| Network File System Version 4 | C. Lever, Ed. |
| Internet-Draft | Oracle |
| Obsoletes: 5667 (if approved) | January 20, 2017 |
| Intended status: Standards Track | |
| Expires: July 24, 2017 |
Network File System (NFS) Upper Layer Binding To RPC-Over-RDMA
draft-ietf-nfsv4-rfc5667bis-04
This document specifies Upper Layer Bindings of Network File System (NFS) protocol versions to RPC-over-RDMA. Upper Layer Bindings are required to enable RPC-based protocols, such as NFS, to use Direct Data Placement on RPC-over-RDMA. 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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An RPC-over-RDMA transport, such as the one defined in [I-D.ietf-nfsv4-rfc5666bis], may employ direct data placement to convey data payloads associated with RPC transactions. To enable successful interoperation, RPC client and server implementations must agree as to which XDR data items in what particular RPC procedures are eligible for direct data placement (DDP).
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 specified in this document apply to all versions of RPC-over-RDMA.
Definitions of terminology and a general discussion of how RPC-over-RDMA is used to convey RPC transactions can be found in [I-D.ietf-nfsv4-rfc5666bis]. In this section, these general principles are applied in the context of conveying NFS procedures on RPC-over-RDMA. Some issues common to all NFS protocol versions are introduced.
The Read list in each RPC-over-RDMA transport header represents a set of memory regions containing DDP-eligible NFS argument data. Large data items, such as the data payload of an NFS version 3 WRITE procedure, can be referenced by the Read list. The NFS server pulls such payloads from the client and places them directly into its own memory.
Exactly which XDR data items may be conveyed in this fashion is detailed later in this document.
The Write list in each RPC-over-RDMA transport header represents a set of memory regions that can receive DDP-eligible NFS result data. Large data items, such as the payload of an NFS version 3 READ procedure, can be referenced by the Write list. The NFS server pushes such payloads to the client, placing them directly into the client's memory.
Each Write chunk corresponds to a specific XDR data item in an NFS reply. This document specifies how NFS client and server implementations identify the correspondence between Write chunks and XDR results.
Exactly which XDR data items may be conveyed in this fashion is detailed later in this document.
Small RPC messages are conveyed using RDMA Send operations which are of limited size. If an NFS request is too large to be conveyed within the NFS server's responder inline threshold, and there are no DDP-eligible data items that can be removed, an NFS client must send the request in the form of a Long Call. The entire NFS request is sent in a special Read chunk called a Position Zero Read chunk.
If an NFS client determines that the maximum size of an NFS reply could be too large to be conveyed within it's own responder inline threshold, it provides a Reply chunk in the RPC-over-RDMA transport header conveying the NFS request. The server places the entire NFS reply in the Reply chunk.
When the RPC authentication flavor requires that DDP-eligible data items are never removed from RPC messages, an NFS client can provide both a Position Zero Read chunk and a Reply chunk for the same RPC.
These special chunks are discussed in further detail in [I-D.ietf-nfsv4-rfc5666bis].
A chunk typically corresponds to exactly one XDR data item. Each Read chunk is represented as a list of segments at the same XDR Position. Each Write chunk is represented as an array of segments. An NFS client thus has the flexibility to advertise a set of discontiguous memory regions in which to convey a single DDP-eligible XDR data item.
To report a DDP-eligibity violation, an NFS server MUST return one of:
Subsequent sections of this document describe further considerations particular to specific NFS protocols or procedures.
During the construction of each RPC Call message, an NFS client is responsible for allocating appropriate resources for receiving the matching Reply message. A Reply buffer overrun can result in corruption of the Reply message or termination of the transport connection. Therefore reliable reply size estimation is necessary to ensure successful interoperation.
In many cases the Upper Layer Protocol's XDR definition provides enough information to enable the client to make a reliable prediction of the maximum size of the expected Reply message. If there are variable-size data items in the result, the maximum size of the RPC Reply message can be reliably estimated in most cases:
It is occasionally not possible to determine the maximum Reply message size based solely on the above criteria. NFS client implementers can choose to provide the largest possible Reply buffer in those cases, based on, for instance, the largest possible NFS READ or WRITE payload (which is negotiated at mount time).
In rare cases, a client may encounter a reply for which no a priori determination of reply size bound is possible. The client SHOULD expect a transport error to indicate that it must either terminate that RPC transaction, or retry it with a larger Reply chunk.
The use of NFS COMPOUND operations raises the possibility of non-idempotent requests that combine a non-idempotent operation with an operation whose reply size is uncertain. This causes potential difficulties with retrying the transaction. Note however that many operations normally considered non-idempotent (e.g WRITE, SETATTR) are actually idempotent. Truly non-idempotent operations are quite unusual in COMPOUNDs that include operations with uncertain reply sizes.
This Upper Layer Binding specification applies to NFS Version 2 [RFC1094] and NFS Version 3 [RFC1813]. For brevity, in this section a "legacy NFS client" refers to an NFS client using NFS version 2 or NFS version 3 to communicate with an NFS server. Likewise, a "legacy NFS server" is an NFS server communicating with clients using NFS version 2 or NFS version 3.
The following XDR data items in NFS versions 2 and 3 are DDP-eligible:
All other argument or result data items in NFS versions 2 and 3 are not DDP-eligible.
A legacy server's response to a DDP-eligibility violation (described in Section 2.5) does not give an indication to legacy clients 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 basic criteria outlined in Section 2.6. Such clients provide a Reply chunk when the maximum possible reply size, exclusive of any data items represented by Write chunks, is larger than the client's responder inline threshold.
NFS versions 2 and 3 are typically deployed with several other protocols, sometimes referred to as "NFS auxiliary protocols." These are separate RPC programs that define procedures which are not part of the NFS version 2 or version 3 RPC programs. These include:
RPC-over-RDMA considers these programs as distinct Upper Layer Protocols [I-D.ietf-nfsv4-rfc5666bis]. To enable the use of these ULPs on an RPC-over-RDMA transport, an Upper Layer Binding specification is provided here for each.
Typically MOUNT, NLM, and NSM are conveyed via TCP, even in deployments where NFS operations on RPC-over-RDMA. When a legacy server supports these programs on RPC-over-RDMA, it advertises the port address via the usual rpcbind service [RFC1833].
No operation in these protocols conveys a significant data payload, and the size of RPC messages in these protocols is uniformly small. Therefore, no XDR data items in these protocols are DDP-eligible. The largest variable-length XDR data item is an xdr_netobj. In most implementations this data item is not larger than 1024 bytes, making reliable reply size estimation straightforward using the criteria outlined in Section 2.6.
Legacy clients and servers that support the NFSACL RPC program typically convey NFSACL procedures on the same connection as the NFS RPC program. This obviates the need for separate rpcbind queries to discover server support for this RPC program.
ACLs are typically small, but even large ACLs must be encoded and decoded to some degree. Thus no data item in this Upper Layer Protocol is DDP-eligible.
For procedures whose replies do not include an ACL object, the size of a reply is determined directly from the NFSACL program's XDR definition.
There is no protocol-wide size limit for NFS version 3 ACLs, and there is no mechanism in either the NFSACL or NFS programs for a legacy client to ascertain the largest ACL a legacy server can store. Legacy client implementations should choose a maximum size for ACLs based on their own internal limits. A recommended lower bound for this maximum is 32,768 bytes, though a larger Reply chunk (up to the negotiated rsize setting) can be provided.
This Upper Layer Binding specification applies to all protocols defined in NFS Version 4.0 [RFC7530], NFS Version 4.1 [RFC5661], and NFS Version 4.2 [RFC7862].
Only the following XDR data items in the COMPOUND procedure of all NFS version 4 minor versions are DDP-eligible:
The NFS version 4.2 READ_PLUS operation returns a complex data type [RFC7862]. The rpr_contents field in the result of this operation is an array of read_plus_content unions, one arm of which contains an opaque byte stream (d_data).
The size of d_data is limited to the value of the rpa_count field, but the protocol does not bound the number of elements which can be returned in the rpr_contents array. In order to make the size of READ_PLUS replies predictable by NFS version 4.2 clients, the following restrictions are placed on the use of the READ_PLUS operation on RPC-over-RDMA transports:
An NFS version 4 client provides a Reply chunk when the maximum possible reply size is larger than the client's responder inline threshold.
There are certain NFS version 4 data items whose size cannot be estimated by clients reliably, however, because there is no protocol-specified size limit on these structures. These include:
The items enumerated above in Section 4.2 make it difficult to predict the maximum size of GETATTR replies that interrogate variable-length attributes. As discussed in Section 2.6, client implementations can rely on their own internal architectural limits to bound the reply size, but such limits are not guaranteed to be reliable.
If a client implementation is equipped to recognize that a transport error could mean that it provisioned an inadequately sized Reply chunk, it can retry the operation with a larger Reply chunk. Otherwise, the client must terminate the RPC transaction.
It is best to avoid issuing single COMPOUNDs that contain both non-idempotent operations and operations where the maximum reply size cannot be reliably predicted.
In NFS version 4.1 and newer minor versions, the csa_fore_chan_attrs argument of the CREATE_SESSION operation contains a ca_maxresponsesize field. The value in this field can be taken as the absolute maximum size of replies generated by a replying NFS version 4 server.
This value can be used in cases where it is not possible to estimate a reply size upper bound precisely. In practice, objects such as ACLs, named attributes, layout bodies, and security labels are much smaller than this maximum.
The NFS version 4 COMPOUND procedure allows the transmission of more than one DDP-eligible data item per Call and Reply message. An NFS version 4 client provides XDR Position values in each Read chunk to disambiguate which chunk is associated with which argument data item. However NFS version 4 server and client implementations must agree in advance on how to pair Write chunks with returned result data items.
The mechanism specified in Section 4.3.2 of [I-D.ietf-nfsv4-rfc5666bis]) is applied here, with additional restrictions that appear below. In the following list, an "NFS Read" operation refers to any NFS Version 4 operation which has a DDP-eligible result data item (i.e., either a READ, READ_PLUS, or READLINK operation).
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 protocols support server-initiated callbacks to notify 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 a RPC-over-RDMA transport.
No operation in the NFS version 4.0 callback RPC program conveys a significant data payload. Therefore, no XDR data items in this RPC program is DDP-eligible.
A CB_RECALL reply is small and fixed in size. The CB_GETATTR reply contains a variable-length fattr4 data item. See Section 4.2.1 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 mechanism described in [I-D.ietf-nfsv4-rpcrdma-bidirection] when backchannel operations are conveyed on RPC-over-RDMA 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 reports a backchannel ca_maxrequestsize that is larger than the connection's inline thresholds, the NFS version 4 client can support Long Calls. Otherwise an NFS version 4 server MUST use Short messages to convey backchannel operations.
Typically the presence of an NFS session [RFC5661] has no effect on the operation of RPC-over-RDMA. None of the operations introduced to support NFS sessions contain DDP-eligible data items. There is no need to match the number of session slots with the number of available RPC-over-RDMA credits.
However, there are some rare error conditions which require special handling when an NFS session is operating on an RPC-over-RDMA transport. For example, a requester might receive, in response to an RPC request, an RDMA_ERROR message with an rdma_err value of ERR_CHUNK, or an RDMA_MSG containing an RPC_GARBAGEARGS reply. Within RPC-over-RDMA Version One, this class of error can be generated for two different reasons:
These two situations, which arise due to incorrect implementations or underestimation of reply size, have different implications with regard to Exactly-Once Semantics. An XDR error in decoding the request precludes the execution of the request on the responder, but failure to send a reply indicates that some or all of the operations were executed.
In both instances, the client SHOULD NOT retry the operation without addressing reply resource inadequacy. Such a retry can result in the same sort of error seen previously. Instead, it is best to consider the operation as completed unsuccessfully and report an error to the consumer who requested the RPC.
In addition, within the error response, the requester does not have the result of the execution of the SEQUENCE operation, which identifies the session, slot, and sequence id for the request which has failed. The xid associated with the request, obtained from the rdma_xid field of the RDMA_ERROR or RDMA_MSG message, must be used to determine the session and slot for the request which failed, and the slot must be properly retired. If this is not done, the slot could be rendered permanently unavailable.
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.
RPC programs such as NFS are required to have an Upper Layer Binding specification to interoperate on RPC-over-RDMA transports [I-D.ietf-nfsv4-rfc5666bis]. Via standards action, the Upper Layer Binding specified in this document can be extended to cover versions of the NFS version 4 protocol specified after NFS version 4 minor version 2, or separately published extensions to an existing NFS version 4 minor version, as described in [I-D.ietf-nfsv4-versioning].
NFS use of direct data placement introduces a need for an additional NFS port number assignment for networks that share traditional UDP and TCP port spaces with RDMA services. The iWARP [RFC5041] [RFC5040] protocol is such an example (InfiniBand is not).
NFS servers for versions 2 and 3 [RFC1094] [RFC1813] traditionally listen for clients on UDP and TCP port 2049, and additionally, they register these with the portmapper and/or rpcbind [RFC1833] service. However, [RFC7530] requires NFS version 4 servers to listen on TCP port 2049, and they are not required to register.
An NFS version 2 or version 3 server supporting RPC-over-RDMA on such a network and registering itself with the RPC portmapper MAY choose an arbitrary port, or MAY use the alternative well-known port number for its RPC-over-RDMA service. The chosen port MAY be registered with the RPC portmapper under the netid assigned by the requirement in [I-D.ietf-nfsv4-rfc5666bis].
An NFS version 4 server supporting RPC-over-RDMA on such a network MUST use the alternative well-known port number for its RPC-over-RDMA service. Clients SHOULD connect to this well-known port without consulting the RPC portmapper (as for NFS version 4 on TCP transports).
The port number assigned to an NFS service over an RPC-over-RDMA transport is available from the IANA port registry [RFC3232].
RPC-over-RDMA supports all RPC security models, including RPCSEC_GSS security and transport-level security [RFC2203]. The choice of RDMA Read and RDMA Write to convey RPC argument and results does not affect this, since it changes only the method of data transfer. Specifically, the requirements of [I-D.ietf-nfsv4-rfc5666bis] ensure that this choice does not introduce new vulnerabilities.
Because this document defines only the binding of the NFS protocols atop [I-D.ietf-nfsv4-rfc5666bis], all relevant security considerations are therefore to be described at that layer.
| [I-D.ietf-nfsv4-rfc5666bis] | Lever, C., Simpson, W. and T. Talpey, "Remote Direct Memory Access Transport for Remote Procedure Call, Version One", Internet-Draft draft-ietf-nfsv4-rfc5666bis-09, January 2017. |
| [I-D.ietf-nfsv4-rpcrdma-bidirection] | Lever, C., "Bi-directional Remote Procedure Call On RPC-over-RDMA Transports", Internet-Draft draft-ietf-nfsv4-rpcrdma-bidirection-06, January 2017. |
| [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. |
| [RFC2203] | Eisler, M., Chiu, A. and L. Ling, "RPCSEC_GSS Protocol Specification", RFC 2203, DOI 10.17487/RFC2203, September 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. |
| [RFC7530] | Haynes, T. and D. Noveck, "Network File System (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, March 2015. |
| [RFC7862] | Haynes, T., "Network File System (NFS) Version 4 Minor Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862, November 2016. |
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. Some 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, including discussion of how each NFS version properly estimates the maximum size of RPC replies.
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 with NFS version 4 CREATE(NF4LNK).
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 complete discussion of reply size estimation has been introduced for all protocols covered by the Upper Layer Bindings in this document.
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 4.5
Thanks to Karen Deitke for her sharp observations about idempotency, and the clarity of the discussion of NFS COMPOUNDs.
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.