WDIFF

draft-ietf-avt-rtcp-feedback-00.txt --> draft-ietf-avt-rtcp-feedback-01.txt

INTERNET-DRAFT J÷rg Ott/Universit„t Bremen TZI
draft-ietf-avt-rtcp-feedback-00.txt
draft-ietf-avt-rtcp-feedback-01.txt Stephan Wenger/TU Berlin
Shigeru Fukunaga/Oki
Noriyuki Sato/Oki
Koichi Yano/Fast Forward Networks
Akihiro Miyazaki/Matsushita
Koichi Hata/Matsushita
Rolf Hakenberg/Matsushita
Carsten Burmeister/Matsushita

13 July,

21 November, 2001
Expires January May 2002

Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

Real-time media streams are not resilient against packet losses. RTP
[1] provides all the necessary mechanisms to restore ordering and
timing to properly reproduce a media stream at the recipient. RTP
also provides continuous feedback about the overall reception quality
from all receivers -- thereby allowing the sender(s) in the mid-term
(in the order of several seconds to minutes) to adapt their coding
scheme and transmission behavior to the observed network QoS.
However, except for a few payload specific mechanisms [10], RTP makes
no provision for timely feedback that would allow a sender to repair
the media stream immediately: through retransmissions, retro-active
FEC, or media-specific mechanisms such as reference picture
selection.

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Generally, real-time transport of media streams across IP networks
follows RTP[1] in conjunction with the RTP Profile for Audio and
Video Conferences with Minimal Control [2]. This document modifies
the profile defined in [2] in two ways:

. by providing additional RTCP messages that enable a receiver to
convey more precise feedback to a sender and

. by adapting the timing algorithm for scheduling RTCP packets in
order to allow for occasional timely feedback about events
observed by a receiver (such as lost packets).

The result is an RTP Profile for Audio and Video Conferences with
Minimal Control that allows for more explicit and more immediate
receiver feedback but shares all other properties (including all
other message types and formats, all code points for codecs, payload
formats, scaling capabilities, etc. of [2]). Therefore, this
document only specifies the additions and modifications to [2] rather
than the repeating the entire specification.

1. Introduction

Real-time media streams are not resilient against packet losses. RTP
[1] provides all the necessary mechanisms to restore ordering and
timing present at the sender to properly reproduce a media stream at
a recipient. RTP also provides continuous feedback about the overall
reception quality from all receivers -- thereby allowing the
sender(s) in the mid-term (in the order of several seconds to
minutes) to adapt their coding scheme and transmission behavior to
the observed network QoS. However, except for a few payload specific
mechanisms [10], RTP makes no provision for timely feedback that
would allow a sender to repair the media stream immediately: through
retransmissions, retro-active FEC, or media-specific mechanisms such
as reference picture selection.

Current mechanisms available with RTP to improve error resilience
include audio redundancy coding [7], video redundancy coding [11],
RTP-level FEC [5], and general considerations on more robust media
streams transmission [6]. Particularly in small groups, however,
virtually all kinds of real-time media streams could benefit from a
mechanism that would enable a sender to perform media stream repair -
- including but not limited to audio, video, DTMF, and text chat
streams. In some cases of networks with acceptable round-trip times
but scarce bandwidth, occasional retransmissions may be much
preferred over continuous transmission of redundant information.

For example, predictive video coding is not loss resilient. Any loss
of coded data leads to annoying artifacts not only in the reproduced
picture in which the loss occurred, but also in subsequent pictures.
Error resilience can be achieved by allocating bits to convey
redundant information using source coding based These mechanisms or
transport based mechanisms. This can may be done without applied pro-
actively (thereby increasing the use bandwidth of any
feedback between the decoder(s) and the encoder. Similar
consideration apply to protecting e.g. DTMF (and other tones) carried
in an RTP stream [9].

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Alternatively, where applicable, receivers can inform the sender
through a feedback channel about a loss situation, and the sender can
react accordingly. This approach provides better given media quality and
is more efficient stream).
Alternatively, in sufficiently small groups with respect to the bandwidth used by short RTTs, the sender to
achieve a given media quality. However,
senders may perform repair on-demand, using feedback the above mechanisms is
limited to certain application scenarios identified by encoder
characteristics, delay constraints,
and/or the number of recipients. media-encoding-specific approaches. Note that "small group"
and "sufficiently short RTT" are both highly application dependent.

This document specifies a modified RTP Profile for Audio and Video
conferences with minimal control based upon [1] and [2] by means of
two modifications/additions: To achieve timely feedback the concepts
of Immediate Feedback messages and Early RTCP messages as well as
algorithms allowing for low delay feedback in small multicast groups
(and preventing feedback implosion in large ones) are introduced.
Special consideration is given to point-to-point scenarios.

In addition, various types of And a
small number general-purpose feedback messages as well as a format

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for codec and application-specific feedback information are is defined as
specific RTCP payloads.

1.1 Definitions

The definitions from [1] and [2] apply. In addition, the following
definitions are used in this document:

Early RTCP mode:
The mode of operation in which a receiver of a media stream
is, statistically, often (but not always) capable of
reporting events of interest back to the sender close to
their occurrence. In Early RTCP mode, RTCP feedback messages
are transmitted according to the timing rules defined in this
document.

Early RTCP packet:
An Early RTCP packet is a packet which is transmitted earlier
than would be allowed following the scheduling algorithm of
[1], the reason being that an event observed by a receiver.
Early RTCP packets may be sent in Immediate feedback and in
Early RTCP mode.

Event:
An observation made by the receiver of a media stream that is
(potentially) of interest to the sender -- such as a packet
loss or packet reception, frame loss, etc. -- and thus to be
reported back to the sender by means of a Feedback message.

Feedback (FB) message:
An RTCP message as defined in this document used to convey
events observed at a receiver -- in addition to long term
receiver status information which is carried in RTCP RRs û
back to the sender of the media stream.

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Feedback (FB) threshold:
The FB threshold indicates the "borderline" between Immediate
Feedback and Early RTCP mode. For a multicast scenario, the
FB threshold indicates the maximum group size at which, on
average, each receiver is able to report each event back to
the sender(s) immediately, i.e. without having to wait for
its regularly scheduled RTCP interval. This threshold is
highly dependent on network QoS (e.g. packet loss probability
and distribution), codec and packetization in use, and
application requirements. Hence, no formal definition is
presented in this document.

Immediate Feedback mode:
Mode of operation in which each receiver of a media is,
statistically, capable of reporting each event of interest
immediately back to the media stream sender. In Immediate
Feedback mode, RTCP feedback messages are transmitted
according to the timing rules defined in this document.

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Regular RTCP mode:
Mode of operation in which no preferred transmission of
feedback messages is allowed. Instead, RTCP messages are
sent following the rules of [1] and may contain feedback
messages information as defined in this document.

Regularly Scheduled RTCP packet:
An RTCP packet that is not sent as an Early RTCP packet.

1.2 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 [8]

2. RTP and RTCP Packet Formats and Protocol Behavior

The rules defined in [2] also apply to this profile except for those
rules mentioned in the following:

RTCP packet types:
Three additional RTCP packet types to convey feedback
information are defined in section 4.

RTCP report intervals:
This memo describes three modes of operation which influence
the RTCP report intervals (see section 3.2). In regular
RTCP mode, all rules from [1] apply. In both Immediate
Feedback and Early RTCP modes the minimal interval of 5
seconds between 2 RTCP reports is dropped and the rules
specified in section 3 apply if RTCP packets containing
feedback messages (defined in section 4) are to be
transmitted.

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The rules set forth in [1] may be overridden by session
descriptions specifying different parameters (e.g. for the
bandwidth share assigned to RTCP for senders and receivers,
respectively. For sessions defined using the Session
Description Protocol (SDP) [3], the rules of [4] apply.

Congestion control:
The same basic rules as detailed in [2] apply. Beyond this,
in section 5, further consideration is given to the impact of
feedback and a sender's reaction to feedback messages.

3. Rules for RTCP Feedback

3.1 Compound RTCP Feedback Packets

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Two components constitute RTCP-based feedback as described in this
memo:

. Status reports are contained in SR/RR messages and are transmitted
at regular intervals as part of compound RTCP packets (which also
include SDES and possibly other messages); these status reports
provide an overall indication for the recent reception quality of
a media stream.

. Feedback messages as defined in this document that indicate loss
or reception of particular pieces of a media stream (or provide
some other form of rather immediate feedback on the data
received). Rules for the transmission of feedback messages are
newly introduced in this memo.

RTCP Feedback (FB) messages are just another RTCP packet type (see
section 4). Therefore, multiple FB messages MAY be combined in a
single compound RTCP packet and they MAY also be sent combined with
other RTCP packets.

RTCP packets containing Feedback packets as defined in this document
MUST contain RTCP packets in the order as defined in [1]:

. OPTIONAL encryption prefix that MUST be present if the RTCP
message is to be encrypted.
. MANDATORY SR or RR.
. MANDATORY SDES which MUST contain the CNAME item; all other SDES
items are OPTIONAL.
. One or more FB messages.

The FB MUST be placed in the compound packet after all RR and SDES RTCP
packets defined in [1]. The ordering with respect to other RTCP
extensions is not defined.

Two types of compound RTCP packets carrying feedback packets are used
in this document:

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a) Minimal compound RTCP feedback packet

A minimal compound RTCP feedback packet MUST contain only the
mandatory information as listed above: encryption prefix if
necessary, exactly one RR or SR, exactly one SDES with only the
CNAME item present, and the feedback message(s). This is to
minimize the size of the RTCP packet transmitted to convey
feedback and thus to maximize the frequency at which feedback can
be provided while still adhering to the RTCP bandwidth
limitations.

This packet format SHOULD be used whenever an RTCP feedback
message is sent as part of an Early RTCP packet.

b) (Full) compound RTCP feedback packet

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A (full) compound RTCP feedback packet MAY contain any additional
number of RTCP packets (additional RRs, further SDES items,
etc.).

This packet format MUST be used whenever an RTCP feedback message
is sent as part of a regularly scheduled RTCP packet or in
Regular RTCP mode. This packet format MAY also be used to send
RTCP feedback messages in Immediate Feedback or Early RTCP mode.

RTCP packets that do not contain FB messages are referred to as non-
FB RTCP packets.

3.2 Algorithm Outline

FB messages are part of the RTCP control streams and are thus subject
to the same bandwidth constraints as other RTCP traffic. This means
in particular that it may not be possible to report an event observed
at a receiver immediately back to the sender. However, the value of
feedback given to a sender typically decreases over time -- in terms
of the media quality as perceived by the user at the receiving end
and/or the cost required to achieve media stream repair.

RTP [1] and the commonly used RTP profile [2] specify rules when
compound RTCP packets should be sent. This document modifies those
rules in order to allow applications to timely report media loss or
reception events to accommodate algorithms that use FB messages and
are sensitive to the feedback timing.

The modified algorithm can be outlined as follows: Normally, when no
FB messages have to be conveyed, compound RTCP packets are sent
following the rules of RTP [1] -- except that the 5s minimum interval
between RTCP reports is not enforced. If a receiver detects the need
for an FB message, the receiver waits for a short, random dithering
interval (in case of multicast) and then checks whether it has
already seen a corresponding FB message from any other receiver
(which it can do with all FB messages that are transmitted via
multicast; for unicast sessions, there is no such delay). If this is
the case then the receiver refrains from sending the FB message and
continues to follow the regular RTCP sending schedule. If the

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receiver has not yet seen a similar FB message from any other
receiver, it checks whether it has recently exceeded its RTCP bit
rate budget to transmit another FB message (without waiting for its
regularly scheduled RTCP transmission time). Only if this is not the
case, it sends the FB message as part of a (minimal) compound RTCP
packet.

FB messages may also be sent as part of full compound RTCP packets
which are interspersed as per [1] in regular intervals.

3.3 Modes of Operation

RTCP-based feedback may operate in one of three modes (figure 1):

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a) Immediate feedback mode: the group size is below the FB threshold
which gives each receiving party sufficient bandwidth to transmit
the feedback traffic for the intended purpose. This means, for
each receiver there is enough bandwidth to report each event it is
supposed/expected to by means of a virtually "immediate" RTCP
feedback packet.

The group size threshold is a function of a number of parameters
including (but not necessarily limited to) the type of feedback
used (e.g. ACK vs. NACK), bandwidth, packet rate, packet loss
probability and distribution, media type, codec, and -- again
depending on the type of FB used -- the (worst case or observed)
frequency of events to report (e.g. frame received, packet lost).

A special case of this is the ACK mode (where positive
acknowledgements are used to confirm reception of data) which is
restricted to point-to-point communications.

b) Early RTCP mode: In this mode, the group size and other parameters
no longer allow each receiver to react to each event that would be
worth (or needed) to report. But feedback can still be given
sufficiently often so that it allows the sender to adapt the media
stream transmission accordingly and thereby increase the overall
reproduced media quality.

c) From some group size upwards, it is no longer useful to provide
feedback from individual receivers at all -- because of the time
scale in which the feedback could be provided and/or because in
large groups the sender(s) have no chance to react to individual
feedback anymore.

As the feedback algorithm described in this memo scales smoothly,
there is no need for an agreement among the participants on the
precise values of the respective "thresholds" within the group.
Hence the borders between all these modes are allowed to be fluent.

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ACK
feedback
V
:<- - - - NACK feedback - - - ->//
:
: Immediate ||
: Feedback mode ||Early RTCP mode Regular RTCP mode
:<=============>||<=============>//<=================>
: ||
-+---------------||---------------//------------------> group size
2 ||
Application-specific FB Threshold
= f(data rate, packet loss, codec, ...)

Figure 1: Modes of operation

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The respective thresholds depend on a number of technical parameters
(of the codec, the transport, the feedback used, etc.) but also on
the respective application scenarios. Section 3.5 provides some
useful hints (but no complete precise calculations) on estimating
these thresholds.

3.4 Definitions

The following pieces of state information need to be maintained per
receiver (largely taken from [1]): [1]). Note that all variables (except
for h) are calculated independently at each receiver and so their
local values may differ at a given point in time.

a) Let senders be the number of active senders in the RTP session.

b) Let members be the current estimate of the number of receivers
in the RTP session.

c) Let T_rtt be the maximum round trip time as measured by RTCP
(if available to the receiver). Note that this may be asymmetric.

d) Let tn and tp be the time for the next (last) scheduled
RTCP RR transmission calculated prior to reconsideration.

e) Let T_rr be the interval after which, having just sent a regularly
scheduled RTCP packet, a receiver would schedule the transmission
of its next RTCP packet following the rules of [1]: T_rr = tn û -
tp. Note that the 5s minimum interval between two report as
defined in [1] SHOULD NOT be enforced.

f) Let t0 be the time at which an event that is to be reported is
detected by a receiver.

g) Let T_dither_max be the maximum interval for which an RTCP
feedback packet may be additionally delayed (to prevent
implosions).

h) Let T_max_fb_delay be the upper bound within which feedback to
an event needs to be reported back to the sender to be useful at
all. Note that this value is application-specific.

i) Let te be the time for which a feedback packet is scheduled.

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j) Let T_fd be the actual (randomized) delay for the transmission of
feedback message in response to an event that a certain packet P
caused.

k) Let allow_early be a Boolean variable that indicates whether a the
receiver currently may transmit feedback messages prior to its
next regularly scheduled RTCP interval tn. This variable is used
to throttle the feedback sent by a single receiver. allow_early
is adjusted (set to FALSE) after early feedback transmission and
is reset to TRUE as soon as the next regular RTCP transmission is
scheduled.

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l) Let avg_rtcp_size be the moving average on the RTCP packet size as
defined in [1].

The feedback situation for an event to report at a receiver is
depicted in figure 2 below. At time t0, such an event (e.g. a packet
loss) is detected at the receiver. The receiver decides -- based
upon current T_rtt, group size, and other (application-specific)
parameters -- that a feedback message needs to be sent back to the
sender.

To avoid an implosion of immediate feedback packets, the receiver
MUST delay the transmission of the compound feedback packet by a
random amount T_fd (with the random number evenly distributed in the
interval [0, T_dither_max]. Transmission of the compound RTCP packet
is then scheduled for te = t0 + T_fd.

The T_dither_max parameter is chosen based upon the group size, the
RTCP bandwidth constraints, and, round-trip time
or, if available, the round-trip time. time is not available, based upon the group
size.

Based upon the parameters influencing T_dither_max and a number of
other parameters (such as the type of feedback to be provided) the
receiver may determine T_max_fb_delay (as static value or dynamically
adjusted) as the upper bound for the feedback information to be
useful when it reaches the sender.

If a compound RTCP feedback packet is scheduled, the time slot for
the next scheduled compound RTCP packet is updated accordingly to a
new tn.

event to
report
detected
|
| RTCP feedback range
| (T_max_fb_delay)
vXXXXXXXXXXXXXXXXXXXXXXXXXXX ) )
|---+--------+-------------+-----+------------| |--------+--------->
| | | | ( ( |
| t0 te |
tp tn
\_______ ________/
\/
T_dither_max

Figure 2: Event report and parameters for Early RTCP scheduling

3.5 Early RTCP Algorithm

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3.5 Early RTCP Algorithm

Assume an active sender S0 (out of S senders) and a number N of
receivers with R being one of these receivers.

Assume further that R has verified that using feedback mechanisms is
reasonable at the current constellation (which is highly application
specific and hence not specified in this memo).

Then, receiver R MUST use the following rules for transmitting a one or
more Feedback messages as minimal or full compound RTCP packet:

Initially, R MUST set allow_early := TRUE.

R has transmitted the last RTCP RR packet at tp and has scheduled the
next transmission (prior to reconsideration) for tn.

At time t0, R detects the need to transmit a one or more feedback message
messages (e.g. because a media "unit" "units" needs to be ACKed or NACKed) and
finds that sending the feedback message information is useful for the sender.

R first checks whether there is still an a compound RTCP feedback packet
waiting for transmission. transmission (scheduled as early or regular RTCP packet).
If so, the new feedback message MUST be appended to the packet; the
schedule for the waiting RTCP feedback packet MUST remain unchanged.
When appending, the feedback information of several RTCP feedback
packets SHOULD be merged as few packets as possible.

If no RTCP feedback message is already awaiting transmission (as part
of an Early RTCP packet), transmission, a new
(minimal) compound RTCP feedback packet MUST be created and the
minimal interval for T_dither_max MUST be chosen as follows:

i) If the session is a unicast session (group size = 2) then
T_dither_max := 0.

ii) If the receiver has an RTT estimate to the originator of the
media unit to provide feedback about, then

T_dither_max := k * T_rtt/2 * members

with k=1.

iii) If the receiver does not have an RTT estimate to the originator,
then

T_dither_max := l * T_rr

with l=0.5.

(Application-specific

The values given above for T_dither_max are minimal values.
Application-specific feedback considerations may make it worthwhile
to increase T_dither_max beyond this value. This is up to the
discretion of the implementer.) implementer.

Then, R MUST check whether its next regularly scheduled RTCP packet
is within the time bounds for the RTCP FB (t0 + T_dither_max > tn).

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If so, an Early RTCP packet MUST NOT be scheduled; instead the FB

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message
message(s) MUST be stored to be appended to the regular RTCP packet
scheduled for tn.

Otherwise, R MUST check whether it is allowed to transmit an Early
RTCP packet (allow_early == TRUE).

If so, R MUST schedule an Early RTCP packet for te := t0 + RND *
T_dither_max with the RND function evenly distributed between 0
and 1.

If, while waiting for te, R receives an RTCP feedback packet packets
contained in one or more (minimal) compound RTCP packets, R MUST
act as follows: follows for each of the RTCP feedback packets in the one or
more compound RTCP packets received:

1. If R understands the received feedback message's semantics and
the message contents is a superset of the feedback R wanted to
send then R MUST discard its own feedback message and MUST re-
schedule the next regular RTCP message transmission for tn (as
calculated before).

2. If R understands the received feedback message's semantics and
the message contents is not a superset of the feedback R
wanted to send then R SHOULD transmit its own feedback message
as scheduled. If there is an overlap between the feedback
information to send and the feedback information to receive,
the amount of feedback transmitted is up to R: R MAY send its
feedback information unchanged, R MAY as well eliminate any
redundancy between its own feedback and the feedback received
so far.

3. If R does not understand the received feedback message's
semantics
semantics, R checks whether the compound RTCP packet contains
a Generic INFO message. If a Generic INFO message is present
R performs the comparison based upon this information and
proceeds with alternative 1. or 2. above depending on the
outcome of the comparison. If no Generic INFO message is
present, then R MAY send its own feedback message as or Early
RTCP packet. Alternatively, R MAY re-schedule the next
regular RTCP message transmission for tn (as calculated
before) and MAY append the feedback message to the now
regularly scheduled RTCP message.

Refer to section 4 on the comparison of feedback messages and for
which feedback messages must MUST be understood by a receiver.

Otherwise, when te is reached, R MUST transmit the RTCP packet
containing the FB message. R then MUST set allow_early := FALSE
and MUST recalculate tn := tp + 2*T_rr. As soon as R sends its
next regularly scheduled RTCP RR (at the new tn), it MUST set
allow_early := TRUE again.

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If allow_early == FALSE then R MUST check the time for the next
scheduled RR:

1. If tn û t0 < T_max_fb_delay (i.e. if, despite late reception, the
feedback could still be useful for the sender) then R MAY create
an RTCP FB message for transmission along with the RTCP packet at
tn.

2. Otherwise, R MUST discard the RTCP feedback message.

In regular RTCP intervals as specified by [1] (except for the five
second minimum), a full compound RTCP packet is sent (which may also
contain a feedback message if one has been created according to the
above rules and scheduled for transmission along the full compound
RTCP message).

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The E bit in the message header is used upon reception to detect
whether this

Whenever an RTCP feedback message was sent as Early RTCP or not.
Hence, a feedback message that is sent as an Immediate or Early RTCP
packet MUST set the E bit in the message header to "1". Feedback
messages piggy-backed on regularly scheduled RTCP packets MUST set
the E bit to "0". If a receiver R receives an Early RTCP packet
(E=1), then it MAY set allow_early := TRUE.

Whenever an RTCP packet is packet is sent or received -- minimal or full
compound, early or regularly scheduled -- the avg_rtcp_size variable
is updated accordingly (see [1]) and the tn is calculated using the
new avg_rtcp_size.

3.6 Considerations on the Group Size

This section provides guidelines to the group sizes at which the
various feedback modes may be used.

3.6.1 ACK mode

The group size MUST be exactly two participants, i.e. point-to-point
communications. Unicast addresses SHOULD be used in the session
description.

For unidirectional as well as bi-directional communication between
two parties, 2.5% of the RTP session bandwidth are available for RTCP
traffic from the receivers including feedback. , Assuming that out of
ten RTCP packets, nine are sent as minimal compound RTCP packets and
one as full compound RTCP packet, at 64kbit/s unidirectional
communication scenario, a receiver can report 1.5 events per second
back to the sender, at 256kbit/s 6 events and so forth.

From 1 Mbit/s upwards, a receiver would be able to acknowledge each
individual frame (not packet!) in a 25 fps video stream.

ACK strategies should MUST be defined accordingly to work properly with
these bandwidth limitations. An indication whether or not ACKs are
allowed for a session and, if so, which ACK strategy should be used,
MAY be conveyed by out-of-band mechanisms, e.g. media-specific
attributes in a session description using SDP.

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3.6.2 NACK mode

Negative acknowledgements (or similar types of feedback) MUST be
used for all groups larger than two. Of course, NACKs MAY be used
for point-to-point communications as well.

Whether or not the use of Immediate or Early RTCP packets should be
considered depends upon a number of parameters including session
bandwidth, codec, special type of feedback, number of senders and
receivers, among many others.

The crucial parameters -- to which virtually all of the above can be
reduced -- is the allowed minimal interval between two RTCP reports
and the (average) number of events that presumably need reporting per
time interval (plus their distribution over time, of course). The
minimum interval is derived from the available RTCP bandwidth and the

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expected average size of an RTCP packet. The number of events to
report e.g. per second may be derived from the packet loss rate and
sender's rate of transmitting packets. From these two values, the
allowable group size for the Immediate feedback mode can be
calculated.

The upper bound for the Early RTCP mode then solely depends on the
acceptable quality degradation, i.e. how many events per time
interval may go unreported.

Example: If a 256kbit/s video with 30 fps is transmitted through a
network with an MTU size of some 1500 bytes, then, in most cases,
each frame would fit in its own packet leading to a packet rate of 30
packets per second. If 5% packet loss occurs in the network (equally
distributed, no inter-dependence between receivers), then each
receiver will have to report 3 packets lost each two seconds.
Assuming a single sender and more then than three receivers, this yields
3.75% of the RTCP bandwidth allocated to the receivers and thus
9.6kbit/s. Assuming further a size of 120 bytes for the average
compound RTCP packet allows 10 RTCP packets to be sent per second or
20 in two seconds. If every receiver needs to report three packets,
this yields a maximum group size of 6-7 receivers if all loss events
shall be reported. The rules for transmission of immediate RTCP
packets should provide sufficient flexibility for most of this
reporting to occur in a timely fashion.

Extending this example to determine the upper bound for Early RTCP
mode leads to the following considerations: assume that the
underlying coding scheme and the application (as well as the tolerant
users) allow in on the order of one loss without repair per two seconds.
Thus the number of packets to be reported by each receiver decreases
to two per two seconds second and increases the group size to 10.
Assuming further that some number of packet losses are correlated,
feedback traffic is further reduced and group sizes of some 12 to 16
(maybe even 20) can be reasonably well supported using Early RTCP
mode.

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3.7 Summary of decision steps

3.7.1 General Hints

Before even considering whether or not to send RTCP feedback
information an application has to determine whether this mechanism is
applicable:

1) An application has to decide whether -- for the current ratio of
packet rate with the associated (application-specific) maximum
feedback delay and the currently observed round-trip time (if
available) -- feedback mechanisms can be applied at all.

This decision may obviously be based upon (and dynamically revised
following) regular RTCP reception statistics.

2) The application has to decide whether -- for a certain observed
error rate, assigned bandwidth, frame rate, and group size -- (and
which) feedback mechanisms can be applied.

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Regular RTCP provides valuable input to this step, too.

3) If these tests pass, the application has to follow the rules for
transmitting Early RTCP packets or regularly scheduled RTCP
packets with piggybacked feedback.

3.7.2 Media Session Description Attributes

A number

Media sessions are typically described using out-of-band mechanisms
to convey transport addresses, codec information, etc. between
sender(s) and receiver(s). Such a mechanisms is composed of additional SDP parameters MAY be a format
used to describe a
session. These are defined as media level attributes.

3.7.2.1 Profile identification

The AV profile defined in [4] is referred to as "AVP" in session and another mechanism for
transporting this description.

In the context
of e.g. IETF, the Session Description Protocol (SDP) [3]. The profile
specified in this document is referred currently used
to describe media sessions while protocols such as "AVPF".

Feedback information following SIP, SAP, RTSP,
and HTTP are used to convey the modified timing rules as specified
in this document MUST NOT be sent for a particular media session
unless description.

A present media session description format MAY include parameters to
indicate that RTCP feedback mechanisms are supported in this session
and which of the feedback mechanisms may be applied.

To do so, the profile "AVPF" MUST be indicated instead of "AVP".
Further attributes may be defined to show which type(s) of feedback
are supported.

Section 4 contains the syntax specification to support RTCP feedback
with SDP. Similar specifications for this other media session indicates description
formats are outside the use scope of this specification.

4. SDP Definitions

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This section defines a number of additional SDP parameters that are
used to describe a session. All of these are defined as media level
attributes.

4.1 Profile identification

The AV profile defined in [4] is referred to as "AVP" in the context
of e.g. the "AVPF"
profile. Session Description Protocol (SDP) [3]. The profile
specified in this document is referred to as "AVPF".

Feedback information as part of regularly scheduled compound RTCP
packets following the modified timing rules of [1] and [2] MAY as specified
in this document MUST NOT be sent for a particular media sessions for which session
unless the "AVP" profile is specified. In for this
case, however, the receiver providing feedback MUST NOT rely on session indicates the
sender reacting to use of the feedback at all.

3.7.2.2 "AVPF"
profile.

4.2 RTCP Feedback Capability Attribute

A new payload format-specific SDP attribute (for use with "a=fmtp:")
is defined to indicate the capability of using RTCP feedback as
specified in this document: "rtcp-fb". The "rtcp-fb" attribute MAY
only be used as an SDP media attribute and MUST NOT be provided at
the session level. The rtcp-fb attribute MUST only be used in media
sessions for which the "AVPF" is specified.

The rtcp-fb attribute is used to indicate which RTCP feedback
messages MAY be used in this media session for the indicated payload
type. If several types of feedback are supported, several a=rtcp-fb:
lines MUST be used.

If no rtcp-fb attribute is specified the RTP receivers SHOULD assume
that the RTP senders only support generic NACKs. In addition, the
RTP receivers MAY send feedback using other suitable RTCP feedback
packets as defined for the respective media type. The RTP receivers
MUST NOT rely on the RTP senders reacting to any of the feedback
messages.

If one or more rtcp-fb attributes are present in a media session
description, the RTP receivers for the media session(s) containing
the "rtcp-fb"

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. MUST ignore all rtcp-fb attributes of which they do not fully
understand the semantics (i.e. understand the meaning of all
values in the a=fmtp:rtcp-fb line);

. SHOULD provide feedback information as specified in this document
using any of the RTCP feedback packets as specified in one of the
rtcp-fb attributes for this media session; and

. MUST NOT use other feedback messages than those listed in one of
the rtcp-fb attribute lines.

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RTP senders MUST be prepared to receive any kind of RTCP feedback
messages and MUST silently discard all those RTCP feedback messages
that they do not understand.

The syntax of the rtcp-fb attribute is as follows (the feedback types
and optional parameters are all case sensitive):

rtcp-fb-syntax = "a=fmtp:" <format> WS "rtcp-fb" WS rtcp-fb-value

rtcp-fb-value = "ack" rtcp-fb-param
| "nack" rtcp-fb-nack-param
| rtcp-fb-id rtcp-fb-param

rtcp-fb-id = 1*(alpha-numeric | "-" | "_")

rtcp-fb-param = "app"
| byte-string
| ; empty

The literals of the above grammar have the following semantics:

Feedback type "ack":

This feedback type indicates that positive acknowledgements for
feedback are supported.

The feedback type "ack" MUST only be used if the media session
is allowed to operate in ACK mode as defined in 3.6.1.2.

Parameters may be provided to further distinguish different
types of positive acknowledgement feedback. If no parameters
are present, the Generic ACK as specified in section 4.1.2 is
implied.

If the parameter "app" is specified, this indicates the use of
application layer feedback. In this case, additional parameters
following "app" MAY be used to further differentiate various

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types of application layer feedback. This document does not
define any parameters specific to "app".

Further parameters for "ack" MAY be defined in other documents.

Feedback type "nack":

This feedback type indicates that negative acknowledgements for
feedback are supported.

The feedback type "nack", without parameters, indicates use of

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The feedback type "nack", without parameters, indicates use of
the General NACK feedback format as defined in section 4.2.1.

The following three parameters are defined in this document for
use with "nack" in conjunction with the media type "video":

. "pli" indicates the use of Picture Loss Indication feedback
as defined in section 4.3.1.
. "sli" indicates the use of Slice Loss Indication feedback as
defined in section 4.3.2.
. "rpsi" indicates the use of Reference Picture Selection
Indication feedback as defined in section 4.3.3.
. "app" indicates the use of application layer feedback.
Additional parameters after "app" MAY be provided to
differentiate different types of application layer feedback.
No parameters specific to "app" are defined in this document.

Further parameters for "nack" MAY be defined in other documents.

Other feedback types <rtcp-fb-id>:

Other documents MAY define additional types of feedback; to keep
the grammar extensible for those cases, the rtcp-fb-id is
introduced as a placeholder. A new feedback scheme name needs
to be unique (and thus has to be registered with IANA). Along
with a new name, its semantics, packet formats (if necessary),
and rules for its operation need to be specified.

Note that it is assumed that more specific information about
application layer feedback (as defined in section 4.2.3) will be
conveyed as feedback types and parameters defined elsewhere. Hence,
no further provision for any types and parameters is made in this
document.

Further types of feedback as well as further parameters may be
defined in other documents.

It is up to the recipients whether or not they send feedback
information and up to the sender(s) to make use of feedback provided.

3.7.2.3

4.3 Unicasting

If an m= line in the SDP describing a session indicates unicast
addresses for a particular media type (and does not operate in multi-
unicast mode with all recipients listed explicitly but still

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addressed via unicast), the RTCP feedback MAY operate in ACK feedback
mode.

3.7.2.4

4.4 RTCP Bandwidth Modifiers

The standard RTCP bandwidth assignments as defined in [1] and [2] may
be overridden by bandwidth modifiers as specified in [4]: b=RS:<bw>

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and b=RR:<bw> MAY be used to assign a different bandwidth (measured
in bits per second) to RTP senders and receivers, respectively. The
precedence rules of [4] apply to determine the actual bandwidth to be
used by senders and receivers.

Applications operating knowingly over highly asymmetric links (such
as satellite links) SHOULD use this mechanism to reduce the feedback
rate for high bandwidth streams to prevent deterministic congestion
of the feedback path(s).

3.7.2.5

4.5 Examples

Example 1: The following session description indicates a session made
up from an audio and a DTMF for point-to-point communication in which
the DTMF stream uses Generic ACKs. This session description could be
contained in a SIP INVITE, 200 OK, or ACK message to indicate that
its sender is capable of and willing to receive feedback for the DTMF
stream it transmits.

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 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=fmtp:96 rtcp-fb ack

Example 2: The following session description indicates a multicast
video-only session (using H.263+) with the video source accepting
Generic NACKs and Reference Picture Selection. Such a description
may have been conveyed using Reference Picture Selection. Such a description
may have been conveyed using the Session Announcement Protocol (SAP).

v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multicast video with feedback
t=3203130148 3203137348
m=audio 49170 RTP/AVP 0
c=IN IP4 224.2.1.183
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=fmtp:98 rtcp-fb nack
a=fmtp:98 rtcp-fb nack rpsi

5. Interworking and Co-Existence of AVP and AVPF Entities

The AVPF profile defined in this document is an extension of the AVP
profile as defined in [2]. Both profiles follow the same basic rules

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(including the upper bandwidth limit for RTCP and the bandwidth
assignments to senders and receivers. Therefore, senders and
receivers of using either of the two profiles can be mixed in a
single session.

AVP and AVPF are defined in a way that, from a robustness point of
view,, the RTP entities do not need to be aware of entities of the
respective other profile: they will not disturb each other's
functioning. However, the quality of the media presented may suffer.

The following considerations apply to senders and receivers when used
in a combined session.

. AVP entities (senders and receivers)

AVP senders will receive RTCP feedback packets from AVPF receivers
and ignore these packets. They will see occasional closer spacing
of RTCP messages (e.g. violating the 5s rule) by AVPF entities.
As the overall bandwidth constraints are adhered to by both types
of entities, they will still get their share of the RTCP
bandwidth. However, while AVP entities are bound by the 5s rule,
depending on the group size and session bandwidth, AVPF entities
may provide more frequent RTCP reports than AVP ones will. Also,
the overall reporting may decrease slightly as AVPF entities are
may to send bigger RTCP packets (due to the extra fields).

. AVPF senders

AVPF senders will receive feedback information only from AVPF
receivers. If they rely on feedback to provide the target media
quality, the quality achieved for AVP receivers may be sub-
optimal.

. AVPF receivers

AVPF receivers SHOULD send immediate or early RTCP feedback
packets only if all (sending) entities in the media session
support AVPF. AVPF receivers MAY send feedback information as
part of regularly scheduled compound RTCP packets following the
timing rules of [1] and [2] also in media sessions operating in
mixed mode. In this case, however, the Session Announcement Protocol (SAP).

v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multicast video with receiver providing
feedback
t=3203130148 3203137348
m=audio 49170 RTP/AVP 0
c=IN IP4 224.2.1.183
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVP 98
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=fmtp:98 rtcp-fb nack
a=fmtp:98 rtcp-fb nack rpsi

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4. MUST NOT rely on the sender reacting to the feedback at
all.

6. Format of RTCP Feedback messages Messages

This section defines the format of the low delay RTCP feedback
messages. These messages classified into three categories as
follows:

- Transport layer feedback messages
- Payload-specific feedback messages
- Application layer feedback messages

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Transport layer feedback messages are intended to transmit general
purpose feedback information, i.e. information independent of the
particular codec or the application in use. The information is
expected to be generated and processed at the transport/RTP layer.
Currently, only a general positive acknowledgement (ACK) and negative
acknowledgement (NACK) message are defined.

Payload-specific feedback messages transport information that is
specific to a certain payload and will be generated and acted upon at
the codec "layer". This document defines a common header to be used
in conjunction with all payload-specific feedback messages. The
definition of specific messages is left to either RTP Payload Format
specifications or to additional feedback format documents.

Application layer feedback messages provide a means to transparently
convey feedback from the receiver's to the sender's application. The
information contained in such a message is not expected to be acted
upon at the transport/RTP or the codec layer. The data to be
exchanged between two application instances is usually defined in the
application protocol's specification and thus can be identified by
the application so that there is no need for additional external
information. Hence, this document defines only a common header to be
used along with all application layer feedback messages. From a
protocol point of view, an application layer feedback message is
treated as a special case of a payload-specific feedback message.

This document defines two transport layer feedback and three (video)
payload-specific feedback messages as well as a container for
application layer feedback messages. Additional transport layer and
payload specific feedback messages may be defined in other documents
and are registered through IANA (see section IANA considerations).

The general syntax and semantics for the above RTCP feedback message
types is described in the following subsections.

4.1

6.1 Common Packet Format for Feedback Message

All feedback message share a common packet format that is depicted in
figure 3:

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|E|
|V=2|P|0| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :

Figure 3: Common Packet Format for Feedback Messages

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The various fields V, P, SSRC and length are defined in the RTP
specification [2], the respective meaning being summarized below:

version (V): 2 bits
This field identifies the RTP version. The current version is 2.

padding (P): 1 bit
If set, the padding bit indicates that the packet contains
additional padding octets at the end which are not part of the
control information but are included in the length field.

Early RTCP (E): 1 bit
This bit MUST be set if the packet is sent as an Immediate
Feedback or as an Early RTCP packet.

Feedback message type (FMT): 4 bits
This field identifies the type of the feedback message and is
interpreted relative to the RTCP message type (transport,
payload-specific, or application feedback). The values for each
of the three feedback types are defined in the respective
sections below.

Payload type (PT): 8 bits
This is the RTCP packet type which identifies the packet as being
an RTCP Feedback Message. Two values are defined (TBA. By IANA):

Length: 16 bits
The length of this packet in 32-bit words minus one, including
the header and any padding. This is in line with the definition
of the length field used in RTCP sender and receiver reports [3].

SSRC of packet sender: 32 bits
The synchronization source identifier for the originator of this
packet.

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SSRC of media source: 32 bits
The synchronization source identifier of the media source that
this piece of feedback information is related to.

Feedback Control Information (FCI): variable length
The following three sections define which additional information
is included in the feedback message for each type of feedback.

4.2
Each RTCP feedback packet MUST contain exactly one FCI field of
the types defined in sections 6.2 and 6.3. If multiple FCI
fields (even of the same type) need to be conveyed, then several
RTCP feedback packets MUST be generated and concatenated in the
same compound RTCP packet.

6.2 Transport Layer Feedback Messages

Transport Layer Feedback messages are identified by the value RTPFB
as RTCP message type.

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Two general purpose transport layer feedback messages are defined so
far: General ACK and General NACK. They are identified by means of
the FMT parameter as follows:

0: forbidden
1: General Generic NACK
2: General Generic ACK
3-15:
3: Generic INFO
4-15: reserved

The following two subsections define the packet formats for these
messages.

4.2.1

6.2.1 Generic NACK

The Generic NACK message is identified by PT=RTPFB and FMT=1.

The Generic NACK packet is used to indicate the loss of one or more
RTP packets. The lost packet(s) are identified by the means of a
packet identifier and a bit mask.

The Feedback control information (FCI) field has the following
Syntax (figure 4):

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | BLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Syntax for the Generic NACK message

Packet ID (PID): 16 bits
The PID field is used to specify a lost packet. Typically, the
RTP sequence number is used for PID as the default format, but
RTP Payload Formats may decide to identify a packet differently.

bitmask of following lost packets (BLP): 16 bits
The BLP allows for reporting losses of any of the 16 RTP packets
immediately following the RTP packet indicated by the PID. The
BLP's definition is identical to that given in [10]. Denoting
the BLP's least significant bit as bit 1, and its most

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significant bit as bit 16, then bit i of the bit mask is set to 1
if the sender has not received RTP packet number PID+i (modulo
2^16) and the receiver decides this packet is lost; bit i is set
to 0 otherwise. Note that the sender MUST NOT assume that a
receiver has received a packet because its bit mask was set to 0.
For example, the least significant bit of the BLP would be set to
1 if the packet corresponding to the PID and the following packet
have been lost. However, the sender cannot infer that packets
PID+2 through PID+16 have been received simply because bits 2

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through 15 of the BLP are 0; all the sender knows is that the
receiver has not reported them as lost at this time.

4.2.2

6.2.2 Generic ACK

The Generic ACK message is identified by PT=RTPFB and FMT=2.

The Generic ACK packet is used to indicate that one or several RTP
packets were received correctly. The received packet(s) are
identified by the means of a packet identifier and a bit mask.
ACKing of a range of consecutive packets is also possible.

The Feedback control information (FCI) field has the following
syntax:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID |R| BLP/#packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Syntax for the Generic ACK message

Packet ID (1st PID): 16 bits
This PID field is used to specify a correctly received packet.
Typically, the RTP sequence number is used for PID as the default
format, but RTP Payload Formats may decide to identify a packet
differently.

Range of ACKs (R): 1 bit
The R-bit indicates that a range of consecutive packets are
received correctly. If R=1 then the PID field specifies the
first packet of that range and the next field (BLP/#packets) will
carry the number of packets being acknowledged. If R=0 then PID
specifies the first packet to be acknowledged and BLP/#packets
provides a bit mask to selectively indicate individual packets
that are acknowledged.

Bit mask of lost packets (BLP)/#packets (PID): 15 bits
The semantics of this field depends on the value of the R-bit.

If R=1, this field is used to identify the number of additional
packets of to be acknowledged:

#packets = <highest seq# to be ACKed> - <PID>

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That is, #packets MUST indicate the number of packet to be ACKed
minus one. In particular, if only a single packet is to be ACKed
and R=1 then #packets MUST be set to 0x0000.

Example: If all packets between and including PIDx=380 and PIDy =
422 have been received, the Generic ACK would contain PID = PIDx
= 380 and #packets = PIDy û PID = 42. In case the PID wraps

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around, modulo arithmetic is used to calculate the number of
packets.

If R=0, this field carries a bit mask. The BLP allows for
reporting reception of any of the 15 RTP packets immediately
following the RTP packet indicated by the PID. The BLP's
definition is identical to that given in [10] except that, here,
BLP is only 15 bits wide. Denoting the BLP's least significant
bit as bit 1, and its most significant bit as bit 15, then bit i
of the bitmask is set to 1 if the sender has received RTP packet
number PID+i (modulo 2^16) and the receiver decides to ACK this
packet; bit i is set to 0 otherwise. If only the packet
indicated by PID is to be ACKed and R=0 then BLP MUST be set to
0x0000.

4.3

6.2.3 Generic INFO

The Generic INFO message is identified by PT=RTPFB and FMT=3.

The Generic INFO packet MUST only be used in conjunction with an
application-specific feedback message. The Generic INFO message
indicates which RTP packets the payload-specific message is about.
The packet(s) in question are identified by the means of a packet
identifier and a bit mask.

The sole purpose of the Generic INFO packet is to avoid unnecessary
feedback suppression when payload-specific feedback messages are
mixed with generic ones.

The packet format is the same as for the Generic NACK message defined
in section 6.2.3.

6.3 Payload Specific Feedback Messages

Payload-Specific Feedback Messages are identified by the value PSFB
as RTCP message type.

Three payload-specific feedback messages are defined so far. They
are identified by means of the FMT parameter as follows:

0: forbidden
1: Picture Loss Indication (PLI)
2: Slice Lost Indication (SLI)
3: Reference Picture Selection Indication (RPSI)
4-14: reserved
15: Application layer feedback message

The following subsections define the packet formats for these
messages.

4.3.1

AVPF entities MUST include Generic INFO messages along with any
payload-specific ones in compound RTCP packets (early as well as
regularly scheduled ones). The INFO message(s) MUST cover all the

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RTP packets to which the payload-specific message(s) apply. This is
to avoid that AVPF entities that do not understand the payload-
specific messages unnecessarily suppress their feedback messages.

6.3.1 Picture Loss Indication (PLI)

The PLI feedback message is identified by PT=PSFB and FMT=1.

4.3.1.1

6.3.1.1 Semantics

With the Picture Loss Indication message a decoder informs the
encoder about the loss of one or more full pictures.

4.3.1.2

6.3.1.2 Message Format

PLI does not require parameters. Therefore, the length field MUST be
2, and there MUST NOT be any Feedback Control Information.

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4.3.1.3

6.3.1.3 Timing Rules

The timing follows the rules outlined in section 3. In systems that
employ both PLI and other types of feedback it may be advisable to
follow the regular RTCP RR timing rules for PLI, since PLI is not as
delay critical as other FB types.

4.3.1.4

6.3.1.4 Remarks

PLI messages typically trigger the sending of full Intra pictures.
Intra Pictures 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 (not an unrealistic
assumption, see [14] for details), then a full second of latency has
to be accepted. In such an environment there is no need for a
particular short delay in sending the feedback message. Hence
waiting for the next possible time slot allowed by RTCP timing rules
as per [2] does not have a negative impact on the system performance.

4.3.2

6.3.2 Slice Lost Indication (SLI)

The SLI feedback message is identified by PT=PSFB and FMT=2.

4.3.2.1

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6.3.2.1 Semantics

With the Slice Lost Indication a decoder can inform an encoder that
it was unable to decode one, or several consecutive, macroblocks.
The encoder can take appropriate action in order to re-synchronize
encoder and decoder by means of its choice, typically by sending the
lost macroblocks in Intra mode. This feedback message SHALL NOT be
used for video codecs with non-uniform, dynamically changeable
macroblock sizes such as H.263 with enabled Annex Q. In such a case,
an encoder cannot always identify the corrupted spatial region.

4.3.2.2

6.3.2.2 Format

When FBT indicates a Slice Lost Indication, then there is one
additional PCI field the content of which is depicted in figure 6.
The length of the feedback message MUST be set to 3.

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First | Number | TR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: Syntax of the Slice Lost Indication (SLI)

First: 13 bits
The macroblock (MB) address of the first lost macroblock. The MB
numbering is done such that the macroblock in the upper left
corner of the picture is considered macroblock number 1 and the
number for each macroblock increases from left to right and then
from top to bottom in raster-scan order (such that if there is a
total of N macroblocks in a picture, the bottom right macroblock
is considered macroblock number N).

Number: 13 bits
The number of lost macroblocks, in scan order as discussed above.

TR: 6 bits
The six least significant bits of the Temporal Reference of the
picture.

4.3.2.3

6.3.2.3 Timing Rules

The efficiency of algorithms using the Slice Lost Indication is
reduced greatly when the Indication is not transmitted in a timely
fashion. Motion compensation propagates corrupted pixels that are
not reported as being corrupted. Therefore, the use of the algorithm
discussed in section 3 is highly recommended.

4.3.2.4

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6.3.2.4 Remarks

The First field of the UCI defines the first macroblock of a picture
as 1 and not, as one could suspect, as 0. This was done to align
this specification with the comparable mechanism available in H.245.
The maximum number of macroblocks in a picture (2**13 or 8192)
corresponds to the maximum picture sizes of the ITU-T and ISO/IEC
video codecs. If future video codecs offer larger picture sizes
and/or smaller macroblock sizes, then an additional feedback message
has to be defined. The six least significant bits of the Temporal
Reference field are deemed to be sufficient to indicate the picture
in which the loss occurred.

Algorithms were reported that keep track of the regions effected by
motion compensation, in order to allow for a transmission of Intra
macroblocks to all those areas, regardless of the timing of the FB
(see H.263 (2000) Appendix I [13]] and [15]. While, when those
algorithms are used, the timing of the FB is less critical then
without, it has to be observed that those algorithms correct large
parts of the picture and, therefore, have to transmit many for bits
in case of delayed FBs.

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4.3.3

6.3.3 Reference Picture Selection Indication (RPSI)

The RPSI feedback message is identified by PT=PSFB and FMT=3.

4.3.3.1

6.3.3.1 Semantics

Modern video coding standards such as MPEG-4 visual version 2 [12] or
H.263 version 2 [13] allow the use of older reference pictures then
the most recent one. Typically, a first-in-first-out queue of
reference pictures is maintained. If an encoder has learned about a
loss of encoder-decoder synchronicity, a known-as-correct reference
picture can be used. As this reference picture is temporally further
away then usual, the resulting predictively coded picture will use
more bits.

Both MPEG-4 and H.263 define a binary format for the ôpayloadö of an
RPSI message that includes information such as the temporal ID of the
damaged picture and the size of the damaged region. This bit string
is typically small û- a couple of dozen bits -û, of variable length,
and self-contained, i.e. contains all information that is necessary
to perform reference picture selection.

Note that both MPEG-4 and H.263 allow the use of RPSI with positive
feedback information as well. That is, all corrected pictures are
reported. Any form of positive feedback MUST NOT be used when in a
multicast environment (reporting positive feedback about individual
reference pictures at RTCP intervals is not expected to be of much
use anyway). For point-to-point communication, positive feedback MAY
be used but, again, the bit rate budget of RTCP feedback will prevent
the use in most scenarios anyway.

4.3.3.2

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6.3.3.2 Format

When FB indicates an RPSI, then the length field is set to the number
of bits of the following bit string that contains the RPS
information. This bit string follows byte aligned in the UCI field.
Bit padding is used to achieve 32-bit word alignment of the UCI
message (and the whole packet).

4.3.3.3

6.3.3.3 Timing Rules

RPS is even more critical to delay then algorithms using SLI. This
is due to the fact that the older the RPS message is, the more bits
the encoder has to spend to achieve encoder-decoder synchronicity.
See [14] and [15] for some information about the overhead of RPS for
certain bit rate/frame rate/loss rate scenarios.

Therefore, RPS messages should typically be sent as soon as possible,
employing the algorithm of section 3.

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4.4

6.4 Application Layer Feedback Messages

Payload-Specific Feedback Messages are a special case of payload-
specific messages and identified by PT=PSFB and FMT=15.

These messages are used to transport application defined data
directly from the receiver's to the sender's application. The data
that is transported is not identified by the feedback message.
Therefore the application must be able to identify the messages
payload.

Usually applications define their own set of messages, e.g. NEWPRED
messages in MPEG-4 or feedback messages in H.263/Annex N,U. These
messages do not need any additional information from the RTCP
message. Thus the application message is simply placed into the FCI
field as follows and the length field is set accordingly.

Application Message (FCI): variable length
This field contains the original application message that should
be transported from the receiver to the source. The format is
application dependent. The length of this field is variable. If
the application data is not four-byte aligned, padding must be
added.

As there is no need for additional identification at the RTCP level,
the FMT field is unused and MUST be set to zero:

7. Early Feedback and Congestion Control

In the previous sections, the feedback messages were defined as well
as the timing rules according to which to send these messages. The
way to react to the feedback received depends on the application
using the feedback mechanisms and hence is beyond the scope of this
document.

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However, across all applications, there is a common requirement for
(TCP-friendly) congestion control on the media stream as defined in
[1] and [2] when operating in a best-effort network environment.

Low delay feedback supports the use of congestion control algorithms
in two ways:

. The potentially more frequent RTCP messagesallow messages allow the sender to
monitor the network state more closely than with regular RTCP
and therefore enable reacting to upcoming congestion in a more
timely fashion.

. The feedback messages themselves may convey additional
information as input to congestion control algorithms and thus
improve reaction over conventional RTCP. (For example, ACK-based
feedback may even allow to construct closed loop algorithms and

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NACK-based systems may provide further information on the packet
loss distribution.)

A congestion control algorithm that shares the available bandwidth
fair with competing TCP connections, e.g. TFRC [16], SHOULD be used
to determine the data rate for the media stream (if the low delay RTP
session is transmitted in a best effort environment).

RTCP feedback messages or RTCP SR/RR packets that indicate recent
packet loss MUST NOT lead to a (mid-term) increase in the
transmission data rate and SHOULD lead to a (short-term) decrease of
the transmission data rate. Such messages SHOULD cause the sender to
adjust the transmission data rate to the order of the throughput TCP
would achieve under similar conditions (e.g. using TFRC).

RTCP feedback messages or RTCP SR/RR packets that indicate no recent
packet loss MAY cause the sender to increase the transmission data
rate to roughly the throughput TCP would achieve under similar
conditions (e.g. using TFRC).

8. Security Considerations

RTP packets transporting information with the proposed payload for
mat are subject to the security considerations discussed in the RTP
specification [1]. [1] and in the RTP/AVP profile specification [2].
This implies that confidentiality of profile does not specify any different security services.

This profile modifies the media
streams is achieved by encryption.

If timing behavior of RTCP and eliminates the entire stream (extension data
minimum RTCP interval of 5 seconds and AU data) is allows for earlier feedback to
be secured provided by receivers. This approach does not increase the
potential for denial-of-service attacks beyond those discussed in [1]
and all [2].

Feedback information is suppressed if unknown RTCP feedback packets
are received. This introduces the participants risk of a malicious group member
eliminating all early feedback by simply transmitting payload-
specific RTCP feedback packets with random contents that are expected to have neither

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recognized by any receiver (so they will suppress feedback) nor by
the keys to decode sender (so no repair actions will be taken).

A malicious group member can also report arbitrary high loss rates in
the
entire stream, then feedback information to make the encryption is performed in sender throttle the usual manner, data
transmission and there is no conflict between increase the two operations (encapsulation
and encryption).

The need for a portion amount of stream (e.g. extension data) redundancy information or
take other action to be
encrypted deal with the pretended packet loss. This may
result in a different key, or not to be encrypted, would require
application level signaling protocols to be aware degradation of the usage quality of the XT field, reproduced media
stream.
Finally, a malicious group member can act as a large number of group
members and to exchange keys thereby obtain an artificially large share of the early
feedback bandwidth and negotiate their usage on reduce the
media reactivity of the other group
members -- possibly even causing them to no longer operate in
immediate or early feedback mode and extension data separately.

7. thus undermining the whole
purpose of this profile.

9. IANA Considerations

The feedback profile as an extension to the profile for audio-visual
conferences with minimal control needs to be registered: "AVPF". "RTP/AVPF".

For the Session Description Protocol, the following "fmtp:" attribute
needs to be registered: "rtcp-fb".

Along with "rtcp-fb", the feedback types "ack" and "nack" need to be
registered.

Along with "nack", the feedback type parameters "sli", "pli", and
"rpsi" need to be registered.

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Two RTCP Control Packet Types: for the class of transport layer
feedback messages ("RTPFB") and for the class of payload-specific
feedback messages ("PSFB").

Within the RTPFB range, three format (FMT) values need to be
registered:

0: forbidden
1: General NACK
2: General ACK

Within the PSFB range, five format (FMT) values need to be
registered:

0: forbidden
1: Picture Loss Indication (PLI)
2: Slice Loss Indication (SLI)
3: Reference Picture Selection Indication (SLI)
15: Application layer feedback (AFB)

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10. Acknowledgements

This document is a product of the Audio-Visual Transport (AVT)
Working Group of the IETF. The authors would like to thank Steve
Casner and Colin Perkins for their comments and suggestions as well
as for their responsiveness to numerous questions.

11. Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works.

However, this document itself may not be modified in any way, such as
by removing the copyright notice or references to the Internet Soci-
ety or other Internet organizations, except as needed for the purpose
of developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be fol-
lowed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

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10.

12. Authors' Addresses

J÷rg Ott {sip,mailto}:jo@tzi.uni-bremen.de {sip,mailto}:jo@tzi.org
Universit„t Bremen TZI
MZH 5180
Bibliothekstr. 1
D-28359 Bremen
Germany

Stephan Wenger stewe@cs.tu-berlin.de
TU Berlin
Sekr. FR 6-3
Franklinstr. 28-29
D-10587 Berlin
Germany

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Shigeru Fukunaga
Oki Electric Industry Co., Ltd.
1-2-27 Shiromi, Chuo-ku, Osaka 540-6025 Japan
Tel. +81 6 6949 5101
Fax. +81 6 6949 5108
Mail fukunaga444@oki.co.jp fukunaga444@oki.com

Noriyuki Sato
Oki Electric Industry Co., Ltd.
1-2-27 Shiromi, Chuo-ku, Osaka 540-6025 Japan
Tel. +81 6 6949 5101
Fax. +81 6 6949 5108
Mail sato652@oki.co.jp sato652@oki.com

Koichi Yano
FastForward Networks,
75 Hawthorne St. #601
San Francisco, CA 94105
Tel. +1.415.430.2500

Akihiro Miyazaki
Matsushita Electric Industrial Co., Ltd
1006, Kadoma, Kadoma City, Osaka, Japan
Tel. +81-6-6900-9192
Fax. +81-6-6900-9193
Mail akihiro@isl.mei.co.jp

Koichi Hata
Matsushita Electric Industrial Co., Ltd
1006, Kadoma, Kadoma City, Osaka, Japan
Tel. +81-6-6900-9192
Fax. +81-6-6900-9193
Mail hata@isl.mei.co.jp

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Rolf Hakenberg
Panasonic European Laboratories GmbH
Monzastr. 4c, 63225 Langen, Germany
Tel. +49-(0)6103-766-162
Fax. +49-(0)6103-766-166
Mail hakenberg@panasonic.de

Carsten Burmeister
Panasonic European Laboratories GmbH
Monzastr. 4c, 63225 Langen, Germany
Tel. +49-(0)6103-766-263
Fax. +49-(0)6103-766-166
Mail burmeister@panasonic.de

11. Bibliography

[1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP -
A Transport Protocol for Real-time Applications," Internet
Draft, draft-ietf-avt-rtp-new-09.txt, draft-ietf-avt-rtp-new-10.txt, Work in Progress, March July
2001.

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[2] H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control," Internet Draft draft-ietf-
avt-profile-new-10.txt, March
avt-profile-new-11.txt, July 2001.

[3] M. Handley and V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.

[4] S. Casner, "SDP Bandwidth Modifiers for RTCP Bandwidth",
Internet Draft draft-ietf-avt-rtcp-bw-03.txt, March July 2001.

[5] C. Perkins and O. Hodson, "2354 Options for Repair of Streaming
Media," RFC 2354, June 1998.

[6] J. Rosenberg and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction,", RFC 2733, December 1999.

[7] C. Perkins, I. Kouvelas, O. Hodson, V. Hardman, M. Handley, J.C.
Bolot, A. Vega-Garcia, and S. Fosse-Parisis, "RTP Payload for
Redundant Audio Data," RFC 2198, September 1997.

[8] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119, March 1997.

[9] H. Schulzrinne and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals," RFC 2833, May 2000.

[10] T. Turletti and C. Huitema, "RTP Payload Format for H.261 Video
Streams, RFC 2032, October 1996.

[11] C. Bormann, L. Cline, G. Deisher, T. Gardos, C. Maciocco, D.
Newell, J. Ott, G. Sullivan, S. Wenger, and C. Zhu, "RTP Payload
Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+),"
RFC 2429, October 1998.

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[12] ISO/IEC 14496-2:1999/Amd.1:2000, "Information technology -
Coding of audio-visual objects - Part2: Visual", July 2000.

[13] ITU-T Recommendation H.263, "Video Coding for Low Bit Rate
Communication," November 2000.

[14] S. Wenger, "Media-aware Protocols -- transport aware Media
Coding," Habilitation thesis, in preparation, 2001.

[15] B. Girod, N. Faerber, "Feedback-based error control for mobile
video transmission," Proceedings IEEE, Vol. 87, No. 10, pp. 1707
û 1723, October, 1999.

[16] M. Handley, J. Padhye, S. Floyd, J. Widmer, "TCP friendly Rate
Control (TFRC): Protocol Specification," Internet Draft, draft-
ietf-tsvwg-02.txt, Work in Progress, May 2001.

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Appendix A. Some Background and Motivation (Informative)

A.1 Example: Predictive Video Coding

A.1.1 Video Encoder-decoder synchronicity

Most current video coding schemes for compressed video, such as the
ITU-T H.261 and H.263 and ISO/IEC MPEG[124] employ a mechanism known
as Inter Picture Prediction. Each picture is divided into
macroblocks of uniform size. For each macroblock, one or more
motion vectors may be identified and transmitted. The residual
signal after motion compensation is DCT-transformed, quantized,
entropy coded, and transmitted as well. The encoder reconstructs,
based on this information, a so-called reference picture, which is
used to perform the motion compensation and residual signal coding
steps for the subsequent picture. Since the reference picture is
generated using only such information that is also available at the
decoder, the reference picture is identical to the reconstructed
picture at the decoder. Having identical reference pictures at the
encoder and decoder is referred to as encoder-decoder-synchronicity.

Whenever data is damaged or lost on the way between the encoder and
the decoder, the reconstructed picture at the decoder is no more
identical with the encoder's reference picture -- the encoder-decoder
synchronicity is lost.

Any loss of the encoder-decoder synchronicity results in annoying
artifacts at the decoder. Because the prediction of subsequent
pictures in the decoder is based on a damaged reference picture, the
annoying artifacts are present not only in the picture in which the
loss occurred; they propagate to all subsequent pictures, until,
through source coding based mechanisms, the encoder-decoder
synchronicity is restored. Therefore, the goal of systems employing
predictive video coding in a lossy environment must be to keep the
encoder-decoder synchronicity, or, if this is not possible, to regain
that synchronicity as quickly as possible.

A.1.2. Non-feedback based mechanisms

Avoiding the loss of the encoder-decoder synchronicity corresponds to
avoiding the loss of coded picture data. Such a task can be
performed on the transport layer. In RTP environments, the use of
packet-based FEC is a good example for such a technique. (The use of
TCP or reliable multicast as the transport for media streams would be
an even better one but is inappropriate for low-delay (interactive)
real-time systems.) FEC schemes, interleaving, and other means for
repairing real-time media streams may also add additional delay and
significant bit rate overhead without being able to guarantee
compensation of virtually all packet losses.

Once the encoder-decoder synchronicity is lost, only source coding
oriented mechanisms can help to regain it. One common way is to send
a non-predictively coded picture (known as Intra picture). Intra
pictures have the disadvantage of being several times bigger than

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predictively coded pictures (Inter pictures). Therefore, sending
Intra pictures has negative implications both on the bandwidth and

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(in bandwidth limited environments) delay. Another way is to use
Intra macroblock refresh. Here, certain parts of the picture (those
affected by a packet loss) are coded non-predictively in order to
resynchronize the encoder and decoder over time. Intra macroblock
refresh has better delay characteristics then full Intra pictures
because the picture size can be kept constant, but is less efficient
in terms of bit rate/distortion than full Intra pictures. More
sophisticated means such as Reference Picture Selection (RPS) are
also available in modern video coding standards.

Systems not employing feedback channels may use any combination of
the mechanisms described above to add error resilience -- at the cost
of added bit rate and, sometimes, added delay. The number of
additional bits spent for error resilience can be adapted using the
long-term packet loss rate information in the RTCP receiver reports.
But, even when using such adaptive means, it is still likely that
systems spend many more bits then theoretically necessary to achieve
error resilience in order to be on the safe side. Plus, as regular
RTCP feedback is aimed at longer terms, reactivity to sudden losses
is limited. In all practical applications today this means that
fewer bits are available for non redundant picture data, and hence
the overall picture quality suffers.

A.1.3 Feedback based systems

Feedback-based systems try to avoid spending too many bits for
redundant information by informing the encoder about a loss situation
at the decoder(s). The encoder can then react accordingly and spend
redundant bits only when needed possibly only for the part of the
picture that was effected by the loss -- thereby reducing the number
of redundant bits and leaving more bits for useful information. As a
result, a higher reproduced picture quality can generally be expected
when feedback channels are available.

Similar to the observations of section 2.1.2, transport and source
coding based mechanisms can be distinguished that react on loss
situations reported by feedback.

Transport based systems employing feedback react media unaware, by
re-transmitting lost packets. TCP is a good example for a protocol
following such a scheme. Transport-based feedback in real-time
and/or multicast environments is a complex matter and subject of a
lot of engineering and research in and outside of the IETF. This
specification is not concerned with pure transport-based feedback.

Source coding based mechanisms may react upon the arrival of a
feedback message indicating a loss situation by adding bits that
restore, or at least make an effort to restore, the encoder-decoder
synchronicity. This process has to be performed by a real-time
encoder. However, schemes were reported, that allow the use of
feedback also for non-real-time encoders by storing multiple

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representations of the same data (e.g. Inter and Intra coded), and
dynamically switching between those representations.

Several types of feedback messages, called Feedback Messages or FB
messages, can be defined for such a case. An FB message can be as

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simple as a Boolean condition, indicating for example the loss of a
full picture (and, therefore, the need of a full Intra picture
transmission). Other feedback messages may contain more complex
information such as information about the damage of a spatial region
of the picture. A special form consists of a message the format and
semantics of which are not known at the transport level, because they
are defined in the video codec standards.

A.2 Feedback Messages

Most FB messages contain negative acknowledge information, indicating
an erroneous situation at the decoder. In others, the nature of the
acknowledge (positive, negative, or both) is part of the feedback
message itself. When used in multicast environments, positive
acknowledge must not be used.

This document assumes that feedback messages are transmitted using
RTCP packets. RTCP messages from the receivers to the sender cannot
be sent at any possible time, in order to prevent traffic explosion
in case of large multicast groups. Instead, the bit rate for all
RTCP messages of all receivers together has to obey a maximum
fraction of the total RTP session bit rate, yielding a very limited
bit rate budget for a single receiver when having a large multicast
group. This, in turn, leads to an increased average delay when the
size of the receiving multicast group grows. (see section 6 of [1]
for details)

This specification defines an algorithm that adheres to the bit rate
limitations for the feedback channel on the long term, but allows
short-term overdrafting for any receiver (but not all of them
simultaneously). Thus, the algorithm allows for better real-time
performance then the one specified in [1]. Traffic explosion in such
cases in which many receivers identify a picture damage
simultaneously is prevented by dithering.

As this specification assumes a sender that has full control over its
transmission bit rate (e.g. a real-time encoder), there is no scaling
problem on the forward channel. Any reaction to negative feedback
generates additional bits, which have to be conveyed but this is
taken from the senderÆs total bit rate budget. The encoder can take
this into account by, for example, changing the encoding mode, packet
size, and so forth. The sender is also free to simply ignore
feedback messages. Adjusting the tradeoff between the reproduced
media quality of all receivers of a multicast group and the amount of
additional repair traffic is a media-dependent, very complex task and
is not covered in this specification.

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Finally, frequent RTCP-based feedback messages may provide additional
input to the sender(s)'s congestion control algorithms and thus
improve its reactivity towards network congestion.

Feedback messages as well as sender and receiver behavior are to be
specified in separate documents (such as [7]). Such specifications
need to consider that, frequently, packet loss is an indication of
network congestion and thus define mechanisms for media-specific

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congestion control in the presence of feedback as defined in this
memo.

A.3. Applications and Relationships to other Standards

This specification is based on RTCP, which implies its use in an RTP
environment. RTP itself is used in a variety of systems such as in
SIP- or H.323-based multimedia conferencing/telephony, SAP-announced
Mbone conferences, and RTSP-based media streaming.

As for the video codecs, there is currently a small set of standards
that are, for the purpose of this discussion, roughly comparable.
Many mechanisms for regaining encoder-decoder synchronicity are
applicable to all video codecs. Others require certain tools (such
as Reference Picture Selection, aka NEWPRED) that are available only
in certain versions of the standards, and/or optional tools whose use
must be negotiated prior to being used.

A few RTP payload specifications such as RFC 2032 [10] already define
a feedback mechanism for some of the coding algorithms considered in
this specification. An application capable of performing both
schemes MUST use the feedback mechanism defined in this
specification, although, for backward compatibility reasons, it MUST
also be capable to conform to the feedback scheme defined in the
respective RTP payload format, if this is required by that payload
format.

Also, audio, DTMF, and text streams could benefit from more immediate
feedback even though the redundancy payload formats work well for
these media.

All kinds of non-interactive media streams (such as RTSP-controlled
media streaming applications) could benefit significantly as without
interactivity there is more time available for media repair.

A.4 Remarks on the size of the multicast group

This specification prevents traffic explosion on the feedback channel
in a very similar way as RTP does, with the exception of allowing
individual receivers to overdraft their bit rate budget from time to
time. This is necessary in order to allow for low delay, which is
needed by the algorithms reacting to Feedback messages.

This scaling, however, limits the usefulness of this mechanism in
multicast groups from a certain size upwards (where the size

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threshold depends on a number of parameters including loss rate,
frame rate, number of packets per frame, and session bandwidth). The
maximum size of the multicast group is soft and also depends on
application requirements and is therefore not specified here.
Considerations on the multicast group sizes are presented in section
3.5.

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