NFSv4 Working Group Tom Talpey
Internet-Draft NetApp
Intended status: Standards Track Brent Callaghan
Expires: August 23, 2008 Apple
February 22, 2008
Remote Direct Memory Access Transport for Remote Procedure Call
draft-ietf-nfsv4-rpcrdma-07
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
A protocol is described providing Remote Direct Memory Access
(RDMA) as a new transport for Computing Remote Procedure Call
(RPC). The RDMA transport binding conveys the benefits of
efficient, bulk data transport over high speed networks, while
providing for minimal change to RPC applications and with no
required revision of the application RPC protocol, or the RPC
protocol itself.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Abstract RDMA Requirements . . . . . . . . . . . . . . . . . 3
3. Protocol Outline . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Short Messages . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Data Chunks . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Flow Control . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. XDR Encoding with Chunks . . . . . . . . . . . . . . . . . 7
3.5. XDR Decoding with Read Chunks . . . . . . . . . . . . . 10
3.6. XDR Decoding with Write Chunks . . . . . . . . . . . . . 11
3.7. XDR Roundup and Chunks . . . . . . . . . . . . . . . . . 12
3.8. RPC Call and Reply . . . . . . . . . . . . . . . . . . . 13
3.9. Padding . . . . . . . . . . . . . . . . . . . . . . . . 16
4. RPC RDMA Message Layout . . . . . . . . . . . . . . . . . 17
4.1. RPC over RDMA Header . . . . . . . . . . . . . . . . . . 17
4.2. RPC over RDMA header errors . . . . . . . . . . . . . . 19
4.3. XDR Language Description . . . . . . . . . . . . . . . . 20
5. Long Messages . . . . . . . . . . . . . . . . . . . . . . 22
5.1. Message as an RDMA Read Chunk . . . . . . . . . . . . . 22
5.2. RDMA Write of Long Replies (Reply Chunks) . . . . . . . 24
6. Connection Configuration Protocol . . . . . . . . . . . . 25
6.1. Initial Connection State . . . . . . . . . . . . . . . . 26
6.2. Protocol Description . . . . . . . . . . . . . . . . . . 26
7. Memory Registration Overhead . . . . . . . . . . . . . . . 28
8. Errors and Error Recovery . . . . . . . . . . . . . . . . 28
9. Node Addressing . . . . . . . . . . . . . . . . . . . . . 28
10. RPC Binding . . . . . . . . . . . . . . . . . . . . . . . 29
11. Security Considerations . . . . . . . . . . . . . . . . . 30
12. IANA Considerations . . . . . . . . . . . . . . . . . . . 31
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . 32
14. Normative References . . . . . . . . . . . . . . . . . . 32
15. Informative References . . . . . . . . . . . . . . . . . 33
16. Authors' Addresses . . . . . . . . . . . . . . . . . . . 34
17. Intellectual Property and Copyright Statements . . . . . 35
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 36
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
1. Introduction
Remote Direct Memory Access (RDMA) [RFC5040, RFC5041] [IB] is a
technique for efficient movement of data between end nodes, which
becomes increasingly compelling over high speed transports. By
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directing data into destination buffers as it is sent on a network,
and placing it via direct memory access by hardware, the double
benefit of faster transfers and reduced host overhead is obtained.
Open Network Computing Remote Procedure Call (ONC RPC, or simply,
RPC) [RFC1831bis] is a remote procedure call protocol that has been
run over a variety of transports. Most RPC implementations today
use UDP or TCP. RPC messages are defined in terms of an eXternal
Data Representation (XDR) [RFC4506] which provides a canonical data
representation across a variety of host architectures. An XDR data
stream is conveyed differently on each type of transport. 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 unique fashion
that must be fully described if client and server implementations
are to interoperate.
RDMA transports present new semantics unlike the behaviors of
either UDP or TCP alone. They retain message delineations like UDP
while also providing a reliable, sequenced data transfer like TCP.
And, they provide the new efficient, bulk transfer service of RDMA.
RDMA transports are therefore naturally viewed as a new transport
type by RPC.
RDMA as a transport will benefit the performance of RPC protocols
that move large "chunks" of data, since RDMA hardware excels at
moving data efficiently between host memory and a high speed
network with little or no host CPU involvement. In this context,
the NFS protocol, in all its versions [RFC1094] [RFC1813] [RFC3530]
[NFSv4.1], is an obvious beneficiary of RDMA. A complete problem
statement is discussed in [NFSRDMAPS], and related NFSv4 issues are
discussed in [NFSv4.1]. Many other RPC-based protocols will also
benefit.
Although the RDMA transport described here provides relatively
transparent support for any RPC application, the proposal goes
further in describing mechanisms that can optimize the use of RDMA
with more active participation by the RPC application.
2. Abstract RDMA Requirements
An RPC transport is responsible for conveying an RPC message from a
sender to a receiver. An RPC message is either an RPC call from a
client to a server, or an RPC reply from the server back to the
client. An RPC message contains an RPC call header followed by
arguments if the message is an RPC call, or an RPC reply header
followed by results if the message is an RPC reply. The call
header contains a transaction ID (XID) followed by the program and
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procedure number as well as a security credential. An RPC reply
header begins with an XID that matches that of the RPC call
message, followed by a security verifier and results. All data in
an RPC message is XDR encoded. For a complete description of the
RPC protocol and XDR encoding, see [RFC1831bis] and [RFC4506].
This protocol assumes the following abstract model for RDMA
transports. These terms, common in the RDMA lexicon, are used in
this document. A more complete glossary of RDMA terms can be found
in [RFC5040].
o Registered Memory
All data moved via tagged RDMA operations is resident in
registered memory at its destination. This protocol assumes
that each segment 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.
o RDMA Send
The RDMA provider supports an RDMA Send operation with
completion signalled at the receiver when data is placed in a
pre-posted buffer. The amount of transferred data is limited
only by the size of the receiver's buffer. Sends complete at
the receiver in the order they were issued at the sender.
o RDMA Write
The RDMA provider supports an RDMA Write operation to directly
place data in the receiver's buffer. An RDMA Write is
initiated by the sender and completion is signalled at the
sender. No completion is signalled at the receiver. The
sender uses a steering tag, memory address and length of the
remote destination buffer. RDMA Writes are not necessarily
ordered with respect to one another, but are ordered with
respect to RDMA Sends; a subsequent RDMA Send completion
obtained at the receiver guarantees that prior RDMA Write data
has been successfully placed in the receiver's memory.
o RDMA Read
The RDMA provider supports an RDMA Read operation to directly
place peer source data in the requester's buffer. An RDMA
Read is initiated by the receiver and completion is signalled
at the receiver. The receiver provides steering tags, memory
addresses and a length for the remote source and local
destination buffers. Since the peer at the data source
receives no notification of RDMA Read completion, there is an
assumption that on receiving the data the receiver will signal
completion with an RDMA Send message, so that the peer can
free the source buffers and the associated steering tags.
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This protocol is designed to be carried over all RDMA transports
meeting the stated requirements. This protocol conveys to the RPC
peer, information sufficient for that RPC peer to direct an RDMA
layer to perform transfers containing RPC data, and to communicate
their result(s). For example, it is readily carried over RDMA
transports such as iWARP [RFC5040, RFC5041] or Infiniband [IB].
3. Protocol Outline
An RPC message can be conveyed in identical fashion, whether it is
a call or reply message. In each case, the transmission of the
message proper is preceded by transmission of a transport-specific
header for use by RPC over RDMA transports. This header is
analogous to the record marking used for RPC over TCP, but is more
extensive, since RDMA transports support several modes of data
transfer and it is important to allow the client and server to use
the most efficient mode for any given transfer. Multiple segments
of a message may be transferred in different ways to different
remote memory destinations.
All transfers of a call or reply begin with an RDMA Send which
transfers at least the RPC over RDMA header, usually with the call
or reply message appended, or at least some part thereof. Because
the size of what may be transmitted via RDMA Send is limited by the
size of the receiver's pre-posted buffer, the RPC over RDMA
transport provides a number of methods to reduce the amount
transferred by means of the RDMA Send, when necessary, by
transferring various parts of the message using RDMA Read and RDMA
Write.
RPC over RDMA framing replaces all other RPC framing (such as TCP
record marking) when used atop an RPC/RDMA association, even though
the underlying RDMA protocol may itself be layered atop a protocol
with a defined RPC framing (such as TCP). An upper layer may
however define an exchange to dynamically enable RPC/RDMA on an
existing RPC association. Any such exchange must be carefully
architected so as to prevent any ambiguity as to the framing in use
for each side of the connection. Because RPC/RDMA framing delimits
an entire RPC request or reply, any such shift must occur between
distinct RPC messages.
3.1. Short Messages
Many RPC messages are quite short. For example, the NFS version 3
GETATTR request, is only 56 bytes: 20 bytes of RPC header, plus a
32 byte file handle argument and 4 bytes of length. The reply to
this common request is about 100 bytes.
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There is no benefit in transferring such small messages with an
RDMA Read or Write operation. The overhead in transferring
steering tags and memory addresses is justified only by large
transfers. The critical message size that justifies RDMA transfer
will vary depending on the RDMA implementation and network, but is
typically of the order of a few kilobytes. It is appropriate to
transfer a short message with an RDMA Send to a pre-posted buffer.
The RPC over RDMA header with the short message (call or reply)
immediately following is transferred using a single RDMA Send
operation.
Short RPC messages over an RDMA transport:
RPC Client RPC Server
| RPC Call |
Send | ------------------------------> |
| |
| RPC Reply |
| <------------------------------ | Send
3.2. Data Chunks
Some protocols, like NFS, have RPC procedures that can transfer
very large "chunks" of data in the RPC call or reply and would
cause the maximum send size to be exceeded if one tried to transfer
them as part of the RDMA Send. These large chunks typically range
from a kilobyte to a megabyte or more. An RDMA transport can
transfer large chunks of data more efficiently via the direct
placement of an RDMA Read or RDMA Write operation. Using direct
placement instead of inline transfer not only avoids expensive data
copies, but provides correct data alignment at the destination.
3.3. Flow Control
It is critical to provide RDMA Send flow control for an RDMA
connection. RDMA receive operations will fail if a pre-posted
receive buffer is not available to accept an incoming RDMA Send,
and repeated occurrences of such errors can be fatal to the
connection. This is a departure from conventional TCP/IP
networking where buffers are allocated dynamically on an as-needed
basis, and where pre-posting is not required.
It is not practical to provide for fixed credit limits at the RPC
server. Fixed limits scale poorly, since posted buffers are
dedicated to the associated connection until consumed by receive
operations. Additionally for protocol correctness, the RPC server
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must always be able to reply to client requests, whether or not new
buffers have been posted to accept future receives. (Note that the
RPC server may in fact be a client at some other layer. For
example, NFSv4 callbacks are processed by the NFSv4 client, acting
as an RPC server. The credit discussions apply equally in either
case.)
Flow control for RDMA Send operations is implemented as a simple
request/grant protocol in the RPC over RDMA header associated with
each RPC message. The RPC over RDMA header for RPC call messages
contains a requested credit value for the RPC server, which MAY be
dynamically adjusted by the caller to match its expected needs.
The RPC over RDMA header for the RPC reply messages provides the
granted result, which MAY have any value except it MUST NOT be zero
when no in-progress operations are present at the server, since
such a value would result in deadlock. The value MAY be adjusted
up or down at each opportunity to match the server's needs or
policies.
The RPC client MUST NOT send unacknowledged requests in excess of
this granted RPC server credit limit. If the limit is exceeded,
the RDMA layer may signal an error, possibly terminating the
connection. Even if an error does not occur, it is OPTIONAL that
the server handle the excess request(s), and it MAY return an RPC
error to the client. Also note that the never-zero requirement
implies that an RPC server MUST always provide at least one credit
to each connected RPC client from which no requests are
outstanding. The client would deadlock otherwise, unable to send
another request.
While RPC calls complete in any order, the current flow control
limit at the RPC server is known to the RPC client from the Send
ordering properties. It is always the most recent server-granted
credit value minus the number of requests in flight.
Certain RDMA implementations may impose additional flow control
restrictions, such as limits on RDMA Read operations in progress at
the responder. Because these operations are outside the scope of
this protocol, they are not addressed and SHOULD be provided for by
other layers. For example, a simple upper layer RPC consumer might
perform single-issue RDMA Read requests, while a more
sophisticated, multithreaded RPC consumer might implement its own
FIFO queue of such operations. For further discussion of possible
protocol implementations capable of negotiating these values, see
section 6 "Connection Configuration Protocol" of this draft, or
[NFSv4.1].
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3.4. XDR Encoding with Chunks
The data comprising an RPC call or reply message is marshaled or
serialized into a contiguous stream by an XDR routine. XDR data
types such as integers, strings, arrays and linked lists are
commonly implemented over two very simple functions that encode
either an XDR data unit (32 bits) or an array of bytes.
Normally, the separate data items in an RPC call or reply are
encoded as a contiguous sequence of bytes for network transmission
over UDP or TCP. However, in the case of an RDMA transport, local
routines such as XDR encode can determine that (for instance) an
opaque byte array is large enough to be more efficiently moved via
an RDMA data transfer operation like RDMA Read or RDMA Write.
Semantically speaking, the protocol has no restriction regarding
data types which may or may not be represented by a read or write
chunk. In practice however, efficiency considerations lead to the
conclusion that certain data types are not generally "chunkable".
Typically, only those opaque and aggregate data types that may
attain substantial size are considered to be eligible. With
today's hardware this size may be a kilobyte or more. However any
object MAY be chosen for chunking in any given message.
The eligibility of XDR data items to be candidates for being moved
as data chunks (as opposed to being marshaled inline) is not
specified by the RPC over RDMA protocol. Chunk eligibility
criteria MUST be determined by each upper layer in order to provide
for an interoperable specification. One such example with
rationale, for the NFS protocol family, is provided in [NFSDDP].
The interface by which an upper layer implementation communicates
the eligibility of a data item locally to RPC for chunking is out
of scope for this specification. In many implementations, it is
possible to implement a transparent RPC chunking facility.
However, such implementations may lead to inefficiencies, either
because they require the RPC layer to perform expensive
registration and deregistration of memory "on the fly", or they may
require using RDMA chunks in 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 is that
the resulting RPC RDMA protocol sent to the peer is valid for the
upper layer. See for example [NFSDDP].
When sending any message (request or reply) that contains an
eligible large data chunk, the XDR encoding routine avoids moving
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the data into the XDR stream. Instead, it does not encode the data
portion, but records the address and size of each chunk in a
separate "read chunk list" encoded within RPC RDMA transport-
specific headers. Such chunks will be transferred via RDMA Read
operations initiated by the receiver.
When the read chunks are to be moved via RDMA, the memory for each
chunk is registered. This registration may take place within XDR
itself, providing for full transparency to upper layers, or it may
be performed by any other specific local implementation.
Additionally, when making an RPC call that can result in bulk data
transferred in the reply, write chunks MAY be provided to accept
the data directly via RDMA Write. These write chunks will
therefore be pre-filled by the RPC server prior to responding, and
XDR decode of the data at the client will not be required. These
chunks undergo a similar registration and advertisement via "write
chunk lists" built as a part of XDR encoding.
Some RPC client implementations are not able to determine where an
RPC call's results reside during the "encode" phase. This makes it
difficult or impossible for the RPC client layer to encode the
write chunk list at the time of building the request. In this
case, it is difficult for the RPC implementation to provide
transparency to the RPC consumer, which may require recoding to
provide result information at this earlier stage.
Therefore if the RPC client does not make a write chunk list
available to receive the result, then the RPC server MAY return
data inline in the reply, or if the upper layer specification
permits, it MAY be returned via a read chunk list. It is NOT
RECOMMENDED that upper layer RPC client protocol specifications
omit write chunk lists for eligible replies, due to the lower
performance of the additional handshaking to perform data transfer,
and the requirement that the RPC server must expose (and preserve)
the reply data for a period of time. In the absence of a server-
provided read chunk list in the reply, if the encoded reply
overflows the posted receive buffer, the RPC will fail with an RDMA
transport error.
When any data within a message is provided via either read or write
chunks, the chunk itself refers only to the data portion of the XDR
stream element. In particular, for counted fields (e.g., a "<>"
encoding) the byte count which is encoded as part of the field
remains in the XDR stream, and is also encoded in the chunk list.
The data portion is however elided from the encoded XDR stream, and
is transferred as part of chunk list processing. This is important
to maintain upper layer implementation compatibility - both the
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count and the data must be transferred as part of the logical XDR
stream. While the chunk list processing results in the data being
available to the upper layer peer for XDR decoding, the length
present in the chunk list entries is not. Any byte count in the
XDR stream MUST match the sum of the byte counts present in the
corresponding read or write chunk list. If they do not agree, an
RPC protocol encoding error results.
The following items are contained in a chunk list entry.
Handle
Steering tag or handle obtained when the chunk memory is
registered for RDMA.
Length
The length of the chunk in bytes.
Offset
The offset or beginning memory address of the chunk. In order
to support the widest array of RDMA implementations, as well
as the most general steering tag scheme, this field is
unconditionally included in each chunk list entry.
While zero-based offset schemes are available in many RDMA
implementations, their use by RPC requires individual
registration of each read or write chunk. On many 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, and by
using a single, universal chunk representation, the RPC RDMA
protocol implementation is simplified to its most general
form.
Position
For data which is to be encoded, the position in the XDR
stream where the chunk would normally reside. Note that the
chunk therefore inserts its data into the XDR stream at this
position, but its transfer is no longer "inline". Also note
therefore that all chunks belonging to a single RPC argument
or result will have the same position. For data which is to
be decoded, no position is used.
When XDR marshaling is complete, the chunk list is XDR encoded,
then sent to the receiver prepended to the RPC message. Any source
data for a read chunk, or the destination of a write chunk, remain
behind in the sender's registered memory and their actual payload
is not marshaled into the request or reply.
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+----------------+----------------+-------------
| RPC over RDMA | |
| header w/ | RPC Header | Non-chunk args/results
| chunks | |
+----------------+----------------+-------------
Read chunk lists and write chunk lists are structured somewhat
differently. This is due to the different usage - read chunks are
decoded and indexed by their argument's or result's position in the
XDR data stream; their size is always known. Write chunks on the
other hand are used only for results, and have neither a
preassigned offset in the XDR stream, nor a size until the results
are produced, since the buffers may be only partially filled, or
may not be used for results at all. Their presence in the XDR
stream is therefore not known until the reply is processed. The
mapping of Write chunks onto designated NFS procedures and their
results is described in [NFSDDP].
Therefore, read chunks are encoded into a read chunk list as a
single array, with each entry tagged by its (known) size and its
argument's or result's position in the XDR stream. Write chunks
are encoded as a list of arrays of RDMA buffers, with each list
element (an array) providing buffers for a separate result.
Individual write chunk list elements MAY thereby result in being
partially or fully filled, or in fact not being filled at all.
Unused write chunks, or unused bytes in write chunk buffer lists,
are not returned as results, and their memory is returned to the
upper layer as part of RPC completion. However, the RPC layer MUST
NOT assume that the buffers have not been modified.
3.5. XDR Decoding with Read Chunks
The XDR decode process moves data from an XDR stream into a data
structure provided by the RPC client or server application. Where
elements of the destination data structure are buffers or strings,
the RPC application can either pre-allocate storage to receive the
data, or leave the string or buffer fields null and allow the XDR
decode stage of RPC processing to automatically allocate storage of
sufficient size.
When decoding a message from an RDMA transport, the receiver first
XDR decodes the chunk lists from the RPC over RDMA header, then
proceeds to decode the body of the RPC message (arguments or
results). Whenever the XDR offset in the decode stream matches
that of a chunk in the read chunk list, the XDR routine initiates
an RDMA Read to bring over the chunk data into locally registered
memory for the destination buffer.
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When processing an RPC request, the RPC receiver (RPC server)
acknowledges its completion of use of the source buffers by simply
replying to the RPC sender (client), and the peer may then free all
source buffers advertised by the request.
When processing an RPC reply, after completing such a transfer the
RPC receiver (client) MUST issue an RDMA_DONE message (described in
Section 3.8) to notify the peer (server) that the source buffers
can be freed.
The read chunk list is constructed and used entirely within the
RPC/XDR layer. Other than specifying the minimum chunk size, the
management of the read chunk list is automatic and transparent to
an RPC application.
3.6. XDR Decoding with Write Chunks
When a "write chunk list" is provided for the results of the RPC
call, the RPC server MUST provide any corresponding data via RDMA
Write to the memory referenced in the chunk list entries. The RPC
reply conveys this by returning the write chunk list to the client
with the lengths rewritten to match the actual transfer. The XDR
"decode" of the reply therefore performs no local data transfer but
merely returns the length obtained from the reply.
Each decoded result consumes one entry in the write chunk list,
which in turn consists of an array of RDMA segments. The length is
therefore the sum of all returned lengths in all segments
comprising the corresponding list entry. As each list entry is
"decoded", the entire entry is consumed.
The write chunk list is constructed and used by the RPC
application. The RPC/XDR layer simply conveys the list between
client and server and initiates the RDMA Writes back to the client.
The mapping of write chunk list entries to procedure arguments MUST
be determined for each protocol. An example of a mapping is
described in [NFSDDP].
3.7. XDR Roundup and Chunks
The XDR protocol requires 4-byte alignment of each new encoded
element in any XDR stream. This requirement is for efficiency and
ease of decode/unmarshaling at the receiver - if the XDR stream
buffer begins on a native machine boundary, then the XDR elements
will lie on similarly predictable offsets in memory.
Within XDR, when non-4-byte encodes (such as an odd-length string
or bulk data) are marshaled, their length is encoded literally,
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while their data is padded to begin the next element at a 4-byte
boundary in the XDR stream. For TCP or RDMA inline encoding, this
minimal overhead is required because the transport-specific framing
relies on the fact that the relative offset of the elements in the
XDR stream from the start of the message determines the XDR
position during decode.
On the other hand, RPC/RDMA Read chunks carry the XDR position of
each chunked element and length of the Chunk segment, and can be
placed by the receiver exactly where they belong in the receiver's
memory without regard to the alignment of their position in the XDR
stream. Since any rounded-up data is not actually part of the
upper layer's message, the receiver will not reference it, and
there is no reason to set it to any particular value in the
receiver's memory.
When roundup is present at the end of a sequence of chunks, the
length of the sequence will terminate it at a non-4-byte XDR
position. When the receiver proceeds to decode the remaining part
of the XDR stream, it inspects the XDR position indicated by the
next chunk. Because this position will not match (else roundup
would not have occurred), the receiver decoding will fall back to
inspecting the remaining inline portion. If in turn, no data
remains to be decoded from the inline portion, then the receiver
MUST conclude that roundup is present, and therefore advances the
XDR decode position to that indicated by the next chunk (if any).
In this way, roundup is passed without ever actually transferring
additional XDR bytes.
Some protocol operations over RPC/RDMA, for instance NFS writes of
data encountered at the end of a file or in direct i/o situations,
commonly yield these roundups within RDMA Read Chunks. Because any
roundup bytes are not actually present in the data buffers being
written, memory for these bytes would come from noncontiguous
buffers, either as an additional memory registration segment, or as
an additional Chunk. The overhead of these operations can be
significant to both the sender to marshal them, and even higher to
the receiver which to transfer them. Senders SHOULD therefore
avoid encoding individual RDMA Read Chunks for roundup whenever
possible. It is acceptable, but not necessary, to include roundup
data in an existing RDMA Read Chunk, but only if it is already
present in the XDR stream to carry upper layer data.
Note that there is no exposure of additional data at the sender due
to eliding roundup data from the XDR stream, since any additional
sender buffers are never exposed to the peer. The data is
literally not there to be transferred.
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For RDMA Write Chunks, a simpler encoding method applies. Again,
roundup bytes are not transferred, instead the chunk length sent to
the receiver in the reply is simply increased to include any
roundup. Because of the requirement that the RDMA Write chunks are
filled sequentially without gaps, this situation can only occur on
the final chunk receiving data. Therefore there is no opportunity
for roundup data to insert misalignment or positional gaps into the
XDR stream.
3.8. RPC Call and Reply
The RDMA transport for RPC provides three methods of moving data
between RPC client and server:
Inline
Data are moved between RPC client and server within an RDMA
Send.
RDMA Read
Data are moved between RPC client and server via an RDMA Read
operation via steering tag, address and offset obtained from a
read chunk list.
RDMA Write
Result data is moved from RPC server to client via an RDMA
Write operation via steering tag, address and offset obtained
from a write chunk list or reply chunk in the client's RPC
call message.
These methods of data movement may occur in combinations within a
single RPC. For instance, an RPC call may contain some inline data
along with some large chunks to be transferred via RDMA Read to the
server. The reply to that call may have some result chunks that
the server RDMA Writes back to the client. The following protocol
interactions illustrate RPC calls that use these methods to move
RPC message data:
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An RPC with write chunks in the call message:
RPC Client RPC Server
| RPC Call + Write Chunk list |
Send | ------------------------------> |
| |
| Chunk 1 |
| <------------------------------ | Write
| : |
| Chunk n |
| <------------------------------ | Write
| |
| RPC Reply |
| <------------------------------ | Send
In the presence of write chunks, RDMA ordering provides the
guarantee that all data in the RDMA Write operations has been
placed in memory prior to the client's RPC reply processing.
An RPC with read chunks in the call message:
RPC Client RPC Server
| RPC Call + Read Chunk list |
Send | ------------------------------> |
| |
| Chunk 1 |
| +------------------------------ | Read
| v-----------------------------> |
| : |
| Chunk n |
| +------------------------------ | Read
| v-----------------------------> |
| |
| RPC Reply |
| <------------------------------ | Send
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An RPC with read chunks in the reply message:
RPC Client RPC Server
| RPC Call |
Send | ------------------------------> |
| |
| RPC Reply + Read Chunk list |
| <------------------------------ | Send
| |
| Chunk 1 |
Read | ------------------------------+ |
| <-----------------------------v |
| : |
| Chunk n |
Read | ------------------------------+ |
| <-----------------------------v |
| |
| Done |
Send | ------------------------------> |
The final Done message allows the RPC client to signal the server
that it has received the chunks, so the server can de-register and
free the memory holding the chunks. A Done completion is not
necessary for an RPC call, since the RPC reply Send is itself a
receive completion notification. In the event that the client
fails to return the Done message within some timeout period, the
server MAY conclude that a protocol violation has occurred and
close the RPC connection, or it MAY proceed with a de-register and
free its chunk buffers. This may result in a fatal RDMA error if
the client later attempts to perform an RDMA Read operation, which
amounts to the same thing.
The use of read chunks in RPC reply messages is much less efficient
than providing write chunks in the originating RPC calls, due to
the additional message exchanges, the need for the RPC server to
advertise buffers to the peer, the necessity of the server
maintaining a timer for the purpose of recovery from misbehaving
clients, and the need for additional memory registration. Their
use is NOT RECOMMENDED by upper layers where efficiency is a
primary concern. [NFSDDP] However, they MAY be employed by upper
layer protocol bindings which are primarily concerned with
transparency, since they can frequently be implemented completely
within the RPC lower layers.
It is important to note that the Done message consumes a credit at
the RPC server. The RPC server SHOULD provide sufficient credits
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to the client to allow the Done message to be sent without deadlock
(driving the outstanding credit count to zero). The RPC client
MUST account for its required Done messages to the server in its
accounting of available credits, and the server SHOULD replenish
any credit consumed by its use of such exchanges at its earliest
opportunity.
Finally, it is possible to conceive of RPC exchanges that involve
any or all combinations of write chunks in the RPC call, read
chunks in the RPC call, and read chunks in the RPC reply. Support
for such exchanges is straightforward from a protocol perspective,
but in practice such exchanges would be quite rare, limited to
upper layer protocol exchanges which transferred bulk data in both
the call and corresponding reply.
3.9. Padding
Alignment of specific opaque data enables certain scatter/gather
optimizations. Padding leverages the useful property that RDMA
transfers preserve alignment of data, even when they are placed
into pre-posted receive buffers by Sends.
Many servers can make good use of such padding. Padding allows the
chaining of RDMA receive buffers such that any data transferred by
RDMA on behalf of RPC requests will be placed into appropriately
aligned buffers on the system that receives the transfer. In this
way, the need for servers to perform RDMA Read to satisfy all but
the largest client writes is obviated.
The effect of padding is demonstrated below showing prior bytes on
an XDR stream (XXX) followed by an opaque field consisting of four
length bytes (LLLL) followed by data bytes (DDDD). The receiver of
the RDMA Send has posted two chained receive buffers. Without
padding, the opaque data is split across the two buffers. With the
addition of padding bytes ("ppp" in the figure below) prior to the
first data byte, the data can be forced to align correctly in the
second buffer.
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Buffer 1 Buffer 2
Unpadded -------------- --------------
XXXXXXXLLLLDDDDDDDDDDDDDD ---> XXXXXXXLLLLDDD DDDDDDDDDDD
Padded
XXXXXXXLLLLpppDDDDDDDDDDDDDD ---> XXXXXXXLLLLppp DDDDDDDDDDDDDD
Padding is implemented completely within the RDMA transport
encoding, flagged with a specific message type. Where padding is
applied, two values are passed to the peer: an "rdma_align" which
is the padding value used, and "rdma_thresh", which is the opaque
data size at or above which padding is applied. For instance, if
the server is using chained 4 KB receive buffers, then up to (4 KB
- 1) padding bytes could be used to achieve alignment of the data.
The XDR routine at the peer MUST consult these values when decoding
opaque values. Where the decoded length exceeds the rdma_thresh,
the XDR decode MUST skip over the appropriate padding as indicated
by rdma_align and the current XDR stream position.
4. RPC RDMA Message Layout
RPC call and reply messages are conveyed across an RDMA transport
with a prepended RPC over RDMA header. The RPC over RDMA header
includes data for RDMA flow control credits, padding parameters and
lists of addresses that provide direct data placement via RDMA Read
and Write operations. The layout of the RPC message itself is
unchanged from that described in [RFC1831bis] except for the
possible exclusion of large data chunks that will be moved by RDMA
Read or Write operations. If the RPC message (along with the RPC
over RDMA header) is too long for the posted receive buffer (even
after any large chunks are removed), then the entire RPC message
MAY be moved separately as a chunk, leaving just the RPC over RDMA
header in the RDMA Send.
4.1. RPC over RDMA Header
The RPC over RDMA header begins with four 32-bit fields that are
always present and which control the RDMA interaction including
RDMA-specific flow control. These are then followed by a number of
items such as chunk lists and padding which MAY or MUST NOT be
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present depending on the type of transmission. The four fields
which are always present are:
1. Transaction ID (XID).
The XID generated for the RPC call and reply. Having the XID
at the beginning of the message makes it easy to establish the
message context. This XID MUST be the same as the XID in the
RPC header. 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 header, if it so chooses.
2. Version number.
This version of the RPC RDMA message protocol is 1. The
version number MUST be increased by one whenever the format of
the RPC RDMA messages is changed.
3. Flow control credit value.
When sent in an RPC call message, the requested value is
provided. When sent in an RPC reply message, the granted
value is returned. RPC calls SHOULD not be sent in excess of
the currently granted limit.
4. Message type.
o RDMA_MSG = 0 indicates that chunk lists and RPC message
follow.
o RDMA_NOMSG = 1 indicates that after the chunk lists there
is no RPC message. In this case, the chunk lists provide
information to allow the message proper to be transferred
using RDMA Read or write and thus is not appended to the
RPC over RDMA header.
o RDMA_MSGP = 2 indicates that a chunk list and RPC message
with some padding follow.
0 RDMA_DONE = 3 indicates that the message signals the
completion of a chunk transfer via RDMA Read.
o RDMA_ERROR = 4 is used to signal any detected error(s) in
the RPC RDMA chunk encoding.
Because the version number is encoded as part of this header, and
the RDMA_ERROR message type is used to indicate errors, these first
four fields and the start of the following message body MUST always
remain aligned at these fixed offsets for all versions of the RPC
over RDMA header.
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For a message of type RDMA_MSG or RDMA_NOMSG, the Read and Write
chunk lists follow. If the Read chunk list is null (a 32 bit word
of zeros), then there are no chunks to be transferred separately
and the RPC message follows in its entirety. If non-null, then
it's the beginning of an XDR encoded sequence of Read chunk list
entries. If the Write chunk list is non-null, then an XDR encoded
sequence of Write chunk entries follows.
If the message type is RDMA_MSGP, then two additional fields that
specify the padding alignment and threshold are inserted prior to
the Read and Write chunk lists.
A header of message type RDMA_MSG or RDMA_MSGP MUST be followed by
the RPC call or RPC reply message body, beginning with the XID.
The XID in the RDMA_MSG or RDMA_MSGP header MUST match this.
+--------+---------+---------+-----------+-------------+----------
| | | | Message | NULLs | RPC Call
| XID | Version | Credits | Type | or | or
| | | | | Chunk Lists | Reply Msg
+--------+---------+---------+-----------+-------------+----------
Note that in the case of RDMA_DONE and RDMA_ERROR, no chunk list or
RPC message follows. As an implementation hint: a gather operation
on the Send of the RDMA RPC message can be used to marshal the
initial header, the chunk list, and the RPC message itself.
4.2. RPC over RDMA header errors
When a peer receives an RPC RDMA message, it MUST perform the
following basic validity checks on the header and chunk contents.
If such errors are detected in the request, an RDMA_ERROR reply
MUST be generated.
Two types of errors are defined, version mismatch and invalid chunk
format. When the peer detects an RPC over RDMA header version
which it does not support (currently this draft defines only
version 1), it replies with an error code of ERR_VERS, and provides
the low and high inclusive version numbers it does, in fact,
support. The version number in this reply MUST be any value
otherwise valid at the receiver. When other decoding errors are
detected in the header or chunks, either an RPC decode error MAY be
returned, or the ROC/RDMA error code ERR_CHUNK MUST be returned.
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4.3. XDR Language Description
Here is the message layout in XDR language.
struct xdr_rdma_segment {
uint32 handle; /* Registered memory handle */
uint32 length; /* Length of the chunk in bytes */
uint64 offset; /* Chunk virtual address or offset */
};
struct xdr_read_chunk {
uint32 position; /* Position in XDR stream */
struct xdr_rdma_segment target;
};
struct xdr_read_list {
struct xdr_read_chunk entry;
struct xdr_read_list *next;
};
struct xdr_write_chunk {
struct xdr_rdma_segment target<>;
};
struct xdr_write_list {
struct xdr_write_chunk entry;
struct xdr_write_list *next;
};
struct rdma_msg {
uint32 rdma_xid; /* Mirrors the RPC header xid */
uint32 rdma_vers; /* Version of this protocol */
uint32 rdma_credit; /* Buffers requested/granted */
rdma_body rdma_body;
};
enum rdma_proc {
RDMA_MSG=0, /* An RPC call or reply msg */
RDMA_NOMSG=1, /* An RPC call or reply msg - separate body */
RDMA_MSGP=2, /* An RPC call or reply msg with padding */
RDMA_DONE=3, /* Client signals reply completion */
RDMA_ERROR=4 /* An RPC RDMA encoding error */
};
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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:
rpc_rdma_header_padded rdma_msgp;
case RDMA_DONE:
void;
case RDMA_ERROR:
rpc_rdma_error rdma_error;
};
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;
};
struct rpc_rdma_header_padded {
uint32 rdma_align; /* Padding alignment */
uint32 rdma_thresh; /* Padding threshold */
struct xdr_read_list *rdma_reads;
struct xdr_write_list *rdma_writes;
struct xdr_write_chunk *rdma_reply;
/* rpc body follows */
};
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enum rpc_rdma_errcode {
ERR_VERS = 1,
ERR_CHUNK = 2
};
union rpc_rdma_error switch (rpc_rdma_errcode err) {
case ERR_VERS:
uint32 rdma_vers_low;
uint32 rdma_vers_high;
case ERR_CHUNK:
void;
default:
uint32 rdma_extra[8];
};
5. Long Messages
The receiver of RDMA Send messages is required by RDMA to have
previously posted one or more adequately sized buffers. The RPC
client can inform the server of the maximum size of its RDMA Send
messages via the Connection Configuration Protocol described later
in this document.
Since RPC messages are frequently small, memory savings can be
achieved by posting small buffers. Even large messages like NFS
READ or WRITE will be quite small once the chunks are removed from
the message. However, there may be large messages that would
demand a very large buffer be posted, where the contents of the
buffer may not be a chunkable XDR element. A good example is an
NFS READDIR reply which may contain a large number of small
filename strings. Also, the NFS version 4 protocol [RFC3530]
features COMPOUND request and reply messages of unbounded length.
Ideally, each upper layer will negotiate these limits. However, it
is frequently necessary to provide a transparent solution.
5.1. Message as an RDMA Read Chunk
One relatively simple method is to have the client identify any RPC
message that exceeds the RPC server's posted buffer size and move
it separately as a chunk, i.e., reference it as the first entry in
the read chunk list with an XDR position of zero.
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Normal Message
+--------+---------+---------+------------+-------------+----------
| | | | | | RPC Call
| XID | Version | Credits | RDMA_MSG | Chunk Lists | or
| | | | | | Reply Msg
+--------+---------+---------+------------+-------------+----------
Long Message
+--------+---------+---------+------------+-------------+
| | | | | |
| XID | Version | Credits | RDMA_NOMSG | Chunk Lists |
| | | | | |
+--------+---------+---------+------------+-------------+
|
| +----------
| | Long RPC Call
+->| or
| Reply Message
+----------
If the receiver gets an RPC over RDMA header with a message type of
RDMA_NOMSG and finds an initial read chunk list entry with a zero
XDR position, it allocates a registered buffer and issues an RDMA
Read of the long RPC message into it. The receiver then proceeds
to XDR decode the RPC message as if it had received it inline with
the Send data. Further decoding may issue additional RDMA Reads to
bring over additional chunks.
Although the handling of long messages requires one extra network
turnaround, in practice these messages will be rare if the posted
receive buffers are correctly sized, and of course they will be
non-existent for RDMA-aware upper layers.
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A long call RPC with request supplied via RDMA Read
RPC Client RPC Server
| RDMA over RPC Header |
Send | ------------------------------> |
| |
| Long RPC Call Msg |
| +------------------------------ | Read
| v-----------------------------> |
| |
| RDMA over RPC Reply |
| <------------------------------ | Send
An RPC with long reply returned via RDMA Read
RPC Client RPC Server
| RPC Call |
Send | ------------------------------> |
| |
| RDMA over RPC Header |
| <------------------------------ | Send
| |
| Long RPC Reply Msg |
Read | ------------------------------+ |
| <-----------------------------v |
| |
| Done |
Send | ------------------------------> |
It is possible for a single RPC procedure to employ both a long
call for its arguments, and a long reply for its results. However,
such an operation is atypical, as few upper layers define such
exchanges.
5.2. RDMA Write of Long Replies (Reply Chunks)
A superior method of handling long RPC replies is to have the RPC
client post a large buffer into which the server can write a large
RPC reply. This has the advantage that an RDMA Write may be
slightly faster in network latency than an RDMA Read, and does not
require the server to wait for the completion as it must for RDMA
Read. Additionally, for a reply it removes the need for an
RDMA_DONE message if the large reply is returned as a Read chunk.
This protocol supports direct return of a large reply via the
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inclusion of an OPTIONAL rdma_reply write chunk after the read
chunk list and the write chunk list. The client allocates a buffer
sized to receive a large reply and enters its steering tag, address
and length in the rdma_reply write chunk. If the reply message is
too long to return inline with an RDMA Send (exceeds the size of
the client's posted receive buffer), even with read chunks removed,
then the RPC server performs an RDMA Write of the RPC reply message
into the buffer indicated by the rdma_reply chunk. If the client
doesn't provide an rdma_reply chunk, or if it's too small, then if
the upper layer specification permits, the message MAY be returned
as a Read chunk.
An RPC with long reply returned via RDMA Write
RPC Client RPC Server
| RPC Call with rdma_reply |
Send | ------------------------------> |
| |
| Long RPC Reply Msg |
| <------------------------------ | Write
| |
| RDMA over RPC Header |
| <------------------------------ | Send
The use of RDMA Write to return long replies requires that the
client applications anticipate a long reply and have some knowledge
of its size so that an adequately sized buffer can be allocated.
This is certainly true of NFS READDIR replies; where the client
already provides an upper bound on the size of the encoded
directory fragment to be returned by the server.
The use of these "reply chunks" is highly efficient and convenient
for both RPC client and server. Their use is encouraged for
eligible RPC operations such as NFS READDIR, which would otherwise
require extensive chunk management within the results or use of
RDMA Read and a Done message. [NFSDDP]
6. Connection Configuration Protocol
RDMA Send operations require the receiver to post one or more
buffers at the RDMA connection endpoint, each large enough to
receive the largest Send message. Buffers are consumed as Send
messages are received. If a buffer is too small, or if there are
no buffers posted, the RDMA transport MAY return an error and break
the RDMA connection. The receiver MUST post sufficient, adequately
buffers to avoid buffer overrun or capacity errors.
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The protocol described above includes only a mechanism for managing
the number of such receive buffers, and no explicit features to
allow the RPC client and server to provision or control buffer
sizing, nor any other session parameters.
In the past, this type of connection management has not been
necessary for RPC. RPC over UDP or TCP does not have a protocol to
negotiate the link. The server can get a rough idea of the maximum
size of messages from the server protocol code. However, a
protocol to negotiate transport features on a more dynamic basis is
desirable.
The Connection Configuration Protocol allows the client to pass its
connection requirements to the server, and allows the server to
inform the client of its connection limits.
Use of the Connection Configuration Protocol by an upper layer is
OPTIONAL.
6.1. Initial Connection State
This protocol MAY be used for connection setup prior to the use of
another RPC protocol that uses the RDMA transport. It operates in-
band, i.e., it uses the connection itself to negotiate the
connection parameters. To provide a basis for connection
negotiation, the connection is assumed to provide a basic level of
interoperability: the ability to exchange at least one RPC message
at a time that is at least 1 KB in size. The server MAY exceed
this basic level of configuration, but the client MUST NOT assume
more than one, and MUST receive a valid reply from the server
carrying the actual number of available receive messages, prior to
sending its next request.
6.2. Protocol Description
Version 1 of the Connection Configuration protocol consists of a
single procedure that allows the client to inform the server of its
connection requirements and the server to return connection
information to the client.
The maxcall_sendsize argument is the maximum size of an RPC call
message that the client MAY send inline in an RDMA Send message to
the server. The server MAY return a maxcall_sendsize value that is
smaller or larger than the client's request. The client MUST NOT
send an inline call message larger than what the server will
accept. The maxcall_sendsize limits only the size of inline RPC
calls. It does not limit the size of long RPC messages transferred
as an initial chunk in the Read chunk list.
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The maxreply_sendsize is the maximum size of an inline RPC message
that the client will accept from the server.
The maxrdmaread is the maximum number of RDMA Reads which may be
active at the peer. This number correlates to the RDMA incoming
RDMA Read count ("IRD") configured into each originating endpoint
by the client or server. If more than this number of RDMA Read
operations by the connected peer are issued simultaneously,
connection loss or suboptimal flow control may result, therefore
the value SHOULD be observed at all times. The peers' values need
not be equal. If zero, the peer MUST NOT issue requests which
require RDMA Read to satisfy, as no transfer will be possible.
The align value is the value recommended by the server for opaque
data values such as strings and counted byte arrays. The client
MAY use this value to compute the number of prepended pad bytes
when XDR encoding opaque values in the RPC call message.
typedef unsigned int uint32;
struct config_rdma_req {
uint32 maxcall_sendsize;
/* max size of inline RPC call */
uint32 maxreply_sendsize;
/* max size of inline RPC reply */
uint32 maxrdmaread;
/* max active RDMA Reads at client */
};
struct config_rdma_reply {
uint32 maxcall_sendsize;
/* max call size accepted by server */
uint32 align;
/* server's receive buffer alignment */
uint32 maxrdmaread;
/* max active RDMA Reads at server */
};
program CONFIG_RDMA_PROG {
version VERS1 {
/*
* Config call/reply
*/
config_rdma_reply CONF_RDMA(config_rdma_req) = 1;
} = 1;
} = 100400;
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7. Memory Registration Overhead
RDMA requires that all data be transferred between registered
memory regions at the source and destination. All protocol headers
as well as separately transferred data chunks use registered
memory. Since the cost of registering and de-registering memory
can be a large proportion of the RDMA transaction cost, it is
important to minimize registration activity. This is easily
achieved within RPC controlled memory by allocating chunk list data
and RPC headers in a reusable way from pre-registered pools.
The data chunks transferred via RDMA MAY occupy memory that
persists outside the bounds of the RPC transaction. Hence, the
default behavior of an RPC over RDMA transport is to register and
de-register these chunks on every transaction. However, this is
not a limitation of the protocol - only of the existing local RPC
API. The API is easily extended through such functions as
rpc_control(3) to change the default behavior so that the
application can assume responsibility for controlling memory
registration through an RPC-provided registered memory allocator.
8. Errors and Error Recovery
RPC RDMA protocol errors are described in section 4. RPC errors
and RPC error recovery are not affected by the protocol, and
proceed as for any RPC error condition. RDMA Transport error
reporting and recovery are outside the scope of this protocol.
It is assumed that the link itself will provide some degree of
error detection and retransmission. iWARP's MPA layer (when used
over TCP), SCTP, as well as the Infiniband link layer all provide
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 link level and recover via
retransmission. RPC recovery can handle complete loss and re-
establishment of the link.
See section 11 for further discussion of the use of RPC-level
integrity schemes to detect errors, and related efficiency issues.
9. Node Addressing
In setting up a new RDMA connection, the first action by an RPC
client will be to obtain a transport address for the server. The
mechanism 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.
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10. RPC Binding
RPC services normally register with a portmap or rpcbind [RFC1833]
service, which associates an RPC program number with a service
address. (In the case of UDP or TCP, the service address for NFS
is normally port 2049.) This policy is no different with RDMA
interconnects, although it may require the allocation of port
numbers appropriate to each upper layer binding which uses the RPC
framing defined here.
When mapped atop the iWARP [RFC5040, RFC5041] transport, 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.
When mapped atop Infiniband [IB], which uses a GID-based service
endpoint naming scheme, a translation MUST be employed. One such
translation is defined in the Infiniband Port Addressing Annex
[IBPORT], which is appropriate for translating IP port addressing
to the Infiniband network. Therefore, in this case, IP port
addressing may be readily employed by the upper layer.
When a mapping standard or convention exists for IP ports on an
RDMA interconnect, there are several possibilities for each upper
layer to consider:
One possibility is to have an upper layer server register its
mapped IP port with the rpcbind service, under the netid (or
netid's) defined here. An RPC/RDMA-aware client 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 which are defined to use the rpcbind
service.
A second possibility is to have the server's portmapper
register itself on the RDMA interconnect at a "well known"
service address. (On UDP or TCP, this corresponds to port
111.) A client 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.
Alternatively, the client could simply connect to the mapped
well-known port for the service itself, if it is appropriately
defined.
Historically, different RPC protocols have taken different
approaches to their port assignment, therefore the specific method
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is left to each RPC/RDMA-enabled upper layer binding, and not
addressed here.
This specification defines a new "netid", to be used for
registration of upper layers atop iWARP [RFC5040, RFC5041] and
(when a suitable port translation service is available) Infiniband
[IB] in section 12, "IANA Considerations." Additional RDMA-capable
networks MAY define their own netids, or if they provide a port
translation, MAY share the one defined here.
11. Security Considerations
RPC provides its own security via the RPCSEC_GSS framework
[RFC2203]. RPCSEC_GSS can provide message authentication,
integrity checking, and privacy. This security mechanism will be
unaffected by the RDMA transport. The data integrity and privacy
features alter the body of the message, presenting it as a single
chunk. For large messages the chunk may be large enough to qualify
for RDMA Read transfer. However, there is much data movement
associated with computation and verification of integrity, or
encryption/decryption, so certain performance advantages may be
lost.
For efficiency, a more appropriate security mechanism for RDMA
links may be link-level protection, such as certain configurations
of IPsec, which may be co-located in the RDMA hardware. The use of
link-level protection MAY be negotiated through the use of the new
RPCSEC_GSS mechanism defined in [RPCSECGSSV2] in conjunction with
the Channel Binding mechanism [RFC5056] and IPsec Channel
Connection Latching [BTNSLATCH]. Use of such mechanisms is
REQUIRED where integrity and/or privacy is desired, and where
efficiency is required.
An additional consideration is the protection of the integrity and
privacy of local memory by the RDMA transport itself. The use of
RDMA by RPC MUST NOT introduce any vulnerabilities to system memory
contents, or to memory owned by user processes. These protections
are provided by the RDMA layer specifications, and specifically
their security models. It is REQUIRED that any RDMA provider used
for RPC transport be conformant to the requirements of [RFC5042] in
order to satisfy these protections.
Once delivered securely by the RDMA provider, any RDMA-exposed
addresses will contain only RPC payloads in the chunk lists,
transferred under the protection of RPCSEC_GSS integrity and
privacy. By these means, the data will be protected end-to-end, as
required by the RPC layer security model.
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Where results are supplied to the requester via Read chunks, a
server resource deficit can arise if the client does not promptly
acknowledge their status via the RDMA_DONE message. This can
potentially lead to a denial of service situation, with a single
client unfairly (and unnecessarily) consuming server RDMA
resources. Servers MUST protect against this situation,
originating from one or many clients. For example, a time-based
window of buffer availability may be offered, if the client fails
to obtain the data within the window, it will simply retry using
ordinary RPC retry semantics. Or, a more severe method would be
for the server to simply close the client's RDMA connection,
freeing the RDMA resources and allowing the server to reclaim them.
A fairer and more useful method is provided by the protocol itself.
The server MAY use the rdma_credit value to limit the number of
outstanding requests for each client. By including the number of
outstanding RDMA_DONE completions in the computation of available
client credits, the server can limit its exposure to each client,
and therefore provide uninterrupted service as its resources
permit.
However, the server must ensure that it does not decrease the
credit count to zero with this method, since the RDMA_DONE message
is not acknowledged. If the credit count were to drop to zero
solely due to outstanding RDMA_DONE messages, the client would
deadlock since it would never obtain a new credit with which to
continue. Therefore, if the server adjusts credits to zero for
outstanding RDMA_DONE, it MUST withhold its reply to at least one
message in order to provide the next credit. The time-based window
(or any other appropriate method) SHOULD be used by the server to
recover resources in the event that the client never returns.
The "Connection Configuration Protocol", when used, MUST be
protected by an appropriate RPC security flavor, to ensure it is
not attacked in the process of initiating an RPC/RDMA connection.
12. IANA Considerations
The new RPC transport is to be assigned a new 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 ports.
The following "nc_proto" registry string is hereby defined for this
purpose:
NC_RDMA "rdma"
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This netid MAY be used for any RDMA network satisfying the
requirements of section 2, and able to identify service endpoints
using IP port addressing, possibly through use of a translation
service as described above in section 10, RPC Binding.
As a new RPC transport, this protocol has no effect on RPC program
numbers or existing registered port numbers. However, new port
numbers MAY be registered for use by RPC/RDMA-enabled services, as
appropriate to the new networks over which the services will
operate.
The OPTIONAL Connection Configuration protocol described herein
requires an RPC program number assignment. The value "100400" is
hereby assigned:
rdmaconfig 100400 rpc.rdmaconfig
Currently, neither the nc_proto netid's nor the RPC program numbers
are are assigned by IANA. The list in [RFC1833] has served as the
netid registry, and the republication declared in [IANA-RPC] has
served as the program number registry. Ideally, IANA will create
explicit registries for these objects. However, in the absence of
new registries, this document would serve as the repository for the
RPC program number assignment, and the protocol netid.
13. Acknowledgements
The authors wish to thank Rob Thurlow, John Howard, Chet Juszczak,
Alex Chiu, Peter Staubach, Dave Noveck, Brian Pawlowski, Steve
Kleiman, Mike Eisler, Mark Wittle, Shantanu Mehendale, David
Robinson and Mallikarjun Chadalapaka for their contributions to
this document.
14. Normative References
[RFC2119]
S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", Best Current Practice, BCP 14, RFC 2119, March 1997.
[RFC1094]
Sun Microsystems, "NFS: Network File System Protocol
Specification", (NFS version 2) Informational RFC,
http://www.ietf.org/rfc/rfc1094.txt
[RFC1831bis]
R. Thurlow, Ed., "RPC: Remote Procedure Call Protocol
Specification Version 2", Standards Track RFC
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[RFC4506]
M. Eisler Ed., "XDR: External Data Representation Standard",
Standards Track RFC, http://www.ietf.org/rfc/rfc4506.txt
[RFC1813]
B. Callaghan, B. Pawlowski, P. Staubach, "NFS Version 3
Protocol Specification", Informational RFC,
http://www.ietf.org/rfc/rfc1813.txt
[RFC1833]
R. Srinivasan, "Binding Protocols for ONC RPC Version 2",
Standards Track RFC, http://www.ietf.org/rfc/rfc1833.txt
[RFC3530]
S. Shepler, B. Callaghan, D. Robinson, R. Thurlow, C. Beame,
M. Eisler, D. Noveck, "NFS version 4 Protocol", Standards
Track RFC, http://www.ietf.org/rfc/rfc3530.txt
[RFC2203]
M. Eisler, A. Chiu, L. Ling, "RPCSEC_GSS Protocol
Specification", Standards Track RFC,
http://www.ietf.org/rfc/rfc2203.txt
[RPCSECGSSV2]
M. Eisler, "RPCSEC_GSS Version 2", Internet Draft Work in
Progress draft-ietf-nfsv4-rpcsec-gss-v2
[RFC5056]
N. Williams, "On the Use of Channel Bindings to Secure
Channels", Standards Track RFC
[BTNSLATCH]
N. Williams, "IPsec Channels: Connection Latching", Internet
Draft Work in Progress draft-ietf-btns-connection-latching
[RFC5042]
J. Pinkerton, E. Deleganes, "Direct Data Placement Protocol
(DDP) / Remote Direct Memory Access Protocol (RDMAP) Security"
Standards Track RFC
15. Informative References
[NFSDDP]
B. Callaghan, T. Talpey, "NFS Direct Data Placement" Internet
Draft Work in Progress, draft-ietf-nfsv4-nfsdirect
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[RFC5040]
R. Recio et al., "A Remote Direct Memory Access Protocol
Specification", Standards Track RFC
[RFC5041]
H. Shah et al., "Direct Data Placement over Reliable
Transports", Standards Track RFC
[NFSRDMAPS]
T. Talpey, C. Juszczak, "NFS RDMA Problem Statement", Internet
Draft Work in Progress, draft-ietf-nfsv4-nfs-rdma-problem-
statement
[NFSv4.1]
S. Shepler et al., ed., "NFSv4 Minor Version 1" Internet Draft
Work in Progress, draft-ietf-nfsv4-minorversion1
[IB]
Infiniband Architecture Specification, available from
http://www.infinibandta.org
[IBPORT]
Infiniband Trade Association, "IP Addressing Annex", available
from http://www.infinibandta.org
[IANA-RPC]
IANA Sun RPC number statement,
http://www.iana.org/assignments/sun-rpc-numbers
16. Authors' Addresses
Tom Talpey
Network Appliance, Inc.
1601 Trapelo Road, #16
Waltham, MA 02451 USA
Phone: +1 781 768 5329
EMail: thomas.talpey@netapp.com
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Brent Callaghan
Apple Computer, Inc.
MS: 302-4K
2 Infinite Loop
Cupertino, CA 95014 USA
EMail: brentc@apple.com
17. Intellectual Property and Copyright Statements
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
Intellectual Property
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Information on the procedures with respect to rights in RFC
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specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
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Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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