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rfc:rfc3388

Network Working Group G. Camarillo Request for Comments: 3388 G. Eriksson Category: Standards Track J. Holler

                                                              Ericsson
                                                        H. Schulzrinne
                                                   Columbia University
                                                         December 2002
 Grouping of Media Lines in the Session Description Protocol (SDP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This document defines two Session Description Protocol (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) that are
 encoded in different formats during a particular session, on
 different ports and host interfaces.

Table of Contents

 1. Introduction..................................................  2
 2. Terminology...................................................  2
 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.............................................  5
 7. Flow Identification (FID).....................................  5
    7.1 SIP and Cellular Access...................................  6
    7.2 DTMF Tones................................................  6
    7.3 Media Flow Definition.....................................  6
    7.4 FID Semantics.............................................  7
        7.4.1 Examples of FID.....................................  8
    7.5 Scenarios that FID does not Cover........................  11

Camarillo et. al. Standards Track [Page 1] RFC 3388 Grouping of Media Lines in SDP December 2002

        7.5.1 Parallel Encoding Using Different Codecs...........  11
        7.5.2 Layered Encoding...................................  12
        7.5.3 Same IP Address and Port Number....................  12
 8. Usage of the "group" Attribute in SIP........................  13
    8.1 Mid Value in Answers.....................................  13
        8.1.1 Example............................................  14
    8.2 Group Value in Answers...................................  15
        8.2.1 Example............................................  15
    8.3 Capability Negotiation...................................  16
        8.3.1 Example............................................  17
    8.4 Backward Compatibility...................................  17
        8.4.1 Offerer does not Support "group"...................  17
        8.4.2 Answerer does not Support "group"..................  17
 9.    Security Considerations...................................  18
 10.   IANA Considerations.......................................  18
 11.   Acknowledgements..........................................  19
 12.   References................................................  19
 13.   Authors' Addresses........................................  20
 14.   Full Copyright Statement..................................  21

1. Introduction

 An SDP session description typically contains one or more 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 allow the
 expression of how different media streams within a session
 description relate to each other.  This document defines such
 extensions.

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 BCP 14, RFC 2119
 [1] and indicate requirement levels for compliant implementations.

Camarillo et. al. Standards Track [Page 2] RFC 3388 Grouping of Media Lines in SDP December 2002

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).  Further semantics
 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 a "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 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.

Camarillo et. al. Standards Track [Page 3] RFC 3388 Grouping of Media Lines in SDP December 2002

 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.

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 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.

Camarillo et. al. Standards Track [Page 4] RFC 3388 Grouping of Media Lines in SDP December 2002

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
 Note that although the third media stream is not present in the group
 line, it still MUST contain a 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 [5] 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 [6] 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 [3] and systems receiving DTMF tones on a different host
 than the voice.

Camarillo et. al. Standards Track [Page 5] RFC 3388 Grouping of Media Lines in SDP December 2002

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).

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
 [5] 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.

Camarillo et. al. Standards Track [Page 6] RFC 3388 Grouping of Media Lines in SDP December 2002

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 particular 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.

Camarillo et. al. Standards Track [Page 7] RFC 3388 Grouping of Media Lines in SDP December 2002

7.4.1 Examples of FID

 The session description below might be 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, 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
 (The linebreak in the fmtp line accommodates RFC formatting
 restrictions; SDP does not have continuation lines.)
 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.

Camarillo et. al. Standards Track [Page 8] RFC 3388 Grouping of Media Lines in SDP December 2002

      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 linebreak in the fmtp line accomodates RFC formatting
 restrictions; SDP does not have continuation lines.)
 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 how the "group" attribute used with FID
 semantics can indicate the use of two different codecs in the two
 directions of a bidirectional media stream.
     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).

Camarillo et. al. Standards Track [Page 9] RFC 3388 Grouping of Media Lines in SDP December 2002

 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 in 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 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).
 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.

Camarillo et. al. Standards Track [Page 10] RFC 3388 Grouping of Media Lines in SDP December 2002

 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 [7].  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 is not.

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,
 in effect 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
 context to group both media streams since such a system is not using

Camarillo et. al. Standards Track [Page 11] RFC 3388 Grouping of Media Lines in SDP December 2002

 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

Camarillo et. al. Standards Track [Page 12] RFC 3388 Grouping of Media Lines in SDP December 2002

 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
 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 [3] 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, as described in [4].

8.1 Mid Value in Answers

 The "mid" attribute is an identifier for a particular media stream.
 Therefore, the "mid" value in the offer MUST be the same as the "mid"
 value in the answer.  Besides, subsequent offers (e.g., in a re-
 INVITE) SHOULD use the same "mid" value for the already existing
 media streams.
 RFC 3264 [4] describes the usage of SDP in relation to SIP.  The
 offerer and the answerer align their media description so that the
 nth media stream ("m=" line) in the offerer's session description
 corresponds to the nth media stream in the answerer's 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

Camarillo et. al. Standards Track [Page 13] RFC 3388 Grouping of Media Lines in SDP December 2002

 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
 offer contains a different identifier in the answer the application
 ignores all the "mid" and "group" lines that might appear in the
 session description.  The following example illustrates this
 scenario.

8.1.1 Example

 Two SIP entities exchange SDPs during session establishment. The
 INVITE contains 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.

Camarillo et. al. Standards Track [Page 14] RFC 3388 Grouping of Media Lines in SDP December 2002

 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 Answers

 A SIP entity that receives an offer that contains an "a=group" line
 with semantics that it does not understand MUST return an answer
 without the "group" line.  Note that, as it was described in the
 previous section, the "mid" lines MUST still be present in the
 answer.
 A SIP entity that receives an offer that contains an "a=group" line
 with semantics that are understood MUST return an answer 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 offer or a subset of them (zero
 identification-tags is a valid subset).  When the identification-tags
 in the answer are a subset, the "group" value to be used in the
 session MUST be the one present in the answer.
 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
 offerer, never by the answerer.  Since SIP provides a two-way SDP
 exchange, an answerer that requested grouping would not know whether
 the "group" attribute was accepted by the offerer or not.  An
 answerer that wants to group media lines SHOULD issue another offer
 after having responded to the first one (in a re-INVITE for
 instance).

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 answer.

Camarillo et. al. Standards Track [Page 15] RFC 3388 Grouping of Media Lines in SDP December 2002

 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
     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 to indicate that it
 supports them.  If a client decides to do that, it SHOULD add an
 "a=group" line with no identification-tags for every semantics it
 understands.
 If a server receives an offer that contains empty "a=group" lines, it
 SHOULD add its capabilities also in the form of empty "a=group" lines
 to its answer.

Camarillo et. al. Standards Track [Page 16] RFC 3388 Grouping of Media Lines in SDP December 2002

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

8.4.1 Offerer does not Support "group"

 This situation does not represent a problem because grouping requests
 are always performed by offerers, not by answerers.  If the offerer
 does not support "group" this attribute will just not be used.

8.4.2 Answerer does not Support "group"

 The answerer will ignore the "group" attribute, since it does not
 understand it (it will also ignore the "mid" attribute).  For LS
 semantics, the answerer might decide to perform or to not perform
 synchronization between media streams.
 For FID semantics, the answerer will consider that the session
 comprises several media streams.
 Different implementations would behave in different ways.

Camarillo et. al. Standards Track [Page 17] RFC 3388 Grouping of Media Lines in SDP December 2002

 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. Security Considerations

 Using the "group" parameter with FID semantics, an entity that
 managed to modify the session descriptions exchanged between the
 participants to establish a multimedia session could force the
 participants to send a copy of the media to any particular
 destination.
 Integrity mechanism provided by protocols used to exchange session
 descriptions and media encryption can be used to prevent this attack.

10. 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.
 This document defines a framework to group media lines in SDP using
 different semantics. Semantics to be used with this framework are
 registered by the IANA when they are published in standards track
 RFCs.
 The IANA Considerations section of the RFC MUST include the following
 information, which appears in the IANA registry along with the RFC
 number of the publication.
    o  A brief description of the semantics.
    o  Token to be used within the group attribute. This token may be
       of any length, but SHOULD be no more than four characters long.
    o  Reference to an standards track RFC.

Camarillo et. al. Standards Track [Page 18] RFC 3388 Grouping of Media Lines in SDP December 2002

 The only entries in the registry for the time being are:
 Semantics            Token  Reference
 -------------------  -----  -----------
 Lip synchronization  LS     RFC 3388
 Flow identification  FID    RFC 3388

11. Acknowledgments

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

12. References

12.1 Normative References

 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.
 [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
     RFC 2327, April 1998.
 [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
     Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
     Session Initiation Protocol", RFC 3261, June 2002.
 [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the
     Session Description Protocol (SDP)", RFC 3264, June 2002.

12.2 Informative References

 [5] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
     Protocol (RTSP)", RFC 2326, April 1998.
 [6] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
     A Transport Protocol for Real-Time Applications", RFC 1889,
     January 1996.
 [7] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
     Telephony Tones and Telephony Signals", RFC 2833, May 2000.

Camarillo et. al. Standards Track [Page 19] RFC 3388 Grouping of Media Lines in SDP December 2002

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

Camarillo et. al. Standards Track [Page 20] RFC 3388 Grouping of Media Lines in SDP December 2002

14. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  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
 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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Camarillo et. al. Standards Track [Page 21]

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