Internet DRAFT - draft-ietf-nfsv4-rpcrdma-version-two
draft-ietf-nfsv4-rpcrdma-version-two
Network File System Version 4 C. Lever, Ed.
Internet-Draft Oracle
Intended status: Standards Track D. Noveck
Expires: 30 December 2022 NetApp
28 June 2022
RPC-over-RDMA Version 2 Protocol
draft-ietf-nfsv4-rpcrdma-version-two-07
Abstract
This document specifies the second version of a transport protocol
that conveys Remote Procedure Call (RPC) messages using Remote Direct
Memory Access (RDMA). This version of the protocol is extensible.
Note
This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the NFSv4 working group
mailing list (nfsv4@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/nfsv4/. Working Group
information can be found at https://datatracker.ietf.org/wg/nfsv4/
about/.
The source for this draft is maintained in GitHub. Suggested changes
can be submitted as pull requests at https://github.com/chucklever/
i-d-rpcrdma-version-two. Instructions are on that page as well.
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 https://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 30 December 2022.
Lever & Noveck Expires 30 December 2022 [Page 1]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Copyright Notice
Copyright (c) 2022 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 (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Motivation for a New Version . . . . . . . . . . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Remote Procedure Calls . . . . . . . . . . . . . . . . . 6
3.2. Remote Direct Memory Access . . . . . . . . . . . . . . . 11
4. RPC-over-RDMA Framework . . . . . . . . . . . . . . . . . . . 13
4.1. Message Framing . . . . . . . . . . . . . . . . . . . . . 13
4.2. Reliable Message Delivery . . . . . . . . . . . . . . . . 14
4.3. Initial Connection State . . . . . . . . . . . . . . . . 17
4.4. Using Direct Data Placement . . . . . . . . . . . . . . . 18
4.5. Encoding Chunks . . . . . . . . . . . . . . . . . . . . . 19
4.6. Reverse-Direction Operation . . . . . . . . . . . . . . . 23
4.7. Call-Only Operation . . . . . . . . . . . . . . . . . . . 26
5. Transport Properties . . . . . . . . . . . . . . . . . . . . 26
5.1. Transport Properties Model . . . . . . . . . . . . . . . 27
5.2. Current Transport Properties . . . . . . . . . . . . . . 28
6. Transport Messages . . . . . . . . . . . . . . . . . . . . . 31
6.1. Transport Header Types . . . . . . . . . . . . . . . . . 32
Lever & Noveck Expires 30 December 2022 [Page 2]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.2. Headers and Chunks . . . . . . . . . . . . . . . . . . . 33
6.3. Header Types . . . . . . . . . . . . . . . . . . . . . . 34
6.4. Transport Header Prefix . . . . . . . . . . . . . . . . . 42
6.5. Remote Invalidation . . . . . . . . . . . . . . . . . . . 42
6.6. Payload Formats . . . . . . . . . . . . . . . . . . . . . 43
7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 49
7.1. Basic Transport Stream Parsing Errors . . . . . . . . . . 50
7.2. XDR Errors . . . . . . . . . . . . . . . . . . . . . . . 51
7.3. Responder RDMA Operational Errors . . . . . . . . . . . . 52
7.4. Other Operational Errors . . . . . . . . . . . . . . . . 54
7.5. RDMA Transport Errors . . . . . . . . . . . . . . . . . . 55
8. XDR Protocol Definition . . . . . . . . . . . . . . . . . . . 56
8.1. Code Component License . . . . . . . . . . . . . . . . . 56
8.2. Extraction of the XDR Definition . . . . . . . . . . . . 58
8.3. XDR Definition for RPC-over-RDMA Version 2 Core
Structures . . . . . . . . . . . . . . . . . . . . . . . 59
8.4. XDR Definition for RPC-over-RDMA Version 2 Base Header
Types . . . . . . . . . . . . . . . . . . . . . . . . . . 61
8.5. Use of the XDR Description . . . . . . . . . . . . . . . 64
9. RPC Bind Parameters . . . . . . . . . . . . . . . . . . . . . 65
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 66
11. Security Considerations . . . . . . . . . . . . . . . . . . . 66
11.1. Memory Protection . . . . . . . . . . . . . . . . . . . 67
11.2. RPC Message Security . . . . . . . . . . . . . . . . . . 68
11.3. Transport Properties . . . . . . . . . . . . . . . . . . 71
11.4. Host Authentication . . . . . . . . . . . . . . . . . . 72
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 72
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 72
13.1. Normative References . . . . . . . . . . . . . . . . . . 72
13.2. Informative References . . . . . . . . . . . . . . . . . 74
Appendix A. ULB Specifications . . . . . . . . . . . . . . . . . 76
A.1. DDP-Eligibility . . . . . . . . . . . . . . . . . . . . . 76
A.2. Maximum Reply Size . . . . . . . . . . . . . . . . . . . 78
A.3. Reverse-Direction Operation . . . . . . . . . . . . . . . 78
A.4. Additional Considerations . . . . . . . . . . . . . . . . 78
A.5. ULP Extensions . . . . . . . . . . . . . . . . . . . . . 79
Appendix B. Extending RPC-over-RDMA Version 2 . . . . . . . . . 79
B.1. Documentation Requirements . . . . . . . . . . . . . . . 80
B.2. Adding New Header Types to RPC-over-RDMA Version 2 . . . 80
B.3. Adding New Transport properties to the Protocol . . . . . 81
B.4. Adding New Error Codes to the Protocol . . . . . . . . . 82
Appendix C. Differences from RPC-over-RDMA Version 1 . . . . . . 82
C.1. Changes to the XDR Definition . . . . . . . . . . . . . . 83
C.2. Transport Properties . . . . . . . . . . . . . . . . . . 84
C.3. Credit Management Changes . . . . . . . . . . . . . . . . 84
C.4. Inline Threshold Changes . . . . . . . . . . . . . . . . 85
C.5. Message Continuation Changes . . . . . . . . . . . . . . 86
C.6. Host Authentication Changes . . . . . . . . . . . . . . . 86
Lever & Noveck Expires 30 December 2022 [Page 3]
Internet-Draft RPC-over-RDMA Version 2 June 2022
C.7. Support for Remote Invalidation . . . . . . . . . . . . . 87
C.8. Integration of Reverse-Direction Operation . . . . . . . 88
C.9. Error Reporting Changes . . . . . . . . . . . . . . . . . 89
C.10. Changes in Terminology . . . . . . . . . . . . . . . . . 89
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 90
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 90
1. Introduction
Remote Direct Memory Access (RDMA) [RFC5040] [RFC5041] [IBA] is a
technique for moving data efficiently between network nodes. By
placing transferred data directly into destination buffers using
Direct Memory Access, RDMA delivers the reciprocal benefits of faster
data transfer and reduced host CPU overhead.
Open Network Computing Remote Procedure Call (ONC RPC, often
shortened in NFSv4 documents to RPC) [RFC5531] is a Remote Procedure
Call protocol that runs over a variety of transports. Most RPC
implementations today use UDP [RFC0768] or TCP [RFC0793]. On UDP, a
datagram encapsulates each RPC message. Within a TCP byte stream, a
record marking protocol delineates RPC messages.
An RDMA transport, too, conveys RPC messages in a fashion that must
be fully defined if RPC implementations are to interoperate when
using RDMA to transport RPC transactions. Although RDMA transports
encapsulate messages like UDP, they deliver them reliably and in
order, like TCP. Further, they implement a bulk data transfer
service not provided by traditional network transports. Therefore,
we treat RDMA as a novel transport type for RPC.
1.1. Design Goals
The general mission of RPC-over-RDMA transports is to leverage
network hardware capabilities to reduce host CPU needs related to the
transport of RPC messages. In particular, this includes mitigating
host interrupt rates and limiting the necessity to copy RPC payload
bytes on receivers.
These hardware capabilities benefit both RPC clients and servers. On
balance, however, the RPC-over-RDMA protocol design approach has been
to bolster clients more than servers, as the client is typically
where applications are most hungry for CPU resources.
Additionally, RPC-over-RDMA transports are designed to support RPC
applications transparently. However, such transports can also
provide mechanisms that enable further optimization of data transfer
when RPC applications are structured to exploit direct data
placement. In this context, the Network File System (NFS) family of
Lever & Noveck Expires 30 December 2022 [Page 4]
Internet-Draft RPC-over-RDMA Version 2 June 2022
protocols (as described in [RFC1094], [RFC1813], [RFC7530],
[RFC7862], [RFC8881], and subsequent NFSv4 minor versions) are all
potential beneficiaries of RPC-over-RDMA.
A complete problem statement appears in [RFC5532].
1.2. Motivation for a New Version
Storage administrators have broadly deployed the RPC-over-RDMA
version 1 protocol specified in [RFC8166]. However, there are known
shortcomings to this protocol:
* The protocol's default size of Receive buffers forces the use of
RDMA Read and Write transfers for small payloads, and limits the
size of reverse-direction messages.
* It is difficult to make optimizations or protocol fixes that
require changes to on-the-wire behavior.
* For some RPC procedures, the maximum reply size is difficult or
impossible for an RPC client to estimate in advance.
To address these issues in a way that preserves interoperation with
existing RPC-over-RDMA version 1 deployments, the current document
presents an updated version of the RPC-over-RDMA transport protocol.
This version of RPC-over-RDMA is extensible, enabling the
introduction of OPTIONAL extensions without impacting existing
implementations. See Appendix C.1 for further discussion. It
introduces a mechanism to exchange implementation properties to
automatically provide further optimization of data transfer.
This version also contains incremental changes that relieve
performance constraints and enable recovery from unusual corner
cases. These changes are outlined in Appendix C and include a larger
default inline threshold, the ability to convey a single RPC message
using multiple RDMA Send operations, support for authentication of
connection peers, richer error reporting, improved credit-based flow
control, and support for Remote Invalidation.
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.
Lever & Noveck Expires 30 December 2022 [Page 5]
Internet-Draft RPC-over-RDMA Version 2 June 2022
3. Terminology
3.1. Remote Procedure Calls
This section highlights critical elements of the RPC protocol
[RFC5531] and the External Data Representation (XDR) [RFC4506] it
uses. RPC-over-RDMA version 2 enables the transmission of RPC
messges built using XDR and also uses XDR internally to describe its
header format.
3.1.1. Upper-Layer Protocols
RPCs are an abstraction used to implement the operations of an Upper-
Layer Protocol (ULP). For RPC-over-RDMA, "ULP" refers to an RPC
Program and Version tuple, which is a versioned set of procedure
calls that comprise a single well-defined API. One example of a ULP
is the Network File System Version 4.0 [RFC7530]. In the current
document, the term "RPC consumer" refers to an implementation of a
ULP running on an RPC client.
3.1.2. RPC Procedures
Like a local procedure call, every RPC procedure has a set of
"arguments" and a set of "results". A calling context invokes an RPC
procedure, passing arguments to it, and the procedure subsequently
returns a set of results. Unlike a local procedure call, an RPC
procedure is executed remotely rather than in the local application's
execution context.
3.1.3. RPC Transactions
The RPC protocol as described in [RFC5531] is fundamentally a
message-passing protocol between one or more clients, where RPC
consumers are running, and a server, where a remote execution context
is available to process RPC transactions on behalf of these
consumers.
ONC RPC transactions consist of two types of messages:
* A CALL message, or "Call", requests work. An RPC Call message is
designated by the value zero (0) in the message's msg_type field.
* A REPLY message, or "Reply", reports the results of work requested
by an RPC Call message. An RPC Reply message is designated by the
value one (1) in the message's msg_type field.
Lever & Noveck Expires 30 December 2022 [Page 6]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Section 9 of [RFC5531] introduces the RPC transaction identifier, or
"XID" for short. Each connection endpoint interprets the value of an
XID in the context of the message's msg_type field.
* The sender of a Call message generates an arbitrary XID value for
each RPC that is unique among outstanding Calls from that sender.
* The sender of a Reply message copies the XID of the initiating
Call to the Reply containing the results of that procedure.
After receiving a Reply, a Requester then matches the XID value in
that Reply with a Call it previously sent.
The ratio of Call messages to Reply messages is typically but not
always one-to-one.
The most common operational paradigm is when a Requester sends a Call
message to a Responder, who then sends a Reply message back to the
Requester with the results of that procedure. One Call message
elicits a single Reply message in response. A Responder never sends
more than one Reply for each received Call message.
A "retransmission" occurs when a Requester sends exactly the same
Call message, with the same arguments and XID, more than once. A
Requester can retransmit if it believes the network layer or
Responder has dropped a Call message, or if the Responder's Reply has
been likewise lost. To prevent unnecessary network traffic or the
execution of non-idempotent procedures multiple times, Requesters
avoid retransmitting needlessly.
In rare cases, an RPC procedure may not require any results or even
acknowledgement that the Responder has executed the procedure. In
that case, the Requester sends a Call message but no Reply is
returned. This document refers to that case as "Call-only".
3.1.4. Message Serialization
RPC messages are always transmitted atomically. RPC peers may
interleave messages, but the contents of individual messages cannot
be broken up or interleaved without making the messages illegible.
An RPC peer acting as a "Requester" serializes the procedure's
arguments and conveys them to a "Responder" endpoint via an RPC Call
message. A Call message contains an RPC protocol header with a
unique XID, a header describing the requested upper-layer operation,
and all arguments.
Lever & Noveck Expires 30 December 2022 [Page 7]
Internet-Draft RPC-over-RDMA Version 2 June 2022
An RPC peer acting as a "Responder" deserializes these arguments and
processes the requested procedure. It then serializes the
procedure's results into an RPC Reply message. An RPC Reply message
contains an RPC protocol header with the same XID, a header
describing the upper-layer reply, and all results.
The Requester deserializes the results and allows the RPC consumer to
proceed. At this point, the RPC transaction designated by the XID in
the RPC Call message is complete, and the XID is retired.
3.1.5. RPC Transports
The role of an "RPC transport" is to mediate the exchange of RPC
messages between Requesters and Responders, bridging the gap between
the RPC message abstraction and the native operations of a network
transport (e.g., a socket).
When an RPC transport type is connection-oriented, RPC client
endpoints initiate transport connections, while RPC server endpoints
wait passively to accept incoming connection requests. RPC messages
may also be exchanged without a connection association. Because RPC-
over-RDMA is a connection-oriented RPC transport, connectionless
operation is not discussed further in the current document.
3.1.5.1. Transport Failure Recovery
So that appropriate and timely recovery action can be taken, the
transport implementation is responsible for notifying a Requester
when an RPC Call or Reply was not able to be conveyed. Recovery can
take the form of establishing a new connection, re-sending RPC Calls,
or terminating RPC transactions pending on the Requester.
For instance, a connection loss may occur after a Responder has
received an RPC Call but before it can send the matching RPC Reply.
Once the transport notifies the Requester of the connection loss, the
Requester can re-send all pending RPC Calls on a fresh connection.
3.1.5.2. Forward Direction
Traditionally, an RPC client acts as a Requester, while an RPC
service acts as a Responder. The current document refers to this
direction of RPC message passing as "forward-direction" operation.
Lever & Noveck Expires 30 December 2022 [Page 8]
Internet-Draft RPC-over-RDMA Version 2 June 2022
3.1.5.3. Reverse-Direction
The RPC specification [RFC5531] does not forbid performing RPC
transactions in the other direction. An RPC service endpoint can act
as a Requester, in which case an RPC client endpoint acts as a
Responder. This direction of RPC message passing is known as
"reverse-direction" operation.
During reverse-direction operation, an RPC client is responsible for
establishing transport connections, even though the RPC server
originates RPC Calls.
RPC clients and servers are usually optimized to perform and scale
well when handling traffic in the forward direction. They might not
be prepared to handle operation in the reverse direction. Not until
NFS version 4.1 [RFC8881] has there been a strong need to handle
reverse-direction operation.
3.1.5.4. Bi-directional Operation
A pair of connected RPC endpoints may choose to use only forward-
direction or only reverse-direction operation on a particular
transport connection. Or, these endpoints may send Calls in both
directions concurrently on the same transport connection.
"Bi-directional operation" occurs when both transport endpoints act
as a Requester and a Responder at the same time on a single
connection.
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.
During bi-directional operation, forward- and reverse- direction XIDs
are typically generated on distinct hosts by possibly different
algorithms. There is no coordination between the generation of XIDs
used in forward-direction and reverse-direction operation.
Therefore, a forward-direction Requester MAY use the same XID value
at the same time as a reverse-direction Requester on the same
transport connection. Although such concurrent requests use the same
XID value, they represent distinct RPC transactions.
Lever & Noveck Expires 30 December 2022 [Page 9]
Internet-Draft RPC-over-RDMA Version 2 June 2022
3.1.6. External Data Representation
One cannot assume that all Requesters and Responders represent data
objects in the same way internally. RPC uses External Data
Representation (XDR) to translate native data types and serialize
arguments and results [RFC4506].
XDR encodes data independently of the endianness or size of host-
native data types, enabling unambiguous decoding of data by a
receiver.
XDR assumes only that the number of bits in a byte (octet) and their
order are the same on both endpoints and the physical network. The
smallest indivisible unit of XDR encoding is a group of four octets.
XDR can also flatten lists, arrays, and other complex data types into
a stream of bytes.
We refer to a serialized stream of bytes that is the result of XDR
encoding as an "XDR stream". A sender encodes native data into an
XDR stream and then transmits that stream to a receiver. The
receiver decodes incoming XDR byte streams into its native data
representation format.
3.1.6.1. XDR Opaque Data
Sometimes, a data item is to be transferred as-is, without encoding
or decoding. We refer to the contents of such a data item as "opaque
data". XDR encoding places the content of opaque data items directly
into an XDR stream without altering it in any way. ULPs or
applications perform any needed data translation in this case.
Examples of opaque data items include the content of files or generic
byte strings.
3.1.6.2. XDR Roundup
The number of octets in a variable-length data item precedes that
item in an XDR stream. If the size of an encoded data item is not a
multiple of four octets, the sender appends octets containing zero
after the end of the data item. These zero octets shift the next
encoded data item in the XDR stream so that it always starts on a
four-octet boundary. The addition of extra octets does not change
the encoded size of the data item. Receivers do not expose the extra
octets to ULPs.
We refer to this technique as "XDR roundup", and the extra octets as
"XDR roundup padding".
Lever & Noveck Expires 30 December 2022 [Page 10]
Internet-Draft RPC-over-RDMA Version 2 June 2022
3.2. Remote Direct Memory Access
When a third party transfers large RPC payloads, RPC Requesters and
Responders can become more efficient. An example of such a third
party might be an intelligent network interface (data movement
offload), which places data in the receiver's memory so that no
additional adjustment of data alignment is necessary (direct data
placement or "DDP"). RDMA transports enable both of these
optimizations.
In the current document, the standalone term "RDMA" refers to the
physical mechanism an RDMA transport utilizes when moving data.
3.2.1. Direct Data Placement
Typically, RPC implementations copy the contents of RPC messages into
a buffer before being sent. An efficient RPC implementation sends
bulk data without first copying it into a separate send buffer.
However, socket-based RPC implementations are often unable to receive
data directly into its final place in memory. Receivers often need
to copy incoming data to finish an RPC operation, if only to adjust
data alignment.
Although it may not be efficient, before an RDMA transfer, a sender
may copy data into an intermediate buffer. After an RDMA transfer, a
receiver may copy that data again to its final destination. In this
document, the term "DDP" refers to any optimized data transfer where
a receiving host's CPU does not move transferred data to another
location after arrival.
RPC-over-RDMA version 2 enables the use of RDMA Read and Write
operations to achieve both data movement offload and DDP. However,
note that not all RDMA-based data transfer qualifies as DDP, and some
mechanisms that do not employ explicit RDMA can place data directly.
3.2.2. RDMA Transport Operation
RDMA transports require that RDMA consumers provision resources in
advance to achieve good performance during receive operations. An
RDMA consumer might provide Receive buffers in advance by posting an
RDMA Receive Work Request for every expected RDMA Send from a remote
peer. These buffers are provided before the remote peer posts RDMA
Send Work Requests. Thus this is often referred to as "pre-posting"
buffers.
Lever & Noveck Expires 30 December 2022 [Page 11]
Internet-Draft RPC-over-RDMA Version 2 June 2022
An RDMA Receive Work Request remains outstanding until the RDMA
provider matches it to an inbound Send operation. The resources
associated with that Receive must be retained in host memory, or
"pinned", until the Receive completes.
Given these tenets of operation, the RPC-over-RDMA version 2 protocol
assumes each transport provides the following abstract operations. A
more complete discussion of these operations appears in [RFC5040].
3.2.2.1. Memory Registration
Memory registration assigns a steering tag to a region of memory,
permitting the RDMA provider to perform data-transfer operations.
The RPC-over-RDMA version 2 protocol assumes that a steering tag of
no more than 32 bits and memory addresses of up to 64 bits in length
identifies each registered memory region.
3.2.2.2. RDMA Send
The RDMA provider supports an RDMA Send operation, with completion
signaled on the receiving peer after the RDMA provider has placed
data in a pre-posted buffer. Sends complete at the receiver in the
order they were posted at the sender. The size of the remote peer's
pre-posted buffers limits the amount of data that can be transferred
by a single RDMA Send operation.
3.2.2.3. RDMA Receive
The RDMA provider supports an RDMA Receive operation to receive data
conveyed by incoming RDMA Send operations. To reduce the amount of
memory that must remain pinned awaiting incoming Sends, the amount of
memory posted per Receive is limited. The RDMA consumer (in this
case, the RPC-over-RDMA version 2 protocol) provides flow control to
prevent overrunning receiver resources.
3.2.2.4. RDMA Write
The RDMA provider supports an RDMA Write operation to place data
directly into a remote memory region. The local host initiates an
RDMA Write and the RDMA provider signals completion there. The
remote RDMA provider does not signal completion on the remote peer.
The local host provides the steering tag, the memory address, and the
length of the remote peer's memory region.
RDMA Writes are not ordered relative to one another, but are ordered
relative to RDMA Sends. Thus, a subsequent RDMA Send completion
signaled on the local peer guarantees that prior RDMA Write data has
been successfully placed in the remote peer's memory.
Lever & Noveck Expires 30 December 2022 [Page 12]
Internet-Draft RPC-over-RDMA Version 2 June 2022
3.2.2.5. RDMA Read
The RDMA provider supports an RDMA Read operation to place remote
source data directly into local memory. The local host initiates an
RDMA Read and and the RDMA provider signals completion there. The
remote RDMA provider does not signal completion on the remote peer.
The local host provides the steering tags, the memory addresses, and
the lengths for the remote source and local destination memory
regions.
The RDMA consumer (in this case, the RPC-over-RDMA version 2
protocol) signals Read completion to the remote peer as part of a
subsequent RDMA Send message. The remote peer can then invalidate
steering tags and subsequently free associated source memory regions.
4. RPC-over-RDMA Framework
Before an RDMA data transfer can occur, an endpoint first exposes
regions of its memory to a remote endpoint. The remote endpoint then
initiates RDMA Read and Write operations against the exposed memory.
A "transfer model" designates which endpoint exposes its memory and
which is responsible for initiating the transfer of data.
In RPC-over-RDMA version 2, only Requesters expose their memory to
the Responder, and only Responders initiate RDMA Read and Write
operations. Read access to memory regions enables the Responder to
pull RPC arguments or whole RPC Calls from each Requester. The
Responder pushes RPC results or whole RPC Replies to a Requester's
memory regions to which it has write access.
4.1. Message Framing
Each RPC-over-RDMA version 2 message consists of at most two XDR
streams:
* The "Transport stream" contains a header that describes and
controls the transfer of the Payload stream in this RPC-over-RDMA
message. Every RDMA Send on an RPC-over-RDMA version 2 connection
MUST begin with a Transport stream.
* The "Payload stream" contains part or all of a single RPC message.
The sender MAY divide an RPC message at any convenient boundary
but MUST send RPC message fragments in XDR stream order and MUST
NOT interleave Payload streams from multiple RPC messages.
The RPC-over-RDMA framing mechanism described in this section
replaces all other RPC framing mechanisms. Connection peers use RPC-
over-RDMA framing even when the underlying RDMA protocol runs on a
Lever & Noveck Expires 30 December 2022 [Page 13]
Internet-Draft RPC-over-RDMA Version 2 June 2022
transport type with well-defined RPC framing, such as TCP. However,
a ULP can negotiate the use of RDMA, dynamically enabling the use of
RPC-over-RDMA on a connection established on some other transport
type. Because RPC framing delimits an entire RPC request or reply,
the resulting shift in framing must occur between distinct RPC
messages, and in concert with the underlying transport.
4.2. Reliable Message Delivery
RPC-over-RDMA provides a reliable and in-order data transport service
for RPC Calls and Replies.
RPC-over-RDMA transports MUST operate only on a reliable Queue Pair
(QP) such as the RDMA RC (Reliable Connected) QP type as defined in
Section 9.7.7 of [IBA]. The Marker PDU Aligned (MPA) protocol
[RFC5044], when deployed on a reliable transport such as TCP,
provides similar functionality. Using a reliable QP type ensures in-
transit data integrity and proper recovery from packet loss in the
lower layers.
If any pre-posted Receive buffer on the connection is not large
enough to contain an incoming message, the receiving RDMA provider
cannot deliver that message to the upper-layer consumer. Likewise,
if no pre-posted Receive buffer is available to accept an incoming
message, the receiving RDMA provide cannot pass that message to the
consumer. Exceeding these limits results in a transition to a QP
error state, the loss of an in-flight message, and the potential loss
of the connection.
Therefore, senders need to respect peer receiver resource limits to
ensure that the transport service can deliver every message reliably.
Two operational parameters communicate these limits between RPC-over-
RDMA peers: credits and inline threshold.
4.2.1. Flow Control
RPC-over-RDMA version 2 employs end-to-end credit-based flow control
on each connection to prevent a sender from transmitting more
messages than a receiver is prepared to accept [CBFC]. Credit-based
flow control is relatively simple, providing automated management of
receive buffer allocation and robust operation in the face of bursty
traffic while enabling effective pipelining. The RPC-over-RDMA
version 2 flow control mechanism relies on reliable and in-order
message delivery guarantees provided by the underlying RDMA transport
service.
Lever & Noveck Expires 30 December 2022 [Page 14]
Internet-Draft RPC-over-RDMA Version 2 June 2022
An RPC-over-RDMA version 2 credit represents the capability to convey
exactly one RPC-over-RDMA version 2 message, regardless of its size,
via an RDMA Send/Receive pair. Because an RPC-over-RDMA version 2
connection is full-duplex, each connection peer has its own set of
credits. The two peers manage their credit limits independently,
although they communicate these values by piggy-backing them on a
message in the opposite direction.
Each peer tracks four critical values for each connection. The peer
uses these values to determine when it is safe to send a message on
the connection.
Sent message count: The total number of RDMA Send channel operations
the peer has posted on the connection. The peer MUST set this
value to zero (0) when a connection is first established.
Received message count: The total number of RDMA Receive channel
operations that have completed on the connection. The peer MUST
set this value to zero (0) when a connection is first established.
Advertised credits: The number of messages the peer is prepared to
receive. This value is typically equal to or less than the number
of RDMA Receive channel operations that are currently pending on
the connection.
Received credits: The value in the rdma_start.rdma_credit field in
the most recently received message on the connection. The peer
MUST set this value to one (1) when a connection is first
established.
When constructing an RPC-over-RDMA header to be sent, a sender MUST
set the header's rdma_start.rdma_credit field to the sum of the
connection's "Sent message count" and "Advertised credits" values.
The sender MUST NOT post this message if the connection's "Sent
message counter" is greater than the connection's current "Received
credits" value. To handle counter wrapping, the sender uses
appropriate modulo arithmetic to perform this comparison.
Because the rdma_start.rdma_credit field is 32 bits wide, credit
limit values MUST be less than 2^31 - 1. For a given bandwidth-delay
product, a peer selects an "Advertised credits" value that is large
enough to maximize throughput while not overwhelming its local memory
resources.
A peer MAY adjust its "Advertised credits" to match the needs or
policies in effect on either peer. For instance, a peer may reduce
its "Advertised credits" to accommodate the available resources in a
Shared Receive Queue. Certain RDMA implementations may impose
Lever & Noveck Expires 30 December 2022 [Page 15]
Internet-Draft RPC-over-RDMA Version 2 June 2022
additional flow-control restrictions, such as limits on RDMA Read
operations in progress at the Responder. Accommodation of such
checks is considered the responsibility of each RPC-over-RDMA version
2 implementation.
4.2.1.1. Asynchronous Credit Grants
Credit accounting information is usually piggy-backed on payload-
bearing messages. However, on occasion, a peer might need to reset
its credit limit without sending an RPC payload. A receiving peer
can send a message using a special header type when the sender's
credit limit approaches exhaustion during a stream of unacknowledged
messages. See Section 6.3.2 for information about this header type.
Unlike RPC-over-RDMA version 1, the credit grant on an RPC-over-RDMA
version 2 connection MAY be zero. In that case, the sender waits
until the receiver sends it an asynchronous credit refresh. To
prevent a sender from ever having to wait for a credit refresh, a
good receiver implementation provides a credit refresh before half
its credit limit is exceeded.
To prevent transport deadlock, receivers MUST always be in a position
to receive one asynchronous credit update message, in addition to
payload-bearing messages. A receiver can do this is by posting one
more RDMA Receive than its "Advertised credits" value.
4.2.2. Inline Threshold
An "inline threshold" value is the largest message size (in octets)
that can be conveyed in one direction between peer implementations
using RDMA Send and Receive channel operations. An inline threshold
value is less than or equal to the largest number of octets the
sender can post in a single RDMA Send operation. It is also less
than or equal to the largest number of octets the receiver can
reliably accept via a single RDMA Receive operation.
Each connection has two inline threshold values. There is one for
messages flowing from Requester-to-Responder (referred to as the
"call inline threshold"), and one for messages flowing from
Responder-to-Requester (referred to as the "reply inline threshold").
Peers can advertise their inline threshold values via RPC-over-RDMA
version 2 Transport Properties (see Section 5). In the absence of an
exchange of Transport Properties, connection peers MUST assume both
inline thresholds are 4096 octets.
Lever & Noveck Expires 30 December 2022 [Page 16]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.3. Initial Connection State
Immediately upon connection establishment, both peers MUST allow only
one outstanding RPC-over-RDMA message on the connection at a time
until both the transport protocol version is established and both
peers have received an initial credit limit. Note that because RPC-
over-RDMA versions 1 and 2 each use a different flow control
mechanism, the meaning of the value in the rdma_start.rdma_credit
field depends on the value in the rdma_start.vers field.
The second word of each transport header conveys the transport
protocol version. Immediately after the client establishes a
connection, it sends a single valid RPC-over-RDMA message with the
value two (2) in the rdma_start.rdma_vers field. Because the server
might support only RPC-over-RDMA version 1, this initial message MUST
NOT be larger than the version 1 default inline threshold of 1024
octets.
4.3.1. Server Supports RPC-over-RDMA Version 2
If the server supports RPC-over-RDMA version 2, it sends RPC-over-
RDMA messages back to the client with the value two (2) in the
rdma_start.rdma_vers field. Both peers may assume the default inline
threshold value for RPC-over-RDMA version 2 connections (4096
octets).
4.3.2. Server Does Not Support RPC-over-RDMA Version 2
If the server does not support RPC-over-RDMA version 2, it MUST send
an RPC-over-RDMA message to the client with an XID that matches the
client's first message, RDMA2_ERROR in the rdma_start.rdma_htype
field, and with the error code RDMA2_ERR_VERS. This message also
reports the range of RPC-over-RDMA protocol versions that the server
supports. To continue operation, the client selects a protocol
version in that range for subsequent messages on this connection.
If the connection is dropped immediately after an RDMA2_ERROR/
RDMA2_ERR_VERS message is received, the client should try to avoid a
version negotiation loop when re-establishing another connection. It
can assume that the server does not support RPC-over-RDMA version 2.
A client can assume the same situation (i.e., no server support for
RPC-over-RDMA version 2) if the initial negotiation message is lost
or dropped. Once the version negotiation exchange is complete, both
peers may use the default inline threshold value for the negotiated
transport protocol version.
Lever & Noveck Expires 30 December 2022 [Page 17]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.3.3. Client Does Not Support RPC-over-RDMA Version 2
The server examines the RPC-over-RDMA protocol version used in the
first RPC-over-RDMA message it receives. If it supports this
protocol version, it MUST use it in all subsequent messages it sends
on that connection. The client MUST NOT change the protocol version
for the duration of the connection.
4.4. Using Direct Data Placement
RPC-over-RDMA version 2 provides a mechanism for moving part of an
RPC message via a data transfer distinct from RDMA Send and Receive.
For example, a sender can remove one or more XDR data items from the
Payload stream. These items are then conveyed via other mechanisms,
such as one or more RDMA Read or Write operations.
4.4.1. Chunks and Segments
A Requester records the location information for each registered
memory region associated with an RPC payload in the transport header
of an RPC-over-RDMA message. With this information, the Responder
uses RDMA Read and Write operations to retrieve arguments contained
in the specified region of the Requester's memory or place results in
that region.
A "segment" is a transport header data object that contains the
precise coordinates of a contiguous registered memory region. Each
segment contains the following information:
Handle: A steering Tag (STag) or R_key generated by registering this
memory with the RDMA provider.
Length: The length of the segment's memory region, in octets. The
length of a segment MAY be aligned to a single octet. An "empty
segment" is defined as a segment with the value zero (0) in its
length field.
Offset: The offset or beginning memory address of the segment's
memory region.
The meaning of the values contained in these fields is elaborated in
[RFC5040].
A "chunk" is simply a set of segments that have a related purpose. A
Requester MAY divide a chunk into segments using any convenient
boundaries. The length of a chunk is defined as the sum of the
lengths of the segments that comprise it.
Lever & Noveck Expires 30 December 2022 [Page 18]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.4.2. Reducing a Payload Stream
We refer to a data item that a sender removes from a Payload stream
to transmit separately as a "reduced" data item. After a sender has
finished removing XDR data items from a Payload stream, we refer to
it as a "reduced" Payload stream. A set of segments that describe
memory regions containing a single reduced data item is categorized
as a "data item chunk."
Not all XDR data items benefit from Direct Data Placement. For
example, small data items or data items that require XDR unmarshaling
by the receiver do not benefit from DDP. Moreover, it is impractical
for receivers to prepare for every possible XDR data item in a
protocol to appear in a data item chunk.
Specifying which data items are DDP-eligible is done in separate
standards track documents known as "Upper Layer Bindings". A ULB
identifies which XDR data items a peer MAY transfer using DDP. We
refer to such data items as "DDP-eligible." Senders MUST NOT reduce
any other XDR data items. Detailed requirements for ULB
specifications appear in Appendix A of the current document.
4.4.3. Moving Whole RPC Messages using Explicit RDMA
RPC-over-RDMA version 2 also enables the movement of a whole RPC
message via data transfer distinct from RDMA Send and Receive. A
sender registers the memory containing a Payload stream without
regard to data item boundaries or DDP-eligibility. The Payload
stream is then conveyed via other mechanisms, such as one or more
RDMA Read or Write operations. A set of segments that describe
memory regions containing a Payload stream is categorized as a "body
chunk".
A sender may first reduce that Payload stream if it contains one or
more DDP-eligible data items. The sender moves these data items
using data items chunks, and the reduced Payload stream using a body
chunk.
4.5. Encoding Chunks
The RPC-over-RDMA version 2 transport protocol does not place a limit
on chunk size. However, each ULP may cap the amount of data that can
be transferred by a single RPC transaction. For example, NFS
implementations typically have settings that restrict the payload
size of NFS READ and WRITE operations. The Responder can use such
limits to sanity check chunk sizes before using them in RDMA
operations.
Lever & Noveck Expires 30 December 2022 [Page 19]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.5.1. Read Chunks
A "Read chunk" contains data that its receiver pulls from the sender.
Each Read chunk is a set of one or more "Read segments" encoded as a
list. A Read segment consists of a Position field followed by a
segment, as defined in Section 4.4.1.
Position: The byte offset in the unreduced Payload stream where the
receiver reinserts the data item conveyed in the chunk. The
sender MUST compute the Position value from the beginning of the
unreduced Payload stream, which begins at Position zero. All
segments in the same Read chunk share the same Position value,
even if one or more of the segments have a non-four-byte-aligned
length. The value in this field MUST be a multiple of four.
When constructing an RPC-over-RDMA message, the sender registers
memory regions containing data intended for RDMA Read operations. It
advertises the coordinates of these regions in Read chunks added to
the transport header of an RPC-over-RDMA message.
The receiver of this message then pulls the chunk's data from the
sender using RDMA Read operations. When receiving a Read chunk, the
receiver inserts the first Read segment in a Read chunk into the
Payload stream at the byte offset indicated by its Position field.
The receiver concatenates Read segments whose Position field value
matches this offset until there are no more Read segments at that
Position value.
4.5.1.1. The Read List
Each RPC-over-RDMA message carries a list of Read segments that make
up the set of Read chunks for that message. When no RDMA Read
operations are needed to complete the transmission of the message's
Payload stream, the message's Read list is empty.
If a Responder receives a Read list whose segment position values do
not appear in monotonically increasing order, it MUST discard the
message without processing it and respond with an RDMA2_ERROR message
with the rdma_xid field set to the XID of the malformed message and
the rdma_err field set to RDMA2_ERR_BAD_XDR.
4.5.1.2. The Call Chunk
The Call chunk is a Read chunk that acts as a body chunk containing
an RPC Call message. A Requester can utilize a Call chunk at any
time. However, using a Call chunk is less efficient than an RDMA
Send.
Lever & Noveck Expires 30 December 2022 [Page 20]
Internet-Draft RPC-over-RDMA Version 2 June 2022
A Read chunk may act as either a data item chunk or a body chunk.
When the chunk's position is zero, it acts as a body chunk.
Otherwise, it is a data item chunk containing exactly one XDR data
item.
4.5.1.3. Read Completion
A Responder acknowledges that it is finished with the Requester's
Read chunk memory regions when it sends the corresponding RPC Reply
message. The Requester may then invalidate memory regions belonging
to Read chunks associated with the associated RPC Call message.
4.5.2. Write Chunks
Each "Write chunk" consists of a counted array of zero or more
segments, as defined in Section 4.4.1. The function of a Write chunk
depends on the direction of the containing RPC-over-RDMA message. In
a Call message, a Write chunk advertises registered memory regions
into which the Responder may push data. In a Reply message, a Write
chunk reports how much data has been pushed.
A Requester provisions Write chunks for an RPC transaction long
before the Responder has constructed a corresponding Reply message.
A Requester typically does not know the actual length of the result
data items or Reply to be returned, since the Reply does not yet
exist. Thus, a Requester MUST provision Write chunks large enough to
accommodate the maximum possible size of each returned data item.
An "empty Write chunk" is a Write chunk with a zero segment count.
By definition, the length of an empty Write chunk is zero. An
"unused Write chunk" has a non-zero segment count, but all of its
segments are empty segments.
4.5.2.1. The Write List
Each RPC-over-RDMA message carries a list of Write chunks. When no
DDP-eligible data items are to appear in the Reply to an RPC
transaction, the Requester provides an empty Write list in the RPC
Call, and the Responder leaves the Write list empty in the matching
RPC Reply. When a Write chunk appears in the Write list, it acts
only as a data item chunk.
Lever & Noveck Expires 30 December 2022 [Page 21]
Internet-Draft RPC-over-RDMA Version 2 June 2022
For each Write chunk in the Write list, the Responder pushes one DDP-
eligible data item to the Requester. It fills the chunk contiguously
and in segment array order until the Responder has written that data
item to the Requester in its entirety. The Responder MUST copy the
segment count and all segments from the Requester-provided Write
chunk into the RPC Reply message's transport header. As it does so,
the Responder updates each segment length field to reflect the actual
amount of data returned in that segment.
The Responder then sends the RPC Reply message via an RDMA Send
operation.
4.5.2.2. The Reply Chunk
The Reply chunk is a single Write chunk that acts as a body chunk.
that contains an RPC Reply message. When a Requester estimates that
the Reply message can exceed the connection's ability to convey that
Reply using RDMA Send operations, it should provision a Reply chunk.
4.5.2.3. Write Completion
A Responder acknowledges that it is finished updating the Requester's
Write chunk memory regions when it sends the corresponding RPC Reply
message. The RDMA provider guarantees that the written data is at
rest before the next Receive operation, which typically contains the
corresponding RPC Reply, completes. The Requester may then
invalidate memory regions belonging to Write chunks associated with
the associated RPC Call message.
4.5.2.4. Write Chunk Roundup
When provisioning a Write chunk for a variable-length result data
item, the Requester MUST NOT include additional space for XDR roundup
padding. A Responder MUST NOT write XDR roundup padding into a Write
chunk, even if the result is shorter than the available space in the
chunk.
4.5.3. Reducing Complex XDR Data Types
XDR data items may appear in body chunks without regard to their DDP-
eligibility. As body chunks contain a Payload stream, they MUST
include all appropriate XDR roundup padding to maintain proper XDR
alignment of their contents.
However, a data item chunk MUST contain only one XDR data item, and
the chunk MUST occupy a four-byte aligned length in the Payload
stream so that subsequent data items remain properly aligned once the
reduced data item is removed from the Payload stream.
Lever & Noveck Expires 30 December 2022 [Page 22]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.5.3.1. Variable-Length Data Items
When a sender reduces a variable-length XDR data item, the length of
the item MUST remain in the Payload stream. The sender MUST omit the
item's XDR roundup padding from the Payload stream and the chunk.
The chunk's total length MUST be the same as the encoded length of
the data item.
4.5.3.2. Counted Arrays
When reducing a data item that is a counted array data type, the
count of array elements MUST remain in the Payload stream. The
sender MUST move the array elements into the chunk. For example,
when encoding an opaque byte array as a chunk, the count of bytes
stays in the Payload stream, and the sender places the bytes in the
array in the chunk.
Individual array elements appear in a chunk in their entirety. For
example, when encoding an array of arrays as a chunk, the count of
items in the enclosing array stays in the Payload stream. But each
enclosed array, including its item count, is transferred as part of
the chunk.
4.5.3.3. Optional-Data
Similar to a counted array, when reducing an optional-data data type,
the discriminator field MUST remain in the Payload stream. The
sender MUST place the data, when present, in the chunk.
4.5.3.4. XDR Unions
A union data type MUST NOT be made DDP-eligible. However, one or
more of its arms MAY be made DDP-eligible, subject to the other
requirements in this section.
4.6. Reverse-Direction Operation
The terminology used in this section is introduced in
Section 3.1.5.3.
4.6.1. Sending a Reverse-Direction RPC Call
An RPC-over-RDMA server endpoint constructs the transport header for
a reverse-direction RPC Call as follows:
Lever & Noveck Expires 30 December 2022 [Page 23]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The server generates a new XID value (see Section 3.1.3 for full
requirements) and places it in the rdma_xid field of the transport
header and the xid field of the RPC Call message. The RPC Call
header MUST start with the same XID value that is present in the
transport header.
* The rdma_vers field of each reverse-direction Call MUST contain
the same value as forward-direction Calls on the same connection.
* The server fills in the rdma_credit field with the credit values
for the connection, as described in Section 4.2.1.
* The server determines the Payload format for the RPC message and
fills in the rdma_htype field as appropriate (see Sections 6.6 and
4.6.4). Section 4.6.4 also covers the disposition of the chunk
lists.
4.6.2. Sending a Reverse-Direction RPC Reply
An RPC-over-RDMA client endpoint constructs the transport header for
a reverse-direction RPC Reply as follows:
* The client copies the XID value from the matching RPC Call and
places it in the rdma_xid field of the transport header and the
xid field of the RPC Reply message. The RPC Reply header MUST
start with the same XID value that is present in the transport
header.
* The rdma_vers field of each reverse-direction Call MUST contain
the same value as forward-direction Replies on the same
connection.
* The client fills in the rdma_credit field with the credit values
for the connection, as described in Section 4.2.1.
* The client determines the Payload format for the RPC message and
fills in the rdma_htype field as appropriate (see Sections 6.6 and
4.6.4). Section 4.6.4 also covers the disposition of the chunk
lists.
4.6.3. When Reverse-Direction Operation is Not Supported
An RPC-over-RDMA transport endpoint does not have to support reverse-
direction operation. There might be no mechanism in the transport
implementation to do so. Or, the transport implementation might
support operation in the reverse direction, but the Upper-Layer
Protocol might not configure the transport to handle reverse-
direction traffic.
Lever & Noveck Expires 30 December 2022 [Page 24]
Internet-Draft RPC-over-RDMA Version 2 June 2022
If an endpoint is unprepared to receive a reverse-direction message,
loss of the RDMA connection might result. Thus a denial of service
can occur if an RPC server continues to send reverse-direction
messages after a client that is not prepared to receive them
reconnects to that server.
Connection peers indicate their support for reverse-direction
operation as part of the exchange of Transport Properties just after
a connection is established (see Section 5.2.5).
When dealing with the possibility that the remote peer has no
transport level support for reverse-direction operation, the Upper-
Layer Protocol is responsible for informing peers when reverse-
direction operation is supported. Otherwise, even a simple reverse-
direction RPC NULL procedure from a peer could result in a lost
connection. Therefore, an Upper-Layer Protocol MUST NOT perform
reverse-direction RPC operations until the RPC client indicates
support for them.
4.6.4. Using Chunks During Reverse-Direction Operation
Reverse-direction operations can use chunks for DDP-eligible data
items and Special payload formats the same way chunks are used in
forward-direction operation. Connection peers indicate their support
for using chunks in the reverse direction as part of the exchange of
Transport Properties just after a connection is established (see
Section 5.2.5).
However, an implementation might support only Upper-Layer Protocols
that have no DDP-eligible data items. Such Upper-Layer Protocols can
use only small messages, or they might have a native mechanism for
restricting the size of reverse-direction RPC messages, obviating the
need to handle chunks in the reverse direction.
When there is no Upper-Layer Protocol need for chunks in the reverse
direction, implementers MAY choose not to provide support for chunks
in the reverse direction, thus avoiding the complexity of
implementing support for RDMA Reads and Writes in the reverse
direction. When an RPC-over-RDMA transport implementation does not
support chunks in the reverse direction, RPC endpoints use only the
Simple Payload format without data item chunks or the Continued
Payload format without data item chunks to send RPC messages in the
reverse direction.
If a reverse-direction Requester provides a non-empty chunk list to a
Responder that does not support chunks, the Responder MUST report its
lack of support using one of the error values defined in Section 7.3.
Lever & Noveck Expires 30 December 2022 [Page 25]
Internet-Draft RPC-over-RDMA Version 2 June 2022
4.6.5. Reverse-Direction Retransmission
In rare cases, an RPC server cannot complete an RPC transaction or
cannot send a Reply. In these cases, the Requester may send the RPC
transaction again using the same RPC XID.
In the forward direction, an RPC client is the Requester. The client
is always responsible for ensuring a transport connection is in place
before sending a dropped Call again.
With reverse-direction operation, an RPC server is the Requester.
Because an RPC server is not responsible for establishing transport
connections with clients, the Requester is unable to retransmit a
reverse-direction Call whenever there is no transport connection. In
this case, the RPC server must wait for the RPC client to re-
establish a transport connection before it can retransmit reverse-
direction RPC Calls.
If the forward-direction Requester has no work to do, it can be some
time before the RPC client re-establishes a transport connection. An
RPC server may need to abandon a pending reverse-direction RPC Call
to avoid waiting indefinitely for the client to re-establish a
transport connection.
Therefore forward-direction Requesters SHOULD maintain a transport
connection as long as the RPC server might send reverse-direction
Calls. For example, while an NFS version 4.1 client has open
delegated files or active pNFS layouts, it maintains one or more
transport connections to enable the NFS server to perform callback
operations.
4.7. Call-Only Operation
There is no corresponding Reply to a Call-only procedure. Thus there
is no opportunity for the Responder to indicate it has completed its
use of Read or Call chunks that hold arguments or the whole Call. In
addition, Write and Reply chunks are not necessary because there are
no results and no Reply message. Therefore, Requesters MUST NOT use
chunks when sending Call-only RPC procedures.
5. Transport Properties
RPC-over-RDMA version 2 enables connection endpoints to exchange
information about implementation properties. Compatible endpoints
use this information to optimize data transfer. Initially, only a
small set of transport properties are defined. The protocol provides
header types to exchange transport properties (see 6.3.3 and 6.3.4).
Lever & Noveck Expires 30 December 2022 [Page 26]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Both the set of transport properties and the operations used to
communicate them may be extended. Within RPC-over-RDMA version 2,
such extensions are OPTIONAL. A discussion of extending the set of
transport properties appears in Appendix B.3.
5.1. Transport Properties Model
The current document specifies a basic set of receiver and sender
properties. Such properties are specified using a code point that
identifies the particular transport property and a nominally opaque
array containing the XDR encoding of the property.
The following XDR types handle transport properties:
<CODE BEGINS>
typedef rpcrdma2_propid uint32;
struct rpcrdma2_propval {
rpcrdma2_propid rdma_which;
opaque rdma_data<>;
};
typedef rpcrdma2_propval rpcrdma2_propset<>;
typedef uint32 rpcrdma2_propsubset<>;
<CODE ENDS>
The rpcrdma2_propid type specifies a distinct transport property.
The property code points are defined as const values rather than
elements in an enum type to enable the extension by concatenating XDR
definition files.
The rpcrdma2_propval type carries the value of a transport property.
The rdma_which field identifies the particular property, and the
rdma_data field contains the associated value of that property. A
zero-length rdma_data field represents the default value of the
property specified by rdma_which.
Although the rdma_data field is opaque, receivers interpret its
contents using the XDR type associated with the property specified by
rdma_which. When the contents of the rdma_data field do not conform
to that XDR type, the receiver MUST return the error
RDMA2_ERR_BAD_PROPVAL using the header type RDMA2_ERROR, as described
in Section 6.3.1.
For example, the receiver of a message containing a valid
rpcrdma2_propval returns this error if the length of rdma_data is
greater than the length of the transferred message. Also, when the
Lever & Noveck Expires 30 December 2022 [Page 27]
Internet-Draft RPC-over-RDMA Version 2 June 2022
receiver recognizes the rpcrdma2_propid contained in rdma_which, it
MUST report the error RDMA2_ERR_BAD_PROPVAL if either of the
following occurs:
* The nominally opaque data within rdma_data is not valid when
interpreted using the property-associated typedef.
* The length of rdma_data is insufficient to contain the data
represented by the property-associated typedef.
A receiver does not report an error if it does not recognize the
value contained in rdma_which. In that case, the receiver does not
process that rpcrdma2_propval. Processing continues with the next
rpcrdma2_propval, if any.
The rpcrdma2_propset type specifies a set of transport properties.
The protocol does not impose a particular ordering of the
rpcrdma2_propval items within it.
The rpcrdma2_propsubset type identifies a subset of the properties in
a rpcrdma2_propset. Each bit in the mask denotes a particular
element in a previously specified rpcrdma2_propset. If a particular
rpcrdma2_propval is at position N in the array, then bit number N mod
32 in word N div 32 specifies whether the defined subset includes
that particular rpcrdma2_propval. Words beyond the last one
specified are assumed to contain zero.
5.2. Current Transport Properties
Table 1 specifies a basic set of transport properties. The columns
contain the following information:
* The column labeled "Property" contains a name of the transport
property described by the current row.
* The column labeled "Code" specifies the code point that identifies
this property.
* The column labeled "XDR type" gives the XDR type of the data used
to communicate the value of this property. This data type
overlays the data portion of the nominally opaque rdma_data field.
* The column labeled "Default" gives the default value for the
property.
* The column labeled "Section" indicates the section within the
current document that explains the use of this property.
Lever & Noveck Expires 30 December 2022 [Page 28]
Internet-Draft RPC-over-RDMA Version 2 June 2022
+===========================+======+==========+=========+=========+
| Property | Code | XDR type | Default | Section |
+===========================+======+==========+=========+=========+
| Maximum Send Size | 1 | uint32 | 4096 | 5.2.1 |
+---------------------------+------+----------+---------+---------+
| Receive Buffer Size | 2 | uint32 | 4096 | 5.2.2 |
+---------------------------+------+----------+---------+---------+
| Maximum Segment Size | 3 | uint32 | 1048576 | 5.2.3 |
+---------------------------+------+----------+---------+---------+
| Maximum Segment Count | 4 | uint32 | 16 | 5.2.4 |
+---------------------------+------+----------+---------+---------+
| Reverse-Direction Support | 5 | uint32 | 0 | 5.2.5 |
+---------------------------+------+----------+---------+---------+
| Host Auth Message | 6 | opaque<> | N/A | 5.2.6 |
+---------------------------+------+----------+---------+---------+
Table 1
5.2.1. Maximum Send Size
The value of this property specifies the maximum size, in octets, of
Send payloads. The endpoint receiving this value can size its
Receive buffers based on the value of this property.
<CODE BEGINS>
const uint32 RDMA2_PROPID_SBSIZ = 1;
typedef uint32 rpcrdma2_prop_sbsiz;
<CODE ENDS>
5.2.2. Receive Buffer Size
The value of this property specifies the minimum size, in octets, of
pre-posted receive buffers.
<CODE BEGINS>
const uint32 RDMA2_PROPID_RBSIZ = 2;
typedef uint32 rpcrdma2_prop_rbsiz;
<CODE ENDS>
A sender can subsequently use this value to determine when a message
to be sent fits in pre-posted receive buffers that the receiver has
set up. In particular:
* Requesters may use the value to determine when to use a Call chunk
or Message Continuation when sending a Call.
Lever & Noveck Expires 30 December 2022 [Page 29]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* Requesters may use the value to determine when to provide a Reply
chunk when sending a Call, based on the maximum possible size of
the Reply.
* Responders may use the value to determine when to use a Reply
chunk provided by the Requester, given the actual size of a Reply.
5.2.3. Maximum Segment Size
The value of this property specifies the maximum size, in octets, of
a segment this endpoint is prepared to send or receive.
<CODE BEGINS>
const uint32 RDMA2_PROPID_RSSIZ = 3;
typedef uint32 rpcrdma2_prop_rssiz;
<CODE ENDS>
5.2.4. Maximum Segment Count
The value of this property specifies the maximum number of segments
that can appear in a Requester's transport header.
<CODE BEGINS>
const uint32 RDMA2_PROPID_RCSIZ = 4;
typedef uint32 rpcrdma2_prop_rcsiz;
<CODE ENDS>
5.2.5. Reverse-Direction Support
The value of this property specifies a client implementation's
readiness to process messages that are part of reverse-direction RPC
requests.
<CODE BEGINS>
const uint32 RDMA_RVRSDIR_NONE = 0;
const uint32 RDMA_RVRSDIR_SIMPLE = 1;
const uint32 RDMA_RVRSDIR_CONT = 2;
const uint32 RDMA_RVRSDIR_GENL = 3;
const uint32 RDMA2_PROPID_BRS = 5;
typedef uint32 rpcrdma2_prop_brs;
<CODE ENDS>
Multiple levels of support are distinguished:
* The value RDMA2_RVRSDIR_NONE indicates that the sender does not
support reverse-direction operation.
Lever & Noveck Expires 30 December 2022 [Page 30]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The value RDMA2_RVRSDIR_SIMPLE indicates that the sender supports
using only Simple Format messages without data item chunks for
reverse-direction messages.
* The value RDMA2_RVRSDIR_CONT indicates that the sender supports
using either Simple Format without data item chunks or Continued
Format messages without data item chunks for reverse-direction
messages.
* The value RDMA2_RVRSDIR_GENL indicates that the sender supports
reverse-direction messages in the same way as forward-direction
messages.
When a peer does not provide this property, the default is the peer
does not support reverse-direction operation.
5.2.6. Host Authentication Message
The value of this transport property enables the exchange of host
authentication material. This property can accommodate
authentication handshakes that require multiple challenge-response
interactions and potentially large amounts of material.
<CODE BEGINS>
const uint32 RDMA2_PROPID_HOSTAUTH = 6;
typedef opaque rpcrdma2_prop_hostauth<>;
<CODE ENDS>
When this property is not present, the peer(s) remain
unauthenticated. Local security policy on each peer determines
whether the connection is permitted to continue.
6. Transport Messages
Each transport message consists of multiple sections.
* A transport header prefix, as defined in Section 6.4. Among other
things, this structure indicates the header type.
* The transport header proper, as defined by one of the sub-sections
below. See Section 6.1 for the mapping between header types and
the corresponding header structure.
* Potentially, all or part of an RPC message payload.
This organization differs from that presented in the definition of
RPC-over-RDMA version 1 [RFC8166], which defined the first and second
of the items above as a single XDR data structure. The new
Lever & Noveck Expires 30 December 2022 [Page 31]
Internet-Draft RPC-over-RDMA Version 2 June 2022
organization is in keeping with RPC-over-RDMA version 2's
extensibility model, which enables the definition of new header types
without modifying the XDR definition of existing header types.
6.1. Transport Header Types
Table 2 lists the RPC-over-RDMA version 2 header types. The columns
contain the following information:
* The column labeled "Operation" names the particular operation.
* The column labeled "Code" specifies the value of the header type
for this operation.
* The column labeled "XDR type" gives the XDR type of the data
structure used to organize the information in this new header
type. This data immediately follows the universal portion on the
transport header present in every RPC-over-RDMA transport header.
* The column labeled "Msg" indicates whether this operation is
followed (or not) by an RPC message payload.
* The column labeled "Section" refers to the section within the
current document that explains the use of this header type.
+==============+======+=============================+=====+=========+
| Operation | Code | XDR type | Msg | Section |
+==============+======+=============================+=====+=========+
| Report | 4 | rpcrdma2_hdr_error | No | 6.3.1 |
| Transport | | | | |
| Error | | | | |
+--------------+------+-----------------------------+-----+---------+
| Grant | 5 | void | No | 6.3.2 |
| Credits | | | | |
+--------------+------+-----------------------------+-----+---------+
| Specify | 6 | rpcrdma2_hdr_connprop | No | 6.3.3 |
| Properties | | | | |
| (Middle) | | | | |
+--------------+------+-----------------------------+-----+---------+
| Specify | 7 | rpcrdma2_hdr_connprop | No | 6.3.4 |
| Properties | | | | |
| (Final) | | | | |
+--------------+------+-----------------------------+-----+---------+
| Convey | 8 | rpcrdma2_hdr_call_external | No | 6.3.5 |
| External | | | | |
| RPC Call | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
Lever & Noveck Expires 30 December 2022 [Page 32]
Internet-Draft RPC-over-RDMA Version 2 June 2022
| Convey | 9 | rpcrdma2_hdr_call_middle | Yes | 6.3.6 |
| Continued | | | | |
| RPC Call | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
| Convey | 10 | rpcrdma2_hdr_call_inline | Yes | 6.3.7 |
| Inline RPC | | | | |
| Call | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
| Convey | 11 | rpcrdma2_hdr_reply_external | No | 6.3.8 |
| External | | | | |
| RPC Reply | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
| Convey | 12 | rpcrdma2_hdr_reply_middle | Yes | 6.3.9 |
| Continued | | | | |
| RPC Reply | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
| Convey | 13 | rpcrdma2_hdr_reply_inline | Yes | 6.3.10 |
| Inline RPC | | | | |
| Reply | | | | |
| Message | | | | |
+--------------+------+-----------------------------+-----+---------+
Table 2
RPC-over-RDMA version 2 peers are REQUIRED to support all message
header types in Table 2. RPC-over-RDMA version 2 implementations
that receive an unrecognized header type MUST respond with an
RDMA2_ERROR message with an rdma_err field containing
RDMA2_ERR_INVAL_HTYPE and drop the incoming message without
processing it further.
6.2. Headers and Chunks
Most RPC-over-RDMA version 2 data structures have antecedents in
corresponding structures in RPC-over-RDMA version 1. As is typical
for new versions of an existing protocol, the XDR data structures
have new names, and there are a few small changes in content. In
some cases, there have been structural re-organizations to enable
protocol extensibility.
Lever & Noveck Expires 30 December 2022 [Page 33]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.2.1. Common Transport Header Prefix
The rpcrdma_common structure defines the initial part of each RPC-
over-RDMA transport header for RPC-over-RDMA version 2 and subsequent
versions.
<CODE BEGINS>
struct rpcrdma_common {
uint32 rdma_xid;
uint32 rdma_vers;
uint32 rdma_credit;
uint32 rdma_htype;
};
<CODE ENDS>
RPC-over-RDMA version 2's use of these first four words aligns with
that of version 1 as required by Section 4.2 of [RFC8166]. However,
there are crucial structural differences in the XDR definition of
RPC-over-RDMA version 2: in the way that these words are described by
the respective XDR descriptions:
* The header type is represented as a uint32 rather than as an enum
type. An enum would need to be modified to reflect additions to
the set of header types made by later extensions.
* The header type field is part of an XDR structure devoted to
representing the transport header prefix, rather than being part
of a discriminated union, that includes the body of each transport
header type.
* There is now a prefix structure (see Section 6.4) of which the
rpcrdma_common structure is the initial segment. This prefix is a
newly defined XDR object within the protocol description, which
constrains the universal portion of all header types to the four
words in rpcrdma_common.
These changes are part of a more considerable structural change in
the XDR definition of RPC-over-RDMA version 2 that facilitates a
cleaner treatment of protocol extension. The XDR appearing in
Section 8 reflects these changes, which Appendix C.1 discusses in
further detail.
6.3. Header Types
The header types defined and used in RPC-over-RDMA version 1 are not
carried over into RPC-over-RDMA version 2, although there are easy
equivalents to the version 1 procedures:
Lever & Noveck Expires 30 December 2022 [Page 34]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The RDMA2_ERROR header (defined in Section 6.3.1) has an XDR
definition that differs from that in RPC-over-RDMA version 1, and
its modifications are all compatible extensions.
* Senders use RDMA2_CALL_INLINE or RDMA2_REPLY_INLINE (defined in
Sections 6.3.7 and 6.3.10) in place of RDMA_MSG. There are minor
differences in the on-the-wire format between the version 1
procedure and the version 2 header types.
* Senders use RDMA2_CALL_EXTERNAL or RDMA2_REPLY_EXTERNAL (defined
in Sections 6.3.5 and 6.3.8) in place of RDMA_NOMSG. There are
minor differences in the on-the-wire format between the version 1
procedure and the version 2 header types.
* RDMA2_CONNPROP_MIDDLE and RDMA2_CONNPROP_FINAL (defined in
Sections 6.3.3 and 6.3.4) are new header types devoted to enabling
connection peers to exchange information about their transport
properties.
6.3.1. RDMA2_ERROR: Report Transport Error
RDMA2_ERROR reports a transport layer error on a previous
transmission.
Lever & Noveck Expires 30 December 2022 [Page 35]
Internet-Draft RPC-over-RDMA Version 2 June 2022
<CODE BEGINS>
const rpcrdma2_proc RDMA2_ERROR = 4;
struct rpcrdma2_err_vers {
uint32 rdma_vers_low;
uint32 rdma_vers_high;
};
struct rpcrdma2_err_write {
uint32 rdma_chunk_index;
uint32 rdma_length_needed;
};
union rpcrdma2_hdr_error switch (rpcrdma2_errcode rdma_err) {
case RDMA2_ERR_VERS:
rpcrdma2_err_vers rdma_vrange;
case RDMA2_ERR_READ_CHUNKS:
uint32 rdma_max_chunks;
case RDMA2_ERR_WRITE_CHUNKS:
uint32 rdma_max_chunks;
case RDMA2_ERR_SEGMENTS:
uint32 rdma_max_segments;
case RDMA2_ERR_WRITE_RESOURCE:
rpcrdma2_err_write rdma_writeres;
case RDMA2_ERR_REPLY_RESOURCE:
uint32 rdma_length_needed;
default:
void;
};
<CODE ENDS>
See Section 7 for details on the use of this header type.
6.3.2. RDMA2_GRANT: Grant Credits
The RDMA2_GRANT header type enables a connection peer to update
credit information without conveying a payload.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_GRANT = 5;
<CODE ENDS>
This message carries no payload except for a struct
rpcrdma2_hdr_prefix. The rdma_xid field is unused. Senders MUST set
the rdma_xid field to zero and receivers MUST ignore the value in
this field.
Lever & Noveck Expires 30 December 2022 [Page 36]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.3.3. RDMA2_CONNPROP_MIDDLE: Exchange Transport Properties
The RDMA2_CONNPROP_MIDDLE header type enables a connection peer to
publish the properties of its implementation to its remote peer.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_CONNPROP_MIDDLE = 6;
struct rpcrdma2_hdr_connprop {
rpcrdma2_propset rdma_props;
};
<CODE ENDS>
A peer sends an RDMA2_CONNPROP_MIDDLE header type when it has one or
more properties to send that do not fit within the default inline
threshold for the RPC-over-RDMA version that is in effect.
A peer may encounter properties that it does not recognize or
support. In such cases, the receiver ignores unsupported properties
without generating an error response.
If a peer sends follows an RDMA2_CONNPROP_MIDDLE header type with
anything other than another RDMA2_CONNPROP_MIDDLE message or an
RDMA2_CONNPROP_FINAL message, the receiver MUST respond with an
RDMA2_ERROR header type and set its rdma_err field to
RDMA2_ERR_INVAL_CONT and drop the incoming message without processing
it further.
6.3.4. RDMA2_CONNPROP_FINAL: Exchange Transport Properties
The RDMA2_CONNPROP_FINAL header type enables a connection peer to
publish the properties of its implementation to its remote peer.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_CONNPROP_FINAL = 7;
struct rpcrdma2_hdr_connprop {
rpcrdma2_propset rdma_props;
};
<CODE ENDS>
Each peer sends an RDMA2_CONNPROP_FINAL header type as the final
CONNPROP-type message after the client has established a connection.
The size of this message is limited to the default inline threshold
for the RPC-over-RDMA version that is in effect.
Lever & Noveck Expires 30 December 2022 [Page 37]
Internet-Draft RPC-over-RDMA Version 2 June 2022
A peer may encounter properties that it does not recognize or
support. In such cases, the receiver ignores unsupported properties
without generating an error response.
If a peer sends a CONNPROP-type message on a connection after it has
sent an RDMA2_CONNPROP_FINAL message, the receiver MUST respond with
an RDMA2_ERROR header type and set its rdma_err field to
RDMA2_ERR_INVAL_CONT and drop the incoming message without processing
it further.
6.3.5. RDMA2_CALL_EXTERNAL: Convey External RPC Call Message
RDMA2_CALL_EXTERNAL conveys an RPC Call message payload using
explicit RDMA operations. The Responder reads the Payload stream
from a memory area specified by the Call chunk. The sender MUST set
the rdma_xid field to the same value as the xid of the RPC Reply
message payload.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_CALL_EXTERNAL = 8;
struct rpcrdma2_hdr_call_external {
uint32 rdma_inv_handle;
struct rpcrdma2_read_list *rdma_call;
struct rpcrdma2_read_list *rdma_reads;
struct rpcrdma2_write_list *rdma_provisional_writes;
struct rpcrdma2_write_chunk *rdma_provisional_reply;
};
<CODE ENDS>
rdma_inv_handle: The rdma_inv_handle field contains a 32-bit RDMA
handle that the Responder may use in a Send With Invalidation
operation. See Section 6.5.
rdma_call: The rdma_call field anchors a list of one or more Read
segments that contain the RPC Call's Payload stream.
rdma_reads: The rdma_reads field anchors a list of zero or more Read
segments that contain data item chunks.
rdma_provisional_writes: The rdma_writes field anchors a list of
zero or more provisional Write chunks.
rdma_provisional_reply: The rdma_reply field is a list containing
zero or one provisional Reply chunk.
Lever & Noveck Expires 30 December 2022 [Page 38]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.3.6. RDMA2_CALL_MIDDLE: Convey Continued RPC Call Message
RDMA2_CALL_MIDDLE conveys a beginning or middle portion of an RPC
Call message immediately following the transport header in the send
buffer. The sender MUST set the rdma_xid field to the same value as
the xid of the RPC Reply message payload. The sender sets the
rdma_remaining field to the number of bytes in the RPC Call message
payload that remain to be sent. The rdma_rpc_first_word field
demarks the first word of the Payload stream.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_CALL_MIDDLE = 9;
struct rpcrdma2_hdr_call_middle {
uint32 rdma_remaining;
/* The rpc message starts here and continues
* through the end of the transmission. */
uint32 rdma_rpc_first_word;
};
<CODE ENDS>
If a peer sends follows an RDMA2_CALL_MIDDLE header type with
anything other than an RDMA2_CALL_MIDDLE message or an
RDMA2_CALL_INLINE message, the receiver MUST respond with an
RDMA2_ERROR header type and set its rdma_err field to
RDMA2_ERR_INVAL_CONT and drop the incoming message without processing
it further.
6.3.7. RDMA2_CALL_INLINE: Convey Inline RPC Call Message
RDMA2_CALL_INLINE conveys the only or final portion of an RPC Call
message. The rdma_rpc_first_word field demarks the first word of
this Payload stream.
Lever & Noveck Expires 30 December 2022 [Page 39]
Internet-Draft RPC-over-RDMA Version 2 June 2022
<CODE BEGINS>
const rpcrdma2_proc RDMA2_CALL_INLINE = 10;
struct rpcrdma2_hdr_call_inline {
uint32 rdma_inv_handle;
struct rpcrdma2_read_list *rdma_reads;
struct rpcrdma2_write_list *rdma_provisional_writes;
struct rpcrdma2_write_chunk *rdma_provisional_reply;
/* The rpc message starts here and continues
* through the end of the transmission. */
uint32 rdma_rpc_first_word;
};
<CODE ENDS>
rdma_inv_handle: The rdma_inv_handle field contains a 32-bit RDMA
handle that the Responder may use in a Send With Invalidation
operation. See Section 6.5.
rdma_reads: The rdma_reads field anchors a list of zero or more Read
segments that contain only data item chunks. A Requester MUST NOT
insert Position-zero Read chunks in this list.
rdma_provisional_writes: The rdma_writes field anchors a list of
zero or more provisional Write chunks.
rdma_provisional_reply: The rdma_reply field is a list containing
zero or one provisional Reply chunk.
6.3.8. RDMA2_REPLY_EXTERNAL: Convey External RPC Reply Message
RDMA2_REPLY_EXTERNAL conveys an RPC Reply message payload using
explicit RDMA operations. In particular, it is referred to as a
Special Format Reply when the Responder writes the RPC payload into a
memory area specified by a Reply chunk. The sender MUST set the
rdma_xid field to the same value as the xid of the RPC Reply message
payload.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_REPLY_EXTERNAL = 11;
struct rpcrdma2_hdr_reply_external {
struct rpcrdma2_write_list *rdma_writes;
struct rpcrdma2_write_chunk *rdma_reply;
};
<CODE ENDS>
Lever & Noveck Expires 30 December 2022 [Page 40]
Internet-Draft RPC-over-RDMA Version 2 June 2022
rdma_writes: The rdma_writes field anchors a list of zero or more
Write chunks that are either empty or contain reduced data items.
rdma_reply: The rdma_reply field is a list that MUST contain exactly
one Reply chunk.
6.3.9. RDMA2_REPLY_MIDDLE: Convey Continued RPC Reply Message
RDMA2_REPLY_MIDDLE conveys a beginning or middle portion of an RPC
Reply message immediately following the transport header in the send
buffer. The sender MUST set the rdma_xid field to the same value as
the xid of the RPC Reply message payload. The sender sets the
rdma_remaining field to the number of bytes in the RPC Call message
payload that remain to be sent. The rdma_rpc_first_word field
demarks the first word of the Payload stream.
<CODE BEGINS>
const rpcrdma2_proc RDMA2_REPLY_MIDDLE = 12;
struct rpcrdma2_hdr_reply_middle {
uint32 rdma_remaining;
/* The rpc message starts here and continues
* through the end of the transmission. */
uint32 rdma_rpc_first_word;
};
<CODE ENDS>
If a peer sends follows an RDMA2_REPLY_MIDDLE header type with
anything other than an RDMA2_REPLY_MIDDLE message or an
RDMA2_REPLY_INLINE message, the receiver MUST respond with an
RDMA2_ERROR header type and set its rdma_err field to
RDMA2_ERR_INVAL_CONT and drop the incoming message without processing
it further.
6.3.10. RDMA2_REPLY_INLINE: Convey RPC Reply Message Inline
RDMA2_REPLY_INLINE conveys the only or final portion of an RPC Reply
message immediately following the transport header in the send
buffer. If the Reply message payload has been reduced, the
rdma_chunks object carries the reduced data item chunks.
Lever & Noveck Expires 30 December 2022 [Page 41]
Internet-Draft RPC-over-RDMA Version 2 June 2022
<CODE BEGINS>
const rpcrdma2_proc RDMA2_REPLY_INLINE = 13;
struct rpcrdma2_hdr_reply_inline {
struct rpcrdma2_write_list *rdma_writes;
/* The rpc message starts here and continues
* through the end of the transmission. */
uint32 rdma_rpc_first_word;
};
<CODE ENDS>
rdma_writes: The rdma_writes field anchors a list of zero or more
Write chunks that are either empty or contain reduced data items.
6.4. Transport Header Prefix
The following prefix structure appears at the start of each RPC-over-
RDMA version 2 transport header.
<CODE BEGINS>
struct rpcrdma2_hdr_prefix {
struct rpcrdma_common rdma_start;
};
<CODE ENDS>
6.5. Remote Invalidation
To solicit the use of Remote Invalidation, a Requester sets the value
of the rdma_inv_handle field in an RPC Call's transport header to a
non-zero value that matches one of the rdma_handle fields in that
header. If the Responder may invalidate none of the rdma_handle
values in the header conveying the Call, the Requester sets the RPC
Call's rdma_inv_handle field to the value zero.
If the Responder chooses not to use remote invalidation for this
particular RPC Reply, or the RPC Call's rdma_inv_handle field
contains the value zero, the Responder simply uses RDMA Send to
transmit the matching RPC reply. However, if the Responder chooses
to use Remote Invalidation, it uses RDMA Send With Invalidate to
transmit the RPC Reply. It MUST use the value in the corresponding
Call's rdma_inv_handle field to construct the Send With Invalidate
Work Request.
A Responder never uses a Send With Invalidate Work Request when
sending a control plane header type. This includes the RDMA2_ERROR
header type, the RDMA2_GRANT header type, the RDMA2_CONNPROP_MIDDLE
header type, and the RDMA2_CONNPROP_FINAL header type.
Lever & Noveck Expires 30 December 2022 [Page 42]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.6. Payload Formats
RPC-over-RDMA version 2 provides several ways, known as "payload
formats", to convey an RPC-over-RDMA message. A sender chooses the
payload format for each message based on several factors:
* The existence of DDP-eligible data items in the RPC message
payload
* The size of the RPC message payload
* The direction of the RPC message (i.e., Call or Reply)
* The available hardware resources
* The arrangement of source and sink memory buffers
The following subsections describe in detail how Requesters and
Responders format RPC-over-RDMA message payloads.
6.6.1. Simple Format
All RPC messages conveyed via RPC-over-RDMA version 2 need at least
one RDMA Send operation to convey. Thus, the most efficient way to
send an RPC message that is smaller than the inline threshold is to
append the Payload stream directly to the Transport stream and use an
RDMA Send to convey both. When no chunks are present, senders
construct Calls and Replies the same way, and no other operations are
needed.
6.6.1.1. Simple Format with Data Item Chunks
If DDP-eligible data items are present in a Payload stream, a sender
MAY reduce some or all of these items, removing them from the Payload
stream. The sender then uses a separate mechanism to transfer the
reduced data items. The Transport stream immediately followed by the
reduced Payload stream is then transferred using one RDMA Send
operation.
When data item chunks are present, senders construct Calls
differently than Replies.
Simple Call
After receiving the Transport and Payload streams of an RPC Call
message with Read chunks, the Responder uses RDMA Read operations
to move the reduced data items contained in the Read chunks. RPC-
over-RDMA Calls can carry Write chunks for the Responder to use
when sending the matching Reply.
Lever & Noveck Expires 30 December 2022 [Page 43]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Simple Reply
The Responder uses RDMA Write operations to move reduced data
items contained in Write chunks. Afterward, it sends the
Transport and Payload streams of the RPC Reply message using one
RDMA Send. RPC-over-RDMA Replies always carry an empty Read chunk
list.
6.6.1.2. Simple Format Examples
Requester Responder
| RDMA Send (RDMA2_CALL_INLINE) |
Call | ----------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- | Reply
Figure 1: A Simple Call without data item chunks and a Simple
Reply without data item chunks
Requester Responder
| RDMA Send (RDMA2_CALL_INLINE) |
Call | ----------------------------------> |
| RDMA Read |
| <---------------------------------- |
| RDMA Response (arg data) |
| ----------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- | Reply
Figure 2: A Simple Call with a Read chunk and a Simple Reply
without data item chunks
Requester Responder
| RDMA Send (RDMA2_CALL_INLINE) |
Call | ----------------------------------> |
| |
| | Processing
| |
| RDMA Write (result data) |
| <---------------------------------- |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- | Reply
Lever & Noveck Expires 30 December 2022 [Page 44]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Figure 3: A Simple Call without data item chunks and a Simple
Reply with a Write chunk
6.6.2. Continued Format
For various reasons, a sender can choose to split a message payload
over multiple RPC-over-RDMA messages. The Payload stream of each
RPC-over-RDMA message contains a part of the RPC message. The
receiver reconstructs the original RPC message by concatenating the
Payload stream of each RPC-over-RDMA message in received order. A
sender MAY split the Payload stream on any convenient boundary.
6.6.2.1. Continued Format with Data Item Chunks
If DDP-eligible data items are present in the Payload stream, a
sender MAY reduce some or all of these items, removing them from the
Payload stream. The sender then uses a separate mechanism to
transfer the reduced data items. The Transport stream immediately
follwed by the reduced Payload stream is then transferred using one
RDMA Send operation.
As with Simple Format messages, when chunks are present, senders
construct Calls differently than Replies.
Continued Call
After receiving the Transport and Payload streams of an RPC Call
message with Read chunks, the Responder uses RDMA Read operations
to move the reduced data items contained in Read chunks. RPC-
over-RDMA Calls can carry Write chunks for the Responder to use
when sending the matching Reply.
Continued Reply
The Responder uses RDMA Write operations to move reduced data
items contained in Write chunks. Afterward, it sends the
Transport and Payload streams of the RPC Reply message using
multiple RDMA Sends. RPC-over-RDMA Replies always carry an empty
Read chunk list.
6.6.2.2. Continued Format Examples
Lever & Noveck Expires 30 December 2022 [Page 45]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Requester Responder
| RDMA Send (RDMA2_CALL_MIDDLE) |
Call | ----------------------------------> |
| RDMA Send (RDMA2_CALL_MIDDLE) |
| ----------------------------------> |
| RDMA Send (RDMA2_CALL_INLINE) |
| ----------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA2_REPLY_MIDDLE) |
| <---------------------------------- | Reply
| RDMA Send (RDMA2_REPLY_MIDDLE) |
| <---------------------------------- |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- |
Figure 4: A Continued Call without data item chunks and a
Continued Reply without data item chunks
Requester Responder
| RDMA Send (RDMA2_CALL_MIDDLE) |
Call | ----------------------------------> |
| RDMA Send (RDMA2_CALL_MIDDLE) |
| ----------------------------------> |
| RDMA Send (RDMA2_CALL_INLINE) |
| ----------------------------------> |
| RDMA Read |
| <---------------------------------- |
| RDMA Response (arg data) |
| ----------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- | Reply
Figure 5: A Continued Call with a Read chunk and a Simple Reply
without data item chunks
Lever & Noveck Expires 30 December 2022 [Page 46]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Requester Responder
| RDMA Send (RDMA2_CALL_INLINE) |
Call | ----------------------------------> |
| |
| | Processing
| |
| RDMA Write (result data) |
| <---------------------------------- |
| RDMA Send (RDMA2_REPLY_MIDDLE) |
| <---------------------------------- | Reply
| RDMA Send (RDMA2_REPLY_MIDDLE) |
| <---------------------------------- |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- |
Figure 6: A Simple Call without data item chunks and a Continued
Reply with a Write chunk
6.6.3. Special Format
Even after DDP-eligible data items have been removed, a Payload
stream can sometimes be too large to send using only RDMA Send
operations. In those cases, the sender can use RDMA Read or Write
operations to convey the entire RPC message. We refer to this as a
"Special Format" message.
To transmit a Special Format message, the sender transmits only the
Transport stream with an RDMA Send operation. The sender does not
include the Payload stream in the send buffer. Instead, the
Requester provides a body chunk that the Responder uses to move the
Payload stream.
Because chunks are always present in Special Format messages, the
sender always handles Calls and Replies differently.
Special Call
The Requester provides a Read chunk that contains the RPC Call
message's Payload stream. Every Read segment in this chunk MUST
contain zero (0) in its Position field. This type of Read chunk
is a body chunk known as a Call chunk.
Special Reply
The Requester provisions a Reply chunk in advance. This body
chunk is a Write chunk into which the Responder places the RPC
Reply message's Payload stream. The Requester provisions the
Reply chunk to accommodate the maximum expected reply size for
that upper-layer operation.
Lever & Noveck Expires 30 December 2022 [Page 47]
Internet-Draft RPC-over-RDMA Version 2 June 2022
One purpose of a Special Format message is to handle large RPC
messages. However, Requesters MAY use a Special Format message at
any time to convey an RPC Call message.
When it has alternatives, a Responder chooses which Format to use
based on the chunks provided by the Requester. If a Requester
provided a Write chunk and the Responder has a DDP-eligible result,
it first reduces the reply Payload stream. If a Requester provided a
Reply chunk and the reduced Payload stream is larger than the reply
inline threshold, the Responder MUST use the Requester-provided Reply
chunk for the reply.
6.6.3.1. Special Format Examples
Requester Responder
| RDMA Send (RDMA2_CALL_EXTERNAL) |
Call | ----------------------------------> |
| RDMA Read |
| <---------------------------------- |
| RDMA Response (RPC call) |
| ----------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA2_REPLY_INLINE) |
| <---------------------------------- | Reply
Figure 7: A Special Call and a Simple Reply without data item chunks
Requester Responder
| RDMA Send (RDMA2_CALL_INLINE) |
Call | ----------------------------------> |
| |
| | Processing
| |
| RDMA Write (RPC reply) |
| <---------------------------------- |
| RDMA Send (RDMA2_REPLY_EXTERNAL) |
| <---------------------------------- | Reply
Figure 8: A Simple Call without data item chunks and a Special Reply
Lever & Noveck Expires 30 December 2022 [Page 48]
Internet-Draft RPC-over-RDMA Version 2 June 2022
6.6.4. Choosing a Reply Payload Format
A Requester provisions all necessary registered memory resources for
both an RPC Call and its matching RPC Reply. A Requester constructs
each RPC Call, thus it can compute the exact memory resources needed
to send every Call. However, the Requester allocates memory
resources to receive the corresponding Reply before the Responder has
constructed it. Occasionally, it is challenging for the Requester to
know in advance precisely what resources are needed to receive the
Reply.
In RPC-over-RDMA version 2, a Requester can provide a Reply chunk for
any transaction. The Responder can use the provided Reply chunk or
it can decide to use another means to convey the RPC Reply. If the
combination of the provided Write chunk list and Reply chunk is not
adequate to convey a Reply, the Responder SHOULD use Message
Continuation to send that Reply. If even that is not possible, the
Responder sends an RDMA2_ERROR message to the Requester, as described
in Section 6.3.1:
* If the Write chunk list cannot accommodate the ULP's DDP-eligible
data payload, the Responder sends an RDMA2_ERR_WRITE_RESOURCE
error.
* If the Reply chunk cannot accommodate the parts of the Reply that
are not DDP-eligible, the Responder sends an
RDMA2_ERR_REPLY_RESOURCE error.
When receiving such errors, the Requester can retry the ULP call
using more substantial reply resources. In cases where retrying the
ULP request is not possible (e.g., the request is non-idempotent),
the Requester terminates the RPC transaction and presents an error to
the RPC consumer.
7. Error Handling
A receiver performs validity checks on each ingress RPC-over-RDMA
message before it assembles that message's Payload stream and passes
it to the RPC layer. For example, if an ingress RPC-over-RDMA
message is not as long as the size of struct rpcrdma2_hdr_prefix (20
octets), the receiver cannot trust the value of the rdma_xid field.
In this case, the receiver MUST silently discard the ingress message
without processing it further, and without a response to the sender.
When a request (for instance, an RPC Call or a control plane
operation) is made, typically an RPC consumer blocks while waiting
for the response. Thus when an incoming message conveys a request
and that request cannot be acted upon, the receiver of that request
Lever & Noveck Expires 30 December 2022 [Page 49]
Internet-Draft RPC-over-RDMA Version 2 June 2022
needs to report the problem to its sender in order to unblock
waiters. Likewise, if, after processing a request, a sender is
unable to transmit the response on an otherwise healthy connection,
the sender needs to report that problem for the same reason.
The RDMA2_ERROR header type is used for this purpose. To form an
RDMA2_ERROR type header:
* The rdma_xid field MUST contain the same XID that was in the
rdma_xid field in the ingress request.
* The rdma_vers field MUST contain the same version that was in the
rdma_vers field in the ingress request.
* The sender sets the rdma_credit field to the credit values in
effect for this connection.
* The rdma_htype field MUST contain the value RDMA2_ERROR.
* The rdma_err field contains a value that reflects the type of
error that occurred, as described in the subsections below.
When a peer receives an RDMA2_ERROR message type with an unrecognized
or unsupported value in its rdma_err field, it MUST silently discard
the message without processing it further.
7.1. Basic Transport Stream Parsing Errors
7.1.1. RDMA2_ERR_VERS
When a Responder detects an RPC-over-RDMA header version that it does
not support (the current document defines version 2), it MUST respond
with an RDMA2_ERROR message type and set its rdma_err field to
RDMA2_ERR_VERS. The Responder then fills in the rpcrdma2_err_vers
structure with the RPC-over-RDMA versions it supports. The Responder
MUST silently discard the ingress message without passing it to the
RPC layer.
When a Requester receives this error message, it uses the information
in the rpcrdma2_err_vers structure to select an RPC-over-RDMA version
that both peers support for subsequent operations on the connection.
A Requester MUST NOT subsequently send a message that uses a version
that the Responder has indicated it does not support. RDMA2_ERR_VERS
indicates a permanent error. Receipt of this error completes the RPC
transaction associated with XID in the rdma_xid field.
Lever & Noveck Expires 30 December 2022 [Page 50]
Internet-Draft RPC-over-RDMA Version 2 June 2022
7.1.2. RDMA2_ERR_VERS_MISMATCH
When a Responder receives a message with a transport protocol version
that does not match the protocol version that was used in previous
successful exchanges on the same connection, it MUST respond with an
RDMA2_ERROR message type and set its rdma_err field to
RDMA2_ERR_VERS_MISMATCH. The Responder MUST silently discard the
ingress message without passing it to the RPC layer.
A Requester MUST NOT subsequently send a message that uses a protocol
version that the Responder has indicated it does not recognize on
this connection. The Requester can recover by sending the message
again using a corrected protocol version, or it can terminate the RPC
transaction associated with the XID in the rdma_xid field with an
error.
7.1.3. RDMA2_ERR_INVAL_HTYPE
If a Responder recognizes the value in an ingress rdma_vers field,
but it does not recognize the value in the rdma_htype field or does
not support that header type, it MUST set the rdma_err field to
RDMA2_ERR_INVAL_HTYPE. The Responder MUST silently discard the
incoming message without passing it to the RPC layer.
A Requester MUST NOT subsequently send a message on the connection
that uses an htype that the Responder has indicated it does not
support. RDMA2_ERR_INVAL_HTYPE indicates a permanent error. Receipt
of this error completes the RPC transaction associated with XID in
the rdma_xid field.
7.1.4. RDMA2_ERR_INVAL_CONT
If a Responder detects a problem with an ingress RPC-over-RDMA
message that is part of a Message Continuation sequence, the
Responder MUST set the rdma_err field to RDMA2_ERR_INVAL_CONT. The
Responder MUST silently discard all ingress messages with an rdma_xid
field that matches the failing message without reassembling the
payload.
RDMA2_ERR_INVAL_CONT indicates a permanent error. Receipt of this
error completes the RPC transaction associated with XID in the
rdma_xid field.
7.2. XDR Errors
A receiver might encounter an XDR parsing error that prevents it from
processing an ingress Transport stream. Examples of such errors
include:
Lever & Noveck Expires 30 December 2022 [Page 51]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The value of the rdma_xid field does not match the value of the
XID field in the accompanying RPC message.
* The receive buffer ends before the end of a data object contained
in the Transport stream.
Moreover, when a Responder receives a valid RPC-over-RDMA header but
the Responder's ULP implementation cannot parse the RPC arguments in
the RPC Call, the Responder returns an RPC Reply with status
GARBAGE_ARGS, using an RDMA2_REPLY_INLINE message type. This type of
parsing failure might be due to mismatches between chunk sizes or
offsets and the contents of the Payload stream, for example. In this
case, the error is permanent, but the Requester has no way to know
how much processing the Responder has completed for this RPC
transaction.
7.2.1. RDMA2_ERR_BAD_XDR
If a Responder recognizes the values in the rdma_vers field, but it
cannot otherwise parse the ingress Transport stream, it MUST set the
rdma_err field to RDMA2_ERR_BAD_XDR. The Responder MUST silently
discard the ingress message without passing it to the RPC layer.
RDMA2_ERR_BAD_XDR indicates a permanent error. Receipt of this error
completes the RPC transaction associated with XID in the rdma_xid
field.
7.2.2. RDMA2_ERR_BAD_PROPVAL
If a receiver recognizes the value in an ingress rdma_which field,
but it cannot parse the accompanying propval, it MUST set the
rdma_err field to RDMA2_ERR_BAD_PROPVAL (see Section 5.1). The
receiver MUST silently discard the ingress message without applying
any of its property settings.
7.3. Responder RDMA Operational Errors
In RPC-over-RDMA version 2, the Responder initiates RDMA Read and
Write operations that target the Requester's memory. Problems might
arise as the Responder attempts to use Requester-provided resources
for RDMA operations. For example:
* Usually, chunks can be validated only by using their contents to
perform data transfers. If chunk contents are invalid (e.g., a
memory region is no longer registered or a chunk length exceeds
the end of the registered memory region), a Remote Access Error
occurs.
Lever & Noveck Expires 30 December 2022 [Page 52]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* If a Requester's Receive buffer is too small, the Responder's Send
operation completes with a Local Length Error.
* If the Requester-provided Reply chunk is too small to accommodate
a large RPC Reply message, a Remote Access Error occurs. A
Responder might detect this problem before attempting to write
past the end of the Reply chunk.
RDMA operational errors can be fatal to the connection. To avoid a
retransmission loop and repeated connection loss that deadlocks the
connection, once the Requester has re-established a connection, the
Responder SHOULD send an RDMA2_ERROR response to indicate that no
RPC-level reply is possible for that transaction.
7.3.1. RDMA2_ERR_READ_CHUNKS
If a Requester presents more DDP-eligible arguments than a Responder
is prepared to Read, the Responder MUST set the rdma_err field to
RDMA2_ERR_READ_CHUNKS and set the rdma_max_chunks field to the
maximum number of Read chunks the Responder can process. If the
Responder implementation cannot handle any Read chunks for a request,
it MUST set the rdma_max_chunks to zero in this response. The
Responder MUST silently discard the ingress message without
processing it further.
The Requester can reconstruct the Call using Message Continuation or
a Special Format payload and resend it. If the Requester chooses not
to resend the Call, it MUST terminate this RPC transaction with an
error.
7.3.2. RDMA2_ERR_WRITE_CHUNKS
If a Requester has constructed an RPC Call with more DDP-eligible
results than the Responder is prepared to Write, the Responder MUST
set the rdma_err field to RDMA2_ERR_WRITE_CHUNKS and set the
rdma_max_chunks field to the maximum number of Write chunks the
Responder can return. The Requester can reconstruct the Call with no
Write chunks and a Reply chunk of appropriate size. If the Requester
does not resend the Call, it MUST terminate this RPC transaction with
an error.
If the Responder implementation cannot handle any Write chunks for a
request and cannot send the Reply using Message Continuation, it MUST
return a response of RDMA2_ERR_REPLY_RESOURCE instead (see below).
Lever & Noveck Expires 30 December 2022 [Page 53]
Internet-Draft RPC-over-RDMA Version 2 June 2022
7.3.3. RDMA2_ERR_SEGMENTS
If a Requester has constructed an RPC Call with a chunk that contains
more segments than the Responder supports, the Responder MUST set the
rdma_err field to RDMA2_ERR_SEGMENTS and set the rdma_max_segments
field to the maximum number of segments the Responder can process.
The Requester can reconstruct the Call and resend it. If the
Requester does not resend the Call, it MUST terminate this RPC
transaction with an error.
7.3.4. RDMA2_ERR_WRITE_RESOURCE
If a Requester has provided a Write chunk that is not large enough to
contain a DDP-eligible result, the Responder MUST set the rdma_err
field to RDMA2_ERR_WRITE_RESOURCE. The Responder MUST set the
rdma_chunk_index field to point to the first Write chunk in the
transport header that is too short, or to zero to indicate that it
was not possible to determine which chunk is too small. Indexing
starts at one (1), which represents the first Write chunk. The
Responder MUST set the rdma_length_needed to the number of bytes
needed in that chunk to convey the result data item.
The Requester can reconstruct the Call with more reply resources and
resend it. If the Requester does not resend the Call (for instance,
if the Responder set the index and length fields to zero), it MUST
terminate this RPC transaction with an error.
7.3.5. RDMA2_ERR_REPLY_RESOURCE
If a Responder cannot send an RPC Reply using Message Continuation
and the Reply does not fit in the Reply chunk, the Responder MUST set
the rdma_err field to RDMA2_ERR_REPLY_RESOURCE. The Responder MUST
set the rdma_length_needed to the number of Reply chunk bytes needed
to convey the reply. The Requester can reconstruct the Call with
more reply resources and resend it. If the Requester does not resend
the Call (for instance, if the Responder set the length field to
zero), it MUST terminate this RPC transaction with an error.
7.4. Other Operational Errors
While a Requester is constructing an RPC Call message, an
unrecoverable problem might occur that prevents the Requester from
posting further RDMA Work Requests on behalf of that message. As
with other transports, if a Requester is unable to construct and
transmit an RPC Call, the associated RPC transaction fails
immediately.
Lever & Noveck Expires 30 December 2022 [Page 54]
Internet-Draft RPC-over-RDMA Version 2 June 2022
After a Requester has received a Reply, if it is unable to invalidate
a memory region due to an unrecoverable problem, the Requester MUST
close the connection to protect that memory from Responder access
before the associated RPC transaction is complete.
While a Responder is constructing an RPC Reply message or error
message, an unrecoverable problem might occur that prevents the
Responder from posting further RDMA Work Requests on behalf of that
message. If a Responder is unable to construct and transmit an RPC
Reply or RPC-over-RDMA error message, the Responder MUST close the
connection to signal to the Requester that a reply was lost.
7.4.1. RDMA2_ERR_SYSTEM
If some problem occurs on a Responder that does not fit into the
above categories, the Responder MAY report it to the Requester by
setting the rdma_err field to RDMA2_ERR_SYSTEM. The Responder MUST
silently discard the message(s) associated with the failing
transaction without further processing.
RDMA2_ERR_SYSTEM is a permanent error. This error does not indicate
how much of the transaction the Responder has processed, nor does it
indicate a particular recovery action for the Requester. A Requester
that receives this error MUST terminate the RPC transaction
associated with the XID value in the RDMA2_ERROR message's rdma_xid
field.
7.5. RDMA Transport Errors
The RDMA connection and physical link provide some degree of error
detection and retransmission. The Marker PDU Aligned Framing (MPA)
protocol (as described in Section 7.1 of [RFC5044]) as well as the
InfiniBand link layer [IBA] provide Cyclic Redundancy Check (CRC)
protection of RDMA payloads. CRC-class protection is a general
attribute of such transports.
Additionally, the RPC layer itself can accept errors from the
transport and recover via retransmission. RPC recovery can typically
handle complete loss and re-establishment of a transport connection.
The details of reporting and recovery from RDMA link-layer errors are
described in specific link-layer APIs and operational specifications
and are outside the scope of this protocol specification. See
Section 11 for further discussion of RPC-level integrity schemes.
Lever & Noveck Expires 30 December 2022 [Page 55]
Internet-Draft RPC-over-RDMA Version 2 June 2022
8. XDR Protocol Definition
This section contains a description of the core features of the RPC-
over-RDMA version 2 protocol expressed in the XDR language [RFC4506].
It organizes the description to make it simple to extract into a form
that is ready to compile or combine with similar descriptions
published later as extensions to RPC-over-RDMA version 2.
8.1. Code Component License
Code Components extracted from the current document must include the
following license text. When combining the extracted XDR code with
other XDR code which has an identical license, only a single copy of
the license text needs to be retained.
Lever & Noveck Expires 30 December 2022 [Page 56]
Internet-Draft RPC-over-RDMA Version 2 June 2022
<CODE BEGINS>
/// /*
/// * Copyright (c) 2010, 2020 IETF Trust and the persons
/// * identified as authors of the code. All rights reserved.
/// *
/// * The authors of the code are:
/// * B. Callaghan, T. Talpey, C. Lever, and D. Noveck.
/// *
/// * Redistribution and use in source and binary forms, with
/// * or without modification, are permitted provided that the
/// * following conditions are met:
/// *
/// * - Redistributions of source code must retain the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer.
/// *
/// * - Redistributions in binary form must reproduce the above
/// * copyright notice, this list of conditions and the
/// * following disclaimer in the documentation and/or other
/// * materials provided with the distribution.
/// *
/// * - Neither the name of Internet Society, IETF or IETF
/// * Trust, nor the names of specific contributors, may be
/// * used to endorse or promote products derived from this
/// * software without specific prior written permission.
/// *
/// * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
/// * AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
/// * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
/// * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
/// * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
/// * EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
/// * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
/// * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
/// * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
/// * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
/// * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
/// * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
/// * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
/// * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
/// * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
/// */
///
<CODE ENDS>
Lever & Noveck Expires 30 December 2022 [Page 57]
Internet-Draft RPC-over-RDMA Version 2 June 2022
8.2. Extraction of the XDR Definition
Implementers can apply the following sed script to the current
document to produce a machine-readable XDR description of the base
RPC-over-RDMA version 2 protocol.
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p'
<CODE ENDS>
That is, if this document is in a file called "spec.txt", then
implementers can do the following to extract an XDR description file
and store it in the file rpcrdma-v2.x.
<CODE BEGINS>
sed -n -e 's:^ */// ::p' -e 's:^ *///$::p' \
< spec.txt > rpcrdma-v2.x
<CODE ENDS>
Although this file is a usable description of the base protocol, when
extensions are to be supported, it may be desirable to divide the
description into multiple files. The following script achieves that
purpose:
<CODE BEGINS>
#!/usr/local/bin/perl
open(IN,"rpcrdma-v2.x");
open(OUT,">temp.x");
while(<IN>)
{
if (m/FILE ENDS: (.*)$/)
{
close(OUT);
rename("temp.x", $1);
open(OUT,">temp.x");
}
else
{
print OUT $_;
}
}
close(IN);
close(OUT);
<CODE ENDS>
Running the above script results in two files:
Lever & Noveck Expires 30 December 2022 [Page 58]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The file common.x, containing the license plus the shared XDR
definitions that need to be made available to both the base
protocol and any subsequent extensions.
* The file baseops.x containing the XDR definitions for the base
protocol defined in this document.
Extensions to RPC-over-RDMA version 2, published as Standards Track
documents, should have similarly structured XDR definitions. Once an
implementer has extracted the XDR for all desired extensions and the
base XDR definition contained in the current document, she can
concatenate them to produce a consolidated XDR definition that
reflects the set of extensions selected for her RPC-over-RDMA version
2 implementation.
Alternatively, the XDR descriptions can be compiled separately. In
that case, the combination of common.x and baseops.x defines the base
transport. The combination of common.x and the XDR description of
each extension produces a full XDR definition of that extension.
8.3. XDR Definition for RPC-over-RDMA Version 2 Core Structures
<CODE BEGINS>
/// /***************************************************************
/// * Transport Header Prefixes
/// ***************************************************************/
///
/// struct rpcrdma_common {
/// uint32 rdma_xid;
/// uint32 rdma_vers;
/// uint32 rdma_credit;
/// uint32 rdma_htype;
/// };
///
/// struct rpcrdma2_hdr_prefix {
/// struct rpcrdma_common rdma_start;
/// };
///
/// /***************************************************************
/// * Chunks and Chunk Lists
/// ***************************************************************/
///
/// struct rpcrdma2_segment {
/// uint32 rdma_handle;
/// uint32 rdma_length;
/// uint64 rdma_offset;
/// };
///
Lever & Noveck Expires 30 December 2022 [Page 59]
Internet-Draft RPC-over-RDMA Version 2 June 2022
/// struct rpcrdma2_read_segment {
/// uint32 rdma_position;
/// struct rpcrdma2_segment rdma_target;
/// };
///
/// struct rpcrdma2_read_list {
/// struct rpcrdma2_read_segment rdma_entry;
/// struct rpcrdma2_read_list *rdma_next;
/// };
///
/// struct rpcrdma2_write_chunk {
/// struct rpcrdma2_segment rdma_target<>;
/// };
///
/// struct rpcrdma2_write_list {
/// struct rpcrdma2_write_chunk rdma_entry;
/// struct rpcrdma2_write_list *rdma_next;
/// };
///
/// /***************************************************************
/// * Transport Properties
/// ***************************************************************/
///
/// /*
/// * Types for transport properties model
/// */
/// typedef rpcrdma2_propid uint32;
///
/// struct rpcrdma2_propval {
/// rpcrdma2_propid rdma_which;
/// opaque rdma_data<>;
/// };
///
/// typedef rpcrdma2_propval rpcrdma2_propset<>;
/// typedef uint32 rpcrdma2_propsubset<>;
///
/// /*
/// * Transport propid values for basic properties
/// */
/// const RDMA2_PROPID_SBSIZ = 1;
/// const RDMA2_PROPID_RBSIZ = 2;
/// const RDMA2_PROPID_RSSIZ = 3;
/// const RDMA2_PROPID_RCSIZ = 4;
/// const RDMA2_PROPID_BRS = 5;
/// const RDMA2_PROPID_HOSTAUTH = 6;
///
/// /*
/// * Types specific to particular properties
Lever & Noveck Expires 30 December 2022 [Page 60]
Internet-Draft RPC-over-RDMA Version 2 June 2022
/// */
/// typedef uint32 rpcrdma2_prop_sbsiz;
/// typedef uint32 rpcrdma2_prop_rbsiz;
/// typedef uint32 rpcrdma2_prop_rssiz;
/// typedef uint32 rpcrdma2_prop_rcsiz;
/// typedef uint32 rpcrdma2_prop_brs;
/// typedef opaque rpcrdma2_prop_hostauth<>;
///
/// const RDMA2_RVRSDIR_NONE = 0;
/// const RDMA2_RVRSDIR_SIMPLE = 1;
/// const RDMA2_RVRSDIR_CONT = 2;
/// const RDMA2_RVRSDIR_GENL = 3;
///
/// /* FILE ENDS: common.x; */
<CODE ENDS>
8.4. XDR Definition for RPC-over-RDMA Version 2 Base Header Types
<CODE BEGINS>
/// /***************************************************************
/// * Descriptions of RPC-over-RDMA Header Types
/// ***************************************************************/
///
/// /*
/// * Header Type Codes: Control plane operations.
/// */
/// const RDMA2_ERROR = 4;
/// const RDMA2_GRANT = 5;
/// const RDMA2_CONNPROP_MIDDLE = 6;
/// const RDMA2_CONNPROP_FINAL = 7;
///
/// /*
/// * Header Type Codes: Call messages.
/// */
/// const RDMA2_CALL_EXTERNAL = 8;
/// const RDMA2_CALL_MIDDLE = 9;
/// const RDMA2_CALL_INLINE = 10;
///
/// /*
/// * Header Type Codes: Reply messages.
/// */
/// const RDMA2_REPLY_EXTERNAL = 11;
/// const RDMA2_REPLY_MIDDLE = 12;
/// const RDMA2_REPLY_INLINE = 13;
///
/// /*
/// * Header Type to Report Errors.
/// */
Lever & Noveck Expires 30 December 2022 [Page 61]
Internet-Draft RPC-over-RDMA Version 2 June 2022
/// const RDMA2_ERR_VERS = 1;
/// const RDMA2_ERR_BAD_XDR = 2;
/// const RDMA2_ERR_BAD_PROPVAL = 3;
/// const RDMA2_ERR_INVAL_HTYPE = 4;
/// const RDMA2_ERR_INVAL_CONT = 5;
/// const RDMA2_ERR_READ_CHUNKS = 6;
/// const RDMA2_ERR_WRITE_CHUNKS = 7;
/// const RDMA2_ERR_SEGMENTS = 8;
/// const RDMA2_ERR_WRITE_RESOURCE = 9;
/// const RDMA2_ERR_REPLY_RESOURCE = 10;
/// const RDMA2_ERR_VERS_MISMATCH = 11;
/// const RDMA2_ERR_SYSTEM = 100;
///
/// struct rpcrdma2_err_vers {
/// uint32 rdma_vers_low;
/// uint32 rdma_vers_high;
/// };
///
/// struct rpcrdma2_err_write {
/// uint32 rdma_chunk_index;
/// uint32 rdma_length_needed;
/// };
///
/// union rpcrdma2_hdr_error switch (rpcrdma2_errcode rdma_err) {
/// case RDMA2_ERR_VERS:
/// rpcrdma2_err_vers rdma_vrange;
/// case RDMA2_ERR_READ_CHUNKS:
/// uint32 rdma_max_chunks;
/// case RDMA2_ERR_WRITE_CHUNKS:
/// uint32 rdma_max_chunks;
/// case RDMA2_ERR_SEGMENTS:
/// uint32 rdma_max_segments;
/// case RDMA2_ERR_WRITE_RESOURCE:
/// rpcrdma2_err_write rdma_writeres;
/// case RDMA2_ERR_REPLY_RESOURCE:
/// uint32 rdma_length_needed;
/// default:
/// void;
/// };
///
/// /*
/// * Header Type to Exchange Transport Properties.
/// */
/// struct rpcrdma2_hdr_connprop {
/// rpcrdma2_propset rdma_props;
/// };
///
/// /*
Lever & Noveck Expires 30 December 2022 [Page 62]
Internet-Draft RPC-over-RDMA Version 2 June 2022
/// * Header Types to Convey RPC Messages.
/// */
/// struct rpcrdma2_hdr_call_external {
/// uint32 rdma_inv_handle;
///
/// struct rpcrdma2_read_list *rdma_call;
/// struct rpcrdma2_read_list *rdma_reads;
/// struct rpcrdma2_write_list *rdma_provisional_writes;
/// struct rpcrdma2_write_chunk *rdma_provisional_reply;
/// };
///
/// struct rpcrdma2_hdr_call_middle {
/// uint32 rdma_remaining;
///
/// /* The rpc message starts here and continues
/// * through the end of the transmission. */
/// uint32 rdma_rpc_first_word;
/// };
///
/// struct rpcrdma2_hdr_call_inline {
/// uint32 rdma_inv_handle;
///
/// struct rpcrdma2_read_list *rdma_reads;
/// struct rpcrdma2_write_list *rdma_provisional_writes;
/// struct rpcrdma2_write_chunk *rdma_provisional_reply;
///
/// /* The rpc message starts here and continues
/// * through the end of the transmission. */
/// uint32 rdma_rpc_first_word;
/// };
///
/// struct rpcrdma2_hdr_reply_external {
/// struct rpcrdma2_write_list *rdma_writes;
/// struct rpcrdma2_write_chunk *rdma_reply;
/// };
///
/// struct rpcrdma2_hdr_reply_middle {
/// uint32 rdma_remaining;
///
/// /* The rpc message starts here and continues
/// * through the end of the transmission. */
/// uint32 rdma_rpc_first_word;
/// };
///
/// struct rpcrdma2_hdr_reply_inline {
/// struct rpcrdma2_write_list *rdma_writes;
///
/// /* The rpc message starts here and continues
Lever & Noveck Expires 30 December 2022 [Page 63]
Internet-Draft RPC-over-RDMA Version 2 June 2022
/// * through the end of the transmission. */
/// uint32 rdma_rpc_first_word;
/// };
///
/// /* FILE ENDS: baseops.x; */
<CODE ENDS>
8.5. Use of the XDR Description
The files common.x and baseops.x, when combined with the XDR
descriptions for extension defined later, produce a human-readable
and compilable description of the RPC-over-RDMA version 2 protocol
with the included extensions.
Although this XDR description can generate encoders and decoders for
the Transport and Payload streams, there are elements of the
operation of RPC-over-RDMA version 2 that cannot be expressed within
the XDR language. Implementations that use the output of an
automated XDR processor need to provide additional code to bridge
these gaps.
* The Transport stream is not a single XDR object. Instead, the
header prefix is one XDR data item, and the rest of the header is
a separate XDR data item. Table 2 expresses the mapping between
the header type in the header prefix and the XDR object
representing the header type.
* The relationship between the Transport stream and the Payload
stream is not specified using XDR. Comments within the XDR text
make clear where transported messages, described by their own XDR
definitions, need to appear. Such data is opaque to the
transport.
* Continuation of RPC messages across transport message boundaries
requires that message assembly facilities not specifiable within
XDR are part of transport implementations.
* Transport properties are constant integer values. Table 1
expresses the mapping between each property's code point and the
XDR typedef that represents the structure of the property's value.
XDR does not possess the facility to express that mapping in an
extensible way.
Lever & Noveck Expires 30 December 2022 [Page 64]
Internet-Draft RPC-over-RDMA Version 2 June 2022
The role of XDR in RPC-over-RDMA specifications is more limited than
for protocols where the totality of the protocol is expressible
within XDR. XDR lacks the facility to represent the embedding of
XDR-encoded payload material. Also, the need to cleanly accommodate
extensions has meant that those using rpcgen in their applications
need to take an active role to provide the facilities that cannot be
expressed within XDR.
9. RPC Bind Parameters
Before establishing a new connection, an RPC client obtains a
transport address for the RPC server. The means used to obtain this
address and to open an RDMA connection is dependent on the type of
RDMA transport and is the responsibility of each RPC protocol binding
and its local implementation.
RPC services typically register with a portmap or rpcbind service
[RFC1833], which associates an RPC Program number with a service
address. This policy is no different with RDMA transports. However,
a distinct service address (port number) is sometimes required for
operation on RPC-over-RDMA.
When mapped atop MPA [RFC5044], which uses IP port addressing due to
its layering on TCP or SCTP, port mapping is trivial and consists
merely of issuing the port in the connection process. The NFS/RDMA
protocol service address has been assigned port 20049 by IANA for
this deployment scenario [RFC8267].
When mapped atop InfiniBand [IBA], which uses a service endpoint
naming scheme based on a Group Identifier (GID), a translation MUST
be employed. One such translation is described in Annexes A3
(Application Specific Identifiers), A4 (Sockets Direct Protocol
(SDP)), and A11 (RDMA IP CM Service) of [IBA], which is appropriate
for translating IP port addressing to the InfiniBand network.
Therefore, in this case, IP port addressing may be readily employed
by the upper layer.
When a mapping standard or convention exists for IP ports on an RDMA
interconnect, there are several possibilities for each upper layer to
consider:
* One possibility is to have the server register its mapped IP port
with the rpcbind service under the netid (or netids) defined in
[RFC8166]. An RPC-over-RDMA-aware RPC client can then resolve its
desired service to a mappable port and proceed to connect. This
method is the most flexible and compatible approach for those
upper layers that are defined to use the rpcbind service.
Lever & Noveck Expires 30 December 2022 [Page 65]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* A second possibility is to have the RPC server's portmapper
register itself on the RDMA interconnect at a "well-known" service
address (on UDP or TCP, this corresponds to port 111). An RPC
client can connect to this service address and use the portmap
protocol to obtain a service address in response to a program
number (e.g., a TCP port number or an InfiniBand GID).
* Alternately, an RPC client can connect to the mapped well-known
port for the service itself, if it is appropriately defined. By
convention, the NFS/RDMA service, when operating atop an
InfiniBand fabric, uses the same 20049 assignment as for MPA.
Historically, different RPC protocols have taken different approaches
to their port assignments. The current document leaves the specific
method for each RPC-over-RDMA-enabled ULB.
[RFC8166] defines two new netid values to be used for registration of
upper layers atop MPA and (when a suitable port translation service
is available) InfiniBand. Additional RDMA-capable networks MAY
define their own netids, or if they provide a port translation, they
MAY share the one defined in [RFC8166].
10. Implementation Status
This section is to be removed before publishing as an RFC.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors. This is not intended as, and must not be
construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
At this time, no known implementations of the protocol described in
the current document exist.
11. Security Considerations
Lever & Noveck Expires 30 December 2022 [Page 66]
Internet-Draft RPC-over-RDMA Version 2 June 2022
11.1. Memory Protection
A primary consideration is the protection of the integrity and
confidentiality of host memory by an RPC-over-RDMA transport. The
use of an RPC-over-RDMA transport protocol MUST NOT introduce
vulnerabilities to system memory contents nor memory owned by user
processes. Any RDMA provider used for RPC transport MUST conform to
the requirements of [RFC5042] to satisfy these protections.
11.1.1. Protection Domains
The use of a Protection Domain to limit the exposure of memory
regions to a single connection is critical. Any attempt by an
endpoint not participating in that connection to reuse memory handles
needs to result in immediate failure of that connection. Because ULP
security mechanisms rely on this aspect of Reliable Connected
behavior, implementations SHOULD cryptographically authenticate
connection endpoints.
11.1.2. Handle (STag) Predictability
Implementations should use unpredictable memory handles for any
operation requiring exposed memory regions. Exposing a continuously
registered memory region allows a remote host to read or write to
that region even when an RPC involving that memory is not underway.
Therefore, implementations should avoid the use of persistently
registered memory.
11.1.3. Memory Protection
Requesters should register memory regions for remote access only when
they are about to be the target of an RPC transaction that involves
an RDMA Read or Write.
Requesters should invalidate memory regions as soon as related RPC
operations are complete. Invalidation and DMA unmapping of memory
regions should complete before the receiver checks message integrity,
and before the RPC consumer can use or alter the contents of the
exposed memory region.
An RPC transaction on a Requester can terminate before a Reply
arrives, for example, if the RPC consumer is signaled, or a
segmentation fault occurs. When an RPC terminates abnormally, memory
regions associated with that RPC should be invalidated before the
Requester reuses those regions for other purposes.
Lever & Noveck Expires 30 December 2022 [Page 67]
Internet-Draft RPC-over-RDMA Version 2 June 2022
11.1.4. Denial of Service
A detailed discussion of denial-of-service exposures that can result
from the use of an RDMA transport appears in Section 6.4 of
[RFC5042].
A Responder is not obliged to pull unreasonably large Read chunks. A
Responder can use an RDMA2_ERROR response to terminate RPCs with
unreadable Read chunks. If a Responder transmits more data than a
Requester is prepared to receive in a Write or Reply chunk, the RDMA
provider typically terminates the connection. For further
discussion, see Section 6.3.1. Such repeated connection termination
can deny service to other users sharing the connection from the
errant Requester.
An RPC-over-RDMA transport implementation is not responsible for
throttling the RPC request rate, other than to keep the number of
concurrent RPC transactions within the per connection credit limits
(see Section 4.2.1). A sender can trigger a self-denial of service
by exceeding the credit limit repeatedly.
When an RPC transaction terminates due to a signal or premature exit
of an application process, a Requester should invalidate the RPC's
Write and Reply chunks. Invalidation prevents the subsequent arrival
of the Responder's Reply from altering the memory regions associated
with those chunks after the Requester has released that memory.
On the Requester, a malfunctioning application or a malicious user
can create a situation where RPCs initiate and abort continuously,
resulting in Responder replies that terminate the underlying RPC-
over-RDMA connection repeatedly. Such situations can deny service to
other users sharing the connection from that Requester.
11.2. RPC Message Security
ONC RPC provides cryptographic security via the RPCSEC_GSS framework
[RFC7861]. RPCSEC_GSS implements message authentication
(rpc_gss_svc_none), per-message integrity checking
(rpc_gss_svc_integrity), and per-message confidentiality
(rpc_gss_svc_privacy) in a layer above the RPC-over-RDMA transport.
The integrity and privacy services require significant computation
and movement of data on each endpoint host. Some performance
benefits enabled by RDMA transports can be lost.
Lever & Noveck Expires 30 December 2022 [Page 68]
Internet-Draft RPC-over-RDMA Version 2 June 2022
11.2.1. RPC-over-RDMA Protection at Other Layers
For any RPC transport, utilizing RPCSEC_GSS integrity or privacy
services has performance implications. Protection below the RPC
implementation is often a better choice in performance-sensitive
deployments, especially if it, too, can be offloaded. Certain
implementations of IPsec can be co-located in RDMA hardware, for
example, without change to RDMA consumers and with little loss of
data movement efficiency. Such arrangements can also provide a
higher degree of privacy by hiding endpoint identity or altering the
frequency at which messages are exchanged, at a performance cost.
Implementations MAY negotiate the use of protection in another layer
through the use of an RPCSEC_GSS security flavor defined in [RFC7861]
in conjunction with the Channel Binding mechanism [RFC5056] and IPsec
Channel Connection Latching [RFC5660].
11.2.2. RPCSEC_GSS on RPC-over-RDMA Transports
Not all RDMA devices and fabrics support the above protection
mechanisms. Also, NFS clients, where multiple users can access NFS
files, still require per-message authentication. In these cases,
RPCSEC_GSS can protect NFS traffic conveyed on RPC-over-RDMA
connections.
RPCSEC_GSS extends the ONC RPC protocol without changing the format
of RPC messages. By observing the conventions described in this
section, an RPC-over-RDMA transport can convey RPCSEC_GSS-protected
RPC messages interoperably.
Senders MUST NOT reduce protocol elements of RPCSEC_GSS that appear
in the Payload stream of an RPC-over-RDMA message. Such elements
include control messages exchanged as part of establishing or
destroying a security context, or data items that are part of
RPCSEC_GSS authentication material.
11.2.2.1. RPCSEC_GSS Context Negotiation
Some NFS client implementations use a separate connection to
establish a Generic Security Service (GSS) context for NFS operation.
Such clients use TCP and the standard NFS port (2049) for context
establishment. Therefore, an NFS server MUST also provide a TCP-
based NFS service on port 2049 to enable the use of RPCSEC_GSS with
NFS/RDMA.
Lever & Noveck Expires 30 December 2022 [Page 69]
Internet-Draft RPC-over-RDMA Version 2 June 2022
11.2.2.2. RPC-over-RDMA with RPCSEC_GSS Authentication
The RPCSEC_GSS authentication service has no impact on the DDP-
eligibility of data items in a ULP.
However, RPCSEC_GSS authentication material appearing in an RPC
message header can be larger than, say, an AUTH_SYS authenticator.
In particular, when an RPCSEC_GSS pseudoflavor is in use, a Requester
needs to accommodate a larger RPC credential when marshaling RPC
Calls and needs to provide for a maximum size RPCSEC_GSS verifier
when allocating reply buffers and Reply chunks.
RPC messages, and thus Payload streams, are larger on average as a
result. ULP operations that fit in a Simple Format message when a
simpler form of authentication is in use might need to be reduced or
conveyed via a Special Format message when RPCSEC_GSS authentication
is in use. It is therefore more likely that a Requester provisions
both a Read list and a Reply chunk in the same RPC-over-RDMA
Transport header to convey a Special Format Call and provision a
receptacle for a Special Format Reply.
In addition to this cost, the XDR encoding and decoding of each RPC
message using RPCSEC_GSS authentication requires per-message host
compute resources to construct the GSS verifier.
11.2.2.3. RPC-over-RDMA with RPCSEC_GSS Integrity or Privacy
The RPCSEC_GSS integrity service enables endpoints to detect the
modification of RPC messages in flight. The RPCSEC_GSS privacy
service prevents all but the intended recipient from viewing the
cleartext content of RPC arguments and results. RPCSEC_GSS integrity
and privacy services are end-to-end. They protect RPC arguments and
results from application to server endpoint, and back.
The RPCSEC_GSS integrity and encryption services operate on whole RPC
messages after they have been XDR encoded, and before they have been
XDR decoded after receipt. Connection endpoints use intermediate
buffers to prevent exposure of encrypted or unverified cleartext data
to RPC consumers. After a sender has verified, encrypted, and
wrapped a message, the transport layer MAY use RDMA data transfer
between these intermediate buffers.
Lever & Noveck Expires 30 December 2022 [Page 70]
Internet-Draft RPC-over-RDMA Version 2 June 2022
The process of reducing a DDP-eligible data item removes the data
item and its XDR padding from an encoded Payload stream. In a non-
protected RPC-over-RDMA message, a reduced data item does not include
XDR padding. After reduction, the Payload stream contains fewer
octets than the whole XDR stream did beforehand. XDR padding octets
are often zero bytes, but they don't have to be. Thus, reducing DDP-
eligible items affects the result of message integrity verification
and encryption.
Therefore, a sender MUST NOT reduce a Payload stream when RPCSEC_GSS
integrity or encryption services are in use. Effectively, no data
item is DDP-eligible in this situation. Senders can use only Simple
and Continued Formats without data item chunks, or Special Format.
In this mode, an RPC-over-RDMA transport operates in the same manner
as a transport that does not support DDP.
11.2.2.4. Protecting RPC-over-RDMA Transport Headers
Like the header fields in an RPC message (e.g., the xid and mtype
fields), RPCSEC_GSS does not protect the RPC-over-RDMA Transport
stream. XIDs, connection credit limits, and chunk lists (though not
the content of the data items they refer to) are exposed to malicious
behavior, which can redirect data that is transferred by the RPC-
over-RDMA message, result in spurious retransmits, or trigger
connection loss.
In particular, if an attacker alters the information contained in the
chunk lists of an RPC-over-RDMA Transport header, data contained in
those chunks can be redirected to other registered memory regions on
Requesters. An attacker might alter the arguments of RDMA Read and
RDMA Write operations on the wire to gain a similar effect. If such
alterations occur, the use of RPCSEC_GSS integrity or privacy
services enables a Requester to detect unexpected material in a
received RPC message.
Encryption at other layers, as described in Section 11.2.1, protects
the content of the Transport stream. RDMA transport implementations
should conform to [RFC5042] to address attacks on RDMA protocols
themselves.
11.3. Transport Properties
Like other fields that appear in the Transport stream, transport
properties are sent in the clear with no integrity protection, making
them vulnerable to man-in-the-middle attacks.
Lever & Noveck Expires 30 December 2022 [Page 71]
Internet-Draft RPC-over-RDMA Version 2 June 2022
For example, if a man-in-the-middle were to change the value of the
Receive buffer size, it could reduce connection performance or
trigger loss of connection. Repeated connection loss can impact
performance or even prevent a new connection from being established.
The recourse is to deploy on a private network or use transport layer
encryption.
11.4. Host Authentication
[ cel: This subsection is unfinished. ]
Wherein we use the relevant sections of [RFC3552] to analyze the
addition of host authentication to this RPC-over-RDMA transport.
The authors refer readers to Appendix C of [RFC8446] for information
on how to design and test a secure authentication handshake
implementation.
12. IANA Considerations
The RPC-over-RDMA family of transports have been assigned RPC netids
by [RFC8166]. A netid is an rpcbind [RFC1833] string used to
identify the underlying protocol in order for RPC to select
appropriate transport framing and the format of the service addresses
and ports.
The following netid registry strings are already defined for this
purpose:
NC_RDMA "rdma"
NC_RDMA6 "rdma6"
The "rdma" netid is to be used when IPv4 addressing is employed by
the underlying transport, and "rdma6" when IPv6 addressing is
employed. The netid assignment policy and registry are defined in
[RFC5665]. The current document does not alter these netid
assignments.
These netids MAY be used for any RDMA network that satisfies the
requirements of Section 3.2.2 and that is able to identify service
endpoints using IP port addressing, possibly through use of a
translation service as described in Section 9.
13. References
13.1. Normative References
Lever & Noveck Expires 30 December 2022 [Page 72]
Internet-Draft RPC-over-RDMA Version 2 June 2022
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
RFC 1833, DOI 10.17487/RFC1833, August 1995,
<https://www.rfc-editor.org/info/rfc1833>.
[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>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[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>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <https://www.rfc-editor.org/info/rfc5531>.
[RFC5660] Williams, N., "IPsec Channels: Connection Latching",
RFC 5660, DOI 10.17487/RFC5660, October 2009,
<https://www.rfc-editor.org/info/rfc5660>.
[RFC5665] Eisler, M., "IANA Considerations for Remote Procedure Call
(RPC) Network Identifiers and Universal Address Formats",
RFC 5665, DOI 10.17487/RFC5665, January 2010,
<https://www.rfc-editor.org/info/rfc5665>.
[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>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
Lever & Noveck Expires 30 December 2022 [Page 73]
Internet-Draft RPC-over-RDMA Version 2 June 2022
[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>.
[RFC8267] Lever, C., "Network File System (NFS) Upper-Layer Binding
to RPC-over-RDMA Version 1", RFC 8267,
DOI 10.17487/RFC8267, October 2017,
<https://www.rfc-editor.org/info/rfc8267>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
13.2. Informative References
[CBFC] Kung, H.T., Blackwell, T., and A. Chapman, "Credit-Based
Flow Control for ATM Networks: Credit Update Protocol,
Adaptive Credit Allocation, and Statistical Multiplexing",
Proc. ACM SIGCOMM '94 Symposium on Communications
Architectures, Protocols and Applications, pp. 101-114.,
August 1994.
[I-D.ietf-nfsv4-rpc-tls]
Myklebust, T. and C. Lever, "Towards Remote Procedure Call
Encryption By Default", Work in Progress, Internet-Draft,
draft-ietf-nfsv4-rpc-tls-11, 23 November 2020,
<https://datatracker.ietf.org/doc/html/draft-ietf-nfsv4-
rpc-tls-11>.
[IBA] InfiniBand Trade Association, "InfiniBand Architecture
Specification Volume 1", Release 1.3, March 2015.
Available from https://www.infinibandta.org/
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1094] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <https://www.rfc-editor.org/info/rfc1094>.
Lever & Noveck Expires 30 December 2022 [Page 74]
Internet-Draft RPC-over-RDMA Version 2 June 2022
[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>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[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>.
[RFC5041] Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
Data Placement over Reliable Transports", RFC 5041,
DOI 10.17487/RFC5041, October 2007,
<https://www.rfc-editor.org/info/rfc5041>.
[RFC5044] Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
Carrier, "Marker PDU Aligned Framing for TCP
Specification", RFC 5044, DOI 10.17487/RFC5044, October
2007, <https://www.rfc-editor.org/info/rfc5044>.
[RFC5532] Talpey, T. and C. Juszczak, "Network File System (NFS)
Remote Direct Memory Access (RDMA) Problem Statement",
RFC 5532, DOI 10.17487/RFC5532, May 2009,
<https://www.rfc-editor.org/info/rfc5532>.
[RFC5662] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
External Data Representation Standard (XDR) Description",
RFC 5662, DOI 10.17487/RFC5662, January 2010,
<https://www.rfc-editor.org/info/rfc5662>.
[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>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
Lever & Noveck Expires 30 December 2022 [Page 75]
Internet-Draft RPC-over-RDMA Version 2 June 2022
[RFC8167] Lever, C., "Bidirectional Remote Procedure Call on RPC-
over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
June 2017, <https://www.rfc-editor.org/info/rfc8167>.
[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.
Appendix A. ULB Specifications
Typically, an Upper-Layer Protocol (ULP) is defined without regard to
a particular RPC transport. An Upper-Layer Binding (ULB)
specification provides guidance that helps a ULP interoperate
correctly and efficiently over a particular transport. For RPC-over-
RDMA version 2, a ULB may provide:
* A taxonomy of XDR data items that are eligible for DDP
* Constraints on which upper-layer procedures a sender may reduce,
and on how many chunks may appear in a single RPC message
* A method enabling a Requester to determine the maximum size of the
reply Payload stream for all procedures in the ULP
* An rpcbind port assignment for the RPC Program and Version when
operating on the particular transport
Each RPC Program and Version tuple that operates on RPC-over-RDMA
version 2 needs to have a ULB specification.
A.1. DDP-Eligibility
A ULB designates specific XDR data items as eligible for DDP. As a
sender constructs an RPC-over-RDMA message, it can remove DDP-
eligible data items from the Payload stream so that the RDMA provider
can place them directly in the receiver's memory. An XDR data item
should be considered for DDP-eligibility if there is a clear benefit
to moving the contents of the item directly from the sender's memory
to the receiver's memory.
Criteria for DDP-eligibility include:
* The XDR data item is frequently sent or received, and its size is
often much larger than typical inline thresholds.
* If the XDR data item is a result, its maximum size must be
predictable in advance by the Requester.
Lever & Noveck Expires 30 December 2022 [Page 76]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* Transport-level processing of the XDR data item is not needed.
For example, the data item is an opaque byte array, which requires
no XDR encoding and decoding of its content.
* The content of the XDR data item is sensitive to address
alignment. For example, a data copy operation would be required
on the receiver to enable the message to be parsed correctly, or
to enable the data item to be accessed.
* The XDR data item itself does not contain DDP-eligible data items.
In addition to defining the set of data items that are DDP-eligible,
a ULB may limit the use of chunks to particular upper-layer
procedures. If more than one data item in a procedure is DDP-
eligible, the ULB may limit the number of chunks that a Requester can
provide for a particular upper-layer procedure.
Senders never reduce data items that are not DDP-eligible. Such data
items can, however, be part of a Special Format payload.
The programming interface by which an upper-layer implementation
indicates the DDP-eligibility of a data item to the RPC transport is
not described by this specification. The only requirements are that
the receiver can re-assemble the transmitted RPC-over-RDMA message
into a valid XDR stream and that DDP-eligibility rules specified by
the ULB are respected.
There is no provision to express DDP-eligibility within the XDR
language. The only definitive specification of DDP-eligibility is a
ULB.
In general, a DDP-eligibility violation occurs when:
* A Requester reduces a non-DDP-eligible argument data item. The
Responder reports the violation as described in Section 6.3.1.
* A Responder reduces a non-DDP-eligible result data item. The
Requester terminates the pending RPC transaction and reports an
appropriate permanent error to the RPC consumer.
* A Responder does not reduce a DDP-eligible result data item into
an available Write chunk. The Requester terminates the pending
RPC transaction and reports an appropriate permanent error to the
RPC consumer.
Lever & Noveck Expires 30 December 2022 [Page 77]
Internet-Draft RPC-over-RDMA Version 2 June 2022
A.2. Maximum Reply Size
When expecting small and moderately-sized Replies, a Requester should
rely on Message Continuation rather than provision a Reply chunk.
For each ULP procedure where there is no clear Reply size maximum and
the maximum can be substantial, the ULB should specify a dependable
means for determining the maximum Reply size.
A.3. Reverse-Direction Operation
The direction of operation does not preclude the need for DDP-
eligibility statements.
Reverse-direction operation occurs on an already-established
connection. Specification of RPC binding parameters is usually not
necessary in this case.
Other considerations may apply when distinct RPC Programs share an
RPC-over-RDMA transport connection concurrently.
A.4. Additional Considerations
There may be other details provided in a ULB.
* A ULB may recommend inline threshold values or other transport-
related parameters for RPC-over-RDMA version 2 connections bearing
that ULP.
* A ULP may provide a means to communicate transport-related
parameters between peers.
* Multiple ULPs may share a single RPC-over-RDMA version 2
connection when their ULBs allow the use of RPC-over-RDMA version
2 and the rpcbind port assignments for those protocols permit
connection sharing. In this case, the same transport parameters
(such as inline threshold) apply to all ULPs using that
connection.
Each ULB needs to be designed to allow correct interoperation without
regard to the transport parameters actually in use. Furthermore,
implementations of ULPs must be designed to interoperate correctly
regardless of the connection parameters in effect on a connection.
Lever & Noveck Expires 30 December 2022 [Page 78]
Internet-Draft RPC-over-RDMA Version 2 June 2022
A.5. ULP Extensions
An RPC Program and Version tuple may be extensible. For instance,
the RPC version number may not reflect a ULP minor versioning scheme,
or the ULP may allow the specification of additional features after
the publication of the original RPC Program specification. ULBs are
provided for interoperable RPC Programs and Versions by extending
existing ULBs to reflect the changes made necessary by each addition
to the existing XDR.
[ cel: The final sentence is unclear, and may be inaccurate. I
believe I copied this section directly from RFC 8166. Is there more
to be said, now that we have some experience? ]
Appendix B. Extending RPC-over-RDMA Version 2
This Appendix is not addressed to protocol implementers, but rather
to authors of documents that extend the protocol specified in the
current document.
RPC-over-RDMA version 2 extensibility facilitates limited extensions
to the base protocol presented in the current document so that new
optional capabilities can be introduced without a protocol version
change while maintaining robust interoperability with existing RPC-
over-RDMA version 2 implementations. It allows extensions to be
defined, including the definition of new protocol elements, without
requiring modification or recompilation of the XDR for the base
protocol.
Standards Track documents may introduce extensions to the base RPC-
over-RDMA version 2 protocol in two ways:
* They may introduce new OPTIONAL transport header types.
Appendix B.2 covers such transport header types.
* They may define new OPTIONAL transport properties. Appendix B.3
describes such transport properties.
These documents may also add the following sorts of ancillary
protocol elements to the protocol to support the addition of new
transport properties and header types:
* They may create new error codes, as described in Appendix B.4.
New capabilities can be proposed and developed independently of each
other. Implementers can choose among them, making it straightforward
to create and document experimental features and then bring them
through the standards process.
Lever & Noveck Expires 30 December 2022 [Page 79]
Internet-Draft RPC-over-RDMA Version 2 June 2022
B.1. Documentation Requirements
As described earlier, a Standards Track document introduces a set of
new protocol elements. Together these elements are considered an
OPTIONAL feature. Each implementation is either aware of all the
protocol elements introduced by that feature or is aware of none of
them.
Documents specifying extensions to RPC-over-RDMA version 2 should
contain:
* An explanation of the purpose and use of each new protocol
element.
* An XDR description including all of the new protocol elements, and
a script to extract it.
* A discussion of interactions with other extensions. This
discussion includes requirements for other OPTIONAL features to be
present, or that a particular level of support for an OPTIONAL
facility is required.
Implementers combine the XDR descriptions of the new features they
intend to use with the XDR description of the base protocol in the
current document. This combination is necessary to create a valid
XDR input file because extensions are free to use XDR types defined
in the base protocol, and later extensions may use types defined by
earlier extensions.
The XDR description for the RPC-over-RDMA version 2 base protocol
combined with that for any selected extensions should provide a
human-readable and compilable definition of the extended protocol.
B.2. Adding New Header Types to RPC-over-RDMA Version 2
New transport header types are defined similar to Sections 6.3.5
through 6.3.10. In particular, what is needed is:
* A description of the function and use of the new header type.
* A complete XDR description of the new header type.
* A description of how receivers report errors, including mechanisms
for reporting errors outside the available choices already
available in the base protocol or other extensions.
Lever & Noveck Expires 30 December 2022 [Page 80]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* An indication of whether a Payload stream must be present, and a
description of its contents and how receivers use such Payload
streams to reconstruct RPC messages.
* As appropriate, a statement of whether a Responder may use Remote
Invalidation when sending messages that contain the new header
type.
There needs to be additional documentation that is made necessary due
to the OPTIONAL status of new transport header types:
* The document should discuss constraints on support for the new
header types. For example, if support for one header type is
implied or foreclosed by another one, this needs to be documented.
* The document should describe the preferred method by which a
sender determines whether its peer supports a particular header
type. It is always possible to send a test invocation of a
particular header type to see if support is available. However,
when more efficient means are available (e.g., the value of a
transport property), this should be noted.
B.3. Adding New Transport properties to the Protocol
A Standards Track document defining a new transport property should
include the following information paralleling that provided in this
document for the transport properties defined herein:
* The rpcrdma2_propid value identifying the new property.
* The XDR typedef specifying the structure of its property value.
* A description of the new property.
* An explanation of how the receiver can use this information.
* The default value if a peer never receives the new property.
There is no requirement that propid assignments occur in a continuous
range of values. Implementations should not rely on all such values
being small integers.
Before the defining Standards Track document is published, the nfsv4
Working Group should select a unique propid value, and ensure that:
* rpcrdma2_propid values specified in the document do not conflict
with those currently assigned or in use by other pending working
group documents defining transport properties.
Lever & Noveck Expires 30 December 2022 [Page 81]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* rpcrdma2_propid values specified in the document do not conflict
with the range reserved for experimental use, as defined in
Section 8.2.
[ cel: There is no longer a section 8.2 or an experimental range
of propid values. Should we request the creation of an IANA
registry for propid values? ].
When a Standards Track document proposes additional transport
properties, reviewers should deal with possible security issues
exposed by those new transport properties.
B.4. Adding New Error Codes to the Protocol
The same Standards Track document that defines a new header type may
introduce new error codes used to support it. A Standards Track
document may similarly define new error codes that an existing header
type can return.
For error codes that do not require the return of additional
information, a peer can use the existing RDMA_ERR2 header type to
report the new error. The sender sets the new error code as the
value of rdma_err with the result that the default switch arm of the
rpcrdma2_error (i.e., void) is selected.
For error codes that do require the return of related information
together with the error, a new header type should be defined that
returns the error together with the related information. The sender
of a new header type needs to be prepared to accept header types
necessary to report associated errors.
Appendix C. Differences from RPC-over-RDMA Version 1
The primary goal of RPC-over-RDMA version 2 is to relieve constraints
that have become evident in RPC-over-RDMA version 1 with deployment
experience:
* RPC-over-RDMA version 1 has been challenging to update to address
shortcomings or improve data transfer efficiency.
* The average size of NFSv4 COMPOUNDs is significantly greater than
NFSv3 requests, requiring the use of Long messages for frequent
operations.
* Reply size estimation is difficult more often than first expected.
Lever & Noveck Expires 30 December 2022 [Page 82]
Internet-Draft RPC-over-RDMA Version 2 June 2022
This section details specific changes in RPC-over-RDMA version 2 that
address these constraints directly, in addition to other changes to
make implementation easier.
C.1. Changes to the XDR Definition
Several XDR structural changes enable within-version protocol
extensibility.
[RFC8166] defines the RPC-over-RDMA version 1 transport header as a
single XDR object, with an RPC message potentially following it. In
RPC-over-RDMA version 2, there are separate XDR definitions of the
transport header prefix (see Section 6.4), which specifies the
transport header type to be used, and the transport header itself
(defined within one of the subsections of Section 6.3). This
construction is similar to an RPC message, which consists of an RPC
header (defined in [RFC5531]) followed by a message defined by an
Upper-Layer Protocol.
As a new version of the RPC-over-RDMA transport protocol, RPC-over-
RDMA version 2 exists within the versioning rules defined in
[RFC8166]. In particular, it maintains the first four words of the
protocol header, as specified in Section 4.2 of [RFC8166], even
though, as explained in Section 6.2.1 of the current document, the
XDR definition of those words is structured differently.
Although each of the first four fields retains its semantic function,
there are differences in interpretation:
* The first word of the header, the rdma_xid field, retains the
format and function that it had in RPC-over-RDMA version 1.
Because RPC-over-RDMA version 2 messages can convey non-RPC
messages, a receiver should not use the contents of this field
without consideration of the protocol version and header type.
* The second word of the header, the rdma_vers field, retains the
format and function that it had in RPC-over-RDMA version 1. To
clearly distinguish version 1 and version 2 messages, senders need
to fill in the correct version (fixed after version negotiation).
Receivers should check that the content of the rdma_vers is
correct before using the content of any other header field.
* The third word of the header, the rdma_credit field, retains the
size and general purpose that it had in RPC-over-RDMA version 1.
However, RPC-over-RDMA version 2 divides this field into two
16-bit subfields. See Section 4.2.1 for further details.
Lever & Noveck Expires 30 December 2022 [Page 83]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* The fourth word of the header, previously the union discriminator
field rdma_proc, retains its format and general function even
though the set of valid values has changed. Within RPC-over-RDMA
version 2, this word is the rdma_htype field of the structure
rdma_start. The value of this field is now an unsigned 32-bit
integer rather than an enum type, to facilitate header type
extension.
Beyond conforming to the restrictions specified in [RFC8166], RPC-
over-RDMA version 2 attempts to limit the scope of the changes made
to ensure interoperability. Although it introduces the Call chunk
and splits the two version 1 workhorse procedure types RDMA_MSG and
RDMA_NOMSG into several variants, RPC-over-RDMA version 2 otherwise
expresses chunks in the same format and utilizes them the same way.
C.2. Transport Properties
RPC-over-RDMA version 2 provides a mechanism for exchanging an
implementation's operational properties. The purpose of this
exchange is to help endpoints improve the efficiency of data transfer
by exploiting the characteristics of both peers rather than falling
back on the lowest common denominator default settings. A full
discussion of transport properties appears in Section 5.
C.3. Credit Management Changes
RPC-over-RDMA transports employ credit-based flow control to ensure
that a Requester does not emit more RDMA Sends than the Responder is
prepared to receive.
Section 3.3.1 of [RFC8166] explains the operation of RPC-over-RDMA
version 1 credit management in detail. In that design, each RDMA
Send from a Requester contains an RPC Call with a credit request, and
each RDMA Send from a Responder contains an RPC Reply with a credit
grant. The credit grant implies that enough Receives have been
posted on the Responder to handle the credit grant minus the number
of pending RPC transactions (the number of remaining Receive buffers
might be zero).
Each RPC Reply acts as an implicit ACK for a previous RPC Call from
the Requester. Without an RPC Reply message, the Requester has no
way to know that the Responder is ready for subsequent RPC Calls.
Because version 1 embeds credit management in each message, there is
a strict one-to-one ratio between RDMA Send and RPC message. There
are interesting use cases that might be enabled if this relationship
were more flexible:
Lever & Noveck Expires 30 December 2022 [Page 84]
Internet-Draft RPC-over-RDMA Version 2 June 2022
* RPC-over-RDMA operations that do not carry an RPC message, e.g.,
control plane operations.
* A single RDMA Send that conveys more than one RPC message, e.g.,
for interrupt mitigation.
* An RPC message that requires several sequential RDMA Sends, e.g.,
to reduce the use of explicit RDMA operations for moderate-sized
RPC messages.
* An RPC transaction that requires multiple exchanges or an odd
number of RPC-over-RDMA operations to complete.
RPC-over-RDMA version 2 provides a more sophisticated credit
accounting mechanism to address these shortcomings. Section 4.2.1
explains the new mechanism in detail.
C.4. Inline Threshold Changes
An "inline threshold" value is the largest message size (in octets)
that can be conveyed on an RDMA connection using only RDMA Send and
Receive. Each connection has two inline threshold values: one for
messages flowing from client-to-server (referred to as the "client-
to-server inline threshold") and one for messages flowing from
server-to-client (referred to as the "server-to-client inline
threshold").
A connection's inline thresholds determine, among other things, when
RDMA Read or Write operations are required because an RPC message
cannot be conveyed via a single RDMA Send and Receive pair. When an
RPC message does not contain DDP-eligible data items, a Requester can
prepare a Special Format Call or Reply to convey the whole RPC
message using RDMA Read or Write operations.
RDMA Read and Write operations require that data payloads reside in
memory registered with the local RNIC. When an RPC completes, that
memory is invalidated to fence it from the Responder. Memory
registration and invalidation typically have a latency cost that is
insignificant compared to data handling costs.
When a data payload is small, however, the cost of registering and
invalidating memory where the payload resides becomes a significant
part of total RPC latency. Therefore the most efficient operation of
an RPC-over-RDMA transport occurs when the peers use explicit RDMA
Read and Write operations for large payloads but avoid those
operations for small payloads.
Lever & Noveck Expires 30 December 2022 [Page 85]
Internet-Draft RPC-over-RDMA Version 2 June 2022
When the authors of [RFC8166] first conceived RPC-over-RDMA version
1, the average size of RPC messages that did not involve a
significant data payload was under 500 bytes. A 1024-byte inline
threshold adequately minimized the frequency of inefficient Long
messages.
With NFS version 4 [RFC7530], the increased size of NFS COMPOUND
operations resulted in RPC messages that are, on average, larger than
previous versions of NFS. With a 1024-byte inline threshold,
frequent operations such as GETATTR and LOOKUP require RDMA Read or
Write operations, reducing the efficiency of data transport.
To reduce the frequency of Special Format messages, RPC-over-RDMA
version 2 increases the default size of inline thresholds. This
change also increases the maximum size of reverse-direction RPC
messages.
C.5. Message Continuation Changes
In addition to a larger default inline threshold, RPC-over-RDMA
version 2 introduces Message Continuation. Message Continuation is a
mechanism that enables the transmission of a data payload using more
than one RDMA Send. The purpose of Message Continuation is to
provide relief in several essential cases:
* If a Requester finds that it is inefficient to convey a
moderately-sized data payload using Read chunks, the Requester can
use Message Continuation to send the RPC Call.
* If a Requester has provided insufficient Reply chunk space for a
Responder to send an RPC Reply, the Responder can use Message
Continuation to send the RPC Reply.
* If a sender has to convey a sizeable non-RPC data payload (e.g., a
large transport property), the sender can use Message Continuation
to avoid having to register memory.
C.6. Host Authentication Changes
For the general operation of NFS on open networks, we eventually
intend to rely on RPC-on-TLS [I-D.ietf-nfsv4-rpc-tls] to provide
cryptographic authentication of the two ends of each connection. In
turn, this can improve the trustworthiness of AUTH_SYS-style user
identities that flow on TCP, which are not cryptographically
protected. We do not have a similar solution for RPC-over-RDMA,
however.
Lever & Noveck Expires 30 December 2022 [Page 86]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Here, the RDMA transport layer already provides a strong guarantee of
message integrity. On some network fabrics, IPsec or TLS can protect
the privacy of in-transit data. However, this is not the case for
all fabrics (e.g., InfiniBand [IBA]).
Thus, RPC-over-RDMA version 2 introduces a mechanism for
authenticating connection peers (see Section 5.2.6). And like GSS
channel binding, there is also a way to determine when the use of
host authentication is unnecessary.
C.7. Support for Remote Invalidation
When an RDMA consumer uses FRWR or Memory Windows to register memory,
that memory may be invalidated remotely [RFC5040]. These mechanisms
are available when a Requester's RNIC supports MEM_MGT_EXTENSIONS.
For this discussion, there are two classes of STags. Dynamically-
registered STags appear in a single RPC, then are invalidated.
Persistently-registered STags survive longer than one RPC. They may
persist for the life of an RPC-over-RDMA connection or even longer.
An RPC-over-RDMA Requester can provide more than one STag in a
transport header. It may provide a combination of dynamically- and
persistently-registered STags in one RPC message, or any combination
of these in a series of RPCs on the same connection. Only
dynamically-registered STags using Memory Windows or FRWR may be
invalidated remotely.
There is no transport-level mechanism by which a Responder can
determine how a Requester-provided STag was registered, nor whether
it is eligible to be invalidated remotely. A Requester that mixes
persistently- and dynamically-registered STags in one RPC, or mixes
them across RPCs on the same connection, must, therefore, indicate
which STag the Responder may invalidate remotely via a mechanism
provided in the Upper-Layer Protocol. RPC-over-RDMA version 2
provides such a mechanism.
A sender uses the RDMA Send With Invalidate operation to invalidate
an STag on the remote peer. It is available only when both peers
support MEM_MGT_EXTENSIONS (can send and process an IETH).
Existing RPC-over-RDMA transport protocol specifications [RFC8166]
[RFC8167] do not forbid direct data placement in the reverse
direction. Moreover, there is currently no Upper-Layer Protocol that
makes data items in reverse-direction operations eligible for direct
data placement.
Lever & Noveck Expires 30 December 2022 [Page 87]
Internet-Draft RPC-over-RDMA Version 2 June 2022
When chunks are present in a reverse-direction RPC request, Remote
Invalidation enables the Responder to trigger invalidation of a
Requester's STags as part of sending an RPC Reply, the same way as is
done in the forward direction.
However, in the reverse direction, the server acts as the Requester,
and the client is the Responder. The server's RNIC, therefore, must
support receiving an IETH, and the server must have registered its
STags with an appropriate registration mechanism.
C.8. Integration of Reverse-Direction Operation
Because [RFC5666] did not include specification of reverse-direction
operation, [RFC8166] does not include it either. Reverse-direction
operation in RPC-over-RDMA version 1 is specified by a separate
standards track document [RFC8167].
Reverse-direction operation in RPC-over-RDMA version 1 was
constrained by the limited ability to extend that version of the
protocol. The most awkward issue is that a receiver needs to peek at
ingress RPC message payloads to determine whether it is a Call or
Reply message. This is necessary because the meaning of several
fields in the RPC-over-RDMA transport header is determined by the
direction of the RPC message payload:
* The meaning of the value in the rdma_xid field is determined by
the direction of the message because the XID spaces in the forward
and reverse directions are distinct.
* The meaning of the value in the rdma_credit field is determined by
the direction of the message because credits are granted
separately for forward and reverse direction operation.
* The purpose of Write chunks and the meaning of their length fields
is determined by the direction of the message because in Call
messages, they are provisional, but in Reply messages, they
represent returned results.
The current document remedies this awkwardness by integrating
reverse-direction operation into RPC-over-RDMA version 2 so that it
can make use of all facilities that are available in the forward-
direction, including body chunks, remote invalidation, and message
continuation. To enable this integration, the direction of the RPC
message payload is encoded in each RPC-over-RDMA version 2 transport
header.
Lever & Noveck Expires 30 December 2022 [Page 88]
Internet-Draft RPC-over-RDMA Version 2 June 2022
C.9. Error Reporting Changes
RPC-over-RDMA version 2 expands the repertoire of errors that
connection peers may report to each other. The goals of this
expansion are:
* To fill in details of peer recovery actions.
* To enable retrying certain conditions caused by mis-estimation of
the maximum reply size.
* To minimize the likelihood of a Requester waiting forever for a
Reply when there are communications problems that prevent the
Responder from sending it.
C.10. Changes in Terminology
The RPC-over-RDMA version 2 specification makes the following changes
in terminology. These changes do not result in changes in the
behavior or operation of the protocol.
* The current document explicitly acknowledges the different
semantics and purpose of Write chunks appearing in Call messages
and those appearing in Reply messages.
* The current document introduces the term "payload format" to
describe the selection of a mechanism for reducing and conveying
an RPC message payload. It replaces the terms "short message" and
"long message" with the terms "simple format" and "special format"
because this selection is not based only on the size of the
payload.
* The current document introduces the terms "data item chunk" and
"body chunk" in order to distinguish the purpose and operation of
these two categories of chunk.
* For improved readability, the current document replaces the terms
"RDMA segment" and "plain segment" with the term "segment", and
the term "RDMA read segment" with the term "Read segment".
* The current document refers specifically to the RDMAP, DDP, and
MPA standards track protocols rather than using the nebulous term
"iWARP".
Lever & Noveck Expires 30 December 2022 [Page 89]
Internet-Draft RPC-over-RDMA Version 2 June 2022
Acknowledgments
The authors gratefully acknowledge the work of Brent Callaghan and
Tom Talpey on the original RPC-over-RDMA version 1 specification
[RFC5666].
We are deeply indebted to Jana Igeyar for contributing the RPC-over-
RDMA version 2 flow control mechanism described in Section 4.2.1.
The authors also wish to thank Bill Baker, Greg Marsden, and Matt
Benjamin for their support of this work.
The XDR extraction conventions were first described by the authors of
the NFS version 4.1 XDR specification [RFC5662]. Herbert van den
Bergh suggested the replacement sed script used in this document.
Special thanks go to Transport Area Director Magnus Westerlund, NFSV4
Working Group Chairs Spencer Shepler, and Brian Pawlowski, and NFSV4
Working Group Secretary Thomas Haynes for their support.
Authors' Addresses
Charles Lever (editor)
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
David Noveck
NetApp
1601 Trapelo Road
Waltham, MA 02451
United States of America
Phone: +1 781 572 8038
Email: davenoveck@gmail.com
Lever & Noveck Expires 30 December 2022 [Page 90]
