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Versions: (RFC 5666) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 8166

Network File System Version 4                              C. Lever, Ed.
Internet-Draft                                                    Oracle
Obsoletes: 5666 (if approved)                                 W. Simpson
Intended status: Standards Track                              DayDreamer
Expires: July 28, 2016                                         T. Talpey
                                                               Microsoft
                                                        January 25, 2016


    Remote Direct Memory Access Transport for Remote Procedure Call
                     draft-ietf-nfsv4-rfc5666bis-03

Abstract

   This document specifies a protocol for conveying Remote Procedure
   Call (RPC) messages on physical transports capable of Remote Direct
   Memory Access (RDMA).  It requires no revision to application RPC
   protocols or the RPC protocol itself.  This document obsoletes RFC
   5666.

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 http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on July 28, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Remote Procedure Calls On RDMA Transports . . . . . . . .   3
   2.  Changes Since RFC 5666  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Changes To The Specification  . . . . . . . . . . . . . .   4
     2.2.  Changes To The Protocol . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Remote Procedure Calls  . . . . . . . . . . . . . . . . .   5
     3.2.  Remote Direct Memory Access . . . . . . . . . . . . . . .   8
   4.  RPC-Over-RDMA Protocol Framework  . . . . . . . . . . . . . .  10
     4.1.  Transfer Models . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Message Framing . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Managing Receiver Resources . . . . . . . . . . . . . . .  12
     4.4.  XDR Encoding With Chunks  . . . . . . . . . . . . . . . .  14
     4.5.  Message Size  . . . . . . . . . . . . . . . . . . . . . .  20
   5.  RPC-Over-RDMA In Operation  . . . . . . . . . . . . . . . . .  21
     5.1.  XDR Protocol Definition . . . . . . . . . . . . . . . . .  22
     5.2.  Fixed Header Fields . . . . . . . . . . . . . . . . . . .  24
     5.3.  Chunk Lists . . . . . . . . . . . . . . . . . . . . . . .  26
     5.4.  Memory Registration . . . . . . . . . . . . . . . . . . .  28
     5.5.  Error Handling  . . . . . . . . . . . . . . . . . . . . .  30
     5.6.  Protocol Elements No Longer Supported . . . . . . . . . .  32
     5.7.  XDR Examples  . . . . . . . . . . . . . . . . . . . . . .  33
   6.  RPC Bind Parameters . . . . . . . . . . . . . . . . . . . . .  34
   7.  Bi-Directional RPC-Over-RDMA  . . . . . . . . . . . . . . . .  35
     7.1.  RPC Direction . . . . . . . . . . . . . . . . . . . . . .  36
     7.2.  Backward Direction Flow Control . . . . . . . . . . . . .  37
     7.3.  Conventions For Backward Operation  . . . . . . . . . . .  38
     7.4.  Backward Direction Upper Layer Binding  . . . . . . . . .  40
   8.  Upper Layer Binding Specifications  . . . . . . . . . . . . .  41
     8.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .  41
     8.2.  Maximum Reply Size  . . . . . . . . . . . . . . . . . . .  42
     8.3.  Additional Considerations . . . . . . . . . . . . . . . .  43
     8.4.  Upper Layer Protocol Extensions . . . . . . . . . . . . .  43
   9.  Extensibility Guidelines  . . . . . . . . . . . . . . . . . .  43
     9.1.  Extending RPC-over-RDMA Header XDR  . . . . . . . . . . .  44
     9.2.  RPC-over-RDMA Version Numbering . . . . . . . . . . . . .  45
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  46
     10.1.  Memory Protection  . . . . . . . . . . . . . . . . . . .  46
     10.2.  Using GSS With RPC-Over-RDMA . . . . . . . . . . . . . .  46
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48



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   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  48
     13.2.  Informative References . . . . . . . . . . . . . . . . .  49
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

   This document obsoletes RFC 5666.  However, the protocol specified by
   this document is based on existing interoperating implementations of
   the RPC-over-RDMA Version One protocol.

   The new specification clarifies text that is subject to multiple
   interpretations, and removes support for unimplemented RPC-over-RDMA
   Version One protocol elements.  It makes the role of Upper Layer
   Bindings an explicit part of the protocol specification.

   In addition, this document introduces conventions that enable bi-
   directional RPC-over-RDMA operation, enabling operation of NFSv4.1
   [RFC5661] on RDMA transports.

1.1.  Requirements Language

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

1.2.  Remote Procedure Calls On RDMA Transports

   Remote Direct Memory Access (RDMA) [RFC5040] [RFC5041] [IB] is a
   technique for moving data efficiently between end nodes.  By
   directing data into destination buffers as it is sent on a network,
   and placing it via direct memory access by hardware, the benefits of
   faster transfers and reduced host overhead are obtained.

   Open Network Computing Remote Procedure Call (ONC RPC, or simply,
   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, RPC messages are encapsulated
   inside datagrams, while on a TCP byte stream, RPC messages are
   delineated by a record marking protocol.  An RDMA transport also
   conveys RPC messages in a specific fashion that must be fully
   described if RPC implementations are to interoperate.

   RDMA transports present semantics different from either UDP or TCP.
   They retain message delineations like UDP, but provide a reliable and
   sequenced data transfer like TCP.  They also provide an offloaded
   bulk transfer service not provided by UDP or TCP.  RDMA transports
   are therefore appropriately viewed as a new transport type by RPC.



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   In this context, the Network File System (NFS) protocols as described
   in [RFC1094], [RFC1813], [RFC7530], [RFC5661], and future NFSv4 minor
   verions are obvious beneficiaries of RDMA transports.  A complete
   problem statement is discussed in [RFC5532], and NFSv4-related issues
   are discussed in [RFC5661].  Many other RPC-based protocols can also
   benefit.

   Although the RDMA transport described here can provide relatively
   transparent support for any RPC application, this document also
   describes mechanisms that can optimize data transfer further, given
   more active participation by RPC applications.

2.  Changes Since RFC 5666

2.1.  Changes To The Specification

   The following alterations have been made to the RPC-over-RDMA Version
   One specification.  The section numbers below refer to [RFC5666].

   o  Section 2 has been expanded to introduce and explain key RPC, XDR,
      and RDMA terminology.  These terms are now used consistently
      throughout the specification.  This change was necesssary because
      implementers familiar with RDMA are often not familiar with the
      mechanics of RPC, and vice versa.

   o  Section 3 has been re-organized and split into sub-sections to
      help readers locate specific requirements and definitions.

   o  Sections 4 and 5 have been combined to improve the organization of
      this information.

   o  The XDR definition of RPC-over-RDMA Version One has been updated
      (without on-the-wire changes) to align with the terms and concepts
      introduced in this document.

   o  The specification of the optional Connection Configuration
      Protocol has been removed from the specification, as there are no
      known implementations of this protocol.

   o  A section consolidating requirements for Upper Layer Bindings has
      been added.

   o  A section discussing RPC-over-RDMA protocol extensibility has been
      added.







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2.2.  Changes To The Protocol

   Although the protocol described herein interoperates with existing
   implementations of [RFC5666], the following changes have been made
   relative to the protocol described in that document:

   o  Support for the Read-Read transfer model has been removed.  Read-
      Read is a slower transfer model than Read-Write, thus implementers
      have chosen not to support it.  Removal simplifies explanatory
      text, and support for the RDMA_DONE procedure is no longer
      necessary.

   o  The specification of RDMA_MSGP in [RFC5666] and current
      implementations of it are incomplete.  Even if completed, benefit
      for protocols such as NFSv4.0 [RFC7530] is doubtful.  Therefore
      the RDMA_MSGP message type is no longer supported.

   o  Technical errors with regard to handling RPC-over-RDMA header
      errors have been corrected.

   o  Specific requirements related to handling XDR round-up and complex
      XDR data types have been added.  Responders are now forbidden from
      writing Write chunk round-up bytes.

   o  Explicit guidance is provided for sizing Write chunks, managing
      multiple chunks in the Write list, and handling unused Write
      chunks.

   o  Clear guidance about Send and Receive buffer size has been added.
      This enables better decisions about when to provide and use the
      Reply chunk.

   o  A section specifying bi-directional RPC operation on RPC-over-RDMA
      has been added.  This enables the NFSv4.1 [RFC5661] backchannel on
      RPC-over-RDMA Version One transports when both endpoints support
      the new functionality.

   The protocol version number has not been changed because the protocol
   specified in this document fully interoperates with implementations
   of the RPC-over-RDMA Version One protocol specified in [RFC5666].

3.  Terminology

3.1.  Remote Procedure Calls

   This section introduces key elements of the Remote Procedure Call
   [RFC5531] and External Data Representation [RFC4506] protocols, upon
   which RPC-over-RDMA Version One is constructed.



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3.1.1.  Upper Layer Protocols

   Remote Procedure Calls are an abstraction used to implement the
   operations of an "Upper Layer Protocol," or ULP.  The term Upper
   Layer Protocol 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 an Upper Layer Protocol is the Network File
   System Version 4.0 [RFC7530].

3.1.2.  Requesters And Responders

   Like a local procedure call, every Remote Procedure Call (RPC) has a
   set of "arguments" and a set of "results".  A calling context is not
   allowed to proceed until the procedure's results are available to it.
   Unlike a local procedure call, the called procedure is executed
   remotely rather than in the local application's context.

   The RPC protocol as described in [RFC5531] is fundamentally a
   message-passing protocol between one server and one or more clients.
   ONC RPC transactions are made up of two types of messages:

   CALL Message
      A CALL message, or "Call", requests that work be done.  A Call is
      designated by the value zero (0) in the message's msg_type field.
      An arbitrary unique value is placed in the message's xid field in
      order to match this CALL message to a corresponding REPLY message.

   REPLY Message
      A REPLY message, or "Reply", reports the results of work requested
      by a Call.  A Reply is designated by the value one (1) in the
      message's msg_type field.  The value contained in the message's
      xid field is copied from the Call whose results are being
      reported.

   The RPC client endpoint, or "requester", serializes an RPC Call's
   arguments and conveys them to a server endpoint via an RPC Call
   message.  This message contains an RPC protocol header, a header
   describing the requested upper layer operation, and all arguments.

   The RPC server endpoint, or "responder", deserializes the arguments
   and processes the requested operation.  It then serializes the
   operation's results into another byte stream.  This byte stream is
   conveyed back to the requester via an RPC Reply message.  This
   message contains an RPC protocol header, a header describing the
   upper layer reply, and all results.






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   The requester deserializes the results and allows the original caller
   to proceed.  At this point the RPC transaction designated by the xid
   in the Call message is complete, and the xid is retired.

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 particular network transport.

   RPC-over-RDMA is a connection-oriented RPC transport.  When a
   connection-oriented transport is used, requesters initiate transport
   connections, while responders wait passively for incoming connection
   requests.

3.1.4.  External Data Representation

   One cannot assume that all requesters and responders internally
   represent data objects the same way.  RPC uses eXternal Data
   Representation, or XDR, to translate data types and serialize
   arguments and results [RFC4506].

   The XDR protocol encodes data independent of the endianness or size
   of host-native data types, allowing unambiguous decoding of data on
   the receiving end.  RPC Programs are specified by writing an XDR
   definition of their procedures, argument data types, and result data
   types.

   XDR assumes that the number of bits in a byte (octet) and their order
   are the same on both endpoints and on the physical network.  The
   smallest indivisible unit of XDR encoding is a group of four octets
   in little-endian order.  XDR also flattens lists, arrays, and other
   complex data types so they can be conveyed as a stream of bytes.

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

3.1.4.1.  XDR Opaque Data

   Sometimes a data item must be transferred as-is, without encoding or
   decoding.  Such a data item is referred to as "opaque data."  XDR
   encoding places opaque data items directly into an XDR stream without
   altering their content in any way.  Upper Layer Protocols or
   applications perform any needed data translation in this case.



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   Examples of opaque data items include the content of files, or
   generic byte strings.

3.1.4.2.  XDR Round-up

   The number of octets in a variable-size opaque data item precedes
   that item in an XDR stream.  If the size of an encoded data item is
   not a multiple of four octets, octets containing zero are added to
   the end of the item as it is encoded so that the next encoded data
   item starts on a four-octet boundary.  The encoded size of the item
   is not changed by the addition of the extra octets, and the zero
   bytes are not exposed to the Upper Layer.

   This technique is referred to as "XDR round-up," and the extra octets
   are referred to as "XDR padding".

3.2.  Remote Direct Memory Access

   RPC requesters and responders can be made more efficient if large RPC
   messages are transferred by a third party such as intelligent network
   interface hardware (data movement offload), and placed in the
   receiver's memory so that no additional adjustment of data alignment
   has to be made (direct data placement).  Remote Direct Memory Access
   enables both optimizations.

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.

   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, only to
   adjust data alignment.

   In this document, "RDMA" refers to the physical mechanism an RDMA
   transport utilizes when moving data.  Although this may not be
   efficient, before an RDMA transfer a sender may copy data into an
   intermediate buffer before an RDMA transfer.  After an RDMA transfer,
   a receiver may copy that data again to its final destination.

   This document uses the term "direct data placement" (or DDP) to refer
   specifically to an optimized data transfer where it is unnecessary
   for a receiving host's CPU to copy transferred data to another
   location after it has been received.  Not all RDMA-based data
   transfer qualifies as Direct Data Placement, and DDP can be achieved
   using non-RDMA mechanisms.



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3.2.2.  RDMA Transport Requirements

   The RPC-over-RDMA Version One protocol assumes the physical transport
   provides the following abstract operations.  A more complete
   discussion of these operations is found in [RFC5040].

   Registered Memory
      Registered memory is a segment of memory that is assigned a
      steering tag that temporarily permits access by the RDMA provider
      to perform data transfer operations.  The RPC-over-RDMA Version
      One protocol assumes that each segment of registered memory MUST
      be identified with a steering tag of no more than 32 bits and
      memory addresses of up to 64 bits in length.

   RDMA Send
      The RDMA provider supports an RDMA Send operation, with completion
      signaled on the receiving peer after data has been placed in a
      pre-posted memory segment.  Sends complete at the receiver in the
      order they were issued at the sender.  The amount of data
      transferred by an RDMA Send operation is limited by the size of
      the remote pre-posted memory segment.

   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 pre-posted memory is limited.  Flow-control to
      prevent overrunning receiver resources is provided by the RDMA
      consumer (in this case, the RPC-over-RDMA Version One protocol).

   RDMA Write
      The RDMA provider supports an RDMA Write operation to directly
      place data in remote memory.  The local host initiates an RDMA
      Write, and completion is signaled there.  No completion is
      signaled on the remote.  The local host provides a steering tag,
      memory address, and length of the remote's memory segment.

      RDMA Writes are not necessarily ordered with respect to one
      another, but are ordered with respect to RDMA Sends.  A subsequent
      RDMA Send completion obtained at the write initiator guarantees
      that prior RDMA Write data has been successfully placed in the
      remote peer's memory.

   RDMA Read
      The RDMA provider supports an RDMA Read operation to directly
      place peer source data in the read initiator's memory.  The local
      host initiates an RDMA Read, and completion is signaled there; no
      completion is signaled on the remote.  The local host provides



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      steering tags, memory addresses, and a length for the remote
      source and local destination memory segments.

      The remote peer receives no notification of RDMA Read completion.
      The local host signals completion as part of a subsequent RDMA
      Send message so that the remote peer can release steering tags and
      subsequently free associated source memory segments.

   The RPC-over-RDMA Version One protocol is designed to be carried over
   RDMA transports that support the above abstract operations.  This
   protocol conveys to the RPC peer information sufficient for that RPC
   peer to direct an RDMA layer to perform transfers containing RPC data
   and to communicate their result(s).  For example, it is readily
   carried over RDMA transports such as Internet Wide Area RDMA Protocol
   (iWARP) [RFC5040] [RFC5041].

4.  RPC-Over-RDMA Protocol Framework

4.1.  Transfer Models

   A "transfer model" designates which endpoint is responsible for
   performing RDMA Read and Write operations.  To enable these
   operations, the peer endpoint first exposes segments of its memory to
   the endpoint performing the RDMA Read and Write operations.

   Read-Read
      Requesters expose their memory to the responder, and the responder
      exposes its memory to requesters.  The responder employs RDMA Read
      operations to pull RPC arguments or whole RPC calls from the
      requester.  Requesters employ RDMA Read operations to pull RPC
      results or whole RPC relies from the responder.

   Write-Write
      Requesters expose their memory to the responder, and the responder
      exposes its memory to requesters.  Requesters employ RDMA Write
      operations to push RPC arguments or whole RPC calls to the
      responder.  The responder employs RDMA Write operations to push
      RPC results or whole RPC relies to the requester.

   Read-Write
      Requesters expose their memory to the responder, but the responder
      does not expose its memory.  The responder employs RDMA Read
      operations to pull RPC arguments or whole RPC calls from the
      requester.  The responder employs RDMA Write operations to push
      RPC results or whole RPC relies to the requester.

   Write-Read




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      The responder exposes its memory to requesters, but requesters do
      not expose their memory.  Requesters employ RDMA Write operations
      to push RPC arguments or whole RPC calls to the responder.
      Requesters employ RDMA Read operations to pull RPC results or
      whole RPC relies from the responder.

   [RFC5666] specifies the use of both the Read-Read and the Read-Write
   Transfer Model.  All current RPC-over-RDMA Version One
   implementations use only the Read-Write Transfer Model.  Therefore
   the use of the Read-Read Transfer Model by RPC-over-RDMA Version One
   implementations is no longer supported.  Other Transfer Models may be
   used by a future version of RPC-over-RDMA.

4.2.  Message Framing

   On an RPC-over-RDMA transport, each RPC message is encapsulated by an
   RPC-over-RDMA message.  An RPC-over-RDMA message consists of two XDR
   streams.

   RPC Payload Stream
      The "Payload stream" contains the encapsulated RPC message being
      transferred by this RPC-over-RDMA message.  This stream always
      begins with the XID field of the encapsulated RPC message.

   Transport-Specific Stream
      The "Transport stream" contains a header that describes and
      controls the transfer of the Payload stream in this RPC-over-RDMA
      message.  This header is analogous to the record marking used for
      RPC over TCP but is more extensive, since RDMA transports support
      several modes of data transfer.

   In its simplest form, an RPC-over-RDMA message consists of a
   Transport stream followed immediately by a Payload stream conveyed
   together in a single RDMA Send.  To transmit large RPC messages, a
   combination of one RDMA Send operation and one or more RDMA Read or
   Write operations is employed.

   RPC-over-RDMA framing replaces all other RPC framing (such as TCP
   record marking) when used atop an RPC-over-RDMA association, even
   when the underlying RDMA protocol may itself be layered atop a
   transport with a defined RPC framing (such as TCP).

   It is however possible for RPC-over-RDMA to be dynamically enabled in
   the course of negotiating the use of RDMA via an Upper Layer Protocol
   exchange.  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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4.3.  Managing Receiver Resources

   It is critical to provide RDMA Send flow control for an RDMA
   connection.  If no pre-posted receive buffer is large enough to
   accept an incoming RDMA Send, the RDMA Send operation fails.  If a
   pre-posted receive buffer is not available to accept an incoming RDMA
   Send, the RDMA Send operation can fail.  Repeated occurrences of such
   errors can be fatal to the connection.  This is a departure from
   conventional TCP/IP networking where buffers are allocated
   dynamically as part of receiving messages.

   The longevity of an RDMA connection requires that sending endpoints
   respect the resource limits of peer receivers.  To ensure messages
   can be sent and received reliably, there are two operational
   parameters for each connection.

4.3.1.  Credit Limit

   The number of pre-posted RDMA Receive operations is sometimes
   referred to as a peer's "credit limit."  Flow control for RDMA Send
   operations directed to the responder is implemented as a simple
   request/grant protocol in the RPC-over-RDMA header associated with
   each RPC message.  Section 5.2.3 has further detail.

   o  The RPC-over-RDMA header for RPC Call messages contains a
      requested credit value for the responder.  This is the maximum
      number of RPC replies the requester can handle at once,
      independent of how many RPCs are in flight at that moment.  The
      requester MAY dynamically adjust the requested credit value to
      match its expected needs.

   o  The RPC-over-RDMA header for RPC Reply messages provides the
      granted result.  This is the maximum number of RPC calls the
      responder can handle at once, without regard to how many RPCs are
      in flight at that moment.  The granted value MUST NOT be zero,
      since such a value would result in deadlock.  The responder MAY
      dynamically adjust the granted credit value to match its needs or
      policies (e.g. to accommodate the available resources in a shared
      receive queue).

   The requester MUST NOT send unacknowledged requests in excess of this
   granted responder credit limit.  If the limit is exceeded, the RDMA
   layer may signal an error, possibly terminating the connection.  If
   an RDMA layer error does not occur, the responder MAY handle excess
   requests or return an RPC layer error to the requester.

   While RPC calls complete in any order, the current flow control limit
   at the responder is known to the requester from the Send ordering



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   properties.  It is always the lower of the requested and granted
   credit values, minus the number of requests in flight.  Advertised
   credit values are not altered when individual RPCs are started or
   completed.

   On occasion a requester or responder may need to adjust the amount of
   resources available to a connection.  When this happens, the
   responder needs to ensure that a credit increase is effected (i.e.
   RDMA Receives are posted) before the next reply is sent.

   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 One implementation.

4.3.2.  Inline Threshold

   A receiver's "inline threshold" value is the largest message size (in
   octets) that the receiver can accept via an RDMA Receive operation.
   Each connection has two inline threshold values, one for each peer
   receiver.

   Unlike credit limits, inline threshold values are not advertised to
   peers via the RPC-over-RDMA Version One protocol, and there is no
   provision for the inline threshold value to change during the
   lifetime of an RPC-over-RDMA Version One connection.

4.3.3.  Initial Connection State

   When a connection is first established, peers might not know how many
   receive buffers the other has, nor how large these buffers are.

   As a basis for an initial exchange of RPC requests, each RPC-over-
   RDMA Version One connection provides the ability to exchange at least
   one RPC message at a time that is 1024 bytes in size.  A responder
   MAY exceed this basic level of configuration, but a requester MUST
   NOT assume more than one credit is available, and MUST receive a
   valid reply from the responder carrying the actual number of
   available credits, prior to sending its next request.

   Receiver implementations MUST support an inline threshold of 1024
   bytes, but MAY support larger inline thresholds values.  A mechanism
   for discovering a peer's inline threshold value before a connection
   is established may be used to optimize the use of RDMA Send
   operations.  In the absense of such a mechanism, senders MUST assume
   a receiver's inline threshold is 1024 bytes.





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4.4.  XDR Encoding With Chunks

   When RDMA is available, during XDR encoding it can be determined that
   an XDR data item is large enough that it might be more efficient if
   the transport placed the content of the data item directly in the
   receiver's memory.

4.4.1.  Reducing An XDR Stream

   RPC-over-RDMA Version One provides a mechanism for moving part of an
   RPC message via a data transfer separate from an RDMA Send/Receive.
   The sender removes one or more XDR data items from the Payload
   stream.  They are conveyed via one or more RDMA Read or Write
   operations.  The receiver inserts the data items into the Payload
   stream before passing it to the Upper Layer.

   A contiguous piece of a Payload stream that is split out and moved
   via separate RDMA operations is known as a "chunk."  A Payload stream
   after chunks have been removed is referred to as a "reduced" Payload
   stream.

4.4.2.  DDP-Eligibility

   Only an XDR data item that might benefit from Direct Data Placement
   may be reduced.  The eligibility of particular XDR data items to be
   reduced is not specified by this document.

   To maintain interoperability on an RPC-over-RDMA transport, a
   determination must be made of which XDR data items in each Upper
   Layer Protocol are allowed to use Direct Data Placement.  Therefore
   an additional specification is needed that describes how an Upper
   Layer Protocol enables Direct Data Placement.  The set of
   requirements for an Upper Layer Protocol to use an RPC-over-RDMA
   transport is known as an "Upper Layer Binding specification," or ULB.

   An Upper Layer Binding specification states which specific individual
   XDR data items in an Upper Layer Protocol MAY be transferred via
   Direct Data Placement.  This document will refer to XDR data items
   that are permitted to be reduced as "DDP-eligible".  All other XDR
   data items MUST NOT be reduced.  RPC-over-RDMA Version One uses RDMA
   Read and Write operations to transfer DDP-eligible data that has been
   reduced.

   Detailed requirements for Upper Layer Bindings are discussed in full
   in Section 8.






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4.4.3.  RDMA Segments

   When encoding a Payload stream that contains a DDP-eligible data
   item, a sender may choose to reduce that data item.  It does not
   place the item into the Payload stream.  Instead, the sender records
   in the RPC-over-RDMA header the actual address and size of the memory
   region containing that data item.

   The requester provides location information for DDP-eligible data
   items in both RPC Calls and Replies.  The responder uses this
   information to initiate RDMA Read and Write operations to retrieve or
   update the specified region of the requester's memory.

   An "RDMA segment", or a "plain segment", is an RPC-over-RDMA header
   data object that contains the precise co-ordinates of a contiguous
   memory region that is to be conveyed via one or more RDMA Read or
   RDMA Write operations.  The following fields are contained in each
   segment.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Handle                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Length                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Offset                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Handle
      Steering tag (STag) or handle obtained when the segment's memory
      is registered for RDMA.  Also known as an R_key, this value is
      generated by registering this memory with the RDMA provider.

   Length
      The length of the memory segment, in octets.

   Offset
      The offset or beginning memory address of the segment.

   See [RFC5040] for further discussion of the meaning of these fields.








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4.4.4.  Chunks

   In RPC-over-RDMA Version One, a "chunk" refers to a portion of the
   Payload stream that is moved via RDMA Read or Write operations.
   Chunk data is removed from the sender's Payload stream, transferred
   by separate RDMA operations, and then re-inserted into the receiver's
   Payload stream.

   Each chunk consists of one or more RDMA segments.  Each segment
   represents a single contiguous piece of that chunk.  Segments MAY
   divide a chunk on any boundary that is convenient to the requester.

   Except in special cases, a chunk contains exactly one XDR data item.
   This makes it straightforward to remove chunks from an XDR stream
   without affecting XDR alignment.  Not every RPC-over-RDMA message has
   chunks associated with it.

4.4.4.1.  Counted Arrays

   If a chunk contains a counted array data type, the count of array
   elements MUST remain in the Payload stream, while the array elements
   MUST be moved to the chunk.  For example, when encoding an opaque
   byte array as a chunk, the count of bytes stays in the Payload
   stream, while the bytes in the array are removed from the Payload
   stream and transferred within the chunk.

   Any byte count left in the Payload stream MUST match the sum of the
   lengths of the segments making up the chunk.  If they do not agree,
   an RPC protocol encoding error results.

   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.

4.4.4.2.  Optional-data

   If a chunk contains an optional-data data type, the "is present"
   field MUST remain in the Payload stream, while the data, if present,
   MUST be moved to the chunk.

4.4.4.3.  XDR Unions

   A union data type should never be made DDP-eligible, but one or more
   of its arms may be DDP-eligible.





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4.4.5.  Read Chunks

   A "Read chunk" represents an XDR data item that is to be pulled from
   the requester to the responder using RDMA Read operations.

   A Read chunk is a list of one or more RDMA segments.  Each RDMA
   segment in a Read chunk is a plain segment which has an additional
   Position field.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Position                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Handle                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Length                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Offset                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Position
      The byte offset in the Payload stream where the receiver re-
      inserts the data item conveyed in a chunk.  The Position value
      MUST be computed from the beginning of the Payload stream, which
      begins at Position zero.  All RDMA segments belonging to the same
      Read chunk have the same value in their Position field.

   While constructing an RPC-over-RDMA Call message, a requester
   registers memory segments containing data in Read chunks.  It
   advertises these chunks in the RPC-over-RDMA header of the RPC Call.

   After receiving an RPC Call sent via an RDMA Send operation, a
   responder transfers the chunk data from the requester using RDMA Read
   operations.  The responder reconstructs the transferred chunk data by
   concatenating the contents of each segment, in list order, into the
   received Payload stream at the Position value recorded in the
   segment.

   Put another way, a receiver inserts the first segment in a Read chunk
   into the Payload stream at the byte offset indicated by its Position
   field.  Segments whose Position field value match this offset are
   concatenated afterwards, until there are no more segments at that
   Position value.  The next XDR data item in the Payload stream
   follows.




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4.4.5.1.  Read Chunk Round-up

   XDR requires each encoded data item to start on four-byte alignment.
   When an odd-length data item is encoded, its length is encoded
   literally, while the data is padded so the next data item in the XDR
   stream can start on a four-byte boundary.  Receivers ignore the
   content of the pad bytes.

   After an XDR data item has been reduced, all data items remaining in
   the Payload stream must continue to adhere to these padding
   requirements.  Thus when an XDR data item is moved from the Payload
   stream into a Read chunk, the requester MUST remove XDR padding for
   that data item from the Payload stream as well.

   The length of a Read chunk is the sum of the lengths of the read
   segments that comprise it.  If this sum is not a multiple of four,
   the requester MAY choose to send a Read chunk without any XDR
   padding.  If the requester provides no actual round-up in a Read
   chunk, the responder MUST be prepared to provide appropriate round-up
   in the reconstructed call XDR stream

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

4.4.5.2.  Decoding Read Chunks

   While decoding a received Payload stream, whenever the XDR offset in
   the Payload stream matches that of a Read chunk, the transport
   initiates an RDMA Read to pull the chunk's data content into
   registered memory on the responder.

   The responder acknowledges its completion of use of Read chunk source
   buffers when it sends an RPC Reply to the requester.  The requester
   may then release Read chunks advertised in the request.

4.4.6.  Write Chunks

   A "Write chunk" represents an XDR data item that is to be pushed from
   a responder to a requester using RDMA Write operations.

   A Write chunk is an array of one or more plain RDMA segments.  Write
   chunks are provided by a requester long before the responder has
   prepared the reply Payload stream.  Therefore RDMA segments in a
   Write chunk do not have a Position field.




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   While constructing an RPC Call message, a requester also prepares
   memory regions to catch DDP-eligible reply data items.  A requester
   does not know the actual length of the result data item to be
   returned, thus it MUST register a Write chunk long enough to
   accommodate the maximum possible size of the returned data item.

   A responder copies the requester-provided Write chunk segments into
   the RPC-over-RDMA header that it returns with the reply.  The
   responder MUST NOT change the number of segments in the Write chunk.

   The responder fills the segments in array order until the data item
   has been completely written.  The responder updates the segment
   length fields to reflect the actual amount of data that is being
   returned in each segment.  If a Write chunk segment is not filled by
   the responder, the updated length of the segment SHOULD be zero.

   The responder then sends the RPC Reply via an RDMA Send operation.
   After receiving the RPC Reply, the requester reconstructs the
   transferred data by concatenating the contents of each segment, in
   array order, into RPC Reply XDR stream.

4.4.6.1.  Write Chunk Round-up

   XDR requires each encoded data item to start on four-byte alignment.
   When an odd-length data item is encoded, its length is encoded
   literally, while the data is padded so the next data item in the XDR
   stream can start on a four-byte boundary.  Receivers ignore the
   content of the pad bytes.

   After a data item is reduced, data items remaining in the Payload
   stream must continue to adhere to these padding requirements.  Thus
   when an XDR data item is moved from a reply Payload stream into a
   Write chunk, the responder MUST remove XDR padding for that data item
   from the reply Payload stream as well.

   A requester SHOULD NOT provide extra length in a Write chunk to
   accommodate XDR pad bytes.  A responder MUST NOT write XDR pad bytes
   for a Write chunk.

4.4.6.2.  Unused Write Chunks

   There are occasions when a requester provides a Write chunk but the
   responder does not use it.

   For example, an Upper Layer Protocol may define a union result where
   some arms of the union contain a DDP-eligible data item while other
   arms do not.  The requester is required to provide a Write chunk in
   this case, but if the responder returns a result that uses an arm of



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   the union that has no DDP-eligible data item, the Write chunk remains
   unused.

   When forming an RPC-over-RDMA Reply message with an unused Write
   chunk, the responder MUST set the length of all segments in the chunk
   to zero.

   Unused write chunks, or unused bytes in write chunk segments, are not
   returned as results.  Their memory is returned to the Upper Layer as
   part of RPC completion.  However, the RPC layer MUST NOT assume that
   the buffers have not been modified.

4.5.  Message Size

   A receiver of RDMA Send operations is required by RDMA to have
   previously posted one or more adequately sized buffers.  Memory
   savings can be achieved on both requesters and responders by leaving
   the inline threshold small.  However, not all RPC messages are small.

4.5.1.  Short Messages

   RPC messages are frequently smaller than typical inline thresholds.
   For example, the NFS version 3 GETATTR request is only 56 bytes: 20
   bytes of RPC header, plus a 32-byte file handle argument and 4 bytes
   for its length.  The reply to this common request is about 100 bytes.

   Since all RPC messages conveyed via RPC-over-RDMA require an RDMA
   Send operation, the most efficient way to send an RPC message that is
   smaller than the receiver's inline threshold is to append the Payload
   stream directly to the Transport stream.  An RPC-over-RDMA header
   with a small RPC Call or Reply message immediately following is
   transferred using a single RDMA Send operation.  No RDMA Read or
   Write operations are needed.

4.5.2.  Chunked Messages

   If DDP-eligible data items are present in a Payload stream, a sender
   MAY reduce the Payload stream and use RDMA Read or Write operations
   to move the reduced data items.  The Transport stream with the
   reduced Payload stream immediately following is transferred using a
   single RDMA Send operation.

   After receiving the Transport and Payload streams of a Chunked RPC-
   over-RDMA Call message, the responder uses RDMA Read operations to
   move reduced data items in Read chunks.  Before sending the Transport
   and Payload streams of a Chunked RPC-over-RDMA Reply message, the
   responder uses RDMA Write operations to move reduced data items in
   Write and Reply chunks.



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4.5.3.  Long Messages

   When a Payload stream is larger than the receiver's inline threshold,
   the Payload stream is reduced by removing DDP-eligible data items and
   placing them in chunks to be moved separately.  If there are no DDP-
   eligible data items in the Payload stream, or the Payload stream is
   still too large after it has been reduced, the RDMA transport MUST
   use RDMA Read or Write operations to convey the Payload stream
   itself.  This mechanism is referred to as a "Long Message."

   To transmit a Long Message, the sender conveys only the Transport
   stream with an RDMA Send operation.  The Payload stream is not
   included in the Send buffer in this instance.  Instead, the requester
   provides chunks that the responder uses to move the Payload stream.

   Long RPC Call
      To send a Long RPC-over-RDMA Call message, the requester provides
      a special Read chunk that contains the RPC Call's Payload stream.
      Every segment in this Read chunk MUST contain zero in its Position
      field.  Thus this chunk is known as a "Position Zero Read chunk."

   Long RPC Reply
      To send a Long RPC-over-RDMA Reply message, the requester provides
      a single special Write chunk in advance, known as the "Reply
      chunk", that will contain the RPC Reply's Payload stream.  The
      requester sizes the Reply chunk to accommodate the maximum
      expected reply size for that Upper Layer operation.

   Though the purpose of a Long Message is to handle large RPC messages,
   requesters MAY use a Long Message at any time to convey an RPC Call.
   Responders MUST send a Long reply whenever a Reply chunk has been
   provided by a requester.

   Because these special chunks contain a whole RPC message, any XDR
   data item MAY appear in one of these special chunks without regard to
   its DDP-eligibility.  DDP-eligible data items MAY be removed from
   these special chunks and conveyed via normal chunks, but non-eligible
   data items MUST NOT appear in normal chunks.

5.  RPC-Over-RDMA In Operation

   Every RPC-over-RDMA Version One message has a header that includes a
   copy of the message's transaction ID, data for managing RDMA flow
   control credits, and lists of RDMA segments used for RDMA Read and
   Write operations.  All RPC-over-RDMA header content is contained in
   the Transport stream, and thus MUST be XDR encoded.





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   RPC message layout is unchanged from that described in [RFC5531]
   except for the possible reduction of data items that are moved by
   RDMA Read or Write operations.

5.1.  XDR Protocol Definition

   Code components extracted from this document must include the
   following license boilerplate.


   <CODE BEGINS>

      /*
       * Copyright (c) 2010, 2015 IETF Trust and the persons
       * identified as authors of the code.  All rights reserved.
       *
       * The authors of the code are:
       * B. Callaghan, T. Talpey, and C. Lever.
       *
       * 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



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       *   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.
       */

      struct rpcrdma1_segment {
              uint32 rdma_handle;
              uint32 rdma_length;
              uint64 rdma_offset;
      };

      struct rpcrdma1_read_segment {
              uint32                  rdma_position;
              struct rpcrdma1_segment rdma_target;
      };

      struct rpcrdma1_read_list {
              struct rpcrdma1_read_segment rdma_entry;
              struct rpcrdma1_read_list    *rdma_next;
      };

      struct rpcrdma1_write_chunk {
              struct rpcrdma1_segment rdma_target<>;
      };

      struct rpcrdma1_write_list {
              struct rpcrdma1_write_chunk rdma_entry;
              struct rpcrdma1_write_list  *rdma_next;
      };

      struct rpcrdma1_header {
              uint32        rdma_xid;
              uint32        rdma_vers;
              uint32        rdma_credit;
              rpcrdma1_body rdma_body;
      };

      enum rpcrdma1_proc {
              RDMA_MSG = 0,
              RDMA_NOMSG = 1,
              RDMA_MSGP = 2,  /* Reserved */
              RDMA_DONE = 3,  /* Reserved */
              RDMA_ERROR = 4
      };

      struct rpcrdma1_chunks {
              struct rpcrdma1_read_list   *rdma_reads;



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              struct rpcrdma1_write_list  *rdma_writes;
              struct rpcrdma1_write_chunk *rdma_reply;
      };

      enum rpcrdma1_errcode {
              RDMA_ERR_VERS = 1,
              RDMA_ERR_CHUNK = 2
      };

      union rpcrdma1_error switch (rpcrdma1_errcode rdma_err) {
              case RDMA_ERR_VERS:
                uint32 rdma_vers_low;
                uint32 rdma_vers_high;
              case RDMA_ERR_CHUNK:
                void;
      };

      union rpcrdma1_body switch (rpcrdma1_proc rdma_proc) {
              case RDMA_MSG:
              case RDMA_NOMSG:
                rpcrdma1_chunks rdma_chunks;
              case RDMA_MSGP:
                uint32          rdma_align;
                uint32          rdma_thresh;
                rpcrdma1_chunks rdma_achunks;
              case RDMA_DONE:
                void;
              case RDMA_ERROR:
                rpcrdma1_error rdma_error;
      };

   <CODE ENDS>


5.2.  Fixed Header Fields

   The RPC-over-RDMA header begins with four fixed 32-bit fields that
   control the RDMA interaction.  These four fields, which must remain
   with the same meanings and in the same positions in all subsequent
   versions of the RPC-over-RDMA protocol, are described below.











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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             XID                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Version Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Credit Value                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Procedure Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


5.2.1.  Transaction ID (XID)

   The XID generated for the RPC Call and Reply.  Having the XID at a
   fixed location in the header makes it easy for the receiver to
   establish context as soon as each RPC-over-RDMA message arrives.
   This XID MUST be the same as the XID in the RPC message.  The
   receiver MAY perform its processing based solely on the XID in the
   RPC-over-RDMA header, and thereby ignore the XID in the RPC message,
   if it so chooses.

5.2.2.  Version Number

   For RPC-over-RDMA Version One, this field MUST contain the value one
   (1).  Rules regarding changes to this transport protocol version
   number can be found in Section 9.2.

5.2.3.  Credit Value

   When sent in an RPC Call message, the requested credit value is
   provided.  When sent in an RPC Reply message, the granted credit
   value is returned.  RPC Calls SHOULD NOT be sent in excess of the
   currently granted limit.  Further discussion of how the credit value
   is determined can be found in Section 4.3.

5.2.4.  Procedure number

   o  RDMA_MSG = 0 indicates that chunk lists and a Payload stream
      follow.  The format of the chunk lists is discussed below.

   o  RDMA_NOMSG = 1 indicates that after the chunk lists there is no
      Payload stream.  In this case, the chunk lists provide information
      to allow the responder to transfer the Payload stream using RDMA
      Read or Write operations.

   o  RDMA_MSGP = 2 is reserved.

   o  RDMA_DONE = 3 is reserved.



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   o  RDMA_ERROR = 4 is used to signal an encoding error in the RPC-
      over-RDMA header.

   An RDMA_MSG procedure conveys the Transport stream and the Payload
   stream via an RDMA Send operation.  The Transport stream contains the
   four fixed fields, followed by the Read and Write lists and the Reply
   chunk, though any or all three MAY be marked as not present.  The
   Payload stream then follows, beginning with its XID field.  If a Read
   or Write chunk list is present, a portion of the Payload stream has
   been excised and is conveyed separately via RDMA Read or Write
   operations.

   An RDMA_NOMSG procedure conveys the Transport stream via an RDMA Send
   operation.  The Transport stream contains the four fixed fields,
   followed by the Read and Write chunk lists and the Reply chunk.
   Though any of these MAY be marked as not present, one MUST be present
   and MUST hold the Payload stream for this RPC-over-RDMA message.  If
   a Read or Write chunk list is present, a portion of the Payload
   stream has been excised and is conveyed separately via RDMA Read or
   Write operations.

   An RDMA_ERROR procedure conveys the Transport stream via an RDMA Send
   operation.  The Transport stream contains the four fixed fields,
   followed by formatted error information.  No Payload stream is
   conveyed in this type of RPC-over-RDMA message.

   A gather operation on each RDMA Send operation can be used to combine
   the Transport and Payload streams, which might have been constructed
   in separate buffers.  However, the total length of the gathered send
   buffers MUST NOT exceed the peer receiver's inline threshold.

5.3.  Chunk Lists

   The chunk lists in an RPC-over-RDMA Version One header are three XDR
   optional-data fields that follow the fixed header fields in RDMA_MSG
   and RDMA_NOMSG procedures.  Read Section 4.19 of [RFC4506] carefully
   to understand how optional-data fields work.  Examples of XDR encoded
   chunk lists are provided in Section 5.7 as an aid to understanding.

5.3.1.  Read List

   Each RDMA_MSG or RDMA_NOMSG procedure has one "Read list."  The Read
   list is a list of zero or more Read segments, provided by the
   requester, that are grouped by their Position fields into Read
   chunks.  Each Read chunk advertises the location of argument data the
   responder is to retrieve via RDMA Read operations.  The requester has
   removed the data in these chunks from the call's Payload stream.




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   Via a Position Zero Read Chunk, a requester may provide an RPC Call
   message as a chunk in the Read list.

   If the RPC Call has no argument data that is DDP-eligible and the
   Position Zero Read Chunk is not being used, the requester leaves the
   Read list empty.

5.3.2.  Write List

   Each RDMA_MSG or RDMA_NOMSG procedure has one "Write list."  The
   Write list is a list of zero or more Write chunks, provided by the
   requester.  Each Write chunk is an array of RDMA segments, thus the
   Write list is a list of counted arrays.  Each Write chunk advertises
   receptacles for DDP-eligible data to be pushed by the responder via
   RDMA Write operations.  If the RPC Reply has no possible DDP-eligible
   result data items, the requester leaves the Write list empty.

   *** This section needs to specify when a requester must provide Write
   chunks, and how many chunks must be provided. ***

   When a Write list is provided for the results of an RPC Call, the
   responder MUST provide data corresponding to DDP-eligible XDR data
   items via RDMA Write operations to the memory referenced in the Write
   list.  The responder removes the data in these chunks from the
   reply's Payload stream.

   When multiple Write chunks are present, the responder fills in each
   Write chunk with a DDP-eligible result until either there are no more
   results or no more Write chunks.  The requester may not be able to
   predict which DDP-eligible data item goes in which chunk.  Thus the
   requester is responsible for allocating and registering Write chunks
   large enough to accommodate the largest XDR data item that might be
   associated with each chunk in the list.

   The RPC Reply conveys the size of result data items by returning each
   Write chunk to the requester with the segment lengths rewritten to
   match the actual data transferred.  Decoding the reply therefore
   performs no local data copying but merely returns the length obtained
   from the reply.

   Each decoded result consumes one entry in the Write list, which in
   turn consists of an array of RDMA segments.  The length of a Write
   chunk is therefore the sum of all returned lengths in all segments
   comprising the corresponding list entry.  As each Write chunk is
   decoded, the entire Write list entry is consumed.

   A requester constructs the Write list for an RPC transaction before
   the responder has formulated its reply.  When there is only one DDP-



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   eligible result data item, the requester inserts only a single Write
   chunk in the Write list.  If the responder populates that chunk with
   data, the requester knows with certainty which result data item is
   contained in it.

   However, Upper Layer Protocol procedures may allow replies where more
   than one result data item is DDP-eligible.  For example, an NFSv4
   COMPOUND procedure is composed of individual NFSv4 operations, more
   than one of which may have a reply containing a DDP-eligible result.

   As stated above, when multiple Write chunks are present, the
   responder reduces DDP-eligible result until either there are no more
   results or no more Write chunks.  Then, as the requester decodes the
   reply Payload stream, it is clear from the contents of the reply
   which Write chunk contains which data item.

5.3.3.  Reply Chunk

   Each RDMA_MSG or RDMA_NOMSG procedure has one "Reply chunk."  The
   Reply chunk is a Write chunk, provided by the requester.  The Reply
   chunk is a single counted array of RDMA segments.

   A requester MUST provide a Reply chunk whenever the maximum possible
   size of the reply is larger than its own inline threshold.  The Reply
   chunk MUST be large enough to contain a Payload stream (RPC message)
   of this maximum size.  If the actual reply Payload stream is smaller
   than the requester's inline threshold, the responder MAY return it as
   a Short message rather than using the Reply chunk.

5.4.  Memory Registration

   RDMA requires that data is transferred between only registered memory
   segments at the source and destination.  All protocol headers as well
   as separately transferred data chunks must reside in registered
   memory.

   Since the cost of registering and de-registering memory can be a
   significant proportion of the RDMA transaction cost, it is important
   to minimize registration activity.  For memory that is targeted by
   RDMA Send and Receive operations, a local-only registration is
   sufficient and can be left in place during the life of a connection
   without any risk of data exposure.

5.4.1.  Registration Longevity

   Data transferred via RDMA Read and Write can reside in a memory
   allocation not in the control of the RPC-over-RDMA transport.  These
   memory allocations can persist outside the bounds of an RPC



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   transaction.  They are registered and invalidated as needed, as part
   of each RPC transaction.

   The requester endpoint must ensure that memory segments associated
   with each RPC transaction are properly fenced from responders before
   allowing Upper Layer access to the data contained in them.  Moreover,
   the requester must not access these memory segments while the
   responder has access to them.

   This includes segments that are associated with canceled RPCs.  A
   responder cannot know that the requester is no longer waiting for a
   reply, and might proceed to read or even update memory that the
   requester might have released for other use.

5.4.2.  Communicating DDP-Eligibility

   The interface by which an Upper Layer Protocol implementation
   communicates the eligibility of a data item locally to its local RPC-
   over-RDMA endpoint is not described by this specification.

   Depending on the implementation and constraints imposed by Upper
   Layer Bindings, it is possible to implement reduction transparently
   to upper layers.  Such implementations may lead to inefficiencies,
   either because they require the RPC layer to perform expensive
   registration and de-registration of memory "on the fly", or they may
   require using RDMA chunks in reply messages, along with the resulting
   additional handshaking with the RPC-over-RDMA peer.

   However, these issues are internal and generally confined to the
   local interface between RPC and its upper layers, one in which
   implementations are free to innovate.  The only requirement is that
   the resulting RPC-over-RDMA protocol sent to the peer is valid for
   the upper layer.

5.4.3.  Registration Strategies

   The choice of which memory registration strategies to employ is left
   to requester and responder implementers.  To support the widest array
   of RDMA implementations, as well as the most general steering tag
   scheme, an Offset field is included in each segment.

   While zero-based offset schemes are available in many RDMA
   implementations, their use by RPC requires individual registration of
   each segment.  For such implementations, this can be a significant
   overhead.  By providing an offset in each chunk, many pre-
   registration or region-based registrations can be readily supported.
   By using a single, universal chunk representation, the RPC-over-RDMA
   protocol implementation is simplified to its most general form.



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5.5.  Error Handling

   A receiver performs basic validity checks on the RPC-over-RDMA header
   and chunk contents before it passes the RPC message to the RPC
   consumer.  If errors are detected in an RPC-over-RDMA header, an
   RDMA_ERROR procedure MUST be generated.  Because the transport layer
   may not be aware of the direction of a problematic RPC message, an
   RDMA_ERROR procedure MAY be generated by either a requester or a
   responder.

   To form an RDMA_ERROR procedure: The rdma_xid field MUST contain the
   same XID that was in the rdma_xid field in the failing request; The
   rdma_vers field MUST contain the same version that was in the
   rdma_vers field in the failing request; The rdma_proc field MUST
   contain the value RDMA_ERROR; The rdma_err field contains a value
   that reflects the type of error that occurred, as described below.

   An RDMA_ERROR procedure indicates a permanent error.  When receiving
   an RDMA_ERROR procedure, a requester should attempt to terminate the
   RPC transaction if it recognizes the XID in the reply's rdma_xid
   field, and return an error to the application to prevent retrying the
   failed RPC transaction.

   To avoid an infinite loop, a receiver should drop an RDMA_ERROR
   procedure that is malformed.

5.5.1.  Header Version Mismatch

   When a receiver detects an RPC-over-RDMA header version that it does
   not support (currently this document defines only Version One), it
   MUST reply with an RDMA_ERROR procedure and set the rdma_err value to
   RDMA_ERR_VERS, also providing the low and high inclusive version
   numbers it does, in fact, support.

5.5.2.  XDR Errors

   A receiver might encounter an XDR parsing error that prevents it from
   processing the incoming Transport stream.  Examples of such errors
   include an invalid value in the rdma_proc field, an RDMA_NOMSG
   message that has no chunk lists, or the contents of the rdma_xid
   field might not match the contents of the XID field in the
   accompanying RPC message.  If the rdma_vers field contains a
   recognized value, but an XDR parsing error occurs, the responder MUST
   reply with an RDMA_ERROR procedure and set the rdma_err value to
   RDMA_ERR_CHUNK.

   When a responder receives a valid RPC-over-RDMA header but the
   responder's Upper Layer Protocol implementation cannot parse the RPC



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   arguments in the RPC Call message, the responder SHOULD return a
   RPC_GARBAGEARGS reply, using an RDMA_MSG procedure.  This type of
   parsing failure might be due to mismatches between chunk sizes or
   offsets and the contents of the Payload stream, for example.  A
   responder MAY also report the presence of a non-DDP-eligible data
   item in a Read or Write chunk using RPC_GARBAGEARGS.

5.5.3.  Responder Operational Errors

   Problems can arise as a responder attempts to use requester-provided
   resources for RDMA Read or Write operations.  For example:

   o  Chunks can be validated only by using their contents to form RDMA
      Read or Write operations.  If chunk contents are invalid (say, a
      segment is no longer registered, or a chunk length is too long), a
      Remote Access error occurs.

   o  If a requester's receive buffer is too small, the responder's Send
      operation completes with a Local Length Error.

   o  If the requester-provided Reply chunk is too small to accommodate
      a large RPC Reply, a Remote Access error occurs.  A responder can
      detect this problem before attempting to write past the end of the
      Reply chunk.

   Operational errors are typically 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 RDMA_ERROR reply with an rdma_err value of
   RDMA_ERR_CHUNK to indicate that no RPC-level reply is possible for
   that XID.

5.5.4.  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), Stream Control Transmission Protocol (SCTP), as
   well as the InfiniBand link layer all provide Cyclic Redundancy Check
   (CRC) protection of the RDMA payload, and CRC-class protection is a
   general attribute of such transports.

   Additionally, the RPC layer itself can accept errors from the link
   level and recover via retransmission.  RPC recovery can handle
   complete loss and re-establishment of the link.

   The details of reporting and recovery from RDMA link layer errors are
   outside the scope of this protocol specification.  See Section 10 for




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   further discussion of the use of RPC-level integrity schemes to
   detect errors.

5.6.  Protocol Elements No Longer Supported

   The following protocol elements are no longer supported in RPC-over-
   RDMA Version One.  Related enum values and structure definitions
   remain in the RPC-over-RDMA Version One protocol for backwards
   compatibility.

5.6.1.  RDMA_MSGP

   The specification of RDMA_MSGP in Section 3.9 of [RFC5666] is
   incomplete.  To fully specify RDMA_MSGP would require:

   o  Updating the definition of DDP-eligibility to include data items
      that may be transferred, with padding, via RDMA_MSGP procedures

   o  Adding full operational descriptions of the alignment and
      threshold fields

   o  Discussing how alignment preferences are communicated between two
      peers without using CCP

   o  Describing the treatment of RDMA_MSGP procedures that convey Read
      or Write chunks

   The RDMA_MSGP message type is beneficial only when the padded data
   payload is at the end of an RPC message's argument or result list.
   This is not typical for NFSv4 COMPOUND RPCs, which often include a
   GETATTR operation as the final element of the compound operation
   array.

   Without a full specification of RDMA_MSGP, there has been no fully
   implemented prototype of it.  Without a complete prototype of
   RDMA_MSGP support, it is difficult to assess whether this protocol
   element has benefit, or can even be made to work interoperably.

   Therefore, senders MUST NOT send RDMA_MSGP procedures.  When
   receiving an RDMA_MSGP procedure, receivers SHOULD reply with an
   RDMA_ERROR procedure, setting the rdma_err field to RDMA_ERR_CHUNK.

5.6.2.  RDMA_DONE

   Because no implementation of RPC-over-RDMA Version One uses the Read-
   Read transfer model, there is never a need to send an RDMA_DONE
   procedure.




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   Therefore, senders MUST NOT send RDMA_DONE messages.  When receiving
   an RDMA_DONE procedure, receivers SHOULD reply with an RDMA_ERROR
   procedure, setting the rdma_err field to RDMA_ERR_CHUNK.

5.7.  XDR Examples

   RPC-over-RDMA chunk lists are complex data types.  In this appendix,
   illustrations are provided to help readers grasp how chunk lists are
   represented inside an RPC-over-RDMA header.

   An RDMA segment is the simplest component, being made up of a 32-bit
   handle (H), a 32-bit length (L), and 64-bits of offset (OO).  Once
   flattened into an XDR stream, RDMA segments appear as


      HLOO


   A Read segment has an additional 32-bit position field.  Read
   segments appear as


      PHLOO


   A Read chunk is a list of Read segments.  Each segment is preceded by
   a 32-bit word containing a one if there is a segment, or a zero if
   there are no more segments (optional-data).  In XDR form, this would
   look like


      1 PHLOO 1 PHLOO 1 PHLOO 0


   where P would hold the same value for each segment belonging to the
   same Read chunk.

   The Read List is also a list of Read segments.  In XDR form, this
   would look like a Read chunk, except that the P values could vary
   across the list.  An empty Read List is encoded as a single 32-bit
   zero.

   One Write chunk is a counted array of segments.  In XDR form, the
   count would appear as the first 32-bit word, followed by an HLOO for
   each element of the array.  For instance, a Write chunk with three
   elements would look like





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      3 HLOO HLOO HLOO


   The Write List is a list of counted arrays.  In XDR form, this is a
   combination of optional-data and counted arrays.  To represent a
   Write List containing a Write chunk with three segments and a Write
   chunk with two segments, XDR would encode


      1 3 HLOO HLOO HLOO 1 2 HLOO HLOO 0


   An empty Write List is encoded as a single 32-bit zero.

   The Reply chunk is a Write chunk.  Since it is an optional-data
   field, however, there is a 32-bit field in front of it that contains
   a one if the Reply chunk is present, or a zero if it is not.  After
   encoding, a Reply chunk with 2 segments would look like


      1 2 HLOO HLOO


   Frequently a requester does not provide any chunks.  In that case,
   after the four fixed fields in the RPC-over-RDMA header, there are
   simply three 32-bit fields that contain zero.

6.  RPC Bind Parameters

   In setting up a new RDMA connection, the first action by a requester
   is to obtain a transport address for the responder.  The mechanism
   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 normally register with a portmap or rpcbind [RFC1833]
   service, which associates an RPC Program number with a service
   address.  (In the case of UDP or TCP, the service address for NFS is
   normally port 2049.)  This policy is no different with RDMA
   transports, although it may require the allocation of port numbers
   appropriate to each Upper Layer Protocol that uses the RPC framing
   defined here.

   When mapped atop the iWARP transport [RFC5040] [RFC5041], which uses
   IP port addressing due to its layering on TCP and/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.



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   When mapped atop InfiniBand [IB], which uses a Group Identifier
   (GID)-based service endpoint naming scheme, a translation MUST be
   employed.  One such translation is defined in the InfiniBand Port
   Addressing Annex [IBPORT], 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:

   o  One possibility is to have responder register its mapped IP port
      with the rpcbind service, under the netid (or netid's) defined
      here.  An RPC-over-RDMA-aware requester can then resolve its
      desired service to a mappable port, and proceed to connect.  This
      is the most flexible and compatible approach, for those upper
      layers that are defined to use the rpcbind service.

   o  A second possibility is to have the responder's portmapper
      register itself on the RDMA interconnect at a "well known" service
      address (on UDP or TCP, this corresponds to port 111).  A
      requester could 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.

   o  Alternatively, the requester could simply 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, will use the same 20049 assignment as
      for iWARP.

   Historically, different RPC protocols have taken different approaches
   to their port assignment; therefore, the specific method is left to
   each RPC-over-RDMA-enabled Upper Layer binding, and not addressed
   here.

   In Section 11, this specification 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 [IB].  Additional RDMA-capable networks MAY define their
   own netids, or if they provide a port translation, MAY share the one
   defined here.

7.  Bi-Directional RPC-Over-RDMA







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7.1.  RPC Direction

7.1.1.  Forward Direction

   A traditional ONC RPC client is always a requester.  A traditional
   ONC RPC service is always a responder.  This traditional form of ONC
   RPC message passing is referred to as operation in the "forward
   direction."

   During forward direction operation, the ONC RPC client is responsible
   for establishing transport connections.

7.1.2.  Backward Direction

   The ONC RPC standard does not forbid passing messages in the other
   direction.  An ONC RPC service endpoint can act as a requester, in
   which case an ONC RPC client endpoint acts as a responder.  This form
   of message passing is referred to as operation in the "backward
   direction."

   During backward direction operation, the ONC RPC client is
   responsible for establishing transport connections, even though ONC
   RPC Calls come from the ONC RPC server.

7.1.3.  Bi-direction

   A pair of endpoints may choose to use only forward or only backward
   direction operations on a particular transport.  Or, the endpoints
   may send operations in both directions concurrently on the same
   transport.

   Bi-directional operation occurs when both transport endpoints act as
   a requester and a responder at the same time.  As above, the ONC RPC
   client is responsible for establishing transport connections.

7.1.4.  XIDs with Bi-direction

   During bi-directional operation, the forward and backward directions
   use independent xid spaces.

   In other words, a forward direction requester MAY use the same xid
   value at the same time as a backward direction requester on the same
   transport connection, but such concurrent requests represent distinct
   ONC RPC transactions.







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7.2.  Backward Direction Flow Control

7.2.1.  Backward RPC-over-RDMA Credits

   Credits work the same way in the backward direction as they do in the
   forward direction.  However, forward direction credits and backward
   direction credits are accounted separately.

   In other words, the forward direction credit value is the same
   whether or not there are backward direction resources associated with
   an RPC-over-RDMA transport connection.  The backward direction credit
   value MAY be different than the forward direction credit value.  The
   rdma_credit field in a backward direction RPC-over-RDMA message MUST
   NOT contain the value zero.

   A backward direction requester (an RPC-over-RDMA service endpoint)
   requests credits from the responder (an RPC-over-RDMA client
   endpoint).  The responder reports how many credits it can grant.
   This is the number of backward direction Calls the responder is
   prepared to handle at once.

   When an RPC-over-RDMA server endpoint is operating correctly, it
   sends no more outstanding requests at a time than the client
   endpoint's advertised backward direction credit value.

7.2.2.  Receive Buffer Management

   An RPC-over-RDMA transport endpoint must pre-post receive buffers
   before it can receive and process incoming RPC-over-RDMA messages.
   If a sender transmits a message for a receiver which has no posted
   receive buffer, the RDMA provider MAY drop the RDMA connection.

7.2.2.1.  Client Receive Buffers

   Typically an RPC-over-RDMA caller posts only as many receive buffers
   as there are outstanding RPC Calls.  A client endpoint without
   backward direction support might therefore at times have no pre-
   posted receive buffers.

   To receive incoming backward direction Calls, an RPC-over-RDMA client
   endpoint must pre-post enough additional receive buffers to match its
   advertised backward direction credit value.  Each outstanding forward
   direction RPC requires an additional receive buffer above this
   minimum.

   When an RDMA transport connection is lost, all active receive buffers
   are flushed and are no longer available to receive incoming messages.
   When a fresh transport connection is established, a client endpoint



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   must re-post a receive buffer to handle the Reply for each
   retransmitted forward direction Call, and a full set of receive
   buffers to handle backward direction Calls.

7.2.2.2.  Server Receive Buffers

   A forward direction RPC-over-RDMA service endpoint posts as many
   receive buffers as it expects incoming forward direction Calls.  That
   is, it posts no fewer buffers than the number of RPC-over-RDMA
   credits it advertises in the rdma_credit field of forward direction
   RPC replies.

   To receive incoming backward direction replies, an RPC-over-RDMA
   server endpoint must pre-post a receive buffer for each backward
   direction Call it sends.

   When the existing transport connection is lost, all active receive
   buffers are flushed and are no longer available to receive incoming
   messages.  When a fresh transport connection is established, a server
   endpoint must re-post a receive buffer to handle the Reply for each
   retransmitted backward direction Call, and a full set of receive
   buffers for receiving forward direction Calls.

7.3.  Conventions For Backward Operation

7.3.1.  In the Absense of Backward Direction Support

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

   A loss of the RDMA connection may result if the receiver is not
   prepared to receive an incoming message.  Thus a denial-of-service
   could result if a sender continues to send backchannel messages after
   every transport reconnect to an endpoint that is not prepared to
   receive them.

   For RPC-over-RDMA Version One transports, the Upper Layer Protocol is
   responsible for informing its peer when it has established a backward
   direction capability.  Otherwise even a simple backward direction
   NULL probe from a peer would result in a lost connection.

   An Upper Layer Protocol consumer MUST NOT perform backward direction
   ONC RPC operations unless the peer consumer has indicated it is
   prepared to handle them.  A description of Upper Layer Protocol




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   mechanisms used for this indication is outside the scope of this
   document.

7.3.2.  Backward Direction Retransmission

   In rare cases, an ONC RPC transaction cannot be completed within a
   certain time.  This can be because the transport connection was lost,
   the Call or Reply message was dropped, or because the Upper Layer
   consumer delayed or dropped the ONC RPC request.  Typically, the
   requester sends the transaction again, reusing the same RPC XID.
   This is known as an "RPC retransmission".

   In the forward direction, the Caller is the ONC RPC client.  The
   client is always responsible for establishing a transport connection
   before sending again.

   In the backward direction, the Caller is the ONC RPC server.  Because
   an ONC RPC server does not establish transport connections with
   clients, it cannot send a retransmission if there is no transport
   connection.  It must wait for the ONC RPC client to re-establish the
   transport connection before it can retransmit ONC RPC transactions in
   the backward direction.

   If an ONC RPC client has no work to do, it may be some time before it
   re-establishes a transport connection.  Backward direction Callers
   must be prepared to wait indefinitely before a connection is
   established before a pending backward direction ONC RPC Call can be
   retransmitted.

7.3.3.  Backward Direction Message Size

   RPC-over-RDMA backward direction messages are transmitted and
   received using the same buffers as messages in the forward direction.
   Therefore they are constrained to be no larger than receive buffers
   posted for forward messages.

   It is expected that the Upper Layer Protocol consumer establishes an
   appropriate payload size limit for backward direction operations,
   either by advertising that size limit to its peers, or by convention.
   If that is done, backward direction messages do not exceed the size
   of receive buffers at either endpoint.

   If a sender transmits a backward direction message that is larger
   than the receiver is prepared for, the RDMA provider drops the
   message and the RDMA connection.






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7.3.4.  Sending A Backward Direction Call

   To form a backward direction RPC-over-RDMA Call message on an RPC-
   over-RDMA Version One transport, an ONC RPC service endpoint
   constructs an RPC-over-RDMA header containing a fresh RPC XID in the
   rdma_xid field.

   The rdma_vers field MUST contain the value one.  The number of
   requested credits is placed in the rdma_credit field.

   The rdma_proc field in the RPC-over-RDMA header MUST contain the
   value RDMA_MSG.  All three chunk lists MUST be empty.

   The ONC RPC Call header MUST follow immediately, starting with the
   same XID value that is present in the RPC-over-RDMA header.  The Call
   header's msg_type field MUST contain the value CALL.

7.3.5.  Sending A Backward Direction Reply

   To form a backward direction RPC-over-RDMA Reply message on an RPC-
   over-RDMA Version One transport, an ONC RPC client endpoint
   constructs an RPC-over-RDMA header containing a copy of the matching
   ONC RPC Call's RPC XID in the rdma_xid field.

   The rdma_vers field MUST contain the value one.  The number of
   granted credits is placed in the rdma_credit field.

   The rdma_proc field in the RPC-over-RDMA header MUST contain the
   value RDMA_MSG.  All three chunk lists MUST be empty.

   The ONC RPC Reply header MUST follow immediately, starting with the
   same XID value that is present in the RPC-over-RDMA header.  The
   Reply header's msg_type field MUST contain the value REPLY.

7.4.  Backward Direction Upper Layer Binding

   RPC programs that operate on RPC-over-RDMA Version One only in the
   backward direction do not require an Upper Layer Binding
   specification.  Because RPC-over-RDMA Version One operation in the
   backward direction does not allow reduction, there can be no DDP-
   eligible data items in such a program.  Backward direction operation
   occurs on an already-established connection, thus there is no need to
   specify RPC bind parameters.








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8.  Upper Layer Binding Specifications

   An Upper Layer Protocol is typically defined independently of any
   particular RPC transport.  An Upper Layer Binding specification (ULB)
   provides guidance that helps the Upper Layer Protocol interoperate
   correctly and efficiently over a particular transport.  For RPC-over-
   RDMA Version One, a ULB provides:

   o  A taxonomy of XDR data items that are eligible for Direct Data
      Placement

   o  A method for determining the maximum size of the reply Payload
      stream for all procedures in the Upper Layer Protocol

   o  An rpcbind port assignment for operation of the RPC Program and
      Version on an RPC-over-RDMA transport

   Each RPC Program and Version tuple that utilizes RPC-over-RDMA
   Version One needs to have an Upper Layer Binding specification.
   Requesters MUST NOT send RPC-over-RDMA messages for Upper Layer
   Protocols that do not have a Upper Layer Binding.  Responders MUST
   NOT reply to RPC-over-RDMA messages for Upper Layer Protocols that do
   not have a Upper Layer Binding.

8.1.  DDP-Eligibility

   An Upper Layer Binding designates some XDR data items as eligible for
   Direct Data Placement.  As an RPC-over-RDMA message is formed, DDP-
   eligible data items can be removed from the Payload stream and placed
   directly in the receiver's memory (reduced).

   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:

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

   o  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.

   o  The content of the XDR data item is sensitive to address
      alignment.  For example, pullup would be required on the receiver
      before the content of the item can be used.

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



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   Senders MUST NOT reduce data items that are not DDP-eligible.  Such
   data items MAY, however, be moved as part of a Position Zero Read
   Chunk or a Reply chunk.

   The 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 Upper
   Layer Binding are respected.

   There is no provision to express DDP-eligibility within the XDR
   language.  The only definitive specification of DDP-eligibility is
   the Upper Layer Binding itself.

8.1.1.  DDP-Eligibility Violation

   A DDP-eligibility violation occurs when a requester forms a Call
   message with a non-DDP-eligible data item in a Read chunk.  A
   violation occurs when a responder forms a Reply message without
   reducing a DDP-eligible data item when there is a Write list provided
   by the requester.

   In the first case, a responder MUST NOT process the Call message.

   In the second case, as a requester parses a Reply message, it must
   assume that the responder has correctly reduced a DDP-eligible result
   data item.  If the responder has not done so, it is likely that the
   requester cannot finish parsing the Payload stream and that an XDR
   error would result.

   Both types of violations MUST be reported as described in
   Section 5.5.2.

8.2.  Maximum Reply Size

   A requester provides resources for both a Call message and its
   matching Reply message.  A requester forms the Call message itself,
   thus can compute the exact resources needed for it.

   A requester must allocate resources for the Reply message (an RPC-
   over-RDMA credit, a Receive buffer, and possibly a Write list and
   Reply chunk) before the responder has formed the actual reply.  To
   accommodate all possible replies for the procedure in the Call
   message, a requester must allocate reply resources based on the
   maximum possible size of the expected Reply message.





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   If there are procedures in the Upper Layer Protocol for which there
   is no clear reply size maximum, the Upper Layer Binding needs to
   specify a dependable means for determining the maximum.

8.3.  Additional Considerations

   There may be other details provided in an Upper Layer Binding.

   o  An Upper Layer Binding may recommend an inline threshold value or
      other transport-related parameters for RPC-over-RDMA Version One
      connections bearing that Upper Layer Protocol.

   o  An Upper Layer Protocol may provide a means to communicate these
      transport-related parameters between peers.  Note that RPC-over-
      RDMA Version One does not specify any mechanism for changing any
      transport-related parameter after a connection has been
      established.

   o  Multiple Upper Layer Protocols may share a single RPC-over-RDMA
      Version One connection when their Upper Layer Bindings allow the
      use of RPC-over-RDMA Version One and the rpcbind port assignments
      for the Protocols allow connection sharing.  In this case, the
      same transport parameters (such as inline threshold) apply to all
      Protocols using that connection.

   Given the above, Upper Layer Bindings and Upper Layer Protocols must
   be designed to interoperate correctly no matter what connection
   parameters are in effect on a connection.

8.4.  Upper Layer Protocol Extensions

   An RPC Program and Version tuple may be extensible.  For instance,
   there may be a minor versioning scheme that is not reflected in the
   RPC version number.  Or, the Upper Layer Protocol may allow
   additional features to be specified after the original RPC program
   specification was ratified.

   Upper Layer Bindings are provided for interoperable RPC Programs and
   Versions by extending existing Upper Layer Bindings to reflect the
   changes made necessary by each addition to the existing XDR.

9.  Extensibility Guidelines

   The RPC-over-RDMA header format is specified using XDR, unlike other
   RPC transport protocols such as TCP or UDP.  This creates
   opportunities for addressing minor issues with the transport protocol
   and for introducing optional features, all without having to
   increment the RPC-over-RDMA protocol version number.  When more



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   invasive changes to the protocol are needed, a protocol version
   number change is required.  In either case, no changes to the RPC-
   over-RDMA protocol can be made without Working Group discussion and
   approval by the IESG.

   Unlike the rest of this document, which defines the base of RPC-over-
   RDMA Version One, Section 9 applies to all versions of RPC-over-RDMA.
   New versions of RPC-over-RDMA may be published as separate protocols
   without updating this document, but any change to the extensibility
   model defined here requires updating this document.

9.1.  Extending RPC-over-RDMA Header XDR

   The first four fields in the RPC-over-RDMA header must remain aligned
   at the same fixed offsets for all versions of the RPC-over-RDMA
   protocol.  The version number must be in a fixed place in order for
   version mismatches to be detected.  For version mismatches to be
   reported in a fashion that all future version implementations can
   reliably decode, the rdma_proc field must be in a fixed place, the
   value of RDMA_ERR_VERS must always remain the same, and the field
   placement of the RDMA_ERR_VERS arm of the rpcrdma1_error union must
   always remain the same.

   Given these constraints, one way to extend RPC-over-RDMA is to add
   new values to the rdma_proc enumerated type and new components (arms)
   to the rpcrdma1_body union.  New argument and result types may be
   introduced for each new procedure defined this way.  These extensions
   would be specified by new Internet Drafts with appropriate Working
   Group and IESG review to ensure continued interoperation with
   existing implementations.

   XDR extensions may introduce only optional features to an existing
   RPC-over-RDMA protocol version.  To detect when an optional rdma_proc
   value is supported by a receiver, it is desirable to have a specific
   value of the rdma_err field, say, RDMA_ERR_PROC, that indicates when
   the receiver does not recognize an rdma_proc value.

   In RPC-over-RDMA Version One, a receiver can indicate that it does
   not recognize an rdma_proc enum value only by returning an RDMA_ERROR
   procedure with the rdma_err field set to RDMA_ERR_CHUNK (see
   Section 5.5.2).  This is indistinguishable from a situation where the
   receiver does indeed support the procedure, but the XDR is malformed.

   To resolve this problem, an RPC-over-RDMA Version One sender uses the
   following convention.  If the first time the sender uses an optional
   rdma_proc value the receiver returns an RDMA_ERROR procedure with
   RDMA_ERR_CHUNK in the rdma_err field, the sender simply marks that
   feature as unsupported and does not send it again on the current



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   connection instance.  Subsequent to an initial successful result,
   receiving RDMA_ERR_CHUNK retains its more relaxed meaning of "generic
   XDR parsing error."

   To ensure backwards compatibility when such an extension mechanism is
   in place, the value of RDMA_ERR_CHUNK must remain the same for all
   versions of the RPC-over-RDMA protocol.

9.2.  RPC-over-RDMA Version Numbering

   Before becoming REQUIRED, features created by XDR extension will
   often need a significant period of optional general use to ensure
   they are mature.  This is especially true for infrastructural
   features that others will build upon.  When optional features become
   REQUIRED, that would be an occasion to bump the RPC-over-RDMA
   protocol version.

9.2.1.  Incrementing The Version Number

   The value of the RPC-over-RDMA header's version field has to be
   updated when the protocol is altered in a way that prevents
   interoperability with current implementations.  Two examples of such
   changes include:

   o  Whenever the RPC-over-RDMA header XDR definition is changed to add
      a REQUIRED protocol element, or whenever a REQUIRED protocol
      element is removed

   o  Whenever the use of a new abstract RDMA operation is specified as
      REQUIRED, or the use of an existing REQUIRED abstract RDMA
      operation is removed

   When a version number bump is forced (e.g. a REQUIRED feature is to
   be introduced), the Working Group can:

   o  Document the whole protocol as amended

   o  Normatively reference all features added since the previous
      version

   o  Include all REQUIRED functionality, and normatively reference
      optional functionality

   The Working Group retains all these options but the last is typically
   preferred.






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10.  Security Considerations

10.1.  Memory Protection

   A primary consideration is the protection of the integrity and
   privacy of local memory by an RPC-over-RDMA transport.  The use of
   RPC-over-RDMA MUST NOT introduce any vulnerabilities to system memory
   contents, nor to memory owned by user processes.

   It is REQUIRED that any RDMA provider used for RPC transport be
   conformant to the requirements of [RFC5042] in order to satisfy these
   protections.  These protections are provided by the RDMA layer
   specifications, and in particular, their security models.

10.1.1.  Protection Domains

   The use of Protection Domains to limit the exposure of memory
   segments to a single connection is critical.  Any attempt by an
   endpoint not participating in that connection to re-use memory
   handles should result in immediate failure of that connection.
   Because Upper Layer Protocol security mechanisms rely on this aspect
   of Reliable Connection behavior, strong authentication of remote
   endpoints is recommended.

10.1.2.  Handle Predictability

   Unpredictable memory handles should be used for any operation
   requiring advertised memory segments.  Advertising 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 under way.
   Therefore implementations should avoid advertising persistently
   registered memory.

10.1.3.  Memory Fencing

   Advertised memory segments should be invalidated as soon as related
   RPC operations are complete.  Invalidation and DMA unmapping of
   segments should be complete before the Upper Layer is allowed to
   continue execution and use or alter the contents of a memory region.

10.2.  Using GSS With RPC-Over-RDMA

   ONC RPC provides its own security via the RPCSEC_GSS framework
   [RFC2203].  RPCSEC_GSS can provide message authentication, integrity
   checking, and privacy.  This security mechanism is unaffected by the
   RDMA transport.  However, there is much host data movement associated
   with the computation and verification of integrity and with
   encryption/decryption, so performance advantages can be lost.



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   For efficiency, a more appropriate security mechanism for RDMA links
   may be link-level protection, such as certain configurations of
   IPsec, which may be co-located in the RDMA hardware.  The use of
   link-level protection MAY be negotiated through the use of the
   RPCSEC_GSS mechanism defined in [RFC5403] in conjunction with the
   Channel Binding mechanism [RFC5056] and IPsec Channel Connection
   Latching [RFC5660].  Use of such mechanisms is REQUIRED where
   integrity and/or privacy is desired, and where efficiency is
   required.

   Once delivered securely by the RDMA provider, any RDMA-exposed memory
   will contain only RPC payloads in the chunk lists, transferred under
   the protection of RPCSEC_GSS integrity and privacy.  By these means,
   the data will be protected end-to-end, as required by the RPC layer
   security model.

11.  IANA Considerations

   Three assignments are specified by this document:

   o  A set of RPC "netids" for resolving RPC-over-RDMA services

   o  Optional service port assignments for Upper Layer Bindings

   o  An RPC program number assignment for the configuration protocol

   These assignments have been established, as below.

   The new RPC transport has been assigned an RPC "netid", which is an
   rpcbind [RFC1833] string used to describe the underlying protocol in
   order for RPC to select the appropriate transport framing, as well as
   the format of the service addresses and ports.

   The following "Netid" registry strings are defined for this purpose:


      NC_RDMA "rdma"
      NC_RDMA6 "rdma6"


   These netids MAY be used for any RDMA network satisfying the
   requirements of Section 2, and able to identify service endpoints
   using IP port addressing, possibly through use of a translation
   service as described above in Section 6.  The "rdma" netid is to be
   used when IPv4 addressing is employed by the underlying transport,
   and "rdma6" for IPv6 addressing.

   The netid assignment policy and registry are defined in [RFC5665].



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   As a new RPC transport, this protocol has no effect on RPC Program
   numbers or existing registered port numbers.  However, new port
   numbers MAY be registered for use by RPC-over-RDMA-enabled services,
   as appropriate to the new networks over which the services will
   operate.

   For example, the NFS/RDMA service defined in [RFC5667] has been
   assigned the port 20049, in the IANA registry:


      nfsrdma 20049/tcp Network File System (NFS) over RDMA
      nfsrdma 20049/udp Network File System (NFS) over RDMA
      nfsrdma 20049/sctp Network File System (NFS) over RDMA


   The RPC program number assignment policy and registry are defined in
   [RFC5531].

12.  Acknowledgments

   The editor gratefully acknowledges the work of Brent Callaghan and
   Tom Talpey on the original RPC-over-RDMA Version One specification
   [RFC5666].

   Dave Noveck provided excellent review, constructive suggestions, and
   consistent navigational guidance throughout the process of drafting
   this document.  Dave also contributed much of the organization and
   content of Section 9.

   The comments and contributions of Karen Deitke, Dai Ngo, Chunli
   Zhang, Dominique Martinet, and Mahesh Siddheshwar are accepted with
   great thanks.  The editor also wishes to thank Bill Baker for his
   unwavering support of this work.

   Special thanks go to nfsv4 Working Group Chair Spencer Shepler and
   nfsv4 Working Group Secretary Thomas Haynes for their support.

13.  References

13.1.  Normative References

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







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

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, DOI 10.17487/RFC2203, September
              1997, <http://www.rfc-editor.org/info/rfc2203>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <http://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, <http://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,
              <http://www.rfc-editor.org/info/rfc5056>.

   [RFC5403]  Eisler, M., "RPCSEC_GSS Version 2", RFC 5403, DOI
              10.17487/RFC5403, February 2009,
              <http://www.rfc-editor.org/info/rfc5403>.

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

   [RFC5660]  Williams, N., "IPsec Channels: Connection Latching", RFC
              5660, DOI 10.17487/RFC5660, October 2009,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5665>.

13.2.  Informative References

   [IB]       InfiniBand Trade Association, "InfiniBand Architecture
              Specifications", <http://www.infinibandta.org>.

   [IBPORT]   InfiniBand Trade Association, "IP Addressing Annex",
              <http://www.infinibandta.org>.





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

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

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

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

   [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, <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5661>.

   [RFC5666]  Talpey, T. and B. Callaghan, "Remote Direct Memory Access
              Transport for Remote Procedure Call", RFC 5666, DOI
              10.17487/RFC5666, January 2010,
              <http://www.rfc-editor.org/info/rfc5666>.

   [RFC5667]  Talpey, T. and B. Callaghan, "Network File System (NFS)
              Direct Data Placement", RFC 5667, DOI 10.17487/RFC5667,
              January 2010, <http://www.rfc-editor.org/info/rfc5667>.






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

Authors' Addresses

   Charles Lever (editor)
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   USA

   Phone: +1 734 274 2396
   Email: chuck.lever@oracle.com


   William Allen Simpson
   DayDreamer
   1384 Fontaine
   Madison Heights, MI  48071
   USA

   Email: william.allen.simpson@gmail.com


   Tom Talpey
   Microsoft Corp.
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 704-9945
   Email: ttalpey@microsoft.com


















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