One document matched: draft-ietf-avt-avpf-ccm-02.txt
Differences from draft-ietf-avt-avpf-ccm-01.txt
Network Working Group Stephan Wenger
INTERNET-DRAFT Umesh Chandra
Expires: April 2007 Nokia
Magnus Westerlund
Bo Burman
Ericsson
October 20, 2006
Codec Control Messages in the
Audio-Visual Profile with Feedback (AVPF)
draft-ietf-avt-avpf-ccm-02.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies a few extensions to the messages defined in
the Audio-Visual Profile with Feedback (AVPF). They are helpful
primarily in conversational multimedia scenarios where centralized
multipoint functionalities are in use. However some are also usable
in smaller multicast environments and point-to-point calls. The
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extensions discussed are H.271 video back channel, Full Intra
Request, Temporary Maximum Media Bit-rate and Temporal Spatial Trade-
off.
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TABLE OF CONTENTS
1. Introduction....................................................5
2. Definitions.....................................................7
2.1. Glossary...................................................7
2.2. Terminology................................................8
2.3. Topologies.................................................9
3. Motivation (Informative)........................................9
3.1. Use Cases.................................................10
3.2. Using the Media Path......................................12
3.3. Using AVPF................................................12
3.3.1. Reliability..........................................12
3.4. Multicast.................................................13
3.5. Feedback Messages.........................................13
3.5.1. Full Intra Request Command...........................13
3.5.1.1. Reliability.....................................14
3.5.2. Temporal Spatial Trade-off Request and Announcement..15
3.5.2.1. Point-to-point..................................15
3.5.2.2. Point-to-Multipoint using Multicast or Translators16
3.5.2.3. Point-to-Multipoint using RTP Mixer.............16
3.5.2.4. Reliability.....................................16
3.5.3. H.271 Video Back Channel Message conforming to ITU-T Rec.
H.271.......................................................17
3.5.3.1. Reliability.....................................19
3.5.4. Temporary Maximum Media Bit-rate Request.............19
3.5.4.1. MCU based Multi-point operation.................20
3.5.4.2. Point-to-Multipoint using Multicast or Translators22
3.5.4.3. Point-to-point operation........................22
3.5.4.4. Reliability.....................................22
4. RTCP Receiver Report Extensions................................24
4.1. Design Principles of the Extension Mechanism..............24
4.2. Transport Layer Feedback Messages.........................25
4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR).....25
4.2.1.1. Semantics.......................................25
4.2.1.2. Message Format..................................27
4.2.1.3. Timing Rules....................................28
4.2.2. Temporary Maximum Media Bit-rate Notification (TMMBN) 28
4.2.2.1. Semantics.......................................28
4.2.2.2. Message Format..................................29
4.2.2.3. Timing Rules....................................30
4.3. Payload Specific Feedback Messages........................30
4.3.1. Full Intra Request (FIR) command.....................30
4.3.1.1. Semantics.......................................30
4.3.1.2. Message Format..................................32
4.3.1.3. Timing Rules....................................33
4.3.1.4. Remarks.........................................33
4.3.2. Temporal-Spatial Trade-off Request (TSTR)............34
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4.3.2.1. Semantics.......................................34
4.3.2.2. Message Format..................................34
4.3.2.3. Timing Rules....................................35
4.3.2.4. Remarks.........................................35
4.3.3. Temporal-Spatial Trade-off Announcement (TSTA).......36
4.3.3.1. Semantics.......................................36
4.3.3.2. Message Format..................................36
4.3.3.3. Timing Rules....................................37
4.3.3.4. Remarks.........................................37
4.3.4. H.271 VideoBackChannelMessage (VBCM).................37
5. Congestion Control.............................................40
6. Security Considerations........................................41
7. SDP Definitions................................................41
7.1. Extension of rtcp-fb attribute............................42
7.2. Offer-Answer..............................................43
7.3. Examples..................................................43
8. IANA Considerations............................................46
9. Acknowledgements...............................................47
10. References....................................................48
10.1. Normative references.....................................48
10.2. Informative references...................................48
11. Authors' Addresses............................................49
12. List of Changes relative to previous drafts...................49
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1. Introduction
When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
developed, the main emphasis lied 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 efficiently support MCU-based
multipoint conferencing. 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] bitstrings for video back
channel messages.
In RTP [RFC3550] terminology, MCUs comprise mixers and translators.
Most MCUs also include signalling 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 were
not envisioned (or at least not documented in sufficient detail) in
RTP. 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 acknowledgement. Other messages require
acknowledgement, leading to a two way communication model that could
suggest to some to be useful for control purposes. It is not the
intention of this memo to open up 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 SIP re-invites, undesirable. Furthermore,
all messages are of a very simple format that can be easily processed
by an RTP/RTCP sender/receiver. Finally, all messages infer only to
the RTP stream they are related to, and not to any other property of
a communication system.
The Full Intra Request (FIR) Command requires the receiver of the
message (and sender of the stream) to immediately insert a decoder
refresh point. In video coding, one commonly used form of a decoder
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refresh point is an IDR or Intra picture. Other codecs may have
other forms of decoder refresh points. In order to fulfil congestion
control constraints, sending a decoder refresh point may imply a
significant drop in frame rate, as they are commonly much larger than
regular predicted content. The use of this message is restricted to
cases where no other means of decoder refresh can be employed, e.g.
during the join-phase of a new participant in a multipoint
conference. It is explicitly disallowed to use the FIR command for
error resilience purposes, and instead it is referred to AVPF's
[RFC4585] PLI message, which reports lost pictures and has been
included in AVPF for precisely that purpose. The message does not
require an acknowledgement, as the presence of a decoder refresh
point can be easily derived from the media bit stream. Today, the
FIR message appears to be useful primarily with video streams, but in
the future it may become helpful also in conjunction with other media
codecs that support prediction across RTP packets.
The Temporary Maximum Media Bandwidth Request (TMMBR) Message allows
to signal, from media receiver to media sender, the current maximum
supported media bit-rate for a given media stream. Once a bandwidth
limitation is established by the media sender, that sender notifies
the initiator of the request, and all other session participants, by
sending a TMMBN notification message. One usage scenarios can be
seen as limiting media senders in multiparty conferencing to the
slowest receiver's maximum media bandwidth reception/handling
capability. Such a use is helpful, for example, because the
receiver's situation may have changed due to computational load, or
because the receiver has just joined the conference and it is helpful
to inform media sender(s) about its constraints, without waiting for
congestion induced bandwidth reduction. Another application involves
graceful bandwidth adaptation in scenarios where the upper limit
connection bandwidth to a receiver changes, but is known in the
interval between these dynamic changes. The TMMBR message is useful
for all media types that are not inherently of constant bit rate.
The Video back channel message (VBCM) allows conveying bit streams
conforming to ITU-T Rec. H.271 [H.271], from a video receiver to
video sender. This ITU-T Recommendation defines codepoints for a
number of video-specific feedback messages. Examples include
messages to signal:
- the corruption of reference pictures or parts thereof,
- the corruption of decoder state information, e.g. parameter sets,
- the suggestion of using a reference picture other than the one
typically used, e.g. to support the NEWPRED algorithm [NEWPRED].
The ITU-T plans to add codepoints to H.271 every time a need arises,
e.g. with the introduction of new video codecs or new tools into
existing video codecs.
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There exists some overlap between H.271 messages and "native"
messages specified in this memo and in AVPF. Examples include the
PLI message of [RFC4585] and the FIR message specified herein. As a
general rule, the "native" messages should be prefered over the
sending of VBCM messages when all senders and receivers implement
this memo. However, if gateways are in the picture, it may be more
advisable to utilize VBCM. Similarly, for feedback message types
that exist in H.271 but do not exist in this memo or AVPF, there is
no other choice but using VBCM.
Video feedback channel messages according to H.271 do not require
acknowledgements on a protocol level, because the appropriate
reaction of the video encoder and sender can be derived from the
forward video bit stream.
Finally, the Temporal-Spatial Trade-off Request (TSTR) Message
enables a video receiver to signal to the video sender its preference
for spatial quality or high temporal resolution (frame rate). The
receiver of the video stream generates this signal typically based on
input from its user interface, so to react to explicit requests of
the user. However, some implicit use forms are also known. For
example, the trade-offs commonly used for live video and document
camera content are different. Obviously, this indication is relevant
only with respect to video transmission. The message is acknowledged
by an announcement message indicating the newly chosen tradeoff, so
to allow immediate user feedback.
2. Definitions
2.1. Glossary
ASM - Asynchronous Multicast
AVPF - The Extended RTP Profile for RTCP-based Feedback
FEC - Forward Error Correction
FIR - Full Intra Request
MCU - Multipoint Control Unit
MPEG - Moving Picture Experts Group
PtM - Point to Multipoint
PtP - Point to Point
TMMBN - Temporary Maximum Media Bit-rate Notification
TMMBR - Temporary Maximum Media Bit-rate Request
PLI - Picture Loss Indication
TSTA - Temporal Spatial Trade-off Announcement
TSTR - Temporal Spatial Trade-off Request
VBCM - Video Back Channel Message indication.
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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:
Codepoint defined by this specification, of one of the
following types:
Request:
Message that requires Acknowledgement
Acknowledgment:
Message that answers a Request
Command:
Message that forces the receiver to an action
Indication:
Message that reports a situation
Notification:
See Indication.
Note that, with the exception of "Notification", this terminology
is in alignment with ITU-T Rec. H.245.
Decoder Refresh Point:
A bit string, packetised in one or more RTP packets, which
completely resets the decoder to a known state. Typical
examples of Decoder Refresh Points are H.261 Intra pictures
and H.264 IDR pictures. However, there are also much more
complex decoder refresh points.
Typical examples for "hard" decoder refresh points are Intra
pictures in H.261, H.263, MPEG 1, MPEG 2, and MPEG-4 part 2,
and 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
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H.264, for example, a decoder refresh point contains
parameter set 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).
To the best of the author's knowledge, the term "Decoder
Refresh Point" has been formally defined only in H.264; hence
we are referring here to this video compression standard.
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 performed media
aware, implying that media packets are removed in the order
of their 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.
2.3. Topologies
Please refer to [Topologies] for an in depth discussion. the
topologies referred to throughout this memo are labeled (consistent
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
Topo-Video-switch-MCU . . . . video switching MCU,
Topo-RTCP-terminating-MCU . . mixer but terminating RTCP
3. Motivation (Informative)
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 do quote
relevant sections of it to provide motivation and requirements.
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3.1. Use Cases
There are a number of possible usages for the proposed feedback
messages. Let's begin with looking through the use cases Basso et al.
[Basso] proposed. Some of the use cases have been reformulated and
commented:
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 that 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 a request of
change of the desired trade-off in temporal/spatial resolution.
For example, one user may prefer a higher frame rate and a lower
spatial quality, and another use may prefer the opposite. This
choice is also highly content dependent. Many current video
conferencing systems offer in the user interface a mechanism to
make this selection, usually in the form of a slider. The
mechanism is helpful in point-to-point, centralized multipoint and
non-centralized multipoint uses.
4. Use case 4 of the Basso draft applies only to AVPF's PLI [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
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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 transrating. One way of achieving this is to setup sessions
with endpoints using the maximum bit rate accepted by that
endpoint, and by the call admission method used by the mixer. By
means of commands that allow reducing the maximum media bitrate
beyond what has been negotiated during session setup, the mixer
can then reduce the maximum bit rate sent by endpoints to the
lowest common denominator of all received streams. As the lowest
common denominator changes due to endpoints joining, leaving, or
network congestion, the mixer can adjust the limits to which
endpoints can send their streams to match the new limit. The mixer
then would request a new maximum bit rate, which is equal 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. The used congestion control algorithms (AMID and TFRC) probe for
more bandwidth as long as there is something to send. With
congestion control 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 link
also commonly has some type of QoS negotiation enabling the
cellular device to learn the maximal bit-rate available over this
last hop. Thus indicating the maximum available bit-rate to the
transmitting part can be beneficial to prevent it from even trying
to exceed the known hard limit that exists. For cellular or other
mobile devices the available known bit-rate can also quickly
change due to handover to another transmission technology, QoS
renegotiation due to congestion, etc. To enable minimal disruption
of service a possibility for quick convergence, especially in
cases of reduced bandwidth, a media path signalling method is
desired.
8. The use of reference picture selection as an error resilience tool
has been introduced in 1997 as NEWPRED [NEWPRED], and is now
widely deployed. It operates the receiver sending a feedback
message to the sender, indicating a reference picture that should
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be used for future prediction. AVPF contains a mechanism for
conveying such a message, but did not specify for which codec and
according to which syntax the message conforms to. 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 enjoy wide support and fairly quickly. 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 propose to use the media path for
the codec control messages. First, systems employing MCUs are often
separating the control and media processing parts. As these messages
are intended or generated by the media part rather than the
signalling part of the MCU, having them on the media path avoids
interfaces and unnecessary control traffic between signalling and
processing. If the MCU is physically decomposite, the use of the
media path avoids the need for media control protocol extensions
(e.g. in MEGACO [RFC3525]).
Secondly, the signalling path quite commonly contains several
signalling entities, e.g. SIP-proxies and application servers.
Avoiding signalling 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
signalling entities are also commonly not optimized for minimal
delay, rather other architectural goals. Thus the signalling path can
be significantly longer in both geographical and delay sense.
3.3. Using AVPF
The AVPF feedback message framework [RFC4585] provides a simple way
of implementing the new messages. Furthermore, AVPF implements rules
controlling the timing of feedback messages so to avoid congestion
through network flooding. We re-use these rules by referencing to
AVPF.
The signalling setup for AVPF allows each individual type of function
to be configured or negotiated on a 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.
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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 media related requests might be used with multicast. The RTCP
timing rules specified in [RFC3550] and [RFC4585] ensure that the
messages do not cause overload of the RTCP connection. The use of
multicast may result in the reception of messages with inconsistent
semantics. The reaction to inconsistencies depends on the message
type, and is discussed for each message type separately.
3.5. Feedback Messages
This section describes the semantics of the different feedback
messages and how they apply to the different use cases.
3.5.1. Full Intra Request Command
A Full Intra Request (FIR) command, when received by the designated
media sender, requires that the media sender sends a "decoder refresh
point" (see 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 bandwidth 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
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times the longest Round Trip Time (RTT) 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 delaying 2 times the
longest known RTT is to avoid sending unnecessary decoder refresh
points. A session participant may have sent its own request while
another participants request was in-flight to them. Thus suppressing
those requests that may have been sent without knowledge about the
other request avoids this issue.
Full Intra Request is applicable in use-case 1, 2, and 5.
3.5.1.1. Reliability
The FIR message results in the delivery of a decoder refresh point,
unless the message is lost. Decoder refresh points are easily
identifiable from the bit stream. Therefore, there is no need for
protocol-level acknowledgement, 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 a response has been received. The repetition
interval is determined by the RTCP timing rules the session operates
under. 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. To combat loss of the decoder refresh points sent, the
sender that receives repetitions of the FIR 2*RTT after the
transmission of the decoder refresh point shall send a new decoder
refresh point. Two round trip times allow time for the request to
arrive at the media sender and the decoder refresh point to arrive
back to the requestor. 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.
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 to generate FIR commands
by the MCU. The two legs (FIR-requesting endpoint to MCU, and MCU to
decoder refresh point generating MCU) are handled independently from
each other from a reliability perspective.
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3.5.2. Temporal Spatial Trade-off Request and Announcement
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. 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 if the request
results in a change of the trade off. An acknowledgement process has
been defined to provide feedback of the trade-off that is used
henceforth.
Informative note: TSTR and TSTA 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 would require at least 2 round-trips more (compared to the
TSTR/TSTA mechanism) and may involve proxies and other complex
mechanisms. Even in a well-designed system, it may take a second
or so until finally the new trade-off is selected.
Furthermore the use of RTCP solves very efficiently the multicast
use case.
The use of TSTR and TSTA in multipoint scenarios is a non-trivial
subject, and can be solved in many implementation specific ways.
Problems are stemming 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
MCU's or endpoint's reaction to the reception of a suggested trade-
off as conveyed in the TSTR -- we only require the receiver of a TSTR
message to reply to it by sending a TSTA, carrying the new trade-off
chosen by its own criteria (which may or may not be based on the
trade-off conveyed by TSTR). In other words, the trade-off sent in
TSTR is a non-binding recommendation; nothing more.
With respect to TSTR/TSTA, four scenarios based on the topologies
described in [Topologies] need to be distinguished. 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
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requested value in TSTR, and within its capabilities. The TSTA
message conveys back the new trade-off value (which may be identical
to the old one if, for example, the sender is not capable to adjust
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, TSTR messages from different
receivers may be received unsynchronized, and 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, prioritize certain participants, or
continue using 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 TSTA, 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 MCU 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). The strategy of forming this "consensus" is open for the
implementation, and can, for example, encompass averaging the
participant's request values, prioritizing certain participants, or
use session default values. If the Mixer changes its trade-off, it
needs to request from the media sender(s) the use of 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.
Even if a Mixer or Translator performs transcoding, it is very
difficult to deliver media with the requested trade-off, unless the
content the MCU receives is already close to that trade-off. Only in
cases where the original source has substantially higher quality (and
bit-rate), it is likely that transcoding 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 Announcement (TSTA) 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 information of 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 codepoint defined in
this memo is used to transparently convey such a message from media
receiver to media sender.
We refrain from an in-depth discussion of the available codepoints
within H.271 in this memo for a number of reasons. The perhaps most
important reason is that we expect backward-compatible additions of
codepoints to H.271 outside the update/maturity cycle of this memo.
Another reason lies in the complexity of the H.271 specification: it
is a dense document with currently 16 pages of content. It does not
make any sense to try to summarize its content in a few sentences of
IETF lingo -- oversimplification and misguidance would be inevitable.
Finally, please note that H.271 contains many statements of
applicability and interpretation of its various messages in
conjunction with specific video compression standards. This type of
discussion would overload the present memo.
In so far, this memo follows the guidance of a decade of RTP payload
format specification work -- the details of the media format carried
is normally not described in any significant detail.
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 currenty defined in H.271, their similar 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 AVPF PLI Yes
2 AVPF SLI Yes
3 N/A Yes (no required sender action)
4 N/A Yes (no required sender action)
Table 1: H.271 messages and their AVPF/CCM equivalents
Note: H.271 message type 0 is not a strict equivalent to
AVPF's 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 native messages as defined in
[RFC4585] and in this memo instead of similar messages defined in
[H.271]. Our current understanding of similar messages is
documented in Table 1 above. One good reason to divert from the
SHOULD statement above would be if it is clearly understood that,
for a given application and video compression standard, the
aforementioned "similarity" is not given, in contrast to what
the table indicates.
3. It has been observed that some of the H.271 codepoints 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 deocumented below, the implementation of such a
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mixer and cooperative endpoints is a very difficult and tedious
task. Therefore, H.271 message 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 with point to Multi-point:
1. Different state established on different receivers. One example is
the reference picture feedback message, which, when sent to
receivers in which the video codecs are at different state due to
previous losses or stream switches, the results can be
unpredictable and annoying.
2. Combination of multiple messages/requests by a media sender into
an action and or response.
3. Suppression of requests may need to go beyond the basic mechanism
described in AVPF. For example forward messages may be need to
suppress the generation of requests.
Issues with translators and mixers
1. Combination of multiple message or requests into an action or
response.
2.
3.5.3.1. Reliability
H.271 video back channel messages do not require reliable
transmission, and 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 Bit-rate Request
A receiver, translator or mixer uses the Temporary Maximum Media Bit-
rate Request (TMMBR, "timber") to request a sender to limit the
maximum bit-rate for a media stream to, or below, the provided value.
The primary usage for this is a scenario with MCU (use case 6),
corresponding to Topo-Translator or Topo-Mixer, but also Topo-Point-
to-Point.
The temporary maximum media bit-rate messages are generic messages
that can be applied to any media.
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The reasoning below assumes that the participants have negotiated a
session maximum bit-rate, using the signalling 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 both cases, the bit-rate negotiated in signalling
is the one that the participant guarantees to be able to handle
(encode and decode). In practice, the connectivity of the
participant also bears an influence to the negotiated value -- it
does not necessarily make much sense to negotiate a media bit rate
that one's network interface does not support.
An already established temporary bit-rate value may be changed at any
time (subject to the timing rules of the feedback message sending),
and to any value between zero and the session maximum, as negotiated
during signalling. Even if a sender has received a TMMBR message
increasing the bit-rate, all increases must be governed by a
congestion control algorithm. TMMBR only indicates known limitations,
usually in the local environment, and does not provide any
guarantees.
If it is likely that the new bit-rate indicated by TMMBR will be
valid for the remainder of the session, the TMMBR sender can perform
a renegotiation of the session upper limit using the session
signalling protocol.
3.5.4.1. MCU based Multi-point operation
Assume a small mixer-based multiparty conference is ongoing, as
depicted in Topo-Mixer of [Topologies]. All participants (A-D) have
negotiated a common maximum bit-rate that this session can use. The
conference operates over a number of unicast links between the
participants and the MCU. The congestion situation on each of these
links can easily be monitored by the participant in question and by
the MCU, utilizing, for example, RTCP Receiver Reports. 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 MCU (who is aware of
the congestion situation on all connections it manages) has no
standardized means to inform participants to slow down, short of
forging its own receiver reports (which is undesirable). In
principle, an MCU confronted with such a situation is obliged to thin
or transcode streams intended for connections that detected
congestion.
In practice, stream thinning - if performed media aware - is
unfortunately a very difficult and cumbersome operation and adds
undesirable delay. If done media unaware, it leads very quickly to
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unacceptable reproduced media quality. Hence, means to slow down
senders even in the absence of congestion on their connections to the
MCU are desirable.
To allow the MCU to perform congestion control on the individual
links, without performing transcoding, there is a need for a
mechanism that enables the MCU to request the participant's media
encoders to limit their maximum media bit-rate currently used. The
MCU handles the detection of a congestion state between itself and a
participant as follows:
1. Start thinning the media traffic to the supported bit-rate.
2. Use the TMMBR to request the media sender(s) to reduce the media
bit-rate sent by them to the MCU, to a value that is in compliance
with congestion control principles for the slowest link. Slow
refers here to the available bandwidth and packet rate after
congestion control.
3. As soon as the bit-rate has been reduced by the sending part, the
MCU stops stream thinning implicitly, because there is no need for
it any more as the stream is in compliance with congestion
control.
Above algorithms may suggest to some that there is no need for the
TMMBR - it should be sufficient to solely rely on stream thinning.
As much as this is desirable from a network protocol designer's
viewpoint, it has the disadvantage that it doesn't work very
well - the reproduced media quality quickly becomes unusable.
It appears to be a reasonable compromise to rely on stream thinning
as an immediate reaction tool to combat congestions, and have a quick
control mechanism that instructs the original sender to reduce its
bitrate.
Note also that the standard RTCP receiver report cannot serve for the
purpose mentioned. In an environment with RTP Mixers, the RTCP RR is
being sent between the RTP receiver in the endpoint and the RTP
sender in the Mixer only - as there is no multicast transmission.
The stream that needs to be bandwidth-reduced, however, is the one
between the original sending endpoint and the Mixer. This endpoint
doesn't see the aforementioned RTCP RRs, and hence needs explicitly
informed about desired bandwidth adjustments.
In this topology it is the Mixer's responsibility to collect, and
consider jointly, the different bit-rates which the different links
may support, into the bit rate requested. This aggregation may also
take into account that the Mixer may contain certain transcoding
capabilities (as discussed in under Topo-Mixer in [Topologies]),
which can be employed for those few of the session participants that
have the lowest available bit-rates.
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3.5.4.2. Point-to-Multipoint using Multicast or Translators
In these topologies, corresponding to Topo-Multicast or Topo-
Translator RTCP RRs are transmitted globally which allows for the
detection of 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 of the use of TMMBR
of section 3.5.4.1 does not apply. However, even in this case the
congestion control response can be improved when the unicast links
are employing congestion controlled transport protocols (such as TCP
or DCCP). A peer may also report local limitation to the media
sender.
3.5.4.3. Point-to-point operation
In use case 7 it is possible to use TMMBR to improve the performance
at times of changes in the known upper limit of the bit-rate. In
this use case the signalling protocol has established an upper limit
for the session and media bit-rates. However at the time of
transport link bit-rate reduction, a receiver could avoid serious
congestion by sending a TMMBR to the sending side.
3.5.4.4. 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 does not
allow for easy suppression of other participants requests. For the
reasons mentioned, a mechanism based on explicit notification is
used.
Upon the reception of a request a media sender sends a notification
containing the current applicable limitation of the bit-rate, and
which session participants that own that limit. That allows all other
participants to suppress any request they may have, with limitation
value equal or higher to the current one. The identity of the owner
allows for small message sizes and media sender states. A media
sender only keeps state for the SSRC of the current owner of the
limitation; all other requests and their sources are not saved. Only
the participant with the lowest value is allowed to remove or change
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its limitation. Otherwise anyone that ever set a limitation would
need to remove it to allow the maximum bit-rate to be raised beyond
that value.
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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 Announcement (TSTA), and Video Back Channel Message (VBCM)
are "Payload Specific Feedback Messages" in the sense of section 6.3
of AVPF [RFC4585]. The Temporary Maximum Media Bit-rate Request
(TMMBR) and Temporary Maximum Media Bit-rate Notification (TMMBN) are
"Transport Layer Feedback Messages" in the sense of section 6.2 of
AVPF.
In the following subsections, the new feedback messages are defined,
following a similar structure as in the AVPF specification's sections
6.2 and 6.3, respectively.
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 have been 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 one of its messages. There is danger that
this introduction could be misunderstood as the precedence for the
use of RTCP as an RTP session control protocol. In order to prevent
these misunderstandings, 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 which
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
c) are directly related to activities of a certain media codec, class
of media codecs (e.g. video codecs), or the given media stream.
In this memo, a two-way handshake is only introduced for such
messages that
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a) require a notification or acknowledgement due to their nature,
which is motivated 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 follow a number of
common design principles. In particular:
a) Media receivers are not always implementing higher control
protocol functionalities (SDP, XML parsers and such) in their
media path. Therefore, simple binary representations are used in
the feedback messages and not an (otherwise desirable) flexible
format such as, for example, XML.
4.2. Transport Layer Feedback Messages
Transport Layer FB messages are identified by the value RTPFB (205)
as RTCP packet type.
In AVPF, one message of this category had been defined. This memo
specifies two more messages for a total of three messages of this
type. They are identified by means of the FMT parameter as follows:
0: unassigned
1: Generic NACK (as per AVPF)
2: Temporary Maximum Media Bit-rate Request
3: Temporary Maximum Media Bit-rate Notification
4-30: unassigned
31: reserved for future expansion of the identifier number space
The following subsection defines the formats of the FCI field for
this type of FB message.
4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR)
The FCI field of a TMMBR Feedback message SHALL contain one or more
FCI entries.
4.2.1.1. Semantics
The TMMBR is used to indicate the highest bit-rate per sender of a
media, which the receiver currently supports in this RTP session.
The media sender MAY use any lower bit-rate, as it may need to
address a congestion situation or other limiting factors. See
section 5 (congestion control) for more discussion.
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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 SSRC of media sender in the FCI field denotes the media
sender the message applies to. This is useful in the multicast or
translator topologies where each media sender may be addressed in a
single TMMBR message using multiple FCIs.
A TMMBR FCI MAY be repeated in subsequent TMMBR messages if no
applicable TMMBN FCI has been received at the time of transmission of
the next RTCP packet. The bit-rate value of a TMMBR FCI MAY be
changed from a previous TMMBR message and the next, regardless of the
eventual reception of an applicable TMMBN FCI.
Please note that a TMMBN message SHALL be sent by the media sender at
the earliest possible point in time, as a result of any TMMBR
messages received since the last sending of TMMBN. The TMMBN message
indicates the limit and the owner of that limit at the time of the
transmission of the message. The limit is the lowest of the previous
value and all values received in TMMBR FCI's since the last TMMBN was
transmitted.
A media receiver who is not the owner of the bandwidth limit when
planning to send a TMMBR, SHOULD request a bandwidth lower than their
knowledge of currently established bandwidth limit for this media
sender, or suppres their transmission for TMMBR. The exception to
the above rule is when a receiver either doesn't know the limit or
are certain that their local representation of the value is in error.
All received requests for bandwidth limits greater or equal to the
one currently established are ignored, with the exception of them
resulting in the transmission of a TMMBN. A media receiver who is
the owner of the current bandwidth limit, MAY lower the value
further, raise the value or remove the restriction completely by
setting the bandwidth limit equal to the session limit.
Once a session participant receives the TMMBN in response to its
TMMBR, with its own SSRC, it knows that it "owns" the bandwidth
limitation. Only the "owner" of a bandwidth limitation can raise it
or reset it to the session limit.
Note that, due to the unreliable nature of transport of TMMBR and
TMMBN, the above rules may lead to the sending of TMMBR messages
disobeying the rules above. Furthermore, in multicast scenarios it
can happen that more than one session participants believes it "owns"
the current bandwidth limitation. This is not critical for a number
of reasons:
a) If a TMMBR message is lost in transmission, the media sender does
not learn about the restrictions imposed on it. However, it also
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does not send a TMMBN message notifying reception of a request it
has never received. Therefore, no new limit is established, the
media receiver sending the more restrictive TMMBR is not the
owner. Since this media receiver has not seen a notification
corresponding to its request, it is free to re-send it.
b) Similarly, if a TMMBN message gets lost, the media receiver that
has sent the corresponding TMMBR request does not receive
acknowledgement. In that case, it is also not the "owner" of the
restriction and is free to re-send the request.
c) If multiple competing TMMBR messages are sent by different session
participants, then the resulting TMMBN indicates the lowest
bandwidth requested; the owner is set to the sender of the TMMBR
with the lowest requested bandwidth value.
TMMBR feedback SHOULD NOT be used if the underlying transport
protocol is capable of providing similar feedback information from
the receiver to the sender.
It also important to consider the security risks involved with faked
TMMBRs. See security considerations in Section 6.
The feedback messages may be used in both multicast and unicast
sessions of any of the specified topologies.
For sessions with a larger number of participants using the lowest
common denominator, as required by this mechanism, may not be the
most suitable course of action. Larger session may need to consider
other ways to support adapted bit-rate to participants, such as
partitioning the session in different quality tiers, or use some
other method of achieving bit-rate scalability.
If the value set by a TMMBR message is expected to be permanent the
TMMBR setting party is RECOMMENDED to renegotiate the session
parameters to reflect that using the setup signalling.
An SSRC may time out according to the default rules for RTP session
participants, i.e. the media sender has not received any RTCP packet
from the owner for the last five regular reporting intervals. An SSRC
may also leave the session, indicating this through the transmission
of an RTCP BYE packet or an external signalling channel. In all of
these cases the entity is considered to have left the session. In the
case the "owner" leaves the session, the value SHALL be set to the
session maximum and the transmission of a TMMBN is scheduled.
4.2.1.2. Message Format
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum bit-rate in units of 128 bits/s |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 - Syntax for the TMMBR message
SSRC: The SSRC value of the target of this specific maximum bit-
rate request.
Maximum bit-rate: The temporary maximum media bit-rate value in
units of 128 bit/s. This provides range from 0 to
549755813888 bits/s (~550 Tbit/s) with a granularity of 128
bits/s.
The length of the FB message is be set to 2+2*N where N is the number
of TMMBR FCI entries.
4.2.1.3. Timing Rules
The first transmission of the 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.
4.2.2. Temporary Maximum Media Bit-rate Notification (TMMBN)
The FCI field of the TMMBN Feedback message SHALL contain one TMMBN
FCI entry.
4.2.2.1. Semantics
This feedback message is used to notify the senders of any TMMBR
message that one or more TMMBR messages have been received. It
indicates to all participants the currently employed maximum bit-rate
value and the "owner" of the current limitation. The "owner" of a
limitation is the sender of the last (most restrictive) TMMBR message
received by the media sender.
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The "SSRC of the packet sender" field indicates the source of the
notification. The "SSRC of media source" SHALL be set to the SSRC of
the media receiver that currently owns the bit-rate limitation.
A TMMBN message SHALL be scheduled for transmission after the
reception of a TMMBR message with a FCI including the session
participant's SSRC. Only a single TMMBN SHALL be sent, even if more
than one TMMBR messages are received between the scheduling of the
transmission and the actual transmission of the TMMBN message. The
TMMBN message indicates the limit and the owner of that limit at the
time of transmitting the message. The limit SHALL be the lowest of
the existing and all values received in TMMBR messages since the last
TMMBN was transmitted. The one sending that request SHALL become the
owner of the limit.
The reception of a TMMBR message with a transmission limit greater or
equal than the current limit SHALL still result in the transmission
of a TMMBN message. However the limit and owner is not changed,
unless it was from the same owner, and the current limit and owner is
indicated in the TMMBN message. This procedure allows session
participants that haven't seen the last TMMBN message to get a
correct view of this media sender's state.
When a media sender determines an "owner" of a limitation has left
the session, then the current limitation is removed, and the media
sender SHALL send a TMMBN message indicating the maximum session
bandwidth.
4.2.2.2. Message Format
The TMMBN Feedback control information (FCI) entry has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum bit-rate in units of 128 bits/s |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 - Syntax for the TMMBN message
Maximum bit-rate: The current temporary maximum media bit-rate
value in units of 128 bit/s.
The length field value of the FB message SHALL be 3.
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4.2.2.3. Timing Rules
The 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.
4.3. Payload Specific Feedback Messages
Payload-Specific FB messages are identified by the value PT=PSFB
(206) as RTCP packet type.
AVPF defines three payload-specific FB messages and one application
layer FB message. This memo specifies four additional payload
specific feedback messages. All are identified by means of the FMT
parameter as follows:
0: unassigned
1: Picture Loss Indication (PLI)
2: Slice Lost Indication (SLI)
3: Reference Picture Selection Indication (RPSI)
4: Full Intra Request Command (FIR)
5: Temporal-Spatial Trade-off Request (TSTR)
6: Temporal-Spatial Trade-off Announcement (TSTA)
7: Video Back Channel Message (VBCM)
8-14: unassigned
15: Application layer FB message
16-30: unassigned
31: reserved for future expansion of the number space
The following subsections define the new FCI formats for the payload-
specific FB messages.
4.3.1. Full Intra Request (FIR) command
The FIR command FB message is identified by PT=PSFB and FMT=4.
There MUST be one or more FIR entry contained in the FCI field.
4.3.1.1. Semantics
Upon reception of a FIR message, an encoder MUST send a decoder
refresh point (see Section 2.2) as soon as possible.
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Note: Currently, video appears to be the only useful application
for FIR, as it appears to be the only RTP payloads 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, 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 here.
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 others. However a
session which predominately handles pre-coded content shouldn't use
the 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, a Picture
Loss Indication informs the decoder about the loss of a picture and
hence the likeliness of misalignment of the reference pictures in
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 an encoder has is to send a decoder refresh point. However,
there are other options including ignoring the PLI, for example if
only one receiver of many has sent a PLI or when 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 disallows the encoder to take into account any considerations
such as the ones mentioned above.
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Note: Mandating a maximum delay for completing the sending of a
decoder refresh point would be desirable from an application
viewpoint, but may be 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 such
situations where not sending a decoder refresh point would render the
video unusable for the users.
Note: a typical example where sending FIR is adequate 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 includes
normally 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.
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 SSRC of media sender to which the FIR command applies to is
in the FCI.
4.3.1.2. Message Format
Full Intra Request uses one additional FCI field, the content of
which is depicted in Figure 3 The length of the FB 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 3 - Syntax for the FIR message
SSRC: The SSRC value of the media sender of this specific FIR
command.
Seq. nr: Command sequence number. The sequence number space is
unique for each tuple consisting 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. Initial value is arbitrary.
Reserved: All bits SHALL be set to 0 and SHALL be ignored on
reception.
The semantics of this FB message is independent of the RTP payload
type.
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 onee RTT before being sent. In
early or regular RTCP mode the repetition is sent in the next regular
RTCP packet.
4.3.1.4. Remarks
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 multitude of the typical frame duration. An example: If
the sending frame rate is 10 fps, and an intra picture is assumed to
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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
particular short delay in sending the FIR message. Hence waiting for
the next possible time slot allowed by RTCP timing rules as per
[RFC4585] may not have an overly negative impact on the system
performance.
4.3.2. Temporal-Spatial Trade-off Request (TSTR)
The TSTR FB message is identified by PT=PSFB and FMT=5.
There MUST be one or more TSTR entry contained in the FCI field.
4.3.2.1. Semantics
A decoder can suggest the use of a temporal-spatial trade-off 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 values from 0 to 31 indicate monotonically a desire
for higher frame rate. Actual 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 TSTA message.
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 SSRC of media sender to which the TSTR applies to is in the
FCI entries.
A TSTR message may contain multiple requests to different media
senders, using multiple FCI entries.
4.3.2.2. Message Format
The Temporal-Spatial Trade-off Request uses one FCI field, the
content of which is depicted in Figure 4. The length of the FB
message MUST be set to 2+2*N, where N is the number of FCI entries
included.
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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 | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 - Syntax of the TSTR
SSRC: The SSRC value of the target (or the media sender) of this
specific TSTR request.
Seq. nr: Request sequence number. The sequence number space is
unique for each tuple consisting 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. Initial value is arbitrary.
Index: 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.
Reserved: All bits SHALL be set to 0 and SHALL be ignored on
reception.
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. 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,
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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, frame
rate, and QP is intentionally left open here, as it depends on
factors such as compression standard employed, spatial resolution,
content, bit rate, and many more.
4.3.3. Temporal-Spatial Trade-off Announcement (TSTA)
The TSTA FB message is identified by PT=PSFB and FMT=6.
There SHALL be one or more TSTA contained in the FCI field.
4.3.3.1. Semantics
This feedback message is used to acknowledge the reception of a TSTR.
A TSTA entry in a TSTA 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 TSTA message MAY acknowledge multiple requests using
multiple FCI entries. The index value included SHALL be the same in
all FCI's part of the TSTA message. Including a FCI for each
requestor allows each requesting entity to determine that the media
sender targeted have received the request. The announcement SHALL be
sent also for 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 TSTA 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 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.
The "SSRC of the packet sender" field indicates the source of the
announcement, and the "SSRC of media source" is not used and SHALL be
set to 0. The SSRC of the requesting entity to which the announcement
applies to is in the FCI.
4.3.3.2. Message Format
The Temporal-Spatial Trade-off Announcement uses one additional FCI
field, the content of which is depicted in Figure 5. The length of
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the FB 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 5 - Syntax of the TSTA
SSRC: The SSRC of the source of the TSTA request that is
acknowledged.
Seq. nr: The sequence number value from the TSTA request that is
being acknowledged.
Index: The trade-off value the media sender is using henceforth.
Reserved: All bits SHALL be set to 0 and SHALL be ignored on
reception.
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.
4.3.3.3. 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. Remarks
None
4.3.4. H.271 VideoBackChannelMessage (VBCM)
The VBCM FB message is identified by PT=PSFB and FMT=7.
There MUST be one or more VBCM entry contained in the FCI field.
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4.3.4.1. Semantics
The "payload" of VBCM indication carries codec specific, different
types of feedback information. The type of feedback information can
be classified as "status report" such as receiving bit stream
without errors, loss of partial or complete picture or block or
"update requests" such as complete refresh of the bit stream.
Note: There are possible overlap between the VBCM sub-messages
and CCM/AVPF feedback messages, such FIR. Please see section
3.5.3 for further discussions.
The different types of feedback sub-messages carried in the VBCM are
indicated by the "payloadType" as defined in [VBCM]. The different
sub-message types as defined in [VBCM] are re-produced 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 Type Message Content
0 One or more pictures without detected bitstream 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 bitstream as if no prior bitstream data had
been received
> 5 Reserved for future use by ITU-T
Table 2: H.271 message types
The bit string or the "payload" of 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
Each of the different types of sub-messages (indicated by
payloadType) may have different semantic based on the codec used.
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
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to 0. The SSRC of the media sender to which the VBCM message applies
to is in the FCI.
4.3.4.2. Message Format
The VBCM indication uses one FCI field and the syntax is depicted in
Figure 6.
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 6 - Syntax for VBCM Message
SSRC: The SSRC value of the media sender that is target of the
message, i.e. the media sender whose encoder should to react
to the VBCM message
Seq. nr: Command sequence number. The sequence number space is unique
for each tuple consisting 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. Initial value is
arbitrary.
0: Must be set to 0 and should not be acted upon receiving.
Payload: The RTP payload type for which the VBCM bit stream must be
interpreted.
Length: The length of the VBCM octet string in octets exclusive any
padding octets
VBCM Octet String: This is the octet string generated by the decoder
carrying a specific feedback sub-message. It is of variable
length.
Padding: Bytes set to 0 to make up a 32 bit boundary.
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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
sub-message type with the most stringent requiremnts should be
followed.
Remarks
Please see section 3.5.3 for the applicability of the VBCM message
in relation to messages in both AVPF and this memo with similar
functionality.
Note: There has been some discussion whether the payload type field
in this message is needed. It would be needed if there were
potentially more than one VBCM-capable RTP payload types in the
same session, and that the semantics of a given VBCM message
changes from PT to PT. This appears to be the case. 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.
It was further commented 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.
5. Congestion Control
The correct application of the AVPF timing rules prevents the network
flooding by feedback messages. Hence, assuming a correct
implementation, 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. All
of these modifications MUST only be performed within the bandwidth
limits the applied congestion control provides. For example, when
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 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
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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 signalling 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 signalling 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.
b. The assignment of the ownership of a bit-rate limit with a
TMMBN message to the wrong participant. Thus potentially
freezing the mechanism until a correct TMMBN message reached
the participants.
c. Sending TSTR 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 need to apply authentication and
integrity protection of the feedback messages. This can be
accomplished against group external threats using the RTP profile
that combines SRTP [SRTP] and AVPF into SAVPF [SAVPF]. In the MCU
cases, separate security contexts and filtering can be applied
between the MCU and the participants thus protecting other MCU users
from a misbehaving participant.
7. SDP Definitions
Section 4 of [RFC4585] defines new SDP [RFC2327] attributes that are
used for the capability exchange of the AVPF commands and
indications, such as Reference Picture selection, Picture loss
indication etc. The defined SDP attribute is known as rtcp-fb and its
ABNF is described in section 4.2 of [RFC4585]. In this section we
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extend the rtcp-fb attribute to include the commands and indications
that are described in this document for codec control protocol. We
also discuss the Offer/Answer implications for the codec control
commands and indications.
7.1. Extension of rtcp-fb attribute
As described in [RFC4585], the rtcp-fb attribute is defined to
indicate the capability of using RTCP feedback. As defined in AVPF
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, multiple rtcp-fb attributes in a session description
hold for the new feedback messages for codec control defined in this
document.
The ABNF for rtcp-fb attributed as defined in [RFC4585] is
Rtcp-fb-syntax = "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 type called "ccm"
which indicates the support of codec control using RTCP feedback
messages. The "ccm" feedback value should be used with parameters,
which indicates the support of which codec commands the session may
use. In this draft we define four parameters, which can be used with
the ccm feedback value type.
o "fir" indicates the support of Full Intra Request
o "tmmbr" indicates the support of Temporal Maximum Media Bit-rate
o "tstr" indicates the support of temporal spatial trade-off
request.
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O "vbcm" indicates the support of H.271 video back channel
messages.
In ABNF for rtcp-fb-val defined in [RFC4585], there is a placeholder
called rtcp-fb-id to define new feedback types. The ccm is defined as
a new feedback type in this document and the ABNF for the parameters
for ccm are defined here (please refer section 4.2 of [RFC4585] for
complete ABNF syntax).
Rtcp-fb-param = SP "app" [SP byte-string]
/ SP rtcp-fb-ccm-param
/ ; empty
rtcp-fb-ccm-param = "ccm" SP ccm-param
ccm-param = "fir" ; Full Intra Request
/ "tmmbr" ; Temporary max media bit rate
/ "tstr" ; Temporal Spatial Trade Off
/ "vbcm" 1*[SP subMessageType] ; H.271 VBCM messages
/ token [SP byte-string]
; for future commands/indications
subMessageType = 1*[integer];
byte-string = <as defined in section 4.2 of [RFC4585] >
7.2. Offer-Answer
The Offer/Answer [RFC3264] implications to codec control protocol
feedback messages are similar to as 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.
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 codec control messages - fir, tstr. The SDP is carried in
a high level signalling protocol like SIP
v=0
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o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Point-to-Point call
c=IN IP4 172.11.1.124
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
In the above example the sender when it receives a TSTR message from
the remote party can adjust the trade off as indicated in the RTCP
TSTA feedback message.
Example 2: The following SDP describes a SIP end point joining a
video MCU 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. The
video MCU can send an FIR RTCP feedback message to this end point
when it needs to send this participants video to other participants
of the conference.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multiparty Video Call
c=IN IP4 172.11.1.124
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 mandated 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" messages. The offered SDP is
-------------> Offer
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer
c=IN IP4 172.11.1.124
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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
a=rtcp-fb:98 ccm tmmbr
The answerer only wishes to support FIR and TSTR message as the codec
control messages and the answerer SDP is
<---------------- Answer
v=0
o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
s=Offer/Answer
c=IN IP4 189.13.1.37
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 submessages of payloadType 2(
A set of blocks of one picture that is entirely or partially lost, 3
(CRC for one parameter set) and 4 (CRC for all parameter sets of a
certain type).
-------------> Offer
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Offer/Answer
c=IN IP4 172.11.1.124
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 2 3 4
The answerer only wishes to support sub-messages 3 and 4 only
<---------------- Answer
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INTERNET-DRAFT AVPF RTCP-RR Extensions October 20, 2006
v=0
o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
s=Offer/Answer
c=IN IP4 189.13.1.37
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 vbcm 3 4
So in the above example only VBCM indication comprising of only
"payloadType" 3 and 4 will be supported.
8. IANA Considerations
The new value of ccm for the rtcp-fb attribute needs to be registered
with IANA.
Value name: ccm
Long Name: Codec Control Commands and Indications
Reference: RFC XXXX
For use with "ccm" the following values also needs to be
registered.
Value name: fir
Long name: Full Intra Request Command
Usable with: ccm
Reference: RFC XXXX
Value name: tmmbr
Long name: Temporary Maximum Media 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
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9. Acknowledgements
The authors would like to thank Andrea Basso, Orit Levin, Nermeen
Ismail for their work on the requirement and discussion draft
[Basso].
Funding for the RFC Editor function is currently provided by the
Internet Society.
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10. References
10.1. 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.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[Topologies] M. Westerlund, and S. Wenger, "RTP Topologies", draft-
ietf-avt-topologies-00, work in progress, August 2006
10.2. 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.
[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.
[SAVPF] J. Ott, E. Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)," draft-ietf-avt-profile-
savpf-02.txt, July, 2005.
[RFC3525] Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
"Gateway Control Protocol Version 1", RFC 3525, June
2003.
[VBCM] ITU-T Rec. H.271, "Video Back Channel Messages", June
2006
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11. Authors' Addresses
Stephan Wenger
Nokia Corporation
P.O. Box 100
FIN-33721 Tampere
FINLAND
Phone: +358-50-486-0637
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
12. List of Changes relative to previous drafts
The following changes since draft-wenger-avt-avpf-ccm-01 have been
made:
- The topologies have been rewritten and clarified.
- The TMMBR mechanism has been completely revised to use notification
and suppress messages in deployments with large common SSRC spaces.
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The following changes since draft-wenger-avt-avpf-ccm-02 have been
made:
- Update of section 4.2.2.1 (TMMBN) as per discussions between
Harikishan Desineni and Magnus Westerlund on the AVT list around
Feb 21, 2006
- Section 2.3.4 clarified as per email exchange between Colin Perkins
and Magnus Westerlund around Feb 24
- Section 3.5.2 and other occurrences throughout the draft,
Temporal/Spatial Acknowledgement renamed to Temporal/Spatial
Annoucement
Changes relative to draft-wenger-avt-avpf-ccm-03
- Moved "topologies" out to another draft
- Editorial improvements
- Added new code point VBCM for H.271 Video back channel messages.
Sections 3,4 and 7 were modified in response to H.271 introduction.
- Removed Basso use case referring to forward Freeze command, added
justification.
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Wenger, et al. Standards Track [Page 51]
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