draft-ietf-mmusic-sdp-simulcast-01.txt   draft-ietf-mmusic-sdp-simulcast-02.txt 
Network Working Group B. Burman Network Working Group B. Burman
Internet-Draft M. Westerlund Internet-Draft M. Westerlund
Intended status: Standards Track Ericsson Intended status: Standards Track Ericsson
Expires: January 22, 2016 S. Nandakumar Expires: April 8, 2016 S. Nandakumar
M. Zanaty M. Zanaty
Cisco Cisco
July 21, 2015 October 6, 2015
Using Simulcast in SDP and RTP Sessions Using Simulcast in SDP and RTP Sessions
draft-ietf-mmusic-sdp-simulcast-01 draft-ietf-mmusic-sdp-simulcast-02
Abstract Abstract
In some application scenarios it may be desirable to send multiple In some application scenarios it may be desirable to send multiple
differently encoded versions of the same media source in independent differently encoded versions of the same media source in different
RTP streams. This is called simulcast. This document discusses the RTP streams. This is called simulcast. This document discusses the
best way of accomplishing simulcast in RTP and how to signal it in best way of accomplishing simulcast in RTP and how to signal it in
SDP. A solution is defined by making an extension to SDP, and using SDP. A solution is defined by making an extension to SDP, and using
RTP/RTCP identification methods to relate RTP streams belonging to RTP/RTCP identification methods to relate RTP streams belonging to
the same media source. The SDP extension consists a new media level the same media source. The SDP extension consists of a new media
SDP attribute that express capability to send and/or receive level SDP attribute that expresses capability to send and/or receive
simulcast RTP streams. One part of the RTP/RTCP identification simulcast RTP streams. RTP/RTCP identification using either payload
method is included as a reference to a separate document, since it is types or a separately defined method for RTP stream configuration are
useful also for other purposes. defined.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 22, 2016. This Internet-Draft will expire on April 8, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Reaching a Diverse Set of Receivers . . . . . . . . . . . 5 3.1. Reaching a Diverse Set of Receivers . . . . . . . . . . . 5
3.2. Application Specific Media Source Handling . . . . . . . 6 3.2. Application Specific Media Source Handling . . . . . . . 6
3.3. Receiver Adaptation in Multicast/Broadcast . . . . . . . 6 3.3. Receiver Adaptation in Multicast/Broadcast . . . . . . . 7
3.4. Receiver Media Source Preferences . . . . . . . . . . . . 7 3.4. Receiver Media Source Preferences . . . . . . . . . . . . 8
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Proposed Solution Overview . . . . . . . . . . . . . . . . . 9 5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 9 6. Detailed Description . . . . . . . . . . . . . . . . . . . . 10
6.1. Simulcast Capability . . . . . . . . . . . . . . . . . . 9 6.1. Simulcast Capability . . . . . . . . . . . . . . . . . . 10
6.1.1. Declarative Use . . . . . . . . . . . . . . . . . . . 11 6.1.1. Declarative Use . . . . . . . . . . . . . . . . . . . 12
6.1.2. Offer/Answer Use . . . . . . . . . . . . . . . . . . 11 6.1.2. Offer/Answer Use . . . . . . . . . . . . . . . . . . 12
6.2. Relating Simulcast Versions . . . . . . . . . . . . . . . 13 6.2. Relating Simulcast Streams . . . . . . . . . . . . . . . 14
6.3. Signaling Examples . . . . . . . . . . . . . . . . . . . 13 6.3. Signaling Examples . . . . . . . . . . . . . . . . . . . 14
6.3.1. Unified Plan Client . . . . . . . . . . . . . . . . . 13 6.3.1. Unified Plan Client . . . . . . . . . . . . . . . . . 15
6.3.2. Multi-Source Client . . . . . . . . . . . . . . . . . 15 6.3.2. Multi-Source Client . . . . . . . . . . . . . . . . . 16
7. Network Aspects . . . . . . . . . . . . . . . . . . . . . . . 16 7. Network Aspects . . . . . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 18 12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 19 12.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Changes From Earlier Versions . . . . . . . . . . . 21 Appendix A. Changes From Earlier Versions . . . . . . . . . . . 23
A.1. Modifications Between Individual Version -00 and WG A.1. Modifications Between WG Version -01 and -02 . . . . . . 23
Version -00 . . . . . . . . . . . . . . . . . . . . . . . 21 A.2. Modifications Between WG Version -00 and -01 . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 A.3. Modifications Between Individual Version -00 and WG
Version -00 . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
Most of today's multiparty video conference solutions make use of Most of today's multiparty video conference solutions make use of
centralized servers to reduce the bandwidth and CPU consumption in centralized servers to reduce the bandwidth and CPU consumption in
the endpoints. Those servers receive RTP streams from each the endpoints. Those servers receive RTP streams from each
participant and send some suitable set of possibly modified RTP participant and send some suitable set of possibly modified RTP
streams to the rest of the participants, which usually have streams to the rest of the participants, which usually have
heterogeneous capabilities (screen size, CPU, bandwidth, codec, etc). heterogeneous capabilities (screen size, CPU, bandwidth, codec, etc).
One of the biggest issues is how to perform RTP stream adaptation to One of the biggest issues is how to perform RTP stream adaptation to
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different purposes. This document focuses on the case where it is different purposes. This document focuses on the case where it is
desirable to provide a media source as multiple encoded streams over desirable to provide a media source as multiple encoded streams over
RTP [RFC3550] towards an intermediary so that the intermediary can RTP [RFC3550] towards an intermediary so that the intermediary can
provide the wanted functionality by selecting which RTP stream to provide the wanted functionality by selecting which RTP stream to
forward to other participants in the session, and more specifically forward to other participants in the session, and more specifically
how the identification and grouping of the involved RTP streams are how the identification and grouping of the involved RTP streams are
done. From an RTP perspective, simulcast is a specific application done. From an RTP perspective, simulcast is a specific application
of the aspects discussed in RTP Multiplexing Guidelines of the aspects discussed in RTP Multiplexing Guidelines
[I-D.ietf-avtcore-multiplex-guidelines]. [I-D.ietf-avtcore-multiplex-guidelines].
The purpose of this document is to describe a few scenarios where it This document describes a few scenarios where it is motivated to use
is motivated to use simulcast, and propose a suitable solution for simulcast, and also defines the needed SDP signaling for it.
SDP signaling and performing RTP simulcast.
2. Definitions 2. Definitions
2.1. Terminology 2.1. Terminology
This document makes use of the terminology defined in RTP Taxonomy This document makes use of the terminology defined in RTP Taxonomy
[I-D.ietf-avtext-rtp-grouping-taxonomy], RTP Topology [RFC5117] and [I-D.ietf-avtext-rtp-grouping-taxonomy], RTP Topology [RFC5117] and
RTP Topologies Update [I-D.ietf-avtcore-rtp-topologies-update]. In RTP Topologies Update [I-D.ietf-avtcore-rtp-topologies-update]. In
addition, the following terms are used: addition, the following terms are used:
RTP Mixer: An RTP middle node, defined in [RFC5117] (Section 3.4: RTP Mixer: An RTP middle node, defined in [RFC5117] (Section 3.4:
Topo-Mixer), further elaborated and extended with other topologies Topo-Mixer), further elaborated and extended with other topologies
in [I-D.ietf-avtcore-rtp-topologies-update] (Section 3.6 to 3.9). in [I-D.ietf-avtcore-rtp-topologies-update] (Section 3.6 to 3.9).
RTP Switch: A common short term for the terms "switching RTP mixer", RTP Switch: A common short term for the terms "switching RTP mixer",
"source projecting middlebox", and "video switching MCU" as "source projecting middlebox", and "video switching MCU" as
discussed in [I-D.ietf-avtcore-rtp-topologies-update]. discussed in [I-D.ietf-avtcore-rtp-topologies-update].
Simulcast version: One encoded stream from the set of encoded Simulcast Stream: One Encoded Stream or Dependent Stream from a set
streams that constitutes the simulcast for a single media source. of concurrently transmitted Encoded Streams and optional Dependent
Streams, all sharing a common Media Source, as defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy]. Decoding a Dependent
Stream also requires the related (Dependent and) Encoded
Stream(s), but in the context of simulcast that is considered a
property of the Dependent Stream constituting the simulcast
stream. For example, HD and thumbnail video simulcast versions of
a single Media Source sent concurrently as separate RTP Streams.
Simulcast version alternative: One encoded stream being encoded in Simulcast Format: Different formats of a simulcast stream serve the
one of possibly multiple alternative ways to create a simulcast same purpose as alternative RTP payload types in non-simulcast
version. SDP, to allow multiple alternative media formats for a given RTP
Stream. As for multiple RTP payload types on the m-line, any one
of the alternative formats can be used at a given point in time,
but not more than one (based on RTP timestamp), and what format is
used can change dynamically from one RTP packet to another. For
example, if all participants in a group video call can decode
H.264 and H.265 video, but only some can encode H.265, both H.264
and H.265 can be kept as alternative formats, and the format may
dynamically switch between H.264 and H.265 as different
participants become active speaker.
2.2. Requirements Language 2.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. Use Cases 3. Use Cases
Many use cases of simulcast as described in this document relate to a Many use cases of simulcast as described in this document relate to a
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Mixer and it has very limited impact on media QoE. The main Mixer and it has very limited impact on media QoE. The main
disadvantage is that it can be difficult to combine a subset of disadvantage is that it can be difficult to combine a subset of
received RTP streams into a perfect fit to the resource situation received RTP streams into a perfect fit to the resource situation
of a receiving participant. of a receiving participant.
The use of simulcast relates to the latter approach, where it is more 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 important to reduce the load on the RTP Mixer and/or minimize QoE
impact than to achieve an optimal adaptation of resource usage. impact than to achieve an optimal adaptation of resource usage.
A multicast/broadcast case where the receivers themselves selects the A multicast/broadcast case where the receivers themselves selects the
most appropriate simulcast version and tune in to the right media most appropriate simulcast stream and tune in to the right media
transport to receive that version is also considered (Section 3.3) . transport to receive that stream is also considered (Section 3.3) .
This enables large, heterogeneous receiver populations, when it comes This enables large, heterogeneous receiver populations, when it comes
to capabilities and the use of network path bandwidth resources. to capabilities and the use of network path bandwidth resources.
3.1. Reaching a Diverse Set of Receivers 3.1. Reaching a Diverse Set of Receivers
The media sources provided by a sending participant potentially need The media sources provided by a sending participant potentially need
to reach several receiving participants that differ in terms of to reach several receiving participants that differ in terms of
available resources. The receiver resources that typically differ available resources. The receiver resources that typically differ
include, but are not limited to: include, but are not limited to:
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fit from this set themselves. The end point capabilities will fit from this set themselves. The end point capabilities will
usually result in a single initial choice. However, the network usually result in a single initial choice. However, the network
bandwidth can vary over time, which requires a client to continuously bandwidth can vary over time, which requires a client to continuously
monitor its reception to determine if the received RTP streams still monitor its reception to determine if the received RTP streams still
fit within the available bandwidth. If not, another set of encoded fit within the available bandwidth. If not, another set of encoded
streams from the ones offered in the simulcast will have to be streams from the ones offered in the simulcast will have to be
chosen. chosen.
When using IP multicast, the level of granularity that the receiver When using IP multicast, the level of granularity that the receiver
can select from is decided by its ability to choose different can select from is decided by its ability to choose different
multicast addresses. Thus, different simulcast versions need to be multicast addresses. Thus, different simulcast streams need to be
put on different media transports using different multicast put on different media transports using different multicast
addresses. If these simulcast versions are described using SDP, they addresses. If these simulcast streams are described using SDP, they
need to be part of different SDP media descriptions, as SDP binds to need to be part of different SDP media descriptions, as SDP binds to
transport on media description level. transport on media description level.
3.4. Receiver Media Source Preferences 3.4. Receiver Media Source Preferences
The application logic that controls the communication session may The application logic that controls the communication session may
allow receiving participants to apply preferences to the allow receiving participants to apply preferences to the
characteristics of the RTP stream they receive, for example in terms characteristics of the RTP stream they receive, for example in terms
of the aspects listed in Section 3.1. Sending a simulcast of RTP of the aspects listed in Section 3.1. Sending a simulcast of RTP
streams is one way of accommodating receivers with conflicting or streams is one way of accommodating receivers with conflicting or
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REQ-1.1: In SDP signaling. REQ-1.1: In SDP signaling.
REQ-1.2: On RTP/RTCP level. REQ-1.2: On RTP/RTCP level.
REQ-2: Transport usage. The solution must work when using: REQ-2: Transport usage. The solution must work when using:
REQ-2.1: Legacy SDP with separate media transports per SDP media REQ-2.1: Legacy SDP with separate media transports per SDP media
description. description.
REQ-2.2: Bundled SDP media descriptions. 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: Capability negotiation. It must be possible that:
REQ-3.1: Sender can express capability of sending simulcast. REQ-3.1: Sender can express capability of sending simulcast.
REQ-3.2: Receiver can express capability of receiving simulcast. REQ-3.2: Receiver can express capability of receiving simulcast.
REQ-3.3: Sender can express maximum number of simulcast versions REQ-3.3: Sender can express maximum number of simulcast streams
that can be provided. that can be provided.
REQ-3.4: Receiver can express maximum number of simulcast REQ-3.4: Receiver can express maximum number of simulcast streams
versions that can be received. that can be received.
REQ-3.5: Sender can detail the characteristics of the simulcast REQ-3.5: Sender can detail the characteristics of the simulcast
versions that can be provided. streams that can be provided.
REQ-3.6: Receiver can detail the characteristics of the simulcast REQ-3.6: Receiver can detail the characteristics of the simulcast
versions that it prefers to receive. streams that it prefers to receive.
REQ-4: Distinguishing features. It must be possible to have REQ-4: Distinguishing features. It must be possible to have
different simulcast versions use different codec parameters, as different simulcast streams use different codec parameters, as can
can be expressed by SDP format values and RTP payload types. be expressed by SDP format values and RTP payload types.
REQ-5: Compatibility. It must be possible to use simulcast in REQ-5: Compatibility. It must be possible to use simulcast in
combination with other RTP mechanisms that generate additional RTP combination with other RTP mechanisms that generate additional RTP
streams: streams:
REQ-5.1: RTP Retransmission [RFC4588]. REQ-5.1: RTP Retransmission [RFC4588].
REQ-5.2: RTP Forward Error Correction [RFC5109]. REQ-5.2: RTP Forward Error Correction [RFC5109].
REQ-5.3: Related payload types such as audio Comfort Noise and/or REQ-5.3: Related payload types such as audio Comfort Noise and/or
DTMF. DTMF.
REQ-6: Interoperability. The solution must be possible to use in: REQ-6: Interoperability. The solution must be possible to use in:
REQ-6.1: Interworking with non-simulcast legacy clients using a REQ-6.1: Interworking with non-simulcast legacy clients using a
single media source per media type. single media source per media type.
REQ-6.2: WebRTC "Unified Plan" environment with a single media REQ-6.2: WebRTC "Unified Plan" environment with a single media
source per SDP media description. source per SDP media description.
5. Proposed Solution Overview 5. Overview
The proposed solution consists of signaling simulcast capability and As an overview, the above requirements are met by signaling simulcast
configurations in SDP [RFC4566]: capability and configurations in SDP [RFC4566]:
o An offer or answer can contain a number of simulcast versions, o An offer or answer can contain a number of simulcast streams,
separate for send and receive directions. separate for send and receive directions.
o An offer or answer can contain multiple, alternative simulcast o An offer or answer can contain multiple, alternative simulcast
versions in the same fashion as multiple, alternative codecs can streams in the same fashion as multiple, alternative codecs can be
be offered in a media description. offered in a media description.
o Currently, a single media source per SDP media description is o A single media source per SDP media description is assumed, which
assumed, which makes the solution work in an Unified Plan makes the solution work in an Unified Plan
[I-D.roach-mmusic-unified-plan] context (although different from [I-D.roach-mmusic-unified-plan] context (although different from
what is currently defined there), both with and without BUNDLE what is currently defined there), both with and without BUNDLE
grouping. [I-D.ietf-mmusic-sdp-bundle-negotiation] grouping. This is also
aligned with the concepts defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy].
o The codec configuration for each simulcast version is expressed in o The codec configuration for each simulcast stream is expressed in
terms of existing SDP formats (and typically RTP payload types). terms of existing SDP formats (and typically RTP payload types).
Some codecs may rely on codec configuration based on general Some codecs may rely on codec configuration based on general
attributes that apply for all formats within a media description, attributes that apply for all formats within a media description,
and which could thus not be used to separate different simulcast and which could thus not be used to separate different simulcast
versions. This memo makes no attempt to address such streams. When many different media formats and/or simulcast
shortcomings, but if needed instead encourages that a separate, streams are used in the SDP, the available RTP payload type number
general mechanism is defined for that purpose. space may not be sufficient. This can be particularly prominent
when BUNDLE is used. To mitigate those limitations, this memo
also allows optional use of a separately specified RTP-level
identification mechanism [I-D.pthatcher-mmusic-rid], which
complements and effectively extends the available simulcast stream
identification number space. This also specifies a number of
codec agnostic constraint attributes that may be used to define
simulcast streams.
o It is possible, but not required to use source-specific signaling o It is possible, but not required to use source-specific signaling
[RFC5576] with the proposed solution. [RFC5576] with the proposed solution.
6. Proposed Solution 6. Detailed Description
This section further details the signaling solution outlined above This section further details the overview above (Section 5).
(Section 5).
6.1. Simulcast Capability 6.1. Simulcast Capability
Simulcast capability is expressed as a new media level SDP attribute, Simulcast capability is expressed as a new media level SDP attribute,
"a=simulcast". For each desired direction (send/recv/sendrecv), the "a=simulcast". For each desired direction (send/recv/sendrecv), the
simulcast attribute defines a list of simulcast versions (separated simulcast attribute defines a list of simulcast streams (separated by
by semicolons), each of which is a list of alternative RTP payload semicolons), each of which is a list of simulcast formats (separated
types (separated by commas) for that simulcast version. The meaning by commas). The meaning of the attribute on SDP session level is
of the attribute on SDP session level is undefined and MUST NOT be undefined and MUST NOT be used. There MUST NOT be more than one
used. There MUST be at most one "a=simulcast" attribute per media "a=simulcast" attribute per media description. The ABNF [RFC5234]
description. The ABNF [RFC5234] for this attribute is: for this attribute is:
simulcast-attribute = "a=simulcast" 1*3( WSP sc-dir-list ) sc-attr = "a=simulcast:" 1*3( WSP sc-dir-list )
sc-dir-list = sc-dir WSP sc-fmt-list *( ";" sc-fmt-list ) sc-dir-list = sc-dir WSP sc-id-type "=" sc-alt-list *( ";" sc-alt-list )
sc-dir = "send" / "recv" / "sendrecv" sc-dir = "send" / "recv" / "sendrecv"
sc-fmt-list = sc-fmt *( "," sc-fmt ) sc-id-type = "pt" / "rid" / token
sc-fmt = fmt sc-alt-list = sc-id *( "," sc-id )
; WSP defined in [RFC5234] sc-id = fmt / rid-value / token
; fmt defined in [RFC4566] ; WSP defined in [RFC5234]
; fmt, token defined in [RFC4566]
; rid-value defined in [I-D.pthatcher-mmusic-rid]
Figure 1: ABNF for Simulcast Figure 1: ABNF for Simulcast
There are separate and independent sets of parameters for simulcast There are separate and independent sets of parameters for simulcast
in send and receive directions. When listing multiple directions, in send and receive directions. When listing multiple directions,
each direction MUST NOT occur more than once. each direction MUST NOT occur more than once on the same line.
Two simulcast stream identification methods are defined; "pt" using
RTP payload type (SDP format), and "rid" using an additional RTP-
level identification mechanism [I-D.pthatcher-mmusic-rid].
Attribute parameters are grouped by direction and consist of a Attribute parameters are grouped by direction and consist of a
listing of SDP format tokens (usually corresponding to RTP payload listing of simulcast stream identifications to be used. The number
types), which describe the simulcast versions to be used. The number of (non-alternative, see below) identifications in the list sets a
of (non-alternative, see below) formats in the list sets a limit to limit to the number of supported simulcast streams in that direction.
the number of supported simulcast versions in that direction. The Simulcast stream identifications present in "sendrecv" direction MUST
order of the listed simulcast versions in the "send" direction is not NOT be present also in "send" or "recv" directions, since the meaning
significant. The order of the listed simulcast versions in the of that would be ambiguous. The order of the listed simulcast
"recv" direction expresses a preference which simulcast versions that streams in the "send" direction is not significant. The order of the
are preferred, with the leftmost being most preferred, if the number listed simulcast streams in the "recv" direction expresses a
of actually sent simulcast versions have to be reduced for some preference 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. reason.
Formats that have explicit dependencies [RFC5583] to other formats Editor's note: Consider to remove the sendrecv definitions, as
(even in the same media description) MAY be listed as different they don't match PTs and RIDs unidirectionality.
simulcast versions.
Alternative simulcast versions MAY be specified as part of the Formats that have explicit dependencies [RFC5583]
attribute parameters by expressing each simulcast version format as a [I-D.pthatcher-mmusic-rid] to other formats (even in the same media
comma-separated list of alternative values. In this case, all description) MAY be listed as different simulcast streams.
combinations of those alternatives MUST be supported. The order of
the alternatives within a simulcast version is not significant; codec
preference is expressed by format type ordering on the m-line, using
regular SDP rules.
A simulcast version can use a codec defined such that the same RTP Alternative simulcast formats MAY be specified as part of the
attribute parameters by expressing each simulcast stream as a comma-
separated list of alternative format identifiers. In this case,
there MUST NOT be any capability restriction in what alternatives can
be used across different simulcast streams, like requiring all
simulcast streams to use the same codec alternative. The order of
the alternatives within a simulcast stream is significant; the
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 codec preferences.
A simulcast stream can use a codec defined such that the same RTP
SSRC can change RTP payload type multiple times during a session, SSRC can change RTP payload type multiple times during a session,
possibly even on a per-packet basis. A typical example can be a 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 speech codec that makes use of Comfort Noise [RFC3389] and/or DTMF
[RFC4733] formats. In those cases, such "related" formats MUST NOT [RFC4733] formats. In those cases, such "related" formats MUST NOT
be listed explicitly in the attribute parameters, since they are not be listed explicitly in the attribute parameters, since they are not
strictly simulcast versions of the media source, but rather a strictly simulcast streams of the media source, but rather a specific
specific way of generating the RTP stream of a single simulcast way of generating the RTP stream of a single simulcast stream with
version with varying RTP payload type. Instead, only a single codec varying RTP payload type. Instead, only a single simulcast stream
format MUST be used per simulcast version or simulcast version identification MUST be used per simulcast stream or alternative
alternative (if there are such). The codec format SHOULD be the simulcast format (if there are such) in the SDP. The used simulcast
codec most relevant to the media description, if possible to stream identification SHOULD be the codec format most relevant to the
identify, for example the audio codec rather than the DTMF. What media description, if possible to identify, for example the audio
codec format to choose in the case of switching between multiple codec rather than the DTMF. What codec format to choose in the case
equally "important" formats is left open, but it is assumed that in of switching between multiple equally "important" formats is left
the presence of such strong relation it does not matter which is open, but it is assumed that in the presence of such strong relation
chosen. it does not matter which is chosen.
Use of the redundant audio data [RFC2198] format could be seen as a Use of the redundant audio data [RFC2198] format could be seen as a
form of simulcast for loss protection purposes, but is not considered form of simulcast for loss protection purposes, but is not considered
conflicting with the mechanisms described in this memo and MAY conflicting with the mechanisms described in this memo and MAY
therefore be used as any other format. In this case the "red" 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 format, rather than the carried formats, SHOULD be the one to list as
a simulcast version on the "a=simulcast" line. a simulcast stream on the "a=simulcast" line.
Editor's note: Consider adding the possibility to put an RTP Editor's note: Consider adding the possibility to put an RTP
stream in "paused" state [I-D.ietf-avtext-rtp-stream-pause] from stream in "paused" state [I-D.ietf-avtext-rtp-stream-pause] from
the beginning of the session, possibly starting it at a later the beginning of the session, possibly starting it at a later
point in time by applying RTP/RTCP level procedures from that point in time by applying RTP/RTCP level procedures from that
specification. specification.
6.1.1. Declarative Use 6.1.1. Declarative Use
When used as a declarative media description, a=simulcast "recv" When used as a declarative media description, a=simulcast "recv"
direction formats indicates the configured end point's required direction formats indicates the configured end point's required
capability to recognize and receive a specified set of RTP streams as capability to recognize and receive a specified set of RTP streams as
simulcast streams. In the same fashion, a=simulcast "send" direction simulcast streams. In the same fashion, a=simulcast "send" direction
requests the end point to send a specified set of RTP streams as requests the end point to send a specified set of RTP streams as
simulcast streams. The "sendrecv" direction combines "send" and simulcast streams. The "sendrecv" direction combines "send" and
"recv" requirements, using the same format values for both. "recv" requirements, using the same format values for both.
If simulcast version alternatives are listed, it means that the If multiple simulcast formats are listed, it means that the
configured end point MUST be prepared to receive any of the "recv" configured end point MUST be prepared to receive any of the "recv"
formats, and MAY send any of the "send" formats for that simulcast formats, and MAY send any of the "send" formats for that simulcast
version. stream.
Editor's note: The external RTP identification mechanism currently
lacks a declarative use definition. As declarative use may also
not follow unified plan with a single media source per m= line, it
is uncertain if declarative can be defined for the mechanism in
its current shape.
6.1.2. Offer/Answer Use 6.1.2. Offer/Answer Use
An offerer wanting to use simulcast SHALL include the "a=simulcast" An offerer wanting to use simulcast SHALL include the "a=simulcast"
attribute in the offer. An offerer that receives an answer without attribute in the offer. An offerer that receives an answer without
"a=simulcast" MUST NOT use simulcast towards the answerer. An "a=simulcast" MUST NOT use simulcast towards the answerer. An
offerer that receives an answer with "a=simulcast" not listing a offerer that receives an answer with "a=simulcast" not listing a
direction or without any formats in a specified direction MUST NOT direction or without any simulcast stream identifications in a
use simulcast in that direction. specified direction MUST NOT use simulcast in that direction.
An answerer that does not understand the concept of simulcast will 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 also not know the attribute and will remove it in the SDP answer, as
defined in existing SDP Offer/Answer [RFC3264] procedures. An defined in existing SDP Offer/Answer [RFC3264] procedures. An
answerer that does understand the attribute and that wants to support answerer that does understand the attribute and that wants to support
simulcast in an indicated direction SHALL reverse directionality of simulcast in an indicated direction SHALL reverse directionality of
the unidirectional direction parameters; "send" becomes "recv" and the unidirectional direction parameters; "send" becomes "recv" and
vice versa, and include it in the answer. If the offered direction vice versa, and include it in the answer. If the offered direction
is "sendrecv", the answerer MAY keep it, but MAY also change it to is "sendrecv", the answerer MAY keep it, but MAY also change it to
"send" or "recv" to indicate that it is only interested in simulcast "send" or "recv" to indicate that it is only interested in simulcast
for a single direction. Note that, like all other use of SDP format for a single direction. Note that, like all other use of SDP format
tags for the send direction in Offer/Answer, format tags related to tags ("pt:") for the send direction in Offer/Answer, format tags
the simulcast send direction in an offer ("send" or "sendrecv") are related to the simulcast stream identification send direction in an
placeholders that refer to information in the offer SDP, and the offer ("send" or "sendrecv") are placeholders that refer to
actual formats that will be used on the wire (including RTP Payload information in the offer SDP, and the actual formats that will be
Format numbers) depends on information included in the SDP answer. used on the wire (including RTP Payload Format numbers) depends on
information included in the SDP answer.
An offerer listing a set of receive simulcast versions and/or An offerer listing a set of receive simulcast streams and/or
alternatives in the offer MUST be prepared to receive RTP streams for alternative formats in the offer MUST be prepared to receive RTP
any of those simulcast versions and/or alternatives from the streams for any of those simulcast streams and/or alternative formats
answerer. from the answerer.
An answerer that receives an offer with simulcast containing an An answerer that receives an offer with simulcast containing an
"a=simulcast" attribute listing alternative formats for simulcast "a=simulcast" attribute listing alternative formats for simulcast
versions MAY keep all the alternatives in the answer, but it MAY also streams MAY keep all the alternatives in the answer, but it MAY also
choose to remove any non-desirable alternatives per simulcast version choose to remove any non-desirable alternatives per simulcast stream
in the answer. The answerer MUST NOT add any alternatives that were in the answer. The answerer MUST NOT add any alternatives that were
not present in the offer. not present in the offer.
An answerer that receives an offer with simulcast that lists a number An answerer that receives an offer with simulcast that lists a number
of simulcast versions, MAY reduce the number of simulcast versions in of simulcast streams, MAY reduce the number of simulcast streams in
the answer, but MUST NOT add simulcast versions. the answer, but MUST NOT add simulcast streams.
An offerer that receives an answer where some simulcast version An offerer that receives an answer where some simulcast formats are
alternatives are kept MUST be prepared to receive any of the kept kept MUST be prepared to receive any of the kept send direction
send direction alternatives, and MAY send any of the kept receive alternatives, and MAY send any of the kept receive direction
direction alternatives from the answer. This is similar to the case alternatives from the answer. Similarly, the answerer MUST be
when the answer includes multiple formats on the m-line. prepared to receive any of the kept receive direction alternatives,
and MAY send any of the kept send direction alternatives in the
answer.
The offerer and answerer MUST NOT send more than a single alternative
format at a time (based on RTP timestamps) per simulcast stream, but
MAY change format 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.
An offerer that receives an answer where some of the simulcast An offerer that receives an answer where some of the simulcast
versions are removed MAY release the corresponding resources (codec, streams are removed MAY release the corresponding resources (codec,
transport, etc) in its receive direction and MUST NOT send any RTP transport, etc) in its receive direction and MUST NOT send any RTP
streams corresponding to the removed simulcast versions. streams corresponding to the removed simulcast streams.
Simulcast streams or formats using undefined simulcast stream
identifications MUST NOT be used as valid simulcast streams by an RTP
stream receiver.
The media formats and corresponding characteristics of encoded The media formats and corresponding characteristics of encoded
streams used in a simulcast SHOULD be chosen such that they are streams used in a simulcast SHOULD be chosen such that they are
different. If this difference is not required, RTP duplication different. If this difference is not required, RTP duplication
[RFC7104] procedures SHOULD be considered instead of simulcast. [RFC7104] procedures SHOULD be considered instead of simulcast.
Note: The inclusion of "a=simulcast" or the use of simulcast does Note: The inclusion of "a=simulcast" or the use of simulcast does
not change any of the interpretation or Offer/Answer procedures not change any of the interpretation or Offer/Answer procedures
for other SDP attributes, like "a=fmtp". for other SDP attributes, like "a=fmtp" or "a=rid".
6.2. Relating Simulcast Versions 6.2. Relating Simulcast Streams
As long as there is only a single media source per SDP media As long as there is only a single media source per SDP media
description, simulcast RTP streams can be related on RTP level description, simulcast RTP streams can be related on RTP level
through the RTP payload type, as specified in the SDP "a=simulcast" through the RTP payload type and (optionally) RID
[I-D.pthatcher-mmusic-rid], as specified in the SDP "a=simulcast"
attribute (Section 6.1) parameters. When using BUNDLE attribute (Section 6.1) parameters. When using BUNDLE
[I-D.ietf-mmusic-sdp-bundle-negotiation] to use multiple SDP media [I-D.ietf-mmusic-sdp-bundle-negotiation] with multiple SDP media
descriptions to specify a single RTP session, there is an descriptions to specify a single RTP session, there is an
identification mechanism that allows relating RTP streams back to identification mechanism that allows relating RTP streams back to
individual media descriptions, after which the above RTP payload type individual media descriptions, after which the above RTP payload type
relation can be used. and RID relations can be used.
BUNDLE's MID is an RTCP source description (SDES) item. To ensure
rapid initial reception, required to correctly process the RTP
streams, it is also defined as an RTP header extension [RFC5285].
Editor's note: Consider making RID an SDES item too, for the same
reasons as MID.
6.3. Signaling Examples 6.3. Signaling Examples
These examples are for a case of client to video conference service These examples describe a client to video conference service, using a
using a centralized media topology with an RTP mixer. centralized media topology with an RTP mixer.
+---+ +-----------+ +---+ +---+ +-----------+ +---+
| A |<---->| |<---->| B | | A |<---->| |<---->| B |
+---+ | | +---+ +---+ | | +---+
| Mixer | | Mixer |
+---+ | | +---+ +---+ | | +---+
| F |<---->| |<---->| J | | F |<---->| |<---->| J |
+---+ +-----------+ +---+ +---+ +-----------+ +---+
Figure 2: Four-party Mixer-based Conference Figure 2: Four-party Mixer-based Conference
6.3.1. Unified Plan Client 6.3.1. Unified Plan Client
Alice is calling in to the mixer with a simulcast-enabled Unified Alice is calling in to the mixer with a simulcast-enabled Unified
Plan client capable of a single media source per media type. The Plan client capable of a single media source per media type. The
client can send a simulcast of 2 video resolutions and frame rates: client can send a simulcast of 2 video resolutions and frame rates:
HD 1280x720p 30fps and thumbnail 320x180p 15fps. Alice's Offer: HD 1280x720p 30fps and thumbnail 320x180p 15fps. This is defined
below using the "imageattr" [RFC6236]. Media formats (RTP payload
types) are used as simulcast stream identification. Alice's Offer:
v=0 v=0
o=alice 2362969037 2362969040 IN IP4 192.0.2.156 o=alice 2362969037 2362969040 IN IP4 192.0.2.156
s=Simulcast Enabled Unified Plan Client s=Simulcast Enabled Unified Plan Client
t=0 0 t=0 0
c=IN IP4 192.0.2.156 c=IN IP4 192.0.2.156
m=audio 49200 RTP/AVP 0 m=audio 49200 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 49300 RTP/AVP 97 98 m=video 49300 RTP/AVP 97 98
a=rtpmap:97 H264/90000 a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000 a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000 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: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: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=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
a=simulcast send 97;98 recv 97 a=simulcast: send pt=97;98 recv pt=97
Figure 3: Unified Plan Simulcast Offer Figure 3: Unified Plan Simulcast Offer
The only thing in the SDP that indicates simulcast capability is the The only thing in the SDP that indicates simulcast capability is the
line in the video media description containing the "simulcast" line in the video media description containing the "simulcast"
attribute. The included format parameters indicates that sent attribute. The included format parameters indicates that sent
simulcast versions can differ in video resolution and framerate. simulcast streams can differ in video resolution.
The Answer from the server indicates that it too is simulcast The Answer from the server indicates that it too is simulcast
capable. Should it not have been simulcast capable, the capable. Should it not have been simulcast capable, the
"a=simulcast" line would not have been present and communication "a=simulcast" line would not have been present and communication
would have started with the media negotiated in the SDP. would have started with the media negotiated in the SDP.
v=0 v=0
o=server 823479283 1209384938 IN IP4 192.0.2.2 o=server 823479283 1209384938 IN IP4 192.0.2.2
s=Answer to Simulcast Enabled Unified Plan Client s=Answer to Simulcast Enabled Unified Plan Client
t=0 0 t=0 0
c=IN IP4 192.0.2.43 c=IN IP4 192.0.2.43
m=audio 49672 RTP/AVP 0 m=audio 49672 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 49674 RTP/AVP 97 98 m=video 49674 RTP/AVP 97 98
a=rtpmap:97 H264/90000 a=rtpmap:97 H264/90000
a=rtpmap:98 H264/90000 a=rtpmap:98 H264/90000
a=fmtp:97 profile-level-id=42c01f; max-fs=3600; max-mbps=108000 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: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: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=imageattr:98 send [x=320,y=180] recv [x=320,y=180]
a=simulcast recv 97;98 send 97 a=simulcast: recv pt=97;98 send pt=97
Figure 4: Unified Plan Simulcast Answer Figure 4: Unified Plan Simulcast Answer
Since the server is the simulcast media receiver, it reverses the Since the server is the simulcast media receiver, it reverses the
direction of the "simulcast" attribute. direction of the "simulcast" attribute parameters.
6.3.2. Multi-Source Client 6.3.2. Multi-Source Client
Fred is calling in to the same conference as in the example above 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 with a two-camera, two-display system, thus capable of handling two
separate media sources in each direction, where each media source is separate media sources in each direction, where each media source is
simulcast-enabled in the send direction. Fred's client is a Unified simulcast-enabled in the send direction. Fred's client is a Unified
Plan client, restricted to a single media source per media Plan client, restricted to a single media source per media
description. description.
The first two simulcast versions for the first media source use The first two simulcast streams for the first media source use
different codecs, H264-SVC [RFC6190] and H264 [RFC6184]. These two different codecs, H264-SVC [RFC6190] and H264 [RFC6184]. These two
simulcast versions also have a temporal dependency. Two different simulcast streams also have a temporal dependency. Two different
video codecs, VP8 [I-D.ietf-payload-vp8] and H264, are offered as video codecs, VP8 [I-D.ietf-payload-vp8] and H264, are offered as
alternatives for the third simulcast version for the first media alternatives for the third simulcast stream for the first media
source. source. RID is used as simulcast stream identification, reducing the
number of media formats needed.
The second media source is offered with three different simulcast The second media source is offered with three different simulcast
versions. All video streams of this second media source are loss streams. All video streams of this second media source are loss
protected by RTP retransmission [RFC4588]. protected by RTP retransmission [RFC4588]. RID is used as simulcast
stream identification.
Fred's client is also using BUNDLE to send all RTP streams from all 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 media descriptions in the same RTP session on a single media
transport. There are not so many RTP payload types in this example transport. Although using many different simulcast streams in this
that there is any risk of running out of payload types, but for the example, use of RID as simulcast stream identification enables use of
sake of making an example, it is assumed that one of the payload a low number of RTP payload types. Note that the use of both BUNDLE
types cannot be kept unique across all media descriptions. and RID recommends using the RTP header extension [RFC5285] for
Therefore, the SDP makes use of the mechanism (work in progress) in carrying these fields.
BUNDLE that identifies which media description an RTP stream belongs
to (a new RTCP SDES item and RTP header extension [RFC5285] type
carrying the a=mid value). That identification will make it possible
to identify unambiguously also on RTP level which media source it is
and thus what the related simulcast versions are, even though two
separate RTP streams in the joint RTP session share RTP payload type.
v=0 v=0
o=fred 238947129 823479223 IN IP4 192.0.2.125 o=fred 238947129 823479223 IN IP4 192.0.2.125
s=Offer from Simulcast Enabled Multi-Source Client s=Offer from Simulcast Enabled Multi-Source Client
t=0 0 t=0 0
c=IN IP4 192.0.2.125 c=IN IP4 192.0.2.125
a=group:BUNDLE foo bar zen a=group:BUNDLE foo bar zen
m=audio 49200 RTP/AVP 99 m=audio 49200 RTP/AVP 99
a=mid:foo a=mid:foo
a=rtpmap:99 G722/8000 a=rtpmap:99 G722/8000
m=video 49600 RTP/AVP 100 101 102 103
m=video 49600 RTP/AVP 100 101 103
a=mid:bar a=mid:bar
a=rtpmap:100 H264-SVC/90000 a=rtpmap:100 H264-SVC/90000
a=rtpmap:101 H264/90000 a=rtpmap:101 H264/90000
a=rtpmap:102 H264/90000
a=rtpmap:103 VP8/90000 a=rtpmap:103 VP8/90000
a=fmtp:100 profile-level-id=42400d; max-fs=3600; max-mbps=108000; \ a=fmtp:100 profile-level-id=42400d; max-fs=3600; max-mbps=108000; \
mst-mode=NI-TC mst-mode=NI-TC
a=fmtp:101 profile-level-id=42c00d; max-fs=3600; max-mbps=54000 a=fmtp:101 profile-level-id=42c00d; max-fs=3600; max-mbps=54000
a=fmtp:102 profile-level-id=42c00d; max-fs=900; max-mbps=27000
a=fmtp:103 max-fs=900; max-fr=30 a=fmtp:103 max-fs=900; max-fr=30
a=imageattr:100 send [x=1280,y=720] recv [x=1280,y=720] a=rid:1 send pt=100 max-width=1280;max-height=720;max-fr=60;depend=2
a=imageattr:101 send [x=1280,y=720] recv [x=1280,y=720] a=rid:2 send pt=101 max-width=1280;max-height=720;max-fr=30
a=imageattr:102 send [x=640,y=360] recv [x=640,y=360] a=rid:3 send pt=101 max-width=640;max-height=360
a=imageattr:103 send [x=640,y=360] recv [x=640,y=360] a=rid:4 send pt=103 max-width=640;max-height=360
a=depend:100 lay bar:101 a=depend:100 lay bar:101
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=simulcast sendrecv 100;101 send 103,102 a=extmap:2 urn:ietf:params:rtp-hdrext:rid
a=simulcast: send rid=1;2;4,3
m=video 49602 RTP/AVP 96 103 97 104 105 106 m=video 49602 RTP/AVP 96 104
a=mid:zen a=mid:zen
a=rtpmap:96 VP8/90000 a=rtpmap:96 VP8/90000
a=fmtp:96 max-fs=3600; max-fr=30 a=fmtp:96 max-fs=3600; max-fr=30
a=rtpmap:104 rtx/90000 a=rtpmap:104 rtx/90000
a=fmtp:104 apt=96;rtx-time=200 a=fmtp:104 apt=96;rtx-time=200
a=rtpmap:103 VP8/90000 a=rid:5 send pt=96 max-fs=921600;max-fr=30
a=fmtp:103 max-fs=900; max-fr=30 a=rid:6 send pt=96 max-fs=614400;max-fr=15
a=rtpmap:105 rtx/90000 a=rid:7 send pt=96 max-fs=230400;max-fr=30
a=fmtp:105 apt=103;rtx-time=200
a=rtpmap:97 VP8/90000
a=fmtp:97 max-fs=240; max-fr=15
a=rtpmap:106 rtx/90000
a=fmtp:106 apt=97;rtx-time=200
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=simulcast send 97;96;103 a=extmap:2 urn:ietf:params:rtp-hdrext:rid
a=simulcast: send rid=6;5;7
Figure 5: Fred's Multi-Source Simulcast Offer Figure 5: Fred's Multi-Source Simulcast Offer
Note: Empty lines in the SDP above are added only for readability Note: Empty lines in the SDP above are added only for readability
and would not be present in an actual SDP. and would not be present in an actual SDP.
7. Network Aspects 7. Network Aspects
Simulcast is in this memo defined as the act of sending multiple Simulcast is in this memo defined as the act of sending multiple
alternative encoded streams of the same underlying media source. alternative encoded streams of the same underlying media source.
skipping to change at page 17, line 14 skipping to change at page 18, line 24
ways using RTP. A general discussion on considerations for use of ways using RTP. A general discussion on considerations for use of
the different RTP multiplexing alternatives can be found in the different RTP multiplexing alternatives can be found in
Guidelines for Multiplexing in RTP Guidelines for Multiplexing in RTP
[I-D.ietf-avtcore-multiplex-guidelines]. Discussion and [I-D.ietf-avtcore-multiplex-guidelines]. Discussion and
clarification on how to handle multiple streams in an RTP session can clarification on how to handle multiple streams in an RTP session can
be found in [I-D.ietf-avtcore-rtp-multi-stream]. be found in [I-D.ietf-avtcore-rtp-multi-stream].
The network aspects that are relevant for simulcast are: The network aspects that are relevant for simulcast are:
Quality of Service: When using simulcast it might be of interest to Quality of Service: When using simulcast it might be of interest to
prioritize a particular simulcast version, rather than applying prioritize a particular simulcast stream, rather than applying
equal treatment to all versions. For example, lower bit-rate equal treatment to all streams. For example, lower bit-rate
versions may be prioritized over higher bit-rate versions to streams may be prioritized over higher bit-rate streams to
minimize congestion or packet losses in the low bit-rate versions. minimize congestion or packet losses in the low bit-rate streams.
Thus, there is a benefit to use a simulcast solution that supports Thus, there is a benefit to use a simulcast solution that supports
QoS as good as possible. By separating simulcast versions into QoS as good as possible. By separating simulcast streams into
different RTP sessions and send those RTP sessions over different different RTP sessions and send those RTP sessions over different
media transports, a simulcast version can be prioritized by media transports, a simulcast stream can be prioritized by
existing flow based QoS mechanisms. When using unicast, QoS existing flow based QoS mechanisms. When using unicast, QoS
mechanisms based on individual packet marking are also feasible, mechanisms based on individual packet marking are also feasible,
which do not require separation of simulcast versions into which do not require separation of simulcast streams into
different RTP sessions to apply different QoS. The proposed different RTP sessions to apply different QoS. The proposed
solution can be extended to support this functionality with an solution can be extended to support this functionality with an
optional mid: prefix before the RTP payload types of a simulcast optional mid: prefix before the RTP payload types of a simulcast
version, to describe simulcast across multiple media descriptions. stream, to describe simulcast across multiple media descriptions.
Editor's note: With the chosen approach, it is not possible to
use different simulcast streams on different transports, so
either that description should be removed, or the solution has
to be amended to cater also for that case.
NAT/FW Traversal: Using multiple RTP sessions will incur more cost NAT/FW Traversal: Using multiple RTP sessions will incur more cost
for NAT/FW traversal unless they can re-use the same transport for NAT/FW traversal unless they can re-use the same transport
flow, which can be achieved by either one of multiplexing multiple flow, which can be achieved by either one of multiplexing multiple
RTP sessions on a single lower layer transport RTP sessions on a single lower layer transport
[I-D.westerlund-avtcore-transport-multiplexing] or Multiplexing [I-D.westerlund-avtcore-transport-multiplexing] or Multiplexing
Negotiation Using SDP Port Numbers Negotiation Using SDP Port Numbers
[I-D.ietf-mmusic-sdp-bundle-negotiation]. If flow based QoS with [I-D.ietf-mmusic-sdp-bundle-negotiation]. If flow based QoS with
any differentiation is desirable, the cost for additional any differentiation is desirable, the cost for additional
transport flows is likely necessary. transport flows is likely necessary.
Multicast: Multiple RTP sessions will be required to enable Multicast: Multiple RTP sessions will be required to enable
combining simulcast with multicast. Different simulcast versions combining simulcast with multicast. Different simulcast streams
have to be separated to different multicast groups to allow a have to be separated to different multicast groups to allow a
multicast receiver to pick the version it wants, rather than multicast receiver to pick the stream it wants, rather than
receive all of them. In this case, the only reasonable receive all of them. In this case, the only reasonable
implementation is to use different RTP sessions for each multicast implementation is to use different RTP sessions for each multicast
group so that reporting and other RTCP functions operate as group so that reporting and other RTCP functions operate as
intended. The proposed solution can be extended to support this intended. The proposed solution can be extended to support this
functionality with an optional mid: prefix before the RTP payload functionality with an optional mid: prefix before the RTP payload
types of a simulcast version, to describe simulcast across types of a simulcast stream, to describe simulcast across multiple
multiple media descriptions. media descriptions.
Editor's note: As with QoS above, different simulcast streams on
different multicast groups are not possible with the chosen
approach, and text must be changed accordingly.
8. IANA Considerations 8. IANA Considerations
This document requests to register a new attribute, simulcast. This document requests to register a new SDP attribute, simulcast.
Formal registrations to be written. Formal registrations to be written.
9. Security Considerations 9. Security Considerations
The simulcast capability and configuration attributes and parameters The simulcast capability, configuration attributes and parameters are
are vulnerable to attacks in signaling. vulnerable to attacks in signaling.
A false inclusion of the "a=simulcast" attribute may result in A false inclusion of the "a=simulcast" attribute may result in
simultaneous transmission of multiple RTP streams that would simultaneous transmission of multiple RTP streams that would
otherwise not be generated. The impact is limited by the media otherwise not be generated. The impact is limited by the media
description joint bandwidth, shared by all simulcast versions description joint bandwidth, shared by all simulcast streams
irrespective of their number. There may however be a large number of irrespective of their number. There may however be a large number of
unwanted RTP streams that will impact the share of the bandwidth unwanted RTP streams that will impact the share of bandwidth
allocated for the originally wanted RTP stream. allocated for the originally wanted RTP stream.
A hostile removal of the "a=simulcast" attribute will result in A hostile removal of the "a=simulcast" attribute will result in
simulcast not being used. simulcast not being used.
Neither of the above will likely have any major consequences and can Neither of the above will likely have any major consequences and can
be mitigated by signaling that is at least integrity and source be mitigated by signaling that is at least integrity and source
authenticated to prevent an attacker to change it. authenticated to prevent an attacker to change it.
10. Contributors 10. Contributors
Morgan Lindqvist and Fredrik Jansson, both from Ericsson, have Morgan Lindqvist and Fredrik Jansson, both from Ericsson, have
contributed with important material to the first versions of this contributed with important material to the first versions of this
document. Robert Hansen, from Cisco, contributed significantly to document. Robert Hansen and Cullen Jennings, from Cisco, and Peter
subsequent versions. Thatcher, from Google, contributed significantly to subsequent
versions.
11. Acknowledgements 11. Acknowledgements
12. References 12. References
12.1. Normative References 12.1. Normative References
[I-D.pthatcher-mmusic-rid]
Thatcher, P., Zanaty, M., Nandakumar, S., Roach, A.,
Burman, B., and B. Campen, "RTP Payload Format
Constraints", draft-pthatcher-mmusic-rid-00 (work in
progress), October 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. July 2003, <http://www.rfc-editor.org/info/rfc3550>.
skipping to change at page 19, line 34 skipping to change at page 21, line 16
[I-D.ietf-avtcore-multiplex-guidelines] [I-D.ietf-avtcore-multiplex-guidelines]
Westerlund, M., Perkins, C., and H. Alvestrand, Westerlund, M., Perkins, C., and H. Alvestrand,
"Guidelines for using the Multiplexing Features of RTP to "Guidelines for using the Multiplexing Features of RTP to
Support Multiple Media Streams", draft-ietf-avtcore- Support Multiple Media Streams", draft-ietf-avtcore-
multiplex-guidelines-03 (work in progress), October 2014. multiplex-guidelines-03 (work in progress), October 2014.
[I-D.ietf-avtcore-rtp-multi-stream] [I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins, Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session", "Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-08 (work in progress), draft-ietf-avtcore-rtp-multi-stream-09 (work in progress),
July 2015. September 2015.
[I-D.ietf-avtcore-rtp-topologies-update] [I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft- Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-10 (work in progress), ietf-avtcore-rtp-topologies-update-10 (work in progress),
July 2015. July 2015.
[I-D.ietf-avtext-rtp-grouping-taxonomy] [I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, "A Taxonomy of Semantics and Mechanisms for B. Burman, "A Taxonomy of Semantics and Mechanisms for
Real-Time Transport Protocol (RTP) Sources", draft-ietf- Real-Time Transport Protocol (RTP) Sources", draft-ietf-
avtext-rtp-grouping-taxonomy-08 (work in progress), July avtext-rtp-grouping-taxonomy-08 (work in progress), July
2015. 2015.
[I-D.ietf-avtext-rtp-stream-pause] [I-D.ietf-avtext-rtp-stream-pause]
Burman, B., Akram, A., Even, R., and M. Westerlund, "RTP Burman, B., Akram, A., Even, R., and M. Westerlund, "RTP
Stream Pause and Resume", draft-ietf-avtext-rtp-stream- Stream Pause and Resume", draft-ietf-avtext-rtp-stream-
pause-08 (work in progress), July 2015. pause-10 (work in progress), September 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation] [I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings, Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session "Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-23 (work in progress), July 2015. negotiation-23 (work in progress), July 2015.
[I-D.ietf-payload-vp8] [I-D.ietf-payload-vp8]
Westin, P., Lundin, H., Glover, M., Uberti, J., and F. Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
Galligan, "RTP Payload Format for VP8 Video", draft-ietf- Galligan, "RTP Payload Format for VP8 Video", draft-ietf-
payload-vp8-16 (work in progress), June 2015. payload-vp8-17 (work in progress), September 2015.
[I-D.roach-mmusic-unified-plan] [I-D.roach-mmusic-unified-plan]
Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for Roach, A., Uberti, J., and M. Thomson, "A Unified Plan for
Using SDP with Large Numbers of Media Flows", draft-roach- Using SDP with Large Numbers of Media Flows", draft-roach-
mmusic-unified-plan-00 (work in progress), July 2013. mmusic-unified-plan-00 (work in progress), July 2013.
[I-D.westerlund-avtcore-transport-multiplexing] [I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP
Sessions onto a Single Lower-Layer Transport", draft- Sessions onto a Single Lower-Layer Transport", draft-
westerlund-avtcore-transport-multiplexing-07 (work in westerlund-avtcore-transport-multiplexing-07 (work in
skipping to change at page 21, line 42 skipping to change at page 23, line 24
[RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image [RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image
Attributes in the Session Description Protocol (SDP)", Attributes in the Session Description Protocol (SDP)",
RFC 6236, DOI 10.17487/RFC6236, May 2011, RFC 6236, DOI 10.17487/RFC6236, May 2011,
<http://www.rfc-editor.org/info/rfc6236>. <http://www.rfc-editor.org/info/rfc6236>.
Appendix A. Changes From Earlier Versions Appendix A. Changes From Earlier Versions
NOTE TO RFC EDITOR: Please remove this section prior to publication. NOTE TO RFC EDITOR: Please remove this section prior to publication.
A.1. Modifications Between Individual Version -00 and WG Version -00 A.1. 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.
A.2. Modifications Between WG Version -00 and -01
o No changes. Only preventing expiry.
A.3. Modifications Between Individual Version -00 and WG Version -00
o Added this appendix. o Added this appendix.
Authors' Addresses Authors' Addresses
Bo Burman Bo Burman
Ericsson Ericsson
Kistavagen 25 Kistavagen 25
SE-164 80 Stockholm SE-164 80 Stockholm
Sweden Sweden
Phone: +46 10 714 13 11
Email: bo.burman@ericsson.com Email: bo.burman@ericsson.com
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 6 Farogatan 2
SE-164 80 Stockholm SE-164 80 Stockholm
Sweden Sweden
Phone: +46 10 714 82 87 Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com Email: magnus.westerlund@ericsson.com
Suhas Nandakumar Suhas Nandakumar
Cisco Cisco
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
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