    Network Working Group                                             D. Yon
    Request for Comments: 4145                        Tactical Software, LLC
    Category: Standards Track                                   G. Camarillo
                                                                    Ericsson
                                                              September 2005

      TCP-Based Media Transport in the Session Description Protocol (SDP)

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2005).

    Abstract

       This document describes how to express media transport over TCP 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.

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    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  . . . . . . . . . . . . . . . . . . . .  8
           6.1.  Connection Establishment . . . . . . . . . . . . . . . .  8
           6.2.  Connection Reestablishment . . . . . . . . . . . . . . .  8
           6.3.  Connection Termination . . . . . . . . . . . . . . . . .  8
       7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
           7.1.  Passive/Active . . . . . . . . . . . . . . . . . . . . .  9
           7.2.  Actpass/Passive. . . . . . . . . . . . . . . . . . . . .  9
           7.3.  Existing Connection Reuse. . . . . . . . . . . . . . . . 10
           7.4.  Existing Connection Refusal. . . . . . . . . . . . . . . 10
       8.  Other Connection-Oriented Transport Protocols. . . . . . . . . 11
       9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
       10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
       11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
       12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
           12.1. Normative References . . . . . . . . . . . . . . . . . . 13
           12.2. Informative References . . . . . . . . . . . . . . . . . 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 [11]) to maximize portability among transports.  SDP
       does not define a protocol; it defines 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 TCP is more appropriate.  This document
       defines a new protocol identifier, 'TCP', to describe TCP connections
       in SDP.

       TCP introduces two new factors when describing a session: how and
       when should endpoints perform the TCP connection setup procedure.
       This document defines two new attributes to describe TCP 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 they 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

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       further qualify the application-layer protocol using an fmt
       identifier.  Media described using an 'm' line containing the 'TCP'
       protocol identifier are carried using TCP [1].

    4.  Setup Attribute

       The 'setup' attribute indicates which of the end points should
       initiate the TCP connection establishment (i.e., 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
       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:

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                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 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 is placed 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
       whether a subsequent SDP exchange represents a confirmation that the
       endpoint is to continue using the current TCP connection unchanged,
       or is a request to make a new TCP 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:

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             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 TCP connection should still be used.  When
       an offerer generates an 'm' line that uses TCP, 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.

       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 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'.  Similarly, the
          'connection' attribute in an initial offer (i.e., no transport
          connection has been established yet) takes the value of 'new'.

       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.

       Taking into consideration the rules in Section 5.2, the following are
       the values that the 'connection' attribute can take in an
       offer/answer exchange:

                Offer      Answer
                ________________
                new        new
                existing   existing / new

       If the connection value resulting from an offer/answer exchange is
       'existing', the end-points continue using the existing connection.
       Consequently, the port numbers, IP addresses, and 'setup' attributes
       negotiated in the offer/answer exchange are ignored because there is
       no need to establish a new connection.

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       The previous rule implies that an offerer generating an offer with a
       connection value of 'existing' and a setup value of 'passive' needs
       to be ready (i.e., needs to allocate resources) to receive a
       connection request from the answerer just in case the answerer
       chooses a connection value of 'new' for the answer.  However, if the
       answerer uses a connection value of 'existing' in the answer, the
       offerer would need to deallocate the previously allocated resources
       that were never used because no connection request was received.

       To avoid allocating resources unnecessarily, offerers using a
       connection value of 'existing' in their offers may choose to use a
       setup value of 'holdconn'.  Nevertheless, offerers using this
       strategy should be aware that if the answerer chooses a connection
       value of 'new', a new offer/answer exchange (typically initiated by
       the previous offerer) with setup value different than 'holdconn' will
       be needed to establish the new connection.  This may, of course,
       cause delays in the application using the TCP connection.

       The default value of the connection attribute in both offers and
       answers is 'new'.

    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 TCP
       connection as indicated by the 'setup' attribute.  If a previous TCP
       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 synchronization between the two TCP connections.

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

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    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 TCP 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 TCP connection.

    6.2.  Connection Reestablishment

       If an endpoint determines that the TCP for an 'm' line has been
       closed and 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 TCP connection, but has no impact on
          the TCP connection itself.

    6.3.  Connection Termination

       Typically, endpoints do not close the TCP connection until the
       session has expired, been explicitly terminated, or a new connection
       value has been provided for the 'm' line.  Additionally, specific
       applications can describe further scenarios where an end-point may
       close a given TCP connection (e.g., whenever a connection is in the
       half-close state).  As soon as an end-point notices that it needs to
       terminate a TCP connection, it SHOULD do so.

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

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    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
               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

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       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
                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:passive
                a=connection:existing

       However, this time the answerer is unaware of the old connection and
       thus wishes to establish a new one.  (This could be the result of a
       transfer via third-party call control.)  It is unable to act in the
       'passive' mode and thus responds as 'active':

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                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.

          Note that the endpoint at 192.0.2.2, while using a connection
          value of 'existing', has used a setup value of 'passive'.  Had it
          not done this and instead used a setup value of 'holdconn'
          (probably to avoid allocating resources as described in
          Section 5.1), a new offer/answer exchange would have been needed
          in order to establish the new connection.

    8.  Other Connection-Oriented Transport Protocols

       This document specifies how to describe TCP-based media streams using
       SDP.  Still, some of the attributes defined here could possibly be
       used to describe media streams based on other connection-oriented
       transport protocols as well.  This section provides advice to authors
       of specifications of SDP extensions that deal with connection-
       oriented transport protocols other than TCP.

       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 'setup' and the 'connection' attributes are specified in
       Section 4 and Section 5 respectively.  While both attributes are
       applicable to 'm' lines that use the 'TCP' protocol identifier, they
       are general enough to be reused in 'm' lines with other connection-
       oriented transport protocols.  Therefore, it is recommended that the
       'setup' and 'connection' attributes are reused, as long as it is
       possible, for new proto values associated with connection-oriented
       transport protocols.

       Section 6 deals with TCP connection management.  It should be noted
       that 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 such a 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.  So, specifications dealing with such
       transport protocols may need to specify slightly different procedures
       regarding connection termination.

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    9.  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 in order to have endpoints reestablish connections
       unnecessarily 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.

    10.  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 under "Session Description Protocol (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 under
       "Session Description Protocol (SDP) Parameters" under "proto".

       The SDP specification, RFC2327, states that specifications defining
       new proto values, like the TCP proto value defined in this RFC, 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.

    11.  Acknowledgements

       Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
       Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
       Holmberg provided valuable insights and contributions.

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    12.  References

    12.1.  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.

    12.2.  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 2004.

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    Authors' Addresses

       David Yon
       Tactical Software, LLC
       1750 Elm St., Suite 803
       Manchester, NH  03104
       USA

       EMail: yon-comedia@rfdsoftware.com

       Gonzalo Camarillo
       Ericsson
       Hirsalantie 11
       Jorvas  02420
       Finland

       EMail: Gonzalo.Camarillo@ericsson.com

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    Full Copyright Statement

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    Acknowledgement

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

    Yon & Camarillo             Standards Track                    [Page 15]