MMUSIC Working Group                                              D. Yon
Internet-Draft                                          Dialout.Net, Inc                                    Tactical Software, LLC
Expires: January 14, March 30, 2005                                     G. Camarillo
                                                           July 16,
                                                      September 29, 2004

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

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, I certify each
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   RFC 3668.

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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 connection attribute, which provides a handles connection identifier. reestablishment.

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 Connection Attribute . . . . . . . . . . . . . . . . . . . . .  5
     5.1   Offerer Behaviour  . . . . . . . . . . . . . . . . . . . .  6
     5.2   Answerer Behaviour . . . . . . . . . . . . . . . . . . . .  7
   6.  Connection Management  . . . . . . . . . . . . . . . . . . . .  7
     6.1   Connection Establishment . . . . . . . . . . . . . . . . .  7
     6.2   Connection Reestablishment . . . . . . . . . . . . . . . .  8
     6.3   Connection Termination . . . . . . . . . . . . . . . . . .  8
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1   Passive/Active . . . . . . . . . . . . . . . . . . . . . .  8
     7.2   Passive/Active with Connection Reestablishment   Actpass/Passive  . . . . . . . . . . . . . . . . . . . . .  9
     7.3   Actpass/Passive   Existing Connection Reuse  . . . . . . . . . . . . . . . .  9
     7.4   Existing Connection Refusal  . . . . .  9 . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10 11
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 12
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 10 12
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 10 12
   11.2  Informative References . . . . . . . . . . . . . . . . . . . 11 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 13
       Intellectual Property and Copyright Statements . . . . . . . . 13 14

1.  Introduction

   The Session Description Protocol [3] [4] 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 [10]) [11]) 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 [8], SIP [9], SIP [10], RTSP
   [6], email, HTTP [7], [8], etc.).

   SDP [3] [4] 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.  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 endpoints perform the
   connection setup procedure.  This document defines two new attributes
   to describe connection setups: setup and connid. connection.

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] [3] 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

   This document defines a new 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 described using an m= lines with containing 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 [6] [7] over TCP) start with the string "TCP/"
   (e.g., TCP/TLS).

   The following sections define the setup and the connid connection
   attributes.  While both attributes are applicable to m= lines that
   use the TCP protocol identifier, they are not limited to them.  These
   attributes MAY be used in conjunction with any m= line which uses a connection-oriented
   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], [5], 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 active 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 passive 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 Connection 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 connection attribute, which is charset-independent, is
   used to disambiguate these two scenarios.  The following is the ABNF
   of the
   connid connection attribute:


         connection-attr        = "a=connid:" connection-identifier
         connection-identifier "a=connection:" conn-value
         conn-value             = 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. "new" / "existing"

5.1  Offerer Behaviour

   Offerers and answerers use the connid connection 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.  The
   connection value resulting from an offer/answer exchange is the
   connection value in the answer.  If the connection value in the
   answer is "new", the end-points SHOULD establish a new connection.
   If the connection value in the answer is "existing", the end-points
   SHOULD continue using the exiting connection.

   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, attribute for the m= line
   unless the application using the m= line has other means to deal with
   connection reestablishment.  The
   connid connection 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 takes
   the endpoints will attempt to establish. value of "new".

   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 use a connection value of
   "existing" 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 connection value different than
      the one in use is enough to trigger the establishment of a new
      connection. Still, it is recommended to use a value different than
      all the previous ones used in the m= line to make debugging
      easier. "new".

      Note that, according to the rules in this section, an offer that
      changes the transport address (IP address plus or port number) of an m=
      line will have a new connid connection value for this m= line. of "new".

   The default value of the connection attribute in an offer/answer
   exchange is "new".

5.2  Answerer Behaviour

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

   For an m= line, if  If the offer contains a new connid connection
   value (i.e.,
   different from the one in use) the offer is "new", the answerer MUST also use this a value of
   "new" in the answer.  If the connection value in the offer contains is
   "existing", the connid answerer uses a value of "existing" in use and the
   answerer answer if
   it 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 connection and a value in use but the answerer wishes to
   establish of
   "new" if it wants a new transport-layer connection, connection to be established.

      In some scenarios where third party call control [12] is used, an
      endpoint may receive an initial offer with a connection value of
      "existing".  Following the previous rules, such an answerer MUST would
      use a
   new connid connection value of "new" in the answer.

   If the connid connection value for an m= line resulting from an offer/answer
   exchange is different than the connid in use so far, "new", 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 endpoints 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

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

   This section addresses connection establishment, connection
   reestablishment, and connection termination.

6.1  Connection Establishment

   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 it is completed, able
   to, 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.


      Note that some endpoints do not close the connection until the session
   has expired, been explicitly terminated, or a new connid value has
   been provided need to wait for some event to happen
      before being able to establish the m= line. Additionaly, specific applications can
   describe further scenarios where an end-point connection.  For example, a
      wireless terminal may close need to set up a given
   connection. In case the session is explicitly terminated by one of
   the endpoints (e.g., the endpoint sends radio bearer before being
      able to initiate a SIP [9] BYE), the end point
   terminating the session is responsible for closing the
   transport-connection. connection.

6.2  Connection Reestablishment

   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 connection value of "new"
   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.

6.3  Connection Termination

   Typically, endpoints do not close the connection until the session
   has expired, been explicitly terminated, or a new connection value
   has been provided for the m= line.  Additionaly, specific
   applications can describe further scenarios where an end-point may
   close a given connection.  As soon as an end-point notices that it
   needs to terminate a connection, it SHOULD do so.

   While in TCP both end-points need to close a connection, other
   connection-oriented transport protocols may not have the concept of
   half-close connections.  In this case, a connection would be
   terminated as soon as one of the end-points closed it, making it
   unnecessary for the other end-point to perform any further action to
   terminate the connection.

   In any case, individual applications may provide further
   considerations on how to achieve a graceful connection termination.
   For example, a file application using TCP receiving a FIN from the
   remote endpoint may need to finish the ongoing transmission of a file
   before sending its own FIN.

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
           c=IN IP4

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

7.2  Passive/Active with Connection Reestablishment

   Continuing the preceding  Actpass/Passive

   In another example, consider the scenario where the TCP
   connection fails and the endpoints wish to reestablish the connection
   for the session.  The endpoint an offerer at signals its availability
   for a T.38 fax session at TCP port 54111.  Additionally, this intent with offerer
   is also willing to set up the following SDP: media stream by initiating the TCP

           m=image 54111 TCP t38
           c=IN IP4

   The new connid value informs the endpoint at that this SDP
   represents responds with the following description:

           m=image 54321 TCP t38
           c=IN IP4

   This will cause the intent offerer (at to establish initiate a new connection for media
   transport, rather than continuing with
   to port 54321 at

7.3  Existing Connection Reuse

   Subsequent to the original connection. If exchange in Section 7.2, another offer/answer
   exchange is initiated in the opposite direction.  The endpoint at agrees wishes to continue the session using a new connection, it the existing connection:

            m=image 54321 TCP t38
            c=IN IP4

   The endpoint at also wishes to use the existing connection
   and responds with: with the following description:

            m=image 9 TCP t38
            c=IN IP4

7.3  Actpass/Passive

   In another example, an offerer at

   The existing connection from signals its availability
   for a T.38 fax session to will be reused.

      Note that the endpoint at TCP uses setup:active in response
      to the offer of setup:passive, and uses port 54111. Additionally, this offerer 9 because it is also willing

7.4  Existing Connection Refusal

   Subsequent to set up the media stream exchange in Section 7.3, another offer/answer
   exchange is initiated by initiating the TCP endpoint at, again wishing to
   reuse the existing connection:

            m=image 54111 TCP t38
            c=IN IP4

   The endpoint at responds with

   However, this time the following description: answerer is unaware of the old connection and
   so wishes to establish a new one.  (This could be the result of a
   transfer via 3pcc.) It is unable to act in the passive mode so
   responds as active:

            m=image 54321 9 TCP t38
            c=IN IP4

   This will cause

   The endpoint at then initiates the offerer (at to initiate a TCP connection to port 54321
   54111 at, and the endpoint at closes the old

      Note that the endpoint at, while specifying connection:
      existing has reverted to setup:actpass and its real port number,
      rather than repeating setup:active and port 9 from the previous
      cycle.  Had it not done this, this negotiation would have failed.

8.  Security Considerations

   See RFC 2327 [3] [4] for security and other considerations specific to
   the Session Description Protocol in general.

   An attacker may attempt to modify the values of the connid connection and
   setup attributes to have endpoints reestablish connections
   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 [9],
   [10], S/MIME is the natural choice to provide such end-to-end
   integrity protection, as described in RFC 3261 [9]. [10].  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. connection.  Their formats are defined in Section 4 and
   Section 5 respectively.  These two attributes should be registered by
   the IANA on

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

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

   under "proto".

   Specifications defining new proto values, like this one, must define
   the rules by which their media format (fmt) namespace is managed.
   For the TCP 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 [2] by production
   of, or reference to, a standards-track RFC that defines the transport
   protocol for the format.

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

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


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


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

11.2  Informative References


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


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


   [8]   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.


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


   [10]  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.


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

   [12]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April

Authors' Addresses

   David Yon
   Dialout.Net, Inc
   One Indian Head Plaza
   Tactical Software, LLC
   670 N Commercial St
   Manchester, NH  03060  03101


   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420


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