Remote Direct Data Placement                                  C. Bestler
Working group                                                   Broadcom
Internet-Draft                                                  L. Coene
Expires: March 30, 2006                                          Siemens
                                                      September 26, 2005

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

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes the applicability of Remote Direct Memory
   Access Protocol (RDMAP) and the Direct Data Placement Protocol (DDP).
   It comparese and contrasts the different transport options over IP
   that DDP can use, provides guidance to ULP developers on choosing
   between available transports and/or how to be indifferent to the
   specific transport layer used, compares use of DDP with direct use of

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   the supporting transports, and compares DDP over IP transports with
   non-IP transports that support RDMA functionality.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Direct Placement . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Fewer Required ULP Interactions  . . . . . . . . . . . . .  6
     3.2.  Direct Placement using only the LLP  . . . . . . . . . . .  6
   4.  Tagged Messages  . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Order Independent Reception  . . . . . . . . . . . . . . .  8
     4.2.  Reduced ULP Notifications  . . . . . . . . . . . . . . . .  8
     4.3.  Simplified ULP Exchanges . . . . . . . . . . . . . . . . .  9
     4.4.  Order Independent Sending  . . . . . . . . . . . . . . . . 10
     4.5.  Tagged Buffers as ULP Credits  . . . . . . . . . . . . . . 11
   5.  RDMA Read  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  LLP Comparisons  . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Multistreaming Implications  . . . . . . . . . . . . . . . 14
     6.2.  Out of Order Reception Implications  . . . . . . . . . . . 14
     6.3.  Header and Marker Overhead . . . . . . . . . . . . . . . . 14
     6.4.  Middlebox Support  . . . . . . . . . . . . . . . . . . . . 15
     6.5.  Processing Overhead  . . . . . . . . . . . . . . . . . . . 15
     6.6.  Data Integrity Implications  . . . . . . . . . . . . . . . 15
       6.6.1.  MPA/TCP Specifics  . . . . . . . . . . . . . . . . . . 15
       6.6.2.  SCTP Specifics . . . . . . . . . . . . . . . . . . . . 16
     6.7.  Non-IP Transports  . . . . . . . . . . . . . . . . . . . . 16
       6.7.1.  No RDMA Layer Ack  . . . . . . . . . . . . . . . . . . 16
     6.8.  Other IP Transports  . . . . . . . . . . . . . . . . . . . 17
     6.9.  LLP Independent Session Establishment  . . . . . . . . . . 17
       6.9.1.  RDMA-only Session Establishment  . . . . . . . . . . . 18
       6.9.2.  RDMA-Conditional Session Establishment . . . . . . . . 18
   7.  Local Interface Implications . . . . . . . . . . . . . . . . . 20
   8.  Security considerations  . . . . . . . . . . . . . . . . . . . 21
     8.1.  Connection/Association Setup . . . . . . . . . . . . . . . 21
     8.2.  Tagged Buffer Exposure . . . . . . . . . . . . . . . . . . 21
     8.3.  Impact of Encrypted Transports . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24

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1.  Introduction

   Remote Direct Memory Access Protocol (RDMAP) and Direct Data
   Placement (DDP) work together to provide application independent
   efficient placement of application payload directly into buffers
   specified by the Upper Layer Protocol (ULP).

   The DDP protocol is responsible for direct placement of received
   payload into ULP specified buffers.  The RDMAP protocol provides
   completion notifications to the ULP and support for Data Sink
   initiated fetch of advertised buffers (RDMA Reads).

   DDP and RDMAP are both application independent protocols which allow
   the ULP to perform remote direct data placement.  DDP can use
   multiple standard IP transports including SCTP and TCP.

   By clarifying the situations where the functionality of these
   protocols are applicable, this document can guide implementers,
   application and protocol designers in selecting which protocols to

   The applicability of RDMAP/DDP is driven by their unique

   o  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
      application specific solutions built on top of direct use of the
      underlying transport.

   o  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 sent the data, or what order they
      are received in.  This document will discuss when Data Source
      flexibility is of benefit to applications.

   o  RDMAP consolidates ULP notifications, thereby minimizing the
      number of required ULP interactions.

   o  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.

   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.  Due to the fact that DDP is defined

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   cleanly as a layer over existing IP transports, DDP has simpler
   ordering rules than some prior RDMA protocols.  This may have some
   implications for application designers.

   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.

   Lower Layer Protocol (LLP) The transport protocol that provides
      services to DDP.  This is an IP transport with any required
      adaptation layer.  Adaptation layers are defined for SCTP and TCP.

   Steering Tag (STag) An identifier of a Tagged Buffer on a Node, valid
      as defined within a protocol specification.

   Tagged Message A DDP message that is directed to a ULP specified
      buffer based upon imbedded addressing information.  In the
      immediate sense, the destination buffer is specified by the
      message sender.

   Untagged Message A DDP message that is directed to a ULP specified
      buffer based upon a Message Sequence Number being matched with a
      receiver supplied buffer.  The destination buffer is specified by
      the message receiver.

   Upper Layer Protocol (ULP) The direct user of RDMAP/DDP services.
      This may be an application, or a middleware layer such as Sockets
      Direct Protocol (SDP) or Remote Procedure Calls (RPC).

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3.  Direct Placement

   Direct Data Placement optimizes the placement of ULP payload into the
   correct destination buffers, typically eliminating intermediate
   copying.  Placement is enabled without regard to order of arrival,
   order of transmission or requiring per-placement interaction with the

   RDMAP minimizes the required ULP interactions .  This capability is
   most valuable for applications that require multiple transport layer
   packets for each required ULP interaction.

3.1.  Fewer Required ULP Interactions

   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.

3.2.  Direct Placement using only the LLP

   Direct data placement can be achieved without RDMA.  Pre-posting of
   receive buffers could allow a non-RDMA network stack to place data
   directly to user buffers.

   The degree to which DDP optimizes depends on which transport is being
   compared with, and on the nature of the local interface.  Without
   RDMAP/DDP 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 messages.

   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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   The LLP can also be used directly if ULP specific knowledge is built
   into the protocol stack to allow "parse and place" handling of
   received packets.  Such a solution either requires interaction with
   the ULP, or that the protocol stack have knowledge of ULP specific
   syntax rules.

   DDP achieves the benefits of directly placing incoming payload
   without requiring tight coupling between the ULP and the protocol
   stack.  However, "parse and place" capabilities can certainly provide
   equivalent services to a limited number of ULPs.

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4.  Tagged Messages

   This section covers the major benefits from the use of Tagged

   A more critical advantage of DDP is the ability of the Data Source to
   use tagged buffers.  Tagging messages allows the Data Source to
   choose the ordering and packetization of its payload deliveries.
   With direct data placement based solely upon pre-posted receives, the
   packetization and delivery of payload must be agreed by the ULP peers
   in advance.  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 ULP first.  DDP provides a standardized
   'packing list' which can be interpreted without requiring ULP
   interaction.  Indeed, it is designed to be implementable in hardware.

4.1.  Order Independent Reception

   Tagged messages are directed to a buffer based on an included
   Steering Tag. Additionally, no notice is provided to the ULP for each
   individual Tagged Message's arrival.  Together these allow tagged
   messages received out-of-order to be processed without intermediate
   buffering or additional notifications to the ULP.

4.2.  Reduced ULP Notifications

   RDMAP further reduces required ULP interactions consolidating
   completion notifications of tagged messages with the completion
   notification of a trailing untagged message.  For most ULPs this
   radically reduces the number of ULP required interactions even

   While RDMAP consolidation of notices is beneficial to most
   applications, it may be detrimental to some applications that benefit
   from streamed delivery to enable ULP processing of received data as
   promptly as possible.  A ULP that uses RDMAP cannot begin processing
   any portion of an exchange until it receives notification that the
   entire exchange has been placed.  An "exchange" here is a set of zero
   or more tagged messages and a single terminating untagged message.
   An application that would prefer to begin work on the received
   payload, no matter what order it arrived in, as soon as possible
   might prefer to work directly with the LLP.  RDMAP is optimized for
   applications that are more concerned when the entire exchange is

   An application that benefits from being able to begin processing of

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   each received packet as quickly as possible may find RDMAP interferes
   with that goal.

   Such an application might be able to retain most of the benefits of
   RDMAP by using the DDP layer directly.  However, in addition to
   taking on the responsibilities of the RDMAP layer, the application
   would likely have more difficulty finding support for a DDP-only API.
   Many hardware implementations may choose to tightly couple RDMAP and
   DDP, and might not provide an API directly to DDP services.

   These features minimize 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

4.3.  Simplified ULP Exchanges

   The notification rules for Tagged Messages allows ULPs to create
   multi-message "exchanges" consisting of zero or more tagged messages
   that represent a single step in the ULP interaction.  The receiving
   ULP is notified that the untagged message has arrived, and implicitly
   of any associated tagged messages.

   A ULP where all exchanges would naturally be only the untagged
   message would derive virtually no benefit from the use of RDMAP/DDP
   as opposed to SCTP.  But while tagged buffers are the justification
   for RDMAP/DDP, untagged buffers are still necessary.  Without
   untagged buffers the only method to exchange buffer advertisements
   would involve out-of-band communications and/or sharing of compile
   time constants.  Most RDMA-aware ULPs use untagged buffers for
   requests and responses.  Buffer advertisements are typically done
   within these untagged messages.

   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 RDMA enabled interaction would
   consist of the following:

      Client sends transaction request to server's as an untagged

      This message includes buffer advertisements for the buffers where
      the results are to be placed.

      The Server sends multiple tagged messages to the advertised

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      The Server sends transaction reply as an untagged message to the

      Client receives single notification, indicating completion of the

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

4.4.  Order Independent Sending

   Use of tagged messages 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 RDMA 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).

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   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
   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.

4.5.  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

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   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
   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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5.  RDMA Read

   RDMA Reads are a further service provided by RDMAP.  RDMA Reads allow
   the Data Sink to fetch exactly the portion of the peer ULP buffer
   required on a "just in time" basis.  This can be done without
   requiring per-fetch support from the Data Source ULP.

   Storage servers may wish to limit the maximum write buffer allocated
   to any single session.  The storage server may be a very minimal
   layer between the client and the disk storage media, or the server
   may merely wish to limit the total resources that would be required
   if all clients could push the entire payload they wished written at
   their own convenience.

   In either case, 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.  RDMA Reads allow the Storage Server to
   fetch the payload on a "just in time" basis.  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, or a
   Storage Server with numerous clients can fetch buffers from the
   individual clients in the order that is most convenient to the

   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
   operation.  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 cosnsistency.

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6.  LLP Comparisons

   Normally the choice of underlying IP transport is irrelevant to the
   ULP.  RDMAP and DDP provides the same services over either.  There
   may be performance impacts of the choice, however.  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.

6.1.  Multistreaming Implications

   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, congestion control and acknowledgements.

   Completions are controlled by the DDP Source Sequence Number (DDP-
   SSN) on a per stream basis.  Therefore combining multiple DDP Streams
   into a single SCTP association cannot result in a dropped packet
   carrying data for one stream delaying completions on others.

6.2.  Out of Order Reception Implications

   The use of unordered Data Chunks with SCTP guarantees that the DDP
   layer will be able to perform placements when IP datagrams are
   received out of order.

   Placement of out-of-order DDP Segments carried over MPA/TCP is not
   guaranteed, but certainly allowed.  The ability of the MPA receiver
   to process out-of-order DDP Segments may be impaired when alignment
   of TCP segments and MPA FPDUs is lost.  Using SCTP, each DDP Segment
   is encoded in a single Data Chunk and never spread over multiple IP

6.3.  Header and Marker Overhead

   MPA and TCP headers together are smaller than the headers used by
   SCTP and its adaptation layer.  However, this advantage can be
   considerably reduced by the insertion of MPA markers.  In any event
   the different in ULP payload per IP Datagram is not likely to be a
   signifigant factor.

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6.4.  Middlebox Support

   Even with the MPA adaptation layer, DDP traffic carried over MPA/TCP
   will appear to all network middleboxes as a normal TCP connection.
   In many environments there may be a requirement to use only TCP
   connections to satisfy existing network elements and/or to facilitate
   monitoring and control of connections.  While SCTP is certainly just
   as monitorable and controllable as TCP, there is no guarantee that
   the network management infrastructure has the required support for

6.5.  Processing Overhead

   A DDP stream delivered via MPA/TCP will required more processing
   effort that one delivered over SCTP.  However this extra work may be
   justified for many deployments where full SCTP support is unavailable
   in the endpoints of the network, or where middleboxes impair the
   usability of SCTP.

6.6.  Data Integrity Implications

   Both the SCTP and MPA/TCP adaptation provide end-to-end CRC32c
   protection against data corruption, or its equivalent.

   A ULP that requires a greater degree of protection may add it own.
   However, DDP and RDMAP headers will only be guaranteed to have the
   equivalent of end-to-end CRC32c protection.  A ULP that requires data
   integrity checking more thorough than an end-to-end CRC32c should
   first invalidate all STags that reference a buffer before applying
   their own integrity check.

6.6.1.  MPA/TCP Specifics

   It is mandatory for MPA/TCP implementations to implement CRC32c, but
   it is NOT mandatory to use the CRC32c during an RDMA connection.  The
   activating or deactivating of the CRC in MPA/TCP is an administrative
   configuration operation at the local and remote end.  The
   administration of the CRC(ON/OFF) is invisible to the ULP.

   Applications SHOULD trust that this administrative option will only
   be used when the end-to-end protection is at least as effective as a
   transport layer CRC32c.  Applications SHOULD NOT apply additional
   protection as a guard against this administrative option being turned
   on inadvertently.

   Administrators MUST NOT enable CRC32c suppression unless the end-to-
   end protection is truly equivalent.

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   If the CRC is active/used for one direction/end , then the use of the
   CRC is mandatory in both directions/ends.

   If both ends have been configured NOT to use the CRC, then this is
   allowed as long as an equivalent protection(comparable or better
   than/to CRC) from undetected errors on the connection is provided.

6.6.2.  SCTP Specifics

   SCTP provides CRC32c protection automatically.  The adaptation to
   SCTP provides for no option to suppress SCTP CRC32c protection.

6.7.  Non-IP 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.

6.7.1.  No RDMA Layer Ack

   DDP does not provide for its own acknowledgements.  The only form of
   ack provided at the RDMAP layer is an RDMA Read Response.  DDP and
   RDMAP rely almost entirely upon other layers for flow control and
   pacing.  The LLP is relied upon to guarantee delivery and avoid
   network congestion, and ULP level acking is relied upon for ULP
   pacing and to avoid ULP buffer overruns.

   Previous RDMA protocols, such as InfiniBand, have been able to use
   their integration with the transport layer to provide stronger
   ordering guarantees.  It is important that application designers that
   require such guarantees to provide them through ULP interaction.


      There is no ability for a local interface to "fence" outbound
      messages to guarantee that prior tagged messages have been placed

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      prior to sending a tagged message.  The only guarantees available
      from the other side would be an RDMA Read Response (coming from
      the RDMAP layer) or a response from the ULP layer.  Remember that
      the normal ordering rules only guarantee when the Data Sink ULP
      will be notified of untagged messages, it does not control when
      data is placed into receive buffers.

      Re-use of tagged buffers must be done with extreme care.  The fact
      that an untagged message indicates that all prior tagged messages
      have been placed does not guarantee that no later tagged message
      have.  The best strategy is to only change the state of any given
      advertised buffers with with untagged messages.

      As covered elsewhere in this document, flow control of untagged
      messages MUST be provided by the ULP itself.

6.8.  Other IP Transports

   Both TCP and SCTP provide DDP with reliable transport with TCP
   friendly rate control.  As currently DDP is defined to work over
   reliable transports and implicitly relies upon some form of rate

   DDP is fully compatible with a non-reliable protocol.  Out-of-order
   placement is obviously not dependent on whether the other DDP
   Segments ever actually arrive.

   However, RDMAP requires the LLP to provide reliable service.  An
   alternate completion handling protocol would be required if DDP were
   to be deployed over an unreliable IP transport.

   As noted in the prior section on tagged buffers as ULP credits,
   neither RDMAP or DDP provide any flow control for tagged messages.
   If no transport layer flow control is provided, an RDMAP/DDP
   application would be only limited by the link layer rate, almost
   inevitably resulting in severe network congestion.

   RDMAP encourages applications to be ignorant of the underlying
   transport PMTU.  The ULP is only notified when all messages ending in
   a single untagged message have completed.  The ULP is not aware of
   the granularity or ordering of the underlying message.  This approach
   assumes that the ULP is only interested in the complete set of
   messages, and has no use for a subset of them.

6.9.  LLP Independent Session Establishment

   For an RDMAP/DDP application, the transport services provided by a
   pair of SCTP Streams and by a TCP connection both provide the same

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   service (reliable delivery of DDP Segments between two connected
   RDMAP/DDP endpoints).

6.9.1.  RDMA-only Session Establishment

   It is also possible to allow for transport neutral establishment of
   RDMAP/DDP sessions between endpoints.  Combined, these two features
   would allow most applications to be unconcerned as to which LLP was
   actually in use.

   Specifically, the procedures for DDP Stream Session establishment
   discussed in section 3 of the SCTP mapping, and section 13.3 of the
   MPA/TCP mapping, both allow for the exchange of ULP specific data
   ("Private Data") before enabling the exchange of DDP Segments.  This
   delays can allow for proper selection and/or configuration of the
   endpoints based upon the exchanged data.  For example, each DDP
   Stream Session associated with a single client session might be
   assigned to the same DDP Protection Domain.

   To be transport neutral, the applications should exchange Private
   Data as part of session establishment messages to determine how the
   RDMA endpoints are to be configured.  One side must be the Initiator,
   and the other the Responder.

   With SCTP, a pair of SCTP streams can be used for sequential
   sessions.  With MPA/TCP each connection can be used for at most one
   session.  However, the same source/destination pair of ports can be
   re-used sequentially subject to normal TCP rules.

   Both SCTP and MPA limit the private data size to a maximum of 512

   MPA/TCP requires the end of the TCP connection that initiated the
   conversion to MPA mode to send the first DDP Segment.  SCTP does not
   have this requirement.  ULPs which wish to be transport neutral
   should require the initiating end to send the first message.  A zero-
   length RDMA Write can be used for this purpose if the ULP logic
   itself does naturally support this restriction.

6.9.2.  RDMA-Conditional Session Establishment

   It is sometimes desirable for the active side of a session to connect
   with the passive side before knowing whether the passive side
   supports RDMA.

   This style of session establishment can be supported with either TCP
   or SCTP, but not as transparently as for RDMA-only sessions.  Pre-
   existing non-RDMA servers are also far more likely to be using TCP

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   than SCTP.

   With TCP. a normal TCP connection is established.  It is then used by
   the ULP to determine whether or not to convert to MPA mode and use
   RDMA.  This will typically be integral with other session
   establishment negotiations.

   With SCTP, the establishment of an association tests whether RDMA is
   supported.  If not supported, the application simply requests the
   association without the RDMA adaptation indication.

   In key difference is that with SCTP the determination as to whether
   the peer can support RDMA is made before the transport layer
   association/connection is established while with TCP the established
   connection itself is used to determine whether RDMA is supported.

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7.  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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8.  Security considerations

8.1.  Connection/Association Setup

   Both the SCTP and TCP adaptations allow for existing procedures to be
   followed for the establishment of the SCTP association or TCP
   connection.  Use of DDP does not impair the use of any security
   measures to filter, validate and/or log the remote end of an

8.2.  Tagged Buffer Exposure

   DDP only exposes ULP memory to the extent explicitly allowed by ULP
   actions.  These include posting of receive operations and enabling of
   Steering Tags.

   Neither RDMAP or DDP place requirements on how ULP's advertise
   buffers.  A ULP may use a single Steering Tag for multiple buffer
   advertisements.  However, the ULP should be aware that enforcement on
   STag usage is likely limited to the overall range that is enabled.
   If the remote peer writes into the 'wrong' advertised buffer, neither
   the DDP or RDMAP layer will be aware of this.  Nor is there any
   report to the ULP on how the remote peer specifically used tagged

   Unless the ULP peers have an adequate basis for mutual trust, the
   receiving ULP might be well advised to use a distinct STag for each
   interaction, and to invalidate it after each use or to require its
   peer to use the RDMAP option to invalidate the STag with its
   responding untagged message.

8.3.  Impact of Encrypted Transports

   While DDP is cleanly layered over the LLP, its maximum benefit may be
   limited when the LLP Stream is secured with a streaming cypher, such
   as Transport Layer Security (TLS).  If the LLP must decrypt in order,
   it cannot provide out-of-order DDP Segments to the DDP layer for
   placement purposes.  IPsec tunnel mode encrypts entire IP Datagrams.
   IPsec transport mode encrypts TCP Segments or SCTP packets.  In
   neither case should IPsec preclude providing out-of-order DDP
   Segments to the DDP layer for placement.

   Note that end-to-end use of IPsec cryptographic integrity protection
   may allow suppression of MPA CRC generation and checking under
   certain circumstances.  This is one example where the LLP may be
   judged to have "or equivalent" protection to an end-to-end CRC32c.

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9.  References

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

   [2]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.

   [3]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

   [4]   Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "Stream Control Transmission Protocol", RFC 2960,
         October 2000.

   [5]   Coene, L., "Stream Control Transmission Protocol Applicability
         Statement", RFC 3257, April 2002.

   [6]   Recio, R., "An RDMA Protocol Specification",
         draft-ietf-rddp-rdmap-05 (work in progress), July 2005.

   [7]   Shah, H., "Direct Data Placement over Reliable Transports",
         draft-ietf-rddp-ddp-05 (work in progress), July 2005.

   [8]   Stewart, R., "Stream Control Transmission Protocol (SCTP)
         Remote Direct Memory Access (RDMA) Direct Data Placement (DDP)
         Adaptationn", draft-ietf-rddp-sctp-02 (work in progress),
         August 2005.

   [9]   Culley, P., "Marker PDU Aligned Framing for TCP Specification",
         draft-ietf-rddp-mpa-02 (work in progress), February 2005.

   [10]  "Direct Access File System versino 1.0", September 2001.

   [11]  Pinkerton, J., "Sockets Direct Protocol (SDP) for iWARP over
         TCP 1.0", October 2003.

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Authors' Addresses

   Caitlin Bestler
   49 Discovery
   Irvine, CA  92618

   Phone: 949-926-6383

   Lode Coene
   Atealaan 26
   Herentals,   2200

   Phone: +32-14-252081

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