RFC 4091

RFC 4091: The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework 🦋

okay bestie, let's get into RFC 4091 ✨ It's a Standards Track document from the Network Working Group, published in June 2005. The authors are G. Camarillo (Ericsson) and J. Rosenberg (Cisco Systems). It doesn't list any RFCs it obsoletes or updates, so it stands on its own 💅

Status of This Memo 📋

This document is an Internet standards track protocol for the Internet community, and it asks for discussion and suggestions for improvements. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). 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. ANAT lets alternative types of network addresses be used to establish one particular media stream. It's giving flexibility 🧚

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 💖

okay so an SDP [2] session description holds the media parameters for setting up a bunch of media streams. For each media stream, it includes things like the network addresses and the codec used to send the media. SDP lets you have a set of codecs per media stream, but only one network address 😭

Being able to offer a set of network addresses for a media stream is super useful, for example in environments with both IPv4-only hosts and IPv6-only hosts 🫶

This document defines the ANAT semantics for the SDP grouping framework [4]. ANAT lets you express alternative network addresses (for example, different IP versions) for one particular media stream ✨

1.1. Scope and Relation with Interactive Connectivity Establishment 🎀

ANAT is meant for scenarios with different network address types (like different IP versions). It is not meant to give alternative transport addresses that share the same network type. If a system needs different transport addresses on the same network type, it should use the SDP format defined in ICE (Interactive Connectivity Establishment) [6] instead 💅

ICE is for systems that can't figure out their own transport address as seen from the remote end, but that can come up with several possible options. ICE puts the address most likely to be valid in an 'm' line, and the rest of the addresses go in a= lines right after that 'm' line. That way, systems that don't support ICE just ignore the a= lines and use the address in the 'm' line. It's giving great backward compatibility 🦋

We chose to group 'm' lines with different IP versions at the 'm' level (ANAT) instead of at the a= level (ICE format). The reason is to keep the IPv6 syntax free from ICE parameters used for legacy (IPv4) NATs (Network Address Translators). This gives a syntax much closer to vanilla SDP, where IPv6 addresses get their own 'm' line instead of living in parameters that belong to a different 'm' line 🌈

ICE also only lets you provide a single primary address when the peer doesn't support ICE. ANAT avoids pushing certain address types (like IPv6 addresses) down to being just a secondary alternate to another address type (like IPv4 addresses). Iconic behaviour 👑

Finally, keeping ANAT and ICE separate helps systems that support IPv4 and IPv6 but don't need ICE, like 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. Those capitalised words are the ones that carry the real weight, bestie 💜

3. ANAT Semantics 🌟

We define a new "semantics" attribute within the SDP grouping framework [4], called ANAT (Alternative Network Address Types) 🎀

Media lines grouped with ANAT semantics give alternative network addresses of different types for one 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. ANAT semantics MUST NOT be used to group media streams whose network addresses are of the same type. No mixing those up, periodt 🙈

4. Preference 🍓

The entity generating a session description may have an order of preference for the alternative network address types it offers. 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. So it picks the best one it can actually use and zeroes out the others 😤

6. Example 🎬

This session description has an IPv4 address and an IPv6 address grouped with ANAT. The format for mapping ICE into SDP [6] can be used in both 'm' lines to give extra 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 🚨

⚠️ This part is seriously important, bestie, so read it twice 😭

An attacker who adds group lines using the ANAT semantics to an SDP session description could make an end-point use only one out of all the streams the remote end offered, even though the remote end probably wanted all of them to be established 💀

An attacker who removes 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 🔥

So it is strongly RECOMMENDED that integrity protection be applied to SDP session descriptions. For session descriptions carried in SIP [3], S/MIME is the natural choice for that end-to-end integrity protection, as described in RFC 3261 [3]. Other applications MAY use a different form of integrity protection 🫶

8. IANA Considerations 🗂️

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. Registry goals, bestie 🌈

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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