Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Standards Track November 17, 2019
Expires: May 20, 2020

Network File System (NFS) Upper Layer Binding To RPC-Over-RDMA Version 2


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

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

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 May 20, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Table of Contents

1. Introduction

The RPC-over-RDMA version 2 transport may employ direct data placement to convey data payloads associated with RPC transactions [I-D.cel-nfsv4-rpcrdma-version-two]. To enable successful interoperation, RPC client and server implementations using RPC-over-RDMA version 2 must agree which XDR data items and RPC procedures are eligible to use direct data placement (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 [I-D.cel-nfsv4-rpcrdma-version-two], for the following NFS protocol versions: Section 5).

Upper Layer Bindings are also provided for auxiliary protocols used with NFS versions 2 and 3 (see

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

2. Requirements Language

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

3. 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 MAY provide Write and/or Reply chunks wherein the Responder can place results and the reply's Payload stream.

4. Upper Layer Binding for NFS Versions 2 and 3

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

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

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.

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 service [RFC1833].

A Legacy NFS server supporting RPC-over-RDMA version 2 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 using the netids assigned in [I-D.cel-nfsv4-rpcrdma-version-two].

5. Upper Layer Bindings for NFS Version 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 which are not part of the NFS RPC program (100003). The Upper Layer Bindings in this section apply to:

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.

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 RPC program's XDR definition. Legacy client implementations should choose a maximum size for ACLs based on their own internal limits.

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 version 4.0 [RFC7530] NFS version 4.1 [RFC5661] and NFS version 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:

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 result arrays. These include:

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, an NFS version 4.0 client can provide a Reply chunk to enable a large result to be returned via explicit RDMA. An NFS version 4.0 client can use short Reply chunk retry when an NFS COMPOUND containing a GETATTR operation encounters a transport error.

6.2.2. 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 an NFS version 4.1 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.

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 2 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 which has a DDP-eligible result data item. The mechanism specified in Section 4.3.2 of [I-D.cel-nfsv4-rpcrdma-version-two] is applied to this class of operations:

6.4.2. Chunk List Complexity

The RPC-over-RDMA version 2 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 2.

These implementation limits are especially important when Kerberos integrity or privacy is in use [RFC7861]. GSS services increase the size of credential material in RPC headers, potentially requiring the 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 2 messages. NFS version 4 servers MUST accept and process such requests.

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 2 which cannot be processed due to chunk list complexity limits on the server:

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 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 2 transport.

No operation in the NFS version 4.0 callback RPC program conveys a data payload of significant size. 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 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 2 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 version 4.1 or NFS version 4.2. However, some callback operations, such as messages that convey device ID information, can be large. Message Continuation or a Long message might be used in this situation.

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 2. None of the operations introduced to support NFS sessions (e.g. the SEQUENCE operation) contain DDP-eligible data items. There is no need to match the number of session slots with the number of available RPC-over-RDMA version 2 credits.

However, there are a few new cases where an RPC transaction can fail. For example, a Requester might receive, in response to an RPC request, an RDMA2_ERROR message with 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. RDMA2_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:

Section 2.9.1 of [RFC5661] further states:

RPC-over-RDMA version 2 is constructed on a platform of RDMA Reliable Connected QP type connections [I-D.cel-nfsv4-rpcrdma-version-two] [RFC5041]. RDMA Reliable Connected QPs are reliable, connection-oriented transports that guarantee in-order delivery, 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 the Reliable Connected QP type on InfiniBand) have no keep-alive mechanism. Without a disconnect or new RPC traffic, such connections can remain alive long after an NFS server has become unresponsive. Once an NFS client has consumed all available RPC-over-RDMA version 2 credits on that transport connection, it will forever await a reply before sending another RPC request.

NFS version 4 clients SHOULD reserve one RPC-over-RDMA version 2 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.

In addition to network partition and request loss scenarios, RPC-over-RDMA version 2 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:

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 2 transports [I-D.cel-nfsv4-rpcrdma-version-two]. Via standards action, the Upper Layer Binding specified in this document can be extended to cover versions of the NFS version 4 protocol specified after NFS version 4 minor version 2, or to cover separately published extensions to an existing NFS version 4 minor version, as described in [RFC8178].

8. Security Considerations

RPC-over-RDMA version 2 supports all RPC security models, including RPCSEC_GSS security and transport-level security [RFC7861]. The choice of what Direct Data Placement mechanism to convey RPC argument and results does not affect this, since it changes only the method of data transfer. Because the current document defines only the binding of the NFS protocols atop [I-D.cel-nfsv4-rpcrdma-version-two], 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 [RFC5040] [RFC5041].

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 should be listed as a reference for the nfsrdma port assignments. This document does not alter these assignments.

10. References

10.1. Normative References

[I-D.cel-nfsv4-rpcrdma-version-two] Lever, C. and D. Noveck, "RPC-over-RDMA Version 2 Protocol", Internet-Draft draft-cel-nfsv4-rpcrdma-version-two-09, May 2019.
[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., Eisler, M. and D. Noveck, "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. and D. Noveck, "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.
[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.
[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", February 1998.


Thanks to Tom Talpey, who contributed the text of Section 6.4.2. Dave Noveck contributed the text of Section 6.6 and Section 7.

Special thanks go to Transport Area Director Magnus Westerlund, NFSV4 Working Group Chairs Spencer Shepler and Brian Pawlowski, 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 United States of America EMail: