ...............................     44
     4.2.3.1. Task Management Overview .........................    44
     4.2.3.2. Notion of Affected Tasks .........................    44
     4.2.3.3. Standard Multi-task Abort Semantics ..............    45
     4.2.3.4. FastAbort Multi-task Abort Semantics .............    46
     4.2.3.5. Affected Tasks Shared across Standard and FastAbort
     Sessions ..................................................    48




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     4.2.3.6. Rationale behind the FastAbort Semantics .........   49
   4.2.4. iSCSI Login .........................................    51
   4.2.5. iSCSI Full Feature Phase ............................    52
     4.2.5.1. Command Connection Allegiance ....................   53
     4.2.5.2. Data Transfer Overview ...........................   54
     4.2.5.3. Tags and Integrity Checks ........................   55
     4.2.5.4. Task Management ..................................   56
   4.2.6. iSCSI Connection Termination ........................    56
   4.2.7. iSCSI Names .........................................    57
     4.2.7.1. iSCSI Name Properties ............................   58
     4.2.7.2. iSCSI Name Encoding ..............................   60
     4.2.7.3. iSCSI Name Structure .............................   61
     4.2.7.4. Type "iqn." (iSCSI Qualified Name) ...............   62
     4.2.7.5. Type "eui." (IEEE EUI-64 format) .................   64
     4.2.7.6. Type "naa." - Network Address Authority ..........   64
   4.2.8. Persistent State ....................................    65
   4.2.9. Message Synchronization and Steering ................    66
     4.2.9.1. Sync/Steering and iSCSI PDU Length ...............   67
 4.3. iSCSI Session Types .....................................    67
 4.4. SCSI to iSCSI Concepts Mapping Model ....................    68
   4.4.1. iSCSI Architecture Model ............................    69
   4.4.2. SCSI Architecture Model .............................    72
   4.4.3. Consequences of the Model ...........................    74
     4.4.3.1. I_T Nexus State ..................................   75
     4.4.3.2. Reservations .....................................   75
 4.5. iSCSI UML Model .........................................    76
 4.6. Request/Response Summary ................................    79
   4.6.1. Request/Response Types Carrying SCSI Payload ........    79
     4.6.1.1. SCSI-Command .....................................   79
     4.6.1.2. SCSI-Response ....................................   80
     4.6.1.3. Task Management Function Request .................   80
     4.6.1.4. Task Management Function Response ................   81
     4.6.1.5. SCSI Data-out and SCSI Data-in ...................   81
     4.6.1.6. Ready To Transfer (R2T) ..........................   82
   4.6.2. Requests/Responses carrying SCSI and iSCSI Payload ..    83
     4.6.2.1. Asynchronous Message .............................   83




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   4.6.3. Requests/Responses Carrying iSCSI Only Payload ......    83
     4.6.3.1. Text Request and Text Response ...................   83
     4.6.3.2. Login Request and Login Response .................   84
     4.6.3.3. Logout Request and Response ......................   85
     4.6.3.4. SNACK Request ....................................   85
     4.6.3.5. Reject ...........................................   85
     4.6.3.6. NOP-Out Request and NOP-In Response ..............   86
5. SCSI Mode Parameters for iSCSI................................. 87

6. Login and Full Feature Phase Negotiation....................... 88
 6.1. Text Format ................................................ 89
 6.2. Text Mode Negotiation ...................................... 93
   6.2.1. List negotiations ...................................... 97
   6.2.2. Simple-value Negotiations .............................. 98
 6.3. Login Phase ................................................ 98
   6.3.1. Login Phase Start ..................................... 102
   6.3.2. iSCSI Security Negotiation ............................ 105
   6.3.3. Operational Parameter Negotiation During the Login Phase 106
   6.3.4. Connection Reinstatement .............................. 107
   6.3.5. Session Reinstatement, Closure, and Timeout ........... 107
     6.3.5.1. Loss of Nexus Notification ........................ 108
   6.3.6. Session Continuation and Failure ...................... 108
 6.4. Operational Parameter Negotiation Outside the Login Phase . 109
7. iSCSI Error Handling and Recovery............................. 111
 7.1. Overview ...............................................    111
   7.1.1. Background .........................................    111
   7.1.2. Goals ..............................................    111
   7.1.3. Protocol Features and State Expectations ...........    112
   7.1.4. Recovery Classes ...................................    113
     7.1.4.1. Recovery Within-command .........................   114
     7.1.4.2. Recovery Within-connection ......................   115
     7.1.4.3. Connection Recovery .............................   116
     7.1.4.4. Session Recovery ................................   117
   7.1.5. Error Recovery Hierarchy ...........................    117
 7.2. Retry and Reassign in Recovery .........................    119




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   7.2.1. Usage of Retry .....................................    119
   7.2.2. Allegiance Reassignment ............................    120
 7.3. Usage Of Reject PDU in Recovery ........................    121
 7.4. Error Recovery Considerations for Discovery Sessions ...    122
   7.4.1. ErrorRecoveryLevel for Discovery Sessions ..........    122
   7.4.2. Reinstatement Semantics for Discovery Sessions .....    122
     7.4.2.1. Unnamed Discovery Sessions ......................   123
     7.4.2.2. Named Discovery Session .........................   124
   7.4.3. Target PDUs During Discovery .......................    124
 7.5. Connection Timeout Management ..........................    124
   7.5.1. Timeouts on Transport Exception Events .............    125
   7.5.2. Timeouts on Planned Decommissioning ................    125
 7.6. Implicit Termination of Tasks ..........................    125
 7.7. Format Errors ..........................................    126
 7.8. Digest Errors ..........................................    127
 7.9. Sequence Errors ........................................    129
 7.10. Message Error Checking ................................    129
 7.11. SCSI Timeouts .........................................    130
 7.12. Negotiation Failures ..................................    131
 7.13. Protocol Errors .......................................    131
 7.14. Connection Failures ...................................    132
 7.15. Session Errors ........................................    133
8. State Transitions............................................. 134
 8.1. Standard Connection State Diagrams .....................    134
   8.1.1. State Descriptions for Initiators and Targets ......    134
   8.1.2. State Transition Descriptions for Initiators and Targets135
   8.1.3. Standard Connection State Diagram for an Initiator .    139
   8.1.4. Standard Connection State Diagram for a Target .....    141
 8.2. Connection Cleanup State Diagram for Initiators and Targets 143
   8.2.1. State Descriptions for Initiators and Targets ......    145
   8.2.2. State Transition Descriptions for Initiators and Targets146
 8.3. Session State Diagrams ..................................   148
   8.3.1. Session State Diagram for an Initiator ..............   148
   8.3.2. Session State Diagram for a Target ..................   149
   8.3.3. State Descriptions for Initiators and Targets .......   150




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   8.3.4. State Transition Descriptions for Initiators and Targets 151
9. Security Considerations...................................... 153
 9.1. iSCSI Security Mechanisms .............................    153
 9.2. In-band Initiator-Target Authentication ...............    154
   9.2.1. CHAP Considerations ...............................    156
   9.2.2. SRP Considerations ................................    159
    Kerberos Considerations .................................    159
   9.2.3. ...................................................    159
 9.3. IPsec .................................................    160
   9.3.1. Data Integrity and Authentication .................    160
   9.3.2. Confidentiality ...................................    161
   9.3.3. Policy, Security Associations, and Cryptographic Key
   Management ...............................................    162
 9.4. Security Considerations for the X#NodeArchitecture Key     164
 9.5. SCSI Access Control Considerations ....................    166
10. Notes to Implementers....................................... 167
 10.1. Multiple Network Adapters ............................... 167
   10.1.1. Conservative Reuse of ISIDs ......................... 167
   10.1.2. iSCSI Name, ISID, and TPGT Use ...................... 168
 10.2. Autosense and Auto Contingent Allegiance (ACA) .......... 170
 10.3. iSCSI Timeouts .......................................... 170
 10.4. Command Retry and Cleaning Old Command Instances ........ 171
 10.5. Synch and Steering Layer and Performance ................ 172
 10.6. Considerations for State-dependent Devices and Long-lasting
 SCSI Operations ............................................... 172
   10.6.1. Determining the Proper ErrorRecoveryLevel ........... 173
 10.7. Multi-task Abort Implementation Considerations .......... 174
11. iSCSI PDU Formats........................................... 175
 11.1. iSCSI PDU Length and Padding .........................    175
 11.2. PDU Template, Header, and Opcodes ....................    175
   11.2.1. Basic Header Segment (BHS) .......................    176
     11.2.1.1. I .............................................   177
     11.2.1.2. Opcode ........................................   177
     11.2.1.3. Final (F) bit .................................   179




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     11.2.1.4. Opcode-specific Fields ........................   179
     11.2.1.5. TotalAHSLength ................................   179
     11.2.1.6. DataSegmentLength .............................   179
     11.2.1.7. LUN ...........................................   179
     11.2.1.8. Initiator Task Tag ............................   180
   11.2.2. Additional Header Segment (AHS) ..................    180
     11.2.2.1. AHSType .......................................   180
     11.2.2.2. AHSLength .....................................   181
     11.2.2.3. Extended CDB AHS ..............................   181
     11.2.2.4. Bidirectional Expected Read-Data Length AHS ...   181
   11.2.3. Header Digest and Data Digest ....................    182
   11.2.4. Data Segment .....................................    182
 11.3. SCSI Command .........................................    182
   11.3.1. Flags and Task Attributes (byte 1) ...............    183
   11.3.2. CmdSN - Command Sequence Number ..................    184
   11.3.3. ExpStatSN ........................................    185
   11.3.4. Expected Data Transfer Length ....................    185
   11.3.5. CDB - SCSI Command Descriptor Block ..............    186
   11.3.6. Data Segment - Command Data ......................    186
 11.4. SCSI Response ........................................    186
   11.4.1. Flags (byte 1) ...................................    187
   11.4.2. Status ...........................................    188
   11.4.3. Response .........................................    189
   11.4.4. SNACK Tag ........................................    190
   11.4.5. Residual Count ...................................    190
     11.4.5.1. Field Semantics ...............................   190
     11.4.5.2. Residuals Concepts Overview ...................   191
     11.4.5.3. SCSI REPORT LUNS and Residual Overflow ........   191
   11.4.6. Bidirectional Read Residual Count ................    193
   11.4.7. Data Segment - Sense and Response Data Segment ...    193
     11.4.7.1. SenseLength ...................................   194
     11.4.7.2. Sense Data ....................................   194
   11.4.8. ExpDataSN ........................................    195
   11.4.9. StatSN - Status Sequence Number ..................    195
   11.4.10. ExpCmdSN - Next Expected CmdSN from this Initiator   196
   11.4.11. MaxCmdSN - Maximum CmdSN from this Initiator ....    196




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 11.5. Task Management Function Request .....................    197
   11.5.1. Function .........................................    197
   11.5.2. TotalAHSLength and DataSegmentLength .............    201
   11.5.3. LUN ..............................................    201
   11.5.4. Referenced Task Tag ..............................    201
   11.5.5. RefCmdSN .........................................    201
   11.5.6. ExpDataSN ........................................    202
 11.6. Task Management Function Response ....................    202
   11.6.1. Response .........................................    203
   11.6.2. TotalAHSLength and DataSegmentLength .............    205
 11.7. SCSI Data-out & SCSI Data-in .........................    205
   11.7.1. F (Final) Bit ....................................    208
   11.7.2. A (Acknowledge) bit ..............................    208
   11.7.3. Flags (byte 1) ...................................    209
   11.7.4. Target Transfer Tag and LUN ......................    210
   11.7.5. DataSN ...........................................    210
   11.7.6. Buffer Offset ....................................    210
   11.7.7. DataSegmentLength ................................    211
 11.8. Ready To Transfer (R2T) ..............................    212
   11.8.1. TotalAHSLength and DataSegmentLength .............    214
   11.8.2. R2TSN ............................................    214
   11.8.3. StatSN ...........................................    214
   11.8.4. Desired Data Transfer Length and Buffer Offset ...    214
   11.8.5. Target Transfer Tag ..............................    214
 11.9. Asynchronous Message .................................    215
   11.9.1. AsyncEvent .......................................    216
   11.9.2. AsyncVCode .......................................    219
   11.9.3. LUN ..............................................    219
   11.9.4. Sense Data and iSCSI Event Data ..................    219
     11.9.4.1. SenseLength ...................................   220
 11.10. Text Request ........................................    220
   11.10.1. F (Final) Bit ...................................    222
   11.10.2. C (Continue) Bit ................................    222
   11.10.3. Initiator Task Tag ..............................    222
   11.10.4. Target Transfer Tag .............................    222
   11.10.5. Text ............................................    223




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 11.11. Text Response .......................................    224
   11.11.1. F (Final) Bit ...................................    225
   11.11.2. C (Continue) Bit ................................    226
   11.11.3. Initiator Task Tag ..............................    226
   11.11.4. Target Transfer Tag .............................    226
   11.11.5. StatSN ..........................................    227
   11.11.6. Text Response Data ..............................    227
 11.12. Login Request .......................................    227
   11.12.1. T (Transit) Bit .................................    228
   11.12.2. C (Continue) Bit ................................    229
   11.12.3. CSG and NSG .....................................    229
   11.12.4. Version .........................................    229
     11.12.4.1. Version-max ..................................   229
     11.12.4.2. Version-min ..................................   230
   11.12.5. ISID ............................................    230
   11.12.6. TSIH ............................................    232
   11.12.7. Connection ID - CID .............................    232
   11.12.8. CmdSN ...........................................    232
   11.12.9. ExpStatSN .......................................    233
   11.12.10. Login Parameters ...............................    233
 11.13. Login Response ......................................    233
   11.13.1. Version-max .....................................    234
   11.13.2. Version-active ..................................    235
   11.13.3. TSIH ............................................    235
   11.13.4. StatSN ..........................................    235
   11.13.5. Status-Class and Status-Detail ..................    235
   11.13.6. T (Transit) bit .................................    239
   11.13.7. C (Continue) Bit ................................    240
   11.13.8. Login Parameters ................................    240
 11.14. Logout Request ......................................    240
   11.14.1. Reason Code .....................................    243
   11.14.2. TotalAHSLength and DataSegmentLength ............    244
   11.14.3. CID .............................................    244
   11.14.4. ExpStatSN .......................................    244
   11.14.5. Implicit termination of tasks ...................    244
 11.15. Logout Response .....................................    245




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   11.15.1. Response ........................................     246
   11.15.2. TotalAHSLength and DataSegmentLength ............     247
   11.15.3. Time2Wait .......................................     247
   11.15.4. Time2Retain .....................................     247
 11.16. SNACK Request .......................................     248
   11.16.1. Type ............................................     249
   11.16.2. Data Acknowledgement ............................     250
   11.16.3. Resegmentation ..................................     250
   11.16.4. Initiator Task Tag ..............................     251
   11.16.5. Target Transfer Tag or SNACK Tag ................     251
   11.16.6. BegRun ..........................................     252
   11.16.7. RunLength .......................................     252
 11.17. Reject ..............................................     253
   11.17.1. Reason ..........................................     254
   11.17.2. DataSN/R2TSN ....................................     255
   11.17.3. StatSN, ExpCmdSN and MaxCmdSN ...................     255
   11.17.4. Complete Header of Bad PDU ......................     256
 11.18. NOP-Out .............................................     256
   11.18.1. Initiator Task Tag ..............................     257
   11.18.2. Target Transfer Tag .............................     257
   11.18.3. Ping Data .......................................     258
 11.19. NOP-In ..............................................     259
   11.19.1. Target Transfer Tag .............................     260
   11.19.2. StatSN ..........................................     260
   11.19.3. LUN .............................................     260
12. iSCSI Security Text Keys and Authentication Methods.......... 261
 12.1. AuthMethod ...........................................     261
   12.1.1. Kerberos .........................................     263
   12.1.2. Secure Remote Password (SRP) .....................     264
   12.1.3. Challenge Handshake Authentication Protocol (CHAP)     266
13. Login/Text Operational Text Keys............................. 268
 13.1.   HeaderDigest and DataDigest ..........................   268
 13.2.   MaxConnections .......................................   271
 13.3.   SendTargets ..........................................   271
 13.4.   TargetName ...........................................   271




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 13.5. InitiatorName .........................................   272
 13.6. TargetAlias ...........................................   273
 13.7. InitiatorAlias ........................................   273
 13.8. TargetAddress .........................................   274
 13.9. TargetPortalGroupTag ..................................   275
 13.10. InitialR2T ...........................................   275
 13.11. ImmediateData ........................................   276
 13.12. MaxRecvDataSegmentLength .............................   277
 13.13. MaxBurstLength .......................................   278
 13.14. FirstBurstLength .....................................   278
 13.15. DefaultTime2Wait .....................................   279
 13.16. DefaultTime2Retain ...................................   279
 13.17. MaxOutstandingR2T ....................................   280
 13.18. DataPDUInOrder .......................................   280
 13.19. DataSequenceInOrder ..................................   281
 13.20. ErrorRecoveryLevel ...................................   281
 13.21. SessionType ..........................................   282
 13.22. The Private Extension Key Format .....................   283
 13.23. TaskReporting ........................................   283
 13.24. iSCSIProtocolLevel Negotiation .......................   284
 13.25. Obsoleted Keys .......................................   284
 13.26. X#NodeArchitecture ...................................   285
   13.26.1. Definition .......................................   285
   13.26.2. Implementation Requirements ......................   286
14. Rationale for revised IANA Considerations................. 287

15. IANA Considerations....................................... 289

Appendix A. Examples ......................................... 296
 Read Operation Example ......................................   296
 Write Operation Example .....................................   297
 R2TSN/DataSN Use Examples ...................................   297
 CRC Examples ................................................   301
Appendix B. Login Phase Examples ............................... 303

Appendix C. SendTargets Operation ............................ 313




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Appendix D. Algorithmic Presentation of Error Recovery Classes . 318
   D.2.1. Procedure Descriptions ............................... 320
Appendix E. Clearing Effects of Various Events on Targets ...... 337




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1. Introduction

   The Small Computer Systems Interface (SCSI) is a popular family of
   protocols for communicating with I/O devices, especially storage
   devices. SCSI is a client-server architecture. Clients of a SCSI
   interface are called "initiators". Initiators issue SCSI
   "commands" to request services from components, logical units of a
   server known as a "target". A "SCSI transport" maps the client-
   server SCSI protocol to a specific interconnect. An Initiator is
   one endpoint of a SCSI transport and a target is the other
   endpoint.

   The SCSI protocol has been mapped over various transports,
   including Parallel SCSI, IPI, IEEE-1394 (firewire) and Fibre
   Channel. These transports are I/O specific and have limited
   distance capabilities.

   The iSCSI protocol defined in this document describes a means of
   transporting of the SCSI packets over TCP/IP, providing for an
   interoperable solution which can take advantage of existing
   Internet infrastructure, Internet management facilities and
   address distance limitations.




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2. Acronyms, Definitions and Document Summary

2.1. Acronyms

   Acronym      Definition
   --------------------------------------------------------------
   3DES        Triple Data Encryption Standard
   ACA         Auto Contingent Allegiance
   AEN         Asynchronous Event Notification
   AES         Advanced Encryption Standard
   AH          Additional Header (not the IPsec AH!)
   AHS         Additional Header Segment
   API         Application Programming Interface
   ASC         Additional Sense Code
   ASCII       American Standard Code for Information Interchange
   ASCQ        Additional Sense Code Qualifier
   BHS         Basic Header Segment
   CBC         Cipher Block Chaining
   CD          Compact Disk
   CDB         Command Descriptor Block
   CHAP        Challenge Handshake Authentication Protocol
   CID         Connection ID
   CO          Connection Only
   CRC         Cyclic Redundancy Check
   CRL         Certificate Revocation List
   CSG         Current Stage
   CSM         Connection State Machine
   DES         Data Encryption Standard
   DNS         Domain Name Server
   DOI         Domain of Interpretation
   DVD         Digital Versatile Disk
   EDTL        Expected Data Transfer Length
   ESP         Encapsulating Security Payload
   EUI         Extended Unique Identifier
   FFP         Full Feature Phase
   FFPO        Full Feature Phase Only
   Gbps        Gigabits per Second
   HBA         Host Bus Adapter
   HMAC        Hashed Message Authentication Code
   I_T         Initiator_Target




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  I_T_L       Initiator_Target_LUN
  IANA        Internet Assigned Numbers Authority
  IB          InfiniBand
  ID          Identifier
  IDN         Internationalized Domain Name
  IEEE        Institute of Electrical & Electronics Engineers
  IETF        Internet Engineering Task Force
  IKE         Internet Key Exchange
  I/O         Input-Output
  IO          Initialize Only
  IP          Internet Protocol
  IPsec       Internet Protocol Security
  IPv4        Internet Protocol Version 4
  IPv6        Internet Protocol Version 6
  IQN         iSCSI Qualified Name
  iSCSI       Internet SCSI
  iSER        iSCSI Extensions for RDMA
  ISID        Initiator Session ID
  iSNS        Internet Storage Name Service (see [RFC4171])
  ITN         iSCSI Target Name
  ITT         Initiator Task Tag
  KRB5        Kerberos V5
  LFL         Lower Functional Layer
  LTDS        Logical-Text-Data-Segment
  LO          Leading Only
  LU          Logical Unit
  LUN         Logical Unit Number
  MAC         Message Authentication Codes
  NA          Not Applicable
  NAA         Network Address Authority
  NIC         Network Interface Card
  NOP         No Operation
  NSG         Next Stage
  OS          Operating System
  PDU         Protocol Data Unit
  PKI         Public Key Infrastructure
  R2T         Ready To Transfer
  R2TSN       Ready To Transfer Sequence Number
  RDMA        Remote Direct Memory Access
  RFC         Request For Comments
  SAM         SCSI Architecture Model




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   SAM2        SCSI Architecture Model - 2
   SAN         Storage Area Network
   SAS         Serial Attached SCSI
   SCSI        Small Computer Systems Interface
   SLP         Service Location Protocol
   SN          Sequence Number
   SNACK       Selective Negative Acknowledgment - also
               Sequence Number Acknowledgement for data
   SPDTL       SCSI-Presented Data Transfer Length
   SPKM        Simple Public-Key Mechanism
   SRP         Secure Remote Password
   SSID        Session ID
   SW          Session-Wide
   TCB         Task Control Block
   TCP         Transmission Control Protocol
   TMF         Task Management Function
   TPGT        Target Portal Group Tag
   TSIH        Target Session Identifying Handle
   TTT         Target Transfer Tag
   UA          Unit Attention
   UFL         Upper Functional Layer
   ULP         Upper Level Protocol
   URN         Uniform Resource Names
   UTF         Universal Transformation Format
   WG          Working Group


2.2. Definitions

   - Alias: An alias string can also be associated with an iSCSI
   Node. The alias allows an organization to associate a user-
   friendly string with the iSCSI Name. However, the alias string is
   not a substitute for the iSCSI Name.

   - CID (Connection ID): Connections within a session are identified
   by a connection ID. It is a unique ID for this connection within
   the session for the initiator. It is generated by the initiator
   and presented to the target during login requests and during
   logouts that close connections.

   - Connection: A connection is a TCP connection. Communication
   between the initiator and target occurs over one or more TCP




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  connections. The TCP connections carry control messages, SCSI
  commands, parameters, and data within iSCSI Protocol Data Units
  (iSCSI PDUs).

  - I/O Buffer:A buffer that is used in a SCSI Read or Write
  operation so SCSI data may be sent from or received into that
  buffer. For a read or write data transfer to take place for a
  task, an I/O Buffer is required on the initiator and at least one
  is required on the
  target.

  - INCITS: INCITS stands for InterNational Committee of Information
  Technology Standards. The INCITS has a broad standardization scope
  within the field of Information and Communications Technologies
  (ICT), encompassing storage, processing, transfer, display,
  management, organization, and retrieval of information. INCITS
  serves as ANSI's Technical Advisory Group for the ISO/IEC Joint
  Technical Committee 1 (JTC 1). See http://www.incits.org.

  - InfiniBand: An I/O architecture originally intended to replace
  PCI and to address high performance server interconnectivity [IB].

  - iSCSI Device: A SCSI Device using an iSCSI service delivery
  subsystem. Service Delivery Subsystem is defined by [SAM2] as a
  transport mechanism for SCSI commands and responses.

  - iSCSI Initiator Name: The iSCSI Initiator Name specifies the
  worldwide unique name of the initiator.

  - iSCSI Initiator Node: The "initiator" device. The word
  "initiator" has been appropriately qualified as either a port or a
  device in the rest of the document when the context is ambiguous.
  All unqualified usages of "initiator" refer to an initiator port
  (or device) depending on the context.

  - iSCSI Layer: This layer builds/receives iSCSI PDUs and
  relays/receives them to/from one or more TCP connections that form
  an initiator-target "session".

  - iSCSI Name: The name of an iSCSI initiator or iSCSI target.




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  - iSCSI Node: The iSCSI Node represents a single iSCSI initiator
  or iSCSI target or a single instance of each. There are one or
  more iSCSI Nodes within a Network Entity. The iSCSI Node is
  accessible via one or more Network Portals. An iSCSI Node is
  identified by its iSCSI Name. The separation of the iSCSI Name
  from the addresses used by and for the iSCSI Node allows multiple
  iSCSI nodes to use the same address, and the same iSCSI node to
  use multiple addresses.

  - iSCSI Target Name: The iSCSI Target Name specifies the worldwide
  unique name of the target.

  - iSCSI Target Node: The "target" device. The word "target" has
  been appropriately qualified as either a port or a device in the
  rest of the document when the context is ambiguous. All
  unqualified usages of "target" refer to a target port (or device)
  depending on the context.

  - iSCSI Task: An iSCSI task is an iSCSI request for which a
  response is expected.

  - iSCSI Transfer Direction: The iSCSI transfer direction is
  defined with regard to the initiator. Outbound or outgoing
  transfers are transfers from the initiator to the target, while
  inbound or incoming transfers are from the target to the
  initiator.

  - ISID: The initiator part of the Session Identifier. It is
  explicitly specified by the initiator during Login.

  - I_T nexus: According to [SAM2], the I_T nexus is a relationship
  between a SCSI Initiator Port and a SCSI Target Port. For iSCSI,
  this relationship is a session, defined as a relationship between
  an iSCSI Initiator's end of the session (SCSI Initiator Port) and
  the iSCSI Target's Portal Group. The I_T nexus can be identified
  by the conjunction of the SCSI port names; that is, the I_T nexus
  identifier is the tuple (iSCSI Initiator Name + ',i,'+ ISID, iSCSI
  Target Name + ',t,'+ Portal Group Tag).




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  - I_T_L nexus: An I_T_L nexus is a SCSI concept, and is defined as
  the relationship between a SCSI Initiator Port, a SCSI Target
  Port, and a Logical Unit (LU).


  - NAA: Network Address Authority, a naming format defined by the
  INCITS T11 Fibre Channel protocols [FC-FS3].

  - Network Entity: The Network Entity represents a device or
  gateway that is accessible from the IP network. A Network Entity
  must have one or more Network Portals, each of which can be used
  to gain access to the IP network by some iSCSI Nodes contained in
  that Network Entity.

  - Network Portal: The Network Portal is a component of a Network
  Entity that has a TCP/IP network address and that may be used by
  an iSCSI Node within that Network Entity for the connection(s)
  within one of its iSCSI sessions. A Network Portal in an initiator
  is identified by its IP address. A Network Portal in a target is
  identified by its IP address and its listening TCP port.

  - Originator: In a negotiation or exchange, the party that
  initiates the negotiation or exchange.

  - PDU (Protocol Data Unit): The initiator and target divide their
  communications into messages. The term "iSCSI protocol data unit"
  (iSCSI PDU) is used for these messages.

  - Portal Groups: iSCSI supports multiple connections within the
  same session; some implementations will have the ability to
  combine connections in a session across multiple Network Portals.
  A Portal Group defines a set of Network Portals within an iSCSI
  Network Entity that collectively supports the capability of
  coordinating a session with connections spanning these portals.
  Not all Network Portals within a Portal Group need participate in
  every session connected through that Portal Group. One or more
  Portal Groups may provide access to an iSCSI Node. Each Network
  Portal, as utilized by a given iSCSI Node, belongs to exactly one
  portal group within that node.




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  - Portal Group Tag: This 16-bit quantity identifies a Portal Group
  within an iSCSI Node. All Network Portals with the same portal
  group tag in the context of a given iSCSI Node are in the same
  Portal Group.

  - Recovery R2T: An R2T generated by a target upon detecting the
  loss of one or more Data-Out PDUs through one of the following
  means: a digest error, a sequence error, or a sequence reception
  timeout. A recovery R2T carries the next unused R2TSN, but
  requests all or part of the data burst that an earlier R2T (with a
  lower R2TSN) had already requested.

  - Responder: In a negotiation or exchange, the party that responds
  to the originator of the negotiation or exchange.

  - SAS: Serial Attached SCSI. The Serial Attached SCSI (SAS)
  standard contains both a physical layer compatible with Serial
  ATA, and protocols for transporting SCSI commands to SAS devices
  and ATA commands to SATA devices [SAS].

  - SCSI Device: This is the SAM2 term for an entity that contains
  one or more SCSI ports that are connected to a service delivery
  subsystem and supports a SCSI application protocol. For example, a
  SCSI Initiator Device contains one or more SCSI Initiator Ports
  and zero or more application clients. A Target Device contains one
  or more SCSI Target Ports and one or more device servers and
  associated logical units. For iSCSI, the SCSI Device is the
  component within an iSCSI Node that provides the SCSI
  functionality. As such, there can be, at most, one SCSI Device
  within a given iSCSI Node. Access to the SCSI Device can only be
  achieved in an iSCSI normal operational session. The SCSI Device
  Name is defined to be the iSCSI Name of the node.

  - SCSI Layer: This builds/receives SCSI CDBs (Command Descriptor
  Blocks) and relays/receives them with the remaining command
  execute [SAM2] parameters to/from the iSCSI Layer.

  - Session: The group of TCP connections that link an initiator
  with a target form a session (loosely equivalent to a SCSI I-T
  nexus). TCP connections can be added and removed from a session.




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  Across all connections within a session, an initiator sees one and
  the same target.

  - SCSI Port: This is the SAM2 term for an entity in a SCSI Device
  that provides the SCSI functionality to interface with a service
  delivery subsystem. For iSCSI, the definition of the SCSI
  Initiator Port and the SCSI Target Port are different.

  - SCSI Initiator Port: This maps to the endpoint of an iSCSI
  normal operational session. An iSCSI normal operational session is
  negotiated through the login process between an iSCSI initiator
  node and an iSCSI target node. At successful completion of this
  process, a SCSI Initiator Port is created within the SCSI
  Initiator Device. The SCSI Initiator Port Name and SCSI Initiator
  Port Identifier are both defined to be the iSCSI Initiator Name
  together with (a) a label that identifies it as an initiator port
  name/identifier and (b) the ISID portion of the session
  identifier.

  - SCSI Port Name: A name consisting of UTF-8 [RFC3629] encoding of
  Unicode [UNICODE] characters and includes the iSCSI Name + 'i' or
  't' + ISID or Portal Group Tag.

  - SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the
  aggregate data length of the data that the SCSI layer logically
  "presents" to the iSCSI layer for a Data-In or Data-Out transfer
  in the context of a SCSI task. For a bidirectional task, there are
  two SPDTL values -- one for Data-In and one for Data-Out. Note
  that the notion of "presenting" includes immediate data per the
  data transfer model in [SAM2], and excludes overlapping data
  transfers, if any, requested by the SCSI layer.

  - SCSI Target Port: This maps to an iSCSI Target Portal Group.

  - SCSI Target Port Name and SCSI Target Port Identifier: These are
  both defined to be the iSCSI Target Name together with (a) a label
  that identifies it as a target port name/identifier and (b) the
  portal group tag.

  - SSID (Session ID): A session between an iSCSI initiator and an
  iSCSI target is defined by a session ID that is a tuple composed




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  of an initiator part (ISID) and a target part (Target Portal Group
  Tag). The ISID is explicitly specified by the initiator at session
  establishment. The Target Portal Group Tag is implied by the
  initiator through the selection of the TCP endpoint at connection
  establishment. The TargetPortalGroupTag key must also be returned
  by the target as a confimation during connection establishment.

  - T10: A technical committee within INCITS that develops standards
  and technical reports on I/O interfaces, particularly the series
  of SCSI (Small Computer Systems Interface) standards. See
  http://www.t10.org.

  - T11: A technical committee within INCITS responsible for
  standards development in the areas of Intelligent Peripheral
  Interface (IPI), High-Performance Parallel Interface (HIPPI) and
  Fibre Channel (FC). See http://www.t11.org.

  - Target Portal Group Tag: A numerical identifier (16-bit) for an
  iSCSI Target Portal Group.

  -Target Transfer Tag (TTT): An iSCSI protocol field used in a few
  iSCSI PDUs (e.g. R2T, NOP-In) which is always sent from the target
  to the initiator first and then quoted as a reference in
  initiator-sent PDUs back to the target relating to the same
  task/exchange. So effectively, TTT acts as an opaque handle to an
  existing task/exchange to help target associate the incoming PDUs
  from the initiator to the proper execution context.

  - Third-party: A term used in this document as a qualifier to
  nexus objects (I_T or I_T_L) and iSCSI sessions, to indicate that
  these objects and sessions reap the side effects of actions that
  take place in the context of a separate iSCSI session. One
  example of a third-party session is an iSCSI session discovering
  that its I_T_L nexus to an LU got reset due to an LU Reset
  operation orchestrated via a separate I_T nexus.

  - TSIH (Target Session Identifying Handle): A target assigned tag
  for a session with a specific named initiator. The target
  generates it during session establishment. Other than defining it
  as a 16 bit binary string, its internal format and content are not
  defined by this protocol but for the all 0 value that is reserved




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  and used by the initiator to indicate a new session. It is given
  to the target during additional connection establishment for the
  same session.


2.3. Summary of Changes

  1)   Consolidated RFCs 3720, 3980, 4850 and 5048, and made the
     necessary editorial changes
  2)   iSCSIProtocolLevel is specified as "1" in Section 13.24, and
       added a related normative reference to [iSCSI-SAM] draft
  3)   Markers and related keys were removed
  4)   SPKM authentication and related keys were removed
  5)   Added a new Section 13.25 on responding to obsoleted keys
  6)   Have explicitly allowed initiator+target implementations
       throughout the text
  7)   Clarified in Section 4.2.7 that implementations SHOULD NOT
       rely on SLP-based discovery
  8)   Added UML diagrams and related conventions in Section 3
  9)   FastAbort implementation is made a "SHOULD" requirement in
     Section 4.2.3.4 from the previous "MUST" requirement.
  10) Required in Section 4.2.7.1 that iSCSI Target Name must be
     the same as iSCSI Initiator Name for SCSI (composite) devices
     with both roles
  11) Changed the "MUST NOT" to "should avoid" in Section 4.2.7.2
     regarding usage of characters such as punctuation marks in
     iSCSI Names.
  12) Updated Section 9.3 to require the following: MUST implement
     IPsec, 2400-series RFCs (IPsec v2, IKEv1) and SHOULD implement
     IPsec, 4300-series RFCs (IPsec v3, IKEv2).
  13) Clarified in Section 10.2 that ACA is a SHOULD requirement
     only for iSCSI targets
  14) Prohibited usage of X# name prefix for new public keys in
     Section 6.2
  15) Prohibited usage of Y# name prefix for new digest extensions
     in Section 13.1, and Z# name prefix for new authentication
     method extensions in Section 12.1
  16) Added a SHOULD requirement in Section 6.2 that initiators and
     targets support at least six (6) exchanges during text
     negotiation.
  17) Added a clarification that Appendix.C is normative.




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   18)  Added a normative requirement on [IPSEC-IPS] draft, and made
      a few related changes in Section 9.3 to align the text in this
      document with that of [IPSEC-IPS]
   19) Added a new Section 9.2.3 covering Kerberos authentication
      considerations

2.4. Conventions

   In examples, "I->" and "T->" show iSCSI PDUs sent by the initiator
   and target respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
   in this document are to be interpreted as described in RFC 2119
   [RFC2119].




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3. UML Conventions

3.1. UML Conventions Overview

   The SCSI Architecture Model (SAM) uses class diagrams and object
   diagrams with notation that is based on the Unified Modeling
   Language [UML]. Therefore, this document also uses UML to model
   the relationships for SCSI and iSCSI objects.

   A treatise on the graphical notation used in UML is beyond the
   scope of this document. However, given the use of ASCII drawing
   for UML static class diagrams, a description of the notational
   conventions used in this document is included in the remainder of
   this Section.

3.2. Multiplicity Notion

   Not specified     The number of instances of an attribute is not
   specified.

   1    One instance of the class or attribute exists.

   0..* Zero or more instances of the class or attribute exist.

   1..* One or more instances of the class or attribute exist.

   0..1 Zero or one instance of the class or attribute exists.

   n..m              n to m instances of the class or attribute exist
   (e.g., 2..8).

   x, n..m        Multiple disjoint instances of the class or
   attribute exist (e.g., 2, 8..15).




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3.3. Class Diagram Conventions

  +--------------+    +--------------+       +--------------+
  | Class Name |      | Class Name |         | Class Name |
  +--------------+    +--------------+       +--------------+
  |              |    |              |
  +--------------+    +--------------+
  |              |
  +--------------+
  The previous three diagrams are examples of a class with no
  attributes and with no operations.


  +-------------------+    +-------------------+
  |    Class Name     |    |    Class Name     |
  +-------------------+    +-------------------+
  | attribute 01[1] |      | attribute 01[1] |
  | attribute 02[1] |      | attribute 02[1] |
  +-------------------+    +-------------------+
  |                   |
  +-------------------+
  The preceding two diagrams are examples of a class with attributes
  and with no operations.


  +------------------------+
  |      Class Name        |
  +------------------------+
  |    attribute 01[1..*] |
  |    attribute 02[1]     |
  +------------------------+
  |    operation 01()      |
  |    operation 02()      |
  +------------------------+
  The preceding diagram is an example of a class with attributes
  that have a specified multiplicity and operations.




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3.4. Class Diagram Notation for Associations

  +-----------------+
  |     Class A     |
  +-----------------+ association_name +-----------------+
  | attribute 01[1] |<------------------>|     Class B     |
  | attribute 02[1] | 1..*         0..1 +-----------------+
  +-----------------+                    | attribute 03[1] |
  | operation 1() |                      +-----------------+
  +-----------------+
  The preceding diagram is an example where Class A knows about
  Class B (i.e., read as "Class A association_name ClassB") and
  Class B knows about Class A (i.e., read as "Class B
  association_name Class A"). The use of association_name is
  optional. The multiplicity notation (1..* and 0..1) indicates the
  number of instances of the object.

  +--------------------+
  |      Class A       |
  +--------------------+              +--------------------+
  | attribute 01[1]    |<-------------|      Class B       |
  | attribute 02[1]    | 1      0..1 +--------------------+
  +--------------------+              | attribute 03[1]    |
  | operation 1()      |              +--------------------+
  +--------------------+
  The preceding diagram is an example where Class B knows about
  Class A (i.e., read as "Class B knows about Class A") but Class A
  does not know about Class B.

  +----------------------+
  |       Class A        |
  +----------------------+            +--------------------+
  |   attribute 01[1]    |----------->|      Class B       |
  |   attribute 02[1]    | 0..*     1 +--------------------+
  +----------------------+            | attribute 03[1]    |
  |    operation 1()     |            +--------------------+
  +----------------------+
  The preceding diagram is an example where Class A knows about
  Class B (i.e., read as "Class A knows about Class B") but Class B
  does not know about Class A.




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3.5. Class Diagram Notation for Aggregations

  +---------------+             +--------------+
  | Class whole |o------------| Class part |
  +---------------+             +--------------+
  The preceding diagram is an example where Class whole is an
  aggregate that contains Class part and where Class part may
  continue to exist even if Class whole is removed (i.e., read as
  "the whole contains the part").

  +---------------+             +--------------+
  | Class whole |@------------| Class part |
  +---------------+             +--------------+
  The preceding diagram is an example where Class whole is an
  aggregate that contains Class part where Class part only belongs
  to one Class whole, and the Class part does not continue to exist
  if the Class whole is removed (i.e., read as "the whole contains
  the part").

  +-------------+
  |             |
  +-------------+
     |       |
     + =(a)= +
     |       |
  The preceding diagram is an example where there is a constraint
  between the associations where the (a) footnote describes the
  constraint.

3.6. Class Diagram Notation for Generalizations

  +---------------+
  | Superclass    |
  +-------^-------+
         /_\
          |
  +---------------+
  |    Subclass   |
  +---------------+
  The preceding diagram is an example where the subclass is a kind
  of superclass. A subclass shares all the attributes and




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  operations of the superclass (i.e., the subclass inherits from the
  superclass).




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4. Overview

4.1. SCSI Concepts

   The SCSI Architecture Model-2 [SAM2] describes in detail the
   architecture of the SCSI family of I/O protocols. This Section
   provides a brief background of the SCSI architecture and is
   intended to familiarize readers with its terminology.

   At the highest level, SCSI is a family of interfaces for
   requesting services from I/O devices, including hard drives, tape
   drives, CD and DVD drives, printers, and scanners. In SCSI
   terminology, an individual I/O device is called a "logical unit"
   (LU).

   SCSI is a client-server architecture. Clients of a SCSI interface
   are called "initiators". Initiators issue SCSI "commands" to
   request services from components, logical units, of a server known
   as a "target". The "device server" on the logical unit accepts
   SCSI commands and processes them.

   A "SCSI transport" maps the client-server SCSI protocol to a
   specific interconnect. Initiator is one endpoint of a SCSI
   transport. The "target" is the other endpoint. A target can
   contain multiple Logical Units (LUs). Each Logical Unit has an
   address within a target called a Logical Unit Number (LUN).

   A SCSI task is a SCSI command or possibly a linked set of SCSI
   commands. Some LUs support multiple pending (queued) tasks, but
   the queue of tasks is managed by the logical unit. The target uses
   an initiator provided "task tag" to distinguish between tasks.
   Only one command in a task can be outstanding at any given time.

   Each SCSI command results in an optional data phase and a required
   response phase. In the data phase, information can travel from the
   initiator to target (e.g., WRITE), target to initiator (e.g.,
   READ), or in both directions. In the response phase, the target
   returns the final status of the operation, including any errors.

   Command Descriptor Blocks (CDB) are the data structures used to
   contain the command parameters that an initiator sends to a




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  target. The CDB content and structure is defined by [SAM2] and
  device-type specific SCSI standards.


4.2. iSCSI Concepts and Functional Overview

  The iSCSI protocol is a mapping of the SCSI command, event and
  task management model (see [SAM2]) over the TCP protocol. SCSI
  commands are carried by iSCSI requests and SCSI responses and
  status are carried by iSCSI responses. iSCSI also uses the request
  response mechanism for iSCSI protocol mechanisms.

  For the remainder of this document, the terms "initiator" and
  "target" refer to "iSCSI initiator node" and "iSCSI target node",
  respectively (see iSCSI) unless otherwise qualified.

  As its title suggests, Section 4 presents an overview of the iSCSI
  concepts, and later Sections in the rest of the specification
  contain the normative requirements - in many cases covering the
  same concepts discussed in Section 4. Such normative requirements
  text overrides the overview text in Section 4 if there is a
  disagreement between the two.

  In keeping with similar protocols, the initiator and target divide
  their communications into messages. This document uses the term
  "iSCSI protocol data unit" (iSCSI PDU) for these messages.

  For performance reasons, iSCSI allows a "phase-collapse". A
  command and its associated data may be shipped together from
  initiator to target, and data and responses may be shipped
  together from targets.

  The iSCSI transfer direction is defined with respect to the
  initiator. Outbound or outgoing transfers are transfers from an
  initiator to a target, while inbound or incoming transfers are
  from a target to an initiator.

  An iSCSI task is an iSCSI request for which a response is
  expected.

  In this document "iSCSI request", "iSCSI command", request, or
  (unqualified) command have the same meaning. Also, unless




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  otherwise specified, status, response, or numbered response have
  the same meaning.

4.2.1. Layers and Sessions

  The following conceptual layering model is used to specify
  initiator and target actions and the way in which they relate to
  transmitted and received Protocol Data Units:

     - The SCSI layer builds/receives SCSI CDBs (Command Descriptor
       Blocks) and passes/receives them with the remaining command
       execute parameters ([SAM2]) to/from

     - the iSCSI layer that builds/receives iSCSI PDUs and
       relays/receives them to/from one or more TCP connections;
       the group of connections form an initiator-target "session".


  Communication between the initiator and target occurs over one or
  more TCP connections. The TCP connections carry control messages,
  SCSI commands, parameters, and data within iSCSI Protocol Data
  Units (iSCSI PDUs). The group of TCP connections that link an
  initiator with a target form a session (equivalent to a SCSI I_T
  nexus, see Section 4.4.2). A session is defined by a session ID
  that is composed of an initiator part and a target part. TCP
  connections can be added and removed from a session. Each
  connection within a session is identified by a connection ID
  (CID).

  Across all connections within a session, an initiator sees one
  "target image". All target identifying elements, such as LUN, are
  the same. A target also sees one "initiator image" across all
  connections within a session. Initiator-identifying elements, such
  as the Initiator Task Tag, are global across the session
  regardless of the connection on which they are sent or received.

  iSCSI targets and initiators MUST support at least one TCP
  connection and MAY support several connections in a session. For
  error recovery purposes, targets and initiators that support a
  single active connection in a session SHOULD support two
  connections during recovery.




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4.2.2. Ordering and iSCSI Numbering

  iSCSI uses Command and Status numbering schemes and a Data
  sequencing scheme.

  Command numbering is session-wide and is used for ordered command
  delivery over multiple connections. It can also be used as a
  mechanism for command flow control over a session.

  Status numbering is per connection and is used to enable missing
  status detection and recovery in the presence of transient or
  permanent communication errors.

  Data sequencing is per command or part of a command (R2T-triggered
  sequence) and is used to detect missing data and/or R2T PDUs due
  to header digest errors.

  Typically, fields in the iSCSI PDUs communicate the Sequence
  Numbers between the initiator and target. During periods when
  traffic on a connection is unidirectional, iSCSI NOP-Out/In PDUs
  may be utilized to synchronize the command and status ordering
  counters of the target and initiator.

  The iSCSI session abstraction is equivalent to the SCSI I_T nexus,
  and the iSCSI session provides an ordered command delivery from
  the SCSI initiator to the SCSI target. For detailed design
  considerations that led to the iSCSI session model as it is
  defined here and how it relates the SCSI command ordering features
  defined in SCSI specifications to the iSCSI concepts see
  [RFC3783].

4.2.2.1. Command Numbering and Acknowledging

  iSCSI performs ordered command delivery within a session. All
  commands (initiator-to-target PDUs) in transit from the initiator
  to the target are numbered.

  iSCSI considers a task to be instantiated on the target in
  response to every request issued by the initiator. A set of task
  management operations including abort and reassign (see Section
  11.5) may be performed on an iSCSI task - however an abort
  operation cannot be performed on a task management operation, and




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  usage of reassign operation has certain constraints. See Section
  11.5.1 for the details.

  Some iSCSI tasks are SCSI tasks, and many SCSI activities are
  related to a SCSI task ([SAM2]). In all cases, the task is
  identified by the Initiator Task Tag for the life of the task.

  The command number is carried by the iSCSI PDU as CmdSN (Command-
  Sequence-Number). The numbering is session-wide. Outgoing iSCSI
  PDUs carry this number. The iSCSI initiator allocates CmdSNs with
  a 32-bit unsigned counter (modulo 2**32). Comparisons and
  arithmetic on CmdSN use Serial Number Arithmetic as defined in
  [RFC1982] where SERIAL_BITS = 32.

  Commands meant for immediate delivery are marked with an immediate
  delivery flag; they MUST also carry the current CmdSN. CmdSN MUST
  NOT advance after a command marked for immediate delivery is sent.

  Command numbering starts with the first login request on the first
  connection of a session (the leading login on the leading
  connection) and CmdSN MUST be incremented by 1, in a Serial Number
  Arithmetic sense as defined in [RFC1982], for every non-immediate
  command issued afterwards.

  If immediate delivery is used with task management commands, these
  commands may reach the target before the tasks on which they are
  supposed to act. However their CmdSN serves as a marker of their
  position in the stream of commands. The initiator and target MUST
  ensure that the SCSI task management functions specified in [SAM2]
  act in accordance with the [SAM2] specification. For example, both
  commands and responses appear as if delivered in order. Whenever
  CmdSN for an outgoing PDU is not specified by an explicit rule,
  CmdSN will carry the current value of the local CmdSN variable
  (see later in this Section).

  The means by which an implementation decides to mark a PDU for
  immediate delivery or by which iSCSI decides by itself to mark a
  PDU for immediate delivery are beyond the scope of this document.

  The number of commands used for immediate delivery is not limited
  and their delivery to execution is not acknowledged through the




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  numbering scheme. An iSCSI target MAY reject immediate commands,
  e.g., due to lack of resources to accommodate additional commands.
  An iSCSI target MUST be able to handle at least one immediate task
  management command and one immediate non-task-management iSCSI
  command per connection at any time.

  In this document, delivery for execution means delivery to the
  SCSI execution engine or an iSCSI protocol specific execution
  engine (e.g., for text requests with public or private extension
  keys involving an execution component). With the exception of the
  commands marked for immediate delivery, the iSCSI target layer
  MUST deliver the commands for execution in the order specified by
  CmdSN. Commands marked for immediate delivery may be delivered by
  the iSCSI target layer for execution as soon as detected. iSCSI
  may avoid delivering some commands to the SCSI target layer if
  required by a prior SCSI or iSCSI action (e.g., CLEAR TASK SET
  Task Management request received before all the commands on which
  it was supposed to act).

  On any connection, the iSCSI initiator MUST send the commands in
  increasing order of CmdSN, except for commands that are
  retransmitted due to digest error recovery and connection
  recovery.

  For the numbering mechanism, the initiator and target maintain the
  following three variables for each session:

      - CmdSN - the current command Sequence Number, advanced by 1
       on each command shipped except for commands marked for
       immediate delivery (see Section 4.2.2.1). CmdSN always
       contains the number to be assigned to the next Command PDU.

      - ExpCmdSN - the next expected command by the target. The
       target acknowledges all commands up to, but not including,
       this number. The initiator treats all commands with CmdSN
       less than ExpCmdSN as acknowledged. The target iSCSI layer
       sets the ExpCmdSN to the largest non-immediate CmdSN that it
       can deliver for execution "plus 1" per [RFC1982]. There
       MUST NOT be any holes in the acknowledged CmdSN sequence.




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      - MaxCmdSN - the maximum number to be shipped. The queuing
       capacity of the receiving iSCSI layer is MaxCmdSN - ExpCmdSN
       + 1.

  The initiator's ExpCmdSN and MaxCmdSN are derived from target-to-
  initiator PDU fields. Comparisons and arithmetic on ExpCmdSN and
  MaxCmdSN MUST use Serial Number Arithmetic as defined in [RFC1982]
  where SERIAL_BITS = 32.

  The target MUST NOT transmit a MaxCmdSN that is less than
  ExpCmdSN-1. For non-immediate commands, the CmdSN field can take
  any value from ExpCmdSN to MaxCmdSN inclusive. The target MUST
  silently ignore any non-immediate command outside of this range or
  non-immediate duplicates within the range. The CmdSN carried by
  immediate commands may lie outside the ExpCmdSN to MaxCmdSN range.
  For example, if the initiator has previously sent a non-immediate
  command carrying the CmdSN equal to MaxCmdSN, the target window is
  closed. For group task management commands issued as immediate
  commands, CmdSN indicates the scope of the group action (e.g., on
  ABORT TASK SET indicates which commands are to be aborted).

  MaxCmdSN and ExpCmdSN fields are processed by the initiator as
  follows:

    -If the PDU MaxCmdSN is less than the PDU ExpCmdSN-1 (in
      Serial Arithmetic Sense), they are both ignored.

    -If the PDU MaxCmdSN is greater than the local MaxCmdSN (in
      Serial Arithmetic Sense), it updates the local MaxCmdSN;
      otherwise, it is ignored.

    -If the PDU ExpCmdSN is greater than the local ExpCmdSN (in
      Serial Arithmetic Sense), it updates the local ExpCmdSN;
      otherwise, it is ignored.


  This sequence is required because updates may arrive out of order
  (e.g., the updates are sent on different TCP connections).

  iSCSI initiators and targets MUST support the command numbering
  scheme.




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  A numbered iSCSI request will not change its allocated CmdSN,
  regardless of the number of times and circumstances in which it is
  reissued (see Section 7.2.1). At the target, CmdSN is only
  relevant while the command has not created any state related to
  its execution (execution state); afterwards, CmdSN becomes
  irrelevant. Testing for the execution state (represented by
  identifying the Initiator Task Tag) MUST precede any other action
  at the target. If no execution state is found, it is followed by
  ordering and delivery. If an execution state is found, it is
  followed by delivery if it has not already been delivered.

  If an initiator issues a command retry for a command with CmdSN R
  on a connection when the session CmdSN value is Q, it MUST NOT
  advance the CmdSN past R + 2**31 -1 unless the connection is no
  longer operational (i.e., it has returned to the FREE state, see
  Section 8.1.3), the connection has been reinstated (see Section
  6.3.4), or a non-immediate command with CmdSN equal or greater
  than Q was issued subsequent to the command retry on the same
  connection and the reception of that command is acknowledged by
  the target (see Section 10.4).

  A target command response or Data-in PDU with status MUST NOT
  precede the command acknowledgement. However, the acknowledgement
  MAY be included in the response or the Data-in PDU.

4.2.2.2. Response/Status Numbering and Acknowledging

  Responses in transit from the target to the initiator are
  numbered. The StatSN (Status Sequence Number) is used for this
  purpose. StatSN is a counter maintained per connection. ExpStatSN
  is used by the initiator to acknowledge status. The status
  sequence number space is 32-bit unsigned-integers and the
  arithmetic operations are the regular mod(2**32) arithmetic.

  Status numbering starts with the Login response to the first Login
  request of the connection. The Login response includes an initial
  value for status numbering (any initial value is valid).

  To enable command recovery, the target MAY maintain enough state
  information for data and status recovery after a connection
  failure. A target doing so can safely discard all of the state
  information maintained for recovery of a command after the




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  delivery of the status for the command (numbered StatSN) is
  acknowledged through ExpStatSN.

  A large absolute difference between StatSN and ExpStatSN may
  indicate a failed connection. Initiators MUST undertake recovery
  actions if the difference is greater than an implementation
  defined constant that MUST NOT exceed 2**31-1.

  Initiators and Targets MUST support the response-numbering scheme.

4.2.2.3. Response Ordering

4.2.2.3.1. Need for Response Ordering

  Whenever an iSCSI session is composed of multiple connections, the
  Response PDUs (task responses or TMF responses) originating in
  the target SCSI layer are distributed onto the multiple
  connections by the target iSCSI layer according to iSCSI
  connection allegiance rules. This process generally may not
  preserve the ordering of the responses by the time they are
  delivered to the initiator SCSI layer.

  Since ordering is not expected across SCSI responses anyway, this
  approach works fine in the general case. However, to address the
  special cases where some ordering is desired by the SCSI layer, we
  introduce the notion of a "Response Fence": Response Fence is
  logically the attribute/property of a SCSI response message handed
  off to a target iSCSI layer which indicates that there are special
  SCSI-level ordering considerations associated with this particular
  response message - whenever Response Fence is set or required on a
  SCSI response message, we define the semantics in 4.2.2.3.2 with
  respect to target iSCSI layer's handling of such SCSI response
  messages.

4.2.2.3.2. Response Ordering Model Description

  The target SCSI protocol layer hands off the SCSI response
  messages to the target iSCSI layer by invoking the "Send Command
  Complete" protocol data service ([SAM2], clause 5.4.2) and "Task
  Management Function Executed" ([SAM2], clause 6.9) service. On
  receiving the SCSI response message, the iSCSI layer exhibits the
  Response Fence behavior for certain SCSI response messages




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  (Section 4.2.2.3.4 describes the specific instances where the
  semantics must be realized).

  Whenever the Response Fence behavior is required for a SCSI
  response message, the target iSCSI layer MUST ensure that the
  following conditions are met in delivering the response message to
  the initiator iSCSI layer:

     - Response with Response Fence MUST be delivered
       chronologically after all the "preceding" responses on the
       I_T_L nexus, if the preceding responses are delivered at
       all, to the initiator iSCSI layer.

     - Response with Response Fence MUST be delivered
       chronologically prior to all the "following" responses on
       the I_T_L nexus.


  The "preceding" and "following" notions refer to the order of
  handoff of a response message from the target SCSI protocol layer
  to the target iSCSI layer.

4.2.2.3.3. iSCSI Semantics with the Interface Model

  Whenever the TaskReporting key (Section 13.23) is negotiated to
  ResponseFence or FastAbort for an iSCSI session and the Response
  Fence behavior is required for a SCSI response message, the target
  iSCSI layer MUST perform the actions described in this Section for
  that session.

       a) If it is a single-connection session, no special
          processing is required. The standard SCSI Response PDU
          build and dispatch process happens.
       b) If it is a multi-connection session, the target iSCSI
          layer takes note of the last-sent and unacknowledged
          StatSN on each of the connections in the iSCSI session,
          and waits for an acknowledgement (NOP-In PDUs MAY be used
          to solicit acknowledgements as needed in order to
          accelerate this process) of each such StatSN to clear the
          fence. The SCSI response requiring Response Fence
          behavior MUST NOT be sent to the initiator before




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          acknowledgements are received for each of the
          unacknowledged StatSNs.
       c) The target iSCSI layer must wait for an acknowledgement
          of the SCSI Response PDU that carried the SCSI response
          requiring the Response Fence behavior. The fence MUST be
          considered cleared only after receiving the
          acknowledgement.
       d) All further status processing for the LU is resumed only
          after clearing the fence. If any new responses for the
          I_T_L nexus are received from the SCSI layer before the
          fence is cleared, those Response PDUs MUST be held and
          queued at the iSCSI layer until the fence is cleared.

4.2.2.3.4. Current List of Fenced Response Use Cases

  This Section lists the situations in which fenced response
  behavior is REQUIRED in iSCSI target implementations. Note that
  the following list is an exhaustive enumeration as currently
  identified - it is expected that as SCSI protocol specifications
  evolve, the specifications will specify when response fencing is
  required on a case-by-case basis.

  Whenever the TaskReporting key (Section 13.23) is negotiated to
  ResponseFence or FastAbort for an iSCSI session, the target iSCSI
  layer MUST assume that the Response Fence is required for the
  following SCSI completion messages:

     1. The first completion message carrying the UA after the
        multi-task abort on issuing and third-party sessions. See
        Section 4.2.3.2 for related TMF discussion.

     2. The TMF Response carrying the multi-task TMF Response on
        the issuing session.

     3. The completion message indicating ACA establishment on the
        issuing session.

     4. The first completion message carrying the ACA ACTIVE status
        after ACA establishment on issuing and third-party
        sessions.




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     5. The TMF Response carrying the Clear ACA response on the
        issuing session.

     6. The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
        command.

  Note:
    - Due to the absence of ACA-related fencing requirements in
      [RFC3720], initiator implementations SHOULD NOT use ACA on
      multi-connection iSCSI sessions with targets complying only
      with [RFC3720]. This can be determined via TaskReporting key
      (Section 13.23) negotiation - when the negotiation results
      in either "RFC3720" or "NotUnderstood".

     - Initiators that want to employ ACA on multi-connection iSCSI
       sessions SHOULD first assess response-fencing behavior via
       negotiating for "ResponseFence" or "FastAbort" value for the
       TaskReporting (Section 13.23) key.

4.2.2.4. Data Sequencing

  Data and R2T PDUs transferred as part of some command execution
  MUST be sequenced. The DataSN field is used for data sequencing.
  For input (read) data PDUs, DataSN starts with 0 for the first
  data PDU of an input command and advances by 1 for each subsequent
  data PDU. For output data PDUs, DataSN starts with 0 for the first
  data PDU of a sequence (the initial unsolicited sequence or any
  data PDU sequence issued to satisfy an R2T) and advances by 1 for
  each subsequent data PDU. R2Ts are also sequenced per command. For
  example, the first R2T has an R2TSN of 0 and advances by 1 for
  each subsequent R2T. For bidirectional commands, the target uses
  the DataSN/R2TSN to sequence Data-In and R2T PDUs in one
  continuous sequence (undifferentiated). Unlike command and status,
  data PDUs and R2Ts are not acknowledged by a field in regular
  outgoing PDUs. Data-In PDUs can be acknowledged on demand by a
  special form of the SNACK PDU. Data and R2T PDUs are implicitly
  acknowledged by status for the command. The DataSN/R2TSN field
  enables the initiator to detect missing data or R2T PDUs.

  For any read or bidirectional command, a target MUST issue less
  than 2**32 combined R2T and Data-In PDUs. Any output data sequence
  MUST contain less than 2**32 Data-Out PDUs.




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4.2.3. iSCSI Task Management

4.2.3.1. Task Management Overview

  iSCSI task management features allow an initiator to control the
  active iSCSI tasks on an operational iSCSI session that it has
  with an iSCSI target. Section 11.5 defines the task management
  function types that this specification defines - ABORT TASK, ABORT
  TASK SET, CLEAR ACA, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET
  WARM RESET, TARGET COLD RESET, and TASK REASSIGN.

  Out of these function types, ABORT TASK and TASK REASSIGN
  functions manage a single active task, whereas ABORT TASK SET,
  CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET and TARGET
  COLD RESET functions can each potentially affect multiple active
  tasks.

4.2.3.2. Notion of Affected Tasks

  This Section defines the notion of "affected tasks" in multi-task
  abort scenarios. Scope definitions in this Section apply to both
  the Standard Multi-task Abort semantics (Section 4.2.3.3) and the
  FastAbort Multi-task Abort semantics behavior (Section 4.2.3.4).

  ABORT TASK SET: All outstanding tasks for the I_T_L nexus
  identified by the LUN field in the ABORT TASK SET TMF Request PDU
  (Section 11.5).

  CLEAR TASK SET: All outstanding tasks in the task set for the LU
  identified by the LUN field in the CLEAR TASK SET TMF Request PDU.
  See [SPC3] for the definition of a "task set".

  LOGICAL UNIT RESET: All outstanding tasks from all initiators for
  the LU identified by the LUN field in the LOGICAL UNIT RESET
  Request PDU.

  TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks from
  all initiators across all LUs to which the TMF-issuing session has
  access on the SCSI target device hosting the iSCSI session.




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  Usage: An "ABORT TASK SET TMF Request PDU" in the preceding text
  is an iSCSI TMF Request PDU with the "Function" field set to
  "ABORT TASK SET" as defined in Section 11.5. Similar usage is
  employed for other scope descriptions.

4.2.3.3. Standard Multi-task Abort Semantics

  All iSCSI implementations MUST support the protocol behavior
  defined in this Section as the default behavior. The execution of
  ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM
  RESET, and TARGET COLD RESET TMF Requests consists of the
  following sequence of actions in the specified order on the
  specified party.

  The initiator iSCSI layer:
    a. MUST continue to respond to each TTT received for the
       affected tasks.
    b. SHOULD process any responses received for affected tasks in
       the normal fashion. This is acceptable because the
       responses are guaranteed to have been sent prior to the TMF
       response.
    c. SHOULD receive the TMF Response concluding all the tasks in
       the set of affected tasks unless the initiator has done
       something (e.g., LU reset, connection drop) that may
       prevent the TMF Response from being sent or received. The
       initiator MUST thus conclude all affected tasks as part of
       this step in either case, and MUST discard any TMF Response
       received after the affected tasks are concluded.

  The target iSCSI layer:
    a. MUST wait for responses on currently valid target-transfer
       tags of the affected tasks from the issuing initiator. MAY
       wait for responses on currently valid target-transfer tags
       of the affected tasks from third-party initiators.
    b. MUST wait (concurrent with the wait in Step a) for all
       commands of the affected tasks to be received based on the
       CmdSN ordering. SHOULD NOT wait for new commands on third-
       party affected sessions -- only the instantiated tasks have
       to be considered for the purpose of determining the
       affected tasks. In the case of target-scoped requests
       (i.e., TARGET WARM RESET and TARGET COLD RESET), all of the




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        commands that are not yet received on the issuing session
        in the command stream however can be considered to have
        been received with no command waiting period -- i.e., the
        entire CmdSN space up to the CmdSN of the task management
        function can be "plugged".
     c. MUST propagate the TMF request to and receive the response
        from the target SCSI layer.
     d. MUST provide the Response Fence behavior for the TMF
        Response on the issuing session as specified in Section
        4.2.2.3.2.
     e. MUST provide the Response Fence behavior on the first post-
        TMF Response on third-party sessions as specified in
        Section 4.2.2.3.3. If some tasks originate from non-iSCSI
        I_T_L nexuses, then the means by which the target ensures
        that all affected tasks have returned their status to the
        initiator are defined by the specific non-iSCSI transport
        protocol(s).

  Technically, the TMF servicing is complete in Step d. Data
  transfers corresponding to terminated tasks may however still be
  in progress on third-party iSCSI sessions even at the end of Step
  e. The TMF Response MUST NOT be sent by the target iSCSI layer
  before the end of Step d, and MAY be sent at the end of Step d
  despite these outstanding data transfers until after Step e.

4.2.3.4. FastAbort Multi-task Abort Semantics

  Protocol behavior defined in this Section SHOULD be implemented by
  all iSCSI implementations complying with this document, noting
  that some steps below may not be compatible with [RFC3720]
  semantics. However, protocol behavior defined in this Section MUST
  be exhibited by iSCSI implementations on an iSCSI session when
  they negotiate the TaskReporting (Section 13.23) key to
  "FastAbort" on that session. The execution of ABORT TASK SET,
  CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET, and TARGET
  COLD RESET TMF Requests consists of the following sequence of
  actions in the specified order on the specified party.

  The initiator iSCSI layer:




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    a. MUST NOT send any more Data-Out PDUs for affected tasks on
       the issuing connection of the issuing iSCSI session once
       the TMF is sent to the target.
    b. SHOULD process any responses received for affected tasks in
       the normal fashion. This is acceptable because the
       responses are guaranteed to have been sent prior to the TMF
       response.
    c. MUST respond to each Async Message PDU with FAST_ABORT
       AsyncEvent as defined in Section 11.9.
    d. MUST treat the TMF response as terminating all affected
       tasks for which responses have not been received, and MUST
       discard any responses for affected tasks received after the
       TMF response is passed to the SCSI layer (although the
       semantics defined in this Section ensure that such an out-
       of-order scenario will never happen with a compliant target
       implementation).

  The target iSCSI layer:
    a. MUST wait for all commands of the affected tasks to be
       received based on the CmdSN ordering on the issuing
       session. SHOULD NOT wait for new commands on third-party
       affected sessions - only the instantiated tasks have to be
       considered for the purpose of determining the affected
       tasks. In the case of target-scoped requests (i.e., TARGET
       WARM RESET and TARGET COLD RESET), all the commands that
       are not yet received on the issuing session in the command
       stream can be considered to have been received with no
       command waiting period -- i.e., the entire CmdSN space up
       to the CmdSN of the task management function can be
       "plugged".
    b. MUST propagate the TMF request to and receive the response
       from the target SCSI layer.
    c. MUST leave all active "affected TTTs" (i.e., active TTTs
       associated with affected tasks) valid.
    d. MUST send an Asynchronous Message PDU with AsyncEvent=5
       (Section 11.9) on:
        i)      each connection of each third-party session to
           which at least one affected task is allegiant if
           TaskReporting=FastAbort is operational on that third-
           party session, and,




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          ii)    each connection except the issuing connection of
            the issuing session that has at least one allegiant
            affected task.
         If there are multiple affected LUs (say, due to a target
         reset), then one Async Message PDU MUST be sent for each
         such LU on each connection that has at least one allegiant
         affected task. The LUN field in the Asynchronous Message PDU
         MUST be set to match the LUN for each such LU.
     e. MUST address the Response Fence flag on the TMF Response on
        the issuing session as defined in Section 4.2.2.3.3.
     f. MUST address the Response Fence flag on the first post-TMF
        Response on third-party sessions as defined in Section
        4.2.2.3.3. If some tasks originate from non-iSCSI I_T_L
        nexuses, then the means by which the target ensures that
        all affected tasks have returned their status to the
        initiator are defined by the specific non-iSCSI transport
        protocol(s).
     g. MUST free up the affected TTTs (and STags, if applicable)
        and the corresponding buffers, if any, once it receives
        each associated NOP-Out acknowledgement that the initiator
        generated in response to each Async Message.

  Technically, the TMF servicing is complete in Step e. Data
  transfers corresponding to terminated tasks may however still be
  in progress even at the end of Step f. A TMF Response MUST NOT be
  sent by the target iSCSI layer before the end of Step e, and MAY
  be sent at the end of Step e despite these outstanding Data
  transfers until Step g. Step g specifies an event to free up any
  such resources that may have been reserved to support outstanding
  data transfers.

4.2.3.5. Affected Tasks Shared across Standard and FastAbort Sessions

  If an iSCSI target implementation is capable of supporting
  TaskReporting=FastAbort functionality (Section 13.23), it may end
  up in a situation where some sessions have TaskReporting=RFC3720
  operational (RFC 3720 sessions) while some other sessions have
  TaskReporting=FastAbort operational (FastAbort sessions) even
  while accessing a shared set of affected tasks (Section 4.2.3.2).
  If the issuing session is an RFC 3720 session, the iSCSI target
  implementation is FastAbort-capable, and the third-party affected




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  session is a FastAbort session, the following behavior SHOULD be
  exhibited by the iSCSI target layer:
    a. Between Steps c and d of the target behavior in Section
       4.2.3.3, send an Asynchronous Message PDU with AsyncEvent=5
       (Section 11.9) on each connection of each third-party
       session to which at least one affected task is allegiant.
       If there are multiple affected LUs, then send one Async
       Message PDU for each such LU on each connection that has at
       least one allegiant affected task. When sent, the LUN field
       in the Asynchronous Message PDU MUST be set to match the
       LUN for each such LU.
    b. After Step e of the target behavior in Section 4.2.3.3,
       free up the affected TTTs (and STags, if applicable) and
       the corresponding buffers, if any, once each associated
       NOP-Out acknowledgement is received that the third-party
       initiator generated in response to each Async Message sent
       in Step a.

  If the issuing session is a FastAbort session, the iSCSI target
  implementation is FastAbort-capable, and the third-party affected
  session is an RFC 3720 session, iSCSI target layer MUST NOT send
  Asynchronous Message PDUs on the third-party session to prompt the
  FastAbort behavior.

  If the third-party affected session is a FastAbort session and the
  issuing session is a FastAbort session, the initiator in the
  third-party role MUST respond to each Async Message PDU with
  AsyncEvent=5 as defined in Section 11.9. Note that an initiator
  MAY thus receive these Async Messages on a third-party affected
  session even if the session is a single-connection session.

4.2.3.6. Rationale behind the FastAbort Semantics

  There are fundamentally three basic objectives behind the
  semantics
  specified in Sections 4.2.3.3 and 4.2.3.4.
    1. Maintaining an ordered command flow I_T nexus abstraction
       to the target SCSI layer even with multi-connection
       sessions.
            - Target iSCSI processing of a TMF request must
            maintain the single flow illusion. Target behavior in




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           Step b of Section 4.2.3.3 and Step a of Section 4.2.3.4
           correspond to this objective.
    2. Maintaining a single ordered response flow I_T nexus
       abstraction to the initiator SCSI layer even with multi-
       connection sessions when one response (i.e., TMF response)
       could imply the status of other unfinished tasks from the
       initiator's perspective.
           - The target must ensure that the initiator does not
           see "old" task responses (that were placed on the wire
           chronologically earlier than the TMF Response) after
           seeing the TMF response. The target behavior in Step d
           of Section 4.2.3.3 and Step e of Section 4.2.3.4
           correspond to this objective.
           - Whenever the result of a TMF action is visible across
           multiple I_T_L nexuses, [SAM2] requires the SCSI device
           server to trigger a UA on each of the other I_T_L
           nexuses. Once an initiator is notified of such an UA,
           the application client on the receiving initiator is
           required to clear its task state (clause 5.5 in [SAM2])
           for the affected tasks. It would thus be inappropriate
           to deliver a SCSI Response for a task after the task
           state is cleared on the initiator, i.e., after the UA
           is notified. The UA notification contained in the first
           SCSI Response PDU on each affected Third-party I_T_L
           nexus after the TMF action thus MUST NOT pass the
           affected task responses on any of the iSCSI sessions
           accessing the LU. The target behavior in Step e of
           Section 4.2.3.3 and Step f of Section 4.2.3.4
           correspond to this objective.
    3. Draining all active TTTs corresponding to affected tasks in
       a deterministic fashion.
           - Data-Out PDUs with stale TTTs arriving after the
           tasks are terminated can create a buffer management
           problem even for traditional iSCSI implementations, and
           is fatal for the connection for iSCSI/iSER
           implementations. Either the termination of affected
           tasks should be postponed until the TTTs are retired
           (as in Step a of Section 4.2.3.3), or the TTTs and the
           buffers should stay allocated beyond task termination




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            to be deterministically freed up later (as in Steps c
            and g of Section 4.2.3.4).

   The only other notable optimization is the plugging. If all tasks
   on an I_T nexus will be aborted anyway (as with a target reset),
   there is no need to wait to receive all commands to plug the CmdSN
   holes. The target iSCSI layer can simply plug all missing CmdSN
   slots and move on with TMF processing. The first objective
   (maintaining a single ordered command flow) is still met with this
   optimization because the target SCSI layer only sees ordered
   commands.

4.2.4. iSCSI Login

   The purpose of the iSCSI login is to enable a TCP connection for
   iSCSI use, authentication of the parties, negotiation of the
   session's parameters and marking of the connection as belonging to
   an iSCSI session.

   A session is used to identify to a target all the connections with
   a given initiator that belong to the same I_T nexus. (For more
   details on how a session relates to an I_T nexus, see Section
   4.4.2).

   The targets listen on a well-known TCP port or other TCP port for
   incoming connections. The initiator begins the login process by
   connecting to one of these TCP ports.

   As part of the login process, the initiator and target SHOULD
   authenticate each other and MAY set a security association
   protocol for the session. This can occur in many different ways
   and is subject to negotiation - see Section 12.

   To protect the TCP connection, an IPsec security association MAY
   be established before the Login request. For information on using
   IPsec security for iSCSI see Section 9, [RFC3723] and [IPSEC-IPS}.

   The iSCSI Login Phase is carried through Login requests and
   responses. Once suitable authentication has occurred and
   operational parameters have been set, the session transitions to
   Full Feature Phase and the initiator may start to send SCSI
   commands. The security policy for whether, and by what means, a




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  target chooses to authorize an initiator is beyond the scope of
  this document. For a more detailed description of the Login Phase,
  see Section 6.

  The login PDU includes the ISID part of the session ID (SSID). The
  target portal group that services the login is implied by the
  selection of the connection endpoint. For a new session, the TSIH
  is zero. As part of the response, the target generates a TSIH.

  During session establishment, the target identifies the SCSI
  initiator port (the "I" in the "I_T nexus") through the value pair
  (InitiatorName, ISID). We describe InitiatorName later in this
  Section. Any persistent state (e.g., persistent reservations) on
  the target that is associated with a SCSI initiator port is
  identified based on this value pair. Any state associated with the
  SCSI target port (the "T" in the "I_T nexus") is identified
  externally by the TargetName and portal group tag (see Section
  4.4.1). ISID is subject to reuse restrictions because it is used
  to identify a persistent state (see Section 4.4.3).

  Before the Full Feature Phase is established, only Login Request
  and Login Response PDUs are allowed. Login requests and responses
  MUST be used exclusively during Login. On any connection, the
  login phase MUST immediately follow TCP connection establishment
  and a subsequent Login Phase MUST NOT occur before tearing down a
  connection.

  A target receiving any PDU except a Login request before the Login
  phase is started MUST immediately terminate the connection on
  which the PDU was received. Once the Login phase has started, if
  the target receives any PDU except a Login request, it MUST send a
  Login reject (with Status "invalid during login") and then
  disconnect. If the initiator receives any PDU except a Login
  response, it MUST immediately terminate the connection.

4.2.5. iSCSI Full Feature Phase

  Once the two sides successfully conclude the Login on the first -
  also called the leading - connection in the session, the iSCSI
  session is in the iSCSI Full Feature Phase. A connection is in
  Full Feature Phase if the session is in Full Feature Phase and the




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  connection login has completed successfully. An iSCSI connection
  is not in Full Feature Phase

       a) when it does not have an established transport
          connection,
          OR
       b) when it has a valid transport connection, but a
          successful login was not performed or the connection is
          currently logged out.

  In a normal Full Feature Phase, the initiator may send SCSI
  commands and data to the various LUs on the target by
  encapsulating them in iSCSI PDUs that go over the established
  iSCSI session.

4.2.5.1. Command Connection Allegiance

  For any iSCSI request issued over a TCP connection, the
  corresponding response and/or other related PDU(s) MUST be sent
  over the same connection. We call this "connection allegiance". If
  the original connection fails before the command is completed, the
  connection allegiance of the command may be explicitly reassigned
  to a different transport connection as described in detail in
  Section 7.2.

  Thus, if an initiator issues a READ command, the target MUST send
  the requested data, if any, followed by the status to the
  initiator over the same TCP connection that was used to deliver
  the SCSI command. If an initiator issues a WRITE command, the
  initiator MUST send the data, if any, for that command over the
  same TCP connection that was used to deliver the SCSI command. The
  target MUST return Ready To Transfer (R2T), if any, and the status
  over the same TCP connection that was used to deliver the SCSI
  command. Retransmission requests (SNACK PDUs) and the data and
  status that they generate MUST also use the same connection.

  However, consecutive commands that are part of a SCSI linked
  command-chain task (see [SAM2]) MAY use different connections.
  Connection allegiance is strictly per-command and not per-task.
  During the iSCSI Full Feature Phase, the initiator and target MAY




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  interleave unrelated SCSI commands, their SCSI Data, and responses
  over the session.

4.2.5.2. Data Transfer Overview

  Outgoing SCSI data (initiator to target user data or command
  parameters) is sent as either solicited data or unsolicited data.
  Solicited data are sent in response to R2T PDUs. Unsolicited data
  can be sent as part of an iSCSI command PDU ("immediate data") or
  in separate iSCSI data PDUs.

  Immediate data are assumed to originate at offset 0 in the
  initiator SCSI write-buffer (outgoing data buffer). All other Data
  PDUs have the buffer offset set explicitly in the PDU header.

  An initiator may send unsolicited data up to FirstBurstLength
  (see Section 13.14) as immediate (up to the negotiated maximum PDU
  length), in a separate PDU sequence or both. All subsequent data
  MUST be solicited. The maximum length of an individual data PDU or
  the immediate-part of the first unsolicited burst MAY be
  negotiated at login.

  The maximum amount of unsolicited data that can be sent with a
  command is negotiated at login through the FirstBurstLength (see
  Section 13.14) key. A target MAY separately enable immediate data
  (through the ImmediateData key) without enabling the more general
  (separate data PDUs) form of unsolicited data (through the
  InitialR2T key).

  Unsolicited data on write are meant to reduce the effect of
  latency on throughput (no R2T is needed to start sending data). In
  addition, immediate data is meant to reduce the protocol overhead
  (both bandwidth and execution time).

  An iSCSI initiator MAY choose not to send unsolicited data, only
  immediate data or FirstBurstLength bytes of unsolicited data with
  a command. If any non-immediate unsolicited data is sent, the
  total unsolicited data MUST be either FirstBurstLength, or all of
  the data if the total amount is less than the FirstBurstLength.

  It is considered an error for an initiator to send unsolicited
  data PDUs to a target that operates in R2T mode (only solicited




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  data are allowed). It is also an error for an initiator to send
  more unsolicited data, whether immediate or as separate PDUs, than
  FirstBurstLength.

  An initiator MUST honor an R2T data request for a valid
  outstanding command (i.e., carrying a valid Initiator Task Tag)
  and deliver all the requested data provided the command is
  supposed to deliver outgoing data and the R2T specifies data
  within the command bounds. The initiator action is unspecified for
  receiving an R2T request that specifies data, all or part, outside
  of the bounds of the command.

  A target SHOULD NOT silently discard data and then request
  retransmission through R2T. Initiators SHOULD NOT keep track of
  the data transferred to or from the target (scoreboarding). SCSI
  targets perform residual count calculation to check how much data
  was actually transferred to or from the device by a command. This
  may differ from the amount the initiator sent and/or received for
  reasons such as retransmissions and errors. Read or bidirectional
  commands implicitly solicit the transmission of the entire amount
  of data covered by the command. SCSI data packets are matched to
  their corresponding SCSI commands by using tags specified in the
  protocol.

  In addition, iSCSI initiators and targets MUST enforce some
  ordering rules. When unsolicited data is used, the order of the
  unsolicited data on each connection MUST match the order in which
  the commands on that connection are sent. Command and unsolicited
  data PDUs may be interleaved on a single connection as long as the
  ordering requirements of each are maintained (e.g., command N+1
  MAY be sent before the unsolicited Data-Out PDUs for command N,
  but the unsolicited Data-Out PDUs for command N MUST precede the
  unsolicited Data-Out PDUs of command N+1). A target that receives
  data out of order MAY terminate the session.

4.2.5.3. Tags and Integrity Checks

  Initiator tags for pending commands are unique initiator-wide for
  a session. Target tags are not strictly specified by the protocol.
  It is assumed that target tags are used by the target to tag
  (alone or in combination with the LUN) the solicited data. Target
  tags are generated by the target and "echoed" by the initiator.




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  These mechanisms are designed to accomplish efficient data
  delivery along with a large degree of control over the data flow.

  As the Initiator Task Tag is used to identify a task during its
  execution the iSCSI initiator and target MUST verify that all
  other fields used in task related PDUs have values that are
  consistent with the values used at the task instantiation based on
  Initiator Task Tag (e.g., the LUN used in an R2T PDU MUST be the
  same as the one used in the SCSI command PDU used to instantiate
  the task). Using inconsistent field values is considered a
  protocol error.

4.2.5.4. Task Management

  SCSI task management assumes that individual tasks and task groups
  can be aborted solely based on the task tags (for individual
  tasks) or the timing of the task management command (for task
  groups) and that the task management action is executed
  synchronously - i.e, no message involving an aborted task will be
  seen by the SCSI initiator after receiving the task management
  response. In iSCSI initiators and targets interact asynchronously
  over several connections. iSCSI specifies the protocol mechanism
  and implementation requirements needed to present a synchronous
  view while using an asynchronous infrastructure.

4.2.6. iSCSI Connection Termination

  An iSCSI connection may be terminated by use of a transport
  connection shutdown or a transport reset. Transport reset is
  assumed to be an exceptional event.

  Graceful TCP connection shutdowns are done by sending TCP FINs. A
  graceful transport connection shutdown SHOULD only be initiated by
  either party when the connection is not in iSCSI Full Feature
  Phase. A target MAY terminate a Full Feature Phase connection on
  internal exception events, but it SHOULD announce the fact through
  an Asynchronous Message PDU. Connection termination with
  outstanding commands may require recovery actions.

  If a connection is terminated while in Full Feature Phase,
  connection cleanup (see Section 7) is required prior to recovery.
  By doing connection cleanup before starting recovery, the




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   initiator and target will avoid receiving stale PDUs after
   recovery.

4.2.7. iSCSI Names

   Both targets and initiators require names for the purpose of
   identification. In addition, names enable iSCSI storage resources
   to be managed regardless of location (address). An iSCSI node name
   is also the SCSI device name contained in the iSCSI Node. The
   iSCSI name of a SCSI device is the principal object used in
   authentication of targets to initiators and initiators to targets.
   This name is also used to identify and manage iSCSI storage
   resources.

   iSCSI names must be unique within the operation domain of the end
   user. However, because the operation domain of an IP network is
   potentially worldwide, the iSCSI name formats are architected to
   be worldwide unique. To assist naming authorities in the
   construction of worldwide unique names, iSCSI provides three name
   formats for different types of naming authorities.

   iSCSI names are associated with iSCSI nodes, and not iSCSI network
   adapter cards, to ensure that the replacement of network adapter
   cards does not require reconfiguration of all SCSI and iSCSI
   resource allocation information.

   Some SCSI commands require that protocol-specific identifiers be
   communicated within SCSI CDBs. See SCSI for the definition of the
   SCSI port name/identifier for iSCSI ports.

   An initiator may discover the iSCSI Target Names to which it has
   access, along with their addresses, using the SendTargets text
   request, or other techniques discussed in [RFC3721].

   iSCSI equipment that needs discovery functions beyond SendTargets
   SHOULD implement iSNS (see [RFC4171]) for extended discovery
   management capabilities and interoperability. Although [RFC3721]
   implies an SLP ([RFC2608]) implementation requirement, SLP has not
   been widely implemented or deployed for use with iSCSI in
   practice. iSCSI implementations therefore SHOULD NOT rely on SLP-
   based discovery interoperability.




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4.2.7.1. iSCSI Name Properties

  Each iSCSI node, whether it is an initiator or target or both,
  MUST have an iSCSI name. Whenever an iSCSI Node contains an iSCSI
  Initiator Node and an iSCSI Target Node, the iSCSI Initiator Name
  MUST be the same as the iSCSI Target Name for the contained Nodes
  such that there is only one iSCSI Node Name for the iSCSI Node
  overall. Note the related requirements in Section 9.2.1 on how to
  map CHAP names to iSCSI Names in such a scenario.

  Initiators and targets MUST support the receipt of iSCSI names of
  up to the maximum length of 223 bytes.

  The initiator MUST present both its iSCSI Initiator Name and the
  iSCSI Target Name to which it wishes to connect in the first login
  request of a new session or connection. The only exception is if a
  discovery session (see Section 4.3) is to be established. In this
  case, the iSCSI Initiator Name is still required, but the iSCSI
  Target Name MAY be omitted.

  iSCSI names have the following properties:

     - iSCSI names are globally unique. No two initiators or
       targets can have the same name.

     - iSCSI names are permanent. An iSCSI initiator node or target
       node has the same name for its lifetime.

     - iSCSI names do not imply a location or address. An iSCSI
       initiator or target can move, or have multiple addresses. A
       change of address does not imply a change of name.

     - iSCSI names do not rely on a central name broker; the naming
       authority is distributed.

     - iSCSI names support integration with existing unique naming
       schemes.

     - iSCSI names rely on existing naming authorities. iSCSI does
       not create any new naming authority.




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  The encoding of an iSCSI name has the following properties:

    - iSCSI names have the same encoding method regardless of the
      underlying protocols.

    - iSCSI names are relatively simple to compare. The algorithm
      for comparing two iSCSI names for equivalence does not rely
      on an external server.

    - iSCSI names are composed only of printable ASCII and Unicode
      characters. iSCSI names allow the use of international
      character sets but uppercase characters are prohibited. The
      iSCSI stringprep profile [RFC3722] maps uppercase characters
      to lowercase and SHOULD be used to prepare iSCSI names from
      input that may include uppercase characters. No whitespace
      characters are used in iSCSI names, see [RFC3722] for
      details.

    - iSCSI names may be transported using both binary and ASCII-
      based protocols.


  An iSCSI name really names a logical software entity, and is not
  tied to a port or other hardware that can be changed. For
  instance, an initiator name should name the iSCSI initiator node,
  not a particular NIC or HBA. When multiple NICs are used, they
  should generally all present the same iSCSI initiator name to the
  targets, because they are simply paths to the same SCSI layer. In
  most operating systems, the named entity is the operating system
  image.

  Similarly, a target name should not be tied to hardware interfaces
  that can be changed. A target name should identify the logical
  target and must be the same for the target regardless of the
  physical portion being addressed. This assists iSCSI initiators in
  determining that the two targets it has discovered are really two
  paths to the same target.

  The iSCSI name is designed to fulfill the functional requirements
  for Uniform Resource Names (URN) [RFC1737]. For example, it is
  required that the name have a global scope, be independent of
  address or location, and be persistent and globally unique. Names




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  must be extensible and scalable with the use of naming
  authorities. The name encoding should be both human and machine
  readable. See [RFC1737] for further requirements.

4.2.7.2. iSCSI Name Encoding

  An iSCSI name MUST be a UTF-8 (see [RFC3629]) encoding of a string
  of Unicode characters with the following properties:

     - It is in Normalization Form C (see "Unicode Normalization
       Forms" [UNICODE]).

     - It only contains characters allowed by the output of the
       iSCSI stringprep template (described in [RFC3722]).

     - The following characters are used for formatting iSCSI
       names:



            - dash ('-'=U+002d)
            - dot ('.'=U+002e)
            - colon (':'=U+003a)


     - The UTF-8 encoding of the name is not larger than 223 bytes.


  The stringprep process is described in [RFC3454]; iSCSI's use of
  the stringprep process is described in [RFC3722]. Stringprep is a
  method designed by the Internationalized Domain Name (IDN) working
  group to translate human-typed strings into a format that can be
  compared as opaque strings. iSCSI names are expected to be used by
  administrators for purposes such as system configuration - for
  this reason, characters that may lead to human confusion among
  different iSCSI names (e.g., punctuation, spacing, diacritical
  marks) should be avoided, even when such characters are allowed as
  stringprep processing output by [RFC3722]. The stringprep process
  also converts strings into equivalent strings of lower-case
  characters.




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  The stringprep process does not need to be implemented if the
  names are generated using only characters allowed as output by the
  stringprep processing specified in [RFC3722]. Those allowed
  characters include all ASCII lowercase and numeric characters, as
  well as lowercase Unicode characters as specified in [RFC3722].
  Once iSCSI names encoded in UTF-8 are "normalized" as described in
  this Section, they may be safely compared byte-for-byte.

4.2.7.3. iSCSI Name Structure

  An iSCSI name consists of two parts--a type designator followed by
  a unique name string.

  iSCSI uses three existing naming authorities in constructing
  globally unique iSCSI names. Type designator in an iSCSI name
  indicates the naming authority on which the name is based. The
  three iSCSI name formats are the following:

       a) iSCSI-Qualified Name: it is based on domain names to
          identify a naming authority,
       b) NAA format Name: it is based on a naming format defined
          by [FC-FS3] for constructing globally unique identifiers,
          referred to as the Network Address Authority (NAA), and,
       c) EUI format Name: it is based on EUI names where the IEEE
          Registration Authority assists in the formation of
          worldwide unique names (EUI-64 format).

  The corresponding type designator strings currently defined are:

       a) iqn.   - iSCSI Qualified name

       b) naa. - Remainder of the string is an INCITS T11-defined
          Network Address Authority identifier, in ASCII-encoded
          hexadecimal.

       c) eui. - Remainder of the string is an IEEE EUI-64
          identifier, in ASCII-encoded hexadecimal.



  These three naming authority designators were considered
  sufficient at the time of writing this document. The creation of




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  additional naming type designators for iSCSI may be considered by
  the IETF and detailed in separate RFCs.

  The following table summarizes the current SCSI transport
  protocols and their naming formats.

     SCSI Transport Protocol       Naming Format
  +----------------------------+-------+-----+----+
  |                            | EUI-64| NAA |IQN |
  |----------------------------|-------|-----|----|
  | iSCSI (Internet SCSI)      | X     | X   | X  |
  |----------------------------|-------|-----|----|
  | FCP (Fibre Channel)        |       | X   |    |
  |----------------------------|-------|-----|----|
  | SAS (Serial Attached SCSI) |       | X   |    |
  +----------------------------+-------+-----+----+


4.2.7.4. Type "iqn." (iSCSI Qualified Name)

  This iSCSI name type can be used by any organization that owns a
  domain name. This naming format is useful when an end user or
  service provider wishes to assign iSCSI names for targets and/or
  initiators.

  To generate names of this type, the person or organization
  generating the name must own a registered domain name. This domain
  name does not have to resolve to an address; it just needs to be
  reserved to prevent others from generating iSCSI names using the
  same domain name.

  Since a domain name can expire, be acquired by another entity, or
  may be used to generate iSCSI names by both owners, the domain
  name must be additionally qualified by a date during which the
  naming authority owned the domain name. A date code is provided as
  part of the "iqn." format for this reason.

  The iSCSI qualified name string consists of:

     - The string "iqn.", used to distinguish these names from
       "eui." formatted names.




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    - A date code, in yyyy-mm format. This date MUST be a date
      during which the naming authority owned the domain name used
      in this format, and SHOULD be the first month in which the
      domain name was owned by this naming authority at 00:01 GMT
      of the first day of the month. This date code uses the
      Gregorian calendar. All four digits in the year must be
      present. Both digits of the month must be present, with
      January == "01" and December == "12". The dash must be
      included.

    - A dot "."

    - The reversed domain name of the naming authority (person or
      organization) creating this iSCSI name.

    - An optional, colon (:) prefixed, string within the character
      set and length boundaries that the owner of the domain name
      deems appropriate. This may contain product types, serial
      numbers, host identifiers, or software keys (e.g, it may
      include colons to separate organization boundaries). With
      the exception of the colon prefix, the owner of the domain
      name can assign everything after the reversed domain name as
      desired. It is the responsibility of the entity that is the
      naming authority to ensure that the iSCSI names it assigns
      are worldwide unique. For example, "Example Storage Arrays,
      Inc.", might own the domain name "example.com".


  The following are examples of iSCSI qualified names that might be
  generated by "EXAMPLE Storage Arrays, Inc."

                    Naming     String defined by
       Type Date     Auth      "example.com" naming authority
      +--++-----+ +---------+ +--------------------------------+
      | ||      | |         | |                                |

      iqn.2001-04.com.example:storage:diskarrays-sn-a8675309
      iqn.2001-04.com.example
      iqn.2001-04.com.example:storage.tape1.sys1.xyz
      iqn.2001-04.com.example:storage.disk2.sys1.xyz




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4.2.7.5. Type "eui." (IEEE EUI-64 format)

  The IEEE Registration Authority provides a service for assigning
  globally unique identifiers [EUI]. The EUI-64 format is used to
  build a global identifier in other network protocols. For example,
  Fibre Channel defines a method of encoding it into a
  WorldWideName. For more information on registering for EUI
  identifiers, see [OUI].

  The format is "eui." followed by an EUI-64 identifier (16 ASCII-
  encoded hexadecimal digits).

  Example iSCSI name:

      Type     EUI-64 identifier (ASCII-encoded hexadecimal)

      +--++--------------+

      |   ||               |

      eui.02004567A425678D


  The IEEE EUI-64 iSCSI name format might be used when a
  manufacturer is already registered with the IEEE Registration
  Authority and uses EUI-64 formatted worldwide unique names for its
  products.

  More examples of name construction are discussed in [RFC3721].

4.2.7.6. Type "naa." - Network Address Authority

  The INCITS T11 Framing and Signaling Specification [FC-FS3]
  defines a format called the Network Address Authority (NAA) format
  for constructing worldwide unique identifiers that use various
  identifier registration authorities. This identifier format is
  used by the Fibre Channel and SAS SCSI transport protocols. As FC
  and SAS constitute a large fraction of networked SCSI ports, the
  NAA format is a widely used format for SCSI transports. The
  objective behind iSCSI supporting a direct representation of an
  NAA-format name is to facilitate construction of a target device




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  name that translates easily across multiple namespaces for a SCSI
  storage device containing ports served by different transports.
  More specifically, this format allows implementations wherein one
  NAA identifier can be assigned as the basis for the SCSI device
  name for a SCSI target with both SAS ports and iSCSI ports.

  The iSCSI NAA naming format is "naa.", followed by an NAA
  identifier represented in ASCII-encoded hexadecimal digits.

  An example of an iSCSI name with a 64-bit NAA value follows:

  Type NAA identifier (ASCII-encoded hexadecimal)
  +--++--------------+
  | ||               |
  naa.52004567BA64678D

  An example of an iSCSI name with a 128-bit NAA value follows:

  Type NAA identifier (ASCII-encoded hexadecimal)
  +--++------------------------------+
  | ||                               |
  naa.62004567BA64678D0123456789ABCDEF

  The iSCSI NAA naming format might be used in an implementation
  when the infrastructure for generating NAA worldwide unique names
  is already in place because the device contains both SAS and iSCSI
  SCSI ports.

  The NAA identifier formatted in an ASCII-hexadecimal
  representation has a maximum size of 32 characters (128 bit NAA
  format). As a result, there is no issue with this naming format
  exceeding the maximum size for iSCSI node names.

4.2.8. Persistent State

  iSCSI does not require any persistent state maintenance across
  sessions. However, in some cases, SCSI requires persistent
  identification of the SCSI initiator port name (See Section 4.4.2
  and Section 4.4.3).

  iSCSI sessions do not persist through power cycles and boot
  operations.




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  All iSCSI session and connection parameters are re-initialized on
  session and connection creation.

  Commands persist beyond connection termination if the session
  persists and command recovery within the session is supported.
  However, when a connection is dropped, command execution, as
  perceived by iSCSI (i.e., involving iSCSI protocol exchanges for
  the affected task), is suspended until a new allegiance is
  established by the 'task reassign' task management function. (See
  Section 11.5.)

4.2.9. Message Synchronization and Steering

  iSCSI presents a mapping of the SCSI protocol onto TCP. This
  encapsulation is accomplished by sending iSCSI PDUs of varying
  lengths. Unfortunately, TCP does not have a built-in mechanism for
  signaling message boundaries at the TCP layer. iSCSI overcomes
  this obstacle by placing the message length in the iSCSI message
  header. This serves to delineate the end of the current message as
  well as the beginning of the next message.

  In situations where IP packets are delivered in order from the
  network, iSCSI message framing is not an issue and messages are
  processed one after the other. In the presence of IP packet
  reordering (i.e., frames being dropped), legacy TCP
  implementations store the "out of order" TCP segments in temporary
  buffers until the missing TCP segments arrive, upon which the data
  must be copied to the application buffers. In iSCSI, it is
  desirable to steer the SCSI data within these out of order TCP
  segments into the pre-allocated SCSI buffers rather than store
  them in temporary buffers. This decreases the need for dedicated
  reassembly buffers as well as the latency and bandwidth related to
  extra copies.

  Relying solely on the "message length" information from the iSCSI
  message header may make it impossible to find iSCSI message
  boundaries in subsequent TCP segments due to the loss of a TCP
  segment that contains the iSCSI message length. The missing TCP
  segment(s) must be received before any of the following segments
  can be steered to the correct SCSI buffers (due to the inability
  to determine the iSCSI message boundaries). Since these segments




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  cannot be steered to the correct location, they must be saved in
  temporary buffers that must then be copied to the SCSI buffers.

  Different schemes can be used to recover synchronization. The
  details of any such schemes are beyond this protocol
  specification, but it suffices to note that [RFC4297] provides an
  overview of the direct data placement problem on IP networks, and
  [RFC5046] specifies a protocol extension for iSCSI that
  facilitates this direct data placement objective. Rest of this
  document refers to any such direct data placement protocol usage
  as an example of a "Synch and Steering layer".

  Under normal circumstances (no PDU loss or data reception out of
  order), iSCSI data steering can be accomplished by using the
  identifying tag and the data offset fields in the iSCSI header in
  addition to the TCP sequence number from the TCP header. The
  identifying tag helps associate the PDU with a SCSI buffer address
  while the data offset and TCP sequence number are used to
  determine the offset within the buffer.

4.2.9.1. Sync/Steering and iSCSI PDU Length

  When a large iSCSI message is sent, the TCP segment(s) that
  contain the iSCSI header may be lost. The remaining TCP segment(s)
  up to the next iSCSI message must be buffered (in temporary
  buffers) because the iSCSI header that indicates to which SCSI
  buffers the data are to be steered was lost. To minimize the
  amount of buffering, it is recommended that the iSCSI PDU length
  be restricted to a small value (perhaps a few TCP segments in
  length). During login, each end of the iSCSI session specifies the
  maximum iSCSI PDU length it will accept.

4.3. iSCSI Session Types

  iSCSI defines two types of sessions:

       a) Normal operational session - an unrestricted session.

       b) Discovery-session - a session only opened for target
          discovery. The target MUST ONLY accept text requests with
          the SendTargets key and a logout request with reason
          "close the session". All other requests MUST be rejected.




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  The session type is defined during login with SessionType=value
  parameter in the login command.

4.4. SCSI to iSCSI Concepts Mapping Model

  The following diagram shows an example of how multiple iSCSI Nodes
  (targets in this case) can coexist within the same Network Entity
  and can share Network Portals (IP addresses and TCP ports). Other
  more complex configurations are also possible. For detailed
  descriptions of the components of these diagrams, see Section
  4.4.1.




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                 +-----------------------------------+
                 | Network Entity (iSCSI Client)     |
                 |                                   |
                 |          +-------------+          |
                 |          | iSCSI Node  |          |
                 |          | (Initiator) |          |
                 |          +-------------+          |
                 |              |      |             |
                 | +--------------+ +--------------+ |
                 | |Network Portal| |Network Portal| |
                 | |   192.0.2.4  | |    192.0.2.5 | |
                 +-+--------------+-+--------------+-+
                          |                  |
                          | IP Networks      |
                          |                  |
                 +-+--------------+-+--------------+-+
                 | |Network Portal| |Network Portal| |
                 | |198.51.100.21 | |198.51.100.3  | |
                 | | TCP Port 3260| | TCP Port 3260| |
                 | +--------------+ +--------------+ |
                 |        |                  |       |
                 |         -----------------         |
                 |            |          |           |
                 | +-------------+ +--------------+  |
                 | | iSCSI Node  | | iSCSI Node   |  |
                 | | (Target)    | | (Target)     |  |
                 | +-------------+ +--------------+  |
                 |                                   |
                 |   Network Entity (iSCSI Server)   |
                 +-----------------------------------+

4.4.1. iSCSI Architecture Model

  This Section describes the part of the iSCSI architecture model
  that has the most bearing on the relationship between iSCSI and
  the SCSI Architecture Model.

     - Network Entity - represents a device or gateway that is
       accessible from the IP network. A Network Entity must have
       one or more Network Portals (see a following item), each of
       which can be used by some iSCSI Nodes (see the following
       item) contained in that Network Entity to gain access to the




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      IP network.


    - iSCSI Node - represents a single iSCSI initiator or iSCSI
      target or an instance of each. There are one or more iSCSI
      Nodes within a Network Entity. The iSCSI Node is accessible
      via one or more Network Portals (see item d). An iSCSI Node
      is identified by its iSCSI Name (see Section 4.2.7 and
      Section 13). The separation of the iSCSI Name from the
      addresses used by and for the iSCSI node allows multiple
      iSCSI nodes to use the same addresses, and the same iSCSI
      node to use multiple addresses.


    - An alias string may also be associated with an iSCSI Node.
      The alias allows an organization to associate a user
      friendly string with the iSCSI Name. However, the alias
      string is not a substitute for the iSCSI Name.


    - Network Portal - a component of a Network Entity that has a
      TCP/IP network address and that may be used by an iSCSI Node
      within that Network Entity for the connection(s) within one
      of its iSCSI sessions. In an initiator, it is identified by
      its IP address. In a target, it is identified by its IP
      address and its listening TCP port.


    - Portal Groups - iSCSI supports multiple connections within
      the same session; some implementations will have the ability
      to combine connections in a session across multiple Network
      Portals. A Portal Group defines a set of Network Portals
      within an iSCSI Node that collectively supports the
      capability of coordinating a session with connections that
      span these portals. Not all Network Portals within a Portal
      Group need to participate in every session connected through
      that Portal Group. One or more Portal Groups may provide
      access to an iSCSI Node. Each Network Portal, as utilized by
      a given iSCSI Node, belongs to exactly one portal group
      within that node. Portal Groups are identified within an
      iSCSI Node by a portal group tag, a simple unsigned-integer
      between 0 and 65535 (see Section 13.3). All Network Portals




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      with the same portal group tag in the context of a given
      iSCSI Node are in the same Portal Group.

      Both iSCSI Initiators and iSCSI Targets have portal groups,
      though only the iSCSI Target Portal Groups are used directly
      in the iSCSI protocol (e.g., in SendTargets). For references
      to the Initiator Portal Groups, see Section 10.1.1.


    - Portals within a Portal Group should support similar session
      parameters, because they may participate in a common
      session.

  The following diagram shows an example of one such configuration
  on a target and how a session that shares Network Portals within a
  Portal Group may be established.

    ----------------------------IP Network---------------------
           |                |                   |
      +----|----------------|----+        +----|---------+
      | +---------+ +---------+  |        | +---------+  |
      | | Network | | Network |  |        | | Network |  |
      | | Portal  | | Portal  |  |        | | Portal  |  |
      | +--|------+ +---------+  |        | +---------+  |
      |    |                |    |        |    |         |
      |    |    Portal      |    |        |    | Portal  |
      |    |    Group 1     |    |        |    | Group 2 |
      +--------------------------+        +--------------+
           |                |                  |
  +--------|----------------|------------------|------------------+
  |        |                |                  |                  |
  | +----------------------------+ +----------------------------+ |
  | | iSCSI Session (Target side)| | iSCSI Session (Target side)| |
  | |                            | |                            | |
  | |        (TSIH = 56)         | |       (TSIH = 48)          | |
  | +----------------------------+ +----------------------------+ |
  |                                                               |
  |                      iSCSI Target Node                        |
  |             (within Network Entity, not shown)                |
  +---------------------------------------------------------------+




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4.4.2. SCSI Architecture Model

  This Section describes the relationship between the SCSI
  Architecture Model [SAM2] and constructs of the SCSI device, SCSI
  port and I_T nexus, and the iSCSI constructs described in Section
  4.4.1.

  This relationship implies implementation requirements in order to
  conform to the SAM2 model and other SCSI operational functions.
  These requirements are detailed in Section 4.4.3.

  The following list outlines mappings of SCSI architectural
  elements to iSCSI.

       a) SCSI Device - the SAM2 term for an entity that contains
          one or more SCSI ports that are connected to a service
          delivery subsystem and supports a SCSI application
          protocol. For example, a SCSI Initiator Device contains
          one or more SCSI Initiator Ports and zero or more
          application clients. A SCSI Target Device contains one or
          more SCSI Target Ports and one or more logical units. For
          iSCSI, the SCSI Device is the component within an iSCSI
          Node that provides the SCSI functionality. As such, there
          can be one SCSI Device, at most, within an iSCSI Node.
          Access to the SCSI Device can only be achieved in an
          iSCSI normal operational session (see Section 4.3). The
          SCSI Device Name is defined to be the iSCSI Name of the
          node and MUST be used in the iSCSI protocol.

       b) SCSI Port - the SAM2 term for an entity in a SCSI Device
          that provides the SCSI functionality to interface with a
          service delivery subsystem or transport. For iSCSI, the
          definition of SCSI Initiator Port and SCSI Target Port
          are different.

          SCSI Initiator Port: This maps to one endpoint of an
          iSCSI normal operational session (see Section 4.3). An
          iSCSI normal operational session is negotiated through
          the login process between an iSCSI initiator node and an
          iSCSI target node. At successful completion of this
          process, a SCSI Initiator Port is created within the SCSI




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          Initiator Device. The SCSI Initiator Port Name and SCSI
          Initiator Port Identifier are both defined to be the
          iSCSI Initiator Name together with (a) a label that
          identifies it as an initiator port name/identifier and
          (b) the ISID portion of the session identifier.

          SCSI Target Port: This maps to an iSCSI Target Portal
          Group. The SCSI Target Port Name and the SCSI Target Port
          Identifier are both defined to be the iSCSI Target Name
          together with (a) a label that identifies it as a target
          port name/identifier and (b) the portal group tag.

          The SCSI Port Name MUST be used in iSCSI. When used in
          SCSI parameter data, the SCSI port name MUST be encoded
          as:
              1. The iSCSI Name in UTF-8 format, followed by
              2. a comma separator (1 byte), followed by
              3. the ASCII character 'i' (for SCSI Initiator Port)
                 or the ASCII character 't' (for SCSI Target Port)
                 (1 byte), followed by
              4. a comma separator (1 byte), followed by
              5. a text encoding as a hex-constant (see Section 6.1)
                 of the ISID (for SCSI initiator port) or the
                 portal group tag (for SCSI target port) including
                 the initial 0X or 0x and the terminating null (14
                 bytes).

                The ASCII character 'i' or 't' is the label that
                identifies this port as either a SCSI Initiator
                Port or a SCSI Target Port.

       c) I_T nexus - a relationship between a SCSI Initiator Port
          and a SCSI Target Port, according to [SAM2]. For iSCSI,
          this relationship is a session, defined as a relationship
          between an iSCSI Initiator's end of the session (SCSI
          Initiator Port) and the iSCSI Target's Portal Group. The
          I_T nexus can be identified by the conjunction of the
          SCSI port names or by the iSCSI session identifier SSID.
          iSCSI defines the I_T nexus identifier to be the tuple
          (iSCSI Initiator Name + ",i,0x" + ISID in text format,




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          iSCSI Target Name + ",t,0x" + Portal Group Tag in text
          format) - an upper case hex prefix "0X" may alternatively
          be used in place of "0x".

          NOTE: The I_T nexus identifier is not equal to the
          session identifier (SSID).


4.4.3. Consequences of the Model

  This Section describes implementation and behavioral requirements
  that result from the mapping of SCSI constructs to the iSCSI
  constructs defined above. Between a given SCSI initiator port and
  a given SCSI target port, only one I_T nexus (session) can exist.
  No more than one nexus relationship (parallel nexus) is allowed by
  [SAM2]. Therefore, at any given time, only one session with the
  same session identifier (SSID) can exist between a given iSCSI
  initiator node and an iSCSI target node.

  These assumptions lead to the following conclusions and
  requirements:

  ISID RULE: Between a given iSCSI Initiator and iSCSI Target Portal
  Group (SCSI target port), there can only be one session with a
  given value for ISID that identifies the SCSI initiator port. See
  Section 11.12.5.

  The structure of the ISID that contains a naming authority
  component (see Section 11.12.5 and [RFC3721]) provides a mechanism
  to facilitate compliance with the ISID rule. (See Section 10.1.1)

  The iSCSI Initiator Node should manage the assignment of ISIDs
  prior to session initiation. The "ISID RULE" does not preclude the
  use of the same ISID from the same iSCSI Initiator with different
  Target Portal Groups on the same iSCSI target or on other iSCSI
  targets (see Section 10.1.1). Allowing this would be analogous to
  a single SCSI Initiator Port having relationships (nexus) with
  multiple SCSI target ports on the same SCSI target device or SCSI
  target ports on other SCSI target devices. It is also possible to
  have multiple sessions with different ISIDs to the same Target
  Portal Group. Each such session would be considered to be with a
  different initiator even when the sessions originate from the same




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  initiator device. The same ISID may be used by a different iSCSI
  initiator because it is the iSCSI Name together with the ISID that
  identifies the SCSI Initiator Port.

  NOTE: A consequence of the ISID RULE and the specification for the
  I_T nexus identifier is that two nexus with the same identifier
  should never exist at the same time.

  TSIH RULE: The iSCSI Target selects a non-zero value for the TSIH
  at session creation (when an initiator presents a 0 value at
  Login). After being selected, the same TSIH value MUST be used
  whenever initiator or target refers to the session and a TSIH is
  required.

4.4.3.1. I_T Nexus State

  Certain nexus relationships contain an explicit state (e.g.,
  initiator-specific mode pages) that may need to be preserved by
  the device server [SAM2] in a logical unit through changes or
  failures in the iSCSI layer (e.g., session failures). In order for
  that state to be restored, the iSCSI initiator should reestablish
  its session (re-login) to the same Target Portal Group using the
  previous ISID. That is, it should reinstate the session via iSCSI
  session reinstatement (Section 6.3.5) or continue via session
  continuation (Section 6.3.6). This is because the SCSI initiator
  port identifier and the SCSI target port identifier (or relative
  target port) form the datum that the SCSI logical unit device
  server uses to identify the I_T nexus.

4.4.3.2. Reservations

  There are two reservation management methods defined in the SCSI
  standards, reserve/release reservations, based on the RESERVE and
  RELEASE commands [SPC2], and persistent reservations, based on the
  PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT commands [SPC3].
  Reserve/release reservations are obsolete [SPC3] and should not be
  used; persistent reservations are suggested as an alternative, see
  Annex B of [SPC4].

  State for persistent reservations is required to persist through
  changes and failures at the iSCSI layer that result in I_T nexus
  failures, see [SPC3] for details and specific requirements.




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  In contrast, [SPC2] does not specify detailed persistence
  requirements for reserve/release reservation state after an I_T
  nexus failure. Nonetheless, when reserve/release reservations are
  supported by an iSCSI target, the preferred implementation
  approach is to preserve reserve/release reservation state for
  iSCSI session reinstatement (see Section 6.3.5) or session
  continuation (see Section 6.3.6).

  Two additional caveats apply to reserve/release reservations:

     - Retention of a failed session's reserve/release reservation
       state by an iSCSI target, even after that failed iSCSI
       session is not reinstated or continued, may require an
       initiator to issue a reset (e.g., LOGICAL UNIT RESET, see
       Section 11.5) in order to remove that reservation state.

     - Reserve/release reservations may not behave as expected when
       persistent reservations are also used on the same logical
       unit; see the discussion of "Exceptions to SPC-2 RESERVE and
       RELEASE behavior" in [SPC4].



4.5. iSCSI UML Model

  This Section presents the application of the UML modeling concepts
  discussed in Section 3 to the iSCSI and SCSI architecture model
  discussed in Section 4.4.




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                    +----------------+
                    | Network Entity |
                    +----------------+
                         @ 1     @ 1
                         |       |
  +--------------------+         |
  |                              |
  |                              | 0..*
  |                   +------------------+
  |                   | iSCSI Node       |
  |                   +------------------+
  |                          @       @
  |                          |       |
  |           +-----------+ =(a)= +-----------+
  |           |                               |
  |           | 0..1                          | 0..1
  | +------------------------+       +----------------------+
  | |    iSCSI Target Node   |       | iSCSI Initiator Node |
  | +------------------------+       +----------------------+
  |             @ 1                            @ 1
  |             +--------------+               |
  |                        1..* |              | 1..*
  |                    +-----------------------------+
  |                    |         Portal Group        |
  |                    +-----------------------------+
  |                                     O 1
  |                                     |
  |                                     | 1..*
  |               1..* +------------------------+
  +--------------------|        Network Portal  |
                       +------------------------+

  (a) Each instance of an iSCSI Node class MUST contain one iSCSI
      Target Node instance or one iSCSI Initiator Node instance, or
      both.




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                   +----------------+
                   | Network Entity |
                   +----------------+
                        @ 1         @ 1
                        |           |             +-------------------+
  +---------------------+           |             |    iSCSI Session  |
  |                                 |             +-------------------+
  |                                 | 0..*        |      SSID[1]      |
  |                  +--------------------+       |      ISID[1]      |
  |                  |      iSCSI Node    |       +-------------------+
  |                  +--------------------+                   @ 1
  |                  | iSCSI Node Name[1] |                   |
  |                  |    Alias [0..1]    |                   | 0..*
  |                  +--------------------+         +------------------+
  |                  |                    |         | iSCSI Connection |
  |                  +--------------------+         +------------------+
  |                         @ 1         @ 1         |      CID[1]      |
  |                         |           |           +------------------+
  |           +-------------+ ==(b)==   +----------+            0..* |
  |           | 1                                  | 1               |
  | +------------------------+            +------------------------+ |
  | |   iSCSI Target Node    |            | iSCSI Initiator Node   | |
  | +------------------------+            +------------------------+ |
  | | iSCSI Target name [1] |             |iSCSI Initiator Name [1]| |
  | +------------------------+            +------------------------+ |
  |            @ 1                                   @ 1             |
  |            | 1..*                                | 1..*          |
  | +--------------------------+          +------------------------+ |
  | |   Target Portal Group    |          | Initiator Portal Group | |
  | +--------------------------+          +------------------------+ |
  | |Target Portal Group Tag[1]|          | Portal group tag[1]    | |
  | +--------------------------+          +------------------------+ |
  |            o 1                                   o 1             |
  |            +------------+              +---------+               |
  |                    1..* |              | 1..*                    |
  |                +-------------------------+                       |
  |                |          Network Portal |                       |
  |                +-------------------------+                       |
  |          1..*  |         IP Address [1]  | 1                     |
  +----------------|         TCP Port [0..1] |<----------------------+
                   +-------------------------+




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  (b) Each instance of an iSCSI Node class MUST contain one iSCSI
      Target Node instance or one iSCSI Initiator Node instance, or
      both. However, in all scenarios, note that an iSCSI Node MUST
      only have a single iSCSI Name. Note the related requirement in
      Section 4.2.7.1.

4.6. Request/Response Summary

  This Section lists and briefly describes all the iSCSI PDU types
  (request and responses).

  All iSCSI PDUs are built as a set of one or more header segments
  (basic and auxiliary) and zero or one data segments. The header
  group and the data segment may each be followed by a CRC (digest)
  (see [CRC]).

  The basic header segment has a fixed length of 48 bytes.

4.6.1. Request/Response Types Carrying SCSI Payload

4.6.1.1. SCSI-Command

  This request carries the SCSI CDB and all the other SCSI execute
  command procedure call (see [SAM2]) IN arguments such as task
  attributes, Expected Data Transfer Length for one or both transfer
  directions (the latter for bidirectional commands), and Task Tag
  (as part of the I_T_L_x nexus). The I_T_L nexus is derived by the
  initiator and target from the LUN field in the request and the I_T
  nexus is implicit in the session identification.

  In addition, the SCSI-command PDU carries information required for
  the proper operation of the iSCSI protocol - the command sequence
  number (CmdSN) and the expected status number (ExpStatSN) on the
  connection it is issued.

  All or part of the SCSI output (write) data associated with the
  SCSI command may be sent as part of the SCSI-Command PDU as a data
  segment.




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4.6.1.2. SCSI-Response

  The SCSI-Response carries all the SCSI execute-command procedure
  call (see [SAM2]) OUT arguments and the SCSI execute-command
  procedure call return value.

  The SCSI-Response contains the residual counts from the operation,
  if any, an indication of whether the counts represent an overflow
  or an underflow, and the SCSI status if the status is valid or a
  response code (a non-zero return value for the execute-command
  procedure call) if the status is not valid.

  For a valid status that indicates that the command has been
  processed, but resulted in an exception (e.g., a SCSI CHECK
  CONDITION), the PDU data segment contains the associated sense
  data. The use of Autosense ([SAM2]) is REQUIRED by iSCSI.

  Some data segment content may also be associated (in the data
  segment) with a non-zero response code.

  In addition, the SCSI-Response PDU carries information required
  for the proper operation of the iSCSI protocol:

     - The number of Data-In PDUs that a target has sent (to enable
       the initiator to check that all have arrived).

     - StatSN - the Status Sequence Number on this connection.

     - ExpCmdSN - the next Expected Command Sequence Number at the
       target.

     - MaxCmdSN - the maximum CmdSN acceptable at the target from
       this initiator.


4.6.1.3. Task Management Function Request

  The Task Management function request provides an initiator with a
  way to explicitly control the execution of one or more SCSI Tasks
  or iSCSI functions. The PDU carries a function identifier (which
  task management function to perform) and enough information to
  unequivocally identify the task or task-set on which to perform




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  the action, even if the task(s) to act upon has not yet arrived or
  has been discarded due to an error.

  The referenced tag identifies an individual task if the function
  refers to an individual task.

  The I_T_L nexus identifies task sets. In iSCSI the I_T_L nexus is
  identified by the LUN and the session identification (the session
  identifies an I_T nexus).

  For task sets, the CmdSN of the Task Management function request
  helps identify the tasks upon which to act, namely all tasks
  associated with a LUN and having a CmdSN preceding the Task
  Management function request CmdSN.

  For a Task Management function, the coordination between responses
  to the tasks affected and the Task Management function response is
  done by the target.

4.6.1.4. Task Management Function Response

  The Task Management function response carries an indication of
  function completion for a Task Management function request
  including how it completed (response and qualifier) and additional
  information for failure responses.

  After the Task Management response indicates Task Management
  function completion, the initiator will not receive any additional
  responses from the affected tasks.

4.6.1.5. SCSI Data-out and SCSI Data-in

  SCSI Data-out and SCSI Data-in are the main vehicles by which SCSI
  data payload is carried between initiator and target. Data payload
  is associated with a specific SCSI command through the Initiator
  Task Tag. For target convenience, outgoing solicited data also
  carries a Target Transfer Tag (copied from R2T) and the LUN. Each
  PDU contains the payload length and the data offset relative to
  the buffer address contained in the SCSI execute command procedure
  call.




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  In each direction, the data transfer is split into "sequences". An
  end-of-sequence is indicated by the F bit.

  An outgoing sequence is either unsolicited (only the first
  sequence can be unsolicited) or consists of all the Data-Out PDUs
  sent in response to an R2T.

  Input sequences enable the switching of direction for
  bidirectional commands as required.

  For input, the target may request positive acknowledgement of
  input data. This is limited to sessions that support error
  recovery and is implemented through the A bit in the SCSI Data-in
  PDU header.

  Data-in and Data-out PDUs also carry the DataSN to enable the
  initiator and target to detect missing PDUs (discarded due to an
  error).

  In addition, StatSN is carried by the Data-In PDUs.

  To enable a SCSI command to be processed while involving a minimum
  number of messages, the last SCSI Data-in PDU passed for a command
  may also contain the status if the status indicates termination
  with no exceptions (no sense or response involved).


4.6.1.6. Ready To Transfer (R2T)

  R2T is the mechanism by which the SCSI target "requests" the
  initiator for output data. R2T specifies to the initiator the
  offset of the requested data relative to the buffer address from
  the execute command procedure call and the length of the solicited
  data.

  To help the SCSI target associate the resulting Data-out with an
  R2T, the R2T carries a Target Transfer Tag that will be copied by
  the initiator in the solicited SCSI Data-out PDUs. There are no
  protocol specific requirements with regard to the value of these
  tags, but it is assumed that together with the LUN, they will
  enable the target to associate data with an R2T.




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  R2T also carries information required for proper operation of the
  iSCSI protocol, such as:

     - R2TSN (to enable an initiator to detect a missing R2T)

     - StatSN

     - ExpCmdSN

     - MaxCmdSN


4.6.2. Requests/Responses carrying SCSI and iSCSI Payload

4.6.2.1. Asynchronous Message

  Asynchronous Messages are used to carry SCSI asynchronous events
  (AEN) and iSCSI asynchronous messages.

  When carrying an AEN, the event details are reported as sense data
  in the data segment.

4.6.3. Requests/Responses Carrying iSCSI Only Payload

4.6.3.1. Text Request and Text Response

  Text requests and responses are designed as a parameter
  negotiation vehicle and as a vehicle for future extension.

  In the data segment Text Requests/Responses carry text information
  using a simple "key=value" syntax.

  Text Request/Responses may form extended sequences using the same
  Initiator Task Tag. The initiator uses the F (Final) flag bit in
  the text request header to indicate its readiness to terminate a
  sequence. The target uses the F (Final) flag bit in the text
  response header to indicate its consent to sequence termination.

  Text Request and Responses also use the Target Transfer Tag to
  indicate continuation of an operation or a new beginning. A target
  that wishes to continue an operation will set the Target Transfer
  Tag in a Text Response to a value different from the default




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  0xffffffff. An initiator willing to continue will copy this value
  into the Target Transfer Tag of the next Text Request. If the
  initiator wants to restart the current target negotiation (start
  fresh) will set the Target Transfer Tag to 0xffffffff.

  Although a complete exchange is always started by the initiator,
  specific parameter negotiations may be initiated by the initiator
  or target.

4.6.3.2. Login Request and Login Response

  Login Requests and Responses are used exclusively during the Login
  Phase of each connection to set up the session and connection
  parameters. (The Login Phase consists of a sequence of login
  requests and responses carrying the same Initiator Task Tag.)

  A connection is identified by an arbitrarily selected connection-
  ID (CID) that is unique within a session.

  Similar to the Text Requests and Responses, Login
  Requests/Responses carry key=value text information with a simple
  syntax in the data segment.

  The Login Phase proceeds through several stages (security
  negotiation, operational parameter negotiation) that are selected
  with two binary coded fields in the header -- the "current stage"
  (CSG) and the "next stage" (NSG) with the appearance of the latter
  being signaled by the "transit" flag (T).

  The first Login Phase of a session plays a special role, called
  the leading login, which determines some header fields (e.g., the
  version number, the maximum number of connections, and the session
  identification).

  The CmdSN initial value is also set by the leading login.

  StatSN for each connection is initiated by the connection login.

  A login request may indicate an implied logout (cleanup) of the
  connection to be logged in (a connection restart) by using the
  same Connection ID (CID) as an existing connection as well as the




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   same session identifying elements of the session to which the old
   connection was associated.

4.6.3.3. Logout Request and Response

   Logout Requests and Responses are used for the orderly closing of
   connections for recovery or maintenance. The logout request may be
   issued following a target prompt (through an asynchronous message)
   or at an initiators initiative. When issued on the connection to
   be logged out no other request may follow it.

   The Logout response indicates that the connection or session
   cleanup is completed and no other responses will arrive on the
   connection (if received on the logging out connection). In
   addition, the Logout Response indicates how long the target will
   continue to hold resources for recovery (e.g., command execution
   that continues on a new connection) in the Time2Retain field and
   how long the initiator must wait before proceeding with recovery
   in the Time2Wait field.

4.6.3.4. SNACK Request

   With the SNACK Request, the initiator requests retransmission of
   numbered-responses or data from the target. A single SNACK request
   covers a contiguous set of missing items, called a run, of a given
   type of items. The type is indicated in a type field in the PDU
   header. The run is composed of an initial item (StatSN, DataSN,
   R2TSN) and the number of missed Status, Data, or R2T PDUs. For
   long data-in sequences, the target may request (at predefined
   minimum intervals) a positive acknowledgement for the data sent. A
   SNACK request with a type field that indicates ACK and the number
   of Data-In PDUs acknowledged conveys this positive
   acknowledgement.

4.6.3.5. Reject

   Reject enables the target to report an iSCSI error condition
   (e.g., protocol, unsupported option) that uses a Reason field in
   the PDU header and includes the complete header of the bad PDU in
   the Reject PDU data segment.




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4.6.3.6. NOP-Out Request and NOP-In Response

  This request/response pair may be used by an initiator and target
  as a "ping" mechanism to verify that a connection/session is still
  active and all of its components are operational. Such a ping may
  be triggered by the initiator or target. The triggering party
  indicates that it wants a reply by setting a value different from
  the default 0xffffffff in the corresponding Initiator/Target
  Transfer Tag.

  NOP-In/NOP-Out may also be used "unidirectional" to convey to the
  initiator/target command, status or data counter values when there
  is no other "carrier" and there is a need to update the
  initiator/target.




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5. SCSI Mode Parameters for iSCSI

  There are no iSCSI specific mode pages.




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6. Login and Full Feature Phase Negotiation

  iSCSI parameters are negotiated at session or connection
  establishment by using Login Requests and Responses (see Section
  4.2.4) and during Full Feature Phase (Section 4.2.5) by using Text
  Requests and Responses. In both cases the mechanism used is an
  exchange of iSCSI-text-key=value pairs. For brevity, iSCSI-text-
  keys are called just keys in the rest of this document.

  Keys are either declarative or require negotiation and the key
  description indicates if the key is declarative or requires
  negotiation.

  For the declarative keys the declaring party sets a value for the
  key. The key specification indicates if the key can be declared by
  the initiator, target or both.

  For the keys that require negotiation, one of the parties (the
  proposing party) proposes a value or set of values by including
  the key=value in the data part of a Login or Text Request or
  Response. The other party (the accepting party) makes a selection
  based on the value or list of values proposed and includes the
  selected value in a key=value in the data part of the following
  Login or Text Response or Request. For most of the keys, both the
  initiator and target can be proposing parties.

  The login process proceeds in two stages - the security
  negotiation stage and the operational parameter negotiation stage.
  Both stages are optional but at least one of them has to be
  present to enable setting some mandatory parameters.

  If present, the security negotiation stage precedes the
  operational parameter negotiation stage.

  Progression from stage to stage is controlled by the T
  (Transition) bit in the Login Request/Response PDU header. Through
  the T bit set to 1, the initiator indicates that it would like to
  transition. The target agrees to the transition (and selects the
  next stage) when ready. A field in the Login PDU header indicates
  the current stage (CSG) and during transition, another field
  indicates the next stage (NSG) proposed (initiator) and selected
  (target).




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   The Text negotiation process is used to negotiate or declare
   operational parameters. The negotiation process is controlled by
   the F (final) bit in the PDU header. During text negotiations, the
   F bit is used by the initiator to indicate that it is ready to
   finish the negotiation and by the Target to acquiesce the end of
   negotiation.

   Since some key=value pairs may not fit entirely in a single PDU,
   the C (continuation) bit is used (both in Login and Text) to
   indicate that "more follows".

   The text negotiation uses an additional mechanism by which a
   target may deliver larger amounts of data to an enquiring
   initiator. The target sets a Target Task Tag to be used as a
   bookmark which when returned by the initiator, means "go on". If
   reset to a "neutral value", it means "forget about the rest".

   This Section details types of keys and values used, the syntax
   rules for parameter formation, and the negotiation schemes to be
   used with different types of parameters.

6.1. Text Format

   The initiator and target send a set of key=value pairs encoded in
   UTF-8 Unicode. All the text keys and text values specified in this
   document are to be presented and interpreted in the case in which
   they appear in this document. They are case sensitive.

   The following character symbols are used in this document for text
   items (the hexadecimal values represent Unicode code points):

   (a-z, A-Z) ) (0x61-0x7a, 0x41-0x5a) - letters
   (0-9) (0x30-0x39) - digits
   " " (0x20) - space
   "." (0x2e) - dot
   "-" (0x2d) - minus
   "+" (0x2b) - plus
   "@" (0x40) - commercial at
   "_" (0x5f) - underscore
   "=" (0x3d) - equal
   ":" (0x3a) - colon




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  "/"    (0x2f)   -   solidus or slash
  "["    (0x5b)   -   left bracket
  "]"    (0x5d)   -   right bracket
  null   (0x00)   -   null separator
  ","    (0x2c)   -   comma
  "~"    (0x7e)   -   tilde

  Key=value pairs may span PDU boundaries. An initiator or target
  that sends partial key=value text within a PDU indicates that more
  text follows by setting the C bit in the Text or Login Request or
  Text or Login Response to 1. Data segments in a series of PDUs
  that have the C bit set to 1 and end with a PDU that have the C
  bit set to 0, or include a single PDU that has the C bit set to 0
  have to be considered as forming a single logical-text-data-
  segment (LTDS).

  Every key=value pair, including the last or only pair in a LTDS,
  MUST be followed by one null (0x00) delimiter.

  A key-name is whatever precedes the first = in the key=value pair.
  The term key is used frequently in this document in place of key-
  name.

  A value is whatever follows the first = in the key=value pair up
  to the end of the key=value pair, but not including the null
  delimiter.

  The following definitions will be used in the rest of this
  document:

    - standard-label: A string of one or more characters that
      consist of letters, digits, dot, minus, plus, commercial at,
      or underscore. A standard-label MUST begin with a capital
      letter and must not exceed 63 characters.

    - key-name: A standard-label.

    - text-value: A string of zero or more characters that consist
      of letters, digits, dot, minus, plus, commercial at,
      underscore, slash, left bracket, right bracket, or colon.




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    - iSCSI-name-value: A string of one or more characters that
      consist of minus, dot, colon, or any character allowed by
      the output of the iSCSI string-prep template as specified in
      [RFC3722] (see also Section 4.2.7.2).

    - iSCSI-local-name-value: A UTF-8 string; no null characters
      are allowed in the string. This encoding is to be used for
      localized (internationalized) aliases.

    - boolean-value: The string "Yes" or "No".

    - hex-constant: A hexadecimal constant encoded as a string
      that starts with "0x" or "0X" followed by one or more digits
      or the letters a, b, c, d, e, f, A, B, C, D, E, or F. Hex-
      constants are used to encode numerical values or binary
      strings. When used to encode numerical values, the excessive
      use of leading 0 digits is discouraged. The string following
      0X (or 0x) represents a base16 number that starts with the
      most significant base16 digit, followed by all other digits
      in decreasing order of significance and ending with the
      least-significant base16 digit. When used to encode binary
      strings, hexadecimal constants have an implicit byte-length
      that includes four bits for every hexadecimal digit of the
      constant, including leading zeroes. For example, a hex-
      constant of n hexadecimal digits has a byte-length of (the
      integer part of) (n+1)/2.

    - decimal-constant: An unsigned decimal number with the digit
      0 or a string of one or more digits that start with a non-
      zero digit. Decimal-constants are used to encode numerical
      values or binary strings. Decimal constants can only be used
      to encode binary strings if the string length is explicitly
      specified. There is no implicit length for decimal strings.
      Decimal-constant MUST NOT be used for parameter values if
      the values can be equal or greater than 2**64 (numerical) or
      for binary strings that can be longer than 64 bits.

    - base64-constant: base64 constant encoded as a string that
      starts with "0b" or "0B" followed by 1 or more digits or
      letters or plus or slash or equal. The encoding is done
      according to [RFC4648].




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    - numerical-value: An unsigned integer always less than 2**64
      encoded as a decimal-constant or a hex-constant. Unsigned
      integer arithmetic applies to numerical-values.

    - large-numerical-value: An unsigned integer that can be
      larger than or equal to 2**64 encoded as a hex constant, or
      base64-constant. Unsigned integer arithmetic applies to
      large-numeric-values.

    - numeric-range: Two numerical-values separated by a tilde
      where the value to the right of tilde must not be lower than
      the value to the left.

    - regular-binary-value: A binary string not longer than 64
      bits encoded as a decimal constant, hex constant, or base64-
      constant. The length of the string is either specified by
      the key definition or is the implicit byte-length of the
      encoded string.

    - large-binary-value: A binary string longer than 64 bits
      encoded as a hex-constant or base64-constant. The length of
      the string is either specified by the key definition or is
      the implicit byte-length of the encoded string.

    - binary-value: A regular-binary-value or a large-binary-
      value. Operations on binary values are key specific.

    - simple-value: Text-value, iSCSI-name-value, boolean-value,
      numeric-value, a numeric-range, or a binary-value.

    - list-of-values: A sequence of text-values separated by a
      comma.

  If not otherwise specified, the maximum length of a simple-value
  (not its encoded representation) is 255 bytes not including the
  delimiter (comma or zero byte).

  Any iSCSI target or initiator MUST support receiving at least 8192
  bytes of key=value data in a negotiation sequence. When proposing
  or accepting authentication methods that explicitly require
  support for very long authentication items, the initiator and




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  target MUST support receiving of at least 64 kilobytes of
  key=value data.

6.2. Text Mode Negotiation

  During login, and thereafter, some session or connection
  parameters are either declared or negotiated through an exchange
  of textual information.

  The initiator starts the negotiation and/or declaration through a
  Text or Login request and indicates when it is ready for
  completion (by setting the F bit to 1 and keeping it to 1 in a
  Text Request or the T bit in the Login Request). As negotiation
  text may span PDU boundaries, a Text or Login Request or Text or
  Login Response PDU that have the C bit set to 1 MUST NOT have the
  F/T bit set to 1.

  A target receiving a Text or Login Request with the C bit set to 1
  MUST answer with a Text or Login Response with no data segment
  (DataSegmentLength 0). An initiator receiving a Text or Login
  Response with the C bit set to 1 MUST answer with a Text or Login
  Request with no data segment (DataSegmentLength 0).

  A target or initiator SHOULD NOT use a Text or Login Response or
  Text or Login Request with no data segment (DataSegmentLength 0)
  unless explicitly required by a general or a key-specific
  negotiation rule.

  There MUST NOT be more than one outstanding Text Request, or Text
  Response PDU on an iSCSI connection. An outstanding PDU in this
  context is one that has not been acknowledged by the remote iSCSI
  side.

  The format of a declaration is:

     Declarer-> <key>=<valuex>


  The general format of text negotiation is:

     Proposer-> <key>=<valuex>




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    Acceptor-> <key>={<valuey>|NotUnderstood|Irrelevant|Reject}


  Thus a declaration is a one-way textual exchange (unless the key
  is not understood by the receiver) while a negotiation is a two-
  way exchange.

  The proposer or declarer can either be the initiator or the
  target, and the acceptor can either be the target or initiator,
  respectively. Targets are not limited to respond to key=value
  pairs as proposed by the initiator. The target may propose
  key=value pairs of its own.

  All negotiations are explicit (i.e., the result MUST only be based
  on newly exchanged or declared values). There are no implicit
  proposals. If a proposal is not made, then a reply cannot be
  expected. Conservative design also requires that default values
  should not be relied upon when use of some other value has serious
  consequences.

  The value proposed or declared can be a numerical-value, a
  numerical-range defined by lower and upper value with both
  integers separated by tilde, a binary value, a text-value, an
  iSCSI-name-value, an iSCSI-local-name-value, a boolean-value (Yes
  or No), or a list of comma separated text-values. A range, a
  large-numerical-value, an iSCSI-name-value and an iSCSI-local-
  name-value MAY ONLY be used if it is explicitly allowed. An
  accepted value can be a numerical-value, a large-numerical-value,
  a text-value, or a boolean-value.

  If a specific key is not relevant for the current negotiation, the
  acceptor may answer with the constant "Irrelevant" for all types
  of negotiation. However the negotiation is not considered as
  failed if the answer is "Irrelevant". The "Irrelevant" answer is
  meant for those cases in which several keys are presented by a
  proposing party but the selection made by the acceptor for one of
  the keys makes other keys irrelevant. The following example
  illustrates the use of "Irrelevant":

  I->T InitialR2T=No,ImmediateData=Yes,FirstBurstLength=4192
  T->I InitialR2T=Yes,ImmediateData=No,FirstBurstLength=Irrelevant




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  I->T X-rdname-vkey1=(bla,alb,None), X-rdname-vkey2=(bla,alb)
  T->I X-rdname-vkey1=None, X-rdname-vkey2=Irrelevant

  Any key not understood by the acceptor may be ignored by the
  acceptor without affecting the basic function. However, the answer
  for a key not understood MUST be key=NotUnderstood. Note that
  NotUnderstood is a valid answer for both declarative and
  negotiated keys. The general iSCSI philosophy is that
  comprehension precedes processing for any iSCSI key. A proposer
  of an iSCSI key, negotiated or declarative, in a text key exchange
  MUST thus be able to properly handle a NotUnderstood response.

  The proper way to handle a NotUnderstood response depends on where
  the key is specified and whether the key is declarative vs.
  negotiated. An iSCSI implementation MUST comprehend all text keys
  defined in this document. Returning a NotUnderstood response on
  any of these text keys therefore MUST be considered a protocol
  error and handled accordingly. For all other "later" keys, i.e.
  text keys defined in later specifications, a NotUnderstood answer
  concludes the negotiation for a negotiated key whereas for a
  declarative key, a NotUnderstood answer simply informs the
  declarer of a lack of comprehension by the receiver.

  In either case, a NotUnderstood answer always requires that the
  protocol behavior associated with that key not be used within the
  scope of the key (connection/session) by either side.

  The constants "None", "Reject", "Irrelevant", and "NotUnderstood"
  are reserved and MUST ONLY be used as described here. Violation of
  this rule is a protocol error (in particular the use of "Reject",
  "Irrelevant", and "NotUnderstood" as proposed values).

  Reject or Irrelevant are legitimate negotiation options where
  allowed but their excessive use is discouraged. A negotiation is
  considered complete when the acceptor has sent the key value pair
  even if the value is "Reject", "Irrelevant", or "NotUnderstood".
  Sending the key again would be a re-negotiation and is forbidden
  for many keys.

  If the acceptor sends "Reject" as an answer the negotiated key is
  left at its current value (or default if no value was set). If the




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  current value is not acceptable to the proposer on the connection
  or to the session it is sent, the proposer MAY choose to terminate
  the connection or session.

  All keys in this document MUST be supported by iSCSI initiators
  and targets when used as specified here. If used as specified,
  these keys MUST NOT be answered with NotUnderstood.

  Implementers may introduce new private keys by prefixing them with
  X- followed by their (reversed) domain name, or with new public
  keys registered with IANA. For example, the entity owning the
  domain example.com can issue:

    X-com.example.bar.foo.do_something=3

  Each new public key in the course of standardization MUST define
  the acceptable responses to the key, including NotUnderstood as
  appropriate. Unlike [RFC3720], note that this document prohibits
  the X# prefix for new public keys. Based on iSCSI implementation
  experience, we know that there is no longer a need for a standard
  name prefix for keys that allow NotUnderstood response. Note that
  NotUnderstood will generally have to be allowed for new public
  keys for backwards compatibility, as well as for private X- keys.
  Thus the name prefix "X#" in new public key names does not carry
  any significance. New public key names MUST NOT begin with "X#"
  prefix to avoid confusion.

  Implementers MAY also introduce new values, but ONLY for new keys
  or authentication methods (see Section 12), or digests (see
  Section 13.1).

  Whenever parameter action or acceptance are dependent on other
  parameters, the dependency rules and parameter sequence must be
  specified with the parameters.

  In the Login Phase (see Section 6.3), every stage is a separate
  negotiation. In the FullFeaturePhase, a Text Request Response
  sequence is a negotiation. Negotiations MUST be handled as atomic
  operations. For example, all negotiated values go into effect
  after the negotiation concludes in agreement or are ignored if the
  negotiation fails.




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  Some parameters may be subject to integrity rules (e.g.,
  parameter-x must not exceed parameter-y or parameter-u not 1
  implies parameter-v be Yes). Whenever required, integrity rules
  are specified with the keys. Checking for compliance with the
  integrity rule must only be performed after all the parameters are
  available (the existent and the newly negotiated). An iSCSI target
  MUST perform integrity checking before the new parameters take
  effect. An initiator MAY perform integrity checking.

  An iSCSI initiator or target MAY terminate a negotiation that does
  not terminate within an implementation-specific reasonable time or
  number of exchanges, but SHOULD allow at least six (6) exchanges.

6.2.1. List negotiations

  In list negotiation, the originator sends a list of values (which
  may include "None") in its order of preference.

  The responding party MUST respond with the same key and the first
  value that it supports (and is allowed to use for the specific
  originator) selected from the originator list.

  The constant "None" MUST always be used to indicate a missing
  function. However, "None" is only a valid selection if it is
  explicitly proposed. When "None" is proposed as a selection item
  in a negotiation for a key, it indicates to the responder that not
  supporting any functionality related to that key is legal, and if
  "None" is the negotiation result for such a key, it means that
  key-specific semantics are not operational for the negotiation
  scope (connection or session) of that key.

  If an acceptor does not understand any particular value in a list,
  it MUST ignore it. If an acceptor does not support, does not
  understand, or is not allowed to use any of the proposed options
  with a specific originator, it may use the constant "Reject" or
  terminate the negotiation. The selection of a value not proposed
  MUST be handled by the originator as a protocol error.




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6.2.2. Simple-value Negotiations

   For simple-value negotiations, the accepting party MUST answer
   with the same key. The value it selects becomes the negotiation
   result.

   Proposing a value not admissible (e.g., not within the specified
   bounds) MAY be answered with the constant "Reject", otherwise the
   acceptor MUST select an admissible value.

   The selection, by the acceptor, of a value not admissible under
   the selection rules is considered a protocol error. The selection
   rules are key-specific.

   For a numerical range the value selected MUST be an integer within
   the proposed range or "Reject" (if the range is unacceptable).

   For Boolean negotiations (i.e., keys taking the values Yes or No),
   the accepting party MUST answer with the same key and the result
   of the negotiation when the received value does not determine that
   result by itself. The last value transmitted becomes the
   negotiation result. The rules for selecting the value to answer
   with are expressed as Boolean functions of the value received, and
   the value that the accepting party would have selected if given a
   choice.

   Specifically, the two cases in which answers are OPTIONAL are:

     - The Boolean function is "AND" and the value "No" is
       received. The outcome of the negotiation is "No".

     - The Boolean function is "OR" and the value "Yes" is
       received. The outcome of the negotiation is "Yes".


   Responses are REQUIRED in all other cases, and the value chosen
   and sent by the acceptor becomes the outcome of the negotiation.

6.3. Login Phase

   The Login Phase establishes an iSCSI connection between an
   initiator and a target; it creates also a new session or




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  associates the connection to an existing session. The Login Phase
  sets the iSCSI protocol parameters, security parameters, and
  authenticates the initiator and target to each other.

  The Login Phase is only implemented via Login request and
  responses. The whole Login Phase is considered as a single task
  and has a single Initiator Task Tag (similar to the linked SCSI
  commands).

  There MUST NOT be more than one outstanding Login Request, or
  Login Response on an iSCSI connection. An outstanding PDU in this
  context is one that has not been acknowledged by the remote iSCSI
  side.

  The default MaxRecvDataSegmentLength is used during Login.

  The Login Phase sequence of requests and responses proceeds as
  follows:

    - Login initial request

    - Login partial response (optional)

    - More Login requests and responses (optional)

    - Login Final-Response (mandatory)


  The initial login request of any connection MUST include the
  InitiatorName key=value pair. The initial login request of the
  first connection of a session MAY also include the SessionType
  key=value pair. For any connection within a session whose type is
  not "Discovery", the first login request MUST also include the
  TargetName key=value pair.

  The Login Final-response accepts or rejects the Login request.

  The Login Phase MAY include a SecurityNegotiation stage and a
  LoginOperationalNegotiation stage and MUST include at least one of
  them, but the included stage MAY be empty except for the mandatory
  names.




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  The login requests and responses contain a field (CSG) that
  indicates the current negotiation stage (SecurityNegotiation or
  LoginOperationalNegotiation). If both stages are used, the
  SecurityNegotiation MUST precede the LoginOperationalNegotiation.

  Some operational parameters can be negotiated outside the login
  through Text requests and responses.

  Authentication-related security keys (Section 12 ) MUST be
  completely negotiated within the Login Phase. The use of
  underlying IPsec security is specified in Section 9.3, in
  [RFC3723], and in [IPSEC-IPS}. iSCSI support for security within
  the protocol only consists of authentication in the Login Phase.

  In some environments, a target or an initiator is not interested
  in authenticating its counterpart. It is possible to bypass
  authentication through the Login request and response.

  The initiator and target MAY want to negotiate iSCSI
  authentication parameters. Once this negotiation is completed, the
  channel is considered secure.

  Most of the negotiation keys are only allowed in a specific stage.
  The SecurityNegotiation keys appear in Section 12 and the
  LoginOperationalNegotiation keys appear in Section 13. Only a
  limited set of keys (marked as Any-Stage in Section 13) may be
  used in any of the two stages.

  Any given Login request or response belongs to a specific stage;
  this determines the negotiation keys allowed with the request or
  response. It is considered to be a protocol error to send a key
  not allowed in the current stage.

  Stage transition is performed through a command exchange
  (request/response) that carries the T bit and the same CSG code.
  During this exchange, the next stage is selected by the target
  through the "next stage" code (NSG). The selected NSG MUST NOT
  exceed the value stated by the initiator. The initiator can
  request a transition whenever it is ready, but a target can only
  respond with a transition after one is proposed by the initiator.




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  In a negotiation sequence, the T bit settings in one pair of login
  request-responses have no bearing on the T bit settings of the
  next pair. An initiator that has a T bit set to 1 in one pair and
  is answered with a T bit setting of 0 may issue the next request
  with T bit set to 0.

  When a transition is requested by the initiator and acknowledged
  by the target, both the initiator and target switch to the
  selected stage.

  Targets MUST NOT submit parameters that require an additional
  initiator login request in a login response with the T bit set to
  1.

  Stage transitions during login (including entering and exit) are
  only possible as outlined in the following table:

  +-----------------------------------------------------------+
  |From      To ->  | Security    | Operational | FullFeature |
  | |               |             |             |             |
  | V               |             |             |             |
  +-----------------------------------------------------------+
  | (start)         | yes         | yes         | no          |
  +-----------------------------------------------------------+
  | Security        | no          | yes         | yes         |
  +-----------------------------------------------------------+
  | Operational     | no          | no          | yes         |
  +-----------------------------------------------------------+

  The Login Final-Response that accepts a Login Request can only
  come as a response to a Login request with the T bit set to 1, and
  both the request and response MUST indicate FullFeaturePhase as
  the next phase via the NSG field.

  Neither the initiator nor the target should attempt to declare or
  negotiate a parameter more than once during login except for
  responses to specific keys that explicitly allow repeated key
  declarations (e.g., TargetAddress). An attempt to
  renegotiate/redeclare parameters not specifically allowed MUST be
  detected by the initiator and target. If such an attempt is
  detected by the target, the target MUST respond with Login reject




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  (initiator error); if detected by the initiator, the initiator
  MUST drop the connection.

6.3.1. Login Phase Start

  The Login Phase starts with a login request from the initiator to
  the target. The initial login request includes:

     -Protocol version supported by the initiator.

     -iSCSI Initiator Name and iSCSI Target Name

     -ISID, TSIH, and connection Ids

     -Negotiation stage that the initiator is ready to enter.


  A login may create a new session or it may add a connection to an
  existing session. Between a given iSCSI Initiator Node (selected
  only by an InitiatorName) and a given iSCSI target defined by an
  iSCSI TargetName and a Target Portal Group Tag, the login results
  are defined by the following table:


  +----------------------------------------------------------------+
  |ISID    | TSIH        | CID    |     Target action              |
  +----------------------------------------------------------------+
  |new     | non-zero    | any    |     fail the login             |
  |        |             |        |     ("session does not exist") |
  +----------------------------------------------------------------+
  |new     | zero        | any    |     instantiate a new session |
  +----------------------------------------------------------------+
  |existing| zero        | any    |     do session reinstatement   |
  |        |             |        |    (see Section 6.3.5)         |
  +----------------------------------------------------------------+




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  |existing| non-zero    | new    |     add a new connection to    |
  |        | existing    |        |     the session                |
  +----------------------------------------------------------------+
  |existing| non-zero    |existing|     do connection reinstatement|
  |        | existing    |        |    (see Section 7.1.4.3)       |
  +----------------------------------------------------------------+
  |existing| non-zero    | any    |         fail the login         |
  |        | new         |        |     ("session does not exist") |
  +----------------------------------------------------------------+


  Determination of "existing" or "new" are made by the target.

  Optionally, the login request may include:

    -Security parameters
      OR

    -iSCSI operational parameters
      AND/OR

    -The next negotiation stage that the initiator is ready to
      enter.


  The target can answer the login in the following ways:

    -Login Response with Login reject. This is an immediate
      rejection from the target that causes the connection to
      terminate and the session to terminate if this is the first
      (or only) connection of a new session. The T bit and the CSG
      and NSG fields are reserved.

    -Login Response with Login accept as a final response (T bit
      set to 1 and the NSG in both request and response are set to
      FullFeaturePhase). The response includes the protocol
      version supported by the target and the session ID, and may
      include iSCSI operational or security parameters (that
      depend on the current stage).

    -Login Response with Login Accept as a partial response (NSG
      not set to FullFeaturePhase in both request and response)




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      that indicates the start of a negotiation sequence. The
      response includes the protocol version supported by the
      target and either security or iSCSI parameters (when no
      security mechanism is chosen) supported by the target.


  If the initiator decides to forego the SecurityNegotiation stage,
  it issues the Login with the CSG set to
  LoginOperationalNegotiation and the target may reply with a Login
  Response that indicates that it is unwilling to accept the
  connection (see Section 11.13) without SecurityNegotiation and
  will terminate the connection with a response of Authentication
  failure (see Section 11.13.5).

  If the initiator is willing to negotiate iSCSI security, but is
  unwilling to make the initial parameter proposal and may accept a
  connection without iSCSI security, it issues the Login with the T
  bit set to 1, the CSG set to SecurityNegotiation, and NSG set to
  LoginOperationalNegotiation. If the target is also ready to skip
  security, the login response only contains the
  TargetPortalGroupTag key (see Section 13.9), the T bit set to 1,
  the CSG set to SecurityNegotiation, and NSG set to
  LoginOperationalNegotiation.

  An initiator that chooses to operate without iSCSI security and
  with all the operational parameters taking the default values
  issues the Login with the T bit set to 1, the CSG set to
  LoginOperationalNegotiation, and NSG set to FullFeaturePhase. If
  the target is also ready to forego security and can finish its
  LoginOperationalNegotiation, the Login response has T bit set to
  1, the CSG set to LoginOperationalNegotiation, and NSG set to
  FullFeaturePhase in the next stage.

  During the Login Phase the iSCSI target MUST return the
  TargetPortalGroupTag key with the first Login Response PDU with
  which it is allowed to do so (i.e., the first Login Response
  issued after the first Login Request with the C bit set to 0) for
  all session types. The TargetPortalGroupTag key value indicates
  the iSCSI portal group servicing the Login Request PDU. If the
  reconfiguration of iSCSI portal groups is a concern in a given
  environment, the iSCSI initiator should use this key to ascertain




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  that it had indeed initiated the Login Phase with the intended
  target portal group.

6.3.2. iSCSI Security Negotiation

  The security exchange sets the security mechanism and
  authenticates the initiator user and the target to each other. The
  exchange proceeds according to the authentication method chosen in
  the negotiation phase and is conducted using the login requests'
  and responses' key=value parameters.

  An initiator directed negotiation proceeds as follows:

     -The initiator sends a login request with an ordered list of
       the options it supports (authentication algorithm). The
       options are listed in the initiator's order of preference.
       The initiator MAY also send private or public extension
       options.
       -The target MUST reply with the first option in the list it
       supports and is allowed to use for the specific initiator
       unless it does not support any in which case it MUST answer
       with "Reject" (see Section 6.2). The parameters are encoded
       in UTF8 as key=value. For security parameters, see Section
       12.

     -When the initiator considers that it is ready to conclude the
       SecurityNegotiation stage, it sets the T bit to 1 and the
       NSG to what it would like the next stage to be. The target
       will then set the T bit to 1 and set NSG to the next stage
       in the Login response when it finishes sending its security
       keys. The next stage selected will be the one the target
       selected. If the next stage is FullFeaturePhase, the target
       MUST respond with a Login Response with the TSIH value.


  If the security negotiation fails at the target, then the target
  MUST send the appropriate Login Response PDU. If the security
  negotiation fails at the initiator, the initiator SHOULD close the
  connection.

  It should be noted that the negotiation might also be directed by
  the target if the initiator does support security, but is not




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  ready to direct the negotiation (propose options) - see Appendix B
  for an example.


6.3.3. Operational Parameter Negotiation During the Login Phase

  Operational parameter negotiation during the login MAY be done:

     - Starting with the first Login request if the initiator does
       not propose any security/ integrity option.

     - Starting immediately after the security negotiation if the
       initiator and target perform such a negotiation.

  Operational parameter negotiation MAY involve several Login
  request-response exchanges started and terminated by the
  initiator. The initiator MUST indicate its intent to terminate the
  negotiation by setting the T bit to 1; the target sets the T bit
  to 1 on the last response.

  Even when the initiator indicates its intent to switch stage by
  setting the T bit to 1 in a Login request, the target MAY respond
  with a Login response with the T bit set to 0. In that case, the
  initiator SHOULD continue to set the T bit to 1 in subsequent
  Login requests (even empty) that it sends, until target sends a
  Login response with the T bit set to 1 or sends a key that
  requires initiator to set the T bit to 0.

  Some session specific parameters can only be specified during the
  Login Phase of the first connection of a session (i.e., begun by a
  login request that contains a zero-valued TSIH) - the leading
  Login Phase (e.g., the maximum number of connections that can be
  used for this session).

  A session is operational once it has at least one connection in
  FullFeaturePhase. New or replacement connections can only be added
  to a session after the session is operational.

  For operational parameters, see Section 13.




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6.3.4. Connection Reinstatement

  Connection reinstatement is the process of an initiator logging in
  with a ISID-TSIH-CID combination that is possibly active from the
  target's perspective, which causes the implicit logging out of the
  connection corresponding to the CID and reinstating a new Full
  Feature Phase iSCSI connection in its place (with the same CID).
  Thus, the TSIH in the Login Request PDU MUST be non-zero and CID
  does not change during a connection reinstatement. The Login
  request performs the logout function of the old connection if an
  explicit logout was not performed earlier. In sessions with a
  single connection, this may imply the opening of a second
  connection with the sole purpose of cleaning up the first. Targets
  MUST support opening a second connection even when they do not
  support multiple connections in Full Feature Phase if
  ErrorRecoveryLevel is 2 and SHOULD support opening a second
  connection if ErrorRecoveryLevel is less than 2.

  If the operational ErrorRecoveryLevel is 2, connection
  reinstatement enables future task reassignment. If the operational
  ErrorRecoveryLevel is less than 2, connection reinstatement is the
  replacement of the old CID without enabling task reassignment. In
  this case, all the tasks that were active on the old CID must be
  immediately terminated without further notice to the initiator.

  The initiator connection state MUST be CLEANUP_WAIT (Section
  8.1.3) when the initiator attempts a connection reinstatement.

  In practical terms, in addition to the implicit logout of the old
  connection, reinstatement is equivalent to a new connection login.

6.3.5. Session Reinstatement, Closure, and Timeout

  Session reinstatement is the process of the initiator logging in
  with an ISID that is possibly active from the target's
  perspective. Thus implicitly logging out the session that
  corresponds to the ISID and reinstating a new iSCSI session in its
  place (with the same ISID). Therefore, the TSIH in the Login PDU
  MUST be zero to signal session reinstatement. Session
  reinstatement causes all the tasks that were active on the old
  session to be immediately terminated by the target without further
  notice to the initiator.




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  The initiator session state MUST be FAILED (Section 8.3) when the
  initiator attempts a session reinstatement.

  Session closure is an event defined to be one of the following:

     - A successful "session close" logout.

     - A successful "connection close" logout for the last Full
       Feature Phase connection when no other connection in the
       session is waiting for cleanup (Section 8.2) and no tasks in
       the session are waiting for reassignment.


  Session timeout is an event defined to occur when the last
  connection state timeout expires and no tasks are waiting for
  reassignment. This takes the session to the FREE state (N6
  transition in the session state diagram).

6.3.5.1. Loss of Nexus Notification

  The iSCSI layer provides the SCSI layer with the "I_T nexus loss"
  notification when any one of the following events happens:

     Successful completion of session reinstatement.
     Session closure event.
     Session timeout event.

  Certain SCSI object clearing actions may result due to the
  notification in the SCSI end nodes, as documented in Appendix E.

6.3.6. Session Continuation and Failure

  Session continuation is the process by which the state of a
  preexisting session continues to be used by connection
  reinstatement (Section 6.3.4), or by adding a connection with a
  new CID. Either of these actions associates the new transport
  connection with the session state.

  Session failure is an event where the last Full Feature Phase
  connection reaches the CLEANUP_WAIT state (Section 8.2), or
  completes a successful recovery logout thus causing all active




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  tasks (that are formerly allegiant to the connection) to start
  waiting for task reassignment.

6.4. Operational Parameter Negotiation Outside the Login Phase

  Some operational parameters MAY be negotiated outside (after) the
  Login Phase.

  Parameter negotiation in Full Feature Phase is done through Text
  requests and responses. Operational parameter negotiation MAY
  involve several Text request-response exchanges, which the
  initiator always starts, terminates, and uses the same Initiator
  Task Tag. The initiator MUST indicate its intent to finish the
  negotiation by setting the F bit to 1; the target sets the F bit
  to 1 on the last response.

  If the target responds to a Text request with the F bit set to 1
  with a Text response with the F bit set to 0, the initiator should
  keep sending the Text request (even empty) with the F bit set to
  1, while it still wants to finish the negotiation, until it
  receives the Text response with the F bit set to 1. Responding to
  a Text request with the F bit set to 1 with an empty (no key=value
  pairs) response with the F bit set to 0 is discouraged.

  Even when the initiator indicates its intent to finish the
  negotiation by setting the F bit to 1 in a Text request, the
  target MAY respond with a Text response with the F bit set to 0.
  In that case, the initiator SHOULD continue to set the F bit to 1
  in subsequent Text requests (even empty) that it sends, until
  target sends the final Text response with the F bit set to 1. Note
  that in the same case of Text request with the F bit set to 1,
  target SHOULD NOT respond with an empty (no key=value pairs) Text
  response with the F bit set to 0, because such a response may
  cause the initiator to abandon negotiation.

  Targets MUST NOT submit parameters that require an additional
  initiator Text request in a Text response with the F bit set to 1.

  In a negotiation sequence, the F bit settings in one pair of Text
  request-responses have no bearing on the F bit settings of the
  next pair. An initiator that has the F bit set to 1 in a request




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  and is being answered with an F bit setting of 0 may issue the
  next request with the F bit set to 0.

  Whenever the target responds with the F bit set to 0, it MUST set
  the Target Transfer Tag to a value other than the default
  0xffffffff.

  An initiator MAY reset an operational parameter negotiation by
  issuing a Text request with the Target Transfer Tag set to the
  value 0xffffffff after receiving a response with the Target
  Transfer Tag set to a value other than 0xffffffff. A target may
  reset an operational parameter negotiation by answering a Text
  request with a Reject PDU.

  Neither the initiator nor the target should attempt to declare or
  negotiate a parameter more than once during any negotiation
  sequence, except for responses to specific keys that explicitly
  allow repeated key declarations (e.g., TargetAddress). If detected
  by the target, this MUST result in a Reject PDU with a reason of
  "protocol error". The initiator MUST reset the negotiation as
  outlined above.

  Parameters negotiated by a text exchange negotiation sequence only
  become effective after the negotiation sequence is completed.




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7. iSCSI Error Handling and Recovery

7.1. Overview

7.1.1. Background

   The following two considerations prompted the design of much of
   the error recovery functionality in iSCSI:

     An iSCSI PDU may fail the digest check and be dropped, despite
        being received by the TCP layer. The iSCSI layer must
        optionally be allowed to recover such dropped PDUs.

     A TCP connection may fail at any time during the data
        transfer. All the active tasks must optionally be allowed
        to be continued on a different TCP connection within the
        same session.

   Implementations have considerable flexibility in deciding what
   degree of error recovery to support, when to use it and by which
   mechanisms to achieve the required behavior. Only the externally
   visible actions of the error recovery mechanisms must be
   standardized to ensure interoperability.

   This Section describes a general model for recovery in support of
   interoperability. See Appendix D for further detail on how the
   described model may be implemented. Compliant implementations do
   not have to match the implementation details of this model as
   presented, but the external behavior of such implementations must
   correspond to the externally observable characteristics of the
   presented model.

7.1.2. Goals

   The major design goals of the iSCSI error recovery scheme are as
   follows:

      Allow iSCSI implementations to meet different requirements by
       defining a collection of error recovery mechanisms that
       implementations may choose from.




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     Ensure interoperability between any two implementations
       supporting different sets of error recovery capabilities.

      Define the error recovery mechanisms to ensure command
       ordering even in the face of errors, for initiators that
       demand ordering.

     Do not make additions in the fast path, but allow moderate
       complexity in the error recovery path.

      Prevent both the initiator and target from attempting to
       recover the same set of PDUs at the same time. For example,
       there must be a clear "error recovery functionality
       distribution" between the initiator and target.


7.1.3. Protocol Features and State Expectations

  The initiator mechanisms defined in connection with error recovery
  are:

       a) NOP-OUT to probe sequence numbers of the target (Section
          11.18)
       b) Command retry (Section 7.2.1)
       c) Recovery R2T support (Section 7.8)
       d) Requesting retransmission of status/data/R2T using the
          SNACK facility (Section 11.16)
       e) Acknowledging the receipt of the data (Section 11.16)
       f) Reassigning the connection allegiance of a task to a
          different TCP connection (Section 7.2.2)
       g) Terminating the entire iSCSI session to start afresh
          (Section 7.1.4.4)

  The target mechanisms defined in connection with error recovery
  are:

     a) NOP-IN to probe sequence numbers of the initiator (Section
        11.19)
     b) Requesting retransmission of data using the recovery R2T
        feature (Section 7)
     c) SNACK support (Section 11.16)




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     d) Requesting that parts of read data be acknowledged (Section
        11.7.2)
     e) Allegiance reassignment support (Section 7.2.2)
     f) Terminating the entire iSCSI session to force the initiator
        to start over (Section 7.1.4.4)

  For any outstanding SCSI command, it is assumed that iSCSI, in
  conjunction with SCSI at the initiator, is able to keep enough
  information to be able to rebuild the command PDU, and that
  outgoing data is available (in host memory) for retransmission
  while the command is outstanding. It is also assumed that at the
  target, incoming data (read data) MAY be kept for recovery or it
  can be reread from a device server.

  It is further assumed that a target will keep the "status & sense"
  for a command it has executed if it supports status
  retransmission.

  A target that agrees to support data retransmission is expected to
  be prepared to retransmit the outgoing data (i.e., Data-In) on
  request until either the status for the completed command is
  acknowledged, or the data in question has been separately
  acknowledged.

7.1.4. Recovery Classes

  iSCSI enables the following classes of recovery (in the order of
  increasing scope of affected iSCSI tasks):

     - Within a command (i.e., without requiring command restart).

     - Within a connection (i.e., without requiring the connection
       to be rebuilt, but perhaps requiring command restart).

     - Connection recovery (i.e., perhaps requiring connections to
       be rebuilt and commands to be reissued).

     - Session recovery.




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  The recovery scenarios detailed in the rest of this Section are
  representative rather than exclusive. In every case, they detail
  the lowest class recovery that MAY be attempted. The implementer
  is left to decide under which circumstances to escalate to the
  next recovery class and/or what recovery classes to implement.
  Both the iSCSI target and initiator MAY escalate the error
  handling to an error recovery class, which impacts a larger number
  of iSCSI tasks in any of the cases identified in the following
  discussion.

  In all classes, the implementer has the choice of deferring errors
  to the SCSI initiator (with an appropriate response code), in
  which case the task, if any, has to be removed from the target and
  all the side-effects, such as ACA, must be considered.

  Use of within-connection and within-command recovery classes MUST
  NOT be attempted before the connection is in Full Feature Phase.

  In the detailed description of the recovery classes, the mandating
  terms (MUST, SHOULD, MAY, etc.) indicate normative actions to be
  executed if the recovery class is supported (see Section 7.1.5 for
  the related negotiation semantics) and used.

7.1.4.1. Recovery Within-command

  At the target, the following cases lend themselves to within-
  command recovery:

     a) Lost data PDU - realized through one of the following:
     b) Data digest error - dealt with as specified in Section 7.8,
        using the option of a recovery R2T.
     c) Sequence reception timeout (no data or partial-data-and-no-F-
        bit) - considered an implicit sequence error and dealt with
        as specified in Section 7.9, using the option of a recovery
        R2T.
     d) Header digest error, which manifests as a sequence reception
        timeout or a sequence error - dealt with as specified in
        Section 7.9, using the option of a recovery R2T.


  At the initiator, the following cases lend themselves to within-
  command recovery:




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     a) Lost data PDU or lost R2T - realized through one of the
        following:
     b) Data digest error - dealt with as specified in Section 7.8,
        using the option of a SNACK.
     c) Sequence reception timeout (no status) or response reception
        timeout - dealt with as specified in Section 7.9, using the
        option of a SNACK.
     d) Header digest error, which manifests as a sequence reception
        timeout or a sequence error - dealt with as specified in
        Section 7.9, using the option of a SNACK.


  To avoid a race with the target, which may already have a recovery
  R2T or a termination response on its way, an initiator SHOULD NOT
  originate a SNACK for an R2T based on its internal timeouts (if
  any). Recovery in this case is better left to the target.


  The timeout values used by the initiator and target are outside
  the scope of this document. Sequence reception timeout is
  generally a large enough value to allow the data sequence transfer
  to be complete.

7.1.4.2. Recovery Within-connection

  At the initiator, the following cases lend themselves to within-
  connection recovery:

     a) Requests not acknowledged for a long time. Requests are
        acknowledged explicitly through ExpCmdSN or implicitly by
        receiving data and/or status. The initiator MAY retry non-
        acknowledged commands as specified in Section 7.2.
     b) Lost iSCSI numbered Response. It is recognized by either
        identifying a data digest error on a Response PDU or a Data-
        In PDU carrying the status, or by receiving a Response PDU
        with a higher StatSN than expected. In the first case, digest
        error handling is done as specified in Section 7.8 using the
        option of a SNACK. In the second case, sequence error
        handling is done as specified in Section 7.9, using the
        option of a SNACK.




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  At the target, the following cases lend themselves to within-
  connection recovery:

     - Status/Response not acknowledged for a long time. The target
       MAY issue a NOP-IN (with a valid Target Transfer Tag or
       otherwise) that carries the next status sequence number it
       is going to use in the StatSN field. This helps the
       initiator detect any missing StatSN(s) and issue a SNACK for
       the status.


  The timeout values used by the initiator and the target are
  outside the scope of this document.

7.1.4.3. Connection Recovery

  At an iSCSI initiator, the following cases lend themselves to
  connection recovery:

     a) TCP connection failure: The initiator MUST close the
        connection. It then MUST either implicitly or explicitly
        logout the failed connection with the reason code "remove the
        connection for recovery" and reassign connection allegiance
        for all commands still in progress associated with the failed
        connection on one or more connections (some or all of which
        MAY be newly established connections) using the "Task
        reassign" task management function (see Section 11.5.1). For
        an initiator, a command is in progress as long as it has not
        received a response or a Data-In PDU including status.

        Note: The logout function is mandatory. However, a new
        connection establishment is only mandatory if the failed
        connection was the last or only connection in the session.
     b) Receiving an Asynchronous Message that indicates one or all
        connections in a session has been dropped. The initiator
        MUST handle it as a TCP connection failure for the
        connection(s) referred to in the Message.

  At an iSCSI target, the following cases lend themselves to
  connection recovery:




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     - TCP connection failure. The target MUST close the connection
       and, if more than one connection is available, the target
       SHOULD send an Asynchronous Message that indicates it has
       dropped the connection. Then, the target will wait for the
       initiator to continue recovery.


7.1.4.4. Session Recovery

  Session recovery should be performed when all other recovery
  attempts have failed. Very simple initiators and targets MAY
  perform session recovery on all iSCSI errors and rely on recovery
  on the SCSI layer and above.

  Session recovery implies the closing of all TCP connections,
  internally aborting all executing and queued tasks for the given
  initiator at the target, terminating all outstanding SCSI commands
  with an appropriate SCSI service response at the initiator, and
  restarting a session on a new set of connection(s) (TCP connection
  establishment and login on all new connections).

  For possible clearing effects of session recovery on SCSI and
  iSCSI objects, refer to Appendix E.

7.1.5. Error Recovery Hierarchy

  The error recovery classes described so far are organized into a
  hierarchy for ease in understanding and to limit the
  implementation complexity. With few and well defined recovery
  levels interoperability is easier to achieve. The attributes of
  this hierarchy are as follows:

       a) Each level is a superset of the capabilities of the
          previous level. For example, Level 1 support implies
          supporting all capabilities of Level 0 and more.
       b) As a corollary, supporting a higher error recovery level
          means increased sophistication and possibly an increase
          in resource requirements.
       c) Supporting error recovery level "n" is advertised and
          negotiated by each iSCSI entity by exchanging the text
          key "ErrorRecoveryLevel=n". The lower of the two




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          exchanged values is the operational ErrorRecoveryLevel
          for the session.

  The following diagram represents the error recovery hierarchy.

                                +
                               / \
                              / 2 \        <-- Connection recovery
                             +-----+
                           /    1   \     <-- Digest failure recovery
                          +----------+
                        /       0     \   <-- Session failure recovery
                       +---------------+

  The following table lists the error recovery capabilities expected
  from the implementations that support each error recovery level.

  +-------------------+--------------------------------------------+
  |ErrorRecoveryLevel | Associated Error recovery capabilities     |
  +-------------------+--------------------------------------------+
  |        0          | Session recovery class                     |
  |                   | (Session Recovery)         |
  +-------------------+--------------------------------------------+
  |        1          | Digest failure recovery (See Note below.) |
  |                   | plus the capabilities of ER Level 0        |
  +-------------------+--------------------------------------------+
  |        2          | Connection recovery class                  |
  |                   | (Connection Recovery)      |
  |                   | plus the capabilities of ER Level 1        |
  +-------------------+--------------------------------------------+

  Note: Digest failure recovery is comprised of two recovery
  classes: Within-Connection recovery class (Recovery Within-
  connection) and Within-Command recovery class (Recovery Within-
  command).

  When a defined value of ErrorRecoveryLevel is proposed by an
  originator in a text negotiation, the originator MUST support the
  functionality defined for the proposed value and additionally,
  functionality corresponding to any defined value numerically less
  than the proposed. When a defined value of ErrorRecoveryLevel is




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  returned by a responder in a text negotiation, the responder MUST
  support the functionality corresponding to the ErrorRecoveryLevel
  it is accepting.

  When either party attempts to use error recovery functionality
  beyond what is negotiated, the recovery attempts MAY fail unless
  an apriori agreement outside the scope of this document exists
  between the two parties to provide such support.

  Implementations MUST support error recovery level "0", while the
  rest are OPTIONAL to implement. In implementation terms, the
  above striation means that the following incremental
  sophistication with each level is required.

  +-------------------+--------------------------------------------+
  |Level transition   | Incremental requirement                    |
  +-------------------+--------------------------------------------+
  |        0->1       | PDU retransmissions on the same connection|
  +-------------------+--------------------------------------------+
  |        1->2       | Retransmission across connections and      |
  |                   | allegiance reassignment                    |
  +-------------------+--------------------------------------------+

7.2. Retry and Reassign in Recovery

  This Section summarizes two important and somewhat related iSCSI
  protocol features used in error recovery.

7.2.1. Usage of Retry

  By resending the same iSCSI command PDU ("retry") in the absence
  of a command acknowledgement (by way of an ExpCmdSN update) or a
  response, an initiator attempts to "plug" (what it thinks are) the
  discontinuities in CmdSN ordering on the target end. Discarded
  command PDUs, due to digest errors, may have created these
  discontinuities.

  Retry MUST NOT be used for reasons other than plugging command
  sequence gaps, and in particular, cannot be used for requesting
  PDU retransmissions from a target. Any such PDU retransmission
  requests for a currently allegiant command in progress may be made




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  using the SNACK mechanism described in Section 11.16, although the
  usage of SNACK is OPTIONAL.

  If initiators, as part of plugging command sequence gaps as
  described above, inadvertently issue retries for allegiant
  commands already in progress (i.e., targets did not see the
  discontinuities in CmdSN ordering), the duplicate commands are
  silently ignored by targets as specified in Section 4.2.2.1.

  When an iSCSI command is retried, the command PDU MUST carry the
  original Initiator Task Tag and the original operational
  attributes (e.g., flags, function names, LUN, CDB etc.) as well as
  the original CmdSN. The command being retried MUST be sent on the
  same connection as the original command unless the original
  connection was already successfully logged out.

7.2.2. Allegiance Reassignment

  By issuing a "task reassign" task management request (Section
  11.5.1), the initiator signals its intent to continue an already
  active command (but with no current connection allegiance) as part
  of connection recovery. This means that a new connection
  allegiance is requested for the command, which seeks to associate
  it to the connection on which the task management request is being
  issued. Before the allegiance reassignment is attempted for a
  task, an implicit or explicit Logout with the reason code "remove
  the connection for recovery" (see Section 11.14.1) MUST be
  successfully completed for the previous connection to which the
  task was allegiant.

  In reassigning connection allegiance for a command, the targets
  SHOULD continue the command from its current state. For example,
  when reassigning read commands, the target SHOULD take advantage
  of the ExpDataSN field provided by the Task Management function
  request (which must be set to zero if there was no data transfer)
  and bring the read command to completion by sending the remaining
  data and sending (or resending) the status. ExpDataSN
  acknowledges all data sent up to, but not including, the Data-In
  PDU and or R2T with DataSN (or R2TSN) equal to ExpDataSN. However,
  targets may choose to send/receive all unacknowledged data or all
  of the data on a reassignment of connection allegiance if unable
  to recover or maintain accurate state. Initiators MUST NOT




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  subsequently request data retransmission through Data SNACK for
  PDUs numbered less than ExpDataSN (i.e., prior to the acknowledged
  sequence number). For all types of commands, a reassignment
  request implies that the task is still considered in progress by
  the initiator and the target must conclude the task appropriately
  if the target returns the "Function Complete" response to the
  reassignment request. This might possibly involve retransmission
  of data/R2T/status PDUs as necessary, but MUST involve the
  (re)transmission of the status PDU.

  It is OPTIONAL for targets to support the allegiance reassignment.
  This capability is negotiated via the ErrorRecoveryLevel text key
  during the login time. When a target does not support allegiance
  reassignment, it MUST respond with a Task Management response code
  of "Allegiance reassignment not supported". If allegiance
  reassignment is supported by the target, but the task is still
  allegiant to a different connection, or a successful recovery
  Logout of the previously allegiant connection was not performed,
  the target MUST respond with a Task Management response code of
  "Task still allegiant".

  If allegiance reassignment is supported by the target, the Task
  Management response to the reassignment request MUST be issued
  before the reassignment becomes effective.

  If a SCSI Command that involves data input is reassigned, any
  SNACK Tag it holds for a final response from the original
  connection is deleted and the default value of 0 MUST be used
  instead.

7.3. Usage Of Reject PDU in Recovery

  Targets MUST NOT implicitly terminate an active task by sending a
  Reject PDU for any PDU exchanged during the life of the task. If
  the target decides to terminate the task, a Response PDU (SCSI,
  Text, Task, etc.) must be returned by the target to conclude the
  task. If the task had never been active before the Reject (i.e.,
  the Reject is on the command PDU), targets should not send any
  further responses because the command itself is being discarded.

  The above rule means that the initiator can eventually expect a
  response on receiving Rejects, if the received Reject is for a PDU




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  other than the command PDU itself. The non-command Rejects only
  have diagnostic value in logging the errors, and they can be used
  for retransmission decisions by the initiators.

  The CmdSN of the rejected command PDU (if it is a non-immediate
  command) MUST NOT be considered received by the target (i.e., a
  command sequence gap must be assumed for the CmdSN), even though
  the CmdSN of the rejected command PDU may be reliably ascertained.
  Upon receiving the Reject, the initiator MUST plug the CmdSN gap
  in order to continue to use the session. The gap may be plugged
  either by transmitting a command PDU with the same CmdSN, or by
  aborting the task (see SCS on how an abort may plug a CmdSN gap).

  When a data PDU is rejected and its DataSN can be ascertained, a
  target MUST advance ExpDataSN for the current data burst if a
  recovery R2T is being generated. The target MAY advance its
  ExpDataSN if it does not attempt to recover the lost data PDU.

7.4. Error Recovery Considerations for Discovery Sessions

7.4.1. ErrorRecoveryLevel for Discovery Sessions

  The negotiation of the key ErrorRecoveryLevel is not required for
  Discovery sessions -- i.e., for sessions that negotiated
  "SessionType=Discovery" -- because the default value of 0 is
  necessary and sufficient for Discovery sessions. It is however
  possible that some legacy iSCSI implementations might attempt to
  negotiate the ErrorRecoveryLevel key on Discovery sessions. When
  such a negotiation attempt is made by the remote side, a compliant
  iSCSI implementation MUST propose a value of 0 (zero) in response.
  The operational ErrorRecoveryLevel for Discovery sessions thus
  MUST
  be 0. This naturally follows from the functionality constraints
  that Section 4.3 imposes on Discovery sessions.

7.4.2. Reinstatement Semantics for Discovery Sessions

  Discovery sessions are intended to be relatively short-lived.
  Initiators are not expected to establish multiple Discovery
  sessions to the same iSCSI Network Portal. An initiator may use
  the same iSCSI Initiator Name and ISID when establishing different
  unique sessions with different targets and/or different portal




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  groups. This behavior is discussed in Section 10.1.1 and is, in
  fact, encouraged as conservative reuse of ISIDs.

  The ISID RULE in Section 4.4.3 states that there must not be more
  than one session with a matching 4-tuple: <InitiatorName, ISID,
  TargetName, TargetPortalGroupTag>. While the spirit of the ISID
  RULE applies to Discovery sessions the same as it does for Normal
  sessions, note that some Discovery sessions differ from the Normal
  sessions in two important aspects:

       a) Because Appendix C allows a Discovery session to be
          established without specifying a TargetName key in the
          Login Request PDU (let us call such a session an "Unnamed"
          Discovery session), there is no Target Node context to
          enforce the ISID RULE.

       b) Portal Groups are defined only in the context of a Target
          Node. When the TargetName key is NULL-valued (i.e., not
          specified), the TargetPortalGroupTag thus cannot be
          ascertained to enforce the ISID RULE.

  The following two sections describe each of the two scenarios --
  Named Discovery sessions and Unnamed Discovery sessions.

7.4.2.1. Unnamed Discovery Sessions

  For Unnamed Discovery sessions, neither the TargetName nor the
  TargetPortalGroupTag is available to the targets in order to
  enforce the ISID RULE. So the following rule applies.

  UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
  following 4-tuple for Unnamed Discovery sessions: <InitiatorName,
  ISID, NULL, TargetAddress>. The following semantics are implied by
  this uniqueness requirement.

  Targets SHOULD allow concurrent establishment of one Discovery
  session with each of its Network Portals by the same initiator
  port with a given iSCSI Node Name and an ISID. Each of the
  concurrent Discovery sessions, if established by the same
  initiator port to other Network Portals, MUST be treated as
  independent sessions -- i.e., one session MUST NOT reinstate the
  other.




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  A new Unnamed Discovery session that has a matching
  <InitiatorName, ISID, NULL, TargetAddress> to an existing
  Discovery session MUST reinstate the existing Unnamed Discovery
  session. Note thus that only an Unnamed Discovery session may
  reinstate an Unnamed Discovery session.

7.4.2.2. Named Discovery Session

  For a Named Discovery session, the TargetName key is specified by
  the initiator and thus the target can unambiguously ascertain the
  TargetPortalGroupTag as well. Since all the four elements of the
  4-tuple are known, the ISID RULE MUST be enforced by targets with
  no changes from Section 4.4.3 semantics. A new session with a
  matching <InitiatorName, ISID, TargetName, TargetPortalGroupTag>
  thus will reinstate an existing session. Note in this case that
  any new iSCSI session (Discovery or Normal) with the matching 4-
  tuple may reinstate an existing Named Discovery iSCSI session.

7.4.3. Target PDUs During Discovery

  Targets SHOULD NOT send any responses other than a Text Response
  and Logout Response on a Discovery session, once in Full Feature
  Phase.

  Implementation Note: A target may simply drop the connection in a
  Discovery session when it would have requested a Logout via an
  Async Message on Normal sessions.

7.5. Connection Timeout Management

  iSCSI defines two session-global timeout values (in seconds) -
  Time2Wait and Time2Retain - that are applicable when an iSCSI Full
  Feature Phase connection is taken out of service either
  intentionally or by an exception. Time2Wait is the initial
  "respite time" before attempting an explicit/implicit Logout for
  the CID in question or task reassignment for the affected tasks
  (if any). Time2Retain is the maximum time after the initial
  respite interval that the task and/or connection state(s) is/are
  guaranteed to be maintained on the target to cater to a possible
  recovery attempt.   Recovery attempts for the connection and/or
  task(s) SHOULD NOT be made before Time2Wait seconds, but MUST be




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  completed within   Time2Retain seconds after that initial Time2Wait
  waiting period.

7.5.1. Timeouts on Transport Exception Events

  A transport connection shutdown or a transport reset without any
  preceding iSCSI protocol interactions informing the end-points of
  the fact causes a Full Feature Phase iSCSI connection to be
  abruptly terminated. The timeout values to be used in this case
  are the negotiated values of DefaultTime2Wait (Section 13.15) and
  DefaultTime2Retain (Section 13.16) text keys for the session.

7.5.2. Timeouts on Planned Decommissioning

  Any planned decommissioning of a Full Feature Phase iSCSI
  connection is preceded by either a Logout Response PDU, or an
  Async Message PDU. The Time2Wait and Time2Retain field values
  (Section 11.15) in a Logout Response PDU, and the Parameter2 and
  Parameter3 fields of an Async Message (AsyncEvent types "drop the
  connection" or "drop all the connections"; Section 11.9.1) specify
  the timeout values to be used in each of these cases.

  These timeout values are only applicable for the affected
  connection, and the tasks active on that connection. These
  timeout values have no bearing on initiator timers (if any) that
  are already running on connections or tasks associated with that
  session.

7.6. Implicit Termination of Tasks

  A target implicitly terminates the active tasks due to iSCSI
  protocol dynamics in the following cases:

       a) When a connection is implicitly or explicitly logged out
          with the reason code of "Close the connection" and there
          are active tasks allegiant to that connection.

       b) When a connection fails and eventually the connection
          state times out (state transition M1 in Section 8.2.2)
          and there are active tasks allegiant to that connection.




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       c) When a successful Logout with the reason code of "remove
          the connection for recovery" is performed while there are
          active tasks allegiant to that connection, and those
          tasks eventually time out after the Time2Wait and
          Time2Retain periods without allegiance reassignment.

       d) When a connection is implicitly or explicitly logged out
          with the reason code of "Close the session" and there are
          active tasks in that session.

   If the tasks terminated in the above cases a), b), c) and d)are
   SCSI tasks, they must be internally terminated as if with CHECK
   CONDITION status. This status is only meaningful for appropriately
   handling the internal SCSI state and SCSI side effects with
   respect to ordering because this status is never communicated back
   as a terminating status to the initiator. However additional
   actions may have to be taken at SCSI level depending on the SCSI
   context as defined by the SCSI standards (e.g., queued commands
   and ACA, UA for the next command on the I_T nexus in cases a), b),
   and c) etc. - see [SAM2] and [SPC3]).

7.7. Format Errors

   The following two explicit violations of PDU layout rules are
   format errors:

       a) Illegal contents of any PDU header field except the
          Opcode (legal values are specified in Section 11).
       b) Inconsistent field contents (consistent field contents
          are specified in Section 11).

   Format errors indicate a major implementation flaw in one of the
   parties.

   When a target or an initiator receives an iSCSI PDU with a format
   error, it MUST immediately terminate all transport connections in
   the session either with a connection close or with a connection
   reset and escalate the format error to session recovery (see
   Section 7.1.4.4).




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   All initiator-detected PDU construction errors MUST be considered
   as format errors. Some examples of such errors are:
     - NOP-In with a valid TTT but an invalid LUN

     - NOP-In with a valid ITT (i.e., a NOP-In response) and also a
       valid TTT

     - SCSI Response PDU with Status=CHECK CONDITION, but
       DataSegmentLength = 0


7.8. Digest Errors

   The discussion of the legal choices in handling digest errors
   below excludes session recovery as an explicit option, but either
   party detecting a digest error may choose to escalate the error to
   session recovery.

   When a target or an initiator receives any iSCSI PDU, with a
   header digest error, it MUST either discard the header and all
   data up to the beginning of a later PDU or close the connection.
   Because the digest error indicates that the length field of the
   header may have been corrupted, the location of the beginning of a
   later PDU needs to be reliably ascertained by other means such as
   the operation of a sync and steering layer.

   When a target receives any iSCSI PDU with a payload digest error,
   it MUST answer with a Reject PDU with a reason code of Data-
   Digest-Error and discard the PDU.

     - If the discarded PDU is a solicited or unsolicited iSCSI
       data PDU (for immediate data in a command PDU, non-data PDU
       rule below applies), the target MUST do one of the
       following:

         i) Request retransmission with a recovery R2T.
         ii) Terminate the task with a response PDU with a CHECK
           CONDITION Status and an iSCSI Condition of "protocol
           service CRC error" (Section 11.4.7.2). If the target
           chooses to implement this option, it MUST wait to
           receive all the data (signaled by a Data PDU with the
           final bit set for all outstanding R2Ts) before sending




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          the response PDU. A task management command (such as an
          abort task) from the initiator during this wait may
          also conclude the task.
    - No further action is necessary for targets if the discarded
      PDU is a non-data PDU. In case of immediate data being
      present on a discarded command, the immediate data is
      implicitly recovered when the task is retried (see Section
      7.2.1) followed by the entire data transfer for the task.


  When an initiator receives any iSCSI PDU with a payload digest
  error, it MUST discard the PDU.

    - If the discarded PDU is an iSCSI data PDU, the initiator
      MUST do one of the following:

           a) Request the desired data PDU through SNACK. In
              response to the SNACK, the target MUST either resend
              the data PDU or reject the SNACK with a Reject PDU
              with a reason code of "SNACK reject" in which case:
           b) If the status has not already been sent for the
              command, the target MUST terminate the command with a
              CHECK CONDITION Status and an iSCSI Condition of
              "SNACK rejected" (Section 11.4.7.2).
           c) If the status was already sent, no further action is
              necessary for the target. The initiator in this case
              MUST wait for the status to be received and then
              discard it, so as to internally signal the completion
              with CHECK CONDITION Status and an iSCSI Condition of
              "protocol service CRC error" (Section 11.4.7.2).
           d) Abort the task and terminate the command with an
              error.

    - If the discarded PDU is a response PDU or an unsolicited PDU
      (e.g. Async, Reject), the initiator MUST do one of the
      following:

           a) Request PDU retransmission with a status SNACK.
           b) Logout the connection for recovery and continue the
              tasks on a different connection instance as described
              in Section 7.2.




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           c) Logout to close the connection (abort all the
              commands associated with the connection).

        Note that an unsolicited PDU carries the next StatSN value on
        an iSCSI connection, thereby advancing the StatSN. When an
        initiator discards one of these PDUs due to a payload digest
        error, the entire PDU including the header MUST be discarded.
        Consequently, the initiator MUST treat the exception like a
        loss of any other solicited response PDU.

7.9. Sequence Errors

  When an initiator receives an iSCSI R2T/data PDU with an out of
  order R2TSN/DataSN or a SCSI response PDU with an ExpDataSN that
  implies missing data PDU(s), it means that the initiator must have
  detected a header or payload digest error on one or more earlier
  R2T/data PDUs. The initiator MUST address these implied digest
  errors as described in Section 7.8. When a target receives a data
  PDU with an out of order DataSN, it means that the target must
  have hit a header or payload digest error on at least one of the
  earlier data PDUs. The target MUST address these implied digest
  errors as described in Section 7.8.

  When an initiator receives an iSCSI status PDU with an out of
  order StatSN that implies missing responses, it MUST address the
  one or more missing status PDUs as described in Section 7.8. As a
  side effect of receiving the missing responses, the initiator may
  discover missing data PDUs. If the initiator wants to recover the
  missing data for a command, it MUST NOT acknowledge the received
  responses that start from the StatSN of the relevant command,
  until it has completed receiving all the data PDUs of the command.

  When an initiator receives duplicate R2TSNs (due to proactive
  retransmission of R2Ts by the target) or duplicate DataSNs (due to
  proactive SNACKs by the initiator), it MUST discard the
  duplicates.

7.10. Message Error Checking

  In the iSCSI implementations till date, there has been some
  uncertainty on the extent to which incoming messages have to be
  checked for protocol errors, beyond what is strictly required for




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   processing the inbound message. This Section addresses this
   question.

   Unless this document requires it, an iSCSI implementation is not
   required to do an exhaustive protocol conformance check on an
   incoming iSCSI PDU. The iSCSI implementation especially is not
   required to double-check the remote iSCSI implementation's
   conformance to protocol requirements.

7.11. SCSI Timeouts

   An iSCSI initiator MAY attempt to plug a command sequence gap on
   the target end (in the absence of an acknowledgement of the
   command by way of ExpCmdSN) before the ULP timeout by retrying the
   unacknowledged command, as described in Section 7.2.

   On a ULP timeout for a command (that carried a CmdSN of n), if the
   iSCSI initiator intends to continue the session it MUST abort the
   command by either using an appropriate Task Management function
   request for the specific command, or a "close the connection"
   Logout. When using an ABORT TASK, if the ExpCmdSN is still less
   than (n+1), the target may see the abort request while missing the
   original command itself due to one of the following reasons:

     - Original command was dropped due to digest error.

     - Connection on which the original command was sent was
       successfully logged out. On logout, the unacknowledged
       commands issued on the connection being logged out are
       discarded.


   If the abort request is received and the original command is
   missing, targets MUST consider the original command with that
   RefCmdSN to be received and issue a Task Management response with
   the response code: "Function Complete". This response concludes
   the task on both ends. If the abort request is received and the
   target can determine (based on the Referenced Task Tag) that the
   command was received and executed and also that the response was
   sent prior to the abort, then the target MUST respond with the
   response code of "Task Does Not Exist".




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7.12. Negotiation Failures

  Text request and response sequences, when used to set/negotiate
  operational parameters, constitute the negotiation/parameter
  setting. A negotiation failure is considered to be one or more of
  the following:

     - None of the choices, or the stated value, is acceptable to
       one of the sides in the negotiation.

     - The text request timed out and possibly terminated.

     - The text request was answered with a Reject PDU.

  The following two rules should be used to address negotiation
  failures:

       a) During Login, any failure in negotiation MUST be
          considered a login process failure and the Login Phase
          MUST be terminated, and with it, the connection. If the
          target detects the failure, it must terminate the login
          with the appropriate login response code.


       b) A failure in negotiation, while in the Full Feature
          Phase, will terminate the entire negotiation sequence
          that may consist of a series of text requests that use
          the same Initiator Task Tag. The operational parameters
          of the session or the connection MUST continue to be the
          values agreed upon during an earlier successful
          negotiation (i.e., any partial results of this
          unsuccessful negotiation MUST NOT take effect and MUST be
          discarded).


7.13. Protocol Errors

  Mapping framed messages over a "stream" connection, such as TCP,
  makes the proposed mechanisms vulnerable to simple software
  framing errors. On the other hand, the introduction of framing
  mechanisms to limit the effects of these errors may be onerous on
  performance for simple implementations. Command Sequence Numbers




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  and the above mechanisms for connection drop and reestablishment
  help handle this type of mapping errors.

  All violations of iSCSI PDU exchange sequences specified in this
  draft are also protocol errors. This category of errors can only
  be
  addressed by fixing the implementations; iSCSI defines Reject and
  response codes to enable this.

7.14. Connection Failures

  iSCSI can keep a session in operation if it is able to
  keep/establish at least one TCP connection between the initiator
  and the target in a timely fashion. Targets and/or initiators may
  recognize a failing connection by either transport level means
  (TCP), a gap in the command sequence number, a response stream
  that is not filled for a long time, or by a failing iSCSI NOP
  (acting as a ping). The latter MAY be used periodically to
  increase the speed and likelihood of detecting connection
  failures. As an example for transport level means, initiators and
  targets MAY also use the keep-alive option, see [RFC1122], on the
  TCP connection to enable early link failure detection on otherwise
  idle links.

  On connection failure, the initiator and target MUST do one of the
  following:

       a) Attempt connection recovery within the session
          (Connection Recovery).

       b) Logout the connection with the reason code "closes the
          connection" (Section 10.14.5), re-issue missing commands,
          and implicitly terminate all active commands. This option
          requires support for the within-connection recovery class
          (Recovery Within-connection).

       c) Perform session recovery (Session Recovery).

  Either side may choose to escalate to session recovery (via the
  initiator dropping all the connections, or via an Async Message
  that announces the similar intent from a target), and the other
  side MUST give it precedence. On a connection failure, a target




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  MUST terminate and/or discard all of the active immediate commands
  regardless of which of the above options is used (i.e., immediate
  commands are not recoverable across connection failures).

7.15. Session Errors

  If all of the connections of a session fail and cannot be
  reestablished in a short time, or if initiators detect protocol
  errors repeatedly, an initiator may choose to terminate a session
  and establish a new session.

  In this case, the initiator takes the following actions:

     - Resets or closes all the transport connections.

     - Terminates all outstanding requests with an appropriate
       response before initiating a new session. If the same I_T
       nexus is intended to be reestablished, the initiator MUST
       employ session reinstatement (see Section 6.3.5).


  When the session timeout (the connection state timeout for the
  last failed connection) happens on the target, it takes the
  following actions:

     - Resets or closes the TCP connections (closes the session).

     - Terminates all active tasks that were allegiant to the
       connection(s) that constituted the session.


  A target MUST also be prepared to handle a session reinstatement
  request from the initiator that may be addressing session errors.




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8. State Transitions

  iSCSI connections and iSCSI sessions go through several well-
  defined states from the time they are created to the time they are
  cleared.

  The connection state transitions are described in two separate but
  dependent state diagrams for ease in understanding. The first
  diagram, "standard connection state diagram", describes the
  connection state transitions when the iSCSI connection is not
  waiting for, or undergoing, a cleanup by way of an explicit or
  implicit Logout. The second diagram, "connection cleanup state
  diagram", describes the connection state transitions while
  performing the iSCSI connection cleanup.

  The "session state diagram" describes the state transitions an
  iSCSI session would go through during its lifetime, and it depends
  on the states of possibly multiple iSCSI connections that
  participate in the session.

  States and transitions are described in text, tables and diagrams.
  The diagrams are used for illustration. The text and the tables
  are the governing specification.

8.1. Standard Connection State Diagrams

8.1.1. State Descriptions for Initiators and Targets

  State descriptions for the standard connection state diagram are
  as follows:
  -S1: FREE
       -initiator: State on instantiation, or after successful
       connection closure.
       -target: State on instantiation, or after successful
       connection closure.
  -S2: XPT_WAIT
       -initiator: Waiting for a response to its transport
       connection establishment request.
       -target: Illegal
  -S3: XPT_UP
       -initiator: Illegal
       -target: Waiting for the Login process to commence.




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   -S4: IN_LOGIN
        -initiator: Waiting for the Login process to conclude,
        possibly involving several PDU exchanges.
        -target: Waiting for the Login process to conclude,
        possibly involving several PDU exchanges.
   -S5: LOGGED_IN
        -initiator: In Full Feature Phase, waiting for all
        internal, iSCSI, and transport events.
        -target: In Full Feature Phase, waiting for all internal,
        iSCSI, and transport events.
   -S6: IN_LOGOUT
        -initiator: Waiting for a Logout response.
        -target: Waiting for an internal event signaling completion
        of logout processing.
   -S7: LOGOUT_REQUESTED
        -initiator: Waiting for an internal event signaling
        readiness to proceed with Logout.
        -target: Waiting for the Logout process to start after
        having requested a Logout via an Async Message.
   -S8: CLEANUP_WAIT
        -initiator: Waiting for the context and/or resources to
        initiate the cleanup processing for this CSM.
        -target: Waiting for the cleanup process to start for this
        CSM.
8.1.2. State Transition Descriptions for Initiators and Targets

  -T1:
         -initiator: Transport connect request was made (e.g., TCP
         SYN sent).
         -target: Illegal
  -T2:
         -initiator: Transport connection request timed out, a
         transport reset was received, or an internal event of
         receiving a Logout response (success) on another connection
         for a "close the session" Logout request was received.
         -target:Illegal
  -T3:
         -initiator: Illegal
         -target: Received a valid transport connection request that
         establishes the transport connection.
  -T4:




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         -initiator: Transport connection established, thus
         prompting the initiator to start the iSCSI Login.
         -target: Initial iSCSI Login request was received.
  -T5:
         -initiator: The final iSCSI Login response with a Status-
         Class of zero was received.
         -target: The final iSCSI Login request to conclude the
         Login Phase was received, thus prompting the target to send
         the final iSCSI Login response with a Status-Class of zero.
  -T6:
         -initiator: Illegal
         -target: Timed out waiting for an iSCSI Login, transport
         disconnect indication was received, transport reset was
         received, or an internal event indicating a transport
         timeout was received. In all these cases, the connection is
         to be closed.
  -T7:
         -initiator - one of the following events caused the
         transition:
              a) The final iSCSI Login response was received with a
                 non-zero Status-Class.
              b) Login timed out.
              c) A transport disconnect indication was received.
              d) A transport reset was received.
              e) An internal event indicating a transport timeout was
                 received.
              f) An internal event of receiving a Logout response
                 (success) on another connection for a "close the
                 session" Logout request was received.

         In all these cases, the transport connection is closed.

         -target - one of the following events caused the
         transition:
              a) The final iSCSI Login request to conclude the Login
                 Phase was received, prompting the target to send the
                 final iSCSI Login response with a non-zero Status-
                 Class.
              b) Login timed out.
              c) Transport disconnect indication was received.




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             d) Transport reset was received.
             e) An internal event indicating a transport timeout was
                received.
             f) On another connection, a "close the session" Logout
                request was received.

         In all these cases, the connection is to be closed.
  -T8:
        -initiator: An internal event of receiving a Logout
        response (success) on another connection for a "close the
        session" Logout request was received, thus closing this
        connection requiring no further cleanup.
        -target: An internal event of sending a Logout response
        (success) on another connection for a "close the session"
        Logout request was received, or an internal event of a
        successful connection/session reinstatement is received,
        thus prompting the target to close this connection cleanly.
  -T9, T10:
        -initiator: An internal event that indicates the readiness
        to start the Logout process was received, thus prompting an
        iSCSI Logout to be sent by the initiator.
        -target: An iSCSI Logout request was received.
  -T11, T12:
        -initiator: Async PDU with AsyncEvent "Request Logout" was
        received.
        -target: An internal event that requires the
        decommissioning of the connection is received, thus causing
        an Async PDU with an AsyncEvent "Request Logout" to be
        sent.
  -T13:
        -initiator: An iSCSI Logout response (success) was
        received, or an internal event of receiving a Logout
        response (success) on another connection for a "close the
        session" Logout request was received.
        -target: An internal event was received that indicates
        successful processing of the Logout, which prompts an iSCSI
        Logout response (success) to be sent; an internal event of
        sending a Logout response (success) on another connection
        for a "close the session" Logout request was received; or
        an internal event of a successful connection/session




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          reinstatement is received. In all these cases, the
          transport connection is closed.

  -T14:
       -initiator: Async PDU with AsyncEvent "Request Logout" was
       received again.
       -target: Illegal
  -T15, T16:
       -initiator: One or more of the following events caused this
       transition:
            a) Internal event that indicates a transport connection
               timeout was received thus prompting transport RESET
               or transport connection closure.
            b) A transport RESET.
            c) A transport disconnect indication.
            d) Async PDU with AsyncEvent "Drop connection" (for
               this CID).
            e) Async PDU with AsyncEvent "Drop all connections".
       -target: One or more of the following events caused this
       transition:
            a) Internal event that indicates a transport connection
               timeout was received, thus prompting transport RESET
               or transport connection closure.
            b) An internal event of a failed connection/session
               reinstatement is received.
            c) A transport RESET.
            d) A transport disconnect indication.
            e) Internal emergency cleanup event was received which
               prompts an Async PDU with AsyncEvent "Drop
               connection" (for this CID), or event "Drop all
               connections".

  -T17:
          -initiator: One or more of the following events caused this
          transition:
               a) Logout response, (failure i.e., a non-zero status)
                  was received, or Logout timed out.
               b) Any of the events specified for T15 and T16.
          -target: One or more of the following events caused this
          transition:




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              a) Internal event that indicates a   failure of the
                 Logout processing was received,   which prompts a
                 Logout response (failure, i.e.,   a non-zero status)
                 to be sent.
              b) Any of the events specified for   T15 and T16.
  -T18:
          -initiator: An internal event of receiving a Logout
          response (success) on another connection for a "close the
          session" Logout request was received.

          -target: An internal event of sending a Logout response
          (success) on another connection for a "close the session"
          Logout request was received, or an internal event of a
          successful connection/session reinstatement is received.
          In both these cases, the connection is closed.




  The CLEANUP_WAIT state (S8) implies that there are possible iSCSI
  tasks that have not reached conclusion and are still considered
  busy.

8.1.3. Standard Connection State Diagram for an Initiator

  Symbolic names for States:

      S1: FREE

      S2: XPT_WAIT

      S4: IN_LOGIN

      S5: LOGGED_IN

      S6: IN_LOGOUT

      S7: LOGOUT_REQUESTED

      S8: CLEANUP_WAIT




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  States S5, S6, and S7 constitute the Full Feature Phase operation
  of the connection.

  The state diagram is as follows:

                     -------<-------------+
         +--------->/ S1    \<----+       |
      T13|       +->\       /<-+   \      |
         |      /    ---+---    \   \     |
         |     /        |     T2 \   |    |
         |  T8 |        |T1       |  |    |
         |     |        |        /   |T7  |
         |     |        |       /    |    |
         |     |        |      /     |    |
         |     |        V     /     /     |
         |     |     ------- /     /      |
         |     |    / S2    \     /       |
         |     |    \       /    /        |
         |     |     ---+---    /         |
         |     |        |T4    /          |
         |     |        V     /           | T18
         |     |     ------- /            |
         |     |    / S4    \             |
         |     |    \       /             |
         |     |     ---+---              |         T15
         |     |        |T5      +--------+---------+
         |     |        |       /T16+-----+------+  |
         |     |        |      /   -+-----+--+   |  |
         |     |        |     /   /  S7   \  |T12|  |
         |     |        |    / +->\       /<-+   V  V
         |     |        |   / /    -+-----       -------
         |     |        |  / /T11   |T10        /  S8   \
         |     |        V / /       V  +----+   \       /
         |     |      ---+-+-      ----+--  |    -------
         |     |     / S5    \T9  / S6    \<+    ^
         |     +-----\       /--->\       / T14  |
         |            -------      --+----+------+T17
         +---------------------------+

  The following state transition table represents the above diagram.
  Each row represents the starting state for a given transition,




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  which after taking a transition marked in a table cell would end
  in the state represented by the column of the cell. For example,
  from state S1, the connection takes the T1 transition to arrive at
  state S2. The fields marked "-" correspond to undefined
  transitions.

     +----+---+---+---+---+----+---+
     |S1  |S2 |S4 |S5 |S6 |S7  |S8 |
  ---+----+---+---+---+---+----+---+
   S1| -  |T1 | - | - | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S2|T2  |-  |T4 | - | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S4|T7  |-  |-  |T5 | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S5|T8  |-  |-  | - |T9 |T11 |T15|
  ---+----+---+---+---+---+----+---+
   S6|T13 |-  |-  | - |T14|-   |T17|
  ---+----+---+---+---+---+----+---+
   S7|T18 |-  |-  | - |T10|T12 |T16|
  ---+----+---+---+---+---+----+---+
   S8| -  |-  |-  | - | - | -  | - |
  ---+----+---+---+---+---+----+---+

8.1.4. Standard Connection State Diagram for a Target

  Symbolic names for States:
     S1: FREE

      S3: XPT_UP

      S4: IN_LOGIN

      S5: LOGGED_IN

      S6: IN_LOGOUT

      S7: LOGOUT_REQUESTED

      S8: CLEANUP_WAIT




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  States S5, S6, and S7 constitute the Full Feature Phase operation
  of the connection.

  The state diagram is as follows:

                        -------<-------------+
            +--------->/ S1    \<----+       |
         T13|       +->\       /<-+   \      |
            |      /    ---+---    \   \     |
            |     /        |     T6 \   |    |
            |  T8 |        |T3       |  |    |
            |     |        |        /   |T7  |
            |     |        |       /    |    |
            |     |        |      /     |    |
            |     |        V     /     /     |
            |     |     ------- /     /      |
            |     |    / S3    \     /       |
            |     |    \       /    /        | T18
            |     |     ---+---    /         |
            |     |        |T4    /          |
            |     |        V     /           |
            |     |     ------- /            |
            |     |    / S4    \             |
            |     |    \       /             |
            |     |     ---+---         T15  |
            |     |        |T5      +--------+---------+
            |     |        |       /T16+-----+------+  |
            |     |        |      /  -+-----+---+   |  |
            |     |        |     /   /  S7   \  |T12|  |
            |     |        |    / +->\       /<-+   V  V
            |     |        |   / /    -+-----       -------
            |     |        |  / /T11   |T10        /  S8   \
            |     |        V / /       V           \       /
            |     |      ---+-+-      -------       -------
            |     |     / S5    \T9  / S6    \        ^
            |     +-----\       /--->\       /        |
            |            -------      --+----+--------+T17
            +---------------------------+


  The following state transition table represents the above diagram,
  and follows the conventions described for the initiator diagram.




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     +----+---+---+---+---+----+---+
     |S1  |S3 |S4 |S5 |S6 |S7  |S8 |
  ---+----+---+---+---+---+----+---+
   S1| -  |T3 | - | - | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S3|T6  |-  |T4 | - | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S4|T7  |-  |-  |T5 | - | -  | - |
  ---+----+---+---+---+---+----+---+
   S5|T8  |-  |-  | - |T9 |T11 |T15|
  ---+----+---+---+---+---+----+---+
   S6|T13 |-  |-  | - |-  |-   |T17|
  ---+----+---+---+---+---+----+---+
   S7|T18 |-  |-  | - |T10|T12 |T16|
  ---+----+---+---+---+---+----+---+
   S8| -  |-  |-  | - | - | -  | - |
  ---+----+---+---+---+---+----+---+

8.2. Connection Cleanup State Diagram for Initiators and Targets

  Symbolic names for states:

     R1: CLEANUP_WAIT (same as S8)

     R2: IN_CLEANUP

     R3: FREE (same as S1)



  Whenever a connection state machine in cleanup (let's call it CSM-
  C) enters the CLEANUP_WAIT state (S8), it must go through the
  state transitions described in the connection cleanup state
  diagram either a) using a separate full-feature phase connection
  (let's call it CSM-E, for explicit) in the LOGGED_IN state in the
  same session, or b) using a new transport connection (let's call
  it CSM-I, for implicit) in the FREE state that is to be added to
  the same session. In the CSM-E case, an explicit logout for the
  CID that corresponds to CSM-C (either as a connection or session
  logout) needs to be performed to complete the cleanup. In the CSM-
  I case, an implicit logout for the CID that corresponds to CSM-C




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  needs to be performed by way of connection reinstatement (Section
  6.3.4) for that CID. In either case, the protocol exchanges on
  CSM-E or CSM-I determine the state transitions for CSM-C.
  Therefore, this cleanup state diagram is only applicable to the
  instance of the connection in cleanup (i.e., CSM-C). In the case
  of an implicit logout for example, CSM-C reaches FREE (R3) at the
  time CSM-I reaches LOGGED_IN. In the case of an explicit logout,
  CSM-C reaches FREE (R3) when CSM-E receives a successful logout
  response while continuing to be in the LOGGED_IN state.

  An initiator must initiate an explicit or implicit connection
  logout for a connection in the CLEANUP_WAIT state, if the
  initiator intends to continue using the associated iSCSI session.

  The following state diagram applies to both initiators and
  targets.




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                          -------
                        / R1      \
                     +--\         /<-+
                   /      ---+---    \
                 /           |        \ M3
              M1 |           |M2      |
                 |           |        /
                 |           |       /
                 |           |      /
                 |           V     /
                 |        ------- /
                 |      / R2      \
                 |      \         /
                 |        -------
                 |           |
                 |           |M4
                 |           |
                 |           |
                 |           |
                 |           V
                 |       -------
                 |     / R3     \
                 +---->\        /
                         -------

  The following state transition table represents the above diagram,
  and follows the same conventions as in earlier sections.

       +----+----+----+
       |R1  |R2  |R3  |
  -----+----+----+----+
   R1  | -  |M2  |M1  |
  -----+----+----+----+
   R2  |M3  | -  |M4  |
  -----+----+----+----+
   R3  | -  | -  | -  |
  -----+----+----+----+


8.2.1. State Descriptions for Initiators and Targets

  -R1: CLEANUP_WAIT (Same as S8)




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       -initiator: Waiting for the internal event to initiate the
       cleanup processing for CSM-C.
       -target: Waiting for the cleanup process to start for CSM-
       C.
  -R2: IN_CLEANUP
       -initiator: Waiting for the connection cleanup process to
       conclude for CSM-C.
       -target: Waiting for the connection cleanup process to
       conclude for CSM-C.
  -R3: FREE (Same as S1)
       -initiator: End state for CSM-C.
       -target: End state for CSM-C.

8.2.2. State Transition Descriptions for Initiators and Targets

  -M1:    One or more of the following events was received:
         -initiator:
             -An internal event that indicates connection state
             timeout.
             -An internal event of receiving a successful Logout
             response on a different connection for a "close the
             session" Logout.
         -target:
             -An internal event that indicates connection state
             timeout.
             -An internal event of sending a Logout response
             (success) on a different connection for a "close the
             session" Logout request.

  -M2: An implicit/explicit logout process was initiated by the
  initiator.
       -In CSM-I usage:
           -initiator: An internal event requesting the connection
           (or session) reinstatement was received, thus prompting
           a connection (or session) reinstatement Login to be
           sent transitioning CSM-I to state IN_LOGIN.
           -target: A connection/session reinstatement Login was
           received while in state XPT_UP.
       -In CSM-E usage:




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           -initiator: An internal event that indicates that an
           explicit logout was sent for this CID in state
           LOGGED_IN.
           -target: An explicit logout was received for this CID
           in state LOGGED_IN.
  -M3: Logout failure detected
       -In CSM-I usage:
           -initiator: CSM-I failed to reach LOGGED_IN and arrived
           into FREE instead.
           -target: CSM-I failed to reach LOGGED_IN and arrived
           into FREE instead.
       -In CSM-E usage:
           -initiator: CSM-E either moved out of LOGGED_IN, or
           Logout timed out and/or aborted, or Logout response
           (failure) was received.
           -target: CSM-E either moved out of LOGGED_IN, Logout
           timed out and/or aborted, or an internal event that
           indicates a failed Logout processing was received. A
           Logout response (failure) was sent in the last case.


  -M4: Successful implicit/explicit logout was performed.
       - In CSM-I usage:
           -initiator: CSM-I reached state LOGGED_IN, or an
           internal event of receiving a Logout response (success)
           on another connection for a "close the session" Logout
           request was received.
           -target: CSM-I reached state LOGGED_IN, or an internal
           event of sending a Logout response (success) on a
           different connection for a "close the session" Logout
           request was received.
       - In CSM-E usage:
           -initiator: CSM-E stayed in LOGGED_IN and received a
           Logout response (success), or an internal event of
           receiving a Logout response (success) on another
           connection for a "close the session" Logout request was
           received.
           -target: CSM-E stayed in LOGGED_IN and an internal
           event indicating a successful Logout processing was
           received, or an internal event of sending a Logout




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            response (success) on a different connection for a
            "close the session" Logout request was received.

8.3. Session State Diagrams

8.3.1. Session State Diagram for an Initiator


  Symbolic Names for States:

     Q1: FREE

     Q3: LOGGED_IN

     Q4: FAILED

  State Q3 represents the Full Feature Phase operation of the
  session.

  The state diagram is as follows:

                                 -------
                               / Q1      \
                       +------>\         /<-+
                     /           ---+---    |
                   /                |       |N3
             N6  |                  |N1     |
                 |                  |       |
                 |       N4         |       |
                 | +----------+     |      /
                 | |            |   |     /
                 | |            |   |    /
                 | |            V   V   /
                -+--+--          -----+-
              / Q4     \ N5    / Q3      \
              \        /<------\         /
                -------          -------

  The state transition table is as follows:




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       +---+---+---+
       |Q1 |Q3 |Q4 |
  -----+---+---+---+
   Q1  | - |N1 | - |
  -----+---+---+---+
   Q3  |N3 | - |N5 |
  -----+---+---+---+
   Q4  |N6 |N4 | - |
  -----+---+---+---+


8.3.2. Session State Diagram for a Target

  Symbolic Names for States:

     Q1: FREE

     Q2: ACTIVE

     Q3: LOGGED_IN

     Q4: FAILED

     Q5: IN_CONTINUE


  State Q3 represents the Full Feature Phase operation of the
  session.


  The state diagram is as follows:




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                                         -------
                   +------------------>/ Q1     \
                 /     +-------------->\        /<-+
                 |     |                ---+---     |
                 |     |                 ^ |        |N3
             N 6 |     |N11            N9| V N1     |
                 |     |                 +------    |
                 |     |                / Q2      \ |
                 |     |                \         / |
                 |   --+----             +--+---    |
                 | / Q5      \              |       |
                 | \         / N10          |       |
                 |   +-+---+------------+   | N2   /
                 |   ^ |                |   |     /
                 | N7| |N8              |   |    /
                 |   | |                |   V   /
                -+---+-V                V------+-
              / Q4      \ N5           / Q3      \
              \         /<-------------\         /
                -------                  -------

  The state transition table is as follows:

       +----+----+----+----+----+
       |Q1  |Q2  |Q3  |Q4  |Q5  |
  -----+----+----+----+----+----+
   Q1  | -  |N1  | -  | -  | -  |
  -----+----+----+----+----+----+
   Q2  |N9  | -  |N2  | -  | -  |
  -----+----+----+----+----+----+
   Q3  |N3  | -  | -  |N5  | -  |
  -----+----+----+----+----+----+
   Q4  |N6  | -  | -  | -  |N7  |
  -----+----+----+----+----+----+
   Q5  |N11 | -  |N10 |N8  | -  |
  -----+----+----+----+----+----+


8.3.3. State Descriptions for Initiators and Targets

  -Q1: FREE
       -initiator: State on instantiation or after cleanup.




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         -target: State on instantiation or after cleanup.
  -Q2:   ACTIVE
         -initiator: Illegal.
         -target: The first iSCSI connection in the session
         transitioned to IN_LOGIN, waiting for it to complete the
         login process.
  -Q3:   LOGGED_IN
         -initiator: Waiting for all session events.
         -target: Waiting for all session events.
  -Q4:   FAILED
         -initiator: Waiting for session recovery or session
         continuation.
         -target: Waiting for session recovery or session
         continuation.
  -Q5:   IN_CONTINUE
         -initiator: Illegal.
         -target: Waiting for session continuation attempt to reach
         a conclusion.


8.3.4. State Transition Descriptions for Initiators and Targets

  -N1:
         -initiator: At least one transport connection reached the
         LOGGED_IN state.
         -target: The first iSCSI connection in the session had
         reached the IN_LOGIN state.
  -N2:
         -initiator: Illegal.
         -target: At least one iSCSI connection reached the
         LOGGED_IN state.
  -N3:
         -initiator: Graceful closing of the session via session
         closure (Section 6.3.6).
         -target: Graceful closing of the session via session
         closure (Section 6.3.6) or a successful session
         reinstatement cleanly closed the session.
  -N4:
         -initiator: A session continuation attempt succeeded.
         -target: Illegal.
  -N5:




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          -initiator: Session failure (Section 6.3.6) occurred.
          -target: Session failure (Section 6.3.6) occurred.
  -N6:
          -initiator: Session state timeout occurred, or a session
          reinstatement cleared this session instance. This results
          in the freeing of all associated resources and the session
          state is discarded.
          -target: Session state timeout occurred, or a session
          reinstatement cleared this session instance. This results
          in the freeing of all associated resources and the session
          state is discarded.
  -N7:
          -initiator: Illegal.
          -target: A session continuation attempt is initiated.
  -N8:
          -initiator: Illegal.
          -target: The last session continuation attempt failed.
  -N9:
          -initiator: Illegal.
          -target: Login attempt on the leading connection failed.
  -N10:
          -initiator: Illegal.
          -target: A session continuation attempt succeeded.
  -N11:
          -initiator: Illegal.
          -target: A successful session reinstatement cleanly closed
          the session.




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9. Security Considerations

  Historically, native storage systems have not had to consider
  security because their environments offered minimal security
  risks. That is, these environments consisted of storage devices
  either directly attached to hosts or connected via a Storage Area
  Network (SAN) distinctly separate from the communications network.
  The use of storage protocols, such as SCSI, over IP-networks
  requires that security concerns be addressed. iSCSI
  implementations must provide means of protection against active
  attacks (e.g., pretending to be another identity, message
  insertion, deletion, modification, and replaying) and passive
  attacks (e.g.,eavesdropping, gaining advantage by analyzing the
  data sent over the line).

  Although technically possible, iSCSI SHOULD NOT be configured
  without security, specifically in-band authentication, see Section
  9.2. iSCSI configured without security should be confined to
  closed environments that have very limited and well controlled
  security risks. [RFC3723] specifies the mechanisms that must be
  used in order to mitigate risks fully described in that document.

  The following Section describes the security mechanisms provided
  by an iSCSI implementation.

9.1. iSCSI Security Mechanisms

  The entities involved in iSCSI security are the initiator, target,
  and the IP communication end points. iSCSI scenarios in which
  multiple initiators or targets share a single communication end
  point are expected. To accommodate such scenarios, iSCSI uses two
  separate security mechanisms: In-band authentication between the
  initiator and the target at the iSCSI connection level (carried
  out by exchange of iSCSI Login PDUs), and packet protection
  (integrity, authentication, and confidentiality) by IPsec at the
  IP level. The two security mechanisms complement each other. The
  in-band authentication provides end-to-end trust (at login time)
  between the iSCSI initiator and the target while IPsec provides a
  secure channel between the IP communication end points. iSCSI can
  be used to access sensitive information for which significant
  security protection is appropriate. As further specified in the
  rest of this security considerations section, both iSCSI security




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  mechanisms are mandatory to implement (MUST). Use of in-band
  authentication is strongly recommended (SHOULD). In contrast, use
  of IPsec is optional (MAY) as the security risks that it addresses
  may only be present over a subset of the networks used by an iSCSI
  connection or a session; a specific example is that when an iSCSI
  session spans data centers, IPsec VPN gateways at the data center
  boundaries to protect the WAN connectivity between data centers
  may be appropriate in combination with in-band iSCSI
  authentication.

  Further details on typical iSCSI scenarios and the relation
  between the initiators, targets, and the communication end points
  can be found in [RFC3723].


9.2. In-band Initiator-Target Authentication

  During login, the target MAY authenticate the initiator and the
  initiator MAY authenticate the target. The authentication is
  performed on every new iSCSI connection by an exchange of iSCSI
  Login PDUs using a negotiated authentication method.

  The authentication method cannot assume an underlying IPsec
  protection, because IPsec is optional to use. An attacker should
  gain as little advantage as possible by inspecting the
  authentication phase PDUs. Therefore, a method using clear text
  (or equivalent) passwords MUST NOT be used; on the other hand,
  identity protection is not strictly required.

  The authentication mechanism protects against an unauthorized
  login to storage resources by using a false identity (spoofing).
  Once the authentication phase is completed, if the underlying
  IPsec is not used, all PDUs are sent and received in clear. The
  authentication mechanism alone (without underlying IPsec) should
  only be used when there is no risk of eavesdropping, message
  insertion, deletion, modification, and replaying.

  Section 11 defines several authentication methods and the exact
  steps that must be followed in each of them, including the iSCSI-
  text-keys and their allowed values in each step. Whenever an iSCSI
  initiator gets a response whose keys, or their values, are not
  according to the step definition, it MUST abort the connection.




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  Whenever an iSCSI target gets a response whose keys, or their
  values, are not according to the step definition, it MUST answer
  with a Login reject with the "Initiator Error" or "Missing
  Parameter" status. These statuses are not intended for
  cryptographically incorrect values such as the CHAP response, for
  which "Authentication Failure" status MUST be specified. The
  importance of this rule can be illustrated in CHAP with target
  authentication (see Section 12.1.3) where the initiator would have
  been able to conduct a reflection attack by omitting his response
  key (CHAP_R) using the same CHAP challenge as the target and
  reflecting the target's response back to the target. In CHAP, this
  is prevented because the target must answer the missing CHAP_R key
  with a Login reject with the "Missing Parameter" status.

  For some of the authentication methods, a key specifies the
  identity of the iSCSI initiator or target for authentication
  purposes. The value associated with that key MAY be different from
  the iSCSI Name and SHOULD be configurable. (CHAP_N, see Section
  12.1.3 and SRP_U, see Section 12.1.2). For this reason, iSCSI
  implementations SHOULD manage authentication in a way that
  impersonation across iSCSI Names via these authentication
  identities is not possible. Specifically, implementations SHOULD
  allow configuration of an authentication identity for a Name if
  different, and authentication credentials for that identity.
  During the login time, implementations SHOULD verify the Name-to-
  identity relationship in addition to authenticating the identity
  through the negotiated authentication method.

  When an iSCSI session has multiple TCP connections, either
  concurrently or sequentially, the authentication method and
  identities should not vary among the connections. Therefore, all
  connections in an iSCSI session SHOULD use the same authentication
  method, iSCSI Name and authentication identity (for authentication
  methods that use an authentication identity). Implementations
  SHOULD check this and cause an authentication failure on a new
  connection that uses a different authentication method, iSCSI
  Name or authentication identity from those already used in the
  session. In addition, implementations SHOULD NOT support both
  authenticated and unauthenticated TCP connections in the same
  iSCSI session, added either concurrently or sequentially to the
  session.




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9.2.1. CHAP Considerations

  Compliant iSCSI initiators and targets MUST implement the CHAP
  authentication method [RFC1994] (according to Section 12.1.3
  including the target authentication option).

  When CHAP is performed over a non-encrypted channel, it is
  vulnerable to an off-line dictionary attack. Implementations MUST
  support use of up to 128 bit random CHAP secrets, including the
  means to generate such secrets and to accept them from an external
  generation source. Implementations MUST NOT provide secret
  generation (or expansion) means other than random generation.

  An administrative entity of an environment in which CHAP is used
  with a secret that has less than 96 random bits MUST enforce IPsec
  encryption (according to the implementation requirements in
  Confidentiality) to protect the connection. Moreover, in this case
  IKE authentication with group pre-shared cryptographic keys SHOULD
  NOT be used unless it is not essential to protect group members
  against off-line dictionary attacks by other members.

  CHAP secrets MUST be an integral number of bytes (octets). A
  compliant implementation SHOULD NOT continue with the login step
  in which it should send a CHAP response (CHAP_R, Section 12.1.3)
  unless it can verify that the CHAP secret is at least 96 bits, or
  that IPsec encryption is being used to protect the connection.

  Any CHAP secret used for initiator authentication MUST NOT be
  configured for authentication of any target, and any CHAP secret
  used for target authentication MUST NOT be configured for
  authentication of any initiator. If the CHAP response received by
  one end of an iSCSI connection is the same as the CHAP response
  that the receiving endpoint would have generated for the same CHAP
  challenge, the response MUST be treated as an authentication
  failure and cause the connection to close (this ensures that the
  same CHAP secret is not used for authentication in both
  directions). Also, if an iSCSI implementation can function as
  both initiator and target, different CHAP secrets and identities
  MUST be configured for these two roles. The following is an
  example of the attacks prevented by the above requirements:




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       a) Rogue wants to impersonate Storage to Alice, and knows
          that a single secret is used for both directions of
          Storage-Alice authentication.


       b) Rogue convinces Alice to open two connections to Rogue,
          and Rogue identifies itself as Storage on both
          connections.


       c) Rogue issues a CHAP challenge on connection 1, waits for
          Alice to respond, and then reflects Alice's challenge as
          the initial challenge to Alice on connection 2.


       d) If Alice doesn't check for the reflection across
          connections, Alice's response on connection 2 enables
          Rogue to impersonate Storage on connection 1, even though
          Rogue does not know the Alice-Storage CHAP secret.


  Originators MUST NOT reuse the CHAP challenge sent by the
  Responder for the other direction of a bidirectional
  authentication. Responders MUST check for this condition and close
  the iSCSI TCP connection if it occurs.

  The same CHAP secret SHOULD NOT be configured for authentication
  of multiple initiators or multiple targets, as this enables any of
  them to impersonate any other one of them, and compromising one of
  them enables the attacker to impersonate any of them. It is
  recommended that iSCSI implementations check for use of identical
  CHAP secrets by different peers when this check is feasible, and
  take appropriate measures to warn users and/or administrators when
  this is detected.

  When an iSCSI initiator or target authenticates itself to
  counterparts in multiple administrative domains, it SHOULD use a
  different CHAP secret for each administrative domain to avoid
  propagating security compromises across domains.

  Within a single administrative domain:




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    - A single CHAP secret MAY be used for authentication of an
      initiator to multiple targets.

    - A single CHAP secret MAY be used for an authentication of a
      target to multiple initiators when the initiators use an
      external server (e.g., RADIUS, [RFC2865]) to verify the
      target's CHAP responses and do not know the target's CHAP
      secret.


  If an external response verification server (e.g., RADIUS) is not
  used, employing a single CHAP secret for authentication of a
  target to multiple initiators requires that all such initiators
  know that target secret. Any of these initiators can impersonate
  the target to any other such initiator, and compromise of such an
  initiator enables an attacker to impersonate the target to all
  such initiators. Targets SHOULD use separate CHAP secrets for
  authentication to each initiator when such risks are of concern;
  in this situation it may be useful to configure a separate logical
  iSCSI target with its own iSCSI Node Name for each initiator or
  group of initiators among which such separation is desired.

   The above requirements strengthen the security properties of CHAP
  authentication for iSCSI by comparison to the basic CHAP
  authentication mechanism [RFC1994]. It is very important to
  adhere to these requirements, especially the requirements for
  strong (large randomly generated) CHAP secrets, as iSCSI
  implementations and deployments that fail to use strong CHAP
  secrets are likely to be highly vulnerable to off-line dictionary
  attacks on CHAP secrets.

  Replacement of CHAP with a better authentication mechanism is
  anticipated in a future version of iSCSI. The FC-SP-2 standard
  [FC-SP-2] has specified the EAP-GPSK authentication mechanism
  [RFC5433] as an alternative to (and possible future replacement
  for) Fibre Channel's similar usage of strengthened CHAP. Another
  possible replacement for CHAP is a secure password mechanism,
  e.g., an updated version of iSCSI's current SRP authentication
  mechanism.




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9.2.2. SRP Considerations

  The strength of the SRP authentication method (specified in
  [RFC2945]) is dependent on the characteristics of the group being
  used (i.e., the prime modulus N and generator g). As described in
  [RFC2945], N is required to be a Sophie-German prime (of the form
  N = 2q + 1, where q is also prime) and the generator g is a
  primitive root of GF(n). In iSCSI authentication, the prime
  modulus N MUST be at least 768 bits.

  The list of allowed SRP groups is provided in [RFC3723].

9.2.3. Kerberos Considerations

  iSCSI uses raw Kerberos V5 [RFC4120] for authenticating a client
  (iSCSI initiator) principal to a service (iSCSI target) principal.
  Note that iSCSI does not use the Generic Security Services
  Application Programming Interface (GSS-API) [RFC2743] nor the
  Kerberos V5 GSS-API security mechanism [RFC4121]. This means that
  iSCSI implementations supporting the KRB5 AuthMethod (Section
  12.1) are directly involved in the Kerberos protocol. When
  Kerberos V5 is used for authentication, the following actions MUST
  be performed as specified in [RFC4120]:

     Target MUST validate the KRB_AP_REQ to ensure that the
       initiator can be trusted

     When mutual authentication is selected, the initiator MUST
       validate KRB_AP_REP to determine the outcome of mutual
       authentication

  As Kerberos V5 is capable of providing mutual authentication,
  implementations SHOULD support mutual authentication by default
  for login authentication.

  Note however that Kerberos authentication only assures that the
  server (iSCSI target) can be trusted by the Kerberos client
  (initiator) and vice-versa; an initiator should employ
  appropriately secured service discovery techniques (e.g. iSNS,
  Section 4.2.7) to ensure that it is talking to the intended target
  principal.




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   iSCSI does not use Kerberos v5 for either integrity or
   confidentiality protection of the iSCSI protocol. iSCSI uses IPsec
   for those purposes as specified in Section 9.3.

9.3. IPsec

   iSCSI uses the IPsec mechanism for packet protection
   (cryptographic integrity, authentication, and confidentiality) at
   the IP level between the iSCSI communicating end points. The
   following sections describe the IPsec protocols that must be
   implemented for data integrity and authentication,
   confidentiality, and cryptographic key management.

   An iSCSI initiator or target may provide the required IPsec
   support fully integrated or in conjunction with an IPsec front-end
   device. In the latter case, the compliance requirements with
   regard to IPsec support apply to the "combined device". Only the
   "combined device" is to be considered an iSCSI device.

   Detailed considerations and recommendations for using IPsec for
   iSCSI are provided in [RFC3723] as updated by [IPSEC-IPS]. The
   IPsec requirements are reproduced here for convenience and are
   intended to match those in [IPSEC-IPS]; in the event of a
   discrepancy, the requirements in [IPSEC-IPS] apply.


9.3.1. Data Integrity and Authentication

   Data authentication and integrity is provided by a cryptographic
   keyed Message Authentication Code in every sent packet. This code
   protects against message insertion, deletion, and modification.
   Protection against message replay is realized by using a sequence
   counter.

   An iSCSI-compliant initiator or target MUST provide data integrity
   and authentication by implementing IPsec v2 [RFC2401] with ESPv2
   [RFC2406] in tunnel mode, SHOULD provide data integrity and
   authentication by implementing IPsec v3 [RFC4301] with ESPv3
   [RFC4303] in tunnel mode, and MAY provide data integrity and
   authentication by implementing either IPsec v2 or v3 with the
   appropriate version of ESP in transport mode. The IPsec




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  implementation MUST fulfill the following iSCSI-specific
  requirements:

     - HMAC-SHA1 MUST be implemented in the specific form of HMAC-
       SHA-1-96 [RFC2404].

     - AES CBC MAC with XCBC extensions using 128-bit keys SHOULD
       be implemented [RFC3566].

     - Implementations that support IKEv2 [RFC5996] SHOULD also
       implement AES GMAC [RFC4543] using 128-bit keys.


  The ESP anti-replay service MUST also be implemented.

  At the high speeds iSCSI is expected to operate, a single IPsec SA
  could rapidly cycle through the ESP 32-bit sequence number space.
  In view of this, an iSCSI implementation that is capable of
  operating at speeds of 1 Gbps and that implements both IKEv2
  [RFC5996] and ESPv3 [RFC4303] MUST also implement extended (64-
  bit) sequence numbers for ESPv3 and SHOULD use ESPv3 extended
  sequence numbers for all security associations that use ESPv3 to
  protect iSCSI connections.

9.3.2. Confidentiality

  Confidentiality is provided by encrypting the data in every
  packet. When confidentiality is used it MUST be accompanied by
  data integrity and authentication to provide comprehensive
  protection against eavesdropping, message insertion, deletion,
  modification, and replaying.

  An iSCSI-compliant initiator or target MUST provide
  confidentiality by implementing IPsec v2 [RFC2401] with ESPv2
  [RFC2406] in tunnel mode, SHOULD provide confidentiality by
  implementing IPsec v3 [RFC4301] with ESPv3 [RFC4303] in tunnel
  mode and MAY provide confidentiality by implementing either IPsec
  v2 or v3 with the appropriate version of ESP in transport mode,
  with the following iSCSI specific requirements that apply to IPsec
  v2 and IPsec v3:
    - 3DES in CBC mode MAY be implemented [RFC2451].




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     - AES in CBC mode with 128-bit keys MUST be implemented
       [RFC3602]; other key sizes MAY be supported.

     - AES in Counter mode MAY be implemented [RFC3686].

     - Implementations that support IKEv2 [RFC5996] SHOULD also
       implement AES GCM with 128-bit keys [RFC4106] ]; other key
       sizes MAY be supported.


  DES in CBC mode MUST NOT be used due to its inherent weakness.

  The NULL encryption algorithm MUST also be implemented.

9.3.3. Policy, Security Associations, and Cryptographic Key
        Management

  A compliant iSCSI implementation MUST meet the cryptographic key
  management requirements of the IPsec protocol suite.
  Authentication, security association negotiation, and
  cryptographic key management MUST be provided by implementing IKE
  [RFC2409] using the IPsec DOI [RFC2407], and SHOULD be provided by
  implementing IKEv2 [RFC5996], with the following iSCSI-specific
  requirements:

     a) Peer authentication using a pre-shared cryptographic key MUST
        be supported. Certificate-based peer authentication using
        digital signatures MAY be supported. For IKEv1 ([RFC2409]),
        peer authentication using the public key encryption methods
        outlined in IKE sections 5.2 and 5.3 of [RFC2409] SHOULD NOT
        be used.
     b) When digital signatures are used to achieve authentication,
        an IKE negotiator SHOULD use IKE Certificate Request
        Payload(s) to specify the certificate authority. IKE
        negotiators SHOULD check the pertinent Certificate Revocation
        List (CRL) before accepting a PKI certificate for use in IKE
        authentication procedures. These checks may not be needed in
        environments where a small number of certificates are
        statically configured as trust anchors.
     c) Conformant iSCSI implementations of IKEv1 MUST support Main
        Mode and SHOULD support Aggressive Mode. Main Mode with pre-
        shared key authentication method SHOULD NOT be used when




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        either the initiator or the target uses dynamically assigned
        addresses. While in many cases pre-shared keys offer good
        security, situations in which dynamically assigned addresses
        are used force the use of a group pre-shared key, which
        creates vulnerability to a man-in-the-middle attack.
     d) In the IKEv1 Phase 2 Quick Mode, exchanges for creating the
        Phase 2 SA, the Identification Payload MUST be present.
     e) The following identification type requirements apply to
        IKEv1. ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack
        supports IPv6) and ID_FQDN Identification Types MUST be
        supported; ID_USER_FQDN SHOULD be supported. The IP Subnet,
        IP Address Range, ID_DER_ASN1_DN, and ID_DER_ASN1_GN
        Identification Types SHOULD NOT be used. The ID_KEY_ID
        Identification Type MUST NOT be used.
     f) If IKEv2 is supported, the following identification
        requirements apply. ID_IPV4_ADDR, ID_IPV6_ADDR (if the
        protocol stack supports IPv6) and ID_FQDN Identification
        Types MUST be supported; ID_RFC822_ADDR SHOULD be supported.
        The ID_DER_ASN1_DN, and ID_DER_ASN1_GN Identification Types
        SHOULD NOT be used. The ID_KEY_ID Identification Type MUST
        NOT be used.

  The reasons for the "MUST NOT" and "SHOULD NOT" requirements for
  identification type requirements in preceding bullets e) and f)
  are:

    - IP Subnet and IP Address Range are too broad to usefully
      identify an iSCSI endpoint.

    - The DN and GN types are X.500 identities; it is usually
      better to use an identity from subjectAltName in a PKI
      certificate.

    - ID_KEY_ID is not interoperable as specified.


  Manual cryptographic keying MUST NOT be used because it does not
  provide the necessary re-keying support.

  When DH groups are used, a DH group of at least 2048 bits SHOULD
  be offered as a part of all proposals to create IPsec Security
  Associations to protect iSCSI traffic, with both IKEv1 and IKEv2.




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  When IPsec is used, the receipt of an IKEv1 Phase 2 delete message
  or an IKEv2 INFORMATIONAL exchange that deletes the SA SHOULD NOT
  be interpreted as a reason for tearing down the iSCSI TCP
  connection. If additional traffic is sent on it, a new IKE SA will
  be created to protect it.

  The method used by the initiator to determine whether the target
  should be connected using IPsec is regarded as an issue of IPsec
  policy administration, and thus not defined in the iSCSI standard.

  The method used by an initiator that supports both IPsec v2 and v3
  to determine which versions of IPsec are supported by the target
  is also regarded as an issue of IPsec policy administration, and
  thus not defined in the iSCSI standard. If both IPsec v2 and v3
  are supported by both the initiator and target, use of IPsec v3 is
  recommended.

  If an iSCSI target is discovered via a SendTargets request in a
  discovery session not using IPsec, the initiator should assume
  that it does not need IPsec to establish a session to that target.
  If an iSCSI target is discovered using a discovery session that
  does use IPsec, the initiator SHOULD use IPsec when establishing a
  session to that target.

9.4. Security Considerations for the X#NodeArchitecture Key

  The security considerations in this Section are specific to the
  X#NodeArchitecture discussed in Section 13.26.

  This extension key transmits specific implementation details about
  the node that sends it; such details may be considered sensitive
  in some environments. For example, if a certain software or
  firmware version is known to contain security weaknesses,
  announcing the presence of that version via this key may not be
  desirable. The countermeasures for this security concern are:

     a) sending less detailed information in the key values,
     b) not sending the extension key, or
     c) using IPsec ([RFC4303]) to provide confidentiality for the
        iSCSI connection on which the key is sent




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  To support the first and second countermeasures, all
  implementations of this extension key MUST provide an
  administrative mechanism to disable sending the key. In addition,
  all implementations SHOULD provide an administrative mechanism to
  configure a verbosity level of the key value, thereby controlling
  the amount of information sent.

  For example, a lower verbosity might enable transmission of node
  architecture component names only, but no version numbers. The
  choice of which countermeasure is most appropriate depends on the
  environment. However, sending less detailed information in the key
  values may be an acceptable countermeasure in many environments,
  since it provides a compromise between sending too much
  information and the other more complete countermeasures of not
  sending the key at all or using IPsec.

  In addition to security considerations involving transmission of
  the key contents, any logging method(s) used for the key values
  MUST keep the information secure from intruders. For all
  implementations, the requirements to address this security concern
  are:

     a) Display of the log MUST only be possible with administrative
        rights to the node.
     b) Options to disable logging to disk and to keep logs for a
        fixed duration SHOULD be provided.

  Finally, it is important to note that different nodes may have
  different levels of risk, and these differences may affect the
  implementation. The components of risk include assets, threats,
  and vulnerabilities. Consider the following example iSCSI nodes,
  which demonstrate differences in assets and vulnerabilities of the
  nodes, and as a result, differences in implementation:

       a) One iSCSI target based on a special-purpose operating
          system: Since the iSCSI target controls access to the
          data storage containing company assets, the asset level
          is seen as very high. Also, because of the special-
          purpose operating system, in which vulnerabilities are
          less well-known, the vulnerability level is viewed as
          low.




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       b) Multiple iSCSI initiators in a blade farm, each running a
          general purpose operating system: The asset level of each
          node is viewed as low, since blades are replaceable and
          low cost. However, the vulnerability level is viewed as
          high, since there may be many well-known vulnerabilities
          to that general-purpose operating system. For this
          target, an appropriate implementation might be logging of
          received key values, but no transmission of the key. For
          this initiator, an appropriate implementation might be
          transmission of the key, but no logging of received key
          values.


9.5. SCSI Access Control Considerations

  iSCSI is a SCSI transport protocol and as such does not apply any
  access controls on SCSI-level operations such as SCSI task
  management functions (e.g. LU Reset, see Section 11.5.1). SCSI-
  level access controls (e.g. ACCESS CONTROL OUT, see [SPC3]) have
  to be appropriately deployed in practice to address SCSI-level
  security considerations, in addition to security at iSCSI
  connection and packet protection mechanisms that were already
  discussed in preceding Sections.




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10. Notes to Implementers

  This Section notes some of the performance and reliability
  considerations of the iSCSI protocol. This protocol was designed
  to allow efficient silicon and software implementations. The iSCSI
  task tag mechanism was designed to enable Direct Data Placement
  (DDP - a DMA form) at the iSCSI level or lower.

  The guiding assumption made throughout the design of this protocol
  is that targets are resource constrained relative to initiators.

  Implementers are also advised to consider the implementation
  consequences of the iSCSI to SCSI mapping model as outlined in
  Section 4.4.3.

10.1. Multiple Network Adapters

  The iSCSI protocol allows multiple connections, not all of which
  need to go over the same network adapter. If multiple network
  connections are to be utilized with hardware support, the iSCSI
  protocol command-data-status allegiance to one TCP connection
  ensures that there is no need to replicate information across
  network adapters or otherwise require them to cooperate.

  However, some task management commands may require some loose form
  of cooperation or replication at least on the target.

10.1.1. Conservative Reuse of ISIDs

  Historically, the SCSI model (and implementations and applications
  based on that model) has assumed that SCSI ports are static,
  physical entities. Recent extensions to the SCSI model have taken
  advantage of persistent worldwide unique names for these ports. In
  iSCSI however, the SCSI initiator ports are the endpoints of
  dynamically created sessions, so the presumptions of "static and
  physical" do not apply. In any case, the model clauses
  (particularly, Section 4.4.1) provide for persistent, reusable
  names for the iSCSI-type SCSI initiator ports even though there
  does not need to be any physical entity bound to these names.

  To both minimize the disruption of legacy applications and to
  better facilitate the SCSI features that rely on persistent names




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  for SCSI ports, iSCSI implementations SHOULD attempt to provide a
  stable presentation of SCSI Initiator Ports (both to the upper OS-
  layers and to the targets to which they connect). This can be
  achieved in an initiator implementation by conservatively reusing
  ISIDs. In other words, the same ISID should be used in the Login
  process to multiple target portal groups (of the same iSCSI Target
  or different iSCSI Targets). The ISID RULE (Section 4.4.3) only
  prohibits reuse to the same target portal group. It does not
  "preclude" reuse to other target portal groups.
  The principle of conservative reuse "encourages" reuse to other
  target portal groups. When a SCSI target device sees the same
  (InitiatorName, ISID) pair in different sessions to different
  target portal groups, it can identify the underlying SCSI
  Initiator Port on each session as the same SCSI port. In effect,
  it can recognize multiple paths from the same source.

10.1.2. iSCSI Name, ISID, and TPGT Use

  The designers of the iSCSI protocol are aware that legacy SCSI
  transports rely on initiator identity to assign access to storage
  resources. Although newer techniques are available and simplify
  access control, support for configuration and authentication
  schemes that are based on initiator identity is deemed important
  in order to support legacy systems and administration software.
  iSCSI thus supports the notion that it should be possible to
  assign access to storage resources based on "initiator device"
  identity.

  When there are multiple hardware or software components
  coordinated as a single iSCSI Node, there must be some (logical)
  entity that represents the iSCSI Node that makes the iSCSI Node
  Name available to all components involved in session creation and
  login. Similarly, this entity that represents the iSCSI Node must
  be able to coordinate session identifier resources (ISID for
  initiators) to enforce both the ISID and TSIH RULES (see Section
  4.4.3).

  For targets, because of the closed environment, implementation of
  this entity should be straightforward. However, vendors of iSCSI
  hardware (e.g., NICs or HBAs) intended for targets, SHOULD provide
  mechanisms for configuration of the iSCSI Node Name across the




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  portal groups instantiated by multiple instances of these
  components within a target.

  However, complex targets making use of multiple Target Portal
  Group Tags may reconfigure them to achieve various quality goals.
  The initiators have two mechanisms at their disposal to discover
  and/or check reconfiguring targets - the discovery session type
  and a key returned by the target during login to confirm the TPGT.
  An initiator should attempt to "rediscover" the target
  configuration anytime a session is terminated unexpectedly.

  For initiators, in the long term, it is expected that operating
  system vendors will take on the role of this entity and provide
  standard APIs that can inform components of their iSCSI Node Name
  and can configure and/or coordinate ISID allocation, use, and
  reuse.

  Recognizing that such initiator APIs are not available today,
  other implementations of the role of this entity are possible. For
  example, a human may instantiate the (common) Node name as part of
  the installation process of each iSCSI component involved in
  session creation and login. This may be done either by pointing
  the component to a vendor-specific location for this datum or to a
  system-wide location. The structure of the ISID namespace (see
  Section 11.12.5 and [RFC3721]) facilitates implementation of the
  ISID coordination by allowing each component vendor to
  independently (of other vendor's components) coordinate
  allocation, use, and reuse of its own partition of the ISID
  namespace in a vendor-specific manner. Partitioning of the ISID
  namespace within initiator portal groups managed by that vendor
  allows each such initiator portal group to act independently of
  all other portal groups when selecting an ISID for a login; this
  facilitates enforcement of the ISID RULE (see Section 4.4.3) at
  the initiator.

  A vendor of iSCSI hardware (e.g., NICs or HBAs) intended for use
  in initiators MUST implement a mechanism for configuring the iSCSI
  Node Name. Vendors, and administrators must ensure that iSCSI Node
  Names are unique worldwide. It is therefore important that when
  one chooses to reuse the iSCSI Node Name of a disabled unit, not




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  to re-assign that name to the original unit unless its worldwide
  uniqueness can be ascertained again.

  In addition, a vendor of iSCSI hardware must implement a mechanism
  to configure and/or coordinate ISIDs for all sessions managed by
  multiple instances of that hardware within a given iSCSI Node.
  Such configuration might be either permanently pre-assigned at the
  factory (in a necessarily globally unique way), statically
  assigned (e.g., partitioned across all the NICs at initialization
  in a locally unique way), or dynamically assigned (e.g., on-line
  allocator, also in a locally unique way). In the latter two cases,
  the configuration may be via public APIs (perhaps driven by an
  independent vendor's software, such as the OS vendor) or via
  private APIs driven by the vendor's own software.

  The process of name assignment and coordination has to be as
  encompassing and automated as possible as years of legacy usage
  have shown it to be highly error-prone. It is to be mentioned
  that SCSI has today alternative schemes of access control that can
  be used by all transports and their security is not dependent on
  strict naming coordination.

10.2. Autosense and Auto Contingent Allegiance (ACA)

  Autosense refers to the automatic return of sense data to the
  initiator in case a command did not complete successfully. iSCSI
  initiators and targets MUST support and use autosense.

  ACA helps preserve ordered command execution in the presence of
  errors. As there can be many commands in-flight between an
  initiator and a target, SCSI initiator functionality in some
  operating systems depends on ACA to enforce ordered command
  execution during error recovery, and hence iSCSI initiator
  implementations for those operating systems need to support ACA.
  In order to support error recovery for these operating systems and
  iSCSI initiators, iSCSI targets SHOULD support ACA.

10.3. iSCSI Timeouts

  iSCSI recovery actions are often dependent on iSCSI time-outs
  being recognized and acted upon before SCSI time-outs. Determining
  the right time-outs to use for various iSCSI actions (command




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  acknowledgements expected, status acknowledgements, etc.) is very
  much dependent on infrastructure (hardware, links, TCP/IP stack,
  iSCSI driver). As a guide, the implementer may use an average Nop-
  Out/Nop-In turnaround delay multiplied by a "safety factor" (e.g.,
  4) as a good estimate for the basic delay of the iSCSI stack for a
  given connection. The safety factor should account for the network
  load variability. For connection teardown the implementer may
  want to consider also TCP common practice for the given
  infrastructure.

  Text negotiations MAY also be subject to either time-limits or
  limits in the number of exchanges. Those SHOULD be generous enough
  to avoid affecting interoperability (e.g., allowing each key to be
  negotiated on a separate exchange).

  The relation between iSCSI timeouts and SCSI timeouts should also
  be considered. SCSI timeouts should be longer than iSCSI timeouts
  plus the time required for iSCSI recovery whenever iSCSI recovery
  is planned. Alternatively, an implementer may choose to interlock
  iSCSI timeouts and recovery with SCSI timeouts so that SCSI
  recovery will become active only where iSCSI is not planned to, or
  failed to, recover.

  The implementer may also want to consider the interaction between
  various iSCSI exception events - such as a digest failure - and
  subsequent timeouts. When iSCSI error recovery is active, a digest
  failure is likely to result in discovering a missing command or
  data PDU. In these cases, an implementer may want to lower the
  timeout values to enable faster initiation for recovery
  procedures.

10.4. Command Retry and Cleaning Old Command Instances

  To avoid having old, retried command instances appear in a valid
  command window after a command sequence number wrap around, the
  protocol requires (see Section 4.2.2.1) that on every connection
  on which a retry has been issued, a non-immediate command be
  issued and acknowledged within a 2**31-1 commands interval from
  the CmdSN of the retried command. This requirement can be
  fulfilled by an implementation in several ways.




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  The simplest technique to use is to send a (non-retry) non-
  immediate SCSI command (or a NOP if no SCSI command is available
  for a while) after every command retry on the connection on which
  the retry was attempted. As errors are deemed rare events, this
  technique is probably the most effective, as it does not involve
  additional checks at the initiator when issuing commands.

10.5. Synch and Steering Layer and Performance

  While a synch and steering layer is optional, an initiator/target
  that does not have it working against a target/initiator that
  demands synch and steering may experience performance degradation
  caused by packet reordering and loss. Providing a synch and
  steering mechanism is recommended for all high-speed
  implementations.

10.6. Considerations for State-dependent Devices and Long-lasting
     SCSI Operations

  Sequential access devices operate on the principle that the
  position of the device is based on the last command processed. As
  such, command processing order and knowledge of whether or not the
  previous command was processed is of the utmost importance to
  maintain data integrity. For example, inadvertent retries of SCSI
  commands when it is not known if the previous SCSI command was
  processed is a potential data integrity risk.

  For a sequential access device, consider the scenario in which a
  SCSI SPACE command to backspace one filemark is issued and then
  re-issued due to no status received for the command. If the first
  SPACE command was actually processed, the re-issued SPACE command,
  if processed, will cause the position to change. Thus, a
  subsequent write operation will write data to the wrong position
  and any previous data at that position will be overwritten.

  For a medium changer device, consider the scenario in which an
  EXCHANGE MEDIUM command (the SOURCE ADDRESS and DESTINATION
  ADDRESS are the same thus performing a swap) is issued and then
  re-issued due to no status received for the command. If the first
  EXCHANGE MEDIUM command was actually processed, the re-issued
  EXCHANGE MEDIUM command, if processed, will perform the swap




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  again. The net effect is no swap was performed thus leaving a data
  integrity exposure.

  All commands that change the state of the device (as in SPACE
  commands for sequential access devices, and EXCHANGE MEDIUM for
  medium changer device), MUST be issued as non-immediate commands
  for deterministic and in order delivery to iSCSI targets.

  For many of those state changing commands, the execution model
  also assumes that the command is executed exactly once. Devices
  implementing READ POSITION and LOCATE provide a means for SCSI
  level command recovery and new tape-class devices should support
  those commands. In their absence a retry at SCSI level is
  difficult and error recovery at iSCSI level is advisable.

  Devices operating on long latency delivery subsystems and
  performing long lasting SCSI operations may need mechanisms that
  enable connection replacement while commands are running (e.g.,
  during an extended copy operation).

10.6.1. Determining the Proper ErrorRecoveryLevel

  The implementation and use of a specific ErrorRecoveryLevel should
  be determined based on the deployment scenarios of a given iSCSI
  implementation. Generally, the following factors must be
  considered before deciding on the proper level of recovery:

       a) Application resilience to I/O failures.
       b) Required level of availability in the face of transport
          connection failures.
       c) Probability of transport layer "checksum escape" (message
          error undetected by TCP checksum - see [RFC3385] for
          related discussion). This in turn decides the iSCSI
          digest failure frequency, and thus the criticality of
          iSCSI-level error recovery. The details of estimating
          this probability are outside the scope of this document.

  A consideration of the above factors for SCSI tape devices as an
  example suggests that implementations SHOULD use
  ErrorRecoveryLevel=1 when transport connection failure is not a
  concern and SCSI level recovery is unavailable, and




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  ErrorRecoveryLevel=2 when the connection failure is also of high
  likelihood during a backup/retrieval.

  For extended copy operations, implementations SHOULD use
  ErrorRecoveryLevel=2 whenever there is a relatively high
  likelihood of connection failure.

10.7. Multi-task Abort Implementation Considerations

  Multi-task abort operations are typically issued in emergencies -
  such as clearing a device lock-up, HA failover/failback etc. In
  these circumstances, it is desirable to rapidly go through the
  error handling process as opposed to the target waiting on
  multiple third-party initiators who may not even be functional
  anymore - especially if this emergency is triggered because of one
  such initiator failure. Therefore, both iSCSI target and
  initiator implementations SHOULD support FastAbort multi-task
  abort semantics (Section 4.2.3.4).

  Note that both in standard semantics (Section 4.2.3.3) and
  FastAbort semantics (Section 4.2.3.4), there may be outstanding
  data transfers even after the TMF completion is reported on the
  issuing session. In the case of iSCSI/iSER [iSER], these would be
  tagged data transfers for STags not owned by any active tasks.
  Whether or not real buffers support these data transfers is
  implementation-dependent. However, the data transfers logically
  MUST be silently discarded by the target iSCSI layer in all cases.
  A target MAY, on an implementation-defined internal timeout, also
  choose to drop the connections on which it did not receive the
  expected Data-Out sequences (Section 4.2.3.3) or NOP-Out
  acknowledgements (Section 4.2.3.4) so as to reclaim the associated
  buffer, STag, and TTT resources as appropriate.




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11. iSCSI PDU Formats

  All multi-byte integers that are specified in formats defined in
  this document are to be represented in network byte order (i.e.,
  big endian). Any field that appears in this document assumes that
  the most significant byte is the lowest numbered byte and the most
  significant bit (within byte or field) is the lowest numbered bit
  unless specified otherwise.

  Any compliant sender MUST set all bits not defined and all
  reserved fields to zero unless specified otherwise. Any compliant
  receiver MUST ignore any bit not defined and all reserved fields
  unless specified otherwise. Receipt of reserved code values in
  defined fields MUST be reported as a protocol error.

  Reserved fields are marked by the word "reserved", some
  abbreviation of "reserved", or by "." for individual bits when no
  other form of marking is technically feasible.

11.1. iSCSI PDU Length and Padding

  iSCSI PDUs are padded to the closest integer number of four byte
  words. The padding bytes SHOULD be sent as 0.

11.2. PDU Template, Header, and Opcodes

  All iSCSI PDUs have one or more header segments and, optionally, a
  data segment. After the entire header segment group a header-
  digest MAY follow. The data segment MAY also be followed by a
  data-digest.

  The Basic Header Segment (BHS) is the first segment in all of the
  iSCSI PDUs. The BHS is a fixed-length 48-byte header segment. It
  MAY be followed by Additional Header Segments (AHS), a Header-
  Digest, a Data Segment, and/or a Data-Digest.

  The overall structure of an iSCSI PDU is as follows:




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  Byte/     0       |        1      |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0/ Basic Header Segment (BHS)                                   /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48/ Additional Header Segment 1 (AHS) (optional)                 /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    / Additional Header Segment 2 (AHS) (optional)                 /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    +---------------+---------------+---------------+--------------+
    / Additional Header Segment n (AHS) (optional)                 /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
   k/ Header-Digest (optional)                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
   l/ Data Segment(optional)                                       /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
   m/ Data-Digest (optional)                                       /
   +/                                                              /
    +---------------+---------------+---------------+--------------+

  All PDU segments and digests are padded to the closest integer
  number of four byte words. For example, all PDU segments and
  digests start at a four byte word boundary and the padding ranges
  from 0 to 3 bytes. The padding bytes SHOULD be sent as 0.

  iSCSI response PDUs do not have AH Segments.

11.2.1. Basic Header Segment (BHS)

  The BHS is 48 bytes long. The Opcode and DataSegmentLength fields
  appear in all iSCSI PDUs. In addition, when used, the Initiator
  Task Tag and Logical Unit Number always appear in the same
  location in the header.

  The format of the BHS is:




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   Byte/     0       |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|I| Opcode    |F| Opcode-specific fields                     |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +---------------+---------------+---------------+--------------+
    8| LUN or Opcode-specific fields                                |
     +                                                              +
   12|                                                              |
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag                                           |
     +---------------+---------------+---------------+--------------+
   20/ Opcode-specific fields                                       /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   48

11.2.1.1. I

   For request PDUs, the I bit set to 1 is an immediate delivery
   marker.

11.2.1.2. Opcode

   The Opcode indicates the type of iSCSI PDU the header
   encapsulates.

   The Opcodes are divided into two categories: initiator opcodes and
   target opcodes. Initiator opcodes are in PDUs sent by the
   initiator (request PDUs). Target opcodes are in PDUs sent by the
   target (response PDUs).

   Initiators MUST NOT use target opcodes and targets MUST NOT use
   initiator opcodes.

   Initiator opcodes defined in this specification are:




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    0x00 NOP-Out

    0x01 SCSI Command (encapsulates a SCSI Command Descriptor
      Block)

    0x02 SCSI Task Management function request

    0x03 Login Request

    0x04 Text Request

    0x05 SCSI Data-out (for WRITE operations)

    0x06 Logout Request

    0x10 SNACK Request

    0x1c-0x1e Vendor specific codes


  Target opcodes are:


    0x20 NOP-In

    0x21 SCSI Response - contains SCSI status and possibly sense
      information or other response information.

    0x22 SCSI Task Management function response

    0x23 Login Response

    0x24 Text Response

    0x25 SCSI Data-in - for READ operations.

    0x26 Logout Response

    0x31 Ready To Transfer (R2T) - sent by target when it is ready
      to receive data.

    0x32 Asynchronous Message - sent by target to indicate certain
      special conditions.




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     0x3c-0x3e Vendor specific codes

     0x3f Reject


   All other opcodes are reserved.

11.2.1.3. Final (F) bit

   When set to 1 it indicates the final (or only) PDU of a sequence.

11.2.1.4. Opcode-specific Fields

   These fields have different meanings for different opcode types.

11.2.1.5. TotalAHSLength

   Total length of all AHS header segments in units of four byte
   words including padding, if any.

   The TotalAHSLength is only used in PDUs that have an AHS and MUST
   be 0 in all other PDUs.

11.2.1.6. DataSegmentLength

   This is the data segment payload length in bytes (excluding
   padding). The DataSegmentLength MUST be 0 whenever the PDU has no
   data segment.

11.2.1.7. LUN

   Some opcodes operate on a specific Logical Unit. The Logical Unit
   Number (LUN) field identifies which Logical Unit. If the opcode
   does not relate to a Logical Unit, this field is either ignored or
   may be used in an opcode specific way. The LUN field is 64-bits
   and should be formatted in accordance with [SAM2]. For example,
   LUN[0] from [SAM2] is BHS byte 8 and so on up to LUN[7] from
   [SAM2], which is BHS byte 15.




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11.2.1.8. Initiator Task Tag

   The initiator assigns a Task Tag to each iSCSI task it issues.
   While a task exists, this tag MUST uniquely identify the task
   session-wide. SCSI may also use the initiator task tag as part of
   the SCSI task identifier when the timespan during which an iSCSI
   initiator task tag must be unique extends over the timespan during
   which a SCSI task tag must be unique. However, the iSCSI Initiator
   Task Tag must exist and be unique even for untagged SCSI commands.

   An ITT value of 0xffffffff is reserved and MUST NOT be assigned
   for a task by the initiator. The only instance in which it may be
   seen on the wire is in a target-initiated NOP-In PDU (Section
   11.19) and in the initiator response to that PDU, if necessary.

11.2.2. Additional Header Segment (AHS)

   The general format of an AHS is:

   Byte/      0       |       1       |       2       |       3      |
       /              |               |               |              |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
      +---------------+---------------+---------------+--------------+
    0| AHSLength                      | AHSType       | AHS-Specific |
      +---------------+---------------+---------------+--------------+
    4/ AHS-Specific                                                  /
    +/                                                               /
      +---------------+---------------+---------------+--------------+
    x

11.2.2.1. AHSType

   The AHSType field is coded as follows:

     bit 0-1 - Reserved

     bit 2-7 - AHS code

      0 - Reserved

      1 - Extended CDB




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      2 - Expected Bidirectional Read Data Length

      3 - 63 Reserved


11.2.2.2. AHSLength

   This field contains the effective length in bytes of the AHS
   excluding AHSType and AHSLength and padding, if any. The AHS is
   padded to the smallest integer number of 4 byte words (i.e., from
   0 up to 3 padding bytes).

11.2.2.3. Extended CDB AHS

   The format of the Extended CDB AHS is:

   Byte/      0       |       1       |       2       |       3      |
       /              |               |               |              |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
      +---------------+---------------+---------------+--------------+
    0| AHSLength (CDBLength-15)       | 0x01          | Reserved     |
      +---------------+---------------+---------------+--------------+
    4/ ExtendedCDB...+padding                                        /
    +/                                                               /
      +---------------+---------------+---------------+--------------+
    x

   This type of AHS MUST NOT be used if the CDBLength is less than
   17.
   The length includes the reserved byte 3.

11.2.2.4. Bidirectional Expected Read-Data Length AHS

   The format of the Bidirectional Read Expected Data Transfer Length
   AHS is:




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   Byte/      0       |       1       |       2       |       3      |
       /              |               |               |              |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
      +---------------+---------------+---------------+--------------+
    0| AHSLength (0x0005)             | 0x02          | Reserved     |
      +---------------+---------------+---------------+--------------+
    4| Expected Read-Data Length                                     |
      +---------------+---------------+---------------+--------------+
    8

11.2.3. Header Digest and Data Digest

   Optional header and data digests protect the integrity of the
   header and data, respectively. The digests, if present, are
   located, respectively, after the header and PDU-specific data, and
   cover respectively the header and the PDU data, each including
   the padding bytes, if any.

   The existence and type of digests are negotiated during the Login
   Phase.

   The separation of the header and data digests is useful in iSCSI
   routing applications, in which only the header changes when a
   message is forwarded. In this case, only the header digest should
   be recalculated.

   Digests are not included in data or header length fields.

   A zero-length Data Segment also implies a zero-length data-digest.

11.2.4. Data Segment

   The (optional) Data Segment contains PDU associated data. Its
   payload effective length is provided in the BHS field -
   DataSegmentLength. The Data Segment is also padded to an integer
   number of 4 byte words.

11.3. SCSI Command

   The format of the SCSI Command PDU is:




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  Byte/     0      |        1       |       2      |       3       |
    /              |                |              |               |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|I| 0x01       |F|R|W|. .|ATTR | Reserved                    |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| Logical Unit Number (LUN)                                    |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Expected Data Transfer Length                                |
    +---------------+---------------+---------------+--------------+
  24| CmdSN                                                        |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN                                                    |
    +---------------+---------------+---------------+--------------+
  32/ SCSI Command Descriptor Block (CDB)                          /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48/ AHS (Optional)                                               /
    +---------------+---------------+---------------+--------------+
   x/ Header Digest (Optional)                                     /
    +---------------+---------------+---------------+--------------+
   y/ (DataSegment, Command Data) (Optional)                       /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
   z/ Data Digest (Optional)                                       /
    +---------------+---------------+---------------+--------------+

11.3.1. Flags and Task Attributes (byte 1)

     The flags for a SCSI Command are:


     bit 0   (F) is set to 1 when no unsolicited SCSI Data-Out PDUs
       follow this PDU. When F=1 for a write and if Expected Data
       Transfer Length is larger than the DataSegmentLength, the
       target may solicit additional data through R2T.




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     bit 1   (R) is set to 1 when the command is expected to input
       data.

     bit 2   (W) is set to 1 when the command is expected to output
       data.

     bit 3-4 Reserved.

     bit 5-7 contains Task Attributes.


  Task Attributes (ATTR) have one of the following integer values
  (see [SAM2] for details):

     0 - Untagged

     1 - Simple

     2 - Ordered

     3 - Head of Queue

     4 - ACA

     5-7 - Reserved


  At least one of the W and F bits MUST be set to 1.
  Either or both of R and W MAY be 1 when either the Expected Data
  Transfer Length and/or Bidirectional Read Expected Data Transfer
  Length are 0, but they MUST NOT both be 0 when the Expected Data
  Transfer Length and/or Bidirectional Read Expected Data Transfer
  Length are not 0 (i.e., when some data transfer is expected the
  transfer direction is indicated by the R and/or W bit).


11.3.2. CmdSN - Command Sequence Number

  Enables ordered delivery across multiple connections in a single
  session.




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11.3.3. ExpStatSN

   Command responses up to ExpStatSN-1 (mod 2**32) have been received
   (acknowledges status) on the connection.

11.3.4. Expected Data Transfer Length

   For unidirectional operations, the Expected Data Transfer Length
   field contains the number of bytes of data involved in this SCSI
   operation. For a unidirectional write operation (W flag set to 1
   and R flag set to 0), the initiator uses this field to specify the
   number of bytes of data it expects to transfer for this operation.
   For a unidirectional read operation (W flag set to 0 and R flag
   set to 1), the initiator uses this field to specify the number of
   bytes of data it expects the target to transfer to the initiator.
   It corresponds to the SAM2 byte count.

   For bidirectional operations (both R and W flags are set to 1),
   this field contains the number of data bytes involved in the write
   transfer. For bidirectional operations, an additional header
   segment MUST be present in the header sequence that indicates the
   Bidirectional Read Expected Data Transfer Length. The Expected
   Data Transfer Length field and the Bidirectional Read Expected
   Data Transfer Length field correspond to the SAM2 byte count.

   If the Expected Data Transfer Length for a write and the length of
   the immediate data part that follows the command (if any) are the
   same, then no more data PDUs are expected to follow. In this
   case, the F bit MUST be set to 1.

   If the Expected Data Transfer Length is higher than the
   FirstBurstLength (the negotiated maximum amount of unsolicited
   data the target will accept), the initiator MUST send the maximum
   amount of unsolicited data OR ONLY the immediate data, if any.

   Upon completion of a data transfer, the target informs the
   initiator (through residual counts) of how many bytes were
   actually processed (sent and/or received) by the target.




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11.3.5. CDB - SCSI Command Descriptor Block

   There are 16 bytes in the CDB field to accommodate the commonly
   used CDBs. Whenever the CDB is larger than 16 bytes, an Extended
   CDB AHS MUST be used to contain the CDB spillover.

11.3.6. Data Segment - Command Data

   Some SCSI commands require additional parameter data to accompany
   the SCSI command. This data may be placed beyond the boundary of
   the iSCSI header in a data segment. Alternatively, user data
   (e.g., from a WRITE operation) can be placed in the data segment
   (both cases are referred to as immediate data). These data are
   governed by the rules for solicited vs. unsolicited data outlined
   in Section 4.2.5.2.

11.4. SCSI Response

   The format of the SCSI Response PDU is:




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  Byte/      0      |       1       |       2       |        3     |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x21      |1|. .|o|u|O|U|.| Response      | Status       |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| Reserved                                                     |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| SNACK Tag or Reserved                                        |
    +---------------+---------------+---------------+--------------+
  24| StatSN                                                       |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36| ExpDataSN or Reserved                                        |
    +---------------+---------------+---------------+--------------+
  40| Bidirectional Read Residual Count or Reserved                |
    +---------------+---------------+---------------+--------------+
  44| Residual Count or Reserved                                   |
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / Data Segment (Optional)                                      /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+

11.4.1. Flags (byte 1)

     bit 1-2 Reserved.

     bit 3 - (o) set for Bidirectional Read Residual Overflow. In
       this case, the Bidirectional Read Residual Count indicates




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       the number of bytes that were not transferred to the
       initiator because the initiator's Expected Bidirectional
       Read Data Transfer Length was not sufficient.

     bit 4 - (u) set for Bidirectional Read Residual Underflow. In
       this case, the Bidirectional Read Residual Count indicates
       the number of bytes that were not transferred to the
       initiator out of the number of bytes expected to be
       transferred.

     bit 5 - (O) set for Residual Overflow. In this case, the
       Residual Count indicates the number of bytes that were not
       transferred because the initiator's Expected Data Transfer
       Length was not sufficient. For a bidirectional operation,
       the Residual Count contains the residual for the write
       operation.

     bit 6 - (U) set for Residual Underflow. In this case, the
       Residual Count indicates the number of bytes that were not
       transferred out of the number of bytes that were expected to
       be transferred. For a bidirectional operation, the Residual
       Count contains the residual for the write operation.

     bit 7 - (0) Reserved.


   Bits O and U and bits o and u are mutually exclusive (i.e., having
   both o and u or O and U set to 1 is a protocol error).

   For a response other than "Command Completed at Target", bits 3-6
   MUST be 0.

11.4.2. Status

   The Status field is used to report the SCSI status of the command
   (as specified in [SAM2]) and is only valid if the Response Code is
   Command Completed at target.

   Some of the status codes defined in [SAM2] are:

     0x00 GOOD




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     0x02 CHECK CONDITION

     0x08 BUSY

     0x18 RESERVATION CONFLICT

     0x28 TASK SET FULL

     0x30 ACA ACTIVE

     0x40 TASK ABORTED


   See [SAM2] for the complete list and definitions.

   If a SCSI device error is detected while data from the initiator
   is still expected (the command PDU did not contain all the data
   and the target has not received a Data PDU with the final bit
   Set), the target MUST wait until it receives a Data PDU with the F
   bit set in the last expected sequence before sending the Response
   PDU.

11.4.3. Response

   This field contains the iSCSI service response.

   iSCSI service response codes defined in this specification are:

     0x00 - Command Completed at Target

     0x01 - Target Failure

     0x80-0xff - Vendor specific

   All other response codes are reserved.

   The Response is used to report a Service Response. The mapping of
   the response code into a SCSI service response code value, if
   needed, is outside the scope of this document. However, in
   symbolic terms response value 0x00 maps to the SCSI service
   response (see [SAM2] and [SPC3]) of TASK COMPLETE or LINKED
   COMMAND COMPLETE. All other Response values map to the SCSI
   service response of SERVICE DELIVERY OR TARGET FAILURE.




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   If a SCSI Response PDU does not arrive before the session is
   terminated, the SCSI service response is SERVICE DELIVERY OR
   TARGET FAILURE.

   A non-zero response field indicates a failure to execute the
   command in which case the Status and Flag fields are undefined and
   MUST be ignored on reception.

11.4.4. SNACK Tag

   This field contains a copy of the SNACK Tag of the last SNACK Tag
   accepted by the target on the same connection and for the command
   for which the response is issued. Otherwise it is reserved and
   should be set to 0.

   After issuing a R-Data SNACK the initiator must discard any SCSI
   status unless contained in an SCSI Response PDU carrying the same
   SNACK Tag as the last issued R-Data SNACK for the SCSI command on
   the current connection.

   For a detailed discussion on R-Data SNACK see 11.16.3.

11.4.5. Residual Count

11.4.5.1. Field Semantics

   The Residual Count field MUST be valid in the case where either
   the U bit or the O bit is set. If neither bit is set, the Residual
   Count field MUST be ignored on reception and SHOULD be set to 0
   when sending. Targets may set the residual count and initiators
   may use it when the response code is "completed at target" (even
   if the status returned is not GOOD). If the O bit is set, the
   Residual Count indicates the number of bytes that were not
   transferred because the initiator's Expected Data Transfer Length
   was not sufficient. If the U bit is set, the Residual Count
   indicates the number of bytes that were not transferred out of the
   number of bytes expected to be transferred.




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11.4.5.2. Residuals Concepts Overview

  SCSI-Presented Data Transfer Length (SPDTL) is the term this
  document uses (see Section 2.1 for definition) to represent the
  aggregate data length that the target SCSI layer attempts to
  transfer using the local iSCSI layer for a task. Expected Data
  Transfer Length (EDTL) is the iSCSI term that represents the
  length of data that the iSCSI layer expects to transfer for a
  task. EDTL is specified in the SCSI Command PDU.

  When SPDTL = EDTL for a task, the target iSCSI layer completes the
  task with no residuals. Whenever SPDTL differs from EDTL for a
  task, that task is said to have a residual.

  If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
  SCSI Response PDU as specified in Section 11.4.5.1. The Residual
  Count MUST be set to the numerical value of (SPDTL - EDTL).

  If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in
  the SCSI Response PDU as specified in Section 11.4.5.1. The
  Residual Count MUST be set to the numerical value of (EDTL -
  SPDTL).

  Note that the Overflow and Underflow scenarios are independent of
  Data-In and Data-Out. Either scenario is logically possible in
  either direction of data transfer.

11.4.5.3. SCSI REPORT LUNS and Residual Overflow

  This Section discusses the residual overflow issues citing the
  example of the SCSI REPORT LUNS command. Note however that there
  are several SCSI commands (e.g., INQUIRY) with ALLOCATION LENGTH
  fields following the same underlying rules. The semantics in the
  rest of the Section apply to all such SCSI commands.

  The specification of the SCSI REPORT LUNS command requires that
  the SCSI target limit the amount of data transferred to a maximum
  size (ALLOCATION LENGTH) provided by the initiator in the REPORT
  LUNS CDB.

  If the Expected Data Transfer Length (EDTL) in the iSCSI header of
  the SCSI Command PDU for a REPORT LUNS command is set to at least




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  as large as that ALLOCATION LENGTH, the SCSI layer truncation
  prevents an iSCSI Residual Overflow from occurring. A SCSI
  initiator can detect that such truncation has occurred via other
  information at the SCSI layer. The rest of the Section elaborates
  this required behavior.

  The SCSI REPORT LUNS command requests a target SCSI layer to
  return a logical unit inventory (LUN list) to the initiator SCSI
  layer (see Section 6.21 of [SPC3]). The size of this LUN list may
  not be known to the initiator SCSI layer when it issues the REPORT
  LUNS command; to avoid transferring more LUN list data than the
  initiator is prepared for, the REPORT LUNS CDB contains an
  ALLOCATION LENGTH field to specify the maximum amount of data to
  be transferred to the initiator for this command. If the initiator
  SCSI layer has underestimated the number of logical units at the
  target, it is possible that the complete logical unit inventory
  does not fit in the specified ALLOCATION LENGTH. In this
  situation, Section 4.3.3.6 in [SPC3] requires that the target SCSI
  layer "shall terminate transfers to the Data-In Buffer" when the
  number of bytes specified by the ALLOCATION LENGTH field have been
  transferred.

  Therefore, in response to a REPORT LUNS command, the SCSI layer at
  the target presents at most ALLOCATION LENGTH bytes of data
  (logical unit inventory) to iSCSI for transfer to the initiator.
  For a REPORT LUNS command, if the iSCSI EDTL is at least as large
  as the ALLOCATION LENGTH, the SCSI truncation ensures that the
  EDTL will accommodate all of the data to be transferred. If all of
  the logical unit inventory data presented to the iSCSI layer --
  i.e., the data remaining after any SCSI truncation -- is
  transferred to the initiator by the iSCSI layer, an iSCSI Residual
  Overflow has not occurred and the iSCSI (O) bit MUST NOT be set in
  the SCSI Response or final SCSI Data-Out PDU. Note that this
  behavior is implied by the combination of Section 11.4.5.1 along
  with the specification of the REPORT LUNS command in [SPC3].
  However, if the iSCSI EDTL is larger than the ALLOCATION LENGTH in
  this scenario, note that the iSCSI Underflow MUST be signaled in
  the SCSI Response PDU. An iSCSI Underflow MUST also be signaled
  when the iSCSI EDTL is equal to the ALLOCATION LENGTH but the
  logical unit inventory data presented to the iSCSI layer is
  smaller than the ALLOCATION LENGTH.




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  The LUN LIST LENGTH field in the logical unit inventory (the first
  field in the inventory) is not affected by truncation of the
  inventory to fit in ALLOCATION LENGTH; this enables a SCSI
  initiator to determine that the received inventory is incomplete
  by noticing that the LUN LIST LENGTH in the inventory is larger
  than the ALLOCATION LENGTH that was sent in the REPORT LUNS CDB. A
  common initiator behavior in this situation is to re-issue the
  REPORT LUNS command with a larger ALLOCATION LENGTH.

11.4.6. Bidirectional Read Residual Count

  The Bidirectional Read Residual Count field MUST be valid in the
  case where either the u bit or the o bit is set. If neither bit is
  set, the Bidirectional Read Residual Count field is reserved.
  Targets may set the Bidirectional Read Residual Count and
  initiators may use it when the response code is "completed at
  target". If the o bit is set, the Bidirectional Read Residual
  Count indicates the number of bytes that were not transferred to
  the initiator because the initiator's Expected Bidirectional Read
  Transfer Length was not sufficient. If the u bit is set, the
  Bidirectional Read Residual Count indicates the number of bytes
  that were not transferred to the initiator out of the number of
  bytes expected to be transferred.

11.4.7. Data Segment - Sense and Response Data Segment

  iSCSI targets MUST support and enable autosense. If Status is
  CHECK CONDITION (0x02), then the Data Segment MUST contain sense
  data for the failed command.

  For some iSCSI responses, the response data segment MAY contain
  some response related information, (e.g., for a target failure, it
  may contain a vendor specific detailed description of the
  failure).

  If the DataSegmentLength is not 0, the format of the Data Segment
  is as follows:




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  Byte/     0       |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|SenseLength                    | Sense Data                   |
    +---------------+---------------+---------------+--------------+
   x/ Sense Data                                                   /
    +---------------+---------------+---------------+--------------+
   y/ Response Data                                                /
    /                                                              /
    +---------------+---------------+---------------+--------------+

11.4.7.1. SenseLength

  Length of Sense Data.

11.4.7.2. Sense Data

  The Sense Data contains detailed information about a check
  condition and [SPC3] specifies the format and content of the Sense
  Data.

  Certain iSCSI conditions result in the command being terminated at
  the target (response Command Completed at Target) with a SCSI
  Check Condition Status as outlined in the next table:




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   +--------------------------+----------+--------------------------+
   | iSCSI Condition          |Sense     | Additional Sense Code & |
   |                          |Key       | Qualifier                |
   +--------------------------+----------+--------------------------+
   | Unexpected unsolicited   |Aborted   | ASC = 0x0c ASCQ = 0x0c   |
   | data                     |Command-0B| Write Error              |
   +--------------------------+----------+--------------------------+
   | Incorrect amount of data |Aborted   | ASC = 0x0c ASCQ = 0x0d   |
   |                          |Command-0B| Write Error              |
   +--------------------------+----------+--------------------------+
   | Protocol Service CRC     |Aborted   | ASC = 0x47 ASCQ = 0x05   |
   | error                    |Command-0B| CRC Error Detected       |
   +--------------------------+----------+--------------------------+
   | SNACK rejected           |Aborted   | ASC = 0x11 ASCQ = 0x13   |
   |                          |Command-0B| Read Error               |
   +--------------------------+----------+--------------------------+

   The target reports the "Incorrect amount of data" condition if
   during data output the total data length to output is greater than
   FirstBurstLength and the initiator sent unsolicited non-immediate
   data but the total amount of unsolicited data is different than
   FirstBurstLength. The target reports the same error when the
   amount of data sent as a reply to an R2T does not match the amount
   requested.

11.4.8. ExpDataSN

   The number of Data-In (read) PDUs the target has sent for the
   command.

   This field MUST be 0 if the response code is not Command Completed
   at Target or the target sent no Data-In PDUs for the command.

11.4.9. StatSN - Status Sequence Number

   StatSN is a Sequence Number that the target iSCSI layer generates
   per connection and that in turn, enables the initiator to
   acknowledge status reception. StatSN is incremented by 1 for every
   response/status sent on a connection except for responses sent as
   a result of a retry or SNACK. In the case of responses sent due to
   a retransmission request, the StatSN MUST be the same as the first




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   time the PDU was sent unless the connection has since been
   restarted.
11.4.10. ExpCmdSN - Next Expected CmdSN from this Initiator

  ExpCmdSN is a Sequence Number that the target iSCSI returns to the
  initiator to acknowledge command reception. It is used to update a
  local variable with the same name. An ExpCmdSN equal to MaxCmdSN+1
  indicates that the target cannot accept new commands.

11.4.11. MaxCmdSN - Maximum CmdSN from this Initiator

  MaxCmdSN is a Sequence Number that the target iSCSI returns to the
  initiator to indicate the maximum CmdSN the initiator can send. It
  is used to update a local variable with the same name. If MaxCmdSN
  is equal to ExpCmdSN-1, this indicates to the initiator that the
  target cannot receive any additional commands. When MaxCmdSN
  changes at the target while the target has no pending PDUs to
  convey this information to the initiator, it MUST generate a NOP-
  IN to carry the new MaxCmdSN.




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11.5. Task Management Function Request

   Byte/     0       |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|I| 0x02      |1| Function    | Reserved                     |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +---------------+---------------+---------------+--------------+
    8| Logical Unit Number (LUN) or Reserved                        |
     +                                                              +
   12|                                                              |
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag                                           |
     +---------------+---------------+---------------+--------------+
   20| Referenced Task Tag or 0xffffffff                            |
     +---------------+---------------+---------------+--------------+
   24| CmdSN                                                        |
     +---------------+---------------+---------------+--------------+
   28| ExpStatSN                                                    |
     +---------------+---------------+---------------+--------------+
   32| RefCmdSN or Reserved                                         |
     +---------------+---------------+---------------+--------------+
   36| ExpDataSN or Reserved                                        |
     +---------------+---------------+---------------+--------------+
   40/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+

11.5.1. Function

   The Task Management functions provide an initiator with a way to
   explicitly control the execution of one or more Tasks (SCSI and
   iSCSI tasks). The Task Management function codes are listed below.
   For a more detailed description of SCSI task management, see
   [SAM2].

     1    - ABORT TASK - aborts the task identified by the
         Referenced Task Tag field.




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    2    - ABORT TASK SET - aborts all Tasks issued via this
        session on the logical unit.

    3    - CLEAR ACA - clears the Auto Contingent Allegiance
        condition.

    4    - CLEAR TASK SET - aborts all Tasks in the appropriate
        task set as defined by the TST field in the Control mode
        page (see [SPC3]).

    5    -   LOGICAL UNIT RESET

    6    -   TARGET WARM RESET

    7   -    TARGET COLD RESET

    8    - TASK REASSIGN - reassigns connection allegiance for the
        task identified by the Initiator Task Tag field to this
        connection, thus resuming the iSCSI exchanges for the task.

  All other possible values for Function field are reserved.

  For all these functions, the Task Management function response
  MUST be returned as detailed in Section 11.6. All these functions
  apply to the referenced tasks regardless of whether they are
  proper SCSI tasks or tagged iSCSI operations. Task management
  requests must act on all the commands from the same session having
  a CmdSN lower than the task management CmdSN. LOGICAL UNIT RESET,
  TARGET WARM RESET and TARGET COLD RESET may affect commands from
  other sessions or commands from the same session regardless of
  their CmdSN value.

  If the task management request is marked for immediate delivery,
  it must be considered immediately for execution, but the
  operations involved (all or part of them) may be postponed to
  allow the target to receive all relevant tasks. According to
  [SAM2], for all the tasks covered by the Task Management response
  (i.e., with CmdSN lower than the task management command CmdSN)
  but except the Task Management response to a TASK REASSIGN,
  additional responses MUST NOT be delivered to the SCSI layer after
  the Task Management response. The iSCSI initiator MAY deliver to
  the SCSI layer all responses received before the Task Management




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  response (i.e., it is a matter of implementation if the SCSI
  responses, received before the Task Management response but after
  the task management request was issued, are delivered to the SCSI
  layer by the iSCSI layer in the initiator). The iSCSI target MUST
  ensure that no responses for the tasks covered by a task
  management function are delivered to the iSCSI initiator after the
  Task Management response except for a task covered by a TASK
  REASSIGN.

  For ABORT TASK SET and CLEAR TASK SET, the issuing initiator MUST
  continue to respond to all valid target transfer tags (received
  via R2T, Text Response, NOP-In, or SCSI Data-in PDUs) related to
  the affected task set, even after issuing the task management
  request. The issuing initiator SHOULD however terminate (i.e., by
  setting the F-bit to 1) these response sequences as quickly as
  possible. The target on its part MUST wait for responses on all
  affected target transfer tags before acting on either of these two
  task management requests. In case all or part of the response
  sequence is not received (due to digest errors) for a valid TTT,
  the target MAY treat it as a case of within-command error recovery
  class (see Section 7.1.4.1) if it is supporting ErrorRecoveryLevel
  >= 1, or alternatively may drop the connection to complete the
  requested task set function.

  If an ABORT TASK is issued for a task created by an immediate
  command then RefCmdSN MUST be that of the Task Management request
  itself (i.e. CmdSN and RefCmdSN are equal); otherwise RefCmdSN
  MUST be set to the CmdSN of the task to be aborted (lower than
  CmdSN).

  If the connection is still active (it is not undergoing an
  implicit or explicit logout), ABORT TASK MUST be issued on the
  same connection to which the task to be aborted is allegiant at
  the time the Task Management Request is issued. If the connection
  is implicitly or explicitly logged out (i.e., no other request
  will be issued on the failing connection and no other response
  will be received on the failing connection), then an ABORT TASK
  function request may be issued on another connection. This Task
  Management request will then establish a new allegiance for the
  command to be aborted as well as abort it (i.e., the task to be
  aborted will not have to be retried or reassigned, and its status,




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  if sent but not acknowledged, will be resent followed by the Task
  Management response).

  At the target an ABORT TASK function MUST NOT be executed on a
  Task Management request; such a request MUST result in Task
  Management response of "Function rejected".

  For the LOGICAL UNIT RESET function, the target MUST behave as
  dictated by the Logical Unit Reset function in [SAM2].

  The implementation of the TARGET WARM RESET function and the
  TARGET COLD RESET function is OPTIONAL and when implemented,
  should act as described below. The TARGET WARM RESET is also
  subject to SCSI access controls on the requesting initiator as
  defined in [SPC3]. When authorization fails at the target, the
  appropriate response as described in Section 11.6.1 MUST be
  returned by the target. The TARGET COLD RESET function is not
  subject to SCSI access controls, but its execution privileges may
  be managed by iSCSI mechanisms such as login authentication.

  When executing the TARGET WARM RESET and TARGET COLD RESET
  functions, the target cancels all pending operations on all
  Logical Units known by the issuing initiator. Both functions are
  equivalent to the Target Reset function specified by [SAM2]. They
  can affect many other initiators logged in with the servicing SCSI
  target port.

  The target MUST treat the TARGET COLD RESET function additionally
  as a power on event, thus terminating all of its TCP connections
  to all initiators (all sessions are terminated). For this reason,
  the Service Response (defined by [SAM2]) for this SCSI task
  management function may not be reliably delivered to the issuing
  initiator port.

  For the TASK REASSIGN function, the target should reassign the
  connection allegiance to this new connection (and thus resume
  iSCSI exchanges for the task). TASK REASSIGN MUST ONLY be received
  by the target after the connection on which the command was
  previously executing has been successfully logged-out. The Task
  Management response MUST be issued before the reassignment becomes
  effective.




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   For additional usage semantics see Section 7.2.

   At the target a TASK REASSIGN function request MUST NOT be
   executed to reassign the connection allegiance of a Task
   Management function request, an active text negotiation task, or a
   Logout task; such a request MUST result in Task Management
   response of "Function rejected".

   TASK REASSIGN MUST be issued as an immediate command.

11.5.2. TotalAHSLength and DataSegmentLength

   For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.5.3. LUN

   This field is required for functions that address a specific LU
   (ABORT TASK, CLEAR TASK SET, ABORT TASK SET, CLEAR ACA, LOGICAL
   UNIT RESET) and is reserved in all others.

11.5.4. Referenced Task Tag

   The Initiator Task Tag of the task to be aborted for the ABORT
   TASK function or reassigned for the TASK REASSIGN function. For
   all the other functions this field MUST be set to the reserved
   value 0xffffffff.

11.5.5. RefCmdSN

   If an ABORT TASK is issued for a task created by an immediate
   command then RefCmdSN MUST be that of the Task Management request
   itself (i.e. CmdSN and RefCmdSN are equal).

   For an ABORT TASK of a task created by non-immediate command
   RefCmdSN MUST be set to the CmdSN of the task identified by the
   Referenced Task Tag field. Targets must use this field as
   described in Section 11.6.1 when the task identified by the
   Referenced Task Tag field is not with the target.

   Otherwise, this field is reserved.




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11.5.6. ExpDataSN

   For recovery purposes, the iSCSI target and initiator maintain a
   data acknowledgement reference number - the first input DataSN
   number unacknowledged by the initiator. When issuing a new
   command, this number is set to 0. If the function is TASK
   REASSIGN, which establishes a new connection allegiance for a
   previously issued Read or Bidirectional command, ExpDataSN will
   contain an updated data acknowledgement reference number or the
   value 0; the latter indicating that the data acknowledgement
   reference number is unchanged. The initiator MUST discard any data
   PDUs from the previous execution that it did not acknowledge and
   the target MUST transmit all Data-in PDUs (if any) starting with
   the data acknowledgement reference number. The number of
   retransmitted PDUs may or may not be the same as the original
   transmission depending on if there was a change in
   MaxRecvDataSegmentLength in the reassignment. The target MAY also
   send no more Data-In PDUs if all data has been acknowledged.

   The value of ExpDataSN MUST be 0 or higher than the DataSN of the
   last acknowledged Data-In PDU, but not larger than DataSN+1 of the
   last Data-IN PDU sent by the target. Any other value MUST be
   ignored by the target.

   For other functions this field is reserved.


11.6. Task Management Function Response




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   Byte/      0      |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|.| 0x22      |1| Reserved    | Response      | Reserved     |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +--------------------------------------------------------------+
    8/ Reserved                                                     /
     /                                                              /
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag                                           |
     +---------------+---------------+---------------+--------------+
   20| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   24| StatSN                                                       |
     +---------------+---------------+---------------+--------------+
   28| ExpCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   32| MaxCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   36/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+

   For the functions ABORT TASK, ABORT TASK SET, CLEAR ACA, CLEAR
   TASK SET, LOGICAL UNIT RESET, TARGET COLD RESET, TARGET WARM RESET
   and TASK REASSIGN, the target performs the requested Task
   Management function and sends a Task Management response back to
   the initiator. For TASK REASSIGN, the new connection allegiance
   MUST ONLY become effective at the target after the target issues
   the Task Management Response.

11.6.1. Response

   The target provides a Response, which may take on the following
   values:

       0 - Function complete.
       1 - Task does not exist.




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      2   -   LUN does not exist.
      3   -   Task still allegiant.
      4   -   Task allegiance reassignment not supported.
      5   -   Task management function not supported.
      6   -   Function authorization failed.
    255   -   Function rejected.

  All other values are reserved.

  For a discussion on usage of response codes 3 and 4, see Section
  7.2.2.

  For the TARGET COLD RESET and TARGET WARM RESET functions, the
  target cancels all pending operations across all Logical Units
  known to the issuing initiator. For the TARGET COLD RESET
  function, the target MUST then close all of its TCP connections to
  all initiators (terminates all sessions).

  The mapping of the response code into a SCSI service response code
  value, if needed, is outside the scope of this document. However,
  in symbolic terms, Response values 0 and 1 map to the SCSI service
  response of FUNCTION COMPLETE. Response value 2 maps to SCSI
  service response of INCORRECT LOGICAL UNIT NUMBER. All other
  Response values map to the SCSI service response of FUNCTION
  REJECTED. If a Task Management function response PDU does not
  arrive before the session is terminated, the SCSI service response
  is SERVICE DELIVERY OR TARGET FAILURE.

  The response to ABORT TASK SET and CLEAR TASK SET MUST only be
  issued by the target after all of the commands affected have been
  received by the target, the corresponding task management
  functions have been executed by the SCSI target, and the delivery
  of all responses delivered until the task management function
  completion have been confirmed (acknowledged through ExpStatSN) by
  the initiator on all connections of this session. For the exact
  timeline of events, refer to Section 4.2.3.3 and Section 4.2.3.4.

  For the ABORT TASK function,




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     a) If the Referenced Task Tag identifies a valid task leading
        to a successful termination, then targets must return the
        "Function complete" response.
     b) If the Referenced Task Tag does not identify an existing
        task, but if the CmdSN indicated by the RefCmdSN field in
        the Task Management function request is within the valid
        CmdSN window and less than the CmdSN of the Task Management
        function request itself, then targets must consider the
        CmdSN received and return the "Function complete" response.
     c) If the Referenced Task Tag does not identify an existing
        task and if the CmdSN indicated by the RefCmdSN field in
        the Task Management function request is outside the valid
        CmdSN window, then targets must return the "Task does not
        exist" response.

  For response semantics on function types that can potentially
  impact multiple active tasks on the target, see Section 4.2.3.

11.6.2. TotalAHSLength and DataSegmentLength

  For this PDU TotalAHSLength and DataSegmentLength MUST be 0.


11.7. SCSI Data-out & SCSI Data-in

  The SCSI Data-out PDU for WRITE operations has the following
  format:




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  Byte/      0      |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x05      |F| Reserved                                   |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag or 0xffffffff                            |
    +---------------+---------------+---------------+--------------+
  24| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN                                                    |
    +---------------+---------------+---------------+--------------+
  32| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  36| DataSN                                                       |
    +---------------+---------------+---------------+--------------+
  40| Buffer Offset                                                |
    +---------------+---------------+---------------+--------------+
  44| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / DataSegment                                                  /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+

  The SCSI Data-in PDU for READ operations has the following format:




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  Byte/      0      |       1       |       2       |       3      |
      /              |              |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x25      |F|A|0 0 0|O|U|S| Reserved      |Status or Rsvd|
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag or 0xffffffff                            |
    +---------------+---------------+---------------+--------------+
  24| StatSN or Reserved                                           |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36| DataSN                                                       |
    +---------------+---------------+---------------+--------------+
  40| Buffer Offset                                                |
    +---------------+---------------+---------------+--------------+
  44| Residual Count                                               |
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / DataSegment                                                  /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+

  Status can accompany the last Data-in PDU if the command did not
  end with an exception (i.e., the status is "good status" - GOOD,
  CONDITION MET or INTERMEDIATE CONDITION MET). The presence of
  status (and of a residual count) is signaled though the S flag
  bit. Although targets MAY choose to send even non-exception




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  status in separate responses, initiators MUST support non-
  exception status in Data-In PDUs.

11.7.1. F (Final) Bit

  For outgoing data, this bit is 1 for the last PDU of unsolicited
  data or the last PDU of a sequence that answers an R2T.

  For incoming data, this bit is 1 for the last input (read) data
  PDU of a sequence. Input can be split into several sequences,
  each having its own F bit. Splitting the data stream into
  sequences does not affect DataSN counting on Data-In PDUs. It MAY
  be used as a "change direction" indication for Bidirectional
  operations that need such a change.

  DataSegmentLength MUST NOT exceed MaxRecvDataSegmentLength for the
  direction it is sent and the total of all the DataSegmentLength of
  all PDUs in a sequence MUST NOT exceed MaxBurstLength (or
  FirstBurstLength for unsolicited data). However the number of
  individual PDUs in a sequence (or in total) may be higher than the
  MaxBurstLength (or FirstBurstLength) to MaxRecvDataSegmentLength
  ratio (as PDUs may be limited in length by the sender
  capabilities). Using DataSegmentLength of 0 may increase beyond
  what is reasonable for the number of PDUs and should therefore be
  avoided.

  For Bidirectional operations, the F bit is 1 for both the end of
  the input sequences and the end of the output sequences.

11.7.2. A (Acknowledge) bit

  For sessions with ErrorRecoveryLevel 1 or higher, the target sets
  this bit to 1 to indicate that it requests a positive
  acknowledgement from the initiator for the data received. The
  target should use the A bit moderately; it MAY only set the A bit
  to 1 once every MaxBurstLength bytes, or on the last Data-In PDU
  that concludes the entire requested read data transfer for the
  task from the target's perspective, and it MUST NOT do so more
  frequently. The target MUST NOT set to 1 the A bit for sessions
  with ErrorRecoveryLevel=0. The initiator MUST ignore the A bit set
  to 1 for sessions with ErrorRecoveryLevel=0.




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  On receiving a Data-In PDU with the A bit set to 1 on a session
  with ErrorRecoveryLevel greater than 0, if there are no holes in
  the read data until that Data-In PDU, the initiator MUST issue a
  SNACK of type DataACK except when it is able to acknowledge the
  status for the task immediately via ExpStatSN on other outbound
  PDUs if the status for the task is also received. In the latter
  case (acknowledgement through ExpStatSN), sending a SNACK of type
  DataACK in response to the A bit is OPTIONAL, but if it is done,
  it must not be sent after the status acknowledgement through
  ExpStatSN. If the initiator has detected holes in the read data
  prior to that Data-In PDU, it MUST postpone issuing the SNACK of
  type DataACK until the holes are filled. An initiator also MUST
  NOT acknowledge the status for the task before those holes are
  filled. A status acknowledgement for a task that generated the
  Data-In PDUs is considered by the target as an implicit
  acknowledgement of the Data-In PDUs if such an acknowledgement was
  requested by the target.

11.7.3. Flags (byte 1)

  The last SCSI Data packet sent from a target to an initiator for a
  SCSI command that completed successfully (with a status of GOOD,
  CONDITION MET, INTERMEDIATE or INTERMEDIATE CONDITION MET) may
  also optionally contain the Status for the data transfer. In this
  case, Sense Data cannot be sent together with the Command Status.
  If the command is completed with an error, then the response and
  sense data MUST be sent in a SCSI Response PDU (i.e., MUST NOT be
  sent in a SCSI Data packet). For Bidirectional commands, the
  status MUST be sent in a SCSI Response PDU.

     bit 2-4 - Reserved.

     bit 5-6 - used the same as in a SCSI Response. These bits are
       only valid when S is set to 1. For details see SNACK .

     bit 7 S (status)- set to indicate that the Command Status
       field contains status. If this bit is set to 1, the F bit
       MUST also be set to 1.

  The fields StatSN, Status, and Residual Count only have meaningful
  content if the S bit is set to 1 and their values are defined in
  SNACK .




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11.7.4. Target Transfer Tag and LUN

   On outgoing data, the Target Transfer Tag is provided to the
   target if the transfer is honoring an R2T. In this case, the
   Target Transfer Tag field is a replica of the Target Transfer Tag
   provided with the R2T.

   On incoming data, the Target Transfer Tag and LUN MUST be provided
   by the target if the A bit is set to 1; otherwise they are
   reserved. The Target Transfer Tag and LUN are copied by the
   initiator into the SNACK of type DataACK that it issues as a
   result of receiving a SCSI Data-in PDU with the A bit set to 1.

   The Target Transfer Tag values are not specified by this protocol
   except that the value 0xffffffff is reserved and means that the
   Target Transfer Tag is not supplied. If the Target Transfer Tag
   is provided, then the LUN field MUST hold a valid value and be
   consistent with whatever was specified with the command;
   otherwise, the LUN field is reserved.

11.7.5. DataSN

   For input (read) or bidirectional Data-In PDUs, the DataSN is the
   input PDU number within the data transfer for the command
   identified by the Initiator Task Tag.

   R2T and Data-In PDUs, in the context of bidirectional commands,
   share the numbering sequence (see Section 4.2.2.4).

   For output (write) data PDUs, the DataSN is the Data-Out PDU
   number within the current output sequence. The current output
   sequence is either identified by the Initiator Task Tag (for
   unsolicited data) or is a data sequence generated for one R2T (for
   data solicited through R2T).

11.7.6. Buffer Offset

   The Buffer Offset field contains the offset of this PDU payload
   data within the complete data transfer. The sum of the buffer
   offset and length should not exceed the expected transfer length
   for the command.




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  The order of data PDUs within a sequence is determined by
  DataPDUInOrder. When set to Yes, it means that PDUs have to be in
  increasing Buffer Offset order and overlays are forbidden.

  The ordering between sequences is determined by
  DataSequenceInOrder. When set to Yes, it means that sequences have
  to be in increasing Buffer Offset order and overlays are
  forbidden.

11.7.7. DataSegmentLength

  This is the data payload length of a SCSI Data-In or SCSI Data-Out
  PDU. The sending of 0 length data segments should be avoided, but
  initiators and targets MUST be able to properly receive 0 length
  data segments.

  The Data Segments of Data-in and Data-out PDUs SHOULD be filled to
  the integer number of 4 byte words (real payload) unless the F bit
  is set to 1.




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11.8. Ready To Transfer (R2T)

  Byte/      0      |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x31      |1| Reserved                                   |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN                                                          |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag                                          |
    +---------------+---------------+---------------+--------------+
  24| StatSN                                                       |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36| R2TSN                                                        |
    +---------------+---------------+---------------+--------------+
  40| Buffer Offset                                                |
    +---------------+---------------+---------------+--------------+
  44| Desired Data Transfer Length                                 |
    +--------------------------------------------------------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+


  When an initiator has submitted a SCSI Command with data that
  passes from the initiator to the target (WRITE), the target may
  specify which blocks of data it is ready to receive. The target
  may request that the data blocks be delivered in whichever order
  is convenient for the target at that particular instant. This
  information is passed from the target to the initiator in the
  Ready To Transfer (R2T) PDU.




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  In order to allow write operations without an explicit initial
  R2T, the initiator and target MUST have negotiated the key
  InitialR2T to No during Login.

  An R2T MAY be answered with one or more SCSI Data-out PDUs with a
  matching Target Transfer Tag. If an R2T is answered with a single
  Data-out PDU, the Buffer Offset in the Data PDU MUST be the same
  as the one specified by the R2T, and the data length of the Data
  PDU MUST be the same as the Desired Data Transfer Length specified
  in the R2T. If the R2T is answered with a sequence of Data PDUs,
  the Buffer Offset and Length MUST be within the range of those
  specified by R2T, and the last PDU MUST have the F bit set to 1.
  If the last PDU (marked with the F bit) is received before the
  Desired Data Transfer Length is transferred, a target MAY choose
  to Reject that PDU with "Protocol error" reason code.
  DataPDUInOrder governs the Data-Out PDU ordering. If
  DataPDUInOrder is set to Yes, the Buffer Offsets and Lengths for
  consecutive PDUs MUST form a continuous non-overlapping range and
  the PDUs MUST be sent in increasing offset order.

  The target may send several R2T PDUs. It, therefore, can have a
  number of pending data transfers. The number of outstanding R2T
  PDUs are limited by the value of the negotiated key
  MaxOutstandingR2T. Within a task, outstanding R2Ts MUST be
  fulfilled by the initiator in the order in which they were
  received.

  R2T PDUs MAY also be used to recover Data Out PDUs. Such an R2T
  (Recovery-R2T) is generated by a target upon detecting the loss of
  one or more Data-Out PDUs due to:

    - Digest error

    - Sequence error

    - Sequence reception timeout

  A Recovery-R2T carries the next unused R2TSN, but requests part of
  or the entire data burst that an earlier R2T (with a lower R2TSN)
  had already requested.

  DataSequenceInOrder governs the buffer offset ordering in




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   consecutive R2Ts. If DataSequenceInOrder is Yes, then consecutive
   R2Ts MUST refer to continuous non-overlapping ranges except for
   Recovery-R2Ts.

11.8.1. TotalAHSLength and DataSegmentLength

   For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

11.8.2. R2TSN

   R2TSN is the R2T PDU input PDU number within the command
   identified by the Initiator Task Tag.

   For bidirectional commands R2T and Data-In PDUs share the input
   PDU numbering sequence (see Section 4.2.2.4).

11.8.3. StatSN

   The StatSN field will contain the next StatSN. The StatSN for this
   connection is not advanced after this PDU is sent.

11.8.4. Desired Data Transfer Length and Buffer Offset

   The target specifies how many bytes it wants the initiator to send
   because of this R2T PDU. The target may request the data from the
   initiator in several chunks, not necessarily in the original order
   of the data. The target, therefore, also specifies a Buffer Offset
   that indicates the point at which the data transfer should begin,
   relative to the beginning of the total data transfer. The Desired
   Data Transfer Length MUST NOT be 0 and MUST NOT exceed
   MaxBurstLength.

11.8.5. Target Transfer Tag

   The target assigns its own tag to each R2T request that it sends
   to the initiator. This tag can be used by the target to easily
   identify the data it receives. The Target Transfer Tag and LUN are
   copied in the outgoing data PDUs and are only used by the target.
   There is no protocol rule about the Target Transfer Tag except
   that the value 0xffffffff is reserved and MUST NOT be sent by a
   target in an R2T.




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11.9. Asynchronous Message

  An Asynchronous Message may be sent from the target to the
  initiator without correspondence to a particular command. The
  target specifies the reason for the event and sense data.

  Byte/      0      |        1      |       2       |        3     |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x32      |1| Reserved                                   |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| 0xffffffff                                                   |
    +---------------+---------------+---------------+--------------+
  20| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  24| StatSN                                                       |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36| AsyncEvent    | AsyncVCode    | Parameter1 or Reserved       |
    +---------------+---------------+---------------+--------------+
  40| Parameter2 or Reserved        | Parameter3 or Reserved       |
    +---------------+---------------+---------------+--------------+
  44| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / DataSegment - Sense Data and iSCSI Event Data                /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+




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   Some Asynchronous Messages are strictly related to iSCSI while
   others are related to SCSI [SAM2].

   StatSN counts this PDU as an acknowledgeable event (StatSN is
   advanced), which allows for initiator and target state
   synchronization.

11.9.1. AsyncEvent

   The codes used for iSCSI Asynchronous Messages (events) are:

     0 (SCSI_ASYNC) - a SCSI Asynchronous Event is reported in the
       sense data. Sense Data that accompanies the report, in the
       data segment, identifies the condition. The sending of a
       SCSI Event (Asynchronous Event Reporting in SCSI
       terminology) is dependent on the target support for SCSI
       asynchronous event reporting (see [SAM2]) as indicated in
       the standard INQUIRY data (see [SPC3]). Its use may be
       enabled by parameters in the SCSI Control mode page (see
       [SPC3]).

     1 (REQUEST_LOGOUT) - target requests Logout. This Async
       Message MUST be sent on the same connection as the one
       requesting to be logged out. The initiator MUST honor this
       request by issuing a Logout as early as possible, but no
       later than Parameter3 seconds.   Initiator MUST send a
       Logout with a reason code of "Close the connection" OR
       "Close the session" to close all the connections. Once this
       message is received, the initiator SHOULD NOT issue new
       iSCSI commands on the connection to be logged out. The
       target MAY reject any new I/O requests that it receives
       after this Message with the reason code "Waiting for
       Logout". If the initiator does not Logout in Parameter3
       seconds, the target should send an Async PDU with iSCSI
       event code "Dropped the connection" if possible, or simply
       terminate the transport connection. Parameter1 and
       Parameter2 are reserved.




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    2 (CONNECTION_DROP) - target indicates it will drop the
      connection.


      The Parameter1 field indicates the CID of the connection
      going to be dropped.


      The Parameter2 field (Time2Wait) indicates, in seconds, the
      minimum time to wait before attempting to reconnect or
      reassign.


      The Parameter3 field (Time2Retain) indicates the maximum
      time allowed to reassign commands after the initial wait (in
      Parameter2).


      If the initiator does not attempt to reconnect and/or
      reassign the outstanding commands within the time specified
      by Parameter3, or if Parameter3 is 0, the target will
      terminate all outstanding commands on this connection. In
      this case, no other responses should be expected from the
      target for the outstanding commands on this connection.


      A value of 0 for Parameter2 indicates that reconnect can be
      attempted immediately.



    3 (SESSION_DROP) - target indicates it will drop all the
      connections of this session.


      Parameter1 field is reserved.


      The Parameter2 field (Time2Wait) indicates, in seconds, the
      minimum time to wait before attempting to reconnect.
      The Parameter3 field (Time2Retain) indicates the maximum
      time allowed to reassign commands after the initial wait (in
      Parameter2).




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      If the initiator does not attempt to reconnect and/or
      reassign the outstanding commands within the time specified
      by Parameter3, or if Parameter3 is 0, the session is
      terminated. In this case, the target will terminate all
      outstanding commands in this session; no other responses
      should be expected from the target for the outstanding
      commands in this session. A value of 0 for Parameter2
      indicates that reconnect can be attempted immediately.



    4 (RENEGOTIATE) - target requests parameter negotiation on
      this connection. The initiator MUST honor this request by
      issuing a Text Request (that can be empty) on the same
      connection as early as possible, but no later than
      Parameter3 seconds, unless a Text Request is already pending
      on the connection, or by issuing a Logout Request. If the
      initiator does not issue a Text Request the target may
      reissue the Asynchronous Message requesting parameter
      negotiation.



    5 (FAST_ABORT) - all active tasks for LU with a matching LUN
      field in the Async Message PDU are being terminated. The
      receiving initiator iSCSI layer MUST respond to this Message
      by taking the following steps in order.

       - Stop Data-Out transfers on that connection for all active
          TTTs for the affected LUN quoted in the Async Message
          PDU.
       - Acknowledge the StatSN of the Async Message PDU via a NOP-
          Out PDU with ITT=0xffffffff (i.e., non-ping flavor),
          while copying the LUN field from the Async Message to
          NOP-Out.

      This value of AsyncEvent however MUST NOT be used on an
      iSCSI session unless the new TaskReporting text key defined
      in Section 13.23 was negotiated to FastAbort on the session.




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     255 - vendor-specific iSCSI Event. The AsyncVCode details the
       vendor code, and data MAY accompany the report.



   All other event codes are reserved.

11.9.2. AsyncVCode

   AsyncVCode is a vendor specific detail code that is only valid if
   the AsyncEvent field indicates a vendor specific event. Otherwise,
   it is reserved.

11.9.3. LUN

   The LUN field MUST be valid if AsyncEvent is 0. Otherwise, this
   field is reserved.

11.9.4. Sense Data and iSCSI Event Data

   For a SCSI event, this data accompanies the report in the data
   segment and identifies the condition.

   For an iSCSI event, additional vendor-unique data MAY accompany
   the Async event. Initiators MAY ignore the data when not
   understood while processing the rest of the PDU.

   If the DataSegmentLength is not 0, the format of the DataSegment
   is as follows:




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   Byte/     0       |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|SenseLength                    | Sense Data                   |
     +---------------+---------------+---------------+--------------+
    x/ Sense Data                                                   /
     +---------------+---------------+---------------+--------------+
    y/ iSCSI Event Data                                             /
     /                                                              /
     +---------------+---------------+---------------+--------------+
    z|

11.9.4.1. SenseLength

   This is the length of Sense Data. When the Sense Data field is
   empty (e.g., the event is not a SCSI event) SenseLength is 0.


11.10. Text Request

   The Text Request is provided to allow for the exchange of
   information and for future extensions. It permits the initiator to
   inform a target of its capabilities or to request some special
   operations.




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  Byte/     0       |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|I| 0x04      |F|C| Reserved                                 |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag or 0xffffffff                            |
    +---------------+---------------+---------------+--------------+
  24| CmdSN                                                        |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN                                                    |
    +---------------+---------------+---------------+--------------+
  32/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / DataSegment (Text)                                           /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+

  An initiator MUST NOT have more than one outstanding Text Request
  on a connection at any given time.

  On a connection failure, an initiator must either explicitly abort
  any active allegiant text negotiation task or must cause such a
  task to be implicitly terminated by the target.




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11.10.1. F (Final) Bit

  When set to 1, indicates that this is the last or only text
  request in a sequence of Text Requests; otherwise, it indicates
  that more Text Requests will follow.

11.10.2. C (Continue) Bit

  When set to 1, indicates that the text (set of key=value pairs) in
  this Text Request is not complete (it will be continued on
  subsequent Text Requests); otherwise, it indicates that this Text
  Request ends a set of key=value pairs. A Text Request with the C
  bit set to 1 MUST have the F bit set to 0.

11.10.3. Initiator Task Tag

  The initiator assigned identifier for this Text Request. If the
  command is sent as part of a sequence of text requests and
  responses, the Initiator Task Tag MUST be the same for all the
  requests within the sequence (similar to linked SCSI commands).
  The I bit for all requests in a sequence also MUST be the same.

11.10.4. Target Transfer Tag

  When the Target Transfer Tag is set to the reserved value
  0xffffffff, it tells the target that this is a new request and the
  target resets any internal state associated with the Initiator
  Task Tag (resets the current negotiation state).

  The target sets the Target Transfer Tag in a text response to a
  value other than the reserved value 0xffffffff whenever it
  indicates that it has more data to send or more operations to
  perform that are associated with the specified Initiator Task Tag.
  It MUST do so whenever it sets the F bit to 0 in the response. By
  copying the Target Transfer Tag from the response to the next Text
  Request, the initiator tells the target to continue the operation
  for the specific Initiator Task Tag. The initiator MUST ignore the
  Target Transfer Tag in the Text Response when the F bit is set to
  1.




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   This mechanism allows the initiator and target to transfer a large
   amount of textual data over a sequence of text-command/text-
   response exchanges, or to perform extended negotiation sequences.

   If the Target Transfer Tag is not 0xffffffff, the LUN field MUST
   be sent by the target in the Text Response.

   A target MAY reset its internal negotiation state if an exchange
   is stalled by the initiator for a long time or if it is running
   out of resources.

   Long text responses are handled as in the following example:

     I->T Text SendTargets=All (F=1,TTT=0xffffffff)

     T->I Text <part 1> (F=0,TTT=0x12345678)

     I->T Text <empty> (F=1, TTT=0x12345678)

     T->I Text <part 2> (F=0, TTT=0x12345678)

     I->T Text <empty> (F=1, TTT=0x12345678)

     ...

     T->I Text <part n> (F=1, TTT=0xffffffff)


11.10.5. Text

   The data lengths of a text request MUST NOT exceed the iSCSI
   target MaxRecvDataSegmentLength (a per connection and per
   direction negotiated parameter). The text format is specified in
   Section 6.2.

   Section 12 and Section 13 list some basic Text key=value pairs,
   some of which can be used in Login Request/Response and some in
   Text Request/Response.

   A key=value pair can span Text request or response boundaries. A
   key=value pair can start in one PDU and continue on the next. In




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  other words the end of a PDU does not necessarily signal the end
  of a key=value pair.

  The target responds by sending its response back to the initiator.
  The response text format is similar to the request text format.
  The text response MAY refer to key=value pairs presented in an
  earlier text request and the text in the request may refer to
  earlier responses.

  Section 6.2 details the rules for the Text Requests and Responses.

  Text operations are usually meant for parameter
  setting/negotiations, but can also be used to perform some long
  lasting operations.

  Text operations that take a long time should be placed in their
  own Text request.

11.11. Text Response

  The Text Response PDU contains the target's responses to the
  initiator's Text request. The format of the Text field matches
  that of the Text request.




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  Byte/      0      |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x24      |F|C| Reserved                                 |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag or 0xffffffff                            |
    +---------------+---------------+---------------+--------------+
  24| StatSN                                                       |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+
    / DataSegment (Text)                                           /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
    | Data-Digest (Optional)                                       |
    +---------------+---------------+---------------+--------------+

11.11.1. F (Final) Bit

  When set to 1, in response to a Text Request with the Final bit
  set to 1, the F bit indicates that the target has finished the
  whole operation. Otherwise, if set to 0 in response to a Text
  Request with the Final Bit set to 1, it indicates that the target
  has more work to do (invites a follow-on text request). A Text
  Response with the F bit set to 1 in response to a Text Request
  with the F bit set to 0 is a protocol error.




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  A Text Response with the F bit set to 1 MUST NOT contain key=value
  pairs that may require additional answers from the initiator.

  A Text Response with the F bit set to 1 MUST have a Target
  Transfer Tag field set to the reserved value of 0xffffffff.

  A Text Response with the F bit set to 0 MUST have a Target
  Transfer Tag field set to a value other than the reserved
  0xffffffff.

11.11.2. C (Continue) Bit

  When set to 1, indicates that the text (set of key=value pairs) in
  this Text Response is not complete (it will be continued on
  subsequent Text Responses); otherwise, it indicates that this Text
  Response ends a set of key=value pairs. A Text Response with the C
  bit set to 1 MUST have the F bit set to 0.

11.11.3. Initiator Task Tag

  The Initiator Task Tag matches the tag used in the initial Text
  Request.

11.11.4. Target Transfer Tag

  When a target has more work to do (e.g., cannot transfer all the
  remaining text data in a single Text Response or has to continue
  the negotiation) and has enough resources to proceed, it MUST set
  the Target Transfer Tag to a value other than the reserved value
  of 0xffffffff. Otherwise, the Target Transfer Tag MUST be set to
  0xffffffff.

  When the Target Transfer Tag is not 0xffffffff, the LUN field may
  be significant.

  The initiator MUST copy the Target Transfer Tag and LUN in its
  next request to indicate that it wants the rest of the data.

  When the target receives a Text Request with the Target Transfer
  Tag set to the reserved value of 0xffffffff, it resets its




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   internal information (resets state) associated with the given
   Initiator Task Tag (restarts the negotiation).

   When a target cannot finish the operation in a single Text
   Response, and does not have enough resources to continue, it
   rejects the Text Request with the appropriate Reject code.

   A target may reset its internal state associated with an Initiator
   Task Tag (the current negotiation state), state expressed through
   the Target Transfer Tag if the initiator fails to continue the
   exchange for some time. The target may reject subsequent Text
   Requests with the Target Transfer Tag set to the "stale" value.

11.11.5. StatSN

   The target StatSN variable is advanced by each Text Response sent.

11.11.6. Text Response Data

   The data lengths of a text response MUST NOT exceed the iSCSI
   initiator MaxRecvDataSegmentLength (a per connection and per
   direction negotiated parameter).

   The text in the Text Response Data is governed by the same rules
   as the text in the Text Request Data (see Section 11.11.2).

   Although the initiator is the requesting party and controls the
   request-response initiation and termination, the target can offer
   key=value pairs of its own as part of a sequence and not only in
   response to the initiator.

11.12. Login Request

   After establishing a TCP connection between an initiator and a
   target, the initiator MUST start a Login Phase to gain further
   access to the target's resources.

   The Login Phase (see Section 6.3) consists of a sequence of Login
   requests and responses that carry the same Initiator Task Tag.

   Login requests are always considered as immediate.




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  Byte/     0        |       1       |       2       |       3      |
      /              |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|1| 0x03       |T|C|.|.|CSG|NSG| Version-max   | Version-min |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                             |
    +---------------+---------------+---------------+--------------+
   8| ISID                                                          |
    +                                +---------------+--------------+
  12|                                | TSIH                         |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                            |
    +---------------+---------------+---------------+--------------+
  20| CID                            | Reserved                     |
    +---------------+---------------+---------------+--------------+
  24| CmdSN                                                         |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN   or    Reserved                                    |
    +---------------+---------------+---------------+--------------+
  32| Reserved                                                      |
    +---------------+---------------+---------------+--------------+
  36| Reserved                                                      |
    +---------------+---------------+---------------+--------------+
  40/ Reserved                                                      /
   +/                                                               /
    +---------------+---------------+---------------+--------------+
  48/ DataSegment - Login Parameters in Text request Format         /
   +/                                                               /
    +---------------+---------------+---------------+--------------+

11.12.1. T (Transit) Bit

  If set to 1, indicates that the initiator is ready to transit to
  the next stage.

  If the T bit is set to 1 and NSG is FullFeaturePhase, then this
  also indicates that the initiator is ready for the Final Login
  Response (see Section 6.3).




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11.12.2. C (Continue) Bit

   When set to 1, this bit indicates that the text (set of key=value
   pairs) in this Login Request is not complete (it will be continued
   on subsequent Login Requests); otherwise, it indicates that this
   Login Request ends a set of key=value pairs. A Login Request with
   the C bit set to 1 MUST have the T bit set to 0.

11.12.3. CSG and NSG

   Through these fields, Current Stage (CSG) and Next Stage (NSG),
   the Login negotiation requests and responses are associated with a
   specific stage in the session (SecurityNegotiation,
   LoginOperationalNegotiation, FullFeaturePhase) and may indicate
   the next stage to which they want to move (see Section 6.3). The
   next stage value is only valid when the T bit is 1; otherwise, it
   is reserved.

   The stage codes are:

     - 0 - SecurityNegotiation

     - 1 - LoginOperationalNegotiation

     - 3 - FullFeaturePhase


   All other codes are reserved.

11.12.4. Version

   The version number of the current draft is 0x00. As such, all
   devices MUST carry version 0x00 for both Version-min and Version-
   max.

11.12.4.1. Version-max

   Maximum Version number supported.

   All Login requests within the Login Phase MUST carry the same
   Version-max.




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   The target MUST use the value presented with the first login
   request.

11.12.4.2. Version-min


   All Login requests within the Login Phase MUST carry the same
   Version-min. The target MUST use the value presented with the
   first login request.

11.12.5. ISID

   This is an initiator-defined component of the session identifier
   and is structured as follows (see Section 10.1.1 for details):


   Byte/     0       |       1       |       2       |       3      |
       /             |                |              |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    8| T |     A     |              B                |      C       |
     +---------------+---------------+---------------+--------------+
   12|               D               |
     +---------------+---------------+

   The T field identifies the format and usage of A, B, C, and D as
   indicated below:

     T

     00b        OUI-Format

                A&B are a 22 bit OUI

                (the I/G & U/L bits are omitted)

                C&D 24 bit qualifier

     01b        EN - Format (IANA Enterprise Number)

                A - Reserved

                B&C EN (IANA Enterprise Number)




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             D - Qualifier

    10b     "Random"

             A - Reserved

             B&C Random

             D - Qualifier

    11b     A,B,C&D Reserved

  For the T field values 00b and 01b, a combination of A and B (for
  00b) or B and C (for 01b) identifies the vendor or organization
  whose component (software or hardware) generates this ISID. A
  vendor or organization with one or more OUIs, or one or more
  Enterprise Numbers, MUST use at least one of these numbers and
  select the appropriate value for the T field when its components
  generate ISIDs. An OUI or EN MUST be set in the corresponding
  fields in network byte order (byte big-endian).

  If the T field is 10b, B and C are set to a random 24-bit unsigned
  integer value in network byte order (byte big-endian). See
  [RFC3721] for how this affects the principle of "conservative
  reuse".

  The Qualifier field is a 16 or 24-bit unsigned integer value that
  provides a range of possible values for the ISID within the
  selected namespace. It may be set to any value within the
  constraints specified in the iSCSI protocol (see Section 4.4.3 and
  Section 10.1.1).

  The T field value of 11b is reserved.

  If the ISID is derived from something assigned to a hardware
  adapter or interface by a vendor, as a preset default value, it
  MUST be configurable to a value assigned according to the SCSI
  port behavior desired by the system in which it is installed (see
  Section 10.1.1 and Section 10.1.2). The resultant ISID MUST also
  be persistent over power cycles, reboot, card swap, etc.




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11.12.6. TSIH

   TSIH must be set in the first Login Request. The reserved value 0
   MUST be used on the first connection for a new session. Otherwise,
   the TSIH sent by the target at the conclusion of the successful
   login of the first connection for this session MUST be used. The
   TSIH identifies to the target the associated existing session for
   this new connection.

   All Login requests within a Login Phase MUST carry the same TSIH.

   The target MUST check the value presented with the first login
   request and act as specified in Section 5.3.1.

11.12.7. Connection ID - CID

   A unique ID for this connection within the session.

   All Login requests within the Login Phase MUST carry the same CID.

   The target MUST use the value presented with the first login
   request.

   A Login request with a non-zero TSIH and a CID equal to that of an
   existing connection implies a logout of the connection followed by
   a Login (see Section 6.3.4). For the details of the implicit
   Logout Request, see Section 11.14.

11.12.8. CmdSN

   CmdSN is either the initial command sequence number of a session
   (for the first Login request of a session - the "leading" login),
   or the command sequence number in the command stream if the login
   is for a new connection in an existing session.

   Examples:

     - Login on a leading connection - if the leading login carries
       the CmdSN 123, all other login requests in the same login
       phase carry the CmdSN 123 and the first non-immediate
       command in FullFeaturePhase also carries the CmdSN 123.




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     - Login on other than a leading connection - if the current
       CmdSN at the time the first login on the connection is
       issued is 500, then that PDU carries CmdSN=500. Subsequent
       login requests that are needed to complete this login phase
       may carry a CmdSN higher than 500 if non-immediate requests
       that were issued on other connections in the same session
       advance CmdSN.


   If the login request is a leading login request, the target MUST
   use the value presented in CmdSN as the target value for ExpCmdSN.

11.12.9. ExpStatSN

   For the first Login Request on a connection this is ExpStatSN for
   the old connection and this field is only valid if the Login
   request restarts a connection (see Section 6.3.4).

   For subsequent Login Requests it is used to acknowledge the Login
   Responses with their increasing StatSN values.

11.12.10. Login Parameters

   The initiator MUST provide some basic parameters in order to
   enable the target to determine if the initiator may use the
   target's resources and the initial text parameters for the
   security exchange.

   All the rules specified in Section 11.10.5 for text requests also
   hold for login requests. Keys and their explanations are listed
   in Section 12 (security negotiation keys) and Section 13
   (operational parameter negotiation keys). All keys in Section 13,
   except for the X extension formats, MUST be supported by iSCSI
   initiators and targets. Keys in Section 12 only need to be
   supported when the function to which they refer is mandatory to
   implement.

11.13. Login Response

   The Login Response indicates the progress and/or end of the Login
   Phase.




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  Byte/      0      |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x23      |T|C|.|.|CSG|NSG| Version-max   |Version-active|
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| ISID                                                         |
    +                               +---------------+--------------+
  12|                               | TSIH                         |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  24| StatSN                                                       |
    +---------------+---------------+---------------+--------------+
  28| ExpCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  32| MaxCmdSN                                                     |
    +---------------+---------------+---------------+--------------+
  36| Status-Class | Status-Detail | Reserved                      |
    +---------------+---------------+---------------+--------------+
  40/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48/ DataSegment - Login Parameters in Text request Format        /
   +/                                                              /
    +---------------+---------------+---------------+--------------+

11.13.1. Version-max

  This is the highest version number supported by the target.

  All Login responses within the Login Phase MUST carry the same
  Version-max.

  The initiator MUST use the value presented as a response to the
  first login request.




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11.13.2. Version-active

   Indicates the highest version supported by the target and
   initiator. If the target does not support a version within the
   range specified by the initiator, the target rejects the login and
   this field indicates the lowest version supported by the target.

   All Login responses within the Login Phase MUST carry the same
   Version-active.

   The initiator MUST use the value presented as a response to the
   first login request.

11.13.3. TSIH

   The TSIH is the target assigned session identifying handle. Its
   internal format and content are not defined by this protocol
   except for the value 0 that is reserved. With the exception of the
   Login Final-Response in a new session, this field should be set to
   the TSIH provided by the initiator in the Login Request. For a
   new session, the target MUST generate a non-zero TSIH and ONLY
   return it in the Login Final-Response (see Section 6.3).

11.13.4. StatSN

   For the first Login Response (the response to the first Login
   Request), this is the starting status Sequence Number for the
   connection. The next response of any kind, including the next
   login response, if any, in the same Login Phase, will carry this
   number + 1. This field is only valid if the Status-Class is 0.

11.13.5. Status-Class and Status-Detail

   The Status returned in a Login Response indicates the execution
   status of the Login Phase. The status includes:

     Status-Class

     Status-Detail


   0 Status-Class indicates success.




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  A non-zero Status-Class indicates an exception. In this case,
  Status-Class is sufficient for a simple initiator to use when
  handling exceptions, without having to look at the Status-Detail.
  The Status-Detail allows finer-grained exception handling for more
  sophisticated initiators and for better information for logging.

  The status classes are as follows:

    0 - Success - indicates that the iSCSI target successfully
      received, understood, and accepted the request. The
      numbering fields (StatSN, ExpCmdSN, MaxCmdSN) are only valid
      if Status-Class is 0.

      1 - Redirection - indicates that the initiator must take
       further action to complete the request. This is usually due
       to the target moving to a different address. All of the
       redirection status class responses MUST return one or more
       text key parameters of the type "TargetAddress", which
       indicates the target's new address. A redirection response
       MAY be issued by a target prior or after completing a
       security negotiation if a security negotiation is required.
       A redirection SHOULD be accepted by an initiator even
       without having the target complete a security negotiation if
       any security negotiation is required, and MUST be accepted
       by the initiator after the completion of the security
       negotiation if any security negotiation is required.

    2 - Initiator Error (not a format error) - indicates that the
      initiator most likely caused the error. This MAY be due to a
      request for a resource for which the initiator does not have
      permission. The request should not be tried again.

    3 - Target Error - indicates that the target sees no errors in
      the initiator's login request, but is currently incapable of
      fulfilling the request. The initiator may re-try the same
      login request later.




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  The table below shows all of the currently allocated status codes.
  The codes are in hexadecimal; the first byte is the status class
  and the second byte is the status detail.




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  -----------------------------------------------------------------
  Status        | Code | Description
                |(hex) |
  -----------------------------------------------------------------
  Success       | 0000 | Login is proceeding OK (*1).
  -----------------------------------------------------------------
  Target moved | 0101 | The requested iSCSI Target Name (ITN)
  temporarily   |      | has temporarily moved
                |      | to the address provided.
  -----------------------------------------------------------------
  Target moved | 0102 | The requested ITN has permanently moved
  permanently   |      | to the address provided.
  -----------------------------------------------------------------
  Initiator     | 0200 | Miscellaneous iSCSI initiator
  error         |      | errors.
  ----------------------------------------------------------------
  Authentication| 0201 | The initiator could not be
  failure       |      | successfully authenticated or target
                |      | authentication is not supported.
  -----------------------------------------------------------------
  Authorization | 0202 | The initiator is not allowed access
  failure       |      | to the given target.
  -----------------------------------------------------------------
  Not found     | 0203 | The requested ITN does not
                |      | exist at this address.
  -----------------------------------------------------------------
  Target removed| 0204 | The requested ITN has been removed and
                |      | no forwarding address is provided.
  -----------------------------------------------------------------
  Unsupported   | 0205 | The requested iSCSI version range is
  version       |      | not supported by the target.
  -----------------------------------------------------------------
  Too many      | 0206 | Too many connections on this SSID.
  connections   |      |
  -----------------------------------------------------------------
  Missing       | 0207 | Missing parameters (e.g., iSCSI
  parameter     |      | Initiator and/or Target Name).
  -----------------------------------------------------------------
  Can't include | 0208 | Target does not support session
  in session    |      | spanning to this connection (address).
  -----------------------------------------------------------------




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  Session type | 0209 | Target does not support this type of
  not supported |      | of session or not from this Initiator.
  -----------------------------------------------------------------
  Session does | 020a | Attempt to add a connection
  not exist     |      | to a non-existent session.
  -----------------------------------------------------------------
  Invalid during| 020b | Invalid Request type during Login.
  login         |      |
  -----------------------------------------------------------------
  Target error | 0300 | Target hardware or software error.
  -----------------------------------------------------------------
  Service       | 0301 | The iSCSI service or target is not
  unavailable   |      | currently operational.
  -----------------------------------------------------------------
  Out of        | 0302 | The target has insufficient session,
  resources     |      | connection, or other resources.
  -----------------------------------------------------------------

  (*1)If the response T bit is 1 in both the request and the
  matching response, and the NSG is FullFeaturePhase in both the
  request and the matching response, the Login Phase is finished and
  the initiator may proceed to issue SCSI commands.

  If the Status Class is not 0, the initiator and target MUST close
  the TCP connection.

  If the target wishes to reject the login request for more than one
  reason, it should return the primary reason for the rejection.

11.13.6. T (Transit) bit

  The T bit is set to 1 as an indicator of the end of the stage. If
  the T bit is set to 1 and NSG is FullFeaturePhase, then this is
  also the Final Login Response (see Section 6.3). A T bit of 0
  indicates a "partial" response, which means "more negotiation
  needed".

  A login response with a T bit set to 1 MUST NOT contain key=value
  pairs that may require additional answers from the initiator
  within the same stage.




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  If the status class is 0, the T bit MUST NOT be set to 1 if the T
  bit in the request was set to 0.

11.13.7. C (Continue) Bit

  When set to 1, indicates that the text (set of key=value pairs) in
  this Login Response is not complete (it will be continued on
  subsequent Login Responses); otherwise, it indicates that this
  Login Response ends a set of key=value pairs. A Login Response
  with the C bit set to 1 MUST have the T bit set to 0.

11.13.8. Login Parameters

  The target MUST provide some basic parameters in order to enable
  the initiator to determine if it is connected to the correct port
  and the initial text parameters for the security exchange.

  All the rules specified in Section 11.11.6 for text responses also
  hold for login responses. Keys and their explanations are listed
  in Section 12(security negotiation keys) and Section 13
  (operational parameter negotiation keys). All keys in Section 13,
  except for the X extension formats, MUST be supported by iSCSI
  initiators and targets. Keys in Section 12, only need to be
  supported when the function to which they refer is mandatory to
  implement.


11.14. Logout Request

  The Logout request is used to perform a controlled closing of a
  connection.

  An initiator MAY use a logout request to remove a connection from
  a session or to close an entire session.

  After sending the Logout request PDU, an initiator MUST NOT send
  any new iSCSI requests on the closing connection. If the Logout
  request is intended to close the session, new iSCSI requests MUST
  NOT be sent on any of the connections participating in the
  session.




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  When receiving a Logout request with the reason code of "close the
  connection" or "close the session", the target MUST terminate all
  pending commands, whether acknowledged via ExpCmdSN or not, on
  that connection or session respectively.

  When receiving a Logout request with the reason code "remove
  connection for recovery", the target MUST discard all requests not
  yet acknowledged via ExpCmdSN that were issued on the specified
  connection, and suspend all data/status/R2T transfers on behalf of
  pending commands on the specified connection.

  The target then issues the Logout response and half-closes the TCP
  connection (sends FIN). After receiving the Logout response and
  attempting to receive the FIN (if still possible), the initiator
  MUST completely close the logging-out connection. For the
  terminated commands, no additional responses should be expected.

  A Logout for a CID may be performed on a different transport
  connection when the TCP connection for the CID has already been
  terminated. In such a case, only a logical "closing" of the iSCSI
  connection for the CID is implied with a Logout.

  All commands that were not terminated or not completed (with
  status) and acknowledged when the connection is closed completely
  can be reassigned to a new connection if the target supports
  connection recovery.

  If an initiator intends to start recovery for a failing
  connection, it MUST use the Logout request to "clean-up" the
  target end of a failing connection and enable recovery to start,
  or the Login request with a non-zero TSIH and the same CID on a
  new connection for the same effect. In sessions with a single
  connection, the connection can be closed and then a new connection
  reopened. A connection reinstatement login can be used for
  recovery (see Section 6.3.4).

  A successful completion of a logout request with the reason code
  of "close the connection" or "remove the connection for recovery"
  results at the target in the discarding of unacknowledged commands
  received on the connection being logged out. These are commands
  that have arrived on the connection being logged out, but have not




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  been delivered to SCSI because one or more commands with a smaller
  CmdSN has not been received by iSCSI. See Section 4.2.2.1. The
  resulting holes in the command sequence numbers will have to be
  handled by appropriate recovery (see Section 7) unless the session
  is also closed.

  The entire logout discussion in this Section is also applicable
  for an implicit Logout realized by way of a connection
  reinstatement or session reinstatement. When a Login Request
  performs an implicit Logout, the implicit Logout is performed as
  if having the reason codes specified below:

    Reason code          Type of implicit Logout

    -------------------------------------------

         0        session reinstatement

         1     connection reinstatement when the operational
               ErrorRecoveryLevel < 2

        2      connection reinstatement when the operational
               ErrorRecoveryLevel = 2




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  Byte/     0       |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|I| 0x06      |1| Reason Code | Reserved                     |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +--------------------------------------------------------------+
   8/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag                                           |
    +---------------+---------------+---------------+--------------+
  20| CID or Reserved               | Reserved                     |
    +---------------+---------------+---------------+--------------+
  24| CmdSN                                                        |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN                                                    |
    +---------------+---------------+---------------+--------------+
  32/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+

11.14.1. Reason Code

  Reason Code indicates the reason for Logout as follows:

     0 - close the session. All commands associated with the
       session (if any) are terminated.

     1 - close the connection. All commands associated with
       connection (if any) are terminated.

     2 - remove the connection for recovery. Connection is closed
       and all commands associated with it, if any, are to be
       prepared for a new allegiance.


  All other values are reserved.




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11.14.2. TotalAHSLength and DataSegmentLength

   For this PDU TotalAHSLength and DataSegmentLength MUST be 0.


11.14.3. CID

   This is the connection ID of the connection to be closed
   (including closing the TCP stream). This field is only valid if
   the reason code is not "close the session".

11.14.4. ExpStatSN

   This is the last ExpStatSN value for the connection to be closed.

11.14.5. Implicit termination of tasks

   A target implicitly terminates the active tasks due to the iSCSI
   protocol in the following cases:

     a) When a connection is implicitly or explicitly logged out
        with the reason code of "Close the connection" and there
        are active tasks allegiant to that connection.

     b) When a connection fails and eventually the connection state
        times out (state transition M1 in Section 8.2.2) and there
        are active tasks allegiant to that connection.

     c) When a successful recovery Logout is performed while there
        are active tasks allegiant to that connection, and those
        tasks eventually time out after the Time2Wait and
        Time2Retain periods without allegiance reassignment.

     d) When a connection is implicitly or explicitly logged out
        with the reason code of "Close the session" and there are
        active tasks in that session.


   If the tasks terminated in any of the above cases are SCSI tasks,
   they must be internally terminated as if with CHECK CONDITION
   status. This status is only meaningful for appropriately handling




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  the internal SCSI state and SCSI side effects with respect to
  ordering because this status is never communicated back as a
  terminating status to the initiator. However additional actions
  may have to be taken at SCSI level depending on the SCSI context
  as defined by the SCSI standards (e.g., queued commands and ACA,
  UA for the next command on the I_T nexus in cases a), b), and c),
  after the tasks are terminated, the target MUST report a Unit
  Attention condition on the next command processed on any
  connection for each affected I_T_L nexus with the status of CHECK
  CONDITION, and the ASC/ASCQ value of 47h/7Fh - "SOME COMMANDS
  CLEARED BY ISCSI PROTOCOL EVENT" - etc. - see [SPC3]).

11.15. Logout Response

  The logout response is used by the target to indicate if the
  cleanup operation for the connection(s) has completed.

  After Logout, the TCP connection referred by the CID MUST be
  closed at both ends (or all connections must be closed if the
  logout reason was session close).




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   Byte/      0      |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|.| 0x26      |1| Reserved    | Response      | Reserved     |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +--------------------------------------------------------------+
    8/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag                                           |
     +---------------+---------------+---------------+--------------+
   20| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   24| StatSN                                                       |
     +---------------+---------------+---------------+--------------+
   28| ExpCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   32| MaxCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   36| Reserved                                                     |
     +--------------------------------------------------------------+
   40| Time2Wait                     | Time2Retain                  |
     +---------------+---------------+---------------+--------------+
   44| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+

11.15.1. Response

   Logout response:

     0 - connection or session closed successfully.

     1 - CID not found.

     2 - connection recovery is not supported. If Logout reason
       code was recovery and target does not support it as
       indicated by the ErrorRecoveryLevel.
       3 - cleanup failed for various reasons.




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11.15.2. TotalAHSLength and DataSegmentLength

   For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

11.15.3. Time2Wait

   If the Logout response code is 0 and if the operational
   ErrorRecoveryLevel is 2, this is the minimum amount of time, in
   seconds, to wait before attempting task reassignment. If the
   Logout response code is 0 and if the operational
   ErrorRecoveryLevel is less than 2, this field is to be ignored.

   This field is invalid if the Logout response code is 1.

   If the Logout response code is 2 or 3, this field specifies the
   minimum time to wait before attempting a new implicit or explicit
   logout.

   If Time2Wait is 0, the reassignment or a new Logout may be
   attempted immediately.

11.15.4. Time2Retain

   If the Logout response code is 0 and if the operational
   ErrorRecoveryLevel is 2, this is the maximum amount of time, in
   seconds, after the initial wait (Time2Wait), the target waits for
   the allegiance reassignment for any active task after which the
   task state is discarded. If the Logout response code is 0 and if
   the operational ErrorRecoveryLevel is less than 2, this field is
   to be ignored.

   This field is invalid if the Logout response code is 1.

   If the Logout response code is 2 or 3, this field specifies the
   maximum amount of time, in seconds, after the initial wait
   (Time2Wait), the target waits for a new implicit or explicit
   logout.

   If it is the last connection of a session, the whole session state
   is discarded after Time2Retain.




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  If Time2Retain is 0, the target has already discarded the
  connection (and possibly the session) state along with the task
  states. No reassignment or Logout is required in this case.

11.16. SNACK Request

  Byte/      0      |       1       |       2       |       3      |
      /             |               |               |              |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
    +---------------+---------------+---------------+--------------+
   0|.|.| 0x10      |1|.|.|.| Type | Reserved                      |
    +---------------+---------------+---------------+--------------+
   4|TotalAHSLength | DataSegmentLength                            |
    +---------------+---------------+---------------+--------------+
   8| LUN or Reserved                                              |
    +                                                              +
  12|                                                              |
    +---------------+---------------+---------------+--------------+
  16| Initiator Task Tag or 0xffffffff                             |
    +---------------+---------------+---------------+--------------+
  20| Target Transfer Tag or SNACK Tag or 0xffffffff               |
    +---------------+---------------+---------------+--------------+
  24| Reserved                                                     |
    +---------------+---------------+---------------+--------------+
  28| ExpStatSN                                                    |
    +---------------+---------------+---------------+--------------+
  32/ Reserved                                                     /
   +/                                                              /
    +---------------+---------------+---------------+--------------+
  40| BegRun                                                       |
    +--------------------------------------------------------------+
  44| RunLength                                                    |
    +--------------------------------------------------------------+
  48| Header-Digest (Optional)                                     |
    +---------------+---------------+---------------+--------------+

  If the implementation supports ErrorRecoveryLevel greater than
  zero, it MUST support all SNACK types.

  The SNACK is used by the initiator to request the retransmission
  of numbered-responses, data, or R2T PDUs from the target. The
  SNACK request indicates the numbered-responses or data "runs"




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   whose retransmission is requested by the target, where the run
   starts with the first StatSN, DataSN, or R2TSN whose
   retransmission is requested and indicates the number of Status,
   Data, or R2T PDUs requested including the first. 0 has special
   meaning when used as a starting number and length:

     - When used in RunLength, it means all PDUs starting with the
       initial.

     - When used in both BegRun and RunLength, it means all
       unacknowledged PDUs.


   The numbered-response(s) or R2T(s), requested by a SNACK, MUST be
   delivered as exact replicas of the ones that the target
   transmitted originally except for the fields ExpCmdSN, MaxCmdSN,
   and ExpDataSN, which MUST carry the current values.
   R2T(s)requested by SNACK MUST also carry the current value of
   StatSN.

   The numbered Data-In PDUs, requested by a Data SNACK MUST be
   delivered as exact replicas of the ones that the target
   transmitted originally except for the fields ExpCmdSN and
   MaxCmdSN, which MUST carry the current values and except for
   resegmentation (see Section 11.16.3).

   Any SNACK that requests a numbered-response, Data, or R2T that was
   not sent by the target or was already acknowledged by the
   initiator, MUST be rejected with a reason code of "Protocol
   error".

11.16.1. Type

   This field encodes the SNACK function as follows:

     0-Data/R2T SNACK - requesting retransmission of one or more
       Data-In or R2T PDUs.

     1-Status SNACK - requesting retransmission of one or more
       numbered responses.




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     2-DataACK - positively acknowledges Data-In PDUs.


     3-R-Data SNACK - requesting retransmission of Data-In PDUs
       with possible resegmentation and status tagging.

  All other values are reserved.

  Data/R2T SNACK, Status SNACK, or R-Data SNACK for a command MUST
  precede status acknowledgement for the given command.

11.16.2. Data Acknowledgement

  If an initiator operates at ErrorRecoveryLevel 1 or higher, it
  MUST issue a SNACK of type DataACK after receiving a Data-In PDU
  with the A bit set to 1. However, if the initiator has detected
  holes in the input sequence, it MUST postpone issuing the SNACK of
  type DataACK until the holes are filled. An initiator MAY ignore
  the A bit if it deems that the bit is being set aggressively by
  the target (i.e.,      before the MaxBurstLength limit is
  reached).

  The DataACK is used to free resources at the target and not to
  request or imply data retransmission.

  An initiator MUST NOT request retransmission for any data it had
  already acknowledged.

11.16.3. Resegmentation

  If the initiator MaxRecvDataSegmentLength changed between the
  original transmission and the time the initiator requests
  retransmission, the initiator MUST issue a R-Data SNACK (see
  Section 11.16.1). With R-Data SNACK, the initiator indicates that
  it discards all the unacknowledged data and expects the target to
  resend it. It also expects resegmentation. In this case, the
  retransmitted Data-In PDUs MAY be different from the ones
  originally sent in order to reflect changes in
  MaxRecvDataSegmentLength. Their DataSN starts with the BegRun of
  the last DataACK received by the target if any was received;
  otherwise it starts with 0 and is increased by 1 for each resent
  Data-In PDU.




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  A target that has received a R-Data SNACK MUST return a SCSI
  Response that contains a copy of the SNACK Tag field from the R-
  Data SNACK in the SCSI Response SNACK Tag field as its last or
  only Response. For example, if it has already sent a response
  containing another value in the SNACK Tag field or had the status
  included in the last Data-In PDU, it must send a new SCSI Response
  PDU. If a target sends more than one SCSI Response PDU due to this
  rule, all SCSI responses must carry the same StatSN (see Section
  11.4.4). If an initiator attempts to recover a lost SCSI Response
  (with a Status-SNACK, see Section 11.16.1) when more than one
  response has been sent, the target will send the SCSI Response
  with the latest content known to the target, including the last
  SNACK Tag for the command.

  For considerations in allegiance reassignment of a task to a
  connection with a different MaxRecvDataSegmentLength, refer to
  Section 7.2.2.

11.16.4. Initiator Task Tag

  For Status SNACK and DataACK, the Initiator Task Tag MUST be set
  to the reserved value 0xffffffff. In all other cases, the
  Initiator Task Tag field MUST be set to the Initiator Task Tag of
  the referenced command.

11.16.5. Target Transfer Tag or SNACK Tag

  For an R-Data SNACK, this field MUST contain a value that is
  different from 0 or 0xffffffff and is unique for the task
  (identified by the Initiator Task Tag). This value MUST be copied
  by the iSCSI target in the last or only SCSI Response PDU it
  issues for the command.

  For DataACK, the Target Transfer Tag MUST contain a copy of the
  Target Transfer Tag and LUN provided with the SCSI Data-In PDU
  with the A bit set to 1.

  In all other cases, the Target Transfer Tag field MUST be set to
  the reserved value of 0xffffffff.




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11.16.6. BegRun

   The DataSN, R2TSN, or StatSN of the first PDU whose retransmission
   is requested (Data/R2T and Status SNACK), or the next expected
   DataSN (DataACK SNACK).

   BegRun 0 when used in conjunction with RunLength 0 means resend
   all unacknowledged Data-In, R2T or Response PDUs.

   BegRun MUST be 0 for a R-Data SNACK.

11.16.7. RunLength

   The number of PDUs whose retransmission is requested.

   RunLength 0 signals that all Data-In, R2T, or Response PDUs
   carrying the numbers equal to or greater than BegRun have to be
   resent.

   The RunLength MUST also be 0 for a DataACK SNACK in addition to R-
   Data SNACK.




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11.17. Reject

   Byte/      0      |       1       |        2      |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|.| 0x3f      |1| Reserved    | Reason        | Reserved     |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +---------------+---------------+---------------+--------------+
    8/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   16| 0xffffffff                                                   |
     +---------------+---------------+---------------+--------------+
   20| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   24| StatSN                                                       |
     +---------------+---------------+---------------+--------------+
   28| ExpCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   32| MaxCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   36| DataSN/R2TSN or Reserved                                     |
     +---------------+---------------+---------------+--------------+
   40| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   44| Reserved                                                     |
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+
   xx/ Complete Header of Bad PDU                                   /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   yy/Vendor specific data (if any)                                 /
     /                                                              /
     +---------------+---------------+---------------+--------------+
   zz| Data-Digest (Optional)                                       |
     +---------------+---------------+---------------+--------------+




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   Reject is used to indicate an iSCSI error condition (protocol,
   unsupported option, etc.).

11.17.1. Reason

   The reject Reason is coded as follows:

   +------+----------------------------------------+----------------+
   | Code | Explanation                            |Can the original|
   | (hex)|                                        |PDU be re-sent? |
   +------+----------------------------------------+----------------+
   | 0x01 | Reserved                               | no             |
   |      |                                        |                |
   | 0x02 | Data (payload) Digest Error            | yes (Note 1) |
   |      |                                        |                |
   | 0x03 | SNACK Reject                           | yes            |
   |      |                                        |                |
   | 0x04 | Protocol Error (e.g., SNACK request for| no             |
   |      | a status that was already acknowledged)|                |
   |      |                                        |                |
   | 0x05 | Command not supported                  | no             |
   |      |                                        |                |
   | 0x06 | Immediate Command Reject - too many    | yes            |
   |      | immediate commands                     |                |
   |      |                                        |                |
   | 0x07 | Task in progress                       | no             |
   |      |                                        |                |
   | 0x08 | Invalid Data ACK                       | no             |
   |      |                                        |                |
   | 0x09 | Invalid PDU field                      | no   (Note 2) |
   |      |                                        |                |
   | 0x0a | Long Operation Reject - Can't generate | yes            |
   |      | Target Transfer Tag - out of resources |                |
   |      |                                        |                |
   | 0x0c | Waiting for Logout                     | no             |
   +------+----------------------------------------+----------------+

   Note 1: For iSCSI, Data-Out PDU retransmission is only done if the
   target requests retransmission with a recovery R2T. However, if
   this is the data digest error on immediate data, the initiator may
   choose to retransmit the whole PDU including the immediate data.




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  Note 2: A target should use this reason code for all invalid
  values of PDU fields that are meant to describe a task, a
  response, or a data transfer. Some examples are invalid TTT/ITT,
  buffer offset, LUN qualifying a TTT, and an invalid sequence
  number in a SNACK.

  Note 3: Reason code 0x0b ("Negotiation reset") defined in
  [RFC3720] is deprecated and MUST NOT be used by implementations.
  An implementation receiving reason code 0x0b MUST treat it as a
  negotiation failure that terminates the Login Phase and the TCP
  connection, as specified in Section 7.12.

  All other values for Reason are reserved.


  In all the cases in which a pre-instantiated SCSI task is
  terminated because of the reject, the target MUST issue a proper
  SCSI command response with CHECK CONDITION as described in Section
  11.4.3. In these cases in which a status for the SCSI task was
  already sent before the reject, no additional status is required.
  If the error is detected while data from the initiator is still
  expected (i.e., the command PDU did not contain all the data and
  the target has not received a Data-out PDU with the Final bit set
  to 1 for the unsolicited data, if any, and all outstanding R2Ts,
  if any), the target MUST wait until it receives the last expected
  Data-out PDUs with the F bit set to 1 before sending the Response
  PDU.

  For additional usage semantics of Reject PDU, see Section 7.3.

11.17.2. DataSN/R2TSN

  This field is only valid if the rejected PDU is a Data/R2T SNACK
  and the Reject reason code is "Protocol error" (see Section
  11.16). The DataSN/R2TSN is the next Data/R2T sequence number
  that the target would send for the task, if any.

11.17.3. StatSN, ExpCmdSN and MaxCmdSN

  These fields carry their usual values and are not related to the
  rejected command. StatSN is advanced after a Reject.




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11.17.4. Complete Header of Bad PDU

   The target returns the header (not including digest) of the PDU in
   error as the data of the response.

11.18. NOP-Out

   Byte/     0       |       1       |       2       |       3      |
       /             |               |               |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|I| 0x00      |1| Reserved                                   |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +---------------+---------------+---------------+--------------+
    8| LUN or Reserved                                              |
     +                                                              +
   12|                                                              |
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag or 0xffffffff                             |
     +---------------+---------------+---------------+--------------+
   20| Target Transfer Tag or 0xffffffff                            |
     +---------------+---------------+---------------+--------------+
   24| CmdSN                                                        |
     +---------------+---------------+---------------+--------------+
   28| ExpStatSN                                                    |
     +---------------+---------------+---------------+--------------+
   32/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+
     / DataSegment - Ping Data (optional)                           /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
     | Data-Digest (Optional)                                       |
     +---------------+---------------+---------------+--------------+

   A NOP-Out may be used by an initiator as a "ping request" to
   verify that a connection/session is still active and all its
   components are operational. The NOP-In response is the "ping
   echo".




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  A NOP-Out is also sent by an initiator in response to a NOP-In.

  A NOP-Out may also be used to confirm a changed ExpStatSN if
  another PDU will not be available for a long time.

  Upon receipt of a NOP-In with the Target Transfer Tag set to a
  valid value (not the reserved 0xffffffff), the initiator MUST
  respond with a NOP-Out. In this case, the NOP-Out Target Transfer
  Tag MUST contain a copy of the NOP-In Target Transfer Tag. The
  initiator SHOULD NOT send a NOP-Out in response to any other
  received NOP-In in order to avoid lengthy sequences of NOP-In and
  NOP-Out PDUs sent in response to each other.

11.18.1. Initiator Task Tag

  The NOP-Out MUST have the Initiator Task Tag set to a valid value
  only if a response in the form of NOP-In is requested (i.e., the
  NOP-Out is used as a ping request). Otherwise, the Initiator Task
  Tag MUST be set to 0xffffffff.

  When a target receives the NOP-Out with a valid Initiator Task
  Tag, it MUST respond with a Nop-In Response (see Section 6).

  If the Initiator Task Tag contains 0xffffffff, the I bit MUST be
  set to 1 and the CmdSN is not advanced after this PDU is sent.

11.18.2. Target Transfer Tag

  A target assigned identifier for the operation.

  The NOP-Out MUST only have the Target Transfer Tag set if it is
  issued in response to a NOP-In with a valid Target Transfer Tag.
  In this case, it copies the Target Transfer Tag from the NOP-In
  PDU. Otherwise, the Target Transfer Tag MUST be set to 0xffffffff.

  When the Target Transfer Tag is set to a value other than
  0xffffffff, the LUN field MUST also be copied from the NOP-In.




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11.18.3. Ping Data

   Ping data is reflected in the NOP-In Response. The length of the
   reflected data is limited to MaxRecvDataSegmentLength. The length
   of ping data is indicated by the DataSegmentLength. 0 is a valid
   value for the DataSegmentLength and indicates the absence of ping
   data.




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11.19. NOP-In

   Byte/      0      |       1       |       2       |       3      |
       /             |                |              |              |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 67|
     +---------------+---------------+---------------+--------------+
    0|.|.| 0x20      |1| Reserved                                   |
     +---------------+---------------+---------------+--------------+
    4|TotalAHSLength | DataSegmentLength                            |
     +---------------+---------------+---------------+--------------+
    8| LUN or Reserved                                              |
     +                                                              +
   12|                                                              |
     +---------------+---------------+---------------+--------------+
   16| Initiator Task Tag or 0xffffffff                             |
     +---------------+---------------+---------------+--------------+
   20| Target Transfer Tag or 0xffffffff                            |
     +---------------+---------------+---------------+--------------+
   24| StatSN                                                       |
     +---------------+---------------+---------------+--------------+
   28| ExpCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   32| MaxCmdSN                                                     |
     +---------------+---------------+---------------+--------------+
   36/ Reserved                                                     /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
   48| Header-Digest (Optional)                                     |
     +---------------+---------------+---------------+--------------+
     / DataSegment - Return Ping Data                               /
    +/                                                              /
     +---------------+---------------+---------------+--------------+
     | Data-Digest (Optional)                                       |
     +---------------+---------------+---------------+--------------+


   NOP-In is either sent by a target as a response to a NOP-Out, as a
   "ping" to an initiator, or as a means to carry a changed ExpCmdSN
   and/or MaxCmdSN if another PDU will not be available for a long
   time (as determined by the target).




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   When a target receives the NOP-Out with a valid Initiator Task Tag
   (not the reserved value 0xffffffff), it MUST respond with a NOP-In
   with the same Initiator Task Tag that was provided in the NOP-Out
   request. It MUST also duplicate up to the first
   MaxRecvDataSegmentLength bytes of the initiator provided Ping
   Data. For such a response, the Target Transfer Tag MUST be
   0xffffffff. The target SHOULD NOT send a NOP-In in response to any
   other received NOP-Out in order to avoid lengthy sequences of NOP-
   In and NOP-Out PDUs sent in response to each other.

   Otherwise, when a target sends a NOP-In that is not a response to
   a Nop-Out received from the initiator, the Initiator Task Tag MUST
   be set to 0xffffffff and the Data Segment MUST NOT contain any
   data (DataSegmentLength MUST be 0).

11.19.1. Target Transfer Tag

   If the target is responding to a NOP-Out, this is set to the
   reserved value 0xffffffff.

   If the target is sending a NOP-In as a Ping (intending to receive
   a corresponding NOP-Out), this field is set to a valid value (not
   the reserved 0xffffffff).

   If the target is initiating a NOP-In without wanting to receive a
   corresponding NOP-Out, this field MUST hold the reserved value of
   0xffffffff.

11.19.2. StatSN

   The StatSN field will always contain the next StatSN. However,
   when the Initiator Task Tag is set to 0xffffffff StatSN for the
   connection is not advanced after this PDU is sent.

11.19.3. LUN

   A LUN MUST be set to a correct value when the Target Transfer Tag
   is valid (not the reserved value 0xffffffff).




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12. iSCSI Security Text Keys and Authentication Methods

   Only the following keys are used during the SecurityNegotiation
   stage of the Login Phase:

     SessionType

     InitiatorName

     TargetName

     TargetAddress

     InitiatorAlias

     TargetAlias

     TargetPortalGroupTag

     AuthMethod and the keys used by the authentication methods
       specified under Section 12.1 along with all of their
       associated keys as well as Vendor-Specific Authentication
       Methods.


   Other keys MUST NOT be used.

   SessionType, InitiatorName, TargetName, InitiatorAlias,
   TargetAlias, and TargetPortalGroupTag are described in Section 13
   as they can be used also in the OperationalNegotiation stage.

   All security keys have connection-wide applicability.

12.1. AuthMethod

   Use: During Login - Security Negotiation
   Senders: Initiator and Target
   Scope: connection

   AuthMethod = <list-of-values>

   The main item of security negotiation is the authentication method
   (AuthMethod).




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  The authentication methods that can be used (appear in the list-
  of-values) are either those listed in the following table or are
  vendor-unique methods:

  +------------------------------------------------------------+
  | Name          | Description                                |
  +------------------------------------------------------------+
  | KRB5          | Kerberos V5 - defined in [RFC4120]         |
  +------------------------------------------------------------+
  | SRP           | Secure Remote Password                     |
  |               | defined in [RFC2945]                       |
  +------------------------------------------------------------+
  | CHAP          | Challenge Handshake Authentication Protocol|
  |               | defined in [RFC1994]                       |
  +------------------------------------------------------------+
  | None          | No authentication                          |
  +------------------------------------------------------------+


  The AuthMethod selection is followed by an "authentication
  exchange" specific to the authentication method selected.

  The authentication method proposal may be made by either the
  initiator or the target. However the initiator MUST make the first
  step specific to the selected authentication method as soon as it
  is selected. It follows that if the target makes the
  authentication method proposal the initiator sends the first
  key(s) of the exchange together with its authentication method
  selection.

  The authentication exchange authenticates the initiator to the
  target, and optionally, the target to the initiator.
  Authentication is OPTIONAL to use but MUST be supported by the
  target and initiator.

  The initiator and target MUST implement CHAP. All other
  authentication methods are OPTIONAL.

  Private or public extension algorithms MAY also be negotiated for
  authentication methods. Whenever a private or public extension




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   algorithm is part of the default offer (the offer made in absence
   of explicit administrative action) the implementer MUST ensure
   that CHAP is listed as an alternative in the default offer and
   "None" is not part of the default offer.


   Extension authentication methods MUST be named using one of the
   following two formats:

         i)        Z-reversed.vendor.dns_name.do_something=
         ii)       New public key with no name prefix constraints

   Authentication methods named using the Z- format are used as
   private extensions. New public keys must be registered with IANA
   using IETF Review process ([RFC5226]). New public extensions for
   authentication methods MUST NOT use the Z# name prefix.

   For all of the public or private extension authentication methods,
   the method specific keys MUST conform to the format specified in
   Section 6.1 for standard-label.

   To identify the vendor for private extension authentication
   methods, we suggest you use the reversed DNS-name as a prefix to
   the proper digest names.

   The part of digest-name following Z- MUST conform to the format
   for standard-label specified in Section 6.1.

   Support for public or private extension authentication methods is
   OPTIONAL.

   The following subsections define the specific exchanges for each
   of the standardized authentication methods. As mentioned earlier
   the first step is always done by the initiator.

12.1.1. Kerberos

   For KRB5 (Kerberos V5) [RFC4120] and [RFC1964], the initiator MUST
   use:

       KRB_AP_REQ=<KRB_AP_REQ>




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  where KRB_AP_REQ is the client message as defined in [RFC4120].

  The default principal name assumed by an iSCSI initiator or target
  (prior to any administrative configuration action) MUST be the
  iSCSI Initiator Name or iSCSI Target Name respectively, prefixed
  by the string "iscsi/".

  If the initiator authentication fails, the target MUST respond
  with a Login reject with "Authentication Failure" status.
  Otherwise, if the initiator has selected the mutual authentication
  option (by setting MUTUAL-REQUIRED in the ap-options field of the
  KRB_AP_REQ), the target MUST reply with:

       KRB_AP_REP=<KRB_AP_REP>

  where KRB_AP_REP is the server's response message as defined in
  [RFC4120].

  If mutual authentication was selected and target authentication
  fails, the initiator MUST close the connection.

  KRB_AP_REQ and KRB_AP_REP are binary-values and their binary
  length (not the length of the character string that represents
  them in encoded form) MUST NOT exceed 65536 bytes. Hex or Base64
  encoding may be used for KRB_AP_REQ and KRB_AP_REP, see Section
  6.1.

12.1.2. Secure Remote Password (SRP)


  For SRP [RFC2945], the initiator MUST use:

      SRP_U=<U> TargetAuth=Yes   /* or TargetAuth=No */

  The target MUST answer with a Login reject with the "Authorization
  Failure" status or reply with:

  SRP_GROUP=<G1,G2...> SRP_s=<s>

  Where G1,G2... are proposed groups, in order of preference.

  The initiator MUST either close the connection or continue with:




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  SRP_A=<A> SRP_GROUP=<G>

  Where G is one of G1,G2... that were proposed by the target.

  The target MUST answer with a Login reject with the
  "Authentication Failure" status or reply with:

     SRP_B=<B>

  The initiator MUST close the connection or continue with:

     SRP_M=<M>

  If the initiator authentication fails, the target MUST answer with
  a Login reject with "Authentication Failure" status. Otherwise, if
  the initiator sent TargetAuth=Yes in the first message (requiring
  target authentication), the target MUST reply with:

    SRP_HM=<H(A | M | K)>

  If the target authentication fails, the initiator MUST close the
  connection.

  Where U, s, A, B, M, and H(A | M | K) are defined in [RFC2945]
  (using the SHA1 hash function, such as SRP-SHA1) and G,Gn (Gn
  stands for G1,G2...) are identifiers of SRP groups specified in
  [RFC3723]. G, Gn, and U are text strings, s,A,B,M, and H(A | M |
  K) are binary-values. The length of s,A,B,M and H(A | M | K) in
  binary form (not the length of the character string that
  represents them in encoded form) MUST NOT exceed 1024 bytes. Hex
  or Base64 encoding may be used for s,A,B,M and H(A | M | K), see
  Section 6.1.

  See Appendix B for the related login example.

  For the SRP_GROUP, all the groups specified in [RFC3723] up to
  1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) must be
  supported by initiators and targets. To guarantee
  interoperability, targets MUST always offer "SRP-1536" as one of
  the proposed groups.




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12.1.3. Challenge Handshake Authentication Protocol (CHAP)

  For CHAP [RFC1994], the initiator MUST use:

     CHAP_A=<A1,A2...>

  Where A1,A2... are proposed algorithms, in order of preference.

  The target MUST answer with a Login reject with the
  "Authentication Failure" status or reply with:

     CHAP_A=<A> CHAP_I=<I> CHAP_C=<C>

  Where A is one of A1,A2... that were proposed by the initiator.

  The initiator MUST continue with:

     CHAP_N=<N> CHAP_R=<R>

  or, if it requires target authentication, with:

     CHAP_N=<N> CHAP_R=<R> CHAP_I=<I> CHAP_C=<C>

  If the initiator authentication fails, the target MUST answer with
  a Login reject with "Authentication Failure" status. Otherwise, if
  the initiator required target authentication, the target MUST
  either answer with a Login reject with "Authentication Failure" or
  reply with:

     CHAP_N=<N> CHAP_R=<R>

  If target authentication fails, the initiator MUST close the
  connection.

  Where N, (A,A1,A2), I, C, and R are (correspondingly) the Name,
  Algorithm, Identifier, Challenge, and Response as defined in
  [RFC1994], N is a text string, A,A1,A2, and I are numbers, and C
  and R are binary-values and their binary length (not the length of
  the character string that represents them in encoded form) MUST
  NOT exceed 1024 bytes. Hex or Base64 encoding may be used for C
  and R, see Section 6.1.




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  See Appendix B for the related login example.

  For the Algorithm, as stated in [RFC1994], one value is required
  to be implemented:

      5       (CHAP with MD5)

  To guarantee interoperability, initiators MUST always offer it as
  one of the proposed algorithms.




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13. Login/Text Operational Text Keys

  Some session specific parameters MUST only be carried on the
  leading connection and cannot be changed after the leading
  connection login (e.g., MaxConnections, the maximum number of
  connections). This holds for a single connection session with
  regard to connection restart. The keys that fall into this
  category have the use: LO (Leading Only).

  Keys that can only be used during login have the use: IO
  (initialize only), while those that can be used in both the Login
  Phase and Full Feature Phase have the use: ALL.

  Keys that can only be used during Full Feature Phase use FFPO
  (Full Feature Phase only).

  Keys marked as Any-Stage may also appear in the
  SecurityNegotiation stage while all other keys described in this
  Section are operational keys.

  Keys that do not require an answer are marked as Declarative.

  Key scope is indicated as session-wide (SW) or connection-only
  (CO).

  Result function, wherever mentioned, states the function that can
  be applied to check the validity of the responder selection.
  Minimum means that the selected value cannot exceed the offered
  value. Maximum means that the selected value cannot be lower than
  the offered value. AND means that the selected value must be a
  possible result of a Boolean "and" function with an arbitrary
  Boolean value (e.g., if the offered value is No the selected value
  must be No). OR means that the selected value must be a possible
  result of a Boolean "or" function with an arbitrary Boolean value
  (e.g., if the offered value is Yes the selected value must be
  Yes).

13.1. HeaderDigest and DataDigest

  Use: IO
  Senders: Initiator and Target
  Scope: CO




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  HeaderDigest = <list-of-values>
  DataDigest = <list-of-values>

  Default is None for both HeaderDigest and DataDigest.

  Digests enable the checking of end-to-end, non-cryptographic data
  integrity beyond the integrity checks provided by the link layers
  and the covering of the whole communication path including all
  elements that may change the network level PDUs such as routers,
  switches, and proxies.

  The following table lists cyclic integrity checksums that can be
  negotiated for the digests and that MUST be implemented by every
  iSCSI initiator and target. These digest options only have error
  detection significance.

  +---------------------------------------------+
  | Name          | Description     | Generator |
  +---------------------------------------------+
  | CRC32C        | 32 bit CRC      |0x11edc6f41|
  +---------------------------------------------+
  | None          | no digest                   |
  +---------------------------------------------+

  The generator polynomial for this digest is given in hex-notation
  (e.g., 0x3b stands for 0011 1011 and the polynomial is
  x**5+X**4+x**3+x+1).

  When the Initiator and Target agree on a digest, this digest MUST
  be used for every PDU in Full Feature Phase.


  Padding bytes, when present in a segment covered by a CRC, SHOULD
  be set to 0 and are included in the CRC.

  The CRC MUST be calculated by a method that produces the same
  results as the following process:

    - The PDU bits are considered as the coefficients of a
      polynomial M(x) of degree n-1; bit 7 of the lowest numbered
      byte is considered the most significant bit (x^n-1),




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      followed by bit 6 of the lowest numbered byte through bit 0
      of the highest numbered byte (x^0).

    - The most significant 32 bits are complemented.

    - The polynomial is multiplied by x^32 then divided by G(x).
      The generator polynomial produces a remainder R(x) of degree
      <= 31.

    - The coefficients of R(x) are considered a 32 bit sequence.

    - The bit sequence is complemented and the result is the CRC.

    - The CRC bits are mapped into the digest word. The x^31
      coefficient in bit 7 of the lowest numbered byte of the
      digest continuing through to the byte up to the x^24
      coefficient in bit 0 of the lowest numbered byte, continuing
      with the x^23 coefficient in bit 7 of next byte through x^0
      in bit 0 of the highest numbered byte.

    - Computing the CRC over any segment (data or header) extended
      to include the CRC built using the generator 0x11edc6f41
      will always get the value 0x1c2d19ed as its final remainder
      (R(x)). This value is given here in its polynomial form
      (i.e., not mapped as the digest word).

  For a discussion about selection criteria for the CRC, see
  [RFC3385]. For a detailed analysis of the iSCSI polynomial, see
  [Castagnoli93].

  Private or public extension algorithms MAY also be negotiated for
  digests. Whenever a private or public digest extension algorithm
  is part of the default offer (the offer made in absence of
  explicit administrative action) the implementer MUST ensure that
  CRC32C is listed as an alternative in the default offer and "None"
  is not part of the default offer.

  Extension digest algorithms MUST be named using one of the
  following two formats:

        i)     Y-reversed.vendor.dns_name.do_something=
        ii)    New public key with no name prefix constraints




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   Digests named using the Y- format are used for private purposes
   (unregistered). New public keys must be registered with IANA using
   IETF Review process ([RFC5226]). New public extensions for digests
   MUST NOT use the Y# name prefix.

   For private extension digests, to identify the vendor, we suggest
   you use the reversed DNS-name as a prefix to the proper digest
   names.

   The part of digest-name following Y- MUST conform to the format
   for standard-label specified in Section 6.1.

   Support for public or private extension digests is OPTIONAL.

13.2. MaxConnections

   Use: LO
   Senders: Initiator and Target
   Scope: SW
   Irrelevant when: SessionType=Discovery

   MaxConnections=<numerical-value-from-1-to-65535>

   Default is 1.
   Result function is Minimum.

   Initiator and target negotiate the maximum number of connections
   requested/acceptable.

13.3. SendTargets

   Use: FFPO
   Senders: Initiator
   Scope: SW

   For a complete description, see Appendix C.

13.4. TargetName

   Use: IO by initiator, FFPO by target - only as response to a
   SendTargets, Declarative, Any-Stage




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   Senders: Initiator and Target
   Scope: SW

   TargetName=<iSCSI-name-value>

   Examples:

     TargetName=iqn.1993-11.com.disk-vendor:diskarrays.sn.45678

     TargetName=eui.020000023B040506

     TargetName=naa.62004567BA64678D0123456789ABCDEF


   The initiator of the TCP connection MUST provide this key to the
   remote endpoint in the first login request if the initiator is not
   establishing a discovery session. The iSCSI Target Name specifies
   the worldwide unique name of the target.

   The TargetName key may also be returned by the "SendTargets" text
   request (which is its only use when issued by a target).

   TargetName MUST NOT be redeclared within the login phase.

13.5. InitiatorName

   Use: IO, Declarative, Any-Stage
   Senders: Initiator
   Scope: SW

   InitiatorName=<iSCSI-name-value>

   Examples:

     InitiatorName=iqn.1992-04.com.os-vendor.plan9:cdrom.12345

     InitiatorName=iqn.2001-02.com.ssp.users:customer235.host90

     InitiatorName=naa.52004567BA64678D




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   The initiator of the TCP connection MUST provide this key to the
   remote endpoint at the first Login of the Login Phase for every
   connection. The InitiatorName key enables the initiator to
   identify itself to the remote endpoint.

   InitiatorName MUST NOT be redeclared within the login phase.

13.6. TargetAlias

   Use: ALL, Declarative, Any-Stage
   Senders: Target
   Scope: SW

   TargetAlias=<iSCSI-local-name-value>

   Examples:

     TargetAlias=Bob-s Disk

     TargetAlias=Database Server 1 Log Disk

     TargetAlias=Web Server 3 Disk 20


   If a target has been configured with a human-readable name or
   description, this name SHOULD be communicated to the initiator
   during a Login Response PDU if SessionType=Normal (see 13.21).
   This string is not used as an identifier, nor is it meant to be
   used for authentication or authorization decisions. It can be
   displayed by the initiator's user interface in a list of targets
   to which it is connected.

13.7. InitiatorAlias

   Use: ALL, Declarative, Any-Stage
   Senders: Initiator
   Scope: SW

   InitiatorAlias=<iSCSI-local-name-value>

   Examples:




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     InitiatorAlias=Web Server 4

     InitiatorAlias=spyalley.nsa.gov

     InitiatorAlias=Exchange Server


   If an initiator has been configured with a human-readable name or
   description, it SHOULD be communicated to the target during a
   Login Request PDU. If not, the host name can be used instead. This
   string is not used as an identifier, nor is meant to be used for
   authentication or authorization decisions. It can be displayed by
   the target's user interface in a list of initiators to which it is
   connected.

13.8. TargetAddress

   Use: ALL, Declarative, Any-Stage
   Senders: Target
   Scope: SW

   TargetAddress=domainname[:port][,portal-group-tag]

   The domainname can be specified as either a DNS host name, a
   dotted-decimal IPv4 address, or a bracketed IPv6 address as
   specified in [RFC3986].

   If the TCP port is not specified, it is assumed to be the IANA-
   assigned default port for iSCSI (see Section 14).

   If the TargetAddress is returned as the result of a redirect
   status in a login response, the comma and portal group tag MUST be
   omitted.

   If the TargetAddress is returned within a SendTargets response,
   the portal group tag MUST be included.

   Examples:

     TargetAddress=10.0.0.1:5003,1

     TargetAddress=[1080:0:0:0:8:800:200C:417A],65




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     TargetAddress=[1080::8:800:200C:417A]:5003,1

     TargetAddress=computingcenter.example.com,23


   Use of the portal-group-tag is described in Appendix C. The
   formats for the port and portal-group-tag are the same as the one
   specified in TargetPortalGroupTag.

13.9. TargetPortalGroupTag

   Use: IO by target, Declarative, Any-Stage
   Senders: Target
   Scope: SW

   TargetPortalGroupTag=<16-bit-binary-value>

   Examples:
   TargetPortalGroupTag=1

   The target portal group tag is a 16-bit binary-value that uniquely
   identifies a portal group within an iSCSI target node. This key
   carries the value of the tag of the portal group that is servicing
   the Login request. The iSCSI target returns this key to the
   initiator in the Login Response PDU to the first Login Request PDU
   that has the C bit set to 0 when TargetName is given by the
   initiator.

   [SAM2] notes in its informative text that TPGT value should be
   non-zero, note that it is incorrect. A zero value is allowed as a
   legal value for TPGT. This discrepancy currently stands corrected
   in [SAM4].

   For the complete usage expectations of this key see Section 6.3.


13.10. InitialR2T

   Use: LO
   Senders: Initiator and Target
   Scope: SW
   Irrelevant when: SessionType=Discovery




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  InitialR2T=<boolean-value>

  Examples:

     I->InitialR2T=No

     T->InitialR2T=No


  Default is Yes.
  Result function is OR.

  The InitialR2T key is used to turn off the default use of R2T for
  unidirectional and the output part of bidirectional commands, thus
  allowing an initiator to start sending data to a target as if it
  has received an initial R2T with Buffer Offset=Immediate Data
  Length and Desired Data Transfer Length=(min(FirstBurstLength,
  Expected Data Transfer Length) - Received Immediate Data Length).

  The default action is that R2T is required, unless both the
  initiator and the target send this key-pair attribute specifying
  InitialR2T=No. Only the first outgoing data burst (immediate data
  and/or separate PDUs) can be sent unsolicited (i.e., not requiring
  an explicit R2T).

13.11. ImmediateData

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery

  ImmediateData=<boolean-value>

  Default is Yes.
  Result function is AND.

  The initiator and target negotiate support for immediate data. To
  turn immediate data off, the initiator or target must state its
  desire to do so. ImmediateData can be turned on if both the
  initiator and target have ImmediateData=Yes.




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  If ImmediateData is set to Yes and InitialR2T is set to Yes
  (default), then only immediate data are accepted in the first
  burst.

  If ImmediateData is set to No and InitialR2T is set to Yes, then
  the initiator MUST NOT send unsolicited data and the target MUST
  reject unsolicited data with the corresponding response code.

  If ImmediateData is set to No and InitialR2T is set to No, then
  the initiator MUST NOT send unsolicited immediate data, but MAY
  send one unsolicited burst of Data-OUT PDUs.

  If ImmediateData is set to Yes and InitialR2T is set to No, then
  the initiator MAY send unsolicited immediate data and/or one
  unsolicited burst of Data-OUT PDUs.

  The following table is a summary of unsolicited data options:

  +----------+-------------+------------------+--------------+
  |InitialR2T|ImmediateData|     Unsolicited  |Immediate Data|
  |          |             |    Data Out PDUs |              |
  +----------+-------------+------------------+--------------+
  | No       | No          | Yes              | No           |
  +----------+-------------+------------------+--------------+
  | No       | Yes         | Yes              | Yes          |
  +----------+-------------+------------------+--------------+
  | Yes      | No          | No               | No           |
  +----------+-------------+------------------+--------------+
  | Yes      | Yes         | No               | Yes          |
  +----------+-------------+------------------+--------------+


13.12. MaxRecvDataSegmentLength

  Use: ALL, Declarative
  Senders: Initiator and Target
  Scope: CO

  MaxRecvDataSegmentLength=<numerical-value-512-to-(2**24-1)>

  Default is 8192 bytes.




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  The initiator or target declares the maximum data segment length
  in bytes it can receive in an iSCSI PDU.

  The transmitter (initiator or target) is required to send PDUs
  with a data segment that does not exceed MaxRecvDataSegmentLength
  of the receiver.

  A target receiver is additionally limited by MaxBurstLength for
  solicited data and FirstBurstLength for unsolicited data. An
  initiator MUST NOT send solicited PDUs exceeding MaxBurstLength
  nor unsolicited PDUs exceeding FirstBurstLength (or
  FirstBurstLength-Immediate Data Length if immediate data were
  sent).

13.13. MaxBurstLength

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery

  MaxBurstLength=<numerical-value-512-to-(2**24-1)>

  Default is 262144 (256 Kbytes).
  Result function is Minimum.

  The initiator and target negotiate maximum SCSI data payload in
  bytes in a Data-In or a solicited Data-Out iSCSI sequence. A
  sequence consists of one or more consecutive Data-In or Data-Out
  PDUs that end with a Data-In or Data-Out PDU with the F bit set to
  one.

13.14. FirstBurstLength

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery
  Irrelevant when: ( InitialR2T=Yes and ImmediateData=No )

  FirstBurstLength=<numerical-value-512-to-(2**24-1)>




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  Default is 65536 (64 Kbytes).
  Result function is Minimum.

  The initiator and target negotiate the maximum amount in bytes of
  unsolicited data an iSCSI initiator may send to the target during
  the execution of a single SCSI command. This covers the immediate
  data (if any) and the sequence of unsolicited Data-Out PDUs (if
  any) that follow the command.

  FirstBurstLength MUST NOT exceed MaxBurstLength.

13.15. DefaultTime2Wait

  Use: LO
  Senders: Initiator and Target
  Scope: SW

  DefaultTime2Wait=<numerical-value-0-to-3600>

  Default is 2.
  Result function is Maximum.

  The initiator and target negotiate the minimum time, in seconds,
  to wait before attempting an explicit/implicit logout or an active
  task reassignment after an unexpected connection termination or a
  connection reset.

  A value of 0 indicates that logout or active task reassignment can
  be attempted immediately.

13.16. DefaultTime2Retain

  Use: LO
  Senders: Initiator and Target
  Scope: SW

  DefaultTime2Retain=<numerical-value-0-to-3600>

  Default is 20.
  Result function is Minimum.




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  The initiator and target negotiate the maximum time, in seconds
  after an initial wait (Time2Wait), before which an active task
  reassignment is still possible after an unexpected connection
  termination or a connection reset.

  This value is also the session state timeout if the connection in
  question is the last LOGGED_IN connection in the session.

  A value of 0 indicates that connection/task state is immediately
  discarded by the target.

13.17. MaxOutstandingR2T

  Use: LO
  Senders: Initiator and Target
  Scope: SW

  MaxOutstandingR2T=<numerical-value-from-1-to-65535>
  Irrelevant when: SessionType=Discovery

  Default is 1.
  Result function is Minimum.

  Initiator and target negotiate the maximum number of outstanding
  R2Ts per task, excluding any implied initial R2T that might be
  part of that task. An R2T is considered outstanding until the last
  data PDU (with the F bit set to 1) is transferred, or a sequence
  reception timeout (Section 7.1.4.1) is encountered for that data
  sequence.

13.18. DataPDUInOrder

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery

  DataPDUInOrder=<boolean-value>

  Default is Yes.
  Result function is OR.




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  No is used by iSCSI to indicate that the data PDUs within
  sequences can be in any order. Yes is used to indicate that data
  PDUs within sequences have to be at continuously increasing
  addresses and overlays are forbidden.

13.19. DataSequenceInOrder

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery

  DataSequenceInOrder=<boolean-value>

  Default is Yes.
  Result function is OR.

  A Data Sequence is a sequence of Data-In or Data-Out PDUs that end
  with a Data-In or Data-Out PDU with the F bit set to one. A Data-
  out sequence is sent either unsolicited or in response to an R2T.
  Sequences cover an offset-range.

  If DataSequenceInOrder is set to No, Data PDU sequences may be
  transferred in any order.

  If DataSequenceInOrder is set to Yes, Data Sequences MUST be
  transferred using continuously non-decreasing sequence offsets
  (R2T buffer offset for writes, or the smallest SCSI Data-In buffer
  offset within a read data sequence).

  If DataSequenceInOrder is set to Yes, a target may retry at most
  the last R2T, and an initiator may at most request retransmission
  for the last read data sequence. For this reason, if
  ErrorRecoveryLevel is not 0 and DataSequenceInOrder is set to Yes
  then MaxOustandingR2T MUST be set to 1.

13.20. ErrorRecoveryLevel

  Use: LO
  Senders: Initiator and Target
  Scope: SW




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   ErrorRecoveryLevel=<numerical-value-0-to-2>

   Default is 0.
   Result function is Minimum.

   The initiator and target negotiate the recovery level supported.

   Recovery levels represent a combination of recovery capabilities.
   Each recovery level includes all the capabilities of the lower
   recovery levels and adds some new ones to them.

   In the description of recovery mechanisms, certain recovery
   classes are specified. Section 7.1.5 describes the mapping between
   the classes and the levels.

13.21. SessionType

   Use: LO, Declarative, Any-Stage
   Senders: Initiator
   Scope: SW

   SessionType= <Discovery|Normal>

   Default is Normal.

   The Initiator indicates the type of session it wants to create.
   The target can either accept it or reject it.

   A Discovery session indicates to the Target that the only purpose
   of this Session is discovery. The only requests a target accepts
   in this type of session are a text request with a SendTargets key
   and a logout request with reason "close the session".

   The Discovery session implies MaxConnections = 1 and overrides
   both the default and an explicit setting. As Section 7.4.1
   states, ErrorRecoveryLevel MUST be 0 (zero) for Discovery
   sessions.

   Depending on the type of the session, a target may decide on
   resources to allocate and the security to enforce, etc. for thion.
   If the SessionType key is thus going to be offered as "Discovery",




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  it SHOULD be offered in the initial Login request by the
  initiator.

13.22. The Private Extension Key Format

  Use: ALL
  Senders: Initiator and Target
  Scope: specific key dependent

  X-reversed.vendor.dns_name.do_something=

  Keys with this format are used for private extension purposes.
  These keys always start with X- if unregistered with IANA
  (private). New public keys (if registered with IANA via an IETF
  Review, [RFC5226]) no longer have an X# name prefix requirement,
  implementers may propose any intuitive unique name.

  For unregistered keys, to identify the vendor, we suggest you use
  the reversed DNS-name as a prefix to the key-proper.

  The part of key-name following X- MUST conform to the format for
  key-name specified in Section 6.1.

  Vendor specific keys MUST ONLY be used in normal sessions.

  Support for public or private extension keys is OPTIONAL.

13.23. TaskReporting

  Use: LO
  Senders: Initiator and Target
  Scope: SW
  Irrelevant when: SessionType=Discovery
  TaskReporting=<list-of-values>

  Default is RFC3720.
  Result function is AND.

  This key is used to negotiate the task completion reporting
  semantics from the SCSI target. The following table describes the
  semantics that an iSCSI target MUST support for respective
  negotiated key values. Whenever this key is negotiated, at least




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  the RFC3720 and ResponseFence values MUST be offered as options by
  the negotiation originator.
  +--------------+------------------------------------------+
  | Name         |             Description                  |
  +--------------+------------------------------------------+
  | RFC3720      | RFC 3720-compliant semantics. Response   |
  |              | fencing is not guaranteed and fast       |
  |              | completion of multi-task aborting is not |
  |              | supported                                |
  +--------------+------------------------------------------+
  | ResponseFence| Response Fence (Section 4.2.2.3.3)       |
  |              | semantics MUST be supported in reporting |
  |              | task completions                         |
  +--------------+------------------------------------------+
  | FastAbort    | Updated fast multi-task abort semantics |
  |              | defined in Section 4.2.3.4 MUST be       |
  |              | supported. Support for Response Fence is |
  |              | implied -- i.e., (Section 4.2.2.3.3)     |
  |              | semantics MUST be supported as well      |
  +--------------+------------------------------------------+
  When TaskReporting is not negotiated to FastAbort, the standard
  multi-task abort semantics in Section 4.2.3.3 MUST be used.

13.24. iSCSIProtocolLevel Negotiation

  The iSCSIProtocolLevel associated with this document is "1". As a
  responder or an originator in a negotiation of this key, an iSCSI
  implementation compliant to this document alone, without any
  future protocol extensions, MUST use this value as defined by the
  [iSCSI-SAM] document.

13.25. Obsoleted Keys

  This document obsoletes the following keys defined in [RFC3720]:
  IFMarker, OFMarker, OFMarkInt, IFMarkInt. However, iSCSI
  implementations compliant to this document may still receive these
  obsoleted keys - i.e. in a responder role - in a text negotiation.

  When IFMarker or OFMarker key is received, a compliant iSCSI
  implementation SHOULD respond with the constant "Reject" value.
  The implementation MAY alternatively respond with a "No" value.




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   However, the implementation MUST NOT respond with a
   "NotUnderstood" value for either of these keys.

   When IFMarkInt or OFMarkInt key is received, a compliant iSCSI
   implementation MUST respond with the constant "Reject" value.
   The implementation MUST NOT respond with a "NotUnderstood" value
   for either of these keys.

13.26. X#NodeArchitecture

13.26.1. Definition

   Use: LO, Declarative
   Senders: Initiator and Target
   Scope: SW

   X#NodeArchitecture=<list-of-values>

   Default is none.

   Examples:
   X#NodeArchitecture=ExampleOS/v1234,ExampleInc_SW_Initiator/1.05a
   X#NodeArchitecture=ExampleInc_HW_Initiator/4010,Firmware/2.0.0.5
   X#NodeArchitecture=ExampleInc_SW_Initiator/2.1,CPU_Arch/i686

   This document does not define the structure or content of the list
   of values.

   The initiator or target declares the details of its iSCSI node
   architecture to the remote endpoint. These details may include,
   but are not limited to, iSCSI vendor software, firmware, or
   hardware versions, the OS version, or hardware architecture. This
   key may be declared on a Discovery session or a Normal session.

   The length of the key value (total length of the list-of-values)
   MUST NOT be greater than 255 bytes.

   X#NodeArchitecture MUST NOT be redeclared during the login phase.




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13.26.2. Implementation Requirements

  Functional behavior of the iSCSI node (this includes the iSCSI
  protocol logic -- the SCSI, iSCSI, and TCP/IP protocols) MUST NOT
  depend on the presence, absence, or content of the
  X#NodeArchitecture key. The key MUST NOT be used by iSCSI nodes
  for interoperability, or exclusion of other nodes. To ensure
  proper use, key values SHOULD be set by the node itself, and there
  SHOULD NOT be provisions for the key values to contain user-
  defined text.

  Nodes implementing this key MUST choose one of the following
  implementation options:

     - only transmit the key,

     - only log the key values received from other nodes, or

     - both transmit and log the key values.


  Each node choosing to implement transmission of the key values
  MUST be prepared to handle the response of iSCSI Nodes that do not
  understand the key.

  Nodes that implement transmission and/or logging of the key values
  may also implement administrative mechanisms that disable and/or
  change the logging and key transmission detail (see Section 9.4).
  Thus, a valid behavior for this key may be that a node is
  completely silent (the node does not transmit any key value, and
  simply discards any key values it receives without issuing a
  NotUnderstood response).




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14. Rationale for revised IANA Considerations

  This document makes rather significant changes in this area, and
  this Section outlines the reasons behind the changes. As
  previously specified in [RFC3720], iSCSI had used text string
  prefixes, such as X- and X#, to distinguish extended login/text
  keys, digest algorithms and authentication methods from their
  standardized counterparts. Based on experience with other
  protocols, [RFC6648] however strongly recommends against this
  practice, in large part because extensions that use such prefixes
  may become standard over time at which point it can be infeasible
  to change their text string names due to widespread usage under
  the existing text string name.

  iSCSI's experience with public extensions supports the
  recommendations in [RFC6648], as the only extension item ever
  registered with IANA, the X#NodeArchitecture key, was specified as
  a standard key in a standards-track RFC [RFC4850], and hence did
  not require the X# prefix. In addition, that key is the only
  public iSCSI extension that has been registered with IANA since
  RFC 3720 was originally published, so there has been effectively
  no use of the X#, Y# and Z# public extension formats.

  Therefore, this document makes the following changes to the IANA
  registration procedures for iSCSI:

     (1) The separate registries for X#, Y# and Z# public
       extensions are removed. The single entry in the registry for
       X# login/text keys(X#NodeArchitecture) is transferred to the
       main login/text key registry. IANA has never created the
       latter two registries because there have been no
       registration requests for them. These public extension
       formats (X#, Y#, Z#) MUST NOT be used with the exception of
       the existing X#NodeArchitecture key.

     (2) The Registration Procedures for the main login/text key,
       digest algorithm and authentication method IANA registries
       are changed to IETF Review [RFC5226] for possible future
       extensions to iSCSI. This change includes a deliberate
       decision to remove the possibility of specifying an IANA-
       registered iSCSI extension in an RFC published via an RFC




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      Editor independent submission, as the level of review in
      that process is insufficient for iSCSI extensions.

    (3) The restriction against registering items using the
      private extension formats (X-, Y-, Z-) in the main IANA
      registries is removed. Extensions using these formats MAY be
      registered under the IETF Review registration procedures,
      but each format is restricted to the type of extension for
      which it is specified in this RFC and MUST NOT be used for
      other types. For example, the X- extension format for
      extension login/text keys MUST NOT be used for digest
      algorithms or authentication methods.




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15. IANA Considerations

  The well-known TCP port number for iSCSI connections assigned by
  IANA is 3260 and this is the default iSCSI port. Implementations
  needing a system TCP port number may use port 860, the port
  assigned by IANA as the iSCSI system port; however in order to use
  port 860, it MUST be explicitly specified - implementations MUST
  NOT default to use of port 860, as 3260 is the only allowed
  default.

  IANA is requested to update all references to RFC 3720, RFC 4850
  and RFC 5048 to instead reference this RFC in all of the iSCSI
  registries that are part of the "Internet Small Computer System
  Interface (iSCSI) Parameters" set of registries. This change
  reflects the fact that those three RFCs are obsoleted by this RFC.
  References to other RFCs that are not being obsoleted (e.g., RFC
  3723, RFC 5046) should not be changed.

  IANA is requested to perform the following actions on the iSCSI
  Login/Text Keys registry:

     - Change the registration procedure to IETF Review from
       Standard Required.

     - Change the RFC 5048 reference for the registry to reference
       this RFC.

     - Add the X#NodeArchitecture Key from the iSCSI extended key
       registry and change its reference to this RFC.

     - Change all references of RFC 3720 and RFC 5048 to reference
       this RFC.

  IANA is requested to change the Registration Procedures for the
  iSCSI authentication methods and iSCSI digests registries to IETF
  Review from RFC Required.

  IANA is requested to remove the iSCSI extended key registry, as
  its one entry is to be added to the iSCSI login/text keys
  registry.




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  IANA is requested to mark obsolete the values 4 and 5, for SPKM1
  and SPKM2 respectively, in the iSCSI authentication methods
  subregistry of the Internet Small Computer System Interface
  (iSCSI) Parameters registries.

  All the other IANA considerations stated in [RFC3720] and
  [RFC5048] remain unchanged.

  This document obsoletes the SPKM1 and SPKM2 key values for the
  AuthMethod text key. Consequently, the SPKM_ text key prefix MUST
  be treated as obsolete and be not reused.

References

  Normative References


    [EUI] "Guidelines for 64-bit Global Identifier (EUI-64)",
      http://standards.ieee.org/regauth/oui/tutorials/EUI64.html

    [FC-FS3] INCITS 470-2011, Fibre Channel - Framing and
      Signaling - 3 (FC-FS-3).

    [IPSEC-IPS] Black, D., Koning, P., "IP Storage: IPsec
      Requirements Update for IPsec v3", draft-ietf-storm-ipsec-
      ips-update-01.txt (work in progress), June 2013

    [iSCSI-SAM] Knight, F., Chadalapaka, M., "Internet Small
      Computer Systems Interface (iSCSI) SCSI Architecture
      Features Update", draft-ietf-storm-iscsi-sam-06.txt (work in
      progress), July 2012

    [OUI] "IEEE OUI and Company_Id Assignments",
      http://standards.ieee.org/regauth/oui

    [RFC1122] R.Braden, "Requirements for Internet Hosts --
      Communication Layers", October 1989.

    [RFC1964] J. Linn, "The Kerberos Version 5 GSS-API Mechanism",
      June 1996.

    [RFC1982] Elz, R., Bush, R., "Serial Number Arithmetic",
      August 1996.




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    [RFC1994] W. Simpson, "PPP Challenge Handshake Authentication
      Protocol (CHAP)", August 1996.

    [RFC2119] Bradner, S. "Key Words for use in RFCs to Indicate
      Requirement Levels", BCP 14, March 1997.

    [RFC2404] C. Madson, R. Glenn, "The Use of HMAC-SHA-1-96
      within ESP and AH", November 1998.

    [RFC2451] R. Pereira, R. Adams "The ESP CBC-Mode Cipher
      Algorithms".

    [RFC2945] Wu, T., "The SRP Authentication and Key Exchange
      System", September 2000.

    [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
      Internationalized Strings ("stringprep")", RFC 3454,
      December 2002.

    [RFC3566] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96
      Algorithm and Its Use With IPsec", RFC 3566, September 2003.

    [RFC3629] Yergeau, F., "UTF-8, a Transformation Format of ISO
      10646", RFC 3629, November 2003

    [RFC3686] Housley, R., "Using Advanced Encryption Standard
      (AES) Counter Mode with IPsec Encapsulating Security Payload
      (ESP)", RFC 3686, January 2004.

      [RFC3722] Bakke, M., "String Profile for Internet Small
       Computer Systems Interface (iSCSI) Names", RFC 3722, March
       2004.

    [RFC3723] Aboba, B., Tseng, J., Walker, J., Rangan, V. and F.
      Travostino, "Securing Block Storage Protocols over IP", RFC
      3723, March 2004.

      [RFC3986] T. Berners-Lee, R. Fielding, L. Masinter "Uniform
       Resource Identifier (URI): Generic Syntax", January 2005.

    [RFC4106] J. Viega, D. McGrew, "The Use of Galois/Counter Mode
      (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC
      4106, June 2005.




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                     iSCSI (Consolidated)                6/23/13


    [RFC4120] Neuman, C., Yu, T., Hartman, S., Raeburn, K, "The
      Kerberos Network Authentication Service (V5)", RFC 4120,
      July 2005.

    [RFC4171] J. Tseng, K. Gibbons, F. Travostino, C. Du Laney, J.
      Souza, "Internet Storage Name Service (iSNS)", September
      2005.

    [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
      Architecture", February 2006.

    [RFC4301] S. Kent, K.Seo, "Security Architecture for the
    Internet Protocol", December 2005.

    [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
      RFC 4303, December 2005

    [RFC4543] D. McGrew, J. Viega, "The Use of Galois Message
      Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
      May 2006

    [RFC4648] S. Josefsson, "The Base16, Base32, and Base64 Data
      Encodings", RFC 4648, October 2006

    [RFC5226] H. Alvestrand, T. Narten, "Guidelines for Writing an
      IANA Considerations Section in RFCs", RFC 5226, May 2008

    [RFC5996] C. Kaufman, P. Hoffman, Y. Nir, P. Eronen, "Internet
      Key Exchange Protocol Version 2 (IKEv2) ", RFC 5996,
      September 2010.

    [SAM2] T10/1157-D, SCSI Architecture Model - 2 (SAM-2).

    [SAM4] T10/1683-D, SCSI Architecture Model - 4 (SAM-4).

    [SPC2] T10/1236-D, SCSI Primary Commands-2.

    [SPC3] T10/1416-D, SCSI Primary Commands-3.

    [UML]   ISO/IEC 19501, Unified Modeling Language
      Specification Version 1.4.2.




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    [UNICODE] Unicode Standard Annex #15, "Unicode Normalization
      Forms", http://www.unicode.org/unicode/reports/tr15


  Informative References

    [FC-SP-2] T11/1835-D, Fibre Channel Security Protocols- 2 (FC-
      SP-2).

    [RFC1737] K. Sollins, L. Masinter "Functional Requirements for
      Uniform Resource Names".

    [RFC5433] T. Clancy, H. Tschofenig "Extensible Authentication
      Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method",
      RFC 5433, February 2009.

    [IB] InfiniBand{tm} Architecture Specification, Vol. 1,
      Rel.1.0.a, InfiniBand Trade
      Association(http://www.infinibandta.org).

    [Castagnoli93] G. Castagnoli, S. Braeuer and M. Herrman
      "Optimization of Cyclic Redundancy-Check Codes with 24 and
      32 Parity Bits", IEEE Transact. on Communications, Vol. 41,
      No. 6, June 1993.

    [CRC] ISO 3309, High-Level Data Link Control (CRC 32).

    [RFC2401] S. Kent, R. Atkinson, "Security Architecture for the
      Internet Protocol ", November 1998.

    [RFC2406] S. Kent, R. Atkinson, "IP Encapsulating Security
      Payload (ESP)", November 1998.

    [RFC2407] D. Piper, "The Internet IP Security Domain of
      Interpretation for ISAKMP", November 1998.

    [RFC2409] D. Harkins, D. Carrel, "The Internet Key Exchange
      (IKE)", November 1998.

    [RFC2608] E. Guttman, C. Perkins, J. Veizades, M. Day,
      "Service Location Protocol, Version 2", June 1999.

    [RFC2743] J.Linn, "Generic Security Service Application Program
       Interface Version 2, Update 1", January 2000.

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                     iSCSI (Consolidated)                6/23/13


    [RFC2865] C. Rigney, S. Willens, A. Rubens, W. Simpson,
      "Remote Authentication Dial In User Service (RADIUS)", June
      2000.

    [RFC3385] Sheinwald, D., Satran, J., Thaler, P. and V.
      Cavanna, "Internet Protocol Small Computer System Interface
      (iSCSI) Cyclic Redundancy Check (CRC)/Checksum
      Considerations", RFC 3385, September 2002.

    [RFC3602] S. Frankel, R. Glenn, S. Kelly, "The AES-CBC Cipher
      Algorithm and Its Use with IPsec", RFC 3602, September 2003.

    [RFC3720] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka,
      M., and E. Zeidner, "Internet Small Computer Systems
      Interface (iSCSI)", RFC 3720, April 2004.

    [RFC3721] Bakke, M., Hafner, J., Hufferd, J., Voruganti, K.
      and M. Krueger, "Internet Small Computer Systems Interface
      (iSCSI) Naming and Discovery", RFC 3721, March 2004

    [RFC3783] M. Chadalapaka, R. Elliott, "Small Computer Systems
      Interface (SCSI) Command Ordering Considerations with
      iSCSI", RFC 3783, May 2004.

    [RFC4121] L. Zhu, K. Jaganathan, S. Hartman, "The Kerberos
      Version 5 Generic Security Service Application Program
      Interface (GSS-API) Mechanism: Version 2", RFC 4121, July
      2005.

    [RFC4297] Romanow, A., Mogul, J., Talpey, T., and S. Bailey,
      "Remote Direct Memory Access (RDMA) over IP Problem
      Statement", RFC 4297, October 2004

    [RFC4850] Wysochanski, D., "Declarative Public Extension Key
      for Internet Small Computer Systems Interface (iSCSI) Node
      Architecture", RFC 4850, April 2007.

    [RFC5046] Ko, M., Chadalapaka, M., Hufferd, J., Elzur, U.,
      Shah, H., and P. Thaler, "Internet Small Computer System
      Interface (iSCSI) Extensions for Remote Direct Memory Access
      (RDMA)", RFC 5046, October 2007




Chadalapaka et al.       Expires December 31, 2013       [Page 294]

                     iSCSI (Consolidated)                6/23/13


    [RFC5048] Chadalapaka, M., "Internet Small Computer Systems
      Interface (iSCSI) Corrections and Clarifications", RFC 5048,
      October 2007.

    [RFC6648] P. Saint-Andre, D. Crocker, M. Nottingham,
      "Deprecating the X- Prefix and Similar Constructs in
      Application Protocols", RFC 6648, June 2012

    [SAS] T10/2125-D, Serial Attached SCSI - 2.1 (SAS-2.1);
      T10/2124-D, SAS Protocol Layer (SPL); T10/2124-M, SAS
      Protocol Layer (SPL) Amendment #1 (SPL AM1).

    [SBC2] NCITS.405-205, SCSI Block Commands - 2 (SBC-2).

    [SPC4] T10/1731-D, SCSI Primary Commands-4.




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Appendix A.    Examples

Read Operation Example

  +------------------+-----------------------+---------------------+
  |Initiator Function|    PDU Type           | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command (READ)>>> |                      |
  | (read)           |                       |                     |
  +------------------+-----------------------+---------------------+
  |                  |                       |Prepare Data Transfer|
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  +------------------+-----------------------+---------------------+
  |                  |   <<< SCSI Response   |Send Status and Sense|
  +------------------+-----------------------+---------------------+
  | Command Complete |                       |                     |
  +------------------+-----------------------+---------------------+




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Write Operation Example

  +------------------+-----------------------+---------------------+
  |Initiator Function|    PDU Type           | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command (WRITE)>>>| Receive command      |
  | (write)          |                       | and queue it        |
  +------------------+-----------------------+---------------------+
  |                  |                       | Process old commands|
  +------------------+-----------------------+---------------------+
  |                  |                       | Ready to process    |
  |                  |   <<< R2T             | WRITE command       |
  +------------------+-----------------------+---------------------+
  |   Send Data      |   SCSI Data-out >>>   |   Receive Data      |
  +------------------+-----------------------+---------------------+
  |                  |   <<< R2T             | Ready for data      |
  +------------------+-----------------------+---------------------+
  |                  |   <<< R2T             | Ready for data      |
  +------------------+-----------------------+---------------------+
  |   Send Data      |   SCSI Data-out >>>   |   Receive Data      |
  +------------------+-----------------------+---------------------+
  |   Send Data      |   SCSI Data-out >>>   |   Receive Data      |
  +------------------+-----------------------+---------------------+
  |                  |   <<< SCSI Response   |Send Status and Sense|
  +------------------+-----------------------+---------------------+
  | Command Complete |                       |                     |
  +------------------+-----------------------+---------------------+

R2TSN/DataSN Use Examples

  Output (write) data DataSN/R2TSN Example




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  +------------------+-----------------------+---------------------+
  |Initiator Function|    PDU Type & Content | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command (WRITE)>>>| Receive command      |
  | (write)          |                       | and queue it        |
  +------------------+-----------------------+---------------------+
  |                  |                       | Process old commands|
  +------------------+-----------------------+---------------------+
  |                  |   <<< R2T             | Ready for data      |
  |                  |   R2TSN = 0           |                     |
  +------------------+-----------------------+---------------------+
  |                  |   <<< R2T             | Ready for more data |
  |                  |   R2TSN = 1           |                     |
  +------------------+-----------------------+---------------------+
  | Send Data        |   SCSI Data-out >>>   |   Receive Data      |
  | for R2TSN 0      |   DataSN = 0, F=0     |                     |
  +------------------+-----------------------+---------------------+
  | Send Data        |   SCSI Data-out >>>   |   Receive Data      |
  | for R2TSN 0      |   DataSN = 1, F=1     |                     |
  +------------------+-----------------------+---------------------+
  | Send Data        |   SCSI Data >>>       |   Receive Data      |
  | for R2TSN 1      |   DataSN = 0, F=1     |                     |
  +------------------+-----------------------+---------------------+
  |                  |   <<< SCSI Response   |Send Status and Sense|
  |                  |   ExpDataSN = 0       |                     |
  +------------------+-----------------------+---------------------+
  | Command Complete |                       |                     |
  +------------------+-----------------------+---------------------+



   Input (read) data DataSN Example




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  +------------------+-----------------------+---------------------+
  |Initiator Function|    PDU Type           | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command (READ)>>> |                      |
  | (read)           |                       |                     |
  +------------------+-----------------------+---------------------+
  |                  |                       |Prepare Data Transfer|
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  |                  |   DataSN = 0, F=0     |                     |
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  |                  |   DataSN = 1, F=0     |                     |
  +------------------+-----------------------+---------------------+
  |   Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  |                  |   DataSN = 2, F=1     |                     |
  +------------------+-----------------------+---------------------+
  |                  |   <<< SCSI Response   |Send Status and Sense|
  |                  |   ExpDataSN = 3       |                     |
  +------------------+-----------------------+---------------------+
  | Command Complete |                       |                     |
  +------------------+-----------------------+---------------------+


   Bidirectional DataSN Example




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  +------------------+-----------------------+---------------------+
  |Initiator Function|    PDU Type           | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command >>>        |                     |
  | (Read-Write)     | Read-Write            |                     |
  +------------------+-----------------------+---------------------+
  |                  |                       | Process old commands|
  +------------------+-----------------------+---------------------+
  |                  |   <<< R2T             | Ready to process    |
  |                  |   R2TSN = 0           | WRITE command       |
  +------------------+-----------------------+---------------------+
  | * Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  |                  |   DataSN = 0, F=0     |                     |
  +------------------+-----------------------+---------------------+
  | * Receive Data   |   <<< SCSI Data-in    |   Send Data         |
  |                  |   DataSN = 1, F=1     |                     |
  +------------------+-----------------------+---------------------+
  | * Send Data      |   SCSI Data-out >>>   |   Receive Data      |
  | for R2TSN 0      |   DataSN = 0, F=1     |                     |
  +------------------+-----------------------+---------------------+
  |                  |   <<< SCSI Response   |Send Status and Sense|
  |                  |   ExpDataSN = 2       |                     |
  +------------------+-----------------------+---------------------+
  | Command Complete |                       |                     |
  +------------------+-----------------------+---------------------+

  *) Send data and Receive Data may be transferred simultaneously as
  in an atomic Read-Old-Write-New or sequentially as in an atomic
  Read-Update-Write (in the latter case the R2T may follow the
  received data).

  Unsolicited and immediate output (write) data with DataSN Example




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  +------------------+-----------------------+---------------------+
  |Initiator Function|     PDU Type & Content | Target Function     |
  +------------------+-----------------------+---------------------+
  | Command request |SCSI Command (WRITE)>>>| Receive command       |
  | (write)          |F=0                     | and data            |
  |+ immediate data |                         | and queue it        |
  +------------------+-----------------------+---------------------+
  | Send Unsolicited |    SCSI Write Data >>> | Receive more Data   |
  | Data             |    DataSN = 0, F=1     |                     |
  +------------------+-----------------------+---------------------+
  |                  |                        | Process old commands|
  +------------------+-----------------------+---------------------+
  |                  |    <<< R2T             | Ready for more data |
  |                  |    R2TSN = 0           |                     |
  +------------------+-----------------------+---------------------+
  | Send Data        |    SCSI Write Data >>> |   Receive Data      |
  | for R2TSN 0      |    DataSN = 0, F=1     |                     |
  +------------------+-----------------------+---------------------+
  |                  |    <<< SCSI Response   |Send Status and Sense|
  |                  |                        |                     |
  +------------------+-----------------------+---------------------+
  | Command Complete |                        |                     |
  +------------------+-----------------------+---------------------+

CRC Examples

  N.B. all Values are Hexadecimal

  32 bytes of zeroes:

    Byte:        0    1   2    3

       0:       00 00 00 00
     ...
      28:       00 00 00 00

     CRC:       aa 36 91 8a

  32 bytes of ones:

    Byte:        0    1   2    3




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       0:       ff ff ff ff
     ...
      28:       ff ff ff ff

     CRC:       43 ab a8 62

  32 bytes of incrementing 00..1f:

    Byte:        0    1    2    3

       0:       00 01 02 03
     ...
      28:       1c 1d 1e 1f

     CRC:       4e 79 dd 46

  32 bytes of decrementing 1f..00:

    Byte:        0    1    2    3

       0:       1f 1e 1d 1c
     ...
      28:       03 02 01 00

     CRC:       5c db 3f 11

  An iSCSI - SCSI Read (10) Command PDU

   Byte:        0    1    2    3

      0:       01    c0   00   00
      4:       00    00   00   00
      8:       00    00   00   00
     12:       00    00   00   00
     16:       14    00   00   00
     20:       00    00   04   00
     24:       00    00   00   14
     28:       00    00   00   18
     32:       28    00   00   00
     36:       00    00   00   00
     40:       02    00   00   00




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     44:       00 00 00 00

    CRC:       56 3a 96 d9

Appendix B.    Login Phase Examples

  In the first example, the initiator and target authenticate each
  other via Kerberos:

    I-> Login (CSG,NSG=0,1 T=1)
      InitiatorName=iqn.1999-07.com.os:hostid.77
      TargetName=iqn.1999-07.com.example:diskarray.sn.88
      AuthMethod=KRB5,SRP,None

    T-> Login (CSG,NSG=0,0 T=0)
      AuthMethod=KRB5


    I-> Login (CSG,NSG=0,1 T=1)
      KRB_AP_REQ=<krb_ap_req>

  (krb_ap_req contains the Kerberos V5 ticket and authenticator with
  MUTUAL-REQUIRED set in the ap-options field)

  If the authentication is successful, the target proceeds with:

    T-> Login (CSG,NSG=0,1 T=1)
      KRB_AP_REP=<krb_ap_rep>

  (krb_ap_rep is the Kerberos V5 mutual authentication reply)

  If the authentication is successful, the initiator may proceed
  with:

    I-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=8192
    T-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=4096
  MaxBurstLength=8192
    I-> Login (CSG,NSG=1,0 T=0) MaxBurstLength=8192
      ... more iSCSI Operational Parameters

    T-> Login (CSG,NSG=1,0 T=0)
      ... more iSCSI Operational Parameters




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    And at the end:

    I-> Login (CSG,NSG=1,3 T=1)
      optional iSCSI parameters

    T-> Login (CSG,NSG=1,3 T=1) "login accept"

  If the initiator's authentication by the target is not successful,
  the target responds with:

    T-> Login "login reject"

  instead of the Login KRB_AP_REP message, and terminates the
  connection.

  If the target's authentication by the initiator is not successful,
  the initiator terminates the connection (without responding to the
  Login KRB_AP_REP message).

  In the next example only the initiator is authenticated by the
  target via Kerberos:

    I-> Login (CSG,NSG=0,1 T=1)
      InitiatorName=iqn.1999-07.com.os:hostid.77
      TargetName=iqn.1999-07.com.example:diskarray.sn.88
      AuthMethod=SRP,KRB5,None

    T-> Login-PR (CSG,NSG=0,0 T=0)
      AuthMethod=KRB5

    I-> Login (CSG,NSG=0,1 T=1)
      KRB_AP_REQ=krb_ap_req

  (MUTUAL-REQUIRED not set in the ap-options field of krb_ap_req)

  If the authentication is successful, the target proceeds with:

    T-> Login (CSG,NSG=0,1 T=1)

    I-> Login (CSG,NSG=1,0 T=0)




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      ... iSCSI parameters

    T-> Login (CSG,NSG=1,0 T=0)
      ... iSCSI parameters

  . . .

    T-> Login (CSG,NSG=1,3 T=1)"login accept"


  In the next example, the initiator and target authenticate each
  other via SRP:

    I-> Login (CSG,NSG=0,1 T=1)
      InitiatorName=iqn.1999-07.com.os:hostid.77
      TargetName=iqn.1999-07.com.example:diskarray.sn.88
      AuthMethod=KRB5,SRP,None

    T-> Login-PR (CSG,NSG=0,0 T=0)
      AuthMethod=SRP

    I-> Login (CSG,NSG=0,0 T=0)
      SRP_U=<user>
      TargetAuth=Yes

    T-> Login (CSG,NSG=0,0 T=0)
      SRP_N=<N>
      SRP_g=<g>
      SRP_s=<s>

    I-> Login (CSG,NSG=0,0 T=0)
      SRP_A=<A>

    T-> Login (CSG,NSG=0,0 T=0)
      SRP_B=<B>

    I-> Login (CSG,NSG=0,1 T=1)
      SRP_M=<M>

  If the initiator authentication is successful, the target
  proceeds:




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    T-> Login (CSG,NSG=0,1 T=1)
      SRP_HM=<H(A | M | K)>

     Where N, g, s, A, B, M, and H(A | M | K) are defined in
  [RFC2945].

  If the target authentication is not successful, the initiator
  terminates the connection; otherwise, it proceeds.

    I-> Login (CSG,NSG=1,0 T=0)
      ... iSCSI parameters

    T-> Login (CSG,NSG=1,0 T=0)
      ... iSCSI parameters


    And at the end:



    I-> Login (CSG,NSG=1,3 T=1)
      optional iSCSI parameters

    T-> Login   (CSG,NSG=1,3 T=1) "login accept"

  If the initiator authentication is not successful, the target
  responds with:

    T-> Login "login reject"

  Instead of the T-> Login SRP_HM=<H(A | M | K)>      message and
  terminates the connection.

  In the next example, only the initiator is authenticated by the
  target via SRP:

    I-> Login (CSG,NSG=0,1 T=1)
      InitiatorName=iqn.1999-07.com.os:hostid.77
      TargetName=iqn.1999-07.com.example:diskarray.sn.88
      AuthMethod=KRB5,SRP,None




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    T-> Login-PR (CSG,NSG=0,0 T=0)
      AuthMethod=SRP

    I-> Login (CSG,NSG=0,0 T=0)
      SRP_U=<user>
      TargetAuth=No


     T-> Login (CSG,NSG=0,0 T=0)
         SRP_N=<N>
         SRP_g=<g>
         SRP_s=<s>

     I-> Login (CSG,NSG=0,0 T=0)
      SRP_A=<A>


      T-> Login (CSG,NSG=0,0 T=0)
         SRP_B=<B>

      I-> Login (CSG,NSG=0,1 T=1)
         SRP_M=<M>


    If the initiator authentication is successful, the target
      proceeds:



    T-> Login (CSG,NSG=0,1 T=1)



    I-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    T-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters




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    And at the end:

    I-> Login (CSG,NSG=1,3 T=1)

         optional iSCSI parameters



    T-> Login (CSG,NSG=1,3 T=1) "login accept"

  In the next example the initiator and target authenticate each
  other via CHAP:

    I-> Login (CSG,NSG=0,0 T=0)

         InitiatorName=iqn.1999-07.com.os:hostid.77

         TargetName=iqn.1999-07.com.example:diskarray.sn.88

         AuthMethod=KRB5,CHAP,None



    T-> Login-PR (CSG,NSG=0,0 T=0)

         AuthMethod=CHAP



    I-> Login (CSG,NSG=0,0 T=0)

         CHAP_A=<A1,A2>



     T-> Login (CSG,NSG=0,0 T=0)
         CHAP_A=<A1>
         CHAP_I=<I>
         CHAP_C=<C>

    I-> Login (CSG,NSG=0,1 T=1)




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         CHAP_N=<N>

         CHAP_R=<R>

         CHAP_I=<I>

         CHAP_C=<C>


    If the initiator authentication is successful, the target
      proceeds:



    T-> Login (CSG,NSG=0,1 T=1)

         CHAP_N=<N>

         CHAP_R=<R>



    If the target authentication is not successful, the initiator
      aborts the connection; otherwise, it proceeds.



    I-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters

    T-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    And at the end:



    I-> Login (CSG,NSG=1,3 T=1)

         optional iSCSI parameters




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    T-> Login (CSG,NSG=1,3 T=1) "login accept"


    If the initiator authentication is not successful, the target
      responds with:



    T-> Login "login reject"



    Instead of the Login CHAP_R=<response> "proceed and change
      stage" message and terminates the connection.



  In the next example, only the initiator is authenticated by the
  target via CHAP:

    I-> Login (CSG,NSG=0,1 T=0)

         InitiatorName=iqn.1999-07.com.os:hostid.77

         TargetName=iqn.1999-07.com.example:diskarray.sn.88

         AuthMethod=KRB5,CHAP,None



    T-> Login-PR (CSG,NSG=0,0 T=0)

         AuthMethod=CHAP



    I-> Login (CSG,NSG=0,0 T=0)

         CHAP_A=<A1,A2>




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     T-> Login (CSG,NSG=0,0 T=0)
         CHAP_A=<A1>
         CHAP_I=<I>
         CHAP_C=<C>

    I-> Login (CSG,NSG=0,1 T=1)

         CHAP_N=<N>

         CHAP_R=<R>



    If the initiator authentication is successful, the target
      proceeds:



    T-> Login (CSG,NSG=0,1 T=1)



    I-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    T-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    And at the end:



    I-> Login (CSG,NSG=1,3 T=1)

         optional iSCSI parameters




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    T-> Login (CSG,NSG=1,3 T=1) "login accept"



  In the next example, the initiator does not offer any security
  parameters. It therefore may offer iSCSI parameters on the Login
  PDU with the T bit set to 1, and the target may respond with a
  final Login Response PDU immediately:

    I-> Login (CSG,NSG=1,3 T=1)

         InitiatorName=iqn.1999-07.com.os:hostid.77

         TargetName=iqn.1999-07.com.example:diskarray.sn.88

         ... iSCSI parameters



    T-> Login (CSG,NSG=1,3 T=1) "login accept"

         ... ISCSI parameters



    In the next example, the initiator does offer security
      parameters on the Login PDU, but the target does not choose
      any (i.e., chooses the "None" values):


    I-> Login (CSG,NSG=0,1 T=1)

         InitiatorName=iqn.1999-07.com.os:hostid.77

         TargetName=iqn.1999-07.com.example:diskarray.sn.88

         AuthMethod=KRB5,SRP,None



    T-> Login-PR (CSG,NSG=0,1 T=1)

         AuthMethod=None




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    I-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    T-> Login (CSG,NSG=1,0 T=0)

         ... iSCSI parameters



    And at the end:



    I-> Login (CSG,NSG=1,3 T=1)

         optional iSCSI parameters



    T-> Login (CSG,NSG=1,3 T=1) "login accept"




Appendix C.    SendTargets Operation

  The text in this Appendix is a normative part of this document.

  To reduce the amount of configuration required on an initiator,
  iSCSI provides the SendTargets text request. The initiator uses
  the SendTargets request to get a list of targets to which it may
  have access, as well as the list of addresses (IP address and TCP
  port) on which these targets may be accessed.

  To make use of SendTargets, an initiator must first establish one
  of         two types of sessions. If the initiator establishes
  the session using the key "SessionType=Discovery", the session is
  a discovery session, and a target name does not need to be




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  specified. Otherwise, the session is a normal, operational
  session. The SendTargets command MUST only be sent during the
  Full Feature Phase of a normal or discovery session.

  A system that contains targets MUST support discovery sessions on
  each of its iSCSI IP address-port pairs, and MUST support the
  SendTargets command on the discovery session. In a discovery
  session, a target MUST return all path information (IP address-
  port pairs and portal group tags) for the targets on the target
  network entity which the requesting initiator is authorized to
  access.

  A target MUST support the SendTargets command on operational
  sessions; these will only return path information about the target
  to which the session is connected, and do not need to return
  information about other target names that may be defined in the
  responding system.

  An initiator MAY make use of the SendTargets as it sees fit.

  A SendTargets command consists of a single Text request PDU.
  This PDU contains exactly one text key and value. The text key
  MUST be SendTargets. The expected response depends upon the
  value, as well as whether the session is a discovery or
  operational session.

  The value must be one of:

    All

    The initiator is requesting that information on all relevant
      targets known to the implementation be returned. This value
      MUST be supported on a discovery session, and MUST NOT be
      supported on an operational session.



    <iSCSI-target-name>

    If an iSCSI target name is specified, the session should
      respond with addresses for only the named target, if
      possible. This value MUST be supported on discovery




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      sessions. A discovery session MUST be capable of returning
      addresses for those targets that would have been returned
      had value=All been designated.



    <nothing>

    The session should only respond with addresses for the target
      to which the session is logged in. This MUST be supported
      on operational sessions, and MUST NOT return targets other
      than the one to which the session is logged in.



  The response to this command is a text response that contains a
  list of zero or more targets and, optionally, their addresses.
  Each target is returned as a target record. A target record
  begins with the TargetName text key, followed by a list of
  TargetAddress text keys, and bounded by the end of the text
  response or the next TargetName key, which begins a new record.
  No text keys other than TargetName and TargetAddress are permitted
  within a SendTargets response.

  For the format of the TargetName, see Section 13.4.

  A discovery session MAY respond to a SendTargets request with its
  complete list of targets, or with a list of targets that is based
  on the name of the initiator logged in to the session.

  A SendTargets response MUST NOT contain target names if there are
  no targets for the requesting initiator to access.

  Each target record returned includes zero or more TargetAddress
  fields.

  Each target record starts with one text key of the form:

    TargetName=<target-name-goes-here>


  Followed by zero or more address keys of the form:




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    TargetAddress=<hostname-or-ipaddress>[:<tcp-port>],<portal-
      group-tag>

  The hostname-or-ipaddress contains a domain name, IPv4 address, or
  IPv6 address ([RFC4291]), as specified for the TargetAddress key.

  A hostname-or-ipaddress duplicated in TargetAddress responses for
  a given node (the port is absent or equal) would probably indicate
  that multiple address families are in use at once (IPv6 and IPv4).

  Each TargetAddress belongs to a portal group, identified by its
  numeric portal group tag (as in Section 13.9). The iSCSI target
  name, together with this tag, constitutes the SCSI port
  identifier; the tag only needs to be unique within a given
  target's name list of addresses.

  Multiple-connection sessions can span iSCSI addresses that belong
  to the same portal group.

  Multiple-connection sessions cannot span iSCSI addresses that
  belong to different portal groups.

  If a SendTargets response reports an iSCSI address for a target,
  it SHOULD also report all other addresses in its portal group in
  the same response.

  A SendTargets text response can be longer than a single Text
  Response PDU, and makes use of the long text responses as
  specified.

  After obtaining a list of targets from the discovery target
  session,
  an iSCSI initiator may initiate new sessions to log in to the
  discovered targets for full operation. The initiator MAY keep the
  discovery session open, and MAY send subsequent SendTargets
  commands to discover new targets.

  Examples:




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  This example is the SendTargets response from a single target that
  has no other interface ports.

  Initiator sends text request that contains:

    SendTargets=All


  Target sends a text response that contains:

    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309


  All the target had to return in the simple case was the target
  name. It is assumed by the initiator that the IP address and TCP
  port for this target are the same as used on the current
  connection to the default iSCSI target.

  The next example has two internal iSCSI targets, each accessible
  via two different ports with different IP addresses. The
  following is the text response:

    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

    TargetAddress=10.1.0.45:3000,1

    TargetAddress=10.1.1.45:3000,2

    TargetName=iqn.1993-11.com.example:diskarray.sn.1234567

    TargetAddress=10.1.0.45:3000,1

    TargetAddress=10.1.1.45:3000,2


  Both targets share both addresses; the multiple addresses are
  likely used to provide multi-path support. The initiator may
  connect to either target name on either address. Each of the
  addresses has its own portal group tag; they do not support
  spanning multiple-connection sessions with each other. Keep in
  mind that the portal group tags for the two named targets are
  independent of one another; portal group "1" on the first target




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  is not necessarily the same as portal group "1" on the second
  target.

  In the above example, a DNS host name or an IPv6 address could
  have been returned instead of an IPv4 address.

  The next text response shows a target that supports spanning
  sessions across multiple addresses, and further illustrates the
  use of the portal group tags:

    TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

    TargetAddress=10.1.0.45:3000,1

    TargetAddress=10.1.1.46:3000,1

    TargetAddress=10.1.0.47:3000,2

    TargetAddress=10.1.1.48:3000,2

    TargetAddress=10.1.1.49:3000,3


  In this example, any of the target addresses can be used to reach
  the same target. A single-connection session can be established
  to any of these TCP addresses. A multiple-connection session
  could span addresses .45 and .46 or .47 and .48, but cannot span
  any other combination. A TargetAddress with its own tag (.49)
  cannot be combined with any other address within the same session.

  This SendTargets response does not indicate whether .49 supports
  multiple connections per session; it is communicated via the
  MaxConnections text key upon login to the target.

Appendix D.    Algorithmic Presentation of Error Recovery Classes

  This Appendix illustrates the error recovery classes using a
  pseudo-programming-language. The procedure names are chosen to be
  obvious to most implementers. Each of the recovery classes
  described has initiator procedures as well as target procedures.
  These algorithms focus on outlining the mechanics of error




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  recovery classes, and do not exhaustively describe all other
  aspects/cases. Examples of this approach are:

    - Handling for only certain Opcode types is shown.

    - Only certain reason codes (e.g., Recovery in Logout command)
      are outlined.

    - Resultant cases, such as recovery of Synchronization on a
      header digest error are considered out-of-scope in these
      algorithms. In this particular example, a header digest
      error may lead to connection recovery if some type of sync
      and steering layer is not implemented.


  These algorithms strive to convey the iSCSI error recovery
  concepts in the simplest terms, and are not designed to be
  optimal.

   D.1. General Data Structure and Procedure Description

  This Section defines the procedures and data structures that are
  commonly used by all the error recovery algorithms. The structures
  may not be the exhaustive representations of what is required for
  a typical implementation.

  Data structure definitions -
  struct TransferContext {
          int TargetTransferTag;
          int ExpectedDataSN;
  };

  struct TCB {              /* task control block */
          Boolean SoFarInOrder;
          int ExpectedDataSN; /* used for both R2Ts, and Data */
          int MissingDataSNList[MaxMissingDPDU];
          Boolean FbitReceived;
          Boolean StatusXferd;
          Boolean CurrentlyAllegiant;
          int ActiveR2Ts;
          int Response;




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          char *Reason;
          struct TransferContext
                      TransferContextList[MaxOutStandingR2T];
          int InitiatorTaskTag;
          int CmdSN;
          int SNACK_Tag;
  };

  struct Connection {
          struct Session SessionReference;
          Boolean SoFarInOrder;
          int CID;
          int State;
          int CurrentTimeout;
          int ExpectedStatSN;
          int MissingStatSNList[MaxMissingSPDU];
          Boolean PerformConnectionCleanup;
  };

  struct Session {
          int NumConnections;
          int CmdSN;
          int Maxconnections;
          int ErrorRecoveryLevel;
          struct iSCSIEndpoint OtherEndInfo;
          struct Connection ConnectionList[MaxSupportedConns];
  };

  Procedure descriptions -
  Receive-a-In-PDU(transport connection, inbound PDU);
  check-basic-validity(inbound PDU);
  Start-Timer(timeout handler, argument, timeout value);
  Build-And-Send-Reject(transport connection, bad PDU, reason
  code);

   D.2. Within-command Error Recovery Algorithms

  D.2.1. Procedure Descriptions

  Recover-Data-if-Possible(last required DataSN, task control
  block);




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  Build-And-Send-DSnack(task control block);
  Build-And-Send-RDSnack(task control block);
  Build-And-Send-Abort(task control block);
  SCSI-Task-Completion(task control block);
  Build-And-Send-A-Data-Burst(transport connection, data-
  descriptor,
                                                task control
  block);
  Build-And-Send-R2T(transport connection, data-descriptor,
                                               task control
  block);
  Build-And-Send-Status(transport connection, task control block);
  Transfer-Context-Timeout-Handler(transfer context);

  Notes:

    - One procedure used in this Section: Handle-Status-SNACK-
      request is defined in Within-connection recovery algorithms.



    - The Response processing pseudo-code, shown in the target
      algorithms, applies to all solicited PDUs that carry StatSN
      - SCSI Response, Text Response etc.


  D.2.2. Initiator Algorithms

  Recover-Data-if-Possible(LastRequiredDataSN, TCB)
  {
      if (operational ErrorRecoveryLevel > 0) {
           if (# of missing PDUs is trackable) {
                 Note the missing DataSNs in TCB.
                 if (the task spanned a change in
                            MaxRecvDataSegmentLength) {
                      if (TCB.StatusXferd is TRUE)
                          drop the status PDU;
                      Build-And-Send-RDSnack(TCB);
                 } else {
                      Build-And-Send-DSnack(TCB);
                 }




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           } else {
               TCB.Reason = "Protocol service CRC error";
                    }
      } else {
            TCB.Reason = "Protocol service CRC error";
      }
      if (TCB.Reason == "Protocol service CRC error") {
            Clear the missing PDU list in the TCB.
            if (TCB.StatusXferd is not TRUE)
               Build-And-Send-Abort(TCB);
      }
  }

  Receive-a-In-PDU(Connection, CurrentPDU)
  {
     check-basic-validity(CurrentPDU);
     if (Header-Digest-Bad) discard, return;
     Retrieve TCB for CurrentPDU.InitiatorTaskTag.
     if ((CurrentPDU.type == Data)
                 or (CurrentPDU.type = R2T)) {
        if (Data-Digest-Bad for Data) {
                  send-data-SNACK = TRUE;
          LastRequiredDataSN = CurrentPDU.DataSN;
                } else {
              if (TCB.SoFarInOrder = TRUE) {
                  if (current DataSN is expected) {
                       Increment TCB.ExpectedDataSN.
                  } else {
                             TCB.SoFarInOrder = FALSE;
                             send-data-SNACK = TRUE;
                          }
              } else {
                           if (current DataSN was considered
  missing) {
                              remove current DataSN from missing
  PDU list.
                           } else if (current DataSN is higher than
  expected) {
                                 send-data-SNACK = TRUE;
                          } else {




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                                  discard, return;
                            }
                            Adjust TCB.ExpectedDataSN if
  appropriate.
                 }
                 LastRequiredDataSN = CurrentPDU.DataSN - 1;
                   }
                   if (send-data-SNACK is TRUE and
                     task is not already considered failed) {
                 Recover-Data-if-Possible(LastRequiredDataSN, TCB);
        }
                if (missing data PDU list is empty) {
                   TCB.SoFarInOrder = TRUE;
                }
        if (CurrentPDU.type == R2T) {
           Increment ActiveR2Ts for this task.
           Create a data-descriptor for the data burst.
           Build-And-Send-A-Data-Burst(Connection, data-
  descriptor,
                                                    TCB);
        }
     } else if (CurrentPDU.type == Response) {
        if (Data-Digest-Bad) {
                   send-status-SNACK = TRUE;
                } else {
           TCB.StatusXferd = TRUE;
           Store the status information in TCB.
           if (ExpDataSN does not match) {
                TCB.SoFarInOrder = FALSE;
                Recover-Data-if-Possible(current DataSN, TCB);
           }
                   if (missing data PDU list is empty) {
                         TCB.SoFarInOrder = TRUE;
                   }
        }
     } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT
  SHOWN */
     }
     if ((TCB.SoFarInOrder == TRUE) and
                                (TCB.StatusXferd == TRUE)) {




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             SCSI-Task-Completion(TCB);
      }
  }

  D.2.3. Target Algorithms

  Receive-a-In-PDU(Connection, CurrentPDU)
  {
    check-basic-validity(CurrentPDU);
    if (Header-Digest-Bad) discard, return;
    Retrieve TCB for CurrentPDU.InitiatorTaskTag.
    if (CurrentPDU.type == Data) {
        Retrieve TContext from CurrentPDU.TargetTransferTag;
        if (Data-Digest-Bad) {
                    Build-And-Send-Reject(Connection, CurrentPDU,
                                 Payload-Digest-Error);
           Note the missing data PDUs in MissingDataRange[].
                    send-recovery-R2T = TRUE;
                 } else {
           if (current DataSN is not expected) {
               Note the missing data PDUs in MissingDataRange[].
                        send-recovery-R2T = TRUE;
                    }
           if (CurrentPDU.Fbit == TRUE) {
               if (current PDU is solicited) {
                       Decrement TCB.ActiveR2Ts.
               }
               if ((current PDU is unsolicited and
                       data received is less than I/O length and
                         data received is less than
  FirstBurstLength)
                    or (current PDU is solicited and the length of
                         this burst is less than expected)) {
                    send-recovery-R2T = TRUE;
                    Note the missing data in MissingDataRange[].
               }
                    }
                 }
                 Increment TContext.ExpectedDataSN.
        if (send-recovery-R2T is TRUE and




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                  task is not already considered failed) {
           if (operational ErrorRecoveryLevel > 0) {
               Increment TCB.ActiveR2Ts.
               Create a data-descriptor for the data burst
                          from MissingDataRange.
               Build-And-Send-R2T(Connection, data-descriptor,
  TCB);
           } else {
                if (current PDU is the last unsolicited)
                    TCB.Reason = "Not enough unsolicited data";
                else
                    TCB.Reason = "Protocol service CRC error";
           }
        }
        if (TCB.ActiveR2Ts == 0) {
           Build-And-Send-Status(Connection, TCB);
        }
    } else if (CurrentPDU.type == SNACK) {
        snack-failure = FALSE;
        if (operational ErrorRecoveryLevel > 0) {
           if (CurrentPDU.type == Data/R2T) {
                if (the request is satisfiable) {
                   if (request for Data) {
                      Create a data-descriptor for the data burst
                          from BegRun and RunLength.
                      Build-And-Send-A-Data-Burst(Connection,
                                    data-descriptor, TCB);
                   } else { /* R2T */
                      Create a data-descriptor for the data burst
                          from BegRun and RunLength.
                      Build-And-Send-R2T(Connection, data-
  descriptor,
                                      TCB);
                    }
                } else {
                      snack-failure = TRUE;
                }
           } else if (CurrentPDU.type == status) {
                Handle-Status-SNACK-request(Connection,
  CurrentPDU);
           } else if (CurrentPDU.type == DataACK) {




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                  Consider all data upto CurrentPDU.BegRun as
                  acknowledged.
                  Free up the retransmission resources for that data.
             } else if (CurrentPDU.type == R-Data SNACK) {
                           Create a data descriptor for a data burst
  covering
                    all unacknowledged data.
                 Build-And-Send-A-Data-Burst(Connection,
                                     data-descriptor, TCB);
                 TCB.SNACK_Tag = CurrentPDU.SNACK_Tag;
                 if (there's no more data to send) {
                    Build-And-Send-Status(Connection, TCB);
                 }
           }
        } else { /* operational ErrorRecoveryLevel = 0 */
                 snack-failure = TRUE;
        }
        if (snack-failure == TRUE) {
             Build-And-Send-Reject(Connection, CurrentPDU,
                                                     SNACK-Reject);
             if (TCB.StatusXferd != TRUE) {
                 TCB.Reason = "SNACK Rejected";
                 Build-And-Send-Status(Connection, TCB);
             }
        }

    } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT
  SHOWN */
    }
  }

  Transfer-Context-Timeout-Handler(TContext)
  {
    Retrieve TCB and Connection from TContext.
    Decrement TCB.ActiveR2Ts.
    if (operational ErrorRecoveryLevel > 0 and
                  task is not already considered failed) {
        Note the missing data PDUs in MissingDataRange[].
        Create a data-descriptor for the data burst
                          from MissingDataRange[].
        Build-And-Send-R2T(Connection, data-descriptor, TCB);




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      } else {
          TCB.Reason = "Protocol service CRC error";
          if (TCB.ActiveR2Ts = 0) {
             Build-And-Send-Status(Connection, TCB);
          }
      }
  }

   D.3. Within-connection Recovery Algorithms

  D.3.1. Procedure Descriptions

  Procedure descriptions:
  Recover-Status-if-Possible(transport connection,
                                      currently received PDU);
  Evaluate-a-StatSN(transport connection, currently received PDU);
  Retransmit-Command-if-Possible(transport connection, CmdSN);
  Build-And-Send-SSnack(transport connection);
  Build-And-Send-Command(transport connection, task control
  block);
  Command-Acknowledge-Timeout-Handler(task control block);
  Status-Expect-Timeout-Handler(transport connection);
  Build-And-Send-Nop-Out(transport connection);
  Handle-Status-SNACK-request(transport connection, status SNACK
  PDU);
  Retransmit-Status-Burst(status SNACK, task control block);
  Is-Acknowledged(beginning StatSN, run length);

  Implementation-specific tunables:
  InitiatorProactiveSNACKEnabled

  Notes:
    - The initiator algorithms only deal with unsolicited Nop-In
      PDUs for generating status SNACKs. A solicited Nop-In PDU
      has an assigned StatSN, which, when out of order, could
      trigger the out of order StatSN handling in Within-command
      algorithms, again leading to Recover-Status-if-Possible.




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    - The pseudo-code shown may result in the retransmission of
      unacknowledged commands in more cases than necessary. This
      will not, however, affect the correctness of the operation
      because the target is required to discard the duplicate
      CmdSNs.



    - The procedure Build-And-Send-Async is defined in the
      Connection recovery algorithms.



    - The procedure Status-Expect-Timeout-Handler describes how
      initiators may proactively attempt to retrieve the Status if
      they so choose. This procedure is assumed to be triggered
      much before the standard ULP timeout.


  D.3.2. Initiator Algorithms

  Recover-Status-if-Possible(Connection, CurrentPDU)
  {
      if ((Connection.state == LOGGED_IN) and
                       connection is not already considered
  failed) {
         if (operational ErrorRecoveryLevel > 0) {
            if (# of missing PDUs is trackable) {
                      Note the missing StatSNs in Connection
                 that were not already requested with SNACK;
              Build-And-Send-SSnack(Connection);
                    } else {
                      Connection.PerformConnectionCleanup = TRUE;
            }
         } else {
                    Connection.PerformConnectionCleanup = TRUE;
         }
         if (Connection.PerformConnectionCleanup == TRUE) {
            Start-Timer(Connection-Cleanup-Handler, Connection,
  0);
                  }
      }




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  }

  Retransmit-Command-if-Possible(Connection, CmdSN)
  {
      if (operational ErrorRecoveryLevel > 0) {
         Retrieve the InitiatorTaskTag, and thus TCB for the
  CmdSN.
         Build-And-Send-Command(Connection, TCB);
      }
  }

  Evaluate-a-StatSN(Connection, CurrentPDU)
  {
      send-status-SNACK = FALSE;
      if (Connection.SoFarInOrder == TRUE) {
         if (current StatSN is the expected) {
              Increment Connection.ExpectedStatSN.
         } else {
                       Connection.SoFarInOrder = FALSE;
                       send-status-SNACK = TRUE;
                  }
      } else {
         if (current StatSN was considered missing) {
              remove current StatSN from the missing list.
         } else {
                       if (current StatSN is higher than expected){
                           send-status-SNACK = TRUE;
                       } else {
                           send-status-SNACK = FALSE;
                   discard the PDU;
              }
         }
         Adjust Connection.ExpectedStatSN if appropriate.
         if (missing StatSN list is empty) {
              Connection.SoFarInOrder = TRUE;
                  }
      }
      return send-status-SNACK;
  }




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  Receive-a-In-PDU(Connection, CurrentPDU)
  {
      check-basic-validity(CurrentPDU);
      if (Header-Digest-Bad) discard, return;
      Retrieve TCB for CurrentPDU.InitiatorTaskTag.
      if (CurrentPDU.type == Nop-In) {
            if (the PDU is unsolicited) {
                  if (current StatSN is not expected) {
                           Recover-Status-if-Possible(Connection,
  CurrentPDU);
                  }
                  if (current ExpCmdSN is not Session.CmdSN) {
                      Retransmit-Command-if-Possible(Connection,
                                     CurrentPDU.ExpCmdSN);
                  }
            }
      } else if (CurrentPDU.type == Reject) {
            if (it is a data digest error on immediate data) {
                  Retransmit-Command-if-Possible(Connection,

  CurrentPDU.BadPDUHeader.CmdSN);
            }
      } else if (CurrentPDU.type == Response) {
           send-status-SNACK = Evaluate-a-StatSN(Connection,
                                          CurrentPDU);
           if (send-status-SNACK == TRUE)
               Recover-Status-if-Possible(Connection, CurrentPDU);
      } else { /* REST UNRELATED TO WITHIN-CONNECTION-RECOVERY,
                * NOT SHOWN */
      }
  }

  Command-Acknowledge-Timeout-Handler(TCB)
  {
      Retrieve the Connection for TCB.
      Retransmit-Command-if-Possible(Connection, TCB.CmdSN);
  }

  Status-Expect-Timeout-Handler(Connection)
  {




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      if (operational ErrorRecoveryLevel > 0) {
          Build-And-Send-Nop-Out(Connection);
      } else if (InitiatorProactiveSNACKEnabled){
          if ((Connection.state == LOGGED_IN) and
                       connection is not already considered
  failed) {
               Build-And-Send-SSnack(Connection);
          }
      }
  }

  D.3.3.Target Algorithms
  Handle-Status-SNACK-request(Connection, CurrentPDU)
  {
      if (operational ErrorRecoveryLevel > 0) {
         if (request for an acknowledged run) {
             Build-And-Send-Reject(Connection, CurrentPDU,
                                                Protocol-Error);
         } else if (request for an untransmitted run) {
             discard, return;
         } else {
             Retransmit-Status-Burst(CurrentPDU, TCB);
         }
      } else {
         Build-And-Send-Async(Connection, DroppedConnection,
                                 DefaultTime2Wait,
  DefaultTime2Retain);
      }
  }

   D.4. Connection Recovery Algorithms

  D.4.1. Procedure Descriptions

  Build-And-Send-Async(transport connection, reason code,
                                     minimum time, maximum time);
  Pick-A-Logged-In-Connection(session);
  Build-And-Send-Logout(transport connection, logout connection
                    identifier, reason code);
  PerformImplicitLogout(transport connection, logout connection
                    identifier, target information);




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  PerformLogin(transport connection, target information);
  CreateNewTransportConnection(target information);
  Build-And-Send-Command(transport connection, task control
  block);
  Connection-Cleanup-Handler(transport connection);
  Connection-Resource-Timeout-Handler(transport connection);
  Quiesce-And-Prepare-for-New-Allegiance(session, task control
  block);
  Build-And-Send-Logout-Response(transport connection,
                           CID of connection in recovery, reason
  code);
  Build-And-Send-TaskMgmt-Response(transport connection,
                         task mgmt command PDU, response code);
  Establish-New-Allegiance(task control block, transport
  connection);
  Schedule-Command-To-Continue(task control block);

  Notes:
    - Transport exception conditions, such as unexpected
      connection termination, connection reset, and hung
      connection while the connection is in the full-feature
      phase, are all assumed to be asynchronously signaled to the
      iSCSI layer using the Transport_Exception_Handler procedure.


  D.4.2. Initiator Algorithms


  Receive-a-In-PDU(Connection, CurrentPDU)
  {
      check-basic-validity(CurrentPDU);
      if (Header-Digest-Bad) discard, return;
      Retrieve TCB from CurrentPDU.InitiatorTaskTag.
      if (CurrentPDU.type == Async) {
          if (CurrentPDU.AsyncEvent == ConnectionDropped) {
             Retrieve the AffectedConnection for
  CurrentPDU.Parameter1.
             AffectedConnection.CurrentTimeout =
  CurrentPDU.Parameter3;
            AffectedConnection.State = CLEANUP_WAIT;
            Start-Timer(Connection-Cleanup-Handler,




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                           AffectedConnection,
  CurrentPDU.Parameter2);
          } else if (CurrentPDU.AsyncEvent == LogoutRequest)) {
            AffectedConnection = Connection;
            AffectedConnection.State = LOGOUT_REQUESTED;
            AffectedConnection.PerformConnectionCleanup = TRUE;
                     AffectedConnection.CurrentTimeout =
  CurrentPDU.Parameter3;
            Start-Timer(Connection-Cleanup-Handler,
                          AffectedConnection, 0);
          } else if (CurrentPDU.AsyncEvent == SessionDropped)) {
            for (each Connection) {
                Connection.State = CLEANUP_WAIT;
                Connection.CurrentTimeout = CurrentPDU.Parameter3;
                Start-Timer(Connection-Cleanup-Handler,
                          Connection, CurrentPDU.Parameter2);
            }
            Session.state = FAILED;
          }

      } else if (CurrentPDU.type == LogoutResponse) {
          Retrieve the CleanupConnection for CurrentPDU.CID.
          if (CurrentPDU.Response = failure) {
             CleanupConnection.State = CLEANUP_WAIT;
          } else {
              CleanupConnection.State = FREE;
          }
      } else if (CurrentPDU.type == LoginResponse) {
           if (this is a response to an implicit Logout) {
              Retrieve the CleanupConnection.
              if (successful) {
                  CleanupConnection.State = FREE;
                  Connection.State = LOGGED_IN;
              } else {
                   CleanupConnection.State = CLEANUP_WAIT;
                   DestroyTransportConnection(Connection);
              }
           }
      } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
                * NOT SHOWN */




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      }
      if (CleanupConnection.State == FREE) {
         for (each command that was active on CleanupConnection) {
         /* Establish new connection allegiance */
              NewConnection = Pick-A-Logged-In-
  Connection(Session);
              Build-And-Send-Command(NewConnection, TCB);
          }
      }
  }

  Connection-Cleanup-Handler(Connection)
  {
      Retrieve Session from Connection.
      if (Connection can still exchange iSCSI PDUs) {
          NewConnection = Connection;
      } else {
          Start-Timer(Connection-Resource-Timeout-Handler,
                Connection, Connection.CurrentTimeout);
          if (there are other logged-in connections) {
               NewConnection = Pick-A-Logged-In-
  Connection(Session);
          } else {
               NewConnection =

  CreateTransportConnection(Session.OtherEndInfo);
               Initiate an implicit Logout on NewConnection for
                                                 Connection.CID.
               return;
          }
      }
      Build-And-Send-Logout(NewConnection, Connection.CID,
                                          RecoveryRemove);
  }

  Transport_Exception_Handler(Connection)
  {
      Connection.PerformConnectionCleanup = TRUE;
      if (the event is an unexpected transport disconnect) {
          Connection.State = CLEANUP_WAIT;




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          Connection.CurrentTimeout = DefaultTime2Retain;
          Start-Timer(Connection-Cleanup-Handler, Connection,
                                            DefaultTime2Wait);

      } else {
          Connection.State = FREE;
      }
  }

  D.4.3. Target Algorithms

  Receive-a-In-PDU(Connection, CurrentPDU)
  {
      check-basic-validity(CurrentPDU);
      if (Header-Digest-Bad) discard, return;
      else if (Data-Digest-Bad) {
                Build-And-Send-Reject(Connection, CurrentPDU,
                                         Payload-Digest-Error);
                discard, return;
      }
      Retrieve TCB and Session.
      if (CurrentPDU.type == Logout) {
         if (CurrentPDU.ReasonCode = RecoveryRemove) {
             Retrieve the CleanupConnection from CurrentPDU.CID).
             for (each command active on CleanupConnection) {
                  Quiesce-And-Prepare-for-New-Allegiance(Session,
  TCB);
                  TCB.CurrentlyAllegiant = FALSE;
             }
             Cleanup-Connection-State(CleanupConnection);
             if ((quiescing successful) and (cleanup successful))
  {
                  Build-And-Send-Logout-Response(Connection,
                                       CleanupConnection.CID,
  Success);
             } else {
                  Build-And-Send-Logout-Response(Connection,
                                       CleanupConnection.CID,
  Failure);
             }




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          }
      } else if ((CurrentPDU.type == Login) and
                           operational ErrorRecoveryLevel == 2) {
              Retrieve the CleanupConnection from CurrentPDU.CID).
              for (each command active on CleanupConnection) {
                    Quiesce-And-Prepare-for-New-Allegiance(Session,
  TCB);
                    TCB.CurrentlyAllegiant = FALSE;
              }
              Cleanup-Connection-State(CleanupConnection);
              if ((quiescing successful) and (cleanup successful))
  {
                    Continue with the rest of the Login processing;
              } else {
                    Build-And-Send-Login-Response(Connection,
                                  CleanupConnection.CID, Target
  Error);
              }
          }
      } else if (CurrentPDU.type == TaskManagement) {
            if (CurrentPDU.function == "TaskReassign") {
                  if (Session.ErrorRecoveryLevel < 2) {
                      Build-And-Send-TaskMgmt-Response(Connection,
                           CurrentPDU, "Allegiance reassignment
                                                  not supported");
                  } else if (task is not found) {
                      Build-And-Send-TaskMgmt-Response(Connection,
                           CurrentPDU, "Task not in task set");
                  } else if (task is currently allegiant) {
                      Build-And-Send-TaskMgmt-Response(Connection,
                                CurrentPDU, "Task still allegiant");
                  } else {
                      Establish-New-Allegiance(TCB, Connection);
                      TCB.CurrentlyAllegiant = TRUE;
                      Schedule-Command-To-Continue(TCB);
                  }
            }
      } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
                 * NOT SHOWN */
      }




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  }

  Transport_Exception_Handler(Connection)
  {
      Connection.PerformConnectionCleanup = TRUE;
      if (the event is an unexpected transport disconnect) {
          Connection.State = CLEANUP_WAIT;
           Start-Timer(Connection-Resource-Timeout-Handler,
  Connection,

  (DefaultTime2Wait+DefaultTime2Retain));
            if (this Session has full-feature phase connections
  left) {
                DifferentConnection =
                   Pick-A-Logged-In-Connection(Session);
                 Build-And-Send-Async(DifferentConnection,
                       DroppedConnection, DefaultTime2Wait,
                         DefaultTime2Retain);
          }
      } else {
            Connection.State = FREE;
      }
  }
  Appendix E. Clearing Effects of Various Events on Targets

      E.1.  Clearing Effects on iSCSI Objects

  The following tables describe the target behavior on receiving the
  events specified in the rows of the table. The second table is
  an extension of the first table and defines clearing actions for
  more objects on the same events. The legend is:

      Y     = Yes (cleared/discarded/reset on the event specified in
          the row). Unless otherwise noted, the clearing action is
          only applicable for the issuing initiator port.

      N     = No (not affected on the event specified in the row,
          i.e., stays at previous value).

      NA     = Not Applicable or Not Defined.




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                        +-----+-----+-----+-----+-----+
                        |IT(1)|IC(2)|CT(5)|ST(6)|PP(7)|
  +---------------------+-----+-----+-----+-----+-----+
  |connection failure(8)|Y    |Y    |N    |N    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |connection state     |NA   |NA   |Y    |N    |NA   |
  |timeout (9)          |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session timeout/     |Y    |Y    |Y    |Y    |Y(14)|
  |closure/reinstatement|     |     |     |     |     |
  |(10)                 |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session continuation |NA   |NA   |N(11)|N    |NA   |
  |(12)                 |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |successful connection|Y    |Y    |Y    |N    |Y(13)|
  |close logout         |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session failure (18) |Y    |Y    |N    |N    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |successful recovery |Y     |Y    |N    |N    |Y(13)|
  |Logout               |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |failed Logout        |Y    |Y    |N    |N    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |connection Login     |NA   |NA   |NA   |Y(15)|NA   |
  |(leading)            |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |connection Login     |NA   |NA   |N(11)|N    |Y    |
  |(non-leading)        |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |target cold reset(16)|Y(20)|Y    |Y    |Y    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |target warm reset(16)|Y(20)|Y    |Y    |Y    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |LU reset(19)         |Y(20)|Y    |Y    |Y    |Y    |
  +---------------------+-----+-----+-----+-----+-----+
  |powercycle(16)       |Y    |Y    |Y    |Y    |Y    |
  +---------------------+-----+-----+-----+-----+-----+




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1. Incomplete TTTs - Target Transfer Tags on which the target is
   still expecting PDUs to be received. Examples include TTTs
   received via R2T, NOP-IN, etc.

2. Immediate Commands - immediate commands, but waiting for
   execution on a target. For example, Abort Task Set.

5. Connection Tasks - tasks that are active on the iSCSI connection
   in question.

6. Session Tasks - tasks that are active on the entire iSCSI
   session. A union of "connection tasks" on all participating
   connections.

7. Partial PDUs (if any) - PDUs that are partially sent and waiting
   for transport window credit to complete the transmission.

8. Connection failure is a connection exception condition - one of
   the transport connections shutdown, transport connections
   reset, or transport connections timed out, which abruptly
   terminated the iSCSI full-feature phase connection. A
   connection failure always takes the connection state machine to
   the CLEANUP_WAIT state.

9. Connection state timeout happens if a connection spends more
   time than agreed upon during Login negotiation in the
   CLEANUP_WAIT state, and this takes the connection to the FREE
   state (M1 transition in connection cleanup state diagram).

10.These are defined in Section 6.3.5.

11.This clearing effect is "Y" only if it is a connection
   reinstatement and the operational ErrorRecoveryLevel is less
   than 2.

12.Session continuation is defined in Section 6.3.5.

13.This clearing effect is only valid if the connection is being
   logged out on a different connection and when the connection
   being logged out on the target may have some partial PDUs
   pending to be sent. In all other cases, the effect is "NA".




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14.This clearing effect is only valid for a "close the session"
   logout in a multi-connection session. In all other cases, the
   effect is "NA".
15.Only applicable if this leading connection login is a session
   reinstatement. If this is not the case, it is "NA".
16.This operation affects all logged-in initiators.
18.Session failure is defined in Section 6.3.5.
19.This operation affects all logged-in initiators and the clearing
   effects are only applicable to the LU being reset.
20.With Standard multi-task abort semantics (Section 4.2.3.3), a
   target warm reset or a target cold reset or an LU reset would
   clear the active TTTs upon completion. However, the FastAbort
   multi-task abort semantics defined by Section 4.2.3.4 do not
   guarantee that the active TTTs are cleared by the end of the
   reset operations. In fact, the FastAbort semantics are designed
   to allow clearing the TTTs in a "lazy" fashion after the TMF
   Response is delivered. Thus, when TaskReporting=FastAbort
   (Section 13.23) is operational on a session, the clearing
   effects of reset operations on "Incomplete TTTs" is "N".




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                        +-----+-----+-----+-----+-----+
                        |DC(1)|DD(2)|SS(3)|CS(4)|DS(5)|
  +---------------------+-----+-----+-----+-----+-----+
  |connection failure   |N    |Y    |N    |N    |N    |
  +---------------------+-----+-----+-----+-----+-----+
  |connection state     |Y    |NA   |Y    |N    |NA   |
  |timeout              |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session timeout/     |Y    |Y    |Y(7) |Y    |NA   |
  |closure/reinstatement|     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session continuation |N(11)|NA*12|NA   |N    |NA*13|
  +---------------------+-----+-----+-----+-----+-----+
  |successful connection|Y    |Y    |Y    |N    |NA   |
  |close Logout         |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |session failure      |N    |Y    |N    |N    |N    |
  +---------------------+-----+-----+-----+-----+-----+
  |successful recovery |Y     |Y    |Y    |N    |N    |
  |Logout               |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |failed Logout        |N    |Y(9) |N    |N    |N    |
  +---------------------+-----+-----+-----+-----+-----+
  |connection Login     |NA   |NA   |N(8) |N(8) |NA   |
  |(leading             |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |connection Login     |N(11)|NA*12|N(8) |N    |NA*13|
  |(non-leading)        |     |     |     |     |     |
  +---------------------+-----+-----+-----+-----+-----+
  |target cold reset    |Y    |Y    |Y    |Y(10)|NA   |
  +---------------------+-----+-----+-----+-----+-----+
  |target warm reset    |Y    |Y    |N    |N    |NA   |
  +---------------------+-----+-----+-----+-----+-----+
  |LU reset             |N    |Y    |N    |N    |N    |
  +---------------------+-----+-----+-----+-----+-----+
  |powercycle           |Y    |Y    |Y    |Y(10)|NA   |
  +---------------------+-----+-----+-----+-----+-----+

  1. Discontiguous Commands - commands allegiant to the connection
  in question and waiting to be reordered in the iSCSI layer. All
  "Y"s in this column assume that the task causing the event (if




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  indeed the event is the result of a task) is issued as an
  immediate command, because the discontiguities can be ahead of the
  task.

  2. Discontiguous Data - data PDUs received for the task in
  question and waiting to be reordered due to prior discontiguities
  in DataSN.

  3. StatSN

  4. CmdSN

  5. DataSN

  7. It clears the StatSN on all the connections.

  8. This sequence number is instantiated on this event.

  9. A logout failure drives the connection state machine to the
  CLEANUP_WAIT state, similar to the connection failure event.
  Hence, it has a similar effect on this and several other protocol
  aspects.

  10. This is cleared by virtue of the fact that all sessions with
  all initiators are terminated.

  11. This clearing effect is "Y" if it is a connection
  reinstatement.

  12. This clearing effect is "Y" only if it is a connection
  reinstatement and the operational ErrorRecoveryLevel is 2.

  13. This clearing effect is "N" only if it is a connection
  reinstatement and the operational ErrorRecoveryLevel is 2.

    E.2.  Clearing Effects on SCSI Objects

  The only iSCSI protocol action that can effect clearing actions on
  SCSI objects is the "I_T nexus loss" notification (Section 6.3.5.1
  Loss of Nexus notification). [SPC3] describes the clearing effects
  of this notification on a variety of SCSI attributes. In addition,




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  SCSI standards documents (such as [SAM2] and [SBC2]) define
  additional clearing actions that may take place for several SCSI
  objects on SCSI events such as LU resets and power-on resets.

  Since iSCSI defines a target cold reset as a protocol-equivalent
  to a target power-cycle, the iSCSI target cold reset must also be
  considered as the power-on reset event in interpreting the actions
  defined in the SCSI standards.

  When the iSCSI session is reconstructed (between the same SCSI
  ports with the same nexus identifier) reestablishing the same I_T
  nexus, all SCSI objects that are defined to not clear on the "I_T
  nexus loss" notification event, such as persistent reservations,
  are automatically associated to this new session.


Acknowledgments

  Several individuals on the original IPS Working Group made
  significant contributions to the original RFCs 3720, 3980, 4850
  and 5048.

  Specifically, the authors of the original RFCs - which this draft
  consolidates into a single document - were the following:

  RFC 3720: Julian Satran, Kalman Meth, Costa Sapuntzakis,
  Mallikarjun Chadalapaka, Efri Zeidner

  RFC 3980: Marjorie Krueger, Mallikarjun Chadalapaka, Rob Elliott

  RFC 4850: David Wysochanski

  RFC 5048: Mallikarjun Chadalapaka

  Many thanks to Fred Knight for contributing to the UML notations
  and drawings in this draft.

  We would in addition like to acknowledge the following individuals
  who contributed to this revised draft: David Harrington, Paul
  Koning, Mark Edwards, Rob Elliott, Martin Stiemerling.




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  Thanks to Yi Zeng and Nico for suggesting and/or reviewing
  Kerberos-related security considerations text.

  Finally, this draft also benefited from significant review
  contributions from the Storm Working Group at large.

Authors' Addresses

  Mallikarjun Chadalapaka
  Microsoft
  One Microsoft Way
  Redmond WA 98052 USA
  E-mail: cbm@chadalapaka.com

  Julian Satran
  Infinidat Ltd.
  E-mail: julians@infinidat.com, julian@satran.net

  Kalman Meth
  IBM Haifa Research Lab
  Haifa University Campus - Mount Carmel
  Haifa 31905, Israel
  Phone +972.4.829.6341
  E-mail: meth@il.ibm.com

  David L. Black,
  EMC Corporation,
  176 South St., Hopkinton, MA    01748
  Phone +1 (508) 293-7953
  Email: david.black@emc.com

  Comments may be sent to Mallikarjun Chadalapaka


Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society




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