draft-ietf-mmusic-fid-06.txt   rfc3388.txt 
Internet Engineering Task Force Gonzalo Camarillo Network Working Group G. Camarillo
Internet draft Jan Holler Request for Comments: 3388 G. Eriksson
Goran AP Eriksson Category: Standards Track J. Holler
Ericsson Ericsson
H. Schulzrinne
Henning Schulzrinne Columbia University
Columbia University December 2002
February 2002
Expires August 2002
<draft-ietf-mmusic-fid-06.txt>
Grouping of media lines in SDP Grouping of Media Lines in the Session Description Protocol (SDP)
Status of this Memo Status of this Memo
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all provisions of Section 10 of RFC2026. Internet community, and requests discussion and suggestions for
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Abstract Abstract
This document defines two SDP attributes: "group" and "mid". They This document defines two Session Description Protocol (SDP)
allow to group together several "m" lines for two different attributes: "group" and "mid". They allow to group together several
purposes: for lip synchronization and for receiving media from a "m" lines for two different purposes: for lip synchronization and for
single flow (several media streams), encoded in different formats receiving media from a single flow (several media streams) that are
during a particular session, in different ports and host interfaces. encoded in different formats during a particular session, on
different ports and host interfaces.
Camarillo/Holler/Eriksson/Schulzrinne 1
Grouping of media lines in SDP
TABLE OF CONTENTS Table of Contents
1 Introduction...............................................2 1. Introduction.................................................. 2
2 Terminology................................................3 2. Terminology................................................... 2
3 Media stream identification attribute......................3 3. Media Stream Identification Attribute......................... 3
4 Group attribute............................................3 4. Group Attribute............................................... 3
5 Use of "group" and "mid"...................................3 5. Use of "group" and "mid"...................................... 3
6 Lip Synchronization (LS)...................................4 6. Lip Synchronization (LS)...................................... 4
6.1 Example of LS..............................................4 6.1 Example of LS............................................. 5
7 Flow Identification (FID)..................................5 7. Flow Identification (FID)..................................... 5
7.1 SIP and cellular access....................................5 7.1 SIP and Cellular Access................................... 6
7.2 DTMF tones.................................................6 7.2 DTMF Tones................................................ 6
7.3 Media flow definition......................................6 7.3 Media Flow Definition..................................... 6
7.4 FID semantics..............................................6 7.4 FID Semantics............................................. 7
7.4.1 Examples of FID............................................6 7.4.1 Examples of FID..................................... 8
7.5 Scenarios that FID does not cover..........................9 7.5 Scenarios that FID does not Cover........................ 11
7.5.1 Parallel encoding using different codecs...................9 7.5.1 Parallel Encoding Using Different Codecs........... 11
7.5.2 Layered encoding..........................................10 7.5.2 Layered Encoding................................... 12
7.5.3 Same IP address and port number...........................10 7.5.3 Same IP Address and Port Number.................... 12
8 Usage of the "group" attribute in SIP.....................11 8. Usage of the "group" Attribute in SIP........................ 13
8.1 Mid value in responses....................................11 8.1 Mid Value in Answers..................................... 13
8.1.1 Example...................................................12 8.1.1 Example............................................ 14
8.2 Group value in responses..................................12 8.2 Group Value in Answers................................... 15
8.2.1 Example...................................................13 8.2.1 Example............................................ 15
8.3 Capability negotiation....................................14 8.3 Capability Negotiation................................... 16
8.3.1 Example...................................................14 8.3.1 Example............................................ 17
8.4 Backward compatibility....................................14 8.4 Backward Compatibility................................... 17
8.4.1 Client does not support "group"...........................15 8.4.1 Offerer does not Support "group"................... 17
8.4.2 Server does not support "group"...........................15 8.4.2 Answerer does not Support "group".................. 17
9 Security considerations...................................15 9. Security Considerations................................... 18
10 IANA considerations.......................................16 10. IANA Considerations....................................... 18
11 Acknowledgements..........................................16 11. Acknowledgements.......................................... 19
12 References................................................16 12. References................................................ 19
13 Authors Addresses........................................16 13. Authors' Addresses........................................ 20
14. Full Copyright Statement.................................. 21
1 Introduction 1. Introduction
An SDP session description typically contains a number (one or more) An SDP session description typically contains one or more media lines
of media lines - they are commonly known as "m" lines. When a - they are commonly known as "m" lines. When a session description
session description contains more than one "m" line, SDP does not contains more than one "m" line, SDP does not provide any means to
provide any means to express a particular relationship between two express a particular relationship between two or more of them. When
or more of them. When an application receives an SDP session an application receives an SDP session description with more than one
description with more than one "m" line it is up to the application "m" line, it is up to the application what to do with them. SDP does
what to do with them. SDP does not carry any information about not carry any information about grouping media streams.
grouping media streams.
While in some environments this information can be carried out of While in some environments this information can be carried out of
band, it would be desirable to have extensions to SDP that allowed band, it would be desirable to have extensions to SDP that allow the
to express how different media streams within a session description expression of how different media streams within a session
relate to each other. This document defines such extensions. description relate to each other. This document defines such
extensions.
Camarillo/Holler/Eriksson/Schulzrinne 2
Grouping of media lines in SDP
2 Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
and indicate requirement levels for compliant implementations. [1] and indicate requirement levels for compliant implementations.
3. Media stream identification attribute 3. Media Stream Identification Attribute
A new "media stream identification" media attribute is defined. It A new "media stream identification" media attribute is defined. It
is used for identifying media streams within a session description. is used for identifying media streams within a session description.
Its formatting in SDP [2] is described by the following BNF: Its formatting in SDP [2] is described by the following BNF:
mid-attribute = "a=mid:" identification-tag mid-attribute = "a=mid:" identification-tag
identification-tag = token identification-tag = token
The identification tag MUST be unique within an SDP session The identification tag MUST be unique within an SDP session
description. description.
4. Group attribute 4. Group Attribute
A new "group" session level attribute is defined. It is used for A new "group" session-level attribute is defined. It is used for
grouping together different media streams. Its formatting in SDP is grouping together different media streams. Its formatting in SDP is
described by the following BNF: described by the following BNF:
group-attribute = "a=group:" semantics group-attribute = "a=group:" semantics
*(space identification-tag) *(space identification-tag)
semantics = "LS" | "FID" semantics = "LS" | "FID"
This document defines two standard semantics: LS (Lip This document defines two standard semantics: LS (Lip
Synchronization) and FID (Flow Identification). If in the future it Synchronization) and FID (Flow Identification). Further semantics
was needed to standardize further semantics they would need to be need to be defined in a standards-track document. However, defining
defined in a standards track document. However, defining new new semantics apart from LS and FID is discouraged. Instead, it is
semantics apart from LS and FID is discouraged. Instead, it is
RECOMMENDED to use other session description mechanisms such as RECOMMENDED to use other session description mechanisms such as
SDPng. SDPng.
5. Use of "group" and "mid" 5. Use of "group" and "mid"
All the "m" lines of a session description that uses "group" MUST be All the "m" lines of a session description that uses "group" MUST be
identified with an "mid" attribute whether they appear in the group 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(s) or not. If a session description contains at least one "m"
line that has no "mid" identification the application MUST NOT line that has no "mid" identification the application MUST NOT
perform any grouping of media lines. perform any grouping of media lines.
"a=group" lines are used to group together several "m" lines that "a=group" lines are used to group together several "m" lines that are
are identified by their "mid" attribute. "a=group" lines that identified by their "mid" attribute. "a=group" lines that contain
contain identification-tags that do not correspond to any "m" line identification-tags that do not correspond to any "m" line within the
within the session description MUST be simply ignored. The session description MUST be ignored. The application acts as if the
application acts as if the "a=group" line did not exist. The "a=group" line did not exist. The behavior of an application
behavior of an application receiving an SDP with grouped "m" lines receiving an SDP with grouped "m" lines is defined by the semantics
is defined by the semantics field in the "a=group" line. field in the "a=group" line.
Camarillo/Holler/Eriksson/Schulzrinne 3
Grouping of media lines in SDP
There MAY be several "a=group" lines in a session description. All 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 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 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" appear in more than one "a=group" line as long as the "a=group" lines
lines use different semantics. An "m" line identified by its "mid" use different semantics. An "m" line identified by its "mid"
attribute MUST NOT appear in more than one "a=group" line using the attribute MUST NOT appear in more than one "a=group" line using the
same semantics. 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) 6. Lip Synchronization (LS)
An application that receives a session description that contains "m" An application that receives a session description that contains "m"
lines that are grouped together using LS semantics MUST synchronize lines that are grouped together using LS semantics MUST synchronize
the playout of the corresponding media streams. Note that LS the playout of the corresponding media streams. Note that LS
semantics not only apply to a video stream that has to be semantics not only apply to a video stream that has to be
synchronized with an audio stream. The playout of two streams of the synchronized with an audio stream. The playout of two streams of the
same type can perfectly be synchronized as well. same type can be synchronized as well.
For RTP streams synchronization is typically performed using RTCP, For RTP streams synchronization is typically performed using RTCP,
which provides enough information to map time stamps from the which provides enough information to map time stamps from the
different streams into a wall clock. However, the concept of media different streams into a wall clock. However, the concept of media
stream synchronization MAY also apply to media streams that do not stream synchronization MAY also apply to media streams that do not
make use of RTP. If this is the case, the application MUST recover make use of RTP. If this is the case, the application MUST recover
the original timing relationship between the streams using whatever the original timing relationship between the streams using whatever
available mechanism. available mechanism.
6.1 Example of LS 6.1 Example of LS
The following example shows a session description of a conference The following example shows a session description of a conference
that is being multicast. The first media stream (mid:1) contains the that is being multicast. The first media stream (mid:1) contains the
voice of the speaker, who speaks in English. The second media stream voice of the speaker who speaks in English. The second media stream
(mid:2) contains the video component and the third (mid:3) media (mid:2) contains the video component and the third (mid:3) media
stream carries the translation to Spanish of what he is saying. The stream carries the translation to Spanish of what he is saying. The
first and the second media streams MUST be synchronized. first and the second media streams MUST be synchronized.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 one.example.com o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0 t=0 0
c=IN IP4 224.2.17.12/127 c=IN IP4 224.2.17.12/127
a=group:LS 1 2 a=group:LS 1 2
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=video 30002 RTP/AVP 31 m=video 30002 RTP/AVP 31
a=mid:2 a=mid:2
m=audio 30004 RTP/AVP 0 m=audio 30004 RTP/AVP 0
i=This media stream contains the Spanish translation i=This media stream contains the Spanish translation
a=mid:3 a=mid:3
Camarillo/Holler/Eriksson/Schulzrinne 4
Grouping of media lines in SDP
Note that although the third media stream is not present in the Note that although the third media stream is not present in the group
group line it still MUST contain an mid attribute (mid:3), as stated line, it still MUST contain a mid attribute (mid:3), as stated
before. before.
7. Flow Identification (FID) 7. Flow Identification (FID)
An "m" line in an SDP session description defines a media stream. An "m" line in an SDP session description defines a media stream.
However, SDP does not define what a media stream is. This definition 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 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 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 video stream as well as a single whiteboard or shared application
group. When using RTP, a stream consists of all RTP and RTCP packets group. When using RTP, a stream consists of all RTP and RTCP packets
created by a source within an RTP session". created by a source within an RTP session".
This definition assumes that a single audio (or video) stream maps This definition assumes that a single audio (or video) stream maps
into an RTP session. The RTP RFC [4] defines an RTP session as into an RTP session. The RTP RFC [6] defines an RTP session as
follows: "For each participant, the session is defined by a follows: "For each participant, the session is defined by a
particular pair of destination transport addresses (one network particular pair of destination transport addresses (one network
address plus a port pair for RTP and RTCP)". address plus a port pair for RTP and RTCP)".
While the previous definitions cover the most common cases, there While the previous definitions cover the most common cases, there are
are situations where a single media instance, (e.g., an audio stream situations where a single media instance, (e.g., an audio stream or a
or a video stream) is sent using more than one RTP session. Two video stream) is sent using more than one RTP session. Two examples
examples (among many others) of this kind of situation are cellular (among many others) of this kind of situation are cellular systems
systems using SIP [5] and systems receiving DTMF tones on a using SIP [3] and systems receiving DTMF tones on a different host
different host than the voice. than the voice.
7.1 SIP and cellular access 7.1 SIP and Cellular Access
Systems using a cellular access and SIP as a signalling protocol Systems using a cellular access and SIP as a signalling protocol need
need to receive media over the air. During a session the media can to receive media over the air. During a session the media can be
be encoded using different codecs. The encoded media has to traverse encoded using different codecs. The encoded media has to traverse
the radio interface. The radio interface is generally characterized the radio interface. The radio interface is generally characterized
by being bit error prone and associated with relatively high packet by being bit error prone and associated with relatively high packet
transfer delays. In addition, radio interface resources in a transfer delays. In addition, radio interface resources in a
cellular environment are scarce and thus expensive, which calls for cellular environment are scarce and thus expensive, which calls for
special measures in providing a highly efficient transport. In order special measures in providing a highly efficient transport. In order
to get an appropriate speech quality in combination with an to get an appropriate speech quality in combination with an efficient
efficient transport, precise knowledge of codec properties are transport, precise knowledge of codec properties are required so that
required so that a proper radio bearer for the RTP session can be a proper radio bearer for the RTP session can be configured before
configured before transferring the media. These radio bearers are transferring the media. These radio bearers are dedicated bearers
dedicated bearers per media type, i.e. codec. per media type, i.e., codec.
Cellular systems typically configure different radio bearers on Cellular systems typically configure different radio bearers on
different port numbers. Therefore, incoming media has to have different port numbers. Therefore, incoming media has to have
different destination port numbers for the different possible codecs different destination port numbers for the different possible codecs
in order to be routed properly to the correct radio bearer. Thus, 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 this is an example in which several RTP sessions are used to carry a
single media instance (the encoded speech from the sender). single media instance (the encoded speech from the sender).
Camarillo/Holler/Eriksson/Schulzrinne 5 7.2 DTMF Tones
Grouping of media lines in SDP
7.2 DTMF tones
Some voice sessions include DTMF tones. Sometimes the voice handling Some voice sessions include DTMF tones. Sometimes the voice handling
is performed by a different host than the DTMF handling. It is is performed by a different host than the DTMF handling. It is
common to have an application server in the network gathering DTMF common to have an application server in the network gathering DTMF
tones for the user while the user receives the encoded speech on his 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 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 sessions: one for the voice and the other for the DTMF tones. Both
RTP sessions are logically part of the same media instance. RTP sessions are logically part of the same media instance.
7.3 Media flow definition 7.3 Media Flow Definition
The previous examples show that the definition of a media stream in 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 [5] do not cover some scenarios. It cannot be assumed that a single
media instance maps into a single RTP session. Therefore, we media instance maps into a single RTP session. Therefore, we
introduce the definition of a media flow: introduce the definition of a media flow:
Media flow consists of a single media instance, e.g., an audio Media flow consists of a single media instance, e.g., an audio stream
stream or a video stream as well as a single whiteboard or shared or a video stream as well as a single whiteboard or shared
application group. When using RTP, a media flow comprises one or application group. When using RTP, a media flow comprises one or
more RTP sessions. more RTP sessions.
7.4 FID semantics 7.4 FID Semantics
Several "m" lines grouped together using FID semantics form a media Several "m" lines grouped together using FID semantics form a media
flow. A media agent handling a media flow that comprises several "m" 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 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 flow as long as the codecs and the direction attribute present in a
particular "m" line allow it. particular "m" line allow it.
It is assumed that the application uses only one codec at a time to It is assumed that the application uses only one codec at a time to
encode the media produced. This codec MAY change dynamically during encode the media produced. This codec MAY change dynamically during
the session, but at any certain moment only one codec is in use. the session, but at any particular moment only one codec is in use.
The application encodes the media using the current codec and checks 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 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 particular "m" line contains the codec being used and the direction
attribute is "sendonly" or "sendrecv" a copy of the encoded media is attribute is "sendonly" or "sendrecv", a copy of the encoded media is
sent to the address/port specified in that particular media stream. 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 If either the "m" line does not contain the codec being used or the
direction attribute is neither "sendonly" nor "sendrecv" nothing is direction attribute is neither "sendonly" nor "sendrecv", nothing is
sent over this media stream. sent over this media stream.
The application typically ends up sending media to different The application typically ends up sending media to different
destinations (IP address/port number) depending on the codec used at destinations (IP address/port number) depending on the codec used at
any moment. any moment.
7.4.1 Examples of FID 7.4.1 Examples of FID
The session description below would be the SDP sent by a SIP user The session description below might be sent by a SIP user agent using
agent using a cellular access. The user agent supports GSM on port a cellular access. The user agent supports GSM on port 30000 and AMR
30000 and AMR on port 30002. When the remote party sends GSM it will on port 30002. When the remote party sends GSM, it will send RTP
send RTP packets to port number 30000. When AMR is the codec chosen, 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
Camarillo/Holler/Eriksson/Schulzrinne 6 between both codecs dynamically in the middle of the session.
Grouping of media lines in SDP
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 However, in this example, only one media stream at a time carries
voice. The other remains "muted" while its corresponding codec is voice. The other remains "muted" while its corresponding codec is
not in use. not in use.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 two.example.com o=Laura 289083124 289083124 IN IP4 two.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 30000 RTP/AVP 3 m=audio 30000 RTP/AVP 3
a=rtpmap:3 GSM/8000 a=rtpmap:3 GSM/8000
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 97 m=audio 30002 RTP/AVP 97
a=rtpmap:97 AMR/8000 a=rtpmap:97 AMR/8000
a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
neighbor; maxframes=1 mode-change-neighbor; maxframes=1
a=mid:2 a=mid:2
In the previous example a system receives media on the same IP (The linebreak in the fmtp line accommodates RFC formatting
address on different port numbers. The following example shows how a 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. system can receive different codecs on different IP addresses.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 three.example.com o=Laura 289083124 289083124 IN IP4 three.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 131.160.1.111 c=IN IP4 131.160.1.111
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 97 m=audio 30002 RTP/AVP 97
a=rtpmap:97 AMR/8000 a=rtpmap:97 AMR/8000
a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2; mode-change- a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
neighbor; maxframes=1 mode-change-neighbor; maxframes=1
a=mid:2 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. The cellular terminal of this example only supports the AMR codec.
However, many current IP phones only support PCM (payload 0). In However, many current IP phones only support PCM (payload 0). In
order to be able to interoperate with them, the cellular terminal order to be able to interoperate with them, the cellular terminal
uses a transcoder whose IP address is 131.160.1.111. The cellular 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. terminal includes in its SDP support for PCM at that IP address.
Remote systems will send AMR directly to the terminal but PCM will Remote systems will send AMR directly to the terminal but PCM will be
be sent to the transcoder. The transcoder will be configured (using sent to the transcoder. The transcoder will be configured (using
whatever method) to convert the incoming PCM audio to AMR and send whatever method) to convert the incoming PCM audio to AMR and send it
it to the terminal. 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.
Camarillo/Holler/Eriksson/Schulzrinne 7 The next example shows how the "group" attribute used with FID
Grouping of media lines in SDP semantics can indicate the use of two different codecs in the two
directions of a bidirectional media stream.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 four.example.com o=Laura 289083124 289083124 IN IP4 four.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 8 m=audio 30002 RTP/AVP 8
a=recvonly a=recvonly
a=mid:2 a=mid:2
A user agent that receives the SDP above knows that at a certain 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- moment it can send either PCM u-law to port number 30000 or PCM A-law
law to port number 30002. However, the media agent also knows that to port number 30002. However, the media agent also knows that the
the other end will only send PCM u-law (payload 0). other end will only send PCM u-law (payload 0).
The following example shows a session description with different "m" The following example shows a session description with different "m"
lines grouped together using FID semantics that contain the same lines grouped together using FID semantics that contain the same
codec. codec.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 five.example.com o=Laura 289083124 289083124 IN IP4 five.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 3 a=group:FID 1 2 3
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 8 m=audio 30002 RTP/AVP 8
a=mid:2 a=mid:2
m=audio 20000 RTP/AVP 0 8 m=audio 20000 RTP/AVP 0 8
c=IN IP4 131.160.1.111 c=IN IP4 131.160.1.111
a=recvonly a=recvonly
a=mid:3 a=mid:3
At a particular point of time, if the media agent is sending PCM u- 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 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 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 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 131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000
(second and third "m" lines). (second and third "m" lines).
The system that generated the SDP above supports PCM u-law on port 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 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 server whose IP address is 131.160.1.111 that records the
conversation. That is why the application server always receives a conversation. That is why the application server always receives a
copy of the audio stream regardless of the codec being used at any copy of the audio stream regardless of the codec being used at any
given moment (it actually performs an RTP dump, so it can given moment (it actually performs an RTP dump, so it can effectively
effectively receive any codec). receive any codec).
Camarillo/Holler/Eriksson/Schulzrinne 8
Grouping of media lines in SDP
Remember that if several "m" lines grouped together using FID Remember that if several "m" lines grouped together using FID
semantics contain the same codec the media agent MUST send media semantics contain the same codec the media agent MUST send media over
over several RTP sessions at the same time. several RTP sessions at the same time.
The last example of this section deals with DTMF tones. DTMF tones The last example of this section deals with DTMF tones. DTMF tones
can be transmitted using a regular voice codec or can be transmitted can be transmitted using a regular voice codec or can be transmitted
as telephony events. The RTP payload for DTMF tones treated as as telephony events. The RTP payload for DTMF tones treated as
telephone events is described in RFC 2833 [6]. Below there is an telephone events is described in RFC 2833 [7]. Below, there is an
example of an SDP session description using FID semantics and this example of an SDP session description using FID semantics and this
payload type. payload type.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 six.example.com o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 20000 RTP/AVP 97 m=audio 20000 RTP/AVP 97
c=IN IP4 131.160.1.111 c=IN IP4 131.160.1.111
a=rtpmap:97 telephone-events a=rtpmap:97 telephone-events
a=mid:2 a=mid:2
The remote party would send PCM encoded voice (payload 0) to The remote party would send PCM encoded voice (payload 0) to
131.160.1.112 and DTMF tones encoded as telephony events to 131.160.1.112 and DTMF tones encoded as telephony events to
131.160.1.111. Note that only voice or DTMF is sent at a particular 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 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 not carry any data and, when voice is sent, there is no data in the
second media stream. FID semantics provide different destinations second media stream. FID semantics provide different destinations
for alternative codecs. for alternative codecs.
7.5 Scenarios that FID does not cover 7.5 Scenarios that FID does not Cover
It is worthwhile mentioning some scenarios where the "group" It is worthwhile mentioning some scenarios where the "group"
attribute using existing semantics (particularly FID) might seem to attribute using existing semantics (particularly FID) might seem to
be applicable but it is not. This section has been included because be applicable but is not.
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 7.5.1 Parallel Encoding Using Different Codecs
FID semantics are useful when the application only uses one codec at FID semantics are useful when the application only uses one codec at
a time. An application that encodes the same media using different a time. An application that encodes the same media using different
codecs simultaneously MUST NOT use FID to group those media lines. codecs simultaneously MUST NOT use FID to group those media lines.
Some systems that handle DTMF tones are a typical example of Some systems that handle DTMF tones are a typical example of parallel
parallel encoding using different codecs. encoding using different codecs.
Some systems implement the RTP payload defined in RFC 2833, but when Some systems implement the RTP payload defined in RFC 2833, but when
they send DTMF tones they do not mute the voice channel. Therefore, they send DTMF tones they do not mute the voice channel. Therefore,
effectively they are sending two copies of the same DTMF tone: in effect they are sending two copies of the same DTMF tone: encoded
encoded as voice and encoded as a telephony event. When the receiver as voice and encoded as a telephony event. When the receiver gets
gets both copies it typically uses the telephony event rather than both copies, it typically uses the telephony event rather than the
the tone encoded as voice. FID semantics MUST NOT be used in this tone encoded as voice. FID semantics MUST NOT be used in this
Camarillo/Holler/Eriksson/Schulzrinne 9
Grouping of media lines in SDP
context to group both media streams since such a system is not using context to group both media streams since such a system is not using
alternative codecs but rather different parallel encodings for the alternative codecs but rather different parallel encodings for the
same information. same information.
7.5.2 Layered encoding 7.5.2 Layered Encoding
Layered encoding schemes encode media in different layers. Quality Layered encoding schemes encode media in different layers. Quality
at the receiver varies depending on the number of layers received. at the receiver varies depending on the number of layers received.
SDP provides a means to group together contiguous multicast SDP provides a means to group together contiguous multicast addresses
addresses that transport different layers. The "c" line below: that transport different layers. The "c" line below:
c=IN IP4 224.2.1.1/127/3 c=IN IP4 224.2.1.1/127/3
is equivalent to the following three "c" lines: is equivalent to the following three "c" lines:
c=IN IP4 224.2.1.1/127 c=IN IP4 224.2.1.1/127
c=IN IP4 224.2.1.2/127 c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127 c=IN IP4 224.2.1.3/127
FID MUST NOT be used to group "m" lines that do not represent the 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 same information. Therefore, FID MUST NOT be used to group "m" lines
that contain the different layers of layered encoding scheme. that contain the different layers of layered encoding scheme.
Besides, we do not define new group semantics to provide a more Besides, we do not define new group semantics to provide a more
flexible way of grouping different layers because the already flexible way of grouping different layers because the already
existing SDP mechanism covers the most useful scenarios. existing SDP mechanism covers the most useful scenarios.
7.5.3 Same IP address and port number 7.5.3 Same IP Address and Port Number
If several codecs have to be sent to the same IP address and port, 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" 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 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 same IP address/port. Therefore, an SDP like the one below MUST NOT
be generated. be generated.
v=0 v=0
o=Laura 289083124 289083124 IN IP4 six.example.com o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 30000 RTP/AVP 8 m=audio 30000 RTP/AVP 8
a=mid:2 a=mid:2
The correct SDP for the session above would be the following one: The correct SDP for the session above would be the following one:
v=0 v=0
o=Laura 289083124 289083124 IN IP4 six.example.com o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
m=audio 30000 RTP/AVP 0 8 m=audio 30000 RTP/AVP 0 8
Camarillo/Holler/Eriksson/Schulzrinne 10
Grouping of media lines in SDP
If two "m" lines are grouped using FID they MUST differ in their If two "m" lines are grouped using FID they MUST differ in their
transport addresses (i.e., IP address plus port). transport addresses (i.e., IP address plus port).
8. Usage of the "group" attribute in SIP 8. Usage of the "group" Attribute in SIP
SDP descriptions are used by several different protocols, SIP among SDP descriptions are used by several different protocols, SIP among
them. We include a section about SIP because the "group" attribute them. We include a section about SIP because the "group" attribute
will most likely be used mainly by SIP systems. will most likely be used mainly by SIP systems.
SIP [5] is an application layer protocol for establishing, SIP [3] is an application layer protocol for establishing,
terminating and modifying multimedia sessions. SIP carries session terminating and modifying multimedia sessions. SIP carries session
descriptions in the bodies of the SIP messages but is independent descriptions in the bodies of the SIP messages but is independent
from the protocol used for describing sessions. SDP [2] is one of from the protocol used for describing sessions. SDP [2] is one of
the protocols that can be used for this purpose. the protocols that can be used for this purpose.
At session establishment SIP provides a three-way handshake (INVITE- At session establishment SIP provides a three-way handshake (INVITE-
200 OK-ACK) between end systems. However, just two of these three 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 messages carry SDP, as described in [4].
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 8.1 Mid Value in Answers
The "mid" attribute is an identifier for a particular media stream. The "mid" attribute is an identifier for a particular media stream.
Therefore, the "mid" value in the response MUST be the same as the Therefore, the "mid" value in the offer MUST be the same as the "mid"
"mid" value in the request. Besides, subsequent requests such as re- value in the answer. Besides, subsequent offers (e.g., in a re-
INVITEs SHOULD use the same "mid" value for the already existing INVITE) SHOULD use the same "mid" value for the already existing
media streams. media streams.
Appendix B of [5] describes the usage of SDP in relation to SIP. It RFC 3264 [4] describes the usage of SDP in relation to SIP. The
states: "The caller and callee align their media description so that offerer and the answerer align their media description so that the
the nth media stream ("m=" line) in the callers session description nth media stream ("m=" line) in the offerer's session description
corresponds to the nth media stream in the callees description." corresponds to the nth media stream in the answerer's description.
The presence of the "group" attribute in an SDP session description The presence of the "group" attribute in an SDP session description
does not modify this behavior. does not modify this behavior.
Since the "mid" attribute provides a means to label "m" lines it Since the "mid" attribute provides a means to label "m" lines, it
would be possible to perform media alignment using "mid" labels would be possible to perform media alignment using "mid" labels
rather than matching nth "m" lines. However this would not bring any rather than matching nth "m" lines. However this would not bring any
gain and would add complexity to implementations. Therefore SIP gain and would add complexity to implementations. Therefore SIP
systems MUST perform media alignment matching nth lines regardless systems MUST perform media alignment matching nth lines regardless of
of the presence of the "group" or "mid" attributes. 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:
Camarillo/Holler/Eriksson/Schulzrinne 11 If a media stream that contained a particular "mid" identifier in the
Grouping of media lines in SDP 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 8.1.1 Example
Two SIP entities exchange SDPs during session establishment. The Two SIP entities exchange SDPs during session establishment. The
INVITE contained the SDP below: INVITE contains the SDP below:
v=0 v=0
o=Laura 289083124 289083124 IN IP4 seven.example.com o=Laura 289083124 289083124 IN IP4 seven.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 a=group:FID 1 2
m=audio 30000 RTP/AVP 0 8 m=audio 30000 RTP/AVP 0 8
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 0 8 m=audio 30002 RTP/AVP 0 8
a=mid:2 a=mid:2
The 200 OK response contains the following SDP: The 200 OK response contains the following SDP:
v=0 v=0
o=Bob 289083122 289083122 IN IP4 eigth.example.com o=Bob 289083122 289083122 IN IP4 eigth.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.113 c=IN IP4 131.160.1.113
a=group:FID 1 2 a=group:FID 1 2
m=audio 25000 RTP/AVP 0 8 m=audio 25000 RTP/AVP 0 8
a=mid:2 a=mid:2
m=audio 25002 RTP/AVP 0 8 m=audio 25002 RTP/AVP 0 8
a=mid:1 a=mid:1
Since alignment of "m" lines is performed based on matching of nth 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 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 200 OK. Therefore, the application MUST ignore every "mid" and
"group" lines contained in the SDP. "group" lines contained in the SDP.
A well-behaved SIP user agent would have returned the SDP below in A well-behaved SIP user agent would have returned the SDP below in
the 200 OK: the 200 OK:
v=0 v=0
o=Bob 289083122 289083122 IN IP4 nine.example.com o=Bob 289083122 289083122 IN IP4 nine.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.113 c=IN IP4 131.160.1.113
a=group:FID 1 2 a=group:FID 1 2
m=audio 25002 RTP/AVP 0 8 m=audio 25002 RTP/AVP 0 8
a=mid:1 a=mid:1
m=audio 25000 RTP/AVP 0 8 m=audio 25000 RTP/AVP 0 8
a=mid:2 a=mid:2
8.2 Group value in responses 8.2 Group Value in Answers
A SIP entity that receives a request that contains an "a=group" line A SIP entity that receives an offer that contains an "a=group" line
with semantics that it does not understand MUST return a response with semantics that it does not understand MUST return an answer
without the "group" line. Note that, as it was described in the without the "group" line. Note that, as it was described in the
previous section, the "mid" lines MUST still be present in the previous section, the "mid" lines MUST still be present in the
response. answer.
Camarillo/Holler/Eriksson/Schulzrinne 12
Grouping of media lines in SDP
A SIP entity that receives a request that contains an "a=group" line A SIP entity that receives an offer that contains an "a=group" line
which semantics that are understood MUST return a response that with semantics that are understood MUST return an answer that
contains an "a=group" line with the same semantics. The contains an "a=group" line with the same semantics. The
identification-tags contained in this "a=group" lines MUST be 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 same that were received in the offer or a subset of them (zero
identification-tags is a valid subset). When the identification-tags identification-tags is a valid subset). When the identification-tags
in the response are a subset the "group" value to be used in the in the answer are a subset, the "group" value to be used in the
session MUST be the one present in the response. session MUST be the one present in the answer.
SIP entities refuse media streams by setting the port to zero in the SIP entities refuse media streams by setting the port to zero in the
corresponding "m" line. "a=group" lines MUST NOT contain corresponding "m" line. "a=group" lines MUST NOT contain
identification-tags that correspond to "m" lines with port zero. identification-tags that correspond to "m" lines with port zero.
Note that grouping of m lines MUST always be requested by the issuer Note that grouping of m lines MUST always be requested by the
of the request (the client), never by the issuer of the response offerer, never by the answerer. Since SIP provides a two-way SDP
(the server). Since SIP provides a two-way SDP exchange, a server exchange, an answerer that requested grouping would not know whether
that requested grouping in a response would not know whether the the "group" attribute was accepted by the offerer or not. An
"group" attribute was accepted by the client or not. A server that answerer that wants to group media lines SHOULD issue another offer
wants to group media lines SHOULD issue another request after having after having responded to the first one (in a re-INVITE for
responded to the first one (a re-INVITE for instance). 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 8.2.1 Example
The example below shows how the callee refuses a media stream The example below shows how the callee refuses a media stream offered
offered by the caller by setting its port number to zero. The "mid" by the caller by setting its port number to zero. The "mid" value
value corresponding to that media stream is removed from the "group" corresponding to that media stream is removed from the "group" value
value in the response. in the answer.
SDP in the INVITE from caller to callee: SDP in the INVITE from caller to callee:
v=0 v=0
o=Laura 289083124 289083124 IN IP4 ten.example.com o=Laura 289083124 289083124 IN IP4 ten.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID 1 2 3 a=group:FID 1 2 3
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 30002 RTP/AVP 8 m=audio 30002 RTP/AVP 8
a=mid:2 a=mid:2
m=audio 30004 RTP/AVP 3 m=audio 30004 RTP/AVP 3
a=mid:3 a=mid:3
SDP in the INVITE from callee to caller: SDP in the INVITE from callee to caller:
v=0 v=0
o=Bob 289083125 289083125 IN IP4 eleven.example.com o=Bob 289083125 289083125 IN IP4 eleven.example.com
t=0 0
Camarillo/Holler/Eriksson/Schulzrinne 13 c=IN IP4 131.160.1.113
Grouping of media lines in SDP a=group:FID 1 3
m=audio 20000 RTP/AVP 0
t=0 0 a=mid:1
c=IN IP4 131.160.1.113 m=audio 0 RTP/AVP 8
a=group:FID 1 3 a=mid:2
m=audio 20000 RTP/AVP 0 m=audio 20002 RTP/AVP 3
a=mid:1 a=mid:3
m=audio 0 RTP/AVP 8
a=mid:2
m=audio 20002 RTP/AVP 3
a=mid:3
8.3 Capability negotiation 8.3 Capability Negotiation
A client that understands "group" and "mid" but does not want to A client that understands "group" and "mid" but does not want to make
make use of them in a particular session MAY want indicate that it 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 supports them. If a client decides to do that, it SHOULD add an
"a=group" line with zero identification-tags for every semantics it "a=group" line with no identification-tags for every semantics it
understands. understands.
If a server receives a request that contains empty "a=group" lines If a server receives an offer that contains empty "a=group" lines, it
it SHOULD add its capabilities also in the form of empty "a=group" SHOULD add its capabilities also in the form of empty "a=group" lines
lines to its response. to its answer.
8.3.1 Example 8.3.1 Example
A system that supports both LS and FID semantics but does not want A system that supports both LS and FID semantics but does not want to
to group any media stream for this particular session generates the group any media stream for this particular session generates the
following SDP: following SDP:
v=0 v=0
o=Bob 289083125 289083125 IN IP4 twelve.example.com o=Bob 289083125 289083125 IN IP4 twelve.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.113 c=IN IP4 131.160.1.113
a=group:LS a=group:LS
a=group:FID a=group:FID
m=audio 20000 RTP/AVP 0 8 m=audio 20000 RTP/AVP 0 8
The server that receives that request supports FID but not LS. It The server that receives that offer supports FID but not LS. It
responds with the SDP below: responds with the SDP below:
v=0 v=0
o=Laura 289083124 289083124 IN IP4 thirteen.example.com o=Laura 289083124 289083124 IN IP4 thirteen.example.com
t=0 0 t=0 0
c=IN IP4 131.160.1.112 c=IN IP4 131.160.1.112
a=group:FID a=group:FID
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
8.4 Backward compatibility 8.4 Backward Compatibility
This document does not define any SIP "Require" header. Therefore, This document does not define any SIP "Require" header. Therefore,
if one of the SIP user agents does not understand the "group" if one of the SIP user agents does not understand the "group"
Camarillo/Holler/Eriksson/Schulzrinne 14
Grouping of media lines in SDP
attribute the standard SDP fall back mechanism MUST be used attribute the standard SDP fall back mechanism MUST be used
(attributes that are not understood are simply ignored). (attributes that are not understood are simply ignored).
8.4.1 Client does not support "group" 8.4.1 Offerer does not Support "group"
This situation does not represent a problem because grouping This situation does not represent a problem because grouping requests
requests is always performed by clients, not by servers. If the are always performed by offerers, not by answerers. If the offerer
client does not support "group" this attribute will just not be does not support "group" this attribute will just not be used.
used.
8.4.2 Server does not support "group" 8.4.2 Answerer does not Support "group"
The server will ignore the "group" attribute, since it does not The answerer will ignore the "group" attribute, since it does not
understand it (it will also ignore the "mid" attribute). For LS understand it (it will also ignore the "mid" attribute). For LS
semantics, the server might decide to perform or to not perform semantics, the answerer might decide to perform or to not perform
synchronization between media streams. synchronization between media streams.
For FID semantics, the server will consider that the session For FID semantics, the answerer will consider that the session
comprises several media streams. comprises several media streams.
Different implementations would behave in different ways. Different implementations would behave in different ways.
In the case of audio and different "m" lines for different codecs an In the case of audio and different "m" lines for different codecs an
implementation might decide to act as a mixer with the different implementation might decide to act as a mixer with the different
incoming RTP sessions, which is the correct behavior. incoming RTP sessions, which is the correct behavior.
An implementation might also decide to refuse the request (e.g. 488 An implementation might also decide to refuse the request (e.g., 488
Not acceptable here or 606 Not Acceptable) because it contains Not acceptable here or 606 Not Acceptable) because it contains
several "m" lines. In this case, the server does not support the several "m" lines. In this case, the server does not support the
type of session that the caller wanted to establish. In case the type of session that the caller wanted to establish. In case the
client is willing to establish a simpler session anyway, he SHOULD client is willing to establish a simpler session anyway, he SHOULD
re-try the request without "group" attribute and only one "m" line re-try the request without "group" attribute and only one "m" line
per flow. per flow.
9. Security considerations 9. Security Considerations
Using the "group" parameter with FID semantics an entity that Using the "group" parameter with FID semantics, an entity that
managed to modify the session descriptions exchanged between the managed to modify the session descriptions exchanged between the
participants to establish a multimedia session could force the participants to establish a multimedia session could force the
participants to send a copy of the media to any particular participants to send a copy of the media to any particular
destination. destination.
Integrity mechanism provided by protocols used to exchange session Integrity mechanism provided by protocols used to exchange session
descriptions and media encryption can be used to prevent this descriptions and media encryption can be used to prevent this attack.
attack.
Camarillo/Holler/Eriksson/Schulzrinne 15
Grouping of media lines in SDP
10. IANA considerations 10. IANA Considerations
This document defines two SDP attributes: "mid" and "group". This document defines two SDP attributes: "mid" and "group".
The "mid" attribute is used to identify media streams within a The "mid" attribute is used to identify media streams within a
session description and its format is defined in Section 3. session description and its format is defined in Section 3.
The "group" attribute is used for grouping together different media The "group" attribute is used for grouping together different media
streams and its format is defined in Section 4. streams and its format is defined in Section 4.
Section 4 also defines two standard semantics related to the "group" This document defines a framework to group media lines in SDP using
attribute: LS (Lip Synchronization) and FID (Flow Identification). different semantics. Semantics to be used with this framework are
If in the future it was needed to standardize further semantics they registered by the IANA when they are published in standards track
would need to be defined in a standards track document. 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.
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 11. Acknowledgments
The authors would like to thank Jonathan Rosenberg, Adam Roach, Orit The authors would like to thank Jonathan Rosenberg, Adam Roach, Orit
Levin and Joerg Ott for their feedback on this document. Levin and Joerg Ott for their feedback on this document.
12. References 12. References
[1] S. Bradner, "Key words for use in RFCs to Indicate Requirement 12.1 Normative References
Levels", RFC 2119, IETF; March 1997.
[2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
2327, IETF; April 1998. Levels", BCP 14, RFC 2119, March 1997.
[3] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
(RTSP)", RFC 2326, IETF; April 1998. RFC 2327, April 1998.
[4] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Transport Protocol for Real-Time Applications", RFC 1889, IETF; Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
January 1996. Session Initiation Protocol", RFC 3261, June 2002.
[5] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP: [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the
Session Initiation Protocol", RFC 2543, IETF; Mach 1999. Session Description Protocol (SDP)", RFC 3264, June 2002.
[6] H. Schulzrinne/S. Petrack, "RTP Payload for DTMF Digits, 12.2 Informative References
Telephony Tones and Telephony Signals", RFC 2833, IETF; May 2000.
13. Authors Addresses [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.
13. Authors' Addresses
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Advanced Signalling Research Lab. Advanced Signalling Research Lab.
FIN-02420 Jorvas FIN-02420 Jorvas
Finland Finland
Phone: +358 9 299 3371 Phone: +358 9 299 3371
Fax: +358 9 299 3052 Fax: +358 9 299 3052
Email: Gonzalo.Camarillo@ericsson.com EMail: Gonzalo.Camarillo@ericsson.com
Camarillo/Holler/Eriksson/Schulzrinne 16
Grouping of media lines in SDP
Jan Holler Jan Holler
Ericsson Research Ericsson Research
S-16480 Stockholm S-16480 Stockholm
Sweden Sweden
Phone: +46 8 58532845 Phone: +46 8 58532845
Fax: +46 8 4047020 Fax: +46 8 4047020
Email: Jan.Holler@era.ericsson.se EMail: Jan.Holler@era.ericsson.se
Goran AP Eriksson Goran AP Eriksson
Ericsson Research Ericsson Research
S-16480 Stockholm S-16480 Stockholm
Sweden Sweden
Phone: +46 8 58531762 Phone: +46 8 58531762
Fax: +46 8 4047020 Fax: +46 8 4047020
Email: Goran.AP.Eriksson@era.ericsson.se EMail: Goran.AP.Eriksson@era.ericsson.se
Henning Schulzrinne Henning Schulzrinne
Dept. of Computer Science Dept. of Computer Science
Columbia University Columbia University
1214 Amsterdam Avenue 1214 Amsterdam Avenue
New York, NY 10027 New York, NY 10027
USA USA
Email: schulzrinne@cs.columbia.edu
Full Copyright Statement EMail: schulzrinne@cs.columbia.edu
Copyright (c) The Internet Society (2002). All Rights Reserved. 14. Full Copyright Statement
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Grouping of media lines in SDP "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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