Network Working Group                                         C. Bestler
Internet-Draft                                         February 21, 2003
Expires: August 22, 2003

    Applicability of Remote Direct Memory Access Protocol (RDMA) and
                      Direct Data Placement (DDP)

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   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document describes the applicability of Remote Direct Memory
   Access Protocol (RDMAP)  and the Direct Data Placement Protocol

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Direct Placement . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Tagged Buffers . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Tagged Buffers as ULP Credits  . . . . . . . . . . . . . . . . 10
   7.  RDMA Read  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Specialized Transports . . . . . . . . . . . . . . . . . . . . 13
   9.  Local Interface Implications . . . . . . . . . . . . . . . . . 14
   10. Comparison of IP Transports  . . . . . . . . . . . . . . . . . 15
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 16
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 17

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1. Introduction

   They provide for application independent efficient placement of
   application payload into Upper Layer Protocol (ULP) specified
   buffers.  DDP can use multiple standard IP transports including SCTP
   and TCP.  RDMAP provides RDMA services, on top of DDP.  This document
   contrasts the applicability of RDMAP/DDP versus direct use of the
   underlying IP transports, and versus non-IP transports designed
   specifically with RDMA capabilities.

   The applicability of RDMAP/DDP is driven by their unique

      The existence of an application independent protocol allows common
      solutions to be implemented in hardware and/or the kernel.  This
      document will discuss when common data placement procedures are of
      the greatest benefit to applications as contrasted with direct use
      of the underlying transport.

      DDP supports both untagged and tagged buffers.  Tagged buffers
      allow the Data Sink ULP to be indifferent to what order (or in
      what packets) the Data Source delivered the data.  This document
      will discuss when Data Source flexibility is of benefit to

   DDP works over standard unmodified IP transports, such as SCTP.  Some
   non-IP transports, such as InfiniBand, directly integrate RDMA
   features.  This document will review the applicability of providing
   RDMA services over ubiquitous IP transports as opposed to the use of
   customized transport protocols.

   RDMAP defines RDMA Reads, which allow remote access to advertised
   buffers.  This document will review the advantages of using RDMA
   Reads as contrasted to alternate solutions.

   The full capabilities of DDP and RDMAP can only be fully realized by
   applications that are designed to exploit them.  The co-existence of
   RDMAP/DDP aware local interfaces with traditional socket interfaces
   will also be explored.

   Finally, DDP support is defined for at least two IP transports: SCTP
   and TCP.  The rationale for supporting both transports is reviewed,
   as well as when each would be the appropriate selection.

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2. Definitions

   Advertisement - the act of informing a Remote Peer that a local RDMA
      Buffer is available to it.  A Node makes available an RDMA Buffer
      for incoming RDMA Read or RDMA Write access by informing its RDMA/
      DDP peer of the Tagged Buffer identifiers (STag, base address, and
      buffer length).  This advertisement of Tagged Buffer information
      is not defined by RDMA/DDP and is left to the ULP.  A typical
      method would be for the Local Peer to embed the Tagged Buffer's
      Steering Tag, base address, and length in a Send Message destined
      for the Remote Peer.

   Data Sink - The peer receiving a data payload.  Note that the Data
      Sink can be required to both send and receive RDMA/DDP Messages to
      transfer a data payload.

   Data Source - The peer sending a data payload.  Note that the Data
      Source can be required to both send and receive RDMA/DDP Messages
      to transfer a data payload.

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3. Conventions

   they appear in this document, are to be interpreted as described in
   RFC2119 [1].

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4. Direct Placement

   Direct Data Placement optimizes the placement of ULP payload into the
   correct destination buffers while minimizing the required ULP
   interactions and typically eliminating intermediate copying.  This
   capability is most valuable for applications that require multiple
   transport layer packets for each required ULP interaction.

   While reducing the number of required ULP interactions is in itself
   desirable, it is critical for high speed connections.  The burst
   packet rate for a high speed interface could easily exceed the host
   systems ability to switch ULP contexts.

   Content access applications are primary examples of applications with
   both high bandwidth and high content to required ULP interaction
   ratios.  These applications include file access protocols (NAS),
   storage access (SAN), database access and other application specific
   forms of content access such as HTTP, XML and email.

   The degree to which this is an optimization depends on which
   transport is being compared with, and on the nature of the local
   interface.  Pre-posting of receive buffers allows direct placement of
   incoming data.  However pre-posting buffers requires the receiving
   side to accurately predict the required buffers and their sizes.
   This is not feasible for all ULPs.  By contrast, DDP only requires
   the ULP to predict the sequence and size of incoming untagged

   Direct Data Placement can be achieved without RDMA.  Pre-posting of
   receive buffers allows any network stack to place data directly to
   user buffers.

   An application that could predict incoming messages and required
   nothing more than direct placement into buffers might be able to do
   so with a properly designed local interface to SCTP or TCP.  Doing so
   for  TCP requires making predictions at a byte level rather than a
   message level.

   The main benefit of DDP for such an application would be that pre-
   posting of receive buffers is a mandated local interface capability,
   and that predictions can be made on a per-message basis (not per

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5. Tagged Buffers

   A more critical advantage of DDP is the ability of the Data Source to
   use tagged buffers.  Tagging transfers allows the Data Source to
   choose the ordering and packetization of its payload deliveries.
   With direct data placement, the packetization and delivery of payload
   must be agreed by the ULP peers.  Even if there is an encoding of
   what is being transferred, as is common with middleware solutions,
   this information is not understood at the application independent
   layers.  The directions on where to place the incoming data cannot be
   accessed without switching to the application layer first.  DDP
   provides a standardized 'packing list' which can be interpreted
   without application layer involvement.  Indeed, it is designed to be
   implementable in hardware.

   The use of a standardized 'packing list' minimizes the required
   interactions with the ULP.  This can be extremely beneficial for
   applications that use multiple transport layer packets to accomplish
   what is a single ULP interaction.

   There are other benefits of tagged buffers covered in later sections.

   An application that had no opportunity to use tagged buffers would
   derive virtually no benefit from the use of DDP as opposed to SCTP.
   But while tagged buffers are the justification for DDP, DDP still
   relies on untagged buffers.  Without them the only method to exchange
   buffer advertisements would involve out-of-band communications and/or
   sharing of compile time constants.  However, most ULP protocols built
   upon RDMA transports continue to use untagged buffers for requests
   and responses.

   Limiting use of untagged buffers to requests and responses by moving
   all bulk data using tagged transfers can greatly simplify the amount
   of prediction that the Data Sink must perform in pre-posting receive
   buffers.  For example, a typical DDP enabled exchange would consist
   of the following operations:

      Client sends transaction request to server's untagged buffer.

      This request includes buffer advertisements for the buffers where
      it wants the results to be placed.

      Server performs multiple tagged puts to the advertised buffers.

      Server sends transaction reply to client's untagged buffer.

   With this type of exchange the pacing and required size of untagged
   buffers is highly predictable.  The variability of response sizes is

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   absorbed by tagged transfers.

   Use of tagged transfers is especially applicable when the Data Sink
   does not know the actual size, structure or location of the content
   it is requesting (or updating).

   For example, suppose the Data Sink ULP needs to fetch four related
   pieces of data into a four separate buffers.  With SCTP the Data Sink
   ULP could receive four messages into four separate buffers, only
   having to predict the maximum size of each.  However it would have to
   dictate the order in which the Data Source supplied the separate
   pieces.  If the Data Source found it advantageous to fetch them in a
   different order it would have to use intermediate buffering to re-
   order the pieces into the expected order even though the application
   only required that all four be delivered and did not truly have an
   ordering requirement.

   Techniques such as RAID striping and mirroring represent this same
   problem, but one step further.  What appears to be a single resource
   to the Data Sink is actually stored in separate locations by the Data
   Source.  Non-DDP protocols would either require the Data Source to
   fetch the material in the desired order or force the Data Source to
   use its own holding buffers to assemble an image of the destination

   While sometimes referred to as a "buffer-to-buffer" solution, RDMA
   more fundamentally enables remote buffer access.  The ULP is free to
   work with larger remote buffers than it has locally.  This reduces
   buffering requirements and the number of times the data must be
   copied in an end-to-end transfer.

   There are numerous reasons why the Data Sink would not know the true
   order or location of the requested data.  It could be different for
   each client, different records selected and/or different sort orders,
   RAID striping, file fragmentation, volume fragmentation, volume
   mirroring and server-side dynamic compositing of content (such as
   server side includes for HTTP).

   In all of these cases the Data Source is free to assemble the desired
   data in the Data Sinks buffer in whatever order the component data
   becomes available to it.  It is not constrained on ordering.  It does
   not have to assemble an image in its own memory before creating it in
   the Data Sink's buffers.

   Note that while DDP enables use of tagged messages for bulk transfer,
   there are some application scenarios where untagged messages would
   still be used for bulk transfer.  For example, under the Direct
   Access File Server (DAFS) protocol the file server does not expose

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   its own memory to its clients.  A client wishing to write may
   advertise a buffer which the server will issue RDMA Reads upon.
   However, when performing a small write it may be preferable to
   include the data in the untagged message rather than incurring an
   additional round trip with the RDMA Read and its response.

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6. Tagged Buffers as ULP Credits

   The handling of end-to-end buffer credits differs considerably with
   DDP than when the ULP directly uses either TCP or SCTP.

   With both TCP and SCTP buffer credits are based upon the receiver
   granting transmit permission based on the total number of bytes.
   These credits reflect system buffering resources and/or simple flow
   control.  They do not represent ULP resources.

   DDP defines no standard flow control, but presumes the existince of a
   ULP mechanism.  The presumed mechanism is that the Data Sink ULP has
   issued credits to the Data Source allowing the Data Source to send a
   specific number of untagged messages.

   The ULP peers must ensure that the sender is aware of the maximum
   size that can be sent to any specific target buffer.  One method of
   doing so is  to use a standard size for all untagged buffers within a
   given connection.  For example, DAFS specifies an initial size
   requirement for session establishment, during which the  untagged
   buffer size for the remainder of the session is negotiated.

   Tagged buffers are ULP resources advertised directly from ULP to ULP.
   A DDP put to a known tagged buffer is constrained only by transport
   level flow control, not by available system buffering.

   Either tagged or untagged buffers allows bypassing of system buffer
   resources.  Use of tagged buffers additionally allows the Data Source
   to choose what order to exercise the credits in.

   To the extent allowed by the ULP, tagged buffers are also divisible
   resources.  The Data Sink can advertise a single 100 KB buffer, and
   then receive notifications from its peer that it had written 50 KB,
   20 KB and 30 KB to that buffer in three successive transactions.

   ULP-management of tagged buffer resources, independent of transport
   and DDP layer credits, is an additional benefit of RDMA protocols.
   Large bulk transfers cannot be blocked by limited general purpose
   buffering capacity.  Applications can flow control  based upon higher
   level abstractions, such as number of outstanding requests,
   independent of the amount of data that must be transferred.

   However, use of system buffering, as offered by direct use of the
   underlying transports, can be preferable under certain circumstances.

   One example would be when the number of target ULP buffers is
   sufficiently large, and the rate at which any writes arrive is
   sufficiently low, that pinning all the target ULP buffers in memory

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   would be undesirable.  The maximum transfer rate, and hence the
   maximum amount of system buffering required,  may be more stable and
   predictable than the total ULP buffer exposure.

   Another would be the Data Sink wishes to receive a stream of data at
   a predictable rate, but does not know in advance what the size of
   each data packet will be.  This is common from streaming media that
   has been encoded with a variable bit rate.  With DDP the Data Sink
   would either have to use untagged buffers large enough for the
   largest packet, or advertise a circular buffer.  If for security or
   other reasons the Data Sink did not want the size of its buffer to be
   publicly known, using the underlying SCTP transport directly may be
   preferable because of their byte-oriented credits.

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7. RDMA Read

   RDMAP/DDP is more than just a "buffer-to-buffer" solution.  A simple
   buffer transfer protocol could have been designed to efficiently
   transfer buffers.  RDMA Reads allow the Data Sink to fetch exactly
   the portion of the peer ULP buffer required on a "just in time"
   basis.  Further, this can be done without requiring per-fetch support
   from the Data Source ULP.

   Storage servers typically have a maximum write buffer.  There is
   little benefit in transferring data from the Data Source far in
   advance of when it will be written to the persistent storage media.
   In this fashion a relatively small number of block sized buffers can
   be used to execute a single transaction that specified writing a
   large file.

   This same capability can be used when the desired portion of the
   advertised buffer is not known in advance.  For example the
   advertised buffer could contain performance statistics.  The data
   sink could request the portions of the data it required, without
   requiring an interaction with the Data Source ULP.

   This is applicable for many applications that publish semi-volatile
   data that does not require transactional validity checking (i.e.,
   authorized users have read access to the entire set of data).  It is
   less applicable when there are ULP consistency checks that must be
   performed upon the data.  Such applications would be better served by
   having the client send a request, and having the server use RDMA
   Writes to publish the requested data.  Neither RDMAP or DDP provide
   mechanisms for bundling multiple disjoint updates into an atomic
   transaction.  Therefore use of an advertised buffer as a data
   resource is subject to the same caveats as any randomly updated data
   resource, such as flat files, that do not enforce their own
   referential integrity.

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8. Specialized Transports

   DDP is defined to operate over ubiquitous IP transports such as SCTP
   and TCP.  This enabled a new DDP-enabled node to be added anywhere to
   an IP network.  No DDP-specific support from middle-boxes is

   There are non-IP transport fabric offering RDMA capabilities.
   Because these capabilities are integrated with the transport protocol
   they have some technical advantages when compared to RDMA over IP.
   For example fencing of RDMA operations can be based upon transport
   level acks.  Because DDP is cleanly layered over an IP transport, any
   explicit RDMA layer ack must be separate from the transport layer

   There may be deployments where the benefits of RDMA/transport
   integration outweigh the benefits of being on an IP network.

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9. Local Interface Implications

   Full utilization of DDP and RDMAP capabilities requires a local
   interface that explicitly requests these services.  Protocols such as
   Sockets Direct Protocol (SDP) can allow applications to keep their
   traditional byte-stream or message-stream interface and still enjoy
   many of the benefits of the optimized wire level  protocols.

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10. Comparison of IP Transports

   It is the responsibility of the ULP to determine which IP transport
   is best suited to its needs.

   SCTP provides for preservation of message boundaries.  Each DDP
   segment will be delivered within a single SCTP packet.  The
   equivalent services are only available with TCP through the use of
   the MPA adaptation layer.

   SCTP also provides multi-streaming.  When the same pair of hosts have
   need for multiple DDP streams this can be a major advantage.  A
   single SCTP association carries multiple DDP streams, consolidating
   connection setup and flow control.

   Even with the MPA adaptation layer, DDP traffic will appear to all
   network traffic as normal TCP connection.  In many environmenets
   there may be a requirement to use only TCP connections to satisfy
   existing network elements and/or to facilitate monitoring and control
   of connections.

   A DDP stream delivered via MPA/TCP will require more processing
   effort than one delivered over SCTP.  However this extra work may be
   justified for many deployments where full SCTP support is unavailable
   in the intermediate network.

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   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

Author's Address

   Caitlin Bestler
   1241 W. North Shore
   # 2G
   Chicago, IL  60626

   Phone: +1-773-743-1594

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