Kumquat: A Generic Bundle Mechanism for the Session Description Protocol (SDP)
draft-worley-sdp-bundle-05
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draft-worley-sdp-bundle-05
rtcweb D. R. Worley
Internet-Draft Ariadne
Intended status: Standards Track March 11, 2013
Expires: September 12, 2013
Kumquat: A Generic Bundle Mechanism for the Session Description Protocol
(SDP)
draft-worley-sdp-bundle-05
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 12, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Desiderata . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Feature Desiderata . . . . . . . . . . . . . . . . . . . 6
3.2. Compatibility Desiderata . . . . . . . . . . . . . . . . 8
4. Tutorial Examples . . . . . . . . . . . . . . . . . . . . . . 10
4.1. One Audio Stream and One Video Stream . . . . . . . . . . 10
4.1.1. Offer without Bundling . . . . . . . . . . . . . . . 10
4.1.2. Offer with Bundling . . . . . . . . . . . . . . . . . 11
4.1.3. Answer from an Answerer that Supports Bundling . . . 13
4.1.4. Answer from an Answerer that Does Not Support
Bundling . . . . . . . . . . . . . . . . . . . . . . 14
4.1.5. Fast-Start Offer . . . . . . . . . . . . . . . . . . 17
4.2. Two Audio Streams and Two Video Streams . . . . . . . . . 18
4.3. Virtual Classroom with One Audio Stream, Two Video
Streams, and a Group of Video Streams . . . . . . . . . . 20
4.4. One Audio Stream and Two SCTP Streams . . . . . . . . . . 22
5. Syntax and Semantics . . . . . . . . . . . . . . . . . . . . 23
5.1. Constructing an Offer . . . . . . . . . . . . . . . . . . 23
5.2. Constructing an Answer . . . . . . . . . . . . . . . . . 23
5.3. Offer/Answer Considerations . . . . . . . . . . . . . . . 23
5.4. Multiplexing and Demultiplexing Media Streams . . . . . . 23
5.4.1. The "kumquat" Payload Format . . . . . . . . . . . . 23
5.5. RTCP, SSRC, and RTP Sessions . . . . . . . . . . . . . . 27
5.6. ICE considerations . . . . . . . . . . . . . . . . . . . 27
5.7. Encryption considerations . . . . . . . . . . . . . . . . 27
6. Compatibility Considerations . . . . . . . . . . . . . . . . 27
6.1. Backward Compatibility during Offer/Answer . . . . . . . 27
6.2. Backward Compatibility with Existing Devices . . . . . . 27
7. Design Features and Comparison . . . . . . . . . . . . . . . 27
7.1. Aggregation of Constituent Media Descriptions . . . . . . 29
7.2. Presence of a Bundle Media Description and Its Media Type 29
7.3. Immediate Update . . . . . . . . . . . . . . . . . . . . 30
7.4. Effective Media Description Ports after Session
Establishment . . . . . . . . . . . . . . . . . . . . . . 30
7.5. Payload Types in the Bundled Media Description . . . . . 30
7.6. Relationship of Payload Types of Constituent Media
Descriptions . . . . . . . . . . . . . . . . . . . . . . 31
7.7. Basis of Demultiplexing . . . . . . . . . . . . . . . . . 31
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7.8. Basis of Rejection of the Bundle MD . . . . . . . . . . . 31
7.9. How Constituent MDs Are Offered . . . . . . . . . . . . . 32
8. Demultiplexing Considerations . . . . . . . . . . . . . . . . 34
9. Security Considerations . . . . . . . . . . . . . . . . . . . 38
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38
12. Revision History . . . . . . . . . . . . . . . . . . . . . . 38
12.1. draft-worley-sdp-bundle-00 . . . . . . . . . . . . . . . 38
12.2. Changes from draft-worley-sdp-bundle-00 to draft-worley-
sdp-bundle-01 . . . . . . . . . . . . . . . . . . . . . 38
12.3. Changes from draft-worley-sdp-bundle-01 to draft-worley-
sdp-bundle-02 . . . . . . . . . . . . . . . . . . . . . 38
12.4. Changes from draft-worley-sdp-bundle-02 to draft-worley-
sdp-bundle-03 . . . . . . . . . . . . . . . . . . . . . 39
12.5. Changes from draft-worley-sdp-bundle-03 to draft-worley-
sdp-bundle-04 . . . . . . . . . . . . . . . . . . . . . 39
12.6. Changes from draft-worley-sdp-bundle-04 to draft-worley-
sdp-bundle-05 . . . . . . . . . . . . . . . . . . . . . 39
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
13.1. Normative References . . . . . . . . . . . . . . . . . . 40
13.2. Informative References . . . . . . . . . . . . . . . . . 40
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
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
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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
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].
The important RFCs in this area use inconsistent terminology. Here,
we use:
media Media is (1) media content, considered in an abstract way,
that is, without consideration of its particular encoding or the
framing information around it, and (2) the particular bits and
octets used to encode and transmit the abstract media content.
media stream (Taken from [RFC3550].) A media stream is a set of RTP
packets that are generated by and interpreted by one codec. The
RTP packets of a media stream are identified by a unique SSRC.
capture (Taken from CLUE's work.) A capture is a set of media
streams that originate from one (physical or virtual) media source
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and should be composed to provide rendering of that source. For
example, media streams from one origin including layered
encodings, forward error correction streams, recovery streams, and
simulcasted media streams of varying bit rates compose one
capture.
transport association (Taken from [I-D.alvestrand-mmusic-msid].) A
transport association is a single data path between two hosts,
such as a TCP connection, or a pair of UDP ports that send packets
to each other. A transport association is identified by the
identity of the protocol being used, the relevant host addresses,
and the relevant port numbers. In the case of unicast
communications, these form a "5-tuple", namely, the protocol, the
host addresses of the two hosts, and the port numbers used on the
two hosts. In the case of multicast sessions, these form a
"3-tuple", namely, the protocol, the multicast address, and the
port number. In SDP, a transport association is specified by the
address and port of a media description (and possibly the same
information from the matching offer/answer SDP). If a media
description specifies multiple addresses or ports, each address or
port specifies one transport association.
transport flow (Taken from
[I-D.ietf-avtcore-multi-media-rtp-session].) (This is called an
"RTP session" by [RFC3264].) A transport flow is the data that
flows across a transport association.
media description (Taken from [RFC4566].) A media description is
one group of lines in a session description demarcated by an m=
line. By synecdoche, a media description is often referred to as
"an m= line".
transport association group A transport association group is the set
of transport associations denoted by one media description.
Usually the m= line specifies only one port and the c= line
specifies only one address, and so the media description's
transport association group contains only one transport
association.
transport flow group A transport flow group is the set of transport
flows of the transport associations of a transport association
group.
session description (Taken from [RFC4566] section 2.) A session
description is an SDP instance.
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multimedia session (Taken from [RFC4566] section 2.) A multimedia
session is the totality of the media that is transmitted/received
as described by a particular session description.
RTP session (Taken from [RFC3550].) An RTP session is a group of
media streams which must not have duplicated SSRC values because
the endpoints share RTCP reporting information. Note that an RTP
session may encompass more than one multimedia session. RTP
sessions are not fully described by session descriptions.
3. Desiderata
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.
3.1. Feature Desiderata
These desiderata describe features that we would like the bundling
mechanism to provide.
DES F1 For each bundle, there is a group of media descriptions which
describe the application-level RTP sessions. This specification
must allow the same granularity of description as when the media
flows were not multiplexed. This description includes identifiers
which connect the media flows with the application and with each
other.
This requirement is taken from [I-D.jennings-mmusic-media-req].
DES F2 For each bundle, there is a media description that describes
the transport-level RTP session.
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.
DES F3 There must be a uniform way to deal with new SDP parameters,
so that newly defined SDP parameters do not require a specific
updating of the bundling procedures.
This desideratum is taken from slides-interim-2013-rtcweb-1-10.pdf.
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DES F4 Multiple separate bundles within one SDP must be supported.
DES F5 Bundles may contain other bundles as constituents.
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.)
DES F6 A bundle may contain zero constituents.
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.)
DES F7 If an answerer that does understand the bundle mechanism
processes an offer that contains a bundle, it must be able to (1)
accept the bundle and selectively accept or reject each
constituent RTP session within it, (2) reject the bundle as a
whole, or (3) reject the bundling and selectively accept or reject
each constituent RTP session as separate RTP sessions.
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.
DES F8 There must be a reliable way to demultiplex incoming RTP into
the separate application-level RTP sessions. Similarly, there
must be a reliable way to demultiplex the associated RTCP
information.
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.
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DES F9 The specification must specify any needed additional
procedures for handling SSRC collisions between media sources
within different application-level RTP sessions, as those can now
collide.
In the terminology of [RFC3550], the constituent media descriptions
are now part of one RTP session.
DES F10 When an offer is constructed, the offerer must not need to
preallocate TURN relays for constituent media descriptions. When
both endpoints support bundling, the mechanism must not require
the offerer to allocate TURN relays for constituent media
descriptions.
This desideratum was suggested by Andrew Hutton.
DES F11 It must be possible to add and remove one way video flows
within the bundle without requiring an additional offer/answer
cycle.
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.
DES F12 Bundling must not interfere with ICE usage, and in
particular, ICE's ability to negotiate both IPv4 and IPv6
addresses simultaneously.
This desideratum was suggested by Andrew Hutton.
3.2. Compatibility Desiderata
These desiderata describe compatibility of the bundling mechanism
with with non-supporting endpoints or with existing entities in the
Internet infrastructure.
DES C1 In offer/answer usage, an endpoint using the bundle mechanism
must interwork correctly with an endpoint that does not understand
the bundle mechanism.
DES C2 Interworking must continue when SDP endpoints are replaced
with other endpoints during a sequence of offer/answer exchanges
(such as happens in 3PCC or call transfers "behind an SBC"),
including when a supporting endpoint is replaced by a non-
supporting endpoint or vice-versa.
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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
[I-D.worley-service-example]). This desideratum was suggested by
Richard Ejzak.
DES C3 Avoid using media types in m= lines other than audio and
video unless required for user media, as some SBCs reject SDP that
uses other media types.
This desideratum was suggested by Hadriel Kaplan.
DES C4 Any additional m= lines prescribed by the bundle mechanism
should be ordered after the constituent m= lines.
Many devices that have only one audio or video channel accept the
first m= line with that media type and reject any further ones
non-DES C5 SBCs generally pass through attributes that they do not
understand. SBCs generally pass through codec specifications that
they do not understand, even if they are configured to transcode
certain specific codecs.
This non-desideratum was suggested by Hadriel Kaplan.
DES C6 After offer/answer processing is finished, if the exchanged
SDP is examined by a non-supporting SBC, the set of transport
associations that it sees being specified for media exchange
should be the set that are actually used for media transfer.
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.
DES C7 In a session description, no endpoint of a transport
association may be used multiple times.
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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.
DES C8 Offer/answer processing between supporting processors must be
completed in one exchange. When interworking between supporting
and non-supporting processors, it is less desirable but admissible
that a second offer/answer exchange may be needed to complete
configuring the multimedia session.
DES C9 If an intermediate entity provides transcoding between
codecs, and if the offer/answer does not specify encryption of a
media stream, the media stream should be able to take advantage of
the transcoding facility.
The non-encrypted case is not expected to be very common. Encrypted
media can't be transcoded by an intermediate entity.
4. Tutorial Examples
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.
4.1. One Audio Stream and One Video Stream
4.1.1. Offer without Bundling
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.
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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).
4.1.2. Offer with Bundling
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
The following group attribute declares 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 processor that does not support
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bundling, the transport information sets up the transport association
for the audio TF.
m=audio 10002 RTP/AVP 0 8 97
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
information is used only by a processor that does not support
bundling.
m=video 10004 RTP/AVP 31 32
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.
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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.
4.1.3. Answer from an Answerer that Supports Bundling
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
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a=candidate:1 1 UDP 1694194431 198.51.100.35 51090 typ srflx \
raddr 10.0.2.1 rport 20000
Because the offer contained real addresses and ports for the
constituent MDs and the answer accepted them, intermediate entities
may expect the offerer to send at least RTCP on the transport
association. To prevent such an intermediate entity from timing-out
the multimedia session (because such RTCP will not be sent), the
offerer needs to update its offer to withdraw the real address and
ports for the constituent MDs, replacing them with a null address and
port 9.
o=- 2890844526 2890844527 IN IP4 host.example.com
c=IN IP4 10.0.1.1
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 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
The answerer responds with the same answer as before.
4.1.4. Answer from an Answerer that Does Not Support Bundling
SDP bundling allows for backward compatibility in case the answerer
does not understand bundling. If the answerer does not understand
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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 10002 RTP/AVP 0 8 97
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10004 RTP/AVP 31 32
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
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
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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 if transport requires ICE
candidates describing TURN relays, the offerer must send an updated
offer containing those ICE candidates 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 ICE candidates for the audio MD. (We can reuse the ICE
candidates (and TURN relay) previously offered for the bundle MD.)
m=audio 10000 RTP/AVP 0 8 97
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
Provide ICE candidates (and a separate TURN relay) for the video MD.
m=video 10002 RTP/AVP 31 32
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
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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
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
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.
4.1.5. Fast-Start Offer
The baseline procedure requires the offerer to update its offer when
it discovers that the answerer does not support SDP bundling if TURN
relays are needed to support the constituent MDs. 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
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m=audio 10000 RTP/AVP 0 8 97
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
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
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
accepts the constituent MDs (with a null address and port 9) and
accepts the bundle MD. If the answerer does not understand bundling,
it accepts the constituent MDs and rejects the bundle MD.
4.2. Two Audio Streams and Two Video Streams
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 10002 RTP/AVP 0 8 97
a=mid:c1
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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 10004 RTP/AVP 0 8 97
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 10006 RTP/AVP 103 104
a=mid:c3
a=label:speaker-video
a=rtcp-mux
a=rtpmap:103 H261/90000
a=rtpmap:104 MPV/90000
m=video 10008 RTP/AVP 103 104
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
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4.3. Virtual Classroom with One Audio Stream, Two Video Streams, and a
Group of Video Streams
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 dynamically
varying 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.
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m=audio 10002 RTP/AVP 0 8 97
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 10004 RTP/AVP 103 104
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 10006 RTP/AVP 105 106
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 10008 RTP/AVP 105 106
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
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4.4. One Audio Stream and Two SCTP Streams
This example contains one audio MD and two SCTP MDs, which are used
for Webrtc datachannels.
o=- 2890844526 2890844526 IN IP4 host.example.com
c=IN IP4 10.0.1.1
The following group attribute declares 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 processor that does not support
bundling, the transport information sets up the transport association
for the audio TF.
m=audio 10002 RTP/AVP 0 8 97
a=mid:con1
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
These MDs provides the the SCTP TFs. Using the Kumquat encapsulation,
the two SCTP TFs can use the same (nominal) SCTP port, since the
encapsulation carries the ordinal number of the constituent MD for
each packet. However, in this example, the two TFs use different port
numbers.
m=application 10004 DTLS/SCTP 5000
a=sctpmap:5000 webrtc-datachannel 16
m=application 10006 DTLS/SCTP 5001
a=sctpmap:5001 webrtc-datachannel 16
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
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a=candidate:1 1 UDP 1694194431 198.51.100.32 51000 typ srflx \
raddr 10.0.1.1 rport 10000
5. Syntax and Semantics
TBD (Here lies the real description.)
5.1. Constructing an Offer
TBD
5.2. Constructing an Answer
TBD
5.3. Offer/Answer Considerations
TBD
5.4. Multiplexing and Demultiplexing Media Streams
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.
5.4.1. The "kumquat" Payload Format
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.
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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 |transport flow |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 media description 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.
transport flow This field contains the index of the transport flow
within the transport flow group of the constituent media
description. That is, it identifies one of the series of ports or
addresses specified by the media description, if the media
description specified multiple ports or addresses. The range of
index values is 0 to 255. (If the media description does not
specify a range of ports or addresses, the transport flow index is
0.)
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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 M field of 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:
V The V field is 2.
P The pad field is unspecified, because the need for padding is
determined only when the RTP packet is considered in the context
of the transport protocol.
X, CC, M, PT These fields are taken from the corresponding fields of
the kumquat payload data.
sequence number, timestamp, SSRC identifier These fields are taken
from the corresponding fields of RTP header before the kumquat
payload.
extension, CSRC identifiers These fields are taken from the
corresponding fields of the kumquat payload data.
payload This field is taken from the encapsulated payload field of
the kumquat payload 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):
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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:
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+=+=+=+==+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 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.
5.5. RTCP, SSRC, and RTP Sessions
TBD
5.6. ICE considerations
TBD
5.7. Encryption considerations
TBD Need to discuss here how the encryption associations are set up.
For SRTP/SRTCP, it would be possible to have either one association
for all multiplexed streams, or one for each constituent MD, because
SRTP preserves the PTs. (Have to verify that and check whether SRTCP
preserves SSRCs.) But SCTP-over-DTLS can't be demultiplexed before
it's decrypted, so there can be only one DTLS crypto association.
6. Compatibility Considerations
6.1. Backward Compatibility during Offer/Answer
TBD
6.2. Backward Compatibility with Existing Devices
TBD
7. Design Features and Comparison
Key:
x = feature present in proposal
- = feature not present in proposal
. = feature not discussed in proposal
N/A = feature is not relevant because of another feature choice
worley-sdp-bundle-05 (KUMQUAT)
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| ietf-mmusic-sdp-bundle-negotiation-03 (BUNDLE)
| | holmberg-mmusic-sdp-mmt-negotiation-00 (MMT)
| | | alvestrand-one-rtp-02 (TOGETHER)
| | | | ejzak-mmusic-bundle-alternatives-01
| | | | | Roach alternative 1a
| | | | | |(roach-mmusic-mlines-00)
| | | | | | Roach alternative 1b
| | | | | | | Roach alternative 2
| | | | | | | | westerlund-avtcore-
| | | | | | | | |transport-multiplexing-05
| | | | | | | | |(SHIM)
V V V V V V V V V
MD grouping:
one - - - - - - x - -
per type - - - - - x - - -
none x x x x x - - x x
Separate bundle MD:
no - x - x x x x x x
m=anymedia - - x - - - - - -
m=audio x - - - - - - - -
m=multipart - - - - - - - - -
Immediate update:
none - - x x x x x x x
for support x x - - - - - - -
for compat. x - - - - - - - -
Constituent MD ports after establishment:
N/A - - - - - x x - -
same - x - x - - - x x
different - - - - - - - - -
null x - - - - - - - -
rejected - - x - x - - - -
Bundle MD payload types:
N/A - - - - - x x - -
one MD - x - x . - - x x
all MDs - - x - . - - - -
encap. x - - - . - - - -
Constituent MD payload types:
N/A - - - - - x x - -
overlapping x - - - . - - x x
distinct - x x x . - - - -
Demultiplexing based on:
N/A - - - - - x x - -
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PT - x x x . - - x -
encap. x - - - . - - - x
Rejection of bundle MD based on:
N/A - x - x x x x x x
media type - - x - - - - - -
proto - - - - - - - - -
codec x - - - - - - - -
Addresses/ports in constituent MDs in offer:
N/A - - - - - x x - -
NA,ZP - - - - - - - - -
NA,NZP - - - - x - - - -
RA,ZP - - - - - - - - -
RA,NZP,U x x x - x - - - -
RA,NZP,S - x - x - - - x x
NA = null address, RA = real address
ZP = zero port, NZP = non-zero port
U = unique ports, S = shared port
7.1. Aggregation of Constituent Media Descriptions
Are the constituent media descriptions combined into grouped media
descriptions?
o All media are combined into one media description.
o All media of each media type are combined into one media
description (which has that type).
o Each constituent media description is separate in the session
description.
This proposal does not aggregate constituent MDs so that attributes
can be provided directly for each constituent MD.
7.2. Presence of a Bundle Media Description and Its Media Type
Is there a separate bundle media description, and if so, what media
type does it have?
o There is no separate bundle media description.
o There is a separate bundle media description of type "anymedia".
o There is a separate bundle media description of type "audio".
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o There is a separate bundle media description of type "multipart".
This proposal has a separate bundle MD so that attributes can be
provided for the bundle MD independently of any constituent MD.
7.3. Immediate Update
Is an immediate updated offer/answer used during session
establishment?
o No.
o Yes, when establishing a session using bundling.
o Yes, when establishing a session not using bundling.
This proposal requires updates for bundled answers (to tell
intermediate entities to not expect media for constituent transport
associations) and for non-bundled answers (to provide TURN ICE
candidates, if needed).
7.4. Effective Media Description Ports after Session Establishment
What are the effective port numbers in MDs after the session is
established?
o There are not multiple media descriptions because constituent MDs
are combined.
o Port numbers in the MDs are the same.
o Port numbers in the MDs are different.
o All but one MD have null addresses.
o All but one MD have a zero port number (and thus are formally
rejected).
This proposal signals null connection addresses for constituent MDs
to prevent intermediate entities from expecting to see media for the
constituent transport associations.
7.5. Payload Types in the Bundled Media Description
What payload types are listed for the bundled MD?
o There is no MD describing the bundle as a whole.
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o The bundle MD lists the payload types of one constituent MD.
o The bundle MD lists the payload types of all constituent MDs.
o The bundle MD lists one payload type for an encapsulation codec.
This proposal uses one payload type in the bundled media description
because (1) only the encapsulated PT is sent on that transport
association, and (2) to ensure that answerers that do not implement
bundling reject this MD.
7.6. Relationship of Payload Types of Constituent Media Descriptions
What is the relationship between the payload types of the constituent
MDs?
o There are not multiple media descriptions because constituent MDs
are combined.
o Different constituent MDs may have overlapping payload type
numbers.
o Different constituent MDs may not have overlapping payload type
numbers.
This proposal allows constituent MDs to use overlapping payload types
because it relies on an encapsulation to demultiplex the constituent
MDs.
7.7. Basis of Demultiplexing
What is the basis for the demultiplexing of RTP?
o Demultiplexing is not done because incoming RTP is not attributed
to specific constituent MDs (possibly because constituent MDs are
combined).
o Demultiplexing is done based on payload type numbers.
o Demultiplexing is done based on data carried in an encapsulation.
This proposal does demultiplexing based on information in the
encapsulated payload format.
7.8. Basis of Rejection of the Bundle MD
We must ensure that the bundle MD is rejected by non-supporting
endpoints. What method is used to ensure rejection?
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o There is no bundle MD.
o The bundle MD uses a special media type value.
o The bundle MD uses a special proto value.
o The bundle MD uses (only) a special codec name.
This proposal uses one encapsulated payload type in the bundled media
description to ensure that answerers that do not implement bundling
reject this MD.
7.9. How Constituent MDs Are Offered
There are a number of alternative ways that the offerer can configure
the constituent media descriptions.
+------------+------+--------+--------+---------+---------+---------+
| Method | 1 | 2 | 3 | 4 | 5 | 6 |
+------------+------+--------+--------+---------+---------+---------+
| Coded in | NA,Z | NA,NZP | RA,ZP | RA,NZP, | RA,NZP, | RA,NZP, |
| chart as | P | | | U | U | S |
| | | | | | | |
| Offered | null | null | real | real | real | real |
| address | | | | | | |
| | | | | | | |
| Offered | zero | non- | zero | non- | non- | non- |
| port | | zero | | zero, | zero, | zero, |
| | | | | unique | unique | shared |
| | | | | | | |
| TURN candi | no | no | no | no | yes | yes |
| dates? | | | | | | |
| | | | | | | |
| Supporting | yes | yes | yes | yes | yes | yes |
| answerer | | | | | | |
| accepts? | | | | | | |
| | | | | | | |
| Update | no | no | possib | yes | yes | no |
| needed for | | | ly | | | |
| supporting | | | | | | |
| answerer? | | | | | | |
| | | | | | | |
| Non- | no | probab | no | yes | yes | yes |
| supporting | | ly | | | | |
| answerer | | | | | | |
| accepts? | | | | | | |
| | | | | | | |
| Update | yes | yes | yes | yes | no | no |
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| needed for | | | | | | |
| non- | | | | | | |
| supporting | | | | | | |
| answerer? | | | | | | |
| | | | | | | |
| Disadvanta | ACE | CD | ABCE | BC | BG | BFG |
| ges | | | | | | |
+------------+------+--------+--------+---------+---------+---------+
Offered address This is the address offered for the MD. There are
two choices, a null address (0.0.0.0 for IPv4 or "invalid." for
IPv6 or mixed IPv4/IPv6) or a real address of the offerer.
Offered port This is the port that is offered. It can be either
non-zero or zero (which indicates a stream that is not being
offered). If the offered port is non-zero and the offered address
is real, the port can be either unique, or shared with the other
media descriptions in the bundle.
TURN candidates? If the offered address is real, are TURN candidates
for the address provided (if they are needed)?
Supporting answerer accepts? If the answerer supports bundling, does
it accept this media description? We assume the answer is Yes, so
that acceptance of the constituent media descriptions can be
signaled to the offerer.
Update needed for supporting answerer? Is an SDP update needed to
complete session setup if the answerer supports bundling? If an
update is needed, it is needed to inform intermediaries that there
will be no media sent based on the connection information in this
media description; the update is not needed to establish
communications and does not delay the application.
Non-supporting answerer accepts? If the answerer does not support
bundling, does it accept this media description (without a further
update)?
Update needed for non-supporting answerer? Is an SDP update needed
to complete session setup if the answerer does not support
bundling? If an update is needed, in all cases, it is needed to
establish communication.
Flaws The disadvantages of each alternative:
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A Media descriptions that are rejected (have zero ports) are
not allowed to be members of a group (in the
offer).[RFC5888]
B An SDP update is needed for a supporting answerer to prevent
intermediaries from timing out the multimedia session.
C An SDP update is needed for a non-supporting answerer to
establish communications.
D Although a media description with a non-zero port but a null
address is formally offered (although shown as on-hold via
the old method), it is possible that the answerer will not
consider it to be offered and many not accept it even if it
otherwise wood.
E If the offered port is zero, the media description is not
formally offered and the answerer should not accept it.
F SDP offers with multiple media descriptions that use the
same port number (for the same real address) may be rejected
by intermediaries.
G A TURN relay must be allocated for the constituent media
description before the offer is sent.
In my estimation, the worst disadvantages are A (zero port in offer),
E (acceptance of offer with zero port), and F (duplicate port
numbers), because they involve violations of [RFC4566] or are known
to trigger limitations of large numbers of intermediate devices.
Disadvantage D (offering a MD with a null address) is nearly as
severe, as we expect it to cause undesired behavior in many non-
supporting answerers. Disadvantages C (update needed to communicate
with non-supporting answerer) and G (TURN relay must be preallocated)
are moderate, and disadvantage B (updated needed to prevent
intermediaries from timing out) is the least severe (because it never
delays the establishment of communication).
Applying these priorities to the possible solutions, methods 4 and 5
(offer real address, non-zero unique port, with/without TURN
candidates) are tied for the best choices, with the choice made based
on the relative importance of minimizing preallocation of TURN relays
compared to quickly establishing communication with non-supporting
answerers.
8. Demultiplexing Considerations
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This section discusses the constraints regarding demultiplexing
datagrams from multiple protocols that are presented on one transport
flow. This is an expansion of the analysis in [RFC5764] section
5.1.2.
The first octets of datagrams generated by particular protocols are:
+--------------+------------+--------------+-----------+------------+
| Protocol | First | Second octet | Third | Fourth |
| | octet | | octet | octet |
+--------------+------------+--------------+-----------+------------+
| STUN | 0x00, 0x01 | 0x00, 0x01 | | |
| | | | | |
| RTP | 0x80 to | 0x00 to | | |
| | 0xBF | 0xC7, 0xCD | | |
| | | to 0xFF | | |
| | | | | |
| RTCP | 0x80 to | 0xC8 to 0xCC | | |
| | 0xBF | | | |
| | | | | |
| RTP/RTCPv3 | 0xC0 to | | | |
| | 0xFF | | | |
| | | | | |
| DTLS | 0x14 to | 0x03 | 0x03 | |
| | 0x17 | | | |
| | | | | |
| SCTP | source | source port | dest. | dest. port |
| | port high | low | port high | low |
+--------------+------------+--------------+-----------+------------+
TBD RFC 5764 specifies that the first octet of a DTLS packet is in
the range 0x14 to 0x3F. RFC 5246 and RFC 6374 together specify the
first octet is a "ContentType", with the range 0x14 to 0x17. Are
additional octet values reserved for expansion? What is the range
that should be reserved in practice?
The most generalized stack of protocols we consider is this:
Layer 6: ... application interfaces ...
||| ||| ||| |||
V V V V
| | | |
| | | |
Layer 5: | | | SCTP
| | | |
| | | |
RTP SRTP | |
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Layer 4: RTCP SRTCP SCTP DTLS [ STUN ]
\ | | | /
--------------- ----------------
V
|
|
Layer 3: [ encapsulation STUN ]
[ \ / ]
[ ---- --------- ]
[ V ]
|
|
Layer 2: [ DTLS STUN ]
[ \ / ]
[ ------- --------- ]
[ V ]
|
|
Layer 1: [ TURN ]
|
|
Layer 0: UDP
Layer 0: UDP This is the base layer of this analysis, where packets
are carried by UDP.
Layer 1: TURN (optional) If a packet arrives from a TURN relay for
which we are a client, the TURN encapsulation must be removed
before further processing. This need can be detected because the
packet arrives from the client-facing address/port of a TURN
server of which this entity is a client.
Layer 2: DTLS applied to the bundle transport flow (optional)
If DTLS is applied to the bundle transport flow as a whole, that
use of DTLS will have been negotiated. However, before
deciphering, STUN packets need to be separated. STUN packets can
be distinguished from DTLS packets by their first or second
octets.
Layer 3: Decapsulation (depending on the bundling method If the
bundling method uses encapsulation, decapsulation is done at this
point in the protocol stack. However, before decapsulating, STUN
packets need to be separated. The encapsulation method must allow
encapsulated packets to be distinguished from STUN packets, if
STUN packets have not been demultiplexed at layer 2 (due to DTLS
encryption of the entire transport stream).
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All proposed encapsulation techniques ensure the first octet of
the encapsulation is in the range allowed for the first octet of
RTP, 0x80 to 0xBF, and thus is distinguishable from STUN.
Layer 4: Core demultiplexing At this layer, most of the protocols
are demultiplexed. RTP/SRTP, SRTP/SRTCP, DTLS, and STUN are
distinguished by the values of the first two octets of the packet.
SRTP/SRTCP is distinguished from RTP/RTCP by negotiation with the
other endpoint -- SRTP/SRTCP is never multiplexed with RTP/RTCP.
SCTP can be distinguished from the other protocols if the other
endpoint agrees to use only SCTP port numbers in the range 0x4000
to 0x7FFF. This restriction can be specified here, because this
limitation is only needed when bundling is implemented, which
happens only when when both endpoints support bundling. (This
particular range is chosen to avoid collision with a future RTP/
RTCP version 3. The range of SCTP ports can be chosen arbitrarily
because the SCTP ports are not used to route packets to this
entity, as they are encapsulated in UDP.)
Layer 5: SCTP over DTLS If the layer 4 protocol is DTLS, the
protocol above it will always be SCTP.
Layer 6: Application interface demultiplexing The method used to
demultiplex the various application interface streams varies
depending whether encapsulation is used and if not, on the layer 4
/5 protocol. If the layer 4 protocol is encapsulated, the
encapsulation determines which constituent media description is to
be assigned to this packet, and then application demultiplexing is
done as normal for for that particular media description.
Otherwise, the constituent media description must be determined by
a method that is specific to the layer 4/5 protocol:
RTP/SRTP An RTP/SRTP packet is assigned to the constituent media
description which specifies its payload type number. (If
encapsulation is not used, the constituent media
descriptions must specify distinct payload type numbers.)
RTCP/SRTCP An RTCP/SRTCP sub-packet is dispatched to the
application which receives the RTP media stream containing
the SSRC that is carried in the RTCP sub-packet.
SCTP An SCTP packet is assigned to the constituent media
description which specifies its destination port number.
(If encapsulation is not used, the constituent media
descriptions must specify distinct SCTP port numbers.)
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9. Security Considerations
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
10. IANA Considerations
TBD
11. Acknowledgments
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
([I-D.alvestrand-one-rtp] (TOGETHER),
[I-D.holmberg-mmusic-sdp-mmt-negotiation] (MMT), and
[I-D.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.
12. Revision History
Note to RFC Editor: Please remove this section before publication.
12.1. draft-worley-sdp-bundle-00
Initial version.
12.2. Changes from draft-worley-sdp-bundle-00 to draft-worley-sdp-
bundle-01
Thoroughly revise the text and structure of the document.
12.3. Changes from draft-worley-sdp-bundle-01 to draft-worley-sdp-
bundle-02
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".
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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 an example of a fast-start offer.
12.4. Changes from draft-worley-sdp-bundle-02 to draft-worley-sdp-
bundle-03
Add design comparison Section 7.
Use bibxml references.
Add DES C9, regarding continued usage of transcoding facilities
offered by intermediate entities.
12.5. Changes from draft-worley-sdp-bundle-03 to draft-worley-sdp-
bundle-04
Add demultiplexing considerations Section 8.
12.6. Changes from draft-worley-sdp-bundle-04 to draft-worley-sdp-
bundle-05
Change recommendation for SCTP port numbers from 0xC000-0xFFFF to
0x4000-0x7FFF to avoid collision with a future RTP/RTCP version 3.
Add the transport flow index to the KUMQUAT encapsulation.
Add section on choices for offering constituent MDs Section 7.9.
Revise the examples to show offering "real address, non-zero port, no
ICE candidates".
Add note to DES C9 (support intermediate transcoding facilities)
saying that intermediate transcoding facilities are not expected to
be very useful, given that encryption will be the normal use case.
Add an exampleSection 4.4 with SCTP MDs.
Add a section for encryption considerations.Section 5.7
Revise generalized demultiplexing diagram to make explicit the
optional RTP encapsulation layer.
Update comparison chartSection 7 for draft-ejzak-mmusic-bundle-
alternatives-01[I-D.ejzak-mmusic-bundle-alternatives] and draft-
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westerlund-avtcore-transport-
multiplexing-05[I-D.westerlund-avtcore-transport-multiplexing].
Update comparison chartSection 7 to discuss alternative address/port/
candidate policies for offering constituent MDs.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
13.2. Informative References
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, 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, April 2004.
[RFC4796] Hautakorpi, J. and G. Camarillo, "The Session Description
Protocol (SDP) Content Attribute", RFC 4796, February
2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
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[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[I-D.alvestrand-mmusic-msid]
Alvestrand, H., "Cross Session Stream Identification in
the Session Description Protocol", draft-alvestrand-
mmusic-msid-02 (work in progress), December 2012.
[I-D.alvestrand-one-rtp]
Alvestrand, H., "SDP Grouping for Single RTP Sessions",
draft-alvestrand-one-rtp-02 (work in progress), September
2011.
[I-D.ejzak-mmusic-bundle-alternatives]
Ejzak, R., "Alternatives to BUNDLE", draft-ejzak-mmusic-
bundle-alternatives-01 (work in progress), February 2013.
[I-D.holmberg-mmusic-sdp-mmt-negotiation]
Holmberg, C., Alvestrand, H., and J. Lennox, "Multiplexed
Media Types (MMT) Using Session Description Protocol (SDP)
Port Numbers", draft-holmberg-mmusic-sdp-mmt-
negotiation-00 (work in progress), October 2012.
[I-D.ietf-avtcore-multi-media-rtp-session]
Westerlund, M., Perkins, C., and J. Lennox, "Multiple
Media Types in an RTP Session", draft-ietf-avtcore-multi-
media-rtp-session-02 (work in progress), February 2013.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
bundle-negotiation-03 (work in progress), February 2013.
[I-D.jennings-mmusic-media-req]
Jennings, C., Uberti, J., and E. Rescorla, "Requirements
from various WG for MMUSIC", draft-jennings-mmusic-media-
req-00 (work in progress), February 2013.
[I-D.roach-mmusic-mlines]
Roach, A., "Thoughts on syntax for representing multiple
media streams", draft-roach-mmusic-mlines-00 (work in
progress), January 2013.
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[I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
Single Lower-Layer Transport", draft-westerlund-avtcore-
transport-multiplexing-05 (work in progress), February
2013.
[I-D.worley-service-example]
Worley, D., "Session Initiation Protocol Service Example
-- Music on Hold", draft-worley-service-example-11 (work
in progress), February 2013.
Author's Address
Dale R. Worley
Ariadne Internet Services, Inc.
738 Main St.
Waltham, MA 02451
US
Phone: +1 781 647 9199
Email: worley@ariadne.com
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