BFCPbis Working Group G. Camarillo
Internet-Draft Ericsson
Obsoletes: 4583 (if approved) T. Kristensen
Intended status: Standards Track Cisco
Expires: April 15, 2013 October 12, 2012
Session Description Protocol (SDP) Format for Binary Floor Control
Protocol (BFCP) Streams
draft-ietf-bfcpbis-rfc4583bis-03
Abstract
This document specifies how to describe Binary Floor Control Protocol
(BFCP) streams in Session Description Protocol (SDP) descriptions.
User agents using the offer/answer model to establish BFCP streams
use this format in their offers and answers.
This document obsoletes RFC 4583. Changes from RFC 4583 are
summarized in section 12.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 15, 2013.
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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 . . . . . . . . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11.1. Registration of SDP 'proto' Values . . . . . . . . . . . 10
11.2. Registration of the SDP 'floorctrl' Attribute . . . . . . 10
11.3. Registration of the SDP 'confid' Attribute . . . . . . . 11
11.4. Registration of the SDP 'userid' Attribute . . . . . . . 11
11.5. Registration of the SDP 'floorid' Attribute . . . . . . . 12
12. Changes from RFC 4583 . . . . . . . . . . . . . . . . . . . . 12
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
14. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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 is to use an offer/
answer [4] exchange. This document specifies how to encode this
information in the SDP session descriptions that 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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document 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 (discussed in Section 7), the port
field contains the port to which the remote endpoint will direct BFCP
messages 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. A port field value of zero has the
standard SDP meaning (i.e., rejection of the media stream).
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We define four new values for the transport field: TCP/BFCP, TCP/TLS/
BFCP, UDP/BFCP, and UDP/TLS/BFCP. TCP/BFCP is used when BFCP runs
directly on top of TCP, TCP/TLS/BFCP is used when BFCP runs on top of
TLS, which in turn runs on top of TCP. Similarly, UDP/BFCP is used
when BFCP runs directly on top of UDP, and UDP/TLS/BFCP is used when
BFCP runs on top of DTLS [12], which in turn runs on top of UDP.
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. In the most common
scenario, a client establishes a BFCP stream with a 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, in 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 act as a floor control
client only.
s-only: The offerer would be willing to act as a floor control
server only.
c-s: The offerer would be willing to act both as a floor control
client and as a floor control server.
If an SDP media description in an offer contains a 'floorctrl'
attribute, the answerer accepting that media MUST include one in the
corresponding media description of 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 descriptions of the roles when they are chosen
by an answerer:
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.
Endpoints that 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 answerer 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
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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:
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 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 decimal integer
representation of a conference ID and a user ID, respectively.
Endpoints that 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 answerer 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 the SDP media description for BFCP
media. 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 that 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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Note: In [15] 'm-stream' was erroneously used in Section 9. Although
the example was non-normative, it is implemented by some vendors.
Therefore, it is RECOMMENDED to support parsing and interpreting
'm-stream' the same way as 'mstrm' when receiving.
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 reset 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 that use the offer/answer model to establish BFCP
connections MUST support the 'setup' and '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 ensure 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
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perform authentication. Within the offer/answer exchange, the
offerer and 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.
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 that 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 with TCP, once the underlying 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 mstrm:10
a=floorid:2 mstrm: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
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backslash and its trailing CRLF and whitespace would not appear in
actual SDP content.
The following is the answer returned by the client.
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
A similar example using unreliable transport and DTLS is shown below,
where the offer is sent from a client.
m=application 50000 UDP/DTLS/BFCP *
a=setup:actpass
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:c-only, s-only
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:10
a=floorid:2 mstrm:11
m=audio 50002 RTP/AVP 0
a=label:10
m=video 50004 RTP/AVP 31
a=label:11
The following is the answer returned by the server.
m=application 9 UDP/DTLS/BFCP *
a=setup:active
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:s-only
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:10
a=floorid:2 mstrm:11
m=audio 55000 RTP/AVP 0
m=video 55002 RTP/AVP 31
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10. Security Considerations
The BFCP [8], SDP [11], and offer/answer [4] specifications discuss
security issues related to BFCP, SDP, and 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.
Furthermore, when using DTLS over UDP, considerations for its use
with RTP and RTCP are presented in [13]. The requirements for the
offer/answer exchange, as listed in Section 5 of that document, MUST
be followed.
BFCP assumes that an initial integrity-protected channel is 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
[Editorial note: The changes in Section 11.1 instruct the IANA to
register the two new values UDP/BFCP and UDP/TLS/BFCP for the SDP
'proto' field, the rest is unchanged from [14].]
11.1. Registration of SDP 'proto' Values
The IANA has registered the following values for the SDP 'proto'
field under the Session Description Protocol (SDP) Parameters
registry:
+--------------+------------+
| Value | Reference |
+--------------+------------+
| TCP/BFCP | [RFC XXXX] |
| TCP/TLS/BFCP | [RFC XXXX] |
| UDP/BFCP | [RFC XXXX] |
| UDP/TLS/BFCP | [RFC XXXX] |
+--------------+------------+
Table 2: Values for the SDP 'proto' field
11.2. Registration of the SDP 'floorctrl' Attribute
The IANA has registered 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: 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
The IANA has registered 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
The IANA has registered the following SDP att-field under the Session
Description Protocol (SDP) Parameters registry:
Contact name: Gonzalo.Camarillo@ericsson.com
Attribute name: userid
Long-form attribute name: User Identifier
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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
The IANA has registered 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. Changes from RFC 4583
Following is the list of technical changes and other fixes from [15].
Main purpose of this work was to add signaling support necessary to
support BFCP over unreliable transport, as described in [8],
resulting in the following changes:
Fields in the 'm' Line (Section 3):
The section is re-written to remove reference to the
exclusivity of TCP as a transport for BFCP streams. The
transport field values UDP/BFCP and UDP/TLS/BFCP added.
Authentication (Section 8):
In last paragraph, made clear that a TCP connection was
described.
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Security Considerations (Section 10):
For the DTLS over UDP case, mention existing considerations and
requirements for the offer/answer exchange in [13].
Registration of SDP 'proto' Values (Section 11.1):
Register the two new values UDP/BFCP and UDP/TLS/BFCP in the
SDP parameters registry.
The clarification and bug fixes:
Errata ID: 712 (Section 4 and Section 6):
Language clarification. Don't use terms like an SDP attribute is
"used in an 'm' line", instead make clear that the attribute is a
media-level attribute.
Fix typo in example (Section 9):
Do not use 'm-stream' in the SDP example, use the correct 'mstrm'
as specified in Section 9. Recommend interpreting 'm-stream' if
it is received, since it is present in some implementations.
13. Acknowledgements
Joerg Ott, Keith Drage, Alan Johnston, Eric Rescorla, Roni Even, and
Oscar Novo provided useful ideas for the original [15]. The authors
also acknowledge contributions to the revision of BFCP for use over
an unreliable transport from Geir Arne Sandbakken, Charles Eckel,
Eoin McLeod and Mark Thompson.
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", STD 68, RFC 5234, January 2008.
[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] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley,
R., and W. Polk, "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile",
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RFC 5280, May 2008.
[6] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.2 Certificate Handling",
RFC 5750, January 2010.
[7] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
Session Description Protocol (SDP)", RFC 4145, September 2005.
[8] Camarillo, G., Drage, K., Kristensen, T., Ott, J., and C.
Eckel, "The Binary Floor Control Protocol (BFCP)",
draft-ietf-bfcpbis-rfc4582bis-06 (work in progress),
October 2012.
[9] Levin, O. and G. Camarillo, "The Session Description Protocol
(SDP) Label Attribute", RFC 4574, August 2006.
[10] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[11] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[12] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[13] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework for
Establishing a Secure Real-time Transport Protocol (SRTP)
Security Context Using Datagram Transport Layer Security
(DTLS)", RFC 5763, May 2010.
[14] Camarillo, G., Ott, J., and K. Drage, "The Binary Floor Control
Protocol (BFCP)", RFC 4582, November 2006.
[15] Camarillo, G., "Session Description Protocol (SDP) Format for
Binary Floor Control Protocol (BFCP) Streams", RFC 4583,
November 2006.
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Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Tom Kristensen
Cisco
Philip Pedersens vei 22
N-1366 Lysaker
Norway
Email: tomkrist@cisco.com, tomkri@ifi.uio.no
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