Internet Engineering Task Force SIP WG Internet Draft G. Camarillo Ericsson J. Rosenberg dynamicsoft draft-camarillo-mmusic-alt-02.txt October 17, 2003 Expires: April 2004 The Alternative IP Versions Semantics for the Session Description Protocol Grouping Framework STATUS OF THIS MEMO 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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract 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. G. Camarillo et. al. [Page 1] Internet Draft SIP August 18, 2003 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 G. Camarillo et. al. [Page 2] Internet Draft SIP August 18, 2003 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 G. Camarillo et. al. [Page 3] Internet Draft SIP August 18, 2003 In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 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 lines. 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. G. Camarillo et. al. [Page 4] Internet Draft SIP August 18, 2003 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. v=0 o=bob 280744730 28977631 IN IP4 host.example.com s= t=0 0 a=group:IPV 1 2 m=audio 6886 RTP/AVP 0 c=IN IP6 2201:056D::112E:144A:1E24 a=mid:1 m=audio 22334 RTP/AVP 0 c=IN IP4 192.0.2.2 a=mid:2 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 G. Camarillo et. al. [Page 5] Internet Draft SIP August 18, 2003 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 Ericsson Advanced Signalling Research Lab. FIN-02420 Jorvas Finland electronic mail: Gonzalo.Camarillo@ericsson.com Jonathan Rosenberg dynamicsoft 72 Eagle Rock Ave East Hanover, NJ 07936 USA 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 G. Camarillo et. al. [Page 6] Internet Draft SIP August 18, 2003 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 2002. 11 Informative References The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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