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Versions: 00 01 02 03 04 05 RFC 3388

Internet Engineering Task Force                     Gonzalo Camarillo
Internet draft                                             Jan Holler
                                                    Goran AP Eriksson
                                                             Ericsson

                                                  Henning Schulzrinne
                                                  Columbia University

                                                       September 2001
                                                   Expires March 2002
                                       <draft-ietf-mmusic-fid-05.txt>


                     Grouping of media 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-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
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   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
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   This document defines two SDP attributes: "group" 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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TABLE OF CONTENTS

   1     Introduction...............................................2
   2     Terminology................................................3
   3     Media stream identification attribute......................3
   4     Group attribute............................................3
   5     Use of "group" and "mid"...................................3
   6     Lip Synchronization (LS)...................................4
   6.1   Example of LS..............................................4
   7     Flow Identification (FID)..................................5
   7.1   SIP and cellular access....................................5
   7.2   DTMF tones.................................................6
   7.3   Media flow definition......................................6
   7.4   FID semantics..............................................6
   7.4.1 Examples of FID............................................6
   7.5   Scenarios that FID does not cover..........................9
   7.5.1 Parallel encoding using different codecs...................9
   7.5.2 Layered encoding..........................................10
   7.5.3 Same IP address and port number...........................10
   8     Usage of the "group" attribute in SIP.....................11
   8.1   Mid value in responses....................................11
   8.1.1 Example...................................................12
   8.2   Group value in responses..................................12
   8.2.1 Example...................................................13
   8.3   Capability negotiation....................................14
   8.3.1 Example...................................................14
   8.4   Backward compatibility....................................14
   8.4.1 Client does not support "group"...........................15
   8.4.2 Server does not support "group"...........................15
   9     IANA considerations.......................................15
   10    Acknowledgements..........................................16
   11    References................................................16
   12    Authors³ Addresses........................................16


1 Introduction

   An SDP session description typically contains a number (one or more)
   of media lines - they are commonly known as "m" lines. When a
   session description contains more than one "m" line, SDP does not
   provide any means to express a particular relationship between two
   or more of them. When an application receives an SDP session
   description with more than one "m" line it is up to the application
   what to do with them. SDP does not carry any information about
   grouping media streams.

   While in some environments this information can be carried out of
   band, it would be desirable to have extensions to SDP that allowed
   to express how different media streams within a session description
   relate to each other. This document defines such extensions.

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2 Terminology

   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 [1]
   and indicate requirement levels for compliant implementations.

3. 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 [2] is described by the following BNF:

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

   The identification tag MUST be unique within an SDP session
   description.

4. 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:

         group-attribute    = "a=group:" semantics
                              *(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.

5. Use of "group" and "mid"

   All the "m" lines of a session description that uses "group" MUST be
   identified with an "mid" attribute whether they appear in the group
   line(s) or not. If a session description contains at least one "m"
   line that has no "mid" identification the application MUST NOT
   perform any grouping of media lines.

   "a=group" lines are used to group together several "m" lines that
   are identified by their "mid" attribute. "a=group" lines that
   contain identification-tags that do not correspond to any "m" line
   within the session description MUST be simply ignored. The
   application acts as if the "a=group" line did not exist. The
   behavior of an application receiving an SDP with grouped "m" lines
   is defined by the semantics field in the "a=group" line.

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   There MAY be several "a=group" lines in a session description. All
   the "a=group" lines of a session description MAY or MAY NOT use the
   same semantics. An "m" line identified by its "mid" attribute MAY
   appear in more than one "a=group" line as long as the "a=group"
   lines use different semantics. An "m" line identified by its "mid"
   attribute MUST NOT appear in more than one "a=group" line using the
   same semantics.

   An application that wants to be compliant to this specification MUST
   support both "group" and "mid". An application that supported just
   one of them would not be compliant.

6. Lip Synchronization (LS)

   An application that receives a session description that contains "m"
   lines that are grouped together using LS semantics MUST synchronize
   the playout of the corresponding media streams. Note that LS
   semantics not only apply to a video stream that has to be
   synchronized with an audio stream. The playout of two streams of the
   same type can perfectly be synchronized as well.

   For RTP streams synchronization is typically performed using RTCP,
   which provides enough information to map time stamps from the
   different streams into a wall clock. However, the concept of media
   stream synchronization MAY also apply to media streams that do not
   make use of RTP. If this is the case, the application MUST recover
   the original timing relationship between the streams using whatever
   available mechanism.

6.1 Example of LS

   The following example shows a session description of a conference
   that is being multicast. The first media stream (mid:1) contains the
   voice of the speaker, who speaks in English. The second media stream
   (mid:2) contains the video component and the third (mid:3) media
   stream carries the translation to Spanish of what he is saying. The
   first and the second media streams MUST be synchronized.

         v=0
         o=Laura 289083124 289083124 IN IP4 one.example.com
         t=0 0
         c=IN IP4 224.2.17.12/127
         a=group:LS 1 2
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=video 30002 RTP/AVP 31
         a=mid:2
         m=audio 30004 RTP/AVP 0
         i=This media stream contains the Spanish translation
         a=mid:3



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   Note that although the third media stream is not present in the
   group line it still MUST contain an mid attribute (mid:3), as stated
   before.



7. Flow Identification (FID)

   An "m" line in an SDP session description defines a media stream.
   However, SDP does not define what a media stream is. This definition
   can be found in the RTSP specification. The RTSP RFC [3] 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 [4] 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)".

   While the previous definitions cover the most common cases, 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 [5] and systems receiving DTMF tones on a
   different host than the voice.

7.1 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
   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. 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).



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7.2 DTMF tones

   Some voice sessions include DTMF tones. Sometimes the voice handling
   is performed by a different host than the DTMF handling. It is
   common to have an application server in the network gathering 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 instance.

7.3 Media flow definition

   The previous examples show that the definition of a media stream in
   [3] do not cover some scenarios. 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.

7.4 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"
   lines MUST send a copy of the media to every "m" line part of the
   flow as long as the codecs and the direction attribute present in a
   particular "m" line allow it.

   It is assumed that the application uses only one codec at a time to
   encode the media produced. This codec MAY change dynamically during
   the session, but at any certain moment only one codec is in use.

   The application encodes the media using the current codec and checks
   one by one all the "m" lines that are part of the flow. If a
   particular "m" line contains the codec being used and the direction
   attribute is "sendonly" or "sendrecv" a copy of the encoded media is
   sent to the address/port specified in that particular media stream.
   If either the "m" line does not contain the codec being used or the
   direction attribute is neither "sendonly" nor "sendrecv" nothing is
   sent over this media stream.

   The application typically ends up sending media to different
   destinations (IP address/port number) depending on the codec used at
   any moment.

7.4.1 Examples of FID

   The session description below would be the SDP sent by a SIP user
   agent using a cellular access. The user agent supports GSM on port
   30000 and AMR on port 30002. When the remote party sends GSM it will
   send RTP packets to port number 30000. When AMR is the codec chosen,

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   packets will be sent to port 30002. Note that the remote party can
   switch between both codecs dynamically in the middle of the session.
   However, in this example, only one media stream at a time carries
   voice. The other remains "muted" while its corresponding codec is
   not in use.

         v=0
         o=Laura 289083124 289083124 IN IP4 two.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 30000 RTP/AVP 3
         a=rtpmap:3 GSM/8000
         a=mid:1
         m=audio 30002 RTP/AVP 97
         a=rtpmap:97 AMR/8000
         a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change-
      neighbor; maxframes=1
         a=mid:2

   In the previous example a system receives media on the same IP
   address on different port numbers. The following example shows how a
   system can receive different codecs on different IP addresses.

         v=0
         o=Laura 289083124 289083124 IN IP4 three.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 20000 RTP/AVP 0
         c=IN IP4 131.160.1.111
         a=rtpmap:0 PCMU/8000
         a=mid:1
         m=audio 30002 RTP/AVP 97
         a=rtpmap:97 AMR/8000
         a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change-
      neighbor; maxframes=1
         a=mid:2

   The cellular terminal of this example only supports the AMR codec.
   However, many current IP phones only support PCM (payload 0). In
   order to be able to interoperate with them, the cellular terminal
   uses a transcoder whose IP address is 131.160.1.111. The cellular
   terminal includes in its SDP support for PCM at that IP address.
   Remote systems will send AMR directly to the terminal but PCM will
   be sent to the transcoder. The transcoder will be configured (using
   whatever method) to convert the incoming PCM audio to AMR and send
   it to the terminal.

   The next example shows that the "group" attribute used with FID
   semantics allows to express uni-directional codecs for a bi-
   directional media flow. That is, a codec that is only used in one
   direction within a sendrecv media stream.

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         v=0
         o=Laura 289083124 289083124 IN IP4 four.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=audio 30002 RTP/AVP 8
         a=recvonly
         a=mid:2

   A user agent that receives the SDP above 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).

   The following example shows a session description with different "m"
   lines grouped together using FID semantics that contain the same
   codec.

         v=0
         o=Laura 289083124 289083124 IN IP4 five.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2 3
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=audio 30002 RTP/AVP 8
         a=mid:2
         m=audio 20000 RTP/AVP 0 8
         c=IN IP4 131.160.1.111
         a=recvonly
         a=mid:3

   At a particular point of time, if the media agent is sending PCM u-
   law (payload 0) it sends RTP packets to 131.160.1.112 on port 30000
   and to 131.160.1.111 on port 20000 (first and third "m" lines). If
   it is sending PCM A-law (payload 8) it sends RTP packets to
   131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000
   (second and third "m" lines).

   The system that generated the SDP above supports PCM u-law on port
   30000 and PCM A-law on port 30002. Besides, it uses an application
   server whose IP address is 131.160.1.111 that records all the
   conversation. That is why the application server always receives a
   copy of the audio stream regardless of the codec being used at any
   given moment (it actually performs an RTP dump, so it can
   effectively receive any codec).



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   Remember that if several "m" lines grouped together using FID
   semantics contain the same codec the media agent MUST send media
   over several RTP sessions at the same time.

   The last example of this section deals with DTMF tones. DTMF tones
   can be transmitted using a regular voice codec or can be transmitted
   as telephony events. The RTP payload for DTMF tones treated as
   telephone events is described in RFC 2833 [6]. Below there is an
   example of an SDP session description using FID semantics and this
   payload type.

         v=0
         o=Laura 289083124 289083124 IN IP4 six.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=audio 20000 RTP/AVP 97
         c=IN IP4 131.160.1.111
         a=rtpmap:97 telephone-events
         a=mid:2

   The remote party would send PCM encoded voice (payload 0) to
   131.160.1.112 and DTMF tones encoded as telephony events to
   131.160.1.111. Note that only voice or DTMF is sent at a particular
   point of time. When DTMF tones are sent the first media stream does
   not carry any data and when voice is sent there is no data in the
   second media stream. FID semantics provide different destinations
   for alternative codecs.

7.5 Scenarios that FID does not cover

   It is worthwhile mentioning some scenarios where the "group"
   attribute using existing semantics (particularly FID) might seem to
   be applicable but it is not. This section has been included because
   we have observed some confusion within the community regarding the
   three scenarios described below. This section helps clarify them.

7.5.1 Parallel encoding using different codecs

   FID semantics are useful when the application only uses one codec at
   a time. An application that encodes the same media using different
   codecs simultaneously MUST NOT use FID to group those media lines.
   Some systems that handle DTMF tones are a typical example of
   parallel encoding using different codecs.

   Some systems implement the RTP payload defined in RFC 2833, but when
   they send DTMF tones they do not mute the voice channel. Therefore,
   effectively they are sending two copies of the same DTMF tone:
   encoded as voice and encoded as a telephony event. When the receiver
   gets both copies it typically uses the telephony event rather than
   the tone encoded as voice. FID semantics MUST NOT be used in this

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                    Grouping of media lines in SDP


   context to group both media streams since such a system is not using
   alternative codecs but rather different parallel encodings for the
   same information.

7.5.2 Layered encoding

   Layered encoding schemes encode media in different layers. Quality
   at the receiver varies depending on the number of layers received.
   SDP provides a means to group together contiguous multicast
   addresses that transport different layers. The "c" line below:

        c=IN IP4 224.2.1.1/127/3

   is equivalent to the following three "c" lines:

        c=IN IP4 224.2.1.1/127
        c=IN IP4 224.2.1.2/127
        c=IN IP4 224.2.1.3/127

   FID MUST NOT be used to group "m" lines that do not represent the
   same information. Therefore, FID MUST NOT be used to group "m" lines
   that contain the different layers of layered encoding scheme.
   Besides, we do not define new group semantics to provide a more
   flexible way of grouping different layers because the already
   existing SDP mechanism covers the most useful scenarios.

7.5.3 Same IP address and port number

   If several codecs have to be sent to the same IP address and port,
   the traditional SDP syntax of listing several codecs in the same "m"
   line MUST be used. FID MUST NOT be used to group "m" lines with the
   same IP address/port. Therefore, an SDP like the one below MUST NOT
   be generated.

         v=0
         o=Laura 289083124 289083124 IN IP4 six.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=audio 30000 RTP/AVP 8
         a=mid:2

   The correct SDP for the session above would be the following one:

         v=0
         o=Laura 289083124 289083124 IN IP4 six.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         m=audio 30000 RTP/AVP 0 8



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   If two "m" lines are grouped using FID they MUST differ in their
   transport addresses (i.e., IP address plus port).

8. Usage of the "group" attribute in SIP

   SDP descriptions are used by several different protocols, SIP among
   them. We include a section about SIP because the "group" attribute
   will most likely be used mainly by SIP systems.

   SIP [5] 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.

   At session establishment SIP provides a three-way handshake (INVITE-
   200 OK-ACK) between end systems. However, just two of these three
   messages carry SDP. SDPs MAY be present in INVITE and 200 OK or in
   200 OK and ACK. The following sections assume that INVITE and 200 OK
   are the ones carrying SDP for the sake of clarity, but everything is
   also applicable to the other possible scenario (200 OK and ACK).

8.1 Mid value in responses

   The "mid" attribute is an identifier for a particular media stream.
   Therefore, the "mid" value in the response MUST be the same as the
   "mid" value in the request. Besides, subsequent requests such as re-
   INVITEs SHOULD use the same "mid" value for the already existing
   media streams.

   Appendix B of [5] 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 "group" attribute in an SDP session description
   does not modify this behavior.

   Since the "mid" attribute provides a means to label "m" lines it
   would be possible to perform media alignment using "mid" labels
   rather than matching nth "m" lines. However this would not bring any
   gain and would add complexity to implementations. Therefore SIP
   systems MUST perform media alignment matching nth lines regardless
   of the presence of the "group" or "mid" attributes.

   If a media stream that contained a particular "mid" identifier in
   the request contains a different identifier in the response the
   application ignores all the "mid" and "group" lines that might
   appear in the session description. The following example illustrates
   this scenario:




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8.1.1 Example

   Two SIP entities exchange SDPs during session establishment. The
   INVITE contained the SDP below:

         v=0
         o=Laura 289083124 289083124 IN IP4 seven.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2
         m=audio 30000 RTP/AVP 0 8
         a=mid:1
         m=audio 30002 RTP/AVP 0 8
         a=mid:2

   The 200 OK response contains the following SDP:

         v=0
         o=Bob 289083122 289083122 IN IP4 eigth.example.com
         t=0 0
         c=IN IP4 131.160.1.113
         a=group:FID 1 2
         m=audio 25000 RTP/AVP 0 8
         a=mid:2
         m=audio 25002 RTP/AVP 0 8
         a=mid:1

   Since alignment of "m" lines is performed based on matching of nth
   lines, the first stream had "mid:1" in the INVITE and "mid:2" in the
   200 OK. Therefore, the application MUST ignore every "mid" and
   "group" lines contained in the SDP.

   A well-behaved SIP user agent would have returned the SDP below in
   the 200 OK:

         v=0
         o=Bob 289083122 289083122 IN IP4 nine.example.com
         t=0 0
         c=IN IP4 131.160.1.113
         a=group:FID 1 2
         m=audio 25002 RTP/AVP 0 8
         a=mid:1
         m=audio 25000 RTP/AVP 0 8
         a=mid:2

8.2 Group value in responses

   A SIP entity that receives a request that contains an "a=group" line
   with semantics that it does not understand MUST return a response
   without the "group" line. Note that, as it was described in the
   previous section, the "mid" lines MUST still be present in the
   response.


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   A SIP entity that receives a request that contains an "a=group" line
   which semantics that are understood MUST return a response that
   contains an "a=group" line with the same semantics. The
   identification-tags contained in this "a=group" lines MUST be the
   same that were received in the request or a subset of them (zero
   identification-tags is a valid subset). When the identification-tags
   in the response are a subset the "group" value to be used in the
   session MUST be the one present in the response.

   SIP entities refuse media streams by setting the port to zero in the
   corresponding "m" line. "a=group" lines MUST NOT contain
   identification-tags that correspond to "m" lines with port zero.

   Note that grouping of m lines MUST always be requested by the issuer
   of the request (the client), never by the issuer of the response
   (the server). Since SIP provides a two-way SDP exchange, a server
   that requested grouping in a response would not know whether the
   "group" attribute was accepted by the client or not. A server that
   wants to group media lines SHOULD issue another request after having
   responded to the first one (a re-INVITE for instance).

        Note that, as we mentioned previously, in this section we are
        assuming that the SDPs are present in the INVITE and in the 200
        OK. Applying the statement above to the scenario where SDPs are
        present in the 200 OK and in the ACK, the entity requesting
        grouping would be the server.

8.2.1 Example

   The example below shows how the callee refuses a media stream
   offered by the caller by setting its port number to zero. The "mid"
   value corresponding to that media stream is removed from the "group"
   value in the response.

   SDP in the INVITE from caller to callee:

         v=0
         o=Laura 289083124 289083124 IN IP4 ten.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID 1 2 3
         m=audio 30000 RTP/AVP 0
         a=mid:1
         m=audio 30002 RTP/AVP 8
         a=mid:2
         m=audio 30004 RTP/AVP 3
         a=mid:3


   SDP in the INVITE from callee to caller:

         v=0
         o=Bob 289083125 289083125 IN IP4 eleven.example.com

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                    Grouping of media lines in SDP


         t=0 0
         c=IN IP4 131.160.1.113
         a=group:FID 1 3
         m=audio 20000 RTP/AVP 0
         a=mid:1
         m=audio 0 RTP/AVP 8
         a=mid:2
         m=audio 20002 RTP/AVP 3
         a=mid:3


8.3 Capability negotiation

   A client that understands "group" and "mid" but does not want to
   make use of them in a particular session MAY want indicate that it
   supports them. If a client decides to do that, it SHOULD add an
   "a=group" line with zero identification-tags for every semantics it
   understands.

   If a server receives a request that contains empty "a=group" lines
   it SHOULD add its capabilities also in the form of empty "a=group"
   lines to its response.

8.3.1 Example

   A system that supports both LS and FID semantics but does not want
   to group any media stream for this particular session generates the
   following SDP:

         v=0
         o=Bob 289083125 289083125 IN IP4 twelve.example.com
         t=0 0
         c=IN IP4 131.160.1.113
         a=group:LS
         a=group:FID
         m=audio 20000 RTP/AVP 0 8

    The server that receives that request supports FID but not LS. It
   responds with the SDP below:


         v=0
         o=Laura 289083124 289083124 IN IP4 thirteen.example.com
         t=0 0
         c=IN IP4 131.160.1.112
         a=group:FID
         m=audio 30000 RTP/AVP 0

8.4 Backward compatibility

   This document does not define any SIP "Require" header. Therefore,
   if one of the SIP user agents does not understand the "group"


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                    Grouping of media lines in SDP


   attribute the standard SDP fall back mechanism MUST be used
   (attributes that are not understood are simply ignored).




8.4.1 Client does not support "group"

   This situation does not represent a problem because grouping
   requests is always performed by clients, not by servers. If the
   client does not support "group" this attribute will just not be
   used.

8.4.2 Server does not support "group"

   The server will ignore the "group" attribute, since it does not
   understand it (it will also ignore the "mid" attribute). For LS
   semantics, the server might decide to perform or to not perform
   synchronization between media streams.

   For FID semantics, the server 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.

   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 server does not support the
   type of session that the caller wanted to establish. In case the
   client is willing to establish a simpler session anyway, he SHOULD
   re-try the request without "group" attribute and only one "m" line
   per flow.

9. IANA considerations

   This document defines two SDP attributes: "mid" and "group".

   The "mid" attribute is used to identify media streams within a
   session description and its format is defined in Section 3.

   The "group" attribute is used for grouping together different media
   streams and its format is defined in Section 4.

   Section 4 also defines two standard semantics related to the "group"
   attribute: 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.



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                    Grouping of media lines in SDP


10. Acknowledgments

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

11. References

   [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement
   Levels", RFC 2119, IETF; March 1997.

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

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

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

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

   [6] H. Schulzrinne/S. Petrack, "RTP Payload for DTMF Digits,
   Telephony Tones and Telephony Signals", RFC 2833, IETF; May 2000.


12. Authors³ Addresses

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

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

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

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                    Grouping of media lines in SDP



   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   USA
   Email: schulzrinne@cs.columbia.edu














































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