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Versions: 00 01                                                         
Internet Engineering Task Force                     Gonzalo Camarillo
Internet draft                                             Jan H÷ller
                                                    G÷ran AP Eriksson
                                                            June 2000
                                                   Expires March 2000

                       SDP media alignment in SIP

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

   The list of current Internet-Drafts can be accessed at
   The list of Internet-Draft Shadow Directories can be accessed at


   This document defines an SDP media attribute. This attribute is
   intended to be used in conjunction with SIP in order to align
   different media streams belonging to a session. The use of this
   attribute allows sending media from a single media stream, encoded
   in different formats during the session, to different ports and host

1. Introduction

   SIP [1] is an application layer protocol for establishing,
   terminating and modifying multimedia sessions. SIP carries session
   descriptions in the bodies of the SIP messages but is independent
   from the protocol used for describing sessions. SDP [2] is one of
   the protocols that can be used for this purpose.

   Appendix B of [1] describes the usage of SDP in relation to SIP. It
   states: "The caller and callee align their media description so that
   the nth media stream ("m=" line) in the callerÆs session description
   corresponds to the nth media stream in the calleeÆs description."

Camarillo/H÷ller/Eriksson                                            1

                      SDP media alignment in SIP

   This way of performing the media alignment implies that all the
   media from one single media stream has to be sent to the same port
   number. This does not suit fine systems that handle different codecs
   in different port numbers. Nor does it suit hosts supporting
   multiple interfaces that handle different codecs on different
   interface types.

2. SIP and cellular access

   Systems using a cellular access (such as UMTS or EDGE) and SIP as a
   signalling protocol need to receive media over the air. During a
   session the media can be encoded using different codecs. The encoded
   media has to traverse the radio interface. The radio interface is
   generally characterized by being bit error prone and associated with
   relatively high packet transfer delays. In addition, radio interface
   resources in a cellular environment are scarce and thus expensive,
   which calls for special measures in providing a highly efficient
   transport [3]. In order to get an appropriate speech quality in
   combination with an efficient transport, precise knowledge of codec
   properties are required so that a proper radio bearer for the RTP
   session can be configured before transferring the media. These radio
   bearers are dedicated bearers per media type, i.e. codec.

   In UMTS, for instance, when the RTP packets shall be delivered over
   the air interface, a packet filtering function routes the packets to
   the proper radio bearer towards the UMTS/SIP terminal. The packet
   filtering function operates using a Traffic Flow Template (TFT) [4],
   which is established when configuring the radio bearer. The TFT
   hence specifies the profile of the data that should be carried by
   the radio bearer. A TFT can contain the following data:

   -Source Address and Subnet Mask.
   -Protocol Number (IPv4) / Next Header (IPv6).
   -Destination Port Range.
   -Source Port Range.
   -IPSec Security Parameter Index (SPI).
   -Type of Service (TOS) (IPv4) / Traffic class (IPv6) and Mask.
   -Flow Label (IPv6).

   It is worth noticing that just certain combinations of these
   parameters are allowed.

   The media has to have different destination port numbers for the
   different possible codecs in order to be filtered and routed
   properly to the correct radio bearer. Examining the payload of every
   packet entering the UMTS network would require too much processing
   power. It would not a viable solution. Demultiplexing of media
   streams should be done at the port level.

   So, when a session is negotiated using SIP, a means for providing
   different port numbers for different codecs is needed. This
   procedure has to be backwards compatible with appendix B of [1].

Camarillo/H÷ller/Eriksson                                            2

                      SDP media alignment in SIP

   Systems using certain architectures have also this requirement in
   order to be able to use different audio libraries. Discussions about
   this can be found in [5].

3. SIP, SDP and multiple host interfaces

   Two typical scenarios apply where a single media stream may be
   received or sent on different host interfaces. The first is when the
   cellular device is equipped with several access technologies. The
   second is the case where media format transcoding may be needed.

   Cellular terminals are traditionally equipped with only one access
   type, the cellular access. Cellular terminals may also be further
   equipped with other higher speed wireless access technologies of
   short range nature such as Wireless LAN and Bluetooth.

   Multiple accesses on the terminal implies that multiple IP addresses
   will be in use for a single terminal.

   Applications based on SIP may rely on the fact that these additional
   interfaces may also be available for use, but of a more sporadic
   nature. The availability and applicability may for instance be based
   on coverage or simply by the userÆs preferences.

   In a session invitation, the caller (on a mobile SIP terminal) may
   be willing to receive a single media stream that could be delivered
   on one of the available interfaces. A typical case could be where a
   stream of high quality audio can be supported over a high bit rate
   interface such as WLAN, and a low rate codec is used over the
   cellular access. Depending on the particular situation, the caller
   may offer both codec types, whereas the callee may select only one
   of the codec types, e.g. depending on what he supports.

   If the caller moves out of coverage of the high bit rate access, an
   ordinary re-invitation may move the media stream to be transported
   over the low bit rate cellular access.

   The second scenario concerns the need for a transcoding service.

   Speech codecs have been developed basically to be robust against the
   high bit error and frame error rate associated with wireless
   accesses. One such codec is GSM. Other encoding formats developed
   for wired accesses with negligible bit errors do not operate
   properly over a lossy link.

   As fixed terminals may not support the wireless codecs, while
   cellular terminals can generally not support wired codecs, there is
   a need for a transcoding function between the two.

   The transcoding function is typically placed in another host
   connected to the internet. This host is placed on the other side of
   the cellular access as viewed from the cellular terminal.

Camarillo/H÷ller/Eriksson                                            3

                      SDP media alignment in SIP

   To make use of the transcoding function, the cellular terminal may
   send an INVITE with an SDP part displaying support for the cellular
   codec as well as support for the codecs supported by the transcoder.
   In this situation, there is one media stream that depending on which
   codec is selected, should be directed to different host interfaces.
   The callee then makes a selection of codec and sends the media to
   the appropriate host interface. The transcoder will also be
   configured to forward the transcoded media stream to the cellular

4. Alignment of SDP in SIP to support multiple port numbers

   A new "flow identification" media attribute is defined. It is used
   for identifying media streams within a session. It provides a means
   for aligning a number of flows within a session between members
   participating in the session. Its formatting in SDP is described by
   the following BNF:

         fid-attribute      = "a=fid:" identification-tag
         identification-tag = token

   The identification tag is unique within the SDP session description.
   The following example illustrates its usage.

5. Example of flow identification attribute

   In the following example John uses a traditional access such as an
   ethernet while Laura has a UMTS/SIP terminal. The caller John sends
   the following session description to the callee Laura.

         o=John 289085535 289085535 IN IP4 first.example.com
         t=0 0
         c=IN IP4
         m=audio 20000 RTP/AVP 0 8
         m=audio 20002 RTP/AVP 0 8

   The callee Laura is on a UMTS/SIP terminal. She configures the
   necessary radio bearers and implements the TFTs:

   For the first media stream with flow identification equal to 1

   All the incoming IP packets with destination port UDP 30000 will be
   carried by the radio access bearer configured for G-711 u-law
   (payload type 0).

   All the incoming IP packets with destination port UDP 30002 will be
   carried by the radio access bearer configured for G-711 A-law
   (payload type 8).

Camarillo/H÷ller/Eriksson                                            4

                      SDP media alignment in SIP

   For the second media stream with flow identification equal to 2

   All the incoming IP packets with destination port UDP 30004 will be
   carried by the radio access bearer configured for G-711 u-law
   (payload type 0).

   All the incoming IP packets with destination port UDP 30006 will be
   carried by the radio access bearer configured for G-711 A-law
   (payload type 8).

   Accordingly, the following SDP is returned to the caller:

         o=Laura 289083124 289083124 IN IP4 second.example.com
         t=0 0
         c=IN IP4
         m=audio 30000 RTP/AVP 0
         m=audio 30002 RTP/AVP 8
         m=audio 30004 RTP/AVP 0
         m=audio 30006 RTP/AVP 8

   In another example, the callee Laura might want to receive single
   flows in different IP addresses depending on the codec. It would
   return an SDP description as follows.

         o=Laura 289083124 289083124 IN IP4 second.example.com
         t=0 0
         c=IN IP4
         m=audio 30000 RTP/AVP 0
         m=audio 30002 RTP/AVP 8
         c=IN IP4
         m=audio 30004 RTP/AVP 0
         m=audio 30006 RTP/AVP 8
         c=IN IP4

6. Media-level versus session-level attribute

   Syntactically fid is a media-level attribute. It provides
   information about a media stream defined by an "m" line.
   Semantically fid would be defined as a session-level attribute since
   it provides flow hierarchy inside a session description.

Camarillo/H÷ller/Eriksson                                            5

                      SDP media alignment in SIP

7. Backward compatibility

   Upon the reception of an SDP description with the fid attribute, the
   callee must add also this attribute to the response. If the callee
   does not understand the fid attribute it will not include fid in the
   response. In this case the callee will proceed as it is described in
   appendix B of [1]. Thus, a flow will be created for every "m" line
   in the session description. The caller will have to issue a re-
   INVITE in order to remove the undesirable media streams. RFC2543bis
   prevents the caller to send an updated session description in an ACK
   request since a complete session description was sent already in the
   INVITE request. Until the re-INVITE is issued, the communication
   between the terminals is possible because they have enough
   information for receiving and sending media.

   It would be possible to use different approaches that would resolve
   these issues (demultiplexing of media stream on port level) and
   provide better backward compatibility. One solution might be to add
   additional codecs supported in different IP addresses or port number
   in a new a attribute. This way, a callee that did not understand the
   attribute would ignore it but still create the correct number of
   media streams.

     o=Laura 289083124 289083124 IN IP4 second.example.com
     t=0 0
     c=IN IP4
     m=audio 30000 RTP/AVP 0
     a=addcodec:8 30002
     m=audio 30004 RTP/AVP 0
     a=addcodec:8 30006

   However, we believe that the fid attribute provides a more general
   mechanism to bundle different media lines into a single flow. This
   mechanism can be used by other applications different than the one
   described in this draft to overcome different problems. We have
   chosen generality over a specific solution.

   The backward compatibility problem might not exist in a future
   depending on the format chosen for SDPng. This draft is also
   intended to serve as input for the SDPng work. Not on the syntax
   level but on the semantics of the fid attribute.

8. Open issues

   How should RTCP be handled?

9. Acronyms

   BNF      Backus-Naur Form
   EDGE     Enhanced Data rates for GSM and TDMA/136 Evolution
   GSM      Global System for Mobile communication

Camarillo/H÷ller/Eriksson                                            6

                      SDP media alignment in SIP

   IP       Internet Protocol
   SDP      Session Description Protocol
   SIP      Session Initiation Protocol
   TFT      Traffic Flow Template
   UMTS     Universal Mobile Telecommunication System
   WLAN     Wireless Local Area Network

10. References

   [1] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP:
   Session Initiation Protocol", RFC 2543, IETF; Mach 1999.

   [2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC
   2327, IETF; April 1998.

   [3] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access
   Networks", draft-westberg-realtime-cellular-02.txt, IETF; May 2000.
   Work in progress.

   [4] 3G TS 23.060 v3.2.1 General Packet Radio Service Description.

   [5] ftp://ftp.isi.edu/confctrl/confctrl.mail ,thread with subject
   "SDP media alignment in SIP".

11. Authors' Addresses

   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Phone: +358 9 299 3371
   Fax: +358 9 299 3118
   Email: Gonzalo.Camarillo@ericsson.com

   Jan H÷ller
   Ericsson Research
   S-16480 Stockholm
   Phone: +46 8 58532845
   Fax: +46 8 4047020
   Email: Jan.Holler@era.ericsson.se

   G÷ran AP Eriksson
   Ericsson Research
   S-16480 Stockholm
   Phone: +46 8 58531762
   Fax: +46 8 4047020
   Email: Goran.AP.Eriksson@era.ericsson.se

Camarillo/H÷ller/Eriksson                                            7