Network File System Version 4                              C. Lever, Ed.
Internet-Draft                                                    Oracle
Obsoletes: 5667 (if approved)                         September 28, 2016                           January 20, 2017
Intended status: Standards Track
Expires: April 1, July 24, 2017

     Network File System (NFS) Upper Layer Binding To RPC-Over-RDMA
                     draft-ietf-nfsv4-rfc5667bis-03
                     draft-ietf-nfsv4-rfc5667bis-04

Abstract

   This document specifies Upper Layer Bindings of Network File System
   (NFS) protocol versions to RPC-over-RDMA transports.  These bindings RPC-over-RDMA.  Upper Layer Bindings are
   required to enable RPC-based protocols protocols, such as NFS NFS, to use direct
   data placement Direct
   Data Placement on RPC-over-RDMA transports. RPC-over-RDMA.  This document obsoletes RFC 5667.

Requirements Language

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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conveying NFS Operations On RPC-Over-RDMA Transports . . . . . . . . . .   3
   3.  Upper Layer Binding For NFS Versions 2 And 3 Upper Layer Binding  .  . . . . . . . . .   4   5
   4.  NFS Version 4  Upper Layer Binding For NFS Version 4 . . . . . . . . . . . . . .   6   7
   5.  Extending NFS Upper Layer Bindings  . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14  15
   Appendix A.  Changes Since RFC 5667 . . . . . . . . . . . . . . .  15  16
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  16  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17  18

1.  Introduction

   An RPC-over-RDMA transport, such as the one defined in
   [I-D.ietf-nfsv4-rfc5666bis], may employ direct data placement to
   convey data payloads associated with RPC transactions.  Each RPC-
   over-RDMA transport header conveys lists of memory locations
   corresponding to XDR data items defined in an Upper Layer Protocol
   (such as NFS).  To facilitate enable
   successful interoperation, RPC client and server implementations must
   agree in advance on what as to which XDR data items in which what particular RPC procedures
   are eligible for direct data placement (DDP).

   This document contains material required of Upper Layer Bindings, as
   specified in [I-D.ietf-nfsv4-rfc5666bis], for the following NFS
   protocol versions:

   o  NFS Version 2 [RFC1094]
   o  NFS Version 3 [RFC1813]

   o  NFS Version 4.0 [RFC7530]

   o  NFS Version 4.1 [RFC5661]

   o  NFS Version 4.2 [I-D.ietf-nfsv4-minorversion2] [RFC7862]

   Upper Layer Bindings specified in this document apply to all versions
   of RPC-over-RDMA.

2.  Conveying NFS Operations On RPC-Over-RDMA Transports

   Definitions of terminology and a general discussion of how RPC-over-
   RDMA is used to convey RPC transactions can be found in
   [I-D.ietf-nfsv4-rfc5666bis].  In this section, these general
   principals
   principles are applied to in the specifics context of the conveying NFS procedures on
   RPC-over-RDMA.  Some issues common to all NFS protocol. protocol versions are
   introduced.

2.1.  Use Of  The Read List

   The Read list in each RPC-over-RDMA transport header represents a set
   of memory regions containing DDP-eligible NFS argument data.  Large
   data items, such as the data payload of an NFS version 3 WRITE
   procedure, are can be referenced by the Read list.  The NFS server pulls
   such payloads from the client and places them directly into its own
   memory.

   Exactly which XDR unmarshaling code on the NFS server identifies the correspondence
   between Read chunks and particular NFS arguments via the chunk
   Position value encoded data items may be conveyed in each Read segment. this fashion is
   detailed later in this document.

2.2.  Use Of  The Write List

   The Write list in each RPC-over-RDMA transport header represents a
   set of memory regions that can receive DDP-eligible NFS result data.
   Large data items, such as the payload of an NFS version 3 READ
   procedure, are can be referenced by the Write list.  The NFS server
   pushes such payloads to the client, placing them directly into the
   client's memory.

   Each Write chunk corresponds to a specific XDR data item in an NFS
   reply.  This document specifies how NFS client and server
   implementations identify the correspondence between Write chunks and
   XDR results.

2.2.1.  Empty Write Chunks

   Section 4.4.6.2 of [I-D.ietf-nfsv4-rfc5666bis] defines the concept of
   unused Write chunks.  An unused Write chunk

   Exactly which XDR data items may be conveyed in this fashion is a Write chunk with
   either zero segments or where all segments
   detailed later in the Write chunk have
   zero length.  In this document these are referred to as "empty" Write
   chunks.  A "non-empty" Write chunk has at least one segment of non-
   zero length.

   An NFS client might wish an NFS server to return a DDP-eligible
   result inline.  If there is only one DDP-eligible result item in the
   reply, the NFS client simply specifies an empty Write list to force
   the NFS server to return that result inline.  If there are multiple
   DDP-eligible results, the NFS client specifies empty Write chunks for
   each DDP-eligible data item that it wishes to be returned inline.

   An NFS server might encounter an XDR union result where there are
   arms that have a DDP-eligible result, and arms that do not.  If the
   NFS client has provided a non-empty Write chunk that matches with a
   DDP-eligible result, but the response does not contain that result,
   the NFS server MUST return an empty Write chunk in that position in
   the Write list. document.

2.3.  Use Of  Long Calls And Replies

   Small RPC messages are conveyed using RDMA Send operations which are
   of limited size.  If an NFS request is too large to be conveyed
   within the NFS server's responder inline threshold, and there are no
   DDP-eligible data items that can be removed, an NFS client must send
   the request using in the form of a Long Call.  The entire NFS request is
   sent in a special Read chunk called a Position-Zero Position Zero Read chunk.

   If an NFS client predicts determines that the maximum size of an NFS reply
   could be too large to be conveyed within it's own responder inline
   threshold, it provides a Reply chunk in the RPC-over-RDMA transport
   header conveying the NFS request.  The server places the entire NFS
   reply in the Reply chunk.

   When the RPC authentication flavor requires that DDP-eligible data
   items are never removed from RPC messages, an NFS client can provide
   both a Position Zero Read chunk and a Reply chunk for the same RPC.

   These special chunks are described discussed in more further detail in
   [I-D.ietf-nfsv4-rfc5666bis].

2.4.  Scatter-Gather Considerations

   A chunk comprises typically corresponds to exactly one XDR data item.  Each
   Read chunk is represented as a list of segments at the same XDR
   Position.  Each Write chunk is represented as an array of segments.
   An NFS client thus has the flexibility to advertise a set of
   discontiguous memory regions in which to send or receive convey a single DDP-eligible
   XDR data item.

3.

2.5.  DDP Eligibility Violations

   To report a DDP-eligibity violation, an NFS Versions 2 And 3 Upper Layer Binding server MUST return one
   of:

   o  An NFS version 2 or version 3 client MAY send a single Read chunk RPC-over-RDMA message of type RDMA_ERROR, with the rdma_xid
      field set to
   supply the opaque file data for an XID of the matching NFS WRITE procedure, or Call, and the
   pathname for an NFS SYMLINK procedure.  For these procedures, NFS
   version 2 or 3 servers MUST ignore Read chunks beyond the first in
   the Read list.  For all other NFS procedures, NFS version 2 rdma_error
      field set to ERR_CHUNK; or 3
   servers MUST ignore Read chunks that have a non-zero value in their
   Position fields.

   Similarly,

   o  An RPC message (via an NFS version 2 or version 3 client MAY provide a single
   Write chunk RDMA_MSG message) with the xid field set to receive either
      the opaque file data from an NFS READ
   procedure, or XID of the pathname from an NFS READLINK procedure.  For these
   procedures, matching NFS version 2 or 3 servers MUST ignore Write chunks
   beyond Call, the first in mtype field set to REPLY,
      the Write list.  For all other NFS procedures,
   NFS version 2 or 3 servers MUST ignore stat field set to MSG_ACCEPTED, and the Write list.

   There are no accept_stat field set
      to GARBAGE_ARGS.

   Subsequent sections of this document describe further considerations
   particular to specific NFS version 2 protocols or 3 procedures that have DDP-eligible
   data items in both their procedures.

2.6.  Reply Size Estimation

   During the construction of each RPC Call and Reply.  However, when message, an NFS
   version 2 or version 3 client sends a Long Call or Reply, it MAY
   provide a combination of a Read list, a Write list, and/or a is
   responsible for allocating appropriate resources for receiving the
   matching Reply
   chunk message.  A Reply buffer overrun can result in
   corruption of the same RPC-over-RDMA header.

   If an NFS version 2 Reply message or version 3 client has not provided termination of the transport
   connection.  Therefore reliable reply size estimation is necessary to
   ensure successful interoperation.

   In many cases the Upper Layer Protocol's XDR definition provides
   enough bytes
   in a Read list information to match enable the client to make a reliable prediction
   of the maximum size of a DDP-eligible NFS argument data
   item, or if an NFS version 2 or version 3 client has not provided
   enough Write list resources to handle an NFS READ or READLINK reply,
   or if the client has not provided a large enough expected Reply chunk to
   convey an NFS reply, the server MUST return one of:

   o  An RPC-over-RDMA message of type RDMA_ERROR, with message.  If there are
   variable-size data items in the rdma_xid
      field set to result, the XID maximum size of the matching NFS Call, and the rdma_error
      field set to ERR_CHUNK; or

   o  An RPC
   Reply message (via an RDMA_MSG message) with the xid field set to
      the XID can be reliably estimated in most cases:

   o  The client requests only a specific portion of an object (for
      example, using the matching NFS Call, the mtype field set to REPLY,
      the stat field set to MSG_ACCEPTED, "count" and the accept_stat field set
      to GARBAGE_ARGS.

   These replies do not give any indication to NFS version 2 or version
   3 clients of whether "offset" fields in an NFS version 2 or 3 server READ).

   o  The client has processed already cached the
   arguments size of the RPC Call, or whether the NFS version 2 or 3 server
   has accessed NFS client memory associated with that RPC. whole object it is
      about to request (say, via a previous NFS version 2 or version 3 clients already successfully estimate GETATTR request).

   It is occasionally not possible to determine the maximum reply Reply
   message size of each operation in order to provide an adequate
   set of buffers to receive each NFS reply.  An based solely on the above criteria.  NFS version 2 or
   version 3 client provides a Reply chunk when
   implementers can choose to provide the maximum largest possible
   reply size is larger than Reply buffer
   in those cases, based on, for instance, the client's responder inline threshold.

3.1.  Auxiliary Protocols

   NFS versions 2 and 3 are typically deployed with several other
   protocols, referred to as "auxiliary" protocols.  These are separate
   RPC protcols which handle operations that are not part of the main largest possible NFS protocol.  These include the MOUNT and NLM protocols, introduced
   in an appendix of [RFC1813]; the NSM protocol, described in Chapter
   11 of [NSM]; and the NFSACL protocol, READ
   or WRITE payload (which is negotiated at mount time).

   In rare cases, a client may encounter a reply for which does not have no a public
   definition.  However NFSACL priori
   determination of reply size bound is treated as possible.  The client SHOULD
   expect a de facto standard and
   there are several interoperating implementations.

   RPC-over-RDMA considers these as individual Upper Layer Protocols
   [I-D.ietf-nfsv4-rfc5666bis].  Therefore transport error to operate on an RPC-over-
   RDMA transport, an Upper Layer Binding indicate that it must be provided for each of
   these.

   Typically MOUNT, NLM, and NSM are conveyed via TCP rather than RPC-
   over-RDMA.  Note either terminate
   that only metadata is conveyed in these protocols,
   thus direct data placement is never necessary, and the size of RPC
   messages is uniformly small. transaction, or retry it with a larger Reply chunk.

   The maximum size use of replies is easily
   determined by examining NFS COMPOUND operations raises the XDR definitions possibility of these protocols.

   Implementations non-
   idempotent requests that support the NFSACL protocol typically send
   NFSACL procedures on the same connection as combine a non-idempotent operation with an
   operation whose reply size is uncertain.  This causes potential
   difficulties with retrying the main transaction.  Note however that many
   operations normally considered non-idempotent (e.g WRITE, SETATTR)
   are actually idempotent.  Truly non-idempotent operations are quite
   unusual in COMPOUNDs that include operations with uncertain reply
   sizes.

3.  Upper Layer Binding For NFS protocol.
   Thus NFSACL does require an Versions 2 And 3

   This Upper Layer Binding.

   No data item Binding specification applies to NFS Version 2
   [RFC1094] and NFS Version 3 [RFC1813].  For brevity, in this protocol is DDP-eligible.  There is no protocol
   size limit for NFS version 3 ACL objects.  The client can have some
   difficulty ascertaining the size of ACLs to be read from servers.
   Practically speaking, ACLs are not large (less than 4KB in most
   cases), but a large Reply chunk may be provided when the client is in
   doubt.  The usual rules apply to the use of Long Messages when the
   size of an NFSACL RPC exceeds section
   a connection's inline thresholds.

4. "legacy NFS Version 4 Upper Layer Binding

   This specification applies client" refers to an NFS Version 4.0 [RFC7530], client using NFS Version
   4.1 [RFC5661], and version 2 or
   NFS Version 4.2 [I-D.ietf-nfsv4-minorversion2].
   It also applies version 3 to the callback protocols associated communicate with each of
   these minor versions defined in the same documents.

4.1.  DDP-Eligibility

   For each WRITE operation in an NFS version 4 COMPOUND procedure, server.  Likewise, a "legacy
   NFS server" is an NFS server communicating with clients using NFS
   version 4 client MAY provide a single Read chunk to supply the 2 or NFS version 3.

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

   o  The opaque file data argument.  For each CREATE(NF4LNK) operation argument in an
   NFS version 4 COMPOUND procedure, An NFS version 4 client MAY provide
   a single Read chunk to supply the NFS WRITE procedure

   o  The pathname argument.

   Similarly, for each READ operation argument in an NFS version 4 COMPOUND
   procedure, an NFS version 4 client MAY provide a single Write chunk
   to receive the NFS SYMLINK procedure

   o  The opaque file data argument.  For each READ_PLUS
   operation result in an the NFS version 4 COMPOUND procedure, an READ procedure

   o  The pathname result in the NFS version 4
   client MAY provide a single Write chunk to receive NFS4_CONTENT_DATA.
   For each READLINK operation procedure

   All other argument or result data items in an NFS version 4 COMPOUND procedure,
   an NFS version 4 client MAY provide versions 2 and 3 are
   not DDP-eligible.

   A legacy server's response to a single Write chunk DDP-eligibility violation (described
   in Section 2.5) does not give an indication to receive legacy clients of
   whether the pathname argument.

   An server has processed the arguments of the RPC Call, or
   whether the server has accessed or modified client memory associated
   with that RPC.

   A legacy NFS version 4 client MUST NOT determines the maximum reply size for each
   operation using the basic criteria outlined in Section 2.6.  Such
   clients provide a Read or Write Reply chunk that
   corresponds with when the maximum possible reply size,
   exclusive of any other XDR data item in any other NFS version 4
   operation in an NFS version 4 COMPOUND procedure, or in an NFS
   version 4 NULL procedure.

   It items represented by Write chunks, is possible for larger
   than the client's responder inline threshold.

3.1.  Auxiliary Protocols

   NFS version 4 COMPOUND procedures versions 2 and 3 are typically deployed with several other
   protocols, sometimes referred to use both as "NFS auxiliary protocols."  These
   are separate RPC programs that define procedures which are not part
   of the
   Read list and Write list simultaneously.  An NFS version 4 client MAY
   provide a Read list 2 or version 3 RPC programs.  These include:

   o  The MOUNT and a Write list NLM protocols, introduced in the same transaction if it is
   sending a Long Call or Reply.

   If an NFS version 4 client has not provided enough bytes appendix of
      [RFC1813]

   o  The NSM protocol, described in a Read
   list to match the size Chapter 11 of a DDP-eligible NFS argument data item, or
   if an NFS version 4 client has [NSM]

   o  The NFSACL protocol, which does not provided enough Write list
   resources to handle have a WRITE or READLINK operation, or if the client
   has not provided public definition
      (NFSACL here is treated as a large enough Reply chunk to convey de facto standard as there are
      several interoperating implementations).

   RPC-over-RDMA considers these programs as distinct Upper Layer
   Protocols [I-D.ietf-nfsv4-rfc5666bis].  To enable the use of these
   ULPs on an RPC-over-RDMA transport, an Upper Layer Binding
   specification is provided here for each.

3.1.1.  MOUNT, NLM, And NSM Protocols

   Typically MOUNT, NLM, and NSM are conveyed via TCP, even in
   deployments where NFS reply,
   the operations on RPC-over-RDMA.  When a legacy
   server MUST return one of:

   o  An RPC-over-RDMA message of type RDMA_ERROR, with the rdma_xid
      field set to supports these programs on RPC-over-RDMA, it advertises the XID of
   port address via the matching NFS Call, usual rpcbind service [RFC1833].

   No operation in these protocols conveys a significant data payload,
   and the rdma_error
      field set to ERR_CHUNK; or

   o  An size of RPC message (via messages in these protocols is uniformly small.
   Therefore, no XDR data items in these protocols are DDP-eligible.
   The largest variable-length XDR data item is an RDMA_MSG message) with xdr_netobj.  In most
   implementations this data item is not larger than 1024 bytes, making
   reliable reply size estimation straightforward using the xid field set to criteria
   outlined in Section 2.6.

3.1.2.  NFSACL Protocol

   Legacy clients and servers that support the XID of NFSACL RPC program
   typically convey NFSACL procedures on the same connection as the matching NFS Call,
   RPC program.  This obviates the stat field set need for separate rpcbind queries to
      MSG_ACCEPTED,
   discover server support for this RPC program.

   ACLs are typically small, but even large ACLs must be encoded and the accept_stat field set
   decoded to GARBAGE_ARGS.

   Such error some degree.  Thus no data item in this Upper Layer
   Protocol is DDP-eligible.

   For procedures whose replies are permanent errors, and constitute both
   completion of do not include an ACL object, the RPC transaction, and size
   of a valid server response.  It reply is not necessary determined directly from the NFSACL program's XDR
   definition.

   There is no protocol-wide size limit for an NFS version 4 3 ACLs, and
   there is no mechanism in either the NFSACL or NFS programs for a
   legacy client to ascertain the largest ACL a legacy server can store.
   Legacy client implementations should choose a maximum size for ACLs
   based on their own internal limits.  A recommended lower bound for
   this maximum is 32,768 bytes, though a larger Reply chunk (up to drop the transport
   connection
   negotiated rsize setting) can be provided.

4.  Upper Layer Binding For NFS Version 4

   This Upper Layer Binding specification applies to all protocols
   defined in this case.

4.1.1.  Session-Related Considerations

   In most cases, NFS Version 4.0 [RFC7530], NFS Version 4.1 [RFC5661], and
   NFS Version 4.2 [RFC7862].

4.1.  DDP-Eligibility

   Only the presence following XDR data items in the COMPOUND procedure of an all
   NFS session [RFC5661] has no effect
   on version 4 minor versions are DDP-eligible:

   o  The opaque data field in the operation WRITE4args structure

   o  The linkdata field of RPC-over-RDMA.  None the NF4LNK arm in the createtype4 union

   o  The opaque data field in the READ4resok structure

   o  The linkdata field in the READLINK4resok structure

   o  In minor version 2 and newer, the rpc_data field of the operations introduced
   to support
      read_plus_content union (further restrictions on the use of this
      data item follow below).

4.1.1.  READ_PLUS Replies

   The NFS sessions contain DDP-eligible version 4.2 READ_PLUS operation returns a complex data items.  There type
   [RFC7862].  The rpr_contents field in the result of this operation is no
   need
   an array of read_plus_content unions, one arm of which contains an
   opaque byte stream (d_data).

   The size of d_data is limited to match the number value of session slots with the rpa_count field,
   but the protocol does not bound the number of
   available RPC-over-RDMA credits.

   However, there are some rare error conditions elements which require special
   handling when an NFS session is operating on an RPC-over-RDMA
   transport.  For example, a requester might receive, can be
   returned in response the rpr_contents array.  In order to an
   RPC request, an RDMA_ERROR message with an rdma_err value make the size of
   ERR_CHUNK, or an RDMA_MSG containing an RPC_GARBAGEARGS reply.
   Within RPC-over-RDMA Version One, this class
   READ_PLUS replies predictable by NFS version 4.2 clients, the
   following restrictions are placed on the use of error can be
   generated for two different reasons:

   o  There was an XDR error detected parsing the READ_PLUS
   operation on RPC-over-RDMA headers. transports:

   o  There was an error sending the response, because,  An NFS version 4.2 client MUST NOT provide more than one Write
      chunk for example, any READ_PLUS operation.  When providing a
      necessary reply Write chunk was not provided or the one provided
      for a READ_PLUS operation, an NFS version 4.2 client MUST provide
      a Write chunk that is of
      insufficient length.

   These two situations, which arise only due either empty (which forces all result data
      items for this operation to incorrect
   implementations, have different implications with regard be returned inline) or large enough to Exactly-
   Once Semantics.  An XDR error
      receive rpa_count bytes in decoding the request precludes the
   execution of the request on the responder, but failure to send a
   reply indicates that some or all single element of the operations were executed.

   In both instances, rpr_contents
      array.

   o  If the Write chunk provided for a READ_PLUS operation by an NFS
      version 4.2 client SHOULD NOT retry the operation.  A
   retry is liable to result in not empty, an NFS version 4.2 server MUST
      use that chunk for the same sort first element of error seen previously.
   Instead, it is best to consider the operation as completed
   unsuccessfully and report rpr_contents array
      that has an error to the consumer who requested the
   RPC.

   In addition, within the error response, the requester does not have rpc_data arm.

   o  An NFS version 4.2 server MUST NOT return more than two elements
      in the result rpr_contents array of the execution any READ_PLUS operation.  It returns
      as much of the SEQUENCE operation, which
   identifies the session, slot, and sequence id for requested byte range as it can fit within these two
      elements.  If the request which NFS version 4.2 server has failed.  The xid associated with the request, obtained from the
   rdma_xid field of not asserted rpr_eof
      in the RDMA_ERROR or RDMA_MSG message, must be used to
   determine reply, the session and slot NFS version 4.2 client SHOULD send additional
      READ_PLUS requests for the request which failed, and the
   slot must be properly retired.  If this is not done, the slot could
   be rendered permanently unavailable. any remaining bytes.

4.2.  NFS Version 4 Reply Size Estimation

   An NFS version 4 client provides a Reply chunk when the maximum
   possible reply size is larger than the client's responder inline
   threshold.  NFS version 4 clients already successfully estimate the
   maximum reply size of most operations in order to provide an adequate
   set of buffers to receive each NFS reply.

   There are certain NFS version 4 data items whose size cannot be
   estimated by clients reliably, however, because there is no protocol-
   specified size limit on these structures.  These include but are not
   limited to opaque types, such as: include:

   o  The attrlist4 field

   o  Fields containing ACLs such as fattr4_acl, fattr4_dacl,
      fattr4_sacl

   o  Fields in the fs_locations4 and fs_locations_info4 data structures

   o  Opaque 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,

   In NFS version 4.1 and later minor versions, the csa_fore_chan_attrs
   argument of the CREATE_SESSION operation contains a
   ca_maxresponsesize field.

4.2.1.  Reply Size Estimation For Minor Version 0

   The value items enumerated above in this field can be taken as Section 4.2 make it difficult to
   predict the absolute maximum size of GETATTR replies generated by a replying NFS
   version 4 server.  This value can be used that interrogate
   variable-length attributes.  As discussed in cases where it is not
   possible to estimate a reply Section 2.6, client
   implementations can rely on their own internal architectural limits
   to bound the reply size, but such limits are not guaranteed to be
   reliable.

   If a client implementation is equipped to recognize that a transport
   error could mean that it provisioned an inadequately sized Reply
   chunk, it can retry the operation with a larger Reply chunk.
   Otherwise, the client must terminate the RPC transaction.

   It is best to avoid issuing single COMPOUNDs that contain both non-
   idempotent operations and operations where the maximum reply size
   cannot be reliably predicted.

4.2.2.  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
   the absolute maximum size of replies generated by a replying NFS
   version 4 server.

   This value can be used in cases where it is not possible 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.

   With regard to

4.3.  NFS version 4.0, things are more troublesome.
   Typically Version 4 COMPOUND Requests

   The NFS version 4.0 client implementations rely on their own
   architectural limits to keep reply buffer sizes reasonable.  For
   instance, although 4 COMPOUND procedure allows the transmission of more
   than one DDP-eligible data item per Call and Reply message.  An NFS
   version 4 protocol is capable of conveying
   a megabyte-sized ACL, nearly all known physical filesystems store
   ACLs client provides XDR Position values in on-disk containers each Read chunk to
   disambiguate which are small in size.

4.2.1.  Managing READ_PLUS Replies

   The chunk is associated with which argument data item.
   However NFS version 4.2 READ_PLUS operation returns a complex 4 server and client implementations must agree in
   advance on how to pair Write chunks with returned result data type
   [I-D.ietf-nfsv4-minorversion2]. items.

   The rpr_contents field mechanism specified in the result Section 4.3.2 of this operation
   [I-D.ietf-nfsv4-rfc5666bis]) is applied here, with additional
   restrictions that appear below.  In the following list, an array of read_plus_content unions, one arm of
   which contains an opaque byte stream (d_data).

   The size of d_data is limited to the value of the rpa_count field,
   but the protocol does not bound the number of elements which can be
   returned in the rpr_contents array.  In order "NFS Read"
   operation refers to make the size of
   READ_PLUS replies predictable by any NFS version 4.2 clients, the
   following restrictions are placed on the use of the READ_PLUS Version 4 operation on RPC-over-RDMA transports:

   o  An NFS version 4.2 client MUST NOT provide more than one Write
      chunk for any READ_PLUS operation.  When providing which has a Write chunk
      for DDP-
   eligible result data item (i.e., either a READ_PLUS operation, READ, READ_PLUS, or
   READLINK operation).

   o  If an NFS version 4.2 4 client MUST provide
      a Write chunk that is either empty (which forces wishes all result data DDP-eligible items for this operation in an NFS
      reply to be returned inline) or large enough to
      receive rpa_count bytes in a single element of conveyed inline, it leaves the rpr_contents
      array. Write list empty.

   o  If  The first chunk in the Write chunk provided for a READ_PLUS operation list MUST be used by the first READ
      operation in an NFS version 4.2 client 4 COMPOUND procedure.  The next Write
      chunk is not empty, used by the next READ operation, and so on.

   o  If an NFS version 4.2 server MUST
      use that chunk for the first element of 4 client has provided a matching non-empty Write
      chunk, then the rpr_contents array corresponding READ operation MUST return its DDP-
      eligible data item using that has an rpc_data arm. chunk.

   o  An  If an NFS version 4.2 server MUST NOT return more than two elements
      in 4 client has provided an empty matching Write
      chunk, then the rpr_contents array of any READ_PLUS operation.  It returns
      as much corresponding READ operation MUST return all of the requested byte range as it can fit within these two
      elements.
      its result data items inline.

   o  If the NFS version 4.2 server has an READ operation returns a union arm which does not asserted rpr_eof
      in the reply, the NFS version 4.2 client SHOULD send additional
      READ_PLUS requests for any remaining bytes.

4.3.  NFS Version 4 COMPOUND Requests

   A single NFS version 4 COMPOUND procedure supplies arguments for contain a
   sequence of operations,
      DDP-eligible result, and returns results from that sequence, all
   in a single round-trip [RFC7530].  An the NFS version 4 client MAY
   construct has provided a
      matching non-empty Write chunk, an NFS version 4 COMPOUND procedure that provides more than
   one server MUST
      return an empty Write chunk in the Read list or that Write list as long as it observes the
   restrictions position.

   o  If there are more READ operations than Write chunks, then
      remaining NFS Read operations in Section 4.1.

   An an NFS version 4 client provides XDR Position values in each Read
   chunk to disambiguate which COMPOUND that
      have no matching Write chunk is associated MUST return their results inline.

4.3.1.  NFS Version 4 COMPOUND Example

   The following example shows a Write list with which argument
   data item.  However three Write chunks, A,
   B, and C.  The NFS version 4 server and client implementations
   must agree in advance on how to pair consumes the provided Write
   chunks with returned
   result data items.

   The mechanism specified in Section 5.3.2 by writing the results of
   [I-D.ietf-nfsv4-rfc5666bis]) is applied here, with some additional
   restrictions.  In the following list, an "NFS Read" operation refers designated operations in the
   compound request (READ and READLINK) back to any each chunk.

      Write list:

         A --> B --> C

      NFS Version version 4 operation which has a DDP-eligible result data
   item (i.e., either a READ, READ_PLUS, or COMPOUND request:

         PUTFH LOOKUP READ PUTFH LOOKUP READLINK operation).

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

   If an the NFS version 4 client wishes all DDP-eligible items in an NFS
      reply does not want to be conveyed inline, it leaves have the READLINK result
   returned via RDMA, it provides an empty Write list empty.

   o  The first chunk in for buffer B to
   indicate that the Write list MUST READLINK result must be used by the first NFS
      Read operation in an returned inline.

4.4.  NFS version Version 4 COMPOUND procedure. Callback

   The next
      Write chunk is used by the next NFS Read operation, and so on.

   o  If an NFS version 4 client has provided a matching non-empty Write
      chunk, then the corresponding protocols support server-initiated callbacks to
   notify clients of events such as recalled delegations.

4.4.1.  NFS Read operation MUST return its
      DDP-eligible data item using that chunk.

   o  If an Version 4.0 Callback

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

   o  If an NFS Read operation returns 4.0 implementations typically employ a union arm which does not
      contain separate TCP
   connection to handle callback operations, even when the forward
   channel uses a DDP-eligible result, and RPC-over-RDMA transport.

   No operation in the NFS version 4 client has
      provided 4.0 callback RPC program conveys a matching non-empty Write chunk, an NFS version 4 server
      MUST return an empty Write chunk
   significant data payload.  Therefore, no XDR data items in that Write list position.

   o  If there are more NFS Read operations than Write chunks, then
      remaining NFS Read operations this RPC
   program is DDP-eligible.

   A CB_RECALL reply is small and fixed in an NFS version 4 COMPOUND that
      have no matching Write chunk MUST return their results inline.

4.3.1.  NFS Version 4 COMPOUND Example size.  The following example shows CB_GETATTR reply
   contains a Write list with three Write chunks, A,
   B, and C.  The variable-length fattr4 data item.  See Section 4.2.1 for a
   discussion of reply size prediction for this data item.

   An NFS version 4 server consumes the provided Write
   chunks by writing the results of the designated operations in the
   compound request (READ 4.0 client advertises netids and READLINK) back to each chunk.

      Write list:

         A --> B --> C ad hoc port addresses
   for contacting its NFS version 4 COMPOUND request:

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

   If 4.0 callback service using the
   SETCLIENTID operation.

4.4.2.  NFS Version 4.1 Callback

   In NFS version 4 client does not want to have 4.1 and newer minor versions, callback operations may
   appear on the READLINK result
   returned via RDMA, it provides an empty Write chunk same connection as is used for buffer B to
   indicate that the READLINK result must be returned inline.

4.4. NFS Version version 4 Callback

   The forward
   channel client requests.  NFS version 4 protocols support server-initiated callbacks to
   notify clients of events such as recalled delegations.

4.4.1.  NFS Version 4.0 Callback

   NFS version 4.0 implementations typically employ a separate TCP
   connection to handle callback operations, even when the forward
   channel uses a RPC-over-RDMA transport.  Therefore no Upper Layer
   Binding for the NFS version 4.0 callback program is provided in this
   document.

4.4.2.  NFS Version 4.1 Callback

   In NFS version 4.1 and later minor versions, callback operations may
   appear on the same connection as is used for NFS version 4 forward
   channel client requests.  NFS version 4 clients and servers MUST use
   the mechanism described in [I-D.ietf-nfsv4-rpcrdma-bidirection] when
   backchannel operations are conveyed on RPC-over-RDMA transports.

   The csa_back_chan_attrs argument and servers MUST use
   the mechanism described in [I-D.ietf-nfsv4-rpcrdma-bidirection] when
   backchannel operations are conveyed on RPC-over-RDMA transports.

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

   There are no DDP-eligible data items in callback protocols associated
   with procedures defined
   in NFS version 4.1 or NFS version 4.2.  However, some callback
   requests,
   operations, such as messages that convey device ID information, may can
   be large, in which case a Long Call or Reply may might be appropriate. required.

   When
   the an NFS version 4 4.1 client reports a backchannel ca_maxresponsesize
   ca_maxrequestsize that is larger than the connection's inline thresholds, connection's inline
   thresholds, the NFS version 4 client can support Long Calls.
   Otherwise an NFS version 4 server MUST use Short messages to convey
   backchannel operations.

4.5.  Session-Related Considerations

   Typically the presence of an NFS session [RFC5661] has no effect on
   the operation of RPC-over-RDMA.  None of the operations introduced to
   support NFS sessions contain DDP-eligible data items.  There is no
   need to match the number of session slots with the number of
   available RPC-over-RDMA credits.

   However, there are some rare error conditions which require special
   handling when an NFS session is operating on an RPC-over-RDMA
   transport.  For example, a requester might receive, in response to an
   RPC request, an RDMA_ERROR message with an rdma_err value of
   ERR_CHUNK, or an RDMA_MSG containing an RPC_GARBAGEARGS reply.
   Within RPC-over-RDMA Version One, this class of error can be
   generated for two different reasons:

   o  There was an XDR error detected parsing the RPC-over-RDMA headers.

   o  There was an error sending the response, because, for example, a
      necessary reply chunk was not provided or the one provided is of
      insufficient length.

   These two situations, which arise due to incorrect implementations or
   underestimation of reply size, have different implications with
   regard to Exactly-Once Semantics.  An XDR error in decoding the
   request precludes the execution of the request on the responder, but
   failure to send a reply indicates that some or all of the operations
   were executed.

   In both instances, the client SHOULD NOT retry the operation without
   addressing reply resource inadequacy.  Such a retry can result in the
   same sort of error seen previously.  Instead, it is best to consider
   the operation as completed unsuccessfully and report an error to the
   consumer who requested the RPC.

   In addition, within the error response, the requester does not have
   the result of the NFS
   version 4 client can support Long messages (i.e., Read chunks execution of the SEQUENCE operation, which
   identifies the session, slot, and
   Reply chunks).  Otherwise an NFS version 4 server MUST use Short
   messages to convey backchannel operations.

   See Section 4.1 sequence id for a discussion of how an NFS version 4 server
   handles situations where an NFS version 4 client the request which
   has provided
   inadequate RDMA resources failed.  The xid associated with the request, obtained from the
   rdma_xid field of the RDMA_ERROR or RDMA_MSG message, must be used to convey a backchannel reply.

4.5.
   determine the session and slot for the request which failed, and the
   slot must be properly retired.  If this is not done, the slot could
   be rendered permanently unavailable.

4.6.  Connection Keep-Alive

   NFS version 4 client implementations often rely on a transport-layer
   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 reconnection and RPC retransmission.

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

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

   To prevent lease expiry, NFS version 4 clients should use a lease-
   extending operation such as RENEW or SEQUENCE, rather than a NULL
   request, when performing a periodic health assessment.

5.  Extending NFS Upper Layer Bindings

   RPC programs such as NFS are required to have an Upper Layer Binding
   specification to interoperate on RPC-over-RDMA transports
   [I-D.ietf-nfsv4-rfc5666bis].  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.  This includes 2, or separately published extensions to an existing NFS
   version 4 extensions that are
   documented separately from a new minor version. version, as described in [I-D.ietf-nfsv4-versioning].

6.  IANA Considerations

   NFS use of direct data placement introduces a need for an additional
   NFS port number assignment for networks that share traditional UDP
   and TCP port spaces with RDMA services.  The iWARP [RFC5041]
   [RFC5040] protocol is such an example (InfiniBand is not).

   NFS servers for versions 2 and 3 [RFC1094] [RFC1813] traditionally
   listen for clients on UDP and TCP port 2049, and additionally, they
   register these with the portmapper and/or rpcbind [RFC1833] service.
   However, [RFC7530] requires NFS version 4 servers to listen on TCP
   port 2049, and they are not required to register.

   An NFS version 2 or version 3 server supporting RPC-over-RDMA on such
   a network and registering itself with the RPC portmapper MAY choose
   an arbitrary port, or MAY use the alternative well-known port number
   for its RPC-over-RDMA service.  The chosen port MAY be registered
   with the RPC portmapper under the netid assigned by the requirement
   in [I-D.ietf-nfsv4-rfc5666bis].

   An NFS version 4 server supporting RPC-over-RDMA on such a network
   MUST use the alternative well-known port number for its RPC-over-RDMA
   service.  Clients SHOULD connect to this well-known port without
   consulting the RPC portmapper (as for NFS version 4 on TCP
   transports).

   The port number assigned to an NFS service over an RPC-over-RDMA
   transport is available from the IANA port registry [RFC3232].

7.  Security Considerations

   RPC-over-RDMA supports all RPC security models, including RPCSEC_GSS
   security and transport-level security [RFC2203].  The choice of RDMA
   Read and RDMA Write to convey RPC argument and results does not
   affect this, since it changes only the method of data transfer.
   Specifically, the requirements of [I-D.ietf-nfsv4-rfc5666bis] ensure
   that this choice does not introduce new vulnerabilities.

   Because this document defines only the binding of the NFS protocols
   atop [I-D.ietf-nfsv4-rfc5666bis], all relevant security
   considerations are therefore to be described at that layer.

8.  References

8.1.  Normative References

   [I-D.ietf-nfsv4-minorversion2]
              Haynes, T., "NFS Version 4 Minor Version 2", draft-ietf-
              nfsv4-minorversion2-41 (work in progress), January 2016.

   [I-D.ietf-nfsv4-rfc5666bis]
              Lever, C., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call, Version
              One", draft-ietf-nfsv4-rfc5666bis-07 draft-ietf-nfsv4-rfc5666bis-09 (work in progress),
              May 2016.
              January 2017.

   [I-D.ietf-nfsv4-rpcrdma-bidirection]
              Lever, C., "Bi-directional Remote Procedure Call On RPC-
              over-RDMA Transports", draft-ietf-nfsv4-rpcrdma-
              bidirection-05
              bidirection-06 (work in progress), June 2016. January 2017.

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

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

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

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

   [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>.

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

8.2.  Informative References

   [I-D.ietf-nfsv4-versioning]
              Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", draft-ietf-nfsv4-versioning-09 (work in
              progress), December 2016.

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

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

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

   [RFC3232]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
              by an On-line Database", RFC 3232, DOI 10.17487/RFC3232,
              January 2002, <http://www.rfc-editor.org/info/rfc3232>.

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

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

   [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>.

Appendix A.  Changes Since RFC 5667

   Corrections and updates made necessary by new language in
   [I-D.ietf-nfsv4-rfc5666bis] have been introduced.  For example,
   references to deprecated features of RPC-over-RDMA Version One, such
   as RDMA_MSGP, and the use of the Read list for handling RPC replies,
   have been removed.  The term "mapping" has been replaced with the
   term "binding" or "Upper Layer Binding" throughout the document.
   Some material that duplicates what is in [I-D.ietf-nfsv4-rfc5666bis]
   has been deleted.

   Material required by [I-D.ietf-nfsv4-rfc5666bis] for Upper Layer
   Bindings that was not present in [RFC5667] has been added, including
   discussion of how each NFS version properly estimates the maximum
   size of RPC replies.

   Technical corrections have been made.  For example, the mention of
   12KB and 36KB inline thresholds have been removed.  The reference to
   a non-existant NFS version 4 SYMLINK operation has been replaced with
   NFS version 4 CREATE(NF4LNK).

   The discussion of NFS version 4 COMPOUND handling has been completed.
   Some changes were made to the algorithm for matching DDP-eligible
   results to Write chunks.

   Requirements to ignore extra Read or Write chunks have been removed
   from the NFS version 2 and 3 Upper Layer Binding, as they conflict
   with [I-D.ietf-nfsv4-rfc5666bis].

   A complete discussion of reply size estimation has been introduced
   for all protocols covered by the Upper Layer Bindings in this
   document.

   The following additional improvements have been made, relative to
   [RFC5667]:

   o  An explicit discussion of NFS version 4.0 and NFS version 4.1
      backchannel operation has replaced the previous treatment of
      callback operations.

   o  A binding for NFS version 4.2 has been added that includes
      discussion of new data-bearing operations like READ_PLUS.

   o  A section suggesting a mechanism for periodically assessing
      connection health has been introduced.

   o  Language inconsistent with or contradictory to
      [I-D.ietf-nfsv4-rfc5666bis] has been removed from Sections 2 and
      3, and both Sections have been combined into Section 2 in the
      present document.

   o  Ambiguous or erroneous uses of RFC2119 terms have been corrected.

   o  References to obsolete RFCs have been updated.

   o  An IANA Considerations Section has replaced the "Port Usage
      Considerations" Section.

   o  Code excerpts have been removed, and figures have been modernized.

Appendix B.  Acknowledgments

   The author gratefully acknowledges the work of Brent Callaghan and
   Tom Talpey on the original NFS Direct Data Placement specification
   [RFC5667].  The author also wishes to thank Bill Baker and Greg
   Marsden for their support of this work.

   Dave Noveck provided excellent review, constructive suggestions, and
   consistent navigational guidance throughout the process of drafting
   this document.  Dave also contributed the text of Section 4.1.1. 4.5

   Thanks to Karen Deitke for her sharp observations about idempotency,
   and the clarity of the discussion of NFS COMPOUNDs.

   Special thanks go to Transport Area Director Spencer Dawkins, nfsv4
   Working Group Chair Spencer Shepler, and nfsv4 Working Group
   Secretary Thomas Haynes for their support.

Author's Address

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

   Phone: +1 734 274 2396 248 816 6463
   Email: chuck.lever@oracle.com