| Network File System Version 4 | C. Lever |
| Internet-Draft | Oracle |
| Intended status: Standards Track | April 8, 2016 |
| Expires: October 10, 2016 |
Size-Limited Bi-directional Remote Procedure Call On Remote Direct Memory Access Transports
draft-ietf-nfsv4-rpcrdma-bidirection-02
Recent minor versions of NFSv4 work best when ONC RPC transports can send ONC RPC transactions in both directions. This document describes conventions that enable RPC-over-RDMA transport endpoints to interoperate when operation in both directions is necessary.
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The purpose of this document is to enable bi-directional RPC transactions on RPC-over-RDMA transports that do not already support backward direction operation. The conventions described herein can be used with the RPC-over-RDMA Version One protocol without changes. Therefore this document does not update [I-D.ietf-nfsv4-rfc5666bis].
Backward direction transactions enable the operation of NFSv4.1, and in particular pNFS, on RPC-over-RDMA. Providing an Upper Layer Binding for NFSv4.x callback operations is outside the scope of this document.
The ONC RPC protocol as described in [RFC5531] is fundamentally a message-passing protocol between one server and one or more clients. ONC RPC transactions are made up of two types of messages.
A CALL message, or "Call", requests work. A Call is designated by the value CALL in the message's msg_type field. An arbitrary unique value is placed in the message's xid field. A host that originates a Call is referred to in this document as a "Requester."
A REPLY message, or "Reply", reports the results of work requested by a Call. A Reply is designated by the value REPLY in the message's msg_type field. The value contained in the message's xid field is copied from the Call whose results are being returned. A host that emits a Reply is referred to as a "Responder."
RPC-over-RDMA is a connection-oriented RPC transport. When a connection-oriented transport is used, ONC RPC client endpoints are responsible for initiating transport connections, while ONC RPC service endpoints wait passively for incoming connection requests.
RPC direction on connectionless RPC transports is not considered in this document.
A traditional ONC RPC client is always a Requester. A traditional ONC RPC service is always a Responder. This traditional form of ONC RPC message passing is referred to as operation in the "forward direction."
During forward direction operation, the ONC RPC client is responsible for establishing transport connections.
The ONC RPC specification [RFC5531] does not forbid passing messages in the other direction. An ONC RPC service endpoint can act as a Requester, in which case an ONC RPC client endpoint acts as a Responder. This form of message passing is referred to as operation in the "backward direction."
During backward direction operation, the ONC RPC client is responsible for establishing transport connections, even though ONC RPC Calls come from the ONC RPC server.
ONC RPC clients and services are optimized to perform and scale well while handling traffic in the forward direction, and may not be prepared to handle operation in the backward direction. Not until recently has there been a need to handle backward direction operation.
A pair of connected RPC endpoints may choose to use only forward or only backward direction operations on a particular transport. Or, these endpoints may send Calls in both directions concurrently on the same transport.
"Bi-directional operation" occurs when both transport endpoints act as a Requester and a Responder at the same time. As above, the ONC RPC client is always responsible for establishing transport connections.
Section 9 of [RFC5531] introduces the ONC RPC transaction identifier, or "xid" for short. The value of an xid is interpreted in the context of the message's msg_type field.
When receiving a Reply, a Requester matches the xid value in the Reply with a Call it previously sent.
During bi-directional operation, the forward and backward directions use independent xid spaces.
In other words, a forward direction Requester MAY use the same xid value at the same time as a backward direction Requester on the same transport connection. Though such concurrent requests use the same xid value, they represent distinct ONC RPC transactions.
An NFSv4.0 client employs a traditional ONC RPC client to send NFS requests to an NFSv4.0 server's traditional ONC RPC service [RFC7530]. NFSv4.0 requests flow in the forward direction on a connection established by the client. This connection is referred to as a "forechannel" connection.
An NFSv4 "delegation" is simply a promise made by a server that it will notify a client when another client requests access to a file. With this guarantee, that client can operate as sole accessor of this file, and manage the file's data and metadata caches aggressively.
To manage file delegation, NFSv4.0 introduces the use of callback operations, or "callbacks", in Section 10.2 of [RFC7530]. An NFSv4.0 server sets up a traditional ONC RPC client, and an NFSv4.0 client sets up a traditional ONC RPC service. Callbacks flow in the forward direction on a connection established between the server's client, and the client's server. This connection is distinct from connections being used as forechannels, and is referred to as a "backchannel connection."
When an RDMA transport is used as a forechannel, an NFSv4.0 client typically provides a TCP callback service. The client's SETCLIENTID operation advertises the callback service endpoint with a "tcp" or "tcp6" netid. The server then connects to this service using a TCP socket.
NFSv4.0 implementations are fully functional without a backchannel in place. In this case, the server does not grant file delegations. This might result in a negative performance effect, but functional correctness is unaffected.
NFSv4.1 supports file delegation in a similar fashion to NFSv4.0, and extends the repertoire of callbacks to manage pNFS layouts, as discussed in Chapter 12 of [RFC5661].
For various reasons, NFSv4.1 requires that all transport connections be initiated by NFSv4.1 clients. Therefore, NFSv4.1 servers send callbacks to clients in the backward direction on connections established by NFSv4.1 clients.
NFSv4.1 clients and servers indicate to their peers that a backchannel capability is available on a given transport in the arguments and results of a CREATE_SESSION or BIND_CONN_TO_SESSION operation.
NFSv4.1 clients may establish distinct transport connections for forechannel and backchannel operation, or they may combine forechannel and backchannel operation on one transport connection using bi-directional operation.
Without a backward direction RPC-over-RDMA capability, an NFSv4.1 client must additionally connect using a transport with backward direction capability to use as a backchannel. TCP is the only choice at present for an NFSv4.1 backchannel connection.
Some implementations find it more convenient to use a single combined transport (ie. a transport that is capable of bi-directional operation). This simplifies connection establishment, and recovery during network partitions or when one endpoint restarts.
As with NFSv4.0, if a backchannel is not in use, an NFSv4.1 server does not grant delegations. But because of its reliance on callbacks to manage pNFS layout state, pNFS operation is not possible without a backchannel.
As of this writing, the only use case for backward direction ONC RPC messages is the NFSv4.1 backchannel. The conventions described in this document take advantage of certain characteristics of NFSv4.1 callbacks, namely:
Existing clients that implement RPC-over-RDMA Version One should interoperate correctly with servers that implement RPC-over-RDMA with backward direction support, and vice versa.
The approach taken here avoids altering the RPC-over-RDMA XDR specification. Keeping the XDR the same enables existing RPC-over-RDMA Version One implementations to interoperate with implementations that support operation in the backward direction.
Support for operation in the backward direction should never impact the performance or scalability of forward direction operation, where the bulk of ONC RPC transport activity typically occurs.
RDMA transfers involve one endpoint exposing a section of its memory to the other endpoint, which then drives RDMA Read and Write operations to access or modify the exposed memory. RPC-over-RDMA client endpoints expose their memory, and RPC-over-RDMA server endpoints initiate RDMA data transfer operations.
If RDMA transfers are not used for backward direction operations, there is no need for servers to expose their memory to clients. Further, this avoids the client complexity required to drive RDMA transfers.
Small RPC-over-RDMA messages are conveyed using only RDMA Send operations. Send is used to transmit both ONC RPC Calls and replies.
To send a large payload, an RPC-over-RDMA client endpoint registers a region of memory (known as a "chunk") and transmits its coordinates to an RPC-over-RDMA server endpoint, who uses an RDMA transfer to move data to or from the client. See Section 4.4 of [I-D.ietf-nfsv4-rfc5666bis].
To transmit RPC-over-RDMA messages larger than the receive buffer size (1024 bytes on an RPC-over-RDMA Version One transport), a chunk must be used. For example, in an RDMA_NOMSG type message, the entire RPC header and Upper Layer payload are contained in one or more chunks. See Section 4.5 of [I-D.ietf-nfsv4-rfc5666bis]. for further details.
If chunks are not allowed to be used for conveying backward direction messages, an RDMA_NOMSG type message cannot be used to convey a backward direction message using the conventions described in this document. Therefore, backward direction messages sent using the conventions in this document can be no larger than a single receive buffer.
Stipulating such a limit on backward direction message size assumes that either Upper Layer Protocol consumers of backward direction messages can advertise this limit to peers, or that ULP consumers can agree by convention on a maximum size of their backchannel payloads.
In addition, using only inline forms of RPC-over-RDMA messages and never populating the RPC-over-RDMA chunk lists means that the RPC header's msg_type field is always at a fixed location in messages flowing in the backward direction, allowing efficient detection of the direction of an RPC-over-RDMA message.
With few exceptions, NFSv4.1 servers can break down callback requests so they fit within this limit. There are potentially large NFSv4.1 callback operations, such as a CB_GETATTR operation where a large ACL must be conveyed. Although we are not aware of any NFSv4.1 implementation that uses CB_GETATTR, this state of affairs is not guaranteed in perpetuity.
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].
Performing backward direction ONC RPC operations over an RPC-over-RDMA transport can be accomplished within limits by observing the conventions described in the following subsections. For reference, the XDR description of RPC-over-RDMA Version One is contained in Section 5.1 of [I-D.ietf-nfsv4-rfc5666bis].
For an RDMA Send operation to work, the receiving peer must have posted an RDMA Receive Work Request (WR) to provide a receive buffer in which to capture the incoming message. If a receiver hasn't posted enough Receive WRs to catch incoming Send operations, the RDMA provider is allowed to drop the RDMA connection.
RPC-over-RDMA protocols provide built-in send flow control to prevent overrunning the number of pre-posted receive buffers on a connection's receive endpoint. This is fully discussed in Section 4.3 of [I-D.ietf-nfsv4-rfc5666bis].
An RPC-over-RDMA credit is the capability to handle one RPC-over-RDMA transaction. Each forward direction RPC-over-RDMA Call requests a number of credits from the Responder. Each forward direction Reply informs the Requester how many credits the Responder is prepared to handle in total. The value of the request and grant are carried in each RPC-over-RDMA message's rdma_credit field.
Practically speaking, the critical value is the value of the rdma_credit field in RPC-over-RDMA replies. When a Requester is operating correctly, it sends no more outstanding requests at a time than the Responder's advertised forward direction credit value.
The credit value is a guaranteed minimum. However, a receiver can post more receive buffers than its credit value. There is no requirement in the RPC-over-RDMA protocol for a receiver to indicate a credit overrun. Operation continues as long as there are enough receive buffers to handle incoming messages.
Credits work the same way in the backward direction as they do in the forward direction. However, forward direction credits and backward direction credits are accounted separately.
In other words, the forward direction credit value is the same whether or not there are backward direction resources associated with an RPC-over-RDMA transport connection. The backward direction credit value MAY be different than the forward direction credit value. The rdma_credit field in a backward direction RPC-over-RDMA message MUST NOT contain the value zero.
A backward direction Requester (ie, an RPC-over-RDMA service endpoint) requests credits from the Responder (ie, an RPC-over-RDMA client endpoint). The Responder reports how many credits it can grant. This is the number of backward direction Calls the Responder is prepared to handle at once.
When an RPC-over-RDMA server endpoint is operating correctly, it sends no more outstanding requests at a time than the client endpoint's advertised backward direction credit value.
An RPC-over-RDMA transport endpoint must pre-post receive buffers before it can receive and process incoming RPC-over-RDMA messages. If a sender transmits a message for a receiver which has no prepared receive buffer, the RDMA provider is allowed to drop the RDMA connection.
Typically an RPC-over-RDMA Requester posts only as many receive buffers as there are outstanding RPC Calls. A client endpoint without backward direction support might therefore at times have no pre-posted receive buffers.
To receive incoming backward direction Calls, an RPC-over-RDMA client endpoint must pre-post enough additional receive buffers to match its advertised backward direction credit value. Each outstanding forward direction RPC requires an additional receive buffer above this minimum.
When an RDMA transport connection is lost, all active receive buffers are flushed and are no longer available to receive incoming messages. When a fresh transport connection is established, a client endpoint must re-post a receive buffer to handle the Reply for each retransmitted forward direction Call, and a full set of receive buffers to handle backward direction Calls.
A forward direction RPC-over-RDMA service endpoint posts as many receive buffers as it expects incoming forward direction Calls. That is, it posts no fewer buffers than the number of RPC-over-RDMA credits it advertises in the rdma_credit field of forward direction RPC replies.
To receive incoming backward direction replies, an RPC-over-RDMA server endpoint must pre-post a receive buffer for each backward direction Call it sends.
When the existing transport connection is lost, all active receive buffers are flushed and are no longer available to receive incoming messages. When a fresh transport connection is established, a server endpoint must re-post a receive buffer to handle the Reply for each retransmitted backward direction Call, and a full set of receive buffers for receiving forward direction Calls.
An RPC-over-RDMA transport endpoint might not support backward direction operation. There might be no mechanism in the transport implementation to do so. Or the Upper Layer Protocol consumer might not yet have configured the transport to handle backward direction traffic.
A loss of the RDMA connection may result if the receiver is not prepared to receive an incoming message. Thus a denial-of-service could result if a sender continues to send backchannel messages after every transport reconnect to an endpoint that is not prepared to receive them.
Generally, for RPC-over-RDMA Version One transports, the Upper Layer Protocol consumer is responsible for informing its peer when it has support for the backward direction. Otherwise even a simple backward direction NULL probe from a peer could result in a lost connection.
An NFSv4.1 server does not send backchannel messages to an NFSv4.1 client before the NFSv4.1 client has sent a CREATE_SESSION or a BIND_CONN_TO_SESSION operation. As long as an NFSv4.1 client has prepared appropriate backchannel resources before sending one of these operations, denial-of-service is avoided. Legacy versions of NFS never send backchannel operations.
Therefore, an Upper Layer Protocol consumer MUST NOT perform backward direction ONC RPC operations unless the peer consumer has indicated it is prepared to handle them. A description of Upper Layer Protocol mechanisms used for this indication is outside the scope of this document.
In rare cases, an ONC RPC transaction cannot be completed within a certain time. This can be because the transport connection was lost, the Call or Reply message was dropped, or because the Upper Layer consumer delayed or dropped the ONC RPC request. Typically, the Requester sends the transaction again, reusing the same RPC XID. This is known as an "RPC retransmission".
In the forward direction, the Requester is the ONC RPC client. The client is always responsible for establishing a transport connection before sending again.
In the backward direction, the Requester is the ONC RPC server. Because an ONC RPC server does not establish transport connections with clients, it cannot send a retransmission if there is no transport connection. It must wait for the ONC RPC client to re-establish the transport connection before it can retransmit ONC RPC transactions in the backward direction.
If an ONC RPC client has no work to do, it may be some time before it re-establishes a transport connection. Backward direction Requesters must be prepared to wait indefinitely before a connection is established before a pending backward direction ONC RPC Call can be retransmitted.
RPC-over-RDMA backward direction messages are transmitted and received using the same buffers as messages in the forward direction. Therefore they are constrained to be no larger than receive buffers posted for forward messages. The default Receive buffer size in RPC-over-RDMA Version One implementations is 1024 bytes.
It is expected that the Upper Layer Protocol consumer establishes an appropriate payload size limit for backward direction operations, either by advertising that size limit to its peers, or by convention. If that is done, backward direction messages will not exceed the size of receive buffers at either endpoint.
If a sender transmits a backward direction message that is larger than the receiver is prepared for, the RDMA provider drops the message and the RDMA connection.
If a sender transmits an RDMA message that is too small to convey a complete and valid RPC-over-RDMA and RPC message in either direction, the receiver MUST NOT use any value in the fields that were transmitted. Namely, the rdma_credit field MUST be ignored, and the message silently discarded.
To form a backward direction RPC-over-RDMA Call message, an ONC RPC service endpoint constructs an RPC-over-RDMA header containing a fresh RPC XID in the rdma_xid field (see Section 1.1.4 for full requirements).
The number of requested backward direction credits is placed in the rdma_credit field (see Section 2.1).
The rdma_proc field in the RPC-over-RDMA header MUST contain the value RDMA_MSG. All three chunk lists MUST be empty.
The ONC RPC Call header MUST follow immediately, starting with the same XID value that is present in the RPC-over-RDMA header. The Call header's msg_type field MUST contain the value CALL.
To form a backward direction RPC-over-RDMA Reply message, an ONC RPC client endpoint constructs an RPC-over-RDMA header containing a copy of the matching ONC RPC Call's RPC XID in the rdma_xid field (see Section 1.1.4 for full requirements).
The number of granted backward direction credits is placed in the rdma_credit field (see Section 2.1).
The rdma_proc field in the RPC-over-RDMA header MUST contain the value RDMA_MSG. All three chunk lists MUST be empty.
The ONC RPC Reply header MUST follow immediately, starting with the same XID value that is present in the RPC-over-RDMA header. The Reply header's msg_type field MUST contain the value REPLY.
RPC programs that operate on RPC-over-RDMA transports using the conventions described in this document do not require an Upper Layer Binding specification. Because backward direction operation using these conventions cannot transfer data via RMDA Read or Write, there can be no RDMA-eligible data items in the Upper Layer Program on this transport.
In addition, since backward direction operation occurs on an already-established connection, there is no need to specify RPC bind parameters.
The major drawback to the approach described in this document is the limit on payload size in backward direction requests.
A second drawback is the exposure of the client transport endpoint to backward direction Calls before it has posted receive buffers to handle them.
Clients that do not support backward direction operation typically drop messages they do not recognize. However, this does not allow bi-direction-capable servers to quickly identify clients that cannot handle backward direction requests.
The conventions in this document rely on Upper Layer Protocol consumers to decide when backward direction transport operation is appropriate.
To address the limitations described in this section in the long run, two approaches are available:
The latter approach would benefit from changes to the XDR definition of the RPC-over-RDMA protocol, and would require significant changes to implementations.
The use of the conventions described in this document to enable backward direction operation should be considered a transitional approach that is appropriate while the predominantly deployed versions of the RPC-over-RDMA protocol do not have native support for large backward direction messages.
When RPCSEC_GSS integrity and confidentiality services (described in [I-D.ietf-nfsv4-rpcsec-gssv3]) are in use, additional RPC header information is included in each message. This increases the size of each message, further limiting the size of backward direction operations.
This document does not require actions by IANA.
Tom Talpey was an indispensable resource, in addition to creating the foundation upon which this work is based. Our warmest regards go to him for his help and support.
Dave Noveck provided excellent review, constructive suggestions, and navigational guidance throughout the process of drafting this document.
Dai Ngo was a solid partner and collaborator. Together we constructed and tested independent prototypes of the conventions described in this document.
The author wishes to thank Bill Baker for his unwavering support of this work. In addition, the author gratefully acknowledges the expert contributions of Karen Deitke, Chunli Zhang, Mahesh Siddheshwar, Steve Wise, and Tom Tucker.
Special thanks go to the nfsv4 Working Group Chair Spencer Shepler and the nfsv4 Working Group Secretary Tom Haynes for their support.
| [I-D.ietf-nfsv4-rfc5666bis] | Lever, C., Simpson, W. and T. Talpey, "Remote Direct Memory Access Transport for Remote Procedure Call", Internet-Draft draft-ietf-nfsv4-rfc5666bis-04, March 2016. |
| [I-D.ietf-nfsv4-rpcsec-gssv3] | Adamson, A. and N. Williams, "Remote Procedure Call (RPC) Security Version 3", Internet-Draft draft-ietf-nfsv4-rpcsec-gssv3-17, January 2016. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5531] | Thurlow, R., "RPC: Remote Procedure Call Protocol Specification Version 2", RFC 5531, May 2009. |
| [RFC5661] | Shepler, S., Eisler, M. and D. Noveck, "Network File System (NFS) Version 4 Minor Version 1 Protocol", RFC 5661, January 2010. |
| [RFC7530] | Haynes, T. and D. Noveck, "Network File System (NFS) Version 4 Protocol", RFC 7530, March 2015. |