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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

Camarillo & Rosenberg Standards Track [Page 1]  RFC 4091 ANAT Semantics June 2005

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).

Camarillo & Rosenberg Standards Track [Page 2]  RFC 4091 ANAT Semantics June 2005

 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.

Camarillo & Rosenberg Standards Track [Page 3]  RFC 4091 ANAT Semantics June 2005

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.

Camarillo & Rosenberg Standards Track [Page 4]  RFC 4091 ANAT Semantics June 2005

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.

Camarillo & Rosenberg Standards Track [Page 5]  RFC 4091 ANAT Semantics June 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

Camarillo & Rosenberg Standards Track [Page 6]  RFC 4091 ANAT Semantics June 2005

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Camarillo & Rosenberg Standards Track [Page 7]