Network Working Group                                              X. Fu
Internet-Draft                                               C. Dickmann
Intended status: Standards Track                University of Goettingen
Expires: September 9, 2009                                  J. Crowcroft
                                                 University of Cambridge
                                                           March 8, 2009

 General Internet Signaling Transport (GIST) over SCTP and Datagram TLS

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   The General Internet Signaling Transport (GIST) protocol currently

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   uses TCP or TLS over TCP for connection mode operation.  This
   document describes the usage of GIST over the Stream Control
   Transmission Protocol (SCTP) and Datagram Transport Layer Security
   (DTLS).  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, the support of multi-homing to
   provide network level fault tolerance, as well as partial reliability
   extension for partially reliable data transmission.  This document
   also specifies how to establish GIST security over datagram transport
   protocols using an extension to DTLS.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  4
   3.  GIST Over SCTP . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Message Association Setup  . . . . . . . . . . . . . . . .  4
       3.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . .  4
       3.1.2.  Protocol-Definition: Forwards-SCTP . . . . . . . . . .  5
     3.2.  Effect on GIST State Maintenance . . . . . . . . . . . . .  5
     3.3.  PR-SCTP Support  . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  API between GIST and NSLP  . . . . . . . . . . . . . . . .  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.  NAT Traversal Issue  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Use of DTLS with GIST  . . . . . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     11.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     11.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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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) [7] 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.  Furthermore, SCTP offers an advantage of
   message-oriented transport instead of using the byte stream oriented
   TCP where one has to provide its own framing mechanisms.  In
   addition, its Partial Reliability extension (PR-SCTP) [3] supports
   partial retransmission based on a programmable retransmission timer.
   Furthermore, Datagram Transport Layer Security (DTLS) [4] provides a
   viable solution for securing datagram transport protocols, e.g., by
   using DTLS over SCTP [5].

   This document defines the use of SCTP as a transport protocol and the
   use of DTLS as a security mechanism for GIST Messaging Associations
   and discusses the implications on GIST State Maintenance and API
   between GIST and NSLPs.  Furthermore, this document shows how GIST
   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) [8] messages as payload).  More
   o  How to use the multiple streams feature of SCTP.
   o  How to use the PR-SCTP extension 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.

   Additionally, this document specifies the use of DTLS for securing
   GIST over datagram transport protocols such as SCTP.

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2.  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [6].  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  SCD - Stack Configuration Data
   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.  All transport
      addresses used by an SCTP endpoint must use the same port number,
      but can use multiple IP addresses.  A transport address used by an
      SCTP endpoint must not be used by another SCTP endpoint.  In other
      words, a transport address is unique to an SCTP endpoint.  Two
      SCTP endpoints MUST NOT have more than one SCTP association
      between them at any given time [2].
   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

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   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 for establishing the
   message association 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

   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-

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   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, which would be particular useful for delivering GIST
   Connection mode messages.  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 using "timed reliability"
   service instead of a standard SCTP association, if available, to
   support more flexible transport features for potential needs of
   different NSLPs.

   In a standard SCTP, instead, if a node has sent the first
   transmission before the lifetime expires, then the message MUST be
   sent as a normal reliable message.  During episodes of congestion
   this is particularly unfortunate, as retransmission wastes bandwidth
   that could have been used for other (non-lifetime expired) messages.
   The "timed reliability" service in PR-SCTP eliminates this issue and
   is hence RECOMMENDED to be used for GIST over PR-SCTP.

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 from [1] are as follows:
   o  The Timeout parameter in API "SendMessage": According to [1], this
      parameter represents the "length of time GIST should attempt to
      send this message before indicating an error."  When used with PR-
      SCTP, this parameter is used as the timeout for the "timed
      reliability" service of PR-SCTP.
   o  "NetworkNotification": According to [1], this primitive "is passed
      from GIST to a signalling application.  It indicates that a
      network event of possible interest to the signalling application

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      occurred."  Here, if SCTP detects a failure of the primary path,
      GIST SHOULD also indicate this event to the NSLP by calling this
      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

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

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   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.  NAT Traversal Issue

   NAT traversal for GIST over SCTP will follow Section 7.2 of [1] and
   the GIST extensibility capabilities defined in [9].  This
   specification does not define NAT traversal procedure for GIST over
   SCTP, although an approach for SCTP NAT traversal is described in

7.  Use of DTLS with GIST

   The MA-Protocol-ID for DTLS denotes a basic use of datagram transport
   layer channel security, initially in conjunction with SCTP.  It
   provides authentication, integrity and optionally replay protection
   for control packets.  The use of DTLS for securing GIST over SCTP
   allows GIST to take the advantage of features provided by SCTP and
   its extensions.  Note replay protection is not available for DTLS
   over SCTP [5].  The usage of DTLS for GIST over SCTP is similar to
   TLS for GIST as specified in [1], where a stack-proposal containing
   both MA-Protocol-IDs for SCTP and DTLS during the GIST handshake

   GIST message associations using DTLS may carry messages with transfer
   attributes requesting confidentiality or integrity protection.  The
   specific DTLS version will be negotiated within the DTLS layer
   itself, but implementations MUST NOT negotiate to protocol versions
   prior to DTLS v1.0 and MUST use the highest protocol version
   supported by both peers.  GIST nodes supporting DTLS MUST be able to
   negotiate the DTLS NULL and block cipher ciphers and SHOULD be able
   to negotiate the new cipher suites.  They MAY negotiate any mutually
   acceptable ciphersuite that provides authentication, integrity, and
   confidentiality.  The same rules for negotiating TLS cipher suites as

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   specified in Section 5.7.3 of [1] apply.

   No MA-protocol-options field is required for DTLS.  The configuration
   information for the transport protocol over which DTLS is running
   (e.g.  SCTP port number) is provided by the MA-protocol-options for
   that protocol.

8.  Security Considerations

   The security considerations of [1], [2] and [4] apply.  Following
   [5], replay detection of DTLS over SCTP is not supported.

   The usage of DTLS [4] for securing GIST over datagram transport
   protocols MUST be implemented and SHOULD be used.  An implementation
   of GIST over SCTP with no PR-SCTP support MAY use TLS for its channel
   security, when DTLS is not available between two GIST peers.

9.  IANA Considerations

   This specification extends [1] by introducing two additional MA-

     | MA-Protocol-ID      | Protocol                                 |
     | 3                   | SCTP opened in the forwards direction    |
     |                     |                                          |
     | 4                   | DTLS initiated in the forwards direction |

10.  Acknowledgments

   The authors would like to thank John Loughney, Robert Hancock, Andrew
   McDonald, Martin Stiemerling, Fang-Chun Kuo, Jan Demter, Lauri
   Liuhto, Michael Tuexen, and Roland Bless for their helpful

11.  References

11.1.  Normative References

   [1]   Schulzrinne, H. and R. Hancock, "GIST: General Internet
         Signalling Transport", draft-ietf-nsis-ntlp-17 (work in

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         progress), October 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]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", RFC 4347, April 2006.

   [5]   Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
         Transport Layer Security for Stream Control Transmission
         Protocol", draft-ietf-tsvwg-dtls-for-sctp-00 (work in
         progress), October 2008.

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

11.2.  Informative References

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

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

   [9]   Manner, J., Bless, R., Loughney, J., and E. Davies, "Using and
         Extending the NSIS Protocol Family", draft-ietf-nsis-ext-01
         (work in progress), March 2009.

   [10]  Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
         Transmission Protocol (SCTP) Network Address Translation",
         draft-ietf-behave-sctpnat-01 (work in progress), February 2009.

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Authors' Addresses

   Xiaoming Fu
   University of Goettingen
   Institute of Computer Science
   Goldschmidtstr. 7
   Goettingen  37077


   Christian Dickmann
   University of Goettingen
   Institute of Computer Science
   Goldschmidtstr. 7
   Goettingen  37077


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


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