Network Working Group Stephan Wenger
INTERNET-DRAFT Umesh Chandra
Expires: October 2007 Nokia
Magnus Westerlund
Bo Burman
Ericsson
May 30, 2007
Codec Control Messages in the
RTP Audio-Visual Profile with Feedback (AVPF)
draft-ietf-avt-avpf-ccm-07.txt>
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Copyright (C) The IETF Trust (2007).
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
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are also usable in smaller multicast environments and point-to-
point calls. The extensions discussed are messages related to the
ITU-T 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....................................................5
2. Definitions.....................................................6
2.1. Glossary...................................................6
2.2. Terminology................................................6
2.3. Topologies.................................................9
3. Motivation (Informative).......................................10
3.1. Use Cases.................................................10
3.2. Using the Media Path......................................12
3.3. Using AVPF................................................13
3.3.1. Reliability..........................................13
3.4. Multicast.................................................13
3.5. Feedback Messages.........................................13
3.5.1. Full Intra Request Command...........................14
3.5.1.1. Reliability.....................................14
3.5.2. Temporal Spatial Trade-off Request and Notification..15
3.5.2.1. Point-to-Point..................................16
3.5.2.2. Point-to-Multipoint Using Multicast or
Translators.....................................17
3.5.2.3. Point-to-Multipoint Using RTP Mixer.............17
3.5.2.4. Reliability.....................................17
3.5.3. H.271 Video Back Channel Message.....................18
3.5.3.1. Reliability.....................................21
3.5.4. Temporary Maximum Media Stream Bit Rate Request and
Notification.........................................21
3.5.4.1. Behavior for media receivers using TMMBR........23
3.5.4.2. Algorithm for establishing current limitations..25
3.5.4.3. Use of TMMBR in a Mixer Based Multipoint
Operation.......................................32
3.5.4.4. Use of TMMBR in Point-to-Multipoint Using
Multicast or Translators........................33
3.5.4.5. Use of TMMBR in Point-to-point operation........33
3.5.4.6. Reliability.....................................33
4. RTCP Receiver Report Extensions................................35
4.1. Design Principles of the Extension Mechanism..............35
4.2. Transport Layer Feedback Messages.........................36
4.2.1. Temporary Maximum Media Stream Bit Rate Request
(TMMBR)..............................................37
4.2.1.1. Message Format..................................37
4.2.1.2. Semantics.......................................38
4.2.1.3. Timing Rules....................................42
4.2.1.4. Handling in Translator and Mixers...............42
4.2.2. Temporary Maximum Media Stream Bit Rate Notification
(TMMBN)..............................................42
4.2.2.1. Message Format..................................42
4.2.2.2. Semantics.......................................43
4.2.2.3. Timing Rules....................................44
4.2.2.4. Handling by Translators and Mixers..............44
4.3. Payload Specific Feedback Messages........................44
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4.3.1. Full Intra Request (FIR).............................45
4.3.1.1. Message Format..................................45
4.3.1.2. Semantics.......................................46
4.3.1.3. Timing Rules....................................48
4.3.1.4. Handling of FIR Message in Mixer and Translators48
4.3.1.5. Remarks.........................................49
4.3.2. Temporal-Spatial Trade-off Request (TSTR)............49
4.3.2.1. Message Format..................................49
4.3.2.2. Semantics.......................................50
4.3.2.3. Timing Rules....................................51
4.3.2.4. Handling of message in Mixers and Translators...51
4.3.2.5. Remarks.........................................51
4.3.3. Temporal-Spatial Trade-off Notification (TSTN).......51
4.3.3.1. Message Format..................................52
4.3.3.2. Semantics.......................................52
4.3.3.3. Timing Rules....................................53
4.3.3.4. Handling of TSTN in Mixer and Translators.......53
4.3.3.5. Remarks.........................................53
4.3.4. H.271 Video Back Channel Message (VBCM)..............53
4.3.4.1. Message Format..................................54
4.3.4.2. Semantics.......................................55
4.3.4.3. Timing Rules....................................56
4.3.4.4. Handling of message in Mixer or Translator......56
4.3.4.5. Remarks.........................................56
5. Congestion Control.............................................57
6. Security Considerations........................................57
7. SDP Definitions................................................58
7.1. Extension of the rtcp-fb Attribute........................58
7.2. Offer-Answer..............................................60
7.3. Examples..................................................60
8. IANA Considerations............................................64
9. Acknowledgements...............................................65
10. References....................................................67
10.1. Normative references.....................................67
10.2. Informative references...................................67
11. Authors' Addresses............................................69
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1.1. 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 [Topologies], and understanding this memo requires
previous or parallel study of [Topologies].
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) re-
INVITEs [RFC3261], undesirable when used for the same purpose.
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
AMID - Additive Increase Multiplicative Decrease
AVPF - The extended RTP profile for RTCP-based feedback
FEC - Forward Error Correction
FCI - Feedback Control Information [RFC4585]
FIR - Full Intra Request
MCU - Multipoint Control Unit
MPEG - Moving Picture Experts Group
TMMBN - Temporary Maximum Media Stream Bit Rate Notification
TMMBR - Temporary Maximum Media Stream Bit Rate Request
PLI - Picture Loss Indication
PR - Packet rate
QP - Quantizer Parameter
RTT - Round trip time
SSRC - Synchronization Source
TSTN - Temporal Spatial Trade-off Notification
TSTR - Temporal Spatial Trade-off Request
VBCM - Video Back Channel Message indication.
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
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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].
Decoder Refresh Point:
A bit string, packetized in one or more RTP packets, which
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 subject to increasing
levels of thinning. Media-unaware stream thinning leads to
even worse quality degradation. In contrast to
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transcoding, stream thinning is typically seen as a
computationally lightweight operation.
Media:
Often used (sometimes in conjunction with terms like bit
rate, stream, sender ...) to identify the content of the
forward RTP packet stream (carrying the codec data), to
which the codec control message applies.
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,
below for example 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 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-
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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 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 [Topologies] for an in depth discussion. The
topologies referred to throughout this memo are labeled
(consistently with [Topologies]) as follows:
Topo-Point-to-Point . . . . point-to-point communication
Topo-Multicast . . . . . . multicast communication as in RFC 3550
Topo-Translator . . . . . . translator based as in RFC 3550
Topo-Mixer . . . . . . . . mixer based as in RFC 3550
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Topo-Video-switch-MCU . . . video 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 draft was
drawn up [Basso]. This draft has expired; however we quote
relevant sections of it to provide motivation and requirements.
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
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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 draft applies only to Picture Loss
Indication (PLI) as defined in AVPF [RFC4585] and is not
reproduced here.
5. Use case 5 of the Basso draft 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.
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 draws 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 (AMID and 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 does generally result 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
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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.
8. The use of reference picture selection (RPS) as an error
resilience tool has been 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 multiple 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 MEGACO [RFC3525]).
Secondly, the signaling path quite commonly contains several
signaling entities, e.g. SIP proxies and application servers.
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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.
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.
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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 2.2) at the earliest opportunity. The
evaluation of such opportunity includes the current encoder coding
strategy and the current available network resources.
FIR is also known as an "instantaneous decoder refresh 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.
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 (within 2 times the longest Round Trip Time (RTT) known,
plus any AVPF-induced RTCP packet sending delays, if those are
known) after sending a decoder refresh point, 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
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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 request 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 request (different request
sequence number) outstanding at any time per media sender in the
session.
The receiver of FIR (i.e. the media sender) behaves in
complementary fashion to ensure delivery of a decoder refresh
point. If it receives repetitions of the FIR more than 2*RTT
after it has sent a decoder refresh point, it shall send a new
decoder refresh point. Two round trip times allow time for the
decoder refresh point to arrive back to the requestor and for the
end of repetitions of FIR to reach and be detected by the media
sender.
An RTP mixer that receives an FIR from a media receiver is
responsible to ensure that a decoder refresh point is delivered to
the requesting receiver. It may be necessary for the mixer to
generate FIR commands. From a reliability perspective, the two
legs (FIR-requesting endpoint to mixer, and mixer 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.
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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
(as it is common in state-of-the-art video conferencing systems).
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
finally the new trade-off is 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.
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, mixer 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 TSTR is a non-binding
recommendation, nothing more.
Four TSTR/TSTN scenarios need to be distinguished, based on the
topologies described in [Topologies]. The scenarios are described
in the following sub-clauses.
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
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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
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
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A request and reception acknowledgement mechanism is specified.
The Temporal Spatial Trade-off Notification (TSTN) message informs
the request-sender that its request has been received, and what
trade-off is used henceforth. This acknowledgment mechanism is
desirable for at least the following reasons:
o A change in the trade-off cannot be directly identified from the
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, oversimplifying
overview of the messages currently defined in H.271, their roughly
corresponding AVPF or CCM messages (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.
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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.
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
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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.
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, clause 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 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 TMMBR requests 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.
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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 [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 (packet rate, net media bit rate) that have the same
value at the sender as at the receiver.
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 can not
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
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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.
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
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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 then 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 in section 2.2), and also sends out a TMMBN notification
to the media receivers indicating the selected bounding set.
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 request
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 request.
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 request 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
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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.
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:
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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 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 TMMBR requests 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 maximum bit rate lies along the Y-axis. The lower envelope of
the set of lines corresponding to the complete set of TMMBR tuples
defines a polygon. Points lying along or below 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)
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 tradeoffs 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
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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.
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
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(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 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 TMMBR requests 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
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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 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 the 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
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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;
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.
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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 through 9 for the new
candidate, 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 request 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.
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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 [Topologies]. 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 (RR) or
the transport protocol, e.g. 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 draft, the mixer (which is
aware of the congestion situation on 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, media-aware stream thinning is unfortunately 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.
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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 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'
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requests. For these reasons, a mechanism based on explicit
notification is used.
Upon the reception of a request a media sender sends a TMMBN
notification 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 were introduced
in early RTP payload formats for video formats, for example in RFC
2032 [RFC2032]. 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
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 which 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 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 drafts 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 draft. As
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there is sufficient numbering space available, we mark
FMT=2 as reserved so to avoid possible interoperability
problems with any such early implementations.
Available for assignment:
0: unassigned
5-30: unassigned
The following subsection defines the formats of the FCI entries
for the TMMBR and TMMBN messages respectively and specify the
associated behaviour at the media sender and receiver.
4.2.1. Temporary Maximum Media Stream Bit Rate Request (TMMBR)
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
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to description in section 4.2.1.2. The value is an
unsigned integer [0..512].
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 the 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
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TMMBR FCI entry, the "SSRC of media sender" in the FCI field
denotes the media sender 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.
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
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least as strict as the corresponding algorithm defined in section
3
.5.4.2. The media sender MAY accumulate TMMBR requests 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 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].
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
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media sender becomes aware of changed limitations, it is required
to change the tuple, and to send a corresponding TMMBN.
Discussion
Due to the unreliable nature of transport of TMMBR and TMMBN, the
above rules may lead to the sending of TMMBR messages which 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 request 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 if either tuple 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 request.
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
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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.
4.2.1.3. Timing Rules
The first transmission of the TMMBR request 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 Translator 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 request 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 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
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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].
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.
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 the 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.
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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.
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.
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 SSRC's
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
PT=PSFB (206).
AVPF [RFC4585] defines three payload-specific feedback messages
and one application layer feedback message. This memo specifies
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four additional payload-specific feedback messages. All are
identified by means of the FMT parameter as follows:
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 Command (FIR)
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.
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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
SSRC (32 bits): The SSRC value of the media sender which 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
Upon reception of FIR, the encoder MUST send a decoder refresh
point (see section 2.2) as soon as possible.
Note: 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.
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Note: In environments where the sender has no control over the
codec (e.g. when streaming pre-recorded and pre-coded content),
the reaction to this command cannot be specified. One suitable
reaction of a sender would be to skip forward in the video bit
stream to the next decoder refresh point. In other scenarios,
it may be preferable not to react to the command at all, e.g.
when streaming to a large multicast group. Other reactions may
also be possible. When deciding on a strategy, a sender could
take into account factors such as the size of the receiving
group, the "importance" of the sender of the FIR message
(however "importance" may be defined in this specific
application), the frequency of decoder refresh points in the
content, and so on. However a session which predominately
handles pre-coded content is not expected to use FIR at all.
The sender MUST consider congestion control as outlined in section
5, which MAY restrict its ability to send a decoder refresh point
quickly.
Note: The relationship between the Picture Loss Indication and
FIR is as follows. As discussed in section 6.3.1 of AVPF
[RFC4585], a Picture Loss Indication informs the decoder about
the loss of a picture and hence the likelihood of misalignment
of the reference pictures between the encoder and decoder. Such
a scenario is normally related to losses in an ongoing
connection. In point-to-point scenarios, and without the
presence of advanced error resilience tools, one possible option
for an encoder consists in sending a decoder refresh point.
However, there are other options. One example is that the media
sender ignores the PLI, because the embedded stream redundancy
is likely to clean up the reproduced picture within a reasonable
amount of time. The FIR, in contrast, leaves a (real-time)
encoder no choice but to send a decoder refresh point. It does
not allow the encoder to take into account any considerations
such as the ones mentioned above.
Note: Mandating a maximum delay for completing the sending of a
decoder refresh point would be desirable from an application
viewpoint, but is problematic from a congestion control point of
view. "As soon as possible" as mentioned above appears to be a
reasonable compromise.
FIR SHALL NOT be sent as a reaction to picture losses -- it is
RECOMMENDED to use PLI instead. FIR SHOULD be used only in
situations where not sending a decoder refresh point would render
the video unusable for the users.
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Note: 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.
Other RTP payload specifications such as RFC 2032 [RFC2032]
already define a feedback mechanism for certain codecs. An
application supporting both schemes MUST use the feedback
mechanism defined in this specification when sending feedback.
For backward compatibility reasons, such an application SHOULD
also be capable to receive and react to the feedback scheme
defined in the respective RTP payload format, if this is required
by that payload format.
Within the common packet header for feedback messages (as defined
in section 6.1 of [RFC4585]), the "SSRC of the 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 TSTR message MAY contain requests to multiple
media senders, using one FCI entry per target media sender.
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 Mixer 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.
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4.3.1.5. Remarks
In conjunction with video codecs, FIR messages typically trigger
the sending of full intra or IDR pictures. Both are several times
larger then predicted (inter) pictures. Their size is independent
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 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.
4.3.2. Temporal-Spatial Trade-off Request (TSTR)
The TSTR feedback message is identified by RTCP packet type value
PT=PSFB and FMT=5.
The FCI field MUST contain one or more TSTR FCI entries.
4.3.2.1. Message Format
The content of the FCI entry for the Temporal-Spatial Trade-off
Request is depicted in Figure 5. The length of the feedback
message MUST be set to 2+2*N, where N is the number of FCI entries
included.
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 | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 - Syntax of an FCI Entry in the TSTR Message
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SSRC (32 bits): The SSRC of the media sender which is requested
to apply the tradeoff value given in Index.
Seq. nr (8 bits): Request sequence number. The sequence number
space is unique for pairing of the SSRC of request
source and the SSRC of the request 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 (19 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception.
Index (5 bits): An integer value between 0 and 31 that indicates
the relative trade off that is requested. An index
value of 0 index highest possible spatial quality, while
31 indicates highest possible temporal resolution.
4.3.2.2. Semantics
A decoder can suggest a temporal-spatial trade-off level by
sending a TSTR message to an encoder. If the encoder is capable
of adjusting its temporal-spatial trade-off, it SHOULD take into
account the received TSTR message for future coding of pictures.
A value of 0 suggests a high spatial quality and a value of 31
suggests a high frame rate. The progression of values from 0 to
31 indicate monotonically a desire for higher frame rate. The
index values do not correspond to precise values of spatial
quality or frame rate.
The reaction to the reception of more than one TSTR message by a
media sender from different media receivers is left open to the
implementation. The selected trade-off SHALL be communicated to
the media receivers by the means of the TSTN message.
Within the common packet header for feedback messages (as defined
in section 6.1 of [RFC4585]), the "SSRC of the 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 TSTR applies to are in the corresponding FCI
entries.
A TSTR message MAY contain requests to multiple media senders,
using one FCI entry per target media sender.
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4.3.2.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585].
This request message is not time critical and SHOULD be sent using
regular RTCP timing. Only if it is known that the user interface
requires a quick feedback, the message MAY be sent with early or
immediate feedback timing.
4.3.2.4. Handling of message in Mixers and Translators
A mixer or media translator that encodes content sent to the
session participant issuing the TSTR SHALL consider the request to
determine if it can fulfill it by changing its own encoding
parameters. A media translator unable to fulfill the request MAY
forward the request unaltered towards the media sender. A mixer
encoding for multiple session participants will need to consider
the joint needs of these participants before generating a TSTR on
its own behalf towards the media sender. See also the discussion
in Section 3
..5.2.
4.3.2.5. Remarks
The term "spatial quality" does not necessarily refer to the
resolution, measured by the number of pixels the reconstructed
video is using. In fact, in most scenarios the video resolution
stays constant during the lifetime of a session. However, all
video compression standards have means to adjust the spatial
quality at a given resolution, often influenced by the Quantizer
Parameter or QP. A numerically low QP results in a good
reconstructed picture quality, whereas a numerically high QP
yields a coarse picture. The typical reaction of an encoder to
this request is to change its rate control parameters to use a
lower frame rate and a numerically lower (on average) QP, or vice
versa. The precise mapping of Index value to frame rate and QP is
intentionally left open here, as it depends on factors such as the
compression standard employed, spatial resolution, content, bit
rate, and so on.
4.3.3. Temporal-Spatial Trade-off Notification (TSTN)
The TSTN message is identified by RTCP packet type value PT=PSFB
and FMT=6.
The FCI field SHALL contain one or more TSTN FCI entries.
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4.3.3.1. Message Format
The content of an FCI entry for the Temporal-Spatial Trade-off
Notification is depicted in Figure 6. The length of the TSTN
message MUST be set to 2+2*N, where N is the number of FCI
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 | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6 - Syntax of the TSTN
SSRC (32 bits): The SSRC of the source of the TSTR request which
resulted in this Notification.
Seq. nr (8 bits): The sequence number value from the TSTN
request that is being acknowledged.
Reserved (19 bits): All bits SHALL be set to 0 by the sender and
SHALL be ignored on reception.
Index (5 bits): The trade-off value the media sender is using
henceforth.
Informative note: The returned trade-off value (Index) may
differ from the requested one, for example in cases where a
media encoder cannot tune its trade-off, or when pre-recorded
content is used.
4.3.3.2. Semantics
This feedback message is used to acknowledge the reception of a
TSTR. One TSTN entry in a TSTN feedback message SHALL be sent for
each TSTR entry targeted to this session participant, i.e. each
TSTR received that in the SSRC field in the entry has the
receiving entities SSRC. A single TSTN message MAY acknowledge
multiple requests using multiple FCI entries. The index value
included SHALL be the same in all FCI entries of the TSTN message.
Including a FCI for each requestor allows each requesting entity
to determine that the media sender received the request. The
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Notification SHALL also be sent in response to TSTR repetitions
received. If the request receiver has received TSTR with several
different sequence numbers from a single requestor it SHALL only
respond to the request with the highest (modulo 256) sequence
number.
The TSTN SHALL include the Temporal-Spatial Trade-off index that
will be used as a result of the request. This is not necessarily
the same index as requested, as the media sender may need to
aggregate requests from several requesting session participants.
It may also have some other policies or rules that limit the
selection.
Within the common packet header for feedback messages (as defined
in section 6.1 of [RFC4585]), the "SSRC of the packet sender"
field indicates the source of the Notification, and the "SSRC of
media source" is not used and SHALL be set to 0. The SSRCs of the
requesting entities to which the Notification applies are in the
corresponding FCI entries.
4.3.3.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585].
This acknowledgement message is not extremely time critical and
SHOULD be sent using regular RTCP timing.
4.3.3.4. Handling of TSTN in Mixer and Translators
A mixer or translator that acts upon a TSTR SHALL also send the
corresponding TSTN. In cases where it needs to forward a TSTR
itself the notification message MAY need to be delayed until the
TSTR has been responded to.
4.3.3.5. Remarks
None
4.3.4. H.271 Video Back Channel Message (VBCM)
The VBCM is identified by RTCP packet type value PT=PSFB and
FMT=7.
The FCI field MUST contain one or more VBCM FCI entries.
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4.3.4.1. Message Format
The syntax of an FCI entry within the VBCM indication is depicted
in Figure 7.
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 |0| Payload Type| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VBCM Octet String.... | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 - Syntax of an FCI Entry in the VBCM Message
SSRC (32 bits): The SSRC value of the media sender that is
requested to instruct its encoder to react to the VBCM
message
Seq. nr (8 bits): Command sequence number. The sequence number
space is unique for 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.
0: Must be set to 0 by the sender and should not be acted upon by
the message receiver.
Payload Type (7 bits): The RTP payload type for which the VBCM bit
stream must be interpreted.
Length (16 bits): The length of the VBCM octet string in octets
exclusive of any padding octets
VBCM Octet String (Variable length): This is the octet string
generated by the decoder carrying a specific feedback sub-
message.
Padding (Variable length): Bits set to 0 to make up a 32 bit
boundary.
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4.3.4.2. Semantics
The "payload" of the VBCM indication carries different types of
codec-specific, feedback information. The type of feedback
information can be classified as a 'status report' (such as an
indication that a bit stream was received without errors, or that
a partial or complete picture or block was lost) or 'update
requests' (such as complete refresh of the bit stream).
Note: There are possible overlaps between the VBCM sub-
messages and CCM/AVPF feedback messages, such FIR. Please
see section 3.5.3 for further discussion.
The different types of feedback sub-messages carried in the VBCM
are indicated by the "payloadType" as defined in [VBCM]. These
sub-message types are reproduced below for convenience.
"payloadType", in ITU-T Rec. H.271 terminology, refers to the sub-
type of the H.271 message and should not be confused with an RTP
payload type.
Payload Message Content
Type
---------------------------------------------------------------------
0 One or more pictures without detected bit stream error
mismatch
1 One or more pictures that are entirely or partially lost
2 A set of blocks of one picture that is entirely or partially
lost
3 CRC for one parameter set
4 CRC for all parameter sets of a certain type
5 A "reset" request indicating that the sender should completely
refresh the video bit stream as if no prior bit stream data
had been received
> 5 Reserved for future use by ITU-T
Table 2: H.271 message types ("payloadTypes")
The bit string or the "payload" of a VBCM message is of variable
length and is self-contained and coded in a variable length,
binary format. The media sender necessarily has to be able to
parse this optimized binary format to make use of VBCM messages.
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Each of the different types of sub-messages (indicated by
payloadType) may have different semantics depending on the codec
used.
Within the common packet header for feedback messages (as defined
in section 6.1 of [RFC4585]), the "SSRC of the 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 VBCM message applies to are in the
corresponding FCI entries. The sender of the VBCM message MAY
send H.271 messages to multiple media senders and MAY send more
than one H.271 message to the same media sender within the same
VBCM message.
4.3.4.3. Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585].
The different sub-message types may have different properties in
regards to the timing of messages that should be used. If several
different types are included in the same feedback packet then the
requirements for the sub-message type with the most stringent
requirements should be followed.
4.3.4.4. Handling of message in Mixer or Translator
The handling of VBCM in a mixer or translator is sub-message type
dependent.
4.3.4.5. Remarks
Please see section 3
.5.3 for a discussion of the usage of H.271
messages and messages defined in AVPF [RFC4585] and this memo with
similar functionality.
Note: There has been some discussion whether the payload type
field in this message is needed. It will be needed if there is
potentially more than one VBCM-capable RTP payload type in the
same session, and the semantics of a given VBCM message changes
between payload types. For example, the picture identification
mechanism in messages of H.271 type 0 is fundamentally different
between H.263 and H.264 (although both use the same syntax).
Therefore, the payload field is justified here. There was a
further comment that for TSTS and FIR such a need does not
exist, because the semantics of TSTS and FIR are either loosely
enough defined, or generic enough, to apply to all video
payloads currently in existence/envisioned.
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5. Congestion Control
The correct application of the AVPF [RFC4585] timing rules
prevents the network from being flooded by feedback messages.
Hence, assuming a correct implementation and configuration, the
RTCP channel cannot break its bit rate commitment and introduce
congestion.
The reception of some of the feedback messages modifies the
behaviour of the media senders or, more specifically, the media
encoders. Thus modified behaviour MUST respect the bandwidth
limits that the application of congestion control provides. For
example, when a media sender is reacting to a FIR, the unusually
high number of packets that form the decoder refresh point have to
be paced in compliance with the congestion control algorithm, even
if the user experience suffers from a slowly transmitted decoder
refresh point.
A change of the Temporary Maximum Media Stream Bit Rate value can
only mitigate congestion, but not cause congestion as long as
congestion control is also employed. An increase of the value by
a request REQUIRES the media sender to use congestion control when
increasing its transmission rate to that value. A reduction of
the value results in a reduced transmission bit rate thus reducing
the risk for congestion.
6. Security Considerations
The defined messages have certain properties that have security
implications. These must be addressed and taken into account by
users of this protocol.
The defined setup signaling mechanism is sensitive to modification
attacks that can result in session creation with sub-optimal
configuration, and, in the worst case, session rejection. To
prevent this type of attack, authentication and integrity
protection of the setup signaling is required.
Spoofed or maliciously created feedback messages of the type
defined in this specification can have the following implications:
a. severely reduced media bit rate due to false TMMBR messages
that sets the maximum to a very low value;
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b. assignment of the ownership of a bounding tuple to the
wrong participant within a TMMBN message, potentially
causing unnecessary oscillation in the bounding set as the
mistakenly identified owner reports a change in its tuple
and the true owner possibly holds back on changes until a
correct TMMBN message reaches the participants;
c. sending TSTR requests that result in a video quality
different from the user's desire, rendering the session
less useful.
d. Frequent FIR commands will potentially reduce the frame-
rate, making the video jerky, due to the frequent usage of
decoder refresh points.
To prevent these attacks there is a need to apply authentication
and integrity protection of the feedback messages. This can be
accomplished against threats external to the current RTP session
using the RTP profile that combines SRTP [SRTP] and AVPF into
SAVPF [SAVPF]. In the mixer cases, separate security contexts and
filtering can be applied between the mixer and the participants
thus protecting other users on the mixer from a misbehaving
participant.
7. SDP Definitions
Section 4 of [RFC4585] defines a new SDP [RFC4566] attribute,
rtcp-fb, that may be used to negotiate the capability to handle
specific AVPF commands and indications, such as Reference Picture
Selection, Picture Loss Indication etc. The ABNF for rtcp-fb is
described in section 4.2 of [RFC4585]. In this section we extend
the rtcp-fb attribute to include the commands and indications that
are described for codec control protocol in the present document.
We also discuss the Offer/Answer implications for the codec
control commands and indications.
7.1. Extension of the rtcp-fb Attribute
As described in AVPF [RFC4585], the rtcp-fb attribute indicates
the capability of using RTCP feedback. AVPF specifies that the
rtcp-fb attribute must only be used as a media level attribute and
must not be provided at session level. All the rules described in
[RFC4585] for rtcp-fb attribute relating to payload type and to
multiple rtcp-fb attributes in a session description also apply to
the new feedback messages defined in this memo.
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The ABNF [RFC4234] for rtcp-fb as defined in [RFC4585] is
"a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF
where rtcp-fb-pt is the payload type and rtcp-fb-val defines the
type of the feedback message such as ack, nack, trr-int and rtcp-
fb-id. For example to indicate the support of feedback of picture
loss indication, the sender declares the following in SDP
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 nack pli
In this document we define a new feedback value "ccm" which
indicates the support of codec control using RTCP feedback
messages. The "ccm" feedback value SHOULD be used with
parameters, which indicate the specific codec control commands
supported. In this draft we define four parameters, which can be
used with the ccm feedback value type.
o "fir" indicates the support of the Full Intra Request (FIR).
o "tmmbr" indicates the support of the Temporary Maximum Media
Stream Bit Rate Request/Notification (TMMBR/TMMBN). It has
an optional sub parameter to indicate the session maximum
packet rate to be used. If not included this defaults to
infinity.
o "tstr" indicates the support of the Temporal-Spatial Trade-
off Request/Notification (TSTR/TSTN).
O "vbcm" indicates the support of H.271 video back channel
messages (VBCM). It has zero or more subparameters
identifying the supported H.271 "payloadType" values.
In the ABNF for rtcp-fb-val defined in [RFC4585], there is a
placeholder called rtcp-fb-id to define new feedback types. "ccm"
is defined as a new feedback type in this document and the ABNF
for the parameters for ccm are defined here (please refer to
section 4.2 of [RFC4585] for complete ABNF syntax).
rtcp-fb-param = SP "app" [SP byte-string]
/ SP rtcp-fb-ccm-param
/ ; empty
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rtcp-fb-ccm-param = "ccm" SP ccm-param
ccm-param = "fir" ; Full Intra Request
/ "tmmbr" [SP "smaxpr=" MaxPacketRateValue]
; Temporary max media bit rate
/ "tstr" ; Temporal Spatial Trade Off
/ "vbcm" *(SP subMessageType) ; H.271 VBCM messages
/ token [SP byte-string]
; for future commands/indications
subMessageType = 1*8DIGIT
byte-string = <as defined in section 4.2 of [RFC4585] >
MaxPacketRateValue = 1*15DIGIT
7.2. Offer-Answer
The Offer/Answer [RFC3264] implications for codec control protocol
feedback messages are similar those described in [RFC4585]. The
offerer MAY indicate the capability to support selected codec
commands and indications. The answerer MUST remove all ccm
parameters which it does not understand or does not wish to use in
this particular media session. The answerer MUST NOT add new ccm
parameters in addition to what has been offered. The answer is
binding for the media session and both offerer and answerer MUST
only use feedback messages negotiated in this way.
The session maximum packet rate parameter part of the TMMBR
indication is declarative and everyone shall use the highest value
indicated in a response. If the session maximum packet rate
parameter is not present in an offer it SHALL NOT be included by
the answerer.
7.3. Examples
Example 1: The following SDP describes a point-to-point video call
with H.263, with the originator of the call declaring its
capability to support the FIR and TSTR/TSTN codec control
messages. The SDP is carried in a high level signaling protocol
like SIP.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Point-to-Point call
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
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m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir
In the above example, when the sender receives a TSTR message from
the remote party it is capable of adjusting the trade off as
indicated in the RTCP TSTN feedback message.
Example 2: The following SDP describes a SIP end point joining a
video mixer that is hosting a multiparty video conferencing
session. The participant supports only the FIR (Full Intra
Request) codec control command and it declares it in its session
description.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multiparty Video Call
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm fir
When the video MCU decides to route the video of this participant
it sends an RTCP FIR feedback message. Upon receiving this
feedback message the end point is required to generate a full
intra request.
Example 3: The following example describes the Offer/Answer
implications for the codec control messages. The Offerer wishes
to support "tstr", "fir" and "tmmbr". The offered SDP is
-------------> Offer
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir
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a=rtcp-fb:* ccm tmmbr smaxpr=120
The answerer wishes to support only the FIR and TSTR/TSTN messages
and the answerer SDP is
<---------------- Answer
v=0
o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
s=Offer/Answer
c=IN IP4 192.0.2.37
m=audio 47190 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 53273 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir
Example 4: The following example describes the Offer/Answer
implications for H.271 Video back channel messages (VBCM). The
Offerer wishes to support VBCM and the sub-messages of payloadType
1 (one or more pictures that are entirely or partially lost) and 2
(a set of blocks of one picture that are entirely or partially
lost).
-------------> Offer
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm vbcm 1 2
The answerer only wishes to support sub-messages of type 1 only
<---------------- Answer
v=0
o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
s=Offer/Answer
c=IN IP4 192.0.2.37
m=audio 47190 RTP/AVP 0
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a=rtpmap:0 PCMU/8000
m=video 53273 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm vbcm 1
So in the above example only VBCM indications comprised of
"payloadType" 1 will be supported.
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8. IANA Considerations
The new value "ccm" needs to be registered with IANA in the "rtcp-
fb" Attribute Values registry located at the time of publication
at:
http://www.iana.org/assignments/sdp-parameters
Value name: ccm
Long Name: Codec Control Commands and Indications
Reference: RFC XXXX
A new registry "Codec Control Messages" needs to be created to
hold "ccm" parameters located at time of publication at:
http://www.iana.org/assignments/sdp-parameters
New registration in this registry follows the "Specification
required" policy as defined by [RFC2434]. In addition they are
required to indicate which, if any additional RTCP feedback types,
such as "nack", "ack".
The initial content of the registry is the following values:
Value name: fir
Long name: Full Intra Request Command
Usable with: ccm
Reference: RFC XXXX
Value name: tmmbr
Long name: Temporary Maximum Media Stream Bit Rate
Usable with: ccm
Reference: RFC XXXX
Value name: tstr
Long name: temporal Spatial Trade Off
Usable with: ccm
Reference: RFC XXXX
Value name: vbcm
Long name: H.271 video back channel messages
Usable with: ccm
Reference: RFC XXXX
The following values need to be registered as FMT values in the
"FMT Values for RTPFB Payload Types" registry located at the time
of publication at: http://www.iana.org/assignments/rtp-parameters
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RTPFB range
Name Long Name Value Reference
-------------- --------------------------------- ----- ---------
Reserved 2 [RFCxxxx]
TMMBR Temporary Maximum Media Stream Bit 3 [RFCxxxx]
Rate Request
TMMBN Temporary Maximum Media Stream Bit 4 [RFCxxxx]
Rate Notification
The following values need to be registered as FMT values in the
"FMT Values for PSFB Payload Types" registry located at the time
of publication at: http://www.iana.org/assignments/rtp-parameters
PSFB range
Name Long Name Value Reference
-------------- --------------------------------- ----- ---------
FIR Full Intra Request Command 4 [RFCxxxx]
TSTR Temporal-Spatial Trade-off Request 5 [RFCxxxx]
TSTN Temporal-Spatial Trade-off Notification 6 [RFCxxxx]
VBCM Video Back Channel Message 7 [RFCxxxx]
9. Contributors
Tom Taylor has made a very significant contribution, for which the
authors are very grateful, to this specification by helping
rewrite the specification. Especially the parts regarding the
algorithm for determining bounding sets for TMMBR have benefited.
10. Acknowledgements
The authors would like to thank Andrea Basso, Orit Levin, Nermeen
Ismail for their work on the requirement and discussion draft
[Basso].
Drafts of this memo were reviewed and extensively commented by
Roni Even, Colin Perkins, Randell Jesup, Keith Lantz, Harikishan
Desineni, Guido Franceschini and others. The authors appreciate
these reviews.
Wenger, et al. Standards Track [Page 65]
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Funding for the RFC Editor function is currently provided by the
Internet Society.
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11. References
11.1. Normative references
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., Rey,
J., "Extended RTP Profile for Real-Time Transport
Control Protocol (RTCP)-Based Feedback (RTP/AVPF)",
RFC 4585, July 2006
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with Session Description Protocol (SDP)", RFC
3264, June 2002.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
11.2. Informative references
[Basso] A. Basso, et. al., "Requirements for transport of
video control commands", draft-basso-avt-videoconreq-
02.txt, expired Internet Draft, October 2004.
[AVC] Joint Video Team of ITU-T and ISO/IEC JTC 1, Draft
ITU-T Recommendation and Final Draft International
Standard of Joint Video Specification (ITU-T Rec.
H.264 | ISO/IEC 14496-10 AVC), Joint Video Team (JVT)
of ISO/IEC MPEG and ITU-T VCEG, JVT-G050, March 2003.
[H245] ITU-T Rec. HG.245, "Control protocol for multimedia
communication", MAY 2006
[NEWPRED] S. Fukunaga, T. Nakai, and H. Inoue, "Error Resilient
Video Coding by Dynamic Replacing of Reference
Pictures," in Proc. Globcom'96, vol. 3, pp. 1503 -
1508, 1996.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC2032] Turletti, T. and C. Huitema, "RTP Payload Format for
H.261 Video Streams", RFC 2032, October 1996.
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[SAVPF] J. Ott, E. Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)," draft-ietf-avt-
profile-savpf-10.txt, February, 2007.
[RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
"Gateway Control Protocol Version 1", RFC 3525, June
2003.
[RFC3448] M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, Jan 2003
[VBCM] ITU-T Rec. H.271, "Video Back Channel Messages", June
2006
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)",
RFC 3890, September 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March
2006.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M.,
and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, June 2002.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC
2198, September 1997.
[Topologies] M. Westerlund, and S. Wenger, "RTP Topologies",
draft-ietf-avt-topologies-04, work in progress, Feb
2007.
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12. Authors' Addresses
Stephan Wenger
Nokia Corporation
975, Page Mill Road,
Palo Alto,CA 94304
USA
Phone: +1-650-862-7368
EMail: stewe@stewe.org
Umesh Chandra
Nokia Research Center
975, Page Mill Road,
Palo Alto,CA 94304
USA
Phone: +1-650-796-7502
Email: Umesh.Chandra@nokia.com
Magnus Westerlund
Ericsson Research
Ericsson AB
SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000
EMail: magnus.westerlund@ericsson.com
Bo Burman
Ericsson Research
Ericsson AB
SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000
EMail: bo.burman@ericsson.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on
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AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
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Wenger, et al. Standards Track [Page 70]
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
RFC Editor Considerations
The RFC editor is requested to replace all occurrences of XXXX
with the RFC number this document receives.
Wenger, et al. Standards Track [Page 71]
