Internet-Draft                                 Brent Callaghan
Expires: January 2005                    Sun Microsystems, Inc.
                                                    Tom Talpey
                                        Network Appliance, Inc.

Document: draft-ietf-nfsv4-rpcrdma-00.txt           July, 2004

                       RDMA Transport for ONC RPC

Status of this Memo

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   patent or other IPR claims of which I am aware have been disclosed,
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   disclosed, in accordance with RFC 3668.

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Copyright Notice

     Copyright (C) The Internet Society (2004).  All Rights Reserved.


   A protocol is described providing RDMA as a new transport for ONC
   RPC.  The RDMA transport binding conveys the benefits of efficient,

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Abstract RDMA Model  . . . . . . . . . . . . . . . . . . . . 3
   3.  Protocol Outline . . . . . . . . . . . . . . . . . . . . . . 5
   3.1.  Short Messages . . . . . . . . . . . . . . . . . . . . . . 5
   3.2.  Data Chunks  . . . . . . . . . . . . . . . . . . . . . . . 6
   3.3.  Flow Control . . . . . . . . . . . . . . . . . . . . . . . 6
   3.4.  XDR Encoding with Chunks . . . . . . . . . . . . . . . . . 7
   3.5.  Padding  . . . . . . . . . . . . . . . . . . . . . . . . . 9
   3.6.  XDR Decoding with Read Chunks  . . . . . . . . . . . . .  10
   3.7.  XDR Decoding with Write Chunks . . . . . . . . . . . . .  11
   3.8.  RPC Call and Reply . . . . . . . . . . . . . . . . . . .  11
   4.  RPC RDMA Message Layout  . . . . . . . . . . . . . . . . .  14
   4.1.  RPC RDMA Transport Header  . . . . . . . . . . . . . . .  14
   4.2.  XDR Language Description . . . . . . . . . . . . . . . .  16
   5.  Large Chunkless Messages . . . . . . . . . . . . . . . . .  18
   5.1.  Message as an RDMA Read Chunk  . . . . . . . . . . . . .  19
   5.2.  RDMA Write of Long Replies . . . . . . . . . . . . . . .  20
   5.3.  RPC RDMA header errors . . . . . . . . . . . . . . . . .  21
   6.  Connection Configuration Protocol  . . . . . . . . . . . .  22
   6.1.  Initial Connection State . . . . . . . . . . . . . . . .  22
   6.2.  Protocol Description . . . . . . . . . . . . . . . . . .  23
   7.  Memory Registration Overhead . . . . . . . . . . . . . . .  24
   8.  Errors and Error Recovery  . . . . . . . . . . . . . . . .  24
   9.  Node Addressing  . . . . . . . . . . . . . . . . . . . . .  25
   10.  RPC Binding . . . . . . . . . . . . . . . . . . . . . . .  25
   11.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  25
   12.  IANA Considerations . . . . . . . . . . . . . . . . . . .  26
   13.  Acknowledgements  . . . . . . . . . . . . . . . . . . . .  26
   14.  Normative References  . . . . . . . . . . . . . . . . . .  26
   15.  Informative References  . . . . . . . . . . . . . . . . .  27
   16.  Authors' Addresses  . . . . . . . . . . . . . . . . . . .  28
   17.  Full Copyright Statement  . . . . . . . . . . . . . . . .  28
   Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   RDMA is a technique for efficient movement of data over high speed
   transports.  It facilitates data movement via direct memory access by

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   hardware, yielding faster transfers of data over a network while
   reducing host CPU overhead.

   ONC RPC [RFC1831] is a remote procedure call protocol that has been
   run over a variety of transports.  Most implementations today use UDP
   or TCP.  RPC messages are defined in terms of an eXternal Data
   Representation (XDR) [RFC1832] 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

   RDMA transports present new semantics unlike the behaviors of either
   UDP and TCP.  They retain message delineations like UDP while also
   providing a reliable, sequenced data transfer like TCP.  All provide
   the new efficient, bulk transfer service of RDMA.  RDMA transports
   are therefore naturally viewed as a new transport type by ONC 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, is an obvious beneficiary of RDMA.
   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 Model

   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 procedure

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   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 [RFC1831] and [RFC1832].

   This protocol assumes an abstract model for RDMA transports.  The
   following terms, common in the RDMA lexicon, are used in this
   document.  A more complete glossary of RDMA terms can be found in

     o Registered Memory

       All data moved via RDMA must be resident in registered
       memory at its source and destination.  Each segment of
       registered memory must be identified with a Steering Tag
       (STag) 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.

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     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 (STag),
       memory address and length of the remote destination
       buffer.  A subsequent completion, provided by RDMA Send,
       must be obtained at the receiver to guarantee that RDMA
       Write data has been successfully placed in the receiver's

     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.

     In its abstract form, this protocol is not an interoperable stan-
     dard.  It becomes a useful, implementable standard only when mapped
     onto a specific RDMA transport, like iWARP [RDDP] or Infiniband

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 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

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   different ways to different remote memory destinations.

   All transfers of a CALL or REPLY begin with an RDMA send which
   transfers at least the transport 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.

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 filehandle argument and 4 bytes of length.  The reply to this
   common request is about 100 bytes.

   There is no benefit in transferring such small messages with an RDMA
   Read or Write operation.  The overhead in transferring STags 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 transport 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 will look like this:

        Client                                Server
           |               RPC Call              |
      Send |   ------------------------------>   |
           |                                     |
           |               RPC Reply             |
           |   <------------------------------   | Send

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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 in-
   line transfer not only avoids expensive data copies, but provides
   correct data alignment at the destination.

3.3.  Flow Control

   It is critical to provide 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.  Such errors are fatal to
   the connection.  This is a departure from conventional TCP/IP
   networking where buffers are allocated dynamically on an as-needed
   basis, and 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 server must
   be able to reply whether or not a new buffer can be posted to accept
   future receives.

   Flow control is implemented as a simple request/grant protocol in the
   transport header associated with each RPC message.  The transport
   header for RPC CALL messages contains a requested credit value for
   the server, which may be dynamically adjusted by the caller to match
   its expected needs.  The transport header for the RPC REPLY messages
   provide the granted result, which may have any value except it may
   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.

   While RPC CALLs may 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
   credits minus the number of requests in flight.

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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 XDR 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 an opaque byte array is large
   enough to be more efficiently moved via an RDMA data transfer
   operation like RDMA Read or RDMA Write.

   When sending any message (request or reply) that contains a candidate
   large data chunk, the XDR encoding routine avoids moving 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

   Since the chunks are to be moved via RDMA, the memory for each chunk
   must be 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, it is desirable to provide chunks to accept
   the data directly via RDMA Write.  These chunks will therefore be
   pre-filled by the server prior to responding, and XDR decode at the
   client will not be required.  These "write chunk lists" undergo a
   similar registration and advertisement to chunks built as a part of
   XDR encoding.  Just as with an encoded read chunk list, the memory
   referenced in an encoded write chunk list must be pre-registered.  If
   the client chooses not to make a write chunk list available, then the
   server must return data inline in the reply, or via a read chunk

   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 "<>"

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   encoding) the byte count which is encoded as part of the field
   remains in the XDR stream, as well as being encoded in the chunk
   list.  Only the data portion is elided.  This is important to
   maintain upper layer implementation compatibility - both the count
   and the data must be transferred as part of the XDR stream.  In
   addition, 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.

             Steering tag or handle obtained when the chunk
             memory is registered for RDMA.
             The length of the chunk in bytes.
             The offset or memory address of the chunk.
             For data which is to be encoded, the position in
             the XDR stream where the chunk would normally
             reside.  It is possible that a contiguous sequence
             of chunks might all have the same position.  For
             data which is to be decoded, no "position" is

     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.

     |                |                |
     | RDMA header w/ |   RPC Header   | Non-chunk args/results
     |     chunks     |                |

     Read chunk lists are structured differently from write chunk lists.
     This is due to the different usage - read chunks are decoded and
     indexed by their position in the XDR data stream, and may be used
     for both arguments and results.  Write chunks on the other hand are

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     used only for results, and have no preassigned offset in the XDR
     stream until the results are produced.  The mapping of Write chunks
     onto designated NFS procedures and results is described in [NFS-

     Therefore, read chunks are encoded as a single array, with each
     entry tagged by its position in the XDR stream.  Write chunks are
     encoded as a list of arrays of RDMA buffers, with each list element
     providing buffers for a separate result.

3.5.  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) prior to the first data byte, the
   data can be forced to align correctly in the second buffer.

                                            Buffer 1       Buffer 2
   Unpadded                              --------------  --------------



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   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.  If padding is to
   apply only to chunks at least 1 KB in size, then the threshold should
   be set to 1 KB.  The XDR routine at the peer will consult these
   values when decoding opaque values.  Where the decoded length exceeds
   the rdma_thresh, the XDR decode will skip over the appropriate
   padding as indicated by rdma_align and the current XDR stream

3.6.  XDR Decoding with Read Chunks

   The XDR decode process moves data from an XDR stream into a data
   structure provided by the 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 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 RDMA transport 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.  After completing such a transfer, the RPC
   receiver must issue an RDMA_DONE message (described in Section 3.8)
   to notify the peer 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.

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3.7.  XDR Decoding with Write Chunks

   When a "write chunk list" is provided for the results of the RPC
   CALL, the 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].

3.8.  RPC Call and Reply

   The RDMA transport for RPC provides three methods of moving data
   between client and server:

        Data are moved between client and server
        within an RDMA Send.

     RDMA Read
        Data are moved between client and server
        via an RDMA Read operation via STag, address
        and offset obtained from a read chunk list.

     RDMA Write
        Result data is moved from server to client
        via an RDMA Write operation via STag, address
        and offset obtained from a write chunk list
        or reply chunk in the client's RPC call message.

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     These methods of data movement may occur in combinations within a
     single RPC.  For instance, an RPC call may contain some in-line
     data along with some large chunks transferred via RDMA Read by 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:

          An RPC with write chunks in the call message looks like this:

             Client                                Server
                |     RPC Call + Write Chunk list     |
           Send |   ------------------------------>   |
                |                                     |
                |               Chunk 1               |
                |   <------------------------------   | Write
                |                  :                  |
                |               Chunk n               |
                |   <------------------------------   | Write
                |                                     |
                |               RPC Reply             |
                |   <------------------------------   | Send

          An RPC with read chunks in the call message looks like this:

             Client                                Server
                |     RPC Call + Read Chunk list      |
           Send |   ------------------------------>   |
                |                                     |
                |               Chunk 1               |
                |   +------------------------------   | Read
                |   v----------------------------->   |
                |                  :                  |
                |               Chunk n               |
                |   +------------------------------   | Read
                |   v----------------------------->   |
                |                                     |
                |               RPC Reply             |
                |   <------------------------------   | Send

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          And an RPC with read chunks in the reply message looks like this:

             Client                                Server
                |               RPC Call              |
           Send |   ------------------------------>   |
                |                                     |
                |     RPC Reply + Read Chunk list     |
                |   <------------------------------   | Send
                |                                     |
                |               Chunk 1               |
           Read |   ------------------------------+   |
                |   <-----------------------------v   |
                |                  :                  |
                |               Chunk n               |
           Read |   ------------------------------+   |
                |   <-----------------------------v   |
                |                                     |
                |               RPC Done              |
           Send |   ------------------------------>   |

     The final RPC Done message allows the client to signal the server
     that it has received the chunks, so the server can de-register and
     free the memory holding the chunks.  An RPC Done completion is not
     necessary for an RPC call, since the RPC reply Send is itself a
     receive completion notification.

     The RPC Done message has no effect on protocol latency since the
     client has no expectation of a reply from the server.  Nor does it
     adversely affect bandwidth since it is only 16 bytes in length.  In
     the event that the client fails to return the Done message, the
     server can proceed with a de-register and free chunk buffers after
     a time-out.

     It is important to note that the RPC Done message consumes a credit
     at the server.  The client must account for this in its accounting
     of available credits, and the server should replenish the credit
     consumed by RPC Done at its earliest oportunity.

     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

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     upper layer protocol exchanges which transferred bulk data in both
     the call and corresponding reply.

4.  RPC RDMA Message Layout

   RPC call and reply messages are conveyed across an RDMA transport
   with a prepended RDMA transport header.  The transport 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 [RFC1831] 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 transport
   header) is too long for the posted receive buffer (even after any
   large chunks are removed), then the entire RPC message can be moved
   separately as a chunk, leaving just the transport header in the RDMA

4.1.  RPC RDMA Transport Header

   The RPC RDMA transport 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 may not be present
   depending on the type of transmission.  The four fields which are
   always present are:

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     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 mirrors the XID
        in the RPC call header, and takes precedence.

     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 must not be sent
        in excess of the currently granted limit.

     4. Message type.
        RDMA_MSG = 0 indicates that chunk lists and RPC message
        follow.  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 RDMA transport header.  RDMA_MSGP =
        2 indicates that a chunk list and RPC message with some
        padding follow.  RDMA_DONE = 3 indicates that the
        message signals the completion of a chunk transfer via
        RDMA Read.  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
     RDMA transport header.

     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.

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     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 transport header of message type RDMA_MSG or RDMA_MSGP will be
     followed by the RPC call or reply message, beginning with the XID.
     This XID should match the one at the beginning of the RPC message

     |        |         |         | 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 ini-
     tial header, the chunk list, and the RPC message itself.

4.2.  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;

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      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 */

      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:
         case RDMA_ERROR:
           rpc_rdma_error rdma_error;

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      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 */

      enum rpc_rdma_errcode {
         ERR_VERS = 1,
         ERR_CHUNK = 2

      union rpc_rdma_error switch (rpc_rdma_errcode) {
         case ERR_VERS:
           uint32               rdma_vers_low;
           uint32               rdma_vers_high;
         case ERR_CHUNK:
           uint32               rdma_extra[8];

5.  Large Chunkless Messages

   The receiver of RDMA Send messages is required to have previously
   posted one or more correctly sized buffers.  The client can inform
   the server of the maximum size of its RDMA Send messages via the
   Connection Configuration Protocol described later in this document.

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   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, chunkless messages that would
   demand a very large buffer be posted.  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 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.

   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 a transport header with a message type of

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   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 in-line 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 should be rare if the posted
   receive buffers are correctly sized, and of course they will be non-
   existent for RDMA-aware upper layers.

   An RPC with long reply returned via RDMA Read looks like this:

        Client                                Server
           |             RPC Call                |
      Send |   ------------------------------>   |
           |                                     |
           |         RPC Transport Header        |
           |   <------------------------------   | Send
           |                                     |
           |          Long RPC Reply Msg         |
      Read |   ------------------------------+   |
           |   <-----------------------------v   |
           |                                     |
           |               RPC Done              |
      Send |   ------------------------------>   |

5.2.  RDMA Write of Long Replies

   An alternative method of handling long, chunkless RPC replies is to
   have the 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.  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
   inclusion of an optional rdma_reply write chunk after the read chunk
   list and the write chunk list.  The client allocates a buffer sized

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   to receive a large reply and enters its STag, address and length in
   the rdma_reply write chunk.  If the reply message is too long to
   return in-line with an RDMA Send (exceeds the size of the client's
   posted receive buffer), even with read chunks removed, then the
   server RDMA writes 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 the message must be returned as a
   Read chunk.

   An RPC with long reply returned via RDMA Write looks like this:

        Client                                Server
           |      RPC Call with rdma_reply       |
      Send |   ------------------------------>   |
           |                                     |
           |          Long RPC Reply Msg         |
           |   <------------------------------   | Write
           |                                     |
           |         RPC Transport Header        |
           |   <------------------------------   | Send

     The use of RDMA Write to return long replies requires that the
     client application anticipate a long reply and have some knowledge
     of its size so that a correctly 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 direc-
     tory fragment to be returned by the server.

5.3.  RPC RDMA header errors

   When a peer receives an RPC RDMA message, it must perform certain
   basic validity checks on the header and chunk contents.  If errors
   are detected in an RPC request, an RDMA_ERROR reply should be

   Two types of errors are defined, version mismatch and invalid chunk
   format.  When the peer detects an RPC 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 can be any value otherwise valid at the

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   receiver.  When other decoding errors are detected in the header or
   chunks, either an RPC decode error may be returned, or the error code

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 will return an error and break the RDMA
   connection.  The receiver must post sufficient, correctly sized
   buffers to avoid buffer overrun or capacity errors.

   The protocol described above includes only a mechanism for managing
   the number of such receive buffers, and no explicit features to allow
   the 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.

6.1.  Initial Connection State

   This protocol will 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 it.

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6.2.  Protocol Description

   Version 1 of the 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 maxcallsize argument is the maximum size of an RPC call message
   that the client will send in-line in an RDMA Send message to the
   server.  The server may return a maxcallsize value that is smaller or
   larger than the client's request.  The client must not send an in-
   line call message larger than what the server will accept.  The
   maxcallsize limits only the size of in-line RPC calls.  It does not
   limit the size of long RPC messages transferred as an initial chunk
   in the Read chunk list.

   The maxreplysize is the maximum size of an in-line 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 can
   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  maxcallsize;  /* max size of in-line RPC call */
           uint32  maxreplysize; /* max size of in-line RPC reply */
           uint32  maxrdmaread;  /* max active RDMA Reads at client */

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      struct config_rdma_reply {
           uint32  maxcallsize;  /* 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;
      } = nnnnnn;  <-- Need program number assigned

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 must 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 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

   Error reporting and recovery is outside the scope of this protocol.
   It is assumed that the link itself will provide some degree of error

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   detection and retransmission.  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

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 outside the scope of
   this protocol.

10.  RPC Binding

   RPC services normally register with a portmap or rpcbind 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 should be no different with RDMA interconnects.

   One 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. a VI discriminator or
   an Infiniband GID.

11.  Security

   ONC RPC provides its own security via the RPCSEC_GSS framework [RFC
   2203].  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 any performance advantage will be lost.

   There should be no new issues here with exposed addresses.  The only

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   exposed addresses here are in the chunk list and in the transport
   packets generated by an RDMA.  The data contained in these addresses
   is adequately protected by RPCSEC_GSS integrity and privacy.
   RPCSEC_GSS security mechanisms are typically implemented by the host
   CPU.  This additional data movement and CPU use may cancel out much
   of the RDMA direct placement and offload benefit.

   A more appropriate security mechanism for RDMA links may be link-
   level protection, like IPSec, which may be co-located in the RDMA
   link hardware.  The use of link-level protection may be negotiated
   through the use of a new RPCSEC_GSS mechanism like the Credential
   Cache GSS Mechanism (CCM) [CCM].

12.  IANA Considerations

   As a new RPC transport, this protocol should have no effect on RPC
   program numbers or registered port numbers.  The new RPC transport
   should be assigned a new RPC "netid".  If adopted, the Connection
   Configuration protocol described herein will require an RPC program
   number assignment.

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 and Shantanu Mehendale for their
   contributions to this document.

14.  Normative References

      R. Srinivasan, "RPC: Remote Procedure Call Protocol Specification
      Version 2",
      Standards Track RFC,

      R. Srinivasan, "XDR: External Data Representation Standard",
      Standards Track RFC,

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      B. Callaghan, B. Pawlowski, P. Staubach, "NFS Version 3 Protocol
      Informational RFC,

      S. Shepler, B. Callaghan, D. Robinson, R. Thurlow, C. Beame, M.
      Eisler, D. Noveck, "NFS version 4 Protocol",
      Standards Track RFC,

      M. Eisler, A. Chiu, L. Ling, "RPCSEC_GSS Protocol Specification",
      Standards Track RFC,

15.  Informative References

   [RDMA] R. Recio et al, "An RDMA Protocol Specification",
      Internet Draft Work in Progress,

   [CCM] M. Eisler, N. Williams, "CCM: The Credential Cache GSS Mechanism",
      Internet Draft Work in Progress,

      T. Talpey, S. Shepler, J. Bauman, "NFSv4 Session Extensions"
      Internet Draft Work in Progress,

      B. Callaghan, T. Talpey, "NFS Direct Data Placement"
      Internet Draft Work in Progress,

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      Remote Direct Data Placement Working Group Charter,

      Remote Direct Data Placement Working Group Problem Statement,
      Internet Draft Work in Progress,
      A. Romanow, J. Mogul, T. Talpey, S. Bailey,

      Infiniband Architecture Specification,

16.  Authors' Addresses

     Brent Callaghan
     Sun Microsystems, Inc.
     17 Network Circle
     Menlo Park, California 94025 USA

     Phone: +1 650 786 5067

     Tom Talpey
     Network Appliance, Inc.
     375 Totten Pond Road
     Waltham, MA 02451 USA

     Phone: +1 781 768 5329

17.  Full Copyright Statement

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     Copyright (C) The Internet Society (2004).  This document is sub-
     ject to the rights, licenses and restrictions contained in BCP 78
     and except as set forth therein, the authors retain all their

     This document and the information contained herein are provided on

Intellectual Property

     The IETF takes no position regarding the validity or scope of any
     Intellectual Property Rights or other rights that might be claimed
     to pertain to the implementation or use of the technology described
     in this document or the extent to which any license under such
     rights might or might not be available; nor does it represent that
     it has made any independent effort to identify any such rights.
     Information on the procedures with respect to rights in RFC docu-
     ments can be found in BCP 78 and BCP 79.

     Copies of IPR disclosures made to the IETF Secretariat and any
     assurances of licenses to be made available, or the result of an
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     fication can be obtained from the IETF on-line IPR repository at

     The IETF invites any interested party to bring to its attention any
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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-


     Funding for the RFC Editor function is currently provided by the
     Internet Society.

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