Network Working Group Stephan Wenger
INTERNET-DRAFT Umesh Chandra
Expires: May 2007 Nokia
Magnus Westerlund
Bo Burman
Ericsson
March 5, 2007
Codec Control Messages in the
RTP Audio-Visual Profile with Feedback (AVPF)
draft-ietf-avt-avpf-ccm-04.txt>
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies a few extensions to the messages defined in
the Audio-Visual Profile with Feedback (AVPF). They are helpful
primarily in conversational multimedia scenarios where centralized
multipoint functionalities are in use. However some are also usable
in smaller multicast environments and point-to-point calls. The
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extensions discussed are messages related to the ITU-T H.271 Video
Back Channel, Full Intra Request, Temporary Maximum Media Stream Bit-
rate and Temporal Spatial Trade-off.
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TABLE OF CONTENTS
1. Introduction....................................................5
2. Definitions.....................................................7
2.1. Glossary...................................................7
2.2. Terminology................................................8
2.3. Topologies.................................................9
3. Motivation (Informative).......................................10
3.1. Use Cases.................................................10
3.2. Using the Media Path......................................12
3.3. Using AVPF................................................13
3.3.1. Reliability..........................................13
3.4. Multicast.................................................13
3.5. Feedback Messages.........................................13
3.5.1. Full Intra Request Command...........................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..................................16
3.5.2.2. Point-to-Multipoint using Multicast or Translators16
3.5.2.3. Point-to-Multipoint using RTP Mixer.............17
3.5.2.4. Reliability.....................................17
3.5.3. H.271 Video Back Channel Message conforming to ITU-T Rec.
H.271.......................................................17
3.5.3.1. Reliability.....................................20
3.5.4. Temporary Maximum Media Bit-rate Request.............20
3.5.4.1. MCU based Multi-point operation.................25
3.5.4.2. Point-to-Multipoint using Multicast or Translators27
3.5.4.3. Point-to-point operation........................27
3.5.4.4. Reliability.....................................28
4. RTCP Receiver Report Extensions................................29
4.1. Design Principles of the Extension Mechanism..............29
4.2. Transport Layer Feedback Messages.........................30
4.2.1. Temporary Maximum Media Bit-rate Request (TMMBR).....30
4.2.1.1. Semantics.......................................31
4.2.1.2. Message Format..................................33
4.2.1.3. Timing Rules....................................34
4.2.2. Temporary Maximum Media Bit-rate Notification (TMMBN) 35
4.2.2.1. Semantics.......................................35
4.2.2.2. Message Format..................................36
4.2.2.3. Timing Rules....................................36
4.3. Payload Specific Feedback Messages........................37
4.3.1. Full Intra Request (FIR) command.....................37
4.3.1.1. Semantics.......................................37
4.3.1.2. Message Format..................................39
4.3.1.3. Timing Rules....................................40
4.3.1.4. Remarks.........................................40
4.3.2. Temporal-Spatial Trade-off Request (TSTR)............41
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4.3.2.1. Semantics.......................................41
4.3.2.2. Message Format..................................41
4.3.2.3. Timing Rules....................................42
4.3.2.4. Remarks.........................................42
4.3.3. Temporal-Spatial Trade-off Announcement (TSTA).......43
4.3.3.1. Semantics.......................................43
4.3.3.2. Message Format..................................44
4.3.3.3. Timing Rules....................................44
4.3.3.4. Remarks.........................................45
4.3.4. H.271 VideoBackChannelMessage (VBCM).................45
5. Congestion Control.............................................48
6. Security Considerations........................................48
7. SDP Definitions................................................49
7.1. Extension of rtcp-fb attribute............................49
7.2. Offer-Answer..............................................51
7.3. Examples..................................................51
8. IANA Considerations............................................54
9. Acknowledgements...............................................54
10. References....................................................56
10.1. Normative references.....................................56
10.2. Informative references...................................56
11. Authors' Addresses............................................57
12. List of Changes relative to previous draftsError! Bookmark not defined.
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1.
Introduction
When the Audio-Visual Profile with Feedback (AVPF) [RFC4585] was
developed, the main emphasis lay in the efficient support of point-
to-point and small multipoint scenarios without centralized
multipoint control. However, in practice, many small multipoint
conferences operate utilizing devices known as Multipoint Control
Units (MCUs). Long standing experience of the conversational video
conferencing industry suggests that there is a need for a few
additional feedback messages, to efficiently support centralized
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 signaling support. During the development of
this memo, it was noticed that there is considerable confusion in the
community related to the use of terms such as mixer, translator, and
MCU. In response to these concerns, a number of topologies have been
identified that are of practical relevance to the industry, but 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 notification to the message emitter
indicating their reception and/or the message receiver's actions.
Other messages require notification, 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 [RFC3261],
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) 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 refresh
point is an IDR or Intra picture, depending on the video compression
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technology in use. Other codecs may have other forms of decoder
refresh points. In order to fulfill 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 a reception
notification, 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 also prove helpful in conjunction with other media codecs that
support prediction across RTP packets.
The Temporary Maximum Media Stream Bitrate Request (TMMBR) allows to
signal, from media receiver to media sender, the current maximum
media stream bit-rate for a given media stream. The maximum media
stream bit-rate is defined as a tuple. The first value is the bit-
rate available for the packet stream at the layer reported on. The
second value is the measured header sizes between the start of the
header for the layer reported on and the beginning of the RTP
payload. Once, the media sender has received the TMMBR request on
the bitrate limitation, it notifies the initiator of the request, and
all other session participants, by sending a Temporal Maximum Media
Stream Bitrate Notification (TMMBN). The TMMBN contains a list of
the current applicable restrictions to help the participants to
suppress TMMBR requests that wouldn't result in further restrictions
for the sender. One usage scenario can be seen as limiting media
senders in multiparty conferencing to the slowest receiver's Maximum
Media Stream bitrate 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 considers it helpful to inform media
sender(s) about its constraints, without waiting for congestion
induced bitrate reduction. Another application involves graceful
bitrate adaptation in scenarios where the upper limit connection
bitrate to a receiver changes, but is known in the interval between
these dynamic changes. The TMMBR/TMMBN messages are useful for all
media types that are not inherently of constant bit rate. However,
TMMBR is not a congestion control mechanism and can't replace the
need to implement one.
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
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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 has the authority 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.
There exists some overlap between VBCM 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 preferred 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 Back Channel Messages according to H.271 do not require a
notification 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) enables a
video receiver to signal to the video sender its preference for
spatial quality or high temporal resolution (frame rate). Typically,
the receiver of the video stream generates this signal based on input
from its user interface, in reaction 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 a
notification message indicating the newly chosen tradeoff, so to
allow immediate user feedback.
2.
Definitions
2.1.
Glossary
AMID - Additive Increase Multiplicative Decrease
ASM - Asynchronous Multicast
AVPF - The Extended RTP Profile for RTCP-based Feedback
FEC - Forward Error Correction
FIR - Full Intra Request
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MCU - Multipoint Control Unit
MPEG - Moving Picture Experts Group
PtM - Point to Multipoint
PtP - Point to Point
TMMBN - Temporary Maximum Media Stream Bitrate Notification
TMMBR - Temporary Maximum Media Stream Bitrate Request
PLI - Picture Loss Indication
TSTN - Temporal Spatial Trade-off Notification
TSTR - Temporal Spatial Trade-off Request
VBCM - Video Back Channel Message indication.
2.2.
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Message:
Codepoint defined by this specification, of one of the
following types:
Request:
Message that requires Acknowledgement
Command:
Message that forces the receiver to an action
Indication:
Message that reports a situation
Notification:
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, as discussed below.
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
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be used; see for example [AVC]. While both "hard" and
"gradual" decoder refresh points are acceptable in the scope
of this specification, in most cases the user experience will
benefit from using a "hard" decoder refresh point.
A decoder refresh point also contains all header information
above the picture layer (or equivalent, depending on the
video compression standard) that is conveyed in-band. In
H.264, for example, a decoder refresh point contains
parameter set NAL units that generate parameter sets
necessary for the decoding of the following slice/data
partition NAL units (and that are not conveyed out of band).
Decoding:
The operation of reconstructing the media stream.
Rendering:
The operation of presenting (parts of) the reconstructed
media stream to the user.
Stream thinning:
The operation of removing some of the packets from a media
stream. Stream thinning, preferably, is media-aware,
implying that media packets are removed in the order of 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. In contrast to transcoding, stream
thinning is typically seen as a computationally lightweight
operation
Media: Often used (sometimes in conjunction with terms like
bitrate, stream, sender, ...) to identify the content of the
forward RTP packet stream carrying the codec data to which
the codec control message applies to.
Media Stream: The stream of packets carrying the media (and in some
case also repair information such as retransmission or
Forward Error Correction (FEC) information). We further
include within this specification the RTP packetization and
the usage of additional protocol headers on these packets to
carry them from sender to receiver.
2.3.
Topologies
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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.
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
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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 user may prefer the opposite. This
choice is also highly content dependent. Many current video
conferencing systems offer in the user interface a mechanism to
make this selection, usually in the form of a slider. The
mechanism is helpful in point-to-point, centralized multipoint and
non-centralized multipoint uses.
4. Use case 4 of the Basso 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
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 Stream
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 [RFC3448])
probe for more available capacity 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
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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 quick convergence is
necessary, and therefore media path signaling is desirable.
8. The use of reference picture selection (RPS) as an error
resilience tool has been introduced in 1997 as NEWPRED [NEWPRED],
and is now widely deployed. When RPS is in use, simplisticly put,
the receiver can send a feedback message to the sender, indicating
a reference picture that should be used for future prediction.
([NEWPRED] mentions other forms of feedback as well.) AVPF
contains a mechanism for conveying such a message, but did not
specify for which codec and according to which syntax the message
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 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 signaling part of the MCU, having
them on the media path avoids interfaces and unnecessary control
traffic between signaling 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 signaling path quite commonly contains several
signaling entities, e.g. SIP-proxies and application servers.
Avoiding going through signaling entities avoids delay for several
reasons. Proxies have less stringent delay requirements than media
processing and due to their complex and more generic nature may
result in significant processing delay. The topological locations of
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the signaling entities are also commonly not optimized for minimal
delay, but rather towards other architectural goals. Thus the
signaling path can be significantly longer in both geographical and
delay sense.
3.3.
Using AVPF
The AVPF feedback message framework [RFC4585] provides 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 by RTCP traffic. We re-use these rules by
referencing AVPF.
The signaling setup for AVPF allows each individual type of function
to be configured or negotiated on 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.
The different messages proposed in this specification have different
requirements in terms of reliability. However, in all cases, the
reaction to an (occasional) loss of a feedback message is specified.
3.4.
Multicast
The codec control messages might be used with multicast. The RTCP
timing rules specified in [RFC3550] and [RFC4585] ensure that the
messages do not cause overload of the RTCP connection. The use of
multicast may result in the reception of messages with inconsistent
semantics. The reaction to inconsistencies depends on the message
type, and is discussed for each message type separately.
3.5.
Feedback Messages
This section describes the semantics of the different feedback
messages and how they apply to the different use cases.
3.5.1.
Full Intra Request Command
A Full Intra Request (FIR) Command, when received by the designated
media sender, requires that the media sender sends a Decoder Refresh
Point (see 2
.2) at the earliest opportunity. The evaluation of such
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opportunity includes the current encoder coding strategy and the
current available network resources.
FIR is also known as an ''instantaneous decoder refresh request''
or ''video fast update request''.
Using a decoder refresh point implies refraining from using any
picture sent prior to that point as a reference for the encoding
process of any subsequent picture sent in the stream. For predictive
media types that are not video, the analogue applies. For example,
if in MPEG-4 systems scene updates are used, the decoder refresh
point consists of the full representation of the scene and is not
delta-coded relative to previous updates.
Decoder Refresh Points, especially Intra or IDR pictures, are in
general several times larger in size than predicted pictures. Thus,
in scenarios in which the available bit-rate is small, the use of a
Decoder Refresh Point implies a delay that is significantly longer
than the typical picture duration.
Usage in multicast is possible; however aggregation of the commands
is recommended. A receiver that receives a request closely (within 2
times the longest Round Trip Time (RTT) known) 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 participant's request was in-flight to them. Suppressing
those requests that may have been sent without knowledge about the
other request avoids this issue.
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 notification, and a simple command repetition
mechanism is sufficient for ensuring the level of reliability
required. However, the potential use of repetition does require a
mechanism to prevent the recipient from responding to messages
already received and responded to.
To ensure the best possible reliability, a sender of FIR may repeat
the FIR request until a response has been received. The repetition
interval is determined by the RTCP timing rules applicable to the
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session. Upon reception of a complete Decoder Refresh Point or the
detection of an attempt to send a Decoder Refresh Point (which got
damaged due to a packet loss), the repetition of the FIR must stop.
If another FIR is necessary, the request sequence number must be
increased. 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 for the mixer to
generate FIR commands. The two legs (FIR-requesting endpoint to
mixer, and mixer to Decoder Refresh Point generating endpoint) are
handled independently from each other from a reliability perspective.
3.5.2.
Temporal Spatial Trade-off Request and Notification
The Temporal Spatial Trade-off Request (TSTR) instructs the video
encoder to change its trade-off between temporal and spatial
resolution. Index values from 0 to 31 indicate monotonically a
desire for higher frame rate. That is, a requester asking for an
index of 0 prefers a high quality and is willing to accept a low
frame rate, whereas a requester asking for 31 wishes a high frame
rate, potentially at the cost of low spatial quality.
In general the encoder reaction time may be significantly longer than
the typical picture duration. See use case 3 for an example. The
encoder decides if the request results in a change of the trade off.
The Temporal Spatial Trade-Off Notification message (TSTN) has been
defined to provide feedback of the trade-off that is used henceforth.
Informative note: TSTR and TSTN have been introduced primarily
because it is believed that control protocol mechanisms, e.g. a SIP
re-invite, are too heavyweight, and too slow to allow for a
reasonable user experience. Consider, for example, a user
interface where the remote user selects the temporal/spatial trade-
off with a slider (as it is common in state-of-the-art video
conferencing systems). An immediate feedback to any slider
movement is required for a reasonable user experience. A SIP re-
invite [RFC3261] would require at least 2 round-trips more
(compared to the TSTR/TSTN mechanism) and may involve proxies and
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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 TSTN 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
translator, mixer or endpoint's reaction to the reception of a
suggested trade-off as conveyed in the TSTR -- we only require the
receiver of a TSTR message to reply to it by sending a TSTN, carrying
the new trade-off chosen by its own criteria (which may or may not be
based on the trade-off conveyed by TSTR). In other words, the trade-
off sent in TSTR is a non-binding recommendation; nothing more.
With respect to TSTR/TSTN, 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
requested value in TSTR, and within its capabilities. The TSTN
message conveys back the new trade-off value (which may be identical
to the old one if, for example, the sender is not capable of
adjusting its trade-off).
3.5.2.2.
Point-to-Multipoint using Multicast or Translators
RTCP Multicast is used either with media multicast according to Topo-
Multicast, or following RFC 3550's translator model according to
Topo-Translator. In these cases, 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 TSTN, and the value conveyed back has to
reflect the decision made.
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3.5.2.3.
Point-to-Multipoint using RTP Mixer
In this scenario (Topo-Mixer) the RTP Mixer receives all TSTR
messages, and has the opportunity to act on them based on its own
criteria. In most cases, the Mixer should form a ''consensus'' of
potentially conflicting TSTR messages arriving from different
participants, and initiate its own TSTR message(s) to the media
sender(s). The strategy of forming this ''consensus'' is open for
the implementation, and can, for example, encompass averaging the
participants 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 Mixer or Translator 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
A request and reception acknowledgement mechanism is specified. The
Temporal Spatial Trade-off Notification (TSTN) message informs the
request-sender that its request has been received, and what trade-off
is used henceforth. This acknowledgment mechanism is desirable for at
least the following reasons:
o A change in the trade-off cannot be directly identified from the
media bit stream,
o User feedback cannot be implemented without 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. In this memo, we refrain from an in-depth
discussion of the available codepoints within H.271 and refer to the
specification text instead [H.271].
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However, we note that some H.271 messages bear similarities with
native messages of AVPF and this memo. Furthermore, we note that
some H.271 message are known to require caution in multicast
environments -- or are plainly not usable in multicast or multipoint
scenarios. Table 1 provides a brief, oversimplifying overview of the
messages currently defined in H.271, their similar AVPF or CCM
messages (the latter as specified in this memo), and an indication of
our current knowledge of their multicast safety.
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.
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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 documented below, the implementation of such a
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 to be taken into account when considering the use of H.271 in
multipoint environments:
1. Different state on different receivers. In many environments it
cannot be guarantied that the decoder state of all media receivers
is identical at any given point in time. The most obvious reason
for such a possible misalignment of state is a loss that occurs on
the link to only one of many media receivers. However, there are
other not so obvious reasons, such as recent joins to the
multipoint conference (be it by joining the multicast group or
through additional mixer output). Different states can lead the
media receivers to issue potentially contradicting H.271 messages
(or one media receiver issuing an H.271 message that, when
observed by the media sender, is not helpful for the other media
receivers). A naive reaction of the media sender to these
contradicting messages can lead to unpredictable and annoying
results.
2. Combining messages from different media receivers in a media
sender is a non-trivial task. As reasons, we note that these
messages may be contradicting each other, and that their transport
is unreliable (there may well be other reasons). In case of many
H.271 messages (i.e. types 0, 2, 3, and 4), the algorithm for
combining must be both aware of the network/protocol environment
(i.e. with respect to congestion) and of the media codec employed,
as H.271 messages of a given type can have different semantics for
different media codecs.
3. The suppression of requests may need to go beyond the basic
mechanism described in AVPF (which are driven exclusively by
timing and transport considerations on the protocol level). For
example, a receiver is often required to refrain from (or delay)
generating requests, based on information it receives from the
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media stream. For instance, it makes no sense for a receiver to
issue a FIR when a transmission of an Intra/IDR picture is
ongoing.
4. When using the non-multicast-safe messages (e.g. H.271 type 0
positive ACK of received pictures/slices) in larger multicast
groups, the media receiver will likely be forced to delay or even
omit sending these messages. For the media sender this looks like
data has not been properly received (although it was received
properly), and a naively implemented media sender reacts to these
perceived problems where it shouldn't.
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 Stream Bit-rate Request and Notification
A receiver, translator or mixer uses the Temporary Maximum Media
Stream Bit-rate Request (TMMBR, "timber") to request a sender to
limit the maximum bit-rate for a media stream to, or below, the
provided value. The Temporary Maximum Media Stream Bit-rate
Notification (TMMBN) advises the media receiver(s) of the changed
bitrate it is not going to exceed henceforth. The primary usage for
this is a scenario with a MCU or Mixer (use case 6), corresponding to
Topo-Translator or Topo-Mixer, but also Topo-Point-to-Point.
The temporary limitation on the media stream is expressed as a tuple;
one value limiting the bit-rate at the layer for which the overhead
is calculated to. A second value provides the per packet header
overhead between the layer for which bit-rate is reported and the
start of the RTP payload. By having both values the media stream
sender can determine the effect of changing the packet rate for the
media stream in an environment which contains translators or mixers
that affect the amount of per packet overhead. For example a gateway
that convert between IPv4 and IPv6 would affect the per packet
overhead commonly with 20 bytes. There exist also other mechanisms,
like tunnels, that change the amount of headers that are present at a
particular bottleneck for which the TMMBR sending entity has
knowledge about. The problem with varying overhead is also discussed
in [RFC3890].
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The above way of measuring allows for one to provide bit-rate and
overhead values for different protocol layers, for example on IP
level, out part of a tunnel protocol, or the link layer. The level a
peer report on, is fully dependent on the level of integration the
peer has, as it needs to be able to extract the information from that
level. It is expected that peers will be able to report values at
least for the IP layer, but in certain implementations link layer may
be available to allow for more precise information.
The temporary maximum media stream bit-rate messages are generic
messages that can be applied to any RTP packet stream. This
separates it a bit from the other codec control messages defined in
this specification that applies only to specific media types or
payload formats. The TMMBR functionality applies to the transport and
the requirements it places on the media encoding.
The reasoning below assumes that the participants have negotiated a
session maximum bit-rate, using a signaling protocol. This value can
be global, for example in case of point-to-point, multicast, or
translators. It may also be local between the participant and the
peer or mixer. In both cases, the bit-rate negotiated in signaling 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.
It is also beneficial to have negotiated a maximum packet rate for
the session or sender. RFC 3890 provides such a SDP [RFC4566]
attribute, however that is not usable in RTP sessions established
using offer/answer [RFC3264]. Therefore a max packet rate signaling
parameter is specified.
An already established temporary limit may be changed at any time
(subject to the timing rules of the feedback message sending), and to
any values between zero and the session maximum, as negotiated during
session establishment signaling. Even if a sender has received a
TMMBR message allowing an increase in the bit-rate, all increases
must be governed by a congestion control mechanism. TMMBR only
indicates known limitations, usually in the local environment, and
does not provide any guarantees about the full path.
If it is likely that the new value 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 signaling
protocol.
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3.5.4.1.
Behavior for media receivers using TMMBR
In multipart scenarios, different receivers likely have different
limits for receiving bitrate. Therefore, an algorithm to identify
the most restrictive TMMBR requests is specified in section 4
..2.2.1.
The general behavior is explaind in this section and the gist of the
algorithm to determine the most restrictive values are explained
informally in the next section.
Immediately after session setup, the bitrate limit is set to the
session limit as established by the session setup signaling (or
equivalent). The overhead value is set to 0. When the session setup
signaling does not specify a limit, then unlimited bitrate is
assumed. Note that many codecs specify their own limits, e.g.
through H.264's level concept.
At any given time, a media receiver can send a TMMBR with a limit
that is lower than the current limit. The media receiver use the
algorithm outlined in the below Section 3.5.4.2 to determine if its
limit is stricter than already existing ones. The media sender upon
receiving the TMMBR request will also excersie the algorithm to
determine the set of most restrictive limitations and then send a
TMMBN containg that set. Once the media sender has sent the TMMBN
message, the receivers indicated in that message becomes ''owners''
of the limitations. Most likely, the owner is the original sender of
the TMMBR -- for the handling of corner-cases (i.e. concurrent TMMBRs
from different receivers, lost TMMBRs and sender side optimisations)
please see the formal specification. ''Owners'' and limits are
usually known session wide, as both TMMBR and TMMBN are forwarded to
all in the session unless a Mixer or Translator separate the session
from RTCP handling point of view.
Only a ''owner'' is allowed to raise the bitrate limit to a value
higher than the session has been notified of, but not higher than the
session limit negotiated by the session setup signaling (see above).
A ''owner'' does not need to take into account TMMBR messages sent by
anyone else (although that may well be a desirable optimization). If
a ''owner'' sets a new session limit that is too high for someone
else's liking, other media receivers can react to the situation by
emmitting their own TMMBR message (and, in the process, become a
''owner''). Limitations belonging to ''owners'' timing out from the
session are removed by the media sender who notifies the session
about the event by sending a TMMBN.
Obviously, when there is only one media receiver, this receiver
becomes ''owner'' once it receives the first TMMBN in response to its
own TMMBR, and stays ''owner'' for the rest of the session.
Therefore, when it is known that there will always be only a single
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media receiver, the above algorithm is not required. Media receivers
that are aware they are the only ones in a session can send TMMBR
messages with bitrate limits both higher and lower than the
previously notified limit at any time (subject to AVPF's RTCP RR send
timing rules). However, it may be difficult for a session
participant to determine if it is the only receiver in the session.
Due to that any one implementing TMMBR are required to implement this
algorithm.
3.5.4.2.
Algorithm for exstablishing current limitations
First it is important to consider the implications of using a tuple
for limiting the media sender's behavior. The bit-rate and the
overhead value results in a 2-dimensional solution space for possible
media streams. Fortunately the two variables are linked. The bit-rate
available for RTP payloads will be equal to the TMMBR reported bit-
rate minus the packet rate used times the TMMBR reported overhead.
This has the result in a session with two different participants
having set limitations, the used packet rate will determine which of
the two that applies.
Example:
Receiver A: TMMBR_BR = 35 kbps, TMMBR_OH = 40
Receiver B: TMMBR_BR = 40 kbps, TMMBR_OH = 60
For a given packet rate (PR) the bit-rate available for media
payloads in RTP will be:
Max_media_BR_A = TMMBR_BR_A - PR * TMMBR_OH_A * 8
Max_media_BR_B = TMMBR_BR_B - PR * TMMBR_OH_B * 8
For a PR = 20 these calculations will yield a Max_media_BR_A = 28600
bps and Max_media_BR_B = 30400 bps, which shows that receiver A is
the limiting one for this packet rate. However there will be a PR
when the difference in bit-rate restriction will be equal to the
difference in packet overheads. This can be found by setting
Max_media_BR_A equal to Max_media_BR_B and breaking out PR:
TMMBR_BR_A - TMMBR_BR_B
PR = ---------------------------
8*(TMMBR_OH_A - TMMBR_OH_B)
Which, for the numbers above yields 31.25 as the intersection point
between the two limits. The implications of this have to be
considered by application implementors that are going to control
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media encoding and its packetization. Because, as exemplified above,
there might be multiple TMMBR limits that applies to the trade-off
between media bit-rate and packet rate. Which limitation that applies
depends on the packet rate considered to be used.
This also has implications for how the TMMBR mechanism needs to work.
First, there is the possibility that multiple TMMBR tuples are
providing limitations on the media sender. Secondly there is a need
for any session participant (meda sender and receivers) to be able to
determine if a given tuple will become a limitation upon the media
sender, or if the set of already given limitations are stricter than
the given values. Otherwise the suppression of TMMBR requests would
not work.
Thus any session participant needs to be able from a given set X of
tuples determine which is the minimal set need to express the
limitations for all packet rates from 0 to highest possible. Where
the highest possible either is application limited and indicated
trough session setup signaling or as a result of the given
limitations when the available bit-rate is fully consumed by headers.
First determine what the highest possible bit-rate given all the
limitations is. If there is provided a session maximum packet rate
(SMAXPR) then this can be used. In addition one needs to calculate
for each tuple in the set what its maximum is by calculating bit-rate
(BR) divided by overhead (OH) per packet converted to bits.
MaxPR = SMAXPR
For i=1 to size(X) {
tmp_pr = X(i).BR / 8*X(i).OH;
If (tmp_pr < MaxPR) then MaxPR = tmp_pr
}
For a zero packet rate the TMMBR signaled bit-rate will be the only
limiting factor, thus the tuple with the smallest available bit-rate
is a limitation at this point of the range and function as a start
value in the algorithm.
Start by finding the element X(l) in X with the lowest bit-rate value
and the highest overhead if there are multiple on the same bit-rate.
The set Y that is the minimal set of tuples that provide restrictions
initially contain only X(l). Then for each other tuple X(i) calculate
if there exist an intersection between the currently selected tuple
X(s) (initially s=l) and which of the tuples within the set that has
this intersection at the lowest packet rate. Having found the lowest
packet rate, compare it with the sessions maximum packet rate. If
lower than that limit this tuple provide a session limit and the
tuple is added to Y. Update the value of s to the found tuple and
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repeat search for the tuple that has the intersection at the lowest
packet rate, but still higher than the previous intersection.
Algorithm has finished when it can't find any new tuple with an
intersection at a packet rate lower than the session maximum.
// Find the element with the lowest bit-rate in X
l=0;
for (i=1:size(X)){
if (X(i).BR <= X(l).BR) & (X(i).OH > X(l).OH) then
l=I;
}
tuple_index = l; // The lowest bit-rate tuple
Y = X(l); // Initilize Y to X(l)
start_pr = 0; // Start from zero bit-rate
do {
current_low = MaxPr; //Reset packet-rate
current_index = tuple_index; // To allow for no intersection
For i=each element in X
pr = (X(i).BR - X(tuple_index).BR) /
(X(i).OH - X(tuple_index).OH)
// Calculate packet rate compared to element i
If (pr < current_low && pr > start_pr) then {
// Update lowest intersection packet rate
current_low = pr;
current_index = i;
}
}
If (current_index != tuple_index) {
// A tuple intersecting below maxpacket rate
Y(size(Y)+1) = X(current_index) // Add to Y
tuple_index = current_index; // Update which to compare with
start_pr = current_low; // Update packet rate to seek from.
}
} while (current_low < MaxPr)
The above algorithm yields the set of applicable restriction Y.
3.5.4.3.
Use of TMMBR in a Mixer based Multi-point operation
Assume a small mixer-based multiparty conference is ongoing, as
depicted in Topo-Mixer of [Topologies]. All participants have
negotiated a common maximum bit-rate that this session can use. The
conference operates over a number of unicast paths between the
participants and the mixer. The congestion situation on each of
these paths can be monitored by the participant in question and by
the mixer, utilizing, for example, RTCP Receiver Reports or the
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transport protocol, e.g. DCCP [RFC4340]. However, any given
participant has no knowledge of the congestion situation of the
connections to the other participants. Worse, without mechanisms
similar to the ones discussed in this draft, the mixer (who is aware
of the congestion situation on all connections it manages) has no
standardized means to inform media senders to slow down, short of
forging its own receiver reports (which is undesirable). In
principle, a mixer confronted with such a situation is obliged to
thin or transcode streams intended for connections that detected
congestion.
In practice, media-aware stream thinning is unfortunately a very
difficult and cumbersome operation and adds undesirable delay. If
media-unaware, it leads very quickly to unacceptable reproduced media
quality. Hence, means to slow down senders even in the absence of
congestion on their connections to the mixer are desirable.
To allow the mixer to perform congestion control on the individual
links, without performing transcoding, there is a need for a
mechanism that enables the mixer to request the participant's media
encoders to limit their Maximum Media Stream bit-rate currently used.
The mixer 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 mixer, to a value that is in
compliance with congestion control principles for the slowest
link. Slow refers here to the available
bandwidth/bitrate/capacity and packet rate after congestion
control.
3. As soon as the bit-rate has been reduced by the sending part, the
mixer stops stream thinning implicitly, because there is no need
for it 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
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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 bitrate-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 to be
explicitly informed about desired bitrate 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.
3.5.4.4.
Use of TMMBR in Point-to-Multipoint using Multicast or
Translators
In these topologies, corresponding to Topo-Multicast or Topo-
Translator RTCP RRs are transmitted globally 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
.5.4.3 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.
3.5.4.5.
Use of TMMBR in 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 signaling 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. Thus TMMBR is
useful for putting restrictions on the application and thus placing
the congestion control mechanism in the right ballpark. However TMMBR
is usually unable to have continuously quick feedback loop required
for real congestion control. Its semantics is also not a match for
congestion control due to its different purpose. Because of these
reasons TMMBR SHALL NOT be used for congestion control.
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3.5.4.6.
Reliability
The reaction of a media sender to the reception of a TMMBR message is
not immediately identifiable through inspection of the media stream.
Therefore, a more explicit mechanism is needed to avoid unnecessary
re-sending of TMMBR messages. Using a statistically based
retransmission scheme would only provide statistical guarantees of
the request being received. It would also not avoid the
retransmission of already received messages. In addition, it does not
allow for easy suppression of other participants requests. For the
reasons mentioned, a mechanism based on explicit notification is
used, as discussed already in section 3.5.4.1.
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. In multicast
scenarios, that allows all other participants to suppress any request
they may have, with limitation values less strict than the current
ones. The identity of the owners allows for small message sizes and
media sender states. A media sender only keeps state for the SSRCs of
the current owners of the limitations; all other requests and their
sources are not saved. Only the owners are allowed to remove or
change 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 Notification (TSTN), and Video Back Channel Message (VBCM)
are "Payload Specific Feedback Messages" as defined in Section 6.3 of
AVPF [RFC4585]. The Temporary Maximum Media Stream Bit-rate Request
(TMMBR) and Temporary Maximum Media Stream Bit-rate Notification
(TMMBN) are "Transport Layer Feedback Messages" as defined in Section
6.2 of AVPF.
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 some 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 a given RTP packet 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 implement their
codepoints in a simple, fixed binary format. The reason behind this
design principle lies in that media receivers do not always implement
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 (see section 6.1 of RFC 4585 [RFC4585].
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: reserved (see note below)
3: Temporary Maximum Media Stream Bit-rate Request (TMMBR)
4: Temporary Maximum Media Stream Bit-rate Notification (TMMBN)
5-30: unassigned
31: reserved for future expansion of the identifier number space
Note: early drafts of AVPF [RFC4585] reserved FMT=2 for a
codepoint that has later been removed. It has been pointed
out that there may be implementations in the field using this
value for according to the expired draft. As there is
sufficient numbering space available, we mark FMT=2 as
reserved so to avoid possible interoperability problems with
implementations that are standard-incompliant with respect to
RFC 4585 in this very point.
The following subsection defines the formats of the FCI field for
this type of FB message.
4.2.1.
Temporary Maximum Media Stream Bit-rate Request and Notification
The FCI field of a Temporary Maximum Media Stream Bit-Rate Request
(TMMBR) message SHALL contain one or more FCI entries.
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4.2.1.1.
Semantics
TMMBR is used to indicate the transport related limitation in the
form of a tuple. The first value is the highest bit-rate per sender
of a media, which the receiver currently supports in this RTP session
observed at a particular protocol layer. The second value is the
measured header overhead in bytes on the packets received for the
stream. Counting from the start of the header on the protocol layer
for which the bit-rate is reported until the RTP payload's start.
The measurement of the overhead is a running averaging that is
updated for each packet received for this particular media source
(SSRC). For each packet received the overhead is calculated (pckt_OH)
and then added to the average overhead (avg_OH) by calculating:
avg_OH = 15/16*avg_OH + 1/16*pckt_OH.
The bit-rate values used in this formats are averaged out over a
reasonable timescale. What reasonable timescales are, depends on the
application. However the goal is be able to ignore any burstiness on
very short timescales, below for example 100 ms, introduced by
scheduling or link layer packetization effects.
The media sender MAY use any combination of packet rate and RTP
payload bit-rate to produce a lower media stream bit-rate, as it may
need to address a congestion situation or other limiting factors.
See section 5
. (congestion control) for more discussion.
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 Temporal Maximum Media Stream Bit-Rate Notification
(TMMBN) FCI has been received at the time of transmission of the next
RTCP packet. A TMMBN is applicable if it either indicate the sender
of the TMMBR as an owner, or contains limitations that are stricter
than one sent in the TMMBR message. 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. The
overhead measurement SHALL be updated to the current value of avg_OH.
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
limits and the owners of those limits at the time of the transmission
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of the message. The limits SHALL be set to the set of the stricts
limits of the previous limits and all limits received in TMMBR FCI's
since the last TMMBN was transmitted.
A media receiver considering sending a TMMBR, who is not a ''owner''
of a limitation, SHOULD request a limitation stricter than their
knowledge of the currently established limits for this media sender,
or suppress their transmission of the TMMBR. The exception to the
above rule is when a receiver either doesn't know the limit or is
certain that their local representation of the set of limitations are
in error. All received requests for limits equally or less strict
compared to the ones currently established MUST BE ignored, with the
exception of them resulting in the transmission of a TMMBN containg
the current set of limitations. A media receiver who is the owner of
a current limitation MAY lower the value further, raise the value or
remove the restriction completely by setting the bit-rate part of the
limit equal to the session bit-rate limit.
A limitation tuple LT can be determined to be stricter or not
compared to the current set of limitations if LT is part of the set Y
produced by the algorithm described in Section 3.5.4.2.
Once a session participant receives the TMMBN in response to its
TMMBR, with its own SSRC, it knows that it "owns" the bitrate
limitation. Only the "owner" of a bitrate 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 bitrate 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
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 a 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 the
Notification. 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 most
restrictive limits requested including its owners.
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d) If more than one session participant incidently send TMMBR
messages at the same time and with the same limit, the media
sender selects one of them and addresses it as the ''owner''.
Session-wide, the correct limit is thereby established.
It is 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 large number of participants using the lowest
common denominator, as required by this mechanism, may not be the
most suitable course of action. Large 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 session setup signaling, e.g. a SIP
re-invite.
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 signaling channel. In all of
these cases the entity is considered to have left the session. In the
case the "owner" leaves the session, the limit SHALL be removed and
the transmission of a TMMBN is scheduled indicating the remaining
limitations.
4.2.1.2.
Message Format
The Feedback Control Information (FCI) consists of one or more TMMBR
FCI entries with the following syntax:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MMBR Exp | MMBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 - Syntax for the TMMBR message
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SSRC: The SSRC value of the media sender that is requested to
obey the new maximum bit-rate).
MMBR Exp (6 bits): The exponential scaling of the mantissa for the
Maximum Media Stream bit-rate value. The value is non
signed integer [0..63].
MMBR Mantissa (17 bits): The mantissa of the Maximum Media Stream
Bit-rate value as a non-signed integer.
Measured Overhead (9 bits): The measured average packet overhead
value in bytes. The measurement SHALL be done according to
above description in Section 4.2.1.1.
The maximum media stream bit-rate (MMBR) value in bits per second is
calculated from the MMBR exponent (exp) and mantissa in the following
way:
MMBR = mantissa * 2^exp
This allows for 17 bits of resolution in the range 0 to 131072*2^63
(approximately 1.2*10^24).
The length of the 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.
4.2.1.4.
Handling in Translator and Mixers
Media Translators and Mixers will need to receive and respond to
TMMBR messages as they are part of the chain that provides a certain
media stream to the receiver. The mixer or translator may act locally
on the TMMBR request and thus generate a TMMBN to indicate that it
has done so. Alternatively it can forward the request in the case of
a media translator, or generate one of itself in the case of the
mixer. In case it generates a TMMBR, it will need to send a TMMBN
back to the original requestor to indicate that it is handling the
request.
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4.2.2.
Temporary Maximum Media Stream Bit-rate Notification (TMMBN)
The FCI field of the TMMBN Feedback message may contain zero, one or
more TMMBN FCI 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 or that a
owner has left the session. It indicates to all participants the set
of currently employed limitations and the ''owners'' of those.
The ''SSRC of the packet sender'' field indicates the source of the
notification. The ''SSRC of media source'' is not used and SHALL be
set to 0.
A TMMBN message SHALL be scheduled for transmission after the
reception of a TMMBR message with a FCI identifying this media
sender. 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 limits and their owners at the time of
transmitting the message. The limits included SHALL be the set of
most restrictive values in the previously established set and
received TMMBR messages since the last TMMBN was transmitted.
The reception of a TMMBR message with a transmission limit equally or
less restrictive than the set of current limits SHALL still result in
the transmission of a TMMBN message. However the limits and their
owners are not changed, unless it was from an owner of a limit within
the current set of limitations. 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 that limitation is removed, and the media sender
SHALL send a TMMBN message indicating the remaining limitations. In
case there are no remaining limitations a TMMBN without any FCI SHALL
be sent to indicate this.
In unicast scenarios (i.e. where a single sender talks to a single
receiver), the aforementioned algorithm to determine ownership
degenerates to the media receiver becoming the ''owner'' as soon as
the media receiver has issued the first TMMBR message.
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4.2.2.2.
Message Format
The Feedback Control Information (FCI) consists of zero, one or more
TMMBN FCI entries with the following syntax:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MMBR Exp | MMBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 - Syntax for the TMMBR message
SSRC: The SSRC value of the ''owner'' of this limitation.
MMBR Exp (6 bits): The exponential scaling of the mantissa for the
Maximum Media Stream bit-rate value. The value is non-
signed integer [0..63].
MMBR Mantissa (17 bits): The mantissa of the Maximum Media Stream
Bit-rate value as non-signed integer.
Measured Overhead (9 bits): The measured average packet overhead
value in bytes represented as non-signed integer.
Thus the FCI contains blocks indicating the applicable limitations as
the owner followed by the applicable maximum media stream bit-rate
and overhead value.
The length of the FB message is be set to 2+2*N where N is the number
of TMMBR FCI entries.
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.
4.2.2.4.
Handling by Translators and Mixers
As discussed in Section 4.2.1.4 mixer or translators may need to
issue TMMBN messages as response to TMMBR messages handled by the
mixer or translator.
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4.3.
Payload Specific Feedback Messages
Payload-Specific FB messages are identified by the value PT=PSFB
(206) as RTCP packet type (see section 6.1 of RFC 4585 [RFC4585]).
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 Notification (TSTN)
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)
The FIR 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 FIR, the encoder MUST send a Decoder Refresh Point
(see Section 2
..2) as soon as possible.
Note: Currently, video appears to be the only useful application
for FIR, as it appears to be the only RTP 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
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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 herein.
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 is
not expected to use FIR at all.
The sender MUST consider congestion control as outlined in section 5
.,
which MAY restrict its ability to send a decoder refresh point
quickly.
Note: The relationship between the Picture Loss Indication and FIR
is as follows. As discussed in section 6.3.1 of AVPF, a Picture
Loss Indication informs the decoder about the loss of a picture and
hence the likeliness of misalignment of the reference pictures in
the encoder and decoder. Such a scenario is normally related to
losses in an ongoing connection. In point-to-point scenarios, and
without the presence of advanced error resilience tools, one
possible option of an encoder consists in sending a Decoder Refresh
Point. However, there are other options. One example is that the
media sender ignores the PLI, because the embedded stream
redundancy is likely to clean up the reproduced picture within a
reasonable amount of time. The FIR, in contrast, leaves a (real-
time) encoder no choice but to send a Decoder Refresh Point. It
disallows the encoder to take into account any considerations such
as the ones mentioned above.
Note: Mandating a maximum delay for completing the sending of a
Decoder Refresh Point would be desirable from an application
viewpoint, but may be problematic from a congestion control point
of view. ''As soon as possible'' as mentioned above appears to be
a reasonable compromise.
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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 appropriate is when,
in a multipoint conference, a new user joins the session and no
regular Decoder Refresh Point interval is established. Another
example would be a video switching MCU that changes streams. Here,
normally, the MCU issues a FIR to the new sender so to force it to
emit a Decoder Refresh Point. The Decoder Refresh Point 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.
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
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SSRC: The SSRC value of the media sender which is requested to
send a Decoder Refresh Point.
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 one RTT before being sent. In
early or regular RTCP mode the repetition is sent in the next regular
RTCP packet.
4.3.1.4.
Handling of message in Mixer and Translators
A media translator or a mixer performing media encoding of the
content for which the session participant has issued a FIR is
responsible for acting upon it. A mixer acting upon a FIR SHOULD NOT
forward the message unaltered, instead it SHOULD issue a FIR itself.
4.3.1.5.
Remarks
In conjunction with video codecs, FIR messages typically trigger the
sending of full intra or IDR pictures. Both are several times larger
then predicted (inter) pictures. Their size is independent of the
time they are generated. In most environments, especially when
employing bandwidth-limited links, the use of an intra picture
implies an allowed delay that is a significant multitude of the
typical frame duration. An example: If the sending frame rate is 10
fps, and an intra picture is assumed to be 10 times as big as an
inter picture, then a full second of latency has to be accepted. In
such an environment there is no need for a particularly short delay
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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 TSTN 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seq nr. | Reserved | Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 - Syntax of the TSTR
SSRC: The SSRC of the media sender which is requested to apply
the tradeoff value in Index.
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.
Handling of message in Mixers and Translators
Mixer or Media translators that encodes content sent to the session
participant issuing the TSTR SHALL consider the request to determine
if it can fulfill it by changing its own encoding parameters. A media
translator unable to fulfill the request MAY forward the request
unaltered towards the media sender. A Mixer encoding for multiple
session participants will need to consider the joint needs before
generating a TSTR for itself towards the media sender. See also
discussion in Section .
3.5.2.
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4.3.2.5.
Remarks
The term "spatial quality" does not necessarily refer to the
resolution, measured by the number of pixels the reconstructed video
is using. In fact, in most scenarios the video resolution stays
constant during the lifetime of a session. However, all video
compression standards have means to adjust the spatial quality at a
given resolution, often influenced by the Quantizer Parameter or QP.
A numerically low QP results in a good reconstructed picture quality,
whereas a numerically high QP yields a coarse picture. The typical
reaction of an encoder to this request is to change its rate control
parameters to use a lower frame rate and a numerically lower (on
average) QP, or vice versa. The precise mapping of Index, 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 Notification (TSTN)
The TSTN message is identified by PT=PSFB and FMT=6.
There SHALL be one or more TSTN contained in the FCI field.
4.3.3.1.
Semantics
This feedback message is used to acknowledge the reception of a TSTR.
A TSTN entry in a TSTN feedback message SHALL be sent for each TSTR
entry targeted to this session participant, i.e. each TSTR received
that in the SSRC field in the entry has the receiving entities SSRC.
A single TSTN message MAY acknowledge multiple requests using
multiple FCI entries. The index value included SHALL be the same in
all FCI's part of the TSTN message. Including a FCI for each
requestor allows each requesting entity to determine that the media
sender targeted have received the request. The Notification 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 TSTN SHALL include the Temporal-Spatial Trade-off index that will
be used as a result of the request. This is not necessarily the same
index as requested, as 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.
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The ''SSRC of the packet sender'' field indicates the source of the
Notification, and the ''SSRC of media source'' is not used and SHALL
be set to 0. The SSRC of the requesting entity to which the
Notification applies to is in the FCI.
4.3.3.2.
Message Format
The Temporal-Spatial Trade-off Notification uses one additional FCI
field, the content of which is depicted in Figure 5. The length of
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 TSTN
SSRC: The SSRC of the source of the TSTR request which resulted
in this Notification.
Seq. nr: The sequence number value from the TSTN 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.
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4.3.3.4.
Handling of message in Mixer and Translators
A Mixer or Translator that act upon a TSTR SHALL also send the
corresponding TSTN. In cases it needs to forward a TSTR itself the
notification message MAY need to be delayed until that request has
been responded to.
4.3.3.5.
Remarks
None
4.3.4.
H.271 Video Back Channel Message (VBCM)
The VBCM is identified by PT=PSFB and FMT=7.
There MUST be one or more VBCM entry contained in the FCI field.
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 a 'status report' (such as receiving bit stream
without errors, or loss of a partial or complete picture or block) or
'update requests' (such as complete refresh of the bit stream).
Note: There are possible overlaps between the VBCM sub-
messages and CCM/AVPF feedback messages, such FIR. Please see
section 3
..5.3 for further 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.
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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 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
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SSRC: The SSRC value of the media sender that is requested to
instruct its encoder 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.
4.3.4.3.
Timing Rules
The timing follows the rules outlined in section 3 of [RFC4585]. The
different sub-message types may have different properties in regards
to the timing of messages that should be used. If several different
types are included in the same feedback packet then the sub-message
type with the most stringent requirements should be followed.
4.3.4.4.
Handling of message in Mixer or Translator
The handling of VBCM in a mixer or translator are sub-message type
dependent.
4.3.4.5.
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.
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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
from being flooded 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 Stream Bit-rate value can
only mitigate congestion, but not cause congestion as long as
congestion control is also employed. An increase of the value by a
request REQUIRES the media sender to use congestion control when
increasing its transmission rate to that value. A reduction of the
value results in a reduced transmission bit-rate thus reducing the
risk for congestion.
6.
Security Considerations
The defined messages have certain properties that have security
implications. These must be addressed and taken into account by users
of this protocol.
The defined setup signaling mechanism is sensitive to modification
attacks that can result in session creation with sub-optimal
configuration, and, in the worst case, session rejection. To prevent
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this type of attack, authentication and integrity protection of the
setup signaling is required.
Spoofed or maliciously created feedback messages of the type defined
in this specification can have the following implications:
a. Severely reduced media bit-rate due to false TMMBR messages
that sets the maximum to a very low value.
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 a need to apply authentication and
integrity protection of the feedback messages. This can be
accomplished against threats external to the current RTP session
using the RTP profile that combines SRTP [SRTP] and AVPF into SAVPF
[SAVPF]. In the Mixer cases, separate security contexts and filtering
can be applied between the Mixer and the participants thus protecting
other users on the Mixer from a misbehaving participant.
7.
SDP Definitions
Section 4 of [RFC4585] defines new SDP [RFC4566] 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
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 and to
multiple rtcp-fb attributes in a session description also apply to
the new feedback messages defined in this memo.
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The ABNF for rtcp-fb 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 Stream
Bit-rate. It has an optional sub parameter to indicate the
session maximum packet rate to be used. If not included it
defaults to infinity.
o "tstr" indicates the support of temporal spatial trade-off
request.
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
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rtcp-fb-ccm-param = "ccm" SP ccm-param
ccm-param = "fir" ; Full Intra Request
/ "tmmbr" [SP "smaxpr=" MaxPacketRateValue]
; Temporary max media bit rate
/ "tstr" ; Temporal Spatial Trade Off
/ "vbcm" *(SP subMessageType] ; H.271 VBCM messages
/ token [SP byte-string]
; for future commands/indications
subMessageType = 1*8DIGIT
byte-string = <as defined in section 4.2 of [RFC4585] >
MaxPacketRateValue = 1*15DIGIT
7.2.
Offer-Answer
The Offer/Answer [RFC3264] implications to codec control protocol
feedback messages are similar those described in [RFC4585]. The
offerer MAY indicate the capability to support selected codec
commands and indications. The answerer MUST remove all ccm parameters
which it does not understand or does not wish to use in this
particular media session. The answerer MUST NOT add new ccm
parameters in addition to what has been offered. The answer is
binding for the media session and both offerer and answerer MUST only
use feedback messages negotiated in this way.
The session maximum packet rate parameter part of the TMMBR
indication is declarative and everyone shall use the highest value
indicated in a response. If not present in a offer is SHALL NOT be
included by the answerer.
7.3.
Examples
Example 1: The following SDP describes a point-to-point video call
with H.263 with the originator of the call declaring its capability
to support codec control messages - fir, tstr. The SDP is carried in
a high level signaling protocol like SIP
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Point-to-Point call
c=IN IP4 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
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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
TSTN feedback message.
Example 2: The following SDP describes a SIP end point joining a
video Mixer that is hosting a multiparty video conferencing session.
The participant supports only the FIR (Full Intra Request) codec
control command and it declares it in its session description. The
video Mixer 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
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:* ccm tmmbr smaxpr=120
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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 1
(One or more pictures that are entirely or partially lost) and 2 (a
set of blocks of one picture that is entirely or partially lost).
-------------> 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 1 2
The answerer only wishes to support sub-messages 1 only
<---------------- 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
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a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm vbcm 1
So in the above example only VBCM indication comprising of only
"payloadType" 1 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 Stream Bit-rate
Usable with: ccm
Reference: RFC XXXX
Value name: tstr
Long name: temporal Spatial Trade Off
Usable with: ccm
Reference: RFC XXXX
Value name: vbcm
Long name: H.271 video back channel messages
Usable with: ccm
Reference: RFC XXXX
9.
Acknowledgements
The authors would like to thank Andrea Basso, Orit Levin, Nermeen
Ismail for their work on the requirement and discussion draft
[Basso].
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Drafts of this memo were reviewed and extensively commented by Roni
Even, Colin Perkins, Randell Jesup, Keith Lantz, Harikishan Desineni,
Guido Franceschini and others. The authors appreciate these reviews.
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.
Wenger, et al. Standards Track [Page 56]
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[RFC3448] M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol Specification", RFC 3448,
Jan 2003
[VBCM] ITU-T Rec. H.271, "Video Back Channel Messages", June
2006
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)", RFC
3890, September 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March
2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
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
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Ericsson Research
Ericsson AB
SE-164 80 Stockholm, SWEDEN
Phone: +46 8 7190000
EMail: bo.burman@ericsson.com
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Wenger, et al. Standards Track [Page 58]
INTERNET-DRAFT AVPF RTCP-RR Extensions March 5, 2007
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
RFC Editor Considerations
The RFC editor is requested to replace all occurrences of XXXX with
the RFC number this document receives.
Wenger, et al. Standards Track [Page 59]
