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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: July 14, 2005                                  January 13, 2005


   Session Description Protocol (SDP) Format for Binary Floor Control
                        Protocol (BFCP) Streams
                   draft-ietf-mmusic-sdp-bfcp-00.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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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.   The confid and userid SDP Attributes . . . . . . . . . . . .   4
   5.   The k line . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.   The nonce Attribute  . . . . . . . . . . . . . . . . . . . .   4
   7.   Association between Streams and Floors . . . . . . . . . . .   5
   8.   Certificate Choice and Presentation  . . . . . . . . . . . .   5
   9.   TCP Connection Management  . . . . . . . . . . . . . . . . .   6
   10.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .   7
   12.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .   7
     12.1   Registration of the confid Attribute . . . . . . . . . .   8
     12.2   Registration of the userid Attribute . . . . . . . . . .   8
     12.3   Registration of the floorid Attribute  . . . . . . . . .   8
     12.4   Registration of the nonce Attribute  . . . . . . . . . .   9
   13.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .   9
   14.  Normative References . . . . . . . . . . . . . . . . . . . .   9
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  10
        Intellectual Property and Copyright Statements . . . . . . .  11





























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1.  Introduction

   As discussed in the BFCP specification [7], 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 [5] exchange.  This document specifies how to encode
   this information in the SDP session descriptions which are part of 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

   According to RFC 2327 [3], the 'm'line format is the following:

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

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

   The port field is set following the rules in [6].  Depending on the
   value of the 'setup' attribute (disccused in Section 9), 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).

   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.




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   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 20000 TCP/TLS/BFCP *

4.  The confid and userid SDP Attributes

   We define the 'confid' and the 'userid' SDP media-level attributes.
   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.

5.  The k line

   The floor control server MAY use an SDP 'k' line to provide clients
   with a shared secret to be used to calculate the value of the DIGEST
   TLVs.  The following is an example of a 'k' line:



   k=base64:c2hhcmVkLXNlY3JldA==

   Endpoints MAY use other mechanisms (including out-of-band mechanisms)
   to come up with a share secret.  However, if the 'k' line is used in
   the way just described, the session description containing the 'k'
   line with the shared secret MUST be encrypted.

6.  The nonce Attribute

   We define the 'nonce' attribute.  Its Augmented BNF syntax [2] is:





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   nonce-attribute       = "a=nonce: " nonce-value
   nonce-value           = token

   The 'nonce' attribute carries the integer representation of the nonce
   to be used by the client in its next BFCP message (typically the
   first message from the client) towards the floor control server.
   This is an optimization so that the client does not need to generate
   an initial BFCP message only to have it rejected by the floor control
   server with an Error response containing a nonce.

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

7.  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 defined 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 [8]

   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.

8.  Certificate Choice and Presentation

   Floor control servers follow the rules in [9] regarding certificate
   choice and presentation.  This implies that unless the floor control
   server includes a 'fingerprint' attribute in its session description,
   the certificate provided by the floor control server must be signed
   by a certificate authority known to the client.

   Endpoints which use the offer/answer model to establish BFCP
   connections MUST support the 'fingerprint' attribute.  Floor control



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   servers SHOULD include this attribute in their session descriptions
   (no matter whether they are offers or answers).

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

9.  TCP Connection Management

   The management of the TCP connection used to transport BFCP is
   performed using the 'setup' and 'connection' attributes as defined in
   [6].

   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 [7] 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
   [7], 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.

10.  Example

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


   m=application 20000 TCP/TLS/BFCP *
   k=base64:c2hhcmVkLXNlY3JldA==
   a=setup:passive
   a=connection:new
   a=fingerprint:SHA-1 \



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        4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=nonce:5678
   a=confid:4321
   a=userid:1234
   a=floorid:1 m-stream:10
   a=floorid:2 m-stream:11
   m=audio 20000 RTP/AVP 0
   a=label:10
   m=video 30000 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 user.


   m=application 9 TCP/BFCP *
   a=setup:active
   a=connection:new
   m=audio 25000 RTP/AVP 0
   m=video 35000 RTP/AVP 31


11.  Security Considerations

   The BFCP [7], SDP [3], and the offer/answer [5] specifications
   discuss security issues related to BFCP, SDP, and the offer/answer
   respectively.  In addition, [6] and [9] discuss security issues
   related to the establishment of TCP and TLS connections using an
   offer/answer model.

   An issue which is discussed in the previous specifications and is of
   particular importance for this specification relates to the usage of
   the 'k' line to provide shared secrets to clients.  When the 'k' line
   is used in this way, the session description carrying it SHOULD be
   encrypted.  Otherwise, an attacker could get access to the shared
   secret and impersonate the client.  For session descriptions carried
   in SIP [4], S/MIME is the natural choice to provide such end-to-end
   encryption.  Other applications MAY use different encryption
   mechanisms.

12.  IANA Considerations

   This document instructs the IANA to register four new media-level SDP



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   attributes: 'confid', 'userid', 'floorid', and 'nonce'.

12.1  Registration of the confid Attribute

   Contact name:          Gonzalo.Camarillo@ericsson.com

   Attribute name:        confid

   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

12.2  Registration of the userid Attribute

   Contact name:          Gonzalo.Camarillo@ericsson.com

   Attribute name:        userid

   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

12.3  Registration of the floorid Attribute

   Contact name:          Gonzalo.Camarillo@ericsson.com

   Attribute name:        floorid

   Type of attribute      Media level

   Subject to charset:    No

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







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   Allowed attribute values:  Tokens

12.4  Registration of the nonce Attribute

   Contact name:          Gonzalo.Camarillo@ericsson.com

   Attribute name:        nonce

   Type of attribute      Media level

   Subject to charset:    No

   Purpose of attribute:  The 'nonce' attribute carried a nonce to be
      used in the media stream (e.g., in the BFCP connection).

   Allowed attribute values:  A token

13.  Acknowledgments

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

14  Normative References

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

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

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

   [4]  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.

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

   [6]  Yon, D., "Connection-Oriented Media Transport in the Session
        Description Protocol  (SDP)", draft-ietf-mmusic-sdp-comedia-09
        (work in progress), September 2004.

   [7]  Camarillo, G., "The Binary Floor Control Protocol (BFCP)",
        draft-ietf-xcon-bfcp-01 (work in progress), October 2004.

   [8]  Levin, O. and G. Camarillo, "The SDP (Session Description



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        Protocol) Label Attribute",
        draft-levin-mmmusic-sdp-media-label-00 (work in progress), July
        2004.

   [9]  Lennox, J., "Connection-Oriented Media Transport over the
        Transport Layer Security (TLS)  Protocol in the Session
        Description Protocol (SDP)", draft-ietf-mmusic-comedia-tls-02
        (work in progress), October 2004.


Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com
































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