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Network Working Group                                       G. CamarilloRequest for Comments: 4091                                      EricssonCategory: Standards Track                                   J. Rosenberg                                                           Cisco Systems                                                               June 2005The Alternative Network Address Types (ANAT) Semanticsfor the Session Description Protocol (SDP) Grouping FrameworkStatus 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .2       1.1.  Scope and Relation with Interactive Connectivity             Establishment. . . . . . . . . . . . . . . . . . . . . .22.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .33.  ANAT Semantics . . . . . . . . . . . . . . . . . . . . . . . .34.  Preference . . . . . . . . . . . . . . . . . . . . . . . . . .35.  Offer/Answer and ANAT  . . . . . . . . . . . . . . . . . . . .36.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .47.  Security Considerations  . . . . . . . . . . . . . . . . . . .48.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .59.  References . . . . . . . . . . . . . . . . . . . . . . . . . .59.1.  Normative References . . . . . . . . . . . . . . . . . .59.2.  Informational References . . . . . . . . . . . . . . . .5Camarillo & Rosenberg       Standards Track                     [Page 1]

RFC 4091 ANAT Semantics June 20051. 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 inBCP 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 inSection 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 20056.  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:27.  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 inRFC 3261 [3].  Other   applications MAY use a different form of integrity protection.Camarillo & Rosenberg       Standards Track                     [Page 4]

RFC 4091                     ANAT Semantics                    June 20058.  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.  References9.1.  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)",RFC4092, 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 2005Authors' 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.comCamarillo & Rosenberg       Standards Track                     [Page 6]

RFC 4091                     ANAT Semantics                    June 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Camarillo & Rosenberg       Standards Track                     [Page 7]

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