INTERNET-DRAFT                                                  D. Yon
Document: draft-ietf-mmusic-sdp-tcpmedia-00.txt            Dialout.Net
Expires April 2001                                        October 2000



                     TCP-Based Media Transport in SDP
                  <draft-ietf-mmusic-sdp-tcpmedia-00.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes how to express TCP-based media transport
   using the Session Description Protocol (SDP).  It defines two new
   protocol identifiers: TCP and RTP/AVP-TCP.  It also defines the
   syntax and semantics for an SDP "direction" attribute that describes
   the TCP connection setup procedure.

















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Introduction

   The Session Description Protocol [SDP] 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]) to maximize portability among transports.  SDP
   does not define a protocol, but only the syntax to describe a
   multimedia session with sufficient information to discover and
   participate in that session.  Session descriptions may be sent using
   any number of existing application protocols for transport (e.g.,
   SAP, SIP, RTSP, email, HTTP, etc.).

Motivation

   [SDP] describes two protocol identifiers: RTP/AVP and UDP, both of
   which are unreliable, connectionless protocols, an appropriate
   choice for multimedia streams.  There are, however, applications for
   which the connection-oriented TCP transport is more appropriate, but
   [SDP] provides no way to describe a session that uses the TCP
   protocol.

   The connection-oriented nature of TCP introduces a new factor when
   describing a session: not only must it be possible to express that a
   protocol will be based on TCP, but it must also describe the
   connection setup procedure.

  Protocol Identifiers

1.1 RTP/AVP-TCP

   [SDP] already specifies the RTP/AVP protocol identifier as a way to
   describe media that uses the Real Time Protocol and the associated
   Audio-Video Profiles.  The assumption in [SDP] is that the
   underlying network transport is UDP.  RTP is transport-neutral, so
   it is possible to transport RTP/RTCP packets using TCP rather than
   UDP.  To describe a media session that uses RTP/AVP over TCP, the
   protocol identifier RTP/AVP-TCP must be specified in the m= line.

1.2 TCP

   The TCP protocol identifier is similar to the UDP protocol
   identifier in that it only describes the transport protocol without
   any connotation as to the upper-layer protocol.  An m= line that
   specifies TCP must further qualify the protocol using a fmt
   identifier (see [SDP] Appendix B).

  Direction Attribute

   An important attribute of a TCP connection is the setup procedure.
   One endpoint needs to initiate the connection and the other endpoint
   needs to accept the connection.  The direction attribute is used to
   describe these roles, and the syntax is as follows:



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          a=direction:<role> <source-port>

   The <role> is one of the following:

   passive:    The endpoint will accept an incoming TCP connection.

   active:     The endpoint will initiate an outgoing TCP connection.

   both:       The endpoint will both accept an incoming TCP connection
               and will initiate an outgoing TCP connection.

   The <source-port> is an optional value that may only be specified in
   the context of direction:active or direction:both.

2.1 Semantics of direction:passive

   By specifying direction:passive, the endpoint indicates that the
   port number specified in the m= line is available to accept a TCP
   connection from the other endpoint.

2.2 Semantics of direction:active

   By specifying direction:active, the endpoint indicates that it will
   initiate a TCP connection to the port number on the m= line of the
   other endpoint.  The port number on its own m= line is irrelevant
   and is to be ignored by the other endpoint.  Nevertheless, since the
   m= line must contain a valid port number, the endpoint specifying
   direction: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 endpoint may optionally specify the port number from which it
   will initiate the TCP connection in the <source-port> position on
   the a= line.

2.3 Semantics of direction:both

   By specifying direction:both, the endpoint indicates that it will
   both accept a TCP connection on the port number of its own m= line,
   and that it will also initiate a TCP connection to the port number
   on the m= line of the other endpoint.  As with direction:active, the
   endpoint may optionally specify the port number from which it will
   initiate the TCP connection in the <source-port> position on the a=
   line.

   Since this attribute describes behavior that is similar to
   connectionless media descriptions in [SDP], it is the default value
   for the direction attribute and is therefore optional.

   Endpoints may choose to specify direction:both for one or more of
   the following reasons:





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      1) The endpoint has no preference as to whether it accepts or
         initiates the TCP connection, and therefore is offering the
         remote endpoint a choice of connection setup procedures.

      2) The endpoints intend to use a single TCP connection to
         transport the media, but it is not known whether firewall
         issues will prevent either endpoint from initiating or
         accepting the TCP connection.  Therefore both endpoints will
         attempt to initiate a TCP connection in hopes that at least
         one will succeed.

      3) The endpoints intend to use two TCP connections to transport
         the media, and one must be initiated by the remote endpoint
         and the other must be initiated by the local endpoint.

   If one endpoint specifies either direction:active or
   direction:passive and the other specifies direction:both, both
   endpoints must behave as if the latter had specified the inverse
   direction of the former.  For example, specifying direction:both
   when the other endpoint specifies direction:active should cause both
   endpoints to behave as if the former had specified
   direction:passive.  Conversely, specifying direction:both when the
   other endpoint specifies direction:passive should cause both
   endpoints to behave as if the former had specified direction:active.

   If both endpoints specify direction:both then each endpoint must
   initiate a TCP connection to the port number specified on the m=
   line of the opposite endpoint.  If only one connection succeeds,
   then that connection will be used to carry the media.  If both
   connections succeed but only one was needed (case #2 above), the
   following rules shall apply:

      a) Each endpoint MUST accept data from either TCP connection.

      b) Once an endpoint has transmitted data to one of the TCP
         connections, it MUST use that TCP connection exclusively for
         transmission.

      c) Once an endpoint has transmitted AND received data, if one of
         the TCP connections is determined to be idle, the endpoint MAY
         close the idle TCP connection.

  Source-Port Considerations

   In the cases where the endpoint is initiating the TCP connection, a
   source port number may optionally be specified on the a= line by
   that endpoint.  In most environments, the source port number can be
   determined by binding the socket before initiating the connect, as
   shown in the sample C code below:

   {
    SOCKET s_id
    SOCKADDR_IN cli_sin;
    int namelen;


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       // Create the socket
       s_id = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);

       // Bind the socket to any IP address and port
       bzero((char *)&cli_sin,sizeof(cli_sin));
       cli_sin.sin_family      = AF_INET;
       cli_sin.sin_addr.s_addr = htonl(INADDR_ANY);
       cli_sin.sin_port        = 0;
       bind(s_id,(SOCKADDR *)&cli_sin,sizeof(cli_sin));

       // Find the port number that was bound
       namelen = sizeof(cli_sin);
       getsockname(s_id,(SOCKADDR *)&cli_sin,&namelen);

       // Print the port number
       printf("Source Port = %d\n",ntohs(cli_sin.sin_port));
   }

   The motivation for specifying the source port is to allow topologies
   where one or more endpoints use a single, fixed TCP port for
   incoming connections.  Non-RTP protocols transported over TCP
   commonly use this technique.  By specifying the source port, an
   endpoint avoids a potential ambiguity when more than one session is
   set up between two endpoints.

   For example, consider two endpoints with IP addresses of 10.1.1.1
   and 10.1.1.2.  The endpoint at 10.1.1.1 signals the availability of
   a session on TCP port 2393 (passive).  Before the endpoint at
   10.1.1.2 has a chance to initiate the connection, events transpire
   that cause the endpoint at 10.1.1.1 to signal the availability of a
   separate session that is also found at TCP port 2393 (passive).
   Shortly thereafter, both entities at 10.1.1.2 initiate connections
   to 10.1.1.1 on port 2393.

   The problem is this: how does the endpoint at 10.1.1.1 differentiate
   the two connections?  To which entity at 10.1.1.2 does each
   connection correspond?  By specifying the source port prior to
   connecting, the entities at 10.1.1.2 can avoid this ambiguity,
   because now the endpoint at 10.1.1.1 can simply inspect the port
   number from which the connection originated to determine which
   entity has initiated the connection.

   Caution must be exercised when designing systems that rely on this
   feature, as not all environments are able to determine the source
   port prior to initiating the connection.

  Examples

   What follows are a number of examples that show the most common
   usage of the direction 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:


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        v=0
        o=Me <me@ietf.org>
        s=Call me using TCP
        t=0 0

4.1 Example: simple passive/active

   An endpoint at 10.1.1.2 signals the availability of an audio session
   at port 54111:

        c=IN IP4 10.1.1.2/127
        m=audio 54111 RTP/AVP-TCP 0
        a=direction:passive

   An endpoint at 10.1.1.1 receiving this description responds with the
   following:

        c=IN IP4 10.1.1.1/127
        m=audio 9 RTP/AVP-TCP 0
        a=direction:active

   The endpoint at 10.1.1.1 then initiates the TCP connection to port
   54111 at 10.1.1.2.  Note that the TCP connection may originate from
   any port.  The endpoint at 10.1.1.1 could have optionally committed
   to a source port with a simple modification:

        c=IN IP4 10.1.1.1/127
        m=audio 9 RTP/AVP-TCP 0
        a=direction:active 1892

   By adding the "1892" to the a= line, the endpoint at 10.1.1.1 must
   now use a source port of 1892 when initiating the TCP connection to
   port 54111 at 10.1.1.2.

4.2 Example: agnostic both

   An endpoint at 10.1.1.2 signals the availability of an audio session
   at TCP port 54111, but is also willing to set up the media stream by
   initiating the TCP connection:

        c=IN IP4 10.1.1.2/127
        m=audio 54111 RTP/AVP-TCP 0
        a=direction:both

   The endpoint at 10.1.1.1 has three choices:

      1) It can respond with either of the two direction:active
         descriptions listed in the previous example.  In this case the
         endpoint at 10.1.1.1 must initiate a connection to port 54111
         at 10.1.1.2.

      2) It can respond with a description similar to the following:



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               c=IN IP4 10.1.1.1/127
               m=audio 54321 RTP/AVP-TCP 0
               a=direction:passive

         In this case the endpoint at 10.1.1.2 must initiate a
         connection to port 54321 at 10.1.1.1.

      3) It can respond with a description that specifies
         direction:both, which is covered in the next example.

4.3 Example: redundant both

   An endpoint at 10.1.1.2 uses the same description as the previous
   example:

        c=IN IP4 10.1.1.2/127
        m=audio 54111 RTP/AVP-TCP 0
        a=direction:both

   Unlike the previous example, the endpoint at 10.1.1.1 responds with
   the following description:

        c=IN IP4 10.1.1.1/127
        m=audio 54321 RTP/AVP-TCP 0
        a=direction:both

   This will cause the endpoint at 10.1.1.2 to initiate a connection to
   port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a
   connection to port 54111 at 10.1.1.2.  Whichever TCP connection
   succeeds will be used.  If both succeed, one of the connections may
   be closed as an optimization, using the rules in section 2.3.

  Security Considerations

   See [SDP] for security and other considerations specific to the
   Session Description Protocol in general.  There are no new security
   considerations introduced by these protocol identifiers and
   attributes.

  IANA Considerations

   As recommended by [SDP] Appendix B, the direction attribute
   described in this document should be registered with IANA, as should
   the TCP and RTP/AVP-TCP protocol identifiers.

Acknowledgements

   The author would like to thank Jonathan Rosenberg, Anders
   Kristensen, and Robert Fairlie-Cuninghame for their valuable
   insights.






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Appendix A: Direction Attribute Syntax

   This appendix provides an Augmented BNF [ABNF] grammar for
   expressing the direction attribute for TCP connection setup.  It is
   intended as an extension to the grammar for the Session Description
   Protocol, as defined in [SDP].  Specifically, it describes the
   syntax for the new "connection-setup" attribute field, which MAY be
   either a session-level or media-level attribute.

   connection-setup =    "direction" ":" direction-spec

   direction-spec =      "passive" | qualified-direction

   qualified-direction = direction-ident | direction-ident port

   direction-ident =     "both" | "active"

References

   [ABNF]      D. Crocker, P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF," RFC 2234, November 1997

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

   [UTF-8]     F. Yergeau, "UTF-8, a transformation format of Unicode
               and ISO 10646," RFC 2044, October 1996

Author's Address

   David Yon
   Dialout.Net, Inc.
   402 Amherst St
   Nashua, NH 03063

   Phone: (603) 577-8708
   EMail: yon@dialout.net

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   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
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