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Versions: (draft-cel-nfsv4-rpcrdma-version-two) 00 01 02

Network File System Version 4                              C. Lever, Ed.
Internet-Draft                                                    Oracle
Intended status: Standards Track                               D. Noveck
Expires: 4 January 2021                                           NetApp
                                                             3 July 2020


                    RPC-over-RDMA Version 2 Protocol
                draft-ietf-nfsv4-rpcrdma-version-two-02

Abstract

   This document specifies the second version of a transport protocol
   that conveys Remote Procedure Call (RPC) messages using Remote Direct
   Memory Access (RDMA).  This version of the protocol is extensible.

Note

   Discussion of this draft takes place on the NFSv4 working group
   mailing list (nfsv4@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/nfsv4/. Working Group
   information can be found at https://datatracker.ietf.org/wg/nfsv4/
   about/.

   This note is to be removed before publishing as an RFC.

   The source for this draft is maintained in GitHub.  Suggested changes
   can be submitted as pull requests at https://github.com/chucklever/
   i-d-rpcrdma-version-two.  Instructions are on that page as well.

Status of This Memo

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

   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 https://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 4 January 2021.





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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Design Goals  . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Motivation for a New Version  . . . . . . . . . . . . . .   6
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   7
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Remote Procedure Calls  . . . . . . . . . . . . . . . . .   7
       3.1.1.  Upper-Layer Protocols . . . . . . . . . . . . . . . .   7
       3.1.2.  Requesters and Responders . . . . . . . . . . . . . .   7
       3.1.3.  RPC Transports  . . . . . . . . . . . . . . . . . . .   8
       3.1.4.  External Data Representation  . . . . . . . . . . . .  10
     3.2.  Remote Direct Memory Access . . . . . . . . . . . . . . .  11
       3.2.1.  Direct Data Placement . . . . . . . . . . . . . . . .  11
       3.2.2.  RDMA Transport Requirements . . . . . . . . . . . . .  12
   4.  RPC-over-RDMA Framework . . . . . . . . . . . . . . . . . . .  14
     4.1.  Message Framing . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Managing Receiver Resources . . . . . . . . . . . . . . .  15
       4.2.1.  Flow Control  . . . . . . . . . . . . . . . . . . . .  15
       4.2.2.  Inline Threshold  . . . . . . . . . . . . . . . . . .  16
       4.2.3.  Initial Connection State  . . . . . . . . . . . . . .  17
     4.3.  XDR Encoding with Chunks  . . . . . . . . . . . . . . . .  18



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       4.3.1.  Reducing an XDR Stream  . . . . . . . . . . . . . . .  18
       4.3.2.  DDP-Eligibility . . . . . . . . . . . . . . . . . . .  18
       4.3.3.  RDMA Segments . . . . . . . . . . . . . . . . . . . .  19
       4.3.4.  Chunks  . . . . . . . . . . . . . . . . . . . . . . .  19
       4.3.5.  Read Chunks . . . . . . . . . . . . . . . . . . . . .  21
       4.3.6.  Write Chunks  . . . . . . . . . . . . . . . . . . . .  22
     4.4.  Payload Formats . . . . . . . . . . . . . . . . . . . . .  23
       4.4.1.  Simple Format . . . . . . . . . . . . . . . . . . . .  24
       4.4.2.  Continued Format  . . . . . . . . . . . . . . . . . .  26
       4.4.3.  Special Format  . . . . . . . . . . . . . . . . . . .  28
     4.5.  Reverse-Direction Operation . . . . . . . . . . . . . . .  30
       4.5.1.  Sending a Reverse-Direction RPC Call  . . . . . . . .  30
       4.5.2.  Sending a Reverse-Direction RPC Reply . . . . . . . .  30
       4.5.3.  In the Absence of Support For Reverse-Direction
               Operation . . . . . . . . . . . . . . . . . . . . . .  31
       4.5.4.  Using Chunks During Reverse-Direction Operation . . .  32
       4.5.5.  Reverse-Direction Retransmission  . . . . . . . . . .  32
   5.  Transport Properties  . . . . . . . . . . . . . . . . . . . .  33
     5.1.  Transport Properties Model  . . . . . . . . . . . . . . .  33
     5.2.  Current Transport Properties  . . . . . . . . . . . . . .  35
       5.2.1.  Maximum Send Size . . . . . . . . . . . . . . . . . .  36
       5.2.2.  Receive Buffer Size . . . . . . . . . . . . . . . . .  36
       5.2.3.  Maximum RDMA Segment Size . . . . . . . . . . . . . .  37
       5.2.4.  Maximum RDMA Segment Count  . . . . . . . . . . . . .  37
       5.2.5.  Reverse-Direction Support . . . . . . . . . . . . . .  37
       5.2.6.  Host Authentication Message . . . . . . . . . . . . .  38
   6.  Transport Messages  . . . . . . . . . . . . . . . . . . . . .  38
     6.1.  Transport Header Types  . . . . . . . . . . . . . . . . .  39
     6.2.  Headers and Chunks  . . . . . . . . . . . . . . . . . . .  40
       6.2.1.  Common Transport Header Prefix  . . . . . . . . . . .  40
       6.2.2.  Transport Header Prefix . . . . . . . . . . . . . . .  41
       6.2.3.  External Data Payloads  . . . . . . . . . . . . . . .  43
       6.2.4.  Remote Invalidation . . . . . . . . . . . . . . . . .  44
     6.3.  Header Types  . . . . . . . . . . . . . . . . . . . . . .  44
       6.3.1.  RDMA2_MSG: Convey RPC Message Inline  . . . . . . . .  45
       6.3.2.  RDMA2_NOMSG: Convey External RPC Message  . . . . . .  45
       6.3.3.  RDMA2_ERROR: Report Transport Error . . . . . . . . .  46
       6.3.4.  RDMA2_CONNPROP: Exchange Transport Properties . . . .  47
       6.3.5.  RDMA2_GRANT: Grant Credits  . . . . . . . . . . . . .  48
     6.4.  Choosing a Reply Mechanism  . . . . . . . . . . . . . . .  48
   7.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  49
     7.1.  Basic Transport Stream Parsing Errors . . . . . . . . . .  50
       7.1.1.  RDMA2_ERR_VERS  . . . . . . . . . . . . . . . . . . .  50
       7.1.2.  RDMA2_ERR_INVAL_HTYPE . . . . . . . . . . . . . . . .  50
       7.1.3.  RDMA2_ERR_INVAL_CONT  . . . . . . . . . . . . . . . .  51
     7.2.  XDR Errors  . . . . . . . . . . . . . . . . . . . . . . .  51
       7.2.1.  RDMA2_ERR_BAD_XDR . . . . . . . . . . . . . . . . . .  51
       7.2.2.  RDMA2_ERR_BAD_PROPVAL . . . . . . . . . . . . . . . .  52



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     7.3.  Responder RDMA Operational Errors . . . . . . . . . . . .  52
       7.3.1.  RDMA2_ERR_READ_CHUNKS . . . . . . . . . . . . . . . .  52
       7.3.2.  RDMA2_ERR_WRITE_CHUNKS  . . . . . . . . . . . . . . .  53
       7.3.3.  RDMA2_ERR_SEGMENTS  . . . . . . . . . . . . . . . . .  53
       7.3.4.  RDMA2_ERR_WRITE_RESOURCE  . . . . . . . . . . . . . .  53
       7.3.5.  RDMA2_ERR_REPLY_RESOURCE  . . . . . . . . . . . . . .  54
     7.4.  Other Operational Errors  . . . . . . . . . . . . . . . .  54
       7.4.1.  RDMA2_ERR_SYSTEM  . . . . . . . . . . . . . . . . . .  54
     7.5.  RDMA Transport Errors . . . . . . . . . . . . . . . . . .  55
   8.  XDR Protocol Definition . . . . . . . . . . . . . . . . . . .  55
     8.1.  Code Component License  . . . . . . . . . . . . . . . . .  55
     8.2.  Extraction of the XDR Definition  . . . . . . . . . . . .  57
     8.3.  XDR Definition for RPC-over-RDMA Version 2 Core
           Structures  . . . . . . . . . . . . . . . . . . . . . . .  58
     8.4.  XDR Definition for RPC-over-RDMA Version 2 Base Header
           Types . . . . . . . . . . . . . . . . . . . . . . . . . .  60
     8.5.  Use of the XDR Description  . . . . . . . . . . . . . . .  62
   9.  RPC Bind Parameters . . . . . . . . . . . . . . . . . . . . .  63
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  64
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  65
     11.1.  Memory Protection  . . . . . . . . . . . . . . . . . . .  65
       11.1.1.  Protection Domains . . . . . . . . . . . . . . . . .  65
       11.1.2.  Handle (STag) Predictability . . . . . . . . . . . .  65
       11.1.3.  Memory Protection  . . . . . . . . . . . . . . . . .  65
       11.1.4.  Denial of Service  . . . . . . . . . . . . . . . . .  66
     11.2.  RPC Message Security . . . . . . . . . . . . . . . . . .  67
       11.2.1.  RPC-over-RDMA Protection at Other Layers . . . . . .  67
       11.2.2.  RPCSEC_GSS on RPC-over-RDMA Transports . . . . . . .  67
     11.3.  Transport Properties . . . . . . . . . . . . . . . . . .  70
     11.4.  Host Authentication  . . . . . . . . . . . . . . . . . .  70
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  70
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  71
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  71
     13.2.  Informative References . . . . . . . . . . . . . . . . .  72
   Appendix A.  ULB Specifications . . . . . . . . . . . . . . . . .  74
     A.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .  74
     A.2.  Maximum Reply Size  . . . . . . . . . . . . . . . . . . .  75
     A.3.  Reverse-Direction Operation . . . . . . . . . . . . . . .  75
     A.4.  Additional Considerations . . . . . . . . . . . . . . . .  76
     A.5.  ULP Extensions  . . . . . . . . . . . . . . . . . . . . .  76
   Appendix B.  Extending RPC-over-RDMA Version 2  . . . . . . . . .  77
     B.1.  Documentation Requirements  . . . . . . . . . . . . . . .  77
     B.2.  Adding New Header Types to RPC-over-RDMA Version 2  . . .  78
     B.3.  Adding New Header Flags to the Protocol . . . . . . . . .  79
     B.4.  Adding New Transport properties to the Protocol . . . . .  79
     B.5.  Adding New Error Codes to the Protocol  . . . . . . . . .  80
   Appendix C.  Differences from RPC-over-RDMA Version 1 . . . . . .  81
     C.1.  Changes to the XDR Definition . . . . . . . . . . . . . .  81



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     C.2.  Transport Properties  . . . . . . . . . . . . . . . . . .  82
     C.3.  Credit Management Changes . . . . . . . . . . . . . . . .  82
     C.4.  Inline Threshold Changes  . . . . . . . . . . . . . . . .  83
     C.5.  Message Continuation Changes  . . . . . . . . . . . . . .  84
     C.6.  Host Authentication Changes . . . . . . . . . . . . . . .  85
     C.7.  Support for Remote Invalidation . . . . . . . . . . . . .  85
     C.8.  Error Reporting Changes . . . . . . . . . . . . . . . . .  86
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  87
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  87

1.  Introduction

   Remote Direct Memory Access (RDMA) [RFC5040] [RFC5041] [IBA] is a
   technique for moving data efficiently between network nodes.  By
   placing transferred data directly into destination buffers using
   Direct Memory Access, RDMA delivers the reciprocal benefits of faster
   data transfer and reduced host CPU overhead.

   Open Network Computing Remote Procedure Call (ONC RPC, often
   shortened in NFSv4 documents to RPC) [RFC5531] is a Remote Procedure
   Call protocol that runs over a variety of transports.  Most RPC
   implementations today use UDP [RFC0768] or TCP [RFC0793].  On UDP, a
   datagram encapsulates each RPC message.  Within a TCP byte stream, a
   record marking protocol delineates RPC messages.

   An RDMA transport, too, conveys RPC messages in a fashion that must
   be fully defined if RPC implementations are to interoperate when
   using RDMA to transport RPC transactions.  Although RDMA transports
   encapsulate messages like UDP, they deliver them reliably and in
   order, like TCP.  Further, they implement a bulk data transfer
   service not provided by traditional network transports.  Therefore,
   we treat RDMA as a novel transport type for RPC.

1.1.  Design Goals

   The general mission of RPC-over-RDMA transports is to leverage
   network hardware capabilities to reduce host CPU needs related to the
   transport of RPC messages.  In particular, this includes mitigating
   host interrupt rates and limiting the necessity to copy RPC payload
   bytes on receivers.

   These hardware capabilities benefit both RPC clients and servers.  On
   balance, however, the RPC-over-RDMA protocol design approach has been
   to bolster clients more than servers, as the client is typically
   where applications are most hungry for CPU resources.






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   Additionally, RPC-over-RDMA transports are designed to support RPC
   applications transparently.  However, such transports can also
   provide mechanisms that enable further optimization of data transfer
   when RPC applications are structured to exploit direct data
   placement.  In this context, the Network File System (NFS) family of
   protocols (as described in [RFC1094], [RFC1813], [RFC7530],
   [RFC5661], [RFC7862], and subsequent NFSv4 minor versions) are all
   potential beneficiaries of RPC-over-RDMA.

   A complete problem statement appears in [RFC5532].

1.2.  Motivation for a New Version

   Storage administrators have broadly deployed the RPC-over-RDMA
   version 1 protocol specified in [RFC8166].  However, there are known
   shortcomings to this protocol:

   *  The protocol's default size of Receive buffers forces the use of
      RDMA Read and Write transfers for small payloads, and limits the
      size of reverse direction messages.

   *  It is difficult to make optimizations or protocol fixes that
      require changes to on-the-wire behavior.

   *  For some RPC procedures, the maximum reply size is difficult or
      impossible for an RPC client to estimate in advance.

   To address these issues in a way that preserves interoperation with
   existing RPC-over-RDMA version 1 deployments, we present a second
   version of the RPC-over-RDMA transport protocol in the current
   document.

   The version of RPC-over-RDMA presented here is extensible, enabling
   the introduction of OPTIONAL extensions without impacting existing
   implementations.  See Appendix C.1, for further discussion.  It
   introduces a mechanism to exchange implementation properties to
   automatically provide further optimization of data transfer.

   This version also contains incremental changes that relieve
   performance constraints and enable recovery from unusual corner
   cases.  These changes are outlined in Appendix C and include a larger
   default inline threshold, the ability to convey a single RPC message
   using multiple RDMA Send operations, support for authentication of
   connection peers, richer error reporting, improved credit-based flow
   control, and support for Remote Invalidation.






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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

3.1.  Remote Procedure Calls

   This section highlights critical elements of the RPC protocol
   [RFC5531] and the External Data Representation (XDR) [RFC4506] it
   uses.  RPC-over-RDMA version 2 enables the transmission of RPC
   messges built using XDR and also uses XDR internally to describe its
   header formats.  The remainder of this document requires an
   understanding of RPC and its use of XDR.

3.1.1.  Upper-Layer Protocols

   RPCs are an abstraction used to implement the operations of an Upper-
   Layer Protocol (ULP).  For RPC-over-RDMA, "ULP" refers to an RPC
   Program and Version tuple, which is a versioned set of procedure
   calls that comprise a single well-defined API.  One example of a ULP
   is the Network File System Version 4.0 [RFC7530].  In the current
   document, the term "RPC consumer" refers to an implementation of a
   ULP running on an RPC client.

3.1.2.  Requesters and Responders

   Like a local procedure call, every RPC procedure has a set of
   "arguments" and a set of "results".  A calling context invokes a
   procedure, passing arguments to it, and the procedure subsequently
   returns a set of results.  Unlike a local procedure call, the called
   procedure is executed remotely rather than in the local application's
   execution context.

   The RPC protocol as described in [RFC5531] is fundamentally a
   message-passing protocol between one or more clients, where RPC
   consumers are running, and a server, where a remote execution context
   is available to process RPC transactions on behalf of these
   consumers.

   ONC RPC transactions consist of two types of messages:






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   *  A CALL message, or "Call", requests work.  An RPC Call message is
      designated by the value zero (0) in the message's msg_type field.
      The sender places a unique 32-bit value in the message's XID field
      to match this RPC Call message to a corresponding RPC Reply
      message.

   *  A REPLY message, or "Reply", reports the results of work requested
      by an RPC Call message.  An RPC Reply message is designated by the
      value one (1) in the message's msg_type field.  The sender copies
      the value contained in an RPC Reply message's XID field from the
      RPC Call message whose results the sender is reporting.

   Each RPC client endpoint acts as a "Requester", which serializes the
   procedure's arguments and conveys them to a server endpoint via an
   RPC Call message.  A Call message contains an RPC protocol header, a
   header describing the requested upper-layer operation, and all
   arguments.

   An RPC server endpoint acts as a "Responder", which deserializes the
   arguments and processes the requested operation.  It then serializes
   the operation's results into an RPC Reply message.  An RPC Reply
   message contains an RPC protocol header, a header describing the
   upper-layer reply, and all results.

   The Requester deserializes the results and allows the RPC consumer to
   proceed.  At this point, the RPC transaction designated by the XID in
   the RPC Call message is complete, and the XID is retired.

   In summary, Requesters send RPC Call messages to Responders to
   initiate RPC transactions.  Responders send RPC Reply messages to
   Requesters to complete the processing on an RPC transaction.

3.1.3.  RPC Transports

   The role of an "RPC transport" is to mediate the exchange of RPC
   messages between Requesters and Responders.  An RPC transport bridges
   the gap between the RPC message abstraction and the native operations
   of a network transport (e.g., a socket).

   RPC-over-RDMA is a connection-oriented RPC transport.  When a
   transport type is connection-oriented, clients initiate transport
   connections, while servers wait passively to accept incoming
   connection requests.








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3.1.3.1.  Transport Failure Recovery

   So that appropriate and timely recovery action can be taken, the
   transport implementation is responsible for notifying a Requester
   when an RPC Call or Reply was not able to be conveyed.  Recovery can
   take the form of establishing a new connection, re-sending RPC Calls,
   or terminating RPC transactions pending on the Requester.

   For instance, a connection loss may occur after a Responder has
   received an RPC Call but before it can send the matching RPC Reply.
   Once the transport notifies the Requester of the connection loss, the
   Requester can re-send all pending RPC Calls on a fresh connection.

3.1.3.2.  Forward Direction

   Traditionally, an RPC client acts as a Requester, while an RPC
   service acts as a Responder.  The current document refers to this
   direction of RPC message passing as "forward-direction" operation.

3.1.3.3.  Reverse-Direction

   The RPC specification [RFC5531] does not forbid performing RPC
   transactions in the other direction.  An RPC service endpoint can act
   as a Requester, in which case an RPC client endpoint acts as a
   Responder.  This direction of RPC message passing is known as
   "reverse-direction" operation.

   During reverse-direction operation, an RPC client is responsible for
   establishing transport connections, even though the RPC server
   originates RPC Calls.

   RPC clients and servers are usually optimized to perform and scale
   well when handling traffic in the forward direction.  They might not
   be prepared to handle operation in the reverse direction.  Not until
   NFS version 4.1 [RFC5661] has there been a strong need to handle
   reverse-direction operation.

3.1.3.4.  Bi-directional Operation

   A pair of connected RPC endpoints may choose to use only forward-
   direction or only reverse-direction operation on a particular
   transport connection.  Or, these endpoints may send Calls in both
   directions concurrently on the same transport connection.

   "Bi-directional operation" occurs when both transport endpoints act
   as a Requester and a Responder at the same time on a single
   connection.




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   Bi-directionality is an extension of RPC transport connection
   sharing.  Two RPC endpoints wish to exchange independent RPC messages
   over a shared connection but in opposite directions.  These messages
   may or may not be related to the same workloads or RPC Programs.

3.1.3.5.  XID Values

   Section 9 of [RFC5531] introduces the RPC transaction identifier, or
   "XID" for short.  A connection peer interprets the value of an XID in
   the context of the message's msg_type field.

   *  The XID of a Call is arbitrary but is unique among outstanding
      Calls from that Requester on that connection.

   *  The XID of a Reply always matches that of the initiating Call.

   After receiving a Reply, a Requester matches the XID value in that
   Reply with a Call it previously sent.

   During bi-directional operation, forward- and reverse- direction XIDs
   are typically generated on distinct hosts by possibly different
   algorithms.  There is no coordination between the generation of XIDs
   used in forward-direction and reverse-direction operation.

   Therefore, a forward-direction Requester MAY use the same XID value
   at the same time as a reverse-direction Requester on the same
   transport connection.  Although such concurrent requests use the same
   XID value, they represent distinct RPC transactions.

3.1.4.  External Data Representation

   One cannot assume that all Requesters and Responders represent data
   objects in the same way internally.  RPC uses External Data
   Representation (XDR) to translate native data types and serialize
   arguments and results [RFC4506].

   XDR encodes data independently of the endianness or size of host-
   native data types, enabling unambiguous decoding of data by a
   receiver.

   XDR assumes only that the number of bits in a byte (octet) and their
   order are the same on both endpoints and the physical network.  The
   smallest indivisible unit of XDR encoding is a group of four octets.
   XDR can also flatten lists, arrays, and other complex data types into
   a stream of bytes.






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   We refer to a serialized stream of bytes that is the result of XDR
   encoding as an "XDR stream".  A sender encodes native data into an
   XDR stream and then transmits that stream to a receiver.  The
   receiver decodes incoming XDR byte streams into its native data
   representation format.

3.1.4.1.  XDR Opaque Data

   Sometimes, a data item is to be transferred as-is, without encoding
   or decoding.  We refer to the contents of such a data item as "opaque
   data".  XDR encoding places the content of opaque data items directly
   into an XDR stream without altering it in any way.  ULPs or
   applications perform any needed data translation in this case.
   Examples of opaque data items include the content of files or generic
   byte strings.

3.1.4.2.  XDR Roundup

   The number of octets in a variable-length data item precedes that
   item in an XDR stream.  If the size of an encoded data item is not a
   multiple of four octets, the sender appends octets containing zero
   after the end of the data item.  These zero octets shift the next
   encoded data item in the XDR stream so that it always starts on a
   four-octet boundary.  The addition of extra octets does not change
   the encoded size of the data item.  Receivers do not expose the extra
   octets to ULPs.

   We refer to this technique as "XDR roundup", and the extra octets as
   "XDR roundup padding".

3.2.  Remote Direct Memory Access

   When a third party transfers large RPC payloads, RPC Requesters and
   Responders can become more efficient.  An example of such a third
   party might be an intelligent network interface (data movement
   offload), which places data in the receiver's memory so that no
   additional adjustment of data alignment is necessary (direct data
   placement or "DDP").  RDMA transports enable both of these
   optimizations.

   In the current document, "RDMA" refers to the physical mechanism an
   RDMA transport utilizes when moving data.

3.2.1.  Direct Data Placement

   Typically, RPC implementations copy the contents of RPC messages into
   a buffer before being sent.  An efficient RPC implementation sends
   bulk data without copying it into a separate send buffer first.



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   However, socket-based RPC implementations are often unable to receive
   data directly into its final place in memory.  Receivers often need
   to copy incoming data to finish an RPC operation sometimes, if only
   to adjust data alignment.

   Although it may not be efficient, before an RDMA transfer, a sender
   may copy data into an intermediate buffer.  After an RDMA transfer, a
   receiver may copy that data again to its final destination.  In this
   document, the term "DDP" refers to any optimized data transfer where
   a receiving host's CPU does not move transferred data to another
   location after arrival.

   RPC-over-RDMA version 2 enables the use of RDMA Read and Write
   operations to achieve both data movement offload and DDP.  However,
   note that not all RDMA-based data transfer qualifies as DDP, and some
   mechanisms that do not employ explicit RDMA can place data directly.

3.2.2.  RDMA Transport Requirements

   RDMA transports require that RDMA consumers provision resources in
   advance to achieve good performance during receive operations.  An
   RDMA consumer might provide Receive buffers in advance by posting an
   RDMA Receive Work Request for every expected RDMA Send from a remote
   peer.  These buffers are provided before the remote peer posts RDMA
   Send Work Requests.  Thus this is often referred to as "pre-posting"
   buffers.

   An RDMA Receive Work Request remains outstanding until the RDMA
   provider matches it to an inbound Send operation.  The resources
   associated with that Receive must be retained in host memory, or
   "pinned", until the Receive completes.

   Given these tenets of operation, the RPC-over-RDMA version 2 protocol
   assumes each transport provides the following abstract operations.  A
   more complete discussion of these operations appears in [RFC5040].

3.2.2.1.  Memory Registration

   Memory registration assigns a steering tag to a region of memory,
   permitting the RDMA provider to perform data-transfer operations.
   The RPC-over-RDMA version 2 protocol assumes that a steering tag of
   no more than 32 bits and memory addresses of up to 64 bits in length
   identifies each registered memory region.








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3.2.2.2.  RDMA Send

   The RDMA provider supports an RDMA Send operation, with completion
   signaled on the receiving peer after RDMA provider has placed data in
   a pre-posted buffer.  Sends complete at the receiver in the order
   they were posted at the sender.  The size of the remote peer's pre-
   posted buffers limits the amount of data that can be transferred by a
   single RDMA Send operation.

3.2.2.3.  RDMA Receive

   The RDMA provider supports an RDMA Receive operation to receive data
   conveyed by incoming RDMA Send operations.  To reduce the amount of
   memory that must remain pinned awaiting incoming Sends, the amount of
   memory posted per Receive is limited.  The RDMA consumer (in this
   case, the RPC-over-RDMA version 2 protocol) provides flow control to
   prevent overrunning receiver resources.

3.2.2.4.  RDMA Write

   The RDMA provider supports an RDMA Write operation to place data
   directly into a remote memory region.  The local host initiates an
   RDMA Write and the RDMA provider signals completion there.  The
   remote RDMA provider does not signal completion on the remote peer.
   The local host provides the steering tag, the memory address, and the
   length of the remote peer's memory region.

   RDMA Writes are not ordered relative to one another, but are ordered
   relative to RDMA Sends.  Thus, a subsequent RDMA Send completion
   signaled on the local peer guarantees that prior RDMA Write data has
   been successfully placed in the remote peer's memory.

3.2.2.5.  RDMA Read

   The RDMA provider supports an RDMA Read operation to place remote
   source data directly into local memory.  The local host initiates an
   RDMA Read and and the RDMA provider signals completion there.  The
   remote RDMA provider does not signal completion on the remote peer.
   The local host provides the steering tags, the memory addresses, and
   the lengths for the remote source and local destination memory
   regions.

   The RDMA consumer (in this case, the RPC-over-RDMA version 2
   protocol) signals Read completion to the remote peer as part of a
   subsequent RDMA Send message.  The remote peer can then invalidate
   steering tags and subsequently free associated source memory regions.





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4.  RPC-over-RDMA Framework

   Before an RDMA data transfer can occur, an endpoint first exposes
   regions of its memory to a remote endpoint.  The remote endpoint then
   initiates RDMA Read and Write operations against the exposed memory.
   A "transfer model" designates which endpoint exposes its memory and
   which is responsible for initiating the transfer of data.

   In RPC-over-RDMA version 2, only Requesters expose their memory to
   the Responder, and only Responders initiate RDMA Read and Write
   operations.  Read access to memory regions enables the Responder to
   pull RPC arguments or whole RPC Calls from each Requester.  The
   Responder pushes RPC results or whole RPC Replies to a Requester's
   memory regions to which it has write access.

4.1.  Message Framing

   Each RPC-over-RDMA version 2 message consists of at most two XDR
   streams:

   *  The "Transport stream" contains a header that describes and
      controls the transfer of the Payload stream in this RPC-over-RDMA
      message.  Every RDMA Send on an RPC-over-RDMA version 2 connection
      MUST begin with a Transport stream.

   *  The "Payload stream" contains part or all of a single RPC message.
      The sender MAY divide an RPC message at any convenient boundary
      but MUST send RPC message fragments in XDR stream order and MUST
      NOT interleave Payload streams from multiple RPC messages.  The
      RPC-over-RDMA version 2 message carrying the final part of an RPC
      message is marked (see Section 6.2.2.2).

   The RPC-over-RDMA framing mechanism described in this section
   replaces all other RPC framing mechanisms.  Connection peers use RPC-
   over-RDMA framing even when the underlying RDMA protocol runs on a
   transport type with well-defined RPC framing, such as TCP.  However,
   a ULP can negotiate the use of RDMA, dynamically enabling the use of
   RPC-over-RDMA on a connection established on some other transport
   type.  Because RPC framing delimits an entire RPC request or reply,
   the resulting shift in framing must occur between distinct RPC
   messages, and in concert with the underlying transport.










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4.2.  Managing Receiver Resources

   If any pre-posted Receive buffer on the connection is not large
   enough to accept an incoming RDMA Send, the RDMA provider can
   terminate the connection.  Likewise, if a pre-posted Receive buffer
   is not available to accept an incoming RDMA Send, the RDMA provider
   can terminate the connection.  Therefore, a sender needs to respect
   the resource limits of its peer receiver to ensure the longevity of
   each connection.  Two operational parameters communicate these limits
   between connection peers: flow control, and inline threshold.

4.2.1.  Flow Control

   RPC-over-RDMA requires reliable and in-order delivery of data
   payloads.  Therefore, RPC-over-RDMA transports MUST use the RDMA RC
   (Reliable Connected) Queue Pair (QP) type.  The use of an RC QP
   ensures in-transit data integrity and proper recovery from packet
   loss or misordering.

   However, RPC-over-RDMA itself provides a flow control mechanism to
   prevent a sender from overwhelming receiver resources.  RPC-over-RDMA
   transports employ end-to-end credit-based flow control for this
   purpose [CBFC].  Credit-based flow control is relatively simple,
   providing robust operation in the face of bursty traffic and
   automated management of receive buffer allocation.

4.2.1.1.  Granting Credits

   An RPC-over-RDMA version 2 credit is the capability to receive one
   RPC-over-RDMA version 2 message.  This arrangement enables RPC-over-
   RDMA version 2 to support asymmetrical operation, where a message in
   one direction might trigger zero, one, or multiple messages in the
   other direction in response.

   To achieve this, each posted Receive buffer on both connection peers
   receives one credit.  Each Requester has a set of Receive credits,
   and each Responder has a set of Receive credits.  These credit values
   are managed independently of one another.

   Section 7 of [RFC8166] requires that the 32-bit field containing the
   credit grant is the third word in the transport header.  To conform
   with that requirement, senders encode the two independent credit
   values into a single 32-bit field in the fixed portion of the
   transport header.  At the receiver, the low-order two bytes are the
   number of credits that are newly granted by the sender.  The granted
   credit value MUST NOT be zero since such a value would result in
   deadlock.  The high-order two bytes are the maximum number of credits
   that can be outstanding at the sender.



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   A sender must avoid posting more RDMA Send messages than the
   receiver's granted credit limit.  If the sender exceeds the granted
   value, the RDMA provider might signal an error, possibly terminating
   the connection.

   The granted credit values MAY be adjusted to match the needs or
   policies in effect on either peer.  For instance, a peer may reduce
   its granted credit value to accommodate the available resources in a
   Shared Receive Queue.

   Certain RDMA implementations may impose additional flow-control
   restrictions, such as limits on RDMA Read operations in progress at
   the Responder.  Accommodation of such restrictions is considered the
   responsibility of each RPC-over-RDMA version 2 implementation.

4.2.1.2.  Asynchronous Credit Grants

   A special header type enables one peer to refresh its credit grant to
   the other peer without sending an RPC payload.  A receiving peer can
   use this when the sender's credit grant is exhausted in the midst of
   a stream of continued messages.  See Section 6.3.5 for information
   about this header type.

   Receivers MUST always be in a position to receive one such credit
   grant update message, in addition to payload-bearing messages, to
   prevent transport deadlock.  One way a receiver can do this is to
   post one more RDMA Receive than the credit value the receiver
   granted.

4.2.2.  Inline Threshold

   An "inline threshold" value is the largest message size (in octets)
   that can be conveyed in one direction between peer implementations
   using RDMA Send and Receive channel operations.  An inline threshold
   value is less than the largest number of octets the sender can post
   in a single RDMA Send operation.  It is also less than the largest
   number of octets the receiver can reliably accept via a single RDMA
   Receive operation.

   Each connection has two inline threshold values.  There is one for
   messages flowing from Requester-to-Responder, referred to as the
   "call inline threshold", and one for messages flowing from Responder-
   to-Requester, referred to as the "reply inline threshold."

   Peers can advertise their inline threshold values via RPC-over-RDMA
   version 2 Transport Properties (see Section 5).  In the absence of an
   exchange of Transport Properties, connection peers MUST assume both
   inline thresholds are 4096 octets.



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4.2.3.  Initial Connection State

   When an RPC-over-RDMA version 2 client establishes a connection to a
   server, its first order of business is to determine the server's
   highest supported protocol version.

   Upon connection establishment, a client MUST send only a single RPC-
   over-RDMA message until it receives a valid RPC-over-RDMA message
   from the server that grants client credits.

   The second word of each transport header conveys the transport
   protocol version.  In the interest of clarity, the current document
   refers to that word as rdma_vers even though in the RPC-over-RDMA
   version 2 XDR definition, it appears as rdma_start.rdma_vers.

   Immediately after the client establishes a connection, it sends a
   single valid RPC-over-RDMA message with the value two (2) in the
   rdma_vers field.  Because the server might support only RPC-over-RDMA
   version 1, this initial message MUST NOT be larger than the version 1
   default inline threshold of 1024 octets.

4.2.3.1.  Server Does Support RPC-over-RDMA Version 2

   If the server supports RPC-over-RDMA version 2, it sends RPC-over-
   RDMA messages back to the client with the value two (2) in the
   rdma_vers field.  Both peers may assume the default inline threshold
   value for RPC-over-RDMA version 2 connections (4096 octets).

4.2.3.2.  Server Does Not Support RPC-over-RDMA Version 2

   If the server does not support RPC-over-RDMA version 2, it MUST send
   an RPC-over-RDMA message to the client with an XID that matches the
   client's first message, RDMA2_ERROR in the rdma_start.rdma_htype
   field, and with the error code RDMA2_ERR_VERS.  This message also
   reports the range of RPC-over-RDMA protocol versions that the server
   supports.  To continue operation, the client selects a protocol
   version in that range for subsequent messages on this connection.

   If the connection is dropped immediately after an RDMA2_ERROR/
   RDMA2_ERR_VERS message is received, the client should try to avoid a
   version negotiation loop when re-establishing another connection.  It
   can assume that the server does not support RPC-over-RDMA version 2.
   A client can assume the same situation (i.e., no server support for
   RPC-over-RDMA version 2) if the initial negotiation message is lost
   or dropped.  Once the version negotiation exchange is complete, both
   peers may use the default inline threshold value for the negotiated
   transport protocol version.




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4.2.3.3.  Client Does Not Support RPC-over-RDMA Version 2

   The server examines the RPC-over-RDMA protocol version used in the
   first RPC-over-RDMA message it receives.  If it supports this
   protocol version, it MUST use it in all subsequent messages it sends
   on that connection.  The client MUST NOT change the protocol version
   for the duration of the connection.

4.3.  XDR Encoding with Chunks

   When a DDP capability is available, an RDMA provider can place the
   contents of one or more XDR data items directly into a receiver's
   memory.  It can do this separately from the transfer of other parts
   of the containing XDR stream.

4.3.1.  Reducing an XDR Stream

   RPC-over-RDMA version 2 provides a mechanism for moving part of an
   RPC message via a data transfer distinct from an RDMA Send/Receive
   pair.  The sender removes one or more XDR data items from the Payload
   stream.  These items are conveyed via other mechanisms, such as one
   or more RDMA Read or Write operations.  As the receiver decodes an
   incoming message, it skips over directly placed data items.

   We refer to a data item that a sender removes from a Payload stream
   to transmit separately as a "reduced" data item.  After a sender has
   finished removing XDR data items from a Payload stream, we refer to
   it as a "reduced" Payload stream.  The data object in a transport
   header that describes memory regions containing reduced data items is
   known as a "chunk."

4.3.2.  DDP-Eligibility

   Not all XDR data items benefit from Direct Data Placement.  For
   example, small data items or data items that require XDR unmarshaling
   by the receiver do not benefit from DDP.  Moreover, it is impractical
   for receivers to prepare for every possible XDR data item in a
   protocol to appear in a chunk.

   Determining which data items are DDP-eligible is done in additional
   specifications that describe how ULPs employ DDP.  A "ULB
   specification" identifies which XDR data items a peer MAY transfer
   using DDP.  Such data items are known as "DDP-eligible."  Senders
   MUST NOT reduce any other XDR data items.  Detailed requirements for
   ULB specifications appear in Appendix A of the current document.






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4.3.3.  RDMA Segments

   When encoding a Payload stream that contains a DDP-eligible data
   item, a sender may choose to reduce that data item.  When it chooses
   to do so, the sender does not place the item into the Payload stream.
   Instead, the sender records in the transport header the location and
   size of the memory region containing that data item.

   The Requester provides location information for DDP-eligible data
   items in both RPC Call and Reply messages.  The Responder uses this
   information to retrieve arguments contained in the specified region
   of the Requester's memory or place results in that memory region.

   An "RDMA segment", or "plain segment", is a transport header data
   object that contains the precise coordinates of a contiguous memory
   region.  This region is conveyed separately from the Payload stream.
   Each RDMA segment contains the following information:

   Handle:  A steering Tag (STag) or R_key generated by registering this
      memory with the RDMA provider.

   Length:  The length of the RDMA segment's memory region, in octets.
      An "empty segment" is an RDMA segment with the value zero (0) in
      its length field.

   Offset:  The offset or beginning memory address of the RDMA segment's
      memory region.

   See [RFC5040] for further discussion.

4.3.4.  Chunks

   In RPC-over-RDMA version 2, a "chunk" refers to a portion of an RPC
   message that is moved independently of the Payload stream.  The
   sender removes chunk data from the Payload stream, transfers it via
   separate operations, and then the receiver reinserts it into the
   received Payload stream to reconstruct the complete RPC message.

   Each chunk consists of RDMA segments.  Each RDMA segment represents a
   piece of a chunk that is contiguous in memory.  A Requester MAY
   divide a chunk into RDMA segments using any convenient boundaries.
   The length of a chunk is precisely the sum of the lengths of the RDMA
   segments that comprise it.








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   The RPC-over-RDMA version 2 transport protocol does not place a limit
   on chunk size.  However, each ULP may cap the amount of data that can
   be transferred by a single RPC transaction.  For example, NFS has
   "rsize" and "wsize", which restrict the payload size of NFS READ and
   WRITE operations.  The Responder can use such limits to sanity check
   chunk sizes before using them in RDMA operations.

4.3.4.1.  Counted Arrays

   If a chunk is to contain a counted array data type, the count of
   array elements MUST remain in the Payload stream.  The sender MUST
   move the array elements into the chunk.  For example, when encoding
   an opaque byte array as a chunk, the count of bytes stays in the
   Payload stream.  The sender removes the bytes in the array from the
   Payload stream and places them in the chunk.

   Individual array elements appear in a chunk in their entirety.  For
   example, when encoding an array of arrays as a chunk, the count of
   items in the enclosing array stays in the Payload stream.  But each
   enclosed array, including its item count, is transferred as part of
   the chunk.

4.3.4.2.  Optional-Data

   If a chunk contains an optional-data data type, the "is present"
   field MUST remain in the Payload stream.  The sender MUST move the
   data, if present, to the chunk.

4.3.4.3.  XDR Unions

   A union data type MUST NOT be made DDP-eligible.  However, one or
   more of its arms MAY be made DDP-eligible, subject to the other
   requirements in this section.

4.3.4.4.  Chunk Roundup

   Except in special cases (covered in Section 4.4.3), a chunk MUST
   contain only one XDR data item.  This restriction makes it
   straightforward to reduce variable-length data items without
   affecting the XDR alignment of other data items in the Payload
   stream.

   When a sender reduces a variable-length XDR data item, data items
   remaining in the Payload stream MUST remain on four-byte alignment.
   Therefore, the sender always removes XDR roundup padding for that
   data item from the Payload stream.





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4.3.5.  Read Chunks

   A "Read chunk" represents an XDR data item that its receiver pulls
   from the sender.  A Read chunk is a list of one or more RDMA read
   segments.  Each RDMA read segment consists of a Position field
   followed by an RDMA segment, as defined in Section 4.3.3.

   Position:  The byte offset in the unreduced Payload stream where the
      receiver reinserts the data item conveyed in the chunk.  The
      sender MUST compute the Position value from the beginning of the
      unreduced Payload stream, which begins at Position zero.  All RDMA
      read segments belonging to the same Read chunk have the same value
      in their Position field.

   While constructing an RPC-over-RDMA message, the sender registers
   memory regions containing data items intended for RDMA Read
   operations.  It advertises the coordinates of these regions in Read
   chunks added to the transport header of the RPC-over-RDMA message.

   The receiver of this message then pulls the chunk data from the
   sender using RDMA Read operations.  The receiver inserts the first
   RDMA segment in a Read chunk into the Payload stream at the byte
   offset indicated by its Position field.  The receiver concatenates
   RDMA segments whose Position field value matches this offset until
   there are no more RDMA segments at that Position value.

   The Position field in an RDMA read segment indicates where the
   containing Read chunk starts in the Payload stream.  The value in
   this field MUST be a multiple of four.  All segments in the same Read
   chunk share the same Position value, even if one or more of the RDMA
   segments have a non-four-byte-aligned length.

4.3.5.1.  The Read List

   Each RPC-over-RDMA message carries a list of RDMA read segments that
   make up the set of Read chunks for that message.  When no RDMA Read
   operations are needed to complete the transmission of the message's
   Payload stream, the message's Read list is empty.  This is typically
   the case for RPC Replies, for instance.












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   If a Responder receives a Read list whose RDMA segment position
   values do not appear in monotonically increasing order, it MUST
   discard the message without processing it and respond with an
   RDMA2_ERROR message with the rdma_xid field set to the XID of the
   malformed message and the rdma_err field set to RDMA2_ERR_BAD_XDR.
   If a Requester receives a Read list whose RDMA segment position
   values do not appear in monotonically increasing order, it MUST
   discard the message without processing it and terminate the RPC
   transaction corresponds to the XID value in the rdma_xid field of the
   malformed message.

4.3.5.2.  Decoding Read Chunks

   The Responder initiates an RDMA Read to pull a Read chunk's data
   content into registered local memory whenever the XDR offset in the
   Payload stream matches that of a Read chunk.  The Responder
   acknowledges that it is finished with Read chunk source buffers when
   it sends the corresponding RPC Reply message to the Requester.  The
   Requester may then release Read chunks advertised in the RPC-over-
   RDMA Call.

4.3.5.3.  Read Chunk Roundup

   When reducing a variable-length argument data item, the Requester
   MUST NOT include the data item's XDR roundup padding in the chunk
   itself.  The chunk's total length MUST be the same as the encoded
   length of the data item.

4.3.6.  Write Chunks

   While constructing an RPC Call message, a Requester prepares memory
   regions in which to receive DDP-eligible result data items.  A "Write
   chunk" represents an XDR data item that a Responder is to push to a
   Requester.  It consists of an array of zero or more plain segments.

   A Requester provisions Write chunks long before the Responder has
   prepared the reply message.  A Requester often does not know the
   actual length of the result data items to be returned, since the
   result does not yet exist.  Thus, it MUST provision Write chunks
   large enough to accommodate the maximum possible size of each
   returned data item.

   Note that the XDR position of DDP-eligible data items in the reply's
   Payload stream is not predictable when a Requester constructs an RPC
   Call message.  Therefore, RDMA segments in a Write chunk do not have
   a Position field.





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   For each Write chunk provided by a Requester, the Responder pushes
   DDP-eligible one data item to the Requester.  It fills the chunk
   contiguously and in segment array order until the Responder has
   written that data item to the Requester in its entirety.  The
   Responder MUST copy the segment count and all segments from the
   Requester-provided Write chunk into the RPC Reply message's transport
   header.  As it does so, the Responder updates each segment length
   field to reflect the actual amount of data returned in that segment.
   The Responder then sends the RPC Reply message via an RDMA Send
   operation.

   An "empty Write chunk" is a Write chunk with a zero segment count.
   By definition, the length of an empty Write chunk is zero.  An
   "unused Write chunk" has a non-zero segment count, but all of its
   segments are empty segments.

4.3.6.1.  The Write List

   Each RPC-over-RDMA message carries a list of Write chunks.  When no
   DDP-eligible data items are to appear in the Reply to an RPC
   transaction, the Requester provides an empty Write list in the RPC
   Call, and the Responder leaves the Write list empty in the matching
   RPC Reply.

4.3.6.2.  Decoding Write Chunks

   After receiving the RPC Reply message, the Requester reconstructs the
   transferred data by concatenating the contents of each segment in
   array order into the RPC Reply message's XDR stream at the known XDR
   position of the associated DDP-eligible result data item.

4.3.6.3.  Write Chunk Roundup

   When provisioning a Write chunk for a variable-length result data
   item, the Requester MUST NOT include additional space for XDR roundup
   padding.  A Responder MUST NOT write XDR roundup padding into a Write
   chunk, even if the result is shorter than the available space in the
   chunk.  Therefore, when returning a single variable-length result
   data item, a returned Write chunk's total length MUST be the same as
   the encoded length of the result data item.

4.4.  Payload Formats

   Unlike RPC-over-TCP and RPC-over-UDP transports, RPC-over-RDMA
   transports are aware of the XDR encoding of each RPC message payload.
   For efficiency, the transport can convey DDP-eligible XDR data items
   separately from the RPC message itself.  Also, receivers are required
   to post adequate receive resources in advance of each RPC message.



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   RPC-over-RDMA version 2 provides several ways to arrange conveyance
   of an RPC-over-RDMA message.  A sender chooses the specific format
   for a message among several factors:

   *  The existence of DDP-eligible data items in the RPC message

   *  The size of the RPC message

   *  The direction of the RPC message (i.e., Call or Reply)

   *  The available hardware resources

   *  The arrangement of source and sink memory buffers

   The following subsections describe in detail how Requesters and
   Responders format RPC-over-RDMA message payloads.

4.4.1.  Simple Format

   All RPC messages conveyed via RPC-over-RDMA version 2 require at
   least one RDMA Send operation to convey.  Thus, the most efficient
   way to send an RPC message that is smaller than the inline threshold
   is to append the Payload stream directly to the Transport stream.
   When no chunks are present, senders construct Calls and Replies the
   same way, and no other operations are needed.

4.4.1.1.  Simple Format with Chunks

   If DDP-eligible data items are present in a Payload stream, a sender
   MAY reduce some or all of these items, removing them from the Payload
   stream.  The sender then uses a separate mechanism to transfer the
   reduced data items.  The Transport stream with the reduced Payload
   stream immediately following it is then transferred using one RDMA
   Send operation.

   When chunks are present, senders construct Calls differently than
   Replies.

   Simple Call
      After receiving the Transport and Payload streams of an RPC Call
      message with Read chunks, the Responder uses RDMA Read operations
      to move the reduced data items contained in Read chunks.  RPC-
      over-RDMA Calls can carry Write chunks for the Responder to use
      when sending the matching Reply.

   Simple Reply
      The Responder uses RDMA Write operations to move reduced data
      items contained in Write chunks.  Afterward, it sends the



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      Transport and Payload streams of the RPC Reply message using one
      RDMA Send.  RPC-over-RDMA Replies always carry an empty Read chunk
      list.

4.4.1.2.  Simple Format Examples

           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply

     Figure 1: A Simple Call without chunks and a Simple Reply without
                                   chunks

           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |        RDMA Read                    |
               |   <------------------------------   |
               |        RDMA Response (arg data)     |
               |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply

        Figure 2: A Simple Call with a Read chunk and a Simple Reply
                               without chunks

           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Write (result data)     |
               |   <------------------------------   |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply

      Figure 3: A Simple Call without chunks and a Simple Reply with a
                                Write chunk




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4.4.2.  Continued Format

   For various reasons, a sender can choose to split a message payload
   over multiple RPC-over-RDMA messages.  The Payload stream of each
   RPC-over-RDMA message contains a part of the RPC message.  The
   receiver reconstructs the original RPC message by concatenating in
   sequence the Payload stream of each RPC-over-RDMA message.  A sender
   MAY split an RPC message payload on any convenient boundary.

4.4.2.1.  Continued Format with Chunks

   If DDP-eligible data items are present in the Payload stream, a
   sender MAY reduce some or all of these items, removing them from the
   Payload stream.  The sender then uses a separate mechanism to
   transfer the reduced data items.  The Transport stream with the
   reduced Payload stream immediately following it is then transferred
   using one RDMA Send operation.

   As with Simple Format messages, when chunks are present, senders
   construct Calls differently than Replies.

   Continued Call
      After receiving the Transport and Payload streams of an RPC Call
      message with Read chunks, the Responder uses RDMA Read operations
      to move the reduced data items contained in Read chunks.  RPC-
      over-RDMA Calls can carry Write chunks for the Responder to use
      when sending the matching Reply.

   Continued Reply
      The Responder uses RDMA Write operations to move reduced data
      items contained in Write chunks.  Afterward, it sends the
      Transport and Payload streams of the RPC Reply message using
      multiple RDMA Sends.  RPC-over-RDMA Replies always carry an empty
      Read chunk list.

4.4.2.2.  Continued Format Examples















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           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |        RDMA Send (RDMA_MSG)         |
               |   ------------------------------>   |
               |        RDMA Send (RDMA_MSG)         |
               |   ------------------------------>   |
               |                                     |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   |

      Figure 4: A Continued Call without chunks and a Continued Reply
                               without chunks

           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |        RDMA Send (RDMA_MSG)         |
               |   ------------------------------>   |
               |        RDMA Send (RDMA_MSG)         |
               |   ------------------------------>   |
               |        RDMA Read                    |
               |   <------------------------------   |
               |        RDMA Response (arg data)     |
               |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply

      Figure 5: A Continued Call with a Read chunk and a Simple Reply
                               without chunks











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           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Write (result data)     |
               |   <------------------------------   |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   |

     Figure 6: A Simple Call without chunks and a Continued Reply with
                               a Write chunk

4.4.3.  Special Format

   Sometimes, after DDP-eligible data items have been removed, a Payload
   stream is still too large to send using only RDMA Send operations.
   In those cases, the sender can use RDMA Read or Write operations to
   convey the entire RPC message.  We refer to this as a "Special
   Format" message.

   To transmit a Special Format message, the sender transmits only the
   Transport stream with an RDMA Send operation.  The sender does not
   include the Payload stream in the send buffer.  Instead, the
   Requester provides chunks that the Responder uses to move the Payload
   stream.

   Because chunks are always present in Special Format messages, the
   sender always handles Calls and Replies differently.

   Special Call
      The Requester provides a Read chunk that contains the RPC Call
      message's Payload stream.  Every read segment in this chunk MUST
      contain zero (0) in its Position field.  This type of Read chunk
      is known as a "Position Zero Read chunk."

   Special Reply
      The Requester provisions a single Write chunk in advance, known as
      a "Reply chunk", in which the Responder places the RPC Reply
      message's Payload stream.  The Requester provisions the Reply
      chunk to accommodate the maximum expected reply size for that
      upper-layer operation.




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   One purpose of a Special Format message is to handle large RPC
   messages.  However, Requesters MAY use a Special Format message at
   any time to convey an RPC Call message.

   When it has alternatives, a Responder chooses which Format to use
   based on the chunks provided by the Requester.  If a Requester
   provided a Write chunk and the Responder has a DDP-eligible result,
   it first reduces the reply Payload stream.  If a Requester provided a
   Reply chunk and the reduced Payload stream is larger than the reply
   inline threshold, the Responder MUST use the Requester-provided Reply
   chunk for the reply.

   XDR data items may appear in these chunks without regard to their
   DDP-eligibility.  As these chunks contain a Payload stream, they MUST
   include appropriate XDR roundup padding to maintain proper XDR
   alignment of their contents.

   If a Responder receives an RPC-over-RDMA message that carries a
   Position Zero Read chunk and whose RDMA2_F_RESPONSE flag is clear but
   the message does not use the RDMA2_NOMSG header type, it MUST discard
   that message without processing it and respond with an RDMA2_ERROR
   message with the rdma_xid field set to the XID of the malformed
   message and the rdma_err field set to RDMA2_ERR_BAD_XDR.  When a
   Requester receives an RPC-over-RDMA message that carries a Reply
   chunk and whose RDMA2_F_RESPONSE flag is set but the message does not
   use the RDMA2_NOMSG header type, it MUST discard that message without
   processing it and terminate the RPC transaction corresponding to the
   XID value in the message's rdma_xid field.

4.4.3.1.  Special Format Examples

           Requester                             Responder
               |        RDMA Send (RDMA_NOMSG)       |
          Call |   ------------------------------>   |
               |        RDMA Read                    |
               |   <------------------------------   |
               |        RDMA Response (RPC call)     |
               |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Send (RDMA_MSG)         |
               |   <------------------------------   | Reply

         Figure 7: A Special Call and a Simple Reply without chunks






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           Requester                             Responder
               |        RDMA Send (RDMA_MSG)         |
          Call |   ------------------------------>   |
               |                                     |
               |                                     | Processing
               |                                     |
               |        RDMA Write (RPC reply)       |
               |   <------------------------------   |
               |        RDMA Send (RDMA_NOMSG)       |
               |   <------------------------------   | Reply

         Figure 8: A Simple Call without chunks and a Special Reply

4.5.  Reverse-Direction Operation

4.5.1.  Sending a Reverse-Direction RPC Call

   An RPC-over-RDMA server endpoint constructs the transport header for
   a reverse-direction RPC Call as follows:

   *  The server generates a new XID value (see Section 3.1.3.5 for full
      requirements) and places it in the rdma_xid field of the transport
      header and the xid field of the RPC Call message.  The RPC Call
      header MUST start with the same XID value that is present in the
      transport header.

   *  The rdma_vers field of each reverse-direction Call MUST contain
      the same value as forward-direction Calls on the same connection.

   *  The server fills in the rdma_credits with the credit values for
      the connection, as described in Section 4.2.1.1.

   *  The server determines the Payload format for the RPC message and
      fills in the rdma_htype field as appropriate (see Sections 4.4 and
      4.5.4).  Section 4.5.4 also covers the disposition of the chunk
      lists.

   *  The server MUST clear the RDMA2_F_RESPONSE flag in the rdma_flags
      field.  It sets the RDMA2_F_MORE flag in the rdma_flags field as
      described in Section 6.2.2.2.

4.5.2.  Sending a Reverse-Direction RPC Reply

   An RPC-over-RDMA server endpoint constructs the transport header for
   a reverse-direction RPC Reply as follows:






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   *  The server copies the XID value from the matching RPC Call and
      places it in the rdma_xid field of the transport header and the
      xid field of the RPC Reply message.  The RPC Reply header MUST
      start with the same XID value that is present in the transport
      header.

   *  The rdma_vers field of each reverse-direction Call MUST contain
      the same value as forward-direction Replies on the same
      connection.

   *  The server fills in the rdma_credits with the credit values for
      the connection, as described in Section 4.2.1.1.

   *  The server determines the Payload format for the RPC message and
      fills in the rdma_htype field as appropriate (see Sections 4.4 and
      4.5.4).  Section 4.5.4 also covers the disposition of the chunk
      lists.

   *  The server MUST set the RDMA2_F_RESPONSE flag in the rdma_flags
      field.  It sets the RDMA2_F_MORE flag in the rdma_flags field as
      described in Section 6.2.2.2.

4.5.3.  In the Absence of Support For Reverse-Direction Operation

   An RPC-over-RDMA transport endpoint does not have to support reverse-
   direction operation.  There might be no mechanism in the transport
   implementation to do so.  Or, the transport implementation might
   support operation in the reverse direction, but the Upper-Layer
   Protocol might not yet have configured the transport to handle
   reverse-direction traffic.

   If an endpoint is unprepared to receive a reverse-direction message,
   loss of the RDMA connection might result.  Thus a denial of service
   can occur if an RPC server continues to send reverse-direction
   messages after a client that is not prepared to receive them
   reconnects to an endpoint.

   Connection peers indicate their support for reverse-direction
   operation as part of the exchange of Transport Properties just after
   a connection is established (see Section 5.2.5).











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   When dealing with the possibility that the remote peer has no
   transport level support for reverse-direction operation, the Upper-
   Layer Protocol is responsible for informing peers when reverse
   direction operation is supported.  Otherwise, even a simple reverse
   direction RPC NULL procedure from a peer could result in a lost
   connection.  Therefore, an Upper-Layer Protocol MUST NOT perform
   reverse-direction RPC operations until the RPC server indicates
   support for them.

4.5.4.  Using Chunks During Reverse-Direction Operation

   Reverse-direction operations can use chunks, as defined in
   Section 4.3.4, for DDP-eligible data items or in Special payload
   formats.  Reverse-direction chunks operate the same way as in
   forward-direction operation.  Connection peers indicate their support
   for reverse-direction chunks as part of the exchange of Transport
   Properties just after a connection is established (see
   Section 5.2.5).

   However, an implementation might support only Upper-Layer Protocols
   that have no DDP-eligible data items.  Such Upper-Layer Protocols can
   use only small messages, or they might have a native mechanism for
   restricting the size of reverse-direction RPC messages, obviating the
   need to handle chunks in the reverse direction.

   When there is no Upper-Layer Protocol need for chunks in the reverse
   direction, implementers MAY choose not to provide support for chunks
   in the reverse direction, thus avoiding the complexity of
   implementing support for RDMA Reads and Writes in the reverse
   direction.

   When an RPC-over-RDMA transport implementation does not support
   chunks in the reverse direction, RPC endpoints use only the Simple
   Payload format without chunks or the Continued Payload format without
   chunks to send RPC messages in the reverse direction.

   If a reverse-direction Requester provides a non-empty chunk list to a
   Responder that does not support chunks, the Responder MUST report its
   lack of support using one of the error values defined in Section 7.3.

4.5.5.  Reverse-Direction Retransmission

   In rare cases, an RPC server cannot complete an RPC transaction and
   cannot send a Reply.  In these cases, the Requester may send the RPC
   transaction again using the same RPC XID.  We refer to this as an
   "RPC retransmission" or a "replay."





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   In the forward direction, the Requester is the RPC client.  The
   client is always responsible for ensuring a transport connection is
   in place before sending a dropped Call again.

   With reverse-direction operation, the Requester is an RPC server.
   Because an RPC server is not responsible for establishing transport
   connections with clients, the Requester is unable to retransmit a
   reverse-direction Call whenever there is no transport connection.  In
   this case, the RPC server must wait for the RPC client to re-
   establish a transport connection before it can retransmit reverse-
   direction RPC Calls.

   If the forward-direction Requester has no work to do, it can be some
   time before the RPC client re-establishes a transport connection.  An
   RPC server may need to abandon a waiting reverse-direction RPC Call
   to avoid waiting indefinitely for the client to re-establish a
   transport connection.

   Therefore forward-direction Requesters SHOULD maintain a transport
   connection as long as the RPC server might send reverse-direction
   Calls.  For example, while an NFS version 4.1 client has open
   delegated files or active pNFS layouts, it maintains one or more
   transport connections to enable the NFS server to perform callback
   operations.

5.  Transport Properties

   RPC-over-RDMA version 2 enables connection endpoints to exchange
   information about implementation properties.  Compatible endpoints
   use this information to optimize data transfer.  Initially, only a
   small set of transport properties are defined.  The protocol provides
   a single message type to exchange transport properties (see
   Section 6.3.4).

   Both the set of transport properties and the operations used to
   communicate them may be extended.  Within RPC-over-RDMA version 2,
   such extensions are OPTIONAL.  A discussion of extending the set of
   transport properties appears in Appendix B.4.

5.1.  Transport Properties Model

   The current document specifies a basic set of receiver and sender
   properties.  Such properties are specified using a code point that
   identifies the particular transport property and a nominally opaque
   array containing the XDR encoding of the property.

   The following XDR types handle transport properties:




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   <CODE BEGINS>
   typedef rpcrdma2_propid uint32;

   struct rpcrdma2_propval {
           rpcrdma2_propid rdma_which;
           opaque          rdma_data<>;
   };

   typedef rpcrdma2_propval rpcrdma2_propset<>;

   typedef uint32 rpcrdma2_propsubset<>;
   <CODE ENDS>

   The rpcrdma2_propid type specifies a distinct transport property.
   The property code points are defined as const values rather than
   elements in an enum type to enable the extension by concatenating XDR
   definition files.

   The rpcrdma2_propval type carries the value of a transport property.
   The rdma_which field identifies the particular property, and the
   rdma_data field contains the associated value of that property.  A
   zero-length rdma_data field represents the default value of the
   property specified by rdma_which.

   Although the rdma_data field is opaque, receivers interpret its
   contents using the XDR type associated with the property specified by
   rdma_which.  When the contents of the rdma_data field do not conform
   to that XDR type, the receiver MUST return the error
   RDMA2_ERR_BAD_PROPVAL using the header type RDMA2_ERROR, as described
   in Section 6.3.3.

   For example, the receiver of a message containing a valid
   rpcrdma2_propval returns this error if the length of rdma_data is
   greater than the length of the transferred message.  Also, when the
   receiver recognizes the rpcrdma2_propid contained in rdma_which, it
   MUST report the error RDMA2_ERR_BAD_PROPVAL if either of the
   following occurs:

   *  The nominally opaque data within rdma_data is not valid when
      interpreted using the property-associated typedef.

   *  The length of rdma_data is insufficient to contain the data
      represented by the property-associated typedef.

   A receiver does not report an error if it does not recognize the
   value contained in rdma_which.  In that case, the receiver does not
   process that rpcrdma2_propval.  Processing continues with the next
   rpcrdma2_propval, if any.



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   The rpcrdma2_propset type specifies a set of transport properties.
   The protocol does not impose a particular ordering of the
   rpcrdma2_propval items within it.

   The rpcrdma2_propsubset type identifies a subset of the properties in
   a rpcrdma2_propset.  Each bit in the mask denotes a particular
   element in a previously specified rpcrdma2_propset.  If a particular
   rpcrdma2_propval is at position N in the array, then bit number N mod
   32 in word N div 32 specifies whether the defined subset includes
   that particular rpcrdma2_propval.  Words beyond the last one
   specified are assumed to contain zero.

5.2.  Current Transport Properties

   Table 1 specifies a basic set of transport properties.  The columns
   contain the following information:

   *  The column labeled "Property" contains a name of the transport
      property described by the current row.

   *  The column labeled "Code" specifies the code point that identifies
      this property.

   *  The column labeled "XDR type" gives the XDR type of the data used
      to communicate the value of this property.  This data type
      overlays the data portion of the nominally opaque rdma_data field.

   *  The column labeled "Default" gives the default value for the
      property.

   *  The column labeled "Section" indicates the section within the
      current document that explains the use of this property.



















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   +============================+======+==========+=========+=========+
   | Property                   | Code | XDR type | Default | Section |
   +============================+======+==========+=========+=========+
   | Maximum Send Size          | 1    | uint32   | 4096    | 5.2.1   |
   +----------------------------+------+----------+---------+---------+
   | Receive Buffer Size        | 2    | uint32   | 4096    | 5.2.2   |
   +----------------------------+------+----------+---------+---------+
   | Maximum RDMA Segment Size  | 3    | uint32   | 1048576 | 5.2.3   |
   +----------------------------+------+----------+---------+---------+
   | Maximum RDMA Segment Count | 4    | uint32   | 16      | 5.2.4   |
   +----------------------------+------+----------+---------+---------+
   | Reverse-Direction Support  | 5    | uint32   | 0       | 5.2.5   |
   +----------------------------+------+----------+---------+---------+
   | Host Auth Message          | 6    | opaque<> | N/A     | 5.2.6   |
   +----------------------------+------+----------+---------+---------+

                                 Table 1

5.2.1.  Maximum Send Size

   The value of this property specifies the maximum size, in octets, of
   Send payloads.  The endpoint receiving this value can size its
   Receive buffers based on the value of this property.

   <CODE BEGINS>
   const uint32 RDMA2_PROPID_SBSIZ = 1;
   typedef uint32 rpcrdma2_prop_sbsiz;
   <CODE ENDS>

5.2.2.  Receive Buffer Size

   The value of this property specifies the minimum size, in octets, of
   pre-posted receive buffers.

   <CODE BEGINS>
   const uint32 RDMA2_PROPID_RBSIZ = 2;
   typedef uint32 rpcrdma2_prop_rbsiz;
   <CODE ENDS>

   A sender can subsequently use this value to determine when a message
   to be sent fits in pre-posted receive buffers that the receiver has
   set up.  In particular:

   *  Requesters may use the value to determine when to provide a
      Position Zero Read chunk or use Message Continuation when sending
      a Call.





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   *  Requesters may use the value to determine when to provide a Reply
      chunk when sending a Call, based on the maximum possible size of
      the Reply.

   *  Responders may use the value to determine when to use a Reply
      chunk provided by the Requester, given the actual size of a Reply.

5.2.3.  Maximum RDMA Segment Size

   The value of this property specifies the maximum size, in octets, of
   an RDMA segment this endpoint is prepared to send or receive.

   <CODE BEGINS>
   const uint32 RDMA2_PROPID_RSSIZ = 3;
   typedef uint32 rpcrdma2_prop_rssiz;
   <CODE ENDS>

5.2.4.  Maximum RDMA Segment Count

   The value of this property specifies the maximum number of RDMA
   segments that can appear in a Requester's transport header.

   <CODE BEGINS>
   const uint32 RDMA2_PROPID_RCSIZ = 4;
   typedef uint32 rpcrdma2_prop_rcsiz;
   <CODE ENDS>

5.2.5.  Reverse-Direction Support

   The value of this property specifies a client implementation's
   readiness to process messages that are part of reverse direction RPC
   requests.

   <CODE BEGINS>
   const uint32 RDMA_RVRSDIR_NONE = 0;
   const uint32 RDMA_RVRSDIR_SIMPLE = 1;
   const uint32 RDMA_RVRSDIR_CONT = 2;
   const uint32 RDMA_RVRSDIR_GENL = 3;

   const uint32 RDMA2_PROPID_BRS = 5;
   typedef uint32 rpcrdma2_prop_brs;
   <CODE ENDS>

   Multiple levels of support are distinguished:

   *  The value RDMA2_RVRSDIR_NONE indicates that the sender does not
      support reverse-direction operation.




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   *  The value RDMA2_RVRSDIR_SIMPLE indicates that the sender supports
      using only Simple Format messages without chunks for reverse-
      direction messages.

   *  The value RDMA2_RVRSDIR_CONT indicates that the sender supports
      using either Simple Format without chunks or Continued Format
      messages without chunks for reverse-direction messages.

   *  The value RDMA2_RVRSDIR_GENL indicates that the sender supports
      reverse-direction messages in the same way as forward-direction
      messages.

   When a peer does not provide this property, the default is the peer
   does not support reverse-direction operation.

5.2.6.  Host Authentication Message

   The value of this transport property enables the exchange of host
   authentication material.  This property can accommodate
   authentication handshakes that require multiple challenge-response
   interactions and potentially large amounts of material.

   <CODE BEGINS>
   const uint32 RDMA2_PROPID_HOSTAUTH = 6;
   typedef opaque rpcrdma2_prop_hostauth<>;
   <CODE ENDS>

   When this property is not present, the peer(s) remain
   unauthenticated.  Local security policy on each peer determines
   whether the connection is permitted to continue.

6.  Transport Messages

   Each transport message consists of multiple sections.

   *  A transport header prefix, as defined in Section 6.2.2.  Among
      other things, this structure indicates the header type.

   *  The transport header proper, as defined by one of the sub-sections
      below.  See Section 6.1 for the mapping between header types and
      the corresponding header structure.

   *  Potentially, all or part of an RPC message payload.








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   This organization differs from that presented in the definition of
   RPC-over-RDMA version 1 [RFC8166], which defined the first and second
   of the items above as a single XDR data structure.  The new
   organization is in keeping with RPC-over-RDMA version 2's
   extensibility model, which enables the definition of new header types
   without modifying the XDR definition of existing header types.

6.1.  Transport Header Types

   Table 2 lists the RPC-over-RDMA version 2 header types.  The columns
   contain the following information:

   *  The column labeled "Operation" names the particular operation.

   *  The column labeled "Code" specifies the value of the header type
      for this operation.

   *  The column labeled "XDR type" gives the XDR type of the data
      structure used to organize the information in this new message
      type.  This data immediately follows the universal portion on the
      transport header present in every RPC-over-RDMA transport header.

   *  The column labeled "Msg" indicates whether this operation is
      followed (or not) by an RPC message payload.

   *  The column labeled "Section" refers to the section within the
      current document that explains the use of this header type.

   +====================+======+===================+=====+=========+
   | Operation          | Code | XDR type          | Msg | Section |
   +====================+======+===================+=====+=========+
   | Convey Appended    | 0    | rpcrdma2_msg      | Yes | 6.3.1   |
   | RPC Message        |      |                   |     |         |
   +--------------------+------+-------------------+-----+---------+
   | Convey External    | 1    | rpcrdma2_nomsg    | No  | 6.3.2   |
   | RPC Message        |      |                   |     |         |
   +--------------------+------+-------------------+-----+---------+
   | Report Transport   | 4    | rpcrdma2_err      | No  | 6.3.3   |
   | Error              |      |                   |     |         |
   +--------------------+------+-------------------+-----+---------+
   | Specify Properties | 5    | rpcrdma2_connprop | No  | 6.3.4   |
   | at Connection      |      |                   |     |         |
   +--------------------+------+-------------------+-----+---------+
   | Grant Credits      | 6    | void              | No  | 6.3.5   |
   +--------------------+------+-------------------+-----+---------+

                                Table 2




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   Suppport for the operations in Table 2 is REQUIRED.  RPC-over-RDMA
   version 2 implementations that receive an unrecognized header type
   MUST respond with an RDMA2_ERROR message with an rdma_err field
   containing RDMA2_ERR_INVAL_HTYPE and drop the incoming message
   without processing it further.

6.2.  Headers and Chunks

   Most RPC-over-RDMA version 2 data structures have antecedents in
   corresponding structures in RPC-over-RDMA version 1.  As is typical
   for new versions of an existing protocol, the XDR data structures
   have new names, and there are a few small changes in content.  In
   some cases, there have been structural re-organizations to enable
   protocol extensibility.

6.2.1.  Common Transport Header Prefix

   The rpcrdma_common structure defines the initial part of each RPC-
   over-RDMA transport header for RPC-over-RDMA version 2 and subsequent
   versions.

   <CODE BEGINS>
   struct rpcrdma_common {
                uint32         rdma_xid;
                uint32         rdma_vers;
                uint32         rdma_credit;
                uint32         rdma_htype;
   };
   <CODE ENDS>

   RPC-over-RDMA version 2's use of these first four words matches that
   of version 1 as required by [RFC8166].  However, there are crucial
   structural differences in the XDR definition of RPC-over-RDMA version
   2: in the way that these words are described by the respective XDR
   descriptions:

   *  The header type is represented as a uint32 rather than as an enum
      type.  An enum would need to be modified to reflect additions to
      the set of header types made by later extensions.

   *  The header type field is part of an XDR structure devoted to
      representing the transport header prefix, rather than being part
      of a discriminated union, that includes the body of each transport
      header type.







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   *  There is now a prefix structure (see Section 6.2.2) of which the
      rpcrdma_common structure is the initial segment.  This prefix is a
      newly defined XDR object within the protocol description, which
      constrains the universal portion of all header types to the four
      words in rpcrdma_common.

   These changes are part of a more considerable structural change in
   the XDR definition of RPC-over-RDMA version 2 that facilitates a
   cleaner treatment of protocol extension.  The XDR appearing in
   Section 8 reflects these changes, which Appendix C.1 discusses in
   further detail.

6.2.2.  Transport Header Prefix

   The following prefix structure appears at the start of each RPC-over-
   RDMA version 2 transport header.

   <CODE BEGINS>
   const RDMA2_F_RESPONSE           0x00000001;
   const RDMA2_F_MORE               0x00000002;
   const RDMA2_F_TPMORE             0x00000004;

   struct rpcrdma2_hdr_prefix {
           struct rpcrdma_common       rdma_start;
           uint32                      rdma_flags;
   };
   <CODE ENDS>

   The rdma_flags field is new to RPC-over-RDMA version 2.  Currently,
   the only flags defined within this word are the RDMA2_F_RESPONSE flag
   and the RDMA2_F_MORE flag.  The other flags are reserved for future
   use as described in Appendix B.3.  The sender MUST set reserved flags
   to zero, and the receiver MUST ignore reserved flags.

6.2.2.1.  RDMA2_F_RESPONSE Flag

   The RDMA2_F_RESPONSE flag qualifies the value contained in the
   transport header's rdma_xid field.  The RDMA2_F_RESPONSE flag enables
   a receiver to avoid performing an XID lookup on incoming reverse
   direction Call messages.  Therefore:

   *  When the rdma_htype field has the value RDMA2_MSG or RDMA2_NOMSG,
      the value of the RDMA2_F_RESPONSE flag MUST be the same as the
      value of the associated RPC message's msg_type field.







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   *  When the header type is anything else and a whole or partial RPC
      message payload is present, the value of the RDMA2_F_RESPONSE flag
      MUST be the same as the value of the associated RPC message's
      msg_type field.

   *  When no RPC message payload is present, a Requester MUST set the
      value of RDMA2_F_RESPONSE to reflect how the receiver is to
      interpret the rdma_xid field.

   *  When the rdma_htype field has the value RDMA2_ERROR, the
      RDMA2_F_RESPONSE flag MUST be set.

6.2.2.2.  RDMA2_F_MORE Flag

   The RDMA2_F_MORE flag signifies that the RPC-over-RDMA message
   payload continues in the next message.

   When the sender asserts the RDMA2_F_MORE flag, the receiver is to
   concatenate the data payload of the next received message to the end
   of the data payload of the received message.  The sender clears the
   RDMA2_F_MORE flag in the final message in the sequence.

   All RPC-over-RDMA messages in such a sequence MUST have the same
   values in the rdma_xid and rdma_htype fields.  Otherwise, the
   receiver MUST drop the message without processing it further.  If the
   receiver is a Responder, it MUST also respond with an RDMA2_ERROR
   message with the rdma_err field set to RDMA2_ERR_INVAL_CONT.

   If a peer receives an RPC-over-RDMA message with the RDMA2_F_MORE
   flag set, and the rdma_htype field does not contain the values
   RDMA2_MSG or RDMA2_CONNPROP, the receiver MUST drop the message
   without processing it further.  If the receiver is a Responder, it
   MUST also respond with an RDMA2_ERROR message with the rdma_err field
   set to RDMA2_ERR_INVAL_CONT.

   The sender includes chunks only in the final message in a sequence,
   in which the RDMA2_F_MORE flag is clear.  If a peer receives an RPC-
   over-RDMA message with the RDMA2_F_MORE flag set, and its chunk lists
   are not empty, the receiver MUST drop the message without processing
   it further.  If the receiver is a Responder, it MUST also respond
   with an RDMA2_ERROR message with the rdma_err field set to
   RDMA2_ERR_INVAL_CONT.

   There is no protocol-defined limit on the number of concatenated
   messages in a sequence.  If the sender exhausts the receiver's credit
   grant before sending the final message in the sequence, the sender
   waits for a further credit grant from the receiver before continuing
   to send messages.



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   Credit exhaustion can occur at the receiver in the middle of a
   sequence of continued messages.  The receiver can grant more credits
   by sending an RPC message payload or an out-of-band credit grant (see
   Section 4.2.1.2) to enable the sender to send the remaining messages
   in the sequence.

6.2.2.3.  RDMA2_F_TPMORE Flag

   The RDMA2_F_TPMORE flag indicates that the sender has additional
   Transport Properties to send in a subsequent RPC-over-RDMA message.
   If a peer receives any message type other than RDMA2_CONNPROP with
   the RDMA2_F_TPMORE flag set, it MUST respond with an RDMA2_ERROR
   message type whose rdma_err field contains RDMA2_ERR_INVAL_HTYPE, and
   then silently discard the ingress message without processing it.

   The RDMA2_F_TPDONE flag is clear in the final RDMA2_CONNPROP message
   type from this peer on this connection.  If a peer receives an
   RDMA2_CONNPROP message type after it has received an RDMA2_CONNPROP
   message type with a clear RDMA2_F_TPDONE flag, it MUST respond with
   an RDMA2_ERROR message type whose rdma_err field contains
   RDMA2_ERR_INVAL_HTYPE, and then silently discard the ingress message
   without processing it.

   After both connection peers have indicated they have finished sending
   their Transport Properties, they may begin passing RPC traffic.

6.2.3.  External Data Payloads

   The rpcrdma2_chunk_lists structure specifies where to find the parts
   of the message payload that are to be conveyed via explicit RDMA
   operations.

   <CODE BEGINS>
   struct rpcrdma2_chunk_lists {
           uint32                      rdma_inv_handle;
           struct rpcrdma2_read_list   *rdma_reads;
           struct rpcrdma2_write_list  *rdma_writes;
           struct rpcrdma2_write_chunk *rdma_reply;
   };
   <CODE ENDS>

   rdma_inv_handle:  The rdma_inv_handle field contains a 32-bit RDMA
      handle that the Responder should use in a Send With Invalidation
      operation.  More detail is provided in Section 6.2.4.

   rdma_reads:  The rdma_reads field anchors a list of zero or more RDMA
      read segments (see Section 4.3.5.1).




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   rdma_writes:  The rdma_writes field anchors a list of zero or more
      Write chunks (see Section 4.3.6.1).

   rdma_reply:  The rdma_reply field is a list containing zero or one
      Reply chunk.  The use of this chunk is explained further in
      Section 4.4.3.

   The content of the chunk lists is determined by the message's Payload
   format, as explained in Section 4.4.

6.2.4.  Remote Invalidation

   To solicit the use of Remote Invalidation, a Requester sets the value
   of the rdma_inv_handle field in an RPC Call's transport header to a
   non-zero value that matches one of the rdma_handle fields in that
   header.  If the Responder may invalidate none of the rdma_handle
   values in the header conveying the Call, the Requester sets the RPC
   Call's rdma_inv_handle field to the value zero.

   If the Responder chooses not to use remote invalidation for this
   particular RPC Reply, or the RPC Call's rdma_inv_handle field
   contains the value zero, the Responder simply uses RDMA Send to
   transmit the matching RPC reply.  However, if the Responder chooses
   to use Remote Invalidation, it uses RDMA Send With Invalidate to
   transmit the RPC Reply.  It MUST use the value in the corresponding
   Call's rdma_inv_handle field to construct the Send With Invalidate
   Work Request.

   A Responder MUST NOT use a Send With Invalidate Work Request when
   sending an RDMA2_ERROR header type (see Section 6.3.3), an
   RDMA2_CONNPROP header type (see Section 6.3.4), or an asynchronous
   credit grant (see Section 4.2.1.2).

6.3.  Header Types

   The header types defined and used in RPC-over-RDMA version 1 are
   carried over into RPC-over-RDMA version 2, although there are some
   limited changes in the definitions of existing header types:

   *  To simplify interoperability with RPC-over-RDMA version 1, only
      the RDMA2_ERROR header (defined in Section 6.3.3) has an XDR
      definition that differs from that in RPC-over-RDMA version 1, and
      its modifications are all compatible extensions.








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   *  RDMA2_MSG and RDMA2_NOMSG (defined in Sections 6.3.1 and 6.3.2)
      have XDR definitions that match the corresponding RPC-over-RDMA
      version 1 header types.  However, because of the changes to the
      header prefix, the version 1 and version 2 header types differ in
      on-the-wire format.

   *  RDMA2_CONNPROP (defined in Section 6.3.4) is an entirely new
      header type devoted to enabling connection peers to exchange
      information about their transport properties.

6.3.1.  RDMA2_MSG: Convey RPC Message Inline

   RDMA2_MSG conveys all or part of an RPC message immediately following
   the transport header in the send buffer.

   <CODE BEGINS>
   const rpcrdma2_proc RDMA2_MSG = 0;

   struct rpcrdma2_msg {
           struct rpcrdma2_chunk_lists rdma_chunks;

           /* The rpc message starts here and continues
            * through the end of the transmission. */
           uint32                      rdma_rpc_first_word;
   };
   <CODE ENDS>

6.3.2.  RDMA2_NOMSG: Convey External RPC Message

   RDMA2_NOMSG conveys an entire RPC message payload using explicit RDMA
   operations.  In particular, it is referred to as a Special Format
   Call when the Responder reads the RPC payload from a memory area
   specified by a Position Zero Read chunk.  It is referred to as a
   Special Format Reply when the Responder writes the RPC payload into a
   memory area specified by a Reply chunk.  In both cases, the sender
   sets the rdma_xid field to the same value as the xid of the RPC
   message payload.

   If all the chunk lists are empty the message conveys a credit grant
   refresh.  The header prefix of this message contains a credit grant
   refresh in the rdma_credit field.  In this case, the sender MUST set
   the rdma_xid field to zero.









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   <CODE BEGINS>
   const rpcrdma2_proc RDMA2_NOMSG = 1;

   struct rpcrdma2_nomsg {
           struct rpcrdma2_chunk_lists rdma_chunks;
   };
   <CODE ENDS>

   In RPC-over-RDMA version 2, a sender should use Message Continuation
   as an alternative to using a Special Format message.

   When a Responder receives an RDMA2_NOMSG header type with a non-empty
   Read list that does not carry a Position Zero Read chunk, it MUST
   discard that message without processing it and send an RDMA2_ERROR
   message with the rdma_err field set to RDMA2_ERR_BAD_XDR.  When a
   Requester receives an RDMA2_NOMSG header type with an empty Reply
   chunk, it discards that message and terminates the RPC transaction
   represented by the XID value in the rdma_xid field of the malformed
   message.

6.3.3.  RDMA2_ERROR: Report Transport Error

   RDMA2_ERROR reports a transport layer error on a previous
   transmission.



























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   <CODE BEGINS>
   const rpcrdma2_proc RDMA2_ERROR = 4;

   struct rpcrdma2_err_vers {
           uint32 rdma_vers_low;
           uint32 rdma_vers_high;
   };

   struct rpcrdma2_err_write {
           uint32 rdma_chunk_index;
           uint32 rdma_length_needed;
   };

   union rpcrdma2_error switch (rpcrdma2_errcode rdma_err) {
           case RDMA2_ERR_VERS:
             rpcrdma2_err_vers rdma_vrange;
           case RDMA2_ERR_READ_CHUNKS:
             uint32 rdma_max_chunks;
           case RDMA2_ERR_WRITE_CHUNKS:
             uint32 rdma_max_chunks;
           case RDMA2_ERR_SEGMENTS:
             uint32 rdma_max_segments;
           case RDMA2_ERR_WRITE_RESOURCE:
             rpcrdma2_err_write rdma_writeres;
           case RDMA2_ERR_REPLY_RESOURCE:
             uint32 rdma_length_needed;
           default:
             void;
   };
   <CODE ENDS>

   See Section 7 for details on the use of this header type.

6.3.4.  RDMA2_CONNPROP: Exchange Transport Properties

   The RDMA2_CONNPROP message type enables a connection peer to publish
   the properties of its implementation to its remote peer.

   <CODE BEGINS>
   const rpcrdma2_proc RDMA2_CONNPROP = 5;

   struct rpcrdma2_connprop {
           rpcrdma2_propset rdma_props;
   };
   <CODE ENDS>






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   Each peer sends an RDMA2_CONNPROP message type as the first message
   after the client has established a connection.  The size of this
   initial message is limited to the default inline threshold for the
   RPC-over-RDMA version that is in effect.  If a peer has more or
   larger Transport Properties than can fit in the initial
   RDMA2_CONNPROP message type, it sets the RDMA2_F_TPMORE flag.  The
   final RDMA2_CONNPROP message type the peer sends on that connection
   must have a clear RDMA2_F_TPMORE flag.

   A peer may encounter properties that it does not recognize or
   support.  In such cases, the receiver ignores unsupported properties
   without generating an error response.

6.3.5.  RDMA2_GRANT: Grant Credits

   The RDMA2_GRANT message type enables a connection peer to grant
   additional credits to its remote peer without conveying a payload.

   <CODE BEGINS>
   const rpcrdma2_proc RDMA2_GRANT = 6;
   <CODE ENDS>

   This message carries no payload except for a struct
   rpcrdma2_hdr_prefix.  The rdma_flags field is unused.  Senders MUST
   set the rdma_flags field to zero (all clear) and receivers MUST
   ignore the value of the bits in this field.

6.4.  Choosing a Reply Mechanism

   A Requester provisions all necessary registered memory resources for
   both an RPC Call and its matching RPC Reply.  A Requester constructs
   each RPC Call, thus it can compute the exact memory resources needed
   to send every Call.  However, the Requester allocates memory
   resources to receive the corresponding Reply before the Responder has
   constructed it.  Occasionally, it is challenging for the Requester to
   know in advance precisely what resources are needed to receive the
   Reply.

   In RPC-over-RDMA version 2, a Requester can provide a Reply chunk for
   any transaction.  The Responder can use the provided Reply chunk or
   it can decide to use another means to convey the RPC Reply.  If the
   combination of the provided Write chunk list and Reply chunk is not
   adequate to convey a Reply, the Responder SHOULD use Message
   Continuation (see Section 6.2.2.2) to send that Reply.  If even that
   is not possible, the Responder sends an RDMA2_ERROR message to the
   Requester, as described in Section 6.3.3:





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   *  If the Write chunk list cannot accommodate the ULP's DDP-eligible
      data payload, the Responder sends an RDMA2_ERR_WRITE_RESOURCE
      error.

   *  If the Reply chunk cannot accommodate the parts of the Reply that
      are not DDP-eligible, the Responder sends an
      RDMA2_ERR_REPLY_RESOURCE error.

   When receiving such errors, the Requester can retry the ULP call
   using more substantial reply resources.  In cases where retrying the
   ULP request is not possible (e.g., the request is non-idempotent),
   the Requester terminates the RPC transaction and presents an error to
   the RPC consumer.

7.  Error Handling

   A receiver performs validity checks on each ingress RPC-over-RDMA
   message before it passes that message to the RPC layer.  For example,
   if an ingress RPC-over-RDMA message is not as long as the size of
   struct rpcrdma2_hdr_prefix (20 octets), the receiver cannot trust the
   value of the rdma_xid field.  In this case, the receiver MUST
   silently discard the ingress message without processing it further,
   and without a response to the sender.

   When a request (for instance, an RPC Call or a control plane
   operation) is made, typically an RPC consumer blocks while waiting
   for the response.  Thus when an incoming message conveys a request
   and that request cannot be acted upon, the receiver of that request
   needs to report the problem to its sender in order to unblock
   waiters.  Likewise, if, after processing a request, a sender is
   unable to transmit the response on an otherwise healthy connection,
   the sender needs to report that problem for the same reason.

   The RDMA2_ERROR header type is used for this purpose.  To form an
   RDMA2_ERROR type header:

   *  The rdma_xid field MUST contain the same XID that was in the
      rdma_xid field in the ingress request.

   *  The rdma_vers field MUST contain the same version that was in the
      rdma_vers field in the ingress request.

   *  The sender sets the rdma_credit field to the credit values in
      effect for this connection.

   *  The rdma_htype field MUST contain the value RDMA2_ERROR.





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   *  The rdma_err field contains a value that reflects the type of
      error that occurred, as described in the subsections below.

   When a peer receives an RDMA2_ERROR message type with an unrecognized
   or unsupported value in its rdma_err field, it MUST silently discard
   the message without processing it further.

7.1.  Basic Transport Stream Parsing Errors

7.1.1.  RDMA2_ERR_VERS

   When a Responder detects an RPC-over-RDMA header version that it does
   not support (the current document defines version 2), it MUST respond
   with an RDMA2_ERROR message type and set its rdma_err field to
   RDMA2_ERR_VERS.  The Responder then fills in the rpcrdma2_err_vers
   structure with the RPC-over-RDMA versions it supports.  The Responder
   MUST silently discard the ingress message without passing it to the
   RPC layer

   When a Requester receives this error, it uses the information in the
   rpcrdma2_err_vers structure to select an RPC-over-RDMA version that
   both peers support.  A Requester MUST NOT subsequently send a message
   that uses a version that the Responder has indciated it does not
   support.  RDMA2_ERR_VERS indicates a permanent error.  Receipt of
   this error completes the RPC transaction associated with XID in the
   rdma_xid field.

7.1.2.  RDMA2_ERR_INVAL_HTYPE

   If a Responder recognizes the value in an ingress rdma_vers field,
   but it does not recognize the value in the rdma_htype field or does
   not support that header type, it MUST set the rdma_err field to
   RDMA2_ERR_INVAL_HTYPE.  The Responder MUST silently discard the
   incoming message without passing it to the RPC layer.

   A Requester MUST NOT subsequently send a message that uses an htype
   that the Responder has indicated it does not support.
   RDMA2_ERR_INVAL_HTYPE indicates a permanent error.  Receipt of this
   error completes the RPC transaction associated with XID in the
   rdma_xid field.











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7.1.3.  RDMA2_ERR_INVAL_CONT

   If a Responder detects a problem with an ingress RPC-over-RDMA
   message that is part of a Message Continuation sequence, the
   Responder MUST set the rdma_err field to RDMA2_ERR_INVAL_CONT.
   Section 6.2.2.2 discusses the types of problems to watch for.  The
   Responder MUST silently discard all ingress messages with an rdma_xid
   field that matches the failing message without reassembling the
   payload.

   RDMA2_ERR_INVAL_CONT indicates a permanent error.  Receipt of this
   error completes the RPC transaction associated with XID in the
   rdma_xid field.

7.2.  XDR Errors

   A receiver might encounter an XDR parsing error that prevents it from
   processing an ingress Transport stream.  Examples of such errors
   include:

   *  An invalid value in the rdma_proc field.

   *  The value of the rdma_xid field does not match the value of the
      XID field in the accompanying RPC message.

   When a Responder receives a valid RPC-over-RDMA header but the
   Responder's ULP implementation cannot parse the RPC arguments in the
   RPC Call, the Responder returns an RPC Reply with status
   GARBAGE_ARGS, using an RDMA2_MSG message type.  This type of parsing
   failure might be due to mismatches between chunk sizes or offsets and
   the contents of the Payload stream, for example.  In this case, the
   error is permanent, but the Requester has no way to know how much
   processing the Responder has completed for this RPC transaction.

7.2.1.  RDMA2_ERR_BAD_XDR

   If a Responder recognizes the values in the rdma_vers field, but it
   cannot otherwise parse the ingress Transport stream, it MUST set the
   rdma_err field to RDMA2_ERR_BAD_XDR.  The Responder MUST silently
   discard the ingress message without passing it to the RPC layer.

   RDMA2_ERR_BAD_XDR indicates a permanent error.  Receipt of this error
   completes the RPC transaction associated with XID in the rdma_xid
   field.







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7.2.2.  RDMA2_ERR_BAD_PROPVAL

   If a receiver recognizes the value in an ingress rdma_which field,
   but it cannot parse the accompanying propval, it MUST set the
   rdma_err field to RDMA2_ERR_BAD_PROPVAL (see Section 5.1).  The
   receiver MUST silently discard the ingress message without applying
   any of its property settings.

7.3.  Responder RDMA Operational Errors

   In RPC-over-RDMA version 2, the Responder initiates RDMA Read and
   Write operations that target the Requester's memory.  Problems might
   arise as the Responder attempts to use Requester-provided resources
   for RDMA operations.  For example:

   *  Usually, chunks can be validated only by using their contents to
      perform data transfers.  If chunk contents are invalid (e.g., a
      memory region is no longer registered or a chunk length exceeds
      the end of the registered memory region), a Remote Access Error
      occurs.

   *  If a Requester's Receive buffer is too small, the Responder's Send
      operation completes with a Local Length Error.

   *  If the Requester-provided Reply chunk is too small to accommodate
      a large RPC Reply message, a Remote Access Error occurs.  A
      Responder might detect this problem before attempting to write
      past the end of the Reply chunk.

   RDMA operational errors can be fatal to the connection.  To avoid a
   retransmission loop and repeated connection loss that deadlocks the
   connection, once the Requester has re-established a connection, the
   Responder should send an RDMA2_ERROR response to indicate that no
   RPC-level reply is possible for that transaction.

7.3.1.  RDMA2_ERR_READ_CHUNKS

   If a Requester presents more DDP-eligible arguments than a Responder
   is prepared to Read, the Responder MUST set the rdma_err field to
   RDMA2_ERR_READ_CHUNKS and set the rdma_max_chunks field to the
   maximum number of Read chunks the Responder can process.  If the
   Responder implementation cannot handle any Read chunks for a request,
   it MUST set the rdma_max_chunks to zero in this response.  The
   Responder MUST silently discard the ingress message without
   processing it further.






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   The Requester can reconstruct the Call using Message Continuation or
   a Special Format payload and resend it.  If the Requester does not
   resend the Call, it MUST terminate this RPC transaction with an
   error.

7.3.2.  RDMA2_ERR_WRITE_CHUNKS

   If a Requester has constructed an RPC Call with more DDP-eligible
   results than the Responder is prepared to Write, the Responder MUST
   set the rdma_err field to RDMA2_ERR_WRITE_CHUNKS and set the
   rdma_max_chunks field to the maximum number of Write chunks the
   Responder can return.  The Requester can reconstruct the Call with no
   Write chunks and a Reply chunk of appropriate size.  If the Requester
   does not resend the Call, it MUST terminate this RPC transaction with
   an error.

   If the Responder implementation cannot handle any Write chunks for a
   request and cannot send the Reply using Message Continuation, it MUST
   return a response of RDMA2_ERR_REPLY_RESOURCE instead (see below).

7.3.3.  RDMA2_ERR_SEGMENTS

   If a Requester has constructed an RPC Call with a chunk that contains
   more segments than the Responder supports, the Responder MUST set the
   rdma_err field to RDMA2_ERR_SEGMENTS and set the rdma_max_segments
   field to the maximum number of segments the Responder can process.
   The Requester can reconstruct the Call and resend it.  If the
   Requester does not resend the Call, it MUST terminate this RPC
   transaction with an error.

7.3.4.  RDMA2_ERR_WRITE_RESOURCE

   If a Requester has provided a Write chunk that is not large enough to
   contain a DDP-eligible result, the Responder MUST set the rdma_err
   field to RDMA2_ERR_WRITE_RESOURCE.  The Responder MUST set the
   rdma_chunk_index field to point to the first Write chunk in the
   transport header that is too short, or to zero to indicate that it
   was not possible to determine which chunk is too small.  Indexing
   starts at one (1), which represents the first Write chunk.  The
   Responder MUST set the rdma_length_needed to the number of bytes
   needed in that chunk to convey the result data item.

   The Requester can reconstruct the Call with more reply resources and
   resend it.  If the Requester does not resend the Call (for instance,
   if the Responder set the index and length fields to zero), it MUST
   terminate this RPC transaction with an error.





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7.3.5.  RDMA2_ERR_REPLY_RESOURCE

   If a Responder cannot send an RPC Reply using Message Continuation
   and the Reply does not fit in the Reply chunk, the Responder MUST set
   the rdma_err field to RDMA2_ERR_REPLY_RESOURCE.  The Responder MUST
   set the rdma_length_needed to the number of Reply chunk bytes needed
   to convey the reply.  The Requester can reconstruct the Call with
   more reply resources and resend it.  If the Requester does not resend
   the Call (for instance, if the Responder set the length field to
   zero), it MUST terminate this RPC transaction with an error.

7.4.  Other Operational Errors

   While a Requester is constructing an RPC Call message, an
   unrecoverable problem might occur that prevents the Requester from
   posting further RDMA Work Requests on behalf of that message.  As
   with other transports, if a Requester is unable to construct and
   transmit an RPC Call, the associated RPC transaction fails
   immediately.

   After a Requester has received a Reply, if it is unable to invalidate
   a memory region due to an unrecoverable problem, the Requester MUST
   close the connection to protect that memory from Responder access
   before the associated RPC transaction is complete.

   While a Responder is constructing an RPC Reply message or error
   message, an unrecoverable problem might occur that prevents the
   Responder from posting further RDMA Work Requests on behalf of that
   message.  If a Responder is unable to construct and transmit an RPC
   Reply or RPC-over-RDMA error message, the Responder MUST close the
   connection to signal to the Requester that a reply was lost.

7.4.1.  RDMA2_ERR_SYSTEM

   If some problem occurs on a Responder that does not fit into the
   above categories, the Responder MAY report it to the Requester by
   setting the rdma_err field to RDMA2_ERR_SYSTEM.  The Responder MUST
   silently discard the message(s) associated with the failing
   transaction without further processing.

   RDMA2_ERR_SYSTEM is a permanent error.  This error does not indicate
   how much of the transaction the Responder has processed, nor does it
   indicate a particular recovery action for the Requester.  A Requester
   that receives this error MUST terminate the RPC transaction
   associated with the XID value in the RDMA2_ERROR message's rdma_xid
   field.





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7.5.  RDMA Transport Errors

   The RDMA connection and physical link provide some degree of error
   detection and retransmission. iWARP's Marker PDU Aligned (MPA) layer
   (when used over TCP), the Stream Control Transmission Protocol
   (SCTP), as well as the InfiniBand link layer [IBA] all provide Cyclic
   Redundancy Check (CRC) protection of RDMA payloads.  CRC-class
   protection is a general attribute of such transports.

   Additionally, the RPC layer itself can accept errors from the
   transport and recover via retransmission.  RPC recovery can typically
   handle complete loss and re-establishment of a transport connection.

   The details of reporting and recovery from RDMA link-layer errors are
   described in specific link-layer APIs and operational specifications
   and are outside the scope of this protocol specification.  See
   Section 11 for further discussion of RPC-level integrity schemes.

8.  XDR Protocol Definition

   This section contains a description of the core features of the RPC-
   over-RDMA version 2 protocol expressed in the XDR language [RFC4506].
   It organizes the description to make it simple to extract into a form
   that is ready to compile or combine with similar descriptions
   published later as extensions to RPC-over-RDMA version 2.

8.1.  Code Component License

   Code Components extracted from the current document must include the
   following license text.  When combining the extracted XDR code with
   other XDR code which has an identical license, only a single copy of
   the license text needs to be retained.



















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   <CODE BEGINS>
   /// /*
   ///  * Copyright (c) 2010-2018 IETF Trust and the persons
   ///  * identified as authors of the code.  All rights reserved.
   ///  *
   ///  * The authors of the code are:
   ///  * B. Callaghan, T. Talpey, C. Lever, and D. Noveck.
   ///  *
   ///  * Redistribution and use in source and binary forms, with
   ///  * or without modification, are permitted provided that the
   ///  * following conditions are met:
   ///  *
   ///  * - Redistributions of source code must retain the above
   ///  *   copyright notice, this list of conditions and the
   ///  *   following disclaimer.
   ///  *
   ///  * - Redistributions in binary form must reproduce the above
   ///  *   copyright notice, this list of conditions and the
   ///  *   following disclaimer in the documentation and/or other
   ///  *   materials provided with the distribution.
   ///  *
   ///  * - Neither the name of Internet Society, IETF or IETF
   ///  *   Trust, nor the names of specific contributors, may be
   ///  *   used to endorse or promote products derived from this
   ///  *   software without specific prior written permission.
   ///  *
   ///  *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
   ///  *   AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
   ///  *   WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   ///  *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   ///  *   FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
   ///  *   EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
   ///  *   LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   ///  *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   ///  *   NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   ///  *   SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   ///  *   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   ///  *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
   ///  *   OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
   ///  *   IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
   ///  *   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   ///  */
   ///
   <CODE ENDS>







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8.2.  Extraction of the XDR Definition

   Implementers can apply the following sed script to the current
   document to produce a machine-readable XDR description of the base
   RPC-over-RDMA version 2 protocol.

   <CODE BEGINS>
   sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
   <CODE ENDS>

   That is, if this document is in a file called "spec.txt", then
   implementers can do the following to extract an XDR description file
   and store it in the file rpcrdma-v2.x.

   <CODE BEGINS> file "rpcrdma-v2.x"
   sed -n -e 's:^ */// ::p' -e 's:^ *///$::p' \
    < spec.txt > rpcrdma-v2.x
   <CODE ENDS>

   Although this file is a usable description of the base protocol, when
   extensions are to be supported, it may be desirable to divide the
   description into multiple files.  The following script achieves that
   purpose:

   <CODE BEGINS>
   #!/usr/local/bin/perl
   open(IN,"rpcrdma-v2.x");
   open(OUT,">temp.x");
   while(<IN>)
   {
     if (m/FILE ENDS: (.*)$/)
       {
         close(OUT);
         rename("temp.x", $1);
         open(OUT,">temp.x");
       }
       else
       {
         print OUT $_;
       }
   }
   close(IN);
   close(OUT);
   <CODE ENDS>

   Running the above script results in two files:





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   *  The file common.x, containing the license plus the shared XDR
      definitions that need to be made available to both the base
      protocol and any subsequent extensions.

   *  The file baseops.x containing the XDR definitions for the base
      protocol defined in this document.

   Extensions to RPC-over-RDMA version 2, published as Standards Track
   documents, should have similarly structured XDR definitions.  Once an
   implementer has extracted the XDR for all desired extensions and the
   base XDR definition contained in the current document, she can
   concatenate them to produce a consolidated XDR definition that
   reflects the set of extensions selected for her RPC-over-RDMA version
   2 implementation.

   Alternatively, the XDR descriptions can be compiled separately.  In
   that case, the combination of common.x and baseops.x defines the base
   transport.  The combination of common.x and the XDR description of
   each extension produces a full XDR definition of that extension.

8.3.  XDR Definition for RPC-over-RDMA Version 2 Core Structures

   <CODE BEGINS>
   /// /***************************************************************
   ///  *    Transport Header Prefixes
   ///  ***************************************************************/
   ///
   /// struct rpcrdma_common {
   ///         uint32         rdma_xid;
   ///         uint32         rdma_vers;
   ///         uint32         rdma_credit;
   ///         uint32         rdma_htype;
   /// };
   ///
   /// const RDMA2_F_RESPONSE           0x00000001;
   /// const RDMA2_F_MORE               0x00000002;
   /// const RDMA2_F_TPMORE             0x00000004;
   ///
   /// struct rpcrdma2_hdr_prefix
   ///         struct rpcrdma_common       rdma_start;
   ///         uint32                      rdma_flags;
   /// };
   ///
   /// /***************************************************************
   ///  *    Chunks and Chunk Lists
   ///  ***************************************************************/
   ///
   /// struct rpcrdma2_segment {



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   ///         uint32 rdma_handle;
   ///         uint32 rdma_length;
   ///         uint64 rdma_offset;
   /// };
   ///
   /// struct rpcrdma2_read_segment {
   ///         uint32                  rdma_position;
   ///         struct rpcrdma2_segment rdma_target;
   /// };
   ///
   /// struct rpcrdma2_read_list {
   ///         struct rpcrdma2_read_segment rdma_entry;
   ///         struct rpcrdma2_read_list    *rdma_next;
   /// };
   ///
   /// struct rpcrdma2_write_chunk {
   ///         struct rpcrdma2_segment rdma_target<>;
   /// };
   ///
   /// struct rpcrdma2_write_list {
   ///         struct rpcrdma2_write_chunk rdma_entry;
   ///         struct rpcrdma2_write_list  *rdma_next;
   /// };
   ///
   /// struct rpcrdma2_chunk_lists {
   ///         uint32                      rdma_inv_handle;
   ///         struct rpcrdma2_read_list   *rdma_reads;
   ///         struct rpcrdma2_write_list  *rdma_writes;
   ///         struct rpcrdma2_write_chunk *rdma_reply;
   /// };
   ///
   /// /***************************************************************
   ///  *    Transport Properties
   ///  ***************************************************************/
   ///
   /// /*
   ///  * Types for transport properties model
   ///  */
   /// typedef rpcrdma2_propid uint32;
   ///
   /// struct rpcrdma2_propval {
   ///         rpcrdma2_propid rdma_which;
   ///         opaque          rdma_data<>;
   /// };
   ///
   /// typedef rpcrdma2_propval rpcrdma2_propset<>;
   /// typedef uint32 rpcrdma2_propsubset<>;
   ///



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   /// /*
   ///  * Transport propid values for basic properties
   ///  */
   /// const uint32 RDMA2_PROPID_SBSIZ = 1;
   /// const uint32 RDMA2_PROPID_RBSIZ = 2;
   /// const uint32 RDMA2_PROPID_RSSIZ = 3;
   /// const uint32 RDMA2_PROPID_RCSIZ = 4;
   /// const uint32 RDMA2_PROPID_BRS = 5;
   /// const uint32 RDMA2_PROPID_HOSTAUTH = 6;
   ///
   /// /*
   ///  * Types specific to particular properties
   ///  */
   /// typedef uint32 rpcrdma2_prop_sbsiz;
   /// typedef uint32 rpcrdma2_prop_rbsiz;
   /// typedef uint32 rpcrdma2_prop_rssiz;
   /// typedef uint32 rpcrdma2_prop_rcsiz;
   /// typedef uint32 rpcrdma2_prop_brs;
   /// typedef opaque rpcrdma2_prop_hostauth<>;
   ///
   /// const uint32 RDMA2_RVRSDIR_NONE = 0;
   /// const uint32 RDMA2_RVRSDIR_SIMPLE = 1;
   /// const uint32 RDMA2_RVRSDIR_CONT = 1;
   /// const uint32 RDMA2_RVRSDIR_GENL = 3;
   ///
   /// /* FILE ENDS: common.x; */
   <CODE ENDS>

8.4.  XDR Definition for RPC-over-RDMA Version 2 Base Header Types

   <CODE BEGINS>
   /// /***************************************************************
   ///  *    Descriptions of RPC-over-RDMA Header Types
   ///  ***************************************************************/
   ///
   /// /*
   ///  * Header Type Codes.
   ///  */
   /// const rpcrdma2_proc RDMA2_MSG = 0;
   /// const rpcrdma2_proc RDMA2_NOMSG = 1;
   /// const rpcrdma2_proc RDMA2_ERROR = 4;
   /// const rpcrdma2_proc RDMA2_CONNPROP = 5;
   /// const rpcrdma2_proc RDMA2_GRANT = 6;
   ///
   /// /*
   ///  * Header Types to Convey RPC Messages.
   ///  */
   /// struct rpcrdma2_msg {



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   ///         struct rpcrdma2_chunk_lists rdma_chunks;
   ///
   ///         /* The rpc message starts here and continues
   ///          * through the end of the transmission. */
   ///         uint32                      rdma_rpc_first_word;
   /// };
   ///
   /// struct rpcrdma2_nomsg {
   ///         struct rpcrdma2_chunk_lists rdma_chunks;
   /// };
   ///
   /// /*
   ///  * Header Type to Report Errors.
   ///  */
   /// const uint32 RDMA2_ERR_VERS = 1;
   /// const uint32 RDMA2_ERR_BAD_XDR = 2;
   /// const uint32 RDMA2_ERR_BAD_PROPVAL = 3;
   /// const uint32 RDMA2_ERR_INVAL_HTYPE = 4;
   /// const uint32 RDMA2_ERR_INVAL_CONT = 5;
   /// const uint32 RDMA2_ERR_READ_CHUNKS = 6;
   /// const uint32 RDMA2_ERR_WRITE_CHUNKS = 7;
   /// const uint32 RDMA2_ERR_SEGMENTS = 8;
   /// const uint32 RDMA2_ERR_WRITE_RESOURCE = 9;
   /// const uint32 RDMA2_ERR_REPLY_RESOURCE = 10;
   /// const uint32 RDMA2_ERR_SYSTEM = 100;
   ///
   /// struct rpcrdma2_err_vers {
   ///         uint32 rdma_vers_low;
   ///         uint32 rdma_vers_high;
   /// };
   ///
   /// struct rpcrdma2_err_write {
   ///         uint32 rdma_chunk_index;
   ///         uint32 rdma_length_needed;
   /// };
   ///
   /// union rpcrdma2_error switch (rpcrdma2_errcode rdma_err) {
   ///         case RDMA2_ERR_VERS:
   ///           rpcrdma2_err_vers rdma_vrange;
   ///         case RDMA2_ERR_READ_CHUNKS:
   ///           uint32 rdma_max_chunks;
   ///         case RDMA2_ERR_WRITE_CHUNKS:
   ///           uint32 rdma_max_chunks;
   ///         case RDMA2_ERR_SEGMENTS:
   ///           uint32 rdma_max_segments;
   ///         case RDMA2_ERR_WRITE_RESOURCE:
   ///           rpcrdma2_err_write rdma_writeres;
   ///         case RDMA2_ERR_REPLY_RESOURCE:



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   ///           uint32 rdma_length_needed;
   ///         default:
   ///           void;
   /// };
   ///
   /// /*
   ///  * Header Type to Exchange Transport Properties.
   ///  */
   /// struct rpcrdma2_connprop {
   ///         rpcrdma2_propset rdma_props;
   /// };
   ///
   /// /* FILE ENDS: baseops.x; */
   <CODE ENDS>

8.5.  Use of the XDR Description

   The files common.x and baseops.x, when combined with the XDR
   descriptions for extension defined later, produce a human-readable
   and compilable description of the RPC-over-RDMA version 2 protocol
   with the included extensions.

   Although this XDR description can generate encoders and decoders for
   the Transport and Payload streams, there are elements of the
   operation of RPC-over-RDMA version 2 that cannot be expressed within
   the XDR language.  Implementations that use the output of an
   automated XDR processor need to provide additional code to bridge
   these gaps.

   *  The Transport stream is not a single XDR object.  Instead, the
      header prefix is one XDR data item, and the rest of the header is
      a separate XDR data item.  Table 2 expresses the mapping between
      the header type in the header prefix and the XDR object
      representing the header type.

   *  The relationship between the Transport stream and the Payload
      stream is not specified using XDR.  Comments within the XDR text
      make clear where transported messages, described by their own XDR
      definitions, need to appear.  Such data is opaque to the
      transport.

   *  Continuation of RPC messages across transport message boundaries
      requires that message assembly facilities not specifiable within
      XDR are part of transport implementations.







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   *  Transport properties are constant integer values.  Table 1
      expresses the mapping between each property's code point and the
      XDR typedef that represents the structure of the property's value.
      XDR does not possess the facility to express that mapping in an
      extensible way.

   The role of XDR in RPC-over-RDMA specifications is more limited than
   for protocols where the totality of the protocol is expressible
   within XDR.  XDR lacks the facility to represent the embedding of
   XDR-encoded payload material.  Also, the need to cleanly accommodate
   extensions has meant that those using rpcgen in their applications
   need to take an active role to provide the facilities that cannot be
   expressed within XDR.

9.  RPC Bind Parameters

   Before establishing a new connection, an RPC client obtains a
   transport address for the RPC server.  The means used to obtain this
   address and to open an RDMA connection is dependent on the type of
   RDMA transport and is the responsibility of each RPC protocol binding
   and its local implementation.

   RPC services typically register with a portmap or rpcbind service
   [RFC1833], which associates an RPC Program number with a service
   address.  This policy is no different with RDMA transports.  However,
   a distinct service address (port number) is sometimes required for
   operation on RPC-over-RDMA.

   When mapped atop the iWARP transport [RFC5040] [RFC5041], which uses
   IP port addressing due to its layering on TCP or SCTP, port mapping
   is trivial and consists merely of issuing the port in the connection
   process.  The NFS/RDMA protocol service address has been assigned
   port 20049 by IANA, for both iWARP/TCP and iWARP/SCTP [RFC8267].

   When mapped atop InfiniBand [IBA], which uses a service endpoint
   naming scheme based on a Group Identifier (GID), a translation MUST
   be employed.  One such translation is described in Annexes A3
   (Application Specific Identifiers), A4 (Sockets Direct Protocol
   (SDP)), and A11 (RDMA IP CM Service) of [IBA], which is appropriate
   for translating IP port addressing to the InfiniBand network.
   Therefore, in this case, IP port addressing may be readily employed
   by the upper layer.

   When a mapping standard or convention exists for IP ports on an RDMA
   interconnect, there are several possibilities for each upper layer to
   consider:





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   *  One possibility is to have the server register its mapped IP port
      with the rpcbind service under the netid (or netids) defined in
      [RFC8166].  An RPC-over-RDMA-aware RPC client can then resolve its
      desired service to a mappable port and proceed to connect.  This
      method is the most flexible and compatible approach for those
      upper layers that are defined to use the rpcbind service.

   *  A second possibility is to have the RPC server's portmapper
      register itself on the RDMA interconnect at a "well-known" service
      address (on UDP or TCP, this corresponds to port 111).  An RPC
      client can connect to this service address and use the portmap
      protocol to obtain a service address in response to a program
      number (e.g., an iWARP port number or an InfiniBand GID).

   *  Alternately, an RPC client can connect to the mapped well-known
      port for the service itself, if it is appropriately defined.  By
      convention, the NFS/RDMA service, when operating atop such an
      InfiniBand fabric, uses the same 20049 assignment as for iWARP.

   Historically, different RPC protocols have taken different approaches
   to their port assignments.  The current document leaves the specific
   method for each RPC-over-RDMA-enabled ULB.

   [RFC8166] defines two new netid values to be used for registration of
   upper layers atop iWARP [RFC5040] [RFC5041] and (when a suitable port
   translation service is available) InfiniBand [IBA].  Additional RDMA-
   capable networks MAY define their own netids, or if they provide a
   port translation, they MAY share the one defined in [RFC8166].

10.  Implementation Status

   This section is to be removed before publishing as an RFC.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.

   Please note that the listing of any individual implementation here
   does not imply endorsement by the IETF.  Furthermore, no effort has
   been spent to verify the information presented here that was supplied
   by IETF contributors.  This is not intended as, and must not be
   construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.




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   At this time, no known implementations of the protocol described in
   the current document exist.

11.  Security Considerations

11.1.  Memory Protection

   A primary consideration is the protection of the integrity and
   confidentiality of host memory by an RPC-over-RDMA transport.  The
   use of an RPC-over-RDMA transport protocol MUST NOT introduce
   vulnerabilities to system memory contents nor memory owned by user
   processes.  Any RDMA provider used for RPC transport MUST conform to
   the requirements of [RFC5042] to satisfy these protections.

11.1.1.  Protection Domains

   The use of a Protection Domain to limit the exposure of memory
   regions to a single connection is critical.  Any attempt by an
   endpoint not participating in that connection to reuse memory handles
   needs to result in immediate failure of that connection.  Because ULP
   security mechanisms rely on this aspect of Reliable Connected
   behavior, implementations SHOULD cryptographically authenticate
   connection endpoints.

11.1.2.  Handle (STag) Predictability

   Implementations should use unpredictable memory handles for any
   operation requiring exposed memory regions.  Exposing a continuously
   registered memory region allows a remote host to read or write to
   that region even when an RPC involving that memory is not underway.
   Therefore, implementations should avoid the use of persistently
   registered memory.

11.1.3.  Memory Protection

   Requesters should register memory regions for remote access only when
   they are about to be the target of an RPC transaction that involves
   an RDMA Read or Write.

   Requesters should invalidate memory regions as soon as related RPC
   operations are complete.  Invalidation and DMA unmapping of memory
   regions should complete before the receiver checks message integrity,
   and before the RPC consumer can use or alter the contents of the
   exposed memory region.







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   An RPC transaction on a Requester can terminate before a Reply
   arrives, for example, if the RPC consumer is signaled, or a
   segmentation fault occurs.  When an RPC terminates abnormally, memory
   regions associated with that RPC should be invalidated before the
   Requester reuses those regions for other purposes.

11.1.4.  Denial of Service

   A detailed discussion of denial-of-service exposures that can result
   from the use of an RDMA transport appears in Section 6.4 of
   [RFC5042].

   A Responder is not obliged to pull unreasonably large Read chunks.  A
   Responder can use an RDMA2_ERROR response to terminate RPCs with
   unreadable Read chunks.  If a Responder transmits more data than a
   Requester is prepared to receive in a Write or Reply chunk, the RDMA
   provider typically terminates the connection.  For further
   discussion, see Section 6.3.3.  Such repeated connection termination
   can deny service to other users sharing the connection from the
   errant Requester.

   An RPC-over-RDMA transport implementation is not responsible for
   throttling the RPC request rate, other than to keep the number of
   concurrent RPC transactions at or under the number of credits granted
   per connection (see Section 4.2.1).  A sender can trigger a self-
   denial of service by exceeding the credit grant repeatedly.

   When an RPC transaction terminates due to a signal or premature exit
   of an application process, a Requester should invalidate the RPC's
   Write and Reply chunks.  Invalidation prevents the subsequent arrival
   of the Responder's Reply from altering the memory regions associated
   with those chunks after the Requester has released that memory.

   On the Requester, a malfunctioning application or a malicious user
   can create a situation where RPCs initiate and abort continuously,
   resulting in Responder replies that terminate the underlying RPC-
   over-RDMA connection repeatedly.  Such situations can deny service to
   other users sharing the connection from that Requester.













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11.2.  RPC Message Security

   ONC RPC provides cryptographic security via the RPCSEC_GSS framework
   [RFC7861].  RPCSEC_GSS implements message authentication
   (rpc_gss_svc_none), per-message integrity checking
   (rpc_gss_svc_integrity), and per-message confidentiality
   (rpc_gss_svc_privacy) in a layer above the RPC-over-RDMA transport.
   The integrity and privacy services require significant computation
   and movement of data on each endpoint host.  Some performance
   benefits enabled by RDMA transports can be lost.

11.2.1.  RPC-over-RDMA Protection at Other Layers

   For any RPC transport, utilizing RPCSEC_GSS integrity or privacy
   services has performance implications.  Protection below the RPC
   implementation is often a better choice in performance-sensitive
   deployments, especially if it, too, can be offloaded.  Certain
   implementations of IPsec can be co-located in RDMA hardware, for
   example, without change to RDMA consumers and with little loss of
   data movement efficiency.  Such arrangements can also provide a
   higher degree of privacy by hiding endpoint identity or altering the
   frequency at which messages are exchanged, at a performance cost.

   Implementations MAY negotiate the use of protection in another layer
   through the use of an RPCSEC_GSS security flavor defined in [RFC7861]
   in conjunction with the Channel Binding mechanism [RFC5056] and IPsec
   Channel Connection Latching [RFC5660].

11.2.2.  RPCSEC_GSS on RPC-over-RDMA Transports

   Not all RDMA devices and fabrics support the above protection
   mechanisms.  Also, NFS clients, where multiple users can access NFS
   files, still require per-message authentication.  In these cases,
   RPCSEC_GSS can protect NFS traffic conveyed on RPC-over-RDMA
   connections.

   RPCSEC_GSS extends the ONC RPC protocol without changing the format
   of RPC messages.  By observing the conventions described in this
   section, an RPC-over-RDMA transport can convey RPCSEC_GSS-protected
   RPC messages interoperably.

   Senders MUST NOT reduce protocol elements of RPCSEC_GSS that appear
   in the Payload stream of an RPC-over-RDMA message.  Such elements
   include control messages exchanged as part of establishing or
   destroying a security context, or data items that are part of
   RPCSEC_GSS authentication material.





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11.2.2.1.  RPCSEC_GSS Context Negotiation

   Some NFS client implementations use a separate connection to
   establish a Generic Security Service (GSS) context for NFS operation.
   Such clients use TCP and the standard NFS port (2049) for context
   establishment.  Therefore, an NFS server MUST also provide a TCP-
   based NFS service on port 2049 to enable the use of RPCSEC_GSS with
   NFS/RDMA.

11.2.2.2.  RPC-over-RDMA with RPCSEC_GSS Authentication

   The RPCSEC_GSS authentication service has no impact on the DDP-
   eligibility of data items in a ULP.

   However, RPCSEC_GSS authentication material appearing in an RPC
   message header can be larger than, say, an AUTH_SYS authenticator.
   In particular, when an RPCSEC_GSS pseudoflavor is in use, a Requester
   needs to accommodate a larger RPC credential when marshaling RPC
   Calls and needs to provide for a maximum size RPCSEC_GSS verifier
   when allocating reply buffers and Reply chunks.

   RPC messages, and thus Payload streams, are larger on average as a
   result.  ULP operations that fit in a Simple Format message when a
   simpler form of authentication is in use might need to be reduced or
   conveyed via a Special Format message when RPCSEC_GSS authentication
   is in use.  It is therefore more likely that a Requester provisions
   both a Read list and a Reply chunk in the same RPC-over-RDMA
   Transport header to convey a Special Format Call and provision a
   receptacle for a Special Format Reply.

   In addition to this cost, the XDR encoding and decoding of each RPC
   message using RPCSEC_GSS authentication requires per-message host
   compute resources to construct the GSS verifier.

11.2.2.3.  RPC-over-RDMA with RPCSEC_GSS Integrity or Privacy

   The RPCSEC_GSS integrity service enables endpoints to detect the
   modification of RPC messages in flight.  The RPCSEC_GSS privacy
   service prevents all but the intended recipient from viewing the
   cleartext content of RPC arguments and results.  RPCSEC_GSS integrity
   and privacy services are end-to-end.  They protect RPC arguments and
   results from application to server endpoint, and back.









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   The RPCSEC_GSS integrity and encryption services operate on whole RPC
   messages after they have been XDR encoded, and before they have been
   XDR decoded after receipt.  Connection endpoints use intermediate
   buffers to prevent exposure of encrypted or unverified cleartext data
   to RPC consumers.  After a sender has verified, encrypted, and
   wrapped a message, the transport layer MAY use RDMA data transfer
   between these intermediate buffers.

   The process of reducing a DDP-eligible data item removes the data
   item and its XDR padding from an encoded Payload stream.  In a non-
   protected RPC-over-RDMA message, a reduced data item does not include
   XDR padding.  After reduction, the Payload stream contains fewer
   octets than the whole XDR stream did beforehand.  XDR padding octets
   are often zero bytes, but they don't have to be.  Thus, reducing DDP-
   eligible items affects the result of message integrity verification
   and encryption.

   Therefore, a sender MUST NOT reduce a Payload stream when RPCSEC_GSS
   integrity or encryption services are in use.  Effectively, no data
   item is DDP-eligible in this situation.  Senders can use only Simple
   and Continued Formats without chunks, or Special Format.  In this
   mode, an RPC-over-RDMA transport operates in the same manner as a
   transport that does not support DDP.

11.2.2.4.  Protecting RPC-over-RDMA Transport Headers

   Like the header fields in an RPC message (e.g., the xid and mtype
   fields), RPCSEC_GSS does not protect the RPC-over-RDMA Transport
   stream.  XIDs, connection credit limits, and chunk lists (though not
   the content of the data items they refer to) are exposed to malicious
   behavior, which can redirect data that is transferred by the RPC-
   over-RDMA message, result in spurious retransmits, or trigger
   connection loss.

   In particular, if an attacker alters the information contained in the
   chunk lists of an RPC-over-RDMA Transport header, data contained in
   those chunks can be redirected to other registered memory regions on
   Requesters.  An attacker might alter the arguments of RDMA Read and
   RDMA Write operations on the wire to gain a similar effect.  If such
   alterations occur, the use of RPCSEC_GSS integrity or privacy
   services enables a Requester to detect unexpected material in a
   received RPC message.

   Encryption at other layers, as described in Section 11.2.1, protects
   the content of the Transport stream.  RDMA transport implementations
   should conform to [RFC5042] to address attacks on RDMA protocols
   themselves.




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11.3.  Transport Properties

   Like other fields that appear in the Transport stream, transport
   properties are sent in the clear with no integrity protection, making
   them vulnerable to man-in-the-middle attacks.

   For example, if a man-in-the-middle were to change the value of the
   Receive buffer size, it could reduce connection performance or
   trigger loss of connection.  Repeated connection loss can impact
   performance or even prevent a new connection from being established.
   The recourse is to deploy on a private network or use transport layer
   encryption.

11.4.  Host Authentication

   [ cel: This subsection is unfinished. ]

   Wherein we use the relevant sections of [RFC3552] to analyze the
   addition of host authentication to this RPC-over-RDMA transport.

   The authors refer readers to Appendix C of [RFC8446] for information
   on how to design and test a secure authentication handshake
   implementation.

12.  IANA Considerations

   The RPC-over-RDMA family of transports have been assigned RPC netids
   by [RFC8166].  A netid is an rpcbind [RFC1833] string used to
   identify the underlying protocol in order for RPC to select
   appropriate transport framing and the format of the service addresses
   and ports.

   The following netid registry strings are already defined for this
   purpose:

      NC_RDMA "rdma"
      NC_RDMA6 "rdma6"

   The "rdma" netid is to be used when IPv4 addressing is employed by
   the underlying transport, and "rdma6" when IPv6 addressing is
   employed.  The netid assignment policy and registry are defined in
   [RFC5665].  The current document does not alter these netid
   assignments.

   These netids MAY be used for any RDMA network that satisfies the
   requirements of Section 3.2.2 and that is able to identify service
   endpoints using IP port addressing, possibly through use of a
   translation service as described in Section 9.



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13.  References

13.1.  Normative References

   [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
              RFC 1833, DOI 10.17487/RFC1833, August 1995,
              <https://www.rfc-editor.org/info/rfc1833>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <https://www.rfc-editor.org/info/rfc4506>.

   [RFC5042]  Pinkerton, J. and E. Deleganes, "Direct Data Placement
              Protocol (DDP) / Remote Direct Memory Access Protocol
              (RDMAP) Security", RFC 5042, DOI 10.17487/RFC5042, October
              2007, <https://www.rfc-editor.org/info/rfc5042>.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
              <https://www.rfc-editor.org/info/rfc5056>.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
              May 2009, <https://www.rfc-editor.org/info/rfc5531>.

   [RFC5660]  Williams, N., "IPsec Channels: Connection Latching",
              RFC 5660, DOI 10.17487/RFC5660, October 2009,
              <https://www.rfc-editor.org/info/rfc5660>.

   [RFC5665]  Eisler, M., "IANA Considerations for Remote Procedure Call
              (RPC) Network Identifiers and Universal Address Formats",
              RFC 5665, DOI 10.17487/RFC5665, January 2010,
              <https://www.rfc-editor.org/info/rfc5665>.

   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <https://www.rfc-editor.org/info/rfc7861>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.




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   [RFC8166]  Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call Version
              1", RFC 8166, DOI 10.17487/RFC8166, June 2017,
              <https://www.rfc-editor.org/info/rfc8166>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8267]  Lever, C., "Network File System (NFS) Upper-Layer Binding
              to RPC-over-RDMA Version 1", RFC 8267,
              DOI 10.17487/RFC8267, October 2017,
              <https://www.rfc-editor.org/info/rfc8267>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

13.2.  Informative References

   [CBFC]     Kung, H.T., Blackwell, T., and A. Chapman, "Credit-Based
              Flow Control for ATM Networks: Credit Update Protocol,
              Adaptive Credit Allocation, and Statistical Multiplexing",
              Proc. ACM SIGCOMM '94 Symposium on Communications
              Architectures, Protocols and Applications, pp. 101-114.,
              August 1994.

   [I-D.ietf-nfsv4-rpc-tls]
              Myklebust, T. and C. Lever, "Towards Remote Procedure Call
              Encryption By Default", Work in Progress, Internet-Draft,
              draft-ietf-nfsv4-rpc-tls-07, 30 April 2020,
              <https://tools.ietf.org/html/draft-ietf-nfsv4-rpc-tls-07>.

   [IBA]      InfiniBand Trade Association, "InfiniBand Architecture
              Specification Volume 1", Release 1.3, March 2015.
              Available from https://www.infinibandta.org/

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1094]  Nowicki, B., "NFS: Network File System Protocol
              specification", RFC 1094, DOI 10.17487/RFC1094, March
              1989, <https://www.rfc-editor.org/info/rfc1094>.



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   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813,
              DOI 10.17487/RFC1813, June 1995,
              <https://www.rfc-editor.org/info/rfc1813>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC5040]  Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
              Garcia, "A Remote Direct Memory Access Protocol
              Specification", RFC 5040, DOI 10.17487/RFC5040, October
              2007, <https://www.rfc-editor.org/info/rfc5040>.

   [RFC5041]  Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
              Data Placement over Reliable Transports", RFC 5041,
              DOI 10.17487/RFC5041, October 2007,
              <https://www.rfc-editor.org/info/rfc5041>.

   [RFC5532]  Talpey, T. and C. Juszczak, "Network File System (NFS)
              Remote Direct Memory Access (RDMA) Problem Statement",
              RFC 5532, DOI 10.17487/RFC5532, May 2009,
              <https://www.rfc-editor.org/info/rfc5532>.

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
              <https://www.rfc-editor.org/info/rfc5661>.

   [RFC5662]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              External Data Representation Standard (XDR) Description",
              RFC 5662, DOI 10.17487/RFC5662, January 2010,
              <https://www.rfc-editor.org/info/rfc5662>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <https://www.rfc-editor.org/info/rfc7530>.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <https://www.rfc-editor.org/info/rfc7862>.

   [RFC8167]  Lever, C., "Bidirectional Remote Procedure Call on RPC-
              over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
              June 2017, <https://www.rfc-editor.org/info/rfc8167>.




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Appendix A.  ULB Specifications

   Typically, an Upper-Layer Protocol (ULP) is defined without regard to
   a particular RPC transport.  An Upper-Layer Binding (ULB)
   specification provides guidance that helps a ULP interoperate
   correctly and efficiently over a particular transport.  For RPC-over-
   RDMA version 2, a ULB may provide:

   *  A taxonomy of XDR data items that are eligible for DDP

   *  Constraints on which upper-layer procedures a sender may reduce,
      and on how many chunks may appear in a single RPC message

   *  A method enabling a Requester to determine the maximum size of the
      reply Payload stream for all procedures in the ULP

   *  An rpcbind port assignment for the RPC Program and Version when
      operating on the particular transport

   Each RPC Program and Version tuple that operates on RPC-over-RDMA
   version 2 needs to have a ULB specification.

A.1.  DDP-Eligibility

   A ULB designates specific XDR data items as eligible for DDP.  As a
   sender constructs an RPC-over-RDMA message, it can remove DDP-
   eligible data items from the Payload stream so that the RDMA provider
   can place them directly in the receiver's memory.  An XDR data item
   should be considered for DDP-eligibility if there is a clear benefit
   to moving the contents of the item directly from the sender's memory
   to the receiver's memory.

   Criteria for DDP-eligibility include:

   *  The XDR data item is frequently sent or received, and its size is
      often much larger than typical inline thresholds.

   *  If the XDR data item is a result, its maximum size must be
      predictable in advance by the Requester.

   *  Transport-level processing of the XDR data item is not needed.
      For example, the data item is an opaque byte array, which requires
      no XDR encoding and decoding of its content.

   *  The content of the XDR data item is sensitive to address
      alignment.  For example, a data copy operation would be required
      on the receiver to enable the message to be parsed correctly, or
      to enable the data item to be accessed.



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   *  The XDR data item itself does not contain DDP-eligible data items.

   In addition to defining the set of data items that are DDP-eligible,
   a ULB may limit the use of chunks to particular upper-layer
   procedures.  If more than one data item in a procedure is DDP-
   eligible, the ULB may limit the number of chunks that a Requester can
   provide for a particular upper-layer procedure.

   Senders never reduce data items that are not DDP-eligible.  Such data
   items can, however, be part of a Special Format payload.

   The programming interface by which an upper-layer implementation
   indicates the DDP-eligibility of a data item to the RPC transport is
   not described by this specification.  The only requirements are that
   the receiver can re-assemble the transmitted RPC-over-RDMA message
   into a valid XDR stream and that DDP-eligibility rules specified by
   the ULB are respected.

   There is no provision to express DDP-eligibility within the XDR
   language.  The only definitive specification of DDP-eligibility is a
   ULB.

   In general, a DDP-eligibility violation occurs when:

   *  A Requester reduces a non-DDP-eligible argument data item.  The
      Responder reports the violation as described in Section 6.3.3.

   *  A Responder reduces a non-DDP-eligible result data item.  The
      Requester terminates the pending RPC transaction and reports an
      appropriate permanent error to the RPC consumer.

   *  A Responder does not reduce a DDP-eligible result data item into
      an available Write chunk.  The Requester terminates the pending
      RPC transaction and reports an appropriate permanent error to the
      RPC consumer.

A.2.  Maximum Reply Size

   When expecting small and moderately-sized Replies, a Requester should
   rely on Message Continuation rather than provision a Reply chunk.
   For each ULP procedure where there is no clear Reply size maximum and
   the maximum can be substantial, the ULB should specify a dependable
   means for determining the maximum Reply size.

A.3.  Reverse-Direction Operation

   The direction of operation does not preclude the need for DDP-
   eligibility statements.



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   Reverse-direction operation occurs on an already-established
   connection.  Specification of RPC binding parameters is usually not
   necessary in this case.

   Other considerations may apply when distinct RPC Programs share an
   RPC-over-RDMA transport connection concurrently.

A.4.  Additional Considerations

   There may be other details provided in a ULB.

   *  A ULB may recommend inline threshold values or other transport-
      related parameters for RPC-over-RDMA version 2 connections bearing
      that ULP.

   *  A ULP may provide a means to communicate transport-related
      parameters between peers.

   *  Multiple ULPs may share a single RPC-over-RDMA version 2
      connection when their ULBs allow the use of RPC-over-RDMA version
      2 and the rpcbind port assignments for those protocols permit
      connection sharing.  In this case, the same transport parameters
      (such as inline threshold) apply to all ULPs using that
      connection.

   Each ULB needs to be designed to allow correct interoperation without
   regard to the transport parameters actually in use.  Furthermore,
   implementations of ULPs must be designed to interoperate correctly
   regardless of the connection parameters in effect on a connection.

A.5.  ULP Extensions

   An RPC Program and Version tuple may be extensible.  For instance,
   the RPC version number may not reflect a ULP minor versioning scheme,
   or the ULP may allow the specification of additional features after
   the publication of the original RPC Program specification.  ULBs are
   provided for interoperable RPC Programs and Versions by extending
   existing ULBs to reflect the changes made necessary by each addition
   to the existing XDR.

   [ cel: The final sentence is unclear, and may be inaccurate.  I
   believe I copied this section directly from RFC 8166.  Is there more
   to be said, now that we have some experience? ]








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Appendix B.  Extending RPC-over-RDMA Version 2

   This Appendix is not addressed to protocol implementers, but rather
   to authors of documents that extend the protocol specified in the
   current document.

   RPC-over-RDMA version 2 extensibility facilitates limited extensions
   to the base protocol presented in the current document so that new
   optional capabilities can be introduced without a protocol version
   change while maintaining robust interoperability with existing RPC-
   over-RDMA version 2 implementations.  It allows extensions to be
   defined, including the definition of new protocol elements, without
   requiring modification or recompilation of the XDR for the base
   protocol.

   Standards Track documents may introduce extensions to the base RPC-
   over-RDMA version 2 protocol in two ways:

   *  They may introduce new OPTIONAL transport header types.
      Appendix B.2 covers such transport header types.

   *  They may define new OPTIONAL transport properties.  Appendix B.4
      describes such transport properties.

   These documents may also add the following sorts of ancillary
   protocol elements to the protocol to support the addition of new
   transport properties and header types:

   *  They may create new error codes, as described in Appendix B.5.

   *  They may define new flags to use within the rdma_flags field, as
      discussed in Appendix B.3.

   New capabilities can be proposed and developed independently of each
   other.  Implementers can choose among them, making it straightforward
   to create and document experimental features and then bring them
   through the standards process.

B.1.  Documentation Requirements

   As described earlier, a Standards Track document introduces a set of
   new protocol elements.  Together these elements are considered an
   OPTIONAL feature.  Each implementation is either aware of all the
   protocol elements introduced by that feature or is aware of none of
   them.

   Documents specifying extensions to RPC-over-RDMA version 2 should
   contain:



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   *  An explanation of the purpose and use of each new protocol
      element.

   *  An XDR description including all of the new protocol elements, and
      a script to extract it.

   *  A discussion of interactions with other extensions.  This
      discussion includes requirements for other OPTIONAL features to be
      present, or that a particular level of support for an OPTIONAL
      facility is required.

   Implementers combine the XDR descriptions of the new features they
   intend to use with the XDR description of the base protocol in the
   current document.  This combination is necessary to create a valid
   XDR input file because extensions are free to use XDR types defined
   in the base protocol, and later extensions may use types defined by
   earlier extensions.

   The XDR description for the RPC-over-RDMA version 2 base protocol
   combined with that for any selected extensions should provide a
   human-readable and compilable definition of the extended protocol.

B.2.  Adding New Header Types to RPC-over-RDMA Version 2

   New transport header types are defined similar to Sections 6.3.1
   through 6.3.4.  In particular, what is needed is:

   *  A description of the function and use of the new header type.

   *  A complete XDR description of the new header type.

   *  A description of how receivers report errors, including mechanisms
      for reporting errors outside the available choices already
      available in the base protocol or other extensions.

   *  An indication of whether a Payload stream must be present, and a
      description of its contents and how receivers use such Payload
      streams to reconstruct RPC messages.

   *  As appropriate, a statement of whether a Responder may use Remote
      Invalidation when sending messages that contain the new header
      type.

   There needs to be additional documentation that is made necessary due
   to the OPTIONAL status of new transport header types:






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   *  The document should discuss constraints on support for the new
      header types.  For example, if support for one header type is
      implied or foreclosed by another one, this needs to be documented.

   *  The document should describe the preferred method by which a
      sender determines whether its peer supports a particular header
      type.  It is always possible to send a test invocation of a
      particular header type to see if support is available.  However,
      when more efficient means are available (e.g., the value of a
      transport property), this should be noted.

B.3.  Adding New Header Flags to the Protocol

   New flag bits are to be defined similarly to Sections 6.2.2.1 and
   6.2.2.2.  Each new flag definition should include:

   *  An XDR description of the new flag.

   *  A description of the function and use of the new flag.

   *  An indication for which header types the flag value is meaningful,
      and for which header types it is an error to set the flag or to
      leave it unset.

   *  A means to determine whether peers are prepared to receive
      transport headers with the new flag set.

   There needs to be additional documentation that is made necessary due
   to the OPTIONAL status of new flags:

   *  The document should discuss constraints on support for the new
      flags.  For example, if support for one flag is implied or
      foreclosed by another one, this needs to be documented.

B.4.  Adding New Transport properties to the Protocol

   A Standards Track document defining a new transport property should
   include the following information paralleling that provided in this
   document for the transport properties defined herein:

   *  The rpcrdma2_propid value identifying the new property.

   *  The XDR typedef specifying the structure of its property value.

   *  A description of the new property.

   *  An explanation of how the receiver can use this information.




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   *  The default value if a peer never receives the new property.

   There is no requirement that propid assignments occur in a continuous
   range of values.  Implementations should not rely on all such values
   being small integers.

   Before the defining Standards Track document is published, the nfsv4
   Working Group should select a unique propid value, and ensure that:

   *  rpcrdma2_propid values specified in the document do not conflict
      with those currently assigned or in use by other pending working
      group documents defining transport properties.

   *  rpcrdma2_propid values specified in the document do not conflict
      with the range reserved for experimental use, as defined in
      Section 8.2.

      [ cel: There is no section 8.2. ]

      [ cel: Should we request the creation of an IANA registry for
      propid values? ].

   When a Standards Track document proposes additional transport
   properties, reviewers should deal with possible security issues
   exposed by those new transport properties.

B.5.  Adding New Error Codes to the Protocol

   The same Standards Track document that defines a new header type may
   introduce new error codes used to support it.  A Standards Track
   document may similarly define new error codes that an existing header
   type can return.

   For error codes that do not require the return of additional
   information, a peer can use the existing RDMA_ERR2 header type to
   report the new error.  The sender sets the new error code as the
   value of rdma_err with the result that the default switch arm of the
   rpcrdma2_error (i.e., void) is selected.

   For error codes that do require the return of related information
   together with the error, a new header type should be defined that
   returns the error together with the related information.  The sender
   of a new header type needs to be prepared to accept header types
   necessary to report associated errors.







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Appendix C.  Differences from RPC-over-RDMA Version 1

   The goal of RPC-over-RDMA version 2 is to relieve certain constraints
   that have become evident in RPC-over-RDMA version 1 with deployment
   experience:

   *  RPC-over-RDMA version 1 has been challenging to update to address
      shortcomings or improve data transfer efficiency.

   *  The average size of NFSv4 COMPOUNDs is significantly greater than
      NFSv3 requests, requiring the use of Long messages for frequent
      operations.

   *  Reply size estimation is awkward more often than expected.

   This section details specific changes in RPC-over-RDMA version 2 that
   address these constraints directly.

C.1.  Changes to the XDR Definition

   Several XDR structural changes enable within-version protocol
   extensibility.

   [RFC8166] defines the RPC-over-RDMA version 1 transport header as a
   single XDR object, with an RPC message potentially following it.  In
   RPC-over-RDMA version 2, there are separate XDR definitions of the
   transport header prefix (see Section 6.2.2), which specifies the
   transport header type to be used, and the transport header itself
   (defined within one of the subsections of Section 6.3).  This
   construction is similar to an RPC message, which consists of an RPC
   header (defined in [RFC5531]) followed by a message defined by an
   Upper-Layer Protocol.

   As a new version of the RPC-over-RDMA transport protocol, RPC-over-
   RDMA version 2 exists within the versioning rules defined in
   [RFC8166].  In particular, it maintains the first four words of the
   protocol header, as specified in Section 4.2 of [RFC8166], even
   though, as explained in Section 6.2.1 of the current document, the
   XDR definition of those words is structured differently.

   Although each of the first four fields retains its semantic function,
   there are differences in interpretation:

   *  The first word of the header, the rdma_xid field, retains the
      format and function that it had in RPC-over-RDMA version 1.
      Because RPC-over-RDMA version 2 messages can convey non-RPC
      messages, a receiver should not use the contents of this field
      without consideration of the protocol version and header type.



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   *  The second word of the header, the rdma_vers field, retains the
      format and function that it had in RPC-over-RDMA version 1.  To
      clearly distinguish version 1 and version 2 messages, senders need
      to fill in the correct version (fixed after version negotiation).
      Receivers should check that the content of the rdma_vers is
      correct before using the content of any other header field.

   *  The third word of the header, the rdma_credit field, retains the
      size and general purpose that it had in RPC-over-RDMA version 1.
      However, RPC-over-RDMA version 2 divides this field into two
      16-bit subfields.  See Section 4.2.1 for further details.

   *  The fourth word of the header, previously the union discriminator
      field rdma_proc, retains its format and general function even
      though the set of valid values has changed.  Within RPC-over-RDMA
      version 2, this word is the rdma_htype field of the structure
      rdma_start.  The value of this field is now an unsigned 32-bit
      integer rather than an enum type, to facilitate header type
      extension.

   Beyond conforming to the restrictions specified in [RFC8166], RPC-
   over-RDMA version 2 tightly limits the scope of the changes made to
   ensure interoperability.  Version 2 retains all existing transport
   header types used in version 1, as defined in [RFC8166].  And, it
   expresses chunks in the same format and uses them the same way.

C.2.  Transport Properties

   RPC-over-RDMA version 2 provides a mechanism for exchanging an
   implementation's operational properties.  The purpose of this
   exchange is to help endpoints improve the efficiency of data transfer
   by exploiting the characteristics of both peers rather than falling
   back on the lowest common denominator default settings.  A full
   discussion of transport properties appears in Section 5.

C.3.  Credit Management Changes

   RPC-over-RDMA transports employ credit-based flow control to ensure
   that a Requester does not emit more RDMA Sends than the Responder is
   prepared to receive.











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   Section 3.3.1 of [RFC8166] explains the operation of RPC-over-RDMA
   version 1 credit management in detail.  In that design, each RDMA
   Send from a Requester contains an RPC Call with a credit request, and
   each RDMA Send from a Responder contains an RPC Reply with a credit
   grant.  The credit grant implies that enough Receives have been
   posted on the Responder to handle the credit grant minus the number
   of pending RPC transactions (the number of remaining Receive buffers
   might be zero).

   Each RPC Reply acts as an implicit ACK for a previous RPC Call from
   the Requester.  Without an RPC Reply message, the Requester has no
   way to know that the Responder is ready for subsequent RPC Calls.

   Because version 1 embeds credit management in each message, there is
   a strict one-to-one ratio between RDMA Send and RPC message.  There
   are interesting use cases that might be enabled if this relationship
   were more flexible:

   *  RPC-over-RDMA operations that do not carry an RPC message, e.g.,
      control plane operations.

   *  A single RDMA Send that conveys more than one RPC message, e.g.,
      for interrupt mitigation.

   *  An RPC message that requires several sequential RDMA Sends, e.g.,
      to reduce the use of explicit RDMA operations for moderate-sized
      RPC messages.

   *  An RPC transaction that requires multiple exchanges or an odd
      number of RPC-over-RDMA operations to complete.

   RPC-over-RDMA version 2 provides a more sophisticated credit
   accounting mechanism to address these shortcomings.  Section 4.2.1
   explains the new mechanism in detail.

C.4.  Inline Threshold Changes

   An "inline threshold" value is the largest message size (in octets)
   that can be conveyed on an RDMA connection using only RDMA Send and
   Receive.  Each connection has two inline threshold values: one for
   messages flowing from client-to-server (referred to as the "client-
   to-server inline threshold") and one for messages flowing from
   server-to-client (referred to as the "server-to-client inline
   threshold").

   A connection's inline thresholds determine, among other things, when
   RDMA Read or Write operations are required because an RPC message
   cannot be conveyed via a single RDMA Send and Receive pair.  When an



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   RPC message does not contain DDP-eligible data items, a Requester can
   prepare a Special Format Call or Reply to convey the whole RPC
   message using RDMA Read or Write operations.

   RDMA Read and Write operations require that data payloads reside in
   memory registered with the local RNIC.  When an RPC completes, that
   memory is invalidated to fence it from the Responder.  Memory
   registration and invalidation typically have a latency cost that is
   insignificant compared to data handling costs.

   When a data payload is small, however, the cost of registering and
   invalidating memory where the payload resides becomes a significant
   part of total RPC latency.  Therefore the most efficient operation of
   an RPC-over-RDMA transport occurs when the peers use explicit RDMA
   Read and Write operations for large payloads but avoid those
   operations for small payloads.

   When the authors of [RFC8166] first conceived RPC-over-RDMA version
   1, the average size of RPC messages that did not involve a
   significant data payload was under 500 bytes.  A 1024-byte inline
   threshold adequately minimized the frequency of inefficient Long
   messages.

   With NFS version 4 [RFC7530], the increased size of NFS COMPOUND
   operations resulted in RPC messages that are, on average, larger than
   previous versions of NFS.  With a 1024-byte inline threshold,
   frequent operations such as GETATTR and LOOKUP require RDMA Read or
   Write operations, reducing the efficiency of data transport.

   To reduce the frequency of Special Format messages, RPC-over-RDMA
   version 2 increases the default size of inline thresholds.  This
   change also increases the maximum size of reverse-direction RPC
   messages.

C.5.  Message Continuation Changes

   In addition to a larger default inline threshold, RPC-over-RDMA
   version 2 introduces Message Continuation.  Message Continuation is a
   mechanism that enables the transmission of a data payload using more
   than one RDMA Send.  The purpose of Message Continuation is to
   provide relief in several essential cases:

   *  If a Requester finds that it is inefficient to convey a
      moderately-sized data payload using Read chunks, the Requester can
      use Message Continuation to send the RPC Call.






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   *  If a Requester has provided insufficient Reply chunk space for a
      Responder to send an RPC Reply, the Responder can use Message
      Continuation to send the RPC Reply.

   *  If a sender has to convey a sizeable non-RPC data payload (e.g., a
      large transport property), the sender can use Message Continuation
      to avoid having to register memory.

C.6.  Host Authentication Changes

   For the general operation of NFS on open networks, we eventually
   intend to rely on RPC-on-TLS [I-D.ietf-nfsv4-rpc-tls] to provide
   cryptographic authentication of the two ends of each connection.  In
   turn, this can improve the trustworthiness of AUTH_SYS-style user
   identities that flow on TCP, which are not cryptographically
   protected.  We do not have a similar solution for RPC-over-RDMA,
   however.

   Here, the RDMA transport layer already provides a strong guarantee of
   message integrity.  On some network fabrics, IPsec or TLS can protect
   the privacy of in-transit data.  However, this is not the case for
   all fabrics (e.g., InfiniBand [IBA]).

   Thus, RPC-over-RDMA version 2 introduces a mechanism for
   authenticating connection peers (see Section 5.2.6).  And like GSS
   channel binding, there is also a way to determine when the use of
   host authentication is unnecessary.

C.7.  Support for Remote Invalidation

   When an RDMA consumer uses FRWR or Memory Windows to register memory,
   that memory may be invalidated remotely [RFC5040].  These mechanisms
   are available when a Requester's RNIC supports MEM_MGT_EXTENSIONS.

   For this discussion, there are two classes of STags.  Dynamically-
   registered STags appear in a single RPC, then are invalidated.
   Persistently-registered STags survive longer than one RPC.  They may
   persist for the life of an RPC-over-RDMA connection or even longer.

   An RPC-over-RDMA Requester can provide more than one STag in a
   transport header.  It may provide a combination of dynamically- and
   persistently-registered STags in one RPC message, or any combination
   of these in a series of RPCs on the same connection.  Only
   dynamically-registered STags using Memory Windows or FRWR may be
   invalidated remotely.






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   There is no transport-level mechanism by which a Responder can
   determine how a Requester-provided STag was registered, nor whether
   it is eligible to be invalidated remotely.  A Requester that mixes
   persistently- and dynamically-registered STags in one RPC, or mixes
   them across RPCs on the same connection, must, therefore, indicate
   which STag the Responder may invalidate remotely via a mechanism
   provided in the Upper-Layer Protocol.  RPC-over-RDMA version 2
   provides such a mechanism.

   A sender uses the RDMA Send With Invalidate operation to invalidate
   an STag on the remote peer.  It is available only when both peers
   support MEM_MGT_EXTENSIONS (can send and process an IETH).

   Existing RPC-over-RDMA transport protocol specifications [RFC8166]
   [RFC8167] do not forbid direct data placement in the reverse
   direction.  Moreover, there is currently no Upper-Layer Protocol that
   makes data items in reverse direction operations eligible for direct
   data placement.

   When chunks are present in a reverse direction RPC request, Remote
   Invalidation enables the Responder to trigger invalidation of a
   Requester's STags as part of sending an RPC Reply, the same way as is
   done in the forward direction.

   However, in the reverse direction, the server acts as the Requester,
   and the client is the Responder.  The server's RNIC, therefore, must
   support receiving an IETH, and the server must have registered its
   STags with an appropriate registration mechanism.

C.8.  Error Reporting Changes

   RPC-over-RDMA version 2 expands the repertoire of errors that
   connection peers may report to each other.  The goals of this
   expansion are:

   *  To fill in details of peer recovery actions.

   *  To enable retrying certain conditions caused by mis-estimation of
      the maximum reply size.

   *  To minimize the likelihood of a Requester waiting forever for a
      Reply when there are communications problems that prevent the
      Responder from sending it.








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Acknowledgments

   The authors gratefully acknowledge the work of Brent Callaghan and
   Tom Talpey on the original RPC-over-RDMA version 1 specification (RFC
   5666).  The authors also wish to thank Bill Baker, Greg Marsden, and
   Matt Benjamin for their support of this work.

   The XDR extraction conventions were first described by the authors of
   the NFS version 4.1 XDR specification [RFC5662].  Herbert van den
   Bergh suggested the replacement sed script used in this document.

   Special thanks go to Transport Area Director Magnus Westerlund, NFSV4
   Working Group Chairs Spencer Shepler, David Noveck, and Brian
   Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes for their
   support.

Authors' Addresses

   Charles Lever (editor)
   Oracle Corporation
   United States of America

   Email: chuck.lever@oracle.com


   David Noveck
   NetApp
   1601 Trapelo Road
   Waltham, MA 02451
   United States of America

   Phone: +1 781 572 8038
   Email: davenoveck@gmail.com


















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