Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Standards Track January 20, 2017
Expires: July 24, 2017
Bi-directional Remote Procedure Call On RPC-over-RDMA Transports
draft-ietf-nfsv4-rpcrdma-bidirection-06
Abstract
Minor versions of NFSv4 newer than NFSv4.0 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.
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].
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 July 24, 2017.
Copyright Notice
Copyright (c) 2017 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Understanding RPC Direction . . . . . . . . . . . . . . . . . 2
3. Immediate Uses Of Bi-Directional RPC-over-RDMA . . . . . . . 4
4. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Sending And Receiving Backward Operations . . . . . . . . . . 8
6. In the Absence of Backward Direction Support . . . . . . . . 10
7. Considerations For Upper Layer Bindings . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Normative References . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The purpose of this document is to enable concurrent operation in
both directions on a single transport connection using RPC-over-RDMA
protocol versions that do not have specific facilities for backward
direction operation.
Backward direction RPC transactions are necessary for the operation
of NFSv4.1, and in particular, of Parallel NFS (pNFS) [RFC5661],
though any Upper Layer Protocol implementation may make use of them.
An Upper Layer Binding for NFSv4.x callback operation is additionally
required (see Section 7), but is not provided in this document.
For example, using the approach described herein, RPC transactions
can be conveyed in both directions on the same RPC-over-RDMA Version
One connection without changes to the XDR description of RPC-over-
RDMA Version One. This document does not modify the XDR or protocol
described in [I-D.ietf-nfsv4-rfc5666bis]. Future versions of RPC-
over-RDMA may adopt the approach described herein, or may replace it
with a different approach.
2. Understanding RPC Direction
The ONC RPC protocol as described in [RFC5531] is architected as a
message-passing protocol between one server and one or more clients.
ONC RPC transactions are made up of two types of messages.
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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 results in a corresponding Reply. A Reply is never
sent without a corresponding Call.
RPC-over-RDMA is a connection-oriented RPC transport. In all cases,
when a connection-oriented transport is used, ONC RPC client
endpoints are responsible for initiating transport connections, while
ONC RPC service endpoints passively await incoming connection
requests.
RPC direction on connectionless RPC transports is not addressed in
this document.
2.1. Forward Direction
Traditionally, an ONC RPC client acts as a Requester, while an ONC
RPC service acts as a Responder. This form of message passing is
referred to as "forward direction" operation.
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 "backward
direction" operation.
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 might not be
prepared to handle operation in the backward direction. Not until
NFSv4.1 [RFC5661] has there been a strong need to handle backward
direction operation.
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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.
Bi-directionality is an extension of RPC transport connection
sharing. Two RPC endpoints wish to exchange independent RPC messages
over a shared connection, but in opposite directions. These messages
may or may not be related to the same workloads or RPC Programs.
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.
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. Immediate Uses Of 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
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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 client or program running on the
server 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 can function without a backchannel in place.
In this case, the server does not grant file delegations. This might
result in a negative performance effect, but correctness is not
affected.
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].
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.
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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.
Implementations often 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. This can
also enable better scaling by using fewer transport connections to
perform the same work.
As with NFSv4.0, if a backchannel is not in use, an NFSv4.1 server
does not grant delegations. Because NFSv4.1 relies 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 properly, the receiving peer must
have posted a receive buffer in which to accept the incoming message.
If a receiver hasn't posted enough buffers to accommodate each
incoming Send operation, the receiving RDMA provider is allowed to
terminate 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. For RPC-over-RDMA Version One, this
is discussed in Section 4.3 of [I-D.ietf-nfsv4-rfc5666bis].
4.1. Backward Credits
RPC-over-RDMA credits work the same way in the backward direction as
they do in the forward direction. However, forward direction credits
and backward direction credits on the same connection are accounted
separately.
The forward direction credit value retains the same meaning whether
or not there are backward direction resources associated with an RPC-
over-RDMA transport connection. This is the number of RPC requests
the forward direction responder (the RPC server) is prepared to
receive concurrently.
The backward direction credit value is the number of RPC requests the
backward direction responder (the RPC client) is prepared to receive
concurrently. The backward direction credit value MAY be different
than the forward direction credit value.
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During bi-directional operation, each receiver has to decide whether
an incoming message contains a credit request (the receiver is acting
as a responder) or a credit grant (the receiver is acting as a
requester) and apply the credit value accordingly.
When message direction is not fully determined by context (e.g.,
suggested by the definition of the RPC-over-RDMA version that is in
use) or by an accompanying RPC message payload with a call direction
field, it is not possible for the receiver to tell with certainty
whether the header credit value is a request or grant. In such
cases, the receiver MUST ignore the header's credit value.
4.2. Inline Thresholds
Forward and backward operation on the same connection share the same
receive buffers. Therefore the inline threshold values for the
forward direction and the backward direction are the same. The call
inline threshold for the backward direction is the same as the reply
inline threshold for the forward direction, and vice versa. For more
information, see Section 4.3.2 of [I-D.ietf-nfsv4-rfc5666bis].
4.3. 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 posted
receive buffer, the RDMA provider is allowed to drop the RDMA
connection.
4.3.1. 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.
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.
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4.3.2. 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 enough additional receive buffers to
handle replies 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. Sending And Receiving Backward Operations
The operation of RPC-over-RDMA transports in the forward direction is
defined in [RFC5531] and [I-D.ietf-nfsv4-rfc5666bis]. In this
section, a mechanism for backward direction operation on RPC-over-
RDMA is defined. Backward operation used in combination with forward
operation enables bi-directional communication on a common RPC
transport connection.
Certain fields in the RPC-over-RDMA header are fixed for all versions
of RPC-over-RDMA. The XDR description of these fields 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-
over-RDMA header, and the RPC header's msg_type field MUST contain
the value CALL.
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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 RPC 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 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.
Forward direction Requesters are responsible for maintaining a
transport connection as long as there is the possibility of backward
direction requests. For example, an NFSv4.1 client with open
delegated files or active pNFS layouts should maintain a transport
connection so the server can send callback operations.
6. In the Absence of Backward Direction Support
An RPC-over-RDMA transport endpoint might not support backward
direction operation (and thus it does not support bi-directional
operation). There might be no mechanism in the transport
implementation to do so. Or in an implementation that can support
operation in the backward direction, the Upper Layer Protocol
consumer might not yet have configured or enabled 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
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.
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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.
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.
For example, 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 announcing support for backchannel operation, denial
of service is avoided.
7. Considerations For Upper Layer Bindings
An Upper Layer Protocol that operates on RPC-over-RDMA transports may
have procedures that include DDP-eligible data items. DDP-
eligibility is specified in an Upper Layer Binding. Direction of
operation does not obviate the need for DDP-eligibility statements.
Backward-only operation requires the client endpoint to establish a
fresh connection. The Upper Layer Binding can specify appropriate
RPC binding parameters for such connections.
Bi-directional operation occurs on an already-established connection.
Specification of RPC binding parameters is usually not necessary in
this case.
For bi-directional operation, other considerations about sharing an
RPC-over-RDMA transport with another ULP may apply. Consult
Section 6 of [I-D.ietf-nfsv4-rfc5666bis] for details about what else
may be contained in an Upper Layer Binding.
8. Security Considerations
Security considerations for operation on RPC-over-RDMA transports are
outlined in Section 9 of [I-D.ietf-nfsv4-rfc5666bis].
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9. IANA Considerations
This document does not require actions by IANA.
10. Normative References
[I-D.ietf-nfsv4-rfc5666bis]
Lever, C., Simpson, W., and T. Talpey, "Remote Direct
Memory Access Transport for Remote Procedure Call, Version
One", draft-ietf-nfsv4-rfc5666bis-09 (work in progress),
January 2017.
[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.
Appendix A. 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.
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 Transport Area Director Spencer Dawkins, nfsv4
Working Group and document shepherd Chair Spencer Shepler, and nfsv4
Working Group Secretary Tom Haynes for their support.
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Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
USA
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
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