Internet DRAFT - draft-ietf-mmusic-sdp-simulcast
draft-ietf-mmusic-sdp-simulcast
Network Working Group B. Burman
Internet-Draft M. Westerlund
Intended status: Standards Track Ericsson
Expires: September 6, 2019 S. Nandakumar
M. Zanaty
Cisco
March 5, 2019
Using Simulcast in SDP and RTP Sessions
draft-ietf-mmusic-sdp-simulcast-14
Abstract
In some application scenarios it may be desirable to send multiple
differently encoded versions of the same media source in different
RTP streams. This is called simulcast. This document describes how
to accomplish simulcast in RTP and how to signal it in SDP. The
described solution uses an RTP/RTCP identification method to identify
RTP streams belonging to the same media source, and makes an
extension to SDP to relate those RTP streams as being different
simulcast formats of that media source. The SDP extension consists
of a new media level SDP attribute that expresses capability to send
and/or receive simulcast RTP streams.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on September 6, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Reaching a Diverse Set of Receivers . . . . . . . . . . . 6
3.2. Application Specific Media Source Handling . . . . . . . 7
3.3. Receiver Media Source Preferences . . . . . . . . . . . . 7
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Detailed Description . . . . . . . . . . . . . . . . . . . . 10
5.1. Simulcast Attribute . . . . . . . . . . . . . . . . . . . 10
5.2. Simulcast Capability . . . . . . . . . . . . . . . . . . 11
5.3. Offer/Answer Use . . . . . . . . . . . . . . . . . . . . 13
5.3.1. Generating the Initial SDP Offer . . . . . . . . . . 13
5.3.2. Creating the SDP Answer . . . . . . . . . . . . . . . 13
5.3.3. Offerer Processing the SDP Answer . . . . . . . . . . 14
5.3.4. Modifying the Session . . . . . . . . . . . . . . . . 15
5.4. Use with Declarative SDP . . . . . . . . . . . . . . . . 15
5.5. Relating Simulcast Streams . . . . . . . . . . . . . . . 16
5.6. Signaling Examples . . . . . . . . . . . . . . . . . . . 16
5.6.1. Single-Source Client . . . . . . . . . . . . . . . . 17
5.6.2. Multi-Source Client . . . . . . . . . . . . . . . . . 18
5.6.3. Simulcast and Redundancy . . . . . . . . . . . . . . 21
6. RTP Aspects . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. Outgoing from Endpoint with Media Source . . . . . . . . 23
6.2. RTP Middlebox to Receiver . . . . . . . . . . . . . . . . 23
6.2.1. Media-Switching Mixer . . . . . . . . . . . . . . . . 24
6.2.2. Selective Forwarding Middlebox . . . . . . . . . . . 26
6.3. RTP Middlebox to RTP Middlebox . . . . . . . . . . . . . 27
7. Network Aspects . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. Bitrate Adaptation . . . . . . . . . . . . . . . . . . . 28
8. Limitation . . . . . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
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13. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . 31
13.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 34
Appendix B. Changes From Earlier Versions . . . . . . . . . . . 35
B.1. Modifications Between WG Version -13 and -14 . . . . . . 35
B.2. Modifications Between WG Version -12 and -13 . . . . . . 36
B.3. Modifications Between WG Version -11 and -12 . . . . . . 36
B.4. Modifications Between WG Version -10 and -11 . . . . . . 36
B.5. Modifications Between WG Version -09 and -10 . . . . . . 37
B.6. Modifications Between WG Version -08 and -09 . . . . . . 37
B.7. Modifications Between WG Version -07 and -08 . . . . . . 37
B.8. Modifications Between WG Version -06 and -07 . . . . . . 38
B.9. Modifications Between WG Version -05 and -06 . . . . . . 38
B.10. Modifications Between WG Version -04 and -05 . . . . . . 38
B.11. Modifications Between WG Version -03 and -04 . . . . . . 39
B.12. Modifications Between WG Version -02 and -03 . . . . . . 39
B.13. Modifications Between WG Version -01 and -02 . . . . . . 40
B.14. Modifications Between WG Version -00 and -01 . . . . . . 40
B.15. Modifications Between Individual Version -00 and WG
Version -00 . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
Most of today's multiparty video conference solutions make use of
centralized servers to reduce the bandwidth and CPU consumption in
the endpoints. Those servers receive RTP streams from each
participant and send some suitable set of possibly modified RTP
streams to the rest of the participants, which usually have
heterogeneous capabilities (screen size, CPU, bandwidth, codec, etc).
One of the biggest issues is how to perform RTP stream adaptation to
different participants' constraints with the minimum possible impact
on both video quality and server performance.
Simulcast is defined in this memo as the act of simultaneously
sending multiple different encoded streams of the same media source,
e.g. the same video source encoded with different video encoder types
or image resolutions. This can be done in several ways and for
different purposes. This document focuses on the case where it is
desirable to provide a media source as multiple encoded streams over
RTP [RFC3550] towards an intermediary so that the intermediary can
provide the wanted functionality by selecting which RTP stream(s) to
forward to other participants in the session, and more specifically
how the identification and grouping of the involved RTP streams are
done.
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The intended scope of the defined mechanism is to support negotiation
and usage of simulcast when using SDP offer/answer and media
transport over RTP. The media transport topologies considered are
point to point RTP sessions as well as centralized multi-party RTP
sessions, where a media sender will provide the simulcasted streams
to an RTP middlebox or endpoint, and middleboxes may further
distribute the simulcast streams to other middleboxes or endpoints.
Simulcast could, as part of a distributed multi-party scenario, be
used point-to-point between middleboxes. Usage of multicast or
broadcast transport is out of scope and left for future extensions.
This document describes a few scenarios that motivate the use of
simulcast, and also defines the needed RTP/RTCP and SDP signaling for
it.
2. Definitions
2.1. Terminology
This document makes use of the terminology defined in RTP Taxonomy
[RFC7656], and RTP Topologies [RFC7667]. The following terms are
especially noted or here defined:
RTP Mixer: An RTP middle node, defined in [RFC7667] (Section 3.6 to
3.9).
RTP Session: An association among a group of participants
communicating with RTP, as defined in [RFC3550] and amended by
[RFC7656].
RTP Stream: A stream of RTP packets containing media data, as
defined in [RFC7656].
RTP Switch: A common short term for the terms "switching RTP mixer",
"source projecting middlebox", and "video switching MCU" as
discussed in [RFC7667].
Simulcast Stream: One encoded stream or dependent stream from a set
of concurrently transmitted encoded streams and optional dependent
streams, all sharing a common media source, as defined in
[RFC7656]. For example, HD and thumbnail video simulcast versions
of a single media source sent concurrently as separate RTP
Streams.
Simulcast Format: Different formats of a simulcast stream serve the
same purpose as alternative RTP payload types in non-simulcast
SDP: to allow multiple alternative media formats for a given RTP
stream. As for multiple RTP payload types on the m-line in offer/
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answer [RFC3264], any one of the negotiated alternative formats
can be used in a single RTP stream at a given point in time, but
not more than one (based on RTP timestamp). What format is used
can change dynamically from one RTP packet to another.
2.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Use Cases
The use cases of simulcast described in this document relate to a
multi-party communication session where one or more central nodes are
used to adapt the view of the communication session towards
individual participants, and facilitate the media transport between
participants. Thus, these cases target the RTP Mixer type of
topology.
There are two principal approaches for an RTP Mixer to provide this
adapted view of the communication session to each receiving
participant:
o Transcoding (decoding and re-encoding) received RTP streams with
characteristics adapted to each receiving participant. This often
include mixing or composition of media sources from multiple
participants into a mixed media source originated by the RTP
Mixer. The main advantage of this approach is that it achieves
close to optimal adaptation to individual receiving participants.
The main disadvantages are that it can be very computationally
expensive to the RTP Mixer, typically degrades media Quality of
Experience (QoE) such as end-to-end delay for the receiving
participants, and requires RTP Mixer access to media content.
o Switching a subset of all received RTP streams or sub-streams to
each receiving participant, where the used subset is typically
specific to each receiving participant. The main advantages of
this approach are that it is computationally cheap to the RTP
Mixer, has very limited impact on media QoE, and does not require
RTP Mixer (full) access to media content. The main disadvantage
is that it can be difficult to combine a subset of received RTP
streams into a perfect fit to the resource situation of a
receiving participant. It is also a disadvantage that sending
multiple RTP streams consumes more network resources from the
sending participant to the RTP Mixer.
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The use of simulcast relates to the latter approach, where it is more
important to reduce the load on the RTP Mixer and/or minimize QoE
impact than to achieve an optimal adaptation of resource usage.
3.1. Reaching a Diverse Set of Receivers
The media sources provided by a sending participant potentially need
to reach several receiving participants that differ in terms of
available resources. The receiver resources that typically differ
include, but are not limited to:
Codec: This includes codec type (such as RTP payload format MIME
type) and can include codec configuration. A couple of codec
resources that differ only in codec configuration will be
"different" if they are somehow not "compatible", like if they
differ in video codec profile, or the transport packetization
configuration.
Sampling: This relates to how the media source is sampled, in
spatial as well as in temporal domain. For video streams, spatial
sampling affects image resolution and temporal sampling affects
video frame rate. For audio, spatial sampling relates to the
number of audio channels and temporal sampling affects audio
bandwidth. This may be used to suit different rendering
capabilities or needs at the receiving endpoints.
Bitrate: This relates to the number of bits sent per second to
transmit the media source as an RTP stream, which typically also
affects the QoE for the receiving user.
Letting the sending participant create a simulcast of a few
differently configured RTP streams per media source can be a good
tradeoff when using an RTP switch as middlebox, instead of sending a
single RTP stream and using an RTP mixer to create individual
transcodings to each receiving participant.
This requires that the receiving participants can be categorized in
terms of available resources and that the sending participant can
choose a matching configuration for a single RTP stream per category
and media source. For example, a set of receiving participants
differ only in screen resolution; some are able to display video with
at most 360p resolution and some support 720p resolution. A sending
participant can then reach all receivers with best possible
resolution by creating a simulcast of RTP streams with 360p and 720p
resolution for each sent video media source.
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The maximum number of simulcasted RTP streams that can be sent is
mainly limited by the amount of processing and uplink network
resources available to the sending participant.
3.2. Application Specific Media Source Handling
The application logic that controls the communication session may
include special handling of some media sources. It is, for example,
commonly the case that the media from a sending participant is not
sent back to itself.
It is also common that a currently active speaker participant is
shown in larger size or higher quality than other participants (the
sampling or bitrate aspects of Section 3.1) in a receiving client.
Many conferencing systems do not send the active speaker's media back
to the sender itself, which means there is some other participant's
media that instead is forwarded to the active speaker; typically the
previous active speaker. This way, the previously active speaker is
needed both in larger size (to current active speaker) and in small
size (to the rest of the participants), which can be solved with a
simulcast from the previously active speaker to the RTP switch.
3.3. Receiver Media Source Preferences
The application logic that controls the communication session may
allow receiving participants to state preferences on the
characteristics of the RTP stream they like to receive, for example
in terms of the aspects listed in Section 3.1. Sending a simulcast
of RTP streams is one way of accommodating receivers with conflicting
or otherwise incompatible preferences.
4. Overview
This memo defines SDP [RFC4566] signaling that covers the above
described simulcast use cases and functionalities. A number of
requirements for such signaling are elaborated in Appendix A.
The RID mechanism, as defined in [I-D.ietf-mmusic-rid], enables an
SDP offerer or answerer to specify a number of different RTP stream
restrictions for a rid-id by using the "a=rid" line. Examples of
such restrictions are maximum bitrate, maximum spatial video
resolution (width and height), maximum video framerate, etc. Each
rid-id may also be restricted to use only a subset of the RTP payload
types in the associated SDP media description. Those RTP payload
types can have their own configurations and parameters affecting what
can be sent or received, using the "a=fmtp" line as well as other SDP
attributes.
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A new SDP media level attribute "a=simulcast" is defined. The
attribute describes, independently for send and receive directions,
the number of simulcast RTP streams as well as potential alternative
formats for each simulcast RTP stream. Each simulcast RTP stream,
including alternatives, is identified using the RID identifier (rid-
id), defined in [I-D.ietf-mmusic-rid].
a=simulcast:send 1;2,3 recv 4
If the above line is included in an SDP offer, the "send" part
indicates the offerer's capability and proposal to send two simulcast
RTP streams. Each simulcast stream is described by one or more RTP
stream identifiers (rid-id), each group of rid-ids for a simulcast
stream is separated by a semicolon (";"). When a simulcast stream
has multiple rid-ids that are separated by a comma (","), they
describe alternative representations for that particular simulcast
RTP stream. Thus, the above "send" part is interpreted as an
intention to send two simulcast RTP streams. The first simulcast RTP
stream is identified and restricted according to rid-id 1. The
second simulcast RTP stream can be sent as two alternatives,
identified and restricted according to rid-ids 2 and 3. The "recv"
part of the above line indicates that the offerer desires to receive
a single RTP stream (no simulcast) according to rid-id 4.
A more complete example SDP offer media description is provided
below:
m=video 49300 RTP/AVP 97 98 99
a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000
a=rtpmap:99 VP8/90000
a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
a=fmtp:99 max-fs=240; max-fr=30
a=rid:1 send pt=97;max-width=1280;max-height=720
a=rid:2 send pt=98;max-width=320;max-height=180
a=rid:3 send pt=99;max-width=320;max-height=180
a=rid:4 recv pt=97
a=simulcast:send 1;2,3 recv 4
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
Figure 1: Example Simulcast Media Description in Offer
The above SDP media description can be interpreted at a high level to
say that the offerer is capable of sending two simulcast RTP streams,
one H.264 encoded stream in up to 720p resolution, and one additional
stream encoded as either H.264 or VP8 with a maximum resolution of
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320x180 pixels. The offerer can receive one H.264 stream with
maximum 720p resolution.
The receiver of this SDP offer can generate an SDP answer that
indicates what it accepts. It uses the "a=simulcast" attribute to
indicate simulcast capability and specify what simulcast RTP streams
and alternatives to receive and/or send. An example of such
answering "a=simulcast" attribute, corresponding to the above offer,
is:
a=simulcast:recv 1;2 send 4
With this SDP answer, the answerer indicates in the "recv" part that
it wants to receive the two simulcast RTP streams. It has removed an
alternative that it doesn't support (rid-id 3). The send part
confirms to the offerer that it will receive one stream for this
media source according to rid-id 4. The corresponding, more complete
example SDP answer media description could look like:
m=video 49674 RTP/AVP 97 98
a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
a=rid:1 recv pt=97;max-width=1280;max-height=720
a=rid:2 recv pt=98;max-width=320;max-height=180
a=rid:4 send pt=97
a=simulcast:recv 1;2 send 4
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
Figure 2: Example Simulcast Media Description in Answer
It is assumed that a single SDP media description is used to describe
a single media source. This is aligned with the concepts defined in
[RFC7656] and will work in a WebRTC context, both with and without
BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] grouping of media
descriptions.
To summarize, the "a=simulcast" line describes send and receive
direction simulcast streams separately. Each direction can in turn
describe one or more simulcast streams, separated by semicolon. The
identifiers describing simulcast streams on the "a=simulcast" line
are rid-id, as defined by "a=rid" lines in [I-D.ietf-mmusic-rid].
Each simulcast stream can be offered as a list of alternative rid-id,
with each alternative separated by comma (not in the examples above).
A detailed specification can be found in Section 5 and more detailed
examples are outlined in Section 5.6.
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5. Detailed Description
This section further details the overview above (Section 4). First,
formal syntax is provided (Section 5.1), followed by the rest of the
SDP attribute definition in Section 5.2. Relating Simulcast Streams
(Section 5.5) provides the definition of the RTP/RTCP mechanisms
used. The section is concluded with a number of examples.
5.1. Simulcast Attribute
This document defines a new SDP media-level "a=simulcast" attribute,
with value according to the following ABNF [RFC5234] syntax and its
update for Case-Sensitive String Support in ABNF [RFC7405]:
sc-value = ( sc-send [SP sc-recv] ) / ( sc-recv [SP sc-send] )
sc-send = %s"send" SP sc-str-list
sc-recv = %s"recv" SP sc-str-list
sc-str-list = sc-alt-list *( ";" sc-alt-list )
sc-alt-list = sc-id *( "," sc-id )
sc-id-paused = "~"
sc-id = [sc-id-paused] rid-id
; SP defined in [RFC5234]
; rid-id defined in [I-D.ietf-mmusic-rid]
Figure 3: ABNF for Simulcast Value
Note to RFC Editor: Replace "I-D.ietf-mmusic-rid" in the above
figure with RFC number of draft-ietf-mmusic-rid before publication
of this document.
The "a=simulcast" attribute has a parameter in the form of one or two
simulcast stream descriptions, each consisting of a direction ("send"
or "recv"), followed by a list of one or more simulcast streams.
Each simulcast stream consists of one or more alternative simulcast
formats. Each simulcast format is identified by a simulcast stream
identifier (rid-id). The rid-id MUST have the form of an RTP stream
identifier, as described by RTP Payload Format Restrictions
[I-D.ietf-mmusic-rid].
In the list of simulcast streams, each simulcast stream is separated
by a semicolon (";"). Each simulcast stream can in turn be offered
in one or more alternative formats, represented by rid-ids, separated
by a comma (","). Each rid-id can also be specified as initially
paused [RFC7728], indicated by prepending a "~" to the rid-id. The
reason to allow separate initial pause states for each rid-id is that
pause capability can be specified individually for each RTP payload
type referenced by an rid-id. Since pause capability specified via
the "a=rtcp-fb" attribute applies only to specified payload types and
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rid-id specified by "a=rid" can refer to multiple different payload
types, it is unfeasible to pause streams with rid-id where any of the
related RTP payload type(s) do not have pause capability.
5.2. Simulcast Capability
Simulcast capability is expressed through a new media level SDP
attribute, "a=simulcast" (Section 5.1). The use of this attribute at
the session level is undefined. Implementations of this
specification MUST NOT use it at the session level and MUST ignore it
if received at the session level. Extensions to this specification
may define such session level usage. Each SDP media description MUST
contain at most one "a=simulcast" line.
There are separate and independent sets of simulcast streams in send
and receive directions. When listing multiple directions, each
direction MUST NOT occur more than once on the same line.
Simulcast streams using undefined rid-id MUST NOT be used as valid
simulcast streams by an RTP stream receiver. The direction for an
rid-id MUST be aligned with the direction specified for the
corresponding RTP stream identifier on the "a=rid" line.
The listed number of simulcast streams for a direction sets a limit
to the number of supported simulcast streams in that direction. The
order of the listed simulcast streams in the "send" direction
suggests a proposed order of preference, in decreasing order: the
rid-id listed first is the most preferred and subsequent streams have
progressively lower preference. The order of the listed rid-id in
the "recv" direction expresses which simulcast streams that are
preferred, with the leftmost being most preferred. This can be of
importance if the number of actually sent simulcast streams have to
be reduced for some reason.
rid-id that have explicit dependencies [RFC5583]
[I-D.ietf-mmusic-rid] to other rid-id (even in the same media
description) MAY be used.
Use of more than a single, alternative simulcast format for a
simulcast stream MAY be specified as part of the attribute parameters
by expressing the simulcast stream as a comma-separated list of
alternative rid-id. The order of the rid-id alternatives within a
simulcast stream is significant; the rid-id alternatives are listed
from (left) most preferred to (right) least preferred. For the use
of simulcast, this overrides the normal codec preference as expressed
by format type ordering on the "m=" line, using regular SDP rules.
This is to enable a separation of general codec preferences and
simulcast stream configuration preferences. However, the choice of
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which alternative to use per simulcast stream is independent, and
there is currently no mechanism to align the choice between
alternative rid-ids between different simulcast streams.
A simulcast stream can use a codec defined such that the same RTP
SSRC can change RTP payload type multiple times during a session,
possibly even on a per-packet basis. A typical example can be a
speech codec that makes use of Comfort Noise [RFC3389] and/or DTMF
[RFC4733] formats.
If RTP stream pause/resume [RFC7728] is supported, any rid-id MAY be
prefixed by a "~" character to indicate that the corresponding
simulcast stream is initially paused already from start of the RTP
session. In this case, support for RTP stream pause/resume MUST also
be included under the same "m=" line where "a=simulcast" is included.
All RTP payload types related to such an initially paused simulcast
stream MUST be listed in the SDP as pause/resume capable as specified
by [RFC7728], e.g. by using the "*" wildcard format for "a=rtcp-fb".
An initially paused simulcast stream in "send" direction for the
endpoint sending the SDP MUST be considered equivalent to an
unsolicited locally paused stream, and be handled accordingly.
Initially paused simulcast streams are resumed as described by the
RTP pause/resume specification. An RTP stream receiver that wishes
to resume an unsolicited locally paused stream needs to know the SSRC
of that stream. The SSRC of an initially paused simulcast stream can
be obtained from an RTP stream sender RTCP Sender Report (SR)
including both the desired SSRC as "SSRC of sender", and the rid-id
value in an RtpStreamId RTCP SDES item [I-D.ietf-avtext-rid].
If the endpoint sending the SDP includes an "recv" direction
simulcast stream that is initially paused, then the remote RTP sender
receiving the SDP SHOULD put its RTP stream in a unsolicited locally
paused state. The simulcast stream sender does not put the stream in
the locally paused state if there are other RTP stream receivers in
the session that do not mark the simulcast stream as initially
paused. However, in centralized conferencing the RTP sender usually
does not see the SDP signalling from RTP receivers and cannot make
this determination. The reason to require an initially paused "recv"
stream to be considered locally paused by the remote RTP sender,
instead of making it equivalent to implicitly sending a pause
request, is because the pausing RTP sender cannot know which
receiving SSRC owns the restriction when Temporary Maximum Media
Stream Bit Rate Request (TMMBR) and Temporary Maximum Media Stream
Bit Rate Notification (TMMBN) are used for pause/resume signaling
(Section 5.6 of [RFC7728]) since the RTP receiver's SSRC in send
direction is sometimes not yet known.
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Use of the redundant audio data [RFC2198] format could be seen as a
form of simulcast for loss protection purposes, but is not considered
conflicting with the mechanisms described in this memo and MAY
therefore be used as any other format. In this case the "red"
format, rather than the carried formats, SHOULD be the one to list as
a simulcast stream on the "a=simulcast" line.
The media formats and corresponding characteristics of simulcast
streams SHOULD be chosen such that they are different, e.g. as
different SDP formats with differing "a=rtpmap" and/or "a=fmtp"
lines, or as differently defined RTP payload format restrictions. If
this difference is not required, it is RECOMMENDED to use RTP
duplication [RFC7104] procedures instead of simulcast. To avoid
complications in implementations, a single rid-id MUST NOT occur more
than once per "a=simulcast" line. Note that this does not eliminate
use of simulcast as an RTP duplication mechanism, since it is
possible to define multiple different rid-id that are effectively
equivalent.
5.3. Offer/Answer Use
Note: The inclusion of "a=simulcast" or the use of simulcast does
not change any of the interpretation or Offer/Answer procedures
for other SDP attributes, like "a=fmtp" or "a=rid".
5.3.1. Generating the Initial SDP Offer
An offerer wanting to use simulcast for a media description SHALL
include one "a=simulcast" attribute in that media description in the
offer. An offerer listing a set of receive simulcast streams and/or
alternative formats as rid-id in the offer MUST be prepared to
receive RTP streams for any of those simulcast streams and/or
alternative formats from the answerer.
5.3.2. Creating the SDP Answer
An answerer that does not understand the concept of simulcast will
also not know the attribute and will remove it in the SDP answer, as
defined in existing SDP Offer/Answer [RFC3264] procedures. Since SDP
session level simulcast is undefined in this memo, an answerer that
receives an offer with the "a=simulcast" attribute on SDP session
level SHALL remove it in the answer. An answerer that understands
the attribute but receives multiple "a=simulcast" attributes in the
same media description SHALL disable use of simulcast by removing all
"a=simulcast" lines for that media description in the answer.
An answerer that does understand the attribute and that wants to
support simulcast in an indicated direction SHALL reverse
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directionality of the unidirectional direction parameters; "send"
becomes "recv" and vice versa, and include it in the answer.
An answerer that receives an offer with simulcast containing an
"a=simulcast" attribute listing alternative rid-id MAY keep all the
alternative rid-id in the answer, but it MAY also choose to remove
any non-desirable alternative rid-id in the answer. The answerer
MUST NOT add any alternative rid-id in send direction in the answer
that were not present in the offer receive direction. The answerer
MUST be prepared to receive any of the receive direction rid-id
alternatives and MAY send any of the send direction alternatives that
are part of the answer.
An answerer that receives an offer with simulcast that lists a number
of simulcast streams, MAY reduce the number of simulcast streams in
the answer, but MUST NOT add simulcast streams.
An answerer that receives an offer without RTP stream pause/resume
capability MUST NOT mark any simulcast streams as initially paused in
the answer.
An RTP stream pause/resume capable answerer that receives an offer
with RTP stream pause/resume capability MAY mark any rid-id that
refer to pause/resume capable formats as initially paused in the
answer.
An answerer that receives indication in an offer of an rid-id being
initially paused SHOULD mark that rid-id as initially paused also in
the answer, regardless of direction, unless it has good reason for
the rid-id not being initially paused. One reason to remove an
initial pause in the answer compared to the offer could, for example,
be that all receive direction simulcast streams for a media source
the answerer accepts in the answer would otherwise be paused.
5.3.3. Offerer Processing the SDP Answer
An offerer that receives an answer without "a=simulcast" MUST NOT use
simulcast towards the answerer. An offerer that receives an answer
with "a=simulcast" without any rid-id in a specified direction MUST
NOT use simulcast in that direction.
An offerer that receives an answer where some rid-id alternatives are
kept MUST be prepared to receive any of the kept send direction rid-
id alternatives, and MAY send any of the kept receive direction rid-
id alternatives.
An offerer that receives an answer where some of the rid-id are
removed compared to the offer MAY release the corresponding resources
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(codec, transport, etc) in its receive direction and MUST NOT send
any RTP packets corresponding to the removed rid-id.
An offerer that offered some of its rid-id as initially paused and
that receives an answer that does not indicate RTP stream pause/
resume capability, MUST NOT initially pause any simulcast streams.
An offerer with RTP stream pause/resume capability that receives an
answer where some rid-id are marked as initially paused, SHOULD
initially pause those RTP streams regardless if they were marked as
initially paused also in the offer, unless it has good reason for
those RTP streams not being initially paused. One such reason could,
for example, be that the answerer would otherwise initially not
receive any media of that type at all.
5.3.4. Modifying the Session
Offers inside an existing session follow the same rules as for
initial SDP offer, with these additions:
1. rid-id marked as initially paused in the offerer's send direction
SHALL reflect the offerer's opinion of the current pause state at
the time of creating the offer. This is purely informational,
and RTP stream pause/resume [RFC7728] signaling in the ongoing
session SHALL take precedence in case of any conflict or
ambiguity.
2. rid-id marked as initially paused in the offerer's receive
direction SHALL (as in an initial offer) reflect the offerer's
desired rid-id pause state. Except for the case where the
offerer already paused the corresponding RTP stream through RTP
stream pause/resume [RFC7728] signaling , this is identical to
the conditions at an initial offer.
Creation of SDP answers and processing of SDP answers inside an
existing session follow the same rules as described above for initial
SDP offer/answer.
Session modification restrictions in section 6.5 of RTP payload
format restrictions [I-D.ietf-mmusic-rid] also apply.
5.4. Use with Declarative SDP
This document does not define the use of "a=simulcast" in declarative
SDP, partly motivated by use of the simulcast format identification
[I-D.ietf-mmusic-rid] not being defined for use in declarative SDP.
If concrete use cases for simulcast in declarative SDP are identified
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in the future, the authors of this memo expect that additional
specifications will address such use.
5.5. Relating Simulcast Streams
Simulcast RTP streams MUST be related on RTP level through
RtpStreamId [I-D.ietf-avtext-rid], as specified in the SDP
"a=simulcast" attribute (Section 5.2) parameters. This is sufficient
as long as there is only a single media source per SDP media
description. When using BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation], where multiple SDP media
descriptions jointly specify a single RTP session, the SDES MID
identification mechanism in BUNDLE allows relating RTP streams back
to individual media descriptions, after which the above described
RtpStreamId relations can be used. Use of the RTP header extension
[RFC8285] for both MID and RtpStreamId identifications can be
important to ensure rapid initial reception, required to correctly
interpret and process the RTP streams. Implementers of this
specification MUST support the RTCP source description (SDES) item
method and SHOULD support RTP header extension method to signal
RtpStreamId on RTP level.
NOTE: For the case where it is clear from SDP that RTP PT uniquely
maps to corresponding RtpStreamId, an RTP receiver can use RTP PT
to relate simulcast streams. This can sometimes enable decoding
even in advance to receiving RtpStreamId information in RTCP SDES
and/or RTP header extensions.
RTP streams MUST only use a single alternative rid-id at a time
(based on RTP timestamps), but MAY change format (and rid-id) on a
per-RTP packet basis. This corresponds to the existing (non-
simulcast) SDP offer/answer case when multiple formats are included
on the "m=" line in the SDP answer, enabling per-RTP packet change of
RTP payload type.
5.6. Signaling Examples
These examples describe a client to video conference service, using a
centralized media topology with an RTP mixer.
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+---+ +-----------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| Mixer |
+---+ | | +---+
| F |<---->| |<---->| J |
+---+ +-----------+ +---+
Figure 4: Four-party Mixer-based Conference
5.6.1. Single-Source Client
Alice is calling in to the mixer with a simulcast-enabled client
capable of a single media source per media type. The client can send
a simulcast of 2 video resolutions and frame rates: HD 1280x720p
30fps and thumbnail 320x180p 15fps. This is defined below using the
"imageattr" [RFC6236]. In this example, only the "pt" "a=rid"
parameter is used, effectively achieving a 1:1 mapping between
RtpStreamId and media formats (RTP payload types), to describe
simulcast stream formats. Alice's Offer:
v=0
o=alice 2362969037 2362969040 IN IP4 192.0.2.156
s=Simulcast Enabled Client
c=IN IP4 192.0.2.156
t=0 0
m=audio 49200 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 49300 RTP/AVP 97 98
a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
a=imageattr:97 send [x=1280,y=720] recv [x=1280,y=720]
a=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
a=rid:1 send pt=97
a=rid:2 send pt=98
a=rid:3 recv pt=97
a=simulcast:send 1;2 recv 3
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
Figure 5: Single-Source Simulcast Offer
The only thing in the SDP that indicates simulcast capability is the
line in the video media description containing the "simulcast"
attribute. The included "a=fmtp" and "a=imageattr" parameters
indicates that sent simulcast streams can differ in video resolution.
The RTP header extension for RtpStreamId is offered to avoid issues
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with the initial binding between RTP streams (SSRCs) and the
RtpStreamId identifying the simulcast stream and its format.
The Answer from the server indicates that it too is simulcast
capable. Should it not have been simulcast capable, the
"a=simulcast" line would not have been present and communication
would have started with the media negotiated in the SDP. Also the
usage of the RtpStreamId RTP header extension is accepted.
v=0
o=server 823479283 1209384938 IN IP4 192.0.2.2
s=Answer to Simulcast Enabled Client
c=IN IP4 192.0.2.43
t=0 0
m=audio 49672 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 49674 RTP/AVP 97 98
a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42c01f;max-fs=3600;max-mbps=108000
a=fmtp:98 profile-level-id=42c00b;max-fs=240;max-mbps=3600
a=imageattr:97 send [x=1280,y=720] recv [x=1280,y=720]
a=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
a=rid:1 recv pt=97
a=rid:2 recv pt=98
a=rid:3 send pt=97
a=simulcast:recv 1;2 send 3
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
Figure 6: Single-Source Simulcast Answer
Since the server is the simulcast media receiver, it reverses the
direction of the "simulcast" and "rid" attribute parameters.
5.6.2. Multi-Source Client
Fred is calling in to the same conference as in the example above
with a two-camera, two-display system, thus capable of handling two
separate media sources in each direction, where each media source is
simulcast-enabled in the send direction. Fred's client is restricted
to a single media source per media description.
The first two simulcast streams for the first media source use
different codecs, H264-SVC [RFC6190] and H264 [RFC6184]. These two
simulcast streams also have a temporal dependency. Two different
video codecs, VP8 [RFC7741] and H264, are offered as alternatives for
the third simulcast stream for the first media source. Only the
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highest fidelity simulcast stream is sent from start, the lower
fidelity streams being initially paused.
The second media source is offered with three different simulcast
streams. All video streams of this second media source are loss
protected by RTP retransmission [RFC4588]. Also here, all but the
highest fidelity simulcast stream are initially paused. Note that
the lower resolution is more prioritized than the medium resolution
simulcast stream.
Fred's client is also using BUNDLE to send all RTP streams from all
media descriptions in the same RTP session on a single media
transport. Although using many different simulcast streams in this
example, the use of RtpStreamId as simulcast stream identification
enables use of a low number of RTP payload types. Note that the use
of both BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] and "a=rid"
[I-D.ietf-mmusic-rid] recommends using the RTP header extension
[RFC8285] for carrying these RTP stream identification fields, which
is consequently also included in the SDP. Note also that for
"a=rid", the corresponding RtpStreamId SDES attribute RTP header
extension is named rtp-stream-id [I-D.ietf-avtext-rid].
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v=0
o=fred 238947129 823479223 IN IP6 2001:db8::c000:27d
s=Offer from Simulcast Enabled Multi-Source Client
c=IN IP6 2001:db8::c000:27d
t=0 0
a=group:BUNDLE foo bar zen
m=audio 49200 RTP/AVP 99
a=mid:foo
a=rtpmap:99 G722/8000
m=video 49600 RTP/AVPF 100 101 103
a=mid:bar
a=rtpmap:100 H264-SVC/90000
a=rtpmap:101 H264/90000
a=rtpmap:103 VP8/90000
a=fmtp:100 profile-level-id=42400d;max-fs=3600;max-mbps=216000; \
mst-mode=NI-TC
a=fmtp:101 profile-level-id=42c00d;max-fs=3600;max-mbps=108000
a=fmtp:103 max-fs=900; max-fr=30
a=rid:1 send pt=100;max-width=1280;max-height=720;max-fps=60;depend=2
a=rid:2 send pt=101;max-width=1280;max-height=720;max-fps=30
a=rid:3 send pt=101;max-width=640;max-height=360
a=rid:4 send pt=103;max-width=640;max-height=360
a=depend:100 lay bar:101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=rtcp-fb:* ccm pause nowait
a=simulcast:send 1;2;~4,3
m=video 49602 RTP/AVPF 96 104
a=mid:zen
a=rtpmap:96 VP8/90000
a=fmtp:96 max-fs=3600; max-fr=30
a=rtpmap:104 rtx/90000
a=fmtp:104 apt=96;rtx-time=200
a=rid:1 send max-fs=921600;max-fps=30
a=rid:2 send max-fs=614400;max-fps=15
a=rid:3 send max-fs=230400;max-fps=30
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
a=rtcp-fb:* ccm pause nowait
a=simulcast:send 1;~3;~2
Figure 7: Fred's Multi-Source Simulcast Offer
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5.6.3. Simulcast and Redundancy
The example in this section looks at applying simulcast with audio
and video redundancy formats. The audio media description uses codec
and bitrate restrictions, combining it with RTP Payload for Redundant
Audio Data [RFC2198] for enhanced packet loss resilience. The video
media description applies both resolution and bitrate restrictions,
combining it with FEC in the form of Flexible FEC
[I-D.ietf-payload-flexible-fec-scheme] and RTP Retransmission
[RFC4588].
The audio source is offered to be sent as two simulcast streams. The
first simulcast stream is encoded with Opus, restricted to 50 kbps
(rid-id=5), and the second simulcast stream is encoded either with
G.711 (rid-id=7) or with G.711 combined with LPC for redundancy (rid-
id=6). In this example, stand-alone LPC is not offered as an
possible payload type for the second simulcast stream's RID, which
could e.g. be motivated by not providing sufficient quality.
The video source is offered to be sent as two simulcast streams, both
with two alternative simulcast formats. Redundancy and repair are
offered in the form of both Flexible FEC and RTP Retransmission. The
Flexible FEC is not bound to any particular RTP streams and is
therefore possible to use across all RTP streams that are being sent
as part of this media description.
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v=0
o=fred 238947129 823479223 IN IP6 2001:db8::c000:27d
s=Offer from Simulcast Enabled Client using Redundancy
c=IN IP6 2001:db8::c000:27d
t=0 0
a=group:BUNDLE foo bar
m=audio 49200 RTP/AVP 97 98 99 100 101 102
a=mid:foo
a=rtpmap:97 G711/8000
a=rtpmap:98 LPC/8000
a=rtpmap:99 OPUS/48000/1
a=rtpmap:100 RED/8000/1
a=rtpmap:101 CN/8000
a=rtpmap:102 telephone-event/8000
a=fmtp:99 useinbandfec=1;usedtx=0
a=fmtp:100 97/98
a=fmtp:102 0-15
a=ptime:20
a=maxptime:40
a=rid:1 send pt=99,102;max-br=64000
a=rid:2 send pt=100,97,101,102
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=simulcast:send 1;2
m=video 49600 RTP/AVPF 103 104 105 106 107
a=mid:bar
a=rtpmap:103 H264/90000
a=rtpmap:104 VP8/90000
a=rtpmap:105 rtx/90000
a=rtpmap:106 rtx/90000
a=rtpmap:107 flexfec/90000
a=fmtp:103 profile-level-id=42c00d;max-fs=3600;max-mbps=108000
a=fmtp:104 max-fs=3600; max-fr=30
a=fmtp:105 apt=103;rtx-time=200
a=fmtp:106 apt=104;rtx-time=200
a=fmtp:107 repair-window=2000
a=rid:1 send pt=103;max-width=1280;max-height=720;max-fps=30
a=rid:2 send pt=104;max-width=1280;max-height=720;max-fps=30
a=rid:3 send pt=103;max-width=640;max-height=360;max-br=300000
a=rid:4 send pt=104;max-width=640;max-height=360;max-br=300000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
a=rtcp-fb:* ccm pause nowait
a=simulcast:send 1,2;3,4
Figure 8: Simulcast and Redundancy Example
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6. RTP Aspects
This section discusses what the different entities in a simulcast
media path can expect to happen on RTP level. This is explored from
source to sink by starting in an endpoint with a media source that is
simulcasted to an RTP middlebox. That RTP middlebox sends media
sources both to other RTP middleboxes (cascaded middleboxes), as well
as selecting some simulcast format of the media source and sending it
to receiving endpoints. Different types of RTP middleboxes and their
usage of the different simulcast formats results in several different
behaviors.
6.1. Outgoing from Endpoint with Media Source
The most straightforward simulcast case is the RTP streams being
emitted from the endpoint that originates a media source. When
simulcast has been negotiated in the sending direction, the endpoint
can transmit up to the number of RTP streams needed for the
negotiated simulcast streams for that media source. Each RTP stream
(SSRC) is identified by associating (Section 5.5) it with an
RtpStreamId SDES item, transmitted in RTCP and possibly also as an
RTP header extension. In cases where multiple media sources have
been negotiated for the same RTP session and thus BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation] is used, also the MID SDES
item will be sent similarly to the RtpStreamId.
Each RTP stream might not be continuously transmitted due to any of
the following reasons; temporarily paused using Pause/Resume
[RFC7728], sender side application logic temporarily pausing it, or
lack of network resources to transmit this simulcast stream.
However, all simulcast streams that have been negotiated have active
and maintained SSRC (at least in regular RTCP reports), even if no
RTP packets are currently transmitted. The relation between an RTP
Stream (SSRC) and a particular simulcast stream is not expected to
change, except in exceptional situations such as SSRC collisions. At
SSRC changes, the usage of MID and RtpStreamId should enable the
receiver to correctly identify the RTP streams even after an SSRC
change.
6.2. RTP Middlebox to Receiver
RTP streams in a multi-party RTP session can be used in multiple
different ways, when the session utilizes simulcast at least on the
media source to middlebox legs. This is to a large degree due to the
different RTP middlebox behaviors, but also the needs of the
application. This text assumes that the RTP middlebox will select a
media source and choose which simulcast stream for that media source
to deliver to a specific receiver. In many cases, at most one
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simulcast stream per media source will be forwarded to a particular
receiver at any instant in time, even if the selected simulcast
stream may vary. For cases where this does not hold due to
application needs, then the RTP stream aspects will fall under the
middlebox to middlebox case Section 6.3.
The selection of which simulcast streams to forward towards the
receiver, is application specific. However, in conferencing
applications, active speaker selection is common. In case the number
of media sources possible to forward, N, is less than the total
amount of media sources available in an multi-media session, the
current and previous speakers (up to N in total) are often the ones
forwarded. To avoid the need for media specific processing to
determine the current speaker(s) in the RTP middlebox, the endpoint
providing a media source may include meta data, such as the RTP
Header Extension for Client-to-Mixer Audio Level Indication
[RFC6464].
The possibilities for stream switching are media type specific, but
for media types with significant interframe dependencies in the
encoding, like most video coding, the switching needs to be made at
suitable switching points in the media stream that breaks or
otherwise deals with the dependency structure. Even if switching
points can be included periodically, it is common to use mechanisms
like Full Intra Requests [RFC5104] to request switching points from
the endpoint performing the encoding of the media source.
Inclusion of the RtpStreamId SDES item for an SSRC in the middlebox
to receiver direction should only occur when use of RtpStreamId has
been negotiated in that direction. It is worth noting that one can
signal multiple RtpStreamIds when simulcast signalling indicates only
a single simulcast stream, allowing one to use all of the
RtpStreamIds as alternatives for that simulcast stream. One reason
for including the RtpStreamId in the middlebox to receiver direction
for an RTP stream is to let the receiver know which restrictions
apply to the currently delivered RTP stream. In case the RtpStreamId
is negotiated to be used, it is important to remember that the used
identifiers will be specific to each signalling session. Even if the
central entity can attempt to coordinate, it is likely that the
RtpStreamIds need to be translated to the leg specific values. The
below cases will have as base line that RtpStreamId is not used in
the mixer to receiver direction.
6.2.1. Media-Switching Mixer
This section discusses the behavior in cases where the RTP middlebox
behaves like the Media-Switching Mixer (Section 3.6.2) in RTP
Topologies [RFC7667]. The fundamental aspect here is that the media
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sources delivered from the middlebox will be the mixer's conceptual
or functional ones. For example, one media source may be the main
speaker in high resolution video, while a number of other media
sources are thumbnails of each participant.
The above results in that the RTP stream produced by the mixer is one
that switches between a number of received incoming RTP streams for
different media sources and in different simulcast versions. The
mixer selects the media source to be sent as one of the RTP streams,
and then selects among the available simulcast streams for the most
appropriate one. The selection criteria include available bandwidth
on the mixer to receiver path and restrictions based on the
functional usage of the RTP stream delivered to the receiver. As an
example of the latter, it is unnecessary to forward a full HD video
to a receiver if the display area is just a thumbnail. Thus,
restrictions may exist to not allow some simulcast streams to be
forwarded for some of the mixer's media sources.
This will result in a single RTP stream being used for each of the
RTP mixer's media sources. This RTP stream is at any point in time a
selection of one particular RTP stream arriving to the mixer, where
the RTP header field values are rewritten to provide a consistent,
single RTP stream. If the RTP mixer doesn't receive any incoming
stream matched to this media source, the SSRC will not transmit, but
be kept alive using RTCP. The SSRC and thus RTP stream for the
mixer's media source is expected to be long term stable. It will
only be changed by signalling or other disruptive events. Note that
although the above talks about a single RTP stream, there can in some
cases be multiple RTP streams carrying the selected simulcast stream
for the originating media source, including redundancy or other
auxiliary RTP streams.
The mixer may communicate the identity of the originating media
source to the receiver by including the CSRC field with the
originating media source's SSRC value. Note that due to the
possibility that the RTP mixer switches between simulcast versions of
the media source, the CSRC value may change, even if the media source
is kept the same.
It is important to note that any MID SDES item from the originating
media source needs to be removed and not be associated with the RTP
stream's SSRC. That is, there is nothing in the signalling between
the mixer and the receiver that is structured around the originating
media sources, only the mixer's media sources. If they would be
associated with the SSRC, the receiver would likely believe that
there has been an SSRC collision, and that the RTP stream is spurious
as it doesn't carry the identifiers used to relate it to the correct
context. However, this is not true for CSRC values, as long as they
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are never used as SSRC. In these cases one could provide CNAME and
MID as SDES items. A receiver could use this to determine which CSRC
values that are associated with the same originating media source.
If RtpStreamIds are used in the scenario described by this section,
it should be noted that the RtpStreamId on a particular SSRC will
change based on the actual simulcast stream selected for switching.
These RtpStreamId identifiers will be local to this leg's signalling
context. In addition, the defined RtpStreamIds and their parameters
need to cover all the media sources and simulcast streams received by
the RTP mixer that can be switched into this media source, sent by
the RTP mixer.
6.2.2. Selective Forwarding Middlebox
This section discusses the behavior in cases where the RTP middlebox
behaves like the Selective Forwarding Middlebox (Section 3.7) in RTP
Topologies [RFC7667]. Applications for this type of RTP middlebox
results in that each originating media source will have a
corresponding media source on the leg between the middlebox and the
receiver. A Selective Forwarding Middlebox (SFM) could go as far as
exposing all the simulcast streams for an media source, however this
section will focus on having a single simulcast stream that can
contain any of the simulcast formats. This section will assume that
the SFM projection mechanism works on media source level, and maps
one of the media source's simulcast streams onto one RTP stream from
the SFM to the receiver.
This usage will result in that the individual RTP stream(s) for one
media source can switch between being active to paused, based on the
subset of media sources the SFM wants to provide the receiver for the
moment. With SFMs there exist no reasons to use CSRC to indicate the
originating stream, as there is a one to one media source mapping.
If the application requires knowing the simulcast version received to
function well, then RtpStreamId should be negotiated on the SFM to
receiver leg. Which simulcast stream that is being forwarded is not
made explicit unless RtpStreamId is used on the leg.
Any MID SDES items being sent by the SFM to the receiver are only
those agreed between the SFM and the receiver, and no MID values from
the originating side of the SFM are to be forwarded.
A SFM could expose corresponding RTP streams for all the media
sources and their simulcast streams, and then for any media source
that is to be provided forward one selected simulcast stream.
However, this is not recommended as it would unnecessarily increase
the number of RTP streams and require the receiver to timely detect
switching between simulcast streams. The above usage requires the
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same SFM functionality for switching, while avoiding the
uncertainties of timely detecting that a RTP stream ends. The
benefit would be that the received simulcast stream would be
implicitly provided by which RTP stream would be active for a media
source. However, using RtpStreamId to make this explicit also
exposes which alternative format is used. The conclusion is that
using one RTP stream per simulcast stream is unnecessary. The issue
with timely detecting end of streams, independent if they are stopped
temporarily or long term, is that there is no explicit indication
that the transmission has intentionally been stopped. The RTCP based
Pause and Resume mechanism [RFC7728] includes a PAUSED indication
that provides the last RTP sequence number transmitted prior to the
pause. Due to usage, the timeliness of this solution depends on when
delivery using RTCP can occur in relation to the transmission of the
last RTP packet. If no explicit information is provided at all, then
detection based on non increasing RTCP SR field values and timers
need to be used to determine pause in RTP packet delivery. This
results in that one can usually not determine when the last RTP
packet arrives (if it arrives) that this will be the last. That it
was the last is something that one learns later.
6.3. RTP Middlebox to RTP Middlebox
This relates to the transmission of simulcast streams between RTP
middleboxes or other usages where one wants to enable the delivery of
multiple simultaneous simulcast streams per media source, but the
transmitting entity is not the originating endpoint. For a
particular direction between middlebox A and B, this looks very
similar to the originating to middlebox case on a media source basis.
However, in this case there is usually multiple media sources,
originating from multiple endpoints. This can create situations
where limitations in the number of simultaneously received media
streams can arise, for example due to limitation in network
bandwidth. In this case, a subset of not only the simulcast streams,
but also media sources can be selected. This results in that
individual RTP streams can be become paused at any point and later
being resumed based on various criteria.
The MIDs used between A and B are the ones agreed between these two
identities in signalling. The RtpStreamId values will also be
provided to ensure explicit information about which simulcast stream
they are. The RTP stream to MID and RtpStreamId associations should
here be long term stable.
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7. Network Aspects
Simulcast is in this memo defined as the act of sending multiple
alternative encoded streams of the same underlying media source.
When transmitting multiple independent streams that originate from
the same source, it could potentially be done in several different
ways using RTP. A general discussion on considerations for use of
the different RTP multiplexing alternatives can be found in
Guidelines for Multiplexing in RTP
[I-D.ietf-avtcore-multiplex-guidelines]. Discussion and
clarification on how to handle multiple streams in an RTP session can
be found in [RFC8108].
The network aspects that are relevant for simulcast are:
Quality of Service: When using simulcast it might be of interest to
prioritize a particular simulcast stream, rather than applying
equal treatment to all streams. For example, lower bitrate
streams may be prioritized over higher bitrate streams to minimize
congestion or packet losses in the low bitrate streams. Thus,
there is a benefit to use a simulcast solution with good QoS
support.
NAT/FW Traversal: Using multiple RTP sessions incurs more cost for
NAT/FW traversal unless they can re-use the same transport flow,
which can be achieved by Multiplexing Negotiation Using SDP Port
Numbers [I-D.ietf-mmusic-sdp-bundle-negotiation].
7.1. Bitrate Adaptation
Use of multiple simulcast streams can require a significant amount of
network resources. The aggregate bandwidth for all simulcast streams
for a media source (and thus SDP media description) is bounded by any
SDP "b=" line applicable to that media source. It is assumed that a
suitable congestion control mechanism is used by the application to
ensure that it doesn't cause persistent congestion. If the amount of
available network resources varies during an RTP session such that it
does not match what is negotiated in SDP, the bitrate used by the
different simulcast streams may have to be reduced dynamically. When
a simulcasting media source uses a single media transport for all of
the simulcast streams, it is likely that a joint congestion control
across all simulcast streams is used for that media source. What
simulcast streams to prioritize when allocating available bitrate
among the simulcast streams in such adaptation SHOULD be taken from
the simulcast stream order on the "a=simulcast" line and ordering of
alternative simulcast formats Section 5.2. Simulcast streams that
have pause/resume capability and that would be given such low bitrate
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by the adaptation process that they are considered not really useful
can be temporarily paused until the limiting condition clears.
8. Limitation
The chosen approach has a limitation that relates to the use of a
single RTP session for all simulcast formats of a media source, which
comes from sending all simulcast streams related to a media source
under the same SDP media description.
It is not possible to use different simulcast streams on different
media transports, limiting the possibilities to apply different QoS
to different simulcast streams. When using unicast, QoS mechanisms
based on individual packet marking are feasible, since they do not
require separation of simulcast streams into different RTP sessions
to apply different QoS.
It is also not possible to separate different simulcast streams into
different multicast groups to allow a multicast receiver to pick the
stream it wants, rather than receive all of them. In this case, the
only reasonable implementation is to use different RTP sessions for
each multicast group so that reporting and other RTCP functions
operate as intended. Such simulcast usage in multicast context is
out of scope for the current document and would require additional
specification.
9. IANA Considerations
This document requests to register a new media-level SDP attribute,
"simulcast", in the "att-field (media level only)" registry within
the SDP parameters registry, according to the procedures of [RFC4566]
and [I-D.ietf-mmusic-sdp-mux-attributes].
Contact name, email: The IESG (iesg@ietf.org)
Attribute name: simulcast
Long-form attribute name: Simulcast stream description
Charset dependent: No
Attribute value: sc-value; see Section 5.1 of RFC XXXX.
Purpose: Signals simulcast capability for a set of RTP streams
MUX category: NORMAL
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Note to RFC Editor: Please replace "RFC XXXX" with the assigned
number of this RFC.
10. Security Considerations
The simulcast capability, configuration attributes, and parameters
are vulnerable to attacks in signaling.
A false inclusion of the "a=simulcast" attribute may result in
simultaneous transmission of multiple RTP streams that would
otherwise not be generated. The impact is limited by the media
description joint bandwidth, shared by all simulcast streams
irrespective of their number. There may however be a large number of
unwanted RTP streams that will impact the share of bandwidth
allocated for the originally wanted RTP stream.
A hostile removal of the "a=simulcast" attribute will result in
simulcast not being used.
Neither of the above will likely have any major consequences and can
be mitigated by signaling that is at least integrity and source
authenticated to prevent an attacker to change it.
Security considerations related to the use of "a=rid" and the
RtpStreamId SDES item is covered in [I-D.ietf-mmusic-rid] and
[I-D.ietf-avtext-rid]. There are no additional security concerns
related to their use in this specification.
11. Contributors
Morgan Lindqvist and Fredrik Jansson, both from Ericsson, have
contributed with important material to the first versions of this
document. Robert Hansen and Cullen Jennings, from Cisco, Peter
Thatcher, from Google, and Adam Roach, from Mozilla, contributed
significantly to subsequent versions.
12. Acknowledgements
The authors would like to thank Bernard Aboba, Thomas Belling, Roni
Even, Adam Roach, Inaki Baz Castillo, Paul Kyzivat, and Arun
Arunachalam for the feedback they provided during the development of
this document.
13. References
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13.1. Normative References
[I-D.ietf-avtext-rid]
Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
Identifier Source Description (SDES)", draft-ietf-avtext-
rid-09 (work in progress), October 2016.
[I-D.ietf-mmusic-rid]
Roach, A., "RTP Payload Format Restrictions", draft-ietf-
mmusic-rid-15 (work in progress), May 2018.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-54 (work in progress), December 2018.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17
(work in progress), February 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC7728] Burman, B., Akram, A., Even, R., and M. Westerlund, "RTP
Stream Pause and Resume", RFC 7728, DOI 10.17487/RFC7728,
February 2016, <https://www.rfc-editor.org/info/rfc7728>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[I-D.ietf-avtcore-multiplex-guidelines]
Westerlund, M., Burman, B., Perkins, C., Alvestrand, H.,
and R. Even, "Guidelines for using the Multiplexing
Features of RTP to Support Multiple Media Streams", draft-
ietf-avtcore-multiplex-guidelines-08 (work in progress),
December 2018.
[I-D.ietf-payload-flexible-fec-scheme]
Zanaty, M., Singh, V., Begen, A., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction
(FEC)", draft-ietf-payload-flexible-fec-scheme-17 (work in
progress), February 2019.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997,
<https://www.rfc-editor.org/info/rfc2198>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, DOI 10.17487/RFC3389,
September 2002, <https://www.rfc-editor.org/info/rfc3389>.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<https://www.rfc-editor.org/info/rfc4588>.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
DOI 10.17487/RFC4733, December 2006,
<https://www.rfc-editor.org/info/rfc4733>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/info/rfc5104>.
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[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <https://www.rfc-editor.org/info/rfc5109>.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
Dependency in the Session Description Protocol (SDP)",
RFC 5583, DOI 10.17487/RFC5583, July 2009,
<https://www.rfc-editor.org/info/rfc5583>.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<https://www.rfc-editor.org/info/rfc6184>.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
DOI 10.17487/RFC6190, May 2011,
<https://www.rfc-editor.org/info/rfc6190>.
[RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image
Attributes in the Session Description Protocol (SDP)",
RFC 6236, DOI 10.17487/RFC6236, May 2011,
<https://www.rfc-editor.org/info/rfc6236>.
[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
<https://www.rfc-editor.org/info/rfc6464>.
[RFC7104] Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
Semantics in the Session Description Protocol", RFC 7104,
DOI 10.17487/RFC7104, January 2014,
<https://www.rfc-editor.org/info/rfc7104>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<https://www.rfc-editor.org/info/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/rfc7667>.
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[RFC7741] Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
Galligan, "RTP Payload Format for VP8 Video", RFC 7741,
DOI 10.17487/RFC7741, March 2016,
<https://www.rfc-editor.org/info/rfc7741>.
[RFC8108] Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple RTP Streams in a Single RTP Session",
RFC 8108, DOI 10.17487/RFC8108, March 2017,
<https://www.rfc-editor.org/info/rfc8108>.
[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
<https://www.rfc-editor.org/info/rfc8285>.
Appendix A. Requirements
The following requirements are met by the defined solution to support
the use cases (Section 3):
REQ-1: Identification:
REQ-1.1: It must be possible to identify a set of simulcasted RTP
streams as originating from the same media source in SDP
signaling.
REQ-1.2: An RTP endpoint must be capable of identifying the
simulcast stream a received RTP stream is associated with,
knowing the content of the SDP signalling.
REQ-2: Transport usage. The solution must work when using:
REQ-2.1: Legacy SDP with separate media transports per SDP media
description.
REQ-2.2: Bundled [I-D.ietf-mmusic-sdp-bundle-negotiation] SDP
media descriptions.
REQ-3: Capability negotiation. It must be possible that:
REQ-3.1: Sender can express capability of sending simulcast.
REQ-3.2: Receiver can express capability of receiving simulcast.
REQ-3.3: Sender can express maximum number of simulcast streams
that can be provided.
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REQ-3.4: Receiver can express maximum number of simulcast streams
that can be received.
REQ-3.5: Sender can detail the characteristics of the simulcast
streams that can be provided.
REQ-3.6: Receiver can detail the characteristics of the simulcast
streams that it prefers to receive.
REQ-4: Distinguishing features. It must be possible to have
different simulcast streams use different codec parameters, as can
be expressed by SDP format values and RTP payload types.
REQ-5: Compatibility. It must be possible to use simulcast in
combination with other RTP mechanisms that generate additional RTP
streams:
REQ-5.1: RTP Retransmission [RFC4588].
REQ-5.2: RTP Forward Error Correction [RFC5109].
REQ-5.3: Related payload types such as audio Comfort Noise and/or
DTMF.
REQ-5.4: A single simulcast stream can consist of multiple RTP
streams, to support codecs where a dependent stream is
dependent on a set of encoded and dependent streams, each
potentially carried in their own RTP stream.
REQ-6: Interoperability. The solution must be possible to use in:
REQ-6.1: Interworking with non-simulcast legacy clients using a
single media source per media type.
REQ-6.2: WebRTC environment with a single media source per SDP
media description.
Appendix B. Changes From Earlier Versions
NOTE TO RFC EDITOR: Please remove this section prior to publication.
B.1. Modifications Between WG Version -13 and -14
o c= and t= line order corrected in SDP examples
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B.2. Modifications Between WG Version -12 and -13
o Examples corrected to follow RID ABNF
o Example Figure 7 now comments on priority for second media source.
o Clarified a SHOULD limitation.
o Added urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id in
examples with RTX.
o ABNF now uses RFC 7405 to indicate case sensitivity
o Various minor editorials and nits.
B.3. Modifications Between WG Version -11 and -12
o Modified Normative statement regarding RTP stream duplication in
Section 5.2.
o Clarified assumption about use of congestion control by
applications.
o Changed to use RFC 8174 boilerplate instead of RFC 2119.
o Clarified explanation of syntax for simulcast attribute in
Section 4.
o Editorial clarification in Section 5.2 and 5.3.2.
o Various minor editorials and nits.
B.4. Modifications Between WG Version -10 and -11
o Added new SDP example section on Simulcast and Redundancy,
including both RED (RFC2198), RTP RTX (RFC4588), and FEC (draft-
ietf-payload-flexible-fec-scheme).
o Removed restriction that "related" payload formats in an RTP
stream (such as CN and DTMF) must not have their own rid-id, since
there is no reason to forbid this and corresponding clarification
is made in draft-ietf-mmusic-rid.
o Removed any mention of source-specific signaling and the reference
to RFC5576, since draft-ietf-mmusic-rid is not defined for source-
specific signaling.
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o Changed some SDP examples to use a=rid restrictions instead of
a=imageattr.
o Changed reference from the obsoleted RFC 5285 to RFC 8285.
B.5. Modifications Between WG Version -09 and -10
o Amended overview section with a bit more explanation on the
examples, and added an rid-id alternative for one of the streams.
o Removed SCID also from the Terminology section, which was
forgotten in -09 when changing SCID to rid-id.
B.6. Modifications Between WG Version -08 and -09
o Changed SCID to rid-id, to align with ietf-draft-mmusic-rid
naming.
o Changed Overview to be based on examples and shortened it.
o Changed semantics of initially paused rid-id in modified SDP
offers from requiring it to follow actual RFC 7728 pause state to
an informational offerer's opinion at the time of offer creation,
not in any way overriding or amending RFC 7728 signaling.
o Replaced text on ignoring all but the first of multiple
"a=simulcast" lines in a media description with mandating that at
most one "a=simulcast" line is included.
o Clarified with a note that, for the case it is clear from the SDP
that RTP PT uniquely maps to RtpStreamId, an RTP receiver can use
RTP PT to relate simulcast streams.
o Moved Section 4 Requirements to become Appendix A.
o Editorial corrections and clarifications.
B.7. Modifications Between WG Version -07 and -08
o Correcting syntax of SDP examples in section 6.6.1, as found by
Inaki Baz Castillo.
o Changing ABNF to only define the sc-value, not the SDP attribute
itself, as suggested by Paul Kyzivat.
o Changing I-D reference to newly published RFC 8108.
o Adding list of modifications between -06 and -07.
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B.8. Modifications Between WG Version -06 and -07
o A scope clarification, as result of the discussion with Roni Even.
o A reformulation of the identification requirements for simulcast
stream.
o Correcting the statement related to source specific signalling
(RFC 5576) to address Roni Even's comment.
o Update of the last paragraph in Section 6.2 regarding simulcast
stream differences as well as forbidding multiple instances of the
same SCID within a single a=simulcast line.
o Removal of note in Section 6.4 as result of issue raised by Roni
Even.
o Use of "m=" has been changed to media description and a few other
editorial improvements and clarifications.
B.9. Modifications Between WG Version -05 and -06
o Added section on RTP Aspects
o Added a requirement (5-4) on that capability exchange must be
capable of handling multi RTP stream cases.
o Added extmap attribute also on first signalling example as it is a
recommended to use mechanism.
o Clarified the definition of the simulcast attribute and how
simulcast streams relates to simulcast formats and SCIDs.
o Updated References list and moved around some references between
informative and normative categories.
o Editorial improvements and corrections.
B.10. Modifications Between WG Version -04 and -05
o Aligned with recent changes in draft-ietf-mmusic-rid and draft-
ietf-avtext-rid.
o Modified the SDP offer/answer section to follow the generally
accepted structure, also adding a brief text on modifying the
session that is aligned with draft-ietf-mmusic-rid.
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o Improved text around simulcast stream identification (as opposed
to the simulcast stream itself) to consistently use the acronym
SCID and defined that in the Terminology section.
o Changed references for RTP-level pause/resume and VP8 payload
format that are now published as RFC.
o Improved IANA registration text.
o Removed unused reference to draft-ietf-payload-flexible-fec-
scheme.
o Editorial improvements and corrections.
B.11. Modifications Between WG Version -03 and -04
o Changed to only use RID identification, as was consensus during
IETF 94.
o ABNF improvements.
o Clarified offer-answer rules for initially paused streams.
o Changed references for RTP topologies and RTP taxonomy documents
that are now published as RFC.
o Added reference to the new RID draft in AVTEXT.
o Re-structured section 6 to provide an easy reference by the
updated IANA section.
o Added a sub-section 7.1 with a discussion of bitrate adaptation.
o Editorial improvements.
B.12. Modifications Between WG Version -02 and -03
o Removed text on multicast / broadcast from use cases, since it is
not supported by the solution.
o Removed explicit references to unified plan draft.
o Added possibility to initiate simulcast streams in paused mode.
o Enabled an offerer to offer multiple stream identification (pt or
rid) methods and have the answerer choose which to use.
o Added a preference indication also in send direction offers.
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o Added a section on limitations of the current proposal, including
identification method specific limitations.
B.13. Modifications Between WG Version -01 and -02
o Relying on the new RID solution for codec constraints and
configuration identification. This has resulted in changes in
syntax to identify if pt or RID is used to describe the simulcast
stream.
o Renamed simulcast version and simulcast version alternative to
simulcast stream and simulcast format respectively, and improved
definitions for them.
o Clarification that it is possible to switch between simulcast
version alternatives, but that only a single one be used at any
point in time.
o Changed the definition so that ordering of simulcast formats for a
specific simulcast stream do have a preference order.
B.14. Modifications Between WG Version -00 and -01
o No changes. Only preventing expiry.
B.15. Modifications Between Individual Version -00 and WG Version -00
o Added this appendix.
Authors' Addresses
Bo Burman
Ericsson
Gronlandsgatan 31
SE-164 60 Stockholm
Sweden
Email: bo.burman@ericsson.com
Magnus Westerlund
Ericsson
Torshamnsgatan 23
SE-164 83 Stockholm
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
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Suhas Nandakumar
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: snandaku@cisco.com
Mo Zanaty
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: mzanaty@cisco.com
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