[Search] [txt|pdf|bibtex] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02                                                      
Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                            J. Rosenberg
June 17, 2003
Expires: December 2003

                The Alternative 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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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

   To view the list Internet-Draft Shadow Directories, see


   This document defines the alternative (ALT) semantics for the SDP
   grouping framework. The ALT semantics allow offering alternative
   transport addresses to establish a particular media stream. This is
   useful in scenarios that involve IPv4 and IPv6 and/or network address

G. Camarillo et. al.                                          [Page 1]

Internet Draft                    SIP                      June 17, 2003

                           Table of Contents

   1          Introduction ........................................    3
   1.1        Scope ...............................................    3
   1.2        Terminology .........................................    4
   2          ALT Semantics .......................................    4
   2.1        Preference ..........................................    4
   2.2        Media Stream Establishment Attempts .................    4
   2.2.1      ICE .................................................    5
   2.2.2      Public Addresses ....................................    5
   2.3        Backward Compatibility and the "ice" SIP Option
              Tag .................................................    5
   2.4        ALT and Media Configurations ........................    6
   3          Example .............................................    6
   4          IANA Considerations .................................    7
   5          Security Considerations .............................    7
   6          Authors' Addresses ..................................    8
   7          Normative References ................................    8
   8          Informative References ..............................    9

G. Camarillo et. al.                                          [Page 2]

Internet Draft                    SIP                      June 17, 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 provide a set of transport addresses to be tried to
   establish a media stream is useful when a system cannot determine its
   own transport address as seen from the remote end. This happens when
   the end-points support both IPv4 and IPv6 or when there is one or
   more NATs (Network Address Translator) between them. In these cases,
   although the end-point cannot provide a definitive transport address
   to be used for the session at establishment time, it can provide a
   set of possible candidates. Those candidates include local IPv4 and
   IPv6 addresses and addresses obtained using STUN [7], RSIP [8] and/or
   TURN [9].

   This document defines the alternative (ALT) semantics for the SDP
   grouping framework [2]. The ALT semantics allow expressing
   alternative transport addresses for a particular media stream.

1.1 Scope

   The ALT semantics are intended to address scenarios where a system
   cannot determine its own transport address as seen from the remote
   end but can provide a set of possible candidates. When a transport
   address that lets the end points communicate to each other is found,
   it is supposed to be used for the rest of the session.

   Therefore, the set of addresses grouped with ALT is not intended to
   be used as a fail-over mechanism (in case the address in use becomes
   unreachable). Maintaining some addresses (e.g., addresses obtained
   using TURN) throughout the duration of the session would imply to
   keep state in the network that would be used only in case of failure.

   ALT is not intended to address the pick-one-codec-out-of-N problem
   either. In the offer/answer model [3], when an offerer provides a set
   of codecs for a media stream, the answerer can use any of them at a
   given time. Therefore, the offerer needs to be ready to receive any
   of those codecs during the whole session. The offer/answer model does
   not have the concept of offering a set of codecs so that the answerer
   chooses one and sticks to it for the duration of the session.

   New semantics (different than ALT) may be developed in the future to
   address the fail-over and the pick-one-codec-out-of-N problems.

G. Camarillo et. al.                                          [Page 3]

Internet Draft                    SIP                      June 17, 2003

1.2 Terminology

   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 ALT Semantics

   We define a new "semantics" attribute within the SDP grouping
   framework [2]: ALT (Alternative).

   Media lines grouped using ALT semantics provide alternative transport
   addresses for a single logical media stream. The entity receiving a
   session description with an ALT group MUST be ready to receive media
   over any of the grouped m lines until connectivity is achieved.

2.1 Preference

   The entity generating a session description may have an order of
   preference for all the alternative transport addresses offered. We
   define the q media level attribute to provide this information.

        q     =  "a=q:" qvalue

   The q attribute contains a qvalue, as defined in RFC 3261 [5]:

     qvalue   =  ( "0" [ "." 0*3DIGIT ] )
                 / ( "1" [ "." 0*3("0") ] )

   The qvalue reflects the desire that the end point has to receive
   media on that address, and is assigned as a value from 0 to 1 (1
   being most preferred). For example:


2.2 Media Stream Establishment Attempts

   An entity receiving a set of streams grouped using ALT semantics
   cannot assume that it will be possible to successfully use all the
   alternative transport addresses offered. Some of the m lines may

G. Camarillo et. al.                                          [Page 4]

Internet Draft                    SIP                      June 17, 2003

   contain transport addresses that are unreachable for the recipient of
   the session description.

   Such entity needs to discover which is the transport address with the
   highest preference (i.e., highest qvalue) that can be used for the
   session. There are two mechanisms to perform this discovery; ICE
   (Interactive Connectivity Establishment) [6] and public addresses.

2.2.1 ICE

   ICE is intended to be used within the offer/answer model [3]. If the
   media lines grouped with ALT in an offer contain "a=stun" attributes,
   the answerer MUST use ICE [6] to determine the particular transport
   address to be used. This will involve sending STUN requests in
   parallel to all the transport addresses, as described in ICE.

2.2.2 Public Addresses

   The public addresses mechanism MUST only be used if the generator of
   the session description can ensure somehow that the addresses that
   appear in the session description are all public IP addresses. For
   example, a server could group a multicast IPv4 address and a
   multicast IPv6 address using ALT and distribute the session
   description using a web page, email or SAP [10].

   An entity receiving a session description with an ALT group without
   "a=stun" lines SHOULD try to establish the m line with higher
   preference whose address understands. How an m line is established
   depends on the type of the media stream. Establishing an RTP-based m
   line involves sending or waiting for RTP or RTCP packets.

   It is RECOMMENDED) that the public address mechanism is only used
   with multicast addresses or when the remote party does not support

2.3 Backward Compatibility and the "ice" SIP Option Tag

   The receiver of a session description with an ALT group is supposed
   to establish only one media stream. However, if the entity receiving
   such a session description does not understand the ALT semantics or
   the grouping framework, it will establish all the streams of the ALT
   group. If this entity sends media in parallel over all the streams at
   the same time, the resulting session bandwidth will be much higher
   than the expected by the creator of the session. If this entity
   listens to all the streams (e.g., a session multicast on an IPv4 and
   an IPv6 address), the incoming bandwidth as seen by that entity will
   be equally high. The ALT semantics MUST NOT be used when this
   situation is unacceptable.

G. Camarillo et. al.                                          [Page 5]

Internet Draft                    SIP                      June 17, 2003

   Note, that if the offer/answer model [3] is being used the offerer
   will notice that the answerer did not understand ALT upon reception
   of the answer, as described in RFC 3388 [2]. Such an offerer MAY
   reissue a new offer with only one m line immediately.

   The use of ICE requires further considerations. If an answerer that
   does not support ICE receives an m line with a private address and
   with an "a=stun" line it will try to send media packets directly to
   that address (i.e., it will not use STUN). Those media packets might
   arrive to an end-point that happens to use the same private IP
   address as the offerer but is in a different realm.

   Therefore, the offerer MUST ensure that the answerer supports ICE
   before using private IP addresses in a session description. In
   scenarios where SIP [5] and the offer/answer model [3] are involved,
   the "ice" SIP option tag (defined below) MUST be used to ensure that
   the answerer understands the ALT semantics and supports ICE.
   Therefore, we define the option tag "ice" for use in the Require and
   Supported header fields. A SIP entity that includes the "ice" option
   tag in a Supported header field understands the ALT semantics defined
   in this document and supports ICE as defined in [6].

2.4 ALT 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 ALT 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 receiver follows the steps described in
   Section 2.2 using only those m lines that were found, in principle,

   If the session description is part of an offer/answer exchange, the
   answerer will, following normal SIP procedures, set their ports to
   zero in the answer.

3 Example

   An answerer receiving the offer below will start the ICE procedures
   using and

      o=bob 280744730 28977631 IN IP4 host2.example.com
      t=0 0

G. Camarillo et. al.                                          [Page 6]

Internet Draft                    SIP                      June 17, 2003

      a=group:ALT 1 2
      m=audio 6886 RTP/AVP 0
      c=IN IP4
      a=stun:user asd8866
      m=audio 22334 RTP/AVP 0
      c=IN IP4
      a=stun:user asd8866

4 IANA Considerations

   IANA needs to register the following new "semantics" attribute for
   the SDP grouping framework [2]:

   Semantics                  Token      Reference
   -------------------        -----      ---------
   Alternative                ALT        [RFCxxxx]

   It should be registered in the SDP parameters registry
   (http://www.iana.org/assignments/sdp-parameters) under Semantics for
   the "group" SDP Attribute.

   This document defines a SIP option tag (alt) in Section 2.3. It
   should be registered in the SIP parameters registry
   (http://www.iana.org/assignments/sip-parameters) under "Option Tags",
   with the description below.

        A SIP entity that includes the "ice" option tag in a
        Supported header field understands the ALT semantics and
        supports ICE.

5 Security Considerations

   An attacker adding group lines using the ALT 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 ALT 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

G. Camarillo et. al.                                          [Page 7]

Internet Draft                    SIP                      June 17, 2003

   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.

6 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

7 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.

   [3] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   session description protocol (SDP)," RFC 3264, Internet Engineering
   Task Force, June 2002.

   [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

   [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 8]

Internet Draft                    SIP                      June 17, 2003

   Engineering Task Force, Feb.  2003.  Work in progress.

8 Informative References

   [7] J. Rosenberg, J. Weinberger, C. Huitema, and R. Mahy, "STUN -
   simple traversal of user datagram protocol (UDP) through network
   address translators (nats)," RFC 3489, Internet Engineering Task
   Force, Mar. 2003.

   [8] M. Borella, D. Grabelsky, J. Lo, and K. Taniguchi, "Realm
   specific IP:  protocol specification," RFC 3103, Internet Engineering
   Task Force, Oct.  2001.

   [9] J. Rosenberg, "Traversal using relay NAT (TURN)," internet draft,
   Internet Engineering Task Force, Mar. 2003.  Work in progress.

   [10] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
   protocol," RFC 2974, Internet Engineering Task Force, Oct. 2000.

   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. Copies of
   claims of rights made available for publication 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 implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard. Please address the information to the IETF Executive

   Full Copyright Statement

   Copyright (c) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published

G. Camarillo et. al.                                          [Page 9]

Internet Draft                    SIP                      June 17, 2003

   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

G. Camarillo et. al.                                         [Page 10]