Internet DRAFT - draft-ietf-mmusic-rfc3388bis
draft-ietf-mmusic-rfc3388bis
MMUSIC Working Group G. Camarillo
Internet-Draft Ericsson
Obsoletes: 3388 (if approved) H. Schulzrinne
Intended status: Standards Track Columbia University
Expires: May 15, 2010 November 11, 2009
The SDP (Session Description Protocol) Grouping Framework
draft-ietf-mmusic-rfc3388bis-04.txt
Abstract
In this specification, we define a framework to group "m" lines in
SDP (Session Description Protocol) for different purposes. This
framework uses the "group" and "mid" SDP attributes, both of which
are defined in this specification. Additionally, we specify how to
use the framework for two different purposes: for lip synchronization
and for receiving a media flow consisting of several media streams on
different transport addresses. This document obsoletes RFC 3388.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 15, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 1]
Internet-Draft SDP Grouping Framework November 2009
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 3
4. Media Stream Identification Attribute . . . . . . . . . . . . 4
5. Group Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
6. Use of "group" and "mid" . . . . . . . . . . . . . . . . . . . 4
7. Lip Synchronization (LS) . . . . . . . . . . . . . . . . . . . 5
7.1. Example of LS . . . . . . . . . . . . . . . . . . . . . . 5
8. Flow Identification (FID) . . . . . . . . . . . . . . . . . . 6
8.1. SIP and Cellular Access . . . . . . . . . . . . . . . . . 6
8.2. DTMF Tones . . . . . . . . . . . . . . . . . . . . . . . . 7
8.3. Media Flow Definition . . . . . . . . . . . . . . . . . . 7
8.4. FID Semantics . . . . . . . . . . . . . . . . . . . . . . 7
8.4.1. Examples of FID . . . . . . . . . . . . . . . . . . . 8
8.5. Scenarios that FID does not Cover . . . . . . . . . . . . 11
8.5.1. Parallel Encoding Using Different Codecs . . . . . . . 11
8.5.2. Layered Encoding . . . . . . . . . . . . . . . . . . . 12
8.5.3. Same IP Address and Port Number . . . . . . . . . . . 12
9. Usage of the "group" Attribute in SIP . . . . . . . . . . . . 13
9.1. Mid Value in Answers . . . . . . . . . . . . . . . . . . . 13
9.1.1. Example . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. Group Value in Answers . . . . . . . . . . . . . . . . . . 15
9.2.1. Example . . . . . . . . . . . . . . . . . . . . . . . 15
9.3. Capability Negotiation . . . . . . . . . . . . . . . . . . 16
9.3.1. Example . . . . . . . . . . . . . . . . . . . . . . . 16
9.4. Backward Compatibility . . . . . . . . . . . . . . . . . . 17
9.4.1. Offerer does not Support "group" . . . . . . . . . . . 17
9.4.2. Answerer does not Support "group" . . . . . . . . . . 17
10. Changes from RFC 3388 . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
14.2. Informational References . . . . . . . . . . . . . . . . . 20
Camarillo & Schulzrinne Expires May 15, 2010 [Page 2]
Internet-Draft SDP Grouping Framework November 2009
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Camarillo & Schulzrinne Expires May 15, 2010 [Page 3]
Internet-Draft SDP Grouping Framework November 2009
1. Introduction
RFC 3388 [RFC3388] specified a media-line grouping framework for SDP
(Session Description Protocol) [RFC4566]. This specification
obsoletes RFC 3388 [RFC3388].
An SDP [RFC4566] session description typically contains one or more
media lines, which are commonly known as "m" lines. When a session
description contains more than one "m" line, SDP does not provide any
means to express a particular relationship between two or more of
them. When an application receives an SDP session description with
more than one "m" line, it is up to the application what to do with
them. SDP does not carry any information about grouping media
streams.
While in some environments this information can be carried out of
band, it is necessary to have a mechanism in SDP to express how
different media streams within a session description relate to each
other. The framework defined in this specification is such a
mechanism.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Overview of Operation
This section provides a non-normative description 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).
Camarillo & Schulzrinne Expires May 15, 2010 [Page 4]
Internet-Draft SDP Grouping Framework November 2009
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
This document defines the "media stream identification" media
attribute, which is used for identifying media streams within a
session description. Its formatting in SDP [RFC4566] is described by
the following Augmented BNF (Backus-Naur Form) [RFC5234]:
mid-attribute = "a=mid:" identification-tag
identification-tag = token
; token is defined in RFC 4566
The identification tag MUST be unique within an SDP session
description.
5. Group Attribute
This document defines the "group" session-level attribute, which is
used for grouping together different media streams. Its formatting
in SDP is described by the following Augmented BNF [RFC5234]:
group-attribute = "a=group:" semantics
*(SP identification-tag)
semantics = "LS" / "FID" / semantics-extension
semantics-extension = token
; token is defined in RFC 4566
This document defines two standard semantics: LS (Lip
Synchronization) and FID (Flow Identification). Semantics extensions
follow the Standards Action policy [RFC5226].
6. Use of "group" and "mid"
All the "m" lines of a session description that uses "group" MUST be
Camarillo & Schulzrinne Expires May 15, 2010 [Page 5]
Internet-Draft SDP Grouping Framework November 2009
identified with a "mid" attribute whether they appear in the group
line(s) or not. If a session description contains at least one "m"
line that has no "mid" identification the application MUST NOT
perform any grouping of media lines.
"a=group" lines are used to group together several "m" lines that are
identified by their "mid" attribute. "a=group" lines that contain
identification-tags that do not correspond to any "m" line within the
session description MUST be ignored. The application acts as if the
"a=group" line did not exist. The behavior of an application
receiving an SDP with grouped "m" lines is defined by the semantics
field in the "a=group" line.
There MAY be several "a=group" lines in a session description. The
"a=group" lines of a session description can use the same or
different semantics. An "m" line identified by its "mid" attribute
MAY appear in more than one "a=group" line.
7. Lip Synchronization (LS)
An application that receives a session description that contains "m"
lines that are grouped together using LS semantics MUST synchronize
the playout of the corresponding media streams. Note that LS
semantics not only apply to a video stream that has to be
synchronized with an audio stream. The playout of two streams of the
same type can be synchronized as well.
For RTP streams, synchronization is typically performed using RTCP,
which provides enough information to map time stamps from the
different streams into a local absolute time value. However, the
concept of media stream synchronization MAY also apply to media
streams that do not make use of RTP. If this is the case, the
application MUST recover the original timing relationship between the
streams using whatever available mechanism.
7.1. Example of LS
The following example shows a session description of a conference
that is being multicast. The first media stream (mid:1) contains the
voice of the speaker who speaks in English. The second media stream
(mid:2) contains the video component and the third (mid:3) media
stream carries the translation to Spanish of what he is saying. The
first and the second media streams have to be synchronized.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 6]
Internet-Draft SDP Grouping Framework November 2009
v=0
o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0
c=IN IP4 233.252.0.1/127
a=group:LS 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=video 30002 RTP/AVP 31
a=mid:2
m=audio 30004 RTP/AVP 0
i=This media stream contains the Spanish translation
a=mid:3
Note that although the third media stream is not present in the group
line, it still has to contain a mid attribute (mid:3), as stated
before.
8. Flow Identification (FID)
An "m" line in an SDP session description defines a media stream.
However, SDP does not define what a media stream is. This definition
can be found in the RTSP specification. The RTSP RFC [RFC2326]
defines a media stream as "a single media instance, e.g., an audio
stream or a video stream as well as a single whiteboard or shared
application group. When using RTP, a stream consists of all RTP and
RTCP packets created by a source within an RTP session".
This definition assumes that a single audio (or video) stream maps
into an RTP session. The RTP RFC [RFC1889] (at present obsoleted by
[RFC3550]) used to define an RTP session as follows: "For each
participant, the session is defined by a particular pair of
destination transport addresses (one network address plus a port pair
for RTP and RTCP)".
While the previous definitions cover the most common cases, there are
situations where a single media instance, (e.g., an audio stream or a
video stream) is sent using more than one RTP session. Two examples
(among many others) of this kind of situation are cellular systems
using SIP (Session Initiation Protocol; [RFC3261]) and systems
receiving DTMF (Dual-Tone Multi-Frequency) tones on a different host
than the voice.
8.1. SIP and Cellular Access
Systems using a cellular access and SIP as a signalling protocol need
to receive media over the air. During a session the media can be
encoded using different codecs. The encoded media has to traverse
Camarillo & Schulzrinne Expires May 15, 2010 [Page 7]
Internet-Draft SDP Grouping Framework November 2009
the radio interface. The radio interface is generally characterized
by being bit error prone and associated with relatively high packet
transfer delays. In addition, radio interface resources in a
cellular environment are scarce and thus expensive, which calls for
special measures in providing a highly efficient transport. In order
to get an appropriate speech quality in combination with an efficient
transport, precise knowledge of codec properties are required so that
a proper radio bearer for the RTP session can be configured before
transferring the media. These radio bearers are dedicated bearers
per media type (i.e., codec).
Cellular systems typically configure different radio bearers on
different port numbers. Therefore, incoming media has to have
different destination port numbers for the different possible codecs
in order to be routed properly to the correct radio bearer. Thus,
this is an example in which several RTP sessions are used to carry a
single media instance (the encoded speech from the sender).
8.2. DTMF Tones
Some voice sessions include DTMF tones. Sometimes the voice handling
is performed by a different host than the DTMF handling. It is
common to have an application server in the network gathering DTMF
tones for the user while the user receives the encoded speech on his
user agent. In this situations it is necessary to establish two RTP
sessions: one for the voice and the other for the DTMF tones. Both
RTP sessions are logically part of the same media instance.
8.3. Media Flow Definition
The previous examples show that the definition of a media stream in
[RFC2326] do not cover some scenarios. It cannot be assumed that a
single media instance maps into a single RTP session. Therefore, we
introduce the definition of a media flow:
Media flow consists of a single media instance, e.g., an audio stream
or a video stream as well as a single whiteboard or shared
application group. When using RTP, a media flow comprises one or
more RTP sessions.
8.4. FID Semantics
Several "m" lines grouped together using FID semantics form a media
flow. A media agent handling a media flow that comprises several "m"
lines MUST send a copy of the media to every "m" line part of the
flow as long as the codecs and the direction attribute present in a
particular "m" line allow it.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 8]
Internet-Draft SDP Grouping Framework November 2009
It is assumed that the application uses only one codec at a time to
encode the media produced. This codec MAY change dynamically during
the session, but at any particular moment only one codec is in use.
The application encodes the media using the current codec and checks
one by one all the "m" lines that are part of the flow. If a
particular "m" line contains the codec being used and the direction
attribute is "sendonly" or "sendrecv", a copy of the encoded media is
sent to the address/port specified in that particular media stream.
If either the "m" line does not contain the codec being used or the
direction attribute is neither "sendonly" nor "sendrecv", nothing is
sent over this media stream.
The application typically ends up sending media to different
destinations (IP address/port number) depending on the codec used at
any moment.
8.4.1. Examples of FID
The session description below might be sent by a SIP user agent using
a cellular access. The user agent supports GSM (Global System for
Mobile communications) on port 30000 and AMR (Adaptive Multi-Rate) on
port 30002. When the remote party sends GSM, it will send RTP
packets to port number 30000. When AMR is the codec chosen, packets
will be sent to port 30002. Note that the remote party can switch
between both codecs dynamically in the middle of the session.
However, in this example, only one media stream at a time carries
voice. The other remains "muted" while its corresponding codec is
not in use.
v=0
o=Laura 289083124 289083124 IN IP4 two.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 30000 RTP/AVP 3
a=rtpmap:3 GSM/8000
a=mid:1
m=audio 30002 RTP/AVP 97
a=rtpmap:97 AMR/8000
a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
mode-change-neighbor; maxframes=1
a=mid:2
(The linebreak in the fmtp line accommodates RFC formatting
restrictions; SDP does not have continuation lines.)
Camarillo & Schulzrinne Expires May 15, 2010 [Page 9]
Internet-Draft SDP Grouping Framework November 2009
In the previous example, a system receives media on the same IP
address on different port numbers. The following example shows how a
system can receive different codecs on different IP addresses.
v=0
o=Laura 289083124 289083124 IN IP4 three.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.2
a=rtpmap:0 PCMU/8000
a=mid:1
m=audio 30002 RTP/AVP 97
a=rtpmap:97 AMR/8000
a=fmtp:97 mode-set=0,2,5,7; mode-change-period=2;
mode-change-neighbor; maxframes=1
a=mid:2
(The linebreak in the fmtp line accomodates RFC formatting
restrictions; SDP does not have continuation lines.)
The cellular terminal of this example only supports the AMR codec.
However, many current IP phones only support PCM (Pulse-Code
Modulation; payload 0). In order to be able to interoperate with
them, the cellular terminal uses a transcoder whose IP address is
192.0.2.2. The cellular terminal includes in its SDP support for PCM
at that IP address. Remote systems will send AMR directly to the
terminal but PCM will be sent to the transcoder. The transcoder will
be configured (using whatever method) to convert the incoming PCM
audio to AMR and send it to the terminal.
The next example shows how the "group" attribute used with FID
semantics can indicate the use of two different codecs in the two
directions of a bidirectional media stream.
v=0
o=Laura 289083124 289083124 IN IP4 four.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 8
a=recvonly
a=mid:2
Camarillo & Schulzrinne Expires May 15, 2010 [Page 10]
Internet-Draft SDP Grouping Framework November 2009
A user agent that receives the SDP above knows that at a certain
moment it can send either PCM u-law to port number 30000 or PCM A-law
to port number 30002. However, the media agent also knows that the
other end will only send PCM u-law (payload 0).
The following example shows a session description with different "m"
lines grouped together using FID semantics that contain the same
codec.
v=0
o=Laura 289083124 289083124 IN IP4 five.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2 3
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 8
a=mid:2
m=audio 20000 RTP/AVP 0 8
c=IN IP4 192.0.2.2
a=recvonly
a=mid:3
At a particular point in time, if the media agent is sending PCM u-
law (payload 0), it sends RTP packets to 192.0.2.1 on port 30000 and
to 192.0.2.2 on port 20000 (first and third "m" lines). If it is
sending PCM A-law (payload 8), it sends RTP packets to 192.0.2.1 on
port 30002 and to 192.0.2.2 on port 20000 (second and third "m"
lines).
The system that generated the SDP above supports PCM u-law on port
30000 and PCM A-law on port 30002. Besides, it uses an application
server whose IP address is 192.0.2.2 that records the conversation.
That is why the application server always receives a copy of the
audio stream regardless of the codec being used at any given moment
(it actually performs an RTP dump, so it can effectively receive any
codec).
Remember that if several "m" lines grouped together using FID
semantics contain the same codec the media agent MUST send media over
several RTP sessions at the same time.
The last example of this section deals with DTMF tones. DTMF tones
can be transmitted using a regular voice codec or can be transmitted
as telephony events. The RTP payload for DTMF tones treated as
telephone events is described in [RFC4733]. Below, there is an
example of an SDP session description using FID semantics and this
Camarillo & Schulzrinne Expires May 15, 2010 [Page 11]
Internet-Draft SDP Grouping Framework November 2009
payload type.
v=0
o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 20000 RTP/AVP 97
c=IN IP4 192.0.2.2
a=rtpmap:97 telephone-events
a=mid:2
The remote party would send PCM encoded voice (payload 0) to
192.0.2.1 and DTMF tones encoded as telephony events to 192.0.2.2.
Note that only voice or DTMF is sent at a particular point in time.
When DTMF tones are sent, the first media stream does not carry any
data and, when voice is sent, there is no data in the second media
stream. FID semantics provide different destinations for alternative
codecs.
8.5. Scenarios that FID does not Cover
It is worthwhile mentioning some scenarios where the "group"
attribute using existing semantics (particularly FID) might seem to
be applicable but is not.
8.5.1. Parallel Encoding Using Different Codecs
FID semantics are useful when the application only uses one codec at
a time. An application that encodes the same media using different
codecs simultaneously MUST NOT use FID to group those media lines.
Some systems that handle DTMF tones are a typical example of parallel
encoding using different codecs.
Some systems implement the RTP payload defined in RFC 4733 [RFC4733],
but when they send DTMF tones they do not mute the voice channel.
Therefore, in effect they are sending two copies of the same DTMF
tone: encoded as voice and encoded as a telephony event. When the
receiver gets both copies, it typically uses the telephony event
rather than the tone encoded as voice. FID semantics MUST NOT be
used in this context to group both media streams since such a system
is not using alternative codecs but rather different parallel
encodings for the same information.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 12]
Internet-Draft SDP Grouping Framework November 2009
8.5.2. Layered Encoding
Layered encoding schemes encode media in different layers. Quality
at the receiver varies depending on the number of layers received.
SDP provides a means to group together contiguous multicast addresses
that transport different layers. The "c" line below:
c=IN IP4 233.252.0.1/127/3
is equivalent to the following three "c" lines:
c=IN IP4 233.252.0.1/127
c=IN IP4 233.252.0.2/127
c=IN IP4 233.252.0.3/127
FID MUST NOT be used to group "m" lines that do not represent the
same information. Therefore, FID MUST NOT be used to group "m" lines
that contain the different layers of layered encoding scheme.
Besides, we do not define new group semantics to provide a more
flexible way of grouping different layers because the already
existing SDP mechanism covers the most useful scenarios.
8.5.3. Same IP Address and Port Number
If several codecs have to be sent to the same IP address and port,
the traditional SDP syntax of listing several codecs in the same "m"
line MUST be used. FID MUST NOT be used to group "m" lines with the
same IP address/port. Therefore, an SDP like the one below MUST NOT
be generated.
v=0
o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30000 RTP/AVP 8
a=mid:2
The correct SDP for the session above would be the following one:
Camarillo & Schulzrinne Expires May 15, 2010 [Page 13]
Internet-Draft SDP Grouping Framework November 2009
v=0
o=Laura 289083124 289083124 IN IP4 six.example.com
t=0 0
c=IN IP4 192.0.2.1
m=audio 30000 RTP/AVP 0 8
If two "m" lines are grouped using FID they MUST differ in their
transport addresses (i.e., IP address plus port).
9. Usage of the "group" Attribute in SIP
SDP descriptions are used by several different protocols, SIP among
them. We include a section about SIP because the "group" attribute
will most likely be used mainly by SIP systems.
SIP [RFC3261] is an application layer protocol for establishing,
terminating and modifying multimedia sessions. SIP carries session
descriptions in the bodies of the SIP messages but is independent
from the protocol used for describing sessions. SDP [RFC4566] is one
of the protocols that can be used for this purpose.
At session establishment SIP provides a three-way handshake (INVITE-
200 OK-ACK) between end systems. However, just two of these three
messages carry SDP, as described in [RFC3264].
9.1. Mid Value in Answers
The "mid" attribute is an identifier for a particular media stream.
Therefore, the "mid" value in the offer MUST be the same as the "mid"
value in the answer. Besides, subsequent offers (e.g., in a re-
INVITE) SHOULD use the same "mid" value for the already existing
media streams.
[RFC3264] describes the usage of SDP in relation to SIP. The offerer
and the answerer align their media description so that the nth media
stream ("m=" line) in the offerer's session description corresponds
to the nth media stream in the answerer's description.
The presence of the "group" attribute in an SDP session description
does not modify this behavior.
Since the "mid" attribute provides a means to label "m" lines, it
would be possible to perform media alignment using "mid" labels
rather than matching nth "m" lines. However this would not bring any
gain and would add complexity to implementations. Therefore SIP
systems MUST perform media alignment matching nth lines regardless of
the presence of the "group" or "mid" attributes.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 14]
Internet-Draft SDP Grouping Framework November 2009
If a media stream that contained a particular "mid" identifier in the
offer contains a different identifier in the answer the application
ignores all the "mid" and "group" lines that might appear in the
session description. The following example illustrates this
scenario.
9.1.1. Example
Two SIP entities exchange SDPs during session establishment. The
INVITE contains the SDP below:
v=0
o=Laura 289083124 289083124 IN IP4 seven.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2
m=audio 30000 RTP/AVP 0 8
a=mid:1
m=audio 30002 RTP/AVP 0 8
a=mid:2
The 200 OK response contains the following SDP:
v=0
o=Bob 289083122 289083122 IN IP4 eigth.example.com
t=0 0
c=IN IP4 192.0.2.3
a=group:FID 1 2
m=audio 25000 RTP/AVP 0 8
a=mid:2
m=audio 25002 RTP/AVP 0 8
a=mid:1
Since alignment of "m" lines is performed based on matching of nth
lines, the first stream had "mid:1" in the INVITE and "mid:2" in the
200 OK. Therefore, the application ignores every "mid" and "group"
line contained in the SDP.
A well-behaved SIP user agent would have returned the SDP below in
the 200 OK:
Camarillo & Schulzrinne Expires May 15, 2010 [Page 15]
Internet-Draft SDP Grouping Framework November 2009
v=0
o=Bob 289083122 289083122 IN IP4 nine.example.com
t=0 0
c=IN IP4 192.0.2.3
a=group:FID 1 2
m=audio 25002 RTP/AVP 0 8
a=mid:1
m=audio 25000 RTP/AVP 0 8
a=mid:2
9.2. Group Value in Answers
A SIP entity that receives an offer that contains an "a=group" line
with semantics that it does not understand MUST return an answer
without the "group" line. Note that, as it was described in the
previous section, the "mid" lines MUST still be present in the
answer.
A SIP entity that receives an offer that contains an "a=group" line
with semantics that are understood MUST return an answer that
contains an "a=group" line with the same semantics. The
identification-tags contained in this "a=group" lines MUST be the
same that were received in the offer or a subset of them (zero
identification-tags is a valid subset). When the identification-tags
in the answer are a subset, the "group" value to be used in the
session MUST be the one present in the answer.
SIP entities refuse media streams by setting the port to zero in the
corresponding "m" line. "a=group" lines MUST NOT contain
identification-tags that correspond to "m" lines with port zero.
Note that grouping of m lines MUST always be requested by the
offerer, never by the answerer. Since SIP provides a two-way SDP
exchange, an answerer that requested grouping would not know whether
the "group" attribute was accepted by the offerer or not. An
answerer that wants to group media lines SHOULD issue another offer
after having responded to the first one (in a re-INVITE for
instance).
9.2.1. Example
The example below shows how the callee refuses a media stream offered
by the caller by setting its port number to zero. The "mid" value
corresponding to that media stream is removed from the "group" value
in the answer.
SDP in the INVITE from caller to callee:
Camarillo & Schulzrinne Expires May 15, 2010 [Page 16]
Internet-Draft SDP Grouping Framework November 2009
v=0
o=Laura 289083124 289083124 IN IP4 ten.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID 1 2 3
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 8
a=mid:2
m=audio 30004 RTP/AVP 3
a=mid:3
SDP in the INVITE from callee to caller:
v=0
o=Bob 289083125 289083125 IN IP4 eleven.example.com
t=0 0
c=IN IP4 192.0.2.3
a=group:FID 1 3
m=audio 20000 RTP/AVP 0
a=mid:1
m=audio 0 RTP/AVP 8
a=mid:2
m=audio 20002 RTP/AVP 3
a=mid:3
9.3. Capability Negotiation
A client that understands "group" and "mid" but does not want to make
use of them in a particular session MAY want to indicate that it
supports them. If a client decides to do that, it SHOULD add an
"a=group" line with no identification-tags for every semantics value
it understands.
If a server receives an offer that contains empty "a=group" lines, it
SHOULD add its capabilities also in the form of empty "a=group" lines
to its answer.
9.3.1. Example
A system that supports both LS and FID semantics but does not want to
group any media stream for this particular session generates the
following SDP:
Camarillo & Schulzrinne Expires May 15, 2010 [Page 17]
Internet-Draft SDP Grouping Framework November 2009
v=0
o=Bob 289083125 289083125 IN IP4 twelve.example.com
t=0 0
c=IN IP4 192.0.2.3
a=group:LS
a=group:FID
m=audio 20000 RTP/AVP 0 8
The server that receives that offer supports FID but not LS. It
responds with the SDP below:
v=0
o=Laura 289083124 289083124 IN IP4 thirteen.example.com
t=0 0
c=IN IP4 192.0.2.1
a=group:FID
m=audio 30000 RTP/AVP 0
9.4. Backward Compatibility
This document does not define any SIP "Require" header. Therefore,
if one of the SIP user agents does not understand the "group"
attribute the standard SDP fall back mechanism MUST be used
(attributes that are not understood are simply ignored).
9.4.1. Offerer does not Support "group"
This situation does not represent a problem because grouping requests
are always performed by offerers, not by answerers. If the offerer
does not support "group" this attribute will just not be used.
9.4.2. Answerer does not Support "group"
The answerer will ignore the "group" attribute, since it does not
understand it (it will also ignore the "mid" attribute). For LS
semantics, the answerer might decide to perform or to not perform
synchronization between media streams.
For FID semantics, the answerer will consider that the session
comprises several media streams.
Different implementations would behave in different ways.
In the case of audio and different "m" lines for different codecs an
implementation might decide to act as a mixer with the different
incoming RTP sessions, which is the correct behavior.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 18]
Internet-Draft SDP Grouping Framework November 2009
An implementation might also decide to refuse the request (e.g., 488
Not acceptable here or 606 Not Acceptable) because it contains
several "m" lines. In this case, the server does not support the
type of session that the caller wanted to establish. In case the
client is willing to establish a simpler session anyway, he SHOULD
re-try the request without "group" attribute and only one "m" line
per flow.
10. Changes from RFC 3388
Section 3 (Overview of Operation) has been added for clarity. The
AMR and GSM acronyms are now expanded on their first use. The
examples now use IP addresses in the range suitable for examples.
The grouping mechanism is now defined as an extendible framework.
Earlier, RFC 3388 [RFC3388] used to discourage extensions to this
mechanism in favor of using new session description protocols.
Given a semantics value, RFC 3388 [RFC3388] used to restrict "m" line
identifiers to only appear in a single group using that semantics.
That restriction has been lifted in this specification. From
conversations with implementers, existing (i.e., legacy)
implementations enforce this restriction on a per semantics basis.
That is, they only enforce this restriction for supported semantics.
Because of the nature of existing semantics, implementations will
only use a single "m" line identifier across groups using a given
semantics even after the restriction has been lifted by this
specification. Consequently, the lifting of this restriction will
not cause backwards compatibility problems because implementations
supporting new semantics will be updated not to enforce this
restriction at the same time as they are updated to support the new
semantics.
11. Security Considerations
Using the "group" parameter with FID semantics, an entity that
managed to modify the session descriptions exchanged between the
participants to establish a multimedia session could force the
participants to send a copy of the media to any particular
destination.
Integrity mechanism provided by protocols used to exchange session
descriptions and media encryption can be used to prevent this attack.
In SIP, S/MIME [RFC3850] and TLS [RFC5246] can be used to protect
session description exchanges in an end-to-end and a hop-by-hop
fashion respectively.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 19]
Internet-Draft SDP Grouping Framework November 2009
12. IANA Considerations
This document defines two SDP attributes: "mid" and "group".
The "mid" attribute is used to identify media streams within a
session description and its format is defined in Section 4.
The "group" attribute is used for grouping together different media
streams and its format is defined in Section 5.
This document defines a framework to group media lines in SDP using
different semantics. Semantics values to be used with this framework
are registered by the IANA following the Standards Action policy
[RFC5226].
The IANA Considerations section of the RFC MUST include the following
information, which appears in the IANA registry along with the RFC
number of the publication.
o A brief description of the semantics.
o Token to be used within the group attribute. This token may be of
any length, but SHOULD be no more than four characters long.
o Reference to an standards track RFC.
The following are the current entries in the registry:
Semantics Token Reference
--------------------------------- ----- -----------
Lip Synchronization LS [RFCxxxx]
Flow Identification FID [RFCxxxx]
Single Reservation flow SRF [RFC3524]
Alternative Network Address Types ANAT [RFC4091]
Forward Error Correction FEC [RFC4756]
Decoding Dependency DDP [RFC5583]
[Note to the RFC Editor: please replace RFCxxxx above with the number
of this RFC.]
13. Acknowledgments
Goran Eriksson and Jan Holler were coauthors of RFC 3388 [RFC3388].
14. References
Camarillo & Schulzrinne Expires May 15, 2010 [Page 20]
Internet-Draft SDP Grouping Framework November 2009
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3850] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, July 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
14.2. Informational References
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
Camarillo & Schulzrinne Expires May 15, 2010 [Page 21]
Internet-Draft SDP Grouping Framework November 2009
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
Camarillo & Schulzrinne Expires May 15, 2010 [Page 22]
