MMUSIC Working Group G. Camarillo
Internet-Draft Ericsson
Expires: November 30, 2004 J. Rosenberg
dynamicsoft
June 1, 2004
The Alternative Network Address Types Semantics (ANAT) for the
Session Description Protocol (SDP) Grouping Framework
draft-ietf-mmusic-anat-01.txt
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Abstract
This document defines the Alternative Network Address Types (ANAT)
semantics for the SDP grouping framework. The ANAT semantics allow
offering alternative types of network addresses to establish a
particular media stream.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Scope and Relation with ICE . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. ANAT Semantics . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Preference . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Offer/Answer and ANAT . . . . . . . . . . . . . . . . . . . . 4
5.1 ANAT and Media Configurations . . . . . . . . . . . . . . 4
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 6
9.2 Informational References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
An SDP [2] 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 network addresses and the codec to be used to transfer media. SDP
allows providing a set of codecs per media stream, but only one
network address.
Being able to offer a set of network addresses to establish a media
stream is useful in environments with both IPv4-only hosts and
IPv6-only hosts, for instance.
This document defines the Alternative Network Address Types (ANAT)
semantics for the SDP grouping framework [4]. The ANAT semantics
allow expressing alternative network addresses (e.g., different IP
versions) for a particular media stream.
1.1 Scope and Relation with ICE
The ANAT semantics are intended to address scenarios that involve
different network address types (e.g., different IP versions). They
are not intended to provide alternative transport addresses with the
same network type. Systems that need to provide different transport
addresses with the same network type 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 (ANAT 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.
Additionally, ICE only allows us to provide a single primary address
when the peer does not support ICE. The ANAT semantics avoids
relegating addresses of a certain type (e.g., IPv6 addresses) to just
be a secondary alternate to another address type (e.g., IPv4
addresses).
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Furthermore, the separation between ANAT and ICE helps systems that
support IPv4 and IPv6 but that do not need to support ICE (e.g., a
multicast server).
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 [1] and indicate requirement levels for
compliant implementations.
3. ANAT Semantics
We define a new ``semantics'' attribute within the SDP grouping
framework [4]: ANAT (Alternative Network Address Types).
Media lines grouped using ANAT semantics provide alternative network
addresses of different types for a single logical media stream. The
entity creating a session description with an ANAT group MUST be
ready to receive (or send) media over any of the grouped m lines. The
ANAT semantics MUST NOT be used to group media streams whose network
addresses are of the same type.
4. Preference
The entity generating a session description may have an order of
preference for the alternative network address types 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:ANAT 1 2
5. Offer/Answer and ANAT
An offerer using SIP [3] to send its offer SHOULD place the sdp-anat
option-tag [5] in a Require header field.
An answerer receiving a session description that uses the ANAT
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.
5.1 ANAT and Media Configurations
The creator of a session description MAY want to use different media
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configurations (e.g., audio codec) for different network addresses in
the same ANAT 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.
6. Example
The session description below contains an IPv4 address and an IPv6
address grouped using ANAT.
v=0
o=bob 280744730 28977631 IN IP4 host.example.com
s=
t=0 0
a=group:ANAT 1 2
m=audio 6886 RTP/AVP 0
c=IN IP6 2001:0600::1
a=mid:1
m=audio 22334 RTP/AVP 0
c=IN IP4 192.0.2.2
a=mid:2
7. Security Considerations
An attacker adding group lines using the ANAT 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 ANAT 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 [3], 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.
8. IANA Considerations
IANA needs to register the following new ``semantics'' attribute for
the SDP grouping framework [4]:
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Semantics Token Reference
----------------------- ----- ---------
Alternative Network Address Types ANAT [RFCxxxx]
It should be registered in the SDP parameters registry (http://
www.iana.org/assignments/sdp-parameters) under Semantics for the
"group" SDP Attribute.
9. References
9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[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] Camarillo, G., Eriksson, G., Holler, J. and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[5] Camarillo, G. and J. Rosenberg, "The sdp-anat Session Initiation
Protocol (SIP) Option-Tag",
draft-camarillo-sip-anat-option-tag-00.txt (work in progress),
April 2004.
9.2 Informational References
[6] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols",
draft-ietf-mmusic-ice-01 (work in progress), February 2004.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
EMail: jdrosen@dynamicsoft.com
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