Bidirectional RPC-RDMA: Bidirectional Remote Procedure Call on RPC-over-RDMA Transports
How an RPC-over-RDMA connection can carry remote procedure calls in both directions at once, so NFS servers can call clients back.
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>.
@misc{rfc8167,
series = {Request for Comments},
number = 8167,
howpublished = {RFC 8167},
publisher = {RFC Editor},
doi = {10.17487/RFC8167},
url = {https://www.rfc-editor.org/info/rfc8167},
author = {C. Lever},
title = {{Bidirectional Remote Procedure Call on RPC-over-RDMA Transports}},
pagetotal = 13,
year = 2017,
month = jun,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Lever
Request for Comments: 8167 Oracle
Category: Standards Track June 2017
ISSN: 2070-1721
Bidirectional Remote Procedure Call on RPC-over-RDMA Transports
Abstract
Minor versions of Network File System (NFS) version 4 newer than
minor version 0 work best when Remote Procedure Call (RPC) transports
can send RPC transactions in both directions on the same connection.
This document describes how RPC transport endpoints capable of Remote
Direct Memory Access (RDMA) convey RPCs in both directions on a
single connection.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8167.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Understanding RPC Direction . . . . . . . . . . . . . . . . . 3
3. Immediate Uses of Bidirectional RPC-over-RDMA . . . . . . . . 5
4. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Sending and Receiving Operations in the Reverse Direction . . 8
6. In the Absence of Support for Reverse-Direction Operation . . 11
7. Considerations for ULBs . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Normative References . . . . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
RPC-over-RDMA transports, introduced in [RFC8166], efficiently convey
Remote Procedure Call (RPC) transactions on transport layers capable
of Remote Direct Memory Access (RDMA). The purpose of this document
is to enable concurrent operation in both directions on a single
transport connection using RPC-over-RDMA protocol versions that do
not have specific facilities for reverse-direction operation.
Reverse-direction RPC transactions are necessary for the operation of
version 4.1 of the Network File System (NFS), and in particular, of
Parallel NFS (pNFS) [RFC5661], though any Upper-Layer Protocol (ULP)
implementation may make use of them. An Upper-Layer Binding (ULB)
for NFS version 4.x callback operation is additionally required (see
Section 7) but is not provided in this document.
For example, using the approach described herein, RPC transactions
can be conveyed in both directions on the same RPC-over-RDMA version
1 connection without changes to the RPC-over-RDMA version 1 protocol.
This document does not update the protocol specified in [RFC8166].
The remainder of this document assumes familiarity with the
terminology and concepts contained in [RFC8166], especially Sections
2 and 3.
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.
2. Understanding RPC Direction
The Open Network Computing Remote Procedure Call (ONC RPC) protocol
as described in [RFC5531] is architected as a message-passing
protocol between one server and one or more clients. ONC RPC
transactions are made up of two types of messages.
A CALL message, or "Call", requests work. A Call is designated by
the value CALL in the message's msg_type field. An arbitrary unique
value is placed in the message's Transaction ID (XID) field. A host
that originates a Call is referred to in this document as a
"Requester".
A REPLY message, or "Reply", reports the results of work requested by
a Call. A Reply is designated by the value REPLY in the message's
msg_type field. The value contained in the message's XID field is
copied from the Call whose results are being returned. A host that
emits a Reply is referred to as a "Responder".
Typically, a Call results in a corresponding Reply. A Reply is never
sent without a corresponding Call.
RPC-over-RDMA is a connection-oriented RPC transport. In all cases,
when a connection-oriented transport is used, ONC RPC client
endpoints are responsible for initiating transport connections, while
ONC RPC service endpoints passively await incoming connection
requests.
RPC direction on connectionless RPC transports is not addressed in
this document.
2.1. Forward Direction
Traditionally, an ONC RPC client acts as a Requester, while an ONC
RPC service acts as a Responder. This form of message passing is
referred to as "forward-direction" operation.
2.2. Reverse Direction
The ONC RPC specification [RFC5531] does not forbid passing messages
in the other direction. An ONC RPC service endpoint can act as a
Requester, in which case, an ONC RPC client endpoint acts as a
Responder. This form of message passing is referred to as "reverse-
direction" operation.
During reverse-direction operation, the ONC RPC client is responsible
for establishing transport connections, even though RPC Call messages
come from the ONC RPC server.
ONC RPC clients and servers are optimized to perform and scale well
while handling traffic in the forward direction and might not be
prepared to handle operation in the reverse direction. Not until NFS
version 4.1 [RFC5661] has there been a strong need to handle reverse-
direction operation.
2.3. Bidirectional Operation
A pair of connected RPC endpoints may choose to use only forward-
direction or only reverse-direction operations on a particular
transport connection. Or, these endpoints may send Calls in both
directions concurrently on the same transport connection.
"Bidirectional operation" occurs when both transport endpoints act as
a Requester and a Responder at the same time.
Bidirectionality is an extension of RPC transport connection sharing.
Two RPC endpoints wish to exchange independent RPC messages over a
shared connection, but in opposite directions. These messages may or
may not be related to the same workloads or RPC Programs.
2.4. XID Values
Section 9 of [RFC5531] introduces the ONC RPC transaction identifier,
or "XID" for short. The value of an XID is interpreted in the
context of the message's msg_type field.
o The XID of a Call is arbitrary but is unique among outstanding
Calls from that Requester.
o The XID of a Reply always matches that of the initiating Call.
When receiving a Reply, a Requester matches the XID value in the
Reply with a Call it previously sent.
2.4.1. XID Generation
During bidirectional operation, forward- and reverse-direction XIDs
are typically generated on distinct hosts by possibly different
algorithms. There is no coordination between forward- and reverse-
direction XID generation.
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. Though such concurrent requests use the same
XID value, they represent distinct ONC RPC transactions.
3. Immediate Uses of Bidirectional RPC-over-RDMA
3.1. NFS Version 4.0 Callback Operation
An NFS version 4.0 client employs a traditional ONC RPC client to
send NFS requests to an NFS version 4.0 server's traditional ONC RPC
service [RFC7530]. NFS version 4.0 requests flow in the forward
direction on a connection established by the client. This connection
is referred to as a "forechannel" connection.
An NFS version 4.x "delegation" is simply a promise made by a server
that it will notify a client before another client or program running
on the server is allowed access to a file. With this guarantee, that
client can operate as sole accessor of the file. In particular, it
can manage the file's data and metadata caches aggressively.
To administer file delegations, NFS version 4.0 introduces the use of
callback operations, or "callbacks", in Section 10.2 of [RFC7530].
An NFS version 4.0 server sets up a forward-direction ONC RPC client,
and an NFS version 4.0 client sets up a forward-direction ONC RPC
service. Callbacks flow in the forward direction on a connection
established between the server's callback client and the client's
callback service. This connection is distinct from connections being
used as forechannels and is referred to as a "backchannel
connection".
When an RDMA transport is used as a forechannel, an NFS version 4.0
client typically provides a TCP-based callback service. The client's
SETCLIENTID operation advertises the callback service endpoint with a
"tcp" or "tcp6" netid. The server then connects to this service
using a TCP socket.
NFS version 4.0 implementations can function without a backchannel in
place. In this case, the NFS server does not grant file delegations.
This might result in a negative performance effect, but correctness
is not affected.
3.2. NFS Version 4.1 Callback Operation
NFS version 4.1 supports file delegation in a similar fashion to NFS
version 4.0 and extends the callback mechanism to manage pNFS
layouts, as discussed in Section 12 of [RFC5661].
NFS version 4.1 transport connections are initiated by NFS version
4.1 clients. Therefore, NFS version 4.1 servers send callbacks to
clients in the reverse direction on connections established by NFS
version 4.1 clients.
NFS version 4.1 clients and servers indicate to their peers that a
backchannel capability is available on a given transport connection
in the arguments and results of the NFS CREATE_SESSION or
BIND_CONN_TO_SESSION operations.
NFS version 4.1 clients may establish distinct transport connections
for forechannel and backchannel operation, or they may combine
forechannel and backchannel operation on one transport connection
using bidirectional operation.
Without a reverse-direction RPC-over-RDMA capability, an NFS version
4.1 client additionally connects using a transport with reverse-
direction capability to use as a backchannel. Opening an independent
TCP socket is the only choice for an NFS version 4.1 backchannel
connection in this case.
Implementations often find it more convenient to use a single
combined transport (i.e., a transport that is capable of
bidirectional operation). This simplifies connection establishment
and recovery during network partitions or when one endpoint restarts.
This can also enable better scaling by using fewer transport
connections to perform the same work.
As with NFS version 4.0, if a backchannel is not in use, an NFS
version 4.1 server does not grant delegations. Because NFS version
4.1 relies on callbacks to manage pNFS layout state, pNFS operation
is not possible without a backchannel.
4. Flow Control
For an RDMA Send operation to work properly, the receiving peer has
to have already posted a Receive buffer in which to accept the
incoming message. If a receiver hasn't posted enough buffers to
accommodate each incoming Send operation, the receiving RDMA provider
is allowed to terminate the RDMA connection.
RPC-over-RDMA transport protocols provide built-in send flow control
to prevent overrunning the number of pre-posted Receive buffers on a
connection's receive endpoint using a "credit grant" mechanism. The
use of credits in RPC-over-RDMA version 1 is described in
Section 3.3.1 of [RFC8166].
4.1. Reverse-Direction Credits
RPC-over-RDMA credits work the same way in the reverse direction as
they do in the forward direction. However, forward-direction credits
and reverse-direction credits on the same connection are accounted
separately. Direction-independent credit accounting prevents head-
of-line blocking in one direction from impacting operation in the
other direction.
The forward-direction credit value retains the same meaning whether
or not there are reverse-direction resources associated with an RPC-
over-RDMA transport connection. This is the number of RPC requests
the forward-direction Responder (the ONC RPC server) is prepared to
receive concurrently.
The reverse-direction credit value is the number of RPC requests the
reverse-direction Responder (the ONC RPC client) is prepared to
receive concurrently. The reverse-direction credit value MAY be
different than the forward-direction credit value.
During bidirectional operation, each receiver has to decide whether
an incoming message contains a credit request (the receiver is acting
as a Responder) or a credit grant (the receiver is acting as a
requester) and apply the credit value accordingly.
When message direction is not fully determined by context (e.g.,
suggested by the definition of the RPC-over-RDMA version that is in
use) or by an accompanying RPC message payload with a call direction
field, it is not possible for the receiver to tell with certainty
whether the header credit value is a request or grant. In such
cases, the receiver MUST ignore the header's credit value.
4.2. Inline Thresholds
Forward- and reverse-direction operation on the same connection share
the same Receive buffers. Therefore, the inline threshold values for
the forward direction and the reverse direction are the same. The
call inline threshold for the reverse direction is the same as the
reply inline threshold for the forward direction, and vice versa.
For more information, see Section 3.3.2 of [RFC8166].
4.3. Managing Receive Buffers
An RPC-over-RDMA transport endpoint posts Receive buffers before it
can receive and process incoming RPC-over-RDMA messages. If a sender
transmits a message for a receiver that has no posted Receive buffer,
the RDMA provider is allowed to drop the RDMA connection.
4.3.1. Client Receive Buffers
Typically, an RPC-over-RDMA Requester posts only as many Receive
buffers as there are outstanding RPC Calls. Therefore, a client
endpoint without reverse-direction support might, at times, have no
available Receive buffers.
To receive incoming reverse-direction Calls, an RPC-over-RDMA client
endpoint posts enough additional Receive buffers to match its
advertised reverse-direction credit value. Each outstanding forward-
direction RPC requires an additional Receive buffer above this
minimum.
When an RDMA transport connection is lost, all active Receive buffers
are flushed and are no longer available to receive incoming messages.
When a fresh transport connection is established, a client endpoint
posts a Receive buffer to handle the Reply for each retransmitted
forward-direction Call, and it posts enough Receive buffers to handle
reverse-direction Calls.
4.3.2. Server Receive Buffers
A forward-direction RPC-over-RDMA service endpoint posts as many
Receive buffers as it expects incoming forward-direction Calls. That
is, it posts no fewer buffers than the number of credits granted in
the rdma_credit field of forward-direction RPC replies.
To receive incoming reverse-direction replies, an RPC-over-RDMA
server endpoint posts enough additional Receive buffers to handle
replies for each reverse-direction Call it sends.
When the existing transport connection is lost, all active Receive
buffers are flushed and are no longer available to receive incoming
messages. When a fresh transport connection is established, a server
endpoint posts a Receive buffer to handle the Reply for each
retransmitted reverse-direction Call, and it posts enough Receive
buffers to handle incoming forward-direction Calls.
5. Sending and Receiving Operations in the Reverse Direction
The operation of RPC-over-RDMA transports in the forward direction is
defined in [RFC5531] and [RFC8166]. In this section, a mechanism for
reverse-direction operation on RPC-over-RDMA is defined. Reverse-
direction operation used in combination with forward-direction
operation enables bidirectional communication on a common RPC-over-
RDMA transport connection.
Certain fields in the RPC-over-RDMA header have a fixed position in
all versions of RPC-over-RDMA. The normative specification of these
fields is contained in Section 4 of [RFC8166].
5.1. Sending a Call in the Reverse Direction
To form a reverse-direction RPC-over-RDMA Call message, an ONC RPC
service endpoint constructs an RPC-over-RDMA header containing a
fresh RPC XID in the rdma_xid field (see Section 2.4 for full
requirements).
The rdma_vers field MUST contain the same value in reverse- and
forward-direction Call messages on the same connection.
The number of requested reverse-direction credits is placed in the
rdma_credit field (see Section 4).
Whether presented inline or as a separate chunk, the ONC RPC Call
header MUST start with the same XID value that is present in the RPC-
over-RDMA header, and the RPC header's msg_type field MUST contain
the value CALL.
5.2. Sending a Reply in the Reverse Direction
To form a reverse-direction RPC-over-RDMA Reply message, an ONC RPC
client endpoint constructs an RPC-over-RDMA header containing a copy
of the matching ONC RPC Call's RPC XID in the rdma_xid field (see
Section 2.4 for full requirements).
The rdma_vers field MUST contain the same value in a reverse-
direction Reply message as in the matching Call message.
The number of granted reverse-direction credits is placed in the
rdma_credit field (see Section 4).
Whether presented inline or as a separate chunk, the ONC RPC Reply
header MUST start with the same XID value that is present in the RPC-
over-RDMA header, and the RPC header's msg_type field MUST contain
the value REPLY.
5.3. Using Chunks in Reverse-Direction Operations
A "chunk" refers to a portion of a message's Payload stream that is
DDP-eligible and that is placed directly in the receiver's memory by
the transport. Chunk data may be moved by an explicit RDMA
operation, for example. Chunks are defined in Section 3.4.4 and DDP-
eligibility is covered in Section 6.1 of [RFC8166].
Chunks MAY be used in the reverse direction. They operate the same
way as in the forward direction.
An implementation might support only ULPs that have no DDP-eligible
data items. Such ULPs may use only small messages, or they may have
a native mechanism for restricting the size of reverse-direction RPC
messages, obviating the need to handle Long Messages in the reverse
direction.
When there is no ULP requirement for chunks in the reverse direction,
implementers can choose not to provide support for chunks in the
reverse direction. This avoids the complexity of adding support for
performing RDMA Reads and Writes in the reverse direction.
When chunks are not implemented, RPC messages in the reverse
direction are always sent using a Short Message; therefore, they can
be no larger than what can be sent inline (that is, without chunks).
Sending an inline message larger than the inline threshold can result
in loss of connection.
If a reverse-direction requester provides a non-empty chunk list to a
Responder that does not support chunks, the Responder MUST reply with
an RDMA_ERROR message with rdma_err field set to ERR_CHUNK.
5.4. Reverse-Direction Retransmission
In rare cases, an ONC RPC service cannot complete an RPC transaction
and then send a reply. This can be because the transport connection
was lost, because the Call or Reply message was dropped, or because
the ULP delayed or dropped the ONC RPC request. Typically, the
Requester sends the RPC transaction again, reusing the same RPC XID.
This is known as an "RPC retransmission".
In the forward direction, the Requester is the ONC RPC client. The
client is always responsible for establishing a transport connection
before sending again.
With reverse-direction operation, the Requester is the ONC RPC
server. Because an ONC RPC server does not establish transport
connections with clients, it cannot retransmit if there is no
transport connection. It is forced to wait for the ONC RPC client to
re-establish a transport connection before it can retransmit ONC RPC
transactions in the reverse direction.
If the ONC RPC client peer has no work to do, it can be some time
before it re-establishes a transport connection. A waiting reverse-
direction ONC RPC Call may time out to avoid waiting indefinitely for
a connection to be established.
Therefore, forward-direction Requesters SHOULD maintain a transport
connection as long as there is the possibility that the connection
peer can send reverse-direction requests. 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.
6. In the Absence of Support for Reverse-Direction Operation
An RPC-over-RDMA transport endpoint might not support reverse-
direction operation (and thus it does not support bidirectional
operation). There might be no mechanism in the transport
implementation to do so. Or in an implementation that can support
operation in the reverse direction, the ULP might not yet have
configured or enabled the transport to handle reverse-direction
traffic.
If an endpoint is not prepared to receive an incoming reverse-
direction message, loss of the RDMA connection might result. Thus,
denial of service could result if a sender continues to send reverse-
direction messages after every transport reconnect to an endpoint
that is not prepared to receive them.
When dealing with the possibility that the remote peer has no
transport-level support for reverse-direction operation, the ULP
becomes 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, a ULP MUST NOT perform reverse-direction ONC RPC
operations until the peer has indicated it is prepared to handle
them. A description of ULP mechanisms used for this indication is
outside the scope of this document.
For example, an NFS version 4.1 server does not send backchannel
messages to an NFS version 4.1 client before the NFS version 4.1
client has sent a CREATE_SESSION or a BIND_CONN_TO_SESSION operation.
As long as an NFS version 4.1 client has prepared appropriate
resources to receive reverse-direction operations before sending one
of these NFS operations, denial of service is avoided.
7. Considerations for ULBs
A ULP that operates on RPC-over-RDMA transports may have procedures
that include DDP-eligible data items. DDP-eligibility is specified
in an Upper-Layer Binding (ULB). Direction of operation does not
obviate the need for DDP-eligibility statements.
Reverse-direction-only operation requires the client endpoint to
establish a fresh connection. The ULB can specify appropriate RPC
binding parameters for such connections.
Bidirectional operation occurs on an already-established connection.
Specification of RPC binding parameters is usually not necessary in
this case.
For bidirectional operation, other considerations may apply when
distinct RPC Programs share an RPC-over-RDMA transport connection
concurrently. Consult Section 6 of [RFC8166] for details about what
else may be contained in a ULB.
8. Security Considerations
RPC security is handled in the RPC layer, which is above the
transport layer where RPC-over-RDMA operates.
Reverse-direction operations make use of an authentication mechanism
and credentials that are independent of forward-direction operation
but otherwise operate in the same fashion as outlined in Section 8.2
of [RFC8166].
9. IANA Considerations
This document does not require any IANA actions.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <http://www.rfc-editor.org/info/rfc5531>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<http://www.rfc-editor.org/info/rfc5661>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>.
[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,
<http://www.rfc-editor.org/info/rfc8166>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
Acknowledgements
Tom Talpey was an indispensable resource, in addition to creating the
foundation upon which this work is based. The author's warmest
regards go to him for his help and support.
Dave Noveck provided excellent review, constructive suggestions, and
navigational guidance throughout the process of drafting this
document.
Dai Ngo was a solid partner and collaborator. Together we
constructed and tested independent prototypes of the changes
described in this document.
The author wishes to thank Bill Baker and Greg Marsden for their
unwavering support of this work. In addition, the author gratefully
acknowledges the expert contributions of Karen Deitke, Chunli Zhang,
Mahesh Siddheshwar, Steve Wise, and Tom Tucker.
Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chair and Document Shepherd Spencer Shepler, and NFSV4
Working Group Secretary Tom Haynes for their support.
Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
AI ELI5
On this page
§1Why Bidirectional RPC-over-RDMA
RPC-over-RDMA transports, introduced in RFC 8166, carry Remote Procedure Call (RPC) transactions over transports that support Remote Direct Memory Access (RDMA). This document lets both ends of one connection send Calls at the same time, using RPC-over-RDMA protocol versions that have no special facility for the reverse direction.
The need comes from NFS version 4.1 and Parallel NFS (pNFS), which depend on reverse-direction RPC. For example, RPC-over-RDMA version 1 can carry transactions in both directions with no change to that protocol. The RFC does not update RFC 8166. It also says an Upper-Layer Binding (ULB) for NFS version 4.x callbacks is still required and is not given here.
Some file-sharing jobs need the server to ask the client a question too, not just answer. This RFC lets them do that on one connection.
§2RPC Direction and XIDs
ONC RPC messages are either a CALL, which requests work, or a REPLY, which reports the result. The host that sends a Call is the Requester and the host that sends a Reply is the Responder. A Reply is never sent without a matching Call.
Traditionally the client is the Requester and the server is the Responder: this is forward-direction operation. Nothing in RFC 5531 forbids the opposite, where the server is the Requester. That is reverse-direction operation. Even then, the client is still the one that establishes the transport connection. Bidirectional operation means both endpoints act as Requester and Responder at once.
Each Call carries a transaction ID (XID) and a Reply copies it. Forward and reverse XIDs are generated on different hosts, possibly by different algorithms, with no coordination. So a forward Requester MAY use the same XID as a reverse Requester at the same time on one connection, and these are still distinct transactions.
Normally you ask and the shop answers. In two-way mode the shop can ask you things too. Each question has its own number, and the two sides may reuse the same number without mixing things up.
§3Immediate Uses: NFS Callbacks
NFS version 4.x servers grant delegations: a promise to notify a client before anyone else may access a file, so the client can cache its data and metadata aggressively. Callbacks administer these delegations. In NFS version 4.0 the server connects back to a callback service on the client, usually over a separate TCP connection when RDMA is used for the forechannel. Without a backchannel, the server grants no delegations, which may hurt performance but not correctness.
NFS version 4.1 also uses callbacks to manage pNFS layouts. Clients start the connections, so servers send callbacks in the reverse direction on a connection the client made. Clients may keep separate forechannel and backchannel connections or combine them. A combined transport simplifies connection setup and recovery, and can use fewer connections. Without a backchannel, pNFS cannot operate at all.
A file server sometimes needs to tell you, 'someone else wants your file, please hand it back.' This RFC lets it say that on the same line instead of opening a second one.
§4Flow Control and Receive Buffers
An RDMA Send only works if the receiver has already posted a Receive buffer. If it has not, the RDMA provider is allowed to terminate the connection. RPC-over-RDMA prevents this with a credit grant mechanism.
Credits work the same in the reverse direction, but forward and reverse credits on one connection are accounted separately, so a stall in one direction cannot block the other. The reverse credit value is the number of RPC requests the client is ready to receive at once and MAY differ from the forward value. If a receiver cannot tell from context whether a header's credit value is a request or a grant, it MUST ignore it.
Both directions share the same Receive buffers, so the inline thresholds are the same. The reverse call threshold equals the forward reply threshold, and vice versa.
A client posts extra Receive buffers to match its reverse credit value, on top of one per outstanding forward Call. A server posts extra buffers for the replies to each reverse Call it sends. After a lost connection, all posted buffers are flushed, so both sides must post fresh ones on reconnect, including for retransmitted Calls.
Before someone sends you a message, you must hold out a bucket to catch it. Credits say how many buckets are out, and each direction counts its own buckets.
§5Sending Calls and Replies in Reverse
A reverse-direction Call is built by the server endpoint. Its RPC-over-RDMA header holds a fresh XID in rdma_xid. The rdma_vers field MUST match the value used for forward Calls on that connection, and rdma_credit carries the number of requested reverse credits. The ONC RPC Call header, inline or in a chunk, MUST start with the same XID and have msg_type set to CALL.
A reverse-direction Reply is built by the client endpoint. It copies the matching Call's XID into rdma_xid, uses the same rdma_vers, and puts the granted reverse credits in rdma_credit. The RPC header MUST start with that XID and have msg_type set to REPLY.
Chunks MAY be used in the reverse direction and work as they do forward. If no ULP needs them, implementers can skip them. Reverse messages then always go as Short Messages, no larger than the inline threshold, since a larger inline message can cost the connection. A Responder that gets a non-empty chunk list but lacks chunk support MUST reply with an RDMA_ERROR message with rdma_err set to ERR_CHUNK.
A reverse question is written just like a normal one, with its own number, and the answer copies that number back so everyone knows which question it belongs to.
§6Retransmission and Unsupported Peers
When a transaction cannot finish, the Requester normally sends it again with the same XID. In the forward direction the client can always reconnect first. In the reverse direction the Requester is the server, which cannot open connections, so it must wait for the client to reconnect. A waiting reverse Call may time out. Forward-direction Requesters SHOULD therefore keep a connection open as long as the peer might send reverse requests. An NFS version 4.1 client with open delegated files or active pNFS layouts is the RFC's example.
An endpoint may not support reverse operation at all, or the ULP may not have enabled it. Sending it reverse messages could lose the connection, and repeating that after every reconnect could amount to denial of service. So a ULP MUST NOT perform reverse-direction operations until the peer has indicated it is ready. How it indicates this is outside the scope of the RFC. In NFS version 4.1, the server waits for the client's CREATE_SESSION or BIND_CONN_TO_SESSION operation first.
If the line drops, only the client can call back to reconnect, so the server has to wait. And you should never ask a question on a line until the other side says it is ready to hear it.
§7ULBs and Security
A ULP can have procedures with data items eligible for direct data placement (DDP). Which items qualify is stated in its Upper-Layer Binding, and direction of operation does not remove the need for those statements. Reverse-only operation needs the client to make a fresh connection, and the ULB can specify RPC binding parameters for it. Bidirectional operation uses an existing connection, so those are usually unnecessary. If distinct RPC Programs share a connection, other considerations may apply.
On security, the RFC says RPC security is handled in the RPC layer, above RPC-over-RDMA. Reverse-direction operations use an authentication mechanism and credentials independent of the forward direction. The document requires no IANA actions.
Locks and ID checks happen in the layer above the line, and each direction uses its own. This RFC does not need anything added to any official list.