Network File System Version 4                                   C. Lever
Internet-Draft                                                    Oracle
Intended status: Standards Track                             6 July                        16 November 2021
Expires: 7 January 20 May 2022

Network File System (NFS) Upper-Layer Binding To RPC-Over-RDMA Version 2
                     draft-ietf-nfsv4-nfs-ulb-v2-05
                     draft-ietf-nfsv4-nfs-ulb-v2-06

Abstract

   This document specifies Upper-Layer Bindings of Network File System
   (NFS) protocol versions to RPC-over-RDMA version 2.

Note

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

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

   The source for this draft is maintained in GitHub.  Suggested

   Submit suggestions and changes
   can be submitted as pull requests at https://github.com/chucklever/
   i-d-nfs-ulb-v2.
   https://github.com/chucklever/i-d-nfs-ulb-v2.  Instructions are on
   that page as well. page.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Upper-Layer Binding for NFS Versions 2 and 3  . . . . . . . .   4
     3.1.  Reply Size Estimation . . . . . . . . . . . . . . . . . .   4
     3.2.  RPC Binding Considerations  . . . . . . . . . . . . . . .   5
     3.3.  Transport Considerations  . . . . . . . . . . . . . . . .   5
       3.3.1.  Keep-Alive  . . . . . . . . . . . . . . . . . . . . .   5
       3.3.2.  Replay Detection  . . . . . . . . . . . . . . . . . .   6
   4.  Upper-Layer Bindings for NFS Version 2 and 3 Auxiliary
           Protocols . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  MOUNT, NLM, and NSM Protocols . . . . . . . . . . . . . .   7
     4.2.  NFSACL Protocol . . . . . . . . . . . . . . . . . . . . .   7
   5.  Upper-Layer Binding For NFS Version 4 . . . . . . . . . . . .   7
     5.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  The NFSv4.2 READ_PLUS operation . . . . . . . . . . .   8
     5.2.  Reply Size Estimation . . . . . . . . . . . . . . . . . .   8
       5.2.1.  Reply Size Estimation for Minor Version 0 . . . . . .   9
       5.2.2.  Reply Size Estimation for Minor Version 1 and
               Newer . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  RPC Binding Considerations  . . . . . . . . . . . . . . .  10   9
     5.4.  NFS COMPOUND Requests . . . . . . . . . . . . . . . . . .   9
       5.4.1.  Multiple DDP-eligible Data Items  . . . . . . . . . .  10
       5.4.2.  Chunk List Complexity . . . . . . . . . . . . . . . .  10
       5.4.3.  NFS Version 4 COMPOUND Example  . . . . . . . . . . .  11
     5.5.  NFS Callback Requests . . . . . . . . . . . . . . . . . .  11
       5.5.1.  NFS Version 4.0 Callback  . . . . . . . . . . . . . .  12
       5.5.2.  NFS Version 4.1 Callback  . . . . . . . . . . . . . .  12
     5.6.  Session-Related Considerations  . . . . . . . . . . . . .  13  12
     5.7.  Transport Considerations  . . . . . . . . . . . . . . . .  13
       5.7.1.  Congestion Avoidance  . . . . . . . . . . . . . . . .  13
       5.7.2.  Retransmission and Keep-alive . . . . . . . . . . . .  14
   6.  Extending NFS Upper-Layer Bindings  . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17  16

   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18  17

1.  Introduction

   The RPC-over-RDMA version 2 transport may can employ direct data
   placement to convey data payloads associated with RPC transactions,
   as described in [I-D.ietf-nfsv4-rpcrdma-version-two].  As mandated by
   that document, RPC client and server implementations using RPC-over-RDMA RPC-over-
   RDMA version 2 must MUST agree in advance which XDR data items and RPC
   procedures are eligible to use for direct data placement (DDP) to ensure successful interoperation. (DDP).

   An Upper-Layer Binding specifies this agreement for one or more
   versions of one RPC program.  Other operational details, such as RPC
   binding assignments, pairing Write chunks with result data items, and
   reply size estimation, are also specified by such a Binding.

   This document contains material required of Upper-Layer Bindings, as
   specified in Appendix A of [I-D.ietf-nfsv4-rpcrdma-version-two], for
   the following NFS protocol versions:

   *  NFS version 2 [RFC1094]

   *  NFS version 3 [RFC1813]

   *  NFS version 4.0 [RFC7530]

   *  NFS version 4.1 [RFC8881]

   *  NFS version 4.2 [RFC7862]

   The current document also provides Upper-Layer Bindings for auxiliary
   protocols used with NFS versions 2 and 3 (see Section 4).

   This document assumes the reader is already familiar with concepts
   and terminology defined throughout
   [I-D.ietf-nfsv4-rpcrdma-version-two] and the documents it references.

2.  Requirements Language

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

3.  Upper-Layer Binding for NFS Versions 2 and 3

   The Upper-Layer Binding specification in this section applies to NFS
   version 2 [RFC1094] and NFS version 3 [RFC1813].  For brevity, in
   this document, a "Legacy NFS client" refers to an NFS client using
   version 2 or version 3 of the NFS RPC program (100003) to communicate
   with an NFS server.  Likewise, a "Legacy NFS server" is an NFS server
   communicating with clients using NFS version 2 or NFS version 3.

   The following XDR data items in NFS versions 2 and 3 are DDP-
   eligible:

   *  The opaque file data argument in the NFS WRITE procedure

   *  The pathname argument in the NFS SYMLINK procedure

   *  The opaque file data result in the NFS READ procedure

   *  The pathname result in the NFS READLINK procedure

   All other argument or result data items in NFS versions 2 and 3 are
   not DDP-eligible.

   Whether or not

   Regardless of whether an NFS operation is considered non-idempotent,
   a transport error might not indicate whether the server has processed
   the arguments of the RPC Call, Call or whether the server has accessed or
   modified client memory associated with that RPC.

3.1.  Reply Size Estimation

   During the construction of each RPC Call message, a Requester is
   responsible for allocating appropriate transport RDMA resources to receive the
   corresponding Reply message.  These resources must be capable of
   holding the entire Reply, therefore Reply.  Therefore the Requester needs to estimate
   the maximum possible size of the expected Reply message.

   *  In many cases,  Often, the expected Reply can fit in one or a few limited number of RDMA Send
      messages.  The Requester need not provision any RDMA
      resources, resources for
      the Reply, relying instead on message continuation to handle the
      entire Reply message.

   *  In cases where the Requester deems Upper Layer Binding permits direct data
      placement to be of the
      most efficient transfer mechanism, it provisions results (DDP), a Requester can provision Write
      chunks
      wherein the Responder can place to receive those results.  In these cases, the  The Requester must MUST reliably
      estimate the maximum size of each result
      that is to be placed in receive via a Write
      chunk.

   *  When the  A Requester that expects an especially large Reply message, it
      can provision a combination of a Reply chunk and Write chunks for
      result data items.  In such cases, the a large Reply message can provision a
      Reply chunk.  The Requester must MUST reliably estimate the maximum
      size of each result that is to be placed in a
      Write chunk and the maximum size of the remainder to be placed in payload received via the Reply chunk.

   *  If RDMA resources are not available to send a Reply, a Responder
      falls back to message continuation.

   A legacy correctly implemented Legacy NFS client needs to make every effort to avoid thus avoids retransmission
   of non-idempotent NFS requests due to underestimated improperly estimated Reply
   resources.  Thanks to the mechanism of message continuation in
   RPC-over-RDMA version 2, the need for such retransmission is greatly
   reduced.

3.2.  RPC Binding Considerations

   Legacy NFS servers traditionally typically listen for clients on UDP and TCP port
   2049.  Additionally, they register these ports with a local
   portmapper service [RFC1833].

   A Legacy NFS server supporting RPC-over-RDMA version 2 and
   registering itself with the RPC portmapper MAY choose an arbitrary
   port,
   port or MAY use the alternative well-known port number for its RPC-
   over-RDMA service (see Section 8).  The chosen port MAY be registered
   with the RPC portmapper using the netids assigned in Section 12 of
   [I-D.ietf-nfsv4-rpcrdma-version-two].

3.3.  Transport Considerations

3.3.1.  Keep-Alive

   Legacy NFS client implementations often can rely on a transport-layer connection keep-alive mechanism
   to detect when a legacy Legacy NFS server has become unresponsive.  When an
   NFS server is no longer responsive, client-
   side client-side keep-alive terminates
   the connection, which in turn triggers triggering reconnection and retransmission of
   outstanding RPC transactions.

3.3.1.  Keep-Alive

   Some RDMA transports (such as the Reliable Connected QP type on
   InfiniBand) have no keep-alive mechanism.  Without a disconnect or
   new RPC traffic, such connections can remain alive long after an NFS
   server has become unresponsive or unreachable.  Once an NFS client
   has consumed all available RPC-over-RDMA version 2 credits on that
   transport connection, it awaits a reply indefinitely before sending
   another RPC request.

   Legacy NFS clients SHOULD reserve one RPC-over-RDMA version 2 credit
   to use for periodic server or connection health assessment.  Either
   peer can use this credit to drive an RPC request on an otherwise idle
   connection, triggering either an affirmative server response or a
   connection termination.

3.3.2.  Replay Detection

   Like NFSv4.0, Legacy NFS servers typically employ request replay
   detection to reduce the risk of data and file namespace corruption
   that could result when an NFS client retransmits a non-idempotent NFS
   request.  A legacy Legacy NFS server can send a cached response when a
   replay is detected, rather than executing the request again.  Replay
   detection is not perfect, but it is usually adequate.

   For legacy Legacy NFS servers, replay detection commonly utilizes heuristic
   indicators such as the IP address of the NFS client, the source port
   of the connection, the transaction ID of the request, and the
   contents of the request's RPC and upper-layer protocol headers.  In
   short, replay detection is typically based on a connection tuple and
   the request's XID.  A legacy
   Legacy NFS client is careful to re-use the same source port, if practical, port when
   reconnecting so that legacy Legacy NFS servers are can better able to detect retransmissions. RPC
   retransmission.

   However, a legacy Legacy NFS client operating over an RDMA transport has no
   control over connection source ports.  It is almost certain that an
   RPC request that is retransmitted on a new connection can never be detected
   as a replay if the legacy receiving Legacy NFS server includes the
   connection source port in its replay detection heuristics.

   Therefore a legacy Legacy NFS server using an RDMA transport should never
   use a legacy NFS client connection's source port as part of its NFS request replay
   detection mechanism.

4.  Upper-Layer Bindings for NFS Version 2 and 3 Auxiliary Protocols

   Storage administrators typically deploy NFS versions 2 and 3 with
   several other protocols, sometimes referred to as called the "NFS auxiliary
   protocols."  These are distinct RPC programs that define procedures
   that are
   not part of the NFS RPC program (100003).  The Upper-Layer Bindings
   in this section apply to:

   *  Versions 2 and 3 of the MOUNT RPC program (100005) [RFC1813]

   *  Versions 1, 3, and 4 of the NLM RPC program (100021) [RFC1813]

   *  Version 1 of the NSM RPC program (100024), described in Chapter 11
      of [XNFS]

   *  Versions 2 and 3 of the NFSACL RPC program (100227).  The NFSACL
      program does not have a public definition.  In this  This document it is
      treated treats
      the NFSACL program as a de facto standard, as there are several
      interoperating implementations.

4.1.  MOUNT, NLM, and NSM Protocols

   Historically, NFS/RDMA implementations have chosen to convey conveyed the MOUNT, NLM,
   and NSM protocols via TCP.  A legacy Legacy NFS server implementation MUST
   provide support for these auxiliary protocols via TCP to
   enable interoperation of TCP.

   Moreover, there is little benefit from transporting these protocols when NFS/RDMA is in use.
   via RDMA.  Thus this document does not provide an Upper-Layer binding
   for them.

4.2.  NFSACL Protocol

   Often legacy

   Legacy NFS clients and servers that support the NFSACL RPC program convey NFSACL procedures on the same
   transport connection and port as the NFS RPC program (100003).
   Utilizing the same port obviates the need for separate a separate rpcbind
   query to discover server support for this RPC program.

   ACLs are typically small, but even large ACLs must be encoded and
   decoded to some degree before being made available to users. being stored in local
   filesystems.  Thus no data item in this Upper-Layer Protocol is DDP-eligible. DDP-
   eligible.

   For procedures whose replies do not include an ACL object, the size
   of a reply each Reply is determined directly from the NFSACL RPC program's
   XDR definition.  However, legacy client implementations should choose

   The NFSACL protocol does not provide a
   maximum size for ACLs based on internal limits, and can rely on
   message continuation mechanism to handle determine the a priori unknown
   size of large a received ACL
   objects in Replies. advance.  When preparing for responses that
   include ACLs, Legacy NFS clients estimate a maximum reply size based
   on limits within their local file systems.  If that estimation is
   inadequate, a Responder falls back to message continuation.

5.  Upper-Layer Binding For NFS Version 4

   The Upper-Layer Binding specification in this section applies to
   versions of the NFS RPC program defined in NFS version 4.0 [RFC7530] [RFC7530],
   NFS version 4.1 [RFC8881] [RFC8881], and NFS version 4.2 [RFC7862].

5.1.  DDP-Eligibility

   Only the following XDR data items in the COMPOUND procedure of all
   NFS version 4 minor versions are DDP-eligible:

   *  The opaque data field in the WRITE4args structure

   *  The linkdata field of the NF4LNK arm in the createtype4 union

   *  The opaque data field in the READ4resok structure
   *  The linkdata field in the READLINK4resok structure

5.1.1.  The NFSv4.2 READ_PLUS operation

   NFS version 4.2 introduces an enhanced READ operation called
   READ_PLUS [RFC7862].  READ_PLUS enables an NFS server to perform data
   reduction of READ results so that the compact
   returned READ data payloads.  No part of a READ_PLUS Reply is more
   compact. DDP-
   eligible.

   In a READ_PLUS result, returned file content appears as a list of one
   or more of the following items:

   *  Regular data content: content, the same as the result of a traditional READ
      operation.
      operation

   *  Unallocated space in a file: file, where no data has yet been written written, or
      previously-written data has been removed via a hole-punch
      operation.
      operation

   *  A counted pattern. pattern

   Upon receipt of a READ_PLUS result, an NFSv4.2 client expands the
   returned list into the its preferred local representation of the original file
   content.

   Before receiving that result, an NFSv4.2 client typically does not
   know is unaware of how the file's content is
   NFS server has organized on the NFS server. file content.  Thus it is not possible
   to predict the size or structure of a READ_PLUS Reply in advance.
   The use of direct data placement is therefore challenging.

   A READ_PLUS content list containing more than one segment of regular
   file data could be conveyed using multiple Write chunks, but only if
   the client knows in advance where those chunks appear in the Reply
   Payload stream.  Moreover,
   the usual benefits of hardware-assisted data placement are entirely waived
   lost if the client-side transport client must parse the result of each read READ I/O.

   Therefore this Upper Layer Binding does not make any element elements of an
   NFSv4.2 READ_PLUS Reply DDP-eligible.  Further, this Upper Layer
   Binding recommends that implementations NFS client implemenations avoid the use of using the
   READ_PLUS operation on NFS/RDMA mount points.

5.2.  Reply Size Estimation

   Within NFS version 4, there are certain variable-length result data
   items whose maximum size cannot be estimated by clients reliably
   because there is no protocol-specified size limit on these result
   arrays.  These include:

   *  The attrlist4 field

   *  Fields containing ACLs such as fattr4_acl, fattr4_dacl, and
      fattr4_sacl

   *  Fields in the fs_locations4 and fs_locations_info4 data structures

   *  Fields which pertain to pNFS layout metadata, such as loc_body,
      loh_body, da_addr_body, lou_body, lrf_body, fattr_layout_types,
      and fs_layout_types

5.2.1.  Reply Size Estimation for Minor Version 0

   The NFS version 4.0 protocol itself does not impose any bound on the
   size of NFS calls Calls or replies.

   Some of the data items enumerated in Section 5.2 (in particular, the
   items related to ACLs and fs_locations) Replies.

   Variable-length fattr4 attributes make it particularly difficult for
   clients to predict the maximum size of some NFS version 4.0 replies that interrogate
   variable-length fattr4 attributes. Replies.
   Client implementations might rely upon internal architectural limits
   to constrain the reply size, but such limits are not always guaranteed to be reliable.
   When an NFS version 4.0 client expects an especially sizeable fattr4
   result, cannot predict the size of a Reply, it
   can rely on message continuation or provision a Reply
   chunk to enable that server to return that result via explicit RDMA. a Reply under any
   circumstances.

5.2.2.  Reply Size Estimation for Minor Version 1 and Newer

   In NFS version 4.1 and newer minor versions, the csa_fore_chan_attrs
   argument of the CREATE_SESSION operation contains a
   ca_maxresponsesize field.  The value in this field can be taken as is the absolute
   maximum size of replies generated by an NFS version 4.1 server.

   An NFS version 4 client can use this value in cases where when it is not
   possible impossible to
   estimate a reply size upper bound precisely.  In practice, objects
   such as ACLs, named attributes, layout bodies, and security labels
   are much smaller than this maximum.

5.3.  RPC Binding Considerations

   NFS version 4 servers are required to listen on TCP port 2049, 2049 and are
   not required to register with an rpcbind service [RFC7530].
   Therefore, an NFS version 4 server supporting RPC-over-RDMA version 2
   MUST use the alternative well-known port number for its RPC-over-RDMA
   service (see defined in Section 8 Clients SHOULD connect to this well-known port
   without consulting the RPC portmapper (as for NFS version 4 on TCP
   transports). 8.

5.4.  NFS COMPOUND Requests
5.4.1.  Multiple DDP-eligible Data Items

   An NFS version 4 COMPOUND procedure can contain more than one
   operation that carries a DDP-eligible data item.  An NFS version 4
   client provides XDR Position values in each Read chunk to
   disambiguate determine
   which chunk is associated with which argument data item.  However,
   NFS version 4 server and client implementations must agree
   in advance on how to
   pair Write chunks with returned result data items.

   In the following lists, a

   A "READ operation" refers to any NFS version 4 operation that has with a DDP-eligible DDP-
   eligible result data item. item in the following lists.  An NFS version 4
   client applies the mechanism specified in Section 4.3.2 of
   [I-D.ietf-nfsv4-rpcrdma-version-two] to this class of operations as
   follows:

   *  If an NFS version 4 client wishes all DDP-eligible items in an NFS
      reply to be conveyed inline, it leaves the Write list empty.

   An NFS version 4 server acts as follows:

   *  The first READ operation MUST use the first chunk in the Write
      list MUST be used by the first READ
      operation in an NFS version 4 COMPOUND procedure.  The next Write
      chunk is used by READ
      operation uses the next READ operation, Write chunk, and so on.

   *  If an NFS version 4 client has provided a matching non-empty Write
      chunk, then the corresponding READ operation MUST return its DDP-
      eligible data item using that chunk.

   *  If an NFS version 4 client has provided an empty matching Write
      chunk, then the corresponding READ operation MUST return all of
      its result data items inline.

   *  If a READ operation returns a union arm which does not contain a
      DDP-eligible result, and the NFS version 4 client has provided a
      matching non-empty Write chunk, an NFS version 4 server MUST
      return an empty Write chunk in that Write list position.

   *  If there are more READ operations than Write chunks, then
      remaining NFS Read operations in an NFS version 4 COMPOUND that
      have no matching Write chunk MUST return their results inline.

5.4.2.  Chunk List Complexity

   By default, the RPC-over-RDMA version 2 protocol places limits on the number of
   chunks or segments that may appear in Read or Write lists (see
   Section 5.2 of [I-D.ietf-nfsv4-rpcrdma-version-two]).

   These implementation limits are especially important significant when Kerberos integrity
   or privacy is in use [RFC7861].  GSS services increase the size of
   credential material in RPC headers, potentially requiring the more
   frequent use of a Long message, which increases the complexity of chunk lists
   independent of the particular NFS version 4 COMPOUND being conveyed.

   In the absence of an explicit transport property exchange that alters
   these limits, less efficient Special Payload or Continued Payload
   messages.

   NFS version 4 clients SHOULD follow the prescriptions listed below when
   constructing RPC-over-RDMA version 2 messages. messages in the absence of an
   explicit transport property exchange that alters these limits.  NFS
   version 4 servers MUST accept and process all such requests.

   *  The Read list can contain either a Position-Zero Call chunk, no more than one
      Read chunk, or both a Call chunk and one Read chunk with a non-zero Position, or both. chunk.

   *  The Write list can contain no more than one Write chunk.

   NFS version 4 clients wishing to send more complex chunk lists can
   provide configuration interfaces
   use transport properties to bound the complexity of NFS version 4
   COMPOUNDs, limit the number of elements in scatter-gather operations,
   and avoid other sources of chunk overruns at the receiving peer.

   If an NFS version 4 server receives an RPC request via RPC-over-RDMA
   version 2 that it cannot process due to chunk list complexity limits,
   it SHOULD return one of the following responses to the client:

   *  A problem is detected by the transport layer while parsing the
      transport header in an RPC Call message.  The server responds with
      an RDMA2_ERROR message with the err field set to ERR_CHUNK.

   *  A problem is detected during XDR decoding of the RPC Call message
      while the RPC layer reassembles the call's XDR stream.  The server
      responds with an RPC reply with its "reply_stat" field set to
      MSG_ACCEPTED and its "accept_stat" field set to GARBAGE_ARGS.

   After receiving one of these errors, an NFS version 4 client SHOULD
   NOT retransmit the failing request, as the result would be the same
   error.  It SHOULD terminate the RPC transaction associated with the
   XID in the reply without further processing, and report an error to
   the RPC consumer.

5.4.3.  NFS Version 4 COMPOUND Example

   The following example shows a Write list with three Write chunks, A,
   B, and C.  The NFS version 4 server consumes the provided Write
   chunks by writing the results of the designated operations in the
   compound request (READ and READLINK) back to each chunk.

     Write list:

        A --> B --> C

     NFS version 4 COMPOUND request:

        PUTFH LOOKUP READ PUTFH LOOKUP READLINK PUTFH LOOKUP READ
                      |                   |                   |
                      v                   v                   v
                      A                   B                   C

   If the NFS version 4 client does not want to have the READLINK result
   returned via RDMA, it provides an empty Write chunk for buffer B to
   indicate that the READLINK result must be returned inline.

5.5.  NFS Callback Requests

   The NFS version 4 family of protocols support supports server-initiated
   callbacks to notify NFS version 4 clients of events such as recalled
   delegations.

5.5.1.  NFS Version 4.0 Callback

   An NFS version 4.0 client uses the SETCLIENTID operation to advertise for
   advertising the IP address, port, and netid of its NFS version 4.0
   callback service.  When an NFS version 4.0 server provides a
   backchannel service to an NFS version 4.0 client that uses RPC-over-RDMA RPC-over-
   RDMA version 2 for its forward channel, the server MUST advertise the
   backchannel service using either the "tcp" or "tcp6" netid.

   Because the NFSv4.0 backchannel does not operate on RPC-over-RDMA, no XDR
   data item in
   this document does not specify an Upper-Layer binding for the NFS version 4.0 callback NFSv4.0
   backchannel RPC program is DDP-
   eligible. program.

5.5.2.  NFS Version 4.1 Callback

   In NFS version 4.1 and newer minor versions, callback operations may
   appear on the same connection that is in use for NFS version 4
   forward channel client requests.  NFS version 4 clients and servers
   MUST use the mechanisms described in Section 4.5 of
   [I-D.ietf-nfsv4-rpcrdma-version-two] to convey backchannel operations
   on an RPC-over-RDMA version 2 transport.

   The csa_back_chan_attrs argument of the CREATE_SESSION operation
   contains a ca_maxresponsesize field.  The value in this field is the
   absolute maximum size of backchannel replies generated by a replying
   NFS version 4 client.

   There are no DDP-eligible data items in callback procedures defined
   in NFS version 4.1 or NFS version 4.2.  However, some callback
   operations, such as messages that convey device ID information, can
   be sizeable.  A sender can use Message Continuation or a Long Special
   Payload message in this situation.

   When an NFS version 4.1 client can support Long Special Payload Calls in
   its backchannel, it reports a backchannel ca_maxrequestsize that is
   larger than the connection's inline thresholds.  Otherwise, an NFS
   version 4 server MUST use only Short Simple Payload or Continued Payload
   messages to convey backchannel operations.

5.6.  Session-Related Considerations

   The presence of an NFS version 4 session (as defined in [RFC8881])
   does not effect affect the operation of RPC-over-RDMA version 2.  None of
   the operations introduced to support NFS sessions (e.g., the SEQUENCE
   operation) contain DDP-eligible data items.  There is no need to
   match the number of session slots with the number of available RPC-
   over-RDMA RPC-over-RDMA
   version 2 credits.

   However, there are a few new cases where an RPC transaction can fail.
   For example, a Requester might receive, in response to an RPC
   request, an RDMA2_ERROR message with a rdma_err value of ERR_CHUNK.
   RDMA2_ERR_BADXDR.  These situations are not different from existing
   RPC errors, which an NFS session implementation can already handle
   for other transport types.  Moreover, there might be no SEQUENCE
   result available to the Requester to distinguish whether failure
   occurred before or after the Responder executed the requested
   operations.

   When a transport error occurs (e.g., an RDMA2_ERROR type message is
   received), the Requester proceeds, as usual, to match the incoming
   XID value to a waiting RPC Call.  The Requester terminates the RPC
   transaction and reports the result status to the RPC consumer.  The
   Requester's session implementation then determines the session ID and
   slot for the failed request and performs slot recovery to make that
   slot usable again.  Otherwise, that slot could be is rendered permanently
   unavailable.

   When an NFS session is not present (for example, when NFS version 4.0
   is in use), a transport error does not indicate whether the server
   has processed the arguments of the RPC Call, or whether the server
   has accessed or modified client memory associated with that RPC.

5.7.  Transport Considerations

5.7.1.  Congestion Avoidance

   Section 3.1 of [RFC7530] states:

      Where an NFS version 4 implementation supports operation over the
      IP network protocol, the supported transport layer between NFS and
      IP MUST be an IETF standardized transport protocol that is
      specified to avoid network congestion; such transports include TCP
      and the Stream Control Transmission Protocol (SCTP).

   Section 2.9.1 of [RFC8881] further states:

      Even if NFS version 4.1 is used over a non-IP network protocol, it
      is RECOMMENDED that the transport support congestion control.

      It is permissible for a connectionless transport to be used under
      NFS version 4.1; however, reliable and in-order delivery of data
      combined with congestion control by the connectionless transport
      is REQUIRED.  As a consequence, UDP by itself MUST NOT be used as
      an NFS version 4.1 transport.

   RPC-over-RDMA version 2 utilizes only reliable, connection-oriented
   transports that guarantee in-order delivery, meeting all the above
   requirements for NFS version 4.0 and 4.1.  See Section 4.2.1 of
   [I-D.ietf-nfsv4-rpcrdma-version-two] for more details.

5.7.2.  Retransmission and Keep-alive

   NFS version 4 client implementations often rely on a transport-layer
   connection keep-alive mechanism to detect when an NFS version 4
   server has become unresponsive.  When an NFS server is no longer
   responsive, client-side keep-alive terminates the connection, which in turn
   triggers
   triggering reconnection and RPC retransmission.

   Some RDMA transports (such as the Reliable Connected QP type on
   InfiniBand) have no keep-alive mechanism.  Without a disconnect or
   new RPC traffic, such connections can remain alive long after an NFS
   server has become unresponsive.  Once an NFS client has consumed all
   available RPC-over-RDMA version 2 credits on that transport
   connection, it indefinitely awaits a reply before sending another RPC
   request.

   NFS version 4 clients peers SHOULD reserve one RPC-over-RDMA version 2 credit to use
   for periodic server or connection health assessment.  Either peer can
   use this credit to drive an RPC request on an otherwise idle
   connection, triggering either a quick affirmative server response or
   immediate connection termination.

   In addition to network partition and request loss scenarios, RPC-
   over-RDMA version 2 transport connections peers can be terminated terminate a connection when a Transport
   header is malformed, Reply messages exceed receive
   resources, malformed or when too many RPC-over-RDMA messages are sent at once.
   without a credit update.  In such cases:

   *  If a transport error occurs (e.g., an RDMA2_ERROR type message is
      received) just before the disconnect or instead of a disconnect,
      the Requester MUST respond to that error as prescribed by the
      specification of the RPC transport.  Then the NFS version 4 rules
      for handling retransmission apply.

   *  If there is a transport disconnect and the Responder has provided
      no other response for a request, then only the NFS version 4 rules
      for handling retransmission apply.

6.  Extending NFS Upper-Layer Bindings

   RPC programs such as NFS are required to must have an Upper-Layer Binding
   specification to interoperate operate on an RPC-over-RDMA version 2 transports transport
   [I-D.ietf-nfsv4-rpcrdma-version-two].  Via standards action, the
   Upper-Layer Binding specified in this document can be extended to
   cover versions of the NFS version 4 protocol specified after NFS
   version 4 minor version 2, or to cover separately published
   extensions to an existing NFS version 4 minor version, as described
   in [RFC8178].

7.  Security Considerations

   RPC-over-RDMA version 2 supports all RPC security models, including
   RPCSEC_GSS security and transport-level security [RFC7861].  The
   choice of what Direct Data Placement mechanism to convey RPC argument
   and results does not affect this since it changes only the method of
   data transfer.  Because the current document defines only the binding
   of the NFS protocols atop RPC-over-RDMA version 2
   [I-D.ietf-nfsv4-rpcrdma-version-two], all relevant security
   considerations are, therefore, described at that layer.

8.  IANA Considerations

   The use of direct data placement in NFS introduces a need for an
   additional port number assignment for networks that share traditional
   UDP and TCP port spaces with RDMA services.  The iWARP DDP protocol is such
   an example [RFC5040] [RFC5041].

   For this purpose, the current document specifies lists a set of transport
   protocol port number assignments.
   assignments that IANA has already assigned the following
   ports for NFS/RDMA in the IANA
   port registry, according to the guidelines described in [RFC6335].

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

   The author requests that IANA add the current document should be added as a reference
   for the existing nfsrdma port assignments.  The current  This document does not
   alter these assignments.

9.  References

9.1.  Normative References

   [I-D.ietf-nfsv4-rpcrdma-version-two]
              Lever, C. and D. Noveck, "RPC-over-RDMA Version 2
              Protocol", Work in Progress, Internet-Draft, draft-ietf-
              nfsv4-rpcrdma-version-two-05, 6 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-nfsv4-
              rpcrdma-version-two-05>.

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

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

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.
              <https://www.rfc-editor.org/rfc/rfc6335>.

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

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

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

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

   [RFC8881]  Noveck, D., Ed. and C. Lever, "Network File System (NFS)
              Version 4 Minor Version 1 Protocol", RFC 8881,
              DOI 10.17487/RFC8881, August 2020,
              <https://www.rfc-editor.org/info/rfc8881>.
              <https://www.rfc-editor.org/rfc/rfc8881>.

9.2.  Informative References

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

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

   [RFC5040]  Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
              Garcia, "A Remote Direct Memory Access Protocol
              Specification", RFC 5040, DOI 10.17487/RFC5040, October
              2007, <https://www.rfc-editor.org/info/rfc5040>.
              <https://www.rfc-editor.org/rfc/rfc1813>.

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

   [RFC8178]  Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
              <https://www.rfc-editor.org/info/rfc8178>.
              <https://www.rfc-editor.org/rfc/rfc8178>.

   [XNFS]     The Open Group, "Protocols for Interworking: XNFS, Version
              3W", February January 1998.

Acknowledgments

   Thanks to Tom Talpey, who contributed the text of Section 5.4.2.
   David Noveck contributed the text of Section 5.6 and Section 6.  The
   author also wishes to thank Bill Baker and Greg Marsden for their
   support of this work.

   Special thanks go to Transport Area Directors Zaheduzzaman Sarker,
   NFSV4 Working Group Chairs Brian Pawlowski, and David Noveck, and
   NFSV4 Working Group Secretary Thomas Haynes for their support.

Author's Address

   Charles Lever
   Oracle Corporation
   United States of America

   Email: chuck.lever@oracle.com