One document matched: draft-ietf-mmusic-sdp-comedia-09.txt
Differences from draft-ietf-mmusic-sdp-comedia-08.txt
MMUSIC Working Group D. Yon
Internet-Draft Tactical Software, LLC
Expires: March 30, 2005 G. Camarillo
Ericsson
September 29, 2004
Connection-Oriented Media Transport in the Session Description
Protocol (SDP)
draft-ietf-mmusic-sdp-comedia-09.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 (2004).
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 connection attribute, which handles connection reestablishment.
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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 Connection Attribute . . . . . . . . . . . . . . . . . . . 5
5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7
6. Connection Management . . . . . . . . . . . . . . . . . . . . 7
6.1 Connection Establishment . . . . . . . . . . . . . . . . . 7
6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8
6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 9
7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
11.2 Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
The Session Description Protocol [4] 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 [11]) 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 [4] 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. This document defines 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 endpoints perform the
connection setup procedure. This document defines two new attributes
to describe connection setups: setup and connection.
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 [3] and indicate requirement levels for
compliant implementations.
3. Protocol Identifier
The following is the ABNF for an m= line, as specified by RFC 2327
[4].
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
This document defines a new value 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 described using an m= lines containing the TCP protocol
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identifier are carried using TCP [1].
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 connection
attributes. While both attributes are applicable to m= lines that
use the TCP protocol identifier, they are not limited to them. These
attributes MAY be used in conjunction with 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"
/ "holdconn"
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.
Holdconn: The endpoint does not want the connection to be
established for the time being.
4.1 The Setup Attribute in the Offer/answer Model
The offer/answer model, defined in RFC 3264 [5], 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
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follows. The offerer states which role or roles it 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
________________
active passive / holdconn
passive active / holdconn
actpass active / passive / holdconn
holdconn holdconn
The active 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 passive endpoint SHOULD be ready to accept a connection on the
port number specified in the m= line.
A value of 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.
A value of holdconn indicates that the connection should not be
established for the time being.
The default value of the setup attribute in an offer/answer exchange
is active in the offer and passive in the answer.
5. The Connection 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
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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 connection attribute, which is charset-independent, is
used to disambiguate these two scenarios. The following is the ABNF
of the connection attribute:
connection-attr = "a=connection:" conn-value
conn-value = "new" / "existing"
5.1 Offerer Behaviour
Offerers and answerers use the connection 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. The
connection value resulting from an offer/answer exchange is the
connection value in the answer. If the connection value in the
answer is "new", the end-points SHOULD establish a new connection.
If the connection value in the answer is "existing", the end-points
SHOULD continue using the exiting connection.
When an offerer generates an m= line which uses a connection-oriented
transport, it SHOULD provide a connection attribute for the m= line
unless the application using the m= line has other means to deal with
connection reestablishment. The connection attribute in an initial
offer (i.e., no transport connection has been established yet) takes
the value of "new".
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 use a connection value of
"existing" 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 connection value of "new".
Note that, according to the rules in this section, an offer that
changes the transport address (IP address or port number) of an m=
line will have a connection value of "new".
The default value of the connection attribute in an offer/answer
exchange is "new".
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5.2 Answerer Behaviour
The connection value for an m= line is negotiated using the offer/
answer model. The resulting connection value after an offer/answer
exchange is the connection value in the answer. If the connection
value in the offer is "new", the answerer MUST also use a value of
"new" in the answer. If the connection value in the offer is
"existing", the answerer uses a value of "existing" in the answer if
it wishes to continue using the existing connection and a value of
"new" if it wants a new connection to be established.
In some scenarios where third party call control [12] is used, an
endpoint may receive an initial offer with a connection value of
"existing". Following the previous rules, such an answerer would
use a connection value of "new" in the answer.
If the connection value for an m= line resulting from an offer/answer
exchange is "new", the endpoints SHOULD establish a new
transport-layer connection as indicated by the setup attribute. If a
previous connection is still up, the endpoints SHOULD close it as
soon as the offer/answer exchange is completed. It is up to the
application to ensure proper data synchornization between the two
connections.
If the connection value for an m= line resulting from an offer/answer
exchange is "existing", the endpoints SHOULD continue using the
existing connection.
6. Connection Management
This section addresses connection establishment, connection
reestablishment, and connection termination.
6.1 Connection Establishment
An endpoint that according to an offer/answer exchange is supposed to
initiate a new connection SHOULD initiate it as soon as it is able
to, 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.
Note that some endpoints need to wait for some event to happen
before being able to establish the connection. For example, a
wireless terminal may need to set up a radio bearer before being
able to initiate a connection.
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6.2 Connection Reestablishment
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 connection value of "new"
for this m= line.
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.
6.3 Connection Termination
Typically, endpoints do not close the connection until the session
has expired, been explicitly terminated, or a new connection value
has been provided for the m= line. Additionaly, specific
applications can describe further scenarios where an end-point may
close a given connection. As soon as an end-point notices that it
needs to terminate a connection, it SHOULD do so.
While in TCP both end-points need to close a connection, other
connection-oriented transport protocols may not have the concept of
half-close connections. In this case, a connection would be
terminated as soon as one of the end-points closed it, making it
unnecessary for the other end-point to perform any further action to
terminate the connection.
In any case, individual applications may provide further
considerations on how to achieve a graceful connection termination.
For example, a file application using TCP receiving a FIN from the
remote endpoint may need to finish the ongoing transmission of a file
before sending its own FIN.
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
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c=IN IP4 192.0.2.2
a=setup:passive
a=connection:new
An answerer at 192.0.2.1 receiving this offer responds with the
following answer:
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connection:new
The endpoint at 192.0.2.1 then initiates the TCP connection to port
54111 at 192.0.2.2.
7.2 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=connection:new
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=connection:new
This will cause the offerer (at 192.0.2.2) to initiate a connection
to port 54321 at 192.0.2.1.
7.3 Existing Connection Reuse
Subsequent to the exchange in Section 7.2, another offer/answer
exchange is initiated in the opposite direction. The endpoint at
192.0.2.1 wishes to continue using the existing connection:
m=image 54321 TCP t38
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c=IN IP4 192.0.2.1
a=setup:passive
a=connection:existing
The endpoint at 192.0.2.2 also wishes to use the existing connection
and responds with the following description:
m=image 9 TCP t38
c=IN IP4 192.0.2.2
a=setup:active
a=connection:existing
The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.
Note that the endpoint at 192.0.2.2 uses setup:active in response
to the offer of setup:passive, and uses port 9 because it is
active.
7.4 Existing Connection Refusal
Subsequent to the exchange in Section 7.3, another offer/answer
exchange is initiated by the endpoint at 192.0.2.2, again wishing to
reuse the existing connection:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:existing
However, this time the answerer is unaware of the old connection and
so wishes to establish a new one. (This could be the result of a
transfer via 3pcc.) It is unable to act in the passive mode so
responds as active:
m=image 9 TCP t38
c=IN IP4 192.0.2.3
a=setup:active
a=connection:new
The endpoint at 192.0.2.3 then initiates the TCP connection to port
54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old
connection.
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Note that the endpoint at 192.0.2.2, while specifying connection:
existing has reverted to setup:actpass and its real port number,
rather than repeating setup:active and port 9 from the previous
cycle. Had it not done this, this negotiation would have failed.
8. Security Considerations
See RFC 2327 [4] for security and other considerations specific to
the Session Description Protocol in general.
An attacker may attempt to modify the values of the connection and
setup attributes to have endpoints reestablish connections
unnecesaryly or to keep them from establishing a connection. 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 a different form of integrity protection.
9. IANA Considerations
This document defines two session and media level SDP attributes:
setup and connection. 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 value: 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".
Specifications defining new proto values, like this one, must define
the rules by which their media format (fmt) namespace is managed.
For the TCP protocol, new formats SHOULD have an associated MIME
registration. Use of an existing MIME subtype for the format is
encouraged. If no MIME subtype exists, it is RECOMMENDED that a
suitable one is registered through the IETF process [2] by production
of, or reference to, a standards-track RFC that defines the transport
protocol for the format.
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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] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
11.2 Informative References
[6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
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.
[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
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[12] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
Authors' Addresses
David Yon
Tactical Software, LLC
670 N Commercial St
Manchester, NH 03101
USA
EMail: yon-comedia@rfdsoftware.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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