draft-ietf-mmusic-rfc3388bis-00.txt   draft-ietf-mmusic-rfc3388bis-01.txt 
MMUSIC Working Group G. Camarillo MMUSIC Working Group G. Camarillo
Internet-Draft Ericsson Internet-Draft Ericsson
Obsoletes: 3388 (if approved) June 10, 2008 Obsoletes: 3388 (if approved) July 7, 2008
Intended status: Standards Track Intended status: Standards Track
Expires: December 12, 2008 Expires: January 8, 2009
The SDP (Session Description Protocol) Grouping Framework The SDP (Session Description Protocol) Grouping Framework
draft-ietf-mmusic-rfc3388bis-00.txt draft-ietf-mmusic-rfc3388bis-01.txt
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
Abstract Abstract
This document defines two Session Description Protocol (SDP) In this specification, we define a framework to group "m" lines in
attributes: "group" and "mid". They allow to group together several SDP (Session Description Protocol) for different purposes. This
"m" lines for two different purposes: for lip synchronization and for framework uses the "group" and "mid" SDP attributes, both of which
receiving media from a single flow (several media streams) that are are defined in this specification. Additionally, we specify how to
encoded in different formats during a particular session, on use the framework for two different purposes: for lip synchronization
different ports and host interfaces. and for receiving a media flow consisting of several media streams on
different transport addresses.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Media Stream Identification Attribute . . . . . . . . . . . . 3 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 3
4. Group Attribute . . . . . . . . . . . . . . . . . . . . . . . 3 4. Media Stream Identification Attribute . . . . . . . . . . . . 4
5. Use of "group" and "mid" . . . . . . . . . . . . . . . . . . . 4 5. Group Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
6. Lip Synchronization (LS) . . . . . . . . . . . . . . . . . . . 4 6. Use of "group" and "mid" . . . . . . . . . . . . . . . . . . . 4
6.1. Example of LS . . . . . . . . . . . . . . . . . . . . . . 5 7. Lip Synchronization (LS) . . . . . . . . . . . . . . . . . . . 5
7. Flow Identification (FID) . . . . . . . . . . . . . . . . . . 5 7.1. Example of LS . . . . . . . . . . . . . . . . . . . . . . 5
7.1. SIP and Cellular Access . . . . . . . . . . . . . . . . . 6 8. Flow Identification (FID) . . . . . . . . . . . . . . . . . . 6
7.2. DTMF Tones . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. SIP and Cellular Access . . . . . . . . . . . . . . . . . 6
7.3. Media Flow Definition . . . . . . . . . . . . . . . . . . 6 8.2. DTMF Tones . . . . . . . . . . . . . . . . . . . . . . . . 7
7.4. FID Semantics . . . . . . . . . . . . . . . . . . . . . . 7 8.3. Media Flow Definition . . . . . . . . . . . . . . . . . . 7
7.4.1. Examples of FID . . . . . . . . . . . . . . . . . . . 7 8.4. FID Semantics . . . . . . . . . . . . . . . . . . . . . . 7
7.5. Scenarios that FID does not Cover . . . . . . . . . . . . 10 8.4.1. Examples of FID . . . . . . . . . . . . . . . . . . . 8
7.5.1. Parallel Encoding Using Different Codecs . . . . . . . 11 8.5. Scenarios that FID does not Cover . . . . . . . . . . . . 11
7.5.2. Layered Encoding . . . . . . . . . . . . . . . . . . . 11 8.5.1. Parallel Encoding Using Different Codecs . . . . . . . 11
7.5.3. Same IP Address and Port Number . . . . . . . . . . . 11 8.5.2. Layered Encoding . . . . . . . . . . . . . . . . . . . 11
8. Usage of the "group" Attribute in SIP . . . . . . . . . . . . 12 8.5.3. Same IP Address and Port Number . . . . . . . . . . . 12
8.1. Mid Value in Answers . . . . . . . . . . . . . . . . . . . 12 9. Usage of the "group" Attribute in SIP . . . . . . . . . . . . 13
8.1.1. Example . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. Mid Value in Answers . . . . . . . . . . . . . . . . . . . 13
8.2. Group Value in Answers . . . . . . . . . . . . . . . . . . 14 9.1.1. Example . . . . . . . . . . . . . . . . . . . . . . . 14
8.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 15 9.2. Group Value in Answers . . . . . . . . . . . . . . . . . . 15
8.3. Capability Negotiation . . . . . . . . . . . . . . . . . . 16 9.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 15
8.3.1. Example . . . . . . . . . . . . . . . . . . . . . . . 16 9.3. Capability Negotiation . . . . . . . . . . . . . . . . . . 16
8.4. Backward Compatibility . . . . . . . . . . . . . . . . . . 16 9.3.1. Example . . . . . . . . . . . . . . . . . . . . . . . 16
8.4.1. Offerer does not Support "group" . . . . . . . . . . . 17 9.4. Backward Compatibility . . . . . . . . . . . . . . . . . . 17
8.4.2. Answerer does not Support "group" . . . . . . . . . . 17 9.4.1. Offerer does not Support "group" . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17 9.4.2. Answerer does not Support "group" . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 10. Changes from RFC 3388 . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11.2. Informational References . . . . . . . . . . . . . . . . . 19 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . . 19
13.2. Informational References . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20 Intellectual Property and Copyright Statements . . . . . . . . . . 21
1. Introduction 1. Introduction
An SDP session description typically contains one or more media lines An SDP [RFC4566] session description typically contains one or more
- they are commonly known as "m" lines. When a session description media lines, which are commonly known as "m" lines. When a session
contains more than one "m" line, SDP does not provide any means to description contains more than one "m" line, SDP does not provide any
express a particular relationship between two or more of them. When means to express a particular relationship between two or more of
an application receives an SDP session description with more than one them. When an application receives an SDP session description with
"m" line, it is up to the application what to do with them. SDP does more than one "m" line, it is up to the application what to do with
not carry any information about grouping media streams. them. SDP does not carry any information about 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 allow the band, it is necessary to have a mechanism in SDP to express how
expression of how different media streams within a session different media streams within a session description relate to each
description relate to each other. This document defines such other. The framework defined in this specification is such a
extensions. mechanism.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Media Stream Identification Attribute 3. Overview of Operation
This section provides a non-normative description on 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 an "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)
and group for the purpose of lip synchronization (LS).
v=0
o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0
c=IN IP4 192.0.2.1
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
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 [RFC2327] is described by the following BNF: Its formatting in SDP [RFC4566] is described by the following BNF
(Backus-Naur Form):
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 5. 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" / semantics-extension
semantics-extension = token
This document defines two standard semantics: LS (Lip This document defines two standard semantics: LS (Lip
Synchronization) and FID (Flow Identification). Further semantics Synchronization) and FID (Flow Identification). Further semantics
need to be defined in a standards-track document. However, defining MUST be defined in a standards-track document.
new semantics apart from LS and FID is discouraged. Instead, it is
RECOMMENDED to use other session description mechanisms such as
SDPng.
5. Use of "group" and "mid" 6. 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 a "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 are "a=group" lines are used to group together several "m" lines that are
identified by their "mid" attribute. "a=group" lines that contain identified by their "mid" attribute. "a=group" lines that contain
identification-tags that do not correspond to any "m" line within the identification-tags that do not correspond to any "m" line within the
session description MUST be ignored. The application acts as if the session description MUST be ignored. The application acts as if the
"a=group" line did not exist. The behavior of an application "a=group" line did not exist. The behavior of an application
receiving an SDP with grouped "m" lines is defined by the semantics receiving an SDP with grouped "m" lines is defined by the semantics
field in the "a=group" line. field in the "a=group" line.
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" lines appear in more than one "a=group" line.
use different semantics. An "m" line identified by its "mid"
attribute MUST NOT appear in more than one "a=group" line using the
same semantics.
6. Lip Synchronization (LS) 7. 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 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 7.1. 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 have to 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
skipping to change at page 5, line 27 skipping to change at page 6, line 4
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
Note that although the third media stream is not present in the group Note that although the third media stream is not present in the group
line, it still MUST contain a mid attribute (mid:3), as stated line, it still has to contain a mid attribute (mid:3), as stated
before. before.
7. Flow Identification (FID) 8. 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 [RFC2326] can be found in the RTSP specification. The RTSP RFC [RFC2326]
defines a media stream as "a single media instance, e.g., an audio 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 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 application group. When using RTP, a stream consists of all RTP and
RTCP packets created by a source within an RTP session". RTCP packets 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 [RFC1889] defines an RTP session as into an RTP session. The RTP RFC [RFC1889] (at present obsoleted by
follows: "For each participant, the session is defined by a [RFC3550]) used to define an RTP session as follows: "For each
particular pair of destination transport addresses (one network participant, the session is defined by a particular pair of
address plus a port pair for RTP and RTCP)". 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 While the previous definitions cover the most common cases, there are
situations where a single media instance, (e.g., an audio stream or a 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 video stream) is sent using more than one RTP session. Two examples
(among many others) of this kind of situation are cellular systems (among many others) of this kind of situation are cellular systems
using SIP [RFC3261] and systems receiving DTMF tones on a different using SIP (Session Initiation Protocol; [RFC3261]) and systems
host than the voice. receiving DTMF (Dual-Tone Multi-Frequency) tones on a different host
than the voice.
7.1. SIP and Cellular Access 8.1. SIP and Cellular Access
Systems using a cellular access and SIP as a signalling protocol need 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 to receive media over the air. During a session the media can 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 efficient to get an appropriate speech quality in combination with an efficient
transport, precise knowledge of codec properties are required so that transport, precise knowledge of codec properties are required so that
a proper radio bearer for the RTP session can be configured before a proper radio bearer for the RTP session can be configured before
transferring the media. These radio bearers are dedicated bearers transferring the media. These radio bearers are 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).
7.2. DTMF Tones 8.2. 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 8.3. 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
[RFC2326] do not cover some scenarios. It cannot be assumed that a [RFC2326] do not cover some scenarios. It cannot be assumed that a
single media instance maps into a single RTP session. Therefore, we single 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 stream 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 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 8.4. 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 particular moment only one codec is in use. the session, but at any particular moment only one codec is in use.
skipping to change at page 7, line 31 skipping to change at page 8, line 9
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 8.4.1. Examples of FID
The session description below might be sent by a SIP user agent using The session description below might be sent by a SIP user agent using
a cellular access. The user agent supports GSM on port 30000 and AMR a cellular access. The user agent supports GSM (Global System for
on port 30002. When the remote party sends GSM, it will send RTP 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 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 will be sent to port 30002. Note that the remote party can switch
between both codecs dynamically in the middle of the session. 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 192.0.2.1
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; a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
mode-change-neighbor; maxframes=1 mode-change-neighbor; maxframes=1
a=mid:2 a=mid:2
(The linebreak in the fmtp line accommodates RFC formatting (The linebreak in the fmtp line accommodates RFC formatting
restrictions; SDP does not have continuation lines.) restrictions; SDP does not have continuation lines.)
In the previous example, a system receives media on the same IP In the previous example, a system receives media on the same IP
address on different port numbers. The following example shows how a 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 192.0.2.1
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 192.0.2.2
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; a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
mode-change-neighbor; maxframes=1 mode-change-neighbor; maxframes=1
a=mid:2 a=mid:2
(The linebreak in the fmtp line accomodates RFC formatting (The linebreak in the fmtp line accomodates RFC formatting
restrictions; SDP does not have continuation lines.) 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 (Pulse-Code
order to be able to interoperate with them, the cellular terminal Modulation; payload 0). In order to be able to interoperate with
uses a transcoder whose IP address is 131.160.1.111. The cellular them, the cellular terminal uses a transcoder whose IP address is
terminal includes in its SDP support for PCM at that IP address. 192.0.2.2. The cellular terminal includes in its SDP support for PCM
Remote systems will send AMR directly to the terminal but PCM will be at that IP address. Remote systems will send AMR directly to the
sent to the transcoder. The transcoder will be configured (using terminal but PCM will be sent to the transcoder. The transcoder will
whatever method) to convert the incoming PCM audio to AMR and send it be configured (using whatever method) to convert the incoming PCM
to the terminal. audio to AMR and send it to the terminal.
The next example shows how the "group" attribute used with FID The next example shows how the "group" attribute used with FID
semantics can indicate the use of two different codecs in the two semantics can indicate the use of two different codecs in the two
directions of a bidirectional media stream. 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 192.0.2.1
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-law 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 to port number 30002. However, the media agent also knows that 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 192.0.2.1
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 192.0.2.2
a=recvonly a=recvonly
a=mid:3 a=mid:3
At a particular point in 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 192.0.2.1 on port 30000 and
and to 131.160.1.111 on port 20000 (first and third "m" lines). If to 192.0.2.2 on port 20000 (first and third "m" lines). If it is
it is sending PCM A-law (payload 8), it sends RTP packets to sending PCM A-law (payload 8), it sends RTP packets to 192.0.2.1 on
131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000 port 30002 and to 192.0.2.2 on port 20000 (second and third "m"
(second and third "m" lines). 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 the server whose IP address is 192.0.2.2 that records the conversation.
conversation. That is why the application server always receives a That is why the application server always receives a copy of the
copy of the audio stream regardless of the codec being used at any audio stream regardless of the codec being used at any given moment
given moment (it actually performs an RTP dump, so it can effectively (it actually performs an RTP dump, so it can effectively receive any
receive any codec). codec).
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 over semantics contain the same codec the media agent MUST send media 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 [RFC2833]. Below, there is an telephone events is described in [RFC2833]. 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 192.0.2.1
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 192.0.2.2
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 192.0.2.1 and DTMF tones encoded as telephony events to 192.0.2.2.
131.160.1.111. Note that only voice or DTMF is sent at a particular Note that only voice or DTMF is sent at a particular point of time.
point of time. When DTMF tones are sent, the first media stream does When DTMF tones are sent, the first media stream does not carry any
not carry any data and, when voice is sent, there is no data in the data and, when voice is sent, there is no data in the second media
second media stream. FID semantics provide different destinations stream. FID semantics provide different destinations for alternative
for alternative codecs. codecs.
7.5. Scenarios that FID does not Cover 8.5. 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 is not. be applicable but is not.
7.5.1. Parallel Encoding Using Different Codecs 8.5.1. 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 parallel Some systems that handle DTMF tones are a typical example of 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,
in effect they are sending two copies of the same DTMF tone: encoded 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 as voice and encoded as a telephony event. When the receiver gets
both copies, it typically uses the telephony event rather than the both copies, it typically uses the telephony event rather than 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
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 8.5.2. 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 addresses SDP provides a means to group together contiguous multicast 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
skipping to change at page 11, line 45 skipping to change at page 12, line 23
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 8.5.3. 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 192.0.2.1
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 192.0.2.1
m=audio 30000 RTP/AVP 0 8 m=audio 30000 RTP/AVP 0 8
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 9. 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 [RFC3261] is an application layer protocol for establishing, SIP [RFC3261] 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 [RFC2327] is one from the protocol used for describing sessions. SDP [RFC4566] is one
of the protocols that can be used for this purpose. of 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, as described in [RFC3264]. messages carry SDP, as described in [RFC3264].
8.1. Mid Value in Answers 9.1. 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 offer MUST be the same as the "mid" 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- value in the answer. Besides, subsequent offers (e.g., in a re-
INVITE) 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.
[RFC3264] describes the usage of SDP in relation to SIP. The offerer [RFC3264] describes the usage of SDP in relation to SIP. The offerer
and the answerer align their media description so that the nth media and the answerer align their media description so that the nth media
stream ("m=" line) in the offerer's session description corresponds stream ("m=" line) in the offerer's session description corresponds
skipping to change at page 13, line 27 skipping to change at page 14, line 5
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 of systems MUST perform media alignment matching nth lines regardless 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 If a media stream that contained a particular "mid" identifier in the
offer contains a different identifier in the answer the application offer contains a different identifier in the answer the application
ignores all the "mid" and "group" lines that might appear in the ignores all the "mid" and "group" lines that might appear in the
session description. The following example illustrates this session description. The following example illustrates this
scenario. scenario.
8.1.1. Example 9.1.1. Example
Two SIP entities exchange SDPs during session establishment. The Two SIP entities exchange SDPs during session establishment. The
INVITE contains 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 192.0.2.1
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 192.0.2.3
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 192.0.2.3
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 Answers 9.2. Group Value in Answers
A SIP entity that receives an offer 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 an answer 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
answer. answer.
A SIP entity that receives an offer that contains an "a=group" line A SIP entity that receives an offer that contains an "a=group" line
with semantics that are understood MUST return an answer 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
skipping to change at page 15, line 14 skipping to change at page 15, line 34
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 Note that grouping of m lines MUST always be requested by the
offerer, never by the answerer. Since SIP provides a two-way SDP offerer, never by the answerer. Since SIP provides a two-way SDP
exchange, an answerer that requested grouping would not know whether exchange, an answerer that requested grouping would not know whether
the "group" attribute was accepted by the offerer or not. An the "group" attribute was accepted by the offerer or not. An
answerer that wants to group media lines SHOULD issue another offer answerer that wants to group media lines SHOULD issue another offer
after having responded to the first one (in a re-INVITE for after having responded to the first one (in a re-INVITE for
instance). instance).
8.2.1. Example 9.2.1. Example
The example below shows how the callee refuses a media stream offered 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 by the caller by setting its port number to zero. The "mid" value
corresponding to that media stream is removed from the "group" value corresponding to that media stream is removed from the "group" value
in the answer. 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 192.0.2.1
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 t=0 0
c=IN IP4 131.160.1.113 c=IN IP4 192.0.2.3
a=group:FID 1 3 a=group:FID 1 3
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
a=mid:1 a=mid:1
m=audio 0 RTP/AVP 8 m=audio 0 RTP/AVP 8
a=mid:2 a=mid:2
m=audio 20002 RTP/AVP 3 m=audio 20002 RTP/AVP 3
a=mid:3 a=mid:3
8.3. Capability Negotiation 9.3. Capability Negotiation
A client that understands "group" and "mid" but does not want to make A client that understands "group" and "mid" but does not want to make
use of them in a particular session MAY want to indicate that it 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 no identification-tags for every semantics it "a=group" line with no identification-tags for every semantics it
understands. understands.
If a server receives an offer that contains empty "a=group" lines, it 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 SHOULD add its capabilities also in the form of empty "a=group" lines
to its answer. to its answer.
8.3.1. Example 9.3.1. Example
A system that supports both LS and FID semantics but does not want to A system that supports both LS and FID semantics but does not want 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 192.0.2.3
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 offer 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 192.0.2.1
a=group:FID a=group:FID
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
8.4. Backward Compatibility 9.4. 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"
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. Offerer does not Support "group" 9.4.1. Offerer does not Support "group"
This situation does not represent a problem because grouping requests This situation does not represent a problem because grouping requests
are always performed by offerers, not by answerers. If the offerer are always performed by offerers, not by answerers. If the offerer
does not support "group" this attribute will just not be used. does not support "group" this attribute will just not be used.
8.4.2. Answerer does not Support "group" 9.4.2. Answerer does not Support "group"
The answerer 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 answerer 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 answerer 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.
skipping to change at page 17, line 35 skipping to change at page 18, line 13
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 10. Changes from RFC 3388
The grouping mechanism is now defined as an extendible framework.
Earlier, [RFC3388] used to discourage extensions to this mechanism in
favor of using new session description protocols.
Given a semantics value, [RFC3388] used to restrict "m" line
identifiers to only appear in a single group using that semantics.
That restriction has been lifted. From conversations with
implementers, it seems that the lifting of this restriction is
unlikely to cause backwards compatibility problems.
11. 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 attack. descriptions and media encryption can be used to prevent this attack.
In SIP, S/MIME [RFC3850] and TLS [RFC4346] can be used to protect
session description exchanges in an end-to-end and a hop-by-hop
fashion respectively.
10. IANA Considerations 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 3.
The "group" attribute is used for grouping together different media
streams and its format is defined in Section 4.
This document defines a framework to group media lines in SDP using
different semantics. Semantics to be used with this framework are
registered by the IANA when they are published in standards track
RFCs.
The IANA Considerations section of the RFC MUST include the following
information, which appears in the IANA registry along with the RFC
number of the publication.
o A brief description of the semantics.
o Token to be used within the group attribute. This token may be of
any length, but SHOULD be no more than four characters long.
o Reference to an standards track RFC.
The only entries in the registry for the time being are:
Semantics Token Reference [RFC3388] already registered the "mid" and "group" SDP attributes
------------------- ----- ----------- with the IANA and created a registry for semantics. This document
Lip synchronization LS RFC 3388 does not contain any additional actions for the IANA.
Flow identification FID RFC 3388
11. References 13. References
11.1. Normative References 13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Protocol", RFC 2327, April 1998. Description Protocol", RFC 4566, July 2006.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
June 2002. June 2002.
11.2. Informational References [RFC3850] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, July 2004.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
Streaming Protocol (RTSP)", RFC 2326, April 1998. (TLS) Protocol Version 1.1", RFC 4346, April 2006.
13.2. Informational References
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996. Applications", RFC 1889, January 1996.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2833] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF [RFC2833] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF
Digits, Telephony Tones and Telephony Signals", RFC 2833, Digits, Telephony Tones and Telephony Signals", RFC 2833,
May 2000. May 2000.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
Author's Address Author's Address
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Hirsalantie 11 Hirsalantie 11
Jorvas 02420 Jorvas 02420
Finland Finland
Email: Gonzalo.Camarillo@ericsson.com Email: Gonzalo.Camarillo@ericsson.com
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