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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







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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).



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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.  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.









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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.













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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.










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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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RFC 4091                     ANAT Semantics                    June 2005


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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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