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rfc:rfc4296

Network Working Group S. Bailey Request for Comments: 4296 Sandburst Category: Informational T. Talpey

                                                                NetApp
                                                         December 2005
          The Architecture of Direct Data Placement (DDP)
    and Remote Direct Memory Access (RDMA) on Internet Protocols

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.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document defines an abstract architecture for Direct Data
 Placement (DDP) and Remote Direct Memory Access (RDMA) protocols to
 run on Internet Protocol-suite transports.  This architecture does
 not necessarily reflect the proper way to implement such protocols,
 but is, rather, a descriptive tool for defining and understanding the
 protocols.  DDP allows the efficient placement of data into buffers
 designated by Upper Layer Protocols (e.g., RDMA).  RDMA provides the
 semantics to enable Remote Direct Memory Access between peers in a
 way consistent with application requirements.

Bailey & Talpey Informational [Page 1] RFC 4296 DDP and RDMA Architecture December 2005

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................2
    1.2. DDP and RDMA Protocols .....................................3
 2. Architecture ....................................................4
    2.1. Direct Data Placement (DDP) Protocol Architecture ..........4
         2.1.1. Transport Operations ................................6
         2.1.2. DDP Operations ......................................7
         2.1.3. Transport Characteristics in DDP ...................10
    2.2. Remote Direct Memory Access (RDMA) Protocol Architecture ..12
         2.2.1. RDMA Operations ....................................14
         2.2.2. Transport Characteristics in RDMA ..................16
 3. Security Considerations ........................................17
    3.1. Security Services .........................................18
    3.2. Error Considerations ......................................19
 4. Acknowledgements ...............................................19
 5. Informative References .........................................20

1. Introduction

 This document defines an abstract architecture for Direct Data
 Placement (DDP) and Remote Direct Memory Access (RDMA) protocols to
 run on Internet Protocol-suite transports.  This architecture does
 not necessarily reflect the proper way to implement such protocols,
 but is, rather, a descriptive tool for defining and understanding the
 protocols.  This document uses C language notation as a shorthand to
 describe the architectural elements of DDP and RDMA protocols.  The
 choice of C notation is not intended to describe concrete protocols
 or programming interfaces.
 The first part of the document describes the architecture of DDP
 protocols, including what assumptions are made about the transports
 on which DDP is built.  The second part describes the architecture of
 RDMA protocols layered on top of DDP.

1.1. Terminology

 Before introducing the protocols, certain definitions will be useful
 to guide discussion:
 o    Placement - writing to a data buffer.
 o    Operation - a protocol message, or sequence of messages, which
      provide an architectural semantic, such as reading or writing of
      a data buffer.

Bailey & Talpey Informational [Page 2] RFC 4296 DDP and RDMA Architecture December 2005

 o    Delivery - informing any Upper Layer or application that a
      particular message is available for use.  Therefore, delivery
      may be viewed as the "control" signal associated with a unit of
      data.  Note that the order of delivery is defined more strictly
      than it is for placement.
 o    Completion - informing any Upper Layer or application that a
      particular operation has finished.  A completion, for instance,
      may require the delivery of several messages, or it may also
      reflect that some local processing has finished.
 o    Data Sink - the peer on which any placement occurs.
 o    Data Source - the peer from which the placed data originates.
 o    Steering Tag - a "handle" used to identify the buffer that is
      the target of placement.  A "tagged" message is one that
      references such a handle.
 o    RDMA Write - an Operation that places data from a local data
      buffer to a remote data buffer specified by a Steering Tag.
 o    RDMA Read - an Operation that places data to a local data buffer
      specified by a Steering Tag from a remote data buffer specified
      by another Steering Tag.
 o    Send - an Operation that places data from a local data buffer to
      a remote data buffer of the data sink's choice.  Therefore,
      sends are "untagged".

1.2. DDP and RDMA Protocols

 The goal of the DDP protocol is to allow the efficient placement of
 data into buffers designated by protocols layered above DDP (e.g.,
 RDMA).  This is described in detail in [ROM].  Efficiency may be
 characterized by the minimization of the number of transfers of the
 data over the receiver's system buses.
 The goal of the RDMA protocol is to provide the semantics to enable
 Remote Direct Memory Access between peers in a way consistent with
 application requirements.  The RDMA protocol provides facilities
 immediately useful to existing and future networking, storage, and
 other application protocols.  [FCVI, IB, MYR, SDP, SRVNET, VI]
 The DDP and RDMA protocols work together to achieve their respective
 goals.  DDP provides facilities to safely steer payloads to specific
 buffers at the Data Sink.  RDMA provides facilities to Upper Layers
 for identifying these buffers, controlling the transfer of data

Bailey & Talpey Informational [Page 3] RFC 4296 DDP and RDMA Architecture December 2005

 between peers' buffers, supporting authorized bidirectional transfer
 between buffers, and signalling completion.  Upper Layer Protocols
 that do not require the features of RDMA may be layered directly on
 top of DDP.
 The DDP and RDMA protocols are transport independent.  The following
 figure shows the relationship between RDMA, DDP, Upper Layer
 Protocols, and Transport.
        +--------------------------------------------------+
        |               Upper Layer Protocol               |
        +---------+------------+---------------------------+
        |         |            |           RDMA            |
        |         |            +---------------------------+
        |         |                   DDP                  |
        |         +----------------------------------------+
        |                    Transport                     |
        +--------------------------------------------------+

2. Architecture

 The Architecture section is presented in two parts:  Direct Data
 Placement Protocol architecture and Remote Direct Memory Access
 Protocol architecture.

2.1. Direct Data Placement (DDP) Protocol Architecture

 The central idea of general-purpose DDP is that a data sender will
 supplement the data it sends with placement information that allows
 the receiver's network interface to place the data directly at its
 final destination without any copying.  DDP can be used to steer
 received data to its final destination, without requiring layer-
 specific behavior for each different layer.  Data sent with such DDP
 information is said to be `tagged'.
 The central components of the DDP architecture are the `buffer',
 which is an object with beginning and ending addresses, and a method
 (set()), which sets the value of an octet at an address.  In many
 cases, a buffer corresponds directly to a portion of host user
 memory.  However, DDP does not depend on this; a buffer could be a
 disk file, or anything else that can be viewed as an addressable
 collection of octets.  Abstractly, a buffer provides the interface:
      typedef struct {
        const address_t start;
        const address_t end;
        void            set(address_t a, data_t v);
      } ddp_buffer_t;

Bailey & Talpey Informational [Page 4] RFC 4296 DDP and RDMA Architecture December 2005

 address_t
      a reference to local memory
 data_t
      an octet data value.
 The protocol layering and in-line data flow of DDP is:
                       DDP Client Protocol
                (e.g., RDMA or Upper Layer Protocol)
                              |  ^
            untagged messages |  | untagged message delivery
              tagged messages |  | tagged message delivery
                              v  |
                              DDP+---> data placement
                               ^
                               | transport messages
                               v
                           Transport
                  (e.g., SCTP, DCCP, framed TCP)
                               ^
                               | IP datagrams
                               v
                             . . .
 In addition to in-line data flow, the client protocol registers
 buffers with DDP, and DDP performs buffer update (set()) operations
 as a result of receiving tagged messages.
 DDP messages may be split into multiple, smaller DDP messages, each
 in a separate transport message.  However, if the transport is
 unreliable or unordered, messages split across transport messages may
 or may not provide useful behavior, in the same way as splitting
 arbitrary Upper Layer messages across unreliable or unordered
 transport messages may or may not provide useful behavior.  In other
 words, the same considerations apply to building client protocols on
 different types of transports with or without the use of DDP.

Bailey & Talpey Informational [Page 5] RFC 4296 DDP and RDMA Architecture December 2005

 A DDP message split across transport messages looks like:
 DDP message:                Transport messages:
   stag=s, offset=o,          message 1:
   notify=y, id=i               |type=ddp  |
   message=                     |stag=s    |
     |aabbccddee|-------.       |offset=o  |
     ~   ...    ~----.   \      |notify=n  |
     |vvwwxxyyzz|-.   \   \     |id=?      |
                  |    \   `--->|aabbccddee|
                  |     \       ~    ...   ~
                  |      +----->|iijjkkllmm|
                  |      |
                  +      |    message 2:
                   \     |      |type=ddp  |
                    \    |      |stag=s    |
                     \   +      |offset=o+n|
                      \   \     |notify=y  |
                       \   \    |id=i      |
                        \   `-->|nnooppqqrr|
                         \      ~    ...   ~
                          `---->|vvwwxxyyzz|
 Although this picture suggests that DDP information is carried in-
 line with the message payload, components of the DDP information may
 also be in transport-specific fields, or derived from transport-
 specific control information if the transport permits.

2.1.1. Transport Operations

 For the purposes of this architecture, the transport provides:
      void      xpt_send(socket_t s, message_t m);
      message_t xpt_recv(socket_t s);
      msize_t   xpt_max_msize(socket_t s);
 socket_t
      a transport address, including IP addresses, ports and other
      transport-specific identifiers.
 message_t
      a string of octets.

Bailey & Talpey Informational [Page 6] RFC 4296 DDP and RDMA Architecture December 2005

 msize_t (scalar)
      a message size.
 xpt_send(socket_t s, message_t m)
      send a transport message.
 xpt_recv(socket_t s)
      receive a transport message.
 xpt_max_msize(socket_t s)
      get the current maximum transport message size.  Corresponds,
      roughly, to the current path Maximum Transfer Unit (PMTU),
      adjusted by underlying protocol overheads.
 Real implementations of xpt_send() and xpt_recv() typically return
 error indications, but that is not relevant to this architecture.

2.1.2. DDP Operations

 The DDP layer provides:
      void       ddp_send(socket_t s, message_t m);
      void       ddp_send_ddp(socket_t s, message_t m, ddp_addr_t d,
                              ddp_notify_t n);
      void       ddp_post_recv(socket_t s, bdesc_t b);
      ddp_ind_t  ddp_recv(socket_t s);
      bdesc_t    ddp_register(socket_t s, ddp_buffer_t b);
      void       ddp_deregister(bhand_t bh);
      msizes_t   ddp_max_msizes(socket_t s);
 ddp_addr_t
      the buffer address portion of a tagged message:
              typedef struct {
                stag_t stag;
                address_t offset;
              } ddp_addr_t;
 stag_t (scalar)
      a Steering Tag.  A stag_t identifies the destination buffer for
      tagged messages.  stag_ts are generated when the buffer is
      registered, communicated to the sender by some client protocol

Bailey & Talpey Informational [Page 7] RFC 4296 DDP and RDMA Architecture December 2005

      convention and inserted in DDP messages.  stag_t values in this
      DDP architecture are assumed to be completely opaque to the
      client protocol, and implementation-dependent.  However,
      particular implementations, such as DDP on a multicast transport
      (see below), may provide the buffer holder some control in
      selecting stag_ts.
 ddp_notify_t
      the notification portion of a DDP message, used to signal
      that the message represents the final fragment of a
      multi-segmented DDP message:
              typedef struct {
                boolean_t notify;
                ddp_msg_id_t i;
              } ddp_notify_t;
 ddp_msg_id_t (scalar)
      a DDP message identifier.  msg_id_ts are chosen by the DDP
      message receiver (buffer holder), communicated to the sender by
      some client protocol convention and inserted in DDP messages.
      Whether a message reception indication is requested for a DDP
      message is a matter of client protocol convention.  Unlike
      stag_ts, the structure of msg_id_ts is opaque to DDP, and
      therefore, it is completely in the hands of the client protocol.
 bdesc_t
      a description of a registered buffer:
              typedef struct {
                bhand_t bh;
                ddp_addr_t a;
              } bdesc_t;
      `a.offset' is the starting offset of the registered buffer,
      which may have no relationship to the `start' or `end' addresses
      of that buffer.  However, particular implementations, such as
      DDP on a multicast transport (see below), may allow some client
      protocol control over the starting offset.
 bhand_t
      an opaque buffer handle used to deregister a buffer.

Bailey & Talpey Informational [Page 8] RFC 4296 DDP and RDMA Architecture December 2005

 recv_message_t
      a description of a completed untagged receive buffer:
              typedef struct {
                bdesc_t b;
                length_t l;
              } recv_message_t;
 ddp_ind_t
      an untagged message, a tagged message reception indication, or a
      tagged message reception error:
              typedef union {
                recv_message_t m;
                ddp_msg_id_t i;
                ddp_err_t e;
              } ddp_ind_t;
 ddp_err_t
      indicates an error while receiving a tagged message, typically
      `offset' out of bounds, or `stag' is not registered to the
      socket.
 msizes_t
      The maximum untagged and tagged messages that fit in a single
      transport message:
              typedef struct {
                msize_t max_untagged;
                msize_t max_tagged;
              } msizes_t;
 ddp_send(socket_t s, message_t m)
      send an untagged message.
 ddp_send_ddp(socket_t s, message_t m, ddp_addr_t d, ddp_notify_t n)
      send a tagged message to remote buffer address d.

Bailey & Talpey Informational [Page 9] RFC 4296 DDP and RDMA Architecture December 2005

 ddp_post_recv(socket_t s, bdesc_t b)
      post a registered buffer to accept a single received untagged
      message.  Each buffer is returned to the caller in a ddp_recv()
      untagged message reception indication, in the order in which it
      was posted.  The same buffer may be enabled on multiple sockets;
      receipt of an untagged message into the buffer from any of these
      sockets unposts the buffer from all sockets.
 ddp_recv(socket_t s)
      get the next received untagged message, tagged message reception
      indication, or tagged message error.
 ddp_register(socket_t s, ddp_buffer_t b)
      register a buffer for DDP on a socket.  The same buffer may be
      registered multiple times on the same or different sockets.  The
      same buffer registered on different sockets may result in a
      common registration.  Different buffers may also refer to
      portions of the same underlying addressable object (buffer
      aliasing).
 ddp_deregister(bhand_t bh)
      remove a registration from a buffer.
 ddp_max_msizes(socket_t s)
      get the current maximum untagged and tagged message sizes that
      will fit in a single transport message.

2.1.3. Transport Characteristics in DDP

 Certain characteristics of the transport on which DDP is mapped
 determine the nature of the service provided to client protocols.
 Fundamentally, the characteristics of the transport will not be
 changed by the presence of DDP.  The choice of transport is therefore
 driven not by DDP, but by the requirements of the Upper Layer, and
 employing the DDP service.
 Specifically, transports are:
   o    reliable or unreliable,
   o    ordered or unordered,
   o    single source or multisource,

Bailey & Talpey Informational [Page 10] RFC 4296 DDP and RDMA Architecture December 2005

   o    single destination or multidestination (multicast or anycast).
 Some transports support several combinations of these
 characteristics.  For example, SCTP [SCTP] is reliable, single
 source, single destination (point-to-point) and supports both ordered
 and unordered modes.
 DDP messages carried by transport are framed for processing by the
 receiver, and may be further protected for integrity or privacy in
 accordance with the transport capabilities.  DDP does not provide
 such functions.
 In general, transport characteristics equally affect transport and
 DDP message delivery.  However, there are several issues specific to
 DDP messages.
 A key component of DDP is how the following operations on the
 receiving side are ordered among themselves, and how they relate to
 corresponding operations on the sending side:
        o    set()s,
        o    untagged message reception indications, and
        o    tagged message reception indications.
 These relationships depend upon the characteristics of the underlying
 transport in a way that is defined by the DDP protocol.  For example,
 if the transport is unreliable and unordered, the DDP protocol might
 specify that the client protocol is subject to the consequences of
 transport messages being lost or duplicated, rather than requiring
 that different characteristics be presented to the client protocol.
 Buffer access must be implemented consistently across endpoint IP
 addresses on transports allowing multiple IP addresses per endpoint,
 for example, SCTP.  In particular, the Steering Tag must be
 consistently scoped and must address the same buffer across all IP
 address associations belonging to the endpoint.  Additionally,
 operation ordering relationships across IP addresses within an
 association (set(), get(), etc.) depend on the underlying transport.
 If the above consistency relationships cannot be maintained by a
 transport endpoint, then the endpoint is unsuitable for a DDP
 connection.
 Multidestination data delivery is a transport characteristic that may
 require specific consideration in a DDP protocol.  As mentioned
 above, the basic DDP model assumes that buffer address values
 returned by ddp_register() are opaque to the client protocol, and can

Bailey & Talpey Informational [Page 11] RFC 4296 DDP and RDMA Architecture December 2005

 be implementation dependent.  The most natural way to map DDP to a
 multidestination transport is to require that all receivers produce
 the same buffer address when registering a multidestination
 destination buffer.  Restriction of the DDP model to accommodate
 multiple destinations involves engineering tradeoffs comparable to
 those of providing non-DDP multidestination transport capability.
 A registered buffer is identified within DDP by its stag_t, which in
 turn is associated with a socket.  Therefore, this registration
 grants a capability to the DDP peer, and the socket (using the
 underlying properties of its chosen transport and possible security)
 identifies the peer and authenticates the stag_t.
 The same buffer may be enabled by ddp_post_recv() on multiple
 sockets.  In this case any ddp_recv() untagged message reception
 indication may be provided on a different socket from that on which
 the buffer was posted.  Such indications are not ordered among
 multiple DDP sockets.
 When multiple sockets reference an untagged message reception buffer,
 local interfaces are responsible for managing the mechanisms of
 allocating posted buffers to received untagged messages, the handling
 of received untagged messages when no buffer is available, and of
 resource management among multiple sockets.  Where underprovisioning
 of buffers on multiple sockets is allowed, mechanisms should be
 provided to manage buffer consumption on a per-socket or group of
 related sockets basis.
 Architecturally, therefore, DDP is a flexible and general paradigm
 that may be applied to any variety of transports.  Implementations of
 DDP may, however, adapt themselves to these differences in ways
 appropriate to each transport.  In all cases, the layering of DDP
 must continue to express the transport's underlying characteristics.

2.2. Remote Direct Memory Access (RDMA) Protocol Architecture

 Remote Direct Memory Access (RDMA) extends the capabilities of DDP
 with two primary functions.
 First, it adds the ability to read from buffers registered to a
 socket (RDMA Read).  This allows a client protocol to perform
 arbitrary, bidirectional data movement without involving the remote
 client.  When RDMA is implemented in hardware, arbitrary data
 movement can be performed without involving the remote host CPU at
 all.

Bailey & Talpey Informational [Page 12] RFC 4296 DDP and RDMA Architecture December 2005

 In addition, RDMA specifies a transport-independent untagged message
 service (Send) with characteristics that are both very efficient to
 implement in hardware, and convenient for client protocols.
 The RDMA architecture is patterned after the traditional model for
 device programming, where the client requests an operation using
 Send-like actions (programmed I/O), the server performs the necessary
 data transfers for the operation (DMA reads and writes), and notifies
 the client of completion.  The programmed I/O+DMA model efficiently
 supports a high degree of concurrency and flexibility for both the
 client and server, even when operations have a wide range of
 intrinsic latencies.
 RDMA is layered as a client protocol on top of DDP:
                    Client Protocol
                         |  ^
                   Sends |  | Send reception indications
      RDMA Read Requests |  | RDMA Read Completion indications
             RDMA Writes |  | RDMA Write Completion indications
                         v  |
                         RDMA
                         |  ^
       untagged messages |  | untagged message delivery
         tagged messages |  | tagged message delivery
                         v  |
                         DDP+---> data placement
                          ^
                          | transport messages
                          v
                        . . .
 In addition to in-line data flow, read (get()) and update (set())
 operations are performed on buffers registered with RDMA as a result
 of RDMA Read Requests and RDMA Writes, respectively.
 An RDMA `buffer' extends a DDP buffer with a get() operation that
 retrieves the value of the octet at address `a':
         typedef struct {
           const address_t start;
           const address_t end;
           void            set(address_t a, data_t v);
           data_t          get(address_t a);
         } rdma_buffer_t;

Bailey & Talpey Informational [Page 13] RFC 4296 DDP and RDMA Architecture December 2005

2.2.1. RDMA Operations

 The RDMA layer provides:
      void        rdma_send(socket_t s, message_t m);
      void        rdma_write(socket_t s, message_t m, ddp_addr_t d,
                             rdma_notify_t n);
      void        rdma_read(socket_t s, ddp_addr_t s, ddp_addr_t d);
      void        rdma_post_recv(socket_t s, bdesc_t b);
      rdma_ind_t  rdma_recv(socket_t s);
      bdesc_t     rdma_register(socket_t s, rdma_buffer_t b,
                             bmode_t mode);
      void        rdma_deregister(bhand_t bh);
      msizes_t    rdma_max_msizes(socket_t s);
 Although, for clarity, these data transfer interfaces are
 synchronous, rdma_read() and possibly rdma_send() (in the presence of
 Send flow control) can require an arbitrary amount of time to
 complete.  To express the full concurrency and interleaving of RDMA
 data transfer, these interfaces should also be reentrant.  For
 example, a client protocol may perform an rdma_send(), while an
 rdma_read() operation is in progress.
 rdma_notify_t
      RDMA Write notification information, used to signal that the
      message represents the final fragment of a multi-segmented RDMA
      message:
              typedef struct {
                boolean_t notify;
                rdma_write_id_t i;
              } rdma_notify_t;
      identical in function to ddp_notify_t, except that the type
      rdma_write_id_t may not be equivalent to ddp_msg_id_t.
 rdma_write_id_t (scalar)
      an RDMA Write identifier.

Bailey & Talpey Informational [Page 14] RFC 4296 DDP and RDMA Architecture December 2005

 rdma_ind_t
      a Send message, or an RDMA error:
              typedef union {
                recv_message_t m;
                rdma_err_t e;
              } rdma_ind_t;
 rdma_err_t
      an RDMA protocol error indication.  RDMA errors include buffer
      addressing errors corresponding to ddp_err_ts, and buffer
      protection violations (e.g., RDMA Writing a buffer only
      registered for reading).
 bmode_t
      buffer registration mode (permissions).  Any combination of
      permitting RDMA Read (BMODE_READ) and RDMA Write (BMODE_WRITE)
      operations.
 rdma_send(socket_t s, message_t m)
      send a message, delivering it to the next untagged RDMA buffer
      at the remote peer.
 rdma_write(socket_t s, message_t m, ddp_addr_t d, rdma_notify_t n)
      RDMA Write to remote buffer address d.
 rdma_read(socket_t s, ddp_addr_t s, length_t l, ddp_addr_t d)
      RDMA Read l octets from remote buffer address s to local buffer
      address d.
 rdma_post_recv(socket_t s, bdesc_t b)
      post a registered buffer to accept a single Send message, to be
      filled and returned in-order to a subsequent caller of
      rdma_recv().  As with DDP, buffers may be enabled on multiple
      sockets, in which case ordering guarantees are relaxed.  Also as
      with DDP, local interfaces must manage the mechanisms of
      allocation and management of buffers posted to multiple sockets.

Bailey & Talpey Informational [Page 15] RFC 4296 DDP and RDMA Architecture December 2005

 rdma_recv(socket_t s);
      get the next received Send message, RDMA Write completion
      identifier, or RDMA error.
 rdma_register(socket_t s, rdma_buffer_t b, bmode_t mode)
      register a buffer for RDMA on a socket (for read access, write
      access or both).  As with DDP, the same buffer may be registered
      multiple times on the same or different sockets, and different
      buffers may refer to portions of the same underlying addressable
      object.
 rdma_deregister(bhand_t bh)
      remove a registration from a buffer.
 rdma_max_msizes(socket_t s)
      get the current maximum Send (max_untagged) and RDMA Read or
      Write (max_tagged) operations that will fit in a single
      transport message.  The values returned by rdma_max_msizes() are
      closely related to the values returned by ddp_max_msizes(), but
      may not be equal.

2.2.2. Transport Characteristics in RDMA

 As with DDP, RDMA can be used on transports with a variety of
 different characteristics that manifest themselves directly in the
 service provided by RDMA.  Also, as with DDP, the fundamental
 characteristics of the transport will not be changed by the presence
 of RDMA.
 Like DDP, an RDMA protocol must specify how:
        o    set()s,
        o    get()s,
        o    Send messages, and
        o    RDMA Read completions
 are ordered among themselves and how they relate to corresponding
 operations on the remote peer(s).  These relationships are likely to
 be a function of the underlying transport characteristics.

Bailey & Talpey Informational [Page 16] RFC 4296 DDP and RDMA Architecture December 2005

 There are some additional characteristics of RDMA that may translate
 poorly to unreliable or multipoint transports due to attendant
 complexities in managing endpoint state:
   o    Send flow control
   o    RDMA Read
 These difficulties can be overcome by placing restrictions on the
 service provided by RDMA.  However, many RDMA clients, especially
 those that separate data transfer and application logic concerns, are
 likely to depend upon capabilities only provided by RDMA on a point-
 to-point, reliable transport.  In other words, many potential Upper
 Layers, which might avail themselves of RDMA services, are naturally
 already biased toward these transport classes.

3. Security Considerations

 Fundamentally, the DDP and RDMA protocols themselves should not
 introduce additional vulnerabilities.  They are intermediate
 protocols and so should not perform or require functions such as
 authorization, which are the domain of Upper Layers.  However, the
 DDP and RDMA protocols should allow mapping by strict Upper Layers
 that are not permissive of new vulnerabilities; DDP and RDMAP
 implementations should be prohibited from `cutting corners' that
 create new vulnerabilities.  Implementations must ensure that only
 `supplied' resources (i.e., buffers) can be manipulated by DDP or
 RDMAP messages.
 System integrity must be maintained in any RDMA solution.  Mechanisms
 must be specified to prevent RDMA or DDP operations from impairing
 system integrity.  For example, threats can include potential buffer
 reuse or buffer overflow, and are not merely a security issue.  Even
 trusted peers must not be allowed to damage local integrity.  Any DDP
 and RDMA protocol must address the issue of giving end-systems and
 applications the capabilities to offer protection from such
 compromises.
 Because a Steering Tag exports access to a buffer, one critical
 aspect of security is the scope of this access.  It must be possible
 to individually control specific attributes of the access provided by
 a Steering Tag on the endpoint (socket) on which it was registered,
 including remote read access, remote write access, and others that
 might be identified.  DDP and RDMA specifications must provide both
 implementation requirements relevant to this issue, and guidelines to
 assist implementors in making the appropriate design decisions.

Bailey & Talpey Informational [Page 17] RFC 4296 DDP and RDMA Architecture December 2005

 For example, it must not be possible for DDP to enable evasion of
 buffer consistency checks at the recipient.  The DDP and RDMA
 specifications must allow the recipient to rely on its consistent
 buffer contents by explicitly controlling peer access to buffer
 regions at appropriate times.
 The use of DDP and RDMA on a transport connection may interact with
 any security mechanism, and vice-versa.  For example, if the security
 mechanism is implemented above the transport layer, the DDP and RDMA
 headers may not be protected.  Therefore, such a layering may be
 inappropriate, depending on requirements.

3.1. Security Services

 The following end-to-end security services protect DDP and RDMAP
 operation streams:
   o    Authentication of the data source, to protect against peer
        impersonation, stream hijacking, and man-in-the-middle attacks
        exploiting capabilities offered by the RDMA implementation.
        Peer connections that do not pass authentication and
        authorization checks must not be permitted to begin processing
        in RDMA mode with an inappropriate endpoint.  Once associated,
        peer accesses to buffer regions must be authenticated and made
        subject to authorization checks in the context of the
        association and endpoint (socket) on which they are to be
        performed, prior to any transfer operation or data being
        accessed.  The RDMA protocols must ensure that these region
        protections be under strict application control.
   o    Integrity, to protect against modification of the control
        content and buffer content.
        While integrity is of concern to any transport, it is
        important for the DDP and RDMAP protocols that the RDMA
        control information carried in each operation be protected, in
        order to direct the payloads appropriately.
   o    Sequencing, to protect against replay attacks (a special case
        of the above modifications).
   o    Confidentiality, to protect the stream from eavesdropping.
 IPsec, operating to secure the connection on a packet-by-packet
 basis, is a natural fit to securing RDMA placement, which operates in
 conjunction with transport.  Because RDMA enables an implementation
 to avoid buffering, it is preferable to perform all applicable

Bailey & Talpey Informational [Page 18] RFC 4296 DDP and RDMA Architecture December 2005

 security protection prior to processing of each segment by the
 transport and RDMA layers.  Such a layering enables the most
 efficient secure RDMA implementation.
 The TLS record protocol, on the other hand, is layered on top of
 reliable transports and cannot provide such security assurance until
 an entire record is available, which may require the buffering and/or
 assembly of several distinct messages prior to TLS processing.  This
 defers RDMA processing and introduces overheads that RDMA is designed
 to avoid.  In addition, TLS length restrictions on records themselves
 impose additional buffering and processing for long operations that
 must span multiple records.  TLS therefore is viewed as potentially a
 less natural fit for protecting the RDMA protocols.
 Any DDP and RDMAP specification must provide the means to satisfy the
 above security service requirements.
 IPsec is sufficient to provide the required security services to the
 DDP and RDMAP protocols, while enabling efficient implementations.

3.2. Error Considerations

 Resource issues leading to denial-of-service attacks, overwrites and
 other concurrent operations, the ordering of completions as required
 by the RDMA protocol, and the granularity of transfer are all within
 the required scope of any security analysis of RDMA and DDP.
 The RDMA operations require checking of what is essentially user
 information, explicitly including addressing information and
 operation type (read or write), and implicitly including protection
 and attributes.  The semantics associated with each class of error
 resulting from possible failure of such checks must be clearly
 defined, and the expected action to be taken by the protocols in each
 case must be specified.
 In some cases, this will result in a catastrophic error on the RDMA
 association; however, in others, a local or remote error may be
 signalled.  Certain of these errors may require consideration of
 abstract local semantics.  The result of the error on the RDMA
 association must be carefully specified so as to provide useful
 behavior, while not constraining the implementation.

4. Acknowledgements

 The authors wish to acknowledge the valuable contributions of Caitlin
 Bestler, David Black, Jeff Mogul, and Allyn Romanow.

Bailey & Talpey Informational [Page 19] RFC 4296 DDP and RDMA Architecture December 2005

5. Informative References

 [FCVI]   ANSI Technical Committee T11, "Fibre Channel Standard
          Virtual Interface Architecture Mapping", ANSI/NCITS 357-
          2001, March 2001, available from
          http://www.t11.org/t11/stat.nsf/fcproj.
 [IB]     InfiniBand Trade Association, "InfiniBand Architecture
          Specification Volumes 1 and 2", Release 1.1, November 2002,
          available from http://www.infinibandta.org/specs.
 [MYR]    VMEbus International Trade Association, "Myrinet on VME
          Protocol Specification", ANSI/VITA 26-1998, August 1998,
          available from http://www.myri.com/open-specs.
 [ROM]    Romanow, A., Mogul, J., Talpey, T., and S. Bailey, "Remote
          Direct Memory Access (RDMA) over IP Problem Statement", RFC
          4297, December 2005.
 [SCTP]   Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
          Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
          L., and V. Paxson, "Stream Control Transmission Protocol",
          RFC 2960, October 2000.
 [SDP]    InfiniBand Trade Association, "Sockets Direct Protocol
          v1.0", Annex A of InfiniBand Architecture Specification
          Volume 1, Release 1.1, November 2002, available from
          http://www.infinibandta.org/specs.
 [SRVNET] R. Horst, "TNet: A reliable system area network", IEEE
          Micro, pp. 37-45, February 1995.
 [VI]     D. Cameron and G. Regnier, "The Virtual Interface
          Architecture", ISBN 0971288704, Intel Press, April 2002,
          more info at http://www.intel.com/intelpress/via/.

Bailey & Talpey Informational [Page 20] RFC 4296 DDP and RDMA Architecture December 2005

Authors' Addresses

 Stephen Bailey
 Sandburst Corporation
 600 Federal Street
 Andover, MA  01810 USA
 USA
 Phone: +1 978 689 1614
 EMail: steph@sandburst.com
 Tom Talpey
 Network Appliance
 1601 Trapelo Road
 Waltham, MA  02451 USA
 Phone: +1 781 768 5329
 EMail: thomas.talpey@netapp.com

Bailey & Talpey Informational [Page 21] RFC 4296 DDP and RDMA Architecture December 2005

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Bailey & Talpey Informational [Page 22]

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