[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 4145

MMUSIC Working Group                                              D. Yon
Internet-Draft                                    Tactical Software, LLC
Expires: March 30, 2005                                     G. Camarillo
                                                                Ericsson
                                                      September 29, 2004


     Connection-Oriented Media Transport in the Session Description
                             Protocol (SDP)
                  draft-ietf-mmusic-sdp-comedia-09.txt

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, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she 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://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on March 30, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   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 connection attribute, which handles connection reestablishment.



Yon & Camarillo          Expires March 30, 2005                 [Page 1]

Internet-Draft         Connection-Oriented Media          September 2004


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 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   Actpass/Passive  . . . . . . . . . . . . . . . . . . . . .  9
     7.3   Existing Connection Reuse  . . . . . . . . . . . . . . . .  9
     7.4   Existing Connection Refusal  . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 12
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 12
   11.2  Informative References . . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14
























Yon & Camarillo          Expires March 30, 2005                 [Page 2]

Internet-Draft         Connection-Oriented Media          September 2004


1.  Introduction

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

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

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 [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
   [4].


    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 containing the TCP protocol



Yon & Camarillo          Expires March 30, 2005                 [Page 3]

Internet-Draft         Connection-Oriented Media          September 2004


   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] over TCP) start with the string "TCP/"
   (e.g., TCP/TLS).

   The following sections define the setup and the 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 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
   attribute:


         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 [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



Yon & Camarillo          Expires March 30, 2005                 [Page 4]

Internet-Draft         Connection-Oriented Media          September 2004


   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
   choose.

   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 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



Yon & Camarillo          Expires March 30, 2005                 [Page 5]

Internet-Draft         Connection-Oriented Media          September 2004


   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 connection attribute, which is charset-independent, is
   used to disambiguate these two scenarios.  The following is the ABNF
   of the connection attribute:


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


5.1  Offerer Behaviour

   Offerers and answerers use the 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 a connection attribute for the m= line
   unless the application using the m= line has other means to deal with
   connection reestablishment.  The connection attribute in an initial
   offer (i.e., no transport connection has been established yet) takes
   the 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 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 connection value of "new".

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

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






Yon & Camarillo          Expires March 30, 2005                 [Page 6]

Internet-Draft         Connection-Oriented Media          September 2004


5.2  Answerer Behaviour

   The connection value for an m= line is negotiated using the offer/
   answer model.  The resulting connection value after an offer/answer
   exchange is the connection value in the answer.  If the connection
   value in the offer is "new", the answerer MUST also use a value of
   "new" in the answer.  If the connection value in the offer is
   "existing", the answerer uses a value of "existing" in the answer if
   it wishes to continue using the existing connection and a value of
   "new" if it wants a new 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 would
      use a connection value of "new" in the answer.

   If the connection value for an m= line resulting from an offer/answer
   exchange is "new", the endpoints SHOULD establish a new
   transport-layer connection as indicated by the setup attribute.  If a
   previous connection is still up, the 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
   connections.

   If the connection value for an m= line resulting from an offer/answer
   exchange is "existing", the endpoints SHOULD continue using the
   existing connection.

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 it is 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 need to wait for some event to happen
      before being able to establish the connection.  For example, a
      wireless terminal may need to set up a radio bearer before being
      able to initiate a connection.






Yon & Camarillo          Expires March 30, 2005                 [Page 7]

Internet-Draft         Connection-Oriented Media          September 2004


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


           m=image 54111 TCP t38



Yon & Camarillo          Expires March 30, 2005                 [Page 8]

Internet-Draft         Connection-Oriented Media          September 2004


           c=IN IP4 192.0.2.2
           a=setup:passive
           a=connection:new

   An answerer at 192.0.2.1 receiving this offer responds with the
   following answer:


           m=image 9 TCP t38
           c=IN IP4 192.0.2.1
           a=setup:active
           a=connection:new

   The endpoint at 192.0.2.1 then initiates the TCP connection to port
   54111 at 192.0.2.2.

7.2  Actpass/Passive

   In another example, an offerer at 192.0.2.2 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
   connection:


           m=image 54111 TCP t38
           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 TCP t38
           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 a connection
   to port 54321 at 192.0.2.1.

7.3  Existing Connection Reuse

   Subsequent to the exchange in Section 7.2, another offer/answer
   exchange is initiated in the opposite direction.  The endpoint at
   192.0.2.1 wishes to continue using the existing connection:


            m=image 54321 TCP t38



Yon & Camarillo          Expires March 30, 2005                 [Page 9]

Internet-Draft         Connection-Oriented Media          September 2004


            c=IN IP4 192.0.2.1
            a=setup:passive
            a=connection:existing

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


            m=image 9 TCP t38
            c=IN IP4 192.0.2.2
            a=setup:active
            a=connection:existing

   The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.

      Note that the endpoint at 192.0.2.2 uses setup:active in response
      to the offer of setup:passive, and uses port 9 because it is
      active.

7.4  Existing Connection Refusal

   Subsequent to the exchange in Section 7.3, another offer/answer
   exchange is initiated by the endpoint at 192.0.2.2, again wishing to
   reuse the existing connection:


            m=image 54111 TCP t38
            c=IN IP4 192.0.2.2
            a=setup:actpass
            a=connection:existing

   However, this time the 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 9 TCP t38
            c=IN IP4 192.0.2.3
            a=setup:active
            a=connection:new

   The endpoint at 192.0.2.3 then initiates the TCP connection to port
   54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old
   connection.






Yon & Camarillo          Expires March 30, 2005                [Page 10]

Internet-Draft         Connection-Oriented Media          September 2004


      Note that the endpoint at 192.0.2.2, 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 [4] for security and other considerations specific to
   the Session Description Protocol in general.

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

   http://www.iana.org/assignments/sdp-parameters

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

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

   http://www.iana.org/assignments/sdp-parameters

   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.






Yon & Camarillo          Expires March 30, 2005                [Page 11]

Internet-Draft         Connection-Oriented Media          September 2004


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.




Yon & Camarillo          Expires March 30, 2005                [Page 12]

Internet-Draft         Connection-Oriented Media          September 2004


   [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
         2004.


Authors' Addresses

   David Yon
   Tactical Software, LLC
   670 N Commercial St
   Manchester, NH  03101
   USA

   EMail: yon-comedia@rfdsoftware.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com



























Yon & Camarillo          Expires March 30, 2005                [Page 13]

Internet-Draft         Connection-Oriented Media          September 2004


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 procedures with respect to rights in RFC 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
   http://www.ietf.org/ipr.

   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
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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.


Acknowledgment

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




Yon & Camarillo          Expires March 30, 2005                [Page 14]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/