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Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                            J. Rosenberg
October 17, 2003
Expires: April 2004

            The Alternative IP Versions Semantics for the
           Session Description Protocol Grouping Framework


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   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",
   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

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.

      o=bob 280744730 28977631 IN IP4 host.example.com
      t=0 0
      a=group:IPV 1 2
      m=audio 6886 RTP/AVP 0
      c=IN IP6 2201:056D::112E:144A:1E24
      m=audio 22334 RTP/AVP 0
      c=IN IP4

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
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   electronic mail:  Gonzalo.Camarillo@ericsson.com

   Jonathan Rosenberg
   72 Eagle Rock Ave
   East Hanover, NJ 07936
   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

11 Informative References

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