Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
J. Rosenberg
dynamicsoft
draft-camarillo-mmusic-alt-00.txt
February 3, 2003
Expires: August 2003
The Alternative Semantics for the Session
Description Protocol Grouping Framework
STATUS OF THIS MEMO
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Abstract
This document defines the alternative (ALT) semantics for the SDP
grouping framework. The ALT semantics allow offering alternative
media configurations to establish a particular media stream.
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Table of Contents
1 Introduction ........................................ 3
1.1 Terminology ......................................... 3
2 ALT Semantics ....................................... 3
2.1 Preference .......................................... 3
2.2 Media Stream Establishment Attempts ................. 4
2.3 Backward Compatibility and the "alt" SIP Option
Tag ................................................. 5
2.4 ALT and the Offer/Answer Model ...................... 5
3 Example ............................................. 5
4 IANA Considerations ................................. 6
5 Security Considerations ............................. 6
6 Authors' Addresses .................................. 7
7 Normative References ................................ 7
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1 Introduction
An SDP [1] session description contains the media parameters to be
used to establish a number of media streams. For a particular media
stream, an SDP session description contains, among other parameters,
the transport addresses and the codec to be used to transfer media.
SDP allows providing one transport address and a list of codecs per
media stream. The users can choose to use any of those codecs at any
point in time during the session, but they only have a single
transport address to choose from.
Being able to dynamically change transport address during a session
is useful when a system cannot determine its own transport address as
seen from the remote end in presence of a NAT (Network Address
Translator), but it can provide a list of possible candidates. Having
several alternative transport addresses for a particular stream also
provides a fail-over mechanism in case one of the addresses becomes
unreachable.
This document defines the alternative (ALT) semantics for the SDP
grouping framework [2]. The ALT semantics allow expressing
alternative configurations, including transport addresses and codecs,
for a particular media stream.
1.1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and
indicate requirement levels for compliant SIP implementations.
2 ALT Semantics
We define a new "semantics" attribute within the SDP grouping
framework [2]: ALT (Alternative).
Media lines grouped using ALT semantics represent alternative
configurations of a single logical media stream. The entity receiving
a session description with an ALT group MUST be ready to receive
media over any of the grouped m lines.
2.1 Preference
The entity generating a session description may have an order of
preference for all the alternative configurations offered. The
identifiers of the media streams MUST be listed in order of
preference in the group line. In the example below, the m line with
mid=1 has a higher preference than the m line with mid=2.
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a=group:ALT 1 2
In the ALT context, preferred means that the recipient of the SDP
SHOULD send data using the m line with the highest preference that is
acceptable to it.
In SDP, the codecs within a given media line are listed in
order of preference. The ALT semantics effectively stretch
the concept of dynamic changes of codec in the middle of a
session to dynamic changes of transport addresses and other
media parameters in the middle of a session. Therefore, we
have chosen to use the same mechanism (i.e., a list ordered
by preference) to express preferences among grouped media
lines.
2.2 Media Stream Establishment Attempts
An entity receiving a set of streams grouped using ALT semantics
cannot assume that it will be possible to successfully use all the
alternative configurations offered. Some of the m lines may contain
transport addresses that are unreachable for the recipient of the
session description.
Such entity SHOULD try to establish the grouped m lines in order of
preference. How an m line is established depends on the type of the
media stream. Establishing a TCP-based m line involves establishing a
TCP connection whereas establishing an RTP-based m line involves
sending RTP or RTCP packets.
An entity SHOULD use network feedback (e.g., ICMP messages) and/or
timeouts to determine whether or not the establishment of the media
stream has been successful. If the establishment is not successful,
the receiver of the session description SHOULD try to establish the
next m line of the ALT group in order of preference.
An entity MAY try to establish different m lines of the ALT group in
parallel. However, even if more than one m line are established
successfully, an entity MUST only send media over one of the
successfully established m lines.
During the session, a sender MAY choose to use any of the grouped m
lines at a particular point in time to send data. This makes it
possible to use the ALT semantics as a fail-over mechanism for
ongoing sessions. If once a media stream has been successfully
established, there is some type of transmission error, the end-points
can try to use any other configuration from the ALT group to try to
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recover from the error.
2.3 Backward Compatibility and the "alt" SIP Option Tag
The receiver of a session description with an ALT group is supposed
to establish only one media stream. However, if the entity receiving
such a session description does not understand the ALT semantics or
the grouping framework, it will establish all the streams of the ALT
group. If this entity sends media in parallel over all the streams at
the same time, the resulting session bandwidth will be much higher
than the expected by the creator of the session. The ALT semantics
MUST NOT be used when this situation is unacceptable.
Note, however, that there are scenarios where the situation described
above is not problematic. In sendonly sessions, for instance, this
problem is minimized, because the creator of the SDP is the only one
sending media. The receiver that does not understand ALT will be
receiving media over only one m line at a time.
Scenarios that involve SIP [4] and the offer/answer model [5] are not
problematic either, since they can use SIP options tags to ensure
that the answerer understands the ALT semantics. Therefore, we define
the option tag "alt" for use in the Require and Supported header
fields. A SIP entity that includes the "alt" option tag in a
Supported header field understands the ALT semantics defined in this
document.
2.4 ALT and the Offer/Answer Model
An answerer getting a number of m lines grouped using ALT semantics
may find some of them unacceptable. They may contain codecs that the
answerer does not support or contain any other parameter that makes
them unacceptable. Following normal SIP procedures, the answerer will
set their ports to zero in the answer [5].
The answerer follows the steps described in Section 2.2 using only
those m lines that were found, in principle, acceptable.
3 Example
An end-point receiving the SDP description below needs to choose
between the destination ports 20000 and 30000. The end-point will be
able to change dynamically between both ports during the session.
v=0
o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0
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c=IN IP4 192.0.0.1
a=group:ALT 1 2
m=audio 20000 RTP/AVP 0
a=mid:1
m=audio 30000 RTP/AVP 0
a=mid:2
4 IANA Considerations
IANA needs to register the following new "semantics" attribute for
the SDP grouping framework [2]:
Semantics Token Reference
------------------- ----- ---------
Alternative ALT [RFCxxxx]
It should be registered in the SDP parameters registry
(http://www.iana.org/assignments/sdp-parameters) under Semantics for
the "group" SDP Attribute.
This document defines a SIP option tag (alt) in Section 2.3. It
should be registered in the SIP parameters registry
(http://www.iana.org/assignments/sip-parameters) under "Option Tags",
with the description below.
A SIP entity that includes the "alt" option tag in a
Supported header field understands the ALT semantics.
5 Security Considerations
An attacker adding group lines using the ALT semantics to an SDP
session description could make an end-point use only one out of all
the streams offered by the remote end, when the intention of the
remote-end might have been to establish all the streams.
An attacker removing group lines using ALT semantics could make and
end-point establish a higher number of media streams. If the end-
point sends media over all of them, the session bandwidth may
increase dramatically.
It is thus STRONGLY RECOMMENDED that integrity protection be applied
to the SDP session descriptions. For session descriptions carried in
SIP [4], S/MIME is the natural choice to provide such end-to-end
integrity protection, as described in RFC 3261. Other applications
MAY use a different form of integrity protection.
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6 Authors' Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Ave
East Hanover, NJ 07936
USA
electronic mail: jdrosen@dynamicsoft.com
7 Normative References
[1] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[2] G. Camarillo, G. Eriksson, J. Holler, and H. Schulzrinne,
"Grouping of media lines in the session description protocol (SDP),"
RFC 3388, Internet Engineering Task Force, Dec. 2002.
[3] S. Bradner, "Key words for use in rfcs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[4] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[5] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
session description protocol (SDP)," RFC 3264, Internet Engineering
Task Force, June 2002.
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