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

MMUSIC Working Group                                              D. Yon
Internet-Draft                                          Dialout.Net, Inc
Expires: November 12, 2004                                  G. Camarillo
                                                                Ericsson
                                                            May 14, 2004


     Connection-Oriented Media Transport in the Session Description
                             Protocol (SDP)
                  draft-ietf-mmusic-sdp-comedia-06.txt

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Copyright Notice

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

Abstract

   This document describes how to express media transport over
   connection-oriented protocols using the Session Description Protocol
   (SDP). It defines two new protocol identifiers: TCP and TCP/TLS.  It
   also defines the SDP setup attribute, which describes the connection
   setup procedure, and the SDP reconnect attribute.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol Identifiers . . . . . . . . . . . . . . . . . . . . .  3
     3.1   TCP  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.2   TCP/TLS  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Setup Attribute  . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1   The Setup Attribute in the Offer/answer Model  . . . . . .  4
     4.2   Multiple-Connection Avoidance when Using Actpass . . . . .  5
   5.  The Reconnect Attribute  . . . . . . . . . . . . . . . . . . .  6
   6.  Connection Lifetime  . . . . . . . . . . . . . . . . . . . . .  7
     6.1   Session Renegotiation  . . . . . . . . . . . . . . . . . .  7
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1   Passive/Active . . . . . . . . . . . . . . . . . . . . . .  8
     7.2   Passive/Active with Reconnect  . . . . . . . . . . . . . .  9
     7.3   Actpass  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 11
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 11
   11.2  Informational References . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13


























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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 [6]) 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
   [7], 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. We define two new protocol identifiers: TCP and
   TCP/TLS. Both represent connection-oriented reliable transports.

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

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 [2] and indicate requirement levels for
   compliant implementations.

3.  Protocol Identifiers

   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

   We define two new values for the proto field: TCP and TCP/TLS.

3.1  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



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   qualify the application-layer protocol using an fmt identifier.

   Media lines with the TCP protocol identifier are carried using TCP
   [1].

3.2  TCP/TLS

   The TCP/TLS protocol identifier specifies that the session will use
   the Transport Layer Security (TLS) protocol [3] on top on a TCP [1]
   connection.

   An m= line that contain the TCP/TLS protocol identifier MUST further
   qualify the protocol using a fmt identifier.

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"

      Active: The endpoint will initiate an outgoing connection.
      Passive: The endpoint will accept an incoming connection.
      ActPass: The endpoint will both accept an incoming connection and
      will initiate an outgoing connection.

   The default value of the setup attribute is actpass. That is, an m=
   line without an associated setup line is considered to be actpass.

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
   follows. The offerer states which role or roles is willing to perform
   and the answerer, taking the offerer's willingness into
   consideration, chooses which roles both endpoints will actually



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   perform during connection establishment. The following are the values
   that the setup attribute can take in an offer/answer exchange:


            Offer     Answer
            _______________
            active    passive
            passive   active
            actpass   active / passive / actpass

   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, as that carries
   other semantics in SDP.

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

   The value actpass indicates that the endpoint SHOULD initiate a
   connection to the port number on the m= line of the other endpoint
   and that the endpoint SHOULD be ready to accept a connection on the
   port number specified in the m= line. It is RECOMMENDED that, if
   possible, endpoints set the port number on their m= line to the
   source port number which they will use to establish the connection
   towards the remote endpoint. This way, the transport-layer protocol
   (e.g., TCP) can take care of simultaneous opens.

   Endpoints typically use the actpass value for the following reasons:
   1.  The offerer has no preference as to whether it accepts or
       initiates the connection and, so, is letting the answerer choose.
   2.  The endpoints intend to use a single connection to transport the
       media, but it is not known whether NAT (Network Address
       Translator) issues will prevent either endpoint from initiating
       or accepting the connection. So, both endpoints will attempt to
       initiate a connection hoping that at least one will succeed.

4.2  Multiple-Connection Avoidance when Using Actpass

   When an offer/answer exchange results in actpass, each endpoint
   attempts to establish a transport connection towards the other
   endpoint. If only one of the connections succeeds, this connection is
   used to transfer media. Nevertheless, if both connections succeed,
   one of them needs to be terminated so that both endpoints exchange



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   data over a single connection. In this section, we provide rules to
   choose which of the two connections should be terminated (or not even
   initiated).

   First of all, if the endpoints follow the recommendation of setting
   the port number in their m= line to the source port number which they
   will use to establish the connection towards the remote endpoint, the
   transport layer should take care of simultaneous opens (at least if
   TCP is the transport protocol). If, for some reason, any of the
   endpoints does not follow this recommendation, both endpoints should
   follow the rules below.

   If an endpoint is notified about a connection establishment attempt
   from the other endpoint before performing its own connection attempt,
   it SHOULD behave as a passive endpoint and SHOULD NOT attempt to
   establish any other connection.

   In case two connections are established, if an endpoint receives data
   (i.e., media) over one of the connections before having sent any data
   on any of the connections, the endpoint SHOULD terminate the
   connection that has not carried any data.

   When two connections are established and both endpoints start sending
   data before receiving anything from the other endpoint, it may happen
   that each of the endpoints choose a different connection to send
   data. If the answerer receives data over a connection after having
   sent data on the other connection, it SHOULD continue sending data on
   the other connection until an application-layer data boundary. At
   that point, the answerer SHOULD terminate this connection and start
   using the connection on which the offerer was sending data.

   Note that different applications may define application-layer
   boundaries in different ways. A typical suitable point for the
   answerer to change connections is the end of an application-layer
   message and the beginning of the next one.

5.  The Reconnect 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
   reconnect attribute, which is charset-independent and can be a



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   session-level or a media-level attribute, is used to disambiguate
   these two scenarios. The following is the ABNF of the reconnect
   attribute:


         reconnect-attr       =  "a=reconnect"

   On reception of an m= line with a reconnect attribute, the endpoints
   SHOULD close the existing connection, in case it was still up, and
   SHOULD establish a new connection according to the setup attribute in
   the m= line.

   Either the offerer or the answerer can include a reconnect attribute
   in an m= line. In any event, if the offer contained this attribute,
   the answer MUST contain it as well.

6.  Connection Lifetime

   An endpoint that intends to initiate the connection SHOULD initiate
   the connection immediately after it has sufficient information to do
   so, even if it does not intend to immediately begin sending media to
   the remote endpoint.  This allows media to flow from the remote
   endpoint. An endpoint SHOULD NOT close the connection until the
   session has expired, been explicitly terminated, or the media stream
   is redirected to a different address or port.

   If the endpoint determines that the connection has been closed, it
   MAY attempt to re-establish the connection. The decision to do so is
   application and context dependant.

6.1  Session Renegotiation

   There are scenarios where SDP is sent by an endpoint in order to
   renegotiate an existing session.  These include muting/unmuting a
   session, renegotiating the attributes of the media used by the
   session, or extending the length of a session about to expire.
   Connection-oriented media introduces some ambiguities into session
   renegotiation as to when the direction attribute must be obeyed and
   when it is ignored.

   The scenario of extending the duration of an existing session is a
   good example: in order to extend an existing session, endpoints will
   typically resend the original SDP with updated time information. In
   connectionless media the result is no change to the existing media
   streams.  The problem with connection oriented media is that the
   original SDP will contain a setup attribute which can be considered
   as a request to create a new connection, as opposed to a request to
   maintain steady state. The following rule help avoid this ambiguity:



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      If the transport section (the c= and m= lines) of an SDP
      description describes an existing connection between two endpoints
      and the m= line does not contain a reconnect attribute, the
      endpoints SHOULD use that connection to carry the media described
      in the remainder of the message. The endpoints SHOULD NOT attempt
      to set up a new connection, regardless of what is specified in the
      setup attribute.
      Note that if the port number in the m= line changes, there is no
      need to use the reconnect attribute because the new port will
      trigger the establishment of a new connection anyway.

7.  Examples

   What follows are a number of examples that 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 and is assumed to be the
   following:


           v=0
           o=me 2890844526 2890842807 IN IP4 10.1.1.2
           s=Call me using TCP
           t=3034423619 3042462419


7.1  Passive/Active

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


           c=IN IP4 192.0.2.2
           m=image 54111 TCP t38
           a=setup:passive

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


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

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





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7.2  Passive/Active with Reconnect

   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 192.0.2.2 signals this intent with
   the following SDP:


           c=IN IP4 192.0.2.2
           m=image 54111 TCP t38
           a=setup:passive
           a=reconnect

   The reconnect attribute informs the endpoint at 192.0.2.1 that this
   SDP represents the intent to establish a new connection for media
   transport, rather than continuing with the original connection.
   Because the endpoint at 192.0.2.1 may not yet be aware that the TCP
   connection has failed, this eliminates any ambiguity. If 192.0.2.1
   agrees to continue the session using a new connection, it responds
   with:


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


7.3  Actpass

   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:


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

   The endpoint at 192.0.2.1 responds with the following description:


           c=IN IP4 192.0.2.1
           m=image 54321 TCP t38
           a=setup:actpass

   This will cause the offerer (at 192.0.2.2) to initiate a connection
   to port 54321 at 192.0.2.1 and the answerer (at 192.0.2.1) to



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   initiate a connection to port 54111 at 192.0.2.2. Ideally, the
   offerer would use 192.0.2.2:5411 as the source of its connection
   attempt and the answerer would use 192.0.2.1:54321 as its.

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 substitute TCP/TLS with only TCP in a
   session description. 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.

   This document touches upon NAT traversal. Implementers should be
   aware of some issues that relate to the use of private IP addresses
   within the offer/answer model (i.e., they are not specific to this
   document).

   When an endpoint receives a session description with a private IP
   address that belongs to a different address space, in most of the
   cases, the endpoint will not be able to reach such an address.
   Nevertheless, if this particular address also exists in the
   endpoint's address space, the endpoint may end up reaching a
   different peer than the one that generated the session description.
   It is RECOMMENDED that endpoints authenticate their peer somehow
   (e.g., using TLS [3]) or that they encrypt their media.

9.  IANA Considerations

   This document defines two session and media level SDP attributes:
   setup and reconnect. 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 two proto values: TCP and TCP/TLS. Their
   formats are defined in Section 3.1 and Section 3.2 respectively.
   These two proto values should be registered by the IANA on http://
   www.iana.org/assignments/sdp-parameters under "proto".

10.  Acknowledgements

   The authors would like to thank Jonathan Rosenberg, Rohan Mahy,
   Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert



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   Fairlie-Cuninghame, and Colin Perkins for their 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]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [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.

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

11.2  Informational References

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

   [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.











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Authors' Addresses

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

   EMail: yon@dialout.net


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com

































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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.




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