MMUSIC Working Group                                              D. Yon
Internet-Draft                                          Dialout.Net, Inc
Expires: December 10, 2004 January 14, 2005                                   G. Camarillo
                                                           June 11,
                                                           July 16, 2004

     Connection-Oriented Media Transport in the Session Description
                             Protocol (SDP)

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 10, 2004. January 14, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document describes how to express media transport over
   connection-oriented protocols using the Session Description Protocol
   (SDP). It defines the SDP TCP protocol identifier, the SDP setup
   attribute, which describes the connection setup procedure, and the
   SDP connid attribute, which provides a connection identifier.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol Identifier  . . . . . . . . . . . . . . . . . . . . .  3
   4.  Setup Attribute  . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1   The Setup Attribute in the Offer/answer Model  . . . . . .  4
   5.  The Connid Attribute . . . . . . . . . . . . . . . . . . . . .  5
     5.1   Offerer Behaviour  . . . . . . . . . . . . . . . . . . . .  6
     5.2   Answerer Behaviour . . . . . . . . . . . . . . . . . . . .  6  7
   6.  Connection Management  . . . . . . . . . . . . . . . . . . . .  7
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1   Passive/Active . . . . . . . . . . . . . . . . . . . . . .  8
     7.2   Passive/Active with Connection Reestablishment . . . . . .  8  9
     7.3   Actpass/Passive  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 10
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 10
   11.2  Informational  Informative References . . . . . . . . . . . . . . . . . . 10 . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
       Intellectual Property and Copyright Statements . . . . . . . . 12 13

1.  Introduction

   The Session Description Protocol [3] provides a general-purpose
   format for describing multimedia sessions in announcements or
   invitations. SDP uses an entirely textual data format (the US-ASCII
   subset of UTF-8 [5]) [10]) to maximize portability among transports.  SDP
   does not define a protocol, but only the syntax to describe a
   multimedia session with sufficient information to participate in that
   session. Session descriptions may be sent using arbitrary existing
   application protocols for transport (e.g., SAP [9], [8], SIP [10], [9], RTSP
   [5], email, HTTP [8], [7], etc.).

   SDP [3] defines two protocol identifiers: RTP/AVP and UDP, both of
   which represent unreliable connectionless protocols. While these
   transports are appropriate choices for multimedia streams, there are
   applications for which connection-oriented transports, such as TCP,
   are more appropriate. We define This document defines a new protocol
   identifier, TCP, to describe TCP connetions in SDP.

   Connection-oriented protocols introduce two new factor when
   describing a session: how and when should end points perform the
   connection setup procedure. We define This document defines two new attributes
   to describe connection setups: setup and connid.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [2] and indicate requirement levels for
   compliant implementations.

3.  Protocol Identifier

   The following is the ABNF for an m= line, as specified by RFC 2327

    media-field =         "m=" media space port ["/" integer]
                          space proto 1*(space fmt) CRLF

   We define

   This document defines a new values value for the proto field: TCP.

   The TCP protocol identifier is similar to the UDP protocol identifier
   in that it only describes the transport protocol, and not the
   upper-layer protocol.  An m= line that specifies "TCP" MUST further
   qualify the application-layer protocol using an fmt identifier. Media
   lines with the TCP protocol identifier are carried using TCP [1].

   It is RECOMMENDED that documents defining new SDP protocol
   identifiers that involve extra protocol layers between TCP and the
   media itself (e.g., TLS [7] [6] over TCP) start with the string "TCP/"
   (e.g., TCP/TLS).

   The following sections define the setup and the connid attributes.
   While they both attributes are applicable to m= lines that use the TCP
   protocol identifier, they are not limited to them. These attributes SHOULD
   MAY be used in any m= line which uses a connection-oriented transport
   protocol, even if the protocol identifier of the m= line is not TCP.

4.  Setup Attribute

   The setup attribute indicates which of the end points should initiate
   the connection establishment (e.g., send the initial TCP SYN). The
   setup attribute is charset-independent and can be a session-level or
   a media-level attribute. The following is the ABNF of the setup

         setup-attr           =  "a=setup:" role
         role                 =  "active" / "passive" / "actpass"
                                 / "holdconn"

      Active: The endpoint will initiate an outgoing connection.

      Passive: The endpoint will accept an incoming connection.

      ActPass: The endpoint is willing to accept an incoming connection
      or to initiate an outgoing connection.

      Holdconn: The endpoint does not want the connection to be
      established for the time being.

4.1  The Setup Attribute in the Offer/answer Model

   The offer/answer model, defined in RFC 3264 [4], provides endpoints
   with a means to obtain shared view of a session. Some session
   parameters are negotiated (e.g., codecs to use), while others are
   simply communicated from one endpoint to the other (e.g., IP
   addresses). The value of the setup attribute falls into the first
   category. That is, both endpoints negotiate its value using the
   offer/answer model.

   The negotiation of the value of the setup attribute takes places as
   follows. The offerer states which role or roles it is willing to
   perform and the answerer, taking the offerer's willingness into
   consideration, chooses which roles both endpoints will actually
   perform during connection establishment. The following are the values
   that the setup attribute can take in an offer/answer exchange:

            Offer      Answer
            active     passive / holdconn
            passive    active / holdconn
            actpass    active / passive / holdconn
            holdconn   holdconn

   The value active indicates that the endpoint SHOULD initiate a connection to the port number
   on the m= line of the other endpoint.  The port number on its own m=
   line is irrelevant, and the opposite endpoint MUST NOT attempt to
   initiate a connection to the port number specified there.
   Nevertheless, since the m= line must contain a valid port number, the
   endpoint specifying using the value active SHOULD specify a port
   number of 9 (the discard port) on its m= line. The endpoint MUST NOT
   specify a port number of zero, except to denote an m= line that has
   been or is being refused.

   The value passive indicates that the endpoint SHOULD be ready to accept a connection on the
   port number specified in the m= line.


   A value of actpass indicates that the offerer can either initiate a
   connection to the port number on the m= line in the answer or accept
   a connection on the port number specified in the m= line in the
   offer. That is, the offerer has no preference as to whether it
   accepts or initiates the connection and, so, is letting the answerer

   A value of holdconn indicates that the connection should not be
   established for the time being.

   The default value of the setup attribute in an offer/answer exchange
   is active in the offer and passive in the answer.

5.  The Connid Attribute

   The preceding description of the setup attribute has been in the
   context of using SDP to initiate a session. Still, SDP may be
   exchanged between endpoints at various stages of a session to
   accomplish tasks such as terminating a session, redirecting media to
   a new endpoint, or renegotiating the media parameters for a session.
   After the initial session has been established, it may be ambiguous
   as to whether subsequent SDP exchange represents a confirmation that
   the endpoint is to continue using the current media connection
   unchanged, or is a request to make a new media connection. The
   media-level connid attribute, which is charset-independent, is used
   to disambiguate these two scenarios. The following is the ABNF of the
   connid attribute:

         connid                 = "a=connid:" connection-identifier
         connection-identifier  = token

   The connid attribute provides an identifier for the transport-layer
   connection used by the m= line. Connid values are meaningful in the
   context of a particular m= line. So, different m= lines in the same
   session description MAY have the same connid value.

5.1  Offerer Behaviour

   Offerers and answerers use the connid attribute to decide whether a
   new transport connection needs to be established or, on the other
   hand, the existing transport connection should still be used.

   When an offerer generates an m= line which uses a connection-oriented
   transport, it SHOULD provide such an m= line with a connection
   identifier using a connid attribute, unless the application using the
   m= line has other means to deal with connection reestablishment. The
   connid attribute in an initial offer (i.e., no transport connection
   has been established yet) can take any value. This value identifies
   the initial connection that the endpoints will attempt to establish.

   After the initial offer/answer exchange, any of the endpoints can
   generate a new offer to change some characteristics of the session
   (e.g., the direction attribute). If such an offerer wants to continue
   using the previously-established transport-layer connection for the
   m= line, the offerer MUST use the same connid value for the m= line.
   If, on the other hand, the offerer wants to establish a new
   transport-layer connection for the m= line, it MUST use a new connid
   value. This new connid value MUST be different from the current
   connid value in use and SHOULD be different than any connid value
   used previously in the same m= line.

      The connid value in an offer is only compared with the connid
      value currently in use. So, having a connid value different than
      the one in use is enough to trigger the establishment of a new
      connection. Still, we recommend it is recommended to use a value different than
      all the previous ones used in the m= line to make debugging
      Note that, according to the rules in this section, an offer that
      changes the transport address (IP address plus port number) of an
      m= line will have a new connid value for this m=line. m= line.

5.2  Answerer Behaviour

   The connid value for an m= line is negotiated using the offer/answer
   model. The resulting connid value after an offer/answer exchange is
   the connid value in the answer.

   For an m= line, if the offer contains a new connid value (i.e.,
   different from the one in use) the answerer MUST use this value in
   the answer. If the offer contains the connid value in use and the
   answerer wishes to continue using the existing transport-layer
   connection, the answerer MUST use this connid value in the answer. If
   the offer contains the connid value in use but the answerer wishes to
   establish a new transport-layer connection, the answerer MUST use a
   new connid value in the answer.

   If the connid value for an m= line resulting from an offer/answer
   exchange is different than the connid in use so far, the endpoints
   SHOULD establish a new transport-layer connection as indicated by the
   setup attribute. If a previous connection is still up, the endpoint
   responsible for establishing the new connection performing the active
   role SHOULD close it as soon as the offer/answer exchange is
   completed. It is up to the application to ensure proper data
   synchornization between the two connections.

   If the connid value for an m= line resulting from an offer/answer
   exchange is the same as the connid in use so far, the endpoints
   SHOULD continue using the existing connection.

      In the past, it was proposed to use the presence of a media-level
      SDP attribute as a flag to indicate that a new connection needed
      to be established. We chose It was decided not to follow the flag approach
      because an offerer whose intent was to signal "no changes" in a
      session would need to issue a different offer than the previous
      one (i.e., it would need to remove the flag from the m= line). By
      using the connid attribute instead, an offerer signals "no
      changes" in a session by issuing an identical offer to the one in

6.  Connection Management

   An endpoint that according to an offer/answer exchange is supposed to
   initiate a new connection SHOULD initiate it as soon as the offer/
   answer exchange is completed, even if the endpoint does not intend to
   immediately begin sending media to the remote endpoint. This allows
   media to flow from the remote endpoint if needed.

   Typically, endpoints do not close the connection until the session
   has expired, been explicitly terminated, or a new connid value has
   been provided for the m= line. Additionaly, specific applications can
   describe further scenarios where an end-point may close a given
   connection. In case the session is explicitly terminated by one of
   the endpoints (e.g., the endpoint sends a SIP [10] [9] BYE), the end point
   terminating the session is responsible for closing the

   If an endpoint determines that the transport-connection for an m=
   line has been closed and it should be reestablished, it SHOULD
   perform a new offer/answer exchange using a new connid value for this
   m= line.

      Note that the SDP direction attribute (e.g., a=sendonly) deals
      with the media sent over the transport-connection, but has no
      impact on the transport-connection itself.

7.  Examples

   The following examples show the most common usage of the setup
   attribute combined with TCP-based media descriptions. 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).

7.1  Passive/Active

   An offerer at signals its availability for a T.38 fax
   session at port 54111:

           m=image 54111 TCP t38
           c=IN IP4

   An answerer at receiving this offer responds with the
   following answer:

           c=IN IP4
           m=image 9 TCP t38

   The endpoint at then initiates the TCP connection to port
   54111 at

7.2  Passive/Active with Connection Reestablishment

   Continuing the preceding example, consider the scenario where the TCP
   connection fails and the endpoints wish to reestablish the connection
   for the session.  The endpoint at signals this intent with
   the following SDP:

           m=image 54111 TCP t38
           c=IN IP4

   The new connid value informs the endpoint at that this SDP
   represents the intent to establish a new connection for media
   transport, rather than continuing with the original connection. If agrees to continue the session using a new connection, it
   responds with:

           m=image 9 TCP t38
           c=IN IP4

7.3  Actpass/Passive

   In another example, an offerer at signals its availability
   for a T.38 fax session at TCP port 54111. Additionally, this offerer
   is also willing to set up the media stream by initiating the TCP

           m=image 54111 TCP t38
           c=IN IP4

   The endpoint at responds with the following description:

           m=image 54321 TCP t38
           c=IN IP4

   This will cause the offerer (at to initiate a connection
   to port 54321 at

8.  Security Considerations

   See RFC 2327 [3] for security and other considerations specific to
   the Session Description Protocol in general.

   An attacker may attempt to modify the values of the connid and setup
   attributes to have endpoints reestablish connections unnecesaryly. unnecesaryly or
   to keep them from establishing a connection. So, it is STRONGLY
   RECOMMENDED that integrity protection be applied to the SDP session
   descriptions. For session descriptions carried in SIP [10], [9], S/MIME is
   the natural choice to provide such end-to-end integrity protection,
   as described in RFC 3261 [10]. [9]. Other applications MAY use a different
   form of integrity protection.

9.  IANA Considerations

   This document defines two session and media level SDP attributes:
   setup and connid. Their formats are defined in Section 4 and Section
   5 respectively. These two attributes should be registered by the IANA

   under "att-field (both session and media level)".

   This document defines a proto values: TCP. Its format is defined in
   Section 3. This proto value should be registered by the IANA on

   under "proto".

10.  Acknowledgements

   Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
   Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
   Holmberg provided valuable insights and contributions.

11.  References

11.1  Normative References

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

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

   [3]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

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

   [5]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
        63, RFC 3629, November 2003.

11.2  Informational  Informative References


   [5]   Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.


   [6]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.


   [7]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.


   [8]   Handley, M., Perkins, C. and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.


   [9]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [10]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
         63, RFC 3629, November 2003.

Authors' Addresses

   David Yon
   Dialout.Net, Inc
   One Indian Head Plaza
   Nashua, NH  03060

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights. Information
   on the IETF's procedures with respect to rights in IETF Documents can
   be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard. Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.