RFC 5104 - Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)
(Formats: TXT)
Network Working Group S. Wenger
Request for Comments: 5104 U. Chandra
Category: Standards Track Nokia
M. Westerlund
B. Burman
Ericsson
February 2008
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Codec Control Messages in the
RTP Audio-Visual Profile with Feedback (AVPF)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies a few extensions to the messages defined in
the Audio-Visual Profile with Feedback (AVPF). They are helpful
primarily in conversational multimedia scenarios where centralized
multipoint functionalities are in use. However, some are also usable
in smaller multicast environments and point-to-point calls.
The extensions discussed are messages related to the ITU-T Rec. H.271
Video Back Channel, Full Intra Request, Temporary Maximum Media
Stream Bit Rate, and Temporal-Spatial Trade-off.
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Table of Contents
1. Introduction ....................................................4
2. Definitions .....................................................5
2.1. Glossary ...................................................5
2.2. Terminology ................................................5
2.3. Topologies .................................................8
3. Motivation ......................................................8
3.1. Use Cases ..................................................9
3.2. Using the Media Path ......................................11
3.3. Using AVPF ................................................11
3.3.1. Reliability ........................................12
3.4. Multicast .................................................12
3.5. Feedback Messages .........................................12
3.5.1. Full Intra Request Command .........................12
3.5.1.1. Reliability ...............................13
3.5.2. Temporal-Spatial Trade-off Request and
Notification .......................................14
3.5.2.1. Point-to-Point ............................15
3.5.2.2. Point-to-Multipoint Using
Multicast or Translators ..................15
3.5.2.3. Point-to-Multipoint Using RTP Mixer .......15
3.5.2.4. Reliability ...............................16
3.5.3. H.271 Video Back Channel Message ...................16
3.5.3.1. Reliability ...............................19
3.5.4. Temporary Maximum Media Stream Bit Rate
Request and Notification ...........................19
3.5.4.1. Behavior for Media Receivers Using TMMBR ..21
3.5.4.2. Algorithm for Establishing Current
Limitations ...............................23
3.5.4.3. Use of TMMBR in a Mixer-Based
Multipoint Operation ......................29
3.5.4.4. Use of TMMBR in Point-to-Multipoint Using
Multicast or Translators ..................30
3.5.4.5. Use of TMMBR in Point-to-Point Operation ..31
3.5.4.6. Reliability ...............................31
4. RTCP Receiver Report Extensions ................................32
4.1. Design Principles of the Extension Mechanism ..............32
4.2. Transport Layer Feedback Messages .........................33
4.2.1. Temporary Maximum Media Stream Bit Rate
Request (TMMBR) ....................................34
4.2.1.1. Message Format ............................34
4.2.1.2. Semantics .................................35
4.2.1.3. Timing Rules ..............................39
4.2.1.4. Handling in Translators and Mixers ........39
4.2.2. Temporary Maximum Media Stream Bit Rate
Notification (TMMBN) ...............................39
4.2.2.1. Message Format ............................39
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4.2.2.2. Semantics .................................40
4.2.2.3. Timing Rules ..............................41
4.2.2.4. Handling by Translators and Mixers ........41
4.3. Payload-Specific Feedback Messages ........................41
4.3.1. Full Intra Request (FIR) ...........................42
4.3.1.1. Message Format ............................42
4.3.1.2. Semantics .................................43
4.3.1.3. Timing Rules ..............................44
4.3.1.4. Handling of FIR Message in Mixers and
Translators ...............................44
4.3.1.5. Remarks ...................................44
4.3.2. Temporal-Spatial Trade-off Request (TSTR) ..........45
4.3.2.1. Message Format ............................46
4.3.2.2. Semantics .................................46
4.3.2.3. Timing Rules ..............................47
4.3.2.4. Handling of Message in Mixers and
Translators ...............................47
4.3.2.5. Remarks ...................................47
4.3.3. Temporal-Spatial Trade-off Notification (TSTN) .....48
4.3.3.1. Message Format ............................48
4.3.3.2. Semantics .................................49
4.3.3.3. Timing Rules ..............................49
4.3.3.4. Handling of TSTN in Mixers and
Translators ...............................49
4.3.3.5. Remarks ...................................49
4.3.4. H.271 Video Back Channel Message (VBCM) ............50
4.3.4.1. Message Format ............................50
4.3.4.2. Semantics .................................51
4.3.4.3. Timing Rules ..............................52
4.3.4.4. Handling of Message in Mixers or
Translators ...............................52
4.3.4.5. Remarks ...................................52
5. Congestion Control .............................................52
6. Security Considerations ........................................53
7. SDP Definitions ................................................54
7.1. Extension of the rtcp-fb Attribute ........................54
7.2. Offer-Answer ..............................................55
7.3. Examples ..................................................56
8. IANA Considerations ............................................58
9. Contributors ...................................................60
10. Acknowledgements ..............................................60
11. References ....................................................60
11.1. Normative References .....................................60
11.2. Informative References ...................................61
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1. Introduction
When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
developed, the main emphasis lay in the efficient support of point-
to-point and small multipoint scenarios without centralized
multipoint control. However, in practice, many small multipoint
conferences operate utilizing devices known as Multipoint Control
Units (MCUs). Long-standing experience of the conversational video
conferencing industry suggests that there is a need for a few
additional feedback messages, to support centralized multipoint
conferencing efficiently. Some of the messages have applications
beyond centralized multipoint, and this is indicated in the
description of the message. This is especially true for the message
intended to carry ITU-T Rec. H.271 [H.271] bit strings for Video Back
Channel messages.
In Real-time Transport Protocol (RTP) [RFC3550] terminology, MCUs
comprise mixers and translators. Most MCUs also include signaling
support. During the development of this memo, it was noticed that
there is considerable confusion in the community related to the use
of terms such as mixer, translator, and MCU. In response to these
concerns, a number of topologies have been identified that are of
practical relevance to the industry, but are not documented in
sufficient detail in [RFC3550]. These topologies are documented in
[RFC5117], and understanding this memo requires previous or parallel
study of [RFC5117].
Some of the messages defined here are forward only, in that they do
not require an explicit notification to the message emitter that they
have been received and/or indicating the message receiver's actions.
Other messages require a response, leading to a two-way communication
model that one could view as useful for control purposes. However,
it is not the intention of this memo to open up RTP Control Protocol
(RTCP) to a generalized control protocol. All mentioned messages
have relatively strict real-time constraints, in the sense that their
value diminishes with increased delay. This makes the use of more
traditional control protocol means, such as Session Initiation
Protocol (SIP) [RFC3261], undesirable when used for the same purpose.
That is why this solution is recommended instead of "XML Schema for
Media Control" [XML-MC], which uses SIP Info to transfer XML messages
with similar semantics to what are defined in this memo.
Furthermore, all messages are of a very simple format that can be
easily processed by an RTP/RTCP sender/receiver. Finally, and most
importantly, all messages relate only to the RTP stream with which
they are associated, and not to any other property of a communication
system. In particular, none of them relate to the properties of the
access links traversed by the session.
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2. Definitions
2.1. Glossary
AIMD - Additive Increase Multiplicative Decrease
AVPF - The extended RTP profile for RTCP-based feedback
FCI - Feedback Control Information [RFC4585]
FEC - Forward Error Correction
FIR - Full Intra Request
MCU - Multipoint Control Unit
MPEG - Moving Picture Experts Group
PLI - Picture Loss Indication
PR - Packet rate
QP - Quantizer Parameter
RTT - Round trip time
SSRC - Synchronization Source
TMMBN - Temporary Maximum Media Stream Bit Rate Notification
TMMBR - Temporary Maximum Media Stream Bit Rate Request
TSTN - Temporal-Spatial Trade-off Notification
TSTR - Temporal-Spatial Trade-off Request
VBCM - Video Back Channel Message
2.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Message:
An RTCP feedback message [RFC4585] defined by this
specification, of one of the following types:
Request:
Message that requires acknowledgement
Command:
Message that forces the receiver to an action
Indication:
Message that reports a situation
Notification:
Message that provides a notification that an event has
occurred. Notifications are commonly generated in
response to a Request.
Note that, with the exception of "Notification", this terminology is
in alignment with ITU-T Rec. H.245 [H245].
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Decoder Refresh Point:
A bit string, packetized in one or more RTP packets, that
completely resets the decoder to a known state.
Examples for "hard" decoder refresh points are Intra pictures
in H.261, H.263, MPEG-1, MPEG-2, and MPEG-4 part 2, and
Instantaneous Decoder Refresh (IDR) pictures in H.264.
"Gradual" decoder refresh points may also be used; see for
example [AVC]. While both "hard" and "gradual" decoder
refresh points are acceptable in the scope of this
specification, in most cases the user experience will benefit
from using a "hard" decoder refresh point.
A decoder refresh point also contains all header information
above the picture layer (or equivalent, depending on the video
compression standard) that is conveyed in-band. In H.264, for
example, a decoder refresh point contains parameter set
Network Adaptation Layer (NAL) units that generate parameter
sets necessary for the decoding of the following slice/data
partition NAL units (and that are not conveyed out of band).
Decoding:
The operation of reconstructing the media stream.
Rendering:
The operation of presenting (parts of) the reconstructed media
stream to the user.
Stream thinning:
The operation of removing some of the packets from a media
stream. Stream thinning, preferably, is media-aware, implying
that media packets are removed in the order of increasing
relevance to the reproductive quality. However, even when
employing media-aware stream thinning, most media streams
quickly lose quality when subjected to increasing levels of
thinning. Media-unaware stream thinning leads to even worse
quality degradation. In contrast to transcoding, stream
thinning is typically seen as a computationally lightweight
operation.
Media:
Often used (sometimes in conjunction with terms like bit rate,
stream, sender, etc.) to identify the content of the forward
RTP packet stream (carrying the codec data), to which the
codec control message applies.
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Media Stream:
The stream of RTP packets labeled with a single
Synchronization Source (SSRC) carrying the media (and also in
some cases repair information such as retransmission or
Forward Error Correction (FEC) information).
Total media bit rate:
The total bits per second transferred in a media stream,
measured at an observer-selected protocol layer and averaged
over a reasonable timescale, the length of which depends on
the application. In general, a media sender and a media
receiver will observe different total media bit rates for the
same stream, first because they may have selected different
reference protocol layers, and second, because of changes in
per-packet overhead along the transmission path. The goal
with bit rate averaging is to be able to ignore any burstiness
on very short timescales (e.g., below 100 ms) introduced by
scheduling or link layer packetization effects.
Maximum total media bit rate:
The upper limit on total media bit rate for a given media
stream at a particular receiver and for its selected protocol
layer. Note that this value cannot be measured on the
received media stream. Instead, it needs to be calculated or
determined through other means, such as quality of service
(QoS) negotiations or local resource limitations. Also note
that this value is an average (on a timescale that is
reasonable for the application) and that it may be different
from the instantaneous bit rate seen by packets in the media
stream.
Overhead:
All protocol header information required to convey a packet
with media data from sender to receiver, from the application
layer down to a pre-defined protocol level (for example, down
to, and including, the IP header). Overhead may include, for
example, IP, UDP, and RTP headers, any layer 2 headers, any
Contributing Sources (CSRCs), RTP padding, and RTP header
extensions. Overhead excludes any RTP payload headers and the
payload itself.
Net media bit rate:
The bit rate carried by a media stream, net of overhead. That
is, the bits per second accounted for by encoded media, any
applicable payload headers, and any directly associated meta
payload information placed in the RTP packet. A typical
example of the latter is redundancy data provided by the use
of RFC 2198 [RFC2198]. Note that, unlike the total media bit
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rate, the net media bit rate will have the same value at the
media sender and at the media receiver unless any mixing or
translating of the media has occurred.
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 the
per-packet overhead as defined above multiplied by the packet
rate.
Feasible region:
The set of all combinations of packet rate and net media bit
rate that do not exceed the restrictions in maximum media bit
rate placed on a given media sender by the Temporary Maximum
Media Stream Bit Rate Request (TMMBR) messages it has
received. The feasible region will change as new TMMBR
messages are received.
Bounding set:
The set of TMMBR tuples, selected from all those received at a
given media sender, that define the feasible region for that
media sender. The media sender uses an algorithm such as that
in section 3.5.4.2 to determine or iteratively approximate the
current bounding set, and reports that set back to the media
receivers in a Temporary Maximum Media Stream Bit Rate
Notification (TMMBN) message.
2.3. Topologies
Please refer to [RFC5117] for an in-depth discussion. The topologies
referred to throughout this memo are labeled (consistently with
[RFC5117]) as follows:
Topo-Point-to-Point . . . . . Point-to-point communication
Topo-Multicast . . . . . . . Multicast communication
Topo-Translator . . . . . . . Translator based
Topo-Mixer . . . . . . . . . Mixer based
Topo-RTP-switch-MCU . . . . . RTP stream switching MCU
Topo-RTCP-terminating-MCU . . Mixer but terminating RTCP
3. Motivation
This section discusses the motivation and usage of the different
video and media control messages. The video control messages have
been under discussion for a long time, and a requirement document was
drawn up [Basso]. That document has expired; however, we quote
relevant sections of it to provide motivation and requirements.
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3.1. Use Cases
There are a number of possible usages for the proposed feedback
messages. Let us begin by looking through the use cases Basso et al.
[Basso] proposed. Some of the use cases have been reformulated and
comments have been added.
1. An RTP video mixer composes multiple encoded video sources into a
single encoded video stream. Each time a video source is added,
the RTP mixer needs to request a decoder refresh point from the
video source, so as to start an uncorrupted prediction chain on
the spatial area of the mixed picture occupied by the data from
the new video source.
2. An RTP video mixer receives multiple encoded RTP video streams
from conference participants, and dynamically selects one of the
streams to be included in its output RTP stream. At the time of a
bit stream change (determined through means such as voice
activation or the user interface), the mixer requests a decoder
refresh point from the remote source, in order to avoid using
unrelated content as reference data for inter picture prediction.
After requesting the decoder refresh point, the video mixer stops
the delivery of the current RTP stream and monitors the RTP stream
from the new source until it detects data belonging to the decoder
refresh point. At that time, the RTP mixer starts forwarding the
newly selected stream to the receiver(s).
3. An application needs to signal to the remote encoder that the
desired trade-off between temporal and spatial resolution has
changed. For example, one user may prefer a higher frame rate and
a lower spatial quality, and another user may prefer the opposite.
This choice is also highly content dependent. Many current video
conferencing systems offer in the user interface a mechanism to
make this selection, usually in the form of a slider. The
mechanism is helpful in point-to-point, centralized multipoint and
non-centralized multipoint uses.
4. Use case 4 of the Basso document applies only to Picture Loss
Indication (PLI) as defined in AVPF [RFC4585] and is not
reproduced here.
5. Use case 5 of the Basso document relates to a mechanism known as
"freeze picture request". Sending freeze picture requests over a
non-reliable forward RTCP channel has been identified as
problematic. Therefore, no freeze picture request has been
included in this memo, and the use case discussion is not
reproduced here.
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6. A video mixer dynamically selects one of the received video
streams to be sent out to participants and tries to provide the
highest bit rate possible to all participants, while minimizing
stream trans-rating. One way of achieving this is to set up
sessions with endpoints using the maximum bit rate accepted by
each endpoint, and accepted by the call admission method used by
the mixer. By means of commands that reduce the maximum media
stream bit rate below what has been negotiated during session set
up, the mixer can reduce the maximum bit rate sent by endpoints to
the lowest of all the accepted bit rates. As the lowest accepted
bit rate changes due to endpoints joining and leaving or due to
network congestion, the mixer can adjust the limits at which
endpoints can send their streams to match the new value. The
mixer then requests a new maximum bit rate, which is equal to or
less than the maximum bit rate negotiated at session setup for a
specific media stream, and the remote endpoint can respond with
the actual bit rate that it can support.
The picture Basso, et al., draw up covers most applications we
foresee. However, we would like to extend the list with two
additional use cases:
7. Currently deployed congestion control algorithms (AIMD and TCP
Friendly Rate Control (TFRC) [RFC3448]) probe for additional
available capacity as long as there is something to send. With
congestion control algorithms using packet loss as the indication
for congestion, this probing generally results in reduced media
quality (often to a point where the distortion is large enough to
make the media unusable), due to packet loss and increased delay.
In a number of deployment scenarios, especially cellular ones, the
bottleneck link is often the last hop link. That cellular link
also commonly has some type of QoS negotiation enabling the
cellular device to learn the maximal bit rate available over this
last hop. A media receiver behind this link can, in most (if not
all) cases, calculate at least an upper bound for the bit rate
available for each media stream it presently receives. How this
is done is an implementation detail and not discussed herein.
Indicating the maximum available bit rate to the transmitting
party for the various media streams can be beneficial to prevent
that party from probing for bandwidth for this stream in excess of
a known hard limit. For cellular or other mobile devices, the
known available bit rate for each stream (deduced from the link
bit rate) can change quickly, due to handover to another
transmission technology, QoS renegotiation due to congestion, etc.
To enable minimal disruption of service, quick convergence is
necessary, and therefore media path signaling is desirable.
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8. The use of reference picture selection (RPS) as an error
resilience tool was introduced in 1997 as NEWPRED [NEWPRED], and
is now widely deployed. When RPS is in use, simplistically put,
the receiver can send a feedback message to the sender,
indicating a reference picture that should be used for future
prediction. ([NEWPRED] mentions other forms of feedback as
well.) AVPF contains a mechanism for conveying such a message,
but did not specify for which codec and according to which syntax
the message should conform. Recently, the ITU-T finalized Rec.
H.271, which (among other message types) also includes a feedback
message. It is expected that this feedback message will fairly
quickly enjoy wide support. Therefore, a mechanism to convey
feedback messages according to H.271 appears to be desirable.
3.2. Using the Media Path
There are two reasons why we use the media path for the codec control
messages.
First, systems employing MCUs often separate the control and media
processing parts. As these messages are intended for or generated by
the media part rather than the signaling part of the MCU, having them
on the media path avoids transmission across interfaces and
unnecessary control traffic between signaling and processing. If the
MCU is physically decomposed, the use of the media path avoids the
need for media control protocol extensions (e.g., in media gateway
control (MEGACO) [RFC3525]).
Secondly, the signaling path quite commonly contains several
signaling entities, e.g., SIP proxies and application servers.
Avoiding going through signaling entities avoids delay for several
reasons. Proxies have less stringent delay requirements than media
processing, and due to their complex and more generic nature may
result in significant processing delay. The topological locations of
the signaling entities are also commonly not optimized for minimal
delay, but rather towards other architectural goals. Thus, the
signaling path can be significantly longer in both geographical and
delay sense.
3.3. Using AVPF
The AVPF feedback message framework [RFC4585] provides the
appropriate framework to implement the new messages. AVPF implements
rules controlling the timing of feedback messages to avoid congestion
through network flooding by RTCP traffic. We re-use these rules by
referencing AVPF.
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The signaling setup for AVPF allows each individual type of function
to be configured or negotiated on an RTP session basis.
3.3.1. Reliability
The use of RTCP messages implies that each message transfer is
unreliable, unless the lower layer transport provides reliability.
The different messages proposed in this specification have different
requirements in terms of reliability. However, in all cases, the
reaction to an (occasional) loss of a feedback message is specified.
3.4. Multicast
The codec control messages might be used with multicast. The RTCP
timing rules specified in [RFC3550] and [RFC4585] ensure that the
messages do not cause overload of the RTCP connection. The use of
multicast may result in the reception of messages with inconsistent
semantics. The reaction to inconsistencies depends on the message
type, and is discussed for each message type separately.
3.5. Feedback Messages
This section describes the semantics of the different feedback
messages and how they apply to the different use cases.
3.5.1. Full Intra Request Command
A Full Intra Request (FIR) Command, when received by the designated
media sender, requires that the media sender sends a Decoder Refresh
Point (see section 2.2) at the earliest opportunity. The evaluation
of such an opportunity includes the current encoder coding strategy
and the current available network resources.
FIR is also known as an "instantaneous decoder refresh request",
"fast video update request" or "video fast update request".
Using a decoder refresh point implies refraining from using any
picture sent prior to that point as a reference for the encoding
process of any subsequent picture sent in the stream. For predictive
media types that are not video, the analogue applies. For example,
if in MPEG-4 systems scene updates are used, the decoder refresh
point consists of the full representation of the scene and is not
delta-coded relative to previous updates.
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Decoder refresh points, especially Intra or IDR pictures, are in
general several times larger in size than predicted pictures. Thus,
in scenarios in which the available bit rate is small, the use of a
decoder refresh point implies a delay that is significantly longer
than the typical picture duration.
Usage in multicast is possible; however, aggregation of the commands
is recommended. A receiver that receives a request closely after
sending a decoder refresh point -- within 2 times the longest round
trip time (RTT) known, plus any AVPF-induced RTCP packet sending
delays -- should await a second request message to ensure that the
media receiver has not been served by the previously delivered
decoder refresh point. The reason for the specified delay is to
avoid sending unnecessary decoder refresh points. A session
participant may have sent its own request while another participant's
request was in-flight to them. Suppressing those requests that may
have been sent without knowledge about the other request avoids this
issue.
Using the FIR command to recover from errors is explicitly
disallowed, and instead the PLI message defined in AVPF [RFC4585]
should be used. The PLI message reports lost pictures and has been
included in AVPF for precisely that purpose.
Full Intra Request is applicable in use-cases 1 and 2.
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 repeat
the FIR until the desired content has been received. The repetition
interval is determined by the RTCP timing rules applicable to the
session. Upon reception of a complete decoder refresh point or the
detection of an attempt to send a decoder refresh point (which got
damaged due to a packet loss), the repetition of the FIR must stop.
If another FIR is necessary, the request sequence number must be
increased. A FIR sender shall not have more than one FIR (different
request sequence number) outstanding at any time per media sender in
the session.
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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 or RTP switching MCU that receive a FIR from a media
receiver is responsible to ensure that a decoder refresh point is
delivered to the requesting receiver. It may be necessary for the
mixer/MCU to generate FIR commands. From a reliability perspective,
the two legs (FIR-requesting endpoint to mixer/MCU, and mixer/MCU to
decoder refresh point generating endpoint) are handled independently
from each other.
3.5.2. Temporal-Spatial Trade-off Request and Notification
The Temporal-Spatial Trade-off Request (TSTR) instructs the video
encoder to change its trade-off between temporal and spatial
resolution. Index values from 0 to 31 indicate monotonically a
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
frame rate, whereas a requester asking for 31 wishes a high frame
rate, potentially at the cost of low spatial quality.
In general, the encoder reaction time may be significantly longer
than the typical picture duration. See use case 3 for an example.
The encoder decides whether and to what extent the request results in
a change of the trade-off. It returns a Temporal-Spatial Trade-off
Notification (TSTN) message to indicate the trade-off that it will
use henceforth.
TSTR and TSTN have been introduced primarily because it is believed
that control protocol mechanisms, e.g., a SIP re-invite, are too
heavyweight and too slow to allow for a reasonable user experience.
Consider, for example, a user interface where the remote user selects
the temporal/spatial trade-off with a slider. An immediate feedback
to any slider movement is required for a reasonable user experience.
A SIP re-INVITE [RFC3261] would require at least two round-trips more
(compared to the TSTR/TSTN mechanism) and may involve proxies and
other complex mechanisms. Even in a well-designed system, it could
take a second or so until the new trade-off is finally selected.
Furthermore, the use of RTCP solves the multicast use case very
efficiently.
The use of TSTR and TSTN in multipoint scenarios is a non-trivial
subject, and can be achieved in many implementation-specific ways.
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Problems stem from the fact that TSTRs will typically arrive
unsynchronized, and may request different trade-off values for the
same stream and/or endpoint encoder. This memo does not specify 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
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 or may not be
based on the trade-off conveyed by the TSTR). In other words, the
trade-off sent in a TSTR is a non-binding recommendation, nothing
more.
Three TSTR/TSTN scenarios need to be distinguished, based on the
topologies described in [RFC5117]. The scenarios are described in
the following subsections.
3.5.2.1. Point-to-Point
In this most trivial case (Topo-Point-to-Point), the media sender
typically adjusts its temporal/spatial trade-off based on the
requested value in TSTR, subject to its own capabilities. The TSTN
message conveys back the new trade-off value (which may be identical
to the old one if, for example, the sender is not capable of
adjusting its trade-off).
3.5.2.2. Point-to-Multipoint Using Multicast or Translators
RTCP Multicast is used either with media multicast according to
Topo-Multicast, or following RFC 3550's translator model according to
Topo-Translator. In these cases, unsynchronized TSTR messages from
different receivers may be received, possibly with different
requested trade-offs (because of different user preferences). This
memo does not specify how the media sender tunes its trade-off.
Possible strategies include selecting the mean or median of all
trade-off requests received, giving priority to certain participants,
or continuing to use the previously selected trade-off (e.g., when
the sender is not capable of adjusting it). Again, all TSTR messages
need to be acknowledged by TSTN, and the value conveyed back has to
reflect the decision made.
3.5.2.3. Point-to-Multipoint Using RTP Mixer
In this scenario (Topo-Mixer), the RTP mixer receives all TSTR
messages, and has the opportunity to act on them based on its own
criteria. In most cases, the mixer should form a "consensus" of
potentially conflicting TSTR messages arriving from different
participants, and initiate its own TSTR message(s) to the media
sender(s). As in the previous scenario, the strategy for forming
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this "consensus" is up to the implementation, and can, for example,
encompass averaging the participants' request values, giving priority
to certain participants, or using session default values.
Even if a mixer or translator performs transcoding, it is very
difficult to deliver media with the requested trade-off, unless the
content the mixer or translator receives is already close to that
trade-off. Thus, if the mixer changes its trade-off, it needs to
request the media sender(s) to use the new value, by creating a TSTR
of its own. Upon reaching a decision on the used trade-off, it
includes that value in the acknowledgement to the downstream
requestors. Only in cases where the original source has
substantially higher quality (and bit rate) is it likely that
transcoding alone can result in the requested trade-off.
3.5.2.4. Reliability
A request and reception acknowledgement mechanism is specified. The
Temporal-Spatial Trade-off Notification (TSTN) message informs the
requester that its request has been received, and what trade-off is
used henceforth. This acknowledgement mechanism is desirable for at
least the following reasons:
o A change in the trade-off cannot be directly identified from the
media bit stream.
o User feedback cannot be implemented without knowing the chosen
trade-off value, according to the media sender's constraints.
o Repetitive sending of messages requesting an unimplementable
trade-off can be avoided.
3.5.3. H.271 Video Back Channel Message
ITU-T Rec. H.271 defines syntax, semantics, and suggested encoder
reaction to a Video Back Channel Message. The structure defined in
this memo is used to transparently convey such a message from media
receiver to media sender. In this memo, we refrain from an in-depth
discussion of the available code points within H.271 and refer to the
specification text [H.271] instead.
However, we note that some H.271 messages bear similarities with
native messages of AVPF and this memo. Furthermore, we note that
some H.271 message are known to require caution in multicast
environments -- or are plainly not usable in multicast or multipoint
scenarios. Table 1 provides a brief, simplified overview of the
messages currently defined in H.271, their roughly corresponding AVPF
or Codec Control Messages (CCMs) (the latter as specified in this
memo), and an indication of our current knowledge of their multicast
safety.
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H.271 msg type AVPF/CCM msg type multicast-safe
--------------------------------------------------------------------
0 (when used for
reference picture
selection) AVPF RPSI No (positive ACK of pictures)
1 picture loss AVPF PLI Yes
2 partial loss AVPF SLI Yes
3 one 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
Table 1: H.271 messages and their AVPF/CCM equivalents
Note: H.271 message type 0 is not a strict equivalent to
AVPF's Reference Picture Selection Indication (RPSI); it is an
indication of known-as-correct reference picture(s) at the
decoder. It does not command an encoder to use a defined
reference picture (the form of control information envisioned
to be carried in RPSI). However, it is believed and intended
that H.271 message type 0 will be used for the same purpose as
AVPF's RPSI -- although other use forms are also possible.
In response to the opaqueness of the H.271 messages, especially with
respect to the multicast safety, the following guidelines MUST be
followed when an implementation wishes to employ the H.271 video back
channel message:
1. Implementations utilizing the H.271 feedback message MUST stay in
compliance with congestion control principles, as outlined in
section 5.
2. An implementation SHOULD utilize the IETF-native messages as
defined in [RFC4585] and in this memo instead of similar messages
defined in [H.271]. Our current understanding of similar messages
is documented in Table 1 above. One good reason to divert from
the SHOULD statement above would be if it is clearly understood
that, for a given application and video compression standard, the
aforementioned "similarity" is not given, in contrast to what the
table indicates.
3. It has been observed that some of the H.271 code points currently
in existence are not multicast-safe. Therefore, the sensible
thing to do is not to use the H.271 feedback message type in
multicast environments. It MAY be used only when all the issues
mentioned later are fully understood by the implementer, and
properly taken into account by all endpoints. In all other cases,
the H.271 message type MUST NOT be used in conjunction with
multicast.
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4. It has been observed that even in centralized multipoint
environments, where the mixer should theoretically be able to
resolve issues as documented below, the implementation of such a
mixer and cooperative endpoints is a very difficult and tedious
task. Therefore, H.271 messages MUST NOT be used in centralized
multipoint scenarios, unless all the issues mentioned below are
fully understood by the implementer, and properly taken into
account by both mixer and endpoints.
Issues to be taken into account when considering the use of H.271 in
multipoint environments:
1. Different state on different receivers. In many environments, it
cannot be guaranteed that the decoder state of all media receivers
is identical at any given point in time. The most obvious reason
for such a possible misalignment of state is a loss that occurs on
the path to only one of many media receivers. However, there are
other not so obvious reasons, such as recent joins to the
multipoint conference (be it by joining the multicast group or
through additional mixer output). Different states can lead the
media receivers to issue potentially contradicting H.271 messages
(or one media receiver issuing an H.271 message that, when
observed by the media sender, is not helpful for the other media
receivers). A naive reaction of the media sender to these
contradicting messages can lead to unpredictable and annoying
results.
2. Combining messages from different media receivers in a media
sender is a non-trivial task. As reasons, we note that these
messages may be contradicting each other, and that their 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 algorithm for
combining must be aware both of the network/protocol environment
(i.e., with respect to congestion) and of the media codec
employed, as H.271 messages of a given type can have different
semantics for different media codecs.
3. The suppression of requests may need to go beyond the basic
mechanisms described in AVPF (which are driven exclusively by
timing and transport considerations on the protocol level). For
example, a receiver is often required to refrain from (or delay)
generating requests, based on information it receives from the
media stream. For instance, it makes no sense for a receiver to
issue a FIR when a transmission of an Intra/IDR picture is
ongoing.
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4. When using the non-multicast-safe messages (e.g., H.271 type 0
positive ACK of received pictures/slices) in larger multicast
groups, the media receiver will likely be forced to delay or even
omit sending these messages. For the media sender, this looks
like data has not been properly received (although it was received
properly), and a naively implemented media sender reacts to these
perceived problems where it should not.
3.5.3.1. Reliability
H.271 Video Back Channel Messages do not require reliable
transmission, and confirmation of the reception of a message can be
derived from the forward video bit stream. Therefore, no specific
reception acknowledgement is specified.
With respect to re-sending rules, section 3.5.1.1 applies.
3.5.4. Temporary Maximum Media Stream Bit Rate Request and Notification
A receiver, translator, or mixer uses the Temporary Maximum Media
Stream Bit Rate Request (TMMBR, "timber") to request a sender to
limit the maximum bit rate for a media stream (see section 2.2) to,
or below, the provided value. The Temporary Maximum Media Stream Bit
Rate Notification (TMMBN) contains the media sender's current view of
the most limiting subset of the TMMBR-defined limits it has received,
to help the participants to suppress TMMBRs that would not further
restrict the media sender. The primary usage for the TMMBR/TMMBN
messages is in a scenario with an MCU or mixer (use case 6),
corresponding to Topo-Translator or Topo-Mixer, but also to Topo-
Point-to-Point.
Each temporary limitation on the media stream is expressed as a
tuple. The first component of the tuple is the maximum total media
bit rate (as defined in section 2.2) that the media receiver is
currently prepared to accept for this media stream. The second
component is the per-packet overhead that the media receiver has
observed for this media stream at its chosen reference protocol
layer.
As indicated in section 2.2, the overhead as observed by the sender
of the TMMBR (i.e., the media receiver) may differ from the overhead
observed at the receiver of the TMMBR (i.e., the media sender) due to
use of a different reference protocol layer at the other end or due
to the intervention of translators or mixers that affect the amount
of per packet overhead. For example, a gateway in between the two
that converts between IPv4 and IPv6 affects the per-packet overhead
by 20 bytes. Other mechanisms that change the overhead include
tunnels. The problem with varying overhead is also discussed in
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[RFC3890]. As will be seen in the description of the algorithm for
use of TMMBR, the difference in perceived overhead between the
sending and receiving ends presents no difficulty because
calculations are carried out in terms of variables that have the same
value at the sender as at the receiver -- for example, packet rate
and net media rate.
Reporting both maximum total media bit rate and per-packet overhead
allows different receivers to provide bit rate and overhead values
for different protocol layers, for example, at the IP level, at the
outer part of a tunnel protocol, or at the link layer. The protocol
level a peer reports on depends on the level of integration the peer
has, as it needs to be able to extract the information from that
protocol level. For example, an application with no knowledge of the
IP version it is running over cannot meaningfully determine the
overhead of the IP header, and hence will not want to include IP
overhead in the overhead or maximum total media bit rate calculation.
It is expected that most peers will be able to report values at least
for the IP layer. In certain implementations, it may be advantageous
to also include information pertaining to the link layer, which in
turn allows for a more precise overhead calculation and a better
optimization of connectivity resources.
The Temporary Maximum Media Stream Bit Rate messages are generic
messages that can be applied to any RTP packet stream. This
separates them from the other codec control messages defined in this
specification, which apply only to specific media types or payload
formats. The TMMBR functionality applies to the transport, and the
requirements the transport places on the media encoding.
The reasoning below assumes that the participants have negotiated a
session maximum bit rate, using a signaling protocol. This value can
be global, for example, in case of point-to-point, multicast, or
translators. It may also be local between the participant and the
peer or mixer. In either case, the bit rate negotiated in signaling
is the one that the participant guarantees to be able to handle
(depacketize and decode). In practice, the connectivity of the
participant also influences the negotiated value -- it does not make
much sense to negotiate a total media bit rate that one's network
interface does not support.
It is also beneficial to have negotiated a maximum packet rate for
the session or sender. RFC 3890 provides an SDP [RFC4566] attribute
that can be used for this purpose; however, that attribute is not
usable in RTP sessions established using offer/answer [RFC3264].
Therefore, an optional maximum packet rate signaling parameter is
specified in this memo.
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An already established maximum total media bit rate may be changed at
any time, subject to the timing rules governing the sending of
feedback messages. The limit may change to any value between zero
and the session maximum, as negotiated during session establishment
signaling. However, even if a sender has received a TMMBR message
allowing an increase in the bit rate, all increases must be governed
by a congestion control mechanism. TMMBR indicates known limitations
only, usually in the local environment, and does not provide any
guarantees about the full path. Furthermore, any increases in
TMMBR-established bit rate limits are to be executed only after a
certain delay from the sending of the TMMBN message that notifies the
world about the increase in limit. The delay is specified as at
least twice the longest RTT as known by the media sender, plus the
media sender's calculation of the required wait time for the sending
of another TMMBR message for this session based on AVPF timing rules.
This delay is introduced to allow other session participants to make
known their bit rate limit requirements, which may be lower.
If it is likely that the new value indicated by TMMBR will be valid
for the remainder of the session, the TMMBR sender is expected to
perform a renegotiation of the session upper limit using the session
signaling protocol.
3.5.4.1. Behavior for Media Receivers Using TMMBR
This section is an informal description of behaviour described more
precisely in section 4.2.
A media sender begins the session limited by the maximum media bit
rate and maximum packet rate negotiated in session signaling, if any.
Note that this value may be negotiated for another protocol layer
than the one the participant uses in its TMMBR messages. Each media
receiver selects a reference protocol layer, forms an estimate of the
overhead it is observing (or estimating it if no packets has been
seen yet) at that reference level, and determines the maximum total
media bit rate it can accept, taking into account its own limitations
and any transport path limitations of which it may be aware. In case
the current limitations are more restricting than what was agreed on
in the session signaling, the media receiver reports its initial
estimate of these two quantities to the media sender using a TMMBR
message. Overall message traffic is reduced by the possibility of
including tuples for multiple media senders in the same TMMBR
message.
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
most limiting (i.e., the bounding set as defined in section 2.2). It
modifies its operation to stay within the feasible region (as defined
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in section 2.2), and also sends out a TMMBN to the media receivers
indicating the selected bounding set. That notification also
indicates who was responsible for the tuples in the bounding set,
i.e., the "owner"(s) of the limitation. A session participant that
owns no tuple in the bounding set is called a "non-owner".
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 media
sender to determine if its current estimated (maximum total media bit
rate, overhead) tuple would enter the bounding set if known to the
media sender. If so, it issues a TMMBR reporting the tuple value to
the sender. Otherwise, it takes no action for the moment.
Periodically, its estimated tuple values may change or it may receive
a new TMMBN. If so, it reapplies the algorithm to decide whether it
needs to issue a TMMBR.
If, alternatively, a media receiver owns one of the tuples in the
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
TMMBR to the media sender to report the changed values.
A media receiver may change status between owner and non-owner of a
bounding tuple between one TMMBN message and the next. Thus, it must
check the contents of each TMMBN to determine its subsequent actions.
Implementations may use other algorithms of their choosing, as long
as the bit rate limitations resulting from the exchange of TMMBR and
TMMBN messages are at least as strict (at least as low, in the bit
rate dimension) as the ones resulting from the use of the
aforementioned algorithm.
Obviously, in point-to-point cases, when there is only one media
receiver, this receiver becomes "owner" once it receives the first
TMMBN in response to its own TMMBR, and stays "owner" for the rest of
the session. Therefore, when it is known that there will always be
only a single media receiver, the above algorithm is not required.
Media receivers that are aware they are the only ones in a session
can send TMMBR messages with bit rate limits both higher and lower
than the previously notified limit, at any time (subject to the AVPF
[RFC4585] RTCP RR send timing rules). However, it may be difficult
for a session participant to determine if it is the only receiver in
the session. Because of this, any implementation of TMMBR is
required to include the algorithm described in the next section or a
stricter equivalent.
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3.5.4.2. Algorithm for Establishing Current Limitations
This section introduces an example algorithm for the calculation of a
session limit. Other algorithms can be employed, as long as the
result of the calculation is at least as restrictive as the result
that is obtained by this algorithm.
First, it is important to consider the implications of using a tuple
for limiting the media sender's behavior. The bit rate and the
overhead value result in a two-dimensional solution space for the
calculation of the bit rate of media streams. Fortunately, the two
variables are linked. Specifically, the bit rate available for RTP
payloads is equal to the TMMBR reported bit rate minus the packet
rate used, multiplied by the TMMBR reported overhead converted to
bits. As a result, when different bit rate/overhead combinations
need to be considered, the packet rate determines the correct
limitation. This is perhaps best explained by an example:
Example:
Receiver A: TMMBR_max total BR = 35 kbps, TMMBR_OH = 40 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
payloads in RTP will be:
Max_net media_BR_A =
TMMBR_max total BR_A - PR * TMMBR_OH_A * 8 ... (1)
Max_net media_BR_B =
TMMBR_max total BR_B - PR * TMMBR_OH_B * 8 ... (2)
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
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
the limiting one. The switchover point can be identified by setting
Max_media_BR_A equal to Max_media_BR_B and breaking out PR:
TMMBR_max total BR_A - TMMBR_max total BR_B
PR = ------------------------------------------- ... (3)
8*(TMMBR_OH_A - TMMBR_OH_B)
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
second, receiver A is the limiting receiver, and for higher packet
rates, receiver B is more limiting. The implications of this
behavior have to be considered by implementations that are going to
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control media encoding and its packetization. As exemplified above,
multiple TMMBR limits may apply to the trade-off between net media
bit rate and packet rate. Which limitation applies depends on the
packet rate being considered.
This also has implications for how the TMMBR mechanism needs to work.
First, there is the possibility that multiple TMMBR tuples are
providing limitations on the media sender. Secondly, there is a need
for any session participant (media sender and receivers) to be able
to determine if a given tuple will become a limitation upon the media
sender, or if the set of already given limitations is stricter than
the given values. In the absence of the ability to make this
determination, the suppression of TMMBRs would not work.
The basic idea of the algorithm is as follows. Each TMMBR tuple can
be viewed as the equation of a straight line (cf. equations (1) and
(2)) in a space where packet rate lies along the X-axis and net bit
rate along the Y-axis. The lower envelope of the set of lines
corresponding to the complete set of TMMBR tuples, together with the
X and Y axes, defines a polygon. Points lying within this polygon
are combinations of packet rate and bit rate that meet all of the
TMMBR constraints. The highest feasible packet rate within this
region is the minimum of the rate at which the bounding polygon meets
the X-axis or the session maximum packet rate (SMAXPR, measured in
packets per second) provided by signaling, if any. Typically, a
media sender will prefer to operate at a lower rate than this
theoretical maximum, so as to increase the rate at which actual media
content reaches the receivers. The purpose of the algorithm is to
distinguish the TMMBR tuples constituting the bounding set and thus
delineate the feasible region, so that the media sender can select
its preferred operating point within that region
Figure 1 below shows a bounding polygon formed by TMMBR tuples A and
B. A third tuple C lies outside the bounding polygon and is
therefore irrelevant in determining feasible trade-offs between media
rate and packet rate. The line labeled ss..s represents the limit on
packet rate imposed by the session maximum packet rate (SMAXPR)
obtained by signaling during session setup. In Figure 1, the limit
determined by tuple B happens to be more restrictive than SMAXPR.
The situation could easily be the reverse, meaning that the bounding
polygon is terminated on the right by the vertical line representing
the SMAXPR constraint.
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Net ^
Media|a c b s
Bit | a c b s
Rate | a c b s
| a cb s
| a c s
| a bc s
| a b c s
| ab c s
| Feasible b c s
| region ba s
| b a s c
| b s c
| b s a
| bs
+------------------------------>
Packet rate
Figure 1 - Geometric Interpretation of TMMBR Tuples
Note that the slopes of the lines making up the bounding polygon are
increasingly negative as one moves in the direction of increasing
packet rate. Note also that with slight rearrangement, equations (1)
and (2) have the canonical form:
y = mx + b
where
m is the slope and has value equal to the negative of the tuple
overhead (in bits),
and
b is the y-intercept and has value equal to the tuple maximum
total media bit rate.
These observations lead to the conclusion that when processing the
TMMBR tuples to select the initial bounding set, one should sort and
process the tuples by order of increasing overhead. Once a
particular tuple has been added to the bounding set, all tuples not
already selected and having lower overhead can be eliminated, because
the next side of the bounding polygon has to be steeper (i.e., the
corresponding TMMBR must have higher overhead) than the latest added
tuple.
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, the
corresponding TMMBR tuple contains a higher maximum total media bit
rate value. Since line cc..c is outside the bounding polygon, it
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illustrates the conclusion that if two TMMBR tuples have the same
overhead value, the one with higher maximum total media bit rate
value cannot be part of the bounding set and can be set aside.
Two further observations complete the algorithm. Obviously, moving
from the left, the successive corners of the bounding polygon (i.e.,
the intersection points between successive pairs of sides) lie at
successively higher packet rates. On the other hand, again moving
from the left, each successive line making up the bounding set
crosses the X-axis at a lower packet rate.
The complete algorithm can now be specified. The algorithm works
with two lists of TMMBR tuples, the candidate list X and the selected
list Y, both ordered by increasing overhead value. The algorithm
terminates when all members of X have been discarded or removed for
processing. Membership of the selected list Y is probationary until
the algorithm is complete. Each member of the selected list is
associated with an intersection value, which is the packet rate at
which the line corresponding to that TMMBR tuple intersects with the
line corresponding to the previous TMMBR tuple in the selected list.
Each member of the selected list is also associated with a maximum
packet rate value, which is the lesser of the session maximum packet
rate SMAXPR (if any) and the packet rate at which the line
corresponding to that tuple crosses the X-axis.
When the algorithm terminates, the selected list is equal to the
bounding set as defined in section 2.2.
Initial Algorithm
This algorithm is used by the media sender when it has received one
or more TMMBRs and before it has determined a bounding set for the
first time.
1. Sort the TMMBR tuples by order of increasing overhead. This is
the initial candidate list X.
2. When multiple tuples in the candidate list have the same overhead
value, discard all but the one with the lowest maximum total media
bit rate value.
3. Select and remove from the candidate list the TMMBR tuple with the
lowest maximum total media bit rate value. If there is more than
one tuple with that value, choose the one with the highest
overhead value. This is the first member of the selected list Y.
Set its intersection value equal to zero. Calculate its maximum
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packet rate as the minimum of SMAXPR (if available) and the value
obtained from the following formula, which is the packet rate at
which the corresponding line crosses the X-axis.
Max PR = TMMBR max total BR / (8 * TMMBR OH) ... (4)
4. Discard from the candidate list all tuples with a lower overhead
value than the selected tuple.
5. Remove the first remaining tuple from the candidate list for
processing. Call this the current candidate.
6. Calculate the packet rate PR at the intersection of the line
generated by the current candidate with the line generated by the
last tuple in the selected list Y, using equation (3).
7. If the calculated value PR is equal to or lower than the
intersection value stored for the last tuple of the selected list,
discard the last tuple of the selected list and go back to step 6
(retaining the same current candidate).
Note that the choice of the initial member of the selected list Y
in step 3 guarantees that the selected list will never be emptied
by this process, meaning that the algorithm must eventually (if
not immediately) fall through to step 8.
8. (This step is reached when the calculated PR value of the current
candidate is greater than the intersection value of the current
last member of the selected list Y.) If the calculated value PR
of the current candidate is lower than the maximum packet rate
associated with the last tuple in the selected list, add the
current candidate tuple to the end of the selected list. Store PR
as its intersection value. Calculate its maximum packet rate as
the lesser of SMAXPR (if available) and the maximum packet rate
calculated using equation (4).
9. If any tuples remain in the candidate list, go back to step 5.
Incremental Algorithm
The previous algorithm covered the initial case, where no selected
list had previously been created. It also applied only to the media
sender. When a previously created selected list is available at
either the media sender or media receiver, two other cases can be
considered:
o when a TMMBR tuple not currently in the selected list is a
candidate for addition;
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o when the values change in a TMMBR tuple currently in the
selected list.
At the media receiver, these cases correspond, respectively, to those
of the non-owner and owner of a tuple in the TMMBN-reported bounding
set.
In either case, the process of updating the selected list to take
account of the new/changed tuple can use the basic algorithm
described above, with the modification that the initial candidate set
consists only of the existing selected list and the new or changed
tuple. Some further optimization is possible (beyond starting with a
reduced candidate set) by taking advantage of the following
observations.
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
more other tuples to be dropped from the list. However, if more than
one other tuple is dropped, the dropped tuples will be consecutive.
This can be confirmed geometrically by visualizing a new line that
cuts off a series of segments from the previously existing bounding
polygon. The cut-off segments are connected one to the next, the
geometric equivalent of consecutive tuples in a list ordered by
overhead value. Beyond the dropped set in either direction all of
the tuples that were in the earlier selected list will be in the
updated one. The second observation is that, leaving aside the new
candidate, the order of tuples remaining in the updated selected list
is unchanged because their overhead values have not changed.
The consequence of these two observations is that, once the placement
of the new candidate and the extent of the dropped set of tuples (if
any) has been determined, the remaining tuples can be copied directly
from the candidate list into the selected list, preserving their
order. This conclusion suggests the following modified algorithm:
o Run steps 1-4 of the basic algorithm.
o If the new candidate has survived steps 2 and 4 and has become
the new first member of the selected list, run steps 5-9 on
subsequent candidates until another candidate is added to the
selected list. Then move all remaining candidates to the
selected list, preserving their order.
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
moving all tuples in the candidate list with lower overhead
values than that of the new candidate to the selected list,
preserving their order. Run steps 5-9 for the new candidate,
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with the modification that the intersection values and maximum
packet rates for the tuples on the selected list have to be
calculated on the fly because they were not previously stored.
Continue processing only until a subsequent tuple has been
added to the selected list, then move all remaining candidates
to the selected list, preserving their order.
Note that the new candidate could be added to the selected
list only to be dropped again when the next tuple is
processed. It can easily be seen that in this case the new
candidate does not displace any of the earlier tuples in the
selected list. The limitations of ASCII art make this
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 higher
overhead value), but still intersected line aa..a beyond where
line aa..a intersects line bb..b.
The algorithm just described is approximate, because it does not take
account of tuples outside the selected list. To see how such tuples
can become relevant, consider Figure 1 and suppose that the maximum
total media bit rate in tuple A increases to the point that line
aa..a moves outside line cc..c. Tuple A will remain in the bounding
set calculated by the media sender. However, once it issues a new
TMMBN, media receiver C will apply the algorithm and discover that
its tuple C should now enter the bounding set. It will issue a TMMBR
to the media sender, which will repeat its calculation and come to
the appropriate conclusion.
The rules of section 4.2 require that the media sender refrain from
raising its sending rate until media receivers have had a chance to
respond to the TMMBN. In the example just given, this delay ensures
that the relaxation of tuple A does not actually result in an attempt
to send media at a rate exceeding the capacity at C.
3.5.4.3. Use of TMMBR in a Mixer-Based Multipoint Operation
Assume a small mixer-based multiparty conference is ongoing, as
depicted in Topo-Mixer of [RFC5117]. All participants have
negotiated a common maximum bit rate that this session can use. The
conference operates over a number of unicast paths between the
participants and the mixer. The congestion situation on each of
these paths can be monitored by the participant in question and by
the mixer, utilizing, for example, RTCP receiver reports (RRs) or the
transport protocol, e.g., Datagram Congestion Control Protocol (DCCP)
[RFC4340]. However, any given participant has no knowledge of the
congestion situation of the connections to the other participants.
Worse, without mechanisms similar to the ones discussed in this
document, the mixer (which is aware of the congestion situation on
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all connections it manages) has no standardized means to inform media
senders to slow down, short of forging its own receiver reports
(which is undesirable). In principle, a mixer confronted with such a
situation is obliged to thin or transcode streams intended for
connections that detected congestion.
In practice, unfortunately, media-aware streaming thinning is a very
difficult and cumbersome operation and adds undesirable delay. If
media-unaware, it leads very quickly to unacceptable reproduced media
quality. Hence, a means to slow down senders even in the absence of
congestion on their connections to the mixer is desirable.
To allow the mixer to throttle traffic on the individual links,
without performing transcoding, there is a need for a mechanism that
enables the mixer to ask a participant's media encoders to limit the
media stream bit rate they are currently generating. TMMBR provides
the required mechanism. When the mixer detects congestion between
itself and a given participant, it executes the following procedure:
1. It starts thinning the media traffic to the congested participant
to the supported bit rate.
2. It uses TMMBR to request the media sender(s) to reduce the total
media bit rate sent by them to the mixer, to a value that is in
compliance with congestion control principles for the slowest
link. Slow refers here to the available bandwidth / bit rate /
capacity and packet rate after congestion control.
3. As soon as the bit rate has been reduced by the sending part, the
mixer stops stream thinning implicitly, because there is no need
for it once the stream is in compliance with congestion control.
This use of stream thinning as an immediate reaction tool followed up
by a quick control mechanism appears to be a reasonable compromise
between media quality and the need to combat congestion.
3.5.4.4. Use of TMMBR in Point-to-Multipoint Using Multicast or
Translators
In these topologies, corresponding to Topo-Multicast or Topo-
Translator, RTCP RRs are transmitted globally. This allows all
participants to detect transmission problems such as congestion, on a
medium timescale. As all media senders are aware of the congestion
situation of all media receivers, the rationale for the use of TMMBR
in the previous section does not apply. However, even in this case
the congestion control response can be improved when the unicast
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links are using congestion controlled transport protocols (such as
TCP or DCCP). A peer may also report local limitations to the media
sender.
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 performance
when the known upper limit of the bit rate changes. In this use
case, the signaling protocol has established an upper limit for the
session and total media bit rates. However, at the time of transport
link bit rate reduction, a receiver can avoid serious congestion by
sending a TMMBR to the sending side. Thus, TMMBR is useful for
putting restrictions on the application and thus placing the
congestion control mechanism in the right ballpark. However, TMMBR
is usually unable to provide the continuously quick feedback loop
required for real congestion control. Nor do its semantics match
those of congestion control given its different purpose. For these
reasons, TMMBR SHALL NOT be used as a substitute for congestion
control.
3.5.4.6. Reliability
The reaction of a media sender to the reception of a TMMBR message is
not immediately identifiable through inspection of the media stream.
Therefore, a more explicit mechanism is needed to avoid unnecessary
re-sending of TMMBR messages. Using a statistically based
retransmission scheme would only provide statistical guarantees of
the request being received. It would also not avoid the
retransmission of already received messages. In addition, it would
not allow for easy suppression of other participants' requests. For
these reasons, a mechanism based on explicit notification is used.
Upon the reception of a TMMBR, a media sender sends a TMMBN
containing the current bounding set, and indicating which session
participants own that limit. In multicast scenarios, that allows all
other participants to suppress any request they may have, if their
limitations are less strict than the current ones (i.e., define lines
lying outside the feasible region as defined in section 2.2).
Keeping and notifying only the bounding set of tuples allows for
small message sizes and media sender states. A media sender only
keeps state for the SSRCs of the current owners of the bounding set
of tuples; all other requests and their sources are not saved. Once
the bounding set has been established, new TMMBR messages should be
generated only by owners of the bounding tuples and by other entities
that determine (by applying the algorithm of section 3.5.4.2 or its
equivalent) that their limitations should now be part of the bounding
set.
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4. RTCP Receiver Report Extensions
This memo specifies six new feedback messages. The Full Intra
Request (FIR), Temporal-Spatial Trade-off Request (TSTR), Temporal-
Spatial Trade-off Notification (TSTN), and Video Back Channel Message
(VBCM) are "Payload Specific Feedback Messages" as defined in section
6.3 of AVPF [RFC4585]. The Temporary Maximum Media Stream Bit Rate
Request (TMMBR) and Temporary Maximum Media Stream Bit Rate
Notification (TMMBN) are "Transport Layer Feedback Messages" as
defined in section 6.2 of AVPF.
The new feedback messages are defined in the following subsections,
following a similar structure to that in sections 6.2 and 6.3 of the
AVPF specification [RFC4585].
4.1. Design Principles of the Extension Mechanism
RTCP was originally introduced as a channel to convey presence,
reception quality statistics and hints on the desired media coding.
A limited set of media control mechanisms was introduced in early RTP
payload formats for video formats, for example, in RFC 2032 [RFC2032]
(which was obsoleted by RFC 4587 [RFC4587]). However, this
specification, for the first time, suggests a two-way handshake for
some of its messages. There is danger that this introduction could
be misunderstood as a precedent for the use of RTCP as an RTP session
control protocol. To prevent such a misunderstanding, this
subsection attempts to clarify the scope of the extensions specified
in this memo, and it strongly suggests that future extensions follow
the rationale spelled out here, or compellingly explain why they
divert from the rationale.
In this memo, and in AVPF [RFC4585], only such messages have been
included as:
a) have comparatively strict real-time constraints, which prevent the
use of mechanisms such as a SIP re-invite in most application
scenarios (the real-time constraints are explained separately for
each message where necessary);
b) are multicast-safe in that the reaction to potentially
contradicting feedback messages is specified, as necessary for
each message; and
c) are directly related to activities of a certain media codec, class
of media codecs (e.g., video codecs), or a given RTP packet
stream.
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In this memo, a two-way handshake is introduced only for messages for
which:
a) a notification or acknowledgement is required due to their nature.
An analysis to determine whether this requirement exists has been
performed separately for each message.
b) the notification or acknowledgement cannot be easily derived from
the media bit stream.
All messages in AVPF [RFC4585] and in this memo present their
contents in a simple, fixed binary format. This accommodates media
receivers that have not implemented higher control protocol
functionalities (SDP, XML parsers, and such) in their media path.
Messages that do not conform to the design principles just described
are not an appropriate use of RTCP or of the Codec Control Framework
defined in this document.
4.2. Transport Layer Feedback Messages
As specified in section 6.1 of RFC 4585 [RFC4585], transport layer
feedback messages are identified by the RTCP packet type value RTPFB
(205).
In AVPF, one message of this category had been defined. This memo
specifies two more such messages. They are identified by means of
the feedback message type (FMT) parameter as follows:
Assigned in AVPF [RFC4585]:
1: Generic NACK
31: reserved for future expansion of the identifier number space
Assigned in this memo:
2: reserved (see note below)
3: Temporary Maximum Media Stream Bit Rate Request (TMMBR)
4: Temporary Maximum Media Stream Bit Rate Notification (TMMBN)
Note: early versions of AVPF [RFC4585] reserved FMT=2 for a
code point that has later been removed. It has been pointed
out that there may be implementations in the field using this
value in accordance with the expired document. As there is
sufficient numbering space available, we mark FMT=2 as
reserved so to avoid possible interoperability problems with
any such early implementations.
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Available for assignment:
0: unassigned
5-30: unassigned
The following subsection defines the formats of the Feedback Control
Information (FCI) entries for the TMMBR and TMMBN messages,
respectively, and specifies the associated behaviour at the media
sender and receiver.
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
(TMMBR) message SHALL contain one or more FCI entries.
4.2.1.1. Message Format
The Feedback Control Information (FCI) consists of one or more TMMBR
FCI entries with the following syntax:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 - Syntax of an FCI Entry in the TMMBR Message
SSRC (32 bits): The SSRC value of the media sender that is
requested to obey the new maximum bit rate.
MxTBR Exp (6 bits): The exponential scaling of the mantissa for the
maximum total media bit rate value. The value is an
unsigned integer [0..63].
MxTBR Mantissa (17 bits): The mantissa of the maximum total media
bit rate value as an unsigned integer.
Measured Overhead (9 bits): The measured average packet overhead
value in bytes. The measurement SHALL be done according
to the description in section 4.2.1.2. The value is an
unsigned integer [0..511].
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The maximum total media bit rate (MxTBR) value in bits per second is
calculated from the MxTBR exponent (exp) and mantissa in the
following way:
MxTBR = mantissa * 2^exp
This allows for 17 bits of resolution in the range 0 to 131072*2^63
(approximately 1.2*10^24).
The length of the TMMBR feedback message SHALL be set to 2+2*N where
N is the number of TMMBR FCI entries.
4.2.1.2. Semantics
Behaviour at the Media Receiver (Sender of the TMMBR)
TMMBR is used to indicate a transport-related limitation at the
reporting entity acting as a media receiver. TMMBR has the form of a
tuple containing two components. The first value is the highest bit
rate per sender of a media stream, available at a receiver-chosen
protocol layer, which the receiver currently supports in this RTP
session. The second value is the measured header overhead in bytes
as defined in section 2.2 and measured at the chosen protocol layer
in the packets received for the stream. The measurement of the
overhead is a running average that is updated for each packet
received for this particular media source (SSRC), using the following
formula:
avg_OH (new) = 15/16*avg_OH (old) + 1/16*pckt_OH,
where avg_OH is the running (exponentially smoothed) average and
pckt_OH is the overhead observed in the latest packet.
If a maximum bit rate has been negotiated through signaling, the
maximum total media bit rate that the receiver reports in a TMMBR
message MUST NOT exceed the negotiated value converted to a common
basis (i.e., with overheads adjusted to bring it to the same
reference protocol layer).
Within the common packet header for feedback messages (as defined in
section 6.1 of [RFC4585]), the "SSRC of packet sender" field
indicates the source of the request, and the "SSRC of media source"
is not used and SHALL be set to 0. Within a particular TMMBR FCI
entry, the "SSRC of media source" in the FCI field denotes the media
sender that the tuple applies to. This is useful in the multicast or
translator topologies where the reporting entity may address all of
the media senders in a single TMMBR message using multiple FCI
entries.
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The media receiver SHALL save the contents of the latest TMMBN
message received from each media sender.
The media receiver MAY send a TMMBR FCI entry to a particular media
sender under the following circumstances:
o before any TMMBN message has been received from that media
sender;
o when the media receiver has been identified as the source of a
bounding tuple within the latest TMMBN message received from
that media sender, and the value of the maximum total media bit
rate or the overhead relating to that media sender has changed;
o when the media receiver has not been identified as the source
of a bounding tuple within the latest TMMBN message received
from that media sender, and, after the media receiver applies
the incremental algorithm from section 3.5.4.2 or a stricter
equivalent, the media receiver's tuple relating to that media
sender is determined to belong to the bounding set.
A TMMBR FCI entry MAY be repeated in subsequent TMMBR messages if no
Temporary Maximum Media Stream Bit Rate Notification (TMMBN) FCI has
been received from the media sender at the time of transmission of
the next RTCP packet. The bit rate value of a TMMBR FCI entry MAY be
changed from one TMMBR message to the next. The overhead measurement
SHALL be updated to the current value of avg_OH each time the entry
is sent.
If the value set by a TMMBR message is expected to be permanent, the
TMMBR setting party SHOULD renegotiate the session parameters to
reflect that using session setup signaling, e.g., a SIP re-invite.
Behaviour at the Media Sender (Receiver of the TMMBR)
When it receives a TMMBR message containing an FCI entry relating to
it, the media sender SHALL use an initial or incremental algorithm as
applicable to determine the bounding set of tuples based on the new
information. The algorithm used SHALL be at least as strict as the
corresponding algorithm defined in section 3.5.4.2. The media sender
MAY accumulate TMMBRs over a small interval (relative to the RTCP
sending interval) before making this calculation.
Once it has determined the bounding set of tuples, the media sender
MAY use any combination of packet rate and net media bit rate within
the feasible region that these tuples describe to produce a lower
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total media stream bit rate, as it may need to address a congestion
situation or other limiting factors. See section 5 (congestion
control) for more discussion.
If the media sender concludes that it can increase the maximum total
media bit rate value, it SHALL wait before actually doing so, for a
period long enough to allow a media receiver to respond to the TMMBN
if it determines that its tuple belongs in the bounding set. This
delay period is estimated by the formula:
2 * RTT + T_Dither_Max,
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]. Even in
point-to-point sessions, a media sender MUST obey the aforementioned
rule, as it is not guaranteed that a participant is able to determine
correctly whether all the sources are co-located in a single node,
and are coordinated.
A TMMBN message SHALL be sent by the media sender at the earliest
possible point in time, in response to any TMMBR messages received
since the last sending of TMMBN. The TMMBN message indicates the
calculated set of bounding tuples and the owners of those tuples at
the time of the transmission of the message.
An SSRC may time out according to the default rules for RTP session
participants, i.e., the media sender has not received any RTP or RTCP
packets from the owner for the last five regular reporting intervals.
An SSRC may also explicitly leave the session, with the participant
indicating this through the transmission of an RTCP BYE packet or
using an external signaling channel. If the media sender determines
that the owner of a tuple in the bounding set has left the session,
the media sender SHALL transmit a new TMMBN containing the previously
determined set of bounding tuples but with the tuple belonging to the
departed owner removed.
A media sender MAY proactively initiate the equivalent to a TMMBR
message to itself, when it is aware that its transmission path is
more restrictive than the current limitations. As a result, a TMMBN
indicating the media source itself as the owner of a tuple is being
sent, thereby avoiding unnecessary TMMBR messages from other
participants. However, like any other participant, when the media
sender becomes aware of changed limitations, it is required to change
the tuple, and to send a corresponding TMMBN.
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Discussion
Due to the unreliable nature of transport of TMMBR and TMMBN, the
above rules may lead to the sending of TMMBR messages that appear to
disobey those rules. Furthermore, in multicast scenarios it can
happen that more than one "non-owning" session participant may
determine, rightly or wrongly, that its tuple belongs in the bounding
set. This is not critical for a number of reasons:
a) If a TMMBR message is lost in transmission, either the media
sender sends a new TMMBN message in response to some other media
receiver or it does not send a new TMMBN message at all. In the
first case, the media receiver applies the incremental algorithm
and, if it determines that its tuple should be part of the
bounding set, sends out another TMMBR. In the second case, it
repeats the sending of a TMMBR unconditionally. Either way, the
media sender eventually gets the information it needs.
b) Similarly, if a TMMBN message gets lost, the media receiver that
has sent the corresponding TMMBR does not receive the notification
and is expected to re-send the request and trigger the
transmission of another TMMBN.
c) If multiple competing TMMBR messages are sent by different session
participants, then the algorithm can be applied taking all of
these messages into account, and the resulting TMMBN provides the
participants with an updated view of how their tuples compare with
the bounded set.
d) If more than one session participant happens to send TMMBR
messages at the same time and with the same tuple component
values, it does not matter which of those tuples is taken into the
bounding set. The losing session participant will determine,
after applying the algorithm, that its tuple does not enter the
bounding set, and will therefore stop sending its TMMBR.
It is important to consider the security risks involved with faked
TMMBRs. See the security considerations in section 6.
As indicated already, the feedback messages may be used in both
multicast and unicast sessions in any of the specified topologies.
However, for sessions with a large number of participants, using the
lowest common denominator, as required by this mechanism, may not be
the most suitable course of action. Large sessions may need to
consider other ways to adapt the bit rate to participants'
capabilities, such as partitioning the session into different quality
tiers or using some other method of achieving bit rate scalability.
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4.2.1.3. Timing Rules
The first transmission of the TMMBR message MAY use early or
immediate feedback in cases when timeliness is desirable. Any
repetition of a request message SHOULD use regular RTCP mode for its
transmission timing.
4.2.1.4. Handling in Translators and Mixers
Media translators and mixers will need to receive and respond to
TMMBR messages as they are part of the chain that provides a certain
media stream to the receiver. The mixer or translator may act
locally on the TMMBR and thus generate a TMMBN to indicate that it
has done so. Alternatively, in the case of a media translator it can
forward the request, or in the case of a mixer generate one of its
own and pass it forward. In the latter case, the mixer will need to
send a TMMBN back to the original requestor to indicate that it is
handling the request.
4.2.2. Temporary Maximum Media Stream Bit Rate Notification (TMMBN)
The Temporary Maximum Media Stream Bit Rate Notification is
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
The Feedback Control Information (FCI) consists of zero, one, or more
TMMBN FCI entries with the following syntax:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 - Syntax of an FCI Entry in the TMMBN Message
SSRC (32 bits): The SSRC value of the "owner" of this tuple.
MxTBR Exp (6 bits): The exponential scaling of the mantissa for the
maximum total media bit rate value. The value is an
unsigned integer [0..63].
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MxTBR Mantissa (17 bits): The mantissa of the maximum total media
bit rate value as an unsigned integer.
Measured Overhead (9 bits): The measured average packet overhead
value in bytes represented as an unsigned integer
[0..511].
Thus, the FCI within the TMMBN message contains entries indicating
the bounding tuples. For each tuple, the entry gives the owner by
the SSRC, followed by the applicable maximum total media bit rate and
overhead value.
The length of the TMMBN message SHALL be set to 2+2*N where N is the
number of TMMBN FCI entries.
4.2.2.2. Semantics
This feedback message is used to notify the senders of any TMMBR
message that one or more TMMBR messages have been received or that an
owner has left the session. It indicates to all participants the
current set of bounding tuples and the "owners" of those tuples.
Within the common packet header for feedback messages (as defined in
section 6.1 of [RFC4585]), the "SSRC of packet sender" field
indicates the source of the notification. The "SSRC of media source"
is not used and SHALL be set to 0.
A TMMBN message SHALL be scheduled for transmission after the
reception of a TMMBR message with an FCI entry identifying this media
sender. Only a single TMMBN SHALL be sent, even if more than one
TMMBR message is received between the scheduling of the transmission
and the actual transmission of the TMMBN message. The TMMBN message
indicates the bounding tuples and their owners at the time of
transmitting the message. The bounding tuples included SHALL be the
set arrived at through application of the applicable algorithm of
section 3.5.4.2 or an equivalent, applied to the previous bounding
set, if any, and tuples received in TMMBR messages since the last
TMMBN was transmitted.
The reception of a TMMBR message SHALL still result in the
transmission of a TMMBN message even if, after application of the
algorithm, the newly reported TMMBR tuple is not accepted into the
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 within
the previously calculated bounding set. This procedure allows
session participants that did not see the last TMMBN message to get a
correct view of this media sender's state.
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As indicated in section 4.2.1.2, when a media sender determines that
an "owner" of a bounding tuple has left the session, then that tuple
is removed from the bounding set, and the media sender SHALL send a
TMMBN message indicating the remaining bounding tuples. If there are
no remaining bounding tuples, a TMMBN without any FCI SHALL be sent
to indicate this. Without a remaining bounding tuple, the maximum
media bit rate and maximum packet rate negotiated in session
signaling, if any, apply.
Note: if any media receivers remain in the session, this last will
be a temporary situation. The empty TMMBN will cause every
remaining media receiver to determine that its limitation belongs
in the bounding set and send a TMMBR in consequence.
In unicast scenarios (i.e., where a single sender talks to a single
receiver), the aforementioned algorithm to determine ownership
degenerates to the media receiver becoming the "owner" of the one
bounding tuple as soon as the media receiver has issued the first
TMMBR message.
4.2.2.3. Timing Rules
The TMMBN acknowledgement SHOULD be sent as soon as allowed by the
applied timing rules for the session. Immediate or early feedback
mode SHOULD be used for these messages.
4.2.2.4. Handling by Translators and Mixers
As discussed in section 4.2.1.4, mixers or translators may need to
issue TMMBN messages as responses to TMMBR messages for SSRCs handled
by them.
4.3. Payload-Specific Feedback Messages
As specified by section 6.1 of RFC 4585 [RFC4585], Payload-Specific
FB messages are identified by the RTCP packet type value PSFB (206).
AVPF [RFC4585] defines three payload-specific feedback messages and
one application layer feedback message. This memo specifies four
additional payload-specific feedback messages. All are identified by
means of the FMT parameter as follows:
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Assigned in [RFC4585]:
1: Picture Loss Indication (PLI)
2: Slice Lost Indication (SLI)
3: Reference Picture Selection Indication (RPSI)
15: Application layer FB message
31: reserved for future expansion of the number space
Assigned in this memo:
4: Full Intra Request (FIR) Command
5: Temporal-Spatial Trade-off Request (TSTR)
6: Temporal-Spatial Trade-off Notification (TSTN)
7: Video Back Channel Message (VBCM)
Unassigned:
0: unassigned
8-14: unassigned
16-30: unassigned
The following subsections define the new FCI formats for the
payload-specific feedback messages.
4.3.1. Full Intra Request (FIR)
The FIR message is identified by RTCP packet type value PT=PSFB and
FMT=4.
The FCI field MUST contain one or more FIR entries. Each entry
applies to a different media sender, identified by its SSRC.
4.3.1.1. Message Format
The Feedback Control Information (FCI) for the Full Intra Request
consists of one or more FCI entries, the content of which is 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq nr. | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 - Syntax of an FCI Entry in the FIR Message
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SSRC (32 bits): The SSRC value of the media sender that is
requested to send a decoder refresh point.
Seq nr. (8 bits): Command sequence number. The sequence number
space is unique for each pairing of the SSRC of command
source and the SSRC of the command target. The sequence
number SHALL be increased by 1 modulo 256 for each new
command. A repetition SHALL NOT increase the sequence
number. The initial value is arbitrary.
Reserved (24 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception.
The semantics of this feedback message is independent of the RTP
payload type.
4.3.1.2. Semantics
Within the common packet header for feedback messages (as defined in
section 6.1 of [RFC4585]), the "SSRC of packet sender" field
indicates the source of the request, and the "SSRC of media source"
is not used and SHALL be set to 0. The SSRCs of the media senders to
which the FIR command applies are in the corresponding FCI entries.
A FIR message MAY contain requests to multiple media senders, using
one FCI entry per target media sender.
Upon reception of FIR, the encoder MUST send a decoder refresh point
(see section 2.2) as soon as possible.
The sender MUST consider congestion control as outlined in section 5,
which MAY restrict its ability to send a decoder refresh point
quickly.
FIR SHALL NOT be sent as a reaction to picture losses -- it is
RECOMMENDED to use PLI [RFC4585] instead. FIR SHOULD be used only in
situations where not sending a decoder refresh point would render the
video unusable for the users.
A typical example where sending FIR is appropriate is when, in a
multipoint conference, a new user joins the session and no regular
decoder refresh point interval is established. Another example would
be a video switching MCU that changes streams. Here, normally, the
MCU issues a FIR to the new sender so to force it to emit a decoder
refresh point. The decoder refresh point normally includes a Freeze
Picture Release (defined outside this specification), which re-starts
the rendering process of the receivers. Both techniques mentioned
are commonly used in MCU-based multipoint conferences.
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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 of receiving and
reacting to the feedback scheme defined in the respective RTP payload
format, if this is required by that payload format.
4.3.1.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. FIR
commands MAY be used with early or immediate feedback. The FIR
feedback message MAY be repeated. If using immediate feedback mode,
the repetition SHOULD wait at least one RTT before being sent. In
early or regular RTCP mode, the repetition is sent in the next
regular RTCP packet.
4.3.1.4. Handling of FIR Message in Mixers and Translators
A media translator or a mixer performing media encoding of the
content for which the session participant has issued a FIR is
responsible for acting upon it. A mixer acting upon a FIR SHOULD NOT
forward the message unaltered; instead, it SHOULD issue a FIR itself.
4.3.1.5. Remarks
Currently, video appears to be the only useful application for FIR,
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
than predicted (inter) pictures. Their size is independent of the
time they are generated. In most environments, especially when
employing bandwidth-limited links, the use of an intra picture
implies an allowed delay that is a significant multiple of the
typical frame duration. An example: if the sending frame rate is 10
fps, and an intra picture is assumed to be 10 times as big as an
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RFC 5104 Codec Control Messages in AVPF February 2008
inter picture, then a full second of latency has to be accepted. In
such an environment, there is no need for a particularly short delay
in sending the FIR message. Hence, waiting for the next possible
time slot allowed by RTCP timing rules as per [RFC4585] should not
have an overly negative impact 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 that predominantly 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 strea |
