MMUSIC Working Group G. Camarillo
Internet-Draft Ericsson
Obsoletes: 3388 (if approved) January 13, 2009
Intended status: Standards Track
Expires: July 17, 2009
The SDP (Session Description Protocol) Grouping Framework
draft-ietf-mmusic-rfc3388bis-02.txt
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Abstract
In this specification, we define a framework to group "m" lines in
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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.
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 . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . . 19
13.2. Informational References . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
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).
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
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a=mid:2
4. Media Stream Identification Attribute
A new "media stream identification" media attribute is defined. It
is used for identifying media streams within a session description.
Its formatting in SDP [RFC4566] is described by the following BNF
(Backus-Naur Form):
mid-attribute = "a=mid:" identification-tag
identification-tag = token
The identification tag MUST be unique within an SDP session
description.
5. Group Attribute
A new "group" session-level attribute is defined. It is used for
grouping together different media streams. Its formatting in SDP is
described by the following BNF:
group-attribute = "a=group:" semantics
*(space identification-tag)
semantics = "LS" / "FID" / semantics-extension
semantics-extension = token
This document defines two standard semantics: LS (Lip
Synchronization) and FID (Flow Identification). Further semantics
MUST be defined in a standards-track document.
6. Use of "group" and "mid"
All the "m" lines of a session description that uses "group" MUST be
identified with a "mid" attribute whether they appear in the group
line(s) or not. If a session description contains at least one "m"
line that has no "mid" identification the application MUST NOT
perform any grouping of media lines.
"a=group" lines are used to group together several "m" lines that are
identified by their "mid" attribute. "a=group" lines that contain
identification-tags that do not correspond to any "m" line within the
session description MUST be ignored. The application acts as if the
"a=group" line did not exist. The behavior of an application
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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. All
the "a=group" lines of a session description MAY or MAY NOT use the
same semantics. An "m" line identified by its "mid" attribute MAY
appear in more than one "a=group" line.
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 wall clock. However, the concept of media
stream synchronization MAY also apply to media streams that do not
make use of RTP. If this is the case, the application MUST recover
the original timing relationship between the streams using whatever
available mechanism.
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.
v=0
o=Laura 289083124 289083124 IN IP4 one.example.com
t=0 0
c=IN IP4 224.2.17.12/127
a=group:LS 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=video 30002 RTP/AVP 31
a=mid:2
m=audio 30004 RTP/AVP 0
i=This media stream contains the Spanish translation
a=mid:3
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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
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
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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.
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.
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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.)
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.
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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
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).
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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 [RFC2833]. Below, there is an
example of an SDP session description using FID semantics and this
payload type.
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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 of time.
When DTMF tones are sent, the first media stream does not carry any
data and, when voice is sent, there is no data in the second media
stream. FID semantics provide different destinations for alternative
codecs.
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 2833, but when
they send DTMF tones they do not mute the voice channel. Therefore,
in effect they are sending two copies of the same DTMF tone: encoded
as voice and encoded as a telephony event. When the receiver gets
both copies, it typically uses the telephony event rather than the
tone encoded as voice. FID semantics MUST NOT be used in this
context to group both media streams since such a system is not using
alternative codecs but rather different parallel encodings for the
same information.
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.
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SDP provides a means to group together contiguous multicast addresses
that transport different layers. The "c" line below:
c=IN IP4 224.2.1.1/127/3
is equivalent to the following three "c" lines:
c=IN IP4 224.2.1.1/127
c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127
FID MUST NOT be used to group "m" lines that do not represent the
same information. Therefore, FID MUST NOT be used to group "m" lines
that contain the different layers of layered encoding scheme.
Besides, we do not define new group semantics to provide a more
flexible way of grouping different layers because the already
existing SDP mechanism covers the most useful scenarios.
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:
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
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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.
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.
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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 MUST ignore every "mid" and
"group" lines contained in the SDP.
A well-behaved SIP user agent would have returned the SDP below in
the 200 OK:
v=0
o=Bob 289083122 289083122 IN IP4 nine.example.com
t=0 0
c=IN IP4 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
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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:
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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 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:
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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.
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An implementation might also decide to refuse the request (e.g., 488
Not acceptable here or 606 Not Acceptable) because it contains
several "m" lines. In this case, the server does not support the
type of session that the caller wanted to establish. In case the
client is willing to establish a simpler session anyway, he SHOULD
re-try the request without "group" attribute and only one "m" line
per flow.
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
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 [RFC4346] can be used to protect
session description exchanges in an end-to-end and a hop-by-hop
fashion respectively.
12. IANA Considerations
[RFC3388] already registered the "mid" and "group" SDP attributes
with the IANA and created a registry for semantics. This document
does not contain any additional actions for the IANA.
13. References
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13.1. 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.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
13.2. 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.
[RFC2833] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF
Digits, Telephony Tones and Telephony Signals", RFC 2833,
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.
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Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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