RDMA Connection Manager Private Data For RPC-Over-RDMA Version 1
draft-ietf-nfsv4-rpcrdma-cm-pvt-data-00
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| Author | Chuck Lever | ||
| Last updated | 2018-07-25 | ||
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draft-ietf-nfsv4-rpcrdma-cm-pvt-data-00
Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Informational July 25, 2018
Expires: January 26, 2019
RDMA Connection Manager Private Data For RPC-Over-RDMA Version 1
draft-ietf-nfsv4-rpcrdma-cm-pvt-data-00
Abstract
This document specifies the format of RDMA-CM Private Data exchanged
between RPC-over-RDMA version 1 peers as a transport connection is
established. Such private data is used to indicate peer support for
remote invalidation and larger-than-default inline thresholds. The
data format is extensible.
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
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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 January 26, 2019.
Copyright Notice
Copyright (c) 2018 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Advertised Transport Properties . . . . . . . . . . . . . . . 3
4. Private Data Message Format . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The RPC-over-RDMA version 1 transport protocol enables the use of
RDMA data transfer for upper layer protocols based on RPC [RFC8166].
The terms "Remote Direct Memory Access" (RDMA) and "Direct Data
Placement" (DDP) are introduced in [RFC5040].
The two most immediate shortcomings of RPC-over-RDMA version 1 are:
o Setting up an RDMA data transfer (via RDMA Read or Write) can be
costly. The small default size of messages transmitted using RDMA
Send forces the use of RDMA Read or Write operations even for
relatively small messages and data payloads.
The original specification of RPC-over-RDMA version 1 provided an
out-of-band protocol for passing inline threshold values between
connected peers [RFC5666]. However, [RFC8166] eliminated support
for this protocol making it unavailable for this purpose.
o Unlike most other contemporary RDMA-enabled storage protocols,
there is no facility in RPC-over-RDMA version 1 that enables the
use of Remote Invalidation [RFC5042].
RPC-over-RDMA version 1 has no means of extending its XDR definition
in such a way that interoperability with existing implementations is
preserved. As a result, an out-of-band mechanism is needed to help
relieve these constraints for existing RPC-over-RDMA version 1
implementations.
This document specifies a simple, non-XDR-based message format
designed to be passed between RPC-over-RDMA version 1 peers at the
time each RDMA transport connection is first established. The
purpose of this message format is two-fold:
o To provide immediate relief from certain performance constraints
inherent in RPC-over-RDMA version 1
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o To enable experimentation with parameters of the base RDMA
transport over which RPC-over-RDMA runs
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Advertised Transport Properties
3.1. Inline Threshold Size
Section 3.3.2 of [RFC8166] defines the term "inline threshold." An
inline threshold is the maximum number of bytes that can be
transmitted using only one RDMA Send and one RDMA Receive. There are
a pair of inline thresholds per transport connection, one for each
direction of message flow.
If an incoming message exceeds the size of a receiver's inline
threshold, the receive operation fails and the connection is
typically terminated. To convey a message larger than a receiver's
inline threshold, an NFS client uses explicit RDMA data transfer
operations, which are more expensive to use than RDMA Send.
The default value of inline thresholds for RPC-over-RDMA version 1
connections is 1024 bytes in both directions (see Section 3.3.3 of
[RFC8166]). This value is adequate for nearly all NFS version 3
procedures.
NFS version 4 COMPOUND operations [RFC7530] are larger on average
than NFS version 3 procedures [RFC1813], forcing clients to use
explicit RDMA operations for frequently-issued requests such as
LOOKUP and GETATTR. The use of RPCSEC_GSS security also increases
the average size of RPC messages, due to the larger size of
RPCSEC_GSS credential material included in RPC headers [RFC7861].
If a sender and receiver can somehow agree on larger inline
thresholds, frequently-used RPC transactions avoid the cost of
explicit RDMA operations.
3.2. Remote Invalidation
After an RDMA data transfer operation completes, an RDMA peer can use
Remote Invalidation to request that the remote peer RNIC invalidate
an STag associated with the data transfer [RFC5042].
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An RDMA consumer requests Remote Invalidation by posting an RDMA Send
With Invalidate Work Request in place of an RDMA Send Work Request.
Each RDMA Send With Invalidate carries one STag to invalidate. The
receiver of an RDMA Send With Invalidate performs the requested
invalidation, and then reports that invalidation as part of the
completion of a waiting Receive Work Request.
An RPC-over-RDMA responder can use Remote Invalidation when replying
to an RPC request that provided Read or Write chunks. The requester
thus avoids dispatching an extra Work Request, the resulting context
switch, and the invalidation completion interrupt as part of
completing an RPC transaction that uses chunks. The upshot is faster
completion of RPC transactions that involve RDMA data transfer.
There are some important caveats which contraindicate the blanket use
of Remote Invalidation:
o Remote Invalidation is not supported by all RNICs.
o Not all RPC-over-RDMA requester implementations can recognize when
Remote Invalidate has occurred.
o Not all RPC-over-RDMA responder implementations can generate RDMA
Send With Invalidate Work Requests.
o On one connection in different RPC-over-RDMA transactions, or in a
single RPC-over-RDMA transaction, an RPC-over-RDMA requester can
expose a mixture of STags that may be invalidated remotely and
some that must not. No indication is provided at the RDMA layer
as to which is which.
A responder therefore must not employ Remote Invalidation unless it
is aware of support for it in its own RDMA stack, and on the
requester. And, without altering the XDR structure of RPC-over-RDMA
version 1 messages, it is not possible to support Remote Invalidation
with requesters that mix STags that may and must not by invalidated
remotely in a single RPC or on the same connection.
However, it is possible to provide a simple signaling mechanism for a
requester to indicate it can deal with Remote Invalidation of any
STag it has presented to a responder. There are some NFS/RDMA client
implementations that can successfully make use of such a signaling
mechanism.
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4. Private Data Message Format
With an InfiniBand lower layer, for example, RDMA connection setup
uses the InfiniBand Connection Manager to establish a Reliable
Connection [IBARCH]. When an RPC-over-RDMA version 1 transport
connection is established, the client (which actively establishes
connections) and the server (which passively accepts connections)
SHOULD populate the CM Private Data field exchanged as part of CM
connection establishment.
The transport properties exchanged via this mechanism are fixed for
the life of the connection. Each new connection presents an
opportunity for a fresh exchange.
For RPC-over-RDMA version 1, the CM Private Data field is formatted
as described in the following subsection. RPC clients and servers
use the same format. If the capacity of the Private Data field is
too small to contain this message format, the underlying RDMA
transport is not managed by a Connection Manager, or the underlying
RDMA transport uses Private Data for its own purposes, the CM Private
Data field cannot be used on behalf of RPC-over-RDMA version 1.
4.1. Fixed Mandatory Fields
The first 8 octets of the CM Private Data field MUST be formatted as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Flags | Send Size | Receive Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Protocol Number
This field contains a fixed 32-bit value that identifies the
content of the Private Data field as an RPC-over-RDMA version 1 CM
Private Data message. The value of this field MUST be 0xf6ab0e18,
in network byte order. The use of this field is further expanded
upon in Section 4.1.1.
Version
This 8-bit field contains a message format version number. The
value "1" in this field indicates that exactly eight octets are
present, that they appear in the order described in this section,
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and that each has the meaning defined in this section. Further
considerations about the use of this field are discussed in
Section 4.1.2.
Bit Flags
This 8-bit field contains eight bit flags that indicate the
support status of optional features, such as Remote Invalidation.
The meaning of these flags is defined in Section 4.1.3.
Send Size
This 8-bit field contains an encoded value corresponding to the
maximum number of bytes this peer will transmit in a single RDMA
Send. The value is encoded as described in Section 4.1.4.
Receive Size
This 8-bit field contains an encoded value corresponding to the
maximum number of bytes this peer can receive with a single RDMA
Receive. The value is encoded as described in Section 4.1.4.
4.1.1. Interoperability Considerations
RPC-over-RDMA version 1 implementations that support the extension
described in this document are intended to interoperate fully with
RPC-over-RDMA version 1 implementations that do not recognize the
exchange of CM Private Data. When a peer does not receive a CM
Private Data message which conforms to Section 4, it MUST act as if
the remote peer supports only the default RPC-over-RDMA version 1
settings, as defined in [RFC8166]. In other words, the peer is to
behave as if a Private Data message was received in which bit 8 of
the Flags field is zero, and both Size fields contain the value zero.
The Protocol Number field is provided in order to distinguish RPC-
over-RDMA version 1 Private Data from private data inserted by layers
below or above RPC-over RDMA version 1. During connection
establishment, RPC-over-RDMA version 1 implementations check for this
protocol number before decoding subsequent fields. If this protocol
number is not present as the first 4 octets, an RPC-over-RDMA
receiver MUST ignore the CM-Private Data (ie., behave as if no RPC-
over-RDMA version 1 Private Data has been provided).
4.1.2. Extending the Message Format
Because the first 8 octets of the Private Data format described above
contain a Version field, subsequent versions of this data structure
MUST also start with these 8 octets exactly as they appear here.
However, the Private Data format described here can be extended by
adding additional fields which follow the first eight octets, or by
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making use of one of the bits in the Flags field that is marked
reserved in this document.
To introduce such changes while preserving interoperability, a new
Version number is to be allocated, and new fields and bit flags are
to be defined. A description of how receivers should behave if they
do not recognize the new format is to be provided as well. Such
situations may be addressed by specifying the new format in a
document updating this one.
4.1.3. Feature Support Flags
The bits in the Flags field are labeled from bit 8 to bit 15, as
shown in the diagram above. When the Version field contains the
value "1", the bits in the Flags field have the following meaning:
Bit 15
When both connection peers have set this flag in their CM Private
Data, the responder MAY use RDMA Send With Invalidate when
transmitting RPC Replies. Each RDMA Send With Invalidate MUST
invalidate an STag associated only with the XID in the rdma_xid
field of the RPC-over-RDMA Transport Header it carries.
When either peer on a connection clears this flag, the responder
MUST use only RDMA Send when transmitting RPC Replies.
Bits 14 - 8
These bits are reserved and MUST be zero.
4.1.4. Inline Threshold Values
Inline threshold sizes from 1KB to 256KB can be represented in the
Send Size and Receive Size fields. A sender computes the encoded
value by dividing the actual value by 1024 and subtracting one from
the result. A receiver decodes this value by performing a
complementary set of operations.
The requester MUST use the smaller of its own send size and the
responder's reported receive size as the requester-to-responder
inline threshold. The responder MUST use the smaller of its own send
size and the requester's reported receive size as the responder-to-
requester inline threshold.
5. IANA Considerations
In accordance with [RFC8126], the author requests that IANA create a
new registry in the "Remote Direct Data Placement" Protocol Category
Group. The new registry is to be called the "RDMA-CM Private Data
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Identifier Registry". This is a registry of 32-bit numbers that
identify the Upper Layer protocol associated with data that appears
in the RDMA-CM Private Data area.
The information that must be provided to add an entry to this
registry will be an IESG-approved Standards Track specification
defining the semantics and interoperability requirements of the
proposed new value and the fields to be recorded in the registry.
The fields in this registry include: Protocol number, Protocol name,
RFC Reference.
The initial contents of this registry are a single entry:
0xf6ab0e18, RPC-over-RDMA version 1 CM Private Data, this
specification.
All other values are available to IANA for assignment. New protocol
numbers can be assigned at random as long as they do not conflict
with existing entries in this registry.
Allocation Policy: Standards Action [RFC8126]
6. Security Considerations
RDMA-CM Private Data typically traverses the link layer in the clear.
A man-in-the-middle attack could alter the settings exchanged at
connect time such that one or both peers might perform operations
that result in premature termination of the connection.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5040] Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
Garcia, "A Remote Direct Memory Access Protocol
Specification", RFC 5040, DOI 10.17487/RFC5040, October
2007, <https://www.rfc-editor.org/info/rfc5040>.
[RFC5042] Pinkerton, J. and E. Deleganes, "Direct Data Placement
Protocol (DDP) / Remote Direct Memory Access Protocol
(RDMAP) Security", RFC 5042, DOI 10.17487/RFC5042, October
2007, <https://www.rfc-editor.org/info/rfc5042>.
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[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8166] Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
Memory Access Transport for Remote Procedure Call Version
1", RFC 8166, DOI 10.17487/RFC8166, June 2017,
<https://www.rfc-editor.org/info/rfc8166>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[IBARCH] InfiniBand Trade Association, "InfiniBand Architecture
Specification Volume 1", Release 1.3, March 2015,
<http://www.infinibandta.org/content/
pages.php?pg=technology_download>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<https://www.rfc-editor.org/info/rfc1813>.
[RFC5666] Talpey, T. and B. Callaghan, "Remote Direct Memory Access
Transport for Remote Procedure Call", RFC 5666,
DOI 10.17487/RFC5666, January 2010,
<https://www.rfc-editor.org/info/rfc5666>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
[RFC7861] Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
November 2016, <https://www.rfc-editor.org/info/rfc7861>.
Acknowledgments
Thanks to Christoph Hellwig and Devesh Sharma for suggesting this
approach, and to Tom Talpey for his comments and review. The author
also wishes to thank Bill Baker and Greg Marsden for their support of
this work.
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Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
Working Group Secretary Thomas Haynes.
Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
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