Internet Engineering Task Force (IETF)
Request for Comments: 8267
Obsoletes: 5667
Category: Standards Track
ISSN: 2070-1721
C. Lever
October 2017

Network File System (NFS) Upper-Layer Binding to RPC-over-RDMA Version 1


This document specifies Upper-Layer Bindings of Network File System (NFS) protocol versions to RPC-over-RDMA version 1, thus enabling the use of Direct Data Placement. This document obsoletes RFC 5667.

Status of This Memo

This is an Internet Standards Track document.

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

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Reply Size Estimation . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Short Reply Chunk Retry . . . . . . . . . . . . . . . . .   5
   4.  Upper-Layer Binding for NFS Versions 2 and 3  . . . . . . . .   6
     4.1.  Reply Size Estimation . . . . . . . . . . . . . . . . . .   7
     4.2.  RPC Binding Considerations  . . . . . . . . . . . . . . .   7
   5.  Upper-Layer Bindings for NFS Versions 2 and 3 Auxiliary
       Protocols . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  MOUNT, NLM, and NSM Protocols . . . . . . . . . . . . . .   8
     5.2.  NFSACL Protocol . . . . . . . . . . . . . . . . . . . . .   8
   6.  Upper-Layer Binding for NFS Version 4 . . . . . . . . . . . .   8
     6.1.  DDP-Eligibility . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Reply Size Estimation . . . . . . . . . . . . . . . . . .   9
     6.3.  RPC Binding Considerations  . . . . . . . . . . . . . . .  10
     6.4.  NFS COMPOUND Requests . . . . . . . . . . . . . . . . . .  10
     6.5.  NFS Callback Requests . . . . . . . . . . . . . . . . . .  13
     6.6.  Session-Related Considerations  . . . . . . . . . . . . .  14
     6.7.  Transport Considerations  . . . . . . . . . . . . . . . .  15
   7.  Extending NFS Upper-Layer Bindings  . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Changes Since RFC 5667 . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1. Introduction

The RPC-over-RDMA version 1 transport may employ Direct Data Placement (DDP) to convey data payloads associated with RPC transactions [RFC8166]. To enable successful interoperation, RPC client and server implementations using RPC-over-RDMA version 1 must agree which External Data Representation (XDR) data items and RPC procedures are eligible to use DDP.

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

This document contains material required of Upper-Layer Bindings, as specified in [RFC8166], 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 [RFC7862]

Upper-Layer Bindings are also provided for auxiliary protocols used with NFS versions 2 and 3 (see Section 5).

This document assumes the reader is already familiar with concepts and terminology defined in [RFC8166] and the documents it references.

2. Requirements Language

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

3. Reply Size Estimation

During the construction of each RPC Call message, a requester is responsible for allocating appropriate resources for receiving the corresponding Reply message. If the requester expects the RPC Reply message will be larger than its inline threshold, it provides Write and/or Reply chunks wherein the responder can place results and the Reply's Payload stream.

A reply resource overrun occurs if the RPC Reply Payload stream does not fit into the provided Reply chunk or if no Reply chunk was provided and the Payload stream does not fit inline. This prevents the responder from returning the Upper-Layer reply to the requester. Therefore, reliable reply size estimation is necessary to ensure successful interoperation.

In most cases, the NFS protocol's XDR definition provides enough information to enable an NFS client to predict the maximum size of the expected Reply message. If there are variable-size data items in the result, the maximum size of the RPC Reply message can be estimated as follows:

  • The client requests only a specific portion of an object (e.g., using the "count" and "offset" fields in an NFS READ).
  • The client limits the number of results (e.g., using the "count" field of an NFS READDIR request).
  • The client has already cached the size of the whole object it is about to request (e.g., via a previous NFS GETATTR request).
  • The client and server have negotiated a maximum size for all calls and responses (e.g., using a CREATE_SESSION operation).

3.1. Short Reply Chunk Retry

In a few cases, either the size of one or more returned data items or the number of returned data items cannot be known in advance of forming an RPC Call.

If an NFS server finds that the NFS client provided inadequate receive resources to return the whole Reply, it returns an RPC-level error or a transport error, such as ERR_CHUNK.

In response to these errors, an NFS client can choose to:

  • terminate the RPC transaction immediately with an error, or
  • allocate a larger Reply chunk and send the same request as a new RPC transaction (a new Transaction ID (XID) should be assigned to the retransmitted request to avoid matching a cached RPC Reply that caches the original error). The NFS client should avoid retrying the request indefinitely because a responder may return ERR_CHUNK for a variety of reasons.

Subsequent sections of this document discuss exactly which operations might have ultimate difficulty with reply size estimation. These operations are eligible for "short Reply chunk retry". Unless explicitly mentioned as applicable, short Reply chunk retry should not be used since accurate reply size estimation is problematic in only a few cases. In all other cases, reply size underestimation is considered a correctable implementation bug.

NFS server implementations can avoid connection loss by first confirming that target RDMA segments are large enough to receive results before initiating explicit RDMA operations.

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

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

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

  • the opaque file data argument in the NFS WRITE procedure
  • the pathname argument in the NFS SYMLINK procedure
  • the opaque file data result in the NFS READ procedure
  • the pathname result in the NFS READLINK procedure

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

A transport error does not give an indication of whether the server has processed the arguments of the RPC Call or whether the server has accessed or modified client memory associated with that RPC.

4.1. Reply Size Estimation

A Legacy NFS client determines the maximum reply size for each operation using the criteria outlined in Section 3. There are no operations in NFS versions 2 or 3 that benefit from short Reply chunk retry.

4.2. RPC Binding Considerations

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

A Legacy NFS server supporting RPC-over-RDMA version 1 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 (see Section 9). The chosen port MAY be registered with the RPC portmapper under the netids assigned in [RFC8166].

5. Upper-Layer Bindings for NFS Versions 2 and 3 Auxiliary Protocols

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

  • versions 2 and 3 of the MOUNT RPC program (100005) [RFC1813];
  • versions 1, 3, and 4 of the NLM (Network Lock Manager) RPC program (100021) [RFC1813];
  • version 1 of the NSM (Network Status Monitor) RPC program (100024), which is described in Chapter 11 of [XNFS]; and
  • version 1 of the NFSACL RPC program (100227), which does not have a public definition. NFSACL is treated in this document as a de facto standard, as there are several interoperating implementations.

5.1. MOUNT, NLM, and NSM Protocols

Historically, NFS/RDMA implementations have chosen to convey the MOUNT, NLM, and NSM protocols via TCP. To enable interoperation of these protocols when NFS/RDMA is in use, a Legacy NFS server MUST provide support for these protocols via TCP.

5.2. 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 (100003). This obviates the need for separate rpcbind queries to discover server support for this RPC program.

Access Control Lists (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 NFSACL procedures whose Replies do not include an ACL object, the size of a Reply is determined directly from the NFSACL RPC program's XDR definition.

There is no protocol-specified size limit for NFS version 3 ACLs, and there is no mechanism in either the NFSACL or NFS RPC programs for a Legacy client to ascertain the largest ACL a Legacy server can return. Legacy client implementations should choose a maximum size for ACLs based on their own internal limits.

Because an NFSACL client cannot know in advance how large a returned ACL will be, it can use short Reply chunk retry when an NFSACL GETACL operation encounters a transport error.

6. Upper-Layer Binding for NFS Version 4

The Upper-Layer Binding specification in this section applies to versions of the NFS RPC program defined in NFS versions 4.0 [RFC7530], 4.1 [RFC5661], and 4.2 [RFC7862].

6.1. DDP-Eligibility

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

  • The opaque data field in the WRITE4args structure
  • The linkdata field of the NF4LNK arm in the createtype4 union
  • The opaque data field in the READ4resok structure
  • The linkdata field in the READLINK4resok structure

6.2. Reply Size Estimation

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

  • the attrlist4 field;
  • fields containing ACLs such as fattr4_acl, fattr4_dacl, and fattr4_sacl;
  • fields in the fs_locations4 and fs_locations_info4 data structures; and
  • fields opaque to the NFS version 4 protocol that pertain to pNFS (parallel NFS) layout metadata, such as loc_body, loh_body, da_addr_body, lou_body, lrf_body, fattr_layout_types, and fs_layout_types.

6.2.1. Reply Size Estimation for Minor Version 0

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

Some of the data items enumerated in Section 6.2 (in particular, the items related to ACLs and fs_locations) make it difficult to predict the maximum size of NFS version 4.0 Replies that interrogate variable-length fattr4 attributes. Client implementations might rely on their own internal architectural limits to constrain the reply size, but such limits are not always guaranteed to be reliable.

When an especially large fattr4 result is expected, a Reply chunk might be required. An NFS version 4.0 client can use short Reply chunk retry when an NFS COMPOUND containing a GETATTR operation encounters a transport error.

The use of NFS COMPOUND operations raises the possibility of requests that combine a non-idempotent operation (e.g., RENAME) with a GETATTR operation that requests one or more variable-length results. This combination should be avoided by ensuring that any GETATTR operation that requests a result of unpredictable length is sent in an NFS COMPOUND by itself.

6.2.2. Reply Size Estimation for Minor Version 1 and Newer Minor


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 an NFS version 4.1 server.

This value can be used in cases where it is not possible to precisely estimate a reply size upper bound. In practice, objects such as ACLs, named attributes, layout bodies, and security labels are much smaller than this maximum.

6.3. RPC Binding Considerations

NFS version 4 servers are required to listen on TCP port 2049, and they are not required to register with an rpcbind service [RFC7530].

Therefore, an NFS version 4 server supporting RPC-over-RDMA version 1 MUST use the alternative well-known port number for its RPC-over-RDMA service (see Section 9). Clients SHOULD connect to this well-known port without consulting the RPC portmapper (as for NFS version 4 on TCP transports).

6.4. NFS COMPOUND Requests

6.4.1. Multiple DDP-Eligible Data Items

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

In the following list, a "READ operation" refers to any NFS version 4 operation that has a DDP-eligible result data item. The mechanism specified in Section 4.3.2 of [RFC8166] is applied to this class of operations:

  • 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.
  • 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.
  • If an NFS version 4 client has provided a matching non-empty Write chunk, then the corresponding READ operation MUST return its DDP-eligible data item using that chunk.
  • If an NFS version 4 client has provided an empty matching Write chunk, then the corresponding READ operation MUST return all of its result data items inline.
  • If a READ operation returns a union arm that does not contain a DDP-eligible result, and the NFS version 4 client has provided a matching non-empty Write chunk, an NFS version 4 server MUST return an empty Write chunk in that Write list position.
  • If there are more READ operations than Write chunks, then remaining NFS READ operations in an NFS version 4 COMPOUND that have no matching Write chunk MUST return their results inline.

6.4.2. Chunk List Complexity

The RPC-over-RDMA version 1 protocol does not place any limit on the number of chunks or segments that may appear in Read or Write lists. However, for various reasons, NFS version 4 server implementations often have practical limits on the number of chunks or segments they are prepared to process in a single RPC transaction conveyed via RPC-over-RDMA version 1.

These implementation limits are especially important when Kerberos integrity or privacy is in use [RFC7861]. Generic Security Service (GSS) services increase the size of credential material in RPC headers, potentially requiring more frequent use of Long messages. This can increase the complexity of chunk lists independent of the NFS version 4 COMPOUND being conveyed.

In the absence of explicit knowledge of the server's limits, NFS version 4 clients SHOULD follow the prescriptions listed below when constructing RPC-over-RDMA version 1 messages. NFS version 4 servers MUST accept and process such requests.

  • The Read list can contain either a Position Zero Read chunk, one Read chunk with a non-zero Position, or both.
  • The Write list can contain no more than one Write chunk.
  • Any chunk can contain up to sixteen RDMA segments.

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

An NFS version 4 server SHOULD return one of the following responses to a client that has sent an RPC transaction via RPC-over-RDMA version 1, which cannot be processed due to chunk list complexity limits on the server:

  • A problem is detected by the transport layer while parsing the transport header in an RPC Call message. The server responds with an RDMA_ERROR message with the err field set to ERR_CHUNK.
  • A problem is detected during XDR decoding of the RPC Call message while the RPC layer reassembles the call's XDR stream. The server responds with an RPC Reply with its "reply_stat" field set to MSG_ACCEPTED and its "accept_stat" field set to GARBAGE_ARGS.

After receiving one of these errors, an NFS version 4 client SHOULD NOT retransmit the failing request, as the result would be the same error. It SHOULD immediately terminate the RPC transaction associated with the XID in the RPC Reply.

6.4.3. NFS Version 4 COMPOUND Example

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

Write list:

         A --> B --> C

NFS version 4 COMPOUND request:


                       |                   |                   |
                       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.

6.5. NFS Callback Requests

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

6.5.1. 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 an RPC-over-RDMA version 1 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 are 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 6.2.1 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.

6.5.2. 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 approach described in [RFC8167] when backchannel operations are conveyed on RPC-over-RDMA version 1 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 procedures defined in NFS versions 4.1 or 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 can support Long Calls in its backchannel, it reports a backchannel ca_maxrequestsize that is larger than the connection's inline thresholds. Otherwise, an NFS version 4 server MUST use only Short messages to convey backchannel operations.

6.6. Session-Related Considerations

The presence of an NFS session (defined in [RFC5661]) has no effect on the operation of RPC-over-RDMA version 1. None of the operations introduced to support NFS sessions (e.g., the SEQUENCE operation) contain DDP-eligible data items. There is no need to match the number of session slots with the number of available RPC-over-RDMA credits.

However, there are a few new cases where an RPC transaction can fail. For example, in response to an RPC request, a requester might receive an RDMA_ERROR message with an rdma_err value of ERR_CHUNK. These situations are not different from existing RPC errors, which an NFS session implementation is already prepared to handle for other transports. And as with other transports during such a failure, there might be no SEQUENCE result available to the requester to distinguish whether failure occurred before or after the requested operations were executed on the responder.

When a transport error occurs (e.g., RDMA_ERROR), the requester proceeds as usual to match the incoming XID value to a waiting RPC Call. The RPC transaction is terminated, and the result status is reported to the upper-layer protocol. The requester's session implementation then determines the session ID and slot for the failed request and performs slot recovery to make that slot usable again. If this were not done, that slot could be rendered permanently unavailable.

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

6.7. Transport Considerations

6.7.1. Congestion Avoidance

Section 3.1 of [RFC7530] states:

Where an NFSv4 implementation supports operation over the IP network protocol, the supported transport layer between NFS and IP MUST be an IETF standardized transport protocol that is specified to avoid network congestion; such transports include TCP and the Stream Control Transmission Protocol (SCTP).

Section 2.9.1 of [RFC5661] also states:

Even if NFSv4.1 is used over a non-IP network protocol, it is RECOMMENDED that the transport support congestion control.

It is permissible for a connectionless transport to be used under NFSv4.1; however, reliable and in-order delivery of data combined with congestion control by the connectionless transport is REQUIRED. As a consequence, UDP by itself MUST NOT be used as an NFSv4.1 transport.

RPC-over-RDMA version 1 is constructed on a platform of RDMA Reliable Connections [RFC8166] [RFC5041]. RDMA Reliable Connections are reliable, connection-oriented transports that guarantee in-order delivery, thus meeting all above requirements for NFS version 4 transports.

6.7.2. Retransmission and 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 a 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.

In addition to network partition and request loss scenarios, RPC-over-RDMA transport connections can be terminated when a Transport header is malformed, Reply messages are larger than receive resources, or when too many RPC-over-RDMA messages are sent at once. In such cases:

  • If there is a transport error indicated (i.e., RDMA_ERROR) before the disconnect or instead of a disconnect, the requester MUST respond to that error as prescribed by the specification of the RPC transport. Then, the NFS version 4 rules for handling retransmission apply.
  • If there is a transport disconnect and the responder has provided no other response for a request, then only the NFS version 4 rules for handling retransmission apply.

7. 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 version 1 transports [RFC8166]. Via IETF standards action, the Upper-Layer Binding specified in this document can be extended to cover (a) versions of the NFS version 4 protocol specified after NFS version 4 minor version 2 or (b) separately published extensions to an existing NFS version 4 minor version, as described in [RFC8178].

8. Security Considerations

RPC-over-RDMA version 1 supports all RPC security models, including RPCSEC_GSS security and transport-level security [RFC7861]. The choice of what Direct Data Placement mechanism to convey RPC argument and results does not affect this, since it changes only the method of data transfer. Because this document defines only the binding of the NFS protocols atop [RFC8166], all relevant security considerations are, therefore, to be described at that layer.

9. IANA Considerations

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

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

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

This document is listed as the reference for the nfsrdma port assignments.

10. References

10.1. Normative References

   [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
              RFC 1833, DOI 10.17487/RFC1833, August 1995,
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
   [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,
   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <>.
   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <>.
   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <>.
   [RFC8166]  Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call Version
              1", RFC 8166, DOI 10.17487/RFC8166, June 2017,
   [RFC8167]  Lever, C., "Bidirectional Remote Procedure Call on RPC-
              over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
              June 2017, <>.
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

10.2. Informative References

   [RFC1094]  Nowicki, B., "NFS: Network File System Protocol
              specification", RFC 1094, DOI 10.17487/RFC1094, March
              1989, <>.
   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813,
              DOI 10.17487/RFC1813, June 1995,
   [RFC5040]  Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
              Garcia, "A Remote Direct Memory Access Protocol
              Specification", RFC 5040, DOI 10.17487/RFC5040, October
              2007, <>.
   [RFC5041]  Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
              Data Placement over Reliable Transports", RFC 5041,
              DOI 10.17487/RFC5041, October 2007,
   [RFC5666]  Talpey, T. and B. Callaghan, "Remote Direct Memory Access
              Transport for Remote Procedure Call", RFC 5666,
              DOI 10.17487/RFC5666, January 2010,
   [RFC5667]  Talpey, T. and B. Callaghan, "Network File System (NFS)
              Direct Data Placement", RFC 5667, DOI 10.17487/RFC5667,
              January 2010, <>.
   [RFC8178]  Noveck, D., "Rules for NFSv4 Extensions and Minor
              Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
   [XNFS]     The Open Group, "Protocols for Interworking: XNFS, Version
              3W", Document Number C702, ISBN 1-85912-184-5, February

Appendix A. Changes Since RFC 5667

Corrections and updates made necessary by new language in [RFC8166] have been introduced. For example, references to deprecated features of RPC-over-RDMA version 1 (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. Material that duplicates what is in [RFC8166] has been deleted.

Material required by [RFC8166] for Upper-Layer Bindings that was not present in [RFC5667] has been added. A complete discussion of reply size estimation has been introduced for all protocols covered by the Upper-Layer Bindings in this document.

Technical corrections have been made. For example, the mention of 12KB and 36KB inline thresholds has been removed. The reference to a nonexistent NFS version 4 SYMLINK operation has been replaced.

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 versions 2 and 3 Upper-Layer Binding, as they conflict with [RFC8166].

A section discussing NFS version 4 retransmission and connection loss has been added.

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

  • An explicit discussion of NFS versions 4.0 and 4.1 backchannel operation have replaced the previous treatment of callback operations.
  • A section describing considerations when an NFS session is in use has been added.
  • An Upper-Layer Binding for NFS version 4.2 has been added.
  • A section suggesting a mechanism for periodically assessing connection health has been introduced.
  • Ambiguous or erroneous uses of key words from RFC 2119 have been corrected.
  • References to obsolete RFCs have been updated.
  • An IANA Considerations section has been added, which specifies the port assignments for NFS/RDMA. This replaces the example assignment that appeared in [RFC5666].
  • Code excerpts have been removed, and figures have been modernized.


The author gratefully acknowledges the work of Brent Callaghan and Tom Talpey on the original NFS Direct Data Placement specification [RFC5667]. Tom contributed the text of Section 6.4.2.

Dave Noveck provided an excellent review, constructive suggestions, and consistent navigational guidance throughout the process of drafting this document. Dave contributed the text of Sections 6.6 and 7 and insisted on precise discussion of reply size estimation.

Thanks to Karen Deitke for her sharp observations about idempotency, NFS COMPOUNDs, and NFS sessions.

Special thanks go to Transport Area Director Spencer Dawkins, NFSV4 Working Group Chair and Document Shepherd Spencer Shepler, and NFSV4 Working Group Secretary Thomas Haynes for their support. The author also wishes to thank Bill Baker and Greg Marsden for their support of this work.

Author's Address

   Charles Lever
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   United States of America
   Phone: +1 248 816 6463