Bi-directional Remote Procedure Call On RPC-over-RDMA Transports
draft-ietf-nfsv4-rpcrdma-bidirection-03
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| Document | Type | Active Internet-Draft (nfsv4 WG) | |
|---|---|---|---|
| Author | Chuck Lever | ||
| Last updated | 2016-05-02 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-nfsv4-rpcrdma-bidirection-03
Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Standards Track May 2, 2016
Expires: November 3, 2016
Bi-directional Remote Procedure Call On RPC-over-RDMA Transports
draft-ietf-nfsv4-rpcrdma-bidirection-03
Abstract
Recent minor versions of NFSv4 work best when ONC RPC transports can
send Remote Procedure Call transactions in both directions on the
same connection. This document describes how RPC-over-RDMA transport
endpoints convey RPCs in both directions on a single connection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 3, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Understanding RPC Direction . . . . . . . . . . . . . . . . . 3
2.1. Forward Direction . . . . . . . . . . . . . . . . . . . . 3
2.2. Backward Direction . . . . . . . . . . . . . . . . . . . 4
2.3. Bi-directional Operation . . . . . . . . . . . . . . . . 4
2.4. XID Values . . . . . . . . . . . . . . . . . . . . . . . 4
3. Rationale For Bi-Directional RPC-over-RDMA . . . . . . . . . 5
3.1. NFSv4.0 Callback Operation . . . . . . . . . . . . . . . 5
3.2. NFSv4.1 Callback Operation . . . . . . . . . . . . . . . 6
4. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Backward Credits . . . . . . . . . . . . . . . . . . . . 7
4.2. Managing Receive Buffers . . . . . . . . . . . . . . . . 7
5. Protocol For Backward Operation . . . . . . . . . . . . . . . 8
5.1. Sending A Backward Direction Call . . . . . . . . . . . . 8
5.2. Sending A Backward Direction Reply . . . . . . . . . . . 9
5.3. Backward Direction Chunks . . . . . . . . . . . . . . . . 9
5.4. Backward Direction Retransmission . . . . . . . . . . . . 10
6. In the Absence of Backward Direction Support . . . . . . . . 10
7. Backward Direction Upper Layer Binding . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. Normative References . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The purpose of this document is to enable bi-directional RPC
operation on RPC-over-RDMA protocol versions that do not have
specific protocol facilities for backward direction operation.
Backward direction RPC transactions enable the operation of NFSv4.1,
and in particular pNFS.
For example, using the protocol described in this document, RPC
transactions can be conveyed in both directions on the same RPC-over-
RDMA Version One connection without changes to the Version One header
XDR description. Therefore this document does not update
[I-D.ietf-nfsv4-rfc5666bis].
Providing an Upper Layer Binding for NFSv4.x callback operations is
outside the scope of this document.
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1.1. Requirements Language
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].
2. Understanding RPC Direction
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."
Typically, a Call generates a corresponding Reply. A Reply is never
sent without a corresponding Call.
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.
2.1. Forward Direction
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.
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2.2. Backward Direction
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.
2.3. Bi-directional 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.
2.4. XID Values
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.
o The xid of a Call is arbitrary but is unique among outstanding
Calls from that Requester.
o The xid of a Reply always matches that of the initiating Call.
When receiving a Reply, a Requester matches the xid value in the
Reply with a Call it previously sent.
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2.4.1. XID Generation
During bi-directional operation, forward and backward direction XIDs
are typically generated on distinct hosts by possibly different
algorithms. There is no co-ordination between forward and backward
direction XID generation.
Therefore, 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.
3. Rationale For Bi-Directional RPC-over-RDMA
3.1. NFSv4.0 Callback Operation
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 before another agent is allowed access to a
file. With this guarantee, that client can operate as sole accessor
of the file. In particular, it can manage the file's data and
metadata caches aggressively.
To administer file delegations, 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 callback client, and the client's callback 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.
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3.2. NFSv4.1 Callback Operation
NFSv4.1 supports file delegation in a similar fashion to NFSv4.0, and
extends the callback mechanism to manage pNFS layouts, as discussed
in Section 12 of [RFC5661].
To facilitate operation through NAT routers, all NFSv4.1 transport
connections are 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 NFS CREATE_SESSION or BIND_CONN_TO_SESSION
operations.
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
for an NFSv4.1 backchannel connection in this case.
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.
4. Flow Control
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 land the incoming message. If a receiver hasn't posted
enough Receive WRs to land incoming Send operations, the RDMA
provider is allowed to drop the RDMA connection.
RPC-over-RDMA transport 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].
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4.1. Backward Credits
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 has
granted. This is the number of backward direction Calls the
Responder is prepared to handle at once.
When message direction is not fully determined by context or by an
accompanying RPC message with a call direction field, it is not
possible to tell whether the header credit value is a request or
grant, or whether the value applies to the forward direction or
backward direction. In such cases, the receiver MUST NOT use the
header's credit value.
4.2. Managing Receive Buffers
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.
4.2.1. Client Receive Buffers
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.
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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.
4.2.2. Server Receive Buffers
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 credits granted 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.
5. Protocol For Backward Operation
Performing backward direction ONC RPC operations over an RPC-over-
RDMA transport connection can be accomplished by observing the
protocol 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].
5.1. Sending A Backward Direction Call
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 2.4 for full
requirements).
The rdma_vers field MUST contain the same value in backward and
forward direction Call messages on the same connection.
The number of requested backward direction credits is placed in the
rdma_credit field (see Section 4).
Whether presented inline or as a separate chunk, the ONC RPC Call
header MUST start with the same XID value that is present in the RPC-
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over-RDMA header, and the header's msg_type field MUST contain the
value CALL.
5.2. Sending A Backward Direction Reply
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 2.4 for full requirements).
The rdma_vers field MUST contain the same value in a backward
direction Reply message as in the matching Call message.
The number of granted backward direction credits is placed in the
rdma_credit field (see Section 4).
Whether presented inline or as a separate chunk, the ONC RPC Reply
header MUST start with the same XID value that is present in the RPC-
over-RDMA header, and the header's msg_type field MUST contain the
value REPLY.
5.3. Backward Direction Chunks
Chunks MAY be used in the backward direction. They operate the same
way as in the forward direction (see [I-D.ietf-nfsv4-rfc5666bis] for
details).
An implementation might not support any Upper Layer Protocol that has
DDP-eligible data items. The Upper Layer Protocol may also use only
small messages, or it may have a native mechanism for restricting the
size of backward direction RPC messages, obviating the need to handle
Long Messages in the backward direction.
When there is no Upper Layer Protocol requirement for chunks,
implementers can choose not to provide support for chunks in the
backward direction. This avoids the complexity of adding support for
performing RDMA Reads and Writes in the backward direction.
When chunks are not implemented, RPC messages in the backward
direction are always sent using RDMA_MSG, and therefore can be no
larger than what can be sent inline (that is, without chunks).
Sending an inline message larger than the receiver's inline threshold
can result in loss of connection.
If a backward direction requester provides a non-empty chunk list to
a responder that does not support chunks, the responder MUST reply
with an RDMA_ERROR message with rdma_err field set to ERR_CHUNK.
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5.4. Backward Direction Retransmission
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 for a connection to be
established before a pending backward direction ONC RPC Call can be
retransmitted.
6. In the Absence of Backward Direction Support
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.
If an endpoint is not prepared to receive an incoming backward
direction message, loss of the RDMA connection might result. Thus a
denial-of-service could result if a sender continues to send backward
direction messages after every transport reconnect to an endpoint
that is not prepared to receive them.
When dealing with the possibility that the remote peer has no
transport level support for backward direction operation, the Upper
Layer Protocol becomes responsible for informing peers when backward
direction operation is supported. 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
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BIND_CONN_TO_SESSION operation. As long as an NFSv4.1 client has
prepared appropriate backchannel resources before sending one of
these operations announcing support for backchannel operation,
denial-of-service is avoided.
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.
7. Backward Direction Upper Layer Binding
Since backward direction operation occurs on an already-established
connection, there is no need to specify RPC bind parameters.
An Upper Layer Protocol that operates on RPC-over-RDMA transports in
the backward direction may have DDP-eligible data items. These are
specified in an Upper Layer Binding document.
By default, no data items in a ULP are DDP-eligible. If there are no
DDP-eligible data items to document, an explicit Upper Layer Binding
may not be needed for an Upper Layer Protocol that operates only in
the backward direction.
Consult Section 7 of [I-D.ietf-nfsv4-rfc5666bis] for details about
what else may be contained in a binding.
8. Security Considerations
Security considerations for operation on RPC-over-RDMA transports are
outlined in Section 9 of [I-D.ietf-nfsv4-rfc5666bis].
9. IANA Considerations
This document does not require actions by IANA.
10. Acknowledgements
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.
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Dai Ngo was a solid partner and collaborator. Together we
constructed and tested independent prototypes of the changes
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.
11. Normative References
[I-D.ietf-nfsv4-rfc5666bis]
Lever, C., Simpson, W., and T. Talpey, "Remote Direct
Memory Access Transport for Remote Procedure Call", draft-
ietf-nfsv4-rfc5666bis-04 (work in progress), March 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.
Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
USA
Phone: +1 734 274 2396
Email: chuck.lever@oracle.com
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