Network Working Group                                              X. Fu
Internet-Draft                                               C. Dickmann
Expires: May 21, 2008
Intended status: Standards Track                University of Goettingen
Expires: August 17, 2008                                    J. Crowcroft
                                                 University of Cambridge
                                                       November 18, 2007
                                                       February 14, 2008

         General Internet Signaling Transport (GIST) over SCTP

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Copyright Notice

   Copyright (C) The IETF Trust (2007). (2008).


   The General Internet Signaling Transport (GIST) protocol currently
   uses TCP or TLS over TCP for connection mode operation.  This
   document describes the usage of GIST over the Stream Control
   Transmission Protocol (SCTP).  The use of SCTP can take advantage of
   features provided by SCTP, namely streaming-based transport, support
   of multiple streams to avoid head of line blocking, and the support
   of multi-homing to provide network level fault tolerance.
   Additionally, the support for the Partial Reliability Extension of
   SCTP is discussed.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  3
   3.  GIST Over SCTP . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Message Association Setup  . . . . . . . . . . . . . . . .  4
       3.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . .  4
       3.1.2.  Protocol-Definition: Forwards-SCTP . . . . . . . . . .  4
     3.2.  Effect on GIST State Maintenance . . . . . . . . . . . . .  5
     3.3.  PR-SCTP Support  . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  API between GIST and NSLP  . . . . . . . . . . . . . . . .  6
       3.4.1.  SendMessage  . . . . . . . . . . . . . . . . . . . . .  6
       3.4.2.  NetworkNotification  . . . . . . . . . . . . . . . . .  6
   4.  Bit-Level Formats  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  MA-Protocol-Options  . . . . . . . . . . . . . . . . . . .  7
   5.  Application of GIST over SCTP  . . . . . . . . . . . . . . . .  7
     5.1.  Multi-homing support of SCTP . . . . . . . . . . . . . . .  7
     5.2.  Streaming support in SCTP  . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

1.  Introduction

   This document describes the usage of the General Internet Signaling
   Transport (GIST) protocol [1] over the Stream Control Transmission
   Protocol (SCTP) [2].

   GIST, in its initial specification for connection mode operation,
   runs on top of a byte-stream oriented transport protocol providing a
   reliable, in-sequence delivery, i.e., using the Transmission Control
   Protocol (TCP) [5] for signaling message transport.  However, some
   NSLP context information has a definite lifetime, therefore, the GIST
   transport protocol could benefit from flexible retransmission, so
   stale NSLP messages that are held up by congestion can be dropped.
   Together with the head-of-line blocking issue and other issues with
   TCP, these considerations argue that implementations of GIST should
   support the Stream Control Transport Protocol (SCTP)[2] as an
   optional transport protocol for GIST, especially if deployment over
   the public Internet is contemplated.  Like TCP, SCTP supports
   reliability, congestion control and fragmentation.  Unlike TCP, SCTP
   provides a number of functions that are desirable for signaling
   transport, such as multiple streams and multiple IP addresses for
   path failure recovery.  In addition, its Partial Reliability
   extension (PR-SCTP) [3] supports partial retransmission based on a
   programmable retransmission timer.

   This document defines the use of SCTP as a transport protocol for
   GIST Messaging Associations and discusses the implications on GIST
   State Maintenance and API between GIST and NSLPs.  Furturemore, this
   document shows how GIST should SHOULD be used to provide the additional
   features offered by SCTP to deliver the GIST C-mode messages (which
   can in turn carry NSIS Signaling Layer Protocol (NSLP) [6] messages
   as payload).  More specifically:
   o  How to use the multiple streams feature of SCTP.
   o  How to use the PR-SCTP extention of SCTP.
   o  How to take advantage of the multi-homing support of SCTP.

   The method described in this document does not require any changes of
   GIST or SCTP.  However, SCTP implementations MUST support the
   optional feature of fragmentation of SCTP user messages.

2.  Terminology and Abbreviations

   The keywords key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL", "OPTIONAL" in this
   document are to be interpreted as described in
   BCP 14, RFC 2119 [4].  Other
   terminologies and abbreviations used in this document are taken from
   related specifications (e.g., [1] and [2]) as follows:

   o  SCTP - Stream Control Transmission Protocol
   o  PR-SCTP - SCTP Partial Reliability Extension
   o  MRM - Message Routing Method
   o  MRI - Message Routing Information
   o  MRS - Message Routing State
   o  MA - A GIST Messaging Association is a single connection between
      two explicitly identified GIST adjacent peers on the data path.  A
      messaging association may use a specific transport protocol and
      known ports.  If security protection is required, it may use a
      specific network layer security association, or use a transport
      layer security association internally.  A messaging association is
      bidirectional; signaling messages can be sent over it in either
      direction, and can refer to flows of either direction.
   o  SCTP Association - A protocol relationship between SCTP endpoints,
      composed of the two SCTP endpoints and protocol state information.
      An association can be uniquely identified by the transport
      addresses used by the endpoints in the association.  Two SCTP
      endpoints MUST NOT have more than one SCTP association between
      them at any given time.
   o  Stream - A sequence of user messages that are to be delivered to
      the upper-layer protocol in order with respect to other messages
      within the same stream.

3.  GIST Over SCTP

3.1.  Message Association Setup

3.1.1.  Overview

   The basic GIST protocol specification defines two possible protocols
   to be used in Messaging Associations, namely Forwards-TCP and TLS.
   This document adds Forwards-SCTP as another possible protocol.  In
   Forwards-SCTP, analog to Forwards-TCP, connections between peers are
   opened in the forwards direction, from the querying node, towards the

   A new MA-Protocol-ID type, "Forwards-SCTP", is defined in this
   document for using SCTP as GIST transport protocol.  A formal
   definition of Forwards-SCTP is given in the following section.

3.1.2.  Protocol-Definition: Forwards-SCTP

   This MA-Protocol-ID denotes a basic use of SCTP between peers.
   Support for this protocol is OPTIONAL.  If this protocol is offered,
   MA-protocol-options data MUST also be carried in the SCD object.  The
   MA-protocol-options field formats are:

   o  in a Query: no information apart from the field header.
   o  in a Response: 2 byte port number at which the connection will be
      accepted, followed by 2 pad bytes.

   The connection is opened in the forwards direction, from the querying
   node towards the responder.  The querying node MAY use any source
   address and source port.  The destination information MUST be derived
   from information in the Response: the address from the interface-
   address from the Network-Layer-Information object and the port from
   the SCD object as described above.

   Associations using Forwards-SCTP can carry messages with the transfer
   attribute Reliable=True.  If an error occurs on the SCTP connection
   such as a reset, as can be detected for example by a socket exception
   condition, GIST MUST report this to NSLPs as discussed in Section
   4.1.2 of [1].

3.2.  Effect on GIST State Maintenance

   This document defines the use of SCTP as a transport protocol for
   GIST Messaging Associations.  As SCTP provides additional
   functionality over TCP, this section dicusses the implications of
   using GIST over SCTP on GIST State Maintenance.

   While SCTP defines uni-directional streams, for the purpose of this
   document, the concept of a bi-direction stream is used.
   Implementations MUST establish downstream and upstream (uni-
   directional) SCTP streams always together and use the same stream
   identifier in both directions.  Thus, the two uni-directional streams
   (in opposite directions) form a bi-directional stream.

   Due to the multi-streaming support of SCTP, it is possible to use
   different SCTP streams for different resources (e.g., different NSLP
   sessions), rather than maintaining all messages along the same
   transport connection/association in a correlated fashion as TCP
   (which imposes strict (re)ordering and reliability per transport
   level).  However, there are limitations to the use of multi-
   streaming.  All GIST messages for a particular session MUST be sent
   over the same SCTP stream to assure the NSLP assumption of in-order
   delivery.  Multiple sessions MAY share the same SCTP stream based on
   local policy.

   The GIST concept of Messaging Association re-use is not affected by
   this document or the use of SCTP.  All rules defined in the GIST
   specification remain valid in the context of GIST over SCTP.

3.3.  PR-SCTP Support

   A variant of SCTP, PR-SCTP [3] provides a "timed reliability"
   service.  It allows the user to specify, on a per message basis, the
   rules governing how persistent the transport service should be in
   attempting to send the message to the receiver.  Because of the chunk
   bundling function of SCTP, reliable and partial reliable messages can
   be multiplexed over a single PR-SCTP association.  Therefore, a GIST
   over SCTP implementation SHOULD attempt to establish a PR-SCTP
   association instead of a standard SCTP association, if available, to
   support more flexible transport features for potential needs of
   different NSLPs.

3.4.  API between GIST and NSLP

   GIST specification defines an abstract API between GIST and NSLPs.
   While this document does not change the API itself, the semantics of
   some parameters have slightly different interpretation in the context
   of SCTP.  This section only lists those primitives and parameters,
   that need special consideration when used in the context of SCTP.
   The relevant primitives are repeatet from [1] to improve readability,
   but [1] remains authoritative.

3.4.1.  SendMessage

   The SendMessage primitive is used by the NSLP to initiate sending of

   SendMessage ( NSLP-Data, NSLP-Data-Size, NSLP-Message-Handle,
                 NSLP-Id, Session-ID, MRI,
                 SSI-Handle, Transfer-Attributes, Timeout, IP-TTL, GHC )

   The following parameter has changed semantics:

   Timeout: According to [1] this parameter represents the "length of
   time GIST should attempt to send this message before indicating an
   error".  When used with SCTP, this parameter is also used as the
   timeout for the "timed reliability" service of PR-SCTP.

3.4.2.  NetworkNotification

   The NetworkNotification primitive is passed from GIST to an NSLP.  It
   indicates that a network event of possible interest to the NSLP

   NetworkNotification ( MRI, Network-Notification-Type )
   If SCTP detects a failure of the primary path, GIST should SHOULD indicate
   this event to the NSLP by calling the NetworkNotification primitive
   with Network-Notification-Type "Routing Status Change".  This
   notification should be done even if SCTP was able to remain an open
   connection to the peer due to its multi-homing capabilities.

4.  Bit-Level Formats

4.1.  MA-Protocol-Options

   This section provides the bit-level format for the MA-protocol-
   options field that is used for SCTP protocol in the Stack-
   Configuration-Data object of GIST.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   :       SCTP port number        |         Reserved              :

   SCTP port number  = Port number at which the responder will accept
                       SCTP connections

   The SCTP port number is only supplied if sent by the responder.

5.  Application of GIST over SCTP

5.1.  Multi-homing support of SCTP

   In general, the multi-homing support of SCTP can be used to improve
   fault-tolerance in case of a path- or link-failure.  Thus, GIST over
   SCTP would be able to deliver NSLP messages between peers even if the
   primary path is not working anymore.  However, for the Message
   Routing Methods (MRMs) defined in the basic GIST specification such a
   feature is only of limited use.  The default MRM is path-coupled,
   which means, that if the primary path is failing for the SCTP
   association, it most likely is also for the IP traffic that is
   signaled for.  Thus, GIST would need to perform a refresh anyway to
   cope with the route change.  Nevertheless, the use of the multi-
   homing support of SCTP provides GIST and the NSLP with another source
   to detect route changes.  Furthermore, for the time between detection
   of the route change and recovering from it, the alternative path
   offered by SCTP can be used by the NSLP to make the transition more
   smoothly.  Finally, future MRMs might have different properties and
   therefore benefit from multi-homing more broadly.

5.2.  Streaming support in SCTP

   Streaming support in SCTP is advantageous for GIST.  It allows better
   parallel processing, in particular by avoiding head of line blocking
   issue in TCP.  Since a same GIST MA may be reused by multiple
   sessions, using TCP as transport GIST signaling messages belonging to
   different sessions may be blocked if another message is dropped.  In
   the case of SCTP, this can be avoided as different sessions having
   different requirements can belong to different streams, thus a
   message loss or reordering in a stream will only affect the delivery
   of messages within that particular stream, and not any other streams.

6.  Security Considerations

   The security considerations of both [1] and [2] apply.  For securing
   GIST over SCTP channel, it is recommended to use DTLS [7], to take
   the advantage of all the features provided by SCTP and its
   extensions.  DTLS over SCTP is currently being specified in [8].  The
   usage of DTLS for GIST is similar to TLS for GIST as specified in
   [1], and a MA-protocol-ID for DTLS is yet to be defined in another

7.  IANA Considerations

   A new MA-Protocol-ID (Forwards-SCTP) needs to be assigned, with a
   recommended value of 3.

8.  Acknowledgments

   The authors would like to thank John Loughney, Robert Hancock, Andrew
   McDonald, Fang-Chun Kuo and Jan Demter for their helpful suggestions.

9.  References

9.1.  Normative References

   [1]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
        Signalling Transport", draft-ietf-nsis-ntlp-14 draft-ietf-nsis-ntlp-15 (work in
        progress), July 2007. February 2008.

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

   [3]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad,
        "Stream Control Transmission Protocol (SCTP) Partial Reliability
        Extension", RFC 3758, May 2004.

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

9.2.  Informative References

   [5]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
        September 1981.

   [6]  Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
        Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
        June 2005.

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

   [8]  Tuexen, M. and E. Rescorla, "Datagram Transport Layer Security
        for Stream Control Transmission Protocol",
        draft-tuexen-dtls-for-sctp-02 (work in progress), November 2007.

Authors' Addresses

   Xiaoming Fu
   University of Goettingen
   Institute for Informatics of Computer Science
   Lotzestr. 16-18
   Goettingen  37083


   Christian Dickmann
   University of Goettingen
   Institute for Informatics of Computer Science
   Lotzestr. 16-18
   Goettingen  37083

   Jon Crowcroft
   University of Cambridge
   Computer Laboratory
   William Gates Building
   15 JJ Thomson Avenue
   Cambridge  CB3 0FD


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