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Updated by: 7146 INFORMATIONAL

Network Working Group                                    C. Bestler, Ed.
Request for Comments: 5045                                      Neterion
Category: Informational                                         L. Coene
                                                  Nokia Siemens Networks
                                                            October 2007


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

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document describes the applicability of Remote Direct Memory
   Access Protocol (RDMAP) and the Direct Data Placement Protocol (DDP).
   It compares 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
   the supporting transports, and compares DDP over IP transports with
   non-IP transports that support RDMA functionality.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Direct Placement . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Direct Placement Using Only the LLP  . . . . . . . . . . .  5
     3.2.  Fewer Required ULP Interactions  . . . . . . . . . . . . .  6
   4.  Tagged Messages  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Order-Independent Reception  . . . . . . . . . . . . . . .  7
     4.2.  Reduced ULP Notifications  . . . . . . . . . . . . . . . .  7
     4.3.  Simplified ULP Exchanges . . . . . . . . . . . . . . . . .  8
     4.4.  Order-Independent Sending  . . . . . . . . . . . . . . . .  9
     4.5.  Untagged Messages and Tagged Buffers as ULP Credits  . . . 10
   5.  RDMA Read  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  LLP Comparisons  . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Multistreaming Implications  . . . . . . . . . . . . . . . 13
     6.2.  Out-of-Order Reception Implications  . . . . . . . . . . . 13
     6.3.  Header and Marker Overhead . . . . . . . . . . . . . . . . 13
     6.4.  Middlebox Support  . . . . . . . . . . . . . . . . . . . . 14
     6.5.  Processing Overhead  . . . . . . . . . . . . . . . . . . . 14
     6.6.  Data Integrity Implications  . . . . . . . . . . . . . . . 14
       6.6.1.  MPA/TCP Specifics  . . . . . . . . . . . . . . . . . . 15
       6.6.2.  SCTP Specifics . . . . . . . . . . . . . . . . . . . . 15
     6.7.  Non-IP Transports  . . . . . . . . . . . . . . . . . . . . 15
       6.7.1.  No RDMA-Layer Ack  . . . . . . . . . . . . . . . . . . 16
     6.8.  Other IP Transports  . . . . . . . . . . . . . . . . . . . 16
     6.9.  LLP-Independent Session Establishment  . . . . . . . . . . 17
       6.9.1.  RDMA-Only Session Establishment  . . . . . . . . . . . 17
       6.9.2.  RDMA-Conditional Session Establishment . . . . . . . . 18
   7.  Local Interface Implications . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     8.1.  Connection/Association Setup . . . . . . . . . . . . . . . 19
     8.2.  Tagged Buffer Exposure . . . . . . . . . . . . . . . . . . 19
     8.3.  Impact of Encrypted Transports . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19














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1.  Introduction

   Remote Direct Memory Access Protocol (RDMAP) [RFC5040] and Direct
   Data Placement (DDP) [RFC5041] 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 that 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 is applicable, this document can guide implementers and
   application and protocol designers in selecting which protocols to
   use.

   The applicability of RDMAP/DDP is driven by their unique
   capabilities:

   o  This document will discuss when common data placement procedures
      are of more benefit to applications than 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 messages) the Data Source sent the data, or in what order
      packets are received.  Typically, tagged data can be used for
      payload transfer, while untagged is best used for control
      messages.  However each upper-layer protocol can determine the
      optimal use of Tagged and Untagged Messages for itself.  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.

      A more comprehensive introduction to the RDMAP and DDP protocols
      and discussion of their security considerations can be found in
      [RFC5042].



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   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 instead of over
   customized transport protocols.  Due to the fact that DDP is defined
   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 coexistence 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
   [RFC5043] and TCP [RFC5044].  The rationale for supporting both
   transports is reviewed, as well as when each would be the appropriate
   selection.

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.







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   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.  The message receiver is given no independent
      indication that a Tagged Message has been received.

   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.  The message receiver is notified by some
      mechanism that an Untagged Message has been received.

   Upper Layer Protocol (ULP) -  The direct user of RDMAP/DDP services.
      In addition to protocols such as iSER [RFC5046] and NFSv4 over
      RDMA [NFSDIRECT], the ULP may be embedded in an application or a
      middleware layer, as is often the case for the Sockets Direct
      Protocol (SDP) and Remote Procedure Call (RPC) protocols.

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 requirement of per-placement interaction
   with the ULP.

   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.  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 it is
   being compared with, and on the nature of the local interface.
   Without RDMAP/DDP, pre-posting buffers require 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 native SCTP or TCP
   (without RDMA).  This is easier using native SCTP because the




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   application would only have to predict the sequence of messages and
   the maximum size of each message, not the exact size.

   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 always be made on a per-message basis (not
   per byte).

   The Lower Layer Protocol, 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 the protocol stack's 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.

3.2.  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
   system's ability to switch ULP contexts.

   Content access applications are important examples of applications
   that require high bandwidth and can transfer a significant amount of
   content between required ULP interactions.  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.

4.  Tagged Messages

   This section covers the major benefits from the use of Tagged
   Messages.

   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.

   The Upper Layer Protocol can allocate content between Untagged and/or
   Tagged Messages to maximize the potential optimizations.  Placing
   content within an Untagged Message can deliver the content in the



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   same packet that signals completion to the receiver.  This can
   improve latency.  It can even eliminate round trips.  But it requires
   making larger anonymous buffers to be available.

   Some examples of data that typically belongs in the Untagged Message
   would include:

      short fixed-size control data that is inherently part of the
      control message.  This is especially true when the data is a
      required part of the control message.

      relatively short payload that is almost always needed, especially
      when its inclusion would eliminate a round-trip to fetch the data.
      Examples would include the initial data on a write request and
      Advertisements of Tagged Buffers.

   Tagged Messages standardize direct placement of data without per-
   packet interaction with the upper layers.  Even if there is an upper-
   layer protocol 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 offers both Tagged and Untagged Messages.  No receiving-side
   ULP interactions are required for Tagged Messages.  By optimally
   dividing traffic between Tagged and Untagged Messages, the ULP can
   limit the number of events that must be dealt with at the ULP layer.
   This typically reduces the number of context switches required and
   improves performance.

   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
   further.



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   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 as soon as possible, no matter what order it arrived in,
   might prefer to work directly with the LLP.  RDMAP is optimized for
   applications that are more concerned when the entire exchange is
   complete.

   An application that benefits from being able to begin processing of
   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
   interaction.

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
   notified of any associated Tagged Messages.

   If a ULP cannot effectively use Tagged Messages, it would derive
   little benefit from use of RDMAP/DDP by comparison to direct use of
   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 require out-of-
   band communications.  Most RDMA-aware ULPs use Untagged Buffers for
   requests and responses.  Buffer Advertisements are typically done
   within these Untagged Messages.

   More importantly, there would be no reliable method for the upper-
   layer peers to synchronize.  The absence of any guarantees about



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   ordering within or between Tagged Messages is fundamental to allowing
   the DDP layer to optimize transfer of tagged payload.

   Therefore, no ULP can be defined entirely in terms of Tagged
   Messages.  Eventually, a notification that confirms delivery must be
   generated from the RDMAP/DDP layer.

   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:

   1.  Client sends transaction request to server as an Untagged
       Message.

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

   3.  The server sends multiple Tagged Messages to the Advertised
       buffers.

   4.  The server sends transaction reply as an Untagged Message to the
       client.

   5.  Client receives single notification, indicating completion of the
       interaction.

   With this type of exchange, the pacing and required size of Untagged
   Buffers are 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 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.



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   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
   buffer.

   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,
   as well as 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 Sink's 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, a file server may not
   expose its own memory to its clients.  A client wishing to write may
   Advertise a buffer upon which the server will issue RDMA Reads.
   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.

   Generally, the best use of an Untagged Message is to synchronize and
   to deliver data that is naturally tied to the same message as the
   synchronization.  For initial data transfers, this has the additional
   benefit of avoiding the need to Advertise specific Tagged Buffers for
   indefinite time periods.  Instead, anonymous buffers can be used for
   initial data reception.  Because anonymous buffers do not need to be
   tied to specific messages in advance, this can be a major benefit.

4.5.  Untagged Messages and 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.



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   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 existence 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, a ULP may specify an initial Untagged
   Buffer size to be used immediately after session establishment, and
   then optionally specify mechanisms for negotiating changes.

   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 in what order to exercise the credits.

   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
   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.





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   Another example would be when 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 its byte-oriented credits.

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
   server.

   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 nor DDP provide
   mechanisms for bundling multiple disjoint updates into an atomic



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   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 consistency.

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 (Marker PDU Alignment) 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
   datagrams.

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
   reduced by the insertion of MPA markers.  The difference in ULP
   Payload per IP Datagram is not likely to be a significant 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
   both.

6.5.  Processing Overhead

   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 endpoints of the network, or where middleboxes impair the
   usability of SCTP.

6.6.  Data Integrity Implications

   Both the SCTP [RFC4960] and MPA/TCP [RFC5044] adaptation provide end-
   to-end CRC32c protection against data accidental corruption, or its
   equivalent.

   A ULP that requires a greater degree of protection may add its 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
   its own integrity check.

   CRC32c only provides protection against random corruption.  To
   protect against unauthorized alteration or forging of data packets,
   security methods must be applied.  The RDMA security document
   [RFC5042] specifies usage of RFC 2406 [RFC2406] for both adaptation
   layers.  As stated in [RFC5042], note that the IPsec requirements for
   RDDP are based on the version of IPsec specified in RFC 2401
   [RFC2401] and related RFCs, as profiled by RFC 3723 [RFC3723],
   despite the existence of a newer version of IPsec specified in RFC
   4301 [RFC4301] and related RFCs.










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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 assume that disabling CRC32c will only be used
   when the end-to-end protection is at least as effective as a
   transport layer CRC32c.  Applications should not use additional
   integrity checks based solely on the possibility that CRC32c could be
   disabled without equivalent integrity checks at a lower level.

   CRC32c must not be disabled unless equivalent or better end-to-end
   integrity protection is provided.

   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 to or better
   than 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 enables a new DDP-enabled node to be added anywhere to
   an IP network.  No DDP-specific support from middleboxes is required.

   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
   ack.

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






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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 provide them through ULP interaction.

   Specifically:

      There is no ability for a local interface to "fence" outbound
      messages to guarantee that prior Tagged Messages have been placed
      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
      has.  The best strategy is to change only the state of any given
      Advertised Buffers with Untagged Messages.

      As covered elsewhere in this document, flow control of Untagged
      Messages is the responsibility of the ULP.

6.8.  Other IP Transports

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

   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.



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   As noted in the prior section on Tagged Buffers as ULP credits,
   neither RDMAP nor DDP provides any flow control for Tagged Messages.
   If no transport layer flow control is provided, an RDMAP/DDP
   application would be limited only by the link layer rate, almost
   inevitably resulting in severe network congestion.

   RDMAP encourages applications to be ignorant of the underlying
   transport path MTU.  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
   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
   delay 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 successive sessions
   while the SCTP association remains open.  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 for a subsequent TCP
   connection, as allowed by TCP.





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

   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 that 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
   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.

   One 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.

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

   RDMA security considerations are discussed in the RDMA security
   document [RFC5042].  This document will only deal with the more
   usage-oriented aspects, and where there are implications in the
   choice of underlying transport.

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
   association/connection.

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 nor DDP places requirements on how ULPs 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 nor the RDMAP layer will be aware of this.  Nor is there any
   report to the ULP on how the Remote Peer specifically used Tagged
   Buffers.

   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) [RFC4346].  If the LLP must decrypt
   in order, it cannot provide out-of-order DDP Segments to the DDP
   layer for placement purposes.  IPsec [RFC2401] tunnel mode encrypts
   entire IP Datagrams.  IPsec transport mode encrypts TCP Segments or
   SCTP packets, as does use of Datagram TLS (DTLS) [RFC4347] over UDP
   beneath TCP or SCTP.  Neither IPsec nor this use of DTLS precludes
   providing out-of-order DDP Segments to the DDP layer for placement.




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   Note that end-to-end use of 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.

9.  References

9.1.  Normative References

   [RFC2401]    Kent, S. and R. Atkinson, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.

   [RFC2406]    Kent, S. and R. Atkinson, "IP Encapsulating Security
                Payload (ESP)", RFC 2406, November 1998.

   [RFC4960]    Stewart, R., "Stream Control Transmission Protocol",
                RFC 4960, September 2007.

   [RFC5040]    Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
                Garcia, "A Remote Direct Memory Access Protocol
                Specification", RFC 5040, October 2007.

   [RFC5041]    Shah, H., Pinkerton, J., Recio, R., and P. Culley,
                "Direct Data Placement over Reliable Transports",
                RFC 5041, October 2007.

   [RFC5042]    Pinkerton, J. and E. Deleganes, "DDP/RDMAP Security",
                RFC 5042, October 2007.

   [RFC5043]    Bestler, C. and R. Stewart, "Stream Control Transmission
                Protocol (SCTP) Direct Data Placement (DDP) Adaptation",
                RFC 5043, October 2007.

   [RFC5044]    Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
                Carrier, "Marker PDU Aligned Framing for TCP
                Specification", RFC 5044, October 2007.

9.2.  Informative References

   [NFSDIRECT]  Talpey, T., Callaghan, B., and I. Property, "NFS Direct
                Data Placement", Work in Progress, June 2007.

   [RFC3723]    Aboba, B., Tseng, J., Walker, J., Rangan, V., and F.
                Travostino, "Securing Block Storage Protocols over IP",
                RFC 3723, April 2004.

   [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, December 2005.



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   [RFC4346]    Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.1", RFC 4346,
                April 2006.

   [RFC4347]    Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                Security", RFC 4347, April 2006.

   [RFC5046]    Ko, M., Chadalapaka, M., Elzur, U., Shah, H., and P.
                Thaler, "Internet Small Computer System Interface
                (iSCSI) Extensions for Remote Direct Memory Access
                (RDMA)", RFC 5046, October 2007.

Authors' Addresses

   Caitlin Bestler (editor)
   Neterion
   20230 Stevens Creek Blvd.
   Suite C
   Cupertino, CA  95014
   USA

   Phone: 408-366-4639
   EMail: caitlin.bestler@neterion.com


   Lode Coene
   Nokia Siemens Networks
   Atealaan 26
   Herentals  2200
   Belgium

   Phone: +32-14-252081
   EMail: lode.coene@nsn.com


















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Full Copyright Statement

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