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Versions: 00

Network File System Version 4                                  D. Noveck
Internet-Draft                                          February 1, 2017
Intended status: Standards Track
Expires: August 5, 2017


Transport-generic Network File System (NFS) Upper Layer Bindings To RPC-
                               Over-RDMA
                     draft-dnoveck-nfsv4-nfsulb-00

Abstract

   This document specifies Upper Layer Bindings to allow use of RPC-
   over-RDMA by protocols related to the Network File System (NFS).
   Such bindings are required when using RPC-over-RDMA, in order to
   enable use of Direct Data Placement and for a number of other
   reasons.  These bindings are structured to be applicable to all known
   version of the RPC-over-RDMA transport, including optional
   extensions.  All versions of NFS are addressed.

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

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on August 5, 2017.

Copyright Notice

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




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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conveying NFS Operations On RPC-Over-RDMA . . . . . . . . . .   3
     2.1.  Direct Placement of Request Data  . . . . . . . . . . . .   4
     2.2.  Direct Placement of Response Data . . . . . . . . . . . .   4
     2.3.  Scatter-gather when Using DDP . . . . . . . . . . . . . .   5
     2.4.  DDP-eligibility Violations  . . . . . . . . . . . . . . .   5
     2.5.  Long Calls and Replies  . . . . . . . . . . . . . . . . .   6
   3.  Preparatory Material for Multiple Bindings  . . . . . . . . .   6
     3.1.  Reply Size Estimation . . . . . . . . . . . . . . . . . .   7
     3.2.  Retry to Deal with Reply Size Mis-estimation  . . . . . .   7
   4.  Upper Layer Binding for NFS Versions 2 And 3  . . . . . . . .   8
     4.1.  Auxiliary Protocols . . . . . . . . . . . . . . . . . . .   9
   5.  Upper Layer Binding for NFS Version 4 . . . . . . . . . . . .  10
     5.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  NFS Version 4 Reply Size Estimation . . . . . . . . . . .  11
     5.3.  NFS Version 4 COMPOUND Requests . . . . . . . . . . . . .  12
     5.4.  NFS Version 4 Callback  . . . . . . . . . . . . . . . . .  14
     5.5.  Session-Related Considerations  . . . . . . . . . . . . .  15
     5.6.  Connection Keep-Alive . . . . . . . . . . . . . . . . . .  16
   6.  Extending NFS Upper Layer Bindings  . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17



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     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   An RPC-over-RDMA transport, such as the ones defined in
   [I-D.ietf-nfsv4-rfc5666bis] and [rpcrdmav2], may employ direct data
   placement to convey data payloads associated with RPC transactions.
   To enable successful interoperation, RPC client and server
   implementations must agree as to which XDR data items in what
   particular RPC procedures are eligible for direct data placement
   (DDP).  Specifying those data items is a major component of a
   protocol's Upper Layer Binding.

   In addition, Upper Layer Bindings are required to include additional
   information to assure that adequate resources are allocated to
   receive RPC replies, and for a number of other reasons.

   This document contains material required of Upper Layer Bindings, as
   specified in [I-D.ietf-nfsv4-rfc5666bis], for the NFS protocol
   versions listed below.  In addition, bindings are provided, when
   necessary, for auxiliary protocols used together with NFS versions 2
   and 3.

   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 [RFC7862]

2.  Conveying NFS Operations On RPC-Over-RDMA

   This document is written to apply to multiple versions of the RPC-
   over-RDMA transport, only the first of which is currently specified
   by a working group document (in [I-D.ietf-nfsv4-rfc5666bis]).
   However, it is expected that other versions will be created, and this
   document has been structured to support future versions by focusing
   on the functions to be provided by the transport and the transport
   limitations which the Upper Layer Protocols need to accommodate,
   allowing the transport specification and the specifications for
   associated extensions to define how those functions will be provided
   and the details of the transport limitations.




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   In the subsections that follow, we will describe the generic function
   to be provided or limitation to be accommodated and follow it with
   material that describes, in general, how that issue is dealt with in
   Version One.  For more detail about Version One,
   [I-D.ietf-nfsv4-rfc5666bis] should be consulted.  How these issues
   are to be dealt with in future versions is left to the specification
   documents for those versions and associated documents defining
   optional extensions.

   For example:

   o  ULBs within this document define which data items are eligible for
      Direct Data Placement while transport versions might differ as how
      this is to be effected.  See Sections 2.1 and 2.2 for more detail.
      In both cases, Section 2.3 discusses issues connected with the use
      of discontiguous areas for Direct Data Placement.

   o  Section 2.4 defines the concept of a DDP-eligibility violation and
      requires that such violations be reported while transport versions
      might differ as to the manner in which the reporting is to be
      done.

   o  Section 2.5 discusses issues arising from limits on the size of
      messages conveyed using RDMA SENDs.  Different transport version
      may have different size limits, while, in the case of replies the
      ULBs are responsible for specifying how limits on reply sizes are
      to be determined.

2.1.  Direct Placement of Request Data

   When DDP-eligible XDR data items appear in a request the requester
   needs to take special actions in order to provide for the direct
   placement of those items in the responder's memory.  The specific
   actions to be taken are defined by the transport version being used.

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

2.2.  Direct Placement of Response Data

   When a request is such that it is possible for DDP-eligible data
   items to appear in the corresponding reply, the requester needs to
   take special actions in order to provide for the direct placement of
   those items in the requester's memory, if such placement is desired.



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   The specific actions to be taken are defined by the transport version
   being used as is the means to indicate that such direct placement is
   not to be done

   In Version One, 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, can be referenced by the Write list.  The NFS
   server pushes such payloads to the client, placing them directly into
   the client's memory, using target addresses provided by the client
   when sending the request.

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

2.3.  Scatter-gather when Using DDP

   In order to accommodate the storage of multiple data blocks within
   individual cache buffers, the RPC-over-RDMA transport allows the
   addresses to which a DDP-eligible data item to be discontiguous.  How
   these addresses are indicated depends on the transport version.

   Within Version One, a chunk 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 convey a single DDP-
   eligible XDR data item.

2.4.  DDP-eligibility Violations

   When the transport header uses the means defined to directly place on
   an XDR item is applied to an XDR item not define in the ULB as DDP-
   eligible, a DDP-eligibility violation is recognized.  The means by
   which such violations are to be reported is defined by the particular
   transport version being used.

   To report a DDP-eligibility violation within Version One, an NFS
   server returns one of:

   o  An RPC-over-RDMA message of type RDMA_ERROR, with the rdma_xid
      field set to the XID of the matching NFS Call, and the rdma_error
      field set to ERR_CHUNK

   o  An RPC message (via an RDMA_MSG message) with the xid field set to
      the XID of the matching NFS Call, the mtype field set to REPLY,



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      the stat field set to MSG_ACCEPTED, and the accept_stat field set
      to GARBAGE_ARGS.

2.5.  Long Calls and Replies

   Because of the use of pre-posted receive buffers whose size is fixed,
   all RPC-over-RDMA transport versions have limits on the size of
   messages which can be conveyed without use of explicit RDMA
   operations, although different transport versions may have different
   limits.  In particular, when the transport version allows messages to
   be continued across multiple RDMA SENDs, the limit can be
   substantially greater than the receive buffer size.  Also note that
   the size of the messages allowed may be reduced because of space
   taken up by the transport header fields.

   Each transport version is responsible for defining the message size
   limits and the means by which the transfer of messages that exceed
   these limits is to be provided for.  These means may be different in
   the cases of long calls and replies.

   When using Version One, if an NFS request is too large to be conveyed
   within the NFS server's responder inline threshold, even after any
   DDP-eligible data items have been removed, an NFS client must send
   the request in the form of a Long Call.  The entire NFS request is
   sent in a special Read chunk called a Position Zero Read chunk.

   Also when using Version One, if an NFS client determines that the
   maximum size of an NFS reply could be too large to be conveyed within
   its own 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.

   There exist cases in which an NFS client needs to provide both a
   Position Zero Read chunk and a Reply chunk for the same RPC.  One
   common source of such situations is when the RPC authentication
   flavor being used requires that DDP-eligible data items never be
   removed from RPC messages.

3.  Preparatory Material for Multiple Bindings

   Although each of the NFS versions and each of the auxiliary protocols
   discussed in Section 4.1 has its own ULB, there is important
   preparatory material in the subsections below that applies to
   multiple ULPs.  In particular:

   o  The material in Section 3.1 applies to all of the ULPs discussed
      in this document.




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   o  The material in Section 3.2 applies to NFSv2, NFSv3, NFSv4.0, the
      MOUNT protocol, and the NFSACL protocol.

3.1.  Reply Size Estimation

   During the construction of each RPC Call message, a client is
   responsible for allocating appropriate resources for receiving the
   matching Reply message.  The resources required depends on the
   maximum reply size expected, whether DDP-eligible can removed from
   the reply and the transport version being used.  The ULB is
   responsible for defining how the maximum reply size is to be
   determined while the specifiction of the transport version being used
   is responsible for defining how this maximum affects the resources to
   be allocated.  Because the responder may not be able to send the
   required response when these resources have not been allocated,
   reliable reply size estimation is necessary to allow successful
   interoperation.

   In many cases the Upper Layer Protocol's XDR definition provides
   enough information to enable the client to make a reliable prediction
   of the maximum size of the expected Reply message.  However, If there
   are variable-size data items in the result, the maximum size of the
   RPC Reply message can be reliably estimated in many cases:

   o  The client requests only a specific portion of an object (for
      example, using the "count" and "offset" fields in an NFS READ).

   o  The client has already cached the size of the whole object it is
      about to request (e.g., via a previous NFS GETATTR request).

   o  The client specifies a reply size limit for the particular reply,
      as it does by setting the count field of READDIR request.

   It is sometimes not possible to determine the maximum Reply message
   size based solely on the above criteria.  Client implementers can
   choose to provide the largest possible Reply buffer in those cases,
   based on, for instance, the largest possible NFS READ or WRITE
   payload (which is negotiated at mount time).

   There exist cases in which a client cannot be sure any a priori
   determination is fully reliable.  Handling of such cases is discussed
   in Section 3.2.

3.2.  Retry to Deal with Reply Size Mis-estimation

   For some of the protocols discussed in this document, it is possible
   for a compliant responder to send a valid reply whose length exceeds
   the client's a priori estimate.  In such cases, the client needs to



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   expect an error indication that indicates the existence of the
   oversize reply.  When this happens, the client can either terminate
   that RPC transaction, or retry it with a larger reply size estimate.

   In the case of the NFSv4.0, the use of NFS COMPOUND operations raises
   the possibility of non-idempotent requests that combine a non-
   idempotent operation with an operation whose maximum reply size
   cannot be determined with certainty.  This makes retrying the
   operation problematic.  It should be noted 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.

   Depending on the transport version used, the client's choices may be
   restricted as follows:

   o  The client may be required to treat the error as permanent, with
      retry not allowed.

   o  The client may be allowed to reissue the request with a larger
      reply estimate, unless it is a non-idempotent request.  In this
      case, non-idempotent requests may not be retried and will result
      in errors being reported to the issuer in this case.

   o  The client may be allowed to reissue the request with a larger
      reply estimate, in essentially all cases.  In this case, the
      client has sufficient information to avoid re-executing a non-
      idempotent request and may, if it chooses, retry all requests with
      a larger reply size.

   In the case of Version One, the absence of a itinct error code to
   signal a reply chunk of inadequate size meanss that retry in this
   situation is not available.

4.  Upper Layer Binding for NFS Versions 2 And 3

   This Upper Layer Binding specification applies to NFS Version 2
   [RFC1094] and NFS Version 3 [RFC1813].  For brevity, in this section
   a "legacy NFS client" refers to an NFS client using NFS version 2 or
   NFS version 3 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:

   o  The opaque file data argument in the NFS WRITE procedure



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   o  The pathname argument in the NFS SYMLINK procedure

   o  The opaque file data result in the NFS READ procedure

   o  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.

   A legacy NFS client determines the maximum reply size for each
   operation using the basic criteria outlined in Section 3.1.  Such
   clients deal with reply sizes beyond the maximum as escribed in
   Section 2.5.

4.1.  Auxiliary Protocols

   NFS versions 2 and 3 are typically deployed with several other
   protocols, sometimes referred to as "NFS auxiliary protocols."  These
   are separate RPC programs that define procedures which are not part
   of the NFS version 2 or version 3 RPC programs.  These include:

   o  The MOUNT and NLM protocols, introduced in an appendix of
      [RFC1813]

   o  The NSM protocol, described in Chapter 11 of [NSM]

   o  The NFSACL protocol, which does not have a public definition
      (NFSACL here is treated as a de facto standard as there are
      several interoperating implementations).

   RPC-over-RDMA treats 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.

4.1.1.  MOUNT, NLM, And NSM Protocols

   Typically MOUNT, NLM, and NSM are conveyed via TCP, even in
   deployments where NFS operations on RPC-over-RDMA.  When a legacy
   server supports these programs on RPC-over-RDMA, it advertises the
   port address via the usual rpcbind service [RFC1833].

   No operation in these protocols conveys a significant data payload,
   and the size of RPC 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 xdr_netobj.  In most
   implementations this data item is not larger than 1024 bytes, making
   this size a reasonable basis for reply size estimation.  However,



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   since this limit is not specified as part of the protocol, the
   techniques described in Section 3.1 should be used to deal with
   situations where these sizes are exceeded.

4.1.2.  NFSACL Protocol

   Legacy clients and servers that support the NFSACL RPC program
   typically convey NFSACL procedures on the same connection as the NFS
   RPC program.  This obviates the need for separate rpcbind queries to
   discover server support for this RPC program.

   ACLs are typically small, but even large ACLs must be encoded and
   decoded to some degree.  Thus, no data item in this Upper Layer
   Protocol is DDP-eligible.

   For procedures whose replies do not include an ACL object, the size
   of a reply is determined directly from the NFSACL program's XDR
   definition.

   There is no protocol-wide size limit for NFS version 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 the
   negotiated rsize setting) can be provided.  Since no limit is
   specified as part of the protocol, the techniques described in
   Section 3.1 should be used to deal with situations where these
   recommended bounds are exceeded.

5.  Upper Layer Binding for NFS Version 4

   This Upper Layer Binding specification applies to all protocols
   defined in NFS Version 4.0 [RFC7530], NFS Version 4.1 [RFC5661], 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:

   o  The opaque data field in the WRITE4args structure

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

   o  The opaque data field in the READ4resok structure

   o  The linkdata field in the READLINK4resok structure



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

5.1.1.  READ_PLUS Replies

   The NFS version 4.2 READ_PLUS operation returns a complex data type
   [RFC7862].  The rpr_contents field in the result of this operation is
   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 to make the size of
   READ_PLUS replies predictable by NFS version 4.2 clients, the
   following restrictions are placed on the use of the READ_PLUS
   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 a Write chunk
      for a READ_PLUS operation, an NFS version 4.2 client MUST provide
      a Write chunk that is either empty (which forces all result data
      items for this operation to be returned inline) or large enough to
      receive rpa_count bytes in a single element of the rpr_contents
      array.

   o  If the Write chunk provided for a READ_PLUS operation by an NFS
      version 4.2 client is not empty, an NFS version 4.2 server MUST
      use that chunk for the first element of the rpr_contents array
      that has an rpc_data arm.

   o  An NFS version 4.2 server MUST NOT return more than two elements
      in the rpr_contents array of any READ_PLUS operation.  It returns
      as much of the requested byte range as it can fit within these two
      elements.  If the NFS version 4.2 server has not asserted rpr_eof
      in the reply, the NFS version 4.2 client SHOULD send additional
      READ_PLUS requests for any remaining bytes.

5.2.  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.

   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:




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   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,

5.2.1.  Reply Size Estimation for Minor Version 0

   The items enumerated above in Section 5.2 make it difficult to
   predict the maximum size of GETATTR replies that interrogate
   variable-length attributes.  As discussed in Section 3.1, client
   implementations can rely on their own internal architectural limits
   to bound the reply size.  However, since such limits are not
   guaranteed to be reliable, use of the techniques discussed in
   Section 3.2 may sometimes be necessary.

   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.

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
   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.

5.3.  NFS Version 4 COMPOUND Requests

   The NFS version 4 COMPOUND procedure allows the transmission of more
   than one DDP-eligible data item per Call and Reply message.  An NFS
   version 4 client provides XDR Position values in each Read chunk to
   disambiguate 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.




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   The mechanism specified in Section 4.3.2 of
   [I-D.ietf-nfsv4-rfc5666bis]) is applied here, with additional
   restrictions that appear below.  In the following list, an "NFS Read"
   operation refers to any NFS Version 4 operation which has a DDP-
   eligible result data item (i.e., either a READ, READ_PLUS, or
   READLINK operation).

   o  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.

   o  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 the next READ operation, and so on.

   o  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.

   o  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.

   o  If an 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.

   o  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.

5.3.1.  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.














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      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.

5.4.  NFS Version 4 Callback

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

5.4.1.  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.

   No operation in the NFS version 4.0 callback RPC program conveys a
   significant data payload.  Therefore, no XDR data items in this RPC
   program is DDP-eligible.

   A CB_RECALL reply is small and fixed in size.  The CB_GETATTR reply
   contains a variable-length fattr4 data item.  See Section 5.2.1 for a
   discussion of reply size prediction for this data item.

   An NFS version 4.0 client advertises netids and ad hoc port addresses
   for contacting its NFS version 4.0 callback service using the
   SETCLIENTID operation.

5.4.2.  NFS Version 4.1 Callback

   In NFS version 4.1 and newer 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.





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   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 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 large, in which case a Long Call or Reply might be required.

   When an NFS version 4.1 client reports a backchannel
   ca_maxrequestsize that is larger than the 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.

5.5.  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



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   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 execution of the SEQUENCE operation, which
   identifies the session, slot, and sequence id for the request which
   has 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
   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.

5.6.  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, 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.

6.  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, or separately published extensions to an existing NFS
   version 4 minor version, as described in [I-D.ietf-nfsv4-versioning].

7.  IANA Considerations

   NFS use of direct data placement introduces a need for an additional
   NFS port number assignment for networks that share traditional UDP




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   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].

8.  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.

9.  References

9.1.  Normative References

   [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-09 (work in progress),
              January 2017.




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

9.2.  Informative References

   [I-D.ietf-nfsv4-rfc5667bis]
              Lever, C., "Remote Direct Memory Access Transport for
              Remote Procedure Call, Version One", draft-ietf-
              nfsv4-rfc5667bis-04 (work in progress), January 2017.

   [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.






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   [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>.

   [rpcrdmav2]
              Lever, C., Ed. and D. Noveck, "RPC-over-RDMA Version Two",
              December 2016, <http://www.ietf.org/id/
              draft-cel-nfsv4-rpcrdma-version-two-03.txt>.

              Work in progress.

Appendix A.  Acknowledgments

   The author gratefully acknowledges the work of Brent Callaghan and
   Tom Talpey on the original NFS Direct Data Placement specification
   [RFC5667].

   A large part of the material in this doccument is taken from
   [I-D.ietf-nfsv4-rfc5667bis] written by Chuck Lever.  The author
   wishes to acknowlege the debt he owes to Chuck for his work in
   providing an updated Upper Layer Binding for the NFS-related
   protocols.

   The author also wishes to thank Bill Baker and Greg Marsden for their
   support of the work to revive RPC-over-RDMA.



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Author's Address

   David Noveck
   26 Locust Avenue
   Lexington, MA  02421
   USA

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










































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