Internet Engineering Task Force Gonzalo Camarillo
Internet draft Jan Holler
Goran AP Eriksson
Ericsson
July 2001
Expires January 2002
<draft-ietf-mmusic-fid-03.txt>
Grouping of media lines in SDP
Status of this Memo
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Abstract
This document defines two SDP attributes: "group" and "mid". They
allow to group together several "m" lines for two different
purposes: for lip synchronization and for receiving media from a
single flow (several media streams), encoded in different formats
during a particular session, in different ports and host interfaces.
Camarillo/Holler/Eriksson 1
Grouping of media lines in SDP
TABLE OF CONTENTS
1 Terminology................................................2
2 Media stream identification attribute......................2
3 Group attribute............................................2
4 Lip Synchronization (LS)...................................3
5 Flow Identification (FID)..................................3
5.1 SIP and cellular access....................................4
5.2 DTMF tones.................................................4
5.3 Media flow definition......................................5
5.4 FID semantics..............................................5
5.4.1 Interactions of "group" with other media level attributes..6
5.4.2 Media in parallel..........................................7
5.4.3 DTMF tones encoded as telephony events.....................8
6 Usage of the "group" attribute in SIP......................8
6.1 Media alignment............................................9
6.2 Mid value in responses.....................................9
6.3 Group value in responses...................................9
6.4 Backward compatibility....................................10
6.4.1 Client does not support "group"...........................11
6.4.2 Server does not support "group"...........................11
7 Acknoledgements...........................................11
8 References................................................11
9 Authors³ Addresses........................................12
1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1]
and indicate requirement levels for compliant implementations.
2. 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 [2] is described by the following BNF:
mid-attribute = "a=mid:" identification-tag
identification-tag = token
The identification tag MUST be unique within the SDP session
description.
3. 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:
Camarillo/Holler/Eriksson 2
Grouping of media lines in SDP
group-attribute = "a=group:" semantics
2*(space identification-tag)
semantics = "LS" | "FID"
This document defines two standard semantics: LS (Lip
Synchronization) and FID (Flow Identification). If in the future it
was needed to standardize further semantics they would need to be
defined in a standards track document. However, defining new
semantics apart from LS and FID is discouraged. Instead, it is
RECOMMENDED to use other session description mechanisms such as
SDPng [3].
There MAY be several "a=group" lines in a session description.
"a=group" lines that contain identification-tags that are not
present in the session description MUST be simply ignored. The
application acts as if the "a=group" line did not exist.
4. Lip Synchronization (LS)
The play out of media streams that are grouped together using LS
semantics MUST be synchronized. Synchronization is typically
performed using RTCP, which provides enough information to map time
stamps from the different streams into a wall clock.
The following example shows a session description where the audio
and the video stream have to be synchronized.
v=0
o=Laura 289083124 289083124 IN IP4 first.example.com
t=0 0
c=IN IP4 131.160.1.112
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
a=mid:3
Note that although the third media stream is not present in the
group line it still contains an mid attribute (mid:3). All the "m"
lines of a session description that uses "group" MUST be identified
with an "mid" attribute regardless of whether they appear or not in
the group line(s).
5. 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. To find the
Camarillo/Holler/Eriksson 3
Grouping of media lines in SDP
definition of a media stream we have to go to the RTSP
specification. The RTSP RFC [4] 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. To find the definition of an RTP session we go
to the RTP specification. The RTP RFC [5] defines 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 [6] and systems receiving DTMF tones on a
different host than the voice.
5.1 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 [7]. 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).
5.2 DTMF tones
Some voice sessions include DTMF tones. Sometimes the voice handling
is performed by a different host than the DTMF handling. [8]
contains several examples of how application servers in the network
gather 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
Camarillo/Holler/Eriksson 4
Grouping of media lines in SDP
DTMF tones. Both RTP sessions are logically part of the same media
instance.
5.3 Media flow definition
The previous examples show that the definition of a media stream in
[4] 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.
For instance, in a two party call where the voice exchanged can be
encoded using GSM or PCM, the receiver wants to receive GSM on a
port number and PCM on a different port number. Two RTP sessions
will be established, one carrying GSM and the other carrying PCM.
At any particular moment just one codec is in use. Therefore, at any
moment one of the RTP sessions will not transport any voice. Here
the systems are dealing with a single media flow, but two RTP
sessions.
5.4 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 sends media to different destinations (IP address/port number)
depending on the codec used at any moment.
For instance, a SIP user agent receives an INVITE with the following
body:
v=0
o=Laura 289083124 289083124 IN IP4 second.example.com
t=0 0
c=IN IP4 131.160.1.112
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
This would be the SDP sent by a terminal using a cellular access.
The terminal supports GSM on port 30000 and AMR 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
Camarillo/Holler/Eriksson 5
Grouping of media lines in SDP
30002. Note that the remote party can switch between both codecs
dynamically in the middle of the session.
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 third.example.com
t=0 0
c=IN IP4 131.160.1.112
a=group:FID 1 2
m=audio 20000 RTP/AVP 0
c=IN IP4 131.160.1.111
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 cellular terminal of this example only supports the AMR codec.
However, many current IP phones only support PCM (payload 0). In
order to be able to interoperate with them, the cellular terminal
uses a transcoder whose IP address is 131.160.1.111. The cellular
terminal includes in its SDP support for PCM at that IP address.
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.
5.4.1 Interactions of "group" with other media level attributes
Media level attributes affect a media stream defined by an "m" line.
The presence of "group" does not modify this behavior.
This property can be used for different purposes. The example below
shows one possible use of this. A SIP user agent receives an INVITE
with the following body:
v=0
o=Laura 289083124 289083124 IN IP4 forth.example.com
t=0 0
c=IN IP4 131.160.1.112
a=group:FID 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 8
a=recvonly
a=mid:2
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Grouping of media lines in SDP
The media agent 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).
Note that the "group" attribute used with FID semantics allows to
express uni-directional codecs for a bi-directional media flow, as
it is shown in the example above.
5.4.2 Media in parallel
It can happen that different "m" lines grouped together using FID
semantics contain the same codec. The SDP below shows one example of
this situation:
v=0
o=Laura 289083124 289083124 IN IP4 fifth.example.com
t=0 0
c=IN IP4 131.160.1.112
a=groupe: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 131.160.1.111
a=recvonly
a=mid:3
If several "m" lines contain the codec used at a certain point of
time media MUST be sent to different destinations in parallel.
At a particular point of time, if the media agent is sending PCM u-
law (payload 0) it sends RTP packets to 131.160.1.112 on port 30000
and to 131.160.1.111 on port 20000 (first and third "m" lines). If
it is sending PCM A-law (payload 8) it sends RTP packets to
131.160.1.112 on port 30002 and to 131.160.1.111 on port 20000
(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 131.160.1.111 that records all 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 receives both u-law and A-law).
Note 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.
Camarillo/Holler/Eriksson 7
Grouping of media lines in SDP
5.4.3 DTMF tones encoded as telephony events
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 RFC 2833 [9]. Below
there is an example of an SDP session description using FID
semantics and this payload type.
v=0
o=Laura 289083124 289083124 IN IP4 sixth.example.com
t=0 0
c=IN IP4 131.160.1.112
a=group:FID 1 2
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 20000 RTP/AVP 97
c=IN IP4 131.160.1.111
a=rtpmap:97 telephone-events
a=mid:2
The remote party would send PCM encoded voice (payload 0) to
131.160.1.112 and DTMF tones encoded as telephony events to
131.160.1.111. Note that only voice or DTMF is sent at a particular
point of time. When DTMF tones are sent the first media stream does
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.
Some systems implement the RTP payload defined in RFC 2833, but when
they send DTMF tones they do not mute the voice channel. Therefore,
effectively 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.
6. 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 [6] 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 [2] is one of
the protocols that can be used for this purpose.
Camarillo/Holler/Eriksson 8
Grouping of media lines in SDP
6.1 Media alignment
Appendix B of [6] describes the usage of SDP in relation to SIP. It
states: "The caller and callee align their media description so that
the nth media stream ("m=" line) in the caller³s session description
corresponds to the nth media stream in the callee³s 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.
6.2 Mid value in responses
The "mid" attribute is an identifier for a particular media stream.
Therefore, the "mid" value in the response MUST be the same as the
"mid" value in the request. Besides, subsequent requests such as re-
INVITEs MUST use the same "mid" value for the already existing media
streams.
6.3 Group value in responses
The "group" attribute in a response will typically be the same as
the one received in the request. However, there are situations when
both are different. In these situations the "group" value to be used
in the session is the one present in the response.
Note the "group value in the response" really refers to the
"group" value in the last SDP exchanged between both parties.
That is, if in the establishment of a particular session
(INVITE-200 OK-ACK) SDPs are present in the 200 OK and in the
ACK (not in the INVITE), the "group" value to be used during
the session will be the one in the ACK.
The example below shows how the callee refuses a media stream
offered by the caller setting its port number to zero. The "mid"
value corresponding to that media stream is removed from the "group"
value in the response.
SDP in the INVITE from caller to callee:
v=0
o=Laura 289083124 289083124 IN IP4 seventh.example.com
t=0 0
c=IN IP4 131.160.1.112
a=group:FID 1 2 3
m=audio 30000 RTP/AVP 0
a=mid:1
Camarillo/Holler/Eriksson 9
Grouping of media lines in SDP
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 fifth.example.com
t=0 0
c=IN IP4 131.160.1.113
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
Note that although the media stream was refused the "mid" value was
still included.
6.4 Backward compatibility
An application that wants to be compliant to this specification MUST
support both "group" and "mid". Supporting just one of them would be
useless.
A SIP entity that receives a request that contains "group" and "mid"
attributes, understands them and it is willing to use the grouping
semantics offered returns a response that also contains "group" and
"mid" attributes. This way, the client that issued the request knows
that the server understood this extension.
Note that grouping of m lines is always requested by the issuer of
the request (the client), never by the issuer of the response (the
server). Since there is no response to a response in SIP, a server
that requested grouping in a response would not know whether the
"group" attribute was accepted by the client or not. A server that
wants to group media lines should issue another request after having
responded to the first one (a re-INVITE for instance).
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 is used.
A client that does not want to perform grouping of media lines in a
session SHOULD NOT add "mid" lines either. The presence of "mid"
lines would not be of any use for the server. Even if the server can
see that the client supported "mid" (and obviously "group" also) it
would be impossible to know which particular semantics are supported
(LS or/and FID).
Camarillo/Holler/Eriksson 10
Grouping of media lines in SDP
6.4.1 Client does not support "group"
This situation does not represent a problem because grouping
requests is always performed by clients, not by servers. If the
client does not support "group" this attribute will just not be
used.
6.4.2 Server does not support "group"
The server will ignore the "group" attribute, since it does not
understand it (it will also ignore the "mid" attribute). For LS
semantics, the server might decide to perform or to not perform
synchronization between media streams.
For FID semantics, the server 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.
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.
7. Acknowledgments
The authors would like to thank Jonathan Rosenberg, Adam Roach and
Orit Levin for their feedback on this document.
8. References
[1] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, IETF; March 1997.
[2] M. Handley/V. Jacobson, "SDP: Session Description Protocol", RFC
2327, IETF; April 1998.
[3] D. Kutscher/J. Ott/C. Bormann, "Session Description and
Capability Negotiation", draft-ietf-mmusic-sdpng-00.txt, IETF; April
2001. Work in progress.
[4] H. Schulzrinne/A. Rao/R. Lanphier, "Real Time Streaming Protocol
(RTSP)", RFC 2326, IETF; April 1998.
Camarillo/Holler/Eriksson 11
Grouping of media lines in SDP
[5] H. Schulzrinne/S. Casner/R. Frederick/V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", RFC 1889, IETF;
January 1996.
[6] M. Handley/H. Schulzrinne/E. Schooler/J. Rosenberg, "SIP:
Session Initiation Protocol", RFC 2543, IETF; Mach 1999.
[7] L. Westberg/M. Lindqvist, "Realtime Traffic over Cellular Access
Networks", draft-westberg-realtime-cellular-04.txt, IETF; June 2001.
Work in progress.
[8] J. Rosenberg/P.Mataga/H.Schulzrinne, "An Application Server
Component Architecture for SIP", draft-rosenberg-sip-app-components-
00.txt, IETF; November 2000. Work in progress.
[9] H. Schulzrinne/S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, IETF; May 2000.
9. Authors³ Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
Phone: +358 9 299 3371
Fax: +358 9 299 3052
Email: Gonzalo.Camarillo@ericsson.com
Jan Holler
Ericsson Research
S-16480 Stockholm
Sweden
Phone: +46 8 58532845
Fax: +46 8 4047020
Email: Jan.Holler@era.ericsson.se
Goran AP Eriksson
Ericsson Research
S-16480 Stockholm
Sweden
Phone: +46 8 58531762
Fax: +46 8 4047020
Email: Goran.AP.Eriksson@era.ericsson.se
Camarillo/Holler/Eriksson 12
Datatracker
draft-ietf-mmusic-fid-03
Internet-Draft
