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rfc:rfc5045

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.

Bestler & Coene Informational [Page 1] RFC 5045 RDMA/DDP Applicability October 2007

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

Bestler & Coene Informational [Page 2] RFC 5045 RDMA/DDP Applicability October 2007

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].

Bestler & Coene Informational [Page 3] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 4] RFC 5045 RDMA/DDP Applicability October 2007

 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

Bestler & Coene Informational [Page 5] RFC 5045 RDMA/DDP Applicability October 2007

 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

Bestler & Coene Informational [Page 6] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 7] RFC 5045 RDMA/DDP Applicability October 2007

 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

Bestler & Coene Informational [Page 8] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 9] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 10] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 11] RFC 5045 RDMA/DDP Applicability October 2007

 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

Bestler & Coene Informational [Page 12] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 13] RFC 5045 RDMA/DDP Applicability October 2007

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.

Bestler & Coene Informational [Page 14] RFC 5045 RDMA/DDP Applicability October 2007

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.

Bestler & Coene Informational [Page 15] RFC 5045 RDMA/DDP Applicability October 2007

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.

Bestler & Coene Informational [Page 16] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 17] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 18] RFC 5045 RDMA/DDP Applicability October 2007

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.

Bestler & Coene Informational [Page 19] RFC 5045 RDMA/DDP Applicability October 2007

 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.

Bestler & Coene Informational [Page 20] RFC 5045 RDMA/DDP Applicability October 2007

 [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

Bestler & Coene Informational [Page 21] RFC 5045 RDMA/DDP Applicability October 2007

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
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Bestler & Coene Informational [Page 22]

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