RFC8166: Remote Direct Memory Access Transport for Remote Procedure Call Version 1
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Obsoletes:
RFC5666
Related keywords:
(RPC-over-RDMA)
Internet Engineering Task Force (IETF) C. Lever, Ed.
Request for Comments: 8166 Oracle
Obsoletes: 5666 W. Simpson
Category: Standards Track Red Hat
ISSN: 2070-1721 T. Talpey
Microsoft
June 2017
Remote Direct Memory Access Transport for
Remote Procedure Call Version 1
Abstract
This document specifies a protocol for conveying Remote Procedure
Call (RPC) messages on physical transports capable of Remote Direct
Memory Access (RDMA). This protocol is referred to as the RPC-over-
RDMA version 1 protocol in this document. It requires no revision to
application RPC protocols or the RPC protocol itself. This document
obsoletes RFC 5666.
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/rfc8166.
Lever, et al. Standards Track [Page 1]
RFC 8166 RPC-over-RDMA Version 1 June 2017
Copyright Notice
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Lever, et al. Standards Track [Page 2]
RFC 8166 RPC-over-RDMA Version 1 June 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. RPCs on RDMA Transports . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2.2. RPCs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. RDMA . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. RPC-over-RDMA Protocol Framework . . . . . . . . . . . . . . 10
3.1. Transfer Models . . . . . . . . . . . . . . . . . . . . . 10
3.2. Message Framing . . . . . . . . . . . . . . . . . . . . . 11
3.3. Managing Receiver Resources . . . . . . . . . . . . . . . 11
3.4. XDR Encoding with Chunks . . . . . . . . . . . . . . . . 14
3.5. Message Size . . . . . . . . . . . . . . . . . . . . . . 19
4. RPC-over-RDMA in Operation . . . . . . . . . . . . . . . . . 23
4.1. XDR Protocol Definition . . . . . . . . . . . . . . . . . 23
4.2. Fixed Header Fields . . . . . . . . . . . . . . . . . . . 28
4.3. Chunk Lists . . . . . . . . . . . . . . . . . . . . . . . 30
4.4. Memory Registration . . . . . . . . . . . . . . . . . . . 33
4.5. Error Handling . . . . . . . . . . . . . . . . . . . . . 34
4.6. Protocol Elements No Longer Supported . . . . . . . . . . 37
4.7. XDR Examples . . . . . . . . . . . . . . . . . . . . . . 38
5. RPC Bind Parameters . . . . . . . . . . . . . . . . . . . . . 39
6. ULB Specifications . . . . . . . . . . . . . . . . . . . . . 41
6.1. DDP-Eligibility . . . . . . . . . . . . . . . . . . . . . 41
6.2. Maximum Reply Size . . . . . . . . . . . . . . . . . . . 43
6.3. Additional Considerations . . . . . . . . . . . . . . . . 43
6.4. ULP Extensions . . . . . . . . . . . . . . . . . . . . . 43
7. Protocol Extensibility . . . . . . . . . . . . . . . . . . . 44
7.1. Conventional Extensions . . . . . . . . . . . . . . . . . 44
8. Security Considerations . . . . . . . . . . . . . . . . . . . 44
8.1. Memory Protection . . . . . . . . . . . . . . . . . . . . 44
8.2. RPC Message Security . . . . . . . . . . . . . . . . . . 46
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
10.1. Normative References . . . . . . . . . . . . . . . . . . 50
10.2. Informative References . . . . . . . . . . . . . . . . . 51
Appendix A. Changes from RFC 5666 . . . . . . . . . . . . . . . 53
A.1. Changes to the Specification . . . . . . . . . . . . . . 53
A.2. Changes to the Protocol . . . . . . . . . . . . . . . . . 53
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 55
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RFC 8166 RPC-over-RDMA Version 1 June 2017
1. Introduction
This document specifies the RPC-over-RDMA version 1 protocol, based
on existing implementations of RFC 5666 and experience gained through
deployment. This document obsoletes RFC 5666.
This specification clarifies text that was subject to multiple
interpretations and removes support for unimplemented RPC-over-RDMA
version 1 protocol elements. It clarifies the role of Upper-Layer
Bindings (ULBs) and describes what they are to contain.
In addition, this document describes current practice using
RPCSEC_GSS [RFC7861] on RDMA transports.
The protocol version number has not been changed because the protocol
specified in this document fully interoperates with implementations
of the RPC-over-RDMA version 1 protocol specified in [RFC5666].
1.1. RPCs on RDMA Transports
RDMA [RFC5040] [RFC5041] [IBARCH] is a technique for moving data
efficiently between end nodes. By directing data into destination
buffers as it is sent on a network, and placing it via direct memory
access by hardware, the benefits of faster transfers and reduced host
overhead are obtained.
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 [RFC768] or TCP [RFC793]. On UDP, RPC
messages are encapsulated inside datagrams, while on a TCP byte
stream, RPC messages are delineated by a record marking protocol. An
RDMA transport also conveys RPC messages in a specific fashion that
must be fully described if RPC implementations are to interoperate.
RDMA transports present semantics that differ from either UDP or TCP.
They retain message delineations like UDP but provide reliable and
sequenced data transfer like TCP. They also provide an offloaded
bulk transfer service not provided by UDP or TCP. RDMA transports
are therefore appropriately viewed as a new transport type by RPC.
In this context, the Network File System (NFS) protocols, as
described in [RFC1094], [RFC1813], [RFC7530], [RFC5661], and future
NFSv4 minor versions, are all obvious beneficiaries of RDMA
transports. A complete problem statement is presented in [RFC5532].
Many other RPC-based protocols can also benefit.
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Although the RDMA transport described herein can provide relatively
transparent support for any RPC application, this document also
describes mechanisms that can optimize data transfer even further,
when RPC applications are willing to exploit awareness of RDMA as the
transport.
2. Terminology
2.1. 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.
2.2. RPCs
This section highlights key elements of the RPC [RFC5531] and
External Data Representation (XDR) [RFC4506] protocols, upon which
RPC-over-RDMA version 1 is constructed. Strong grounding with these
protocols is recommended before reading this document.
2.2.1. Upper-Layer Protocols
RPCs are an abstraction used to implement the operations of an Upper-
Layer Protocol (ULP). "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 this document, the term "RPC consumer" refers to an implementation
of a ULP running on an RPC client endpoint.
2.2.2. Requesters and Responders
Like a local procedure call, every RPC procedure has a set of
"arguments" and a set of "results". A calling context invokes a
procedure, passing arguments to it, and the procedure subsequently
returns a set of results. Unlike a local procedure call, the called
procedure is executed remotely rather than in the local application's
execution context.
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 those
consumers).
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ONC RPC transactions are made up of two types of messages:
CALL
An "RPC Call message" requests that work be done. This type of
message is designated by the value zero (0) in the message's
msg_type field. An arbitrary unique value is placed in the
message's XID field in order to match this RPC Call message to a
corresponding RPC Reply message.
REPLY
An "RPC Reply message" 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. The value contained in
an RPC Reply message's XID field is copied from the RPC Call
message whose results are being reported.
The RPC client endpoint acts as a "Requester". It serializes the
procedure's arguments and conveys them to a server endpoint via an
RPC Call message. This message contains an RPC protocol header, a
header describing the requested upper-layer operation, and all
arguments.
The RPC server endpoint acts as a "Responder". It deserializes the
arguments and processes the requested operation. It then serializes
the operation's results into another byte stream. This byte stream
is conveyed back to the Requester via an RPC Reply message. This
message contains an RPC protocol header, a header describing the
upper-layer reply, and all results.
The Requester deserializes the results and allows the original caller
to proceed. At this point, the RPC transaction designated by the XID
in the RPC Call message is complete, and the XID is retired.
In summary, RPC Call messages are sent by Requesters to Responders to
initiate RPC transactions. RPC Reply messages are sent by Responders
to Requesters to complete the processing on an RPC transaction.
2.2.3. RPC Transports
The role of an "RPC transport" is to mediate the exchange of RPC
messages between Requesters and Responders. An RPC transport bridges
the gap between the RPC message abstraction and the native operations
of a particular network transport.
RPC-over-RDMA is a connection-oriented RPC transport. When a
connection-oriented transport is used, clients initiate transport
connections, while servers wait passively for incoming connection
requests.
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2.2.4. External Data Representation
One cannot assume that all Requesters and Responders represent data
objects the same way internally. RPC uses External Data
Representation (XDR) to translate native data types and serialize
arguments and results [RFC4506].
The XDR protocol encodes data independently of the endianness or size
of host-native data types, allowing unambiguous decoding of data on
the receiving end. RPC Programs are specified by writing an XDR
definition of their procedures, argument data types, and result data
types.
XDR assumes that the number of bits in a byte (octet) and their order
are the same on both endpoints and on the physical network. The
smallest indivisible unit of XDR encoding is a group of four octets.
XDR also flattens lists, arrays, and other complex data types so they
can be conveyed as a stream of bytes.
A serialized stream of bytes that is the result of XDR encoding is
referred to as an "XDR stream". A sending endpoint encodes native
data into an XDR stream and then transmits that stream to a receiver.
A receiving endpoint decodes incoming XDR byte streams into its
native data representation format.
2.2.4.1. XDR Opaque Data
Sometimes, a data item must be transferred as is: without encoding or
decoding. The contents of such a data item are referred to 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.
2.2.4.2. 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, octets containing zero are added after the
end of the item; this is the case so that the next encoded data item
in the XDR stream starts on a four-octet boundary. The encoded size
of the item is not changed by the addition of the extra octets.
These extra octets are never exposed to ULPs.
This technique is referred to as "XDR roundup", and the extra octets
are referred to as "XDR roundup padding".
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2.3. RDMA
RPC Requesters and Responders can be made more efficient if large RPC
messages are transferred by a third party, such as intelligent
network-interface hardware (data movement offload), and placed in the
receiver's memory so that no additional adjustment of data alignment
has to be made (direct data placement or "DDP"). RDMA transports
enable both optimizations.
2.3.1. DDP
Typically, RPC implementations copy the contents of RPC messages into
a buffer before being sent. An efficient RPC implementation sends
bulk data without copying it into a separate send buffer first.
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: sometimes, only to
adjust data alignment.
In this document, "RDMA" refers to the physical mechanism an RDMA
transport utilizes when moving data. Although this 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 it is unnecessary for a receiving host's CPU to copy
transferred data to another location after it has been received.
Just as [RFC5666] did, this document focuses on the use of RDMA Read
and Write operations to achieve both data movement offload and DDP.
However, not all RDMA-based data transfer qualifies as DDP, and DDP
can be achieved using non-RDMA mechanisms.
2.3.2. RDMA Transport Requirements
To achieve good performance during receive operations, RDMA
transports require that RDMA consumers provision resources in advance
to receive incoming messages.
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.
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An RDMA Receive Work Request remains outstanding until hardware
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 basic tenets of RDMA transport operation, the RPC-over-
RDMA version 1 protocol assumes each transport provides the following
abstract operations. A more complete discussion of these operations
is found in [RFC5040].
Registered Memory
Registered memory is a region of memory that is assigned a
steering tag that temporarily permits access by the RDMA provider
to perform data-transfer operations. The RPC-over-RDMA version 1
protocol assumes that each region of registered memory MUST be
identified with a steering tag of no more than 32 bits and memory
addresses of up to 64 bits in length.
RDMA Send
The RDMA provider supports an RDMA Send operation, with completion
signaled on the receiving peer after data has been placed in a
pre-posted buffer. Sends complete at the receiver in the order
they were issued at the sender. The amount of data transferred by
a single RDMA Send operation is limited by the size of the remote
peer's pre-posted buffers.
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 pre-posted memory is limited. Flow control to
prevent overrunning receiver resources is provided by the RDMA
consumer (in this case, the RPC-over-RDMA version 1 protocol).
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 completion is signaled there. No completion is
signaled on the remote peer. The local host provides a steering
tag, memory address, and length of the remote peer's memory
region.
RDMA Writes are not ordered with respect to one another, but are
ordered with respect to RDMA Sends. A subsequent RDMA Send
completion obtained at the write initiator guarantees that prior
RDMA Write data has been successfully placed in the remote peer's
memory.
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RDMA Read
The RDMA provider supports an RDMA Read operation to place peer
source data directly into the read initiator's memory. The local
host initiates an RDMA Read, and completion is signaled there. No
completion is signaled on the remote peer. The local host
provides steering tags, memory addresses, and a length for the
remote source and local destination memory region.
The local host signals Read completion to the remote peer as part
of a subsequent RDMA Send message. The remote peer can then
release steering tags and subsequently free associated source
memory regions.
The RPC-over-RDMA version 1 protocol is designed to be carried over
RDMA transports that support the above abstract operations. This
protocol conveys information sufficient for an RPC peer to direct an
RDMA provider to perform transfers containing RPC data and to
communicate their result(s).
3. RPC-over-RDMA Protocol Framework
3.1. Transfer Models
A "transfer model" designates which endpoint exposes its memory and
which is responsible for initiating the transfer of data. To enable
RDMA Read and Write operations, for example, an endpoint first
exposes regions of its memory to a remote endpoint, which initiates
these operations against the exposed memory.
Read-Read
Requesters expose their memory to the Responder, and the Responder
exposes its memory to Requesters. The Responder reads, or pulls,
RPC arguments or whole RPC calls from each Requester. Requesters
pull RPC results or whole RPC relies from the Responder.
Write-Write
Requesters expose their memory to the Responder, and the Responder
exposes its memory to Requesters. Requesters write, or push, RPC
arguments or whole RPC calls to the Responder. The Responder
pushes RPC results or whole RPC relies to each Requester.
Read-Write
Requesters expose their memory to the Responder, but the Responder
does not expose its memory. The Responder pulls RPC arguments or
whole RPC calls from each Requester. The Responder pushes RPC
results or whole RPC relies to each Requester.
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Write-Read
The Responder exposes its memory to Requesters, but Requesters do
not expose their memory. Requesters push RPC arguments or whole
RPC calls to the Responder. Requesters pull RPC results or whole
RPC relies from the Responder.
3.2. Message Framing
On an RPC-over-RDMA transport, each RPC message is encapsulated by an
RPC-over-RDMA message. An RPC-over-RDMA message consists of two XDR
streams.
RPC Payload Stream
The "Payload stream" contains the encapsulated RPC message being
transferred by this RPC-over-RDMA message. This stream always
begins with the Transaction ID (XID) field of the encapsulated RPC
message.
Transport Stream
The "Transport stream" contains a header that describes and
controls the transfer of the Payload stream in this RPC-over-RDMA
message. This header is analogous to the record marking used for
RPC on TCP sockets but is more extensive, since RDMA transports
support several modes of data transfer.
In its simplest form, an RPC-over-RDMA message consists of a
Transport stream followed immediately by a Payload stream conveyed
together in a single RDMA Send. To transmit large RPC messages, a
combination of one RDMA Send operation and one or more other RDMA
operations is employed.
RPC-over-RDMA framing replaces all other RPC framing (such as TCP
record marking) when used atop an RPC-over-RDMA association, even
when the underlying RDMA protocol may itself be layered atop a
transport with a defined RPC framing (such as TCP).
However, it is possible for RPC-over-RDMA to be dynamically enabled
in the course of negotiating the use of RDMA via a ULP exchange.
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.
3.3. Managing Receiver Resources
It is critical to provide RDMA Send flow control for an RDMA
connection. If any pre-posted Receive buffer on the connection is
not large enough to accept an incoming RDMA Send, or if a pre-posted
Receive buffer is not available to accept an incoming RDMA Send, the
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RDMA connection can be terminated. This is different than
conventional TCP/IP networking, in which buffers are allocated
dynamically as messages are received.
The longevity of an RDMA connection mandates that sending endpoints
respect the resource limits of peer receivers. To ensure messages
can be sent and received reliably, there are two operational
parameters for each connection.
3.3.1. RPC-over-RDMA Credits
Flow control for RDMA Send operations directed to the Responder is
implemented as a simple request/grant protocol in the RPC-over-RDMA
header associated with each RPC message.
An RPC-over-RDMA version 1 credit is the capability to handle one
RPC-over-RDMA transaction. Each RPC-over-RDMA message sent from
Requester to Responder requests a number of credits from the
Responder. Each RPC-over-RDMA message sent from Responder to
Requester informs the Requester how many credits the Responder has
granted. The requested and granted values are carried in each RPC-
over-RDMA message's rdma_credit field (see Section 4.2.3).
Practically speaking, the critical value is the granted value. A
Requester MUST NOT send unacknowledged requests in excess of the
Responder's granted credit limit. If the granted value is exceeded,
the RDMA layer may signal an error, possibly terminating the
connection. The granted value MUST NOT be zero, since such a value
would result in deadlock.
RPC calls complete in any order, but the current granted credit limit
at the Responder is known to the Requester from RDMA Send ordering
properties. The number of allowed new requests the Requester may
send is then the lower of the current requested and granted credit
values, minus the number of requests in flight. Advertised credit
values are not altered when individual RPCs are started or completed.
The requested and granted credit values MAY be adjusted to match the
needs or policies in effect on either peer. For instance, a
Responder may reduce the granted credit value to accommodate the
available resources in a Shared Receive Queue. The Responder MUST
ensure that an increase in receive resources is effected before the
next RPC Reply message is sent.
A Requester MUST maintain enough receive resources to accommodate
expected replies. Responders have to be prepared for there to be no
receive resources available on Requesters with no pending RPC
transactions.
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Certain RDMA implementations may impose additional flow-control
restrictions, such as limits on RDMA Read operations in progress at
the Responder. Accommodation of such restrictions is considered the
responsibility of each RPC-over-RDMA version 1 implementation.
3.3.2. 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. The inline threshold value is the
smaller of the largest number of bytes the sender can post via a
single RDMA Send operation and the largest number of bytes the
receiver can accept via a single RDMA Receive operation. Each
connection has two inline threshold values: 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").
Unlike credit limits, inline threshold values are not advertised to
peers via the RPC-over-RDMA version 1 protocol, and there is no
provision for inline threshold values to change during the lifetime
of an RPC-over-RDMA version 1 connection.
3.3.3. Initial Connection State
When a connection is first established, peers might not know how many
receive resources the other has, nor how large the other peer's
inline thresholds are.
As a basis for an initial exchange of RPC requests, each RPC-over-
RDMA version 1 connection provides the ability to exchange at least
one RPC message at a time, whose RPC Call and Reply messages are no
more than 1024 bytes in size. A Responder MAY exceed this basic
level of configuration, but a Requester MUST NOT assume more than one
credit is available and MUST receive a valid reply from the Responder
carrying the actual number of available credits, prior to sending its
next request.
Receiver implementations MUST support inline thresholds of 1024 bytes
but MAY support larger inline thresholds values. An independent
mechanism for discovering a peer's inline thresholds before a
connection is established may be used to optimize the use of RDMA
Send and Receive operations. In the absence of such a mechanism,
senders and receives MUST assume the inline thresholds are 1024
bytes.
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3.4. XDR Encoding with Chunks
When a DDP capability is available, the transport places the contents
of one or more XDR data items directly into the receiver's memory,
separately from the transfer of other parts of the containing XDR
stream.
3.4.1. Reducing an XDR Stream
RPC-over-RDMA version 1 provides a mechanism for moving part of an
RPC message via a data transfer distinct from an RDMA Send/Receive
pair. The sender removes one or more XDR data items from the Payload
stream. They are conveyed via other mechanisms, such as one or more
RDMA Read or Write operations. As the receiver decodes an incoming
message, it skips over directly placed data items.
The portion of an XDR stream that is split out and moved separately
is referred to as a "chunk". In some contexts, data in an RPC-over-
RDMA header that describes these split out regions of memory may also
be referred to as a "chunk".
A Payload stream after chunks have been removed is referred to as a
"reduced" Payload stream. Likewise, a data item that has been
removed from a Payload stream to be transferred separately is
referred to as a "reduced" data item.
3.4.2. DDP-Eligibility
Not all XDR data items benefit from DDP. For example, small data
items or data items that require XDR unmarshaling by the receiver do
not benefit from DDP. In addition, it is impractical for receivers
to prepare for every possible XDR data item in a protocol to be
transferred in a chunk.
To maintain interoperability on an RPC-over-RDMA transport, a
determination must be made of which few XDR data items in each ULP
are allowed to use DDP.
This is done by additional specifications that describe how ULPs
employ DDP. A "ULB specification" identifies which specific
individual XDR data items in a ULP MAY be transferred via DDP. Such
data items are referred to as "DDP-eligible". All other XDR data
items MUST NOT be reduced.
Detailed requirements for ULBs are provided in Section 6.
Lever, et al. Standards Track [Page 14]
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3.4.3. RDMA Segments
When encoding a Payload stream that contains a DDP-eligible data
item, a sender may choose to reduce that data item. When it chooses
to do so, the sender does not place the item into the Payload stream.
Instead, the sender records in the RPC-over-RDMA header the location
and size of the memory region containing that data item.
The Requester provides location information for DDP-eligible data
items in both RPC Call and Reply messages. The Responder uses this
information to retrieve arguments contained in the specified region
of the Requester's memory or place results in that memory region.
An "RDMA segment", or "plain segment", is an RPC-over-RDMA Transport
header data object that contains the precise coordinates of a
contiguous memory region that is to be conveyed separately from the
Payload stream. Plain segments contain the following information:
Handle
Steering tag (STag) or R_key generated by registering this memory
with the RDMA provider.
Length
The length of the RDMA segment's memory region, in octets. An
"empty segment" is an RDMA segment with the value zero (0) in its
length field.
Offset
The offset or beginning memory address of the RDMA segment's
memory region.
See [RFC5040] for further discussion.
3.4.4. Chunks
In RPC-over-RDMA version 1, a "chunk" refers to a portion of the
Payload stream that is moved independently of the RPC-over-RDMA
Transport header and Payload stream. Chunk data is removed from the
sender's Payload stream, transferred via separate operations, and
then reinserted into the receiver's Payload stream to form a complete
RPC message.
Each chunk is comprised of RDMA segments. Each RDMA segment
represents a single contiguous piece of that chunk. A Requester MAY
divide a chunk into RDMA segments using any boundaries that are
convenient. The length of a chunk is the sum of the lengths of the
RDMA segments that comprise it.
Lever, et al. Standards Track [Page 15]
RFC 8166 RPC-over-RDMA Version 1 June 2017
The RPC-over-RDMA version 1 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 (for example, NFS has "rsize" and
"wsize", which 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.
3.4.4.1. Counted Arrays
If a chunk contains a counted array data type, the count of array
elements MUST remain in the Payload stream, while the array elements
MUST be moved to the chunk. For example, when encoding an opaque
byte array as a chunk, the count of bytes stays in the Payload
stream, while the bytes in the array are removed from the Payload
stream and transferred within 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.
3.4.4.2. Optional-Data
If a chunk contains an optional-data data type, the "is present"
field MUST remain in the Payload stream, while the data, if present,
MUST be moved to the chunk.
3.4.4.3. XDR Unions
A union data type MUST NOT be made DDP-eligible, but one or more of
its arms MAY be DDP-eligible, subject to the other requirements in
this section.
3.4.4.4. Chunk Roundup
Except in special cases (covered in Section 3.5.3), a chunk MUST
contain exactly one XDR data item. This makes it straightforward to
reduce variable-length data items without affecting the XDR alignment
of data items in the Payload stream.
When a variable-length XDR data item is reduced, the sender MUST
remove XDR roundup padding for that data item from the Payload stream
so that data items remaining in the Payload stream begin on four-byte
alignment.
Lever, et al. Standards Track [Page 16]
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3.4.5. Read Chunks
A "Read chunk" represents an XDR data item that is to be pulled from
the Requester to the Responder.
A Read chunk is a list of one or more RDMA read segments. An RDMA
read segment consists of a Position field followed by a plain
segment. See Section 4.1.2 for details.
Position
The byte offset in the unreduced Payload stream where the receiver
reinserts the data item conveyed in a chunk. The Position value
MUST be computed from the beginning of the unreduced Payload
stream, which begins at Position zero. All RDMA read segments
belonging to the same Read chunk have the same value in their
Position field.
While constructing an RPC Call message, a Requester registers memory
regions that contain data to be transferred via RDMA Read operations.
It advertises the coordinates of these regions in the RPC-over-RDMA
Transport header of the RPC Call message.
After receiving an RPC Call message sent via an RDMA Send operation,
a Responder transfers the chunk data from the Requester using RDMA
Read operations. The Responder reconstructs the transferred chunk
data by concatenating the contents of each RDMA segment, in list
order, into the received Payload stream at the Position value
recorded in that RDMA segment.
Put another way, the Responder inserts the first RDMA segment in a
Read chunk into the Payload stream at the byte offset indicated by
its Position field. RDMA segments whose Position field value match
this offset are concatenated afterwards, until there are no more RDMA
segments at that Position value.
The Position field in a read segment indicates where the containing
Read chunk starts in the Payload stream. The value in this field
MUST be a multiple of four. All segments in the same Read chunk
share the same Position value, even if one or more of the RDMA
segments have a non-four-byte-aligned length.
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3.4.5.1. Decoding Read Chunks
While decoding a received Payload stream, whenever the XDR offset in
the Payload stream matches that of a Read chunk, the Responder
initiates an RDMA Read to pull the chunk's data content into
registered local memory.
The Responder acknowledges its completion of use of Read chunk source
buffers when it sends an RPC Reply message to the Requester. The
Requester may then release Read chunks advertised in the request.
3.4.5.2. Read Chunk Roundup
When reducing a variable-length argument data item, the Requester
SHOULD NOT include the data item's XDR roundup padding in the chunk.
The length of a Read chunk is determined as follows:
o If the Requester chooses to include roundup padding in a Read
chunk, the chunk's total length MUST be the sum of the encoded
length of the data item and the length of the roundup padding.
The length of the data item that was encoded into the Payload
stream remains unchanged.
The sender can increase the length of the chunk by adding another
RDMA segment containing only the roundup padding, or it can do so
by extending the final RDMA segment in the chunk.
o If the sender chooses not to include roundup padding in the chunk,
the chunk's total length MUST be the same as the encoded length of
the data item.
3.4.6. Write Chunks
While constructing an RPC Call message, a Requester prepares memory
regions in which to receive DDP-eligible result data items. A "Write
chunk" represents an XDR data item that is to be pushed from a
Responder to a Requester. It is made up of an array of zero or more
plain segments.
Write chunks are provisioned by a Requester long before the Responder
has prepared the reply Payload stream. A Requester often does not
know the actual length of the result data items to be returned, since
the result does not yet exist. Thus, it MUST register Write chunks
long enough to accommodate the maximum possible size of each returned
data item.
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RFC 8166 RPC-over-RDMA Version 1 June 2017
In addition, the XDR position of DDP-eligible data items in the
reply's Payload stream is not predictable when a Requester constructs
an RPC Call message. Therefore, RDMA segments in a Write chunk do
not have a Position field.
For each Write chunk provided by a Requester, the Responder pushes
one data item to the Requester, filling the chunk contiguously and in
segment array order until that data item has been completely written
to the Requester. 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 that
is being returned in that segment. The Responder then sends the RPC
Reply message via an RDMA Send operation.
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.
3.4.6.1. Decoding Write Chunks
After receiving the RPC Reply message, the Requester reconstructs the
transferred data by concatenating the contents of each segment, in
array order, into the RPC Reply message's XDR stream at the known XDR
position of the associated DDP-eligible result data item.
3.4.6.2. Write Chunk Roundup
When provisioning a Write chunk for a variable-length result data
item, the Requester SHOULD NOT include additional space for XDR
roundup padding. A Responder MUST NOT write XDR roundup padding into
a Write chunk, even if the Requester made space available for it.
Therefore, when returning a single variable-length result data item,
a returned Write chunk's total length MUST be the same as the encoded
length of the result data item.
3.5. Message Size
A receiver of RDMA Send operations is required by RDMA to have
previously posted one or more adequately sized buffers. Memory
savings are achieved on both Requesters and Responders by posting
small Receive buffers. However, not all RPC messages are small.
RPC-over-RDMA version 1 provides several mechanisms that allow
messages of any size to be conveyed efficiently.
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RFC 8166 RPC-over-RDMA Version 1 June 2017
3.5.1. Short Messages
RPC messages are frequently smaller than typical inline thresholds.
For example, the NFS version 3 GETATTR operation is only 56 bytes: 20
bytes of RPC header, a 32-byte file handle argument, and 4 bytes for
its length. The reply to this common request is about 100 bytes.
Since all RPC messages conveyed via RPC-over-RDMA require an RDMA
Send operation, 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. An RPC-over-RDMA header with a
small RPC Call or Reply message immediately following is transferred
using a single RDMA Send operation. No other operations are needed.
An RPC-over-RDMA transaction using Short Messages:
Requester Responder
| RDMA Send (RDMA_MSG) |
Call | ------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA_MSG) |
| <------------------------------ | Reply
3.5.2. Chunked Messages
If DDP-eligible data items are present in a Payload stream, a sender
MAY reduce some or all of these items by removing them from the
Payload stream. The sender uses a separate mechanism to transfer the
reduced data items. The Transport stream with the reduced Payload
stream immediately following is then transferred using a single RDMA
Send operation.
After receiving the Transport and Payload streams of an RPC Call
message accompanied by Read chunks, the Responder uses RDMA Read
operations to move reduced data items in Read chunks. Before sending
the Transport and Payload streams of an RPC Reply message containing
Write chunks, the Responder uses RDMA Write operations to move
reduced data items in Write and Reply chunks.
Lever, et al. Standards Track [Page 20]
RFC 8166 RPC-over-RDMA Version 1 June 2017
An RPC-over-RDMA transaction with a Read chunk:
Requester Responder
| RDMA Send (RDMA_MSG) |
Call | ------------------------------> |
| RDMA Read |
| <------------------------------ |
| RDMA Response (arg data) |
| ------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA_MSG) |
| <------------------------------ | Reply
An RPC-over-RDMA transaction with a Write chunk:
Requester Responder
| RDMA Send (RDMA_MSG) |
Call | ------------------------------> |
| |
| | Processing
| |
| RDMA Write (result data) |
| <------------------------------ |
| RDMA Send (RDMA_MSG) |
| <------------------------------ | Reply
3.5.3. Long Messages
When a Payload stream is larger than the receiver's inline threshold,
the Payload stream is reduced by removing DDP-eligible data items and
placing them in chunks to be moved separately. If there are no DDP-
eligible data items in the Payload stream, or the Payload stream is
still too large after it has been reduced, the RDMA transport MUST
use RDMA Read or Write operations to convey the Payload stream
itself. This mechanism is referred to as a "Long Message".
To transmit a Long Message, the sender conveys only the Transport
stream with an RDMA Send operation. The Payload stream is not
included in the Send buffer in this instance. Instead, the Requester
provides chunks that the Responder uses to move the Payload stream.
Long Call
To send a Long Call message, the Requester provides a special Read
chunk that contains the RPC Call message's Payload stream. Every
RDMA read segment in this chunk MUST contain zero in its Position
field. Thus, this chunk is known as a "Position Zero Read chunk".
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RFC 8166 RPC-over-RDMA Version 1 June 2017
Long Reply
To send a Long Reply, the Requester provides a single special
Write chunk in advance, known as the "Reply chunk", that will
contain the RPC Reply message's Payload stream. The Requester
sizes the Reply chunk to accommodate the maximum expected reply
size for that upper-layer operation.
Though the purpose of a Long Message is to handle large RPC messages,
Requesters MAY use a Long Message at any time to convey an RPC Call
message.
A Responder chooses which form of reply to use based on the chunks
provided by the Requester. If Write chunks were provided and the
Responder has a DDP-eligible result, it first reduces the reply
Payload stream. If a Reply chunk was provided and the reduced
Payload stream is larger than the reply inline threshold, the
Responder MUST use the Requester-provided Reply chunk for the reply.
XDR data items may appear in these special chunks without regard to
their DDP-eligibility. As these chunks contain a Payload stream,
such chunks MUST include appropriate XDR roundup padding to maintain
proper XDR alignment of their contents.
An RPC-over-RDMA transaction using a Long Call:
Requester Responder
| RDMA Send (RDMA_NOMSG) |
Call | ------------------------------> |
| RDMA Read |
| <------------------------------ |
| RDMA Response (RPC call) |
| ------------------------------> |
| |
| | Processing
| |
| RDMA Send (RDMA_MSG) |
| <------------------------------ | Reply
Lever, et al. Standards Track [Page 22]
RFC 8166 RPC-over-RDMA Version 1 June 2017
An RPC-over-RDMA transaction using a Long Reply:
Requester Responder
| RDMA Send (RDMA_MSG) |
Call | ------------------------------> |
| |
| | Processing
| |
| RDMA Write (RPC reply) |
| <------------------------------ |
| RDMA Send (RDMA_NOMSG) |
| <------------------------------ | Reply
4. RPC-over-RDMA in Operation
Every RPC-over-RDMA version 1 message has a header that includes a
copy of the message's transaction ID, data for managing RDMA flow-
control credits, and lists of RDMA segments describing chunks. All
RPC-over-RDMA header content is contained in the Transport stream;
thus, it MUST be XDR encoded.
RPC message layout is unchanged from that described in [RFC5531]
except for the possible reduction of data items that are moved by
separate operations.
The RPC-over-RDMA protocol passes RPC messages without regard to
their type (CALL or REPLY). Apart from restrictions imposed by ULBs,
each endpoint of a connection MAY send RDMA_MSG or RDMA_NOMSG message
header types at any time (subject to credit limits).
4.1. XDR Protocol Definition
This section contains a description of the core features of the RPC-
over-RDMA version 1 protocol, expressed in the XDR language
[RFC4506].
This description is provided in a way that makes it simple to extract
into ready-to-compile form. The reader can apply the following shell
script to this document to produce a machine-readable XDR description
of the RPC-over-RDMA version 1 protocol.
<CODE BEGINS>
#!/bin/sh
grep '^ *///' | sed 's?^ /// ??' | sed 's?^ *///$??'
<CODE ENDS>
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RFC 8166 RPC-over-RDMA Version 1 June 2017
That is, if the above script is stored in a file called "extract.sh"
and this document is in a file called "spec.txt", then the reader can
do the following to extract an XDR description file:
<CODE BEGINS>
sh extract.sh < spec.txt > rpcrdma_corev1.x
<CODE ENDS>
4.1.1. Code Component License
Code components extracted from this document must include the
following license text. When the extracted XDR code is combined with
other complementary XDR code, which itself has an identical license,
only a single copy of the license text need be preserved.
Lever, et al. Standards Track [Page 24]
RFC 8166 RPC-over-RDMA Version 1 June 2017
<CODE BEGINS>
/// /*
/// * Copyright (c) 2010-2017 IETF Trust and the persons
/// * identified as authors of the code. All rights reserved.
/// *
/// * The authors of the code are:
/// * B. Callaghan, T. Talpey, and C. Lever
/// *
/// * 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>
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RFC 8166 RPC-over-RDMA Version 1 June 2017
4.1.2. RPC-over-RDMA Version 1 XDR
XDR data items defined in this section encodes the Transport Header
Stream in each RPC-over-RDMA version 1 message. Comments identify
items that cannot be changed in subsequent versions.
<CODE BEGINS>
/// /*
/// * Plain RDMA segment (Section 3.4.3)
/// */
/// struct xdr_rdma_segment {
/// uint32 handle; /* Registered memory handle */
/// uint32 length; /* Length of the chunk in bytes */
/// uint64 offset; /* Chunk virtual address or offset */
/// };
///
/// /*
/// * RDMA read segment (Section 3.4.5)
/// */
/// struct xdr_read_chunk {
/// uint32 position; /* Position in XDR stream */
/// struct xdr_rdma_segment target;
/// };
///
/// /*
/// * Read list (Section 4.3.1)
/// */
/// struct xdr_read_list {
/// struct xdr_read_chunk entry;
/// struct xdr_read_list *next;
/// };
///
/// /*
/// * Write chunk (Section 3.4.6)
/// */
/// struct xdr_write_chunk {
/// struct xdr_rdma_segment target<>;
/// };
///
/// /*
/// * Write list (Section 4.3.2)
/// */
/// struct xdr_write_list {
/// struct xdr_write_chunk entry;
/// struct xdr_write_list *next;
/// };
///
Lever, et al. Standards Track [Page 26]
RFC 8166 RPC-over-RDMA Version 1 June 2017
/// /*
/// * Chunk lists (Section 4.3)
/// */
/// struct rpc_rdma_header {
/// struct xdr_read_list *rdma_reads;
/// struct xdr_write_list *rdma_writes;
/// struct xdr_write_chunk *rdma_reply;
/// /* rpc body follows */
/// };
///
/// struct rpc_rdma_header_nomsg {
/// struct xdr_read_list *rdma_reads;
/// struct xdr_write_list *rdma_writes;
/// struct xdr_write_chunk *rdma_reply;
/// };
///
/// /* Not to be used */
/// struct rpc_rdma_header_padded {
/// uint32 rdma_align;
/// uint32 rdma_thresh;
/// struct xdr_read_list *rdma_reads;
/// struct xdr_write_list *rdma_writes;
/// struct xdr_write_chunk *rdma_reply;
/// /* rpc body follows */
/// };
///
/// /*
/// * Error handling (Section 4.5)
/// */
/// enum rpc_rdma_errcode {
/// ERR_VERS = 1, /* Value fixed for all versions */
/// ERR_CHUNK = 2
/// };
///
/// /* Structure fixed for all versions */
/// struct rpc_rdma_errvers {
/// uint32 rdma_vers_low;
/// uint32 rdma_vers_high;
/// };
///
/// union rpc_rdma_error switch (rpc_rdma_errcode err) {
/// case ERR_VERS:
/// rpc_rdma_errvers range;
/// case ERR_CHUNK:
/// void;
/// };
///
/// /*
Lever, et al. Standards Track [Page 27]
RFC 8166 RPC-over-RDMA Version 1 June 2017
/// * Procedures (Section 4.2.4)
/// */
/// enum rdma_proc {
/// RDMA_MSG = 0, /* Value fixed for all versions */
/// RDMA_NOMSG = 1, /* Value fixed for all versions */
/// RDMA_MSGP = 2, /* Not to be used */
/// RDMA_DONE = 3, /* Not to be used */
/// RDMA_ERROR = 4 /* Value fixed for all versions */
/// };
///
/// /* The position of the proc discriminator field is
/// * fixed for all versions */
/// union rdma_body switch (rdma_proc proc) {
/// case RDMA_MSG:
/// rpc_rdma_header rdma_msg;
/// case RDMA_NOMSG:
/// rpc_rdma_header_nomsg rdma_nomsg;
/// case RDMA_MSGP: /* Not to be used */
/// rpc_rdma_header_padded rdma_msgp;
/// case RDMA_DONE: /* Not to be used */
/// void;
/// case RDMA_ERROR:
/// rpc_rdma_error rdma_error;
/// };
///
/// /*
/// * Fixed header fields (Section 4.2)
/// */
/// struct rdma_msg {
/// uint32 rdma_xid; /* Position fixed for all versions */
/// uint32 rdma_vers; /* Position fixed for all versions */
/// uint32 rdma_credit; /* Position fixed for all versions */
/// rdma_body rdma_body;
/// };
<CODE ENDS>
4.2. Fixed Header Fields
The RPC-over-RDMA header begins with four fixed 32-bit fields that
control the RDMA interaction.
The first three words are individual fields in the rdma_msg
structure. The fourth word is the first word of the rdma_body union,
which acts as the discriminator for the switched union. The contents
of this field are described in Section 4.2.4.
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RFC 8166 RPC-over-RDMA Version 1 June 2017
These four fields must remain with the same meanings and in the same
positions in all subsequent versions of the RPC-over-RDMA protocol.
4.2.1. Transaction ID (XID)
The XID generated for the RPC Call and Reply messages. Having the
XID at a fixed location in the header makes it easy for the receiver
to establish context as soon as each RPC-over-RDMA message arrives.
This XID MUST be the same as the XID in the RPC message. The
receiver MAY perform its processing based solely on the XID in the
RPC-over-RDMA header, and thereby ignore the XID in the RPC message,
if it so chooses.
4.2.2. Version Number
For RPC-over-RDMA version 1, this field MUST contain the value one
(1). Rules regarding changes to this transport protocol version
number can be found in Section 7.
4.2.3. Credit Value
When sent with an RPC Call message, the requested credit value is
provided. When sent with an RPC Reply message, the granted credit
value is returned. Further discussion of how the credit value is
determined can be found in Section 3.3.
4.2.4. Procedure Number
RDMA_MSG = 0 indicates that chunk lists and a Payload stream
follow. The format of the chunk lists is
discussed below.
RDMA_NOMSG = 1 indicates that after the chunk lists there is no
Payload stream. In this case, the chunk lists
provide information to allow the Responder to
transfer the Payload stream using explicit RDMA
operations.
RDMA_MSGP = 2 is reserved.
RDMA_DONE = 3 is reserved.
RDMA_ERROR = 4 is used to signal an encoding error in the RPC-
over-RDMA header.
An RDMA_MSG procedure conveys the Transport stream and the Payload
stream via an RDMA Send operation. The Transport stream contains the
four fixed fields followed by the Read and Write lists and the Reply
Lever, et al. Standards Track [Page 29]
RFC 8166 RPC-over-RDMA Version 1 June 2017
chunk, though any or all three MAY be marked as not present. The
Payload stream then follows, beginning with its XID field. If a Read
or Write chunk list is present, a portion of the Payload stream has
been reduced and is conveyed via separate operations.
An RDMA_NOMSG procedure conveys the Transport stream via an RDMA Send
operation. The Transport stream contains the four fixed fields
followed by the Read and Write chunk lists and the Reply chunk.
Though any of these MAY be marked as not present, one MUST be present
and MUST hold the Payload stream for this RPC-over-RDMA message. If
a Read or Write chunk list is present, a portion of the Payload
stream has been excised and is conveyed via separate operations.
An RDMA_ERROR procedure conveys the Transport stream via an RDMA Send
operation. The Transport stream contains the four fixed fields
followed by formatted error information. No Payload stream is
conveyed in this type of RPC-over-RDMA message.
A Requester MUST NOT send an RPC-over-RDMA header with the RDMA_ERROR
procedure. A Responder MUST silently discard RDMA_ERROR procedures.
The Transport stream and Payload stream can be constructed in
separate buffers. However, the total length of the gathered buffers
cannot exceed the inline threshold.
4.3. Chunk Lists
The chunk lists in an RPC-over-RDMA version 1 header are three XDR
optional-data fields that follow the fixed header fields in RDMA_MSG
and RDMA_NOMSG procedures. Read Section 4.19 of [RFC4506] carefully
to understand how optional-data fields work. Examples of XDR-encoded
chunk lists are provided in Section 4.7 as an aid to understanding.
Often, an RPC-over-RDMA message has no associated chunks. In this
case, the Read list, Write list, and Reply chunk are all marked "not
present".
4.3.1. Read List
Each RDMA_MSG or RDMA_NOMSG procedure has one "Read list". The Read
list is a list of zero or more RDMA read segments, provided by the
Requester, that are grouped by their Position fields into Read
chunks. Each Read chunk advertises the location of argument data the
Responder is to pull from the Requester. The Requester has reduced
the data items in these chunks from the call's Payload stream.
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RFC 8166 RPC-over-RDMA Version 1 June 2017
A Requester may transmit the Payload stream of an RPC Call message
using a Position Zero Read chunk. If the RPC Call message has no
argument data that is DDP-eligible and the Position Zero Read chunk
is not being used, the Requester leaves the Read list empty.
Responders MUST leave the Read list empty in all replies.
4.3.1.1. Matching Read Chunks to Arguments
When reducing a DDP-eligible argument data item, a Requester records
the XDR stream offset of that data item in the Read chunk's Position
field. The Responder can then tell unambiguously where that chunk is
to be reinserted into the received Payload stream to form a complete
RPC Call message.
4.3.2. Write List
Each RDMA_MSG or RDMA_NOMSG procedure has one "Write list". The
Write list is a list of zero or more Write chunks, provided by the
Requester. Each Write chunk is an array of plain segments; thus, the
Write list is a list of counted arrays.
If an RPC Reply message has no possible DDP-eligible result data
items, the Requester leaves the Write list empty. When a Requester
provides a Write list, the Responder MUST push data corresponding to
DDP-eligible result data items to Requester memory referenced in the
Write list. The Responder removes these data items from the reply's
Payload stream.
4.3.2.1. Matching Write Chunks to Results
A Requester constructs the Write list for an RPC transaction before
the Responder has formulated its reply. When there is only one DDP-
eligible result data item, the Requester inserts only a single Write
chunk in the Write list. If the returned Write chunk is not an
unused Write chunk, the Requester knows with certainty which result
data item is contained in it.
When a Requester has provided multiple Write chunks, the Responder
fills in each Write chunk with one DDP-eligible result until there
are either no more DDP-eligible results or no more Write chunks.
The Requester might not be able to predict in advance which DDP-
eligible data item goes in which chunk. Thus, the Requester is
responsible for allocating and registering Write chunks large enough
to accommodate the largest result data item that might be associated
with each chunk in the Write list.
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As a Requester decodes a reply Payload stream, it is clear from the
contents of the RPC Reply message which Write chunk contains which
result data item.
4.3.2.2. Unused Write Chunks
There are occasions when a Requester provides a non-empty Write chunk
but the Responder is not able to use it. For example, a ULP may
define a union result where some arms of the union contain a DDP-
eligible data item while other arms do not. The Responder is
required to use Requester-provided Write chunks in this case, but if
the Responder returns a result that uses an arm of the union that has
no DDP-eligible data item, that Write chunk remains unconsumed.
If there is a subsequent DDP-eligible result data item in the RPC
Reply message, it MUST be placed in that unconsumed Write chunk.
Therefore, the Requester MUST provision each Write chunk so it can be
filled with the largest DDP-eligible data item that can be placed in
it.
If this is the last or only Write chunk available and it remains
unconsumed, the Responder MUST return this Write chunk as an unused
Write chunk (see Section 3.4.6). The Responder sets the segment
count to a value matching the Requester-provided Write chunk, but
returns only empty segments in that Write chunk.
Unused Write chunks, or unused bytes in Write chunk segments, are
returned to the RPC consumer as part of RPC completion. Even if a
Responder indicates that a Write chunk is not consumed, the Responder
may have written data into one or more segments before choosing not
to return that data item. The Requester MUST NOT assume that the
memory regions backing a Write chunk have not been modified.
4.3.2.3. Empty Write Chunks
To force a Responder to return a DDP-eligible result inline, a
Requester employs the following mechanism:
o When there is only one DDP-eligible result item in an RPC Reply
message, the Requester provides an empty Write list.
o When there are multiple DDP-eligible result data items and a
Requester prefers that a data item is returned inline, the
Requester provides an empty Write chunk for that item (see
Section 3.4.6). The Responder MUST return the corresponding
result data item inline and MUST return an empty Write chunk in
that Write list position in the RPC Reply message.
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As always, a Requester and Responder must prepare for a Long Reply to
be used if the resulting RPC Reply might be too large to be conveyed
in an RDMA Send.
4.3.3. Reply Chunk
Each RDMA_MSG or RDMA_NOMSG procedure has one "Reply chunk" slot. A
Requester MUST provide a Reply chunk whenever the maximum possible
size of the RPC Reply message's Transport and Payload streams is
larger than the inline threshold for messages from Responder to
Requester. Otherwise, the Requester marks the Reply chunk as not
present.
If the Transport stream and Payload stream together are smaller than
the reply inline threshold, the Responder MAY return the RPC Reply
message as a Short message rather than using the Requester-provided
Reply chunk.
When a Requester provides a Reply chunk in an RPC Call message, the
Responder MUST copy that chunk into the Transport header of the RPC
Reply message. As with Write chunks, the Responder modifies the
copied Reply chunk in the RPC Reply message to reflect the actual
amount of data that is being returned in the Reply chunk.
4.4. Memory Registration
The cost of registering and invalidating memory can be a significant
proportion of the cost of an RPC-over-RDMA transaction. Thus, an
important implementation consideration is how to minimize
registration activity without exposing system memory needlessly.
4.4.1. Registration Longevity
Data transferred via RDMA Read and Write can reside in a memory
allocation not in the control of the RPC-over-RDMA transport. These
memory allocations can persist outside the bounds of an RPC
transaction. They are registered and invalidated as needed, as part
of each RPC transaction.
The Requester endpoint must ensure that memory regions associated
with each RPC transaction are protected from Responder access before
allowing upper-layer access to the data contained in them. Moreover,
the Requester must not access these memory regions while the
Responder has access to them.
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This includes memory regions that are associated with canceled RPCs.
A Responder cannot know that the Requester is no longer waiting for a
reply, and it might proceed to read or even update memory that the
Requester might have released for other use.
4.4.2. Communicating DDP-Eligibility
The interface by which a ULP implementation communicates the
eligibility of a data item locally to its local RPC-over-RDMA
endpoint is not described by this specification.
Depending on the implementation and constraints imposed by ULBs, it
is possible to implement reduction transparently to upper layers.
Such implementations may lead to inefficiencies, either because they
require the RPC layer to perform expensive registration and
invalidation of memory "on the fly", or they may require using RDMA
chunks in RPC Reply messages, along with the resulting additional
handshaking with the RPC-over-RDMA peer.
However, these issues are internal and generally confined to the
local interface between RPC and its upper layers, one in which
implementations are free to innovate. The only requirement, beyond
constraints imposed by the ULB, is that the resulting RPC-over-RDMA
protocol sent to the peer be valid for the upper layer.
4.4.3. Registration Strategies
The choice of which memory registration strategies to employ is left
to Requester and Responder implementers. To support the widest array
of RDMA implementations, as well as the most general steering tag
scheme, an Offset field is included in each RDMA segment.
While zero-based offset schemes are available in many RDMA
implementations, their use by RPC requires individual registration of
each memory region. For such implementations, this can be a
significant overhead. By providing an offset in each chunk, many
pre-registration or region-based registrations can be readily
supported.
4.5. Error Handling
A receiver performs basic validity checks on the RPC-over-RDMA header
and chunk contents before it passes the RPC message to the RPC layer.
If an incoming RPC-over-RDMA message is not as long as a minimal size
RPC-over-RDMA header (28 bytes), the receiver cannot trust the value
of the XID field; therefore, it MUST silently discard the message
before performing any parsing. If other errors are detected in the
RPC-over-RDMA header of an RPC Call message, a Responder MUST send an
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RDMA_ERROR message back to the Requester. If errors are detected in
the RPC-over-RDMA header of an RPC Reply message, a Requester MUST
silently discard the message.
To form an RDMA_ERROR procedure:
o The rdma_xid field MUST contain the same XID that was in the
rdma_xid field in the failing request;
o The rdma_vers field MUST contain the same version that was in the
rdma_vers field in the failing request;
o The rdma_proc field MUST contain the value RDMA_ERROR; and
o The rdma_err field contains a value that reflects the type of
error that occurred, as described below.
An RDMA_ERROR procedure indicates a permanent error. Receipt of this
procedure completes the RPC transaction associated with XID in the
rdma_xid field. A receiver MUST silently discard an RDMA_ERROR
procedure that it cannot decode.
4.5.1. Header Version Mismatch
When a Responder detects an RPC-over-RDMA header version that it does
not support (currently this document defines only version 1), it MUST
reply with an RDMA_ERROR procedure and set the rdma_err value to
ERR_VERS, also providing the low and high inclusive version numbers
it does, in fact, support.
4.5.2. XDR Errors
A receiver might encounter an XDR parsing error that prevents it from
processing the incoming Transport stream. Examples of such errors
include an invalid value in the rdma_proc field; an RDMA_NOMSG
message where the Read list, Write list, and Reply chunk are marked
not present; or the value of the rdma_xid field does not match the
value of the XID field in the accompanying RPC message. If the
rdma_vers field contains a recognized value, but an XDR parsing error
occurs, the Responder MUST reply with an RDMA_ERROR procedure and set
the rdma_err value to ERR_CHUNK.
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 message, the Responder SHOULD return an RPC Reply message
with status GARBAGE_ARGS, using an RDMA_MSG procedure. This type of
parsing failure might be due to mismatches between chunk sizes or
offsets and the contents of the Payload stream, for example.
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4.5.3. Responder RDMA Operational Errors
In RPC-over-RDMA version 1, 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:
o 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.
o If a Requester's Receive buffer is too small, the Responder's Send
operation completes with a Local Length Error.
o 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 are typically 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 RDMA_ERROR reply with an
rdma_err value of ERR_CHUNK to indicate that no RPC-level reply is
possible for that XID.
4.5.4. 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 message, the associated RPC transaction fails
immediately.
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.
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4.5.5. RDMA Transport Errors
The RDMA connection and physical link provide some degree of error
detection and retransmission. iWARP's Marker PDU Aligned (MPA) layer
(when used over TCP), the Stream Control Transmission Protocol
(SCTP), as well as the InfiniBand [IBARCH] link layer all provide
Cyclic Redundancy Check (CRC) protection of the RDMA payload, and
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 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 8 for further discussion of the use of RPC-level integrity
schemes to detect errors.
4.6. Protocol Elements No Longer Supported
The following protocol elements are no longer supported in RPC-over-
RDMA version 1. Related enum values and structure definitions remain
in the RPC-over-RDMA version 1 protocol for backwards compatibility.
4.6.1. RDMA_MSGP
The specification of RDMA_MSGP in Section 3.9 of [RFC5666] is
incomplete. To fully specify RDMA_MSGP would require:
o Updating the definition of DDP-eligibility to include data items
that may be transferred, with padding, via RDMA_MSGP procedures
o Adding full operational descriptions of the alignment and
threshold fields
o Discussing how alignment preferences are communicated between two
peers without using CCP
o Describing the treatment of RDMA_MSGP procedures that convey Read
or Write chunks
The RDMA_MSGP message type is beneficial only when the padded data
payload is at the end of an RPC message's argument or result list.
This is not typical for NFSv4 COMPOUND RPCs, which often include a
GETATTR operation as the final element of the compound operation
array.
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Without a full specification of RDMA_MSGP, there has been no fully
implemented prototype of it. Without a complete prototype of
RDMA_MSGP support, it is difficult to assess whether this protocol
element has benefit or can even be made to work interoperably.
Therefore, senders MUST NOT send RDMA_MSGP procedures. When
receiving an RDMA_MSGP procedure, Responders SHOULD reply with an
RDMA_ERROR procedure, setting the rdma_err field to ERR_CHUNK;
Requesters MUST silently discard the message.
4.6.2. RDMA_DONE
Because no implementation of RPC-over-RDMA version 1 uses the Read-
Read transfer model, there is never a need to send an RDMA_DONE
procedure.
Therefore, senders MUST NOT send RDMA_DONE messages. Receivers MUST
silently discard RDMA_DONE messages.
4.7. XDR Examples
RPC-over-RDMA chunk lists are complex data types. In this section,
illustrations are provided to help readers grasp how chunk lists are
represented inside an RPC-over-RDMA header.
A plain segment is the simplest component, being made up of a 32-bit
handle (H), a 32-bit length (L), and 64 bits of offset (OO). Once
flattened into an XDR stream, plain segments appear as
HLOO
An RDMA read segment has an additional 32-bit position field (P).
RDMA read segments appear as
PHLOO
A Read chunk is a list of RDMA read segments. Each RDMA read segment
is preceded by a 32-bit word containing a one if a segment follows or
a zero if there are no more segments in the list. In XDR form, this
would look like
1 PHLOO 1 PHLOO 1 PHLOO 0
where P would hold the same value for each RDMA read segment
belonging to the same Read chunk.
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The Read list is also a list of RDMA read segments. In XDR form,
this would look like a Read chunk, except that the P values could
vary across the list. An empty Read list is encoded as a single
32-bit zero.
One Write chunk is a counted array of plain segments. In XDR form,
the count would appear as the first 32-bit word, followed by an HLOO
for each element of the array. For instance, a Write chunk with
three elements would look like
3 HLOO HLOO HLOO
The Write list is a list of counted arrays. In XDR form, this is a
combination of optional-data and counted arrays. To represent a
Write list containing a Write chunk with three segments and a Write
chunk with two segments, XDR would encode
1 3 HLOO HLOO HLOO 1 2 HLOO HLOO 0
An empty Write list is encoded as a single 32-bit zero.
The Reply chunk is a Write chunk. However, since it is an optional-
data field, there is a 32-bit field in front of it that contains a
one if the Reply chunk is present or a zero if it is not. After
encoding, a Reply chunk with two segments would look like
1 2 HLOO HLOO
Frequently, a Requester does not provide any chunks. In that case,
after the four fixed fields in the RPC-over-RDMA header, there are
simply three 32-bit fields that contain zero.
5. RPC Bind Parameters
In setting up a new RDMA connection, the first action by a Requester
is to obtain a transport address for the Responder. 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 normally 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 different and distinct service address (port number) might
sometimes be required for ULP operation with RPC-over-RDMA.
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When mapped atop the iWARP transport [RFC5040] [RFC5041], which uses
IP port addressing due to its layering on TCP and/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 both iWARP/TCP and iWARP/SCTP
[RFC5667].
When mapped atop InfiniBand [IBARCH], 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 [IBARCH], 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:
o One possibility is to have the Responder register its mapped IP
port with the rpcbind service under the netid (or netids) defined
here. An RPC-over-RDMA-aware Requester can then resolve its
desired service to a mappable port and proceed to connect. This
is the most flexible and compatible approach, for those upper
layers that are defined to use the rpcbind service.
o A second possibility is to have the Responder's portmapper
register itself on the RDMA interconnect at a "well-known" service
address (on UDP or TCP, this corresponds to port 111). A
Requester could connect to this service address and use the
portmap protocol to obtain a service address in response to a
program number, e.g., an iWARP port number or an InfiniBand GID.
o Alternately, the Requester could simply 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
such an InfiniBand fabric, uses the same 20049 assignment as for
iWARP.
Historically, different RPC protocols have taken different approaches
to their port assignment. Therefore, the specific method is left to
each RPC-over-RDMA-enabled ULB and is not addressed in this document.
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In Section 9, this specification defines two new netid values, to be
used for registration of upper layers atop iWARP [RFC5040] [RFC5041]
and (when a suitable port translation service is available)
InfiniBand [IBARCH]. Additional RDMA-capable networks MAY define
their own netids, or if they provide a port translation, they MAY
share the one defined in this document.
6. ULB Specifications
An ULP is typically defined independently of any particular RPC
transport. An ULB (ULB) specification provides guidance that helps
the ULP interoperate correctly and efficiently over a particular
transport. For RPC-over-RDMA version 1, a ULB may provide:
o A taxonomy of XDR data items that are eligible for DDP
o Constraints on which upper-layer procedures may be reduced and on
how many chunks may appear in a single RPC request
o A method for determining the maximum size of the reply Payload
stream for all procedures in the ULP
o An rpcbind port assignment for operation of the RPC Program and
Version on an RPC-over-RDMA transport
Each RPC Program and Version tuple that utilizes RPC-over-RDMA
version 1 needs to have a ULB specification.
6.1. DDP-Eligibility
An ULB designates some XDR data items as eligible for DDP. As an
RPC-over-RDMA message is formed, DDP-eligible data items can be
removed from the Payload stream and placed 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:
o The XDR data item is frequently sent or received, and its size is
often much larger than typical inline thresholds.
o If the XDR data item is a result, its maximum size must be
predictable in advance by the Requester.
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o 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.
o 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.
o The XDR data item does not contain DDP-eligible data items.
In addition to defining the set of data items that are DDP-eligible,
a ULB may also 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 also limit the number of chunks that a
Requester can provide for a particular upper-layer procedure.
Senders MUST NOT reduce data items that are not DDP-eligible. Such
data items MAY, however, be moved as part of a Position Zero Read
chunk or a Reply chunk.
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:
o A Requester reduces a non-DDP-eligible argument data item. The
Responder MUST NOT process this RPC Call message and MUST report
the violation as described in Section 4.5.2.
o A Responder reduces a non-DDP-eligible result data item. The
Requester MUST terminate the pending RPC transaction and report an
appropriate permanent error to the RPC consumer.
o A Responder does not reduce a DDP-eligible result data item into
an available Write chunk. The Requester MUST terminate the
pending RPC transaction and report an appropriate permanent error
to the RPC consumer.
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6.2. Maximum Reply Size
A Requester provides resources for both an RPC Call message and its
matching RPC Reply message. A Requester forms the RPC Call message
itself; thus, the Requester can compute the exact resources needed.
A Requester must allocate resources for the RPC Reply message (an
RPC-over-RDMA credit, a Receive buffer, and possibly a Write list and
Reply chunk) before the Responder has formed the actual reply. To
accommodate all possible replies for the procedure in the RPC Call
message, a Requester must allocate reply resources based on the
maximum possible size of the expected RPC Reply message.
If there are procedures in the ULP for which there is no clear reply
size maximum, the ULB needs to specify a dependable means for
determining the maximum.
6.3. Additional Considerations
There may be other details provided in a ULB.
o An ULB may recommend inline threshold values or other transport-
related parameters for RPC-over-RDMA version 1 connections bearing
that ULP.
o An ULP may provide a means to communicate these transport-related
parameters between peers. Note that RPC-over-RDMA version 1 does
not specify any mechanism for changing any transport-related
parameter after a connection has been established.
o Multiple ULPs may share a single RPC-over-RDMA version 1
connection when their ULBs allow the use of RPC-over-RDMA version
1 and the rpcbind port assignments for the Protocols allow
connection sharing. In this case, the same transport parameters
(such as inline threshold) apply to all Protocols 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.
6.4. ULP Extensions
An RPC Program and Version tuple may be extensible. For instance,
there may be a minor versioning scheme that is not reflected in the
RPC version number, or the ULP may allow additional features to be
specified after the original RPC Program specification was ratified.
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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.
7. Protocol Extensibility
The RPC-over-RDMA header format is specified using XDR, unlike the
message header used with RPC-over-TCP. To maintain a high degree of
interoperability among implementations of RPC-over-RDMA, any change
to this XDR requires a protocol version number change. New versions
of RPC-over-RDMA may be published as separate protocol specifications
without updating this document.
The first four fields in every RPC-over-RDMA header must remain
aligned at the same fixed offsets for all versions of the RPC-over-
RDMA protocol. The version number must be in a fixed place to enable
implementations to detect protocol version mismatches.
For version mismatches to be reported in a fashion that all future
version implementations can reliably decode, the rdma_proc field must
remain in a fixed place, the value of ERR_VERS must always remain the
same, and the field placement in struct rpc_rdma_errvers must always
remain the same.
7.1. Conventional Extensions
Introducing new capabilities to RPC-over-RDMA version 1 is limited to
the adoption of conventions that make use of existing XDR (defined in
this document) and allowed abstract RDMA operations. Because no
mechanism for detecting optional features exists in RPC-over-RDMA
version 1, implementations must rely on ULPs to communicate the
existence of such extensions.
Such extensions must be specified in a Standards Track RFC with
appropriate review by the NFSv4 Working Group and the IESG. An
example of a conventional extension to RPC-over-RDMA version 1 is the
specification of backward direction message support to enable NFSv4.1
callback operations, described in [RFC8167].
8. Security Considerations
8.1. Memory Protection
A primary consideration is the protection of the integrity and
confidentiality of local 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 to memory owned by user
processes.
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It is REQUIRED that any RDMA provider used for RPC transport be
conformant to the requirements of [RFC5042] in order to satisfy these
protections. These protections are provided by the RDMA layer
specifications, and in particular, their security models.
8.1.1. Protection Domains
The use of Protection Domains 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 Connection behavior,
strong authentication of remote endpoints is recommended.
8.1.2. Handle Predictability
Unpredictable memory handles should be used for any operation
requiring advertised memory regions. Advertising 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 under way.
Therefore, implementations should avoid advertising persistently
registered memory.
8.1.3. Memory Protection
Requesters should register memory regions for remote access only when
they are about to be the target of an RPC operation that involves an
RDMA Read or Write.
Registered memory regions should be invalidated as soon as related
RPC operations are complete. Invalidation and DMA unmapping of
memory regions should be complete before message integrity checking
is done and before the RPC consumer is allowed to continue execution
and use or alter the contents of a memory region.
An RPC transaction on a Requester might be terminated before a reply
arrives if the RPC consumer exits unexpectedly (for example, it is
signaled or a segmentation fault occurs). When an RPC terminates
abnormally, memory regions associated with that RPC should be
invalidated appropriately before the regions are released to be
reused for other purposes on the Requester.
8.1.4. Denial of Service
A detailed discussion of denial-of-service exposures that can result
from the use of an RDMA transport is found in Section 6.4 of
[RFC5042].
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A Responder is not obliged to pull Read chunks that are unreasonably
large. The Responder can use an RDMA_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 Network Interface Cards (RNICs) typically terminate the
connection. For further discussion, see Section 4.5. Such repeated
chunk errors 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 at or under the number of credits granted
per connection. This is explained in Section 3.3.1. A sender can
trigger a self denial of service by exceeding the credit grant
repeatedly.
When an RPC has been canceled due to a signal or premature exit of an
application process, a Requester may 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 memory has been reused.
On the Requester, a malfunctioning application or a malicious user
can create a situation where RPCs are continuously initiated and then
aborted, 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.
8.2. 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 the layer above RPC-over-RDMA. The latter
two services require significant computation and movement of data on
each endpoint host. Some performance benefits enabled by RDMA
transports can be lost.
8.2.1. RPC-over-RDMA Protection at Lower Layers
For any RPC transport, utilizing RPCSEC_GSS integrity or privacy
services has performance implications. Protection below the RPC
transport is often more appropriate in performance-sensitive
deployments, especially if it, too, can be offloaded. Certain
configurations of IPsec can be co-located in RDMA hardware, for
example, without change to RDMA consumers and little loss of data
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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.
The use of protection in a lower layer MAY be negotiated 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]. Use of such mechanisms is
REQUIRED where integrity or confidentiality is desired and where
efficiency is required.
8.2.2. RPCSEC_GSS on RPC-over-RDMA Transports
Not all RDMA devices and fabrics support the above protection
mechanisms. Also, per-message authentication is still required on
NFS clients where multiple users access NFS files. In these cases,
RPCSEC_GSS can protect NFS traffic conveyed on RPC-over-RDMA
connections.
RPCSEC_GSS extends the ONC RPC protocol [RFC5531] 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.
As part of the ONC RPC protocol, protocol elements of RPCSEC_GSS that
appear in the Payload stream of an RPC-over-RDMA message (such as
control messages exchanged as part of establishing or destroying a
security context or data items that are part of RPCSEC_GSS
authentication material) MUST NOT be reduced.
8.2.2.1. RPCSEC_GSS Context Negotiation
Some NFS client implementations use a separate connection to
establish a Generic Security Service (GSS) context for NFS operation.
These clients use TCP and the standard NFS port (2049) for context
establishment. To enable the use of RPCSEC_GSS with NFS/RDMA, an NFS
server MUST also provide a TCP-based NFS service on port 2049.
8.2.2.2. 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
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needs to accommodate a larger RPC credential when marshaling RPC Call
messages 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 made larger as a result.
ULP operations that fit in a Short Message when a simpler form of
authentication is in use might need to be reduced, or conveyed via a
Long Message, when RPCSEC_GSS authentication is in use. It is more
likely that a Requester provides both a Read list and a Reply chunk
in the same RPC-over-RDMA header to convey a Long Call and provision
a receptacle for a Long Reply. More frequent use of Long Messages
can impact transport efficiency.
8.2.2.3. RPC-over-RDMA with RPCSEC_GSS Integrity or Privacy
The RPCSEC_GSS integrity service enables endpoints to detect
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 for transmit, and before
they have been XDR decoded after receipt. Both sender and receiver
endpoints use intermediate buffers to prevent exposure of encrypted
data or unverified cleartext data to RPC consumers. After
verification, encryption, and message wrapping has been performed,
the transport layer MAY use RDMA data transfer between these
intermediate buffers.
The process of reducing a DDP-eligible data item removes the data
item and its XDR padding from the encoded XDR stream. XDR padding of
a reduced data item is not transferred in an RPC-over-RDMA message.
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 or 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, and Chunked Messages cannot
be used. In this mode, an RPC-over-RDMA transport operates in the
same manner as a transport that does not support DDP.
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When an RPCSEC_GSS integrity or privacy service is in use, a
Requester provides both a Read list and a Reply chunk in the same
RPC-over-RDMA header to convey a Long Call and provision a receptacle
for a Long Reply.
8.2.2.4. Protecting RPC-over-RDMA Transport Headers
Like the base fields in an ONC RPC message (XID, call direction, and
so on), the contents of an RPC-over-RDMA message's Transport stream
are not protected by RPCSEC_GSS. This exposes XIDs, connection
credit limits, and chunk lists (but not the content of the data items
they refer to) to malicious behavior, which could 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 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 similar effect. If such
alterations occur, the use of RPCSEC_GSS integrity or privacy
services enable a Requester to detect unexpected material in a
received RPC message.
Encryption at lower layers, as described in Section 8.2.1, protects
the content of the Transport stream. To address attacks on RDMA
protocols themselves, RDMA transport implementations should conform
to [RFC5042].
9. IANA Considerations
A set of RPC netids for resolving RPC-over-RDMA services is specified
by this document. This is unchanged from [RFC5666].
The RPC-over-RDMA transport has been assigned an RPC netid, which is
an rpcbind [RFC1833] string used to describe the underlying protocol
in order for RPC to select the appropriate transport framing, as well
as the format of the service addresses and ports.
The following netid registry strings are 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" for IPv6 addressing. The netid
assignment policy and registry are defined in [RFC5665].
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These netids MAY be used for any RDMA network that satisfies the
requirements of Section 2.3.2 and that is able to identify service
endpoints using IP port addressing, possibly through use of a
translation service as described in Section 5.
The use of the RPC-over-RDMA protocol has no effect on RPC Program
numbers or existing registered port numbers. However, new port
numbers MAY be registered for use by RPC-over-RDMA-enabled services,
as appropriate to the new networks over which the services will
operate.
For example, the NFS/RDMA service defined in [RFC5667] has been
assigned the port 20049 in the "Service Name and Transport Protocol
Port Number Registry". This is distinct from the port number defined
for NFS on TCP, which is assigned the port 2049 in the same registry.
NFS clients use the same RPC Program number for NFS (100003) when
using either transport [RFC5531] (see the "Remote Procedure Call
(RPC) Program Numbers" registry).
10. References
10.1. Normative References
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
RFC 1833, DOI 10.17487/RFC1833, August 1995,
<http://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,
<http://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, <http://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, <http://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,
<http://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, <http://www.rfc-editor.org/info/rfc5531>.
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[RFC5660] Williams, N., "IPsec Channels: Connection Latching",
RFC 5660, DOI 10.17487/RFC5660, October 2009,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc7861>.
[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>.
10.2. Informative References
[IBARCH] InfiniBand Trade Association, "InfiniBand Architecture
Specification Volume 1", Release 1.3, March 2015,
<http://www.infinibandta.org/content/
pages.php?pg=technology_download>.
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1094] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <http://www.rfc-editor.org/info/rfc1094>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<http://www.rfc-editor.org/info/rfc1813>.
[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, <http://www.rfc-editor.org/info/rfc5040>.
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RFC 8166 RPC-over-RDMA Version 1 June 2017
[RFC5041] Shah, H., Pinkerton, J., Recio, R., and P. Culley, "Direct
Data Placement over Reliable Transports", RFC 5041,
DOI 10.17487/RFC5041, October 2007,
<http://www.rfc-editor.org/info/rfc5041>.
[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,
<http://www.rfc-editor.org/info/rfc5532>.
[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>.
[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,
<http://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,
<http://www.rfc-editor.org/info/rfc5666>.
[RFC5667] Talpey, T. and B. Callaghan, "Network File System (NFS)
Direct Data Placement", RFC 5667, DOI 10.17487/RFC5667,
January 2010, <http://www.rfc-editor.org/info/rfc5667>.
[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>.
[RFC8167] Lever, C., "Bidirectional Remote Procedure Call on RPC-
over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
June 2017, <http://www.rfc-editor.org/info/rfc8167>.
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Appendix A. Changes from RFC 5666
A.1. Changes to the Specification
The following alterations have been made to the RPC-over-RDMA version
1 specification. The section numbers below refer to [RFC5666].
o Section 2 has been expanded to introduce and explain key RPC
[RFC5531], XDR [RFC4506], and RDMA [RFC5040] terminology. These
terms are now used consistently throughout the specification.
o Section 3 has been reorganized and split into subsections to help
readers locate specific requirements and definitions.
o Sections 4 and 5 have been combined to improve the organization of
this information.
o The optional Connection Configuration Protocol has never been
implemented. The specification of CCP has been deleted from this
specification.
o A section consolidating requirements for ULBs has been added.
o An XDR extraction mechanism is provided, along with full
copyright, matching the approach used in [RFC5662].
o The "Security Considerations" section has been expanded to include
a discussion of how RPC-over-RDMA security depends on features of
the underlying RDMA transport.
o A subsection describing the use of RPCSEC_GSS [RFC7861] with RPC-
over-RDMA version 1 has been added.
A.2. Changes to the Protocol
Although the protocol described herein interoperates with existing
implementations of [RFC5666], the following changes have been made
relative to the protocol described in that document:
o Support for the Read-Read transfer model has been removed. Read-
Read is a slower transfer model than Read-Write. As a result,
implementers have chosen not to support it. Removal of Read-Read
simplifies explanatory text, and the RDMA_DONE procedure is no
longer part of the protocol.
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o The specification of RDMA_MSGP in [RFC5666] is not adequate,
although some incomplete implementations exist. Even if an
adequate specification were provided and an implementation were
produced, benefit for protocols such as NFSv4.0 [RFC7530] is
doubtful. Therefore, the RDMA_MSGP message type is no longer
supported.
o Technical issues with regard to handling RPC-over-RDMA header
errors have been corrected.
o Specific requirements related to implicit XDR roundup and complex
XDR data types have been added.
o Explicit guidance is provided related to sizing Write chunks,
managing multiple chunks in the Write list, and handling unused
Write chunks.
o Clear guidance about Send and Receive buffer sizes has been
introduced. This enables better decisions about when a Reply
chunk must be provided.
Acknowledgments
The editor gratefully acknowledges the work of Brent Callaghan and
Tom Talpey on the original RPC-over-RDMA Version 1 specification
[RFC5666].
Dave Noveck provided excellent review, constructive suggestions, and
consistent navigational guidance throughout the process of drafting
this document. Dave also contributed much of the organization and
content of Section 7 and helped the authors understand the
complexities of XDR extensibility.
The comments and contributions of Karen Deitke, Dai Ngo, Chunli
Zhang, Dominique Martinet, and Mahesh Siddheshwar are accepted with
great thanks. The editor also wishes to thank Bill Baker, Greg
Marsden, and Matt Benjamin for their support of this work.
The extract.sh shell script and formatting conventions were first
described by the authors of the NFSv4.1 XDR specification [RFC5662].
Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
Working Group Chair and Document Shepherd Spencer Shepler, and NFSV4
Working Group Secretary Thomas Haynes for their support.
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Authors' Addresses
Charles Lever (editor)
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
United States of America
Phone: +1 248 816 6463
Email: chuck.lever@oracle.com
William Allen Simpson
Red Hat
1384 Fontaine
Madison Heights, MI 48071
United States of America
Email: william.allen.simpson@gmail.com
Tom Talpey
Microsoft Corp.
One Microsoft Way
Redmond, WA 98052
United States of America
Phone: +1 425 704-9945
Email: ttalpey@microsoft.com
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