]> Oracle Corporation

United States of America chuck.lever@oracle.com

Transport Network File System Version 4 This document specifies Upper-Layer Bindings of Network File System (NFS) protocol versions to RPC-over-RDMA version 2. Note Discussion of this draft takes place on the NFSv4 working group mailing list, archived at . Working Group information is available at . Submit suggestions and changes as pull requests at . Instructions are on that page.

Introduction The RPC-over-RDMA version 2 transport can employ direct data placement to convey data payloads associated with RPC transactions, as described in . As mandated by that document, RPC client and server implementations using RPC-over-RDMA version 2 MUST agree in advance which XDR data items and RPC procedures are eligible for 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 such a Binding. This document contains material required of Upper-Layer Bindings, as specified in Appendix A of , for the following NFS protocol versions:

• NFS version 2
• NFS version 3
• NFS version 4.0
• NFS version 4.1
• NFS version 4.2

The current document also provides Upper-Layer Bindings for auxiliary protocols used with NFS versions 2 and 3 (see ). This document assumes the reader is already familiar with concepts and terminology defined throughout and the documents it references.

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 when, and only when, they appear in all capitals, as shown here.

Upper-Layer Binding for NFS Versions 2 and 3 The Upper-Layer Binding specification in this section applies to NFS version 2 and NFS version 3 . For brevity, in this document, a "Legacy NFS client" refers to an NFS client using version 2 or version 3 of the NFS RPC program (100003) to communicate with an NFS server. Likewise, a "Legacy NFS server" is an NFS server communicating with clients using NFS version 2 or NFS version 3. The following XDR data items in NFS versions 2 and 3 are DDP- eligible:

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

All other argument or result data items in NFS versions 2 and 3 are not DDP-eligible. Regardless of whether an NFS operation is considered non-idempotent, a transport error might not indicate whether the server has processed the arguments of the RPC Call or whether the server has accessed or modified client memory associated with that RPC.

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

• Often, the expected Reply can fit in a limited number of RDMA Send messages. The Requester need not provision any RDMA resources for the Reply, relying instead on message continuation to handle the entire Reply message.
• In cases where the Upper Layer Binding permits direct data placement of the results (DDP), a Requester can provision Write chunks to receive those results. The Requester MUST reliably estimate the maximum size of each result receive via a Write chunk.
• A Requester that expects a large Reply message can provision a Reply chunk. The Requester MUST reliably estimate the maximum size of the payload received via the Reply chunk.
• If RDMA resources are not available to send a Reply, a Responder falls back to message continuation.

A correctly implemented Legacy NFS client thus avoids retransmission of non-idempotent NFS requests due to improperly estimated Reply resources.

RPC Binding Considerations Legacy NFS servers typically listen for clients on UDP and TCP port 2049. Additionally, they register these ports with a local portmapper service . A Legacy NFS server supporting RPC-over-RDMA version 2 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 ). The chosen port MAY be registered with the RPC portmapper using the netids assigned in .

Transport Considerations

Keep-Alive Legacy NFS client implementations can rely on connection keep-alive to detect when a Legacy NFS server has become unresponsive. When an NFS server is no longer responsive, client-side keep-alive terminates the connection, triggering reconnection and retransmission of outstanding RPC transactions. Some RDMA transports (such as the Reliable Connected QP type on InfiniBand) have no keep-alive mechanism. Without a disconnect or new RPC traffic, such connections can remain alive long after an NFS server has become unresponsive or unreachable. Once an NFS client has consumed all available RPC-over-RDMA version 2 credits on that transport connection, it awaits a reply indefinitely before sending another RPC request. Legacy NFS clients SHOULD reserve one RPC-over-RDMA version 2 credit to use for periodic server or connection health assessment. Either peer can use this credit to drive an RPC request on an otherwise idle connection, triggering either an affirmative server response or a connection termination.

Replay Detection Like NFSv4.0, Legacy NFS servers typically employ request replay detection to reduce the risk of data and file namespace corruption that could result when an NFS client retransmits a non-idempotent NFS request. A Legacy NFS server can send a cached response when a replay is detected, rather than executing the request again. Replay detection is not perfect, but it is usually adequate. For Legacy NFS servers, replay detection commonly utilizes heuristic indicators such as the IP address of the NFS client, the source port of the connection, the transaction ID of the request, and the contents of the request's RPC and upper-layer protocol headers. A Legacy NFS client is careful to re-use the same source port when reconnecting so that Legacy NFS servers can better detect RPC retransmission. However, a Legacy NFS client operating over an RDMA transport has no control over connection source ports. It is almost certain that an RPC request retransmitted on a new connection can never be detected as a replay if the receiving Legacy NFS server includes the connection source port in its replay detection heuristics. Therefore a Legacy NFS server using an RDMA transport should never use a connection's source port as part of its NFS request replay detection mechanism.

Upper-Layer Bindings for NFS Version 2 and 3 Auxiliary Protocols Storage administrators typically deploy NFS versions 2 and 3 with several other protocols, sometimes called the "NFS auxiliary protocols." These are distinct RPC programs that define procedures 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)
• Versions 1, 3, and 4 of the NLM RPC program (100021)
• Version 1 of the NSM RPC program (100024), described in Chapter 11 of
• Versions 2 and 3 of the NFSACL RPC program (100227). The NFSACL program does not have a public definition. This document treats the NFSACL program as a de facto standard, as there are several interoperating implementations.

MOUNT, NLM, and NSM Protocols Historically, NFS/RDMA implementations have conveyed the MOUNT, NLM, and NSM protocols via TCP. A Legacy NFS server implementation MUST provide support for these auxiliary protocols via TCP. Moreover, there is little benefit from transporting these protocols via RDMA. Thus this document does not provide an Upper-Layer binding for them.

NFSACL Protocol Legacy NFS clients and servers convey NFSACL procedures on the same transport connection and port as the NFS RPC program (100003). Utilizing the same port obviates the need for a separate rpcbind query to discover server support for this RPC program. ACLs are typically small, but even large ACLs must be encoded and decoded to some degree before being being stored in local filesystems. 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 each Reply is determined directly from the NFSACL RPC program's XDR definition. The NFSACL protocol does not provide a mechanism to determine the size of a received ACL in advance. When preparing for responses that include ACLs, Legacy NFS clients estimate a maximum reply size based on limits within their local file systems. If that estimation is inadequate, a Responder falls back to message continuation.

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 , NFS version 4.1 , and NFS version 4.2 .

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

The NFSv4.2 READ_PLUS operation NFS version 4.2 introduces an enhanced READ operation called READ_PLUS . READ_PLUS enables an NFS server to compact returned READ data payloads. No part of a READ_PLUS Reply is DDP-eligible. In a READ_PLUS result, returned file content appears as a list of one or more of the following items:

• Regular data content, the same as the result of a traditional READ operation
• Unallocated space in a file, where no data has been written, or previously-written data has been removed via a hole-punch operation
• A counted pattern

Upon receipt of a READ_PLUS result, an NFSv4.2 client expands the returned list into its preferred representation of the original file content. Before receiving that result, an NFSv4.2 client is unaware of how the NFS server has organized the file content. Thus it is not possible to predict the size or structure of a READ_PLUS Reply in advance. The use of direct data placement is therefore challenging. Moreover, the usual benefits of hardware-assisted data placement are entirely lost if the client must parse the result of each READ I/O. Therefore this Upper Layer Binding does not make elements of an NFSv4.2 READ_PLUS Reply DDP-eligible. Further, this Upper Layer Binding recommends that NFS client implemenations avoid using the READ_PLUS operation on NFS/RDMA mount points.

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

• The attrlist4 field
• Fields containing ACLs such as fattr4_acl, fattr4_dacl, and fattr4_sacl
• Fields in the fs_locations4 and fs_locations_info4 data structures
• Fields which pertain to pNFS layout metadata, such as loc_body, loh_body, da_addr_body, lou_body, lrf_body, fattr_layout_types, and fs_layout_types

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 Replies. Variable-length fattr4 attributes make it particularly difficult for clients to predict the maximum size of some NFS version 4.0 Replies. Client implementations might rely upon internal architectural limits to constrain the reply size, but such limits are not always reliable. When an NFS version 4.0 client cannot predict the size of a Reply, it can rely on message continuation to enable a Reply under any circumstances.

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 is the absolute maximum size of replies generated by an NFS version 4.1 server. An NFS version 4 client can use this value when it is impossible to estimate a reply size upper bound precisely. In practice, objects such as ACLs, named attributes, layout bodies, and security labels are much smaller than this maximum.

RPC Binding Considerations NFS version 4 servers are required to listen on TCP port 2049 and are not required to register with an rpcbind service . 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 defined in .

NFS COMPOUND Requests

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 determine which chunk is associated with which argument data item. However, NFS version 4 server and client implementations must agree on how to pair Write chunks with returned result data items. A "READ operation" refers to any NFS version 4 operation with a DDP-eligible result data item in the following lists. An NFS version 4 client applies the mechanism specified in to this class of operations as follows:

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

An NFS version 4 server acts as follows:

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

Chunk List Complexity By default, the RPC-over-RDMA version 2 protocol limits the number of chunks or segments that may appear in Read or Write lists (see ). These implementation limits are significant when Kerberos integrity or privacy is in use . GSS services increase the size of credential material in RPC headers, potentially requiring the more frequent use of less efficient Special Payload or Continued Payload messages. NFS version 4 clients follow the prescriptions listed below when constructing RPC-over-RDMA version 2 messages in the absence of an explicit transport property exchange that alters these limits. NFS version 4 servers MUST accept and process all such requests.

• The Read list can contain either a Call chunk, no more than one Read chunk, or both a Call chunk and one Read chunk.
• The Write list can contain no more than one Write chunk.

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

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. 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 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.

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

NFS Version 4.0 Callback An NFS version 4.0 client uses the SETCLIENTID operation for advertising the IP address, port, and netid of its NFS version 4.0 callback service. When an NFS version 4.0 server provides a backchannel service to an NFS version 4.0 client that uses RPC-over-RDMA version 2 for its forward channel, the server MUST advertise the backchannel service using either the "tcp" or "tcp6" netid. Because the NFSv4.0 backchannel does not operate on RPC-over-RDMA, this document does not specify an Upper-Layer binding for the NFSv4.0 backchannel RPC program.

NFS Version 4.1 Callback In NFS version 4.1 and newer minor versions, callback operations may appear on the same connection that is in use for NFS version 4 forward channel client requests. NFS version 4 clients and servers MUST use the mechanisms described in to convey backchannel operations on an RPC-over-RDMA version 2 transport. The csa_back_chan_attrs argument of the CREATE_SESSION operation contains a ca_maxresponsesize field. The value in this field is the absolute maximum size of backchannel replies generated by a replying NFS version 4 client. There are no DDP-eligible data items in callback procedures defined in NFS version 4.1 or NFS version 4.2. However, some callback operations, such as messages that convey device ID information, can be sizeable. A sender can use Message Continuation or a Special Payload message in this situation. When an NFS version 4.1 client can support Special Payload Calls in its backchannel, it reports a backchannel ca_maxrequestsize that is larger than the connection's inline thresholds. Otherwise, an NFS version 4 server MUST use only Simple Payload or Continued Payload messages to convey backchannel operations.

Session-Related Considerations The presence of an NFS version 4 session (as defined in ) does not affect the operation of RPC-over-RDMA version 2. None of the operations introduced to support NFS sessions (e.g., the SEQUENCE operation) contain DDP-eligible data items. There is no need to match the number of session slots with the 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 a rdma_err value of RDMA2_ERR_BADXDR. These situations are not different from existing RPC errors, which an NFS session implementation can already handle for other transport types. Moreover, there might be no SEQUENCE result available to the Requester to distinguish whether failure occurred before or after the Responder executed the requested operations. When a transport error occurs (e.g., an RDMA2_ERROR type message is received), the Requester proceeds, as usual, to match the incoming XID value to a waiting RPC Call. The Requester terminates the RPC transaction and reports the result status to the RPC consumer. The Requester's session implementation then determines the session ID and slot for the failed request and performs slot recovery to make that slot usable again. Otherwise, that slot is rendered permanently unavailable. When an NFS session is not present (for example, when NFS version 4.0 is in use), a transport error does not indicate whether the server has processed the arguments of the RPC Call, or whether the server has accessed or modified client memory associated with that RPC.

Transport Considerations

Congestion Avoidance states:

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

further states:

• Even if NFS version 4.1 is used over a non-IP network protocol, it is RECOMMENDED that the transport support congestion control. It is permissible for a connectionless transport to be used under NFS version 4.1; however, reliable and in-order delivery of data combined with congestion control by the connectionless transport is REQUIRED. As a consequence, UDP by itself MUST NOT be used as an NFS version 4.1 transport.

RPC-over-RDMA version 2 utilizes only reliable, connection-oriented transports that guarantee in-order delivery, meeting all the above requirements for NFS version 4.0 and 4.1. See for more details.

Retransmission and Keep-alive NFS version 4 client implementations often rely on a transport-layer connection keep-alive mechanism to detect when an NFS version 4 server has become unresponsive. When an NFS server is no longer responsive, client-side keep-alive terminates the connection, triggering reconnection and RPC retransmission. Some RDMA transports (such as the Reliable Connected QP type on InfiniBand) have no keep-alive mechanism. Without a disconnect or new RPC traffic, such connections can remain alive long after an NFS server has become unresponsive. Once an NFS client has consumed all available RPC-over-RDMA version 2 credits on that transport connection, it indefinitely awaits a reply before sending another RPC request. NFS version 4 peers SHOULD reserve one RPC-over-RDMA version 2 credit for periodic server or connection health assessment. Either peer can use this credit to drive an RPC request on an otherwise idle connection, triggering either a quick affirmative server response or immediate connection termination. In addition to network partition and request loss scenarios, RPC-over-RDMA version 2 peers can terminate a connection when a Transport header is malformed or when too many RPC-over-RDMA messages are sent without a credit update. In such cases:

• If a transport error occurs (e.g., an RDMA2_ERROR type message is received) just before the disconnect or instead of a disconnect, the Requester MUST respond to that error as prescribed by the specification of the RPC transport. Then the NFS version 4 rules for handling retransmission apply.
• If there is a transport disconnect and the Responder has provided no other response for a request, then only the NFS version 4 rules for handling retransmission apply.

Extending NFS Upper-Layer Bindings RPC programs such as NFS must have an Upper-Layer Binding specification to operate on an RPC-over-RDMA version 2 transport . 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 .

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

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 DDP protocol is such an example . For this purpose, the current document lists a set of port number assignments that IANA has already assigned for NFS/RDMA in the IANA port registry, according to the guidelines described in . The author requests that IANA add the current document as a reference for the existing nfsrdma port assignments. This document does not alter these assignments.

References Normative References Oracle Corporation NetApp This document specifies the second version of a transport protocol that conveys Remote Procedure Call (RPC) messages using Remote Direct Memory Access (RDMA). This version of the protocol is extensible. The Network File System (NFS) version 4 protocol is a distributed file system protocol that builds on the heritage of NFS protocol version 2 (RFC 1094) and version 3 (RFC 1813). Unlike earlier versions, the NFS version 4 protocol supports traditional file access while integrating support for file locking and the MOUNT protocol. In addition, support for strong security (and its negotiation), COMPOUND operations, client caching, and internationalization has been added. Of course, attention has been applied to making NFS version 4 operate well in an Internet environment. This document, together with the companion External Data Representation (XDR) description document, RFC 7531, obsoletes RFC 3530 as the definition of the NFS version 4 protocol. This document describes the Network File System (NFS) version 4 minor version 1, including features retained from the base protocol (NFS version 4 minor version 0, which is specified in RFC 7530) and protocol extensions made subsequently. The later minor version has no dependencies on NFS version 4 minor version 0, and is considered a separate protocol. This document obsoletes RFC 5661. It substantially revises the treatment of features relating to multi-server namespace, superseding the description of those features appearing in RFC 5661. This document describes NFS version 4 minor version 2; it describes the protocol extensions made from NFS version 4 minor version 1. Major extensions introduced in NFS version 4 minor version 2 include the following: Server-Side Copy, Application Input/Output (I/O) Advise, Space Reservations, Sparse Files, Application Data Blocks, and Labeled NFS. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes the binding protocols used in conjunction with the ONC Remote Procedure Call (ONC RPC Version 2) protocols. [STANDARDS-TRACK] This document specifies version 3 of the Remote Procedure Call (RPC) security protocol (RPCSEC_GSS). This protocol provides support for multi-principal authentication of client hosts and user principals to a server (constructed by generic composition), security label assertions for multi-level security and type enforcement, structured privilege assertions, and channel bindings. This document updates RFC 5403. This document defines the procedures that the Internet Assigned Numbers Authority (IANA) uses when handling assignment and other requests related to the Service Name and Transport Protocol Port Number registry. It also discusses the rationale and principles behind these procedures and how they facilitate the long-term sustainability of the registry. This document updates IANA's procedures by obsoleting the previous UDP and TCP port assignment procedures defined in Sections 8 and 9.1 of the IANA Allocation Guidelines, and it updates the IANA service name and port assignment procedures for UDP-Lite, the Datagram Congestion Control Protocol (DCCP), and the Stream Control Transmission Protocol (SCTP). It also updates the DNS SRV specification to clarify what a service name is and how it is registered. This memo documents an Internet Best Current Practice. Informative References This document describes the rules relating to the extension of the NFSv4 family of protocols. It covers the creation of minor versions, the addition of optional features to existing minor versions, and the correction of flaws in features already published as Proposed Standards. The rules relating to the construction of minor versions and the interaction of minor version implementations that appear in this document supersede the minor versioning rules in RFC 5661 and other RFCs defining minor versions. The Open Group This RFC describes a protocol that Sun Microsystems, Inc., and others are using. A new version of the protocol is under development, but others may benefit from the descriptions of the current protocol, and discussion of some of the design issues. This paper describes the NFS version 3 protocol. This paper is provided so that people can write compatible implementations. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. The Direct Data Placement protocol provides information to Place the incoming data directly into an upper layer protocol's receive buffer without intermediate buffers. This removes excess CPU and memory utilization associated with transferring data through the intermediate buffers. [STANDARDS-TRACK]

Acknowledgments Thanks to Tom Talpey, who contributed the text of . David Noveck contributed the text of and . The author also wishes to thank Bill Baker and Greg Marsden for their support of this work. Special thanks go to Transport Area Directors Zaheduzzaman Sarker, NFSV4 Working Group Chairs Brian Pawlowski, and David Noveck, and NFSV4 Working Group Secretary Thomas Haynes for their support.