| Network File System Version 4 | C. Lever, Ed. |
| Internet-Draft | Oracle |
| Obsoletes: 5667 (if approved) | April 12, 2017 |
| Intended status: Standards Track | |
| Expires: October 14, 2017 |
Network File System (NFS) Upper Layer Binding To RPC-Over-RDMA Version One
draft-ietf-nfsv4-rfc5667bis-09
This document specifies Upper Layer Bindings of Network File System (NFS) protocol versions to RPC-over-RDMA Version One, enabling the use of Direct Data Placement. This document obsoletes RFC 5667.
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].
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The RPC-over-RDMA Version One transport may employ direct data placement to convey data payloads associated with RPC transactions [I-D.ietf-nfsv4-rfc5666bis]. To enable successful interoperation, RPC client and server implementations using RPC-over-RDMA Version One 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 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.ietf-nfsv4-rfc5666bis], for the following NFS protocol versions:
Upper Layer Bindings are also provided for auxiliary protocols used with NFS versions 2 and 3.
This document assumes the reader is already familiar with concepts and terminology defined in [I-D.ietf-nfsv4-rfc5666bis] and the documents it references.
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 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:
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:
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.
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.
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 the NFS version 2 or NFS version 3 RPC Programs (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.
A Legacy NFS client determines the maximum reply size for each operation using the criteria outlined in Section 2. There are no operations in NFS version 2 or 3 that benefit from short Reply chunk retry.
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 One 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 8). The chosen port MAY be registered with the RPC portmapper under the netids assigned in [I-D.ietf-nfsv4-rfc5666bis].
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 version 2 or version 3 RPC Programs. The Upper Layer Bindings in this section apply to:
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 TCP-based MOUNT, NLM, and NSM services.
Legacy clients and servers that support the NFSACL RPC Program typically convey NFSACL procedures on the same connection as NFS RPC Programs. 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.
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.
The Upper Layer Binding specification in this section applies to RPC Programs defined in NFS Version 4.0 [RFC7530], NFS Version 4.1 [RFC5661], and NFS Version 4.2 [RFC7862].
Only the following XDR data items in the COMPOUND procedure of all NFS version 4 minor versions are DDP-eligible:
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 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 5.2 (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.
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.
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 One MUST use the alternative well-known port number for its RPC-over-RDMA service (see Section 8). Clients SHOULD connect to this well-known port without consulting the RPC portmapper (as for NFS version 4 on TCP transports).
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.ietf-nfsv4-rfc5666bis]) is applied to this class of operations:
If an NFS version 4 client sends an RPC Call with a Write list that contains more chunks than an NFS version 4 server is prepared to process, the server MUST reject the RPC by responding with an RDMA_ERROR message with the rdma_err value set to ERR_CHUNK.
If an NFS version 4 client sends an RPC Call with a Read list that contains more chunks than an NFS version 4 server is prepared to process, the server MUST reject the RPC by responding with an RDMA_MSG message containing an RPC Reply with an accept status of GARBAGE_ARGS, or with an RDMA_ERROR message with the rdma_err value set to ERR_CHUNK.
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:
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.
The NFS version 4 family of protocols support server-initiated callbacks to notify NFS version 4 clients of events such as recalled delegations.
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 One 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.
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 [I-D.ietf-nfsv4-rpcrdma-bidirection] when backchannel operations are conveyed on RPC-over-RDMA Version One 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, 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.
The presence of an NFS session (defined in [RFC5661]) has no effect on the operation of RPC-over-RDMA Version One. 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, a requester might receive, in response to an RPC request, an RDMA_ERROR message with an rdma_err value of ERR_CHUNK. These situations are no 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 is not done, that slot could be rendered permanently unavailable.
Section 3.1 of [RFC7530] states:
Section 2.9.1 of [RFC5661] also states:
RPC-over-RDMA Version One is constructed on a platform of RDMA Reliable Connections [I-D.ietf-nfsv4-rfc5666bis] [RFC5041]. RDMA Reliable Connections are reliable, connection-oriented transports that guarantee in-order delivery, meeting all above requirements for NFS version 4 transports.
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.
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:
RPC Programs such as NFS are required to have an Upper Layer Binding specification to interoperate on RPC-over-RDMA Version One 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].
RPC-over-RDMA Version One supports all RPC security models, including RPCSEC_GSS security and transport-level security [RFC2203]. 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. 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.
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 should be listed as the reference for the nfsrdma port assignments. This document does not alter these assignments.
Corrections and updates made necessary by new language in [I-D.ietf-nfsv4-rfc5666bis] have been introduced. For example, references to deprecated features of RPC-over-RDMA Version One, such as RDMA_MSGP, and the use of the Read list for handling RPC replies, have been removed. The term "mapping" has been replaced with the term "binding" or "Upper Layer Binding" throughout the document. Material that duplicates what is in [I-D.ietf-nfsv4-rfc5666bis] has been deleted.
Material required by [I-D.ietf-nfsv4-rfc5666bis] for Upper Layer Bindings that was not present in [RFC5667] has been added. 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 have been removed. The reference to a non-existant 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 version 2 and 3 Upper Layer Binding, as they conflict with [I-D.ietf-nfsv4-rfc5666bis].
A section discussing NFS version 4 retransmission and connection loss has been added.
The following additional improvements have been made, relative to [RFC5667]:
The author gratefully acknowledges the work of Brent Callaghan and Tom Talpey on the original NFS Direct Data Placement specification [RFC5667]. The author also wishes to thank Bill Baker and Greg Marsden for their support of this work.
Dave Noveck provided excellent review, constructive suggestions, and consistent navigational guidance throughout the process of drafting this document. Dave also contributed the text of Section 5.6 and Section 6, 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 Spencer Shepler, and nfsv4 Working Group Secretary Thomas Haynes for their support.