General Internet Signaling Transport (GIST) over Stream Control Transmission Protocol (SCTP) and Datagram Transport Layer Security (DTLS)
RFC 6084

Internet Engineering Task Force (IETF)                             X. Fu
Request for Comments: 6084                                   C. Dickmann
Category: Experimental                          University of Goettingen
ISSN: 2070-1721                                             J. Crowcroft
                                                 University of Cambridge
                                                            January 2011

              General Internet Signaling Transport (GIST)
            over Stream Control Transmission Protocol (SCTP)
              and Datagram Transport Layer Security (DTLS)

Abstract

   The General Internet Signaling Transport (GIST) protocol currently
   uses TCP or Transport Layer Security (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).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6084.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
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   the Trust Legal Provisions and are provided without warranty as
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   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  4
   3.  GIST over SCTP . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Message Association Setup  . . . . . . . . . . . . . . . .  5
       3.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Protocol-Definition: Forwards-SCTP . . . . . . . . . .  5
     3.2.  Effect on GIST State Maintenance . . . . . . . . . . . . .  6
     3.3.  PR-SCTP Support  . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  API between GIST and NSLP  . . . . . . . . . . . . . . . .  7
   4.  Bit-Level Formats  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  MA-Protocol-Options  . . . . . . . . . . . . . . . . . . .  7
   5.  Application of GIST over SCTP  . . . . . . . . . . . . . . . .  8
     5.1.  Multihoming Support of SCTP  . . . . . . . . . . . . . . .  8
     5.2.  Streaming Support in SCTP  . . . . . . . . . . . . . . . .  8
   6.  NAT Traversal Issue  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Use of DTLS with GIST  . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     11.2. Informative References . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   This document describes the usage of the General Internet Signaling
   Transport (GIST) protocol [1] and Datagram Transport Layer Security
   (DTLS) [2].

   GIST, in its initial specification for Connection mode (C-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) [9] for signaling message
   transport.  However, some Next Steps in Signaling (NSIS) Signaling
   Layer Protocol (NSLP) [10] 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
   and multihoming issues with TCP, these considerations argue that
   implementations of GIST should support SCTP as an optional transport
   protocol for GIST.  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 the framing
   mechanisms must be provided separately.  In addition, its Partial
   Reliability extension (PR-SCTP) [3] supports partial retransmission
   based on a programmable retransmission timer.  Furthermore, DTLS
   provides a viable solution for securing SCTP [4], which allows SCTP
   to use almost all of its transport features and its extensions.

   This document defines the use of SCTP as the underlying transport
   protocol for GIST and the use of DTLS as a security mechanism for
   protecting GIST Messaging Associations and discusses the implications
   on GIST state maintenance and API between GIST and NSLPs.
   Furthermore, this document describes how GIST is transported over
   SCTP and used by NSLPs in order to exploit the additional
   capabilities offered by SCTP to deliver GIST C-mode messages more
   effectively.  More specifically:

   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 multihoming support of SCTP.

   GIST over SCTP as described in this document does not require any
   changes to the high-level operation and structure of GIST.  However,
   adding new transport options requires additional interface code and
   configuration support to allow applications to exploit the additional

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   transport when appropriate.  In addition, SCTP implementations to
   transport GIST MUST support the optional feature of fragmentation of
   SCTP user messages.

   Additionally, this document also specifies how to establish GIST
   security using DTLS for use in combination with, e.g., SCTP and UDP.

2.  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [5].  Other
   terminologies and abbreviations used in this document are taken from
   related specifications ([1], [2], [3], [6]):

   o  SCTP - Stream Control Transmission Protocol

   o  PR-SCTP - SCTP Partial Reliability Extension

   o  MRM - Message Routing Method

   o  MRI - Message Routing Information

   o  SCD - Stack-Configuration-Data

   o  Messaging Association (MA) - A single connection between two
      explicitly identified GIST adjacent peers, i.e., between a given
      signaling source and destination address.  A messaging association
      may use a transport protocol; 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, referring 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 unidirectional logical channel established from one to
      another associated SCTP endpoint, within which all user messages
      are delivered in sequence except for those submitted to the
      unordered delivery service.

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3.  GIST over SCTP

   This section defines a new MA-Protocol-ID type, "Forwards-SCTP", for
   using SCTP as the GIST transport protocol.  The use of DTLS in GIST
   is defined in Section 7.

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 information is a main part of the Stack Configuration Data (SCD)
   [1].  This section adds Forwards-SCTP (value 3) as another possible
   protocol option.  In Forwards-SCTP, analog to Forwards-TCP,
   connections between peers are opened in the forwards direction, from
   the querying node, towards the responder.

3.1.2.  Protocol-Definition: Forwards-SCTP

   The MA-Protocol-ID "Forwards-SCTP" 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 in 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 reported by an SCTP socket API
   notification [11], GIST MUST report this to NSLPs as discussed in
   Section 4.1.2 of [1].  For the multihoming scenario, when a
   destination address of a GIST-over-SCTP peer encounters a change, the
   SCTP API will notify GIST about the availability of different SCTP
   endpoint addresses and the possible change of the primary path.

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3.2.  Effect on GIST State Maintenance

   As SCTP provides additional functionality over TCP, this section
   discusses the implications of using GIST over SCTP on GIST state
   maintenance.

   While SCTP defines unidirectional streams, for the purpose of this
   document, the concept of a bidirectional stream is used.
   Implementations MUST establish both downstream and upstream
   (unidirectional) SCTP streams and use the same stream identifier in
   both directions.  Thus, the two unidirectional streams (in opposite
   directions) form a bidirectional 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.  When an SCTP implementation is used for GIST transport,
   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 particularly 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
   partially reliable messages can be multiplexed over a single PR-SCTP
   association.  Therefore, an SCTP implementation for GIST transport
   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.

   When using a normally reliable session (as opposed to a partially
   reliable session), 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

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   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

   The 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 interpretations
   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
      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 retain
      an open connection to the peer due to its multihoming
      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.

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5.  Application of GIST over SCTP

5.1.  Multihoming Support of SCTP

   In general, the multihoming 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 failing for the IP traffic that
   is signaled for.  Thus, GIST would need to perform a refresh to the
   NSIS nodes to the alternative path anyway to cope with the route
   change.  When the two endpoints of a multihomed SCTP association (but
   none of the intermediate nodes between them) support NSIS, GIST over
   SCTP provides a robust means for GIST to deliver NSLP messages even
   when the primary path fails but at least one alternative path between
   these (NSIS-enabled) endpoints of the multihomed path is available.
   Additionally, the use of the multihoming 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
   multihoming 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 the head-of-line
   blocking issue in TCP.  Since a single GIST MA may be reused by
   multiple sessions, using TCP as the transport for 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 [12].  This
   specification does not define NAT traversal procedures for GIST over
   SCTP, although an approach for SCTP NAT traversal is described in
   [13].

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7.  Use of DTLS with GIST

   This section specifies a new MA-Protocol-ID "DTLS" (value 4) for the
   use of DTLS in GIST, which denotes a basic use of datagram transport
   layer channel security, initially in conjunction with GIST over SCTP.
   It provides server (i.e., GIST transport receiver) authentication and
   integrity (as long as the NULL ciphersuite is not selected during
   ciphersuite negotiation), as well as 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.  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 phase.

   The usage of DTLS [2] for securing GIST over datagram transport
   protocols MUST be implemented and SHOULD be used.

   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.  NULL authentication and integrity ciphers
   MUST NOT be negotiated for GIST nodes supporting DTLS.  For
   confidentiality ciphers, nodes can negotiate the NULL ciphersuites.
   The same rules for negotiating TLS ciphersuites as specified in
   Section 5.7.3 of [1] apply.

   DTLS renegotiation [7] may cause problems for applications such that
   connection security parameters can change without the application
   knowing it.  Hence, it is RECOMMENDED that renegotiation be disabled
   for GIST over DTLS.

   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], [6], and [2] apply.
   Additionally, although [4] does not support replay detection in DTLS
   over SCTP, the SCTP replay protection mechanisms [6] [8] should be
   able to protect NSIS messages transported using GIST over (DTLS over)
   SCTP from replay attacks.

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9.  IANA Considerations

   According to this specification, IANA has registered the following
   codepoints (MA-Protocol-IDs) in a registry created by [1]:

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

   Note that MA-Protocol-ID "DTLS" is never used alone but always
   coupled with a transport protocol specified in the stack proposal.

10.  Acknowledgments

   The authors would like to thank John Loughney, Jukka Manner, Magnus
   Westerlund, Sean Turner, Lars Eggert, Jeffrey Hutzelman, Robert
   Hancock, Andrew McDonald, Martin Stiemerling, Fang-Chun Kuo, Jan
   Demter, Lauri Liuhto, Michael Tuexen, and Roland Bless for their
   helpful suggestions.

11.  References

11.1.  Normative References

   [1]   Schulzrinne, H. and R. Hancock, "GIST: General Internet
         Signalling Transport", RFC 5971, October 2010.

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

   [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]   Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
         Transport Layer Security (DTLS) for Stream Control Transmission
         Protocol (SCTP)", RFC 6083, January 2011.

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

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

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   [7]   Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport
         Layer Security (TLS) Renegotiation Indication Extension",
         RFC 5746, February 2010.

   [8]   Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
         "Authenticated Chunks for the Stream Control Transmission
         Protocol (SCTP)", RFC 4895, August 2007.

11.2.  Informative References

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

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

   [11]  Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei,
         "Sockets API Extensions for Stream Control Transmission
         Protocol (SCTP)", Work in Progress, January 2011.

   [12]  Manner, J., Bless, R., Loughney, J., and E. Davies, "Using and
         Extending the NSIS Protocol Family", RFC 5978, October 2010.

   [13]  Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
         Transmission Protocol (SCTP) Network Address Translation", Work
         in Progress, December 2010.

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Authors' Addresses

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

   EMail: fu@cs.uni-goettingen.de

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

   EMail: mail@christian-dickmann.de

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

   EMail: jon.crowcroft@cl.cam.ac.uk

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