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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5666

NFSv4 Working Group                                      Tom Talpey
Internet-Draft                                               NetApp
Intended status: Standards Track                    Brent Callaghan
Expires: October 17, 2008                                     Apple
                                                     April 16, 2008

    Remote Direct Memory Access Transport for Remote Procedure Call
                      draft-ietf-nfsv4-rpcrdma-08


Status of this Memo

     By submitting this Internet-Draft, each author represents that any
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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 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.  Acknowledgments . . . . . . . . . . . . . . . . . . . . .  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 upper layer protocol to
     specify the most efficient mode for each of the segments in a
     message.  Multiple segments of a message may thereby 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).  It is however possible
     for RPC/RDMA to be dynamically enabled, in the course of
     negotiating the use of RDMA via an upper layer exchange.  Because
     RPC framing delimits an entire RPC request or reply, the resulting
     shift in framing must occur between distinct RPC messages, and in
     concert with the transport.

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.

     There is no benefit in transferring such small messages with an



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     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
     must always be able to reply to client requests, whether or not new



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     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 upper layer protocols choose to supply results 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 for such
     upper layer protocols 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.  Acknowledgments

     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.

     This document and the information contained herein are provided on
     an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
     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
     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
     documents 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
     attempt made to obtain a general license or permission for the use
     of such proprietary rights by implementers or users of this
     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-
     ipr@ietf.org.

Acknowledgment
     Funding for the RFC Editor function is provided by the IETF
     Administrative Support Activity (IASA).












































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