INTERNET-DRAFT                                                  D. Yon
Document: draft-ietf-mmusic-sdp-comedia-01.txt             Dialout.Net
Expires April 2002                                        October 2001



                Connection-Oriented Media Transport in SDP
                  <draft-ietf-mmusic-sdp-comedia-01.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 (2001).  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 TLS.  It
   also defines the syntax and semantics for an SDP "direction"
   attribute that describes the 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.).

Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
   and indicate requirement levels for compliant implementations.

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 transports such as TCP are more
   appropriate, but [SDP] provides no way to describe a session that
   uses protocols other than RTP or UDP.

   Connection-oriented protocols introduce a new factor when describing
   a session: not only must it be possible to express that a protocol
   will be based on this protocol, but it must also describe the
   connection setup procedure.

  Protocol Identifiers

1.1 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).

1.2 TLS

   The TLS protocol identifier specifies that the session will use the
   Transport Layer Security protocol [TLS] with an implied transport
   protocol of TCP.  To describe a media session that uses TLS over
   TCP, the protocol identifier "TLS" must be specified in the m= line.
   An m= line that specifies TLS MUST further qualify the protocol
   using a fmt identifier.



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  Direction Attribute

   An important attribute of connection-oriented protocols 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:

          a=direction:<role> [<source-address>]

   The <role> is one of the following:

   passive:    The endpoint will accept an incoming connection.

   active:     The endpoint will initiate an outgoing connection.

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

   reuse:      The endpoint will use the connection that has already
               been established with the opposite endpoint.

   The <source-address> is a sequence of values that describe the
   address and port number from where the connection will originate,
   and consists of the following values:

          nettype addrtype unicast-address [port]

   The <source-address> is an optional value that may be specified with
   direction:active, direction:both, or direction:reuse.  Within the
   <source-address>, the source port number is RECOMMENDED but may be
   omitted.

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
   connection from the other endpoint.  The endpoint MUST NOT specify a
   <source-address> after direction:passive.

2.2 Semantics of direction:active

   By specifying direction:active, the endpoint indicates that it will
   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
   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].




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   The endpoint SHOULD specify the address and port number from which
   it will initiate the connection in the <source-address> 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 connection to the port number on
   the m= line of the other endpoint.

   As with direction:active, the endpoint SHOULD specify the address
   and port number from which it will initiate the connection in the
   <source-address> 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:

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

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

      3) The endpoints intend to use two 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 SHALL cause both
   endpoints to behave as if the former had specified
   direction:passive.  Conversely, specifying direction:both when the
   other endpoint specifies direction:passive SHALL 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 connection to the port number specified on the m= line of
   the opposite endpoint.  If a single connection is needed (case #1 or
   #2 above), there is one exception to this requirement: if an
   endpoint receives the incoming connection from the opposite endpoint
   prior to initiating its own outbound connection, then that endpoint
   MAY use that connection rather than attempt to make an outbound
   connection to the opposite endpoint.

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   If only one connection succeeds, then that connection will be used
   to carry the media.  Once it has transmitted data on this
   connection, the initiating endpoint MUST NOT perform another
   connection attempt to the accepting endpoint.  This allows the
   accepting endpoint to release or recycle the listening port for
   another session once it has received data from the initiating
   endpoint.

   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 connection.

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

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

2.4 Semantics of direction:reuse

   By specifying direction:reuse, the endpoint indicates that it is
   changing the parameter(s) of an existing session on a previously
   established connection with the opposite endpoint.  Therefore no new
   connections are to be created.  This is intended for cases where
   media types are added, removed, or changed during a session.  For
   example, an endpoint adding a video stream to an existing audio
   session may elect to multiplex the new stream over the same
   connection that is currently transporting the audio stream.


2.5 Bidirectional versus Unidirectional Media

   In traditional SDP transport types the flow is unidirectional.  If
   the intent is for media to flow in both directions, both endpoints
   must specify SDP that describes where to deliver the media and what
   media type(s) to use.  For example, if only Endpoint A presents SDP
   then media can only flow towards Endpoint A, as Endpoint B has not
   specified where and how to send media to it.

   Because most connection-oriented media is inherently bi-directional,
   endpoints may encounter a situation where only one side presented
   SDP yet there is now a network path that can carry media in either
   direction.  In keeping with traditional SDP semantics, an endpoint
   MUST NOT send data to the other endpoint unless it has specified SDP
   information describing the type of media it can accept.

   It is, however, perfectly acceptable for an endpoint to transmit
   data on the same connection it is using to receive data, so long as
   the other endpoint has advertised its willingness to accept data.
   Likewise, it is perfectly acceptable for an endpoint to receive data

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   on the same connection it is using to transmit data to the
   corresponding remote endpoint.  In other words, for a bi-directional
   application-level session, a connection may be used to send data in
   both directions (contingent to rules outlined in Section 2.3) as
   long as one side of the connection is attached to either of the
   advertised SDP transport addresses.

  Source-Address Considerations

   In the cases where the endpoint is initiating the connection, it is
   RECOMMENDED that a source address be specified on the a= line by
   that endpoint.  It is also RECOMMENDED that the source port be
   included in the source address.  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;

       // 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));
   }

   If the source address is omitted, the receiver of the SDP packet
   MUST NOT make any assumptions in regards to the address or port from
   where the connection will originate.  In particular, the receiver
   MUST NOT assume that the address information listed on the c= line
   has any implication as to where the media connection originates.

   NOTE:
          The motivation for specifying the source address is
          twofold.  First, it aids Application-Level Proxies by
          explicitly announcing the source of the outbound
          connection.  This allows, for example, a dynamic
          firewall pinhole to be created that will allow the
          connection to pass.



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          Second, it allows the passive endpoint to correlate
          the incoming connection with the session being
          negotiated.  Note that great care must be taken when
          using the source address as a means to identify
          incoming connections, as Network Address Translation
          (NAT) can render the source address unreliable.  In
          addition if the originating endpoint omits the source
          port, the source address can be ambiguous if multiple,
          logical endpoints share the same network address.
          Therefore it is NOT RECOMMENDED that the source
          address be used for this purpose unless the SDP occurs
          in the context of a controlled network topology that
          guarantees that the source address is both correct
          (i.e., no NAT, or a NAT with an Application-Level
          Proxy that rewrites the SDP) and unambiguous (i.e.,
          the source port is specified).

3.1 Source Address Timing Considerations

   When used in conjunction with a session signaling protocol such as
   SIP, there may be cases where an endpoint initiates a connection
   prior to the opposite endpoint receiving the SDP that describe the
   source address of the initiating endpoint.  Therefore, an endpoint
   that has advertised an address and port number with direction:both
   or direction:passive MUST be ready to accept a connection on that
   address and port immediately.  If the accepting endpoint requires
   the source address to identify the initiating endpoint, it MUST keep
   the connection active and allow sufficient time for the source
   address to arrive before discarding 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:

        v=0
        o=me 2890844526 2890842807 IN IP4 10.1.1.2
        e=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 a T.38 fax
   session at port 54111:

        c=IN IP4 10.1.1.2
        m=image 54111 TCP t38
        a=direction:passive


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

        c=IN IP4 10.1.1.1
        m=image 9 TCP t38
        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 address or port.  The endpoint at 10.1.1.1 could have optionally
   committed to a source address with a simple modification:

        c=IN IP4 10.1.1.1
        m=image 9 TCP t38
        a=direction:active IN IP4 10.1.1.1 1892

   By adding the source address 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 a T.38 fax
   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
        m=image 54111 TCP t38
        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:

               c=IN IP4 10.1.1.1
               m=image 54321 TCP t38
               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:


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        c=IN IP4 10.1.1.2
        m=image 54111 TCP t38
        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
        m=image 54321 TCP t38
        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 "TLS" protocol identifiers.


Acknowledgements

   The author would like to thank Jonathan Rosenberg, Anders
   Kristensen, Paul Kyzivat, 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 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 source

   direction-ident =     "both" | "active" | "reuse"

   source =              nettype addrtype unicast-address |
                         nettype addrtype unicast-address port

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

   [T38]       International Telecommunication Union, "Procedures for
               Real-Time Group 3 Facsimile Communications over IP
               Networks," Recommendation T.38, June 1998

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

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

AuthorÆs Address

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

   Phone: (603) 324-4100
   EMail: yon@dialout.net

Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.



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