MMUSIC Working Group D. Yon
Internet-Draft Dialout.Net, Inc
Expires: December 10, 2004 G. Camarillo
Ericsson
June 11, 2004
Connection-Oriented Media Transport in the Session Description
Protocol (SDP)
draft-ietf-mmusic-sdp-comedia-07.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes how to express media transport over
connection-oriented protocols 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 connid attribute, which provides a connection identifier.
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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 Connid Attribute . . . . . . . . . . . . . . . . . . . . . 5
5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
6. Connection Management . . . . . . . . . . . . . . . . . . . . 7
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
7.2 Passive/Active with Connection Reestablishment . . . . . . 8
7.3 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 10
11.2 Informational References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
The Session Description Protocol [3] 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 [5]) to maximize portability among transports. SDP
does not define a protocol, but only 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 [3] 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 connection-oriented transports, such as TCP,
are more appropriate. We define a new protocol identifier, TCP, to
describe TCP connetions in SDP.
Connection-oriented protocols introduce two new factor when
describing a session: how and when should end points perform the
connection setup procedure. We define two new attributes to describe
connection setups: setup and connid.
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 [2] and indicate requirement levels for
compliant implementations.
3. Protocol Identifier
The following is the ABNF for an m= line, as specified by RFC 2327
[3].
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
We define a new values 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 further
qualify the application-layer protocol using an fmt identifier. Media
lines with the TCP protocol identifier are carried using TCP [1].
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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 following sections define the setup and the connid attributes.
While they are applicable to m= lines that use the TCP protocol
identifier, they are not limited to them. These attributes SHOULD be
used in any m= line which uses a connection-oriented transport
protocol, even if the protocol identifier of the m= line is not TCP.
4. Setup Attribute
The setup attribute indicates which of the end points should initiate
the connection establishment (e.g., 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"
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.
4.1 The Setup Attribute in the Offer/answer Model
The offer/answer model, defined in RFC 3264 [4], 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 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:
Offer Answer
_______________
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active passive
passive active
actpass active / passive
The value active indicates that the 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 specifying 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 value passive indicates that the endpoint SHOULD be ready to
accept a connection on the port number specified in the m= line.
The value 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.
The default value of the setup attribute in an offer/answer exchange
is active in the offer and passive in the answer.
5. The Connid Attribute
The preceding description of the setup attribute has been 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
as to whether subsequent SDP exchange represents a confirmation that
the endpoint is to continue using the current media connection
unchanged, or is a request to make a new media connection. The
media-level connid attribute, which is charset-independent, is used
to disambiguate these two scenarios. The following is the ABNF of the
connid attribute:
connid = "a=connid:" connection-identifier
connection-identifier = token
The connid attribute provides an identifier for the transport-layer
connection used by the m= line. Connid values are meaningful in the
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context of a particular m= line. So, different m= lines in the same
session description MAY have the same connid value.
5.1 Offerer Behaviour
Offerers and answerers use the connid attribute to decide whether a
new transport connection needs to be established or, on the other
hand, the existing transport connection should still be used.
When an offerer generates an m= line which uses a connection-oriented
transport, it SHOULD provide such an m= line with a connection
identifier using a connid attribute, unless the application using the
m= line has other means to deal with connection reestablishment. The
connid attribute in an initial offer (i.e., no transport connection
has been established yet) can take any value. This value identifies
the initial connection that the endpoints will attempt to establish.
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 the same connid value 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 new connid
value. This new connid value MUST be different from the current
connid value in use and SHOULD be different than any connid value
used previously in the same m= line.
The connid value in an offer is only compared with the connid
value currently in use. So, having a connid value different than
the one in use is enough to trigger the establishment of a new
connection. Still, we recommend to use a value different than all
the previous ones used in the m= line to make debugging easier.
Note that, according to the rules in this section, an offer that
changes the transport address (IP address plus port number) of an
m= line will have a new connid value for this m=line.
5.2 Answerer Behaviour
The connid value for an m= line is negotiated using the offer/answer
model. The resulting connid value after an offer/answer exchange is
the connid value in the answer.
For an m= line, if the offer contains a new connid value (i.e.,
different from the one in use) the answerer MUST use this value in
the answer. If the offer contains the connid value in use and the
answerer wishes to continue using the existing transport-layer
connection, the answerer MUST use this connid value in the answer. If
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the offer contains the connid value in use but the answerer wishes to
establish a new transport-layer connection, the answerer MUST use a
new connid value in the answer.
If the connid value for an m= line resulting from an offer/answer
exchange is different than the connid in use so far, the endpoints
SHOULD establish a new transport-layer connection as indicated by the
setup attribute. If a previous connection is still up, the endpoint
responsible for establishing the new connection performing the active
role SHOULD close it as soon as the offer/answer exchange is
completed.
If the connid value for an m= line resulting from an offer/answer
exchange is the same as the connid in use so far, the endpoints
SHOULD continue using the existing connection.
In the past, it was proposed to use the presence of a media-level
SDP attribute as a flag to indicate that a new connection needed
to be established. We chose not to follow the flag approach
because an offerer whose intent was to signal "no changes" in a
session would need to issue a different offer than the previous
one (i.e., it would need to remove the flag from the m= line). By
using the connid attribute instead, an offerer signals "no
changes" in a session by issuing an identical offer to the one in
use.
6. Connection Management
An endpoint that according to an offer/answer exchange is supposed to
initiate a new connection SHOULD initiate it as soon as the offer/
answer exchange is completed, 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.
Typically, endpoints do not close the connection until the session
has expired, been explicitly terminated, or a new connid value has
been provided for the m= line. Additionaly, specific applications can
describe further scenarios where an end-point may close a given
connection. In case the session is explicitly terminated by one of
the endpoints (e.g., the endpoint sends a SIP [10] BYE), the end
point terminating the session is responsible for closing the
transport-connection.
If an endpoint determines that the transport-connection for an m=
line has been closed and it should be reestablished, it SHOULD
perform a new offer/answer exchange using a new connid value for this
m= line.
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Note that the SDP direction attribute (e.g., a=sendonly) deals
with the media sent over the transport-connection, but has no
impact on the transport-connection itself.
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=connid:1
An answerer at 192.0.2.1 receiving this offer responds with the
following answer:
c=IN IP4 192.0.2.1
m=image 9 TCP t38
a=setup:active
a=connid:1
The endpoint at 192.0.2.1 then initiates the TCP connection to port
54111 at 192.0.2.2.
7.2 Passive/Active with Connection Reestablishment
Continuing the preceding example, consider the scenario where the TCP
connection fails and the endpoints wish to reestablish the connection
for the session. The endpoint at 192.0.2.2 signals this intent with
the following SDP:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connid:2
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The new connid value informs the endpoint at 192.0.2.1 that this SDP
represents the intent to establish a new connection for media
transport, rather than continuing with the original connection. If
192.0.2.1 agrees to continue the session using a new connection, it
responds with:
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connid:2
7.3 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=connid:3
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=connid:3
This will cause the offerer (at 192.0.2.2) to initiate a connection
to port 54321 at 192.0.2.1.
8. Security Considerations
See RFC 2327 [3] for security and other considerations specific to
the Session Description Protocol in general.
An attacker may attempt to modify the values of the connid attributes
to have endpoints reestablish connections unnecesaryly. 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
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a different form of integrity protection.
9. IANA Considerations
This document defines two session and media level SDP attributes:
setup and connid. Their formats are defined in Section 4 and Section
5 respectively. These two attributes should be registered by the IANA
on
http://www.iana.org/assignments/sdp-parameters
under "att-field (both session and media level)".
This document defines a proto values: TCP. Its format is defined in
Section 3. This proto value should be registered by the IANA on
http://www.iana.org/assignments/sdp-parameters
under "proto".
10. Acknowledgements
Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
Holmberg provided valuable insights and contributions.
11. References
11.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
11.2 Informational References
[6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
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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.
Authors' Addresses
David Yon
Dialout.Net, Inc
One Indian Head Plaza
Nashua, NH 03060
USA
EMail: yon@dialout.net
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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