draft-ietf-avt-avpf-ccm-07.txt   draft-ietf-avt-avpf-ccm-08.txt 
Network Working Group Stephan Wenger Network Working Group Stephan Wenger
INTERNET-DRAFT Umesh Chandra INTERNET-DRAFT Umesh Chandra
Expires: October 2007 Nokia Expires: January 2008 Nokia
Magnus Westerlund Intended Status: Proposed Standard Magnus Westerlund
Bo Burman Bo Burman
Ericsson Ericsson
May 30, 2007 July 6, 2007
Codec Control Messages in the Codec Control Messages in the
RTP Audio-Visual Profile with Feedback (AVPF) RTP Audio-Visual Profile with Feedback (AVPF)
draft-ietf-avt-avpf-ccm-07.txt> <draft-ietf-avt-avpf-ccm-08.txt>
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Abstract Abstract
This document specifies a few extensions to the messages defined This document specifies a few extensions to the messages defined in
in the Audio-Visual Profile with Feedback (AVPF). They are the Audio-Visual Profile with Feedback (AVPF). They are helpful
helpful primarily in conversational multimedia scenarios where primarily in conversational multimedia scenarios where centralized
centralized multipoint functionalities are in use. However some multipoint functionalities are in use. However, some are also
are also usable in smaller multicast environments and point-to- usable in smaller multicast environments and point-to-point calls.
point calls. The extensions discussed are messages related to the
ITU-T H.271 Video Back Channel, Full Intra Request, Temporary The extensions discussed are messages related to the ITU-T H.271
Maximum Media Stream Bit Rate and Temporal Spatial Trade-off. Video Back Channel, Full Intra Request, Temporary Maximum Media
Stream Bit Rate and Temporal Spatial Trade-off.
TABLE OF CONTENTS TABLE OF CONTENTS
1. Introduction....................................................5 1. Introduction..................................................5
2. Definitions.....................................................6 2. Definitions...................................................6
2.1. Glossary...................................................6 2.1. Glossary...................................................6
2.2. Terminology................................................6 2.2. Terminology................................................6
2.3. Topologies.................................................9 2.3. Topologies.................................................9
3. Motivation (Informative).......................................10 3. Motivation...................................................10
3.1. Use Cases.................................................10 3.1. Use Cases.................................................10
3.2. Using the Media Path......................................12 3.2. Using the Media Path......................................12
3.3. Using AVPF................................................13 3.3. Using AVPF................................................13
3.3.1. Reliability..........................................13 3.3.1. Reliability..........................................13
3.4. Multicast.................................................13 3.4. Multicast.................................................13
3.5. Feedback Messages.........................................13 3.5. Feedback Messages.........................................13
3.5.1. Full Intra Request Command...........................14 3.5.1. Full Intra Request Command...........................13
3.5.1.1. Reliability.....................................14 3.5.1.1. Reliability.....................................14
3.5.2. Temporal Spatial Trade-off Request and Notification..15 3.5.2. Temporal Spatial Trade-off Request and Notification..15
3.5.2.1. Point-to-Point..................................16 3.5.2.1. Point-to-Point..................................16
3.5.2.2. Point-to-Multipoint Using Multicast or 3.5.2.2. Point-to-Multipoint Using Multicast or
Translators.....................................17 Translators.....................................16
3.5.2.3. Point-to-Multipoint Using RTP Mixer.............17 3.5.2.3. Point-to-Multipoint Using RTP Mixer.............17
3.5.2.4. Reliability.....................................17 3.5.2.4. Reliability.....................................17
3.5.3. H.271 Video Back Channel Message.....................18 3.5.3. H.271 Video Back Channel Message.....................18
3.5.3.1. Reliability.....................................21 3.5.3.1. Reliability.....................................20
3.5.4. Temporary Maximum Media Stream Bit Rate Request and 3.5.4. Temporary Maximum Media Stream Bit Rate Request and
Notification.........................................21 Notification.........................................20
3.5.4.1. Behavior for media receivers using TMMBR........23 3.5.4.1. Behavior for media receivers using TMMBR........23
3.5.4.2. Algorithm for establishing current limitations..25 3.5.4.2. Algorithm for establishing current limitations..24
3.5.4.3. Use of TMMBR in a Mixer Based Multipoint 3.5.4.3. Use of TMMBR in a Mixer Based Multipoint
Operation.......................................32 Operation.......................................31
3.5.4.4. Use of TMMBR in Point-to-Multipoint Using 3.5.4.4. Use of TMMBR in Point-to-Multipoint Using
Multicast or Translators........................33 Multicast or Translators........................32
3.5.4.5. Use of TMMBR in Point-to-point operation........33 3.5.4.5. Use of TMMBR in Point-to-point operation........32
3.5.4.6. Reliability.....................................33 3.5.4.6. Reliability.....................................33
4. RTCP Receiver Report Extensions................................35 4. RTCP Receiver Report Extensions..............................34
4.1. Design Principles of the Extension Mechanism..............35 4.1. Design Principles of the Extension Mechanism..............34
4.2. Transport Layer Feedback Messages.........................36 4.2. Transport Layer Feedback Messages.........................35
4.2.1. Temporary Maximum Media Stream Bit Rate Request 4.2.1. Temporary Maximum Media Stream Bit Rate Request
(TMMBR)..............................................37 (TMMBR)..............................................36
4.2.1.1. Message Format..................................37 4.2.1.1. Message Format..................................36
4.2.1.2. Semantics.......................................38 4.2.1.2. Semantics.......................................37
4.2.1.3. Timing Rules....................................42 4.2.1.3. Timing Rules....................................41
4.2.1.4. Handling in Translator and Mixers...............42 4.2.1.4. Handling in Translator and Mixers...............41
4.2.2. Temporary Maximum Media Stream Bit Rate Notification 4.2.2. Temporary Maximum Media Stream Bit Rate Notification
(TMMBN)..............................................42 (TMMBN)..............................................41
4.2.2.1. Message Format..................................42 4.2.2.1. Message Format..................................41
4.2.2.2. Semantics.......................................43 4.2.2.2. Semantics.......................................42
4.2.2.3. Timing Rules....................................44 4.2.2.3. Timing Rules....................................43
4.2.2.4. Handling by Translators and Mixers..............44 4.2.2.4. Handling by Translators and Mixers..............43
4.3. Payload Specific Feedback Messages........................44 4.3. Payload Specific Feedback Messages........................43
4.3.1. Full Intra Request (FIR).............................45 4.3.1. Full Intra Request (FIR).............................44
4.3.1.1. Message Format..................................45 4.3.1.1. Message Format..................................44
4.3.1.2. Semantics.......................................46 4.3.1.2. Semantics.......................................45
4.3.1.3. Timing Rules....................................48 4.3.1.3. Timing Rules....................................46
4.3.1.4. Handling of FIR Message in Mixer and Translators48 4.3.1.4. Handling of FIR Message in Mixer and Translators46
4.3.1.5. Remarks.........................................49 4.3.1.5. Remarks.........................................46
4.3.2. Temporal-Spatial Trade-off Request (TSTR)............49 4.3.2. Temporal-Spatial Trade-off Request (TSTR)............48
4.3.2.1. Message Format..................................49 4.3.2.1. Message Format..................................48
4.3.2.2. Semantics.......................................50 4.3.2.2. Semantics.......................................49
4.3.2.3. Timing Rules....................................51 4.3.2.3. Timing Rules....................................49
4.3.2.4. Handling of message in Mixers and Translators...51 4.3.2.4. Handling of message in Mixers and Translators...49
4.3.2.5. Remarks.........................................51 4.3.2.5. Remarks.........................................50
4.3.3. Temporal-Spatial Trade-off Notification (TSTN).......51 4.3.3. Temporal-Spatial Trade-off Notification (TSTN).......50
4.3.3.1. Message Format..................................52 4.3.3.1. Message Format..................................50
4.3.3.2. Semantics.......................................52 4.3.3.2. Semantics.......................................51
4.3.3.3. Timing Rules....................................53 4.3.3.3. Timing Rules....................................52
4.3.3.4. Handling of TSTN in Mixer and Translators.......53 4.3.3.4. Handling of TSTN in Mixer and Translators.......52
4.3.3.5. Remarks.........................................53 4.3.3.5. Remarks.........................................52
4.3.4. H.271 Video Back Channel Message (VBCM)..............53 4.3.4. H.271 Video Back Channel Message (VBCM)..............52
4.3.4.1. Message Format..................................54 4.3.4.1. Message Format..................................52
4.3.4.2. Semantics.......................................55 4.3.4.2. Semantics.......................................53
4.3.4.3. Timing Rules....................................56 4.3.4.3. Timing Rules....................................54
4.3.4.4. Handling of message in Mixer or Translator......56 4.3.4.4. Handling of message in Mixer or Translator......55
4.3.4.5. Remarks.........................................56 4.3.4.5. Remarks.........................................55
5. Congestion Control.............................................57 5. Congestion Control...........................................55
6. Security Considerations........................................57 6. Security Considerations......................................56
7. SDP Definitions................................................58 7. SDP Definitions..............................................57
7.1. Extension of the rtcp-fb Attribute........................58 7.1. Extension of the rtcp-fb Attribute........................57
7.2. Offer-Answer..............................................60 7.2. Offer-Answer..............................................58
7.3. Examples..................................................60 7.3. Examples..................................................59
8. IANA Considerations............................................64 8. IANA Considerations..........................................62
9. Acknowledgements...............................................65 9. Contributors.................................................63
10. References....................................................67 10. Acknowledgements.............................................63
10.1. Normative references.....................................67 11. References...................................................64
10.2. Informative references...................................67 11.1. Normative references.....................................64
11. Authors' Addresses............................................69 11.2. Informative references...................................64
12. Authors' Addresses...........................................66
1.1. Introduction 1. Introduction
When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
developed, the main emphasis lay in the efficient support of developed, the main emphasis lay in the efficient support of point-
point-to-point and small multipoint scenarios without centralized to-point and small multipoint scenarios without centralized
multipoint control. However, in practice, many small multipoint multipoint control. However, in practice, many small multipoint
conferences operate utilizing devices known as Multipoint Control conferences operate utilizing devices known as Multipoint Control
Units (MCUs). Long-standing experience of the conversational Units (MCUs). Long-standing experience of the conversational video
video conferencing industry suggests that there is a need for a conferencing industry suggests that there is a need for a few
few additional feedback messages, to support centralized additional feedback messages, to support centralized multipoint
multipoint conferencing efficiently. Some of the messages have conferencing efficiently. Some of the messages have applications
applications beyond centralized multipoint, and this is indicated beyond centralized multipoint, and this is indicated in the
in the description of the message. This is especially true for description of the message. This is especially true for the message
the message intended to carry ITU-T Rec. H.271 [H.271] bit strings intended to carry ITU-T Rec. H.271 [H.271] bit strings for Video
for Video Back Channel messages. Back Channel messages.
In Real-time Transport Protocol (RTP) [RFC3550] terminology, MCUs In Real-time Transport Protocol (RTP) [RFC3550] terminology, MCUs
comprise mixers and translators. Most MCUs also include signaling comprise mixers and translators. Most MCUs also include signaling
support. During the development of this memo, it was noticed that support. During the development of this memo, it was noticed that
there is considerable confusion in the community related to the there is considerable confusion in the community related to the use
use of terms such as mixer, translator, and MCU. In response to of terms such as mixer, translator, and MCU. In response to these
these concerns, a number of topologies have been identified that concerns, a number of topologies have been identified that are of
are of practical relevance to the industry, but are not documented practical relevance to the industry, but are not documented in
in sufficient detail in [RFC3550]. These topologies are sufficient detail in [RFC3550]. These topologies are documented in
documented in [Topologies], and understanding this memo requires [Topologies], and understanding this memo requires previous or
previous or parallel study of [Topologies]. parallel study of [Topologies].
Some of the messages defined here are forward only, in that they Some of the messages defined here are forward only, in that they do
do not require an explicit notification to the message emitter not require an explicit notification to the message emitter that
that they have been received and/or indicating the message they have been received and/or indicating the message receiver's
receiver's actions. Other messages require a response, leading to actions. Other messages require a response, leading to a two way
a two way communication model that one could view as useful for communication model that one could view as useful for control
control purposes. However, it is not the intention of this memo purposes. However, it is not the intention of this memo to open up
to open up RTP Control Protocol (RTCP) to a generalized control RTP Control Protocol (RTCP) to a generalized control protocol. All
protocol. All mentioned messages have relatively strict real-time mentioned messages have relatively strict real-time constraints, in
constraints, in the sense that their value diminishes with the sense that their value diminishes with increased delay. This
increased delay. This makes the use of more traditional control makes the use of more traditional control protocol means, such as
protocol means, such as Session Initiation Protocol (SIP) re- Session Initiation Protocol (SIP) re-INVITEs [RFC3261], undesirable
INVITEs [RFC3261], undesirable when used for the same purpose. when used for the same purpose. Furthermore, all messages are of a
Furthermore, all messages are of a very simple format that can be very simple format that can be easily processed by an RTP/RTCP
easily processed by an RTP/RTCP sender/receiver. Finally, and sender/receiver. Finally, and most importantly, all messages relate
most importantly, all messages relate only to the RTP stream with only to the RTP stream with which they are associated, and not to
which they are associated, and not to any other property of a any other property of a communication system. In particular, none
communication system. In particular, none of them relate to the of them relate to the properties of the access links traversed by
properties of the access links traversed by the session. the session.
2. Definitions 2. Definitions
2.1. Glossary 2.1. Glossary
AMID - Additive Increase Multiplicative Decrease AIMD - Additive Increase Multiplicative Decrease
AVPF - The extended RTP profile for RTCP-based feedback AVPF - The extended RTP profile for RTCP-based feedback
FEC - Forward Error Correction FEC - Forward Error Correction
FCI - Feedback Control Information [RFC4585] FCI - Feedback Control Information [RFC4585]
FIR - Full Intra Request FIR - Full Intra Request
MCU - Multipoint Control Unit MCU - Multipoint Control Unit
MPEG - Moving Picture Experts Group MPEG - Moving Picture Experts Group
TMMBN - Temporary Maximum Media Stream Bit Rate Notification TMMBN - Temporary Maximum Media Stream Bit Rate Notification
TMMBR - Temporary Maximum Media Stream Bit Rate Request TMMBR - Temporary Maximum Media Stream Bit Rate Request
PLI - Picture Loss Indication PLI - Picture Loss Indication
PR - Packet rate PR - Packet rate
QP - Quantizer Parameter QP - Quantizer Parameter
RTT - Round trip time RTT - Round trip time
SSRC - Synchronization Source SSRC - Synchronization Source
TSTN - Temporal Spatial Trade-off Notification TSTN - Temporal Spatial Trade-off Notification
TSTR - Temporal Spatial Trade-off Request TSTR - Temporal Spatial Trade-off Request
VBCM - Video Back Channel Message indication. VBCM - Video Back Channel Message indication.
2.2. Terminology 2.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
"OPTIONAL" in this document are to be interpreted as described in this document are to be interpreted as described in RFC 2119
RFC 2119 [RFC2119]. [RFC2119].
Message: Message:
An RTCP feedback message [RFC4585] defined by this An RTCP feedback message [RFC4585] defined by this
specification, of one of the following types: specification, of one of the following types:
Request: Request:
Message that requires acknowledgement Message that requires acknowledgement
Command: Command:
Message that forces the receiver to an action Message that forces the receiver to an action
skipping to change at page 7, line 16 skipping to change at page 7, line 16
occurred. Notifications are commonly generated in occurred. Notifications are commonly generated in
response to a Request. response to a Request.
Note that, with the exception of "Notification", this Note that, with the exception of "Notification", this
terminology is in alignment with ITU-T Rec. H.245 [H245]. terminology is in alignment with ITU-T Rec. H.245 [H245].
Decoder Refresh Point: Decoder Refresh Point:
A bit string, packetized in one or more RTP packets, which A bit string, packetized in one or more RTP packets, which
completely resets the decoder to a known state. completely resets the decoder to a known state.
Examples for "hard" decoder refresh points are Intra Examples for "hard" decoder refresh points are Intra pictures
pictures in H.261, H.263, MPEG-1, MPEG-2, and MPEG-4 part in H.261, H.263, MPEG-1, MPEG-2, and MPEG-4 part 2, and
2, and Instantaneous Decoder Refresh (IDR) pictures in Instantaneous Decoder Refresh (IDR) pictures in H.264.
H.264. "Gradual" decoder refresh points may also be used; "Gradual" decoder refresh points may also be used; see for
see for example [AVC]. While both "hard" and "gradual" example [AVC]. While both "hard" and "gradual" decoder
decoder refresh points are acceptable in the scope of this refresh points are acceptable in the scope of this
specification, in most cases the user experience will specification, in most cases the user experience will benefit
benefit from using a "hard" decoder refresh point. from using a "hard" decoder refresh point.
A decoder refresh point also contains all header A decoder refresh point also contains all header information
information above the picture layer (or equivalent, above the picture layer (or equivalent, depending on the
depending on the video compression standard) that is video compression standard) that is conveyed in-band. In
conveyed in-band. In H.264, for example, a decoder refresh H.264, for example, a decoder refresh point contains
point contains parameter set Network Adaptation Layer (NAL) parameter set Network Adaptation Layer (NAL) units that
units that generate parameter sets necessary for the generate parameter sets necessary for the decoding of the
decoding of the following slice/data partition NAL units following slice/data partition NAL units (and that are not
(and that are not conveyed out of band). conveyed out of band).
Decoding: Decoding:
The operation of reconstructing the media stream. The operation of reconstructing the media stream.
Rendering: Rendering:
The operation of presenting (parts of) the reconstructed The operation of presenting (parts of) the reconstructed
media stream to the user. media stream to the user.
Stream thinning: Stream thinning:
The operation of removing some of the packets from a media The operation of removing some of the packets from a media
stream. Stream thinning, preferably, is media-aware, stream. Stream thinning, preferably, is media-aware,
implying that media packets are removed in the order of implying that media packets are removed in the order of
increasing relevance to the reproductive quality. However increasing relevance to the reproductive quality. However,
even when employing media-aware stream thinning, most media even when employing media-aware stream thinning, most media
streams quickly lose quality when subject to increasing streams quickly lose quality when subjected to increasing
levels of thinning. Media-unaware stream thinning leads to levels of thinning. Media-unaware stream thinning leads to
even worse quality degradation. In contrast to even worse quality degradation. In contrast to transcoding,
transcoding, stream thinning is typically seen as a stream thinning is typically seen as a computationally
computationally lightweight operation. lightweight operation.
Media: Media:
Often used (sometimes in conjunction with terms like bit Often used (sometimes in conjunction with terms like bit
rate, stream, sender ...) to identify the content of the rate, stream, sender ...) to identify the content of the
forward RTP packet stream (carrying the codec data), to forward RTP packet stream (carrying the codec data), to which
which the codec control message applies. the codec control message applies.
Media Stream: Media Stream:
The stream of RTP packets labeled with a single The stream of RTP packets labeled with a single
Synchronization Source (SSRC) carrying the media (and also Synchronization Source (SSRC) carrying the media (and also in
in some cases repair information such as retransmission or some cases repair information such as retransmission or
Forward Error Correction (FEC) information). Forward Error Correction (FEC) information).
Total media bit rate: Total media bit rate:
The total bits per second transferred in a media stream, The total bits per second transferred in a media stream,
measured at an observer-selected protocol layer and measured at an observer-selected protocol layer and averaged
averaged over a reasonable timescale, the length of which over a reasonable timescale, the length of which depends on
depends on the application. In general, a media sender and the application. In general, a media sender and a media
a media receiver will observe different total media bit receiver will observe different total media bit rates for the
rates for the same stream, first because they may have same stream, first because they may have selected different
selected different reference protocol layers, and second, reference protocol layers, and second, because of changes in
because of changes in per-packet overhead along the per-packet overhead along the transmission path. The goal
transmission path. The goal with bit rate averaging is to with bit rate averaging is to be able to ignore any
be able to ignore any burstiness on very short timescales, burstiness on very short timescales, below for example 100
below for example 100 ms, introduced by scheduling or link ms, introduced by scheduling or link layer packetization
layer packetization effects. effects.
Maximum total media bit rate: Maximum total media bit rate:
The upper limit on total media bit rate for a given media The upper limit on total media bit rate for a given media
stream at a particular receiver and for its selected stream at a particular receiver and for its selected protocol
protocol layer. Note that this value cannot be measured on layer. Note that this value cannot be measured on the
the received media stream, instead it needs to be received media stream, instead it needs to be calculated or
calculated or determined through other means, such as QoS determined through other means, such as QoS negotiations or
negotiations or local resource limitations. Also note that local resource limitations. Also note that this value is an
this value is an average (on a timescale that is reasonable average (on a timescale that is reasonable for the
for the application) and that it may be different from the application) and that it may be different from the
instantaneous bit-rate seen by packets in the media stream. instantaneous bit-rate seen by packets in the media stream.
Overhead: Overhead:
All protocol header information required to convey a packet All protocol header information required to convey a packet
with media data from sender to receiver, from the with media data from sender to receiver, from the application
application layer down to a pre-defined protocol level (for layer down to a pre-defined protocol level (for example down
example down to, and including, the IP header). Overhead to, and including, the IP header). Overhead may include, for
may include, for example, IP, UDP, and RTP headers, any example, IP, UDP, and RTP headers, any layer 2 headers, any
layer 2 headers, any Contributing Sources (CSRCs), RTP- Contributing Sources (CSRCs), RTP-Padding, and RTP header
Padding, and RTP header extensions. Overhead excludes any extensions. Overhead excludes any RTP payload headers and
RTP payload headers and the payload itself. the payload itself.
Net media bit rate: Net media bit rate:
The bit rate carried by a media stream, net of overhead. The bit rate carried by a media stream, net of overhead.
That is, the bits per second accounted for by encoded That is, the bits per second accounted for by encoded media,
media, any applicable payload headers, and any directly any applicable payload headers, and any directly associated
associated meta payload information placed in the RTP meta payload information placed in the RTP packet. A typical
packet. A typical example of the latter is redundancy data example of the latter is redundancy data provided by the use
provided by the use of RFC 2198 [RFC2198]. Note that, of RFC 2198 [RFC2198]. Note that, unlike the total media bit
unlike the total media bit rate, the net media bit rate rate, the net media bit rate will have the same value at the
will have the same value at the media sender and at the media sender and at the media receiver unless any mixing or
media receiver unless any mixing or translating of the translating of the media has occurred.
media has occurred.
For a given observer, the total media bit rate for a media For a given observer, the total media bit rate for a media
stream is equal to the sum of the net media bit rate and stream is equal to the sum of the net media bit rate and the
the per-packet overhead as defined above multiplied by the per-packet overhead as defined above multiplied by the packet
packet rate. rate.
Feasible region: Feasible region:
The set of all combinations of packet rate and net media The set of all combinations of packet rate and net media bit
bit rate that do not exceed the restrictions in maximum rate that do not exceed the restrictions in maximum media bit
media bit rate placed on a given media sender by the rate placed on a given media sender by the Temporary Maximum
Temporary Maximum Media Stream Bit-rate Request (TMMBR) Media Stream Bit-rate Request (TMMBR) messages it has
messages it has received. The feasible region will change received. The feasible region will change as new TMMBR
as new TMMBR messages are received. messages are received.
Bounding set: Bounding set:
The set of TMMBR tuples, selected from all those received The set of TMMBR tuples, selected from all those received at
at a given media sender, that define the feasible region a given media sender, that define the feasible region for
for that media sender. The media sender uses an algorithm that media sender. The media sender uses an algorithm such
such as that in section 3.5.4.2 to determine or iteratively as that in section 3.5.4.2 to determine or iteratively
approximate the current bounding set, and reports that set approximate the current bounding set, and reports that set
back to the media receivers in a Temporary Maximum Media back to the media receivers in a Temporary Maximum Media
Stream Bit-rate Notification (TMMBN) message. Stream Bit-rate Notification (TMMBN) message.
2.3. Topologies 2.3. Topologies
Please refer to [Topologies] for an in depth discussion. The Please refer to [Topologies] for an in depth discussion. The
topologies referred to throughout this memo are labeled topologies referred to throughout this memo are labeled
(consistently with [Topologies]) as follows: (consistently with [Topologies]) as follows:
Topo-Point-to-Point . . . . point-to-point communication Topo-Point-to-Point . . . . . Point-to-point communication
Topo-Multicast . . . . . . multicast communication as in RFC 3550 Topo-Multicast . . . . . . . Multicast communication
Topo-Translator . . . . . . translator based as in RFC 3550 Topo-Translator . . . . . . . Translator based
Topo-Mixer . . . . . . . . mixer based as in RFC 3550 Topo-Mixer . . . . . . . . . Mixer based
Topo-Video-switch-MCU . . . video switching MCU, Topo-Video-switch-MCU . . . . Video switching MCU,
Topo-RTCP-terminating-MCU . mixer but terminating RTCP Topo-RTCP-terminating-MCU . . Mixer but terminating RTCP
3. Motivation 3. Motivation
This section discusses the motivation and usage of the different This section discusses the motivation and usage of the different
video and media control messages. The video control messages have video and media control messages. The video control messages have
been under discussion for a long time, and a requirement draft was been under discussion for a long time, and a requirement draft was
drawn up [Basso]. This draft has expired; however we quote drawn up [Basso]. This draft has expired; however we quote relevant
relevant sections of it to provide motivation and requirements. sections of it to provide motivation and requirements.
3.1. Use Cases 3.1. Use Cases
There are a number of possible usages for the proposed feedback There are a number of possible usages for the proposed feedback
messages. Let us begin by looking through the use cases Basso et messages. Let us begin by looking through the use cases Basso et
al. [Basso] proposed. Some of the use cases have been al. [Basso] proposed. Some of the use cases have been reformulated
reformulated and comments have been added. and comments have been added.
1. An RTP video mixer composes multiple encoded video sources into 1. An RTP video mixer composes multiple encoded video sources into a
a single encoded video stream. Each time a video source is single encoded video stream. Each time a video source is added,
added, the RTP mixer needs to request a decoder refresh point the RTP mixer needs to request a decoder refresh point from the
from the video source, so as to start an uncorrupted prediction video source, so as to start an uncorrupted prediction chain on
chain on the spatial area of the mixed picture occupied by the the spatial area of the mixed picture occupied by the data from
data from the new video source. the new video source.
2. An RTP video mixer receives multiple encoded RTP video streams 2. An RTP video mixer receives multiple encoded RTP video streams
from conference participants, and dynamically selects one of from conference participants, and dynamically selects one of the
the streams to be included in its output RTP stream. At the streams to be included in its output RTP stream. At the time of
time of a bit stream change (determined through means such as a bit stream change (determined through means such as voice
voice activation or the user interface), the mixer requests a activation or the user interface), the mixer requests a decoder
decoder refresh point from the remote source, in order to avoid refresh point from the remote source, in order to avoid using
using unrelated content as reference data for inter picture unrelated content as reference data for inter picture prediction.
prediction. After requesting the decoder refresh point, the After requesting the decoder refresh point, the video mixer stops
video mixer stops the delivery of the current RTP stream and the delivery of the current RTP stream and monitors the RTP
monitors the RTP stream from the new source until it detects stream from the new source until it detects data belonging to the
data belonging to the decoder refresh point. At that time, the decoder refresh point. At that time, the RTP mixer starts
RTP mixer starts forwarding the newly selected stream to the forwarding the newly selected stream to the receiver(s).
receiver(s).
3. An application needs to signal to the remote encoder that the 3. An application needs to signal to the remote encoder that the
desired trade-off between temporal and spatial resolution has desired trade-off between temporal and spatial resolution has
changed. For example, one user may prefer a higher frame rate changed. For example, one user may prefer a higher frame rate
and a lower spatial quality, and another user may prefer the and a lower spatial quality, and another user may prefer the
opposite. This choice is also highly content dependent. Many opposite. This choice is also highly content dependent. Many
current video conferencing systems offer in the user interface current video conferencing systems offer in the user interface a
a mechanism to make this selection, usually in the form of a mechanism to make this selection, usually in the form of a
slider. The mechanism is helpful in point-to-point, slider. The mechanism is helpful in point-to-point, centralized
centralized multipoint and non-centralized multipoint uses. multipoint and non-centralized multipoint uses.
4. Use case 4 of the Basso draft applies only to Picture Loss 4. Use case 4 of the Basso draft applies only to Picture Loss
Indication (PLI) as defined in AVPF [RFC4585] and is not Indication (PLI) as defined in AVPF [RFC4585] and is not
reproduced here. reproduced here.
5. Use case 5 of the Basso draft relates to a mechanism known as 5. Use case 5 of the Basso draft relates to a mechanism known as
"freeze picture request". Sending freeze picture requests "freeze picture request". Sending freeze picture requests
over a non-reliable forward RTCP channel has been identified as over a non-reliable forward RTCP channel has been identified as
problematic. Therefore, no freeze picture request has been problematic. Therefore, no freeze picture request has been
included in this memo, and the use case discussion is not included in this memo, and the use case discussion is not
reproduced here. reproduced here.
6. A video mixer dynamically selects one of the received video 6. A video mixer dynamically selects one of the received video
streams to be sent out to participants and tries to provide the streams to be sent out to participants and tries to provide the
highest bit rate possible to all participants, while minimizing highest bit rate possible to all participants, while minimizing
stream trans-rating. One way of achieving this is to set up stream trans-rating. One way of achieving this is to set up
sessions with endpoints using the maximum bit rate accepted by sessions with endpoints using the maximum bit rate accepted by
each endpoint, and accepted by the call admission method used each endpoint, and accepted by the call admission method used by
by the mixer. By means of commands that reduce the maximum the mixer. By means of commands that reduce the maximum media
media stream bit rate below what has been negotiated during stream bit rate below what has been negotiated during session set
session set up, the mixer can reduce the maximum bit rate sent up, the mixer can reduce the maximum bit rate sent by endpoints
by endpoints to the lowest of all the accepted bit rates. As to the lowest of all the accepted bit rates. As the lowest
the lowest accepted bit rate changes due to endpoints joining accepted bit rate changes due to endpoints joining and leaving or
and leaving or due to network congestion, the mixer can adjust due to network congestion, the mixer can adjust the limits at
the limits at which endpoints can send their streams to match which endpoints can send their streams to match the new value.
the new value. The mixer then requests a new maximum bit rate, The mixer then requests a new maximum bit rate, which is equal to
which is equal to or less than the maximum bit rate negotiated or less than the maximum bit rate negotiated at session setup for
at session setup for a specific media stream, and the remote a specific media stream, and the remote endpoint can respond with
endpoint can respond with the actual bit rate that it can the actual bit rate that it can support.
support.
The picture Basso, et al draws up covers most applications we The picture Basso, et al draws up covers most applications we
foresee. However we would like to extend the list with two foresee. However, we would like to extend the list with two
additional use cases: additional use cases:
7. Currently deployed congestion control algorithms (AMID and TFRC 7. Currently deployed congestion control algorithms (AIMD and TFRC
[RFC3448]) probe for additional available capacity as long as [RFC3448]) probe for additional available capacity as long as
there is something to send. With congestion control algorithms there is something to send. With congestion control algorithms
using packet loss as the indication for congestion, this using packet loss as the indication for congestion, this probing
probing does generally result in reduced media quality (often generally results in reduced media quality (often to a point
to a point where the distortion is large enough to make the where the distortion is large enough to make the media unusable),
media unusable), due to packet loss and increased delay. due to packet loss and increased delay.
In a number of deployment scenarios, especially cellular ones, In a number of deployment scenarios, especially cellular ones,
the bottleneck link is often the last hop link. That cellular the bottleneck link is often the last hop link. That cellular
link also commonly has some type of QoS negotiation enabling link also commonly has some type of QoS negotiation enabling the
the cellular device to learn the maximal bit rate available cellular device to learn the maximal bit rate available over this
over this last hop. A media receiver behind this link can, in last hop. A media receiver behind this link can, in most (if not
most (if not all) cases, calculate at least an upper bound for all) cases, calculate at least an upper bound for the bit rate
the bit rate available for each media stream it presently available for each media stream it presently receives. How this
receives. How this is done is an implementation detail and not is done is an implementation detail and not discussed herein.
discussed herein. Indicating the maximum available bit rate to Indicating the maximum available bit rate to the transmitting
the transmitting party for the various media streams can be party for the various media streams can be beneficial to prevent
beneficial to prevent that party from probing for bandwidth for that party from probing for bandwidth for this stream in excess
this stream in excess of a known hard limit. For cellular or of a known hard limit. For cellular or other mobile devices, the
other mobile devices, the known available bit rate for each known available bit rate for each stream (deduced from the link
stream (deduced from the link bit rate) can change quickly, due bit rate) can change quickly, due to handover to another
to handover to another transmission technology, QoS transmission technology, QoS renegotiation due to congestion,
renegotiation due to congestion, etc. To enable minimal etc. To enable minimal disruption of service, quick convergence
disruption of service, quick convergence is necessary, and is necessary, and therefore media path signaling is desirable.
therefore media path signaling is desirable.
8. The use of reference picture selection (RPS) as an error 8. The use of reference picture selection (RPS) as an error
resilience tool has been introduced in 1997 as NEWPRED resilience tool has been introduced in 1997 as NEWPRED [NEWPRED],
[NEWPRED], and is now widely deployed. When RPS is in use, and is now widely deployed. When RPS is in use, simplistically
simplistically put, the receiver can send a feedback message to put, the receiver can send a feedback message to the sender,
the sender, indicating a reference picture that should be used indicating a reference picture that should be used for future
for future prediction. ([NEWPRED] mentions other forms of prediction. ([NEWPRED] mentions other forms of feedback as
feedback as well.) AVPF contains a mechanism for conveying well.) AVPF contains a mechanism for conveying such a message,
such a message, but did not specify for which codec and but did not specify for which codec and according to which syntax
according to which syntax the message should conform. the message should conform. Recently, the ITU-T finalized Rec.
Recently, the ITU-T finalized Rec. H.271 which (among other H.271 which (among other message types) also includes a feedback
message types) also includes a feedback message. It is message. It is expected that this feedback message will fairly
expected that this feedback message will fairly quickly enjoy quickly enjoy wide support. Therefore, a mechanism to convey
wide support. Therefore, a mechanism to convey feedback feedback messages according to H.271 appears to be desirable.
messages according to H.271 appears to be desirable.
3.2. Using the Media Path 3.2. Using the Media Path
There are multiple reasons why we use the media path for the codec There are multiple reasons why we use the media path for the codec
control messages. control messages.
First, systems employing MCUs often separate the control and media First, systems employing MCUs often separate the control and media
processing parts. As these messages are intended for or generated processing parts. As these messages are intended for or generated
by the media part rather than the signaling part of the MCU, by the media part rather than the signaling part of the MCU, having
having them on the media path avoids transmission across them on the media path avoids transmission across interfaces and
interfaces and unnecessary control traffic between signaling and unnecessary control traffic between signaling and processing. If
processing. If the MCU is physically decomposed, the use of the the MCU is physically decomposed, the use of the media path avoids
media path avoids the need for media control protocol extensions the need for media control protocol extensions (e.g. in MEGACO
(e.g. in MEGACO [RFC3525]). [RFC3525]).
Secondly, the signaling path quite commonly contains several Secondly, the signaling path quite commonly contains several
signaling entities, e.g. SIP proxies and application servers. signaling entities, e.g. SIP proxies and application servers.
Avoiding going through signaling entities avoids delay for several Avoiding going through signaling entities avoids delay for several
reasons. Proxies have less stringent delay requirements than reasons. Proxies have less stringent delay requirements than media
media processing and due to their complex and more generic nature processing and due to their complex and more generic nature may
may result in significant processing delay. The topological result in significant processing delay. The topological locations
locations of the signaling entities are also commonly not of the signaling entities are also commonly not optimized for
optimized for minimal delay, but rather towards other minimal delay, but rather towards other architectural goals. Thus,
architectural goals. Thus the signaling path can be significantly the signaling path can be significantly longer in both geographical
longer in both geographical and delay sense. and delay sense.
3.3. Using AVPF 3.3. Using AVPF
The AVPF feedback message framework [RFC4585] provides the The AVPF feedback message framework [RFC4585] provides the
appropriate framework to implement the new messages. AVPF appropriate framework to implement the new messages. AVPF
implements rules controlling the timing of feedback messages to implements rules controlling the timing of feedback messages to
avoid congestion through network flooding by RTCP traffic. We re- avoid congestion through network flooding by RTCP traffic. We re-
use these rules by referencing AVPF. use these rules by referencing AVPF.
The signaling setup for AVPF allows each individual type of The signaling setup for AVPF allows each individual type of function
function to be configured or negotiated on an RTP session basis. to be configured or negotiated on an RTP session basis.
3.3.1. Reliability 3.3.1. Reliability
The use of RTCP messages implies that each message transfer is The use of RTCP messages implies that each message transfer is
unreliable, unless the lower layer transport provides reliability. unreliable, unless the lower layer transport provides reliability.
The different messages proposed in this specification have The different messages proposed in this specification have different
different requirements in terms of reliability. However, in all requirements in terms of reliability. However, in all cases, the
cases, the reaction to an (occasional) loss of a feedback message reaction to an (occasional) loss of a feedback message is specified.
is specified.
3.4. Multicast 3.4. Multicast
The codec control messages might be used with multicast. The RTCP The codec control messages might be used with multicast. The RTCP
timing rules specified in [RFC3550] and [RFC4585] ensure that the timing rules specified in [RFC3550] and [RFC4585] ensure that the
messages do not cause overload of the RTCP connection. The use of messages do not cause overload of the RTCP connection. The use of
multicast may result in the reception of messages with multicast may result in the reception of messages with inconsistent
inconsistent semantics. The reaction to inconsistencies depends semantics. The reaction to inconsistencies depends on the message
on the message type, and is discussed for each message type type, and is discussed for each message type separately.
separately.
3.5. Feedback Messages 3.5. Feedback Messages
This section describes the semantics of the different feedback This section describes the semantics of the different feedback
messages and how they apply to the different use cases. messages and how they apply to the different use cases.
3.5.1. Full Intra Request Command 3.5.1. Full Intra Request Command
A Full Intra Request (FIR) Command, when received by the A Full Intra Request (FIR) Command, when received by the designated
designated media sender, requires that the media sender sends a media sender, requires that the media sender sends a Decoder Refresh
Decoder Refresh Point (see 2.2) at the earliest opportunity. The Point (see 2
evaluation of such opportunity includes the current encoder coding .2) at the earliest opportunity. The evaluation of such
strategy and the current available network resources. opportunity includes the current encoder coding strategy and the
current available network resources.
FIR is also known as an "instantaneous decoder refresh request" or FIR is also known as an "instantaneous decoder refresh request",
"video fast update request". "fast video update request" or "video fast update request".
Using a decoder refresh point implies refraining from using any Using a decoder refresh point implies refraining from using any
picture sent prior to that point as a reference for the encoding picture sent prior to that point as a reference for the encoding
process of any subsequent picture sent in the stream. For process of any subsequent picture sent in the stream. For
predictive media types that are not video, the analogue applies. predictive media types that are not video, the analogue applies.
For example, if in MPEG-4 systems scene updates are used, the For example, if in MPEG-4 systems scene updates are used, the
decoder refresh point consists of the full representation of the decoder refresh point consists of the full representation of the
scene and is not delta-coded relative to previous updates. scene and is not delta-coded relative to previous updates.
Decoder refresh points, especially Intra or IDR pictures, are in Decoder refresh points, especially Intra or IDR pictures, are in
general several times larger in size than predicted pictures. general several times larger in size than predicted pictures. Thus,
Thus, in scenarios in which the available bit rate is small, the in scenarios in which the available bit rate is small, the use of a
use of a decoder refresh point implies a delay that is decoder refresh point implies a delay that is significantly longer
significantly longer than the typical picture duration. than the typical picture duration.
Usage in multicast is possible; however aggregation of the Usage in multicast is possible; however aggregation of the commands
commands is recommended. A receiver that receives a request is recommended. A receiver that receives a request closely after
closely (within 2 times the longest Round Trip Time (RTT) known, sending a decoder refresh point -- within 2 times the longest Round
plus any AVPF-induced RTCP packet sending delays, if those are Trip Time (RTT) known, plus and AVPF-induced RTCP packet sending
known) after sending a decoder refresh point, should await a delays -- should await a second request message to ensure that the
second request message to ensure that the media receiver has not media receiver has not been served by the previously delivered
been served by the previously delivered decoder refresh point. decoder refresh point. The reason for the specified delay is to
The reason for the specified delay is to avoid sending unnecessary avoid sending unnecessary decoder refresh points. A session
decoder refresh points. A session participant may have sent its participant may have sent its own request while another
own request while another participant's request was in-flight to participant's request was in-flight to them. Suppressing those
them. Suppressing those requests that may have been sent without requests that may have been sent without knowledge about the other
knowledge about the other request avoids this issue. request avoids this issue.
Using the FIR command to recover from errors is explicitly Using the FIR command to recover from errors is explicitly
disallowed, and instead the PLI message defined in AVPF [RFC4585] disallowed, and instead the PLI message defined in AVPF [RFC4585]
should be used. The PLI message reports lost pictures and has should be used. The PLI message reports lost pictures and has been
been included in AVPF for precisely that purpose. included in AVPF for precisely that purpose.
Full Intra Request is applicable in use-cases 1 and 2. Full Intra Request is applicable in use-cases 1 and 2.
3.5.1.1. Reliability 3.5.1.1. Reliability
The FIR message results in the delivery of a decoder refresh
point, unless the message is lost. Decoder refresh points are
easily identifiable from the bit stream. Therefore, there is no
need for protocol-level notification, and a simple command
repetition mechanism is sufficient for ensuring the level of
reliability required. However, the potential use of repetition
does require a mechanism to prevent the recipient from responding
to messages already received and responded to.
To ensure the best possible reliability, a sender of FIR may The FIR message results in the delivery of a decoder refresh point,
repeat the FIR request until the desired content has been unless the message is lost. Decoder refresh points are easily
received. The repetition interval is determined by the RTCP identifiable from the bit stream. Therefore, there is no need for
timing rules applicable to the session. Upon reception of a protocol-level notification, and a simple command repetition
complete decoder refresh point or the detection of an attempt to mechanism is sufficient for ensuring the level of reliability
send a decoder refresh point (which got damaged due to a packet required. However, the potential use of repetition does require a
loss), the repetition of the FIR must stop. If another FIR is mechanism to prevent the recipient from responding to messages
necessary, the request sequence number must be increased. A FIR already received and responded to.
sender shall not have more than one FIR request (different request
sequence number) outstanding at any time per media sender in the
session.
The receiver of FIR (i.e. the media sender) behaves in To ensure the best possible reliability, a sender of FIR may repeat
complementary fashion to ensure delivery of a decoder refresh the FIR request until the desired content has been received. The
point. If it receives repetitions of the FIR more than 2*RTT repetition interval is determined by the RTCP timing rules
after it has sent a decoder refresh point, it shall send a new applicable to the session. Upon reception of a complete decoder
decoder refresh point. Two round trip times allow time for the refresh point or the detection of an attempt to send a decoder
decoder refresh point to arrive back to the requestor and for the refresh point (which got damaged due to a packet loss), the
end of repetitions of FIR to reach and be detected by the media repetition of the FIR must stop. If another FIR is necessary, the
sender. request sequence number must be increased. A FIR sender shall not
have more than one FIR request (different request sequence number)
outstanding at any time per media sender in the session.
The receiver of FIR (i.e. the media sender) behaves in complementary
fashion to ensure delivery of a decoder refresh point. If it
receives repetitions of the FIR more than 2*RTT after it has sent a
decoder refresh point, it shall send a new decoder refresh point.
Two round trip times allow time for the decoder refresh point to
arrive back to the requestor and for the end of repetitions of FIR
to reach and be detected by the media sender.
An RTP mixer that receives an FIR from a media receiver is An RTP mixer that receives an FIR from a media receiver is
responsible to ensure that a decoder refresh point is delivered to responsible to ensure that a decoder refresh point is delivered to
the requesting receiver. It may be necessary for the mixer to the requesting receiver. It may be necessary for the mixer to
generate FIR commands. From a reliability perspective, the two generate FIR commands. From a reliability perspective, the two legs
legs (FIR-requesting endpoint to mixer, and mixer to decoder (FIR-requesting endpoint to mixer, and mixer to decoder refresh
refresh point generating endpoint) are handled independently from point generating endpoint) are handled independently from each
each other. other.
3.5.2. Temporal Spatial Trade-off Request and Notification 3.5.2. Temporal Spatial Trade-off Request and Notification
The Temporal Spatial Trade-off Request (TSTR) instructs the video The Temporal Spatial Trade-off Request (TSTR) instructs the video
encoder to change its trade-off between temporal and spatial encoder to change its trade-off between temporal and spatial
resolution. Index values from 0 to 31 indicate monotonically a resolution. Index values from 0 to 31 indicate monotonically a
desire for higher frame rate. That is, a requester asking for an desire for higher frame rate. That is, a requester asking for an
index of 0 prefers a high quality and is willing to accept a low index of 0 prefers a high quality and is willing to accept a low
frame rate, whereas a requester asking for 31 wishes a high frame frame rate, whereas a requester asking for 31 wishes a high frame
rate, potentially at the cost of low spatial quality. rate, potentially at the cost of low spatial quality.
In general the encoder reaction time may be significantly longer In general the encoder reaction time may be significantly longer
than the typical picture duration. See use case 3 for an example. than the typical picture duration. See use case 3 for an example.
The encoder decides whether and to what extent the request results The encoder decides whether and to what extent the request results
in a change of the trade-off. It returns a Temporal Spatial in a change of the trade-off. It returns a Temporal Spatial Trade-
Trade-Off Notification (TSTN) message to indicate the trade-off Off Notification (TSTN) message to indicate the trade-off that it
that it will use henceforth. will use henceforth.
TSTR and TSTN have been introduced primarily because it is TSTR and TSTN have been introduced primarily because it is believed
believed that control protocol mechanisms, e.g. a SIP re-invite, that control protocol mechanisms, e.g. a SIP re-invite, are too
are too heavyweight and too slow to allow for a reasonable user heavyweight and too slow to allow for a reasonable user experience.
experience. Consider, for example, a user interface where the Consider, for example, a user interface where the remote user
remote user selects the temporal/spatial trade-off with a slider selects the temporal/spatial trade-off with a slider (as it is
(as it is common in state-of-the-art video conferencing systems). common in state-of-the-art video conferencing systems). An
An immediate feedback to any slider movement is required for a immediate feedback to any slider movement is required for a
reasonable user experience. A SIP re-INVITE [RFC3261] would reasonable user experience. A SIP re-INVITE [RFC3261] would require
require at least two round-trips more (compared to the TSTR/TSTN at least two round-trips more (compared to the TSTR/TSTN mechanism)
mechanism) and may involve proxies and other complex mechanisms. and may involve proxies and other complex mechanisms. Even in a
Even in a well-designed system, it could take a second or so until well-designed system, it could take a second or so until the new
finally the new trade-off is selected. trade-off is finally selected. Furthermore the use of RTCP solves
Furthermore the use of RTCP solves the multicast use case very the multicast use case very efficiently.
efficiently.
The use of TSTR and TSTN in multipoint scenarios is a non-trivial The use of TSTR and TSTN in multipoint scenarios is a non-trivial
subject, and can be achieved in many implementation-specific ways. subject, and can be achieved in many implementation-specific ways.
Problems stem from the fact that TSTRs will typically arrive Problems stem from the fact that TSTRs will typically arrive
unsynchronized, and may request different trade-off values for the unsynchronized, and may request different trade-off values for the
same stream and/or endpoint encoder. This memo does not specify a same stream and/or endpoint encoder. This memo does not specify a
translator, mixer or endpoint's reaction to the reception of a translator's, mixer's or endpoint's reaction to the reception of a
suggested trade-off as conveyed in the TSTR. We only require the suggested trade-off as conveyed in the TSTR. We only require the
receiver of a TSTR message to reply to it by sending a TSTN, receiver of a TSTR message to reply to it by sending a TSTN,
carrying the new trade-off chosen by its own criteria (which may carrying the new trade-off chosen by its own criteria (which may or
or may not be based on the trade-off conveyed by the TSTR). In may not be based on the trade-off conveyed by the TSTR). In other
other words, the trade-off sent in TSTR is a non-binding words, the trade-off sent in TSTR is a non-binding recommendation,
recommendation, nothing more. nothing more.
Four TSTR/TSTN scenarios need to be distinguished, based on the Four TSTR/TSTN scenarios need to be distinguished, based on the
topologies described in [Topologies]. The scenarios are described topologies described in [Topologies]. The scenarios are described
in the following sub-clauses. in the following sub-clauses.
3.5.2.1. Point-to-Point 3.5.2.1. Point-to-Point
In this most trivial case (Topo-Point-to-Point), the media sender In this most trivial case (Topo-Point-to-Point), the media sender
typically adjusts its temporal/spatial trade-off based on the typically adjusts its temporal/spatial trade-off based on the
requested value in TSTR, subject to its own capabilities. The requested value in TSTR, subject to its own capabilities. The TSTN
TSTN message conveys back the new trade-off value (which may be message conveys back the new trade-off value (which may be identical
identical to the old one if, for example, the sender is not to the old one if, for example, the sender is not capable of
capable of adjusting its trade-off). adjusting its trade-off).
3.5.2.2. Point-to-Multipoint Using Multicast or Translators 3.5.2.2. Point-to-Multipoint Using Multicast or Translators
RTCP Multicast is used either with media multicast according to RTCP Multicast is used either with media multicast according to
Topo-Multicast, or following RFC 3550's translator model according Topo-Multicast, or following RFC 3550's translator model according
to Topo-Translator. In these cases, unsynchronized TSTR messages to Topo-Translator. In these cases, unsynchronized TSTR messages
from different receivers may be received, possibly with different from different receivers may be received, possibly with different
requested trade-offs (because of different user preferences). requested trade-offs (because of different user preferences). This
This memo does not specify how the media sender tunes its trade- memo does not specify how the media sender tunes its trade-off.
off. Possible strategies include selecting the mean or median of Possible strategies include selecting the mean or median of all
all trade-off requests received, giving priority to certain trade-off requests received, giving priority to certain
participants, or continuing to use the previously selected trade- participants, or continuing to use the previously selected trade-off
off (e.g. when the sender is not capable of adjusting it). Again, (e.g. when the sender is not capable of adjusting it). Again, all
all TSTR messages need to be acknowledged by TSTN, and the value TSTR messages need to be acknowledged by TSTN, and the value
conveyed back has to reflect the decision made. conveyed back has to reflect the decision made.
3.5.2.3. Point-to-Multipoint Using RTP Mixer 3.5.2.3. Point-to-Multipoint Using RTP Mixer
In this scenario (Topo-Mixer) the RTP mixer receives all TSTR In this scenario (Topo-Mixer) the RTP mixer receives all TSTR
messages, and has the opportunity to act on them based on its own messages, and has the opportunity to act on them based on its own
criteria. In most cases, the mixer should form a "consensus" of criteria. In most cases, the mixer should form a "consensus" of
potentially conflicting TSTR messages arriving from different potentially conflicting TSTR messages arriving from different
participants, and initiate its own TSTR message(s) to the media participants, and initiate its own TSTR message(s) to the media
sender(s). As in the previous scenario, the strategy for forming sender(s). As in the previous scenario, the strategy for forming
this "consensus" is up to the implementation, and can, for this "consensus" is up to the implementation, and can, for example,
example, encompass averaging the participants' request values, encompass averaging the participants' request values, giving
giving priority to certain participants, or using session default priority to certain participants, or using session default values.
values.
Even if a mixer or translator performs transcoding, it is very Even if a mixer or translator performs transcoding, it is very
difficult to deliver media with the requested trade-off, unless difficult to deliver media with the requested trade-off, unless the
the content the mixer or translator receives is already close to content the mixer or translator receives is already close to that
that trade-off. Thus if the mixer changes its trade-off, it needs trade-off. Thus, if the mixer changes its trade-off, it needs to
to request the media sender(s) to use the new value, by creating a request the media sender(s) to use the new value, by creating a TSTR
TSTR of its own. Upon reaching a decision on the used trade-off of its own. Upon reaching a decision on the used trade-off it
it includes that value in the acknowledgement to the downstream includes that value in the acknowledgement to the downstream
requestors. Only in cases where the original source has requestors. Only in cases where the original source has
substantially higher quality (and bit rate), is it likely that substantially higher quality (and bit rate) is it likely that
transcoding alone can result in the requested trade-off. transcoding alone can result in the requested trade-off.
3.5.2.4. Reliability 3.5.2.4. Reliability
A request and reception acknowledgement mechanism is specified.
The Temporal Spatial Trade-off Notification (TSTN) message informs A request and reception acknowledgement mechanism is specified. The
the request-sender that its request has been received, and what Temporal Spatial Trade-off Notification (TSTN) message informs the
trade-off is used henceforth. This acknowledgment mechanism is requester that its request has been received, and what trade-off is
desirable for at least the following reasons: used henceforth. This acknowledgment mechanism is desirable for at
least the following reasons:
o A change in the trade-off cannot be directly identified from the o A change in the trade-off cannot be directly identified from the
media bit stream. media bit stream.
o User feedback cannot be implemented without knowing the chosen o User feedback cannot be implemented without knowing the chosen
trade-off value, according to the media sender's constraints. trade-off value, according to the media sender's constraints.
o Repetitive sending of messages requesting an unimplementable o Repetitive sending of messages requesting an unimplementable
trade-off can be avoided. trade-off can be avoided.
3.5.3. H.271 Video Back Channel Message 3.5.3. H.271 Video Back Channel Message
ITU-T Rec. H.271 defines syntax, semantics, and suggested encoder ITU-T Rec. H.271 defines syntax, semantics, and suggested encoder
reaction to a video back channel message. The structure defined reaction to a video back channel message. The structure defined in
in this memo is used to transparently convey such a message from this memo is used to transparently convey such a message from media
media receiver to media sender. In this memo, we refrain from an receiver to media sender. In this memo, we refrain from an in-depth
in-depth discussion of the available code points within H.271 and discussion of the available code points within H.271 and refer to
refer to the specification text [H.271] instead. the specification text [H.271] instead.
However, we note that some H.271 messages bear similarities with However, we note that some H.271 messages bear similarities with
native messages of AVPF and this memo. Furthermore, we note that native messages of AVPF and this memo. Furthermore, we note that
some H.271 message are known to require caution in multicast some H.271 message are known to require caution in multicast
environments -- or are plainly not usable in multicast or environments -- or are plainly not usable in multicast or multipoint
multipoint scenarios. Table 1 provides a brief, oversimplifying scenarios. Table 1 provides a brief, oversimplifed overview of the
overview of the messages currently defined in H.271, their roughly messages currently defined in H.271, their roughly corresponding
corresponding AVPF or CCM messages (the latter as specified in AVPF or CCM messages (the latter as specified in this memo), and an
this memo), and an indication of our current knowledge of their indication of our current knowledge of their multicast safety.
multicast safety.
H.271 msg type AVPF/CCM msg type multicast-safe H.271 msg type AVPF/CCM msg type multicast-safe
--------------------------------------------------------------------- --------------------------------------------------------------------
0 (when used for 0 (when used for
reference picture reference picture
selection) AVPF RPSI No (positive ACK of pictures) selection) AVPF RPSI No (positive ACK of pictures)
1 picture loss AVPF PLI Yes 1 picture loss AVPF PLI Yes
2 partial loss AVPF SLI Yes 2 partial loss AVPF SLI Yes
3 one parameter CRC N/A Yes (no required sender action) 3 one parameter CRC N/A Yes (no required sender action)
4 all parameter CRC N/A Yes (no required sender action) 4 all parameter CRC N/A Yes (no required sender action)
5 refresh point CCM FIR Yes 5 refresh point CCM FIR Yes
Table 1: H.271 messages and their AVPF/CCM equivalents Table 1: H.271 messages and their AVPF/CCM equivalents
Note: H.271 message type 0 is not a strict equivalent to Note: H.271 message type 0 is not a strict equivalent to
AVPF's Reference Picture Selection Indication (RPSI); it is AVPF's Reference Picture Selection Indication (RPSI); it is
an indication of known-as-correct reference picture(s) at an indication of known-as-correct reference picture(s) at the
the decoder. It does not command an encoder to use a decoder. It does not command an encoder to use a defined
defined reference picture (the form of control information reference picture (the form of control information envisioned
envisioned to be carried in RPSI). However, it is believed to be carried in RPSI). However, it is believed and intended
and intended that H.271 message type 0 will be used for the that H.271 message type 0 will be used for the same purpose
same purpose as AVPF's RPSI -- although other use forms are as AVPF's RPSI -- although other use forms are also possible.
also possible.
In response to the opaqueness of the H.271 messages especially In response to the opaqueness of the H.271 messages, especially with
with respect to the multicast safety, the following guidelines respect to the multicast safety, the following guidelines MUST be
MUST be followed when an implementation wishes to employ the H.271 followed when an implementation wishes to employ the H.271 video
video back channel message: back channel message:
1. Implementations utilizing the H.271 feedback message MUST stay 1. Implementations utilizing the H.271 feedback message MUST stay in
in compliance with congestion control principles, as outlined compliance with congestion control principles, as outlined in
in section 5. section 5.
2. An implementation SHOULD utilize the IETF-native messages as 2. An implementation SHOULD utilize the IETF-native messages as
defined in [RFC4585] and in this memo instead of similar defined in [RFC4585] and in this memo instead of similar messages
messages defined in [H.271]. Our current understanding of defined in [H.271]. Our current understanding of similar
similar messages is documented in Table 1 above. One good messages is documented in Table 1 above. One good reason to
reason to divert from the SHOULD statement above would be if it divert from the SHOULD statement above would be if it is clearly
is clearly understood that, for a given application and video understood that, for a given application and video compression
compression standard, the aforementioned "similarity" is not standard, the aforementioned "similarity" is not given, in
given, in contrast to what contrast to what the table indicates.
the table indicates.
3. It has been observed that some of the H.271 code points 3. It has been observed that some of the H.271 code points currently
currently in existence are not multicast-safe. Therefore, the in existence are not multicast-safe. Therefore, the sensible
sensible thing to do is not to use the H.271 feedback message thing to do is not to use the H.271 feedback message type in
type in multicast environments. It MAY be used only when all multicast environments. It MAY be used only when all the issues
the issues mentioned later are fully understood by the mentioned later are fully understood by the implementer, and
implementer, and properly taken into account by all endpoints. properly taken into account by all endpoints. In all other
In all other cases, the H.271 message type MUST NOT be used in cases, the H.271 message type MUST NOT be used in conjunction
conjunction with multicast. with multicast.
4. It has been observed that even in centralized multipoint 4. It has been observed that even in centralized multipoint
environments, where the mixer should theoretically be able to environments, where the mixer should theoretically be able to
resolve issues as documented below, the implementation of such resolve issues as documented below, the implementation of such a
a mixer and cooperative endpoints is a very difficult and mixer and cooperative endpoints is a very difficult and tedious
tedious task. Therefore, H.271 messages MUST NOT be used in task. Therefore, H.271 messages MUST NOT be used in centralized
centralized multipoint scenarios, unless all the issues multipoint scenarios, unless all the issues mentioned below are
mentioned below are fully understood by the implementer, and fully understood by the implementer, and properly taken into
properly taken into account by both mixer and endpoints. account by both mixer and endpoints.
Issues to be taken into account when considering the use of H.271 Issues to be taken into account when considering the use of H.271 in
in multipoint environments: multipoint environments:
1. Different state on different receivers. In many environments 1. Different state on different receivers. In many environments it
it cannot be guaranteed that the decoder state of all media cannot be guaranteed that the decoder state of all media
receivers is identical at any given point in time. The most receivers is identical at any given point in time. The most
obvious reason for such a possible misalignment of state is a obvious reason for such a possible misalignment of state is a
loss that occurs on the path to only one of many media loss that occurs on the path to only one of many media receivers.
receivers. However, there are other not so obvious reasons, However, there are other not so obvious reasons, such as recent
such as recent joins to the multipoint conference (be it by joins to the multipoint conference (be it by joining the
joining the multicast group or through additional mixer multicast group or through additional mixer output). Different
output). Different states can lead the media receivers to states can lead the media receivers to issue potentially
issue potentially contradicting H.271 messages (or one media contradicting H.271 messages (or one media receiver issuing an
receiver issuing an H.271 message that, when observed by the H.271 message that, when observed by the media sender, is not
media sender, is not helpful for the other media receivers). A helpful for the other media receivers). A naive reaction of the
naive reaction of the media sender to these contradicting media sender to these contradicting messages can lead to
messages can lead to unpredictable and annoying results. unpredictable and annoying results.
2. Combining messages from different media receivers in a media 2. Combining messages from different media receivers in a media
sender is a non-trivial task. As reasons, we note that these sender is a non-trivial task. As reasons, we note that these
messages may be contradicting each other, and that their messages may be contradicting each other, and that their
transport is unreliable (there may well be other reasons). In transport is unreliable (there may well be other reasons). In
case of many H.271 messages (i.e. types 0, 2, 3, and 4), the case of many H.271 messages (i.e. types 0, 2, 3, and 4), the
algorithm for combining must be aware both of the algorithm for combining must be aware both of the
network/protocol environment (i.e. with respect to congestion) network/protocol environment (i.e. with respect to congestion)
and of the media codec employed, as H.271 messages of a given and of the media codec employed, as H.271 messages of a given
type can have different semantics for different media codecs. type can have different semantics for different media codecs.
3. The suppression of requests may need to go beyond the basic 3. The suppression of requests may need to go beyond the basic
mechanisms described in AVPF (which are driven exclusively by mechanisms described in AVPF (which are driven exclusively by
timing and transport considerations on the protocol level). timing and transport considerations on the protocol level). For
For example, a receiver is often required to refrain from (or example, a receiver is often required to refrain from (or delay)
delay) generating requests, based on information it receives generating requests, based on information it receives from the
from the media stream. For instance, it makes no sense for a media stream. For instance, it makes no sense for a receiver to
receiver to issue a FIR when a transmission of an Intra/IDR issue a FIR when a transmission of an Intra/IDR picture is
picture is ongoing. ongoing.
4. When using the non-multicast-safe messages (e.g. H.271 type 0 4. When using the non-multicast-safe messages (e.g. H.271 type 0
positive ACK of received pictures/slices) in larger multicast positive ACK of received pictures/slices) in larger multicast
groups, the media receiver will likely be forced to delay or groups, the media receiver will likely be forced to delay or even
even omit sending these messages. For the media sender this omit sending these messages. For the media sender this looks
looks like data has not been properly received (although it was like data has not been properly received (although it was
received properly), and a naively implemented media sender received properly), and a naively implemented media sender reacts
reacts to these perceived problems where it should not. to these perceived problems where it should not.
3.5.3.1. Reliability 3.5.3.1. Reliability
H.271 Video Back Channel messages do not require reliable H.271 Video Back Channel messages do not require reliable
transmission, and confirmation of the reception of a message can transmission, and confirmation of the reception of a message can be
be derived from the forward video bit stream. Therefore, no derived from the forward video bit stream. Therefore, no specific
specific reception acknowledgement is specified. reception acknowledgement is specified.
With respect to re-sending rules, clause 3.5.1.1. applies. With respect to re-sending rules, clause 3.5.1.1 applies.
3.5.4. Temporary Maximum Media Stream Bit Rate Request and 3.5.4. Temporary Maximum Media Stream Bit Rate Request and Notification
Notification
A receiver, translator or mixer uses the Temporary Maximum Media A receiver, translator or mixer uses the Temporary Maximum Media
Stream Bit Rate Request (TMMBR, "timber") to request a sender to Stream Bit Rate Request (TMMBR, "timber") to request a sender to
limit the maximum bit rate for a media stream (see 2.2) to, or limit the maximum bit rate for a media stream (see 2.2) to, or
below, the provided value. The Temporary Maximum Media Stream Bit below, the provided value. The Temporary Maximum Media Stream Bit
Rate Notification (TMMBN) contains the media sender's current view Rate Notification (TMMBN) contains the media sender's current view
of the most limiting subset of the TMMBR-defined limits it has of the most limiting subset of the TMMBR-defined limits it has
received, to help the participants to suppress TMMBR requests that received, to help the participants to suppress TMMBR requests that
would not further restrict the media sender. The primary usage would not further restrict the media sender. The primary usage for
for the TMMBR/TMMBN messages is in a scenario with an MCU or mixer the TMMBR/TMMBN messages is in a scenario with an MCU or mixer (use
(use case 6), corresponding to Topo-Translator or Topo-Mixer, but case 6), corresponding to Topo-Translator or Topo-Mixer, but also to
also to Topo-Point-to-Point. Topo-Point-to-Point.
Each temporary limitation on the media stream is expressed as a Each temporary limitation on the media stream is expressed as a
tuple. The first component of the tuple is the maximum total tuple. The first component of the tuple is the maximum total media
media bit rate (as defined in section 2.2) that the media receiver bit rate (as defined in section 2.2) that the media receiver is
is currently prepared to accept for this media stream. The second currently prepared to accept for this media stream. The second
component is the per-packet overhead that the media receiver has component is the per-packet overhead that the media receiver has
observed for this media stream at its chosen reference protocol observed for this media stream at its chosen reference protocol
layer. layer.
As indicated in section 2.2, the overhead as observed by the As indicated in section 2.2, the overhead as observed by the sender
sender of the TMMBR (i.e. the media receiver) may differ from the of the TMMBR (i.e. the media receiver) may differ from the overhead
overhead observed at the receiver of the TMMBR (i.e. the media observed at the receiver of the TMMBR (i.e. the media sender) due to
sender) due to use of a different reference protocol layer at the use of a different reference protocol layer at the other end or due
other end or due to the intervention of translators or mixers that to the intervention of translators or mixers that affect the amount
affect the amount of per packet overhead. For example, a gateway of per packet overhead. For example, a gateway in between the two
in between the two that converts between IPv4 and IPv6 affects the that converts between IPv4 and IPv6 affects the per-packet overhead
per-packet overhead by 20 bytes. Other mechanisms that change the by 20 bytes. Other mechanisms that change the overhead include
overhead include tunnels. The problem with varying overhead is tunnels. The problem with varying overhead is also discussed in
also discussed in [RFC3890]. As will be seen in the description [RFC3890]. As will be seen in the description of the algorithm for
of the algorithm for use of TMMBR, the difference in perceived use of TMMBR, the difference in perceived overhead between the
overhead between the sending and receiving ends presents no sending and receiving ends presents no difficulty because
difficulty because calculations are carried out in terms of calculations are carried out in terms of variables that have the
variables (packet rate, net media bit rate) that have the same same value at the sender as at the receiver -- for example, packet
value at the sender as at the receiver. rate and net media rate.
Reporting both maximum total media bit rate and per-packet Reporting both maximum total media bit rate and per-packet overhead
overhead allows different receivers to provide bit rate and allows different receivers to provide bit rate and overhead values
overhead values for different protocol layers, for example at the for different protocol layers, for example at the IP level, at the
IP level, at the outer part of a tunnel protocol, or at the link outer part of a tunnel protocol, or at the link layer. The protocol
layer. The protocol level a peer reports on depends on the level level a peer reports on depends on the level of integration the peer
of integration the peer has, as it needs to be able to extract the has, as it needs to be able to extract the information from that
information from that protocol level. For example, an application protocol level. For example, an application with no knowledge of
with no knowledge of the IP version it is running over can not the IP version it is running over can not meaningfully determine the
meaningfully determine the overhead of the IP header, and hence overhead of the IP header, and hence will not want to include IP
will not want to include IP overhead in the overhead or maximum overhead in the overhead or maximum total media bit rate
total media bit rate calculation. calculation.
It is expected that most peers will be able to report values at It is expected that most peers will be able to report values at
least for the IP layer. In certain implementations it may be least for the IP layer. In certain implementations it may be
advantageous to also include information pertaining to the link advantageous to also include information pertaining to the link
layer, which in turn allows for a more precise overhead layer, which in turn allows for a more precise overhead calculation
calculation and a better optimization of connectivity resources. and a better optimization of connectivity resources.
The Temporary Maximum Media Stream Bit Rate messages are generic The Temporary Maximum Media Stream Bit Rate messages are generic
messages that can be applied to any RTP packet stream. This messages that can be applied to any RTP packet stream. This
separates them from the other codec control messages defined in separates them from the other codec control messages defined in this
this specification, which apply only to specific media types or specification, which apply only to specific media types or payload
payload formats. The TMMBR functionality applies to the formats. The TMMBR functionality applies to the transport, and the
transport, and the requirements the transport places on the media requirements the transport places on the media encoding.
encoding.
The reasoning below assumes that the participants have negotiated The reasoning below assumes that the participants have negotiated a
a session maximum bit rate, using a signaling protocol. This session maximum bit rate, using a signaling protocol. This value
value can be global, for example in case of point-to-point, can be global, for example in case of point-to-point, multicast, or
multicast, or translators. It may also be local between the translators. It may also be local between the participant and the
participant and the peer or mixer. In either case, the bit rate peer or mixer. In either case, the bit rate negotiated in signaling
negotiated in signaling is the one that the participant guarantees is the one that the participant guarantees to be able to handle
to be able to handle (depacketize and decode). In practice, the (depacketize and decode). In practice, the connectivity of the
connectivity of the participant also influences the negotiated participant also influences the negotiated value -- it does not make
value -- it does not make much sense to negotiate a total media much sense to negotiate a total media bit rate that one's network
bit rate that one's network interface does not support. interface does not support.
It is also beneficial to have negotiated a maximum packet rate for It is also beneficial to have negotiated a maximum packet rate for
the session or sender. RFC 3890 provides an SDP [RFC4566] the session or sender. RFC 3890 provides an SDP [RFC4566] attribute
attribute that can be used for this purpose; however, that that can be used for this purpose; however, that attribute is not
attribute is not usable in RTP sessions established using usable in RTP sessions established using offer/answer [RFC3264].
offer/answer [RFC3264]. Therefore an optional maximum packet rate Therefore an optional maximum packet rate signaling parameter is
signaling parameter is specified in this memo. specified in this memo.
An already established maximum total media bit rate may be changed An already established maximum total media bit rate may be changed
at any time, subject to the timing rules governing the sending of at any time, subject to the timing rules governing the sending of
feedback messages. The limit may change to any value between zero feedback messages. The limit may change to any value between zero
and the session maximum, as negotiated during session and the session maximum, as negotiated during session establishment
establishment signaling. However, even if a sender has received a signaling. However, even if a sender has received a TMMBR message
TMMBR message allowing an increase in the bit rate, all increases allowing an increase in the bit rate, all increases must be governed
must be governed by a congestion control mechanism. TMMBR by a congestion control mechanism. TMMBR indicates known
indicates known limitations only, usually in the local limitations only, usually in the local environment, and does not
environment, and does not provide any guarantees about the full provide any guarantees about the full path. Furthermore, any
path. Furthermore, any increases in TMMBR-established bit rate increases in TMMBR-established bit rate limits are to be executed
limits are to be executed only after a certain delay from the only after a certain delay from the sending of the TMMBN message
sending of the TMMBN message that notifies the world about the that notifies the world about the increase in limit. The delay is
increase in limit. The delay is specified as at least twice the specified as at least twice the longest RTT as known by the media
longest RTT as known by the media sender, plus the media sender's sender, plus the media sender's calculation of the required wait
calculation of the required wait time for the sending of another time for the sending of another TMMBR message for this session based
TMMBR message for this session based on AVPF timing rules. This on AVPF timing rules. This delay is introduced to allow other
delay is introduced to allow other session participants to make session participants to make known their bit rate limit
known their bit rate limit requirements, which may be lower. requirements, which may be lower.
If it is likely that the new value indicated by TMMBR will be If it is likely that the new value indicated by TMMBR will be valid
valid for the remainder of the session, the TMMBR sender is for the remainder of the session, the TMMBR sender is expected to
expected to perform a renegotiation of the session upper limit perform a renegotiation of the session upper limit using the session
using the session signaling protocol. signaling protocol.
3.5.4.1. Behavior for media receivers using TMMBR 3.5.4.1. Behavior for media receivers using TMMBR
This section is an informal description of behaviour described This section is an informal description of behaviour described more
more precisely in section 4.2. precisely in section 4.2.
A media sender begins the session limited by the maximum media bit A media sender begins the session limited by the maximum media bit
rate and maximum packet rate negotiated in session signaling, if rate and maximum packet rate negotiated in session signaling, if
any. Note that this value may be negotiated for another protocol any. Note that this value may be negotiated for another protocol
layer than the one the participant uses in its TMMBR messages. layer than the one the participant uses in its TMMBR messages. Each
Each media receiver selects a reference protocol layer, forms an media receiver selects a reference protocol layer, forms an estimate
estimate of the overhead it is observing (or estimating it if no of the overhead it is observing (or estimating it if no packets has
packets has been seen yet) at that reference level, and determines been seen yet) at that reference level, and determines the maximum
the maximum total media bit rate it can accept, taking into total media bit rate it can accept, taking into account its own
account its own limitations and any transport path limitations of limitations and any transport path limitations of which it may be
which it may be aware. In case the current limitations are more aware. In case the current limitations are more restricting then
restricting then what was agreed on in the session signaling, the what was agreed on in the session signaling, the media receiver
media receiver reports its initial estimate of these two reports its initial estimate of these two quantities to the media
quantities to the media sender using a TMMBR message. Overall sender using a TMMBR message. Overall message traffic is reduced by
message traffic is reduced by the possibility of including tuples the possibility of including tuples for multiple media senders in
for multiple media senders in the same TMMBR message. the same TMMBR message.
The media sender applies an algorithm such as that specified in The media sender applies an algorithm such as that specified in
section 3.5.4.2 to select which of the tuples it has received are section 3.5.4.2 to select which of the tuples it has received are
most limiting (i.e. the bounding set as defined in section 2.2). most limiting (i.e. the bounding set as defined in section 2.2). It
It modifies its operation to stay within the feasible region (as modifies its operation to stay within the feasible region (as
defined in section 2.2), and also sends out a TMMBN notification defined in section 2.2), and also sends out a TMMBN notification to
to the media receivers indicating the selected bounding set. the media receivers indicating the selected bounding set.
If a media receiver does not own one of the tuples in the bounding If a media receiver does not own one of the tuples in the bounding
set reported by the TMMBN, it applies the same algorithm as the set reported by the TMMBN, it applies the same algorithm as the
media sender to determine if its current estimated (maximum total media sender to determine if its current estimated (maximum total
media bit rate, overhead) tuple would enter the bounding set if media bit rate, overhead) tuple would enter the bounding set if
known to the media sender. If so, it issues a TMMBR request known to the media sender. If so, it issues a TMMBR request
reporting the tuple value to the sender. Otherwise it takes no reporting the tuple value to the sender. Otherwise it takes no
action for the moment. Periodically, its estimated tuple values action for the moment. Periodically, its estimated tuple values may
may change or it may receive a new TMMBN. If so, it reapplies the change or it may receive a new TMMBN. If so, it reapplies the
algorithm to decide whether it needs to issue a TMMBR request. algorithm to decide whether it needs to issue a TMMBR request.
If, alternatively, a media receiver owns one of the tuples in the If, alternatively, a media receiver owns one of the tuples in the
reported bounding set, it takes no action until such time as its reported bounding set, it takes no action until such time as its
estimate of its own tuple values changes. At that time it sends a estimate of its own tuple values changes. At that time it sends a
TMMBR request to the media sender to report the changed values. TMMBR request to the media sender to report the changed values.
A media receiver may change status between owner and non-owner of A media receiver may change status between owner and non-owner of a
a bounding tuple between one TMMBN message and the next. Thus it bounding tuple between one TMMBN message and the next. Thus, it
must check the contents of each TMMBN to determine its subsequent must check the contents of each TMMBN to determine its subsequent
actions. actions.
Implementations may use other algorithms of their choosing, as Implementations may use other algorithms of their choosing, as long
long as the bit rate limitations resulting from the exchange of as the bit rate limitations resulting from the exchange of TMMBR and
TMMBR and TMMBN messages are at least as strict (at least as low, TMMBN messages are at least as strict (at least as low, in the bit
in the bit rate dimension) as the ones resulting from the use of rate dimension) as the ones resulting from the use of the
the aforementioned algorithm. aforementioned algorithm.
Obviously, in point-to-point cases, when there is only one media Obviously, in point-to-point cases, when there is only one media
receiver, this receiver becomes "owner" once it receives the first receiver, this receiver becomes "owner" once it receives the first
TMMBN in response to its own TMMBR, and stays "owner" for the rest TMMBN in response to its own TMMBR, and stays "owner" for the rest
of the session. Therefore, when it is known that there will of the session. Therefore, when it is known that there will always
always be only a single media receiver, the above algorithm is not be only a single media receiver, the above algorithm is not
required. Media receivers that are aware they are the only ones required. Media receivers that are aware they are the only ones in
in a session can send TMMBR messages with bit rate limits both a session can send TMMBR messages with bit rate limits both higher
higher and lower than the previously notified limit, at any time and lower than the previously notified limit, at any time (subject
(subject to the AVPF [RFC4585] RTCP RR send timing rules). to the AVPF [RFC4585] RTCP RR send timing rules). However, it may
However, it may be difficult for a session participant to be difficult for a session participant to determine if it is the
determine if it is the only receiver in the session. Because of only receiver in the session. Because of this any implementation of
this any implementation of TMMBR is required to include the TMMBR is required to include the algorithm described in the next
algorithm described in the next section or a stricter equivalent. section or a stricter equivalent.
3.5.4.2. Algorithm for establishing current limitations 3.5.4.2. Algorithm for establishing current limitations
This section introduces an example algorithm for the calculation This section introduces an example algorithm for the calculation of
of a session limit. Other algorithms can be employed, as long as a session limit. Other algorithms can be employed, as long as the
the result of the calculation is at least as restrictive as the result of the calculation is at least as restrictive as the result
result that is obtained by this algorithm. that is obtained by this algorithm.
First it is important to consider the implications of using a First, it is important to consider the implications of using a tuple
tuple for limiting the media sender's behavior. The bit rate and for limiting the media sender's behavior. The bit rate and the
the overhead value result in a two-dimensional solution space for overhead value result in a two-dimensional solution space for the
the calculation of the bit rate of media streams. Fortunately the calculation of the bit rate of media streams. Fortunately, the two
two variables are linked. Specifically, the bit rate available for variables are linked. Specifically, the bit rate available for RTP
RTP payloads is equal to the TMMBR reported bit rate minus the payloads is equal to the TMMBR reported bit rate minus the packet
packet rate used, multiplied by the TMMBR reported overhead rate used, multiplied by the TMMBR reported overhead converted to
converted to bits. As a result, when different bit rate/overhead bits. As a result, when different bit rate/overhead combinations
combinations need to be considered, the packet rate determines the need to be considered, the packet rate determines the correct
correct limitation. This is perhaps best explained by an example: limitation. This is perhaps best explained by an example:
Example: Example:
Receiver A: TMMBR_max total BR = 35 kbps, TMMBR_OH = 40 bytes Receiver A: TMMBR_max total BR = 35 kbps, TMMBR_OH = 40 bytes
Receiver B: TMMBR_max total BR = 40 kbps, TMMBR_OH = 60 bytes Receiver B: TMMBR_max total BR = 40 kbps, TMMBR_OH = 60 bytes
For a given packet rate (PR) the bit rate available for media For a given packet rate (PR) the bit rate available for media
payloads in RTP will be: payloads in RTP will be:
Max_net media_BR_A = TMMBR_max total BR_A - PR * TMMBR_OH_A * 8 Max_net media_BR_A = TMMBR_max total BR_A - PR * TMMBR_OH_A * 8 ...
... (1) (1)
Max_net media_BR_B = TMMBR_max total BR_B - PR * TMMBR_OH_B * 8 Max_net media_BR_B = TMMBR_max total BR_B - PR * TMMBR_OH_B * 8 ...
... (2) (2)
For a PR = 20 these calculations will yield a Max_net media_BR_A = For a PR = 20 these calculations will yield a Max_net media_BR_A =
28600 bps and Max_net media_BR_B = 30400 bps, which suggests that 28600 bps and Max_net media_BR_B = 30400 bps, which suggests that
receiver A is the limiting one for this packet rate. However at a receiver A is the limiting one for this packet rate. However, at a
certain PR there is a switchover point at which receiver B becomes certain PR there is a switchover point at which receiver B becomes
the limiting one. The switchover point can be identified by the limiting one. The switchover point can be identified by setting
setting Max_media_BR_A equal to Max_media_BR_B and breaking out Max_media_BR_A equal to Max_media_BR_B and breaking out PR:
PR:
TMMBR_max total BR_A - TMMBR_max total BR_B TMMBR_max total BR_A - TMMBR_max total BR_B
PR = ------------------------------------------- ... (3) PR = ------------------------------------------- ... (3)
8*(TMMBR_OH_A - TMMBR_OH_B) 8*(TMMBR_OH_A - TMMBR_OH_B)
which, for the numbers above yields 31.25 as the switchover point which, for the numbers above yields 31.25 as the switchover point
between the two limits. That is, for packet rates below 31.25 per between the two limits. That is, for packet rates below 31.25 per
second, receiver A is the limiting receiver, and for higher packet second, receiver A is the limiting receiver, and for higher packet
rates, receiver B is more limiting. The implications of this rates, receiver B is more limiting. The implications of this
behavior have to be considered by implementations that are going behavior have to be considered by implementations that are going to
to control media encoding and its packetization. As exemplified control media encoding and its packetization. As exemplified above,
above, multiple TMMBR limits may apply to the trade-off between multiple TMMBR limits may apply to the trade-off between net media
net media bit rate and packet rate. Which limitation applies bit rate and packet rate. Which limitation applies depends on the
depends on the packet rate being considered. packet rate being considered.
This also has implications for how the TMMBR mechanism needs to This also has implications for how the TMMBR mechanism needs to
work. First, there is the possibility that multiple TMMBR tuples work. First, there is the possibility that multiple TMMBR tuples
are providing limitations on the media sender. Secondly there is are providing limitations on the media sender. Secondly there is a
a need for any session participant (media sender and receivers) to need for any session participant (media sender and receivers) to be
be able to determine if a given tuple will become a limitation able to determine if a given tuple will become a limitation upon the
upon the media sender, or if the set of already given limitations media sender, or if the set of already given limitations is stricter
is stricter than the given values. In the absence of the ability than the given values. In the absence of the ability to make this
to make this determination the suppression of TMMBR requests would determination the suppression of TMMBR requests would not work.
not work.
The basic idea of the algorithm is as follows. Each TMMBR tuple The basic idea of the algorithm is as follows. Each TMMBR tuple can
can be viewed as the equation of a straight line (cf. equations be viewed as the equation of a straight line (cf. equations (1) and
(1) and (2)) in a space where packet rate lies along the X-axis (2)) in a space where packet rate lies along the X-axis and maximum
and maximum bit rate lies along the Y-axis. The lower envelope of bit rate lies along the Y-axis. The lower envelope of the set of
the set of lines corresponding to the complete set of TMMBR tuples lines corresponding to the complete set of TMMR tuples, together
defines a polygon. Points lying along or below this polygon are with the X and Y axes, defines a polygon. Points lying within this
combinations of packet rate and bit rate that meet all of the polygon are combinations of packet rate and bit rate that meet all
TMMBR constraints. The highest feasible packet rate within this of the TMMBR constraints. The highest feasible packet rate within
region is the minimum of the rate at which the bounding polygon this region is the minimum of the rate at which the bounding polygon
meets the X-axis or the session maximum packet rate (SMAXPR) meets the X-axis or the session maximum packet rate (SMAXPR)
provided by signaling, if any. Typically a media sender will provided by signaling, if any. Typically a media sender will prefer
prefer to operate at a lower rate than this theoretical maximum, to operate at a lower rate than this theoretical maximum, so as to
so as to increase the rate at which actual media content reaches increase the rate at which actual media content reaches the
the receivers. The purpose of the algorithm is to distinguish the receivers. The purpose of the algorithm is to distinguish the TMMBR
TMMBR tuples constituting the bounding set and thus delineate the tuples constituting the bounding set and thus delineate the feasible
feasible region, so that the media sender can select its preferred region, so that the media sender can select its preferred operating
operating point within that region point within that region
Figure 1 below shows a bounding polygon formed by TMMBR tuples A Figure 1 below shows a bounding polygon formed by TMMBR tuples A and
and B. A third tuple C lies outside the bounding polygon and is B. A third tuple C lies outside the bounding polygon and is
therefore irrelevant in determining feasible tradeoffs between therefore irrelevant in determining feasible tradeoffs between media
media rate and packet rate. The line labeled ss..s represents the rate and packet rate. The line labeled ss..s represents the limit
limit on packet rate imposed by the session maximum packet rate on packet rate imposed by the session maximum packet rate (SMAXPR)
(SMAXPR) obtained by signaling during session setup. In Figure 1 obtained by signaling during session setup. In Figure 1 the limit
the limit determined by tuple B happens to be more restrictive determined by tuple B happens to be more restrictive than SMAXPR.
than SMAXPR. The situation could easily be the reverse, meaning The situation could easily be the reverse, meaning that the bounding
that the bounding polygon is terminated on the right by the polygon is terminated on the right by the vertical line representing
vertical line representing the SMAXPR constraint. the SMAXPR constraint.
Net ^ Net ^
Media|a c b s Media|a c b s
Bit | a c b s Bit | a c b s
Rate | a c b s Rate | a c b s
| a cb s | a cb s
| a c s | a c s
| a bc s | a bc s
| a b c s | a b c s
| ab c s | ab c s
| Feasible b c s | Feasible b c s
| region ba s | region ba s
| b a s c | b a s c
| b s c | b s c
| b s a | b s a
| bs |_____________________bs________
+------------------------------> +------------------------------>
Packet rate Packet rate
Figure 1 - Geometric Interpretation of TMMBR Tuples Figure 1 - Geometric Interpretation of TMMBR Tuples
Note that the slopes of the lines making up the bounding polygon Note that the slopes of the lines making up the bounding polygon are
are increasingly negative as one moves in the direction of increasingly negative as one moves in the direction of increasing
increasing packet rate. Note also that with slight rearrangement, packet rate. Note also that with slight rearrangement, equations
equations (1) and (2) have the canonical form: (1) and (2) have the canonical form:
y = mx + b y = mx + b
where where
m is the slope and has value equal to the negative of the tuple m is the slope and has value equal to the negative of the tuple
overhead (in bits), overhead (in bits),
and and
b is the y-intercept and has value equal to the tuple maximum b is the y-intercept and has value equal to the tuple maximum
total media bit rate. total media bit rate.
These observations lead to the conclusion that when processing the These observations lead to the conclusion that when processing the
TMMBR tuples to select the initial bounding set, one should sort TMMBR tuples to select the initial bounding set, one should sort and
and process the tuples by order of increasing overhead. Once a process the tuples by order of increasing overhead. Once a
particular tuple has been added to the bounding set, all tuples particular tuple has been added to the bounding set, all tuples not
not already selected and having lower overhead can be eliminated, already selected and having lower overhead can be eliminated,
because the next side of the bounding polygon has to be steeper because the next side of the bounding polygon has to be steeper
(i.e. the corresponding TMMBR must have higher overhead) than the (i.e. the corresponding TMMBR must have higher overhead) than the
latest added tuple. latest added tuple.
Line cc..c in Figure 1 illustrates another principle. This line is Line cc..c in Figure 1 illustrates another principle. This line is
parallel to line aa..a, but has a higher Y-intercept. That is, parallel to line aa..a, but has a higher Y-intercept. That is, the
the corresponding TMMBR tuple contains a higher maximum total corresponding TMMBR tuple contains a higher maximum total media bit
media bit rate value. Since line cc..c is outside the bounding rate value. Since line cc..c is outside the bounding polygon, it
polygon, it illustrates the conclusion that if two TMMBR tuples illustrates the conclusion that if two TMMBR tuples have the same
have the same overhead value, the one with higher maximum total overhead value, the one with higher maximum total media bit rate
media bit rate value cannot be part of the bounding set and can be value cannot be part of the bounding set and can be set aside.
set aside.
Two further observations complete the algorithm. Obviously, Two further observations complete the algorithm. Obviously, moving
moving from the left, the successive corners of the bounding from the left, the successive corners of the bounding polygon (i.e.
polygon (i.e. the intersection points between successive pairs of the intersection points between successive pairs of sides) lie at
sides) lie at successively higher packet rates. On the other successively higher packet rates. On the other hand, again moving
hand, again moving from the left, each successive line making up from the left, each successive line making up the bounding set
the bounding set crosses the X-axis at a lower packet rate. crosses the X-axis at a lower packet rate.
The complete algorithm can now be specified. The algorithm works The complete algorithm can now be specified. The algorithm works
with two lists of TMMBR tuples, the candidate list X and the with two lists of TMMBR tuples, the candidate list X and the
selected list Y, both ordered by increasing overhead value. The selected list Y, both ordered by increasing overhead value. The
algorithm terminates when all members of X have been discarded or algorithm terminates when all members of X have been discarded or
removed for processing. Membership of the selected list Y is removed for processing. Membership of the selected list Y is
probationary until the algorithm is complete. Each member of the probationary until the algorithm is complete. Each member of the
selected list is associated with an intersection value, which is selected list is associated with an intersection value, which is the
the packet rate at which the line corresponding to that TMMBR packet rate at which the line corresponding to that TMMBR tuple
tuple intersects with the line corresponding to the previous TMMBR intersects with the line corresponding to the previous TMMBR tuple
tuple in the selected list. Each member of the selected list is in the selected list. Each member of the selected list is also
also associated with a maximum packet rate value, which is the associated with a maximum packet rate value, which is the lesser of
lesser of the session maximum packet rate SMAXPR (if any) and the the session maximum packet rate SMAXPR (if any) and the packet rate
packet rate at which the line corresponding to that tuple crosses at which the line corresponding to that tuple crosses the X-axis.
the X-axis.
When the algorithm terminates, the selected list is equal to the When the algorithm terminates, the selected list is equal to the
bounding set as defined in section 2.2. bounding set as defined in section 2.2.
Initial Algorithm Initial Algorithm
This algorithm is used by the media sender when it has received This algorithm is used by the media sender when it has received one
one or more TMMBR requests and before it has determined a bounding or more TMMBR requests and before it has determined a bounding set
set for the first time. for the first time.
1. Sort the TMMBR tuples by order of increasing overhead. This is 1. Sort the TMMBR tuples by order of increasing overhead. This is
the initial candidate list X. the initial candidate list X.
2. When multiple tuples in the candidate list have the same 2. When multiple tuples in the candidate list have the same overhead
overhead value, discard all but the one with the lowest maximum value, discard all but the one with the lowest maximum total media
total media bit rate value. bit rate value.
3. Select and remove from the candidate list the TMMBR tuple with 3. Select and remove from the candidate list the TMMBR tuple with the
the lowest maximum total media bit rate value. If there is more lowest maximum total media bit rate value. If there is more than
than one tuple with that value, choose the one with the highest one tuple with that value, choose the one with the highest
overhead value. This is the first member of the selected list overhead value. This is the first member of the selected list Y.
Y. Set its intersection value equal to zero. Calculate its Set its intersection value equal to zero. Calculate its maximum
maximum packet rate as the minimum of SMAXPR (if available) and packet rate as the minimum of SMAXPR (if available) and the value
the value obtained from the following formula, which is the obtained from the following formula, which is the packet rate at
packet rate at which the corresponding line crosses the X-axis. which the corresponding line crosses the X-axis.
Max PR = TMMBR max total BR / (8 * TMMBR OH) ... (4) Max PR = TMMBR max total BR / (8 * TMMBR OH) ... (4)
4. Discard from the candidate list all tuples with a lower overhead 4. Discard from the candidate list all tuples with a lower overhead
value than the selected tuple. value than the selected tuple.
5. Remove the first remaining tuple from the candidate list for 5. Remove the first remaining tuple from the candidate list for
processing. Call this the current candidate. processing. Call this the current candidate.
6. Calculate the packet rate PR at the intersection of the line 6. Calculate the packet rate PR at the intersection of the line
generated by the current candidate with the line generated by generated by the current candidate with the line generated by the
the last tuple in the selected list Y, using equation (3). last tuple in the selected list Y, using equation (3).
7. If the calculated value PR is equal to or lower than the 7. If the calculated value PR is equal to or lower than the
intersection value stored for the last tuple of the selected intersection value stored for the last tuple of the selected list,
list, discard the last tuple of the selected list and go back to discard the last tuple of the selected list and go back to step 6
step 6 (retaining the same current candidate). (retaining the same current candidate).
Note that the choice of the initial member of the selected list Note that the choice of the initial member of the selected list Y
Y in step 3 guarantees that the selected list will never be in step 3 guarantees that the selected list will never be emptied
emptied by this process, meaning that the algorithm must by this process, meaning that the algorithm must eventually (if
eventually (if not immediately) fall through to the step 8. not immediately) fall through to the step 8.
8. (This step is reached when the calculated PR value of the 8. (This step is reached when the calculated PR value of the current
current candidate is greater than the intersection value of the candidate is greater than the intersection value of the current
current last member of the selected list Y.) If the calculated last member of the selected list Y.) If the calculated value PR
value PR of the current candidate is lower than the maximum of the current candidate is lower than the maximum packet rate
packet rate associated with the last tuple in the selected list, associated with the last tuple in the selected list, add the
add the current candidate tuple to the end of the selected list. current candidate tuple to the end of the selected list. Store PR
Store PR as its intersection value. Calculate its maximum as its intersection value. Calculate its maximum packet rate as
packet rate as the lesser of SMAXPR (if available) and the the lesser of SMAXPR (if available) and the maximum packet rate
maximum packet rate calculated using equation (4). calculated using equation (4).
9. If any tuples remain in the candidate list, go back to step 5. 9. If any tuples remain in the candidate list, go back to step 5.
Incremental Algorithm Incremental Algorithm
The previous algorithm covered the initial case, where no selected The previous algorithm covered the initial case, where no selected
list had previously been created. It also applied only to the list had previously been created. It also applied only to the media
media sender. When a previously-created selected list is sender. When a previously-created selected list is available at
available at either the media sender or media receiver, two other either the media sender or media receiver, two other cases can be
cases can be considered: considered:
o when a TMMBR tuple not currently in the selected list is a o when a TMMBR tuple not currently in the selected list is a
candidate for addition; candidate for addition;
o when the values change in a TMMBR tuple currently in the o when the values change in a TMMBR tuple currently in the
selected list. selected list.
At the media receiver these cases correspond respectively to those At the media receiver these cases correspond respectively to those
of the non-owner and owner of a tuple in the TMMBN-reported of the non-owner and owner of a tuple in the TMMBN-reported bounding
bounding set. set.
In either case, the process of updating the selected list to take In either case, the process of updating the selected list to take
account of the new/changed tuple can use the basic algorithm account of the new/changed tuple can use the basic algorithm
described above, with the modification that the initial candidate described above, with the modification that the initial candidate
set consists only of the existing selected list and the new or set consists only of the existing selected list and the new or
changed tuple. Some further optimization is possible (beyond changed tuple. Some further optimization is possible (beyond
starting with a reduced candidate set) by taking advantage of the starting with a reduced candidate set) by taking advantage of the
following observations. following observations.
The first observation is that if the new/changed candidate becomes The first observation is that if the new/changed candidate becomes
part of the new selected list, the result may be to cause zero or part of the new selected list, the result may be to cause zero or
more other tuples to be dropped from the list. However, if more more other tuples to be dropped from the list. However, if more
than one other tuple is dropped, the dropped tuples will be than one other tuple is dropped, the dropped tuples will be
consecutive. This can be confirmed geometrically by visualizing a consecutive. This can be confirmed geometrically by visualizing a
new line that cuts off a series of segments from the previously- new line that cuts off a series of segments from the previously-
existing bounding polygon. The cut-off segments are connected one existing bounding polygon. The cut-off segments are connected one
to the next, the geometric equivalent of consecutive tuples in a to the next, the geometric equivalent of consecutive tuples in a
list ordered by overhead value. Beyond the dropped set in either list ordered by overhead value. Beyond the dropped set in either
direction all of the tuples that were in the earlier selected list direction all of the tuples that were in the earlier selected list
will be in the updated one. The second observation is that, will be in the updated one. The second observation is that, leaving
leaving aside the new candidate, the order of tuples remaining in aside the new candidate, the order of tuples remaining in the
the updated selected list is unchanged because their overhead updated selected list is unchanged because their overhead values
values have not changed. have not changed.
The consequence of these two observations is that, once the The consequence of these two observations is that, once the
placement of the new candidate and the extent of the dropped set placement of the new candidate and the extent of the dropped set of
of tuples (if any) has been determined, the remaining tuples can tuples (if any) has been determined, the remaining tuples can be
be copied directly from the candidate list into the selected list, copied directly from the candidate list into the selected list,
preserving their order. This conclusion suggests the following preserving their order. This conclusion suggests the following
modified algorithm: modified algorithm:
o Run steps 1-4 of the basic algorithm. o Run steps 1-4 of the basic algorithm.
o If the new candidate has survived steps 2 and 4 and has o If the new candidate has survived steps 2 and 4 and has become
become the new first member of the selected list, run steps the new first member of the selected list, run steps 5-9 on
5-9 on subsequent candidates until another candidate is subsequent candidates until another candidate is added to the
added to the selected list. Then move all remaining selected list. Then move all remaining candidates to the
candidates to the selected list, preserving their order. selected list, preserving their order.
o If the new candidate has survived steps 2 and 4 and has not o If the new candidate has survived steps 2 and 4 and has not
become the new first member of the selected list, start by become the new first member of the selected list, start by
moving all tuples in the candidate list with lower overhead moving all tuples in the candidate list with lower overhead
values than that of the new candidate to the selected list, values than that of the new candidate to the selected list,
preserving their order. Run steps 5 through 9 for the new preserving their order. Run steps 5 through 9 for the new
candidate, with the modification that the intersection candidate, with the modification that the intersection values
values and maximum packet rates for the tuples on the and maximum packet rates for the tuples on the selected list
selected list have to be calculated on the fly because they have to be calculated on the fly because they were not
were not previously stored. Continue processing only until previously stored. Continue processing only until a
a subsequent tuple has been added to the selected list, then subsequent tuple has been added to the selected list, then
move all remaining candidates to the selected list, move all remaining candidates to the selected list, preserving
preserving their order. their order.
Note that the new candidate could be added to the selected Note that the new candidate could be added to the selected
list only to be dropped again when the next tuple is list only to be dropped again when the next tuple is
processed. It can easily be seen that in this case the new processed. It can easily be seen that in this case the new
candidate does not displace any of the earlier tuples in the candidate does not displace any of the earlier tuples in the
selected list. The limitations of ASCII art make this selected list. The limitations of ASCII art make this
difficult to show in a figure. Line cc..c in Figure 1 would difficult to show in a figure. Line cc..c in Figure 1 would
be an example if it had a steeper slope (tuple C had a be an example if it had a steeper slope (tuple C had a higher
higher overhead value), but still intersected line aa..a overhead value), but still intersected line aa..a beyond where
beyond where line aa..a intersects line bb..b. line aa..a intersects line bb..b.
The algorithm just described is approximate, because it does not The algorithm just described is approximate, because it does not
take account of tuples outside the selected list. To see how such take account of tuples outside the selected list. To see how such
tuples can become relevant, consider Figure 1 and suppose that the tuples can become relevant, consider Figure 1 and suppose that the
maximum total media bit rate in tuple A increases to the point maximum total media bit rate in tuple A increases to the point that
that line aa..a moves outside line cc..c. Tuple A will remain in line aa..a moves outside line cc..c. Tuple A will remain in the
the bounding set calculated by the media sender. However, once it bounding set calculated by the media sender. However, once it
issues a new TMMBN, media receiver C will apply the algorithm and issues a new TMMBN, media receiver C will apply the algorithm and
discover that its tuple C should now enter the bounding set. It discover that its tuple C should now enter the bounding set. It
will issue a TMMBR request to the media sender, which will repeat will issue a TMMBR request to the media sender, which will repeat
its calculation and come to the appropriate conclusion. its calculation and come to the appropriate conclusion.
The rules of section 4.2 require that the media sender refrain The rules of section 4.2 require that the media sender refrain from
from raising its sending rate until media receivers have had a raising its sending rate until media receivers have had a chance to
chance to respond to the TMMBN. In the example just given, this respond to the TMMBN. In the example just given, this delay ensures
delay ensures that the relaxation of tuple A does not actually that the relaxation of tuple A does not actually result in an
result in an attempt to send media at a rate exceeding the attempt to send media at a rate exceeding the capacity at C.
capacity at C.
3.5.4.3. Use of TMMBR in a Mixer Based Multipoint Operation 3.5.4.3. Use of TMMBR in a Mixer Based Multipoint Operation
Assume a small mixer-based multiparty conference is ongoing, as Assume a small mixer-based multiparty conference is ongoing, as
depicted in Topo-Mixer of [Topologies]. All participants have depicted in Topo-Mixer of [Topologies]. All participants have
negotiated a common maximum bit rate that this session can use. negotiated a common maximum bit rate that this session can use. The
The conference operates over a number of unicast paths between the conference operates over a number of unicast paths between the
participants and the mixer. The congestion situation on each of participants and the mixer. The congestion situation on each of
these paths can be monitored by the participant in question and by these paths can be monitored by the participant in question and by
the mixer, utilizing, for example, RTCP receiver reports (RR) or the mixer, utilizing, for example, RTCP receiver reports (RR) or the
the transport protocol, e.g. DCCP [RFC4340]. However, any given transport protocol, e.g. DCCP [RFC4340]. However, any given
participant has no knowledge of the congestion situation of the participant has no knowledge of the congestion situation of the
connections to the other participants. Worse, without mechanisms connections to the other participants. Worse, without mechanisms
similar to the ones discussed in this draft, the mixer (which is similar to the ones discussed in this draft, the mixer (which is
aware of the congestion situation on all connections it manages) aware of the congestion situation on all connections it manages) has
has no standardized means to inform media senders to slow down, no standardized means to inform media senders to slow down, short of
short of forging its own receiver reports (which is undesirable). forging its own receiver reports (which is undesirable). In
In principle, a mixer confronted with such a situation is obliged principle, a mixer confronted with such a situation is obliged to
to thin or transcode streams intended for connections that thin or transcode streams intended for connections that detected
detected congestion. congestion.
In practice, media-aware stream thinning is unfortunately a very In practice, unfortunately, media-aware streaming thinning is a very
difficult and cumbersome operation and adds undesirable delay. If difficult and cumbersome operation and adds undesirable delay. If
media-unaware, it leads very quickly to unacceptable reproduced media-unaware, it leads very quickly to unacceptable reproduced
media quality. Hence, a means to slow down senders even in the media quality. Hence, a means to slow down senders even in the
absence of congestion on their connections to the mixer is absence of congestion on their connections to the mixer is
desirable. desirable.
To allow the mixer to throttle traffic on the individual links, To allow the mixer to throttle traffic on the individual links,
without performing transcoding, there is a need for a mechanism without performing transcoding, there is a need for a mechanism that
that enables the mixer to ask a participant's media encoders to enables the mixer to ask a participant's media encoders to limit the
limit the media stream bit rate they are currently generating. media stream bit rate they are currently generating. TMMBR provides
TMMBR provides the required mechanism. When the mixer detects the required mechanism. When the mixer detects congestion between
congestion between itself and a given participant, it executes the itself and a given participant, it executes the following procedure:
following procedure:
1. It starts thinning the media traffic to the congested 1. It starts thinning the media traffic to the congested participant
participant to the supported bit rate. to the supported bit rate.
2. It uses TMMBR to request the media sender(s) to reduce the 2. It uses TMMBR to request the media sender(s) to reduce the total
total media bit rate sent by them to the mixer, to a value that media bit rate sent by them to the mixer, to a value that is in
is in compliance with congestion control principles for the compliance with congestion control principles for the slowest
slowest link. Slow refers here to the available bandwidth / link. Slow refers here to the available bandwidth / bit rate /
bit rate / capacity and packet rate after congestion control. capacity and packet rate after congestion control.
3. As soon as the bit rate has been reduced by the sending part, 3. As soon as the bit rate has been reduced by the sending part, the
the mixer stops stream thinning implicitly, because there is no mixer stops stream thinning implicitly, because there is no need
need for it once the stream is in compliance with congestion for it once the stream is in compliance with congestion control.
control.
This use of stream thinning as an immediate reaction tool followed This use of stream thinning as an immediate reaction tool followed
up by a quick control mechanism appears to be a reasonable up by a quick control mechanism appears to be a reasonable
compromise between media quality and the need to combat compromise between media quality and the need to combat congestion.
congestion.
3.5.4.4. Use of TMMBR in Point-to-Multipoint Using Multicast or 3.5.4.4. Use of TMMBR in Point-to-Multipoint Using Multicast or
Translators Translators
In these topologies, corresponding to Topo-Multicast or Topo- In these topologies, corresponding to Topo-Multicast or Topo-
Translator, RTCP RRs are transmitted globally. This allows all Translator, RTCP RRs are transmitted globally. This allows all
participants to detect transmission problems such as congestion, participants to detect transmission problems such as congestion, on
on a medium timescale. As all media senders are aware of the a medium timescale. As all media senders are aware of the
congestion situation of all media receivers, the rationale for the congestion situation of all media receivers, the rationale for the
use of TMMBR in the previous section does not apply. However, use of TMMBR in the previous section does not apply. However, even
even in this case the congestion control response can be improved in this case the congestion control response can be improved when
when the unicast links are using congestion controlled transport the unicast links are using congestion controlled transport
protocols (such as TCP or DCCP). A peer may also report local protocols (such as TCP or DCCP). A peer may also report local
limitations to the media sender. limitations to the media sender.
3.5.4.5. Use of TMMBR in Point-to-point operation 3.5.4.5. Use of TMMBR in Point-to-point operation
In use case 7 it is possible to use TMMBR to improve the In use case 7 it is possible to use TMMBR to improve the performance
performance when the known upper limit of the bit rate changes. when the known upper limit of the bit rate changes. In this use
In this use case the signaling protocol has established an upper case the signaling protocol has established an upper limit for the
limit for the session and total media bit rates. However, at the session and total media bit rates. However, at the time of
time of transport link bit rate reduction, a receiver can avoid transport link bit rate reduction, a receiver can avoid serious
serious congestion by sending a TMMBR to the sending side. Thus congestion by sending a TMMBR to the sending side. Thus, TMMBR is
TMMBR is useful for putting restrictions on the application and useful for putting restrictions on the application and thus placing
thus placing the congestion control mechanism in the right the congestion control mechanism in the right ballpark. However,
ballpark. However TMMBR is usually unable to provide the TMMBR is usually unable to provide the continuously quick feedback
continuously quick feedback loop required for real congestion loop required for real congestion control. Nor do its semantics
control. Nor do its semantics match those of congestion control match those of congestion control given its different purpose. For
given its different purpose. For these reasons TMMBR SHALL NOT be these reasons TMMBR SHALL NOT be used as a substitute for congestion
used as a substitute for congestion control. control.
3.5.4.6. Reliability 3.5.4.6. Reliability
The reaction of a media sender to the reception of a TMMBR message The reaction of a media sender to the reception of a TMMBR message
is not immediately identifiable through inspection of the media is not immediately identifiable through inspection of the media
stream. Therefore, a more explicit mechanism is needed to avoid stream. Therefore, a more explicit mechanism is needed to avoid
unnecessary re-sending of TMMBR messages. Using a statistically unnecessary re-sending of TMMBR messages. Using a statistically
based retransmission scheme would only provide statistical based retransmission scheme would only provide statistical
guarantees of the request being received. It would also not avoid guarantees of the request being received. It would also not avoid
the retransmission of already received messages. In addition, it the retransmission of already received messages. In addition, it
would not allow for easy suppression of other participants' would not allow for easy suppression of other participants'
requests. For these reasons, a mechanism based on explicit requests. For these reasons, a mechanism based on explicit
notification is used. notification is used.
Upon the reception of a request a media sender sends a TMMBN Upon the reception of a request a media sender sends a TMMBN
notification containing the current bounding set, and indicating notification containing the current bounding set, and indicating
which session participants own that limit. In multicast which session participants own that limit. In multicast scenarios,
scenarios, that allows all other participants to suppress any that allows all other participants to suppress any request they may
request they may have, if their limitations are less strict than have, if their limitations are less strict than the current ones
the current ones (i.e. define lines lying outside the feasible (i.e. define lines lying outside the feasible region as defined in
region as defined in section 2.2). Keeping and notifying only the section 2.2). Keeping and notifying only the bounding set of tuples
bounding set of tuples allows for small message sizes and media allows for small message sizes and media sender states. A media
sender states. A media sender only keeps state for the SSRCs of sender only keeps state for the SSRCs of the current owners of the
the current owners of the bounding set of tuples; all other bounding set of tuples; all other requests and their sources are not
requests and their sources are not saved. Once the bounding set saved. Once the bounding set has been established, new TMMBR
has been established, new TMMBR messages should be generated only messages should be generated only by owners of the bounding tuples
by owners of the bounding tuples and by other entities that and by other entities that determine (by applying the algorithm of
determine (by applying the algorithm of section 3.5.4.2 or its section 3.5.4.2 or its equivalent) that their limitations should now
equivalent) that their limitations should now be part of the be part of the bounding set.
bounding set.
4. RTCP Receiver Report Extensions 4. RTCP Receiver Report Extensions
This memo specifies six new feedback messages. The Full Intra This memo specifies six new feedback messages. The Full Intra
Request (FIR), Temporal-Spatial Trade-off Request (TSTR), Request (FIR), Temporal-Spatial Trade-off Request (TSTR), Temporal-
Temporal-Spatial Trade-off Notification (TSTN), and Video Back Spatial Trade-off Notification (TSTN), and Video Back Channel
Channel Message (VBCM) are "Payload Specific Feedback Messages" as Message (VBCM) are "Payload Specific Feedback Messages" as defined
defined in Section 6.3 of AVPF [RFC4585]. The Temporary Maximum in Section 6.3 of AVPF [RFC4585]. The Temporary Maximum Media
Media Stream Bit Rate Request (TMMBR) and Temporary Maximum Media Stream Bit Rate Request (TMMBR) and Temporary Maximum Media Stream
Stream Bit Rate Notification (TMMBN) are "Transport Layer Feedback Bit Rate Notification (TMMBN) are "Transport Layer Feedback
Messages" as defined in Section 6.2 of AVPF. Messages" as defined in Section 6.2 of AVPF.
The new feedback messages are defined in the following The new feedback messages are defined in the following subsections,
subsections, following a similar structure to that in sections 6.2 following a similar structure to that in sections 6.2 and 6.3 of the
and 6.3 of the AVPF specification [RFC4585]. AVPF specification [RFC4585].
4.1. Design Principles of the Extension Mechanism 4.1. Design Principles of the Extension Mechanism
RTCP was originally introduced as a channel to convey presence, RTCP was originally introduced as a channel to convey presence,
reception quality statistics and hints on the desired media reception quality statistics and hints on the desired media coding.
coding. A limited set of media control mechanisms were introduced A limited set of media control mechanisms were introduced in early
in early RTP payload formats for video formats, for example in RFC RTP payload formats for video formats, for example in RFC 2032
2032 [RFC2032]. However, this specification, for the first time, [RFC2032]. However, this specification, for the first time,
suggests a two-way handshake for some of its messages. There is suggests a two-way handshake for some of its messages. There is
danger that this introduction could be misunderstood as a danger that this introduction could be misunderstood as a precedent
precedent for the use of RTCP as an RTP session control protocol. for the use of RTCP as an RTP session control protocol. To prevent
To prevent such a misunderstanding, this subsection attempts to such a misunderstanding, this subsection attempts to clarify the
clarify the scope of the extensions specified in this memo, and scope of the extensions specified in this memo, and strongly
strongly suggests that future extensions follow the rationale suggests that future extensions follow the rationale spelled out
spelled out here, or compellingly explain why they divert from the here, or compellingly explain why they divert from the rationale.
rationale.
In this memo, and in AVPF [RFC4585], only such messages have been In this memo, and in AVPF [RFC4585], only such messages have been
included as: included as:
a) have comparatively strict real-time constraints, which prevent a) have comparatively strict real-time constraints, which prevent
the use of mechanisms such as a SIP re-invite in most the use of mechanisms such as a SIP re-invite in most application
application scenarios. The real-time constraints are explained scenarios. The real-time constraints are explained separately
separately for each message where necessary. for each message where necessary.
b) are multicast-safe in that the reaction to potentially b) are multicast-safe in that the reaction to potentially
contradicting feedback messages is specified, as necessary for contradicting feedback messages is specified, as necessary for
each message; and each message; and
c) are directly related to activities of a certain media codec, c) are directly related to activities of a certain media codec,
class of media codecs (e.g. video codecs), or a given RTP class of media codecs (e.g. video codecs), or a given RTP packet
packet stream. stream.
In this memo, a two-way handshake is introduced only for messages In this memo, a two-way handshake is introduced only for messages
for which: for which:
a) a notification or acknowledgement is required due to their a) a notification or acknowledgement is required due to their
nature. An analysis to determine whether this requirement nature. An analysis to determine whether this requirement exists
exists has been performed separately for each message. has been performed separately for each message.
b) the notification or acknowledgement cannot be easily derived b) the notification or acknowledgement cannot be easily derived from
from the media bit stream. the media bit stream.
All messages in AVPF [RFC4585] and in this memo present their All messages in AVPF [RFC4585] and in this memo present their
contents in a simple, fixed binary format. This accommodates contents in a simple, fixed binary format. This accommodates media
media receivers which have not implemented higher control protocol receivers which have not implemented higher control protocol
functionalities (SDP, XML parsers and such) in their media path. functionalities (SDP, XML parsers and such) in their media path.
Messages that do not conform to the design principles just Messages that do not conform to the design principles just described
described are not an appropriate use of RTCP or of the Codec are not an appropriate use of RTCP or of the Codec Control Framework
Control Framework defined in this document. defined in this document.
4.2. Transport Layer Feedback Messages 4.2. Transport Layer Feedback Messages
As specified in section 6.1 of RFC 4585 [RFC4585], Transport Layer As specified in section 6.1 of RFC 4585 [RFC4585], Transport Layer
Feedback messages are identified by the RTCP packet type value Feedback messages are identified by the RTCP packet type value RTPFB
RTPFB (205). (205).
In AVPF, one message of this category had been defined. This memo In AVPF, one message of this category had been defined. This memo
specifies two more such messages. They are identified by means of specifies two more such messages. They are identified by means of
the FMT parameter as follows: the FMT parameter as follows:
Assigned in AVPF [RFC4585]: Assigned in AVPF [RFC4585]:
1: Generic NACK 1: Generic NACK
31: reserved for future expansion of the identifier number 31: reserved for future expansion of the identifier number
space space
Assigned in this memo: Assigned in this memo:
2: reserved (see note below) 2: reserved (see note below)
3: Temporary Maximum Media Stream Bit Rate Request (TMMBR) 3: Temporary Maximum Media Stream Bit Rate Request (TMMBR)
4: Temporary Maximum Media Stream Bit Rate Notification (TMMBN) 4: Temporary Maximum Media Stream Bit Rate Notification (TMMBN)
Note: early drafts of AVPF [RFC4585] reserved FMT=2 for a Note: early drafts of AVPF [RFC4585] reserved FMT=2 for a
code point that has later been removed. It has been code point that has later been removed. It has been pointed
pointed out that there may be implementations in the field out that there may be implementations in the field using this
using this value in accordance with the expired draft. As value in accordance with the expired draft. As there is
there is sufficient numbering space available, we mark sufficient numbering space available, we mark FMT=2 as
FMT=2 as reserved so to avoid possible interoperability reserved so to avoid possible interoperability problems with
problems with any such early implementations. any such early implementations.
Available for assignment: Available for assignment:
0: unassigned 0: unassigned
5-30: unassigned 5-30: unassigned
The following subsection defines the formats of the FCI entries The following subsection defines the formats of the FCI entries for
for the TMMBR and TMMBN messages respectively and specify the the TMMBR and TMMBN messages respectively and specify the associated
associated behaviour at the media sender and receiver. behaviour at the media sender and receiver.
4.2.1. Temporary Maximum Media Stream Bit Rate Request (TMMBR) 4.2.1. Temporary Maximum Media Stream Bit Rate Request (TMMBR)
The Temporary Maximum Media Stream Bit Rate Request is identified by
RTCP packet type value PT=RTPFB and FMT=3.
The FCI field of a Temporary Maximum Media Stream Bit-Rate Request The FCI field of a Temporary Maximum Media Stream Bit-Rate Request
(TMMBR) message SHALL contain one or more FCI entries. (TMMBR) message SHALL contain one or more FCI entries.
4.2.1.1. Message Format 4.2.1.1. Message Format
The Feedback Control Information (FCI) consists of one or more The Feedback Control Information (FCI) consists of one or more TMMBR
TMMBR FCI entries with the following syntax: FCI entries with the following syntax:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead| | MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 - Syntax of an FCI entry in the TMMBR message Figure 2 - Syntax of an FCI entry in the TMMBR message
SSRC (32 bits): The SSRC value of the media sender that is SSRC (32 bits): The SSRC value of the media sender that is
requested to obey the new maximum bit rate. requested to obey the new maximum bit rate.
MxTBR Exp (6 bits): The exponential scaling of the mantissa for MxTBR Exp (6 bits): The exponential scaling of the mantissa for
the maximum total media bit rate value. The value is an the maximum total media bit rate value. The value is an
unsigned integer [0..63]. unsigned integer [0..63].
MxTBR Mantissa (17 bits): The mantissa of the maximum total MxTBR Mantissa (17 bits): The mantissa of the maximum total media
media bit rate value as an unsigned integer. bit rate value as an unsigned integer.
Measured Overhead (9 bits): The measured average packet overhead Measured Overhead (9 bits): The measured average packet overhead
value in bytes. The measurement SHALL be done according value in bytes. The measurement SHALL be done according
to description in section 4.2.1.2. The value is an to the description in section 4.2.1.2. The value is an
unsigned integer [0..512]. unsigned integer [0..512].
The maximum total media bit rate (MxTBR) value in bits per second The maximum total media bit rate (MxTBR) value in bits per second is
is calculated from the MxTBR exponent (exp) and mantissa in the calculated from the MxTBR exponent (exp) and mantissa in the
following way: following way:
MxTBR = mantissa * 2^exp MxTBR = mantissa * 2^exp
This allows for 17 bits of resolution in the range 0 to This allows for 17 bits of resolution in the range 0 to 131072*2^63
131072*2^63 (approximately 1.2*10^24). (approximately 1.2*10^24).
The length of the TMMBR feedback message SHALL be set to 2+2*N The length of the TMMBR feedback message SHALL be set to 2+2*N where
where N is the number of TMMBR FCI entries. N is the number of TMMBR FCI entries.
4.2.1.2. Semantics 4.2.1.2. Semantics
Behaviour at the Media Receiver (Sender of the TMMBR) Behaviour at the Media Receiver (Sender of the TMMBR)
TMMBR is used to indicate a transport related limitation at the TMMBR is used to indicate a transport related limitation at the
reporting entity acting as a media receiver. TMMBR has the form reporting entity acting as a media receiver. TMMBR has the form of
of a tuple containing two components. The first value is the a tuple containing two components. The first value is the highest
highest bit rate per sender of a media stream, available at a bit rate per sender of a media stream, available at a receiver-
receiver-chosen protocol layer, which the receiver currently chosen protocol layer, which the receiver currently supports in this
supports in this RTP session. The second value is the measured RTP session. The second value is the measured header overhead in
header overhead in bytes as defined in section 2.2 and measured at bytes as defined in section 2.2 and measured at the chosen protocol
the chosen protocol layer in the packets received for the stream. layer in the packets received for the stream. The measurement of
The measurement of the overhead is a running average that is the overhead is a running average that is updated for each packet
updated for each packet received for this particular media source received for this particular media source (SSRC), using the
(SSRC), using the following formula: following formula:
avg_OH (new) = 15/16*avg_OH (old) + 1/16*pckt_OH, avg_OH (new) = 15/16*avg_OH (old) + 1/16*pckt_OH,
where avg_OH is the running (exponentially smoothed) average and where avg_OH is the running (exponentially smoothed) average and
pckt_OH is the overhead observed in the latest packet. pckt_OH is the overhead observed in the latest packet.
If a maximum bit rate has been negotiated through signaling, the If a maximum bit rate has been negotiated through signaling, the
maximum total media bit rate that the receiver reports in a TMMBR maximum total media bit rate that the receiver reports in a TMMBR
message MUST NOT exceed the negotiated value converted to a common message MUST NOT exceed the negotiated value converted to a common
basis (i.e. with overheads adjusted to bring it to the same basis (i.e. with overheads adjusted to bring it to the same
reference protocol layer). reference protocol layer).
Within the common packet header for feedback messages (as defined Within the common packet header for feedback messages (as defined in
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
field indicates the source of the request, and the "SSRC of media indicates the source of the request, and the "SSRC of media source"
source" is not used and SHALL be set to 0. Within a particular is not used and SHALL be set to 0. Within a particular TMMBR FCI
TMMBR FCI entry, the "SSRC of media sender" in the FCI field entry, the "SSRC of media sender" in the FCI field denotes the media
denotes the media sender the tuple applies to. This is useful in sender the tuple applies to. This is useful in the multicast or
the multicast or translator topologies where the reporting entity translator topologies where the reporting entity may address all of
may address all of the media senders in a single TMMBR message the media senders in a single TMMBR message using multiple FCI
using multiple FCI entries. entries.
The media receiver SHALL save the contents of the latest TMMBN The media receiver SHALL save the contents of the latest TMMBN
message received from each media sender. message received from each media sender.
The media receiver MAY send a TMMBR FCI entry to a particular The media receiver MAY send a TMMBR FCI entry to a particular media
media sender under the following circumstances: sender under the following circumstances:
o before any TMMBN message has been received from that media o before any TMMBN message has been received from that media
sender; sender;
o when the media receiver has been identified as the source of o when the media receiver has been identified as the source of a
a bounding tuple within the latest TMMBN message received bounding tuple within the latest TMMBN message received from
from that media sender, and the value of the maximum total that media sender, and the value of the maximum total media
media bit rate or the overhead relating to that media sender bit rate or the overhead relating to that media sender has
has changed; changed;
o when the media receiver has not been identified as the o when the media receiver has not been identified as the source
source of a bounding tuple within the latest TMMBN message of a bounding tuple within the latest TMMBN message received
received from that media sender, and, after the media from that media sender, and, after the media receiver applies
receiver applies the incremental algorithm from section the incremental algorithm from section 3.5.4.2 or a stricter
3.5.4.2 or a stricter equivalent, the media receiver's tuple equivalent, the media receiver's tuple relating to that media
relating to that media sender is determined to belong to the sender is determined to belong to the bounding set.
bounding set.
A TMMBR FCI entry MAY be repeated in subsequent TMMBR messages if A TMMBR FCI entry MAY be repeated in subsequent TMMBR messages if no
no Temporary Maximum Media Stream Bit-Rate Notification (TMMBN) Temporary Maximum Media Stream Bit-Rate Notification (TMMBN) FCI has
FCI has been received from the media sender at the time of been received from the media sender at the time of transmission of
transmission of the next RTCP packet. The bit rate value of a the next RTCP packet. The bit rate value of a TMMBR FCI entry MAY
TMMBR FCI entry MAY be changed from one TMMBR message to the next. be changed from one TMMBR message to the next. The overhead
The overhead measurement SHALL be updated to the current value of measurement SHALL be updated to the current value of avg_OH each
avg_OH each time the entry is sent. time the entry is sent.
If the value set by a TMMBR message is expected to be permanent, If the value set by a TMMBR message is expected to be permanent, the
the TMMBR setting party SHOULD renegotiate the session parameters TMMBR setting party SHOULD renegotiate the session parameters to
to reflect that using session setup signaling, e.g. a SIP re- reflect that using session setup signaling, e.g. a SIP re-invite.
invite.
Behaviour at the Media Sender (Receiver of the TMMBR) Behaviour at the Media Sender (Receiver of the TMMBR)
When it receives a TMMBR message containing an FCI entry relating When it receives a TMMBR message containing an FCI entry relating to
to it, the media sender SHALL use an initial or incremental it, the media sender SHALL use an initial or incremental algorithm
algorithm as applicable to determine the bounding set of tuples as applicable to determine the bounding set of tuples based on the
based on the new information. The algorithm used SHALL be at new information. The algorithm used SHALL be at least as strict as
least as strict as the corresponding algorithm defined in section the corresponding algorithm defined in section 3.5.4.2. The media
3.5.4.2. The media sender MAY accumulate TMMBR requests over a sender MAY accumulate TMMBR requests over a small interval (relative
small interval (relative to the RTCP sending interval) before to the RTCP sending interval) before making this calculation.
making this calculation.
Once it has determined the bounding set of tuples, the media Once it has determined the bounding set of tuples, the media sender
sender MAY use any combination of packet rate and net media bit MAY use any combination of packet rate and net media bit rate within
rate within the feasible region that these tuples describe to the feasible region that these tuples describe to produce a lower
produce a lower total media stream bit rate, as it may need to total media stream bit rate, as it may need to address a congestion
address a congestion situation or other limiting factors. See situation or other limiting factors. See section 5 (congestion
section 5. (congestion control) for more discussion. control) for more discussion.
If the media sender concludes that it can increase the maximum If the media sender concludes that it can increase the maximum total
total media bit rate value, it SHALL wait before actually doing media bit rate value, it SHALL wait before actually doing so, for a
so, for a period long enough to allow a media receiver to respond period long enough to allow a media receiver to respond to the TMMBN
to the TMMBN if it determines that its tuple belongs in the if it determines that its tuple belongs in the bounding set. This
bounding set. This delay period is estimated by the formula: delay period is estimated by the formula:
2 * RTT + T_Dither_Max, 2 * RTT + T_Dither_Max,
where RTT is the longest round trip time known to the media sender where RTT is the longest round trip time known to the media sender
and T_Dither_Max is defined in section 3.4 of [RFC4585]. and T_Dither_Max is defined in section 3.4 of [RFC4585].
A TMMBN message SHALL be sent by the media sender at the earliest A TMMBN message SHALL be sent by the media sender at the earliest
possible point in time, in response to any TMMBR messages received possible point in time, in response to any TMMBR messages received
since the last sending of TMMBN. The TMMBN message indicates the since the last sending of TMMBN. The TMMBN message indicates the
calculated set of bounding tuples and the owners of those tuples calculated set of bounding tuples and the owners of those tuples at
at the time of the transmission of the message. the time of the transmission of the message.
An SSRC may time out according to the default rules for RTP An SSRC may time out according to the default rules for RTP session
session participants, i.e. the media sender has not received any participants, i.e. the media sender has not received any RTP or RTCP
RTP or RTCP packets from the owner for the last five regular packets from the owner for the last five regular reporting
reporting intervals. An SSRC may also explicitly leave the intervals. An SSRC may also explicitly leave the session, with the
session, with the participant indicating this through the participant indicating this through the transmission of an RTCP BYE
transmission of an RTCP BYE packet or using an external signaling packet or using an external signaling channel. If the media sender
channel. If the media sender determines that the owner of a tuple determines that the owner of a tuple in the bounding set has left
in the bounding set has left the session, the media sender shall the session, the media sender shall transmit a new TMMBN containing
transmit a new TMMBN containing the previously-determined set of the previously-determined set of bounding tuples but with the tuple
bounding tuples but with the tuple belonging to the departed owner belonging to the departed owner removed.
removed.
A media sender MAY proactively initiate the equivalent to a TMMBR A media sender MAY proactively initiate the equivalent to a TMMBR
message to itself, when it is aware that its transmission path is message to itself, when it is aware that its transmission path is
more restrictive than the current limitations. As a result, a more restrictive than the current limitations. As a result, a TMMBN
TMMBN indicating the media source itself as the owner of a tuple indicating the media source itself as the owner of a tuple is being
is being sent, thereby avoiding unnecessary TMMBR messages from sent, thereby avoiding unnecessary TMMBR messages from other
other participants. However, like any other participant, when the participants. However, like any other participant, when the media
media sender becomes aware of changed limitations, it is required sender becomes aware of changed limitations, it is required to
to change the tuple, and to send a corresponding TMMBN. change the tuple, and to send a corresponding TMMBN.
Discussion Discussion
Due to the unreliable nature of transport of TMMBR and TMMBN, the Due to the unreliable nature of transport of TMMBR and TMMBN, the
above rules may lead to the sending of TMMBR messages which appear above rules may lead to the sending of TMMBR messages which appear
to disobey those rules. Furthermore, in multicast scenarios it to disobey those rules. Furthermore, in multicast scenarios it can
can happen that more than one "non-owning" session participant may happen that more than one "non-owning" session participant may
determine, rightly or wrongly, that its tuple belongs in the determine, rightly or wrongly, that its tuple belongs in the
bounding set. This is not critical for a number of reasons: bounding set. This is not critical for a number of reasons:
a) If a TMMBR message is lost in transmission, either the media a) If a TMMBR message is lost in transmission, either the media
sender sends a new TMMBN message in response to some other sender sends a new TMMBN message in response to some other media
media receiver or it does not send a new TMMBN message at all. receiver or it does not send a new TMMBN message at all. In the
In the first case, the media receiver applies the incremental first case, the media receiver applies the incremental algorithm
algorithm and, if it determines that its tuple should be part and, if it determines that its tuple should be part of the
of the bounding set, sends out another TMMBR. In the second bounding set, sends out another TMMBR. In the second case, it
case, it repeats the sending of a TMMBR unconditionally. repeats the sending of a TMMBR unconditionally. Either way, the
Either way, the media sender eventually gets the information it media sender eventually gets the information it needs.
needs.
b) Similarly, if a TMMBN message gets lost, the media receiver b) Similarly, if a TMMBN message gets lost, the media receiver that
that has sent the corresponding TMMBR request does not receive has sent the corresponding TMMBR request does not receive the
the notification and is expected to re-send the request and notification and is expected to re-send the request and trigger
trigger the transmission of another TMMBN. the transmission of another TMMBN.
c) If multiple competing TMMBR messages are sent by different c) If multiple competing TMMBR messages are sent by different
session participants, then the algorithm can be applied taking session participants, then the algorithm can be applied taking
all of these messages into account, and the resulting TMMBN all of these messages into account, and the resulting TMMBN
provides the participants with an updated view of how their provides the participants with an updated view of how their
tuples compare with the bounded set. tuples compare with the bounded set.
d) If more than one session participant happens to send TMMBR d) If more than one session participant happens to send TMMBR
messages at the same time and with the same tuple component messages at the same time and with the same tuple component
values, it does not matter which if either tuple is taken into values, it does not matter which if either tuple is taken into
the bounding set. The losing session participant will the bounding set. The losing session participant will determine
determine after applying the algorithm that its tuple does not after applying the algorithm that its tuple does not enter the
enter the bounding set, and will therefore stop sending its bounding set, and will therefore stop sending its TMMBR request.
TMMBR request.
It is important to consider the security risks involved with faked It is important to consider the security risks involved with faked
TMMBRs. See the security considerations in Section 6. TMMBRs. See the security considerations in Section 6.
As indicated already, the feedback messages may be used in both As indicated already, the feedback messages may be used in both
multicast and unicast sessions in any of the specified topologies. multicast and unicast sessions in any of the specified topologies.
However, for sessions with a large number of participants, using However, for sessions with a large number of participants, using the
the lowest common denominator, as required by this mechanism, may lowest common denominator, as required by this mechanism, may not be
not be the most suitable course of action. Large sessions may the most suitable course of action. Large sessions may need to
need to consider other ways to adapt the bit rate to participants' consider other ways to adapt the bit rate to participants'
capabilities, such as partitioning the session into different capabilities, such as partitioning the session into different
quality tiers, or using some other method of achieving bit rate quality tiers, or using some other method of achieving bit rate
scalability. scalability.
4.2.1.3. Timing Rules 4.2.1.3. Timing Rules
The first transmission of the TMMBR request message MAY use early The first transmission of the TMMBR request message MAY use early or
or immediate feedback in cases when timeliness is desirable. Any immediate feedback in cases when timeliness is desirable. Any
repetition of a request message SHOULD use regular RTCP mode for repetition of a request message SHOULD use regular RTCP mode for its
its transmission timing. transmission timing.
4.2.1.4. Handling in Translator and Mixers 4.2.1.4. Handling in Translator and Mixers
Media translators and mixers will need to receive and respond to Media translators and mixers will need to receive and respond to
TMMBR messages as they are part of the chain that provides a TMMBR messages as they are part of the chain that provides a certain
certain media stream to the receiver. The mixer or translator may media stream to the receiver. The mixer or translator may act
act locally on the TMMBR request and thus generate a TMMBN to locally on the TMMBR request and thus generate a TMMBN to indicate
indicate that it has done so. Alternatively, in the case of a that it has done so. Alternatively, in the case of a media
media translator it can forward the request, or in the case of a translator it can forward the request, or in the case of a mixer
mixer generate one of its own and pass it forward. In the latter generate one of its own and pass it forward. In the latter case,
case, the mixer will need to send a TMMBN back to the original the mixer will need to send a TMMBN back to the original requestor
requestor to indicate that it is handling the request. to indicate that it is handling the request.
4.2.2. Temporary Maximum Media Stream Bit Rate Notification (TMMBN) 4.2.2. Temporary Maximum Media Stream Bit Rate Notification (TMMBN)
The FCI field of the TMMBN Feedback message may contain zero, one The Temporary Maximum Media Stream Bit Rate Notification is
or more TMMBN FCI entries. identified by RTCP packet type value PT=RTPFB and FMT=4.
The FCI field of the TMMBN Feedback message may contain zero, one or
more TMMBN FCI entries.
4.2.2.1. Message Format 4.2.2.1. Message Format
The Feedback Control Information (FCI) consists of zero, one or The Feedback Control Information (FCI) consists of zero, one or more
more TMMBN FCI entries with the following syntax: TMMBN FCI entries with the following syntax:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead| | MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 - Syntax of an FCI entry in the TMMBN message Figure 3 - Syntax of an FCI entry in the TMMBN message
skipping to change at page 43, line 4 skipping to change at page 41, line 46
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead| | MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 - Syntax of an FCI entry in the TMMBN message Figure 3 - Syntax of an FCI entry in the TMMBN message
SSRC (32 bits): The SSRC value of the "owner" of this tuple. SSRC (32 bits): The SSRC value of the "owner" of this tuple.
MxTBR Exp (6 bits): The exponential scaling of the mantissa for MxTBR Exp (6 bits): The exponential scaling of the mantissa for
the maximum total media bit rate value. The value is an the maximum total media bit rate value. The value is an
unsigned integer [0..63]. unsigned integer [0..63].
MxTBR Mantissa (17 bits): The mantissa of the maximum total MxTBR Mantissa (17 bits): The mantissa of the maximum total media
media bit rate value as an unsigned integer. bit rate value as an unsigned integer.
Measured Overhead (9 bits): The measured average packet overhead Measured Overhead (9 bits): The measured average packet overhead
value in bytes represented as an unsigned integer. value in bytes represented as an unsigned integer.
Thus the FCI within the TMMBN message contains entries indicating Thus, the FCI within the TMMBN message contains entries indicating
the bounding tuples. For each tuple, the entry gives the owner by the bounding tuples. For each tuple, the entry gives the owner by
the SSRC, followed by the applicable maximum total media bit rate the SSRC, followed by the applicable maximum total media bit rate
and overhead value. and overhead value.
The length of the TMMBN message SHALL be set to 2+2*N where N is The length of the TMMBN message SHALL be set to 2+2*N where N is the
the number of TMMBN FCI entries. number of TMMBN FCI entries.
4.2.2.2. Semantics 4.2.2.2. Semantics
This feedback message is used to notify the senders of any TMMBR This feedback message is used to notify the senders of any TMMBR
message that one or more TMMBR messages have been received or that message that one or more TMMBR messages have been received or that
an owner has left the session. It indicates to all participants an owner has left the session. It indicates to all participants the
the current set of bounding tuples and the "owners" of those current set of bounding tuples and the "owners" of those tuples.
tuples.
Within the common packet header for feedback messages (as defined Within the common packet header for feedback messages (as defined in
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
field indicates the source of the notification. The "SSRC of indicates the source of the notification. The "SSRC of media
media source" is not used and SHALL be set to 0. source" is not used and SHALL be set to 0.
A TMMBN message SHALL be scheduled for transmission after the A TMMBN message SHALL be scheduled for transmission after the
reception of a TMMBR message with an FCI entry identifying this reception of a TMMBR message with an FCI entry identifying this
media sender. Only a single TMMBN SHALL be sent, even if more media sender. Only a single TMMBN SHALL be sent, even if more than
than one TMMBR message is received between the scheduling of the one TMMBR message is received between the scheduling of the
transmission and the actual transmission of the TMMBN message. transmission and the actual transmission of the TMMBN message. The
The TMMBN message indicates the bounding tuples and their owners TMMBN message indicates the bounding tuples and their owners at the
at the time of transmitting the message. The bounding tuples time of transmitting the message. The bounding tuples included
included SHALL be the set arrived at through application of the SHALL be the set arrived at through application of the applicable
applicable algorithm of section 3.5.4.2 or an equivalent, applied algorithm of section 3.5.4.2 or an equivalent, applied to the
to the previous bounding set if any and tuples received in TMMBR previous bounding set if any and tuples received in TMMBR messages
messages since the last TMMBN was transmitted. since the last TMMBN was transmitted.
The reception of a TMMBR message SHALL still result in the The reception of a TMMBR message SHALL still result in the
transmission of a TMMBN message even if, after application of the transmission of a TMMBN message even if, after application of the
algorithm, the newly reported TMMBR tuple is not accepted into the algorithm, the newly reported TMMBR tuple is not accepted into the
bounding set. In such a case the bounding tuples and their owners bounding set. In such a case the bounding tuples and their owners
are not changed, unless the TMMBR was from an owner of a tuple are not changed, unless the TMMBR was from an owner of a tuple
within the previously calculated bounding set. This procedure within the previously calculated bounding set. This procedure
allows session participants that did not see the last TMMBN allows session participants that did not see the last TMMBN message
message to get a correct view of this media sender's state. to get a correct view of this media sender's state.
As indicated in section 4.2.1.2, when a media sender determines As indicated in section 4.2.1.2, when a media sender determines that
that an "owner" of a bounding tuple has left the session, then an "owner" of a bounding tuple has left the session, then that tuple
that tuple is removed from the bounding set, and the media sender is removed from the bounding set, and the media sender SHALL send a
SHALL send a TMMBN message indicating the remaining bounding TMMBN message indicating the remaining bounding tuples. If there
tuples. If there are no remaining bounding tuples a TMMBN without are no remaining bounding tuples a TMMBN without any FCI SHALL be
any FCI SHALL be sent to indicate this. sent to indicate this.
Note: if any media receivers remain in the session, this last Note: if any media receivers remain in the session, this last will
will be a temporary situation. The empty TMMBN will cause every be a temporary situation. The empty TMMBN will cause every
remaining media receiver to determine that its limitation remaining media receiver to determine that its limitation belongs
belongs in the bounding set and send a TMMBR in consequence. in the bounding set and send a TMMBR in consequence.
In unicast scenarios (i.e. where a single sender talks to a single In unicast scenarios (i.e. where a single sender talks to a single
receiver), the aforementioned algorithm to determine ownership receiver), the aforementioned algorithm to determine ownership
degenerates to the media receiver becoming the "owner" of the one degenerates to the media receiver becoming the "owner" of the one
bounding tuple as soon as the media receiver has issued the first bounding tuple as soon as the media receiver has issued the first
TMMBR message. TMMBR message.
4.2.2.3. Timing Rules 4.2.2.3. Timing Rules
The TMMBN acknowledgement SHOULD be sent as soon as allowed by the The TMMBN acknowledgement SHOULD be sent as soon as allowed by the
skipping to change at page 44, line 46 skipping to change at page 43, line 41
mode SHOULD be used for these messages. mode SHOULD be used for these messages.
4.2.2.4. Handling by Translators and Mixers 4.2.2.4. Handling by Translators and Mixers
As discussed in Section 4.2.1.4 mixers or translators may need to As discussed in Section 4.2.1.4 mixers or translators may need to
issue TMMBN messages as responses to TMMBR messages for SSRC's issue TMMBN messages as responses to TMMBR messages for SSRC's
handled by them. handled by them.
4.3. Payload Specific Feedback Messages 4.3. Payload Specific Feedback Messages
As specified by section 6.1 of RFC 4585 [RFC4585], Payload- As specified by section 6.1 of RFC 4585 [RFC4585], Payload-Specific
Specific FB messages are identified by the RTCP packet type value FB messages are identified by the RTCP packet type value PSFB (206).
PT=PSFB (206).
AVPF [RFC4585] defines three payload-specific feedback messages AVPF [RFC4585] defines three payload-specific feedback messages and
and one application layer feedback message. This memo specifies one application layer feedback message. This memo specifies four
four additional payload-specific feedback messages. All are additional payload-specific feedback messages. All are identified
identified by means of the FMT parameter as follows: by means of the FMT parameter as follows:
Assigned in [RFC4585]: Assigned in [RFC4585]:
1: Picture Loss Indication (PLI) 1: Picture Loss Indication (PLI)
2: Slice Lost Indication (SLI) 2: Slice Lost Indication (SLI)
3: Reference Picture Selection Indication (RPSI) 3: Reference Picture Selection Indication (RPSI)
15: Application layer FB message 15: Application layer FB message
31: reserved for future expansion of the number space 31: reserved for future expansion of the number space
Assigned in this memo: Assigned in this memo:
skipping to change at page 45, line 33 skipping to change at page 44, line 29
0: unassigned 0: unassigned
8-14: unassigned 8-14: unassigned
16-30: unassigned 16-30: unassigned
The following subsections define the new FCI formats for the The following subsections define the new FCI formats for the
payload-specific feedback messages. payload-specific feedback messages.
4.3.1. Full Intra Request (FIR) 4.3.1. Full Intra Request (FIR)
The FIR message is identified by RTCP packet type value PT=PSFB The FIR message is identified by RTCP packet type value PT=PSFB and
and FMT=4. FMT=4.
The FCI field MUST contain one or more FIR entries. Each entry The FCI field MUST contain one or more FIR entries. Each entry
applies to a different media sender, identified by its SSRC. applies to a different media sender, identified by its SSRC.
4.3.1.1. Message Format 4.3.1.1. Message Format
The Feedback Control Information (FCI) for the Full Intra Request The Feedback Control Information (FCI) for the Full Intra Request
consists of one or more FCI entries, the content of which is consists of one or more FCI entries, the content of which is
depicted in Figure 4. The length of the FIR feedback message MUST depicted in Figure 4. The length of the FIR feedback message MUST
be set to 2+2*N, where N is the number of FCI entries. be set to 2+2*N, where N is the number of FCI entries.
skipping to change at page 46, line 33 skipping to change at page 45, line 33
number. The initial value is arbitrary. number. The initial value is arbitrary.
Reserved (24 bits): All bits SHALL be set to 0 by the sender and Reserved (24 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception. SHALL be ignored on reception.
The semantics of this feedback message is independent of the RTP The semantics of this feedback message is independent of the RTP
payload type. payload type.
4.3.1.2. Semantics 4.3.1.2. Semantics
Upon reception of FIR, the encoder MUST send a decoder refresh Within the common packet header for feedback messages (as defined in
point (see section 2.2) as soon as possible. section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
indicates the source of the request, and the "SSRC of media source"
Note: Currently, video appears to be the only useful application is not used and SHALL be set to 0. The SSRCs of the media senders
for FIR, as it appears to be the only RTP payload widely to which the FIR command applies are in the corresponding FCI
deployed that relies heavily on media prediction across RTP entries. A TSTR message MAY contain requests to multiple media
packet boundaries. However, use of FIR could also reasonably be senders, using one FCI entry per target media sender.
envisioned for other media types that share essential properties
with compressed video, namely cross-frame prediction (whatever a
frame may be for that media type). One possible example may be
the dynamic updates of MPEG-4 scene descriptions. It is
suggested that payload formats for such media types refer to FIR
and other message types defined in this specification and in
AVPF [RFC4585], instead of creating similar mechanisms in the
payload specifications. The payload specifications may have to
explain how the payload-specific terminologies map to the video-
centric terminology used herein.
Note: In environments where the sender has no control over the Upon reception of FIR, the encoder MUST send a decoder refresh point
codec (e.g. when streaming pre-recorded and pre-coded content), (see section 2.2) as soon as possible.
the reaction to this command cannot be specified. One suitable
reaction of a sender would be to skip forward in the video bit
stream to the next decoder refresh point. In other scenarios,
it may be preferable not to react to the command at all, e.g.
when streaming to a large multicast group. Other reactions may
also be possible. When deciding on a strategy, a sender could
take into account factors such as the size of the receiving
group, the "importance" of the sender of the FIR message
(however "importance" may be defined in this specific
application), the frequency of decoder refresh points in the
content, and so on. However a session which predominately
handles pre-coded content is not expected to use FIR at all.
The sender MUST consider congestion control as outlined in section The sender MUST consider congestion control as outlined in section
5, which MAY restrict its ability to send a decoder refresh point 5, which MAY restrict its ability to send a decoder refresh point
quickly. quickly.
Note: The relationship between the Picture Loss Indication and
FIR is as follows. As discussed in section 6.3.1 of AVPF
[RFC4585], a Picture Loss Indication informs the decoder about
the loss of a picture and hence the likelihood of misalignment
of the reference pictures between the encoder and decoder. Such
a scenario is normally related to losses in an ongoing
connection. In point-to-point scenarios, and without the
presence of advanced error resilience tools, one possible option
for an encoder consists in sending a decoder refresh point.
However, there are other options. One example is that the media
sender ignores the PLI, because the embedded stream redundancy
is likely to clean up the reproduced picture within a reasonable
amount of time. The FIR, in contrast, leaves a (real-time)
encoder no choice but to send a decoder refresh point. It does
not allow the encoder to take into account any considerations
such as the ones mentioned above.
Note: Mandating a maximum delay for completing the sending of a
decoder refresh point would be desirable from an application
viewpoint, but is problematic from a congestion control point of
view. "As soon as possible" as mentioned above appears to be a
reasonable compromise.
FIR SHALL NOT be sent as a reaction to picture losses -- it is FIR SHALL NOT be sent as a reaction to picture losses -- it is
RECOMMENDED to use PLI instead. FIR SHOULD be used only in RECOMMENDED to use PLI [RFC4585] instead. FIR SHOULD be used only
situations where not sending a decoder refresh point would render in situations where not sending a decoder refresh point would render
the video unusable for the users. the video unusable for the users.
Note: A typical example where sending FIR is appropriate is A typical example where sending FIR is appropriate is when, in a
when, in a multipoint conference, a new user joins the session multipoint conference, a new user joins the session and no regular
and no regular decoder refresh point interval is established. decoder refresh point interval is established. Another example
Another example would be a video switching MCU that changes would be a video switching MCU that changes streams. Here,
streams. Here, normally, the MCU issues a FIR to the new sender normally, the MCU issues a FIR to the new sender so to force it to
so to force it to emit a decoder refresh point. The decoder emit a decoder refresh point. The decoder refresh point normally
refresh point normally includes a Freeze Picture Release includes a Freeze Picture Release (defined outside this
(defined outside this specification), which re-starts the specification), which re-starts the rendering process of the
rendering process of the receivers. Both techniques mentioned receivers. Both techniques mentioned are commonly used in MCU-based
are commonly used in MCU-based multipoint conferences. multipoint conferences.
Other RTP payload specifications such as RFC 2032 [RFC2032]
already define a feedback mechanism for certain codecs. An
application supporting both schemes MUST use the feedback
mechanism defined in this specification when sending feedback.
For backward compatibility reasons, such an application SHOULD
also be capable to receive and react to the feedback scheme
defined in the respective RTP payload format, if this is required
by that payload format.
Within the common packet header for feedback messages (as defined Other RTP payload specifications such as RFC 2032 [RFC2032] already
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" define a feedback mechanism for certain codecs. An application
field indicates the source of the request, and the "SSRC of media supporting both schemes MUST use the feedback mechanism defined in
source" is not used and SHALL be set to 0. The SSRCs of the media this specification when sending feedback. For backward
senders to which the FIR command applies are in the corresponding compatibility reasons such an application SHOULD also be capable of
FCI entries. A TSTR message MAY contain requests to multiple receiving and reacting to the feedback scheme defined in the
media senders, using one FCI entry per target media sender. respective RTP payload format, if this is required by that payload
format.
4.3.1.3. Timing Rules 4.3.1.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. The timing follows the rules outlined in section 3 of [RFC4585].
FIR commands MAY be used with early or immediate feedback. The FIR commands MAY be used with early or immediate feedback. The FIR
FIR feedback message MAY be repeated. If using immediate feedback feedback message MAY be repeated. If using immediate feedback mode
mode the repetition SHOULD wait at least one RTT before being the repetition SHOULD wait at least one RTT before being sent. In
sent. In early or regular RTCP mode the repetition is sent in the early or regular RTCP mode the repetition is sent in the next
next regular RTCP packet. regular RTCP packet.
4.3.1.4. Handling of FIR Message in Mixer and Translators 4.3.1.4. Handling of FIR Message in Mixer and Translators
A media translator or a mixer performing media encoding of the A media translator or a mixer performing media encoding of the
content for which the session participant has issued a FIR is content for which the session participant has issued a FIR is
responsible for acting upon it. A mixer acting upon a FIR SHOULD responsible for acting upon it. A mixer acting upon a FIR SHOULD
NOT forward the message unaltered; instead it SHOULD issue a FIR NOT forward the message unaltered; instead it SHOULD issue a FIR
itself. itself.
4.3.1.5. Remarks 4.3.1.5. Remarks
In conjunction with video codecs, FIR messages typically trigger Currently, video appears to be the only useful application for FIR,
the sending of full intra or IDR pictures. Both are several times as it appears to be the only RTP payload widely deployed that relies
heavily on media prediction across RTP packet boundaries. However,
use of FIR could also reasonably be envisioned for other media types
that share essential properties with compressed video, namely cross-
frame prediction (whatever a frame may be for that media type). One
possible example may be the dynamic updates of MPEG-4 scene
descriptions. It is suggested that payload formats for such media
types refer to FIR and other message types defined in this
specification and in AVPF [RFC4585], instead of creating similar
mechanisms in the payload specifications. The payload
specifications may have to explain how the payload-specific
terminologies map to the video-centric terminology used herein.
In conjunction with video codecs, FIR messages typically trigger the
sending of full intra or IDR pictures. Both are several times
larger then predicted (inter) pictures. Their size is independent larger then predicted (inter) pictures. Their size is independent
of the time they are generated. In most environments, especially of the time they are generated. In most environments, especially
when employing bandwidth-limited links, the use of an intra when employing bandwidth-limited links, the use of an intra picture
picture implies an allowed delay that is a significant multiple of implies an allowed delay that is a significant multiple of the
the typical frame duration. An example: if the sending frame rate typical frame duration. An example: if the sending frame rate is 10
is 10 fps, and an intra picture is assumed to be 10 times as big fps, and an intra picture is assumed to be 10 times as big as an
as an inter picture, then a full second of latency has to be inter picture, then a full second of latency has to be accepted. In
accepted. In such an environment there is no need for a such an environment there is no need for a particularly short delay
particularly short delay in sending the FIR message. Hence in sending the FIR message. Hence, waiting for the next possible
waiting for the next possible time slot allowed by RTCP timing time slot allowed by RTCP timing rules as per [RFC4585] should not
rules as per [RFC4585] should not have an overly negative impact have an overly negative impact on the system performance.
on the system performance.
Mandating a maximum delay for completing the sending of a decoder
refresh point would be desirable from an application viewpoint, but
is problematic from a congestion control point of view. "As soon as
possible" as mentioned above appears to be a reasonable compromise.
In environments where the sender has no control over the codec (e.g.
when streaming pre-recorded and pre-coded content), the reaction to
this command cannot be specified. One suitable reaction of a sender
would be to skip forward in the video bit stream to the next decoder
refresh point. In other scenarios, it may be preferable not to
react to the command at all, e.g. when streaming to a large
multicast group. Other reactions may also be possible. When
deciding on a strategy, a sender could take into account factors
such as the size of the receiving group, the "importance" of the
sender of the FIR message (however "importance" may be defined in
this specific application), the frequency of decoder refresh points
in the content, and so on. However, a session which predominately
handles pre-coded content is not expected to use FIR at all.
The relationship between the Picture Loss Indication and FIR is as
follows. As discussed in section 6.3.1 of AVPF [RFC4585], a Picture
Loss Indication informs the decoder about the loss of a picture and
hence the likelihood of misalignment of the reference pictures
between the encoder and decoder. Such a scenario is normally
related to losses in an ongoing connection. In point-to-point
scenarios, and without the presence of advanced error resilience
tools, one possible option for an encoder consists in sending a
decoder refresh point. However, there are other options. One
example is that the media sender ignores the PLI, because the
embedded stream redundancy is likely to clean up the reproduced
picture within a reasonable amount of time. The FIR, in contrast,
leaves a (real-time) encoder no choice but to send a decoder refresh
point. It does not allow the encoder to take into account any
considerations such as the ones mentioned above.
4.3.2. Temporal-Spatial Trade-off Request (TSTR) 4.3.2. Temporal-Spatial Trade-off Request (TSTR)
The TSTR feedback message is identified by RTCP packet type value The TSTR feedback message is identified by RTCP packet type value
PT=PSFB and FMT=5. PT=PSFB and FMT=5.
The FCI field MUST contain one or more TSTR FCI entries. The FCI field MUST contain one or more TSTR FCI entries.
4.3.2.1. Message Format 4.3.2.1. Message Format
The content of the FCI entry for the Temporal-Spatial Trade-off The content of the FCI entry for the Temporal-Spatial Trade-off
Request is depicted in Figure 5. The length of the feedback Request is depicted in Figure 5. The length of the feedback message
message MUST be set to 2+2*N, where N is the number of FCI entries MUST be set to 2+2*N, where N is the number of FCI entries included.
included.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq nr. | Reserved | Index | | Seq nr. | Reserved | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 - Syntax of an FCI Entry in the TSTR Message Figure 5 - Syntax of an FCI Entry in the TSTR Message
SSRC (32 bits): The SSRC of the media sender which is requested
to apply the tradeoff value given in Index. SSRC (32 bits): The SSRC of the media sender which is requested to
apply the tradeoff value given in Index.
Seq. nr (8 bits): Request sequence number. The sequence number Seq. nr (8 bits): Request sequence number. The sequence number
space is unique for pairing of the SSRC of request space is unique for pairing of the SSRC of request source
source and the SSRC of the request target. The sequence and the SSRC of the request target. The sequence number
number SHALL be increased by 1 modulo 256 for each new SHALL be increased by 1 modulo 256 for each new command.
command. A repetition SHALL NOT increase the sequence A repetition SHALL NOT increase the sequence number. The
number. The initial value is arbitrary. initial value is arbitrary.
Reserved (19 bits): All bits SHALL be set to 0 by the sender and Reserved (19 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception. SHALL be ignored on reception.
Index (5 bits): An integer value between 0 and 31 that indicates Index (5 bits): An integer value between 0 and 31 that indicates
the relative trade off that is requested. An index the relative trade-off that is requested. An index value
value of 0 index highest possible spatial quality, while of 0 indicates highest possible spatial quality, while 31
31 indicates highest possible temporal resolution. indicates highest possible temporal resolution.
4.3.2.2. Semantics 4.3.2.2. Semantics
A decoder can suggest a temporal-spatial trade-off level by A decoder can suggest a temporal-spatial trade-off level by sending
sending a TSTR message to an encoder. If the encoder is capable a TSTR message to an encoder. If the encoder is capable of
of adjusting its temporal-spatial trade-off, it SHOULD take into adjusting its temporal-spatial trade-off, it SHOULD take into
account the received TSTR message for future coding of pictures. account the received TSTR message for future coding of pictures. A
A value of 0 suggests a high spatial quality and a value of 31 value of 0 suggests a high spatial quality and a value of 31
suggests a high frame rate. The progression of values from 0 to suggests a high frame rate. The progression of values from 0 to 31
31 indicate monotonically a desire for higher frame rate. The indicate monotonically a desire for higher frame rate. The index
index values do not correspond to precise values of spatial values do not correspond to precise values of spatial quality or
quality or frame rate. frame rate.
The reaction to the reception of more than one TSTR message by a The reaction to the reception of more than one TSTR message by a
media sender from different media receivers is left open to the media sender from different media receivers is left open to the
implementation. The selected trade-off SHALL be communicated to implementation. The selected trade-off SHALL be communicated to the
the media receivers by the means of the TSTN message. media receivers by the means of the TSTN message.
Within the common packet header for feedback messages (as defined Within the common packet header for feedback messages (as defined in
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
field indicates the source of the request, and the "SSRC of media indicates the source of the request, and the "SSRC of media source"
source" is not used and SHALL be set to 0. The SSRCs of the media is not used and SHALL be set to 0. The SSRCs of the media senders
senders to which the TSTR applies to are in the corresponding FCI to which the TSTR applies are in the corresponding FCI entries.
entries.
A TSTR message MAY contain requests to multiple media senders, A TSTR message MAY contain requests to multiple media senders, using
using one FCI entry per target media sender. one FCI entry per target media sender.
4.3.2.3. Timing Rules 4.3.2.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. The timing follows the rules outlined in section 3 of [RFC4585].
This request message is not time critical and SHOULD be sent using This request message is not time critical and SHOULD be sent using
regular RTCP timing. Only if it is known that the user interface regular RTCP timing. Only if it is known that the user interface
requires a quick feedback, the message MAY be sent with early or requires quick feedback, the message MAY be sent with early or
immediate feedback timing. immediate feedback timing.
4.3.2.4. Handling of message in Mixers and Translators 4.3.2.4. Handling of message in Mixers and Translators
A mixer or media translator that encodes content sent to the session
A mixer or media translator that encodes content sent to the participant issuing the TSTR SHALL consider the request to determine
session participant issuing the TSTR SHALL consider the request to if it can fulfill it by changing its own encoding parameters. A
determine if it can fulfill it by changing its own encoding media translator unable to fulfill the request MAY forward the
parameters. A media translator unable to fulfill the request MAY request unaltered towards the media sender. A mixer encoding for
forward the request unaltered towards the media sender. A mixer multiple session participants will need to consider the joint needs
encoding for multiple session participants will need to consider of these participants before generating a TSTR on its own behalf
the joint needs of these participants before generating a TSTR on towards the media sender. See also the discussion in Section 3.5.2.
its own behalf towards the media sender. See also the discussion
in Section 3 ..5.2.
4.3.2.5. Remarks 4.3.2.5. Remarks
The term "spatial quality" does not necessarily refer to the The term "spatial quality" does not necessarily refer to the
resolution, measured by the number of pixels the reconstructed resolution as measured by the number of pixels the reconstructed
video is using. In fact, in most scenarios the video resolution video is using. In fact, in most scenarios the video resolution
stays constant during the lifetime of a session. However, all stays constant during the lifetime of a session. However, all video
video compression standards have means to adjust the spatial compression standards have means to adjust the spatial quality at a
quality at a given resolution, often influenced by the Quantizer given resolution, often influenced by the Quantizer Parameter or QP.
Parameter or QP. A numerically low QP results in a good A numerically low QP results in a good reconstructed picture
reconstructed picture quality, whereas a numerically high QP quality, whereas a numerically high QP yields a coarse picture. The
yields a coarse picture. The typical reaction of an encoder to typical reaction of an encoder to this request is to change its rate
this request is to change its rate control parameters to use a control parameters to use a lower frame rate and a numerically lower
lower frame rate and a numerically lower (on average) QP, or vice (on average) QP, or vice versa. The precise mapping of Index value
versa. The precise mapping of Index value to frame rate and QP is to frame rate and QP is intentionally left open here, as it depends
intentionally left open here, as it depends on factors such as the on factors such as the compression standard employed, spatial
compression standard employed, spatial resolution, content, bit resolution, content, bit rate, and so on.
rate, and so on.
4.3.3. Temporal-Spatial Trade-off Notification (TSTN) 4.3.3. Temporal-Spatial Trade-off Notification (TSTN)
The TSTN message is identified by RTCP packet type value PT=PSFB The TSTN message is identified by RTCP packet type value PT=PSFB and
and FMT=6. FMT=6.
The FCI field SHALL contain one or more TSTN FCI entries. The FCI field SHALL contain one or more TSTN FCI entries.
4.3.3.1. Message Format 4.3.3.1. Message Format
The content of an FCI entry for the Temporal-Spatial Trade-off The content of an FCI entry for the Temporal-Spatial Trade-off
Notification is depicted in Figure 6. The length of the TSTN Notification is depicted in Figure 6. The length of the TSTN
message MUST be set to 2+2*N, where N is the number of FCI message MUST be set to 2+2*N, where N is the number of FCI entries.
entries.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq nr. | Reserved | Index | | Seq nr. | Reserved | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6 - Syntax of the TSTN Figure 6 - Syntax of the TSTN
SSRC (32 bits): The SSRC of the source of the TSTR request which SSRC (32 bits): The SSRC of the source of the TSTR request which
resulted in this Notification. resulted in this Notification.
Seq. nr (8 bits): The sequence number value from the TSTN Seq. nr (8 bits): The sequence number value from the TSTN request
request that is being acknowledged. that is being acknowledged.
Reserved (19 bits): All bits SHALL be set to 0 by the sender and Reserved (19 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception. SHALL be ignored on reception.
Index (5 bits): The trade-off value the media sender is using Index (5 bits): The trade-off value the media sender is using
henceforth. henceforth.
Informative note: The returned trade-off value (Index) may Informative note: The returned trade-off value (Index) may differ
differ from the requested one, for example in cases where a from the requested one, for example in cases where a media encoder
media encoder cannot tune its trade-off, or when pre-recorded cannot tune its trade-off, or when pre-recorded content is used.
content is used.
4.3.3.2. Semantics 4.3.3.2. Semantics
This feedback message is used to acknowledge the reception of a This feedback message is used to acknowledge the reception of a
TSTR. One TSTN entry in a TSTN feedback message SHALL be sent for TSTR. For each TSTR received targeted at the session participant, a
each TSTR entry targeted to this session participant, i.e. each TSTN entry SHALL be sent included in a TSTN feedback message. A
TSTR received that in the SSRC field in the entry has the single TSTN message MAY acknowledge multiple requests using multiple
receiving entities SSRC. A single TSTN message MAY acknowledge FCI entries. The index value included SHALL be the same in all FCI
multiple requests using multiple FCI entries. The index value entries of the TSTN message. Including a FCI for each requestor
included SHALL be the same in all FCI entries of the TSTN message. allows each requesting entity to determine that the media sender
Including a FCI for each requestor allows each requesting entity received the request. The Notification SHALL also be sent in
to determine that the media sender received the request. The response to TSTR repetitions received. If the request receiver has
Notification SHALL also be sent in response to TSTR repetitions received TSTR with several different sequence numbers from a single
received. If the request receiver has received TSTR with several requestor it SHALL only respond to the request with the highest
different sequence numbers from a single requestor it SHALL only (modulo 256) sequence number. Note that the highest sequence number
respond to the request with the highest (modulo 256) sequence may be a smaller integer value due to the wrapping of the field.
number. Section A.1 of [RFC3550] has an algorithm for keeping track of the
highest received sequence number for RTP packets, this could be
adapted for this usage.
The TSTN SHALL include the Temporal-Spatial Trade-off index that The TSTN SHALL include the Temporal-Spatial Trade-off index that
will be used as a result of the request. This is not necessarily will be used as a result of the request. This is not necessarily
the same index as requested, as the media sender may need to the same index as requested, as the media sender may need to
aggregate requests from several requesting session participants. aggregate requests from several requesting session participants. It
It may also have some other policies or rules that limit the may also have some other policies or rules that limit the selection.
selection.
Within the common packet header for feedback messages (as defined Within the common packet header for feedback messages (as defined in
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
field indicates the source of the Notification, and the "SSRC of indicates the source of the Notification, and the "SSRC of media
media source" is not used and SHALL be set to 0. The SSRCs of the source" is not used and SHALL be set to 0. The SSRCs of the
requesting entities to which the Notification applies are in the requesting entities to which the Notification applies are in the
corresponding FCI entries. corresponding FCI entries.
4.3.3.3. Timing Rules 4.3.3.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. The timing follows the rules outlined in section 3 of [RFC4585].
This acknowledgement message is not extremely time critical and This acknowledgement message is not extremely time critical and
SHOULD be sent using regular RTCP timing. SHOULD be sent using regular RTCP timing.
4.3.3.4. Handling of TSTN in Mixer and Translators 4.3.3.4. Handling of TSTN in Mixer and Translators
skipping to change at page 53, line 43 skipping to change at page 52, line 31
corresponding TSTN. In cases where it needs to forward a TSTR corresponding TSTN. In cases where it needs to forward a TSTR
itself the notification message MAY need to be delayed until the itself the notification message MAY need to be delayed until the
TSTR has been responded to. TSTR has been responded to.
4.3.3.5. Remarks 4.3.3.5. Remarks
None None
4.3.4. H.271 Video Back Channel Message (VBCM) 4.3.4. H.271 Video Back Channel Message (VBCM)
The VBCM is identified by RTCP packet type value PT=PSFB and The VBCM is identified by RTCP packet type value PT=PSFB and FMT=7.
FMT=7.
The FCI field MUST contain one or more VBCM FCI entries. The FCI field MUST contain one or more VBCM FCI entries.
4.3.4.1. Message Format 4.3.4.1. Message Format
The syntax of an FCI entry within the VBCM indication is depicted The syntax of an FCI entry within the VBCM indication is depicted in
in Figure 7. Figure 7.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC | | SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq. nr |0| Payload Type| Length | | Seq. nr |0| Payload Type| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VBCM Octet String.... | Padding | | VBCM Octet String.... | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 - Syntax of an FCI Entry in the VBCM Message Figure 7 - Syntax of an FCI Entry in the VBCM Message
SSRC (32 bits): The SSRC value of the media sender that is requested
SSRC (32 bits): The SSRC value of the media sender that is to instruct its encoder to react to the VBCM message
requested to instruct its encoder to react to the VBCM
message
Seq. nr (8 bits): Command sequence number. The sequence number Seq. nr (8 bits): Command sequence number. The sequence number
space is unique for pairing of the SSRC of command source space is unique for pairing of the SSRC of command source and
and the SSRC of the command target. The sequence number the SSRC of the command target. The sequence number SHALL be
SHALL be increased by 1 modulo 256 for each new command. A increased by 1 modulo 256 for each new command. A repetition
repetition SHALL NOT increase the sequence number. The SHALL NOT increase the sequence number. The initial value is
initial value is arbitrary. arbitrary.
0: Must be set to 0 by the sender and should not be acted upon by 0: Must be set to 0 by the sender and should not be acted upon by
the message receiver. the message receiver.
Payload Type (7 bits): The RTP payload type for which the VBCM bit Payload Type (7 bits): The RTP payload type for which the VBCM bit
stream must be interpreted. stream must be interpreted.
Length (16 bits): The length of the VBCM octet string in octets Length (16 bits): The length of the VBCM octet string in octets
exclusive of any padding octets exclusive of any padding octets
skipping to change at page 55, line 10 skipping to change at page 53, line 35
message. message.
Padding (Variable length): Bits set to 0 to make up a 32 bit Padding (Variable length): Bits set to 0 to make up a 32 bit
boundary. boundary.
4.3.4.2. Semantics 4.3.4.2. Semantics
The "payload" of the VBCM indication carries different types of The "payload" of the VBCM indication carries different types of
codec-specific, feedback information. The type of feedback codec-specific, feedback information. The type of feedback
information can be classified as a 'status report' (such as an information can be classified as a 'status report' (such as an
indication that a bit stream was received without errors, or that indication that a bit stream was received without errors, or that a
a partial or complete picture or block was lost) or 'update partial or complete picture or block was lost) or 'update requests'
requests' (such as complete refresh of the bit stream). (such as complete refresh of the bit stream).
Note: There are possible overlaps between the VBCM sub- Note: There are possible overlaps between the VBCM sub-
messages and CCM/AVPF feedback messages, such FIR. Please messages and CCM/AVPF feedback messages, such FIR. Please
see section 3.5.3 for further discussion. see section 3.5.3 for further discussion.
The different types of feedback sub-messages carried in the VBCM The different types of feedback sub-messages carried in the VBCM are
are indicated by the "payloadType" as defined in [VBCM]. These indicated by the "payloadType" as defined in [VBCM]. These sub-
sub-message types are reproduced below for convenience. message types are reproduced below for convenience. "payloadType",
"payloadType", in ITU-T Rec. H.271 terminology, refers to the sub- in ITU-T Rec. H.271 terminology, refers to the sub-type of the H.271
type of the H.271 message and should not be confused with an RTP message and should not be confused with an RTP payload type.
payload type.
Payload Message Content Payload Message Content
Type Type
--------------------------------------------------------------------- --------------------------------------------------------------------
0 One or more pictures without detected bit stream error 0 One or more pictures without detected bit stream error
mismatch mismatch
1 One or more pictures that are entirely or partially lost 1 One or more pictures that are entirely or partially lost
2 A set of blocks of one picture that is entirely or partially 2 A set of blocks of one picture that is entirely or partially
lost lost
3 CRC for one parameter set 3 CRC for one parameter set
4 CRC for all parameter sets of a certain type 4 CRC for all parameter sets of a certain type
5 A "reset" request indicating that the sender should completely 5 A "reset" request indicating that the sender should completely
refresh the video bit stream as if no prior bit stream data refresh the video bit stream as if no prior bit stream data
had been received had been received
> 5 Reserved for future use by ITU-T > 5 Reserved for future use by ITU-T
Table 2: H.271 message types ("payloadTypes") Table 2: H.271 message types ("payloadTypes")
The bit string or the "payload" of a VBCM message is of variable The bit string or the "payload" of a VBCM message is of variable
length and is self-contained and coded in a variable length, length and is self-contained and coded in a variable length, binary
binary format. The media sender necessarily has to be able to format. The media sender necessarily has to be able to parse this
parse this optimized binary format to make use of VBCM messages. optimized binary format to make use of VBCM messages.
Each of the different types of sub-messages (indicated by Each of the different types of sub-messages (indicated by
payloadType) may have different semantics depending on the codec payloadType) may have different semantics depending on the codec
used. used.
Within the common packet header for feedback messages (as defined Within the common packet header for feedback messages (as defined in
in section 6.1 of [RFC4585]), the "SSRC of the packet sender" section 6.1 of [RFC4585]), the "SSRC of the packet sender" field
field indicates the source of the request, and the "SSRC of media indicates the source of the request, and the "SSRC of media source"
source" is not used and SHALL be set to 0. The SSRCs of the media is not used and SHALL be set to 0. The SSRCs of the media senders
senders to which the VBCM message applies to are in the to which the VBCM message applies to are in the corresponding FCI
corresponding FCI entries. The sender of the VBCM message MAY entries. The sender of the VBCM message MAY send H.271 messages to
send H.271 messages to multiple media senders and MAY send more multiple media senders and MAY send more than one H.271 message to
than one H.271 message to the same media sender within the same the same media sender within the same VBCM message.
VBCM message.
4.3.4.3. Timing Rules 4.3.4.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. The timing follows the rules outlined in section 3 of [RFC4585].
The different sub-message types may have different properties in The different sub-message types may have different properties in
regards to the timing of messages that should be used. If several regards to the timing of messages that should be used. If several
different types are included in the same feedback packet then the different types are included in the same feedback packet then the
requirements for the sub-message type with the most stringent requirements for the sub-message type with the most stringent
requirements should be followed. requirements should be followed.
skipping to change at page 56, line 47 skipping to change at page 55, line 24
similar functionality. similar functionality.
Note: There has been some discussion whether the payload type Note: There has been some discussion whether the payload type
field in this message is needed. It will be needed if there is field in this message is needed. It will be needed if there is
potentially more than one VBCM-capable RTP payload type in the potentially more than one VBCM-capable RTP payload type in the
same session, and the semantics of a given VBCM message changes same session, and the semantics of a given VBCM message changes
between payload types. For example, the picture identification between payload types. For example, the picture identification
mechanism in messages of H.271 type 0 is fundamentally different mechanism in messages of H.271 type 0 is fundamentally different
between H.263 and H.264 (although both use the same syntax). between H.263 and H.264 (although both use the same syntax).
Therefore, the payload field is justified here. There was a Therefore, the payload field is justified here. There was a
further comment that for TSTS and FIR such a need does not further comment that for TSTS and FIR such a need does not exist,
exist, because the semantics of TSTS and FIR are either loosely because the semantics of TSTS and FIR are either loosely enough
enough defined, or generic enough, to apply to all video defined, or generic enough, to apply to all video payloads
payloads currently in existence/envisioned. currently in existence/envisioned.
5. Congestion Control 5. Congestion Control
The correct application of the AVPF [RFC4585] timing rules The correct application of the AVPF [RFC4585] timing rules prevents
prevents the network from being flooded by feedback messages. the network from being flooded by feedback messages. Hence,
Hence, assuming a correct implementation and configuration, the assuming a correct implementation and configuration, the RTCP
RTCP channel cannot break its bit rate commitment and introduce channel cannot break its bit rate commitment and introduce
congestion. congestion.
The reception of some of the feedback messages modifies the The reception of some of the feedback messages modifies the
behaviour of the media senders or, more specifically, the media behaviour of the media senders or, more specifically, the media
encoders. Thus modified behaviour MUST respect the bandwidth encoders. Thus, modified behaviour MUST respect the bandwidth
limits that the application of congestion control provides. For limits that the application of congestion control provides. For
example, when a media sender is reacting to a FIR, the unusually example, when a media sender is reacting to a FIR, the unusually
high number of packets that form the decoder refresh point have to high number of packets that form the decoder refresh point have to
be paced in compliance with the congestion control algorithm, even be paced in compliance with the congestion control algorithm, even
if the user experience suffers from a slowly transmitted decoder if the user experience suffers from a slowly transmitted decoder
refresh point. refresh point.
A change of the Temporary Maximum Media Stream Bit Rate value can A change of the Temporary Maximum Media Stream Bit Rate value can
only mitigate congestion, but not cause congestion as long as only mitigate congestion, but not cause congestion as long as
congestion control is also employed. An increase of the value by congestion control is also employed. An increase of the value by a
a request REQUIRES the media sender to use congestion control when request REQUIRES the media sender to use congestion control when
increasing its transmission rate to that value. A reduction of increasing its transmission rate to that value. A reduction of the
the value results in a reduced transmission bit rate thus reducing value results in a reduced transmission bit rate, thus reducing the
the risk for congestion. risk for congestion.
6. Security Considerations 6. Security Considerations
The defined messages have certain properties that have security The defined messages have certain properties that have security
implications. These must be addressed and taken into account by implications. These must be addressed and taken into account by
users of this protocol. users of this protocol.
The defined setup signaling mechanism is sensitive to modification The defined setup signaling mechanism is sensitive to modification
attacks that can result in session creation with sub-optimal attacks that can result in session creation with sub-optimal
configuration, and, in the worst case, session rejection. To configuration, and, in the worst case, session rejection. To
prevent this type of attack, authentication and integrity prevent this type of attack, authentication and integrity protection
protection of the setup signaling is required. of the setup signaling is required.
Spoofed or maliciously created feedback messages of the type Spoofed or maliciously created feedback messages of the type defined
defined in this specification can have the following implications: in this specification can have the following implications:
a. severely reduced media bit rate due to false TMMBR messages a. severely reduced media bit rate due to false TMMBR messages
that sets the maximum to a very low value; that sets the maximum to a very low value;
b. assignment of the ownership of a bounding tuple to the
wrong participant within a TMMBN message, potentially b. assignment of the ownership of a bounding tuple to the wrong
causing unnecessary oscillation in the bounding set as the participant within a TMMBN message, potentially causing
mistakenly identified owner reports a change in its tuple unnecessary oscillation in the bounding set as the mistakenly
and the true owner possibly holds back on changes until a identified owner reports a change in its tuple and the true
correct TMMBN message reaches the participants; owner possibly holds back on changes until a correct TMMBN
message reaches the participants;
c. sending TSTR requests that result in a video quality c. sending TSTR requests that result in a video quality
different from the user's desire, rendering the session different from the user's desire, rendering the session less
less useful. useful;
d. Frequent FIR commands will potentially reduce the frame- d. sending multiple FIR commands to reduce the frame-rate, and
rate, making the video jerky, due to the frequent usage of make the video jerky, due to the frequent usage of decoder
decoder refresh points. refresh points.
To prevent these attacks there is a need to apply authentication To prevent these attacks there is a need to apply authentication and
and integrity protection of the feedback messages. This can be integrity protection of the feedback messages. This can be
accomplished against threats external to the current RTP session accomplished against threats external to the current RTP session
using the RTP profile that combines SRTP [SRTP] and AVPF into using the RTP profile that combines SRTP [SRTP] and AVPF into SAVPF
SAVPF [SAVPF]. In the mixer cases, separate security contexts and [SAVPF]. In the mixer cases, separate security contexts and
filtering can be applied between the mixer and the participants filtering can be applied between the mixer and the participants,
thus protecting other users on the mixer from a misbehaving thus protecting other users on the mixer from a misbehaving
participant. participant.
7. SDP Definitions 7. SDP Definitions
Section 4 of [RFC4585] defines a new SDP [RFC4566] attribute, Section 4 of [RFC4585] defines a new SDP [RFC4566] attribute, rtcp-
rtcp-fb, that may be used to negotiate the capability to handle fb, that may be used to negotiate the capability to handle specific
specific AVPF commands and indications, such as Reference Picture AVPF commands and indications, such as Reference Picture Selection,
Selection, Picture Loss Indication etc. The ABNF for rtcp-fb is Picture Loss Indication etc. The ABNF for rtcp-fb is described in
described in section 4.2 of [RFC4585]. In this section we extend section 4.2 of [RFC4585]. In this section we extend the rtcp-fb
the rtcp-fb attribute to include the commands and indications that attribute to include the commands and indications that are described
are described for codec control protocol in the present document. for codec control in the present document. We also discuss the
We also discuss the Offer/Answer implications for the codec Offer/Answer implications for the codec control commands and
control commands and indications. indications.
7.1. Extension of the rtcp-fb Attribute 7.1. Extension of the rtcp-fb Attribute
As described in AVPF [RFC4585], the rtcp-fb attribute indicates As described in AVPF [RFC4585], the rtcp-fb attribute indicates the
the capability of using RTCP feedback. AVPF specifies that the capability of using RTCP feedback. AVPF specifies that the rtcp-fb
rtcp-fb attribute must only be used as a media level attribute and attribute must only be used as a media level attribute and must not
must not be provided at session level. All the rules described in be provided at session level. All the rules described in [RFC4585]
[RFC4585] for rtcp-fb attribute relating to payload type and to for rtcp-fb attribute relating to payload type and to multiple rtcp-
multiple rtcp-fb attributes in a session description also apply to fb attributes in a session description also apply to the new
the new feedback messages defined in this memo. feedback messages defined in this memo.
The ABNF [RFC4234] for rtcp-fb as defined in [RFC4585] is The ABNF [RFC4234] for rtcp-fb as defined in [RFC4585] is
"a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF "a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF
where rtcp-fb-pt is the payload type and rtcp-fb-val defines the where rtcp-fb-pt is the payload type and rtcp-fb-val defines the
type of the feedback message such as ack, nack, trr-int and rtcp- type of the feedback message such as ack, nack, trr-int and rtcp-fb-
fb-id. For example to indicate the support of feedback of picture id. For example, to indicate the support of feedback of picture
loss indication, the sender declares the following in SDP loss indication, the sender declares the following in SDP
v=0 v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback s=Media with feedback
t=0 0 t=0 0
c=IN IP4 host.example.com c=IN IP4 host.example.com
m=audio 49170 RTP/AVPF 98 m=audio 49170 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 nack pli a=rtcp-fb:98 nack pli
In this document we define a new feedback value "ccm" which In this document we define a new feedback value "ccm" which
indicates the support of codec control using RTCP feedback indicates the support of codec control using RTCP feedback messages.
messages. The "ccm" feedback value SHOULD be used with The "ccm" feedback value SHOULD be used with parameters that
parameters, which indicate the specific codec control commands indicate the specific codec control commands supported. In this
supported. In this draft we define four parameters, which can be draft we define four such parameters, namely:
used with the ccm feedback value type.
o "fir" indicates the support of the Full Intra Request (FIR). o "fir" indicates support of the Full Intra Request (FIR).
o "tmmbr" indicates the support of the Temporary Maximum Media o "tmmbr" indicates support of the Temporary Maximum Media Stream
Stream Bit Rate Request/Notification (TMMBR/TMMBN). It has Bit Rate Request/Notification (TMMBR/TMMBN). It has an
an optional sub parameter to indicate the session maximum optional sub parameter to indicate the session maximum packet
packet rate to be used. If not included this defaults to rate to be used. If not included this defaults to infinity.
infinity. o "tstr" indicates support of the Temporal-Spatial Trade-off
o "tstr" indicates the support of the Temporal-Spatial Trade- Request/Notification (TSTR/TSTN).
off Request/Notification (TSTR/TSTN). O "vbcm" indicates support of H.271 video back channel messages
O "vbcm" indicates the support of H.271 video back channel (VBCM). It has zero or more subparameters identifying the
messages (VBCM). It has zero or more subparameters supported H.271 "payloadType" values.
identifying the supported H.271 "payloadType" values.
In the ABNF for rtcp-fb-val defined in [RFC4585], there is a In the ABNF for rtcp-fb-val defined in [RFC4585], there is a
placeholder called rtcp-fb-id to define new feedback types. "ccm" placeholder called rtcp-fb-id to define new feedback types. "ccm"
is defined as a new feedback type in this document and the ABNF is defined as a new feedback type in this document and the ABNF for
for the parameters for ccm are defined here (please refer to the parameters for ccm are defined here (please refer to section 4.2
section 4.2 of [RFC4585] for complete ABNF syntax). of [RFC4585] for complete ABNF syntax).
rtcp-fb-param = SP "app" [SP byte-string] rtcp-fb-param = SP "app" [SP byte-string]
/ SP rtcp-fb-ccm-param / SP rtcp-fb-ccm-param
/ ; empty / ; empty
rtcp-fb-ccm-param = "ccm" SP ccm-param rtcp-fb-ccm-param = "ccm" SP ccm-param
ccm-param = "fir" ; Full Intra Request ccm-param = "fir" ; Full Intra Request
/ "tmmbr" [SP "smaxpr=" MaxPacketRateValue] / "tmmbr" [SP "smaxpr=" MaxPacketRateValue]
; Temporary max media bit rate ; Temporary max media bit rate
/ "tstr" ; Temporal Spatial Trade Off / "tstr" ; Temporal Spatial Trade Off
/ "vbcm" *(SP subMessageType) ; H.271 VBCM messages / "vbcm" *(SP subMessageType) ; H.271 VBCM messages
/ token [SP byte-string] / token [SP byte-string]
; for future commands/indications ; for future commands/indications
subMessageType = 1*8DIGIT subMessageType = 1*8DIGIT
skipping to change at page 60, line 20 skipping to change at page 58, line 42
/ "vbcm" *(SP subMessageType) ; H.271 VBCM messages / "vbcm" *(SP subMessageType) ; H.271 VBCM messages
/ token [SP byte-string] / token [SP byte-string]
; for future commands/indications ; for future commands/indications
subMessageType = 1*8DIGIT subMessageType = 1*8DIGIT
byte-string = <as defined in section 4.2 of [RFC4585] > byte-string = <as defined in section 4.2 of [RFC4585] >
MaxPacketRateValue = 1*15DIGIT MaxPacketRateValue = 1*15DIGIT
7.2. Offer-Answer 7.2. Offer-Answer
The Offer/Answer [RFC3264] implications for codec control protocol The Offer/Answer [RFC3264] implications for codec control protocol
feedback messages are similar those described in [RFC4585]. The feedback messages are similar to those described in [RFC4585]. The
offerer MAY indicate the capability to support selected codec offerer MAY indicate the capability to support selected codec
commands and indications. The answerer MUST remove all ccm commands and indications. The answerer MUST remove all ccm
parameters which it does not understand or does not wish to use in parameters that it does not understand or does not wish to use in
this particular media session. The answerer MUST NOT add new ccm this particular media session. The answerer MUST NOT add new ccm
parameters in addition to what has been offered. The answer is parameters in addition to what has been offered. The answer is
binding for the media session and both offerer and answerer MUST binding for the media session and both offerer and answerer MUST
only use feedback messages negotiated in this way. only use feedback messages negotiated in this way.
The session maximum packet rate parameter part of the TMMBR The session maximum packet rate parameter part of the TMMBR
indication is declarative and everyone shall use the highest value indication is declarative and everyone SHALL use the highest value
indicated in a response. If the session maximum packet rate indicated in a response. If the session maximum packet rate
parameter is not present in an offer it SHALL NOT be included by parameter is not present in an offer it SHALL NOT be included by the
the answerer. answerer.
7.3. Examples 7.3. Examples
Example 1: The following SDP describes a point-to-point video call Example 1: The following SDP describes a point-to-point video call
with H.263, with the originator of the call declaring its with H.263, with the originator of the call declaring its capability
capability to support the FIR and TSTR/TSTN codec control to support the FIR and TSTR/TSTN codec control messages. The SDP is
messages. The SDP is carried in a high level signaling protocol carried in a high level signaling protocol like SIP.
like SIP.
v=0 v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Point-to-Point call s=Point-to-Point call
c=IN IP4 192.0.2.124 c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98 m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir a=rtcp-fb:98 ccm fir
In the above example, when the sender receives a TSTR message from In the above example, when the sender receives a TSTR message from
the remote party it is capable of adjusting the trade off as the remote party it is capable of adjusting the trade off as
indicated in the RTCP TSTN feedback message. indicated in the RTCP TSTN feedback message.
Example 2: The following SDP describes a SIP end point joining a Example 2: The following SDP describes a SIP end point joining a
video mixer that is hosting a multiparty video conferencing video mixer that is hosting a multiparty video conferencing session.
session. The participant supports only the FIR (Full Intra The participant supports only the FIR (Full Intra Request) codec
Request) codec control command and it declares it in its session control command and it declares it in its session description.
description.
v=0 v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multiparty Video Call s=Multiparty Video Call
c=IN IP4 192.0.2.124 c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98 m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm fir a=rtcp-fb:98 ccm fir
When the video MCU decides to route the video of this participant it
When the video MCU decides to route the video of this participant sends an RTCP FIR feedback message. Upon receiving this feedback
it sends an RTCP FIR feedback message. Upon receiving this message the end point is required to generate a full intra request.
feedback message the end point is required to generate a full
intra request.
Example 3: The following example describes the Offer/Answer Example 3: The following example describes the Offer/Answer
implications for the codec control messages. The Offerer wishes implications for the codec control messages. The Offerer wishes to
to support "tstr", "fir" and "tmmbr". The offered SDP is support "tstr", "fir" and "tmmbr". The offered SDP is
-------------> Offer -------------> Offer
v=0 v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer s=Offer/Answer
c=IN IP4 192.0.2.124 c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98 m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
skipping to change at page 62, line 24 skipping to change at page 60, line 43
c=IN IP4 192.0.2.37 c=IN IP4 192.0.2.37
m=audio 47190 RTP/AVP 0 m=audio 47190 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 53273 RTP/AVPF 98 m=video 53273 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir a=rtcp-fb:98 ccm fir
Example 4: The following example describes the Offer/Answer Example 4: The following example describes the Offer/Answer
implications for H.271 Video back channel messages (VBCM). The implications for H.271 Video back channel messages (VBCM). The
Offerer wishes to support VBCM and the sub-messages of payloadType Offerer wishes to support VBCM and the sub-messages of payloadType 1
1 (one or more pictures that are entirely or partially lost) and 2 (one or more pictures that are entirely or partially lost) and 2 (a
(a set of blocks of one picture that are entirely or partially set of blocks of one picture that are entirely or partially lost).
lost).
-------------> Offer -------------> Offer
v=0 v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer s=Offer/Answer
c=IN IP4 192.0.2.124 c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98 m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
skipping to change at page 63, line 9 skipping to change at page 61, line 25
v=0 v=0
o=alice 3203093520 3203093524 IN IP4 otherhost.example.com o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
s=Offer/Answer s=Offer/Answer
c=IN IP4 192.0.2.37 c=IN IP4 192.0.2.37
m=audio 47190 RTP/AVP 0 m=audio 47190 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 53273 RTP/AVPF 98 m=video 53273 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000 a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm vbcm 1 a=rtcp-fb:98 ccm vbcm 1
So in the above example only VBCM indications comprised of So, in the above example, only VBCM indications comprised of
"payloadType" 1 will be supported. "payloadType" 1 will be supported.
8. IANA Considerations 8. IANA Considerations
The new value "ccm" needs to be registered with IANA in the "rtcp- The new value "ccm" needs to be registered with IANA in the "rtcp-
fb" Attribute Values registry located at the time of publication fb" Attribute Values registry located at the time of publication at:
at:
http://www.iana.org/assignments/sdp-parameters http://www.iana.org/assignments/sdp-parameters
Value name: ccm Value name: ccm
Long Name: Codec Control Commands and Indications Long Name: Codec Control Commands and Indications
Reference: RFC XXXX Reference: RFC XXXX
A new registry "Codec Control Messages" needs to be created to A new registry "Codec Control Messages" needs to be created to hold
hold "ccm" parameters located at time of publication at: "ccm" parameters located at time of publication at:
http://www.iana.org/assignments/sdp-parameters http://www.iana.org/assignments/sdp-parameters
New registration in this registry follows the "Specification New registration in this registry follows the "Specification
required" policy as defined by [RFC2434]. In addition they are required" policy as defined by [RFC2434]. In addition they are
required to indicate which, if any additional RTCP feedback types, required to indicate which, if any additional RTCP feedback types,
such as "nack", "ack". such as "nack", "ack".
The initial content of the registry is the following values: The initial content of the registry is the following values:
Value name: fir Value name: fir
skipping to change at page 64, line 47 skipping to change at page 62, line 46
Value name: tstr Value name: tstr
Long name: temporal Spatial Trade Off Long name: temporal Spatial Trade Off
Usable with: ccm Usable with: ccm
Reference: RFC XXXX Reference: RFC XXXX
Value name: vbcm Value name: vbcm
Long name: H.271 video back channel messages Long name: H.271 video back channel messages
Usable with: ccm Usable with: ccm
Reference: RFC XXXX Reference: RFC XXXX
The following values need to be registered as FMT values in the The following values need to be registered as FMT values in the "FMT
"FMT Values for RTPFB Payload Types" registry located at the time Values for RTPFB Payload Types" registry located at the time of
of publication at: http://www.iana.org/assignments/rtp-parameters publication at: http://www.iana.org/assignments/rtp-parameters
RTPFB range RTPFB range
Name Long Name Value Reference Name Long Name Value Reference
-------------- --------------------------------- ----- --------- -------------- --------------------------------- ----- ---------
Reserved 2 [RFCxxxx] Reserved 2 [RFCxxxx]
TMMBR Temporary Maximum Media Stream Bit 3 [RFCxxxx] TMMBR Temporary Maximum Media Stream Bit 3 [RFCxxxx]
Rate Request Rate Request
TMMBN Temporary Maximum Media Stream Bit 4 [RFCxxxx] TMMBN Temporary Maximum Media Stream Bit 4 [RFCxxxx]
Rate Notification Rate Notification
The following values need to be registered as FMT values in the The following values need to be registered as FMT values in the "FMT
"FMT Values for PSFB Payload Types" registry located at the time Values for PSFB Payload Types" registry located at the time of
of publication at: http://www.iana.org/assignments/rtp-parameters publication at: http://www.iana.org/assignments/rtp-parameters
PSFB range PSFB range
Name Long Name Value Reference Name Long Name Value Reference
-------------- --------------------------------- ----- --------- -------------- --------------------------------- ----- -------
FIR Full Intra Request Command 4 [RFCxxxx] FIR Full Intra Request Command 4 [RFCxxxx]
TSTR Temporal-Spatial Trade-off Request 5 [RFCxxxx] TSTR Temporal-Spatial Trade-off Request 5 [RFCxxxx]
TSTN Temporal-Spatial Trade-off Notification 6 [RFCxxxx] TSTN Temporal-Spatial Trade-off Notification 6 [RFCxxxx]
VBCM Video Back Channel Message 7 [RFCxxxx] VBCM Video Back Channel Message 7 [RFCxxxx]
9. Contributors 9. Contributors
Tom Taylor has made a very significant contribution, for which the Tom Taylor has made a very significant contribution, for which the
authors are very grateful, to this specification by helping authors are very grateful, to this specification by helping rewrite
rewrite the specification. Especially the parts regarding the the specification. Especially the parts regarding the algorithm for
algorithm for determining bounding sets for TMMBR have benefited. determining bounding sets for TMMBR have benefited.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank Andrea Basso, Orit Levin, Nermeen The authors would like to thank Andrea Basso, Orit Levin, Nermeen
Ismail for their work on the requirement and discussion draft Ismail for their work on the requirement and discussion draft
[Basso]. [Basso].
Drafts of this memo were reviewed and extensively commented by Drafts of this memo were reviewed and extensively commented by Roni
Roni Even, Colin Perkins, Randell Jesup, Keith Lantz, Harikishan Even, Colin Perkins, Randell Jesup, Keith Lantz, Harikishan
Desineni, Guido Franceschini and others. The authors appreciate Desineni, Guido Franceschini and others. The authors appreciate
these reviews. these reviews.
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
11. References 11. References
11.1. Normative references 11.1. Normative references
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., Rey, J.,
J., "Extended RTP Profile for Real-Time Transport "Extended RTP Profile for Real-Time Transport Control
Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
[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, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Session Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
Model with Session Description Protocol (SDP)", RFC with Session Description Protocol (SDP)", RFC 3264, June
3264, June 2002. 2002.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
an IANA Considerations Section in RFCs", BCP 26, RFC IANA Considerations Section in RFCs", BCP 26, RFC 2434,
2434, October 1998. October 1998.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
11.2. Informative references 11.2. Informative references
[Basso] A. Basso, et. al., "Requirements for transport of [Basso] A. Basso, et. al., "Requirements for transport of video
video control commands", draft-basso-avt-videoconreq- control commands", draft-basso-avt-videoconreq-02.txt,
02.txt, expired Internet Draft, October 2004. expired Internet Draft, October 2004.
[AVC] Joint Video Team of ITU-T and ISO/IEC JTC 1, Draft [AVC] Joint Video Team of ITU-T and ISO/IEC JTC 1, Draft ITU-T
ITU-T Recommendation and Final Draft International Recommendation and Final Draft International Standard of
Standard of Joint Video Specification (ITU-T Rec. Joint Video Specification (ITU-T Rec. H.264 | ISO/IEC
H.264 | ISO/IEC 14496-10 AVC), Joint Video Team (JVT) 14496-10 AVC), Joint Video Team (JVT) of ISO/IEC MPEG
of ISO/IEC MPEG and ITU-T VCEG, JVT-G050, March 2003. and ITU-T VCEG, JVT-G050, March 2003.
[H245] ITU-T Rec. HG.245, "Control protocol for multimedia [H245] ITU-T Rec. HG.245, "Control protocol for multimedia
communication", MAY 2006 communication", MAY 2006
[NEWPRED] S. Fukunaga, T. Nakai, and H. Inoue, "Error Resilient [NEWPRED] S. Fukunaga, T. Nakai, and H. Inoue, "Error Resilient
Video Coding by Dynamic Replacing of Reference Video Coding by Dynamic Replacing of Reference
Pictures," in Proc. Globcom'96, vol. 3, pp. 1503 - Pictures," in Proc. Globcom'96, vol. 3, pp. 1503 - 1508,
1508, 1996. 1996.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and [SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004. (SRTP)", RFC 3711, March 2004.
[RFC2032] Turletti, T. and C. Huitema, "RTP Payload Format for [RFC2032] Turletti, T. and C. Huitema, "RTP Payload Format for
H.261 Video Streams", RFC 2032, October 1996. H.261 Video Streams", RFC 2032, October 1996.
[SAVPF] J. Ott, E. Carrara, "Extended Secure RTP Profile for [SAVPF] J. Ott, E. Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)," draft-ietf-avt- RTCP-based Feedback (RTP/SAVPF)," draft-ietf-avt-
profile-savpf-10.txt, February, 2007. profile-savpf-10.txt, February, 2007.
[RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, [RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
skipping to change at page 68, line 21 skipping to change at page 65, line 21
[RFC3448] M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP [RFC3448] M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
[VBCM] ITU-T Rec. H.271, "Video Back Channel Messages", June [VBCM] ITU-T Rec. H.271, "Video Back Channel Messages", June
2006 2006
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth [RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)", Modifier for the Session Description Protocol (SDP)",
RFC 3890, September 2004. RFC 3890, September 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March Congestion Control Protocol (DCCP)", RFC 4340, March
2006. 2006.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
Johnston, A., Peterson, J., Sparks, R., Handley, M., A., Peterson, J., Sparks, R., Handley, M., and E.
and E. Schooler, "SIP: Session Initiation Protocol", Schooler, "SIP: Session Initiation Protocol", RFC 3261,
RFC 3261, June 2002. June 2002.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC Parisis, "RTP Payload for Redundant Audio Data", RFC
2198, September 1997. 2198, September 1997.
[Topologies] M. Westerlund, and S. Wenger, "RTP Topologies", [Topologies] M. Westerlund, and S. Wenger, "RTP Topologies", draft-
draft-ietf-avt-topologies-04, work in progress, Feb ietf-avt-topologies-04, work in progress, Feb 2007.
2007.
12. Authors' Addresses 12. Authors' Addresses
Stephan Wenger Stephan Wenger
Nokia Corporation Nokia Corporation
975, Page Mill Road, 975, Page Mill Road,
Palo Alto,CA 94304 Palo Alto,CA 94304
USA USA
Phone: +1-650-862-7368 Phone: +1-650-862-7368
EMail: stewe@stewe.org EMail: stewe@stewe.org
Umesh Chandra Umesh Chandra
Nokia Research Center Nokia Research Center
975, Page Mill Road, 975, Page Mill Road,
Palo Alto,CA 94304 Palo Alto,CA 94304
USA USA
Phone: +1-650-796-7502 Phone: +1-650-796-7502
Email: Umesh.Chandra@nokia.com Email: Umesh.1.Chandra@nokia.com
Magnus Westerlund Magnus Westerlund
Ericsson Research Ericsson Research
Ericsson AB Ericsson AB
SE-164 80 Stockholm, SWEDEN SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000 Phone: +46 8 7190000
EMail: magnus.westerlund@ericsson.com EMail: magnus.westerlund@ericsson.com
Bo Burman Bo Burman
skipping to change at page 70, line 13 skipping to change at page 67, line 13
EMail: bo.burman@ericsson.com EMail: bo.burman@ericsson.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on This document and the information contained herein are provided on an
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RFC Editor Considerations RFC Editor Considerations
The RFC editor is requested to replace all occurrences of XXXX The RFC editor is requested to replace all occurrences of XXXX with
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