| rtcweb | D. Worley |
| Internet-Draft | Ariadne |
| Intended status: Standards Track | February 8, 2013 |
| Expires: August 12, 2013 |
Kumquat: A Generic Bundle Mechanism for the Session Description Protocol (SDP)
draft-worley-sdp-bundle-02
This document defines a generic bundle mechanism for the Session Description Protocol (SDP) by which the media described by a number of media descriptions ("m= lines") are multiplexed and transmitted over a single transport association. The transport association is described by an additional media description, allowing SDP attributes to be applied to the aggregate, independently of attributes applied to the constituents. In offer/answer usage, the bundle mechanism is backward compatible with SDP processors that do not understand the mechanism. The mechanism is designed to be compatible with the limitations of the existing Internet infrastructure.
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The central idea of bundling is to multiplex the media that would be several RTP sessions into one RTP session, with particular emphasis on allowing one transport association to carry media that are presented to the higher, application layer, as multiple RTP sessions.
At the interface between the SDP-configured layer and the lower, transport layer, the media are organized into a single RTP session. The transport-related properties of the RTP session (e.g., transport 5-tuple, encryption, ICE) are described by the transport-related attributes of a single media description.
At the interface between the SDP-configured layer and the higher, application layer, the media are organized into several RTP sessions. The application-related properties of the RTP session (e.g., media type and label) are described by the application-related attributes of separate media descriptions.
(There are some attributes (e.g., bandwidth limitation) that can apply separately to both the bundled RTP session and the constituent RTP sessions.)
However, we do not include the payload type numbers as information available to the application; only the encoding name and its parameters are accessible to the application. This gives the bundle mechanism freedom to place constraints on the use of payload types.
The bundle is signaled in the session description by a "group" attribute with semantics "KUMQUAT". The first media description listed in the group is the "bundle" media description (MD), whose transport information describes the transport association via which the RTP packets will be sent. The remaining (zero or more) media descriptions listed in the group are the "constituent" MDs. RTP packets received from the applications for these MDs are encapsulated and sent on the transport association for the bundle MD. RTP packets received from the transport association for the bundle MD are deencapsulated and sent to the applications for the constituent MDs.
A new payload type (codec) named "kumquat" is defined to be used for this encapsulation. Section 5.4.1
In offer/answer usage, we must arrange that the bundle mechanism is backward compatible with entities that do not understand the bundle mechanism. This requirement drives many features of this solution. Section 6.1
In addition, many devices in current usage (especially SBCs) apply more restrictions on the usage of SDP than one would expect from abstract consideration of their roles in the network. Some features of this solution are constructed to avoid these restrictions. Section 6.2
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].
The important RFCs in this area use inconsistent terminology. Here, we use:
This section lists desiderata for the bundle mechanism in SDP. (I use the term "desiderata" -- "things that are desired" -- rather than "requirements", because we may discover that we can't optimally satisfy all of these criteria at the same time.) The first section lists desiderata that are arise from considering the ways applications may wish to bundling. The second section lists desiderata that arise from compatibility with existing Internet infrastructure.
This requirement is taken from draft-jennings-mmusic-media-req-00.
DES F1 and DES F2 do not specify whether the transport-level media description may or may not also be one of the application-level media descriptions.
This desideratum is taken from slides-interim-2013-rtcweb-1-10.pdf.
Of course, no bundle may directly or indirectly contain itself. (I don't expect any current implementation to implement bundles within bundles, but we should design the mechanism to allow this, as some day we will likely need it.)
A bundle with no constituents serves no purpose for the transport of media, but we are likely to someday need to describe such a bundle. (Compare that an SDP m= line is syntactically constrained to specify at least one payload type. When SDP was used only to specify multicast sessions, this constraint was common sense. But once SDP offer/answer was invented, when a media description was rejected, the natural representation would be an m= line with a zero port and no payload types. But a payload type was syntactically required, so we now have to provide at least one token payload type in rejected m= lines.)
Presumably answer (3) resembles that which would be produced by an answerer that does not understand the bundle mechanism. It is a lower priority that the answerer can distinguish between accepting the bundle while rejecting all of its constituents, and rejecting the bundle as a whole. But those two conditions differ conceptually regarding whether any "framing" actions of the bundle are performed.
The RTCP information for each media stream is tagged with the SSRC about which it reports, and the SSRC is used to correlate the RTCP reports with the RTP sessions containing media with the same SSRC. So regarding RTCP, this desideratum appears to be straightforward to satisfy.
In the terminology of [RFC3550], the constituent media descriptions are now part of one RTP session.
This desideratum was suggested by Andrew Hutton.
Presumably this can be accomplished as it is now, with a single media description carrying multiple video flows that are distinguished only by their SSRCs. This desideratum is taken from slides-interim-2013-rtcweb-1-10.pdf.
This desideratum was suggested by Andrew Hutton.
SDP features (e.g., the codec set and ICE) are generally designed so that an offerer always offers every facility it is willing to support in the current situation, regardless of whether it was agreed to by the answerer in a preceding exchange. Thus, if the current answerer is a different endpoint than the previous answerer, the new answerer will negotiate a compatible set of facilities without needing knowledge of its predecessor's SDP. The offerer will smoothly transition to the new facilities. This property is required to support 3PCC situations (e.g., [RFC3725] and draft-worley-service-example). This desideratum was suggested by Richard Ejzak.
This desideratum was suggested by Hadriel Kaplan.
Many devices that have only one audio or video channel accept the first m= line with that media type and reject any further ones
This non-desideratum was suggested by Hadriel Kaplan.
This is needed because SBCs monitor the packet traffic on the transport associations and if no media is seen on one of the associations for a significant period of time, the SBC will tear down the call. This desideratum was suggested by Hadriel Kaplan.
Such duplication is not defined by [RFC4566]. Some SBCs do not support such duplication (ultimately, because it was not supported by [RFC2327]), and they reject SDP specifying duplicated transport association endpoints. This desideratum was suggested by Cullen Jennings.
This section is non-normative. (This section was suggested by Charles Eckel.)
This is an introduction to SDP bundling via a series of examples of offer/answer processing. Some mandatory SDP lines have been omitted from the examples for brevity. Long SDP lines have been folded by using trailing backslashes. Blank lines have been inserted for clarity.
Here is a typical, non-bundled SDP example with both audio and video media:
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
This SDP media description (MD) provides the transport information
about the audio and also identifies the role of the audio from the
application's point of view. In this case, the fact that it is the
first audio m= line suffices to tell the application how to treat it.
In more complex cases, label or content attributes might be used to
communicate the proper handling to the application.
m=audio 10000 RTP/AVP 0 8 97
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
This MD provides the transport information about the video and also
identifies the role of the video from the application's point of view.
m=video 10002 RTP/AVP 31 32
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx \
raddr 10.0.1.1 rport 10002
We call the RTP that is described by each media description (MD) a transport flow (TF). The audio and video are carried in separate TFs, which each have a separate transport association (address/port).
With SDP bundling, we add an additional MD to describe a single "bundle" TF to carry both the audio and video information, and a group attribute to show the association of the bundle MD with the constituent MDs:
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
Declare which MDs are included in the multiplexed MD: mid:con1 and
mid:con2 are the constituent MDs whose TFs (from the application
point of view) will be carried by the TF of the first-designed MD,
mid:bundle, which is the bundle MD.
a=group:KUMQUAT bundle con1 con2
This MD provides the application-level description of the audio TF.
As in the previous example, it is the first audio m= line. It
includes any attributes which apply to the audio media from the
application point of view, including the payload type definitions.
When interpreted by a supporting processor, the transport information
is ignored. When interpreted by a non-supporting processor, the
transport information specifies that the TF exists but is currently
"on hold": the association address is null, and the association port
is 9, the discard port.
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=mid:con1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
This MD provides the application-level description of the video TF.
As in the previous example, it is the first video m= line. It
includes any attributes which apply to the video media from the
application point of view. As in the audio MD, the association
address is null, and the association port is 9.
m=video 9 RTP/AVP 31 32
c=IN IP4 0.0.0.0
a=mid:con2
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
This MD provides the transport information for the bundle TF,
including any attributes which apply to the transport. We use RTCP
multiplexing [RFC5761], so only one set of ICE candidates (and only
one TURN relay) is needed for each MD. The MD is artificially given
the media type "audio" (which is ugly, but it avoids rejection by
SBCs) and it is placed after all of the constituent MDs so as to not
affect their positions as "first audio MD", etc. The MD lists a
single payload type for the "kumquat" payload format, which is used to
encapsulate the RTP of the constituent TFs.
m=audio 10000 RTP/AVP 127
a=mid:bundle
a=rtcp-mux
a=rtpmap:127 kumquat
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
If this SDP bundle is accepted, RTP provided by the application for the audio TF will be encapsulated into a kumquat payload and then be sent from port 10000. The encapsulation also contains the ordinal index (i.e., 0) of the audio TF and the payload type of the original audio RTP. RTP provided by the application for the video TF will be encapsulated into a kumquat payload and then be sent from port 10000. The encapsulation also contains the ordinal index (i.e., 1) of the video TF and the payload type of the original video RTP.
RTP that is received on port 10000 is interpreted according to the kumquat payload format: The constituent MD ordinal index is extracted. The encapsulated RTP and its payload type are then interpreted according to the constituent MD.
If the answerer supports SDP bundling, and desires to accept the offered bundle and its constituent MDs, the answerer signals that it accepts the SDP bundling by providing a matching group:KUMQUAT attribute in the answer. As always in offer/answer, the MDs in the answer correspond to the MDs in the offer by ordinal position.
The answerer provides the necessary transport information for the bundle MD. The answerer understands that MDs mid:con1 and mid:con2 are incorporated into MD mid:bundle, and ignores their transport information. It accepts each constituent MD by providing an answer MD for each of them that specifies a null address and port 9 (the discard port).
o=- 2890844526 2890844526 IN IP4 answer.example.com
c=IN IP4 10.0.2.1
a=group:KUMQUAT bundle con1 con2
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=mid:con1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 9 RTP/AVP 31 32
c=IN IP4 0.0.0.0
a=mid:con2
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=audio 20000 RTP/AVP 127
a=mid:bundle
a=rtcp-mux
a=rtpmap:127 kumquat
a=candidate:0 1 UDP 2113601791 10.0.2.1 20000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.35 51090 typ srflx \
raddr 10.0.2.1 rport 20000
SDP bundling allows for backward compatibility in case the answerer does not understand bundling. If the answerer does not understand bundling, it ignores the group attribute, and effectively sees the offer as:
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 9 RTP/AVP 31 32
c=IN IP4 0.0.0.0
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=audio 10000 RTP/AVP 127
a=rtcp-mux
a=rtpmap:127 kumquat
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
If the answerer wishes to accept the first audio and video streams, it assembles this answer:
o=- 2890844526 2890844526 IN IP4 answer.example.com
c=IN IP4 10.0.2.1
The absence of the group attribute informs the offerer that bundling
was rejected.
The audio MD is accepted. Transport information is provided, but it
does not include ICE candidates, because the offer did not provide ICE
candidates for the first and second MDs.
m=audio 20000 RTP/AVP 0 8 97
c=IN IP4 10.0.2.1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
The video MD is accepted. Transport information (using a different
port) is provided.
m=audio 20002 RTP/AVP 31 32
c=IN IP4 10.0.2.1
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
The bundle MD is rejected by the answerer because the only offered
codec was kumquat, and the answerer does not implement it.
m=audio 0 RTP/AVP 127
Because the group attribute is not present in the response, the offerer knows that the answerer does not support bundling (or does not want to consider the offered bundle). The offerer knows that the answerer wants to establish one audio TF and one video TF, and formally, that has been done. But the offerer has not set up its transport for separate audio and video TFs and has not signaled its transport information for those TFs to the answerer.
In order to enable media flow, the offerer sends an updated offer containing transport information for the constituent MDs:
o=- 2890844526 2890844527 IN IP4 host.example.com
c=IN IP4 10.0.1.1
No group attribute is included, to ensure that this update only sets
transport attributes, and does not trigger bundle-supporting behavior
if the answering entity has changed in the meantime.
Provide transport attributes for the audio MD. (We can reuse the ICE
candidates (and TURN relay) offered for the bundle MD.)
m=audio 10000 RTP/AVP 0 8 97
c=IN IP4 10.0.1.1
a=mid:con1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
New ICE candidates (and a separate TURN relay) are needed for the
video MD.
m=video 10002 RTP/AVP 31 32
c=IN IP4 10.0.1.1
a=mid:con2
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx \
raddr 10.0.1.1 rport 10002
The bundle MD must still be listed, but it is disabled.
m=audio 0 RTP/AVP 127
a=mid:bundle
The answerer then provides an answer that contains ICE candidates:
o=- 2890844526 2890844527 IN IP4 answer.example.com
c=IN IP4 10.0.2.1
m=audio 20000 RTP/AVP 0 8 97
c=IN IP4 10.0.2.1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=candidate:0 1 UDP 2113601791 10.0.2.1 20000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.35 51090 typ srflx \
raddr 10.0.2.1 rport 20000
m=audio 20002 RTP/AVP 31 32
c=IN IP4 10.0.2.1
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=candidate:0 1 UDP 2113601791 10.0.2.1 20002 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.35 51092 typ srflx \
raddr 10.0.2.1 rport 20002
m=audio 0 RTP/AVP 127
The ICE negotiations proceed, the transport associations are established, and RTP flows.
The basic procedure requires the offerer to update its offer when it discovers that the answerer does not support SDP bundling. The offerer can avoid this delay by providing transport information for the constituent MDs as well as for the bundle MD. The penalty is that the offerer must preallocate TURN relays for both the constituent MDs as well as the bundle MD.
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
a=group:KUMQUAT bundle con1 con2
m=audio 10000 RTP/AVP 0 8 97
c=IN IP4 10.0.1.1
a=mid:con1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
m=video 10002 RTP/AVP 31 32
c=IN IP4 10.0.1.1
a=mid:con2
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=candidate:0 1 UDP 2113601791 10.0.1.1 10002 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51002 typ srflx \
raddr 10.0.1.1 rport 10002
m=audio 10004 RTP/AVP 127
c=IN IP4 10.0.1.1
a=mid:bundle
a=rtcp-mux
a=rtpmap:127 kumquat
a=candidate:0 1 UDP 2113601791 10.0.1.1 10004 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51004 typ srflx \
raddr 10.0.1.1 rport 10004
If the answerer understands bundling and accepts the bundle, it rejects the constituent MDs and accepts the bundle MD. If the answerer does not understand bundling, it accepts the constituent MDs and rejects the bundle MD.
In this example, a presentation involves four media roles: the speaker's audio, the floor microphone, the video of the speaker, and the video of the speaker's slides. We use separate MDs for each media stream because each TF has a different role; the application will handle each of them in distinctly different ways.
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
a=group:KUMQUAT b c1 c2 c3 c4
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=mid:c1
a=label:speaker-audio
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
Note that different constituent MDs can use the same payload types
(for the same or different codecs), because the kumquat encapsulation
captures the constituent MD ordinal index separately from the payload
type.
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=mid:c2
a=label:floor-mic
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 G722
m=video 9 RTP/AVP 103 104
c=IN IP4 0.0.0.0
a=mid:c3
a=label:speaker-video
a=rtcp-mux
a=rtpmap:103 H261/90000
a=rtpmap:104 MPV/90000
m=video 9 RTP/AVP 103 104
c=IN IP4 0.0.0.0
a=mid:c4
a=label:slides
a=rtcp-mux
a=rtpmap:103 H261/90000
a=rtpmap:104 MPV/90000
m=multipart 10000 RTP/AVP 127
a=mid:b
a=rtcp-mux
a=rtpmap:127 kumquat
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
This example is the teacher's connection to a virtual classroom server. The media descriptions are tagged using the "content" attribute. [RFC4796] The media comprises:
1. one audio channel, for sending the teacher's voice and receiving the voice of a selected student
2. one video channel, for sending the teacher's presentation
3. one video channel, for sending the teacher's face
4. one video channel, for receiving a dynamically varying set of students' faces
The fourth TF (for students' faces) contains a large and variable set of video captures. These can be handled by a single TF because they all have essentially similar roles -- the application will process them as a set. As Adam Roach would say, "no control surfaces are necessary to talk about and/or manipulate the individual streams". In particular, this allows a large number of captures to be handled without mentioning them in the SDP, at the expense of not allowing the SDP to describe any of them individually. Similarly, the number of captures can vary without having to renegotiate the SDP.
(In contrast, the third TF (the teacher's face) is a separate TF because it is processed in a different role than that of the students' faces.)
In unbundled usage, there would be one transport association for the fourth TF. Incoming RTP from that association would be demultiplexed by the application based on the SSRC values, which would be unique for each student. With bundling, once the single transport TF is demultiplexed based on the ordinal index in the kumquat encapsulation, deencapsulated RTP packets destined for the fourth TF (index = 3) would be further demultiplexed by their SSRC values.
The offered SDP is:
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
a=group:KUMQUAT b c1 c2 c3 c4
The audio channel is send/receive.
m=audio 9 RTP/AVP 0 8 97
c=IN IP4 0.0.0.0
a=mid:c1
a=label:speaker-audio
a=content:speaker
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
The teacher's face and presentation are send-only.
m=video 9 RTP/AVP 103 104
c=IN IP4 0.0.0.0
a=mid:c2
a=label:speaker-video
a=content:speaker
a=sendonly
a=rtcp-mux
a=rtpmap:103 H261/90000
a=rtpmap:104 MPV/90000
m=video 9 RTP/AVP 105 106
c=IN IP4 0.0.0.0
a=mid:c3
a=label:presentation
a=content:slides
a=sendonly
a=rtcp-mux
a=rtpmap:105 H261/90000
a=rtpmap:106 MPV/90000
The student video input is receive-only and is limited to 24
simultaneous SSRCs.
m=video 9 RTP/AVP 105 106
c=IN IP4 0.0.0.0
a=mid:c4
a=label:student-thumbnails
a=recvonly
a=max-recv-ssrc:* 24
a=rtcp-mux
a=rtpmap:105 H261/90000
a=rtpmap:106 MPV/90000
m=multipart 10000 RTP/AVP
a=mid:b
a=rtcp-mux
a=candidate:0 1 UDP 2113601791 10.0.1.1 10000 typ host
a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
TBD (Here lies the real description.)
TBD
TBD
TBD
SDP bundling uses a payload type named "kumquat" to encapsulate the RTP packets of several constituent TFs into RTP packets of one TF. Each constituent TF has a distinct index value in the range 0 to 254 (inclusive). When kumquat is used within SDP bundling, the index value is the ordinal index of the MD within the session description. (The indexes start with 0 for the first MD.)
When the application delivers a payload (and associated descriptive information such as SSRC) in the context of a constituent MD to be transmitted, it is encapsulated into a kumquat payload and the kumquat payload is transmitted using the transport association of the bundle MD.
When a kumquat payload arrives on the transport association of the bundle MD, the kumquat payload is interpreted to construct a payload (and associated descriptive information). That payload is delivered to the application in the context of the constituent MD identified by the index value.
The format of a kumquat protocol payload contains a four-octet fixed part followed by zero or more CSRC identifiers, header extension, and the encapsulated payload. Note that this diagram is of the kumquat payload only, and does not include the RTP header before the payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|0|X| CC |M| PT | index | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extension |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| encapsulated payload |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
V: This field contains the value 2.
0 (bit 2): This field contains the value 0.
X: If this field is 1, the extension field is present.
CC: This field contains the count of the number of CSRC identifiers that follow the fixed part.
M: This field contains the "marker" bit associated with the encapsulated payload.
PT: This field contains the payload type number associated with the encapsulated payload. The meaning of PT is defined by the TF identified by the index field.
index: This field contains the index value identifying the constituent TF that the encapsulated payload is associated with. The range of index values is 0 to 254 (inclusive). The value 255 is reserved for further standardization and MUST NOT be used.
0 (bits 24 to 31): This field is reserved for further standardization. It MUST be set to 0 when the payload is created and MUST be ignored when the payload is interpreted.
contributing source (CSRC) identifiers: This variable-length field contains the four-octet CSRC identifiers associated with the encapsulated payload. The number of CSRC identifiers is given by the CC field.
extension: This variable-length field is present only if the X field is 1. If it is present, its format is the same as the extension field of the RTP header. In particular, its length is always a multiple of four octets.
encapsulated payload: This variable-length field contains the payload of the payload type specified by the PT field (interpreted in the context of the constituent MD identified by the index field).
There is no defined meaning for the RTP marker bit in association with a kumquat payload. (Note that this is the marker bit in the RTP header that precedes the kumquat payload, not the M field of the kumquat payload itself.) Its value MUST be 0.
The kumquat payload represents an RTP packet containing the following data:
Graphically, the kumquat encoding sets up the following equivalence between an RTP packet of the constituent TF and an RTP packet of the bundle TF:
RTP packet in the context of the bundle media description (with PT1
specifying kumquat encoding):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
RTP header:
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X1| 0 |0| PT1 | sequence number |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extension (per X1 bit) |
| .... |
+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Payload of kumquat payload type:
+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|V=2|0|X2| CC |M| PT2 | index | 0 |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extension (per X2 bit) |
| .... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers (per CC) |
| .... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| encapsulated payload |
| .... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP packet in the context of the constituent media description
identified by index:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
RTP header:
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X2| CC |M| PT2 | sequence number |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extension (per X2 bit) |
| .... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| contributing source (CSRC) identifiers (per CC) |
| .... |
+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Payload of PT2 payload type:
+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| encapsulated payload |
| .... |
+-+-+-+--+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The kumquat encapsulation usually adds four octets to the length of the encapsulated RTP packet. The encapsulation overhead can be larger if there is a need for a separate RTP header extension for the kumquat RTP packet.
TBD
TBD
TBD
TBD
TBD
If an SBC wishes to prevent positively the transport of certain media types or codecs, and enforces that by examining the content of RTP packets, the use of kumquat encoding may defeat the examination.
TBD
TBD
Many people have provided input for this proposal regarding both the technical aspects and the organization of the presentation. Chief among them are the authors of the predecessor proposals (draft-alvestrand-one-rtp ("TOGETHER"), draft-holmberg-mmusic-sdp-mmt-negotiation ("MMT"), and draft-ietf-mmusic-sdp-bundle-negotiation ("BUNDLE")): Harald Alvestrand, Jonathan Lennox, and Christer Holmberg. In addition, input was provided by Charles Eckel, Andrew Hutton, Cullen Jennings, Hadriel Kaplan, Paul Kyzivat, Adam Roach, and Robert Sparks.
Note to RFC Editor: Please remove this section before publication.
Initial version.
Thoroughly revise the text and structure of the document.
Heavily revise Terminology regarding media flows.
Revise Desiderata, including adding that multiple separate bundles must be possible, and noninterference with ICE negotiation.
Add section on ICE considerations.
Change "fusion" to "bundle".
Use a=rtcp-mux in examples to be more realistic (and to shorten the examples).
Correct the use of ICE in answers; ICE candidates are not provided if an offered MD does not contain ICE candidates.
Add example of fast-start offer.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3264] | Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002. |
| [RFC3550] | Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003. |
| [RFC4566] | Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006. |
| [RFC5245] | Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, DOI 10.17487/RFC5245, April 2010. |
| [RFC2327] | Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998. |
| [RFC3725] | Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004. |
| [RFC4796] | Hautakorpi, J. and G. Camarillo, "The Session Description Protocol (SDP) Content Attribute", RFC 4796, DOI 10.17487/RFC4796, February 2007. |
| [RFC5761] | Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, DOI 10.17487/RFC5761, April 2010. |