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MMUSIC                                                         S. Loreto
Internet-Draft                                              G. Camarillo
Intended status: Standards Track                                Ericsson
Expires: May 7, 2009                                    November 3, 2008


Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
                   Session Description Protocol (SDP)
                    draft-loreto-mmusic-sctp-sdp-02

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Abstract

   SCTP (Stream Control Transmission Protocol) is a transport protocol
   used to establish associations between two endpoints.  This document
   describes how to express media transport over SCTP in SDP (Session
   Description Protocol).  This document defines the 'SCTP' protocol
   identifier for SDP.









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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Protocol Identifier . . . . . . . . . . . . . . . . . . . . . . 3
   4.  The Setup and Connection Attributes and Association
       Management  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Multihoming . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     6.1.  Actpass/Passive . . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  Existing Connection Reuse . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8



































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1.  Introduction

   SDP (Session Description Protocol) [RFC4566] provides a general-
   purpose format for describing multimedia sessions in announcements or
   invitations.  RFC4145 [RFC4145] specifies a general mechanism for
   describing and establishing TCP streams.  RFC 4572 [RFC4572] extends
   RFC4145 [RFC4145] for describing TCP-based media streams that are
   protected using TLS [RFC4346].

   This document defines the a new protocol identifier, 'SCTP', to
   describe SCTP-based [RFC4960] media streams.  Additionally, this
   document specifies the use of the 'setup' and 'connection' SDP
   attributes to establish SCTP associations.  These attributes were
   defined in RFC4145 [RFC4145] for TCP.  This document discusses their
   use with SCTP.

   OPEN ISSUE: we have to specify how to establish secure SCTP-based
   media streams over Datagram Transport Layer Security (DTLS)
   [I-D.ietf-tsvwg-dtls-for-sctp] using the Session Description Protocol
   (SDP).  We have also to define a new SDP protocol identifier, 'SCTP/
   TLS'.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
   levels for compliant implementations.


3.  Protocol Identifier

   The following is the format for an 'm' line, as specified in RFC4566
   [RFC4566]:

       m=<media> <port> <proto> <fmt> ...

   This document defines a new value for the 'proto' field: 'SCTP'.

   The 'SCTP' protocol identifier is similar to both the 'UDP' and 'TCP'
   protocol identifiers in that it only describes the transport protocol
   and not the upper-layer protocol.  An 'm' line that specifies 'SCTP'
   MUST further qualify the application-layer protocol using an fmt
   identifier.  Media described using an 'm' line containing the 'SCTP'
   protocol identifier are carried using SCTP [RFC4960].




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4.  The Setup and Connection Attributes and Association Management

   The use of the 'setup' and 'connection' attributes in the context of
   an SCTP association is identical to the use of these attributes in
   the context of a TCP connection.  That is, SCTP endpoints MUST follow
   the rules in Sections 4 and 5 of RFC 4145 [RFC4145] when it comes to
   the use of the 'setup' and 'connection' attributes in offer/answer
   [RFC3264] exchanges.

   The management of an SCTP association is identical to the management
   of a TCP connection.  That is, SCTP endpoints MUST follow the rules
   in Section 6 of RFC 4145 [RFC4145] to manage SCTP associations.
   Whether to use the SCTP ordered or unordered delivery service is up
   to the applications using the SCTP association.


5.  Multihoming

   An SCTP endpoint, unlike a TCP endpoint, can be multihomed.  An SCTP
   endpoint is considered to be multihomed if it has more than one IP
   address.  A multihomed SCTP endpoint informs a remote SCTP endpoint
   about all its IP addresses using the address parameters of the INIT
   or the INIT-ACK chunk (depending on whether or not the multihomed
   endpoint is the one initiating the establishment of the association).
   Therefore, once the address provided in the 'c' line has been used to
   establish the SCTP association (i.e., to send the INIT chunk),
   address management is performed using SCTP.  This means that two SCTP
   endpoints can use addresses that were not listed in the 'c' line but
   that were negotiated using SCTP mechanisms.

   OPEN ISSUE: that intermediaries such as SBCs will not be aware of
   some of the IP addresses used for media because they will not appear
   in the SDP.  We can RECOMMEND that SCTP endpoints use a main address
   all the time (e.g., not to retransmit to a backup address) and that
   they send a re-INVITE every time they change that address.
   Alternatively (or additionally), we could add SDP attributes with all
   the IP addresses that can be used by the association.


6.  Examples

   The following examples show the use of the 'setup' and 'connection'
   SDP attributes.  As discussed in Section 4, the use of this
   attributes with an SCTP association is identical to their use with a
   TCP connection.  For the purpose of brevity, the main portion of the
   session description is omitted in the examples, which only show 'm'
   lines and their attributes (including 'c' lines).




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6.1.  Actpass/Passive

   An offerer at 192.0.2.2 signals its availability for an SCTP
   association at SCTP port 54111.  Additionally, this offerer is also
   willing to initiate the SCTP association:

              m=image 54111 SCTP *
              c=IN IP4 192.0.2.2
              a=setup:actpass
              a=connection:new

   The endpoint at 192.0.2.1 responds with the following description:

              m=image 54321 SCTP *
              c=IN IP4 192.0.2.1
              a=setup:passive
              a=connection:new

   This will cause the offerer (at 192.0.2.2) to initiate an SCTP
   association to port 54321 at 192.0.2.1.

6.2.  Existing Connection Reuse

   Subsequent to the exchange in Section 6.1, another offer/answer
   exchange is initiated in the opposite direction.  The endpoint at
   192.0.2.1, which now acts as the offerer, wishes to continue using
   the existing association:

              m=application 54321 SCTP *
              c=IN IP4 192.0.2.1
              a=setup:passive
              a=connection:new

                                 Figure 1

   The endpoint at 192.0.2.2 also wishes to use the existing SCTP
   association and responds with the following description:

              m=application 9 SCTP *
              c=IN IP4 192.0.2.2
              a=setup:active
              a=connection:new

                                 Figure 2

   The existing SCTP association between 192.0.2.2 and 192.0.2.1 will be
   reused.




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7.  Security Considerations

   See RFC 4566 [RFC4566] for security considerations on the use of SDP
   in general.  See RFC 3264 [RFC3264] and RFC 4145 [RFC4145] for
   security considerations on establishing media streams using offer/
   answer exchanges.  See RFC 4960 [RFC4960] for security considerations
   on SCTP in general.  This specification does not introduce any new
   security consideration in addition to the ones discussed in those
   specifications.


8.  IANA Considerations

   This document defines a new proto value: SCTP.  Its format is defined
   in Section 3.  This proto value should be registered by the IANA
   under "Session Description Protocol (SDP) Parameters" under "proto".

   The SDP specification, [RFC4566], states that specifications defining
   new proto values, like the SCTP proto value defined in this RFC, must
   define the rules by which their media format (fmt) namespace is
   managed.  For the SCTP protocol, new formats SHOULD have an
   associated MIME registration.  Use of an existing MIME subtype for
   the format is encouraged.  If no MIME subtype exists, it is
   RECOMMENDED that a suitable one is registered through the IETF
   process [RFC2048] by production of, or reference to, a standards-
   track RFC that defines the transport protocol for the format.


9.  Normative References

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.




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   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [I-D.ietf-tsvwg-dtls-for-sctp]
              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.


Authors' Addresses

   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Salvatore.Loreto@ericsson.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com
















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