The Session Description Protocol (SDP) Grouping Framework
RFC 5888

Internet Engineering Task Force (IETF)                      G. Camarillo
Request for Comments: 5888                                      Ericsson
Obsoletes: 3388                                           H. Schulzrinne
Category: Standards Track                            Columbia University
ISSN: 2070-1721                                                June 2010

       The Session Description Protocol (SDP) Grouping Framework

Abstract

   In this specification, we define a framework to group "m" lines in
   the Session Description Protocol (SDP) for different purposes.  This
   framework uses the "group" and "mid" SDP attributes, both of which
   are defined in this specification.  Additionally, we specify how to
   use the framework for two different purposes: for lip synchronization
   and for receiving a media flow consisting of several media streams on
   different transport addresses.  This document obsoletes RFC 3388.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5888.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  3
   4.  Media Stream Identification Attribute  . . . . . . . . . . . .  4
   5.  Group Attribute  . . . . . . . . . . . . . . . . . . . . . . .  4
   6.  Use of "group" and "mid" . . . . . . . . . . . . . . . . . . .  4
   7.  Lip Synchronization (LS) . . . . . . . . . . . . . . . . . . .  5
     7.1.  Example of LS  . . . . . . . . . . . . . . . . . . . . . .  5
   8.  Flow Identification (FID)  . . . . . . . . . . . . . . . . . .  6
     8.1.  SIP and Cellular Access  . . . . . . . . . . . . . . . . .  6
     8.2.  DTMF Tones . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.3.  Media Flow Definition  . . . . . . . . . . . . . . . . . .  7
     8.4.  FID Semantics  . . . . . . . . . . . . . . . . . . . . . .  7
       8.4.1.  Examples of FID  . . . . . . . . . . . . . . . . . . .  8
     8.5.  Scenarios That FID Does Not Cover  . . . . . . . . . . . . 11
       8.5.1.  Parallel Encoding Using Different Codecs . . . . . . . 11
       8.5.2.  Layered Encoding . . . . . . . . . . . . . . . . . . . 12
       8.5.3.  Same IP Address and Port Number  . . . . . . . . . . . 12
   9.  Usage of the "group" Attribute in SIP  . . . . . . . . . . . . 13
     9.1.  Mid Value in Answers . . . . . . . . . . . . . . . . . . . 13
       9.1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . 14
     9.2.  Group Value in Answers . . . . . . . . . . . . . . . . . . 15
       9.2.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . 15
     9.3.  Capability Negotiation . . . . . . . . . . . . . . . . . . 16
       9.3.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . 16
     9.4.  Backward Compatibility . . . . . . . . . . . . . . . . . . 17
       9.4.1.  Offerer Does Not Support "group" . . . . . . . . . . . 17
       9.4.2.  Answerer Does Not Support "group"  . . . . . . . . . . 17
   10. Changes from RFC 3388  . . . . . . . . . . . . . . . . . . . . 18
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     14.2. Informative References . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   RFC 3388 [RFC3388] specified a media-line grouping framework for SDP
   [RFC4566].  This specification obsoletes RFC 3388 [RFC3388].

   An SDP [RFC4566] session description typically contains one or more
   media lines, which 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 to determine 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 is necessary to have a mechanism in SDP to express how
   different media streams within a session description relate to each
   other.  The framework defined in this specification is such a
   mechanism.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Overview of Operation

   This section provides a non-normative description of how the SDP
   Grouping Framework defined in this document works.  In a given
   session description, each "m" line is identified by a token, which is
   carried in a "mid" attribute below the "m" line.  The session
   description carries session-level "group" attributes that group
   different "m" lines (identified by their tokens) using different
   group semantics.  The semantics of a group describe the purpose for
   which the "m" lines are grouped.  For example, the "group" line in
   the session description below indicates that the "m" lines identified
   by tokens 1 and 2 (the audio and the video "m" lines, respectively)
   are grouped for the purpose of lip synchronization (LS).

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          v=0
          o=Laura 289083124 289083124 IN IP4 one.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          a=group:LS 1 2
          m=audio 30000 RTP/AVP 0
          a=mid:1
          m=video 30002 RTP/AVP 31
          a=mid:2

4.  Media Stream Identification Attribute

   This document defines the "media stream identification" media
   attribute, which is used for identifying media streams within a
   session description.  Its formatting in SDP [RFC4566] is described by
   the following Augmented Backus-Naur Form (ABNF) [RFC5234]:

           mid-attribute      = "a=mid:" identification-tag
           identification-tag = token
                                ; token is defined in RFC 4566

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

5.  Group Attribute

   This document defines the "group" session-level attribute, which is
   used for grouping together different media streams.  Its formatting
   in SDP is described by the following ABNF [RFC5234]:

           group-attribute     = "a=group:" semantics
                                 *(SP identification-tag)
           semantics           = "LS" / "FID" / semantics-extension
           semantics-extension = token
                                 ; token is defined in RFC 4566

   This document defines two standard semantics: Lip Synchronization
   (LS) and Flow Identification (FID).  Semantics extensions follow the
   Standards Action policy [RFC5226].

6.  Use of "group" and "mid"

   All of 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.

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   "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 description with grouped "m" lines is defined by the
   semantics field in the "a=group" line.

   There MAY be several "a=group" lines in a session description.  The
   "a=group" lines of a session description can use the same or
   different semantics.  An "m" line identified by its "mid" attribute
   MAY appear in more than one "a=group" line.

7.  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 apply not only 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 the RTP
   Control Protocol (RTCP), which provides enough information to map
   time stamps from the different streams into a local absolute time
   value.  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 mechanism is available.

7.1.  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 she is saying.  The
   first and second media streams have to be synchronized.

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          v=0
          o=Laura 289083124 289083124 IN IP4 two.example.com
          c=IN IP4 233.252.0.1/127
          t=0 0
          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 has to contain a "mid" attribute (mid:3), as stated
   before.

8.  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 Real Time Streaming Protocol (RTSP)
   specification.  The RTSP RFC [RFC2326] 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 [RFC1889] (at present obsoleted by
   [RFC3550]) used to define 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 the Session Initiation Protocol (SIP; [RFC3261]) and systems
   receiving Dual-Tone Multi-Frequency (DTMF) tones on a different host
   than the voice.

8.1.  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

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   the radio interface.  The radio interface is generally characterized
   as being prone to bit errors 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 is 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).

8.2.  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 situation, 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.

8.3.  Media Flow Definition

   The previous examples show that the definition of a media stream in
   [RFC2326] does 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:

      A 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.

8.4.  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 that is part of
   the flow as long as the codecs and the direction attribute present in
   a particular "m" line allow it.

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   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 of 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.

8.4.1.  Examples of FID

   The session description below might be sent by a SIP user agent using
   a cellular access.  The user agent supports GSM (Global System for
   Mobile communications) on port 30000 and AMR (Adaptive Multi-Rate) 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 three.example.com
            c=IN IP4 192.0.2.1
            t=0 0
            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.)

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   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 four.example.com
           c=IN IP4 192.0.2.1
           t=0 0
           a=group:FID 1 2
           m=audio 20000 RTP/AVP 0
           c=IN IP4 192.0.2.2
           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 accommodates RFC formatting
   restrictions; SDP does not have continuation lines.)

   The cellular terminal in this example only supports the AMR codec.
   However, many current IP phones only support PCM (Pulse-Code
   Modulation; payload 0).  In order to be able to interoperate with
   them, the cellular terminal uses a transcoder whose IP address is
   192.0.2.2.  The cellular terminal includes the transcoder IP address
   in its SDP description to provide support for PCM.  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
   is preferred) 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 five.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          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

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   A user agent that receives the SDP description 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 six.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          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 192.0.2.2
          a=recvonly
          a=mid:3

   At a particular point in time, if the media agent receiving the SDP
   message above is sending PCM u-law (payload 0), it sends RTP packets
   to 192.0.2.1 on port 30000 and to 192.0.2.2 on port 20000 (first and
   third "m" lines).  If it is sending PCM A-law (payload 8), it sends
   RTP packets to 192.0.2.1 on port 30002 and to 192.0.2.2 on port 20000
   (second and third "m" lines).

   The system that generated the SDP description above supports PCM
   u-law on port 30000 and PCM A-law on port 30002.  Besides, it uses an
   application server that records the conversation and whose IP address
   is 192.0.2.2.  The application server does not need to understand the
   media content, so it always receives a copy of the media stream,
   regardless of the codec and payload type that is being used.  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 that are grouped together using
   the FID semantics contain the same codec, the media agent MUST send
   copies of the same media stream as several RTP sessions at the same
   time.

   The last example in 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

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   telephone events is described in [RFC4733].  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 seven.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          a=group:FID 1 2
          m=audio 30000 RTP/AVP 0
          a=mid:1
          m=audio 20000 RTP/AVP 97
          c=IN IP4 192.0.2.2
          a=rtpmap:97 telephone-events
          a=mid:2

   The remote party would send PCM encoded voice (payload 0) to
   192.0.2.1 and DTMF tones encoded as telephony events to 192.0.2.2.
   Note that only voice or DTMF is sent at a particular point in 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.

8.5.  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.

8.5.1.  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 4733 [RFC4733], 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 alternative codecs
   but rather different parallel encodings for the same information.

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8.5.2.  Layered Encoding

   Layered encoding schemes encode media in different layers.  The
   quality of the media stream 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 233.252.0.1/127/3

   is equivalent to the following three "c" lines:

          c=IN IP4 233.252.0.1/127
          c=IN IP4 233.252.0.2/127
          c=IN IP4 233.252.0.3/127

   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 schemes.
   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.  Since the
   existing SDP mechanism already covers the most useful scenarios, we
   do not define a new group semantics to define a more flexible way of
   grouping different layers.

8.5.3.  Same IP Address and Port Number

   If media streams using several different 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 description like the one below MUST NOT be generated.

          v=0
          o=Laura 289083124 289083124 IN IP4 eight.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          a=group:FID 1 2
          m=audio 30000 RTP/AVP 0
          a=mid:1
          m=audio 30000 RTP/AVP 8
          a=mid:2

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

          v=0
          o=Laura 289083124 289083124 IN IP4 nine.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          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).

9.  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 [RFC3261] 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 [RFC4566] 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 [RFC3264].

9.1.  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.

   [RFC3264] describes the usage of SDP in text of 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 gain and would add complexity to implementations.  Therefore, SIP

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   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 of the "mid" and "group" lines that might appear in the
   session description.  The following example illustrates this
   scenario.

9.1.1.  Example

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

          v=0
          o=Laura 289083124 289083124 IN IP4 ten.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          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 description:

          v=0
          o=Bob 289083122 289083122 IN IP4 eleven.example.com
          c=IN IP4 192.0.2.3
          t=0 0
          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 ignores every "mid" and "group"
   line contained in the SDP description.

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

          v=0
          o=Bob 289083122 289083122 IN IP4 twelve.example.com
          c=IN IP4 192.0.2.3
          t=0 0
          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

9.2.  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 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" line MUST be the same
   as those 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 the port set to
   zero.

   Note that grouping of "m" lines MUST always be requested by the
   offerer, but 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 issues another offer after
   having responded to the first one (in a re-INVITE, for instance).

9.2.1.  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.

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   SDP description in the INVITE from caller to callee:

          v=0
          o=Laura 289083124 289083124 IN IP4 thirteen.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          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 description in the INVITE from callee to caller:

          v=0
          o=Bob 289083125 289083125 IN IP4 fourteen.example.com
          c=IN IP4 192.0.2.3
          t=0 0
          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

9.3.  Capability Negotiation

   A client that understands "group" and "mid", but does not want to use
   these SDP features in a particular session, may still want to
   indicate that it supports these features.  To indicate this support,
   a client can add an "a=3Dgroup" line with no identification-tags for
   every semantics value 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.

9.3.1.  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 description:

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          v=0
          o=Bob 289083125 289083125 IN IP4 fifteen.example.com
          c=IN IP4 192.0.2.3
          t=0 0
          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 description below:

          v=0
          o=Laura 289083124 289083124 IN IP4 sixteen.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          a=group:FID
          m=audio 30000 RTP/AVP 0

9.4.  Backward Compatibility

   This document does not define any SIP "Require" header field.
   Therefore, if one of the SIP user agents does not understand the
   "group" attribute, the standard SDP fall-back mechanism MUST be used,
   namely, attributes that are not understood are simply ignored.

9.4.1.  Offerer Does Not Support "group"

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

9.4.2.  Answerer Does Not Support "group"

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

   For FID semantics, the answerer will consider the session to consist
   of several media streams.

   Different implementations will 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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   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, the client
   can re-try the request without the "group" attribute and with only
   one "m" line per flow.

10.  Changes from RFC 3388

   Section 3 (Overview of Operation) has been added for clarity.  The
   AMR and GSM acronyms are now expanded on their first use.  The
   examples now use IP addresses in the range suitable for examples.

   The grouping mechanism is now defined as an extensible framework.
   Earlier, RFC 3388 [RFC3388] used to discourage extensions to this
   mechanism in favor of using new session description protocols.

   Given a semantics value, RFC 3388 [RFC3388] used to restrict "m" line
   identifiers to only appear in a single group using that semantics.
   That restriction has been lifted in this specification.  From
   conversations with implementers, existing (i.e., legacy)
   implementations enforce this restriction on a per-semantics basis.
   That is, they only enforce this restriction for supported semantics.
   Because of the nature of existing semantics, implementations will
   only use a single "m" line identifier across groups using a given
   semantics even after the restriction has been lifted by this
   specification.  Consequently, the lifting of this restriction will
   not cause backward-compatibility problems, because implementations
   supporting new semantics will be updated to not enforce this
   restriction at the same time as they are updated to support the new
   semantics.

11.  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 destination of its
   choosing.

   Integrity mechanisms provided by protocols used to exchange session
   descriptions and media encryption can be used to prevent this attack.
   In SIP, Secure/Multipurpose Internet Mail Extensions (S/MIME)
   [RFC5750] and Transport Layer Security (TLS) [RFC5246] can be used to
   protect session description exchanges in an end-to-end and a hop-by-
   hop fashion, respectively.

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12.  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 4.

   The "group" attribute is used for grouping together different media
   streams, and its format is defined in Section 5.

   This document defines a framework to group media lines in SDP using
   different semantics.  Semantics values to be used with this framework
   are registered by the IANA following the Standards Action policy
   [RFC5226].

   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 a standards track RFC.

   The following are the current entries in the registry:

      Semantics                          Token  Reference
      ---------------------------------  -----  -----------
      Lip Synchronization                 LS     [RFC5888]
      Flow Identification                 FID    [RFC5888]
      Single Reservation Flow             SRF    [RFC3524]
      Alternative Network Address Types   ANAT   [RFC4091]
      Forward Error Correction            FEC    [RFC4756]
      Decoding Dependency                 DDP    [RFC5583]

13.  Acknowledgments

   Goran Eriksson and Jan Holler were coauthors of RFC 3388 [RFC3388].

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14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, January 2010.

14.2.  Informative References

   [RFC1889]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 1889, January 1996.

   [RFC2326]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
              Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              December 2006.

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   FINLAND

   EMail: Gonzalo.Camarillo@ericsson.com

   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue
   New York, NY  10027
   USA

   EMail: schulzrinne@cs.columbia.edu

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