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RFC 8166 - Remote Direct Memory Access Transport for Remote Proc

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Internet Engineering Task Force (IETF)                     C. Lever, Ed.
Request for Comments: 8166                                        Oracle
Obsoletes: 5666                                               W. Simpson
Category: Standards Track                                        Red Hat
ISSN: 2070-1721                                                T. Talpey
                                                               June 2017

               Remote Direct Memory Access Transport for
                    Remote Procedure Call Version 1


   This document specifies a protocol for conveying Remote Procedure
   Call (RPC) messages on physical transports capable of Remote Direct
   Memory Access (RDMA).  This protocol is referred to as the RPC-over-
   RDMA version 1 protocol in this document.  It requires no revision to
   application RPC protocols or the RPC protocol itself.  This document
   obsoletes RFC 5666.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  RPCs on RDMA Transports . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     2.2.  RPCs  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  RDMA  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  RPC-over-RDMA Protocol Framework  . . . . . . . . . . . . . .  10
     3.1.  Transfer Models . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Message Framing . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  Managing Receiver Resources . . . . . . . . . . . . . . .  11
     3.4.  XDR Encoding with Chunks  . . . . . . . . . . . . . . . .  14
     3.5.  Message Size  . . . . . . . . . . . . . . . . . . . . . .  19
   4.  RPC-over-RDMA in Operation  . . . . . . . . . . . . . . . . .  23
     4.1.  XDR Protocol Definition . . . . . . . . . . . . . . . . .  23
     4.2.  Fixed Header Fields . . . . . . . . . . . . . . . . . . .  28
     4.3.  Chunk Lists . . . . . . . . . . . . . . . . . . . . . . .  30
     4.4.  Memory Registration . . . . . . . . . . . . . . . . . . .  33
     4.5.  Error Handling  . . . . . . . . . . . . . . . . . . . . .  34
     4.6.  Protocol Elements No Longer Supported . . . . . . . . . .  37
     4.7.  XDR Examples  . . . . . . . . . . . . . . . . . . . . . .  38
   5.  RPC Bind Parameters . . . . . . . . . . . . . . . . . . . . .  39
   6.  ULB Specifications  . . . . . . . . . . . . . . . . . . . . .  41
     6.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .  41
     6.2.  Maximum Reply Size  . . . . . . . . . . . . . . . . . . .  43
     6.3.  Additional Considerations . . . . . . . . . . . . . . . .  43
     6.4.  ULP Extensions  . . . . . . . . . . . . . . . . . . . . .  43
   7.  Protocol Extensibility  . . . . . . . . . . . . . . . . . . .  44
     7.1.  Conventional Extensions . . . . . . . . . . . . . . . . .  44
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
     8.1.  Memory Protection . . . . . . . . . . . . . . . . . . . .  44
     8.2.  RPC Message Security  . . . . . . . . . . . . . . . . . .  46
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     10.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Appendix A.  Changes from RFC 5666  . . . . . . . . . . . . . . .  53
     A.1.  Changes to the Specification  . . . . . . . . . . . . . .  53
     A.2.  Changes to the Protocol . . . . . . . . . . . . . . . . .  53
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

1.  Introduction

   This document specifies the RPC-over-RDMA version 1 protocol, based
   on existing implementations of RFC 5666 and experience gained through
   deployment.  This document obsoletes RFC 5666.

   This specification clarifies text that was subject to multiple
   interpretations and removes support for unimplemented RPC-over-RDMA
   version 1 protocol elements.  It clarifies the role of Upper-Layer
   Bindings (ULBs) and describes what they are to contain.

   In addition, this document describes current practice using
   RPCSEC_GSS [RFC7861] on RDMA transports.

   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 1 protocol specified in [RFC5666].

1.1.  RPCs on RDMA Transports

   RDMA [RFC5040] [RFC5041] [IBARCH] 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, often
   shortened in NFSv4 documents to RPC) [RFC5531] is a remote procedure
   call protocol that runs over a variety of transports.  Most RPC
   implementations today use UDP [RFC768] or TCP [RFC793].  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 that differ from either UDP or TCP.
   They retain message delineations like UDP but provide 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.

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

   Although the RDMA transport described herein can provide relatively
   transparent support for any RPC application, this document also
   describes mechanisms that can optimize data transfer even further,
   when RPC applications are willing to exploit awareness of RDMA as the

2.  Terminology

2.1.  Requirements Language

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

2.2.  RPCs

   This section highlights key elements of the RPC [RFC5531] and
   External Data Representation (XDR) [RFC4506] protocols, upon which
   RPC-over-RDMA version 1 is constructed.  Strong grounding with these
   protocols is recommended before reading this document.

2.2.1.  Upper-Layer Protocols

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

   In this document, the term "RPC consumer" refers to an implementation
   of a ULP running on an RPC client endpoint.

2.2.2.  Requesters and Responders

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

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

   ONC RPC transactions are made up of two types of messages:

      An "RPC Call message" requests that work be done.  This type of
      message 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 RPC Call message to a
      corresponding RPC Reply message.

      An "RPC Reply message" reports the results of work requested by an
      RPC Call message.  An RPC Reply message is designated by the value
      one (1) in the message's msg_type field.  The value contained in
      an RPC Reply message's XID field is copied from the RPC Call
      message whose results are being reported.

   The RPC client endpoint acts as a "Requester".  It serializes the
   procedure'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

   The RPC server endpoint acts as a "Responder".  It 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.

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

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

2.2.3.  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, clients initiate transport
   connections, while servers wait passively for incoming connection

2.2.4.  External Data Representation

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

   The XDR protocol encodes data independently 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

   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.
   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.  XDR Opaque Data

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

   The number of octets in a variable-length data item precedes that
   item in an XDR stream.  If the size of an encoded data item is not a
   multiple of four octets, octets containing zero are added after the
   end of the item; this is the case so that the next encoded data item
   in the XDR stream starts on a four-octet boundary.  The encoded size
   of the item is not changed by the addition of the extra octets.
   These extra octets are never exposed to ULPs.

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

2.3.  RDMA

   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 or "DDP").  RDMA transports
   enable both optimizations.

2.3.1.  DDP

   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.  After an RDMA transfer, a receiver may copy
   that data again to its final destination.

   In this document, the term "DDP" refers to any optimized data
   transfer where it is unnecessary for a receiving host's CPU to copy
   transferred data to another location after it has been received.

   Just as [RFC5666] did, this document focuses on the use of RDMA Read
   and Write operations to achieve both data movement offload and DDP.
   However, not all RDMA-based data transfer qualifies as DDP, and DDP
   can be achieved using non-RDMA mechanisms.

2.3.2.  RDMA Transport Requirements

   To achieve good performance during receive operations, RDMA
   transports require that RDMA consumers provision resources in advance
   to receive incoming messages.

   An RDMA consumer might provide Receive buffers in advance by posting
   an RDMA Receive Work Request for every expected RDMA Send from a
   remote peer.  These buffers are provided before the remote peer posts
   RDMA Send Work Requests; thus, this is often referred to as "pre-
   posting" buffers.

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

   Given these basic tenets of RDMA transport operation, the RPC-over-
   RDMA version 1 protocol assumes each transport provides the following
   abstract operations.  A more complete discussion of these operations
   is found in [RFC5040].

   Registered Memory
      Registered memory is a region 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 1
      protocol assumes that each region 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 buffer.  Sends complete at the receiver in the order
      they were issued at the sender.  The amount of data transferred by
      a single RDMA Send operation is limited by the size of the remote
      peer's pre-posted buffers.

   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 1 protocol).

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

      RDMA Writes are not 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

   RDMA Read
      The RDMA provider supports an RDMA Read operation to place peer
      source data directly into the read initiator's memory.  The local
      host initiates an RDMA Read, and completion is signaled there.  No
      completion is signaled on the remote peer.  The local host
      provides steering tags, memory addresses, and a length for the
      remote source and local destination memory region.

      The local host signals Read completion to the remote peer as part
      of a subsequent RDMA Send message.  The remote peer can then
      release steering tags and subsequently free associated source
      memory regions.

   The RPC-over-RDMA version 1 protocol is designed to be carried over
   RDMA transports that support the above abstract operations.  This
   protocol conveys information sufficient for an RPC peer to direct an
   RDMA provider to perform transfers containing RPC data and to
   communicate their result(s).

3.  RPC-over-RDMA Protocol Framework

3.1.  Transfer Models

   A "transfer model" designates which endpoint exposes its memory and
   which is responsible for initiating the transfer of data.  To enable
   RDMA Read and Write operations, for example, an endpoint first
   exposes regions of its memory to a remote endpoint, which initiates
   these operations against the exposed memory.

      Requesters expose their memory to the Responder, and the Responder
      exposes its memory to Requesters.  The Responder reads, or pulls,
      RPC arguments or whole RPC calls from each Requester.  Requesters
      pull RPC results or whole RPC relies from the Responder.

      Requesters expose their memory to the Responder, and the Responder
      exposes its memory to Requesters.  Requesters write, or push, RPC
      arguments or whole RPC calls to the Responder.  The Responder
      pushes RPC results or whole RPC relies to each Requester.

      Requesters expose their memory to the Responder, but the Responder
      does not expose its memory.  The Responder pulls RPC arguments or
      whole RPC calls from each Requester.  The Responder pushes RPC
      results or whole RPC relies to each Requester.

      The Responder exposes its memory to Requesters, but Requesters do
      not expose their memory.  Requesters push RPC arguments or whole
      RPC calls to the Responder.  Requesters pull RPC results or whole
      RPC relies from the Responder.

3.2.  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

   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 Transaction ID (XID) field of the encapsulated RPC

   Transport 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 on TCP sockets 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 other RDMA
   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).

   However, it is possible for RPC-over-RDMA to be dynamically enabled
   in the course of negotiating the use of RDMA via a ULP 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.

3.3.  Managing Receiver Resources

   It is critical to provide RDMA Send flow control for an RDMA
   connection.  If any pre-posted Receive buffer on the connection is
   not large enough to accept an incoming RDMA Send, or if a pre-posted
   Receive buffer is not available to accept an incoming RDMA Send, the

   RDMA connection can be terminated.  This is different than
   conventional TCP/IP networking, in which buffers are allocated
   dynamically as messages are received.

   The longevity of an RDMA connection mandates 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.

3.3.1.  RPC-over-RDMA Credits

   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.

   An RPC-over-RDMA version 1 credit is the capability to handle one
   RPC-over-RDMA transaction.  Each RPC-over-RDMA message sent from
   Requester to Responder requests a number of credits from the
   Responder.  Each RPC-over-RDMA message sent from Responder to
   Requester informs the Requester how many credits the Responder has
   granted.  The requested and granted values are carried in each RPC-
   over-RDMA message's rdma_credit field (see Section 4.2.3).

   Practically speaking, the critical value is the granted value.  A
   Requester MUST NOT send unacknowledged requests in excess of the
   Responder's granted credit limit.  If the granted value is exceeded,
   the RDMA layer may signal an error, possibly terminating the
   connection.  The granted value MUST NOT be zero, since such a value
   would result in deadlock.

   RPC calls complete in any order, but the current granted credit limit
   at the Responder is known to the Requester from RDMA Send ordering
   properties.  The number of allowed new requests the Requester may
   send is then the lower of the current 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.

   The requested and granted credit values MAY be adjusted to match the
   needs or policies in effect on either peer.  For instance, a
   Responder may reduce the granted credit value to accommodate the
   available resources in a Shared Receive Queue.  The Responder MUST
   ensure that an increase in receive resources is effected before the
   next RPC Reply message is sent.

   A Requester MUST maintain enough receive resources to accommodate
   expected replies.  Responders have to be prepared for there to be no
   receive resources available on Requesters with no pending RPC

   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 1 implementation.

3.3.2.  Inline Threshold

   An "inline threshold" value is the largest message size (in octets)
   that can be conveyed in one direction between peer implementations
   using RDMA Send and Receive.  The inline threshold value is the
   smaller of the largest number of bytes the sender can post via a
   single RDMA Send operation and the largest number of bytes the
   receiver can accept via a single RDMA Receive operation.  Each
   connection has two inline threshold values: one for messages flowing
   from Requester-to-Responder (referred to as the "call inline
   threshold") and one for messages flowing from Responder-to-Requester
   (referred to as the "reply inline threshold").

   Unlike credit limits, inline threshold values are not advertised to
   peers via the RPC-over-RDMA version 1 protocol, and there is no
   provision for inline threshold values to change during the lifetime
   of an RPC-over-RDMA version 1 connection.

3.3.3.  Initial Connection State

   When a connection is first established, peers might not know how many
   receive resources the other has, nor how large the other peer's
   inline thresholds are.

   As a basis for an initial exchange of RPC requests, each RPC-over-
   RDMA version 1 connection provides the ability to exchange at least
   one RPC message at a time, whose RPC Call and Reply messages are no
   more than 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 inline thresholds of 1024 bytes
   but MAY support larger inline thresholds values.  An independent
   mechanism for discovering a peer's inline thresholds before a
   connection is established may be used to optimize the use of RDMA
   Send and Receive operations.  In the absence of such a mechanism,
   senders and receives MUST assume the inline thresholds are 1024

3.4.  XDR Encoding with Chunks

   When a DDP capability is available, the transport places the contents
   of one or more XDR data items directly into the receiver's memory,
   separately from the transfer of other parts of the containing XDR

3.4.1.  Reducing an XDR Stream

   RPC-over-RDMA version 1 provides a mechanism for moving part of an
   RPC message via a data transfer distinct from an RDMA Send/Receive
   pair.  The sender removes one or more XDR data items from the Payload
   stream.  They are conveyed via other mechanisms, such as one or more
   RDMA Read or Write operations.  As the receiver decodes an incoming
   message, it skips over directly placed data items.

   The portion of an XDR stream that is split out and moved separately
   is referred to as a "chunk".  In some contexts, data in an RPC-over-
   RDMA header that describes these split out regions of memory may also
   be referred to as a "chunk".

   A Payload stream after chunks have been removed is referred to as a
   "reduced" Payload stream.  Likewise, a data item that has been
   removed from a Payload stream to be transferred separately is
   referred to as a "reduced" data item.

3.4.2.  DDP-Eligibility

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

   To maintain interoperability on an RPC-over-RDMA transport, a
   determination must be made of which few XDR data items in each ULP
   are allowed to use DDP.

   This is done by additional specifications that describe how ULPs
   employ DDP.  A "ULB specification" identifies which specific
   individual XDR data items in a ULP MAY be transferred via DDP.  Such
   data items are referred to as "DDP-eligible".  All other XDR data
   items MUST NOT be reduced.

   Detailed requirements for ULBs are provided in Section 6.

3.4.3.  RDMA Segments

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

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

   An "RDMA segment", or "plain segment", is an RPC-over-RDMA Transport
   header data object that contains the precise coordinates of a
   contiguous memory region that is to be conveyed separately from the
   Payload stream.  Plain segments contain the following information:

      Steering tag (STag) or R_key generated by registering this memory
      with the RDMA provider.

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

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

   See [RFC5040] for further discussion.

3.4.4.  Chunks

   In RPC-over-RDMA version 1, a "chunk" refers to a portion of the
   Payload stream that is moved independently of the RPC-over-RDMA
   Transport header and Payload stream.  Chunk data is removed from the
   sender's Payload stream, transferred via separate operations, and
   then reinserted into the receiver's Payload stream to form a complete
   RPC message.

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

   The RPC-over-RDMA version 1 transport protocol does not place a limit
   on chunk size.  However, each ULP may cap the amount of data that can
   be transferred by a single RPC (for example, NFS has "rsize" and
   "wsize", which restrict the payload size of NFS READ and WRITE
   operations).  The Responder can use such limits to sanity check chunk
   sizes before using them in RDMA operations.  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.

   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.  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.  XDR Unions

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

   Except in special cases (covered in Section 3.5.3), a chunk MUST
   contain exactly one XDR data item.  This makes it straightforward to
   reduce variable-length data items without affecting the XDR alignment
   of data items in the Payload stream.

   When a variable-length XDR data item is reduced, the sender MUST
   remove XDR roundup padding for that data item from the Payload stream
   so that data items remaining in the Payload stream begin on four-byte

3.4.5.  Read Chunks

   A "Read chunk" represents an XDR data item that is to be pulled from
   the Requester to the Responder.

   A Read chunk is a list of one or more RDMA read segments.  An RDMA
   read segment consists of a Position field followed by a plain
   segment.  See Section 4.1.2 for details.

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

   While constructing an RPC Call message, a Requester registers memory
   regions that contain data to be transferred via RDMA Read operations.
   It advertises the coordinates of these regions in the RPC-over-RDMA
   Transport header of the RPC Call message.

   After receiving an RPC Call message 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 RDMA segment, in list
   order, into the received Payload stream at the Position value
   recorded in that RDMA segment.

   Put another way, the Responder inserts the first RDMA segment in a
   Read chunk into the Payload stream at the byte offset indicated by
   its Position field.  RDMA segments whose Position field value match
   this offset are concatenated afterwards, until there are no more RDMA
   segments at that Position value.

   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.  All segments in the same Read chunk
   share the same Position value, even if one or more of the RDMA
   segments have a non-four-byte-aligned length.  Decoding Read Chunks

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

   The Responder acknowledges its completion of use of Read chunk source
   buffers when it sends an RPC Reply message to the Requester.  The
   Requester may then release Read chunks advertised in the request.  Read Chunk Roundup

   When reducing a variable-length argument data item, the Requester
   SHOULD NOT include the data item's XDR roundup padding in the chunk.
   The length of a Read chunk is determined as follows:

   o  If the Requester chooses to include roundup padding in a Read
      chunk, the chunk's total length MUST be the sum of the encoded
      length of the data item and the length of the roundup padding.
      The length of the data item that was encoded into the Payload
      stream remains unchanged.

      The sender can increase the length of the chunk by adding another
      RDMA segment containing only the roundup padding, or it can do so
      by extending the final RDMA segment in the chunk.

   o  If the sender chooses not to include roundup padding in the chunk,
      the chunk's total length MUST be the same as the encoded length of
      the data item.

3.4.6.  Write Chunks

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

   Write chunks are provisioned by a Requester long before the Responder
   has prepared the reply Payload stream.  A Requester often does not
   know the actual length of the result data items to be returned, since
   the result does not yet exist.  Thus, it MUST register Write chunks
   long enough to accommodate the maximum possible size of each returned
   data item.

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

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

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

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

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

3.5.  Message Size

   A receiver of RDMA Send operations is required by RDMA to have
   previously posted one or more adequately sized buffers.  Memory
   savings are achieved on both Requesters and Responders by posting
   small Receive buffers.  However, not all RPC messages are small.
   RPC-over-RDMA version 1 provides several mechanisms that allow
   messages of any size to be conveyed efficiently.

3.5.1.  Short Messages

   RPC messages are frequently smaller than typical inline thresholds.
   For example, the NFS version 3 GETATTR operation is only 56 bytes: 20
   bytes of RPC header, 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 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 other operations are needed.

   An RPC-over-RDMA transaction using Short Messages:

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

3.5.2.  Chunked Messages

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

   After receiving the Transport and Payload streams of an RPC Call
   message accompanied by Read chunks, the Responder uses RDMA Read
   operations to move reduced data items in Read chunks.  Before sending
   the Transport and Payload streams of an RPC Reply message containing
   Write chunks, the Responder uses RDMA Write operations to move
   reduced data items in Write and Reply chunks.

   An RPC-over-RDMA transaction with a Read chunk:

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

   An RPC-over-RDMA transaction with a Write chunk:

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

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

   Long Reply
      To send a Long Reply, the Requester provides a single special
      Write chunk in advance, known as the "Reply chunk", that will
      contain the RPC Reply message'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

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

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

   An RPC-over-RDMA transaction using a Long Call:

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

   An RPC-over-RDMA transaction using a Long Reply:

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

4.  RPC-over-RDMA in Operation

   Every RPC-over-RDMA version 1 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 describing chunks.  All
   RPC-over-RDMA header content is contained in the Transport stream;
   thus, it MUST be XDR encoded.

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

   The RPC-over-RDMA protocol passes RPC messages without regard to
   their type (CALL or REPLY).  Apart from restrictions imposed by ULBs,
   each endpoint of a connection MAY send RDMA_MSG or RDMA_NOMSG message
   header types at any time (subject to credit limits).

4.1.  XDR Protocol Definition

   This section contains a description of the core features of the RPC-
   over-RDMA version 1 protocol, expressed in the XDR language

   This description is provided in a way that makes it simple to extract
   into ready-to-compile form.  The reader can apply the following shell
   script to this document to produce a machine-readable XDR description
   of the RPC-over-RDMA version 1 protocol.


   grep '^ *///' | sed 's?^ /// ??' | sed 's?^ *///$??'


   That is, if the above script is stored in a file called "extract.sh"
   and this document is in a file called "spec.txt", then the reader can
   do the following to extract an XDR description file:


   sh extract.sh < spec.txt > rpcrdma_corev1.x


4.1.1.  Code Component License

   Code components extracted from this document must include the
   following license text.  When the extracted XDR code is combined with
   other complementary XDR code, which itself has an identical license,
   only a single copy of the license text need be preserved.


   /// /*
   ///  * Copyright (c) 2010-2017 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.
   ///  *
   ///  */


4.1.2.  RPC-over-RDMA Version 1 XDR

   XDR data items defined in this section encodes the Transport Header
   Stream in each RPC-over-RDMA version 1 message.  Comments identify
   items that cannot be changed in subsequent versions.


   /// /*
   ///  * Plain RDMA segment (Section 3.4.3)
   ///  */
   /// struct xdr_rdma_segment {
   ///    uint32 handle;           /* Registered memory handle */
   ///    uint32 length;           /* Length of the chunk in bytes */
   ///    uint64 offset;           /* Chunk virtual address or offset */
   /// };
   /// /*
   ///  * RDMA read segment (Section 3.4.5)
   ///  */
   /// struct xdr_read_chunk {
   ///    uint32 position;        /* Position in XDR stream */
   ///    struct xdr_rdma_segment target;
   /// };
   /// /*
   ///  * Read list (Section 4.3.1)
   ///  */
   /// struct xdr_read_list {
   ///         struct xdr_read_chunk entry;
   ///         struct xdr_read_list  *next;
   /// };
   /// /*
   ///  * Write chunk (Section 3.4.6)
   ///  */
   /// struct xdr_write_chunk {
   ///         struct xdr_rdma_segment target<>;
   /// };
   /// /*
   ///  * Write list (Section 4.3.2)
   ///  */
   /// struct xdr_write_list {
   ///         struct xdr_write_chunk entry;
   ///         struct xdr_write_list  *next;
   /// };

   /// /*
   ///  * Chunk lists (Section 4.3)
   ///  */
   /// struct rpc_rdma_header {
   ///    struct xdr_read_list   *rdma_reads;
   ///    struct xdr_write_list  *rdma_writes;
   ///    struct xdr_write_chunk *rdma_reply;
   ///    /* rpc body follows */
   /// };
   /// struct rpc_rdma_header_nomsg {
   ///    struct xdr_read_list   *rdma_reads;
   ///    struct xdr_write_list  *rdma_writes;
   ///    struct xdr_write_chunk *rdma_reply;
   /// };
   /// /* Not to be used */
   /// struct rpc_rdma_header_padded {
   ///    uint32                 rdma_align;
   ///    uint32                 rdma_thresh;
   ///    struct xdr_read_list   *rdma_reads;
   ///    struct xdr_write_list  *rdma_writes;
   ///    struct xdr_write_chunk *rdma_reply;
   ///    /* rpc body follows */
   /// };
   /// /*
   ///  * Error handling (Section 4.5)
   ///  */
   /// enum rpc_rdma_errcode {
   ///    ERR_VERS = 1,       /* Value fixed for all versions */
   ///    ERR_CHUNK = 2
   /// };
   /// /* Structure fixed for all versions */
   /// struct rpc_rdma_errvers {
   ///    uint32 rdma_vers_low;
   ///    uint32 rdma_vers_high;
   /// };
   /// union rpc_rdma_error switch (rpc_rdma_errcode err) {
   ///    case ERR_VERS:
   ///      rpc_rdma_errvers range;
   ///    case ERR_CHUNK:
   ///      void;
   /// };
   /// /*

   ///  * Procedures (Section 4.2.4)
   ///  */
   /// enum rdma_proc {
   ///    RDMA_MSG = 0,     /* Value fixed for all versions */
   ///    RDMA_NOMSG = 1,   /* Value fixed for all versions */
   ///    RDMA_MSGP = 2,    /* Not to be used */
   ///    RDMA_DONE = 3,    /* Not to be used */
   ///    RDMA_ERROR = 4    /* Value fixed for all versions */
   /// };
   /// /* The position of the proc discriminator field is
   ///  * fixed for all versions */
   /// union rdma_body switch (rdma_proc proc) {
   ///    case RDMA_MSG:
   ///      rpc_rdma_header rdma_msg;
   ///    case RDMA_NOMSG:
   ///      rpc_rdma_header_nomsg rdma_nomsg;
   ///    case RDMA_MSGP:   /* Not to be used */
   ///      rpc_rdma_header_padded rdma_msgp;
   ///    case RDMA_DONE:   /* Not to be used */
   ///      void;
   ///    case RDMA_ERROR:
   ///      rpc_rdma_error rdma_error;
   /// };
   /// /*
   ///  * Fixed header fields (Section 4.2)
   ///  */
   /// struct rdma_msg {
   ///    uint32    rdma_xid;      /* Position fixed for all versions */
   ///    uint32    rdma_vers;     /* Position fixed for all versions */
   ///    uint32    rdma_credit;   /* Position fixed for all versions */
   ///    rdma_body rdma_body;
   /// };


4.2.  Fixed Header Fields

   The RPC-over-RDMA header begins with four fixed 32-bit fields that
   control the RDMA interaction.

   The first three words are individual fields in the rdma_msg
   structure.  The fourth word is the first word of the rdma_body union,
   which acts as the discriminator for the switched union.  The contents
   of this field are described in Section 4.2.4.

   These four fields must remain with the same meanings and in the same
   positions in all subsequent versions of the RPC-over-RDMA protocol.

4.2.1.  Transaction ID (XID)

   The XID generated for the RPC Call and Reply messages.  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.

4.2.2.  Version Number

   For RPC-over-RDMA version 1, this field MUST contain the value one
   (1).  Rules regarding changes to this transport protocol version
   number can be found in Section 7.

4.2.3.  Credit Value

   When sent with an RPC Call message, the requested credit value is
   provided.  When sent with an RPC Reply message, the granted credit
   value is returned.  Further discussion of how the credit value is
   determined can be found in Section 3.3.

4.2.4.  Procedure Number

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

   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 explicit RDMA

   RDMA_MSGP = 2        is reserved.

   RDMA_DONE = 3        is reserved.

   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 reduced and is conveyed via separate 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 via separate 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 Requester MUST NOT send an RPC-over-RDMA header with the RDMA_ERROR
   procedure.  A Responder MUST silently discard RDMA_ERROR procedures.

   The Transport stream and Payload stream can be constructed in
   separate buffers.  However, the total length of the gathered buffers
   cannot exceed the inline threshold.

4.3.  Chunk Lists

   The chunk lists in an RPC-over-RDMA version 1 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 4.7 as an aid to understanding.

   Often, an RPC-over-RDMA message has no associated chunks.  In this
   case, the Read list, Write list, and Reply chunk are all marked "not

4.3.1.  Read List

   Each RDMA_MSG or RDMA_NOMSG procedure has one "Read list".  The Read
   list is a list of zero or more RDMA 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 pull from the Requester.  The Requester has reduced
   the data items in these chunks from the call's Payload stream.

   A Requester may transmit the Payload stream of an RPC Call message
   using a Position Zero Read chunk.  If the RPC Call message 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.

   Responders MUST leave the Read list empty in all replies.  Matching Read Chunks to Arguments

   When reducing a DDP-eligible argument data item, a Requester records
   the XDR stream offset of that data item in the Read chunk's Position
   field.  The Responder can then tell unambiguously where that chunk is
   to be reinserted into the received Payload stream to form a complete
   RPC Call message.

4.3.2.  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 plain segments; thus, the
   Write list is a list of counted arrays.

   If an RPC Reply message has no possible DDP-eligible result data
   items, the Requester leaves the Write list empty.  When a Requester
   provides a Write list, the Responder MUST push data corresponding to
   DDP-eligible result data items to Requester memory referenced in the
   Write list.  The Responder removes these data items from the reply's
   Payload stream.  Matching Write Chunks to Results

   A Requester constructs the Write list for an RPC transaction before
   the Responder has formulated its reply.  When there is only one DDP-
   eligible result data item, the Requester inserts only a single Write
   chunk in the Write list.  If the returned Write chunk is not an
   unused Write chunk, the Requester knows with certainty which result
   data item is contained in it.

   When a Requester has provided multiple Write chunks, the Responder
   fills in each Write chunk with one DDP-eligible result until there
   are either no more DDP-eligible results or no more Write chunks.

   The Requester might not be able to predict in advance 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 result data item that might be associated
   with each chunk in the Write list.

   As a Requester decodes a reply Payload stream, it is clear from the
   contents of the RPC Reply message which Write chunk contains which
   result data item.  Unused Write Chunks

   There are occasions when a Requester provides a non-empty Write chunk
   but the Responder is not able to use it.  For example, a ULP may
   define a union result where some arms of the union contain a DDP-
   eligible data item while other arms do not.  The Responder is
   required to use Requester-provided Write chunks in this case, but if
   the Responder returns a result that uses an arm of the union that has
   no DDP-eligible data item, that Write chunk remains unconsumed.

   If there is a subsequent DDP-eligible result data item in the RPC
   Reply message, it MUST be placed in that unconsumed Write chunk.
   Therefore, the Requester MUST provision each Write chunk so it can be
   filled with the largest DDP-eligible data item that can be placed in

   If this is the last or only Write chunk available and it remains
   unconsumed, the Responder MUST return this Write chunk as an unused
   Write chunk (see Section 3.4.6).  The Responder sets the segment
   count to a value matching the Requester-provided Write chunk, but
   returns only empty segments in that Write chunk.

   Unused Write chunks, or unused bytes in Write chunk segments, are
   returned to the RPC consumer as part of RPC completion.  Even if a
   Responder indicates that a Write chunk is not consumed, the Responder
   may have written data into one or more segments before choosing not
   to return that data item.  The Requester MUST NOT assume that the
   memory regions backing a Write chunk have not been modified.  Empty Write Chunks

   To force a Responder to return a DDP-eligible result inline, a
   Requester employs the following mechanism:

   o  When there is only one DDP-eligible result item in an RPC Reply
      message, the Requester provides an empty Write list.

   o  When there are multiple DDP-eligible result data items and a
      Requester prefers that a data item is returned inline, the
      Requester provides an empty Write chunk for that item (see
      Section 3.4.6).  The Responder MUST return the corresponding
      result data item inline and MUST return an empty Write chunk in
      that Write list position in the RPC Reply message.

   As always, a Requester and Responder must prepare for a Long Reply to
   be used if the resulting RPC Reply might be too large to be conveyed
   in an RDMA Send.

4.3.3.  Reply Chunk

   Each RDMA_MSG or RDMA_NOMSG procedure has one "Reply chunk" slot.  A
   Requester MUST provide a Reply chunk whenever the maximum possible
   size of the RPC Reply message's Transport and Payload streams is
   larger than the inline threshold for messages from Responder to
   Requester.  Otherwise, the Requester marks the Reply chunk as not

   If the Transport stream and Payload stream together are smaller than
   the reply inline threshold, the Responder MAY return the RPC Reply
   message as a Short message rather than using the Requester-provided
   Reply chunk.

   When a Requester provides a Reply chunk in an RPC Call message, the
   Responder MUST copy that chunk into the Transport header of the RPC
   Reply message.  As with Write chunks, the Responder modifies the
   copied Reply chunk in the RPC Reply message to reflect the actual
   amount of data that is being returned in the Reply chunk.

4.4.  Memory Registration

   The cost of registering and invalidating memory can be a significant
   proportion of the cost of an RPC-over-RDMA transaction.  Thus, an
   important implementation consideration is how to minimize
   registration activity without exposing system memory needlessly.

4.4.1.  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
   transaction.  They are registered and invalidated as needed, as part
   of each RPC transaction.

   The Requester endpoint must ensure that memory regions associated
   with each RPC transaction are protected from Responder access before
   allowing upper-layer access to the data contained in them.  Moreover,
   the Requester must not access these memory regions while the
   Responder has access to them.

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

4.4.2.  Communicating DDP-Eligibility

   The interface by which a ULP 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 ULBs, 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
   invalidation of memory "on the fly", or they may require using RDMA
   chunks in RPC 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, beyond
   constraints imposed by the ULB, is that the resulting RPC-over-RDMA
   protocol sent to the peer be valid for the upper layer.

4.4.3.  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 RDMA segment.

   While zero-based offset schemes are available in many RDMA
   implementations, their use by RPC requires individual registration of
   each memory region.  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

4.5.  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 layer.
   If an incoming RPC-over-RDMA message is not as long as a minimal size
   RPC-over-RDMA header (28 bytes), the receiver cannot trust the value
   of the XID field; therefore, it MUST silently discard the message
   before performing any parsing.  If other errors are detected in the
   RPC-over-RDMA header of an RPC Call message, a Responder MUST send an

   RDMA_ERROR message back to the Requester.  If errors are detected in
   the RPC-over-RDMA header of an RPC Reply message, a Requester MUST
   silently discard the message.

   To form an RDMA_ERROR procedure:

   o  The rdma_xid field MUST contain the same XID that was in the
      rdma_xid field in the failing request;

   o  The rdma_vers field MUST contain the same version that was in the
      rdma_vers field in the failing request;

   o  The rdma_proc field MUST contain the value RDMA_ERROR; and

   o  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.  Receipt of this
   procedure completes the RPC transaction associated with XID in the
   rdma_xid field.  A receiver MUST silently discard an RDMA_ERROR
   procedure that it cannot decode.

4.5.1.  Header Version Mismatch

   When a Responder detects an RPC-over-RDMA header version that it does
   not support (currently this document defines only version 1), it MUST
   reply with an RDMA_ERROR procedure and set the rdma_err value to
   ERR_VERS, also providing the low and high inclusive version numbers
   it does, in fact, support.

4.5.2.  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 where the Read list, Write list, and Reply chunk are marked
   not present; or the value of the rdma_xid field does not match the
   value 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 ERR_CHUNK.

   When a Responder receives a valid RPC-over-RDMA header but the
   Responder's ULP implementation cannot parse the RPC arguments in the
   RPC Call message, the Responder SHOULD return an RPC Reply message
   with status GARBAGE_ARGS, 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.

4.5.3.  Responder RDMA Operational Errors

   In RPC-over-RDMA version 1, the Responder initiates RDMA Read and
   Write operations that target the Requester's memory.  Problems might
   arise as the Responder attempts to use Requester-provided resources
   for RDMA operations.  For example:

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

   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 message, a Remote Access Error occurs.  A
      Responder might detect this problem before attempting to write
      past the end of the Reply chunk.

   RDMA operational errors 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 ERR_CHUNK to indicate that no RPC-level reply is
   possible for that XID.

4.5.4.  Other Operational Errors

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

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

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

4.5.5.  RDMA Transport Errors

   The RDMA connection and physical link provide some degree of error
   detection and retransmission.  iWARP's Marker PDU Aligned (MPA) layer
   (when used over TCP), the Stream Control Transmission Protocol
   (SCTP), as well as the InfiniBand [IBARCH] 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
   transport and recover via retransmission.  RPC recovery can handle
   complete loss and re-establishment of a transport connection.

   The details of reporting and recovery from RDMA link-layer errors are
   described in specific link-layer APIs and operational specifications
   and are outside the scope of this protocol specification.  See
   Section 8 for further discussion of the use of RPC-level integrity
   schemes to detect errors.

4.6.  Protocol Elements No Longer Supported

   The following protocol elements are no longer supported in RPC-over-
   RDMA version 1.  Related enum values and structure definitions remain
   in the RPC-over-RDMA version 1 protocol for backwards compatibility.

4.6.1.  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

   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, Responders SHOULD reply with an
   RDMA_ERROR procedure, setting the rdma_err field to ERR_CHUNK;
   Requesters MUST silently discard the message.

4.6.2.  RDMA_DONE

   Because no implementation of RPC-over-RDMA version 1 uses the Read-
   Read transfer model, there is never a need to send an RDMA_DONE

   Therefore, senders MUST NOT send RDMA_DONE messages.  Receivers MUST
   silently discard RDMA_DONE messages.

4.7.  XDR Examples

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

   A plain 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, plain segments appear as


   An RDMA read segment has an additional 32-bit position field (P).
   RDMA read segments appear as


   A Read chunk is a list of RDMA read segments.  Each RDMA read segment
   is preceded by a 32-bit word containing a one if a segment follows or
   a zero if there are no more segments in the list.  In XDR form, this
   would look like

      1 PHLOO 1 PHLOO 1 PHLOO 0

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

   The Read list is also a list of RDMA 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 plain 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


   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.  However, since it is an optional-
   data field, 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 two 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.

5.  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 means used
   to obtain this address, and to open an RDMA connection, is dependent
   on the type of RDMA transport and is the responsibility of each RPC
   protocol binding and its local implementation.

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

   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

   When mapped atop InfiniBand [IBARCH], which uses a service endpoint
   naming scheme based on a Group Identifier (GID), a translation MUST
   be employed.  One such translation is described in Annexes A3
   (Application Specific Identifiers), A4 (Sockets Direct Protocol
   (SDP)), and A11 (RDMA IP CM Service) of [IBARCH], 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

   o  One possibility is to have the Responder register its mapped IP
      port with the rpcbind service under the netid (or netids) 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  Alternately, 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, uses the same 20049 assignment as for

   Historically, different RPC protocols have taken different approaches
   to their port assignment.  Therefore, the specific method is left to
   each RPC-over-RDMA-enabled ULB and is not addressed in this document.

   In Section 9, 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 [IBARCH].  Additional RDMA-capable networks MAY define
   their own netids, or if they provide a port translation, they MAY
   share the one defined in this document.

6.  ULB Specifications

   An ULP is typically defined independently of any particular RPC
   transport.  An ULB (ULB) specification provides guidance that helps
   the ULP interoperate correctly and efficiently over a particular
   transport.  For RPC-over-RDMA version 1, a ULB may provide:

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

   o  Constraints on which upper-layer procedures may be reduced and on
      how many chunks may appear in a single RPC request

   o  A method for determining the maximum size of the reply Payload
      stream for all procedures in the ULP

   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 1 needs to have a ULB specification.

6.1.  DDP-Eligibility

   An ULB designates some XDR data items as eligible for DDP.  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

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

   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, a data copy operation would be required
      on the receiver to enable the message to be parsed correctly, or
      to enable the data item to be accessed.

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

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

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

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

   In general, a DDP-eligibility violation occurs when:

   o  A Requester reduces a non-DDP-eligible argument data item.  The
      Responder MUST NOT process this RPC Call message and MUST report
      the violation as described in Section 4.5.2.

   o  A Responder reduces a non-DDP-eligible result data item.  The
      Requester MUST terminate the pending RPC transaction and report an
      appropriate permanent error to the RPC consumer.

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

6.2.  Maximum Reply Size

   A Requester provides resources for both an RPC Call message and its
   matching RPC Reply message.  A Requester forms the RPC Call message
   itself; thus, the Requester can compute the exact resources needed.

   A Requester must allocate resources for the RPC 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 RPC Call
   message, a Requester must allocate reply resources based on the
   maximum possible size of the expected RPC Reply message.

   If there are procedures in the ULP for which there is no clear reply
   size maximum, the ULB needs to specify a dependable means for
   determining the maximum.

6.3.  Additional Considerations

   There may be other details provided in a ULB.

   o  An ULB may recommend inline threshold values or other transport-
      related parameters for RPC-over-RDMA version 1 connections bearing
      that ULP.

   o  An ULP may provide a means to communicate these transport-related
      parameters between peers.  Note that RPC-over-RDMA version 1 does
      not specify any mechanism for changing any transport-related
      parameter after a connection has been established.

   o  Multiple ULPs may share a single RPC-over-RDMA version 1
      connection when their ULBs allow the use of RPC-over-RDMA version
      1 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

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

6.4.  ULP 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 ULP may allow additional features to be
   specified after the original RPC Program specification was ratified.

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

7.  Protocol Extensibility

   The RPC-over-RDMA header format is specified using XDR, unlike the
   message header used with RPC-over-TCP.  To maintain a high degree of
   interoperability among implementations of RPC-over-RDMA, any change
   to this XDR requires a protocol version number change.  New versions
   of RPC-over-RDMA may be published as separate protocol specifications
   without updating this document.

   The first four fields in every 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 to enable
   implementations to detect protocol version mismatches.

   For version mismatches to be reported in a fashion that all future
   version implementations can reliably decode, the rdma_proc field must
   remain in a fixed place, the value of ERR_VERS must always remain the
   same, and the field placement in struct rpc_rdma_errvers must always
   remain the same.

7.1.  Conventional Extensions

   Introducing new capabilities to RPC-over-RDMA version 1 is limited to
   the adoption of conventions that make use of existing XDR (defined in
   this document) and allowed abstract RDMA operations.  Because no
   mechanism for detecting optional features exists in RPC-over-RDMA
   version 1, implementations must rely on ULPs to communicate the
   existence of such extensions.

   Such extensions must be specified in a Standards Track RFC with
   appropriate review by the NFSv4 Working Group and the IESG.  An
   example of a conventional extension to RPC-over-RDMA version 1 is the
   specification of backward direction message support to enable NFSv4.1
   callback operations, described in [RFC8167].

8.  Security Considerations

8.1.  Memory Protection

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

   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.

8.1.1.  Protection Domains

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

8.1.2.  Handle Predictability

   Unpredictable memory handles should be used for any operation
   requiring advertised memory regions.  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.

8.1.3.  Memory Protection

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

   Registered memory regions should be invalidated as soon as related
   RPC operations are complete.  Invalidation and DMA unmapping of
   memory regions should be complete before message integrity checking
   is done and before the RPC consumer is allowed to continue execution
   and use or alter the contents of a memory region.

   An RPC transaction on a Requester might be terminated before a reply
   arrives if the RPC consumer exits unexpectedly (for example, it is
   signaled or a segmentation fault occurs).  When an RPC terminates
   abnormally, memory regions associated with that RPC should be
   invalidated appropriately before the regions are released to be
   reused for other purposes on the Requester.

8.1.4.  Denial of Service

   A detailed discussion of denial-of-service exposures that can result
   from the use of an RDMA transport is found in Section 6.4 of

   A Responder is not obliged to pull Read chunks that are unreasonably
   large.  The Responder can use an RDMA_ERROR response to terminate
   RPCs with unreadable Read chunks.  If a Responder transmits more data
   than a Requester is prepared to receive in a Write or Reply chunk,
   the RDMA Network Interface Cards (RNICs) typically terminate the
   connection.  For further discussion, see Section 4.5.  Such repeated
   chunk errors can deny service to other users sharing the connection
   from the errant Requester.

   An RPC-over-RDMA transport implementation is not responsible for
   throttling the RPC request rate, other than to keep the number of
   concurrent RPC transactions at or under the number of credits granted
   per connection.  This is explained in Section 3.3.1.  A sender can
   trigger a self denial of service by exceeding the credit grant

   When an RPC has been canceled due to a signal or premature exit of an
   application process, a Requester may invalidate the RPC's Write and
   Reply chunks.  Invalidation prevents the subsequent arrival of the
   Responder's reply from altering the memory regions associated with
   those chunks after the memory has been reused.

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

8.2.  RPC Message Security

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

8.2.1.  RPC-over-RDMA Protection at Lower Layers

   For any RPC transport, utilizing RPCSEC_GSS integrity or privacy
   services has performance implications.  Protection below the RPC
   transport is often more appropriate in performance-sensitive
   deployments, especially if it, too, can be offloaded.  Certain
   configurations of IPsec can be co-located in RDMA hardware, for
   example, without change to RDMA consumers and little loss of data

   movement efficiency.  Such arrangements can also provide a higher
   degree of privacy by hiding endpoint identity or altering the
   frequency at which messages are exchanged, at a performance cost.

   The use of protection in a lower layer MAY be negotiated through the
   use of an RPCSEC_GSS security flavor defined in [RFC7861] in
   conjunction with the Channel Binding mechanism [RFC5056] and IPsec
   Channel Connection Latching [RFC5660].  Use of such mechanisms is
   REQUIRED where integrity or confidentiality is desired and where
   efficiency is required.

8.2.2.  RPCSEC_GSS on RPC-over-RDMA Transports

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

   RPCSEC_GSS extends the ONC RPC protocol [RFC5531] without changing
   the format of RPC messages.  By observing the conventions described
   in this section, an RPC-over-RDMA transport can convey RPCSEC_GSS-
   protected RPC messages interoperably.

   As part of the ONC RPC protocol, protocol elements of RPCSEC_GSS that
   appear in the Payload stream of an RPC-over-RDMA message (such as
   control messages exchanged as part of establishing or destroying a
   security context or data items that are part of RPCSEC_GSS
   authentication material) MUST NOT be reduced.  RPCSEC_GSS Context Negotiation

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

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

   However, RPCSEC_GSS authentication material appearing in an RPC
   message header can be larger than, say, an AUTH_SYS authenticator.
   In particular, when an RPCSEC_GSS pseudoflavor is in use, a Requester

   needs to accommodate a larger RPC credential when marshaling RPC Call
   messages and needs to provide for a maximum size RPCSEC_GSS verifier
   when allocating reply buffers and Reply chunks.

   RPC messages, and thus Payload streams, are made larger as a result.
   ULP operations that fit in a Short Message when a simpler form of
   authentication is in use might need to be reduced, or conveyed via a
   Long Message, when RPCSEC_GSS authentication is in use.  It is more
   likely that a Requester provides both a Read list and a Reply chunk
   in the same RPC-over-RDMA header to convey a Long Call and provision
   a receptacle for a Long Reply.  More frequent use of Long Messages
   can impact transport efficiency.  RPC-over-RDMA with RPCSEC_GSS Integrity or Privacy

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

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

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

   Therefore, a sender MUST NOT reduce a Payload stream when RPCSEC_GSS
   integrity or encryption services are in use.  Effectively, no data
   item is DDP-eligible in this situation, and Chunked Messages cannot
   be used.  In this mode, an RPC-over-RDMA transport operates in the
   same manner as a transport that does not support DDP.

   When an RPCSEC_GSS integrity or privacy service is in use, a
   Requester provides both a Read list and a Reply chunk in the same
   RPC-over-RDMA header to convey a Long Call and provision a receptacle
   for a Long Reply.  Protecting RPC-over-RDMA Transport Headers

   Like the base fields in an ONC RPC message (XID, call direction, and
   so on), the contents of an RPC-over-RDMA message's Transport stream
   are not protected by RPCSEC_GSS.  This exposes XIDs, connection
   credit limits, and chunk lists (but not the content of the data items
   they refer to) to malicious behavior, which could redirect data that
   is transferred by the RPC-over-RDMA message, result in spurious
   retransmits, or trigger connection loss.

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

   Encryption at lower layers, as described in Section 8.2.1, protects
   the content of the Transport stream.  To address attacks on RDMA
   protocols themselves, RDMA transport implementations should conform
   to [RFC5042].

9.  IANA Considerations

   A set of RPC netids for resolving RPC-over-RDMA services is specified
   by this document.  This is unchanged from [RFC5666].

   The RPC-over-RDMA 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"

   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].

   These netids MAY be used for any RDMA network that satisfies the
   requirements of Section 2.3.2 and that is able to identify service
   endpoints using IP port addressing, possibly through use of a
   translation service as described in Section 5.

   The use of the RPC-over-RDMA 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

   For example, the NFS/RDMA service defined in [RFC5667] has been
   assigned the port 20049 in the "Service Name and Transport Protocol
   Port Number Registry".  This is distinct from the port number defined
   for NFS on TCP, which is assigned the port 2049 in the same registry.
   NFS clients use the same RPC Program number for NFS (100003) when
   using either transport [RFC5531] (see the "Remote Procedure Call
   (RPC) Program Numbers" registry).

10.  References

10.1.  Normative References

   [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
              RFC 1833, DOI 10.17487/RFC1833, August 1995,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

   [RFC5665]  Eisler, M., "IANA Considerations for Remote Procedure Call
              (RPC) Network Identifiers and Universal Address Formats",
              RFC 5665, DOI 10.17487/RFC5665, January 2010,

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

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

10.2.  Informative References

   [IBARCH]   InfiniBand Trade Association, "InfiniBand Architecture
              Specification Volume 1", Release 1.3, March 2015,

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [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,

   [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,

   [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,

   [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,

   [RFC5662]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              External Data Representation Standard (XDR) Description",
              RFC 5662, DOI 10.17487/RFC5662, January 2010,

   [RFC5666]  Talpey, T. and B. Callaghan, "Remote Direct Memory Access
              Transport for Remote Procedure Call", RFC 5666,
              DOI 10.17487/RFC5666, January 2010,

   [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>.

   [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>.

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

Appendix A.  Changes from RFC 5666

A.1.  Changes to the Specification

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

   o  Section 2 has been expanded to introduce and explain key RPC
      [RFC5531], XDR [RFC4506], and RDMA [RFC5040] terminology.  These
      terms are now used consistently throughout the specification.

   o  Section 3 has been reorganized and split into subsections 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 optional Connection Configuration Protocol has never been
      implemented.  The specification of CCP has been deleted from this

   o  A section consolidating requirements for ULBs has been added.

   o  An XDR extraction mechanism is provided, along with full
      copyright, matching the approach used in [RFC5662].

   o  The "Security Considerations" section has been expanded to include
      a discussion of how RPC-over-RDMA security depends on features of
      the underlying RDMA transport.

   o  A subsection describing the use of RPCSEC_GSS [RFC7861] with RPC-
      over-RDMA version 1 has been added.

A.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.  As a result,
      implementers have chosen not to support it.  Removal of Read-Read
      simplifies explanatory text, and the RDMA_DONE procedure is no
      longer part of the protocol.

   o  The specification of RDMA_MSGP in [RFC5666] is not adequate,
      although some incomplete implementations exist.  Even if an
      adequate specification were provided and an implementation were
      produced, benefit for protocols such as NFSv4.0 [RFC7530] is
      doubtful.  Therefore, the RDMA_MSGP message type is no longer

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

   o  Specific requirements related to implicit XDR roundup and complex
      XDR data types have been added.

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

   o  Clear guidance about Send and Receive buffer sizes has been
      introduced.  This enables better decisions about when a Reply
      chunk must be provided.


   The editor gratefully acknowledges the work of Brent Callaghan and
   Tom Talpey on the original RPC-over-RDMA Version 1 specification

   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 7 and helped the authors understand the
   complexities of XDR extensibility.

   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, Greg
   Marsden, and Matt Benjamin for their support of this work.

   The extract.sh shell script and formatting conventions were first
   described by the authors of the NFSv4.1 XDR specification [RFC5662].

   Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
   Working Group Chair and Document Shepherd Spencer Shepler, and NFSV4
   Working Group Secretary Thomas Haynes for their support.

Authors' Addresses

   Charles Lever (editor)
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   United States of America

   Phone: +1 248 816 6463
   Email: chuck.lever@oracle.com

   William Allen Simpson
   Red Hat
   1384 Fontaine
   Madison Heights, MI  48071
   United States of America

   Email: william.allen.simpson@gmail.com

   Tom Talpey
   Microsoft Corp.
   One Microsoft Way
   Redmond, WA  98052
   United States of America

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


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