Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
J. Rosenberg
dynamicsoft
draft-camarillo-mmusic-alt-02.txt
October 17, 2003
Expires: April 2004
The Alternative IP Versions Semantics for the
Session Description Protocol Grouping Framework
STATUS OF THIS MEMO
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Abstract
This document defines the alternative IP versions (IPV) semantics for
the SDP grouping framework. The IPV semantics allow offering
alternative transport addresses that use different IP versions to
establish a particular media stream.
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Table of Contents
1 Introduction ........................................ 3
1.1 Scope and Relation with ICE ......................... 3
1.2 Terminology ......................................... 3
2 IPV Semantics ....................................... 4
3 Preference .......................................... 4
4 Offer/Answer and IPV ................................ 4
4.1 IPV and Media Configurations ........................ 4
5 Backwards Compatibility ............................. 5
6 Example ............................................. 5
7 IANA Considerations ................................. 5
8 Security Considerations ............................. 6
9 Authors' Addresses .................................. 6
10 Normative References ................................ 6
11 Informative 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 a set of codecs per media stream, but only one
transport address.
Being able to offer a set of transport addresses to establish a media
stream is useful in environments with both IPv4-only hosts and IPv6-
only hosts.
This document defines the alternative IP versions (IPV) semantics for
the SDP grouping framework [2]. The IPV semantics allow expressing
alternative transport addresses with different IP versions for a
particular media stream.
1.1 Scope and Relation with ICE
The IPV semantics are intended to address scenarios that involve
different IP versions. They are not intended to provide alternative
transport addresses with the same IP version. Systems that need to
provide different transport addresses with the same IP version should
use the SDP format defined in ICE (Interactive Connectivity
Establishment) [6] instead.
ICE is used by systems that cannot determine their own transport
address as seen from the remote end but that can provide several
possible alternatives. ICE encodes the address that is most likely to
be valid in an m= line and the rest of addresses as a= lines after
that m= line. This way, systems that do not support ICE simply ignore
the a= lines and only use the address in the m= line. This achieves
good backwards compatibility.
We have chosen to group m= lines with different IP versions at the m=
level (IPV semantics) rather than at the a= level (ICE format) in
order to keep the IPv6 syntax free from ICE parameters used for
legacy (IPv4) NATs (Network Address Translators). This yields a
syntax much closer to vanilla SDP, where IPv6 addresses are defined
in their own m= line, rather than in parameters belonging to a
different m= line.
In addition to that, the separation between IPV and ICE helps systems
that support IPv4 and IPv6 but that do not need to support ICE (e.g.,
a multicast server).
1.2 Terminology
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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 [4] and
indicate requirement levels for compliant SIP implementations.
2 IPV Semantics
We define a new "semantics" attribute within the SDP grouping
framework [2]: IPV (Alternative IP Versions).
Media lines grouped using IPV semantics provide alternative transport
addresses with different IP versions for a single logical media
stream. The entity creating a session description with an IPV group
MUST be ready to receive (or send) media over any of the grouped m
lines.
3 Preference
The entity generating a session description may have an order of
preference for the alternative IP versions 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.
a=group:IPV 1 2
4 Offer/Answer and IPV
When ICE is used, the ICE spec [6] explains how to choose a
particular IP address among all the alternatives received. When ICE
is not used, an answerer receiving a session description that uses
the IPV semantics SHOULD use the address with highest priority it
understands and set the ports of the rest of the m= lines of the
group to zero.
4.1 IPV and Media Configurations
The creator of a session description MAY want to use different media
configurations (e.g., audio codec) for different transport addresses
in the same IPV group. The receiver of such a session may find some
of the m lines unacceptable. They may contain codecs that the
answerer does not support or contain any other parameter that makes
them unacceptable. The answerer should, following normal SIP
procedures, set their ports to zero in the answer.
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5 Backwards Compatibility
IPv4-only and IPv6-only systems would only understand one of m= lines
of the IPV group. Therefore, they will not have any problem
establishing sessions that use IPV.
It is STRONGLY RECOMMENDED that dual-stack IPv6/IPv4 hosts implement
the IPV semantics. Dual-stack hosts that failed to implement IPV
would need more RTTs to establish a session with a single-stack host.
When acting as answerers, they would establish more media streams
than needed. This could increase the session bandwidth in the first
instants of the session, until the remote end could issue a new offer
with only one m= line.
6 Example
The session description below contains an IPv4 address and an IPv6
address grouped using IPV.
v=0
o=bob 280744730 28977631 IN IP4 host.example.com
s=
t=0 0
a=group:IPV 1 2
m=audio 6886 RTP/AVP 0
c=IN IP6 2201:056D::112E:144A:1E24
a=mid:1
m=audio 22334 RTP/AVP 0
c=IN IP4 192.0.2.2
a=mid:2
7 IANA Considerations
IANA needs to register the following new "semantics" attribute for
the SDP grouping framework [2]:
Semantics Token Reference
----------------------- ----- ---------
Alternative IP Versions IPV [RFCxxxx]
It should be registered in the SDP parameters registry
(http://www.iana.org/assignments/sdp-parameters) under Semantics for
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the "group" SDP Attribute.
8 Security Considerations
An attacker adding group lines using the IPV 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 IPV 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 [5], 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.
9 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
10 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.
[6] J. Rosenberg, "Interactive connectivity establishment (ICE): a
methodology for nettwork address translator (NAT) traversal for the
session initiation protocol (SIP)," internet draft, Internet
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Engineering Task Force, July 2003. Work in progress.
[4] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[5] 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.
11 Informative References
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