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Versions: 00 01 02 03 04 05 06 rfc3388                                  
Internet Engineering Task Force                     Gonzalo Camarillo
Internet draft                                             Jan Holler
                                                    Goran AP Eriksson
                                                            June 2001
                                                Expires December 2001

                       Grouping of m lines in SDP

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-
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   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
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   This document defines two SDP attributes: "groupe" and "mid". They
   allow to group together several "m" lines for two different
   purposes: for lip synchronization and for receiving media from a
   single flow (several media streams), encoded in different formats
   during a particular session, in different ports and host interfaces.

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                      Grouping of m lines in SDP


   1   Media stream identification attribute........................2
   2   Groupe attribute.............................................2
   3   Lip Synchronization (LS).....................................3
   4   Flow Identification (FID)....................................3
   4.1 SIP and cellular access......................................3
   4.2 DTMF tones...................................................4
   5   Media flow definition........................................4
   6   FID semantics................................................4
   7   Interactions of "groupe" with other media level attributes...5
   8   Usage of the "groupe" attribute in SIP.......................6
   8.1 Backward compatibility.......................................6
   8.2 Caller does not support fid..................................6
   8.3 Callee does not support fid..................................6
   9   Acknoledgements..............................................7
   10   References..................................................7
   11   Authors³ Addresses..........................................7

1. Media stream identification attribute

   A new "media stream identification" media attribute is defined. It
   is used for identifying media streams within a session description.
   Its formatting in SDP is described by the following BNF:

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

   The identification tag is unique within the SDP session description.

2. Group attribute

   A new "group" session level attribute is defined. It is used for
   grouping together different media streams. Its formatting in SDP is
   described by the following BNF:

         groupe-attribute   = "a=groupe:" semantics space
                              2*(space identification-tag)
         semantics          = "LS" | "FID"

   This document defines two standard semantics: LS (Lip
   Synchronization) and FID (Flow Identification). If in the future it
   was needed to standardize further semantics they would need to be
   defined in a standards track document. However, defining new
   semantics apart from LS and FID is discouraged. Instead, it is
   RECOMMENDED to use other session description mechanisms such as
   SDPng [1].

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                      Grouping of m lines in SDP

   There might be several "a=groupe" lines in a session description.
   "a=groupe" lines that contain identification-tags that are not
   present in the session description are simply ignored. The
   application acts as if the "a=groupe" line did not exist.

3. Lip Synchronization (LS)

   The play out of media streams that are grouped together using LS
   semantics have to be synchronized. Synchronization is typically
   performed using RTCP, which provides enough information to map time
   stamps from the different streams into a wall clock.

   The following example shows a session description where the audio
   and the video stream have to be synchronized.

         o=Laura 289083124 289083124 IN IP4 first.example.com
         t=0 0
         c=IN IP4
         a=groupe:LS 1 2
         m=audio 30000 RTP/AVP 0
         m=video 30002 RTP/AVP 31

4. Flow Identification (FID)

   The RTSP RFC [2] defines a media stream as "a single media instance,
   e.g., an audio stream or a video stream as well as a single
   whiteboard or shared application group. When using RTP, a stream
   consists of all RTP and RTCP packets created by a source within an
   RTP session".

   This definition assumes that a single audio (or video) stream maps
   into an RTP session. The RTP RFC [3] defines an RTP session as
   follows: "For each participant, the session is defined by a
   particular pair of destination transport addresses (one network
   address plus a port pair for RTP and RTCP)".

   However, there are situations where a single media instance, (e.g.,
   an audio stream or a video stream) is sent using more than one RTP
   session. Two examples (among many others) of this kind of situation
   are cellular systems using SIP [4] and systems receiving DTMF tones
   on a different host than the voice.

4.1 SIP and cellular access

   Systems using a cellular access 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

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                      Grouping of m lines in SDP

   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 [5]. 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.

   Cellular systems typically configure different radio bearers on
   different port numbers. Therefore, incoming media has to have
   different destination port numbers for the different possible codecs
   in order to be routed properly to the correct radio bearer. Thus,
   this is an example in which several RTP sessions are used to carry a
   single media instance (the encoded speech from the sender).

4.2 DTMF tones

   Some voice sessions include DTMF tones. Sometimes the voice handling
   is performed by a different host than the DTMF handling. [6]
   contains several examples of how application servers in the network
   gather DTMF tones for the user while the user receives the encoded
   speech on his user agent. In this situations it is necessary to
   establish two RTP sessions: one for the voice and the other for the
   DTMF tones. Both RTP sessions are logically part of the same media

5. Media flow definition

   The previous examples show that the definition of a media stream in
   [2] has to be updated. It cannot be assumed that a single media
   instance maps into a single RTP session. Therefore, we introduce the
   definition of a media flow:

   Media flow consists of a single media instance, e.g., an audio
   stream or a video stream as well as a single whiteboard or shared
   application group. When using RTP, a media flow comprises one or
   more RTP sessions.

   For instance, in a two party call where the voice exchanged can be
   encoded using GSM or PCM, the receiver wants to receive GSM on a
   port number and PCM on a different port number. Two RTP sessions
   will be established, one carrying GSM and the other carrying PCM.

   At any particular moment just one codec is in use. Therefore, at any
   moment one of the RTP sessions will not transport any voice. Here
   the systems are dealing with a single media flow, but two RTP

6. FID semantics

   Several "m" lines grouped together using FID semantics form a media
   flow. A media agent handling a media flow that comprises several "m"

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                      Grouping of m lines in SDP

   lines sends media to different destinations (IP address/port number)
   depending on the codec used at any moment. If several "m" lines
   contain the codec used media is sent to different destinations in

   For instance, a SIP user agent receives an INVITE with the following

         o=Laura 289083124 289083124 IN IP4 second.example.com
         t=0 0
         c=IN IP4
         a=groupe:FID 1 2 3
         m=audio 30000 RTP/AVP 0
         m=audio 30002 RTP/AVP 8
         m=audio 30004 RTP/AVP 0 8

   At a particular point of time, if the media agent is sending PCM u-
   law (payload 0) it sends RTP packets to ports 30000 and 30004 (first
   and third "m" lines). If it is sending PCM A-law (payload 8) it
   sends RTP packets to ports 30002 and 30004 (second and third "m"

   Note that if several "m" lines with the same fid value contain the
   same codec the media agent MUST send media over several RTP sessions
   at the same time.

7 Interactions of "groupe" with other media level attributes

   Media level attributes affect a media stream defined by an "m" line.
   The presence of "groupe" does not modify this behavior.

   For instance, a SIP user agent receives an INVITE with the following

         o=Laura 289083124 289083124 IN IP4 third.example.com
         t=0 0
         c=IN IP4
         a=groupe:FID 1 2
         m=audio 30000 RTP/AVP 0
         m=audio 30002 RTP/AVP 8

   The media agent knows that at a certain moment it can send either
   PCM u-law to port number 30000 or PCM A-law to port number 30002.
   However, the media agent also knows that the other end will only
   send PCM u-law (payload 0).

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                      Grouping of m lines in SDP

   Note that the "groupe" attribute used with FID semantics allows to
   express uni-directional codecs for a bi-directional media flow, as
   it is shown in the example above.

8. Usage of the "groupe" attribute in SIP

   SIP [4] 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 [7] is one of
   the protocols that can be used for this purpose.

   Appendix B of [4] 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 caller³s session description
   corresponds to the nth media stream in the callee³s description."

   The presence of the "groupe" attribute in an SDP session description
   does not modify this behavior.

8.1 Backward compatibility

   This document does not define any SIP "Require" header. Therefore,
   if one of the SIP user agents does not understand the "groupe"
   attribute the standard SDP fall back mechanism is used.

   A system that understands the "groupe" attribute MUST add an "mid"
   attribute to every "m" line in any SDP session description that it

8.2 Caller does not support "groupe"

   This situation does not represent a problem. The SDP in the INVITE
   will not contain any "mid" attribute. The callee knows that the
   caller does not support "groupe".

8.3 Callee does not support "groupe"

   The callee will ignore the "groupe" attribute, since it does not
   understand it. For LS semantics, the callee might decide to perform
   or to not perform synchronization between media streams.

   For FID semantics, the callee will consider that the session
   comprises several media streams.

   Different implementations would behave in different ways.

   In the case of audio and different "m" lines for different codecs an
   implementation might decide to act as a mixer with the different
   incoming RTP sessions, which is the correct behavior.

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                      Grouping of m lines in SDP

   An implementation might also decide to refuse the request (e.g. 488
   Not acceptable here or 606 Not Acceptable) because it contains
   several "m" lines. In this case, the callee does not support the
   type of session that the caller wanted to establish. In case the
   caller is willing to establish a simpler session anyway, he should
   re-try the request without "groupe" attribute and only one "m" line
   per flow.

9. Acknowledgments

   The authors would like to thank Jonathan Rosenberg, Adam Roach and
   Orit Levin for their feedback on this document.

10. References

   [1] D. Kutscher/J. Ott/C. Bormann, "Session Description and
   Capability Negotiation", draft-ietf-mmusic-sdpng-00.txt, IETF; April
   2001. Work in progress.

   [2] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol
   (RTSP)", RFC 2326, IETF; April 1998.

   [3] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A
   Transport Protocol for Real-Time Applications", RFC 1889, IETF;
   January 1996.

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

   [5] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access
   Networks", draft-westberg-realtime-cellular-03.txt, IETF; November
   2000. Work in progress.

   [6] J. Rosemberg/P.Mataga/H.Schulzrinne, "An Applcation Server
   Component Architecture for SIP", draft-rosenberg-sip-app-components-
   00.txt, IETF; November 2000. Work in progress.

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

11. Authors³ Addresses

   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Phone: +358 9 299 3371
   Fax: +358 9 299 3052
   Email: Gonzalo.Camarillo@ericsson.com

   Jan Holler
   Ericsson Research

Camarillo/Holler/Eriksson                                            7

                      Grouping of m lines in SDP

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

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

Camarillo/Holler/Eriksson                                            8