SDP ANAT Semantics: The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework
Let one media stream offer both IPv4 and IPv6 addresses.
Camarillo, G. and J. Rosenberg, "The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework", RFC 4091, DOI 10.17487/RFC4091, June 2005, <https://www.rfc-editor.org/info/rfc4091>.
@misc{rfc4091,
series = {Request for Comments},
number = 4091,
howpublished = {RFC 4091},
publisher = {RFC Editor},
doi = {10.17487/RFC4091},
url = {https://www.rfc-editor.org/info/rfc4091},
author = {G. Camarillo and J. Rosenberg},
title = {{The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework}},
pagetotal = 7,
year = 2005,
month = jun,
} RFC text
Contents
Network Working Group G. Camarillo
Request for Comments: 4091 Ericsson
Category: Standards Track J. Rosenberg
Cisco Systems
June 2005
The Alternative Network Address Types (ANAT) Semantics
for the Session Description Protocol (SDP) Grouping Framework
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines the Alternative Network Address Types (ANAT)
semantics for the Session Description Protocol (SDP) grouping
framework. The ANAT semantics allow alternative types of network
addresses to establish a particular media stream.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Scope and Relation with Interactive Connectivity
Establishment. . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. ANAT Semantics . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Preference . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Offer/Answer and ANAT . . . . . . . . . . . . . . . . . . . . 3
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informational References . . . . . . . . . . . . . . . . 5
1. Introduction
A Session Description Protocol (SDP) [2] session description contains
the media parameters to be used in establishing 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 in transferring media. SDP allows for a set of codecs per
media stream, but only one network address.
The ability 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 for the expression of alternative network addresses (e.g.,
different IP versions) for a particular media stream.
1.1. Scope and Relation with Interactive Connectivity Establishment
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 backward 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 avoid
relegating certain types of addresses (e.g., IPv6 addresses) to only
be a secondary alternate to another address type (e.g., IPv4
addresses).
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. For example, in the session
description in Section 6, the 'm' line with mid=1 has a higher
preference than the 'm' line with mid=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 the highest priority it
understands and set the ports of the rest of the 'm' lines of the
group to zero.
6. Example
The session description below contains an IPv4 address and an IPv6
address grouped using ANAT. The format corresponding to the mapping
of ICE into SDP [6] can be used in both 'm' lines to provide
additional addresses.
v=0
o=bob 280744730 28977631 IN IP4 host.example.com
s=
t=0 0
a=group:ANAT 1 2
m=audio 25000 RTP/AVP 0
c=IN IP6 2001:DB8::1
a= <ICE-encoded additional IPv6 addresses (and ports)>
a=mid:1
m=audio 22334 RTP/AVP 0
c=IN IP4 192.0.2.1
a= <ICE-encoded additional IPv4 addresses (and ports)>
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 an
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 [3]. Other
applications MAY use a different form of integrity protection.
8. IANA Considerations
The IANA has registered the following new 'semantics' attribute for
the SDP grouping framework [4]:
Semantics Token Reference
--------------------------------- ----- ---------
Alternative Network Address Types ANAT [RFC4091]
ANAT has been registered in the SDP parameters registry 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, "Usage of the Session
Description Protocol (SDP) Alternative Network Address Types
(ANAT) Semantics in the Session Initiation Protocol (SIP)", RFC
4092, June 2005.
9.2. Informative References
[6] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols", Work in Progress,
February 2005.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
EMail: jdrosen@cisco.com
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On this page
§1Introduction
The Session Description Protocol (SDP) describes multimedia sessions. For each media stream (audio, video, etc.), an SDP offer specifies the codecs to use and a network address to send to. SDP allows multiple codecs per stream, but RFC 2327 specifies only one network address per 'm' line.
The problem arises when a caller wants to support both IPv4-only endpoints and IPv6-only endpoints. An IPv4-only device cannot accept an IPv6 address, and an IPv6-only device cannot use an IPv4 address. Without a way to offer multiple address types, a single offer cannot work for both.
This RFC defines the ANAT semantics for the SDP grouping framework. ANAT lets a session description group multiple media lines, each with a different network address type, to represent alternatives for the same logical media stream.
A video call setup document normally says 'send video to this address'. But what if some people can only reach address type A and others only address type B? ANAT lets you say 'try address A first, but if that doesn't work, try address B'.
§2ANAT and the Grouping Framework
The SDP grouping framework [RFC 3388] lets you bundle several media lines into a semantic group. Each group has a 'semantics' attribute that describes the relationship. ANAT defines a new semantics value: ANAT.
Media lines grouped with ANAT semantics all represent alternative network addresses for a single logical media stream. The entity generating the offer MUST be ready to receive (or send) media over any of the grouped 'm' lines. However, ANAT MUST NOT group lines whose network addresses are of the same type (such as two IPv4 addresses)—that is what the ICE protocol is for (see below).
The grouping framework is like a label that says 'these lines go together'. ANAT is a label that says 'these are all ways to reach the same place, just different address types'.
§3Scope and Relation to ICE
ANAT addresses different network address types (IPv4 vs IPv6). It is not designed for alternative transport addresses of the same type. Systems that need multiple addresses of the same type (for NAT traversal or redundancy) should use ICE (Interactive Connectivity Establishment) [RFC 5245] instead.
ICE encodes the primary address in the 'm' line and additional addresses as parameters below it. Endpoints that do not support ICE ignore those parameters and use only the 'm' line address. ANAT uses a different approach: it groups separate 'm' lines with different address types at the media grouping level, keeping IPv6 syntax separate from IPv4-specific NAT parameters. This yields syntax closer to vanilla SDP and avoids relegating IPv6 addresses to secondary status.
Think of ICE as 'here's my main address, plus some backup ones'. ANAT is 'here are completely different address types to choose from'. They serve different purposes.
§4Preference and Offer/Answer
When an offerer includes an ANAT group, the media lines in the group are listed in order of preference. The line listed first has the highest priority. An answerer receiving an ANAT group SHOULD use the address with the highest priority it understands, then set the ports of all other grouped lines to zero (disabling them).
When using ANAT with SIP, the offerer SHOULD include the sdp-anat option-tag in a Require header, signaling that ANAT support is needed.
The order in the group list means 'try this one first'. If the other endpoint understands that type, great. If not, it picks the next one it can use and ignores the rest.
§5Example
Here is a session description with an IPv6 address and an IPv4 address grouped using ANAT:
v=0o=alice 280744730 28977631 IN IP4 host.example.coms=t=0 0a=group:ANAT 1 2m=audio 25000 RTP/AVP 0c=IN IP6 2001:DB8::1a=mid:1m=audio 22334 RTP/AVP 0c=IN IP4 192.0.2.1a=mid:2
The a=group:ANAT 1 2 line groups the two media streams. The first line (mid=1) with the IPv6 address has higher preference. Both lines can include additional ICE addresses (a= lines) if needed.
The SDP says 'I'm offering audio at either IPv6 address 2001:DB8::1 or IPv4 address 192.0.2.1'. The numbering (1 2) means try the IPv6 one first.