draft-ietf-mmusic-sdp-simulcast-03.txt   draft-ietf-mmusic-sdp-simulcast-04.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: April 21, 2016 S. Nandakumar Expires: August 6, 2016 S. Nandakumar
M. Zanaty M. Zanaty
Cisco Cisco
October 19, 2015 February 3, 2016
Using Simulcast in SDP and RTP Sessions Using Simulcast in SDP and RTP Sessions
draft-ietf-mmusic-sdp-simulcast-03 draft-ietf-mmusic-sdp-simulcast-04
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 different 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 describes how
best way of accomplishing simulcast in RTP and how to signal it in to accomplish simulcast in RTP and how to signal it in SDP. The
SDP. A solution is defined by making an extension to SDP, and using described solution uses an RTP/RTCP identification method to identify
RTP/RTCP identification methods to relate RTP streams belonging to RTP streams belonging to the same media source, and makes an
the same media source. The SDP extension consists of a new media extension to SDP to relate those RTP streams as being different
level SDP attribute that expresses capability to send and/or receive simulcast formats of that media source. The SDP extension consists
simulcast RTP streams. RTP/RTCP identification using either payload of a new media level SDP attribute that expresses capability to send
types or a separately defined method for RTP stream configuration are and/or receive simulcast RTP streams.
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 April 21, 2016. This Internet-Draft will expire on August 6, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 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
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 2, line 28 skipping to change at page 2, line 28
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 Media Source Preferences . . . . . . . . . . . . 7 3.3. Receiver Media Source Preferences . . . . . . . . . . . . 7
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Detailed Description . . . . . . . . . . . . . . . . . . . . 9 6. Detailed Description . . . . . . . . . . . . . . . . . . . . 9
6.1. Simulcast Capability . . . . . . . . . . . . . . . . . . 9 6.1. Simulcast Attribute . . . . . . . . . . . . . . . . . . . 9
6.1.1. Declarative Use . . . . . . . . . . . . . . . . . . . 11 6.2. Simulcast Capability . . . . . . . . . . . . . . . . . . 11
6.1.2. Offer/Answer Use . . . . . . . . . . . . . . . . . . 12 6.2.1. Declarative Use . . . . . . . . . . . . . . . . . . . 13
6.2. Relating Simulcast Streams . . . . . . . . . . . . . . . 14 6.2.2. Offer/Answer Use . . . . . . . . . . . . . . . . . . 13
6.3. Signaling Examples . . . . . . . . . . . . . . . . . . . 14 6.3. Relating Simulcast Streams . . . . . . . . . . . . . . . 15
6.3.1. Unified Plan Client . . . . . . . . . . . . . . . . . 14 6.4. Signaling Examples . . . . . . . . . . . . . . . . . . . 15
6.3.2. Multi-Source Client . . . . . . . . . . . . . . . . . 16 6.4.1. Single-Source Client . . . . . . . . . . . . . . . . 16
7. Network Aspects . . . . . . . . . . . . . . . . . . . . . . . 18 6.4.2. Multi-Source Client . . . . . . . . . . . . . . . . . 17
8. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Network Aspects . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 18 7.1. Bitrate Adaptation . . . . . . . . . . . . . . . . . . . 20
8.2. SDP Format Identification . . . . . . . . . . . . . . . . 19 8. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.3. RID Identification . . . . . . . . . . . . . . . . . . . 19 8.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20 10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Normative References . . . . . . . . . . . . . . . . . . 20 13.1. Normative References . . . . . . . . . . . . . . . . . . 22
13.2. Informative References . . . . . . . . . . . . . . . . . 21 13.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Changes From Earlier Versions . . . . . . . . . . . 23 Appendix A. Changes From Earlier Versions . . . . . . . . . . . 25
A.1. Modifications Between WG Version -02 and -03 . . . . . . 23 A.1. Modifications Between WG Version -03 and -04 . . . . . . 25
A.2. Modifications Between WG Version -01 and -02 . . . . . . 24 A.2. Modifications Between WG Version -02 and -03 . . . . . . 26
A.3. Modifications Between WG Version -00 and -01 . . . . . . 24 A.3. Modifications Between WG Version -01 and -02 . . . . . . 26
A.4. Modifications Between Individual Version -00 and WG A.4. Modifications Between WG Version -00 and -01 . . . . . . 26
Version -00 . . . . . . . . . . . . . . . . . . . . . . . 24 A.5. Modifications Between Individual Version -00 and WG
Version -00 . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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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Simulcast is defined in this memo as the act of simultaneously Simulcast is defined in this memo as the act of simultaneously
sending multiple different encoded streams of the same media source, sending multiple different encoded streams of the same media source,
e.g. the same video source encoded with different video encoder types e.g. the same video source encoded with different video encoder types
or image resolutions. This can be done in several ways and for or image resolutions. This can be done in several ways and for
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(s) to provide the wanted functionality by selecting which RTP stream(s) 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.
of the aspects discussed in RTP Multiplexing Guidelines
[I-D.ietf-avtcore-multiplex-guidelines].
This document describes a few scenarios where it is motivated to use This document describes a few scenarios where it is motivated to use
simulcast, and also defines the needed SDP signaling for it. simulcast, and also defines the needed RTP/RTCP and SDP signaling for
it.
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 [RFC7656], and RTP Topologies [RFC7667]. In addition, the following
RTP Topologies Update [I-D.ietf-avtcore-rtp-topologies-update]. In 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 [RFC7667] (Section 3.6 to
Topo-Mixer), further elaborated and extended with other topologies 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 [RFC7667].
Simulcast Stream: One Encoded Stream or Dependent Stream from a set Simulcast Stream: One Encoded Stream or Dependent Stream from a set
of concurrently transmitted Encoded Streams and optional Dependent of concurrently transmitted Encoded Streams and optional Dependent
Streams, all sharing a common Media Source, as defined in Streams, all sharing a common Media Source, as defined in
[I-D.ietf-avtext-rtp-grouping-taxonomy]. Decoding a Dependent [RFC7656]. Decoding a Dependent Stream also requires the related
Stream also requires the related (Dependent and) Encoded (Dependent and) Encoded Stream(s), but in the context of simulcast
Stream(s), but in the context of simulcast that is considered a that is considered a property of the Dependent Stream constituting
property of the Dependent Stream constituting the simulcast the simulcast stream. For example, HD and thumbnail video
stream. For example, HD and thumbnail video simulcast versions of simulcast versions of a single Media Source sent concurrently as
a single Media Source sent concurrently as separate RTP Streams. separate RTP Streams.
Simulcast Format: Different formats of a simulcast stream serve the Simulcast Format: Different formats of a simulcast stream serve the
same purpose as alternative RTP payload types in non-simulcast same purpose as alternative RTP payload types in non-simulcast
SDP, to allow multiple alternative media formats for a given RTP SDP, to allow multiple alternative media formats for a given RTP
Stream. As for multiple RTP payload types on the m-line, any one 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, 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 but not more than one (based on RTP timestamp), and what format is
used can change dynamically from one RTP packet to another. For used can change dynamically from one RTP packet to another. For
example, if all participants in a group video call can decode 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 H.264 and H.265 video, but only some can encode H.265, both H.264
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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
otherwise incompatible preferences. otherwise incompatible preferences.
4. Requirements 4. Requirements
The following requirements need to be met to support the use cases in The following requirements need to be met to support the use cases in
previous sections: previous sections:
Editor's note: Consider adding an explicit requirement that the
solution supports use of simulcast even when using multiple codecs
and multiple redundant RTP streams per defined codec (or something
similar), since this is really an existing requirement and should
also fully motivate the use of RID as identification mechanism.
REQ-1: Identification. It must be possible to identify a set of REQ-1: Identification. It must be possible to identify a set of
simulcasted RTP streams as originating from the same media source: simulcasted RTP streams as originating from the same media source:
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
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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 environment with a single media source per SDP
source per SDP media description. media description.
5. Overview 5. Overview
As an overview, the above requirements are met by signaling simulcast As an overview, the above requirements are met by signaling simulcast
capability and configurations in SDP [RFC4566]: capability and configurations in SDP [RFC4566]:
o An offer or answer can contain a number of simulcast streams, 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
streams in the same fashion as multiple, alternative codecs can be stream formats in the same fashion as multiple, alternative codecs
offered in a media description. can be offered in a media description.
o A single media source per SDP media description is assumed, which o A single media source per SDP media description is assumed, which
is aligned with the concepts defined in is aligned with the concepts defined in [RFC7656] and will
[I-D.ietf-avtext-rtp-grouping-taxonomy] and will specifically work specifically work in a WebRTC context, both with and without
in a WebRTC context, both with and without BUNDLE BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation] grouping.
[I-D.ietf-mmusic-sdp-bundle-negotiation] grouping.
o The codec configuration for a simulcast stream can be expressed in
two alternative ways, with complementing drawbacks and benefits:
* Through existing SDP formats (corresponding to RTP payload
types), enabling the use of simulcast with a minimum set of
additions to existing SDP specifications.
* Through use of a separately specified RTP-level identification o The codec configuration for a simulcast stream is expressed
mechanism [I-D.pthatcher-mmusic-rid], which complements and through use of a separately specified RTP-level identification
effectively extends the available simulcast stream mechanism [I-D.ietf-mmusic-rid][I-D.roach-avtext-rid], which
identification and configuration possibilities provided by complements and effectively extends the available simulcast stream
using SDP formats. identification and configuration possibilities that could be
provided by using only SDP formats.
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. Detailed Description 6. Detailed Description
This section further details the overview above (Section 5). This section further details the overview above (Section 5). First,
formal syntax is provided (Section 6.1), followed by the rest of the
SDP attribute definition in Section 6.2. Relating Simulcast Streams
(Section 6.3) provides the definition of the RTP/RTCP mechanisms
used. The section is concluded with a number of examples.
6.1. Simulcast Capability 6.1. Simulcast Attribute
Simulcast capability is expressed as a new media level SDP attribute, Name: simulcast
"a=simulcast". For each desired direction (send/recv), the simulcast
attribute defines a list of simulcast streams (separated by
semicolons), each of which is a list of simulcast formats (separated
by commas). The meaning of the attribute on SDP session level is
undefined and MUST NOT be used. The ABNF [RFC5234] for this
attribute is:
sc-attr = "a=simulcast:" 1*2( WSP sc-str-list ) [WSP sc-pause-list] Value: sc-value
sc-str-list = sc-dir WSP sc-id-type "=" sc-alt-list *( ";" sc-alt-list )
sc-pause-list = "paused=" sc-alt-list Usage Level: media
sc-dir = "send" / "recv"
sc-id-type = "pt" / "rid" / token Charset Dependent: no
sc-alt-list = sc-id *( "," sc-id )
sc-id = fmt / rid-identifier / token Multiplex Category: NORMAL
; WSP defined in [RFC5234]
; fmt, token defined in [RFC4566] Syntax [RFC5234]:
; rid-identifier defined in [I-D.pthatcher-mmusic-rid]
sc-attr = "a=simulcast:" sc-value
sc-value = sc-str-list [SP sc-str-list]
sc-str-list = sc-dir SP sc-alt-list *( ";" sc-alt-list )
sc-dir = "send" / "recv"
sc-alt-list = sc-id *( "," sc-id )
sc-id-paused = "~"
sc-id = [sc-id-paused] rid-identifier / token
; SP defined in [RFC5234]
; token defined in [RFC4566]
; rid-identifier defined in [I-D.ietf-mmusic-rid]
Figure 1: ABNF for Simulcast Figure 1: ABNF for Simulcast
There are separate and independent sets of parameters for simulcast The "a=simulcast" attribute has a parameter in the form of one or two
in send and receive directions. When listing multiple directions, simulcast stream descriptions, each consisting of a direction ("send"
each direction MUST NOT occur more than once on the same line. or "recv"), followed by a list of one or more simulcast streams.
Each simulcast stream in that list is separated by a semicolon (";").
Each simulcast stream can in turn be offered in one or more
alternative formats, where each simulcast stream alternative is
separated by a comma (","). The simulcast stream alternative MUST be
described in the form of a RID, as described by
[I-D.ietf-mmusic-rid]. Each simulcast stream can be initially paused
[I-D.ietf-avtext-rtp-stream-pause], indicated by prepending a "~" to
the simulcast stream. In case there are simulcast stream
alternatives, pause can be specified individually for each
alternative. The reason to allow separate initial pause states for
each simulcast stream alternative is that pause capability can be
specified individually for each RTP payload type referenced by a RID,
which makes it infeasible to pause RID where any of the related RTP
payload type(s) do not have pause capability.
Two simulcast stream identification methods are defined; "pt" using Examples:
RTP payload type (SDP format), and "rid" using an additional RTP-
level identification mechanism [I-D.pthatcher-mmusic-rid]. Different a=simulcast:send 1,2,3;~4,~5 recv 1;~2,~5
identification methods MUST NOT be used for different directions on a a=simulcast:recv 1;4,5 send 1;2
single "a=simulcast" line. Implementations that support both
identification methods MAY include one "a=simulcast" line for each Figure 2: Simulcast Examples
identification method for the same "m="-line. Multiple "a=simulcast"
lines with the same identification method MUST NOT be used for a Above are two examples of different "a=simulcast" lines.
single "m="-line.
The first line is an example offer to send two simulcast streams and
to receive two simulcast streams. The first simulcast stream in send
direction can be sent as three different alternatives (1, 2, 3), and
the second simulcast stream in send direction can be sent as two
different alternatives (4, 5). All second stream send alternatives
are offered as initially paused. The first simulcast stream in
receive direction has no alternatives (only 1). The second simulcast
stream in receive direction has two alternatives (2, 5) that are both
offered as initially paused.
The second line is an example answer to the first line, accepting to
send and receive the two offered simulcast streams, however send and
receive directions are specified in opposite order compared to the
first line, which lets the answer keep the same order of simulcast
streams in the SDP as in the offer, even though directionality is
reversed. This example answer has removed all offered alternatives
for the first simulcast stream (keeping only 1), but kept alternative
formats for the second simulcast stream in receive direction (4, 5).
The answer accepts to send two simulcast streams, without
alternatives. The answer does not accept initial pause of any
simulcast streams, in either direction. More examples can be found
in Section 6.4.
6.2. Simulcast Capability
Simulcast capability is expressed as a new media level SDP attribute,
"a=simulcast" (Section 6.1), with multiplex category
[I-D.ietf-mmusic-sdp-mux-attributes] NORMAL.
For each desired direction (send/recv), the simulcast attribute
defines a list of simulcast streams (separated by semicolons), each
of which is a list of simulcast formats (separated by commas). The
meaning of the attribute on SDP session level is undefined and MUST
NOT be used.
The meaning of including multiple "a=simulcast" lines in a single SDP
media description is undefined and MUST NOT be used. There are
separate and independent sets of parameters for simulcast in send and
receive directions. When listing multiple directions, each direction
MUST NOT occur more than once on the same line.
The different simulcast streams MUST be identified through the RTP-
level "RID" identification mechanism [I-D.ietf-mmusic-rid].
Attribute parameters are grouped by direction and consist of a Attribute parameters are grouped by direction and consist of a
listing of simulcast stream identifications to be used. The number listing of simulcast stream identifications to be used. The number
of (non-alternative, see below) identifications in the list sets a of (non-alternative, see below) identifications in the list sets a
limit to the number of supported simulcast streams in that direction. limit to the number of supported simulcast streams in that direction.
The order of the listed simulcast versions in the "send" direction The order of the listed simulcast versions in the "send" direction
suggests a proposed order of preference, in decreasing order: the suggests a proposed order of preference, in decreasing order: the
stream listed first is the most preferred Section 3.1, and subsequent stream listed first is the most preferred Section 3.1, and subsequent
streams have progressively lower preference. The order of the listed streams have progressively lower preference. The order of the listed
simulcast streams in the "recv" direction expresses a preference simulcast streams in the "recv" direction expresses a preference
which simulcast streams that are preferred, with the leftmost being which simulcast streams that are preferred, with the leftmost being
most preferred. This can be of importance if the number of actually most preferred. This can be of importance if the number of actually
sent simulcast streams have to be reduced for some reason. sent simulcast streams have to be reduced for some reason.
Formats that have explicit dependencies [RFC5583] Formats that have explicit dependencies [RFC5583]
[I-D.pthatcher-mmusic-rid] to other formats (even in the same media [I-D.ietf-mmusic-rid] to other formats (even in the same media
description) MAY be listed as different simulcast streams. description) MAY be listed as different simulcast streams.
Alternative simulcast formats MAY be specified as part of the Alternative simulcast formats MAY be specified as part of the
attribute parameters by expressing each simulcast stream as a comma- attribute parameters by expressing each simulcast stream as a comma-
separated list of alternative format identifiers. In this case, separated list of alternative format identifiers. In this case, it
there MUST NOT be any capability restriction in what alternative is not possible to align what alternative formats that are used
formats can be used across different simulcast streams, like between different simulcast streams, like requiring all simulcast
requiring all simulcast streams to use the same codec format streams to use alternatives with the same codec format. The order of
alternative. The order of the format alternatives within a simulcast the format alternatives within a simulcast stream is significant; the
stream is significant; the alternatives are listed from (left) most alternatives are listed from (left) most preferred to (right) least
preferred to (right) least preferred. For the use of simulcast, this preferred. For the use of simulcast, this overrides the normal codec
overrides the normal codec preference as expressed by format type preference as expressed by format type ordering on the "m="-line,
ordering on the "m="-line, using regular SDP rules. This is to using regular SDP rules. This is to enable a separation of general
enable a separation of general codec preferences and simulcast stream codec preferences and simulcast stream configuration preferences.
configuration preferences.
A simulcast stream can use a codec defined such that the same RTP 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 streams of the media source, but rather a specific strictly simulcast streams of the media source, but rather a specific
way of generating the RTP stream of a single simulcast stream with way of generating the RTP stream of a single simulcast stream with
varying RTP payload type. Instead, only a single simulcast stream varying RTP payload type. Instead, only a single simulcast stream
identification MUST be used per simulcast stream or alternative identification MUST be used per simulcast stream or alternative
simulcast format (if there are such) in the SDP. The used simulcast simulcast format (if there are such) in the SDP.
stream identification SHOULD be the codec format most relevant to the
media description, if possible to identify, for example the audio
codec rather than the DTMF. What codec format to choose in the case
of switching between multiple equally "important" formats is left
open, but it is assumed that in the presence of such strong relation
it does not matter which is chosen.
If RTP stream pause/resume [I-D.ietf-avtext-rtp-stream-pause] is If RTP stream pause/resume [I-D.ietf-avtext-rtp-stream-pause] is
supported, the optional "paused=" parameter MAY be used in supported, any simulcast stream identification MAY be prefixed by a
conjunction with "rid" simulcast stream identification to specify "~" character to indicate that the corresponding simulcast stream is
that a certain simulcast stream is initially paused already from initially paused already from start of the RTP session. In this
start of the RTP session. In this case, support for RTP stream case, support for RTP stream pause/resume MUST also be included under
pause/resume MUST also be included under the same "m="-line listing the same "m="-line listing "a=simulcast". If the simulcast stream is
"a=simulcast". Initially paused simulcast streams MUST NOT be used specified as a list of alternative formats, the indication is
with "pt" identification. Initially paused simulcast streams are prepended to the first format of the list and applies to whatever
resumed as described by the RTP pause/resume specification. alternative that is eventually chosen. All RTP payload types related
to such initially paused simulcast stream MUST be listed in the SDP
as pause/resume capable as specified by
[I-D.ietf-avtext-rtp-stream-pause].
An initially paused simulcast stream in "send" direction MUST be An initially paused simulcast stream in "send" direction MUST be
considered equivalent to an unsolicited locally paused stream, and be considered equivalent to an unsolicited locally paused stream, and be
handled accordingly. 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 stream RID identification as an RID RTCP SDES item.
An initially paused simulcast stream in "recv" direction SHOULD cause Including an initially paused simulcast stream in "recv" direction in
the remote RTP sender to put the stream as unsolicited locally an SDP towards an RTP sender, SHOULD cause the remote RTP sender to
paused, unless there are other RTP stream receivers that do not mark put the stream as unsolicited locally paused, unless there are other
the simulcast stream as initially paused. The reason to require an RTP stream receivers that do not mark the simulcast stream as
initially paused "recv" stream to be considered locally paused by the initially paused. The reason to require an initially paused "recv"
remote RTP sender, instead of making it equivalent to implicitly stream to be considered locally paused by the remote RTP sender,
sending a pause request, is because the pausing RTP sender cannot instead of making it equivalent to implicitly sending a pause
know which SSRC owns the restriction when TMMBR/TMMBN are used for request, is because the pausing RTP sender cannot know which SSRC
pause/resume signaling since the RTP receiver's SSRC in send owns the restriction when TMMBR/TMMBN are used for pause/resume
direction is not known yet. signaling since the RTP receiver's SSRC in send direction is not
known yet.
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 stream on the "a=simulcast" line. a simulcast stream on the "a=simulcast" line.
6.1.1. Declarative Use The media formats and corresponding characteristics of simulcast
streams SHOULD be chosen such that they are different. If this
difference is not required, RTP duplication [RFC7104] procedures
SHOULD be considered instead of simulcast.
When used as a declarative media description, a=simulcast "recv" 6.2.1. Declarative Use
direction formats indicates the configured end point's required
When used as a declarative media description, "a=simulcast" line
"recv" direction formats indicate 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" line "send"
requests the end point to send a specified set of RTP streams as direction requests the end point to send a specified set of RTP
simulcast streams. streams as simulcast streams.
If multiple simulcast formats 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
stream. stream.
Editor's note: The RID identification mechanism currently lacks a Editor's note: It may not be beneficial for declarative use to be
declarative use definition. As declarative use may also not limited to a single media source per "m=" line, as elaborated
follow unified plan with a single media source per '"m="-line, it further in Section 8.
is uncertain if declarative can be defined for the mechanism in
its current shape.
6.1.2. Offer/Answer Use 6.2.2. 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" without any
direction or without any simulcast stream identifications in a simulcast stream identifications in a specified direction MUST NOT
specified direction MUST NOT use simulcast in that direction. 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. defined in existing SDP Offer/Answer [RFC3264] procedures.
An answerer that does understand the attribute and that wants to An answerer that does understand the attribute and that wants to
support simulcast in an indicated direction SHALL reverse support simulcast in an indicated direction SHALL reverse
directionality of the unidirectional direction parameters; "send" directionality of the unidirectional direction parameters; "send"
becomes "recv" and vice versa, and include it in the answer. Note becomes "recv" and vice versa, and include it in the answer.
that, like all other use of SDP format tags ("pt:") for the send
direction in Offer/Answer, format tags related to the simulcast
stream identification send direction in an offer are placeholders
that refer to information in the offer SDP, and the actual formats
that will be 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 streams and/or An offerer listing a set of receive simulcast streams and/or
alternative formats in the offer MUST be prepared to receive RTP alternative formats in the offer MUST be prepared to receive RTP
streams for any of those simulcast streams and/or alternative formats streams for any of those simulcast streams and/or alternative formats
from the 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
streams 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 stream choose to remove any non-desirable alternatives per simulcast stream
skipping to change at page 13, line 21 skipping to change at page 14, line 35
alternatives, and MAY send any of the kept receive direction alternatives, and MAY send any of the kept receive direction
alternatives from the answer. Similarly, the answerer MUST be alternatives from the answer. Similarly, the answerer MUST be
prepared to receive any of the kept receive direction alternatives, prepared to receive any of the kept receive direction alternatives,
and MAY send any of the kept send direction alternatives in the and MAY send any of the kept send direction alternatives in the
answer. answer.
The offerer and answerer MUST NOT send more than a single alternative The offerer and answerer MUST NOT send more than a single alternative
format at a time (based on RTP timestamps) per simulcast stream, but 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 MAY change format on a per-RTP packet basis. This corresponds to the
existing (non-simulcast) SDP offer/answer case when multiple formats existing (non-simulcast) SDP offer/answer case when multiple formats
are included on the "m="-line in the SDP answer. 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
streams 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 streams. packets corresponding to the removed simulcast streams.
Simulcast streams or formats using undefined simulcast stream Simulcast streams or formats using undefined simulcast stream
identifications MUST NOT be used as valid simulcast streams by an RTP identifications MUST NOT be used as valid simulcast streams by an RTP
stream receiver. stream receiver.
An offerer that is capable of using both simulcast stream An answerer that receives an offer without RTP stream pause/resume
identification methods MAY include one "a=simulcast" line per capability MUST NOT mark any simulcast streams as initially paused in
identification method in the offer. Note that it is in general not the answer.
expected that the "pt" identification method will provide feature
parity with the "rid" method, and the different "a=simulcast" lines
can therefore express different use of simulcast functionality.
However, for some configurations the different identification methods
can be equivalent.
An answerer receiving an offer listing both simulcast stream An answerer that receives an offer with RTP stream pause/resume
identification methods MUST choose only one and remove the other from capability MAY mark any simulcast streams as initially paused in the
the answer. An answerer not supporting a simulcast stream answer.
identification method in the offer MUST remove the non-supported
"a=simulcast" line from the answer, possibly falling back to not
using simulcast at all.
The media formats and corresponding characteristics of encoded An answerer that receives indication in an offer of a simulcast
streams used in a simulcast SHOULD be chosen such that they are stream being initially paused , SHOULD mark that simulcast stream as
different. If this difference is not required, RTP duplication initially paused also in the answer, regardless of direction, unless
[RFC7104] procedures SHOULD be considered instead of simulcast. it has good reason for the stream not being initially paused.
An offerer that offered some of its simulcast streams as initially
paused and that receives an answer that does not indicate RTP stream
pause/resume capability, MUST NOT intially pause any simulcast
streams.
An offerer with RTP stream pause/resume capability that receives an
answer where some simulcast streams are marked as initially paused,
SHOULD initially pause them regardless if they were marked as
initially paused also in the offer, unless it has good reason for
those streams not being initially paused.
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" or "a=rid". for other SDP attributes, like "a=fmtp" or "a=rid".
6.2. Relating Simulcast Streams 6.3. Relating Simulcast Streams
As long as there is only a single media source per SDP media
description, simulcast RTP streams can be related on RTP level
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
[I-D.ietf-mmusic-sdp-bundle-negotiation] with multiple SDP media
descriptions to specify a single RTP session, there is an
identification mechanism that allows relating RTP streams back to
individual media descriptions, after which the above RTP payload type
and RID relations can be used.
BUNDLE's MID is an RTCP source description (SDES) item. To ensure Simulcast RTP streams MUST be related on RTP level through RID
rapid initial reception, required to correctly process the RTP [I-D.roach-avtext-rid], as specified in the SDP "a=simulcast"
streams, it is also defined as an RTP header extension [RFC5285]. attribute (Section 6.2) 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 RID relations can be used. Use of the RTP header extension
[RFC5285] for both MID and RID identifications can be important to
ensure rapid initial reception, required to correctly interpret and
process the RTP streams. Implementers of this specification MUST
support RTCP source description (SDES) item and SHOULD support RTP
header extension method to signal RID on RTP level.
6.3. Signaling Examples 6.4. Signaling Examples
These examples describe a client to video conference service, using a These examples describe a client to video conference service, 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 3: Four-party Mixer-based Conference
6.3.1. Unified Plan Client 6.4.1. Single-Source Client
Alice is calling in to the mixer with a simulcast-enabled Unified Alice is calling in to the mixer with a simulcast-enabled client
Plan client capable of a single media source per media type. The capable of a single media source per media type. The client can send
client can send a simulcast of 2 video resolutions and frame rates: a simulcast of 2 video resolutions and frame rates: HD 1280x720p
HD 1280x720p 30fps and thumbnail 320x180p 15fps. This is defined 30fps and thumbnail 320x180p 15fps. This is defined below using the
below using the "imageattr" [RFC6236]. Media formats (RTP payload "imageattr" [RFC6236]. In this example, only the "pt" RID parameter
types) are used as simulcast stream identification. Alice's Offer: is used, effectively achieving a 1:1 mapping between RID and media
formats (RTP payload types), to describe simulcast stream formats.
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 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 pt=97;98 recv pt=97 a=rid:1 pt=97
a=rid:2 pt=98
a=simulcast:send 1;2 recv 1
Figure 3: Unified Plan Simulcast Offer Figure 4: Single-Source 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 streams can differ in video resolution. 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 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 pt=97;98 send pt=97 a=rid:1 pt=97
a=rid:2 pt=98
a=simulcast:recv 1;2 send 1
Figure 4: Unified Plan Simulcast Answer Figure 5: Single-Source 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 parameters. direction of the "simulcast" attribute parameters.
6.3.2. Multi-Source Client 6.4.2. 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 restricted simulcast-enabled in the send direction. Fred's client is restricted
to a single media source per media description. to a single media source per media description.
The first two simulcast streams 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 streams 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 stream for the first media alternatives for the third simulcast stream for the first media
source. RID is used as simulcast stream identification, reducing the source. Only the highest fidelity simulcast stream are sent from
number of media formats needed. Only the highest fidelity simulcast start, the lower fidelity streams being initially paused.
stream are sent from start, the lower fidelity streams being
initially paused.
The second media source is offered with three different simulcast The second media source is offered with three different simulcast
streams. 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]. RID is used as simulcast protected by RTP retransmission [RFC4588]. Also here, all but the
stream identification. Also here, all but the highest fidelity highest fidelity simulcast stream are initially paused.
simulcast stream are initially paused.
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. Although using many different simulcast streams in this transport. Although using many different simulcast streams in this
example, use of RID as simulcast stream identification enables use of example, the use of RID as simulcast stream identification enables
a low number of RTP payload types. Note that the use of both BUNDLE use of a low number of RTP payload types. Note that the use of both
and RID recommends using the RTP header extension [RFC5285] for BUNDLE and RID recommends using the RTP header extension [RFC5285]
carrying these fields. for carrying these fields, which is consequently also included in the
SDP.
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
skipping to change at page 17, line 31 skipping to change at page 19, line 31
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:103 max-fs=900; max-fr=30 a=fmtp:103 max-fs=900; max-fr=30
a=rid:1 send pt=100;max-width=1280;max-height=720;max-fr=60;depend=2 a=rid:1 send pt=100;max-width=1280;max-height=720;max-fr=60;depend=2
a=rid:2 send pt=101;max-width=1280;max-height=720;max-fr=30 a=rid:2 send pt=101;max-width=1280;max-height=720;max-fr=30
a=rid:3 send pt=101;max-width=640;max-height=360 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=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=extmap:2 urn:ietf:params:rtp-hdrext:rid a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rid
a=rtcp-fb:* ccm pause nowait a=rtcp-fb:* ccm pause nowait
a=simulcast: send rid=1;2;4,3 paused=2,3,4 a=simulcast:send 1;2;~4,3
m=video 49602 RTP/AVPF 96 104 m=video 49602 RTP/AVPF 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=rid:5 send pt=96;max-fs=921600;max-fr=30 a=rid:5 send pt=96;max-fs=921600;max-fr=30
a=rid:6 send pt=96;max-fs=614400;max-fr=15 a=rid:6 send pt=96;max-fs=614400;max-fr=15
a=rid:7 send pt=96;max-fs=230400;max-fr=30 a=rid:7 send pt=96;max-fs=230400;max-fr=30
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:2 urn:ietf:params:rtp-hdrext:rid a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:rid
a=rtcp-fb:* ccm pause nowait a=rtcp-fb:* ccm pause nowait
a=simulcast: send rid=5;6;7 paused=6,7 a=simulcast:send 5;~6;~7
Figure 5: Fred's Multi-Source Simulcast Offer Figure 6: 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.
When transmitting multiple independent streams that originate from When transmitting multiple independent streams that originate from
the same source, it could potentially be done in several different the same source, it could potentially be done in several different
skipping to change at page 18, line 28 skipping to change at page 20, line 28
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 stream, rather than applying prioritize a particular simulcast stream, rather than applying
equal treatment to all streams. For example, lower bit-rate equal treatment to all streams. For example, lower bit-rate
streams may be prioritized over higher bit-rate streams to streams may be prioritized over higher bit-rate streams to
minimize congestion or packet losses in the low bit-rate streams. 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 with good QoS
QoS as good as possible. support.
NAT/FW Traversal: Using multiple RTP sessions incurs more cost for NAT/FW Traversal: Using multiple RTP sessions incurs more cost for
NAT/FW traversal unless they can re-use the same transport flow, NAT/FW traversal unless they can re-use the same transport flow,
which can be achieved by Multiplexing Negotiation Using SDP Port which can be achieved by Multiplexing Negotiation Using SDP Port
Numbers [I-D.ietf-mmusic-sdp-bundle-negotiation]. 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. 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. 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. Simulcast streams that have pause/
resume capability and that would be given such low bitrate by the
adaptation process that they are considered not really useful can be
temporarily paused until the limiting condition clears.
8. Limitations 8. Limitations
The chosen approach has a few limitations that are described in this The chosen approach has a few limitations that are described in this
section. Some relate to the use of a single RTP session for all section. The only one currently described relates to the use of a
simulcast formats of a media source, while others relate to the two single RTP session for all simulcast formats of a media source.
different simulcast stream identification methods.
8.1. Single RTP Session 8.1. Single RTP Session
The limitations in this section come from sending all simulcast The limitations in this section come from sending all simulcast
streams related to a media source under the same SDP media streams related to a media source under the same SDP media
description, which also means they are sent in the same RTP session. description, which also means they are sent in the same RTP session.
It is not possible to use different simulcast streams on different It is not possible to use different simulcast streams on different
transports, limiting the possibilities to apply different QoS to media transports, limiting the possibilities to apply different QoS
different simulcast streams. When using unicast, QoS mechanisms to different simulcast streams. When using unicast, QoS mechanisms
based on individual packet marking are feasible, since they do not based on individual packet marking are feasible, since they do not
require separation of simulcast streams into different RTP sessions require separation of simulcast streams into different RTP sessions
to apply different QoS. to apply different QoS.
It is not possible to separate different simulcast streams into It is not possible to separate different simulcast streams into
different multicast groups to allow a multicast receiver to pick the different multicast groups to allow a multicast receiver to pick the
stream it wants, rather than receive all of them. In this case, the stream it wants, rather than receive all of them. In this case, the
only reasonable implementation is to use different RTP sessions for only reasonable implementation is to use different RTP sessions for
each multicast group so that reporting and other RTCP functions each multicast group so that reporting and other RTCP functions
operate as intended. operate as intended.
8.2. SDP Format Identification
The limitations in this section come from and thus apply only when
using SDP format (RTP payload type) as simulcast stream
identification method.
The available RTP payload type number space may not be sufficient
when many different media formats and/or simulcast streams are used
in the SDP. This can be particularly prominent when BUNDLE is used,
and for any technology that adds to the number of required RTP
payload types in a multiplicative way, such as for example adding RTP
retransmission [RFC4588] and Forward Error Correction [RFC5109].
Flexible FEC Scheme [I-D.ietf-payload-flexible-fec-scheme] can be
used for RTP retransmissions and would avoid the double consumption
of the PT space that RTP Retransmission [RFC4588] causes.
Only existing SDP attributes and parameters can be used to define
codec configuration for a simulcast format. Any codec that does not
define a sufficient set of codec parameters in "a=fmtp", or can make
use of other SDP attributes, may not be capable of expressing the
desired simulcast format dimensions (Section 3.1) with necessary
precision, or not at all. One example of this is the ability to
separate simulcast formats by bandwidth for codecs lacking a codec-
specific bandwidth parameter, since the SDP "b="-line covers all RTP
payload types listed on an "m="-line.
A simulcast stream signaled as initially paused is not possible to
resume by a remote peer, because it cannot know which target SSRC to
use in the RESUME message [I-D.ietf-avtext-rtp-stream-pause].
8.3. RID Identification
The limitations in this section come from and thus apply only when
using RID as simulcast stream identification method.
Use of the additional "a=rid"-line in SDP and the corresponding RID
RTCP SDES item and RTP header extension requires some additional
implementation complexity, and incurs some extra bandwidth cost to
carry the RID RTCP SDES item and RTP header extension.
9. IANA Considerations 9. IANA Considerations
This document requests to register a new SDP attribute, simulcast. This document requests to register a new SDP attribute, simulcast, as
defined in Section 6.1.
Formal registrations to be written.
10. Security Considerations 10. Security Considerations
The simulcast capability, configuration attributes and parameters are The simulcast capability, configuration attributes, and parameters
vulnerable to attacks in signaling. are 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 streams 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 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.
Security considerations related to the use of RID is covered in
[I-D.ietf-mmusic-rid] and [I-D.roach-avtext-rid]. There are no
additional security concerns related to its use in this
specification.
11. Contributors 11. 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 and Cullen Jennings, from Cisco, and Peter document. Robert Hansen and Cullen Jennings, from Cisco, Peter
Thatcher, from Google, contributed significantly to subsequent Thatcher, from Google, and Adam Roach, from Mozilla, contributed
versions. significantly to subsequent versions.
12. Acknowledgements 12. Acknowledgements
13. References 13. References
13.1. Normative References 13.1. Normative References
[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-10 (work in progress), September 2015. pause-10 (work in progress), September 2015.
[I-D.pthatcher-mmusic-rid] [I-D.ietf-mmusic-rid]
Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B., Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B.,
Roach, A., and B. Campen, "RTP Payload Format Roach, A., and B. Campen, "RTP Payload Format
Constraints", draft-pthatcher-mmusic-rid-02 (work in Constraints", draft-ietf-mmusic-rid-01 (work in progress),
progress), October 2015. February 2016.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-12
(work in progress), January 2016.
[I-D.roach-avtext-rid]
Roach, A., Nandakumar, S., and P. Thatcher, "RTP Payload
Format Constraints", draft-roach-avtext-rid-01 (work in
progress), February 2016.
[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 21, line 48 skipping to change at page 23, line 37
13.2. Informative References 13.2. Informative References
[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, Q., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session", "Sending Multiple RTP Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-09 (work in progress), draft-ietf-avtcore-rtp-multi-stream-11 (work in progress),
September 2015. December 2015.
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-10 (work in progress),
July 2015.
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, "A Taxonomy of Semantics and Mechanisms for
Real-Time Transport Protocol (RTP) Sources", draft-ietf-
avtext-rtp-grouping-taxonomy-08 (work in progress), July
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-25 (work in progress), January 2016.
[I-D.ietf-payload-flexible-fec-scheme] [I-D.ietf-payload-flexible-fec-scheme]
Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction Payload Format for Flexible Forward Error Correction
(FEC)", draft-ietf-payload-flexible-fec-scheme-01 (work in (FEC)", draft-ietf-payload-flexible-fec-scheme-01 (work in
progress), October 2015. progress), October 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-
skipping to change at page 23, line 10 skipping to change at page 24, line 35
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588, Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006, DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>. <http://www.rfc-editor.org/info/rfc4588>.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733, Digits, Telephony Tones, and Telephony Signals", RFC 4733,
DOI 10.17487/RFC4733, December 2006, DOI 10.17487/RFC4733, December 2006,
<http://www.rfc-editor.org/info/rfc4733>. <http://www.rfc-editor.org/info/rfc4733>.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
DOI 10.17487/RFC5117, January 2008,
<http://www.rfc-editor.org/info/rfc5117>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>. 2008, <http://www.rfc-editor.org/info/rfc5285>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<http://www.rfc-editor.org/info/rfc5576>. <http://www.rfc-editor.org/info/rfc5576>.
[RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding [RFC5583] Schierl, T. and S. Wenger, "Signaling Media Decoding
skipping to change at page 23, line 43 skipping to change at page 25, line 15
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis, [RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190, "RTP Payload Format for Scalable Video Coding", RFC 6190,
DOI 10.17487/RFC6190, May 2011, DOI 10.17487/RFC6190, May 2011,
<http://www.rfc-editor.org/info/rfc6190>. <http://www.rfc-editor.org/info/rfc6190>.
[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>.
[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,
<http://www.rfc-editor.org/info/rfc7656>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<http://www.rfc-editor.org/info/rfc7667>.
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 WG Version -02 and -03 A.1. 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.
A.2. Modifications Between WG Version -02 and -03
o Removed text on multicast / broadcast from use cases, since it is o Removed text on multicast / broadcast from use cases, since it is
not supported by the solution. not supported by the solution.
o Removed explicit references to unified plan draft. o Removed explicit references to unified plan draft.
o Added possibility to initiate simulcast streams in paused mode. o Added possibility to initiate simulcast streams in paused mode.
o Enabled an offerer to offer multiple stream identification (pt or o Enabled an offerer to offer multiple stream identification (pt or
rid) methods and have the answerer choose which to use. rid) methods and have the answerer choose which to use.
o Added a preference indication also in send direction offers. o Added a preference indication also in send direction offers.
o Added a section on limitations of the current proposal, including o Added a section on limitations of the current proposal, including
identification method specific limitations. identification method specific limitations.
A.2. Modifications Between WG Version -01 and -02 A.3. Modifications Between WG Version -01 and -02
o Relying on the new RID solution for codec constraints and o Relying on the new RID solution for codec constraints and
configuration identification. This has resulted in changes in configuration identification. This has resulted in changes in
syntax to identify if pt or RID is used to describe the simulcast syntax to identify if pt or RID is used to describe the simulcast
stream. stream.
o Renamed simulcast version and simulcast version alternative to o Renamed simulcast version and simulcast version alternative to
simulcast stream and simulcast format respectively, and improved simulcast stream and simulcast format respectively, and improved
definitions for them. definitions for them.
o Clarification that it is possible to switch between simulcast o Clarification that it is possible to switch between simulcast
version alternatives, but that only a single one be used at any version alternatives, but that only a single one be used at any
point in time. point in time.
o Changed the definition so that ordering of simulcast formats for a o Changed the definition so that ordering of simulcast formats for a
specific simulcast stream do have a preference order. specific simulcast stream do have a preference order.
A.3. Modifications Between WG Version -00 and -01 A.4. Modifications Between WG Version -00 and -01
o No changes. Only preventing expiry. o No changes. Only preventing expiry.
A.4. Modifications Between Individual Version -00 and WG Version -00 A.5. 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
Email: bo.burman@ericsson.com Email: bo.burman@ericsson.com
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 2 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
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