[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08

Network Working Group                                            N. Ekiz
Internet-Draft                                                   P. Amer
Intended status: Standards Track                  University of Delaware
Expires: February 19, 2011                                  P. Natarajan
                                                           Cisco Systems
                                                              R. Stewart
                                                              Researcher
                                                              J. Iyengar
                                             Franklin & Marshall College
                                                         August 18, 2010


      Non-Renegable Selective Acknowledgements (NR-SACKs) for SCTP
                draft-natarajan-tsvwg-sctp-nrsack-06.txt

Abstract

   Stream Control Transmission Protocol (SCTP) [RFC4960] specifies
   Selective Acknowledgements (SACKs) to allow an SCTP data receiver to
   acknowledge DATA chunks which arrive out-of-order.  In SCTP, SACK
   information is advisory -- though SACKs notify a data sender about
   the reception of specific out-of-order data, the SCTP data receiver
   is permitted to later discard the data, a.k.a reneging.  Since
   delivery of a SACKed out-of-order DATA chunk is not guaranteed, a
   copy of this DATA chunk MUST be kept in the data sender's
   retransmission queue until this DATA chunk is cumulatively acked.

   By definition, data that has been delivered to the application is
   non-renegable by the SCTP data receiver.  (Recall that, in SCTP, out-
   of-order data can sometimes be delivered.)  Also, SCTP
   implementations can be configured such that the SCTP data receiver is
   not allowed to, and therefore, never reneges on out-of-order data.
   With SCTP's current SACK mechanism, non-renegable out-of-order data
   is selectively acked, and is (wrongly) deemed renegable by the SCTP
   data sender.

   This document specifies an extension to SCTP's acknowledgment
   mechanism called Non-Renegable Selective Acknowledgements (NR-SACKs.)
   NR-SACKs enable a data receiver to explicitly inform the data sender
   of non-renegable out-of-order data.  As opposed to renegable data, a
   data sender can consider non-renegable data as never requiring
   retransmission, and therefore can remove non-renegable data from the
   retransmission queue.

Status of this Memo

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



Ekiz, et al.            Expires February 19, 2011               [Page 1]


Internet-Draft              NR-SACKs for SCTP                August 2010


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on February 19, 2011.

Copyright Notice

   Copyright (c) 2010 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.

























Ekiz, et al.            Expires February 19, 2011               [Page 2]


Internet-Draft              NR-SACKs for SCTP                August 2010


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Negotiation  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  The New Chunk Type: Non-Renegable SACK (NR-SACK) . . . . . . .  6
   5.  An Illustrative Example  . . . . . . . . . . . . . . . . . . . 11
   6.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Sending an NR-SACK chunk . . . . . . . . . . . . . . . . . 14
     6.2.  Receiving an NR-SACK Chunk . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18


































Ekiz, et al.            Expires February 19, 2011               [Page 3]


Internet-Draft              NR-SACKs for SCTP                August 2010


1.  Introduction

   In providing end-to-end reliable data transfer, SCTP specifies
   Cumulative Acknowledgements (ACKs), Selective ACKs (SACKs), and
   Duplicate Selective ACKs (D-SACKs).  These three types of acks are
   carried in the following fields of the SACK chunk, respectively:
   Cumulative TSN Ack, Gap Ack Block, and Duplicate TSN.  In this
   document, we refer to the Cumulative TSN Ack as the "cum-ack", the
   selective Gap Ack Blocks as "gap-acks", and the Duplicate TSN
   selective acks as "dup-TSN reports".

   Gap-acks acknowledge DATA chunks that arrive out-of-order to a
   transport layer data receiver.  A gap-ack in SCTP is advisory, in
   that, while it notifies a data sender about the reception of
   indicated DATA chunks, the data receiver is permitted to later
   discard DATA chunks that it previously had gap-acked.  Discarding a
   previously gap-acked DATA chunk is known as "reneging."  Because of
   the possibility of reneging in SCTP, any gap-acked DATA chunk MUST
   NOT be removed from the data sender's retransmission queue until the
   DATA chunk is later cum-acked.

   Situations exist when a data receiver knows that reneging on a
   particular out-of-order DATA chunk will never take place, such as
   (but not limited to) after an out-of-order DATA chunk is delivered to
   the receiving application.  This document describes an extension to
   SCTP to allow for Non-Renegable Selective Acknowledgments (NR-SACKs).
   A new NR-SACK chunk type is described that allows this extension to
   be implemented.

   The NR-SACK chunk is an extension of the existing SACK chunk.
   Several fields are identical, including the Cumulative TSN Ack, the
   Advertised Receiver Window Credit (a_rwnd), and Duplicate TSNs.
   These fields have the same semantics as described in [RFC4960].

   NR-SACKs extend SACKs by also identifying out-of-order DATA chunks
   that a receiver either: (1) has delivered to its receiving
   application, or (2) takes full responsibility to eventually deliver
   to its receiving application.  These out-of-order DATA chunks are
   "non-renegable."  Non-Renegable data are reported in the NR Gap Ack
   Block field of the NR-SACK chunk as described in Section 4.1.  We
   refer to non-renegable selective acknowledgements as "nr-gap-acks."

   When an out-of-order DATA chunk is nr-gap-acked, the data sender no
   longer needs to keep that particular DATA chunk in its retransmission
   queue, thus allowing the data sender to free up its buffer space
   sooner than if the DATA chunk were only gap-acked.  NR-SACKs have
   been shown to better utilize the data sender's memory and improve
   throughput, at the trade-off of generating and processing additional



Ekiz, et al.            Expires February 19, 2011               [Page 4]


Internet-Draft              NR-SACKs for SCTP                August 2010


   acknowledgement information [Natarajan], [Yilmaz].

   An SCTP message is encapsulated within a single DATA chunk or within
   multiple DATA chunks in case of fragmentation.  In this document
   without loss of generality, each application message maps to a single
   transport layer DATA chunk, and delivering a DATA chunk to a
   receiving application means delivering the message carried within the
   DATA chunk to a receiving application.

   SCTP divides an end-to-end association into independent logical data
   streams (a.k.a. multistreaming.)  A DATA chunk that arrives in-
   sequence within a stream can be delivered to the receiving
   application even if the DATA chunk is out-of-order relative to the
   association's overall flow of data.  These out-of-order DATA chunks
   are "deliverable."  By definition, a DATA chunk marked for unordered
   delivery also is "deliverable" to the receiving application
   immediately upon reception, regardless of its position within the
   overall flow of data.

   With current SACKs in SCTP, it is not possible for a data receiver to
   inform a data sender if or when a particular out-of-order
   "deliverable" DATA chunk has been "delivered" to the receiving
   application.  Thus the data sender MUST keep a copy of every gap-
   acked out-of-order DATA chunk(s) in the data sender's retransmission
   queue until the DATA chunk is cum-acked.  This use of the data
   sender's retransmission queue is wasteful.  Given the receiving
   application has received the data, the data sender has no reason to
   keep this data in its retransmission queue.  Yet, the sending
   transport layer keeps the data because no mechanism currently exists
   to indicate which out-of-order DATA chunks have been delivered.
   (Note: once a DATA chunk is delivered to the receiving application,
   it is impossible for the data receiver to renege on that DATA chunk.)

   If NR-SACKs are used, the data receiver MAY include the TSN of a
   delivered out-of-order DATA chunk in an NR-SACK to inform the data
   sender that the delivery has occurred, allowing the data sender to
   remove the copy of the delivered DATA chunk from the data sender's
   retransmission queue even before the DATA chunk is cum-acked.


2.  Conventions

   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 [RFC2119].






Ekiz, et al.            Expires February 19, 2011               [Page 5]


Internet-Draft              NR-SACKs for SCTP                August 2010


3.  Negotiation

   Before sending/receiving NR-SACKs, both peer endpoints MUST agree on
   using NR-SACKs.  This agreement MUST be negotiated during association
   establishment.  NR-SACK is an extension to the core SCTP, and SCTP
   extensions that an endpoint supports are reported to the peer
   endpoint in Supported Extensions Parameter during association
   establishment (see Section 4.2.7 of [RFC5061].)  The Supported
   Extensions Parameter consists of a list of non-standard Chunk Types
   that are supported by the sender.

   An endpoint supporting the NR-SACK extension MUST list the NR-SACK
   chunk in the Supported Extensions Parameter carried in the INIT or
   INIT-ACK chunk, depending on whether the endpoint initiates or
   responds to the initiation of the association.  If the NR-SACK chunk
   type ID is listed in the Chunk Types List of the Supported Extensions
   Parameter, then the receiving endpoint MUST assume that the NR-SACK
   chunk is supported by the sending endpoint.

   Both endpoints MUST support NR-SACKs for either endpoint to send an
   NR-SACK.  If an endpoint establishes an association with a remote
   endpoint that does not list NR-SACK in the Supported Extensions
   Parameter carried in INIT chunk, then both endpoints of the
   association MUST NOT use NR-SACKs.  After association establishment,
   an endpoint MUST NOT renegotiate the use of NR-SACKs.

   Once both endpoints indicate during association establishment that
   they support the NR-SACK extension, each endpoint SHOULD acknowledge
   received DATA chunks with NR-SACK chunks, and not SACK chunks.  That
   is, throughout an SCTP association, both endpoints SHOULD send either
   SACK chunks or NR-SACK chunks, never a mixture of the two.


4.  The New Chunk Type: Non-Renegable SACK (NR-SACK)

   Table 1 illustrates a new chunk type that will be used to transfer
   NR-SACK information.

      Chunk Type  Chunk Name
      --------------------------------------------------------------
      0x10        Non-Renegable Selective Acknowledgment    (NR-SACK)

      Table 1: NR-SACK Chunk

   As the NR-SACK chunk replaces the SACK chunk, many SACK chunk fields
   are preserved in the NR-SACK chunk.  These preserved fields have the
   same semantics with the corresponding SACK chunk fields, as defined
   in [RFC4960], Section 3.3.4.  The Gap Ack fields from RFC4960 have



Ekiz, et al.            Expires February 19, 2011               [Page 6]


Internet-Draft              NR-SACKs for SCTP                August 2010


   been renamed as R Gap Ack to emphasize their renegable nature.  Their
   semantics are unchanged.  For completeness, we describe all fields of
   the NR-SACK chunk, including those that are identical in the SACK
   chunk.

   Similar to the SACK chunk, the NR-SACK chunk is sent to a peer
   endpoint to (1) acknowledge DATA chunks received in-order, (2)
   acknowledge DATA chunks received out-of-order, and (3) identify DATA
   chunks received more than once (i.e., duplicate.)  In addition, the
   NR-SACK chunk (4) informs the peer endpoint of non-renegable out-of-
   order DATA chunks.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 0x10 |  Chunk Flags  |         Chunk Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Cumulative TSN Ack                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Advertised Receiver Window Credit (a_rwnd)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Number of R Gap Ack Blocks = N |Number of NR Gap Ack Blocks = M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Number of Duplicate TSNs = X  |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | R Gap Ack Block #1 Start      |   R Gap Ack Block #1 End      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  R Gap Ack Block #N Start     |  R Gap Ack Block #N End       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  NR Gap Ack Block #1 Start    |   NR Gap Ack Block #1 End     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   NR Gap Ack Block #M Start   |  NR Gap Ack Block #M End      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN 1                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               /
     \                              ...                              \
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Duplicate TSN X                         |



Ekiz, et al.            Expires February 19, 2011               [Page 7]


Internet-Draft              NR-SACKs for SCTP                August 2010


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


   Type: 8 bits

   This field holds the IANA defined chunk type for NR-SACK chunk.  The
   suggested value of this field for IANA is 0x10.

   Chunk Flags: 8 bits

   Currently not used.  It is recommended a sender set all bits to zero
   on transmit, and a receiver ignore this field.

   Chunk Length: 16 bits (unsigned integer) [Same as SACK chunk]

   This value represents the size of the chunk in bytes including the
   Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.

   Cumulative TSN Ack: 32 bits (unsigned integer) [Same as SACK chunk]

   The value of the Cumulative TSN Ack is the last TSN received before a
   break in the sequence of received TSNs occurs.  The next TSN value
   following the Cumulative TSN Ack has not yet been received at the
   endpoint sending the NR-SACK.

   Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
   integer) [Same as SACK chunk]

   Indicates the updated receive buffer space in bytes of the sender of
   this NR-SACK, see Section 6.2.1 of [RFC4960] for details.

   Number of (R)enegable Gap Ack Blocks (N): 16 bits (unsigned integer)

   Indicates the number of Renegable Gap Ack Blocks included in this NR-
   SACK.

   Number of (N)on(R)enegable Gap Ack Blocks (M): 16 bits (unsigned
   integer)

   Indicates the number of Non-Renegable Gap Ack Blocks included in this
   NR-SACK.

   Number of Duplicate TSNs (X): 16 bits [Same as SACK chunk]

   Contains the number of duplicate TSNs the endpoint has received.
   Each duplicate TSN is listed following the NR Gap Ack Block list.

   Reserved : 16 bits



Ekiz, et al.            Expires February 19, 2011               [Page 8]


Internet-Draft              NR-SACKs for SCTP                August 2010


   Currently not used.  It is recommended a sender set all bits to zero
   on transmit, and a receiver ignore this field.

   (R)enegable Gap Ack Blocks:

   The NR-SACK contains zero or more R Gap Ack Blocks.  Each R Gap Ack
   Block acknowledges a subsequence of renegable out-of-order TSNs.  By
   definition, all TSNs acknowledged by R Gap Ack Blocks are "greater
   than" the value of the Cumulative TSN Ack.

   Because of TSN numbering wraparound, comparisons and all arithmetic
   operations discussed in this document are based on "Serial Number
   Arithmetic" as described in Section 1.6 of [RFC4960].

   R Gap Ack Blocks are repeated for each R Gap Ack Block up to 'N'
   defined in the Number of R Gap Ack Blocks field.  All DATA chunks
   with TSNs >= (Cumulative TSN Ack + R Gap Ack Block Start) and <=
   (Cumulative TSN Ack + R Gap Ack Block End) of each R Gap Ack Block
   are assumed to have been received correctly, and are renegable.

   R Gap Ack Block Start: 16 bits (unsigned integer)

   Indicates the Start offset TSN for this R Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in Cumulative
   TSN Ack field.  To calculate the actual start TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated
   TSN identifies the first TSN in this R Gap Ack Block that has been
   received.

   R Gap Ack Block End: 16 bits (unsigned integer)

   Indicates the End offset TSN for this R Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in the
   Cumulative TSN Ack field.  To calculate the actual TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated
   TSN identifies the TSN of the last DATA chunk received in this R Gap
   Ack Block.

   N(on)R(enegable) Gap Ack Blocks:

   The NR-SACK contains zero or more NR Gap Ack Blocks.  Each NR Gap Ack
   Block acknowledges a continuous subsequence of non-renegable out-of-
   order DATA chunks.  If a TSN is nr-gap-acked in any NR-SACK chunk,
   then all subsequently transmitted NR-SACKs with a smaller cum-ack
   value than that TSN SHOULD also nr-gap-ack that TSN.

   NR Gap Ack Blocks are repeated for each NR Gap Ack Block up to 'M'
   defined in the Number of NR Gap Ack Blocks field.  All DATA chunks



Ekiz, et al.            Expires February 19, 2011               [Page 9]


Internet-Draft              NR-SACKs for SCTP                August 2010


   with TSNs >= (Cumulative TSN Ack + NR Gap Ack Block Start) and <=
   (Cumulative TSN Ack + NR Gap Ack Block End) of each NR Gap Ack Block
   are assumed to be received correctly, and are Non-Renegable.

   NR Gap Ack Block Start: 16 bits (unsigned integer)

   Indicates the Start offset TSN for this NR Gap Ack Block.  This
   number is set relative to the cumulative TSN number defined in
   Cumulative TSN Ack field.  To calculate the actual TSN number, the
   Cumulative TSN Ack is added to this offset number.  The calculated
   TSN identifies the first TSN in this NR Gap Ack Block that has been
   received.

   NR Gap Ack Block End: 16 bits (unsigned integer)

   Indicates the End offset TSN for this NR Gap Ack Block.  This number
   is set relative to the cumulative TSN number defined in Cumulative
   TSN Ack field.  To calculate the actual TSN number, the Cumulative
   TSN Ack is added to this offset number.  The calculated TSN
   identifies the TSN of the last DATA chunk received in this NR Gap Ack
   Block.

   Note:

   NR Gap Ack Blocks and R Gap Ack Blocks in an NR-SACK chunk SHOULD
   acknowledge disjoint sets of TSNs.  That is, an out-of-order TSN
   SHOULD be listed in either an R Gap Ack Block or an NR Gap Ack Block,
   but not the both.  R Gap Ack Blocks and NR Gap Ack Blocks together
   provide the information as do the Gap Ack Block of a SACK chunk, plus
   additional information about non-renegability.

   If all out-of-order data acked by an NR-SACK are renegable, then the
   Number of NR Gap Ack Blocks MUST be set to 0.  If all out-of-order
   data acked by an NR-SACK are non-renegable, then the Number of R Gap
   Ack Blocks SHOULD be set to 0.  TSNs listed in R Gap Ack Block will
   be referred as r-gap-acked.

   Duplicate TSN: 32 bits (unsigned integer) [Same as SACK chunk]

   Indicates a duplicate TSN received since the last NR-SACK was sent.
   Exactly 'X' duplicate TSNs SHOULD be reported where 'X' was defined
   in Number of Duplicate TSNs field.

   Each duplicate TSN is listed in this field as many times as the TSN
   was received since the previous NR-SACK was sent.  For example, if a
   data receiver were to get the TSN 19 three times, the data receiver
   would list 19 twice in the outbound NR-SACK.  After sending the NR-
   SACK if the receiver received one more TSN 19, the receiver would



Ekiz, et al.            Expires February 19, 2011              [Page 10]


Internet-Draft              NR-SACKs for SCTP                August 2010


   list 19 as a duplicate once in the next outgoing NR-SACK.


5.  An Illustrative Example

   Assume the following DATA chunks have arrived at the receiver.

                      --------------------------------
                      | TSN=16| SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      | TSN=15| SID=1 | SSN= 4 | U=0 |
                      --------------------------------
                      | TSN=14| SID=0 | SSN= 4 | U=0 |
                      --------------------------------
                      | TSN=13| SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=11| SID=0 | SSN= 3 | U=0 |
                      -------------------------------
                      |                              |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=8 | SID=2 | SSN=N/A| U=1 |
                      --------------------------------
                      | TSN=7 | SID=1 | SSN= 2 | U=0 |
                      --------------------------------
                      | TSN=6 | SID=1 | SSN= 1 | U=0 |
                      --------------------------------
                      | TSN=5 | SID=0 | SSN= 1 | U=0 |
                      --------------------------------
                      |                              |
                      --------------------------------
                      | TSN=3 | SID=1 | SSN= 0 | U=0 |
                      --------------------------------
                      | TSN=2 | SID=0 | SSN= 0 | U=0 |
                      --------------------------------


   The above figure shows the list of DATA chunks at the receiver.  TSN
   denotes the transmission sequence number of the DATA chunk, SID
   denotes the stream id to which the DATA chunk belongs, SSN denotes
   the sequence number of the DATA chunk within its stream, and the U
   bit denotes whether the DATA chunk requires ordered(=0) or
   unordered(=1) delivery [RFC4960].  Note that TSNs 4,9,10, and 12 have
   not arrived.




Ekiz, et al.            Expires February 19, 2011              [Page 11]


Internet-Draft              NR-SACKs for SCTP                August 2010


   This data can be viewed as three separate streams as follows (assume
   each stream begins with SSN=0.)  Note that in this example, the
   application uses stream 2 for unordered data transfer.  By
   definition, SSN fields of unordered DATA chunks are ignored.



   Stream-0:

   SSN:         0     1     2     3     4
   TSN:      |  2  |  5  |     |  11 |  14 |
   U-Bit:    |  0  |  0  |     |  0  |   0 |

   Stream-1:

   SSN:         0     1     2     3     4
   TSN:      |  3  |  6  |  7  |     |  15 |
   U-Bit:    |  0  |  0  |  0  |     |   0 |

   Stream-2:

   SSN:        N/A   N/A   N/A
   TSN:      |  8  |  13 |  16 |
   U-Bit:    |  1  |   1 |   1 |

   The NR-SACK to acknowledge the above data SHOULD be constructed as
   follows for each of the three cases described below (the a_rwnd is
   arbitrarily set to 4000):

   CASE-1: Minimal Data Receiver Responsibility - no out-of-order
   deliverable data yet delivered

   None of the deliverable out-of-order DATA chunks have been delivered,
   and the receiver of the above data does not take responsibility for
   any of the received out-of-order DATA chunks.  The receiver reserves
   the right to renege any or all of the out-of-order DATA chunks.















Ekiz, et al.            Expires February 19, 2011              [Page 12]


Internet-Draft              NR-SACKs for SCTP                August 2010


       +-----------------------------+-----------------------------+
       | Type = 0x10  |   00000000   |      Chunk Length = 32      |
       +-----------------------------+-----------------------------+
       |                    Cumulative TSN Ack = 3                 |
       +-----------------------------+-----------------------------+
       |                        a_rwnd = 4000                      |
       +-----------------------------+-----------------------------+
       | Num of R Gap Ack Blocks = 3 |Num of NR Gap Ack Blocks = 0 |
       +-----------------------------+-----------------------------+
       |    Num of Duplicates = 0    |            0x00             |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #1 Start = 2 | R Gap Ack Block #1 End = 5  |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #2 Start = 8 | R Gap Ack Block #2 End = 8  |
       +-----------------------------+-----------------------------+
       |R Gap Ack Block #3 Start = 10| R Gap Ack Block #3 End = 13 |
       +-----------------------------+-----------------------------+


   CASE-2: Minimal Data Receiver Responsibility - all out-of-order
   deliverable data delivered

   In this case, the NR-SACK chunk is being sent after the data receiver
   has delivered all deliverable out-of-order DATA chunks to its
   receiving application(i.e., TSNs 5,6,7,8,13, and 16.)  The receiver
   reserves the right to renege on all undelivered out-of-order DATA
   chunks(i.e., TSNs 11,14, and 15.)


      +------------------------------+------------------------------+
      | Type = 0x10  |     0x00      |       Chunk Length = 40      |
      +------------------------------+------------------------------+
      |                     Cumulative TSN Ack = 3                  |
      +------------------------------+------------------------------+
      |                        a_rwnd = 4000                        |
      +------------------------------+------------------------------+
      | Num of R Gap Ack Blocks = 2  | Num of NR Gap Ack Blocks = 3 |
      +------------------------------+------------------------------+
      |    Num of Duplicates = 0     |              0x00            |
      +------------------------------+------------------------------+
      | R Gap Ack Block #1 Start = 8 | R Gap Ack Block #1 End = 8   |
      +------------------------------+------------------------------+
      | R Gap Ack Block #2 Start = 11| R Gap Ack Block #2 End = 12  |
      +------------------------------+------------------------------+
      |NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5  |
      +------------------------------+------------------------------+
      |NR Gap Ack Block #2 Start = 10| NR Gap Ack Block #2 End = 10 |
      +------------------------------+------------------------------+



Ekiz, et al.            Expires February 19, 2011              [Page 13]


Internet-Draft              NR-SACKs for SCTP                August 2010


      |NR Gap Ack Block #3 Start = 13| NR Gap Ack Block #3 End = 13 |
      +------------------------------+------------------------------+

   CASE-3: Maximal Data Receiver Responsibility

   In this special case, all out-of-order data blocks acknowledged are
   non-renegable.  This case would occur when the data receiver is
   programmed never to renege, and takes responsibility to deliver all
   DATA chunks that arrive out-of-order.  In this case Num of R Gap Ack
   Blocks is zero indicating all reported out-of-order TSNs are nr-gap-
   acked.

     +--------------------------------+-------------------------------+
     |  Type = 0x10   |     0x00      |     Chunk Length = 32         |
     +--------------------------------+-------------------------------+
     |                      Cumulative TSN Ack = 3                    |
     +--------------------------------+-------------------------------+
     |                          a_rwnd = 4000                         |
     +--------------------------------+-------------------------------+
     |  Num of R Gap Ack Blocks = 0   |  Num of NR Gap Ack Blocks = 3 |
     +--------------------------------+-------------------------------+
     |     Num of Duplicates = 0      |             0x00              |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #1 Start = 2  | NR Gap Ack Block #1 End = 5   |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #2 Start = 8  | NR Gap Ack Block #2 End = 8   |
     +--------------------------------+-------------------------------+
     | NR Gap Ack Block #3 Start = 10 | NR Gap Ack Block #3 End = 13  |
     +--------------------------------+-------------------------------+


6.  Procedures

   The procedures regarding "when" to send an NR-SACK chunk are
   identical to the procedures regarding when to send a SACK chunk, as
   outlined in Section 6.2 of [RFC4960].

6.1.  Sending an NR-SACK chunk

   All of the NR-SACK chunk fields identical to the SACK chunk MUST be
   formed as described in Section 6.2 of [RFC4960].

   It is up to the data receiver whether or not to take responsibility
   for delivery of each out-of-order DATA chunk.  An out-of-order DATA
   chunk that has already been delivered, or that the receiver takes
   responsibility to deliver (i.e., guarantees not to renege) is Non
   Renegable(NR), and SHOULD be included in an NR Gap Ack Block field of
   the outgoing NR-SACK.  All other out-of-order data is (R)enegable,



Ekiz, et al.            Expires February 19, 2011              [Page 14]


Internet-Draft              NR-SACKs for SCTP                August 2010


   and SHOULD be included in R Gap Ack Block field of the outgoing NR-
   SACK.

   Consider three types of data receiver:

   CASE-1:  Data receiver takes no responsibility for delivery of any
      out-of-order DATA chunks

   CASE-2:  Data receiver takes responsibility for all out-of-order DATA
      chunks that are "deliverable" (i.e., DATA chunks in-sequence
      within the stream they belong to, or DATA chunks whose (U)nordered
      bit is 1)

   CASE-3:  Data receiver takes responsibility for delivery of all out-
      of-order DATA chunks, whether deliverable or not deliverable


   The data receiver SHOULD follow the procedures outlined below for
   building the NR-SACK.

   CASE-1:

   1A)  Identify the TSNs received out-of-order.

   1B)  For these out-of-order TSNs, identify the R Gap Ack Blocks.
      Fill the Number of R Gap Ack Blocks (N) field, R Gap Ack Block #i
      Start, and R Gap Ack Block #i End where i goes from 1 to N.

   1C)  Set the Number of NR Gap Ack Blocks (M) field to 0.


   CASE-2:

   2A)  Identify the TSNs received out-of-order.

   2B)  For the received out-of-order TSNs, check the (U)nordered bit of
      each TSN.  Tag unordered TSNs as NR.

   2C)  For each stream, also identify the TSNs received out-of-order
      but are in-sequence within that stream.  Tag those in-sequence
      TSNs as NR.

   2D)  Tag all out-of-order data that is not NR as (R)enegable.








Ekiz, et al.            Expires February 19, 2011              [Page 15]


Internet-Draft              NR-SACKs for SCTP                August 2010


   2E)  For those TSNs tagged as (R)enegable, identify the (R)enegable
      Blocks.  Fill the Number of R Gap Ack Blocks(N) field, R Gap Ack
      Block #i Start, and R Gap Ack Block #i End where i goes from 1 to
      N.

   2F)  For those TSNs tagged as NR, identify the NR Blocks.  Fill the
      Number of NR Gap Ack Blocks(M) field, NR Gap Ack Block #i Start,
      and NR Gap Ack Block #i End where i goes from 1 to M.


   CASE-3:

   3A)  Identify the TSNs received out-of-order.  All of these TSNs
      SHOULD be nr-gap-acked.

   3B)  Set the Number of R Gap Ack Blocks (N) field to 0.

   3C)  For these out-of-order TSNs, identify the NR Gap Ack Blocks.
      Fill the Number of NR Gap Ack Blocks (M) field, NR Gap Ack Block
      #i Start, and NR Gap Ack Block #i End where i goes from 1 to M.


   RFC4960 states that the SCTP endpoint MUST report as many Gap Ack
   Blocks as can fit in a single SACK chunk limited by the current path
   MTU.  When using NR-SACKs, the SCTP endpoint SHOULD fill as many R
   Gap Ack Blocks and NR Gap Ack Blocks starting from the Cumulative TSN
   Ack value as can fit in a single NR-SACK chunk limited by the current
   path MTU.  If space remains, the SCTP endpoint SHOULD fill as many
   Duplicate TSNs as possible starting from Cumulative TSN Ack value.

6.2.  Receiving an NR-SACK Chunk

   When an NR-SACK chunk is received, all of the NR-SACK fields
   identical to a SACK chunk SHOULD be processed and handled as in SACK
   chunk handling outlined in Section 6.2.1 of [RFC4960].

   The NR Gap Ack Block Start(s) and NR Gap Ack Block End(s) are offsets
   relative to the cum-ack.  To calculate the actual range of nr-gap-
   acked TSNs, the cum-ack MUST be added to the Start and End.

   For example, assume an incoming NR-SACK chunk's cum-ack is 12 and an
   NR Gap Ack Block defines the NR Gap Ack Block Start=5, and the NR Gap
   Ack Block End=7.  This NR Gap Ack block nr-gap-acks TSNs 17 through
   19 inclusive.

   Upon reception of an NR-SACK chunk, all TSNs listed in either R Gap
   Ack Block(s) or NR Gap Ack Block(s) SHOULD be processed as would be
   TSNs included in Gap Ack Block(s) of a SACK chunk.  All TSNs in all



Ekiz, et al.            Expires February 19, 2011              [Page 16]


Internet-Draft              NR-SACKs for SCTP                August 2010


   NR Gap Ack Blocks SHOULD be removed from the data sender's
   retransmission queue as their delivery to the receiving application
   has either already occurred, or is guaranteed by the data receiver.

   Although R Gap Ack Blocks and NR Gap Ack Blocks SHOULD be disjoint
   sets, NR-SACK processing SHOULD work if an NR-SACK chunk has a TSN
   listed in both an R Gap Ack Block and an NR Gap Ack Block.  In this
   case, the TSN SHOULD be treated as Non-Renegable.

   Implementation Note:

   Most of NR-SACK processing at the data sender can be implemented by
   using the same routines as in SACK that process the cum ack and the
   gap ack(s), followed by removal of nr-gap-acked DATA chunks from the
   retransmission queue.  However, with NR-SACKs, as out-of-order DATA
   is sometimes removed from the retransmission queue, the gap ack
   processing routine should recognize that the data sender's
   retransmission queue has some transmitted data removed.  For example,
   while calculating missing reports, the gap ack processing routine
   cannot assume that the highest TSN transmitted is always at the tail
   (right edge) of the retransmission queue.


7.  Security Considerations

   This document does not add any security considerations to those
   specified in [RFC4960].


8.  IANA considerations

   This document defines a new chunk type to transfer the NR-SACK
   information.  Table 2 illustrates the new chunk type.

   The new chunk type must come from the range of chunk types where the
   upper two bits are zero.  We recommend 0x10 but any other available
   code point with the upper two bits set to zero is acceptable.

      Chunk Type  Chunk Name
      --------------------------------------------------------------
      0x10        Non-Renegable Selective Acknowledgment    (NR-SACK)

      Table 2: NR-SACK Chunk








Ekiz, et al.            Expires February 19, 2011              [Page 17]


Internet-Draft              NR-SACKs for SCTP                August 2010


9.  Acknowledgments

   This Internet Draft is the result of a great deal of constructive
   discussion with several people, notably Phillip Conrad, Ertugrul
   Yilmaz, and Jonathan Leighton.


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

10.2.  Informative References

   [Natarajan]
              Natarajan, P., Ekiz, N., Yilmaz, E., Amer, P., Iyengar,
              J., and R. Stewart, "Non-Renegable Selective
              Acknowledgments (NR-SACKs) for SCTP",  International
              Conference on Network Protocols (ICNP) 2008,  Orlando,
              October 2008 .

   [Yilmaz]   Yilmaz, E., Ekiz, N., Natarajan, P., Amer, P., Leighton,
              J., Baker, F., and R. Stewart, "Throughput analysis of
              Non-Renegable Selective Acknowledgments (NR-SACKs) for
              SCTP",  Comput. Commun. (2010), doi:10.1016/
              j.comcom.2010.06.028 .















Ekiz, et al.            Expires February 19, 2011              [Page 18]


Internet-Draft              NR-SACKs for SCTP                August 2010


Authors' Addresses

   Nasif Ekiz
   University of Delaware
   Computer and Information Sciences Department
   Newark, DE  19716
   USA

   Phone:
   Email: nekiz@udel.edu


   Paul D. Amer
   University of Delaware
   Computer and Information Sciences Department
   Newark, DE  19716
   USA

   Phone: 302-831-1944
   Email: amer@cis.udel.edu


   Preethi Natarajan
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone:
   Email: prenatar@cisco.com


   Randall R. Stewart
   Researcher
   Chapi, SC  29036
   USA

   Phone: 202-595-8626
   Email: randall@lakerest.net












Ekiz, et al.            Expires February 19, 2011              [Page 19]


Internet-Draft              NR-SACKs for SCTP                August 2010


   Janardhan Iyengar
   Franklin & Marshall College
   Math and Computer Science
   PO Box 3003
   Lancaster, PA  17604-3003
   USA

   Phone: 717-358-4774
   Email: jiyengar@fandm.edu










































Ekiz, et al.            Expires February 19, 2011              [Page 20]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/