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Versions: (draft-camarillo-mmusic-sdp-bfcp) 00 01 02 03 RFC 4583

MMUSIC Working Group                                        G. Camarillo
Internet-Draft                                                  Ericsson
Expires: June 2, 2006                                  November 29, 2005


   Session Description Protocol (SDP) Format for Binary Floor Control
                        Protocol (BFCP) Streams
                   draft-ietf-mmusic-sdp-bfcp-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies how to describe BFCP streams in SDP session
   descriptions.  User agents using the offer/answer model to establish
   BFCP streams use this format in their offers and their answers.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Fields in the 'm' Line . . . . . . . . . . . . . . . . . . . .  3
   4.  Floor Control Server Determination . . . . . . . . . . . . . .  4
   5.  The 'confid' and 'userid' SDP Attributes . . . . . . . . . . .  6
   6.  Association between Streams and Floors . . . . . . . . . . . .  6
   7.  TCP Connection Management  . . . . . . . . . . . . . . . . . .  7
   8.  Authentication . . . . . . . . . . . . . . . . . . . . . . . .  7
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     11.1.  Registration of the 'TCP/BFCP' and 'TCP/TLS/BFCP' SDP
            'proto' values  . . . . . . . . . . . . . . . . . . . . .  9
     11.2.  Registration of the SDP 'floorctrl' Attribute . . . . . . 10
     11.3.  Registration of the SDP 'confid' Attribute  . . . . . . . 10
     11.4.  Registration of the SDP 'userid' Attribute  . . . . . . . 10
     11.5.  Registration of the SDP 'floorid' Attribute . . . . . . . 11
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   13. Normative References . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14




























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1.  Introduction

   As discussed in the BFCP (Binary Floor Control Protocol)
   specification [8], a given BFCP client needs a set of data in order
   to establish a BFCP connection to a floor control server.  These data
   include the transport address of the server, the conference
   identifier, and the user identifier.

   One way for clients to obtain this information consists of using an
   offer/answer [4] exchange.  This document specifies how to encode
   this information in the SDP session descriptions which are part of
   such an offer/answer exchange.

   User agents typically use the offer/answer model to establish a
   number of media streams of different types.  Following this model, a
   BFCP connection is described as any other media stream by using an
   SDP 'm' line, possibly followed by a number of attributes encoded in
   'a' lines.


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 [1] and indicate requirement levels for
   compliant implementations.


3.  Fields in the 'm' Line

   This Section describes how to generate an 'm' line for a BFCP stream.

   According to the SDP specification [11], the 'm' line format is the
   following:

      m=<media> <port> <transport> <fmt> ...

   The media field MUST have a value of "application".

   The port field is set following the rules in [7].  Depending on the
   value of the 'setup' attribute (disccused in Section 7), the port
   field contains the port the remote endpoint will initiate its TCP
   connection to, or is irrelevant (i.e., the endpoint will initiate the
   connection towards the remote endpoint) and should be set to a value
   of 9, which is the discard port.  Since BFCP only runs on top of TCP,
   the port is always a TCP port.  A port field value of zero has the
   standard SDP meaning (i.e., rejection of the media stream).



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   We define two new values for the transport field: TCP/BFCP and TCP/
   TLS/BFCP.  The former is used when BFCP runs directly on top of TCP
   and the latter is used when BFCP runs on top of TLS, which in turn
   runs on top of TCP.

   The fmt (format) list is ignored for BFCP.  The fmt list of BFCP m
   lines SHOULD contain a single "*" character.

   The following is an example of an 'm' line for a BFCP connection:

   m=application 50000 TCP/TLS/BFCP *


4.  Floor Control Server Determination

   When two endpoints establish a BFCP stream, they need to determine
   which of them acts as a floor control server.  The most common
   scenario consists of a client establishing a BFCP stream with
   conference server that acts as the floor control server.  Floor
   control server determination is straight forward because one endpoint
   can only act as a client and the other can only act as a floor
   control server.

   However, there are scenarios where both endpoints could act as a
   floor control server.  For example, is a two-party session that
   involves an audio stream and a shared whiteboard, the endpoints need
   to decide which party will be acting as the floor control server.

   Furthermore, there are situations where both the offerer and the
   answerer act as both clients and floor control servers in the same
   session.  For example, in a two-party session that involves an audio
   stream and a shared whiteboard, one party acts as the floor control
   server for the audio stream and the other acts as the floor control
   server for the shared whiteboard.

   We define the 'floorctrl' SDP media-level attribute to perform floor
   control determination.  Its Augmented BNF syntax [2] is:



   floor-control-attribute  = "a=floorctrl:" role *(SP role)
   role                     = "c-only" / "s-only" / "c-s"

   The offerer includes this attribute to state all the roles it would
   be willing to perform:






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   c-only: The offerer would be willing to only act as a floor control
      client.
   s-only: The offerer would be willing to only act as a floor control
      server.
   c-s: The offerer would be willing to act both as a floor control
      client and as a floor control server.

   If an 'm' line in an offer contains a 'floorctrl' attribute, the
   answerer MUST include one in the corresponding 'm' line in the
   answer.  The answerer includes this attribute to state which role the
   answerer will perform.  That is, the answerer chooses one of the
   roles the offerer is willing to perform and generates an answer with
   the corresponding role for the answerer.  Table 1 shows the
   corresponding roles for an answerer depending on the offerer's role.

                          +---------+----------+
                          | Offerer | Answerer |
                          +---------+----------+
                          |  c-only |  s-only  |
                          |  s-only |  c-only  |
                          |   c-s   |    c-s   |
                          +---------+----------+

                              Table 1: Roles

   The following are the description of the roles when chosen by an
   answere:.

   c-only: The answerer will act as a floor control client.
      Consequently, the offerer will act as a floor control server.
   s-only: The answerer will act as a floor control server.
      Consequently, the offerer will act as a floor control client.
   c-s: The answerer will act both as a floor control client and as a
      floor control server.  Consequently, the offerer will also act
      both as a floor control client and as a floor control server as
      well.

   Endpoints which use the offer/answer model to establish BFCP
   connections MUST support the 'floorctrl' attribute.  A floor control
   server acting as an offerer or as an answerers SHOULD include this
   attribute in its session descriptions.

   If the 'floorctrl' attribute is not used in an offer/answer exchange,
   by default the offerer and the answerer will act as a floor control
   client and as a floor control server respectively.

   The following is an example of a 'floorctrl' attribute in an offer.
   When this attribute appears in an answer, it only carries one role:



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   a=floorctrl:c-only s-only c-s


5.  The 'confid' and 'userid' SDP Attributes

   We define the 'confid' and the 'userid' SDP media-level attributes.
   These attributes attributes are used by a floor control server to
   provide a client with a conference ID and a user ID respectively.
   Their Augmented BNF syntax [2] is:



   confid-attribute      = "a=confid:" conference-id
   conference-id         = token

   userid-attribute      = "a=userid:" user-id
   user-id               = token

   The confid and the userid attributes carry the integer representation
   of a conference ID and a user ID respectively.

   Endpoints which use the offer/answer model to establish BFCP
   connections MUST support the confid and the userid attributes.  A
   floor control server acting as an offerer or as an answerers SHOULD
   include these attributes in its session descriptions.


6.  Association between Streams and Floors

   We define the floorid SDP media-level attribute.  Its Augmented BNF
   syntax [2] is:



   floor-id-attribute = "a=floorid:" token [" mstrm:" token *(SP token)]

   The floorid attribute is used in BFCP 'm' lines.  It defines a floor
   identifier and, possibly, associates it with one or more media
   streams.  The token representing the floor ID is the integer
   representation of the Floor ID to be used in BFCP.  The token
   representing the media stream is a pointer to the media stream, which
   is identified by an SDP label attribute [9]

   Endpoints which use the offer/answer model to establish BFCP
   connections MUST support the 'floorid' and the 'label' attributes.  A
   floor control server acting as an offerer or as an answerer SHOULD
   include these attributes in its session descriptions.




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7.  TCP Connection Management

   The management of the TCP connection used to transport BFCP is
   performed using the 'setup' and 'connection' attributes as defined in
   [7].

   The 'setup' attribute indicates which of the endpoints (client or
   floor control server) initiates the TCP connection.  The 'connection'
   attribute handles TCP connection reestablishment.

   The BFCP specification [8] describes a number of situations when the
   TCP connection between a client and the floor control server needs to
   be reestablished.  However, that specification does not describe the
   reestablishment process because this process depends on how the
   connection was established in the first place.  BFCP entities using
   the offer/answer model follow the following rules.

   When the existing TCP connection is reseted following the rules in
   [8], the client SHOULD generate an offer towards the floor control
   server in order to reestablish the connection.  If a TCP connection
   cannot deliver a BFCP message and times out, the entity that
   attempted to send the message (i.e., the one that detected the TCP
   timeout) SHOULD generate an offer in order to reestablish the TCP
   connection.

   Endpoints which use the offer/answer model to establish BFCP
   connections MUST support the 'setup' and the 'connection' attributes.


8.  Authentication

   When a BFCP connection is established using the offer/answer model,
   it is assumed that the offerer and the answerer authenticate each
   other using some mechanism.  Once this mutual authentication takes
   place, all the offerer and the answerer need to make sure is that the
   entity they are receiving BFCP messages from is the same as the one
   that generated the previous offer or answer.

   When SIP is used to perform an offer/answer exchange, the initial
   mutual authentication takes place at the SIP level.  Additionally,
   SIP uses S/MIME [6] to provide an integrity protected channel with
   optional confidentiality for the offer/answer exchange.  BFCP takes
   advantage of this integrity protected offer/answer exchange to
   perform authentication.  Within the offer/answer exchange, the
   offerer and the answerer exchange the fingerprints of their self-
   signed certificates.  These self-signed certificates are then used to
   establish the TLS connection that will carry BFCP traffic between the
   offerer and the answerer.



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   BFCP clients and floor control servers follow the rules in [10]
   regarding certificate choice and presentation.  This implies that
   unless a 'fingerprint' attribute is included in the session
   description, the certificate provided at the TLS-level MUST either be
   directly signed by one of the other party's trust anchors or be
   validated using a certification path that terminates at one of the
   other party's trust anchors [5].  Endpoints which use the offer/
   answer model to establish BFCP connections MUST support the
   'fingerprint' attribute and SHOULD include it in their session
   descriptions.

   When TLS is used, once the underlaying TCP connection is established,
   the answerer acts as the TLS server regardless of its role (passive
   or active) in the TCP establishment procedure.


9.  Examples

   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.

   The following is an example of an offer sent by a conference server
   to a client.


   m=application 50000 TCP/TLS/BFCP *
   a=setup:passive
   a=connection:new
   a=fingerprint:SHA-1 \
        4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=floorctrl:s-only
   a=confid:4321
   a=userid:1234
   a=floorid:1 m-stream:10
   a=floorid:2 m-stream:11
   m=audio 50002 RTP/AVP 0
   a=label:10
   m=video 50004 RTP/AVP 31
   a=label:11

   Note that due to RFC formatting conventions, this document splits SDP
   across lines whose content would exceed 72 characters.  A backslash
   character marks where this line folding has taken place.  This
   backslash and its trailing CRLF and whitespace would not appear in
   actual SDP content.

   The following is the answer returned by the client.



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   m=application 9 TCP/TLS/BFCP *
   a=setup:active
   a=connection:new
   a=fingerprint:SHA-1 \
        3D:B4:7B:E3:CC:FC:0D:1B:5D:31:33:9E:48:9B:67:FE:68:40:E8:21
   a=floorctrl:c-only
   m=audio 55000 RTP/AVP 0
   m=video 55002 RTP/AVP 31


10.  Security Considerations

   The BFCP [8], SDP [11], and the offer/answer [4] specifications
   discuss security issues related to BFCP, SDP, and the offer/answer
   respectively.  In addition, [7] and [10] discuss security issues
   related to the establishment of TCP and TLS connections using an
   offer/answer model.

   BFCP assumes an initial integrity-protected channel used to exchange
   self-signed certificates between a client and the floor control
   server.  For session descriptions carried in SIP [3], S/MIME [6] is
   the natural choice to provide such a channel.


11.  IANA Considerations

   The following sections instruct the IANA to perform a set of actions.

11.1.  Registration of the 'TCP/BFCP' and 'TCP/TLS/BFCP' SDP 'proto'
       values

   This section instructs the IANA to register the following two new
   values for the SDP 'proto' field under the Session Description
   Protocol (SDP) Parameters registry:

                       +--------------+-----------+
                       | Value        | Reference |
                       +--------------+-----------+
                       | TCP/BFCP     |  RFCxxxx  |
                       | TCP/TLS/BFCP |  RFCxxxx  |
                       +--------------+-----------+

                 Table 2: Values for the SDP 'proto' field

   [Note to the RFC editor: please, replace RFCxxxx with the RFC number
   that this document will be assigned.]





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11.2.  Registration of the SDP 'floorctrl' Attribute

   This section instructs the IANA to register the following SDP att-
   field under the Session Description Protocol (SDP) Parameters
   registry:

   Contact name:  Gonzalo.Camarillo@ericsson.com

   Attribute name:  floorctrl

   Long-form attribute name:  Floor Control

   Type of attribute  Media level

   Subject to charset:  No

   Purpose of attribute:  The 'floorctrl' attribute is used to perform
      floor control server determination.

   Allowed attribute values:  1*("c-only" / "s-only" / "c-s")

11.3.  Registration of the SDP 'confid' Attribute

   This section instructs the IANA to register the following SDP att-
   field under the Session Description Protocol (SDP) Parameters
   registry:

   Contact name:  Gonzalo.Camarillo@ericsson.com

   Attribute name:  confid

   Long-form attribute name:  Conference Identifier

   Type of attribute  Media level

   Subject to charset:  No

   Purpose of attribute:  The 'confid' attribute carries the integer
      representation of a Conference ID.

   Allowed attribute values:  A token

11.4.  Registration of the SDP 'userid' Attribute

   This section instructs the IANA to register the following SDP att-
   field under the Session Description Protocol (SDP) Parameters
   registry:




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   Contact name:  Gonzalo.Camarillo@ericsson.com

   Attribute name:  userid

   Long-form attribute name:  User Identifier

   Type of attribute  Media level

   Subject to charset:  No

   Purpose of attribute:  The 'userid' attribute carries the integer
      representation of a User ID.

   Allowed attribute values:  A token

11.5.  Registration of the SDP 'floorid' Attribute

   This section instructs the IANA to register the following SDP att-
   field under the Session Description Protocol (SDP) Parameters
   registry:

   Contact name:  Gonzalo.Camarillo@ericsson.com

   Attribute name:  floorid

   Long-form attribute name:  Floor Identifier

   Type of attribute  Media level

   Subject to charset:  No

   Purpose of attribute:  The 'floorid' attribute associates a floor
      with one or more media streams.

   Allowed attribute values:  Tokens


12.  Acknowledgments

   Joerg Ott, Keith Drage, Alan Johnston, Eric Rescorla, Roni Even, and
   Oscar Novo provided useful ideas for this document.

13.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax



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         Specifications: ABNF", RFC 2234, November 1997.

   [3]   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.

   [4]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [5]   Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [6]   Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Certificate Handling", RFC 3850,
         July 2004.

   [7]   Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [8]   Camarillo, G., "The Binary Floor Control Protocol (BFCP)",
         draft-ietf-xcon-bfcp-05 (work in progress), July 2005.

   [9]   Levin, O. and G. Camarillo, "The SDP (Session Description
         Protocol) Label Attribute",
         draft-ietf-mmusic-sdp-media-label-01 (work in progress),
         January 2005.

   [10]  Lennox, J., "Connection-Oriented Media Transport over the
         Transport Layer Security (TLS)  Protocol in the Session
         Description Protocol (SDP)", draft-ietf-mmusic-comedia-tls-05
         (work in progress), September 2005.

   [11]  Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005.
















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Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com










































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   Internet Society.




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