One document matched: draft-ietf-mmusic-sdp-bfcp-00.txt
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
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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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