Internet DRAFT - draft-ietf-avt-rtcp-feedback
draft-ietf-avt-rtcp-feedback
INTERNET-DRAFT Joerg Ott/Uni Bremen TZI
draft-ietf-avt-rtcp-feedback-11.txt Stephan Wenger/TU Berlin
Noriyuki Sato/Oki
Carsten Burmeister/Matsushita
Jose Rey/Matsushita
10 August 2004
Expires February 2005
Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)
Status of this Memo
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Copyright (C) The Internet Society (2004). All Rights Reserved.
This document is a product of the Audio-Visual Transport (AVT)
working group of the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing
list at avt@ietf.org and/or the authors.
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Abstract
Real-time media streams that use RTP are, to some degree, resilient
against packet losses. Receivers may use the base mechanisms of
RTCP to report packet reception statistics and thus allow a sender
to adapt its transmission behavior in the mid-term as sole means
for feedback and feedback-based error repair (besides a few codec-
specific mechanisms). This document defines an extension to the
Audio-visual Profile (AVP) that enables receivers to provide,
statistically, more immediate feedback to the senders and thus
allow for short-term adaptation and efficient feedback-based repair
mechanisms to be implemented. This early feedback profile (AVPF)
maintains the AVP bandwidth constraints for RTCP and preserves
scalability to large groups.
Table of Contents
1 Introduction..................................................3
1.1 Definitions.............................................4
1.2 Terminology.............................................6
2 RTP and RTCP Packet Formats and Protocol Behavior.............6
2.1 RTP.....................................................6
2.2 Underlying Transport Protocols..........................7
3 Rules for RTCP Feedback.......................................8
3.1 Compound RTCP Feedback Packets..........................8
3.2 Algorithm Outline.......................................9
3.3 Modes of Operation.....................................10
3.4 Definitions and Algorithm Overview.....................12
3.5 AVPF RTCP Scheduling Algorithm.........................15
3.5.1 Initialization...................................15
3.5.2 Early Feedback Transmission......................16
3.5.3 Regular RTCP Transmission........................19
3.5.4 Other Considerations.............................20
3.6 Considerations on the Group Size.......................20
3.6.1 ACK mode.........................................20
3.6.2 NACK mode........................................21
3.7 Summary of decision steps..............................22
3.7.1 General Hints....................................22
3.7.2 Media Session Attributes.........................23
4 SDP Definitions..............................................24
4.1 Profile identification.................................24
4.2 RTCP Feedback Capability Attribute.....................24
4.3 RTCP Bandwidth Modifiers...............................28
4.4 Examples...............................................28
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5 Interworking and Co-Existence of AVP and AVPF Entities.......30
6 Format of RTCP Feedback Messages.............................31
6.1 Common Packet Format for Feedback Messages.............32
6.2 Transport Layer Feedback Messages......................34
6.2.1 Generic NACK.....................................34
6.3 Payload Specific Feedback Messages.....................36
6.3.1 Picture Loss Indication (PLI)....................36
6.3.1.1 Semantics...............................36
6.3.1.2 Message Format..........................37
6.3.1.3 Timing Rules............................37
6.3.1.4 Remarks.................................37
6.3.2 Slice Lost Indication (SLI)......................37
6.3.2.1 Semantics...............................38
6.3.2.2 Format..................................38
6.3.2.3 Timing Rules............................39
6.3.2.4 Remarks.................................39
6.3.3 Reference Picture Selection Indication (RPSI)....39
6.3.3.1 Semantics...............................40
6.3.3.2 Format..................................40
6.3.3.3 Timing Rules............................41
6.4 Application Layer Feedback Messages....................41
7 Early Feedback and Congestion Control........................42
8 Security Considerations......................................43
9 IANA Considerations..........................................44
10 Acknowledgements............................................48
11 Authors' Addresses..........................................48
12 Bibliography................................................49
12.1 Normative references...................................49
12.2 Informative References.................................50
13 Disclaimer of Validity......................................51
14 Full Copyright Statement....................................51
15 Acknowledgment..............................................52
1 Introduction
Real-time media streams that use RTP are, to some degree, 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
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coding scheme and transmission behavior to the observed network
QoS. However, except for a few payload specific mechanisms [6],
RTP makes no provision for timely feedback that would allow a
sender to repair the media stream immediately: through
retransmissions, retro-active FEC control, or media-specific
mechanisms for some video codecs, such as reference picture
selection.
Current mechanisms available with RTP to improve error resilience
include audio redundancy coding [13], video redundancy coding [14],
RTP-level FEC [11], and general considerations on more robust media
streams transmission [12]. These mechanisms may be applied pro-
actively (thereby increasing the bandwidth of a given media
stream). Alternatively, in sufficiently small groups with small
RTTs, the senders may perform repair on-demand, using the above
mechanisms and/or media-encoding-specific approaches. Note that
"small group" and "sufficiently small 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
concept of 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. A small number
of general-purpose feedback messages as well as a format for codec
and application-specific feedback information are defined for
transmission in the RTCP payloads.
1.1 Definitions
The definitions from RTP/RTCP [1] and the RTP Profile for Audio and
Video Conferences with Minimal Control [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 often (but not always) capable of reporting events of
interest back to the sender close to their occurrence. In
Early RTCP mode, RTCP packets 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 if following the scheduling
algorithm of [1], the reason being an "event" observed by a
receiver. Early RTCP packets may be sent in Immediate
Feedback and in Early RTCP mode. Sending an Early RTCP
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packet is also referred to as sending Early Feedback in
this document.
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 useful to be reported back to the sender by means of a
Feedback message.
Feedback (FB) message:
An RTCP message as defined in this document is used to
convey information about events observed at a receiver --
in addition to long-term receiver status information which
is carried in RTCP RRs -- back to the sender of the media
stream. For the sake of clarity, feedback message is
referred to as FB message throughout this document.
Feedback (FB) threshold:
The FB threshold indicates the transition between Immediate
Feedback and Early RTCP mode. For a multiparty 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. by means of an Early
RTCP packet without having to wait for its regularly
scheduled RTCP interval. This threshold is highly
dependent on the type of feedback to be provided, network
QoS (e.g. packet loss probability and distribution), codec
and packetization scheme in use, the session bandwidth, and
application requirements. Note that the algorithms do not
depend on all senders and receivers agreeing on the same
value for this threshold. It is merely intended to provide
conceptual guidance to application designers and is not
used in any calculations. For the sake of clarity, the term
feedback threshold is referred to as FB threshold
throughout this document.
Immediate Feedback mode:
A mode of operation in which each receiver of a media
stream is, statistically, capable of reporting each event
of interest immediately back to the media stream sender.
In Immediate Feedback mode, RTCP FB messages are
transmitted according to the timing rules defined in this
document.
Media packet:
A media packet is an RTP packet.
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Regular RTCP mode:
Mode of operation in which no preferred transmission of FB
messages is allowed. Instead, RTCP messages are sent
following the rules of [1]. Nevertheless, such RTCP
messages may contain feedback information as defined in
this document.
Regular RTCP packet:
An RTCP packet that is not sent as an Early RTCP packet.
RTP sender:
An RTP sender is an RTP entity that transmits media packets
as well as RTCP packets and receives Regular as well as
Early RTCP (i.e. feedback) packets. Note that the RTP
sender is a logical role and that the same RTP entity may
at the same time act as an RTP receiver.
RTP receiver:
An RTP receiver is an RTP entity that receives media
packets as well as RTCP packets and transmits Regular as
well as Early RTCP (i.e. feedback) packets. Note that the
RTP receiver is a logical role and that the same RTP entity
may at the same time act as an RTP sender.
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
[5].
2 RTP and RTCP Packet Formats and Protocol Behavior
2.1 RTP
The rules defined in [2] also apply to this profile except for
those rules mentioned in the following:
RTCP packet types:
Two additional RTCP packet types are registered and the
corresponding FB messages to convey feedback information
are defined in section 6 of this memo.
RTCP report intervals:
This document describes three modes of operation which
influence the RTCP report intervals (see section 3.2 of
this memo). In Regular RTCP mode, all rules from [1] apply
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except for the recommended minimal interval of five seconds
between two RTCP reports from the same RTP entity. In both
Immediate Feedback and Early RTCP modes, the minimal
interval of five seconds between two RTCP reports is
dropped and, additionally, the rules specified in section 3
of this memo apply if RTCP packets containing FB messages
(defined in section 4 of this memo) are to be transmitted.
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 7, further consideration is given to the
impact of feedback and a sender's reaction to FB messages.
2.2 Underlying Transport Protocols
RTP is intended to be used on top of unreliable transport
protocols, including UDP and DCCP. This section briefly describes
the specifics beyond plain RTP operation introduced by RTCP
feedback as specified in this memo.
UDP: UDP provides best effort delivery of datagrams for point-
to-point as well as for multicast communications. UDP does
not support congestion control or error repair. The RTCP-
based feedback defined in this memo is able to provide
minimal support for limited error repair. As RTCP feedback
is not guaranteed to operate on sufficiently small
timescales (in the order of RTT), RTCP feedback is not
suitable to support congestion control. This memo
addresses both unicast and multicast operation.
DCCP: DCCP [19] provides for congestion-controlled but unreliable
datagram flows for unicast communications. With TFRC-based
[20] congestion control (CCID 3), DCCP is particularly
suitable for audio and video communications. DCCP's
acknowledgement messages may provide detailed feedback
reporting about received and missed datagrams (and thus
about congestion).
When running RTP over DCCP, congestion control is performed
at the DCCP layer and no additional mechanisms are required
at the RTP layer. Furthermore, an RTCP-feedback capable
sender may leverage the more frequent DCCP-based feedback
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and thus a receiver may abstain from using (additional)
Generic Feedback messages where appropriate.
3 Rules for RTCP Feedback
3.1 Compound RTCP Feedback Packets
Two components constitute RTCP-based feedback as described in this
document:
o Status reports are contained in SR/RR packets 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.
o FB 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 FB messages are newly
introduced in this document.
RTCP 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.
Compound RTCP packets containing FB messages as defined in this
document MUST contain RTCP packets in the order defined in [1]:
o OPTIONAL encryption prefix that MUST be present if the RTCP
packet(s) is to be encrypted according to section 9.1 of [1].
o MANDATORY SR or RR.
o MANDATORY SDES which MUST contain the CNAME item; all other SDES
items are OPTIONAL.
o One or more FB messages.
The FB message(s) MUST be placed in the compound packet after 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:
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
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CNAME item present, and the FB 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 FB message
is sent as part of an Early RTCP packet. This packet type is
referred to as minimal compound RTCP packet in this document.
b) (Full) compound RTCP feedback packet
A (full) compound RTCP feedback packet MAY contain any
additional number of RTCP packets (additional RRs, further SDES
items, etc.). The above ordering rules MUST be adhered to.
This packet format MUST be used whenever an RTCP FB message is
sent as part of a Regular RTCP packet or in Regular RTCP mode.
It MAY also be used to send RTCP FB messages in Immediate
Feedback or Early RTCP mode. This packet type is referred to as
full compound RTCP packet in this document.
RTCP packets that do not contain FB messages are referred to as
non-FB RTCP packets. Such packets MUST follow the format rules in
[1].
3.2 Algorithm Outline
FB messages are part of the RTCP control streams and thus subject
to the RTCP bandwidth constraints. 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 events (e.g.
loss or reception of RTP packets) and to accommodate algorithms
that use FB messages.
The modified RTCP transmission algorithm can be outlined as
follows: As long as no FB messages have to be conveyed, compound
RTCP packets are sent following the rules of RTP [1] -- except that
the five second minimum interval between RTCP reports is not
enforced. Hence, the interval between RTCP reports is only derived
from the average RTCP packet size and the RTCP bandwidth share
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available to the RTP/RTCP entity. Optionally, a minimum interval
between Regular RTCP packets may be enforced.
If a receiver detects the need to send an FB message, it may do so
earlier than the next regular RTCP reporting interval (for which it
would be scheduled following the above regular RTCP algorithm).
Feedback suppression is used to avoid feedback implosion in
multiparty sessions: The receiver waits for a (short) random
dithering interval to check whether it sees a corresponding FB
message from any other receiver reporting the same event. Note
that for point-to-point sessions there is no such delay. If a
corresponding FB message from another member is received, this
receiver refrains from sending the FB message and continues to
follow the Regular RTCP transmission schedule. In case the
receiver has not yet seen a corresponding FB message from any other
member, it checks whether it is allowed to send Early feedback. If
sending Early feedback is permissible , the receiver sends the FB
message as part of a minimal compound RTCP packet. The permission
to send Early feedback depends on the type of the previous RTCP
packet sent by this receiver and the time the previous Early
feedback message was sent.
FB messages may also be sent as part of full compound RTCP packets
which are transmitted as per [1] (except for the five second lower
bound) in regular intervals.
3.3 Modes of Operation
RTCP-based feedback may operate in one of three modes (figure 1) as
described below. The mode of operation is just an indication of
whether or not the receiver will, on average, be able to report all
events to the sender in a timely fashion; the mode does not
influence the algorithm used for scheduling the transmission of FB
messages. And, depending on the reception quality and the locally
monitored state of the RTP session, individual receivers may not
(and not have to) agree on a common perception on the current mode
of operation.
a) Immediate Feedback mode: the group size is below the FB
threshold which gives each receiving party sufficient bandwidth
to transmit the RTCP feedback packets for the intended purpose.
This means that, for each receiver, there is enough bandwidth
to report each event 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,
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and the (worst case or observed) frequency of events to report
(e.g. frame received, packet lost).
As a rough estimate, let N be the average number of events to
be reported per interval T by a receiver, B the RTCP bandwidth
fraction for this particular receiver and R the average RTCP
packet size, then the receiver operates in Immediate Feedback
mode as long as N<=B*T/R.
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 media playback quality.
Using the above notation, Early RTCP mode can be roughly
characterized by N > B*T/R as "lower bound". An estimate for
an upper bound is more difficult. Setting N=1, we obtain for a
given R and B the interval T = R/B as average interval between
events to be reported. This information can be used as a hint
to determine whether or not early transmission of RTCP packets
is useful.
c) Regular RTCP Mode: From some group size upwards, it is no
longer useful to provide feedback for individual events from
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.
No precise group size threshold can be specified at which this
mode starts but, obviously, this boundary matches the upper
bound of the Early RTCP mode as specified in item b).
As the feedback algorithm described in this document scales
smoothly, there is no need for an agreement among the participants
on the precise values of the respective FB thresholds within the
group. Hence the borders between all these modes are soft.
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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
As stated before, the respective FB thresholds depend on a number
of technical parameters (of the codec, the transport, the type of
feedback used, etc.) but also on the respective application
scenarios. Section 3.6 provides some useful hints (but no precise
calculations) on estimating these thresholds.
3.4 Definitions and Algorithm Overview
The following pieces of state information need to be maintained per
receiver (largely taken from [1]). Note that all variables (except
in item h) below) are calculated independently at each receiver.
Therefore, their local values may differ at any 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 tn and tp be the time for the next (last) scheduled
RTCP RR transmission calculated prior to reconsideration.
d) Let Tmin be the minimal interval between RTCP packets as per
[1]. Unlike in [1], the initial Tmin is set to 1 second to
allow for some group size sampling before sending the first RTCP
packet. After the first RTCP packet is sent, Tmin is set to 0.
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 Regular RTCP packet. This value is
obtained following the rules of [1] but with Tmin as defined in
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this document: T_rr = T (the "calculated interval" as defined in
[1]) with tn = tp + T. T_rr always refers to the last value of
T that has been computed (because of reconsideration or to
determine tn). T_rr is also referred to as Regular RTCP interval
in this document.
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 in multiparty sessions; the value for T_dither_max is
dynamically calculated based upon T_rr (or may be derived by
means of another mechanism common across all RTP receivers to be
specified in the future). For point-to-point sessions (i.e.
sessions with exactly two members with no change in the group
size expected, e.g. unicast streaming sessions), T_dither_max is
set to 0.
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. This value is application-specific; and no values are
defined in this document.
i) Let te be the time for which a feedback packet is scheduled.
j) Let T_fd be the actual (randomized) delay for the transmission
of FB message in response to an event at time t0.
k) Let allow_early be a Boolean variable that indicates whether the
receiver currently may transmit FB 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
set to FALSE after Early feedback transmission and is set to
TRUE as soon as the next Regular RTCP transmission takes place.
l) Let avg_rtcp_size be the moving average on the RTCP packet size
as defined in [1].
m) Let T_rr_interval be an OPTIONAL minimal interval to be used
between Regular RTCP packets. If T_rr_interval == 0, then this
variable does not have any impact on the overall operation of
the RTCP feedback algorithm. If T_rr_interval != 0 then the
next Regular RTCP packet will not be scheduled T_rr after the
last Regular RTCP transmission (i.e. at tp+T_rr). Instead, the
next Regular RTCP packet will be delayed until at least
T_rr_interval after the last Regular RTCP transmission, i.e. it
will be scheduled at or later than tp+T_rr_interval. Note that
T_rr_interval does not affect the calculation of T_rr and tp;
instead, Regular RTCP packets scheduled for transmission before
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tp+T_rr_interval will be suppressed if, for example, they do not
contain any FB messages. The T_rr_interval does not affect
transmission scheduling of Early RTCP packets.
NOTE: Providing T_rr_interval as an independent variable is meant
to minimize Regular RTCP feedback (and thus bandwidth consumption)
as needed by the application while additionally allowing the use of
more frequent Early RTCP packets to provide timely feedback. This
goal could not be achieved by reducing the overall RTCP bandwidth
as RTCP bandwidth reduction would also impact the frequency of
Early feedback.
n) Let t_rr_last be the point in time at which the last Regular
RTCP packet has been scheduled and sent, i.e. has not been
suppressed due to T_rr_interval.
o) Let T_retention be the time window for which past FB messages
are stored by an AVPF entity. This is to ensure that feedback
suppression also works for entities that have received FB
messages from other entities prior to noticing the feedback
event itself. T_retention MUST be set to at least 2 seconds.
p) Let M*Td be the timeout value for a receiver to be considered
inactive (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 bandwidth, group size, and other application-
specific parameters -- that a FB message needs to be sent back to
the sender.
To avoid an implosion of feedback packets in multiparty sessions,
the receiver MUST delay the transmission of the RTCP feedback
packet by a random amount of time T_fd (with the random number
evenly distributed in the interval [0, T_dither_max]).
Transmission of the compound RTCP packet MUST then be scheduled for
te = t0 + T_fd.
The T_dither_max parameter is derived from the Regular RTCP
interval, T_rr, which, in turn, is based upon the group size. A
future document may also specify other calculations for
T_dither_max (e.g. based upon RTT) if it can be assured that all
RTP receivers will use the same mechanism for calculating
T_dither_max.
For a certain application scenario, a receiver may determine an
upper bound for the acceptable local delay of FB messages:
T_max_fb_delay. If an a priori estimation or the actual
calculation of T_dither_max indicates that this upper bound MAY be
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violated (e.g. because T_dither_max > T_max_fb_delay), the receiver
MAY decide not to send any feedback at all because the achievable
gain is considered insufficient.
If an Early RTCP packet is scheduled, the time slot for the next
Regular RTCP packet MUST be updated accordingly to a new tn:
tn=tp+2*T_rr and so MUST tp: tp=tp+T_rr afterwards. This is to
ensure that the short-term average RTCP bandwidth used with Early
feedback does not exceed the bandwidth used without Early feedback.
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 AVPF RTCP Scheduling Algorithm
Let S0 be an active sender (out of S senders) and let N be the
number of receivers with R being one of these receivers.
Assume 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 document).
Assume further that T_rr_interval is 0, if no minimal interval
between Regular RTCP packets is to be enforced, or T_rr_interval is
set to some meaningful value, as given by the application. This
value then denotes the minimal interval between Regular RTCP
packets.
With this, a receiver R MUST use the following rules for
transmitting one or more FB messages as minimal or full compound
RTCP packet:
3.5.1 Initialization
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Initially, R MUST set allow_early = TRUE and t_rr_last = NaN (Not-
a-Number, i.e. some invalid value that can be distinguished from a
valid time).
Furthermore, the initialization of the RTCP variables as per [1]
applies except that the initial value for Tmin. For a point-to-
point session, the initial Tmin is set to 0. For a multiparty
session, Tmin is initialized to 1.0 seconds.
3.5.2 Early Feedback Transmission
Assume that R had scheduled the last Regular RTCP RR packet for
transmission at tp (and sent or suppressed this packet at tp) and
has scheduled the next transmission (including possible
reconsideration as per [1]) for tn = tp + T_rr. Assume also that
the last Regular RTCP packet transmission has occurred at
t_rr_last.
The Early Feedback algorithm then comprises the following steps:
1. At time t0, R detects the need to transmit one or more FB
messages, e.g. because media "units" need to be ACKed or NACKed,
and finds that providing the feedback information is potentially
useful for the sender.
2. R first checks whether there is already a compound RTCP packet
containing one or more FB messages scheduled for transmission
(either as Early or as Regular RTCP packet).
2.a) If so, the new FB message MUST be included in the
scheduled packet; the scheduling of the waiting compound RTCP
packet MUST remain unchanged. When doing so, the available
feedback information SHOULD be merged to produce as few FB
messages as possible. This completes the course of immediate
actions to be taken.
2.b) If no compound RTCP packet is already scheduled for
transmission, a new (minimal or full) compound RTCP packet
MUST be created and the minimal interval for T_dither_max MUST
be chosen as follows:
i) If the session is a point-to-point session then
T_dither_max = 0.
ii) If the session is a multiparty session then
T_dither_max = l * T_rr
with l=0.5.
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The value for T_dither_max MAY be calculated differently (e.g.
based upon RTT) which MUST then be specified in a future
document. Such a future specification MUST ensure that all
RTP receivers use the same mechanism to calculate
T_dither_max.
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.
3. Then, R MUST check whether its next Regular RTCP packet would be
within the time bounds for the Early RTCP packet triggered at t0,
i.e. if t0 + T_dither_max > tn.
3.a) If so, an Early RTCP packet MUST NOT be scheduled;
instead the FB message(s) MUST be stored to be included in the
Regular RTCP packet scheduled for tn. This completes the
course of immediate actions to be taken.
3.b) Otherwise, the following steps are carried out.
4. R MUST check whether it is allowed to transmit an Early RTCP
packet, i.e. allow_early == TRUE, or not.
4.a) If allow_early == FALSE then R MUST check the time for the
next scheduled Regular RTCP packet:
1. If tn - t0 < T_max_fb_delay then the feedback could
still be useful for the sender, despite the late
reporting. Hence, R MAY create an RTCP FB message to
be included in the Regular RTCP packet for
transmission at tn.
2. Otherwise, R MUST discard the RTCP FB message.
This completes the immediate course of actions to be taken.
4.b) If allow_early == TRUE then R MUST schedule an Early RTCP
packet for te = t0 + RND * T_dither_max with RND being a pseudo
random function evenly distributed between 0 and 1.
5. R MUST detect overlaps in FB messages received from other
members of the RTP session and the FB messages R wants to send.
Therefore, while member of the RTP session, R MUST continuously
monitor the arrival of (minimal) compound RTCP packets and store
each FB message contained in these RTCP packets for at least
T_retention. When scheduling the transmission of its own FB
message following steps 1. through 4. above, R MUST check each of
the stored and newly received FB messages from the RTCP packets
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received during the interval [t0 - T_retention ; te] and act as
follows:
5.a) If R understands the received FB message's semantics and
the message contents is a superset of the feedback R wanted to
send then R MUST discard its own FB message and MUST re-
schedule the next Regular RTCP packet transmission for tn (as
calculated before).
5.b) If R understands the received FB message's semantics and
the message contents is not a superset of the feedback R wanted
to send then R SHOULD transmit its own FB message as scheduled.
If there is an overlap between the feedback information to send
and the feedback information received, the amount of feedback
transmitted is up to R: R MAY leave its feedback information to
be sent unchanged, R MAY as well eliminate any redundancy
between its own feedback and the feedback received so far from
other session members.
5.c) If R does not understand the received FB message's
semantics, R MAY keep its own FB message scheduled as an Early
RTCP packet, or R MAY re-schedule the next Regular RTCP packet
transmission for tn (as calculated before) and MAY append the
FB message to the now regularly scheduled RTCP message.
Note: With 5.c), receiving unknown FB messages may not lead to
feedback suppression at a particular receiver. As a
consequence, a given event may cause M different types of FB
messages (which are all appropriate but not mutually
understood) to be scheduled, so that a "large" receiver group
may effectively be partitioned into at most M groups. Among
members of each of these M groups, feedback suppression will
occur following 5.a and 5.b but no suppression will happen
across groups. As a result, O(M) RTCP FB messages may be
received by the sender. Hence, there is a chance for a very
limited feedback implosion. However, as sender(s) and all
receivers make up the same application using the same (set of)
codecs in the same RTP session, only little divergence in
semantics for FB messages can safely be assumed and, therefore,
M is assumed to be small in the general case. Given further
that the O(M) FB messages are randomly distributed over a time
interval of T_dither_max we find that the resulting limited
number of extra compound RTCP packets (a) is assumed not to
overwhelm the sender and (b) should be conveyed as all contain
complementary pieces of information.
6. If R's FB message(s) was not suppressed by other receiver FB
messages as per 5., when te is reached, R MUST transmit the
(minimal) compound RTCP packet containing its FB message(s). R
then MUST set allow_early = FALSE, MUST recalculate tn = tp +
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2*T_rr, and MUST set tp to the previous tn. As soon as the newly
calculated tn is reached, regardless whether R sends its next
Regular RTCP packet or suppresses it because of T_rr_interval, it
MUST set allow_early = TRUE again.
3.5.3 Regular RTCP Transmission
Full compound RTCP packets MUST be sent in regular intervals.
These packets MAY also contain one or more FB messages.
Transmission of Regular RTCP packets is scheduled as follows:
If T_rr_interval == 0 then the transmission MUST follow the rules
as specified in section 3.2 and 3.4 of this document and MUST
adhere to the adjustments of tn specified in section 3.5.2, i.e.
skip one regular transmission if an Early RTCP packet transmission
has occurred. Timer reconsideration takes place when tn is reached
as per [1]. The Regular RTCP packet is transmitted after timer
reconsideration. Whenever a Regular RTCP packet is sent or
suppressed, allow_early MUST be set to TRUE and tp, tn MUST be
updated as per [1]. After the first transmission of a Regular RTCP
packet, Tmin MUST be set to 0.
If T_rr_interval != 0 then the calculation for the transmission
times MUST follow the rules as specified in section 3.2 and 3.4 of
this document and MUST adhere to the adjustments of tn specified in
section 3.5.2 (i.e. skip one regular transmission if an Early RTCP
transmission has occurred). Timer reconsideration takes place when
tn is reached as per [1]. After timer reconsideration, the
following actions are taken:
1. If no Regular RTCP packet has been sent before (i.e. if
t_rr_last == NaN) then a Regular RTCP packet MUST be
scheduled. Stored FB messages MAY be included in the
Regular RTCP packet. After the scheduled packet has been
sent, t_rr_last MUST be set to tn. Tmin MUST be set to 0.
2. Otherwise, a temporary value T_rr_current_interval is
calculated as follows:
T_rr_current_interval = RND*T_rr_interval
with RND being a pseudo random function evenly distributed
between 0.5 and 1.5. This dithered value is used to
determine one of the following alternatives:
2a) If t_rr_last + T_rr_current_interval <= tn then a
Regular RTCP packet MUST be scheduled. Stored RTCP FB
messages MAY be included in the Regular RTCP packet.
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After the scheduled packet has been sent, t_rr_last
MUST be set to tn.
2b) If t_rr_last + T_rr_current_interval > tn and RTCP FB
messages have been stored and are awaiting
transmission, an RTCP packet MUST be scheduled for
transmission at tn. This RTCP packet MAY be a minimal
or a Regular RTCP packet (at the discretion of the
implementer) and the compound RTCP packet MUST include
the stored RTCP FB message(s). t_rr_last MUST remain
unchanged.
2c) Otherwise (if t_rr_last + T_rr_current_interval > tn
but no stored RTCP FB messages are awaiting
transmission), the compound RTCP packet MUST be
suppressed (i.e. it MUST NOT be scheduled). t_rr_last
MUST remain unchanged.
In all the four cases above (1, 2a, 2b, and 2c), allow_early MUST
be set to TRUE (possibly after sending the Regular RTCP packet) and
tp and tn MUST be updated following the rules of [1] except for the
five second minimum.
3.5.4 Other Considerations
If T_rr_interval != 0 then the timeout calculation for RTP/AVPF
entities (section 6.3.5 of [1]) MUST be modified to use
T_rr_interval instead of Tmin for computing Td and thus M*Td.
Whenever a compound RTCP packet is sent or received -- minimal or
full compound, Early or Regular -- the avg_rtcp_size variable MUST
be updated accordingly (see [1]) and subsequent computations of tn
MUST use the new avg_rtcp_size.
3.6 Considerations on the Group Size
This section provides some guidelines to the group sizes at which
the various feedback modes may be used.
3.6.1 ACK mode
The RTP session MUST have exactly two members and this group size
MUST NOT grow, i.e. it MUST be point-to-point communications.
Unicast addresses SHOULD be used in the session description.
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For unidirectional as well as bi-directional communication between
two parties, 2.5% of the RTP session bandwidth is available for
RTCP traffic from the receivers including feedback. For a 64
kbit/s stream this yields 1,600 bit/s for RTCP. If we assume an
average of 96 bytes (=768 bits) per RTCP packet a receiver can
report 2 events per second back to the sender. If acknowledgments
for 10 events are collected in each FB message then 20 events can
be acknowledged per second. At 256 kbit/s, 8 events could be
reported per second; thus the ACKs may be sent in a finer
granularity (e.g. only combining three ACKs per FB message).
From 1 Mbit/s upwards, a receiver would be able to acknowledge each
individual frame (not packet!) in a 30 fps video stream.
ACK strategies MUST be defined 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.
3.6.2 NACK mode
Negative acknowledgements (and other types of feedback exhibiting
similar reporting characteristics) MUST be used for all sessions
with a group size that may grow larger than two. Of course, NACKs
MAY be used for point-to-point communications as well.
Whether or not the use of 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.
The most important parameters when determining the mode of
operation are the allowed minimal interval between two compound
RTCP packets (T_rr) and the average number of events that
presumably need reporting per time interval (plus their
distribution over time, of course). The minimum interval can be
derived from the available RTCP bandwidth and the 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.
Let N be the average number of events to be reported per
interval T by a receiver, B the RTCP bandwidth fraction for
this particular receiver and R the average RTCP packet size,
then the receiver operates in Immediate Feedback mode as long
as N<=B*T/R.
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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.
Using the above notation, Early RTCP mode can be roughly
characterized by N > B*T/R as "lower bound". An estimate for
an upper bound is more difficult. Setting N=1, we obtain for a
given R and B the interval T = R/B as average interval between
events to be reported. This information can be used as a hint
to determine whether or not early transmission of RTCP packets
is useful.
Example: If a 256kbit/s video with 30 fps is transmitted through a
network with an MTU size of some 1,500 bytes, then, in most cases,
each frame would fit in into one 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, on average, have to report 3 packets lost each
two seconds. Assuming a single sender and more 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 lost packets per two seconds, this yields a maximum
group size of 6-7 receivers if all loss events shall be reported.
The rules for transmission of Early 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 could lead to the following considerations: assume that the
underlying coding scheme and the application (as well as the
tolerant users) allow 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. Note that all these
considerations are based upon statistics and will fail to hold in
some cases.
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:
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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 be based upon (and dynamically revised
following) RTCP reception statistics as well as out-of-band
mechanisms.
2) The application has to decide -- for a certain observed error
rate, assigned bandwidth, frame/packet rate, and group size --
whether (and which) feedback mechanisms can be applied.
Regular RTCP reception statistics provide valuable input to this
step, too.
3) If the application decides to send feedback, the application has
to follow the rules for transmitting Early RTCP packets or
Regular RTCP packets containing FB messages.
4) The type of RTCP feedback sent should not duplicate information
available to the sender from a lower layer transport protocol.
That is, if the transport protocol provides negative or positive
acknowledgements about packet reception (such as DCCP), the
receiver should avoid repeating the same information at the RTCP
layer (i.e. abstain from sending Generic NACKs).
3.7.2 Media Session Attributes
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 mechanism is two-fold: a format
used to describe a media session and another mechanism for
transporting this description.
In the IETF, the Session Description Protocol (SDP) is currently
used to describe media sessions while protocols such as SIP, SAP,
RTSP, and HTTP (among others) are used to convey the descriptions.
A 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.
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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 other media session
description formats are outside the scope of this document.
4 SDP Definitions
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 Session Description Protocol (SDP) [3]. The
profile specified in this document is referred to as "AVPF".
Feedback information following the modified timing rules as
specified in this document MUST NOT be sent for a particular media
session unless the description for this session indicates the use
of the "AVPF" profile (exclusively or jointly with other AV
profiles).
4.2 RTCP Feedback Capability Attribute
A new payload format-specific SDP attribute is defined to indicate
the capability of using RTCP feedback as specified in this
document: "a=rtcp-fb". The "rtcp-fb" attribute MUST 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 SHOULD be used to indicate which RTCP FB
messages MAY be used in this media session for the indicated
payload type. A wildcard payload type ("*") MAY be used to
indicate that the RTCP feedback attribute applies to all payload
types. If several types of feedback are supported and/or the same
feedback shall be specified for a subset of the payload types,
several "a=rtcp-fb" lines MUST be used.
If no "rtcp-fb" attribute is specified the RTP receivers MAY send
feedback using other suitable RTCP feedback packets as defined for
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the respective media type. The RTP receivers MUST NOT rely on the
RTP senders reacting to any of the FB messages. The RTP sender MAY
choose to ignore some feedback messages.
If one or more "rtcp-fb" attributes are present in a media session
description, the RTCP receivers for the media session(s) containing
the "rtcp-fb"
o MUST ignore all "rtcp-fb" attributes of which they do not fully
understand the semantics (i.e. where they do not understand the
meaning of all values in the "a=rtcp-fb" line);
o 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
o MUST NOT use other FB messages than those listed in one of the
"rtcp-fb" attribute lines.
When used in conjunction with the offer/answer model [9], the
offerer MAY present a set of these AVPF attributes to its peer.
The answerer MUST remove all attributes it does not understand as
well as those it does not support in general or does not wish to
use in this particular media session. The answerer MUST NOT add
feedback parameters to the media description and MUST NOT alter
values of such parameters. The answer is binding for the media
session and both offerer and answerer MUST only use feedback
mechanisms negotiated in this way. Both offerer and answerer MAY
independently decide to send RTCP FB messages of only a subset of
the negotiated feedback mechanisms; but they SHOULD react properly
to all types of the negotiated FB messages when received.
RTP senders MUST be prepared to receive any kind of RTCP FB
messages and MUST silently discard all those RTCP FB 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):
(In the following ABNF, fmt, SP and CRLF are used as defined in
[3].)
rtcp-fb-syntax = "a=rtcp-fb:" rtcp-fb-pt SP rtcp-fb-val CRLF
rtcp-fb-pt = "*" ; wildcard: applies to all formats
/ fmt ; as defined in SDP spec
rtcp-fb-val = "ack" rtcp-fb-ack-param
/ "nack" rtcp-fb-nack-param
/ "trr-int" SP 1*DIGIT
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/ rtcp-fb-id rtcp-fb-param
rtcp-fb-id = 1*(alpha-numeric / "-" / "_")
rtcp-fb-param = SP "app" [SP byte-string]
/ SP token [SP byte-string]
/ ; empty
rtcp-fb-ack-param = SP "rpsi"
/ SP "app" [SP byte-string]
/ SP token [SP byte-string]
/ ; empty
rtcp-fb-nack-param = SP "pli"
/ SP "sli"
/ SP "rpsi"
/ SP "app" [SP byte-string]
/ SP token [SP 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 MUST be provided to further distinguish different
types of positive acknowledgement feedback.
The parameter "rpsi" indicates the use of Reference Picture
Selection Indication feedback as defined in section 6.3.3.
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 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.
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Feedback type "nack":
This feedback type indicates that negative acknowledgements
for feedback are supported.
The feedback type "nack", without parameters, indicates use of
the General NACK feedback format as defined in section 6.2.1.
The following three parameters are defined in this document
for use with "nack" in conjunction with the media type
"video":
o "pli" indicates the use of Picture Loss Indication feedback
as defined in section 6.3.1.
o "sli" indicates the use of Slice Loss Indication feedback
as defined in section 6.3.2.
o "rpsi" indicates the use of Reference Picture Selection
Indication feedback as defined in section 6.3.3.
"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 MUST
to be unique (and thus MUST be registered with IANA). Along
with a new name, its semantics, packet formats (if necessary),
and rules for its operation MUST be specified.
Regular RTCP minimum interval "trr-int":
The attribute "trr-int" is used to specify the minimum
interval T_rr_interval between two Regular (full compound)
RTCP packets in milliseconds for this media session. If "trr-
int" is not specified, a default value of 0 is assumed.
Note that it is assumed that more specific information about
application layer feedback (as defined in section 6.4) will be
conveyed as feedback types and parameters defined elsewhere.
Hence, no further provision for any types and parameters is made in
this document.
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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) (how) to make use of feedback
provided.
4.3 RTCP Bandwidth Modifiers
The standard RTCP bandwidth assignments as defined in [1] and [2]
MAY be overridden by bandwidth modifiers that explicitly define the
maximum RTCP bandwidth. For use with SDP, such modifiers are
specified in [4]: "b=RS:<bw>" 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).
4.4 Examples
Example 1: The following session description indicates a session
made up from audio and DTMF [18] for point-to-point communication
in which the DTMF stream uses Generic NACKs. 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=rtcp-fb:96 nack
This allows sender and receiver to provide reliable transmission of
DTMF events in an audio session. Assuming a 64kbit/s audio stream
with one receiver, the receiver has 2.5% RTCP bandwidth available
for the negative acknowledgment stream, i.e. 250 bytes per second
or some 2 RTCP feedback messages every second. Hence, the receiver
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can individually communicate up to two missing DTMF audio packets
per second.
Example 2: The following session description indicates a multicast
video-only session (using either H.261 or H.263+) with the video
source accepting Generic NACKs for both codecs and Reference
Picture Selection for H.263. 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 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
a=rtcp-fb:* nack
a=rtcp-fb:98 nack rpsi
The sender may use an incoming Generic NACK as a hint to send a new
intra-frame as soon as possible (congestion control permitting).
Receipt of an RPSI message allows the sender to avoid sending a
large intra-frame; instead it may continue to send inter-frames,
however, choosing the indicated frame as new encoding reference.
Example 3: The following session description defines the same media
session as example 2 but allows for mixed mode operation of AVP and
AVPF RTP entities (see also next section). Note that both media
descriptions use the same addresses; however, two m= lines are
needed to convey information about both applicable RTP profiles.
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/AVP 98 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
m=video 51372 RTP/AVPF 98 99
c=IN IP4 224.2.1.184
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a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
a=rtcp-fb:* nack
a=rtcp-fb:98 nack rpsi
Note that these two m= lines SHOULD be grouped by some appropriate
mechanism to indicate that both are alternatives actually conveying
the same contents. A sample mechanism by which this can be
achieved is defined in [7].
In this example, the RTCP feedback-enabled receivers will gain an
occasional advantage to report events earlier back to the sender
(which may benefit the entire group). On average, however, all RTP
receivers will provide the same amount of feedback. The
interworking between AVP and AVPF entities is discussed in depth in
the next section.
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 (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 (see e.g. example 3 in section 4.5).
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.
o 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 five second
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 five second 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 may send bigger compound
RTCP packets (due to the extra RTCP packets).
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If T_rr_interval is used as lower bound between Regular RTCP
packets, T_rr_interval is sufficiently large (e.g. T_rr_interval
> M*Td as per section 6.3.5 of [1]), and no Early RTCP packets
are sent by AVPF entities, AVP entities may accidentally time
out those AVPF group members and hence under-estimate the group
size. Therefore, if AVP entities may be involved in a media
session, T_rr_interval SHOULD NOT be larger than five seconds.
o AVPF entities (senders and receivers)
If the dynamically calculated T_rr is sufficiently small (e.g.
less than one second), AVPF entities may accidentally time out
AVP group members and hence under-estimate the group size.
Therefore, if AVP entities may be involved in a media session,
T_rr_interval SHOULD be used and SHOULD be set to five seconds.
In conclusion, if AVP entities may be involved in a media
session and T_rr_interval is to be used, T_rr_interval SHOULD be
set to five seconds.
o 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.
o AVPF receivers
AVPF receivers SHOULD send 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.
However, the receiver providing feedback MUST NOT rely on the
sender reacting to the feedback at all.
6 Format of RTCP Feedback Messages
This section defines the format of the low delay RTCP feedback
messages. These messages are classified into three categories as
follows:
- Transport layer FB messages
- Payload-specific FB messages
- Application layer FB messages
Transport layer FB messages are intended to transmit general
purpose feedback information, i.e. information independent of the
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particular codec or the application in use. The information is
expected to be generated and processed at the transport/RTP layer.
Currently, only a generic negative acknowledgement (NACK) message
is defined.
Payload-specific FB messages transport information that is specific
to a certain payload type 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 FB messages. The
definition of specific messages is left to either RTP payload
format specifications or to additional feedback format documents.
Application layer FB 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 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 FB messages. From a
protocol point of view, an application layer FB message is treated
as a special case of a payload-specific FB message.
NOTE: Proper processing of some FB messages at the media
sender side may require the sender to know which payload type
the FB message refers to. Most of the time, this knowledge
can likely be derived from a media stream using only a single
payload type. However, if several codecs are used
simultaneously (e.g. with audio and DTMF) or when codec
changes occur, the payload type information may need to be
conveyed explicitly as part of the FB message. This applies
to all payload-specific as well as application layer FB
messages. It is up to the specification of a FB message to
define how payload type information is transmitted.
This document defines two transport layer and three (video)
payload-specific FB messages as well as a single container for
application layer FB messages. Additional transport layer and
payload specific FB messages MAY be defined in other documents and
MUST be registered through IANA (see section IANA considerations).
The general syntax and semantics for the above RTCP FB message
types are described in the following subsections.
6.1 Common Packet Format for Feedback Messages
All FB messages MUST use 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| FMT | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 3: Common Packet Format for Feedback Messages
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.
Feedback message type (FMT): 5 bits
This field identifies the type of the FB message and is
interpreted relative to the type (transport, payload-specific,
or application layer 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 FB message. Two values are defined (TBA. by
IANA):
Name | Value | Brief Description
----------+-------+------------------------------------
RTPFB | 205 | Transport layer FB message
PSFB | 206 | Payload-specific FB message
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].
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SSRC of packet sender: 32 bits
The synchronization source identifier for the originator of
this packet.
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 MAY be included in the FB message for each type of
feedback: transport layer, payload-specific or application
layer feedback. Note that further FCI contents MAY be specified
in further documents.
Each RTCP feedback packet MUST contain at least one FB message in
the FCI field. Sections 6.2 and 6.3 define for each FCI type,
whether or not multiple FB messages MAY be compressed into a single
FCI field. If this is the case, they MUST be of the same type,
i.e. same FMT. If multiple types of feedback messages, i.e.
several FMTs, need to be conveyed, then several RTCP FB messages
MUST be generated and SHOULD be concatenated in the same compound
RTCP packet.
6.2 Transport Layer Feedback Messages
Transport Layer FB messages are identified by the value RTPFB as
RTCP message type.
A single general purpose transport layer FB messages are defined so
far: Generic NACK. It is identified by means of the FMT parameter
as follows:
0: unassigned
1: Generic NACK
2-30: unassigned
31: reserved for future expansion of the identifier number space
The following subsection defines the formats of the FCI field for
this type of FB message. Further generic feedback messages MAY be
defined in the future.
6.2.1 Generic NACK
The Generic NACK message is identified by PT=RTPFB and FMT=1.
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The FCI field MUST contain at least one and MAY contain more than
one Generic NACK.
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.
Generic NACK feedback SHOULD NOT be used if the underlying
transport protocol is capable of providing similar feedback
information to the sender (as may be the case e.g. with DCCP).
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. The PID field
refers to the RTP sequence number of the lost packet.
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 [6].
Denoting the BLP's least significant bit as bit 1, and its most
significant bit as bit 16, then bit i of the bit mask is set to
1 if the receiver has not received RTP packet number (PID+i)
(modulo 2^16) and indicates 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 through 15 of the BLP are 0; all the
sender knows is that the receiver has not reported them as lost
at this time.
The length of the FB message MUST be set to 2+n, with n being the
number of Generic NACKs contained in the FCI field.
The Generic NACK message implicitly references the payload type
through the sequence number(s).
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6.3 Payload Specific Feedback Messages
Payload-Specific FB messages are identified by the value PT=PSFB as
RTCP message type.
Three payload-specific FB messages are defined so far plus an
application layer FB message. They 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)
4-14: unassigned
15: Application layer FB message
16-30: unassigned
31: reserved for future expansion of the sequence number space
The following subsections define the FCI formats for the payload-
specific FB messages, section 6.4 defines FCI format for the
application layer FB message.
6.3.1 Picture Loss Indication (PLI)
The PLI FB message is identified by PT=PSFB and FMT=1.
There MUST be exactly one PLI contained in the FCI field.
6.3.1.1 Semantics
With the Picture Loss Indication message, a decoder informs the
encoder about the loss of an undefined amount of coded video data
belonging to one or more pictures. When used in conjunction with
any video coding scheme that is based on inter-picture prediction,
an encoder that receives a PLI becomes aware that the prediction
chain may be broken. The sender MAY react to a PLI by transmitting
an intra-picture to achieve resynchronization (making effectively
similar to the FIR as defined in [6]); however, the sender MUST
consider congestion control as outlined in section 7 which MAY
restrict its ability to send an intra frame.
Other RTP payload specifications such as RFC 2032 [6] already
define a feedback mechanism for some for certain codecs. An
application supporting both schemes MUST use the feedback mechanism
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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.
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.
The semantics of this FB message is independent of the payload
type.
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.
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,
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 FB 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.
6.3.2 Slice Lost Indication (SLI)
The SLI FB message is identified by PT=PSFB and FMT=2.
The FCI field MUST contain at least one and MAY contain more than
one SLI.
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6.3.2.1 Semantics
With the Slice Lost Indication a decoder can inform an encoder that
it has detected the loss or corruption of one or several
consecutive macroblock(s) in scan order (see below). This FB
message MUST 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.
6.3.2.2 Format
The Slice Lost Indication uses one additional PCI field the
content of which is depicted in figure 6. The length of the FB
message MUST be set to 2+n, with n being the number of SLIs
contained in the FCI field.
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 | PictureID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
PictureID: 6 bits
The six least significant bits of the a codec-specific
identifier that is used to reference the picture in which the
loss of the macroblock (s) has occurred. For many video
codecs, the PictureID is identical to the Temporal Reference.
The applicability of this FB message is limited to a small set of
video codecs and therefore, no explicit payload type information is
provided.
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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.
6.3.2.4 Remarks
The term Slice is defined and used here in the sense of MPEG-1 -- a
consecutive number of macroblocks in scan order. More recent video
coding standards sometimes have a different understanding of the
term Slice. In H.263 (1998), for example, a concept known as
"rectangular Slice" exists. The loss of one Rectangular Slice may
lead to the necessity of sending more than one SLI in order to
precisely identify the region of lost/damaged MBs.
The first field of the FCI 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 most 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
FB 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.
The reaction to a SLI is not part of this specification. One
typical way of reacting to a SLI is to use intra refresh for the
affected spatial region.
Algorithms were reported that keep track of the regions affected 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 [17] 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 much higher
data volume in case of delayed FBs.
6.3.3 Reference Picture Selection Indication (RPSI)
The RPSI FB message is identified by PT=PSFB and FMT=3.
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There MUST be exactly one RPSI contained in the FCI field.
6.3.3.1 Semantics
Modern video coding standards such as MPEG-4 visual version 2 [16]
or H.263 version 2 [17] allow to use older reference pictures than
the most recent one for predictive coding. 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 "payload" 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 -- a couple of dozen bits --, 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, pictures (or Slices) are
reported that were decoded without error. Note that any form of
positive feedback MUST NOT be used when in a multiparty session
(reporting positive feedback about individual reference pictures at
RTCP intervals is not expected to be of much use anyway).
6.3.3.2 Format
The FCI for the RPSI message follows the format depicted in figure
7:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB |0| Payload Type| Native RPSI bit string |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| defined per codec ... | Padding (0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Syntax of the Reference Picture Selection Indication
(RPSI)
PB: 8 bits
The number of unused bits required to pad the length of the
RPSI message to a multiple of 32 bits.
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0: 1 bit
MUST be set to zero upon transmission and ignored upon
reception.
Payload Type: 7 bits
Indicates the RTP payload type in the context of which the
native RPSI bit string MUST be interpreted.
Native RPSI bit string: variable length
The RPSI information as natively defined by the video codec.
Padding: #PB bits
A number of bits set to zero to fill up the contents of the
RPSI message to the next 32 bit boundary. The number of
padding bits MUST be indicated by the PB field.
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 re-establish encoder-decoder
synchronicity. See [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.
6.4 Application Layer Feedback Messages
Application Layer FB messages are a special case of payload-
specific messages and identified by PT=PSFB and FMT=15.
There MUST be exactly one Application Layer FB message contained in
the FCI field, unless the Application Layer FB message structure
itself allows for stacking (e.g. by means of a fixed size or
explicit length indicator).
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 FB 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 [16] (carried in RTP packets according
to RFC 3016 [23]) or FB messages in H.263/Annex N, U [17]
(packetized as per RFC 2429 [14]). These messages do not need any
additional information from the RTCP message. Thus the application
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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 32-bit-aligned,
padding bits and bytes must be added. Identification of
padding is up to the application layer and not defined in this
specification.
The application layer FB message specification MUST define whether
or not the message needs to be interpreted specifically in the
context of a certain codec (identified by the RTP payload type).
If a reference to the payload type is required for proper
processing, the application layer FB message specification MUST
define a way to communicate the payload type information as part of
the application layer FB message itself.
7 Early Feedback and Congestion Control
In the previous sections, the FB 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.
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.
It should be noted that RTCP feedback itself is insufficient for
congestion control purposes as it is likely to operate at much
slower timescales than other transport layer feedback mechanisms
(that usually operate in the order of RTT). Therefore, additional
mechanisms are required to perform proper congestion control.
A congestion control algorithm that shares the available bandwidth
reasonably fairly with competing TCP connections, e.g. TFRC [8],
MUST be used to determine the data rate for the media stream within
the bounds of the RTP sender's and the media session's capabilities
if the RTP/AVPF session is transmitted in a best effort
environment.
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8 Security Considerations
RTP packets transporting information with the proposed payload
format are subject to the security considerations discussed in the
RTP specification [1] and in the RTP/AVP profile specification [2].
This profile does not specify any additional security services.
This profile modifies the timing behavior of RTCP and eliminates
the minimum RTCP interval of five seconds and allows for earlier
feedback to be provided by receivers. Group members of the
associated RTP session (possibly pretending to represent a large
number of entities) may disturb the operation of RTCP by sending
large numbers of RTCP packets thereby reducing the RTCP bandwidth
available for Regular RTCP reporting as well as for Early FB
messages. (Note that an entity need not be member of a multicast
group to cause these effects.) Similarly, malicious members may
send very large RTCP messages, thereby increasing the avg_rtcp_size
variable and reducing the effectively available RTCP bandwidth.
Feedback information may be suppressed if unknown RTCP feedback
packets are received. This introduces the risk of a malicious
group member reducing Early feedback by simply transmitting
payload-specific RTCP feedback packets with random contents that
are neither recognized by any receiver (so they will suppress
feedback) nor by the sender (so no repair actions will be taken).
A malicious group member can also report arbitrary high loss rates
in the feedback information to make the sender throttle the data
transmission and increase the amount of redundancy information or
take other action to deal with the pretended packet loss (e.g. send
fewer frames or decrease audio/video quality). This may result in
a degradation of the quality of the reproduced media stream.
Finally, a malicious group member can act as a large number of
group members and thereby obtain an artificially large share of the
Early feedback bandwidth and reduce the reactivity of the other
group members -- possibly even causing them to no longer operate in
Immediate or Early feedback mode and thus undermining the whole
purpose of this profile.
Senders as well as receivers SHOULD behave conservatively when
observing strange reporting behavior. For excessive failure
reporting from one or a few receivers, the sender MAY decide to no
longer consider this feedback when adapting its transmission
behavior for the media stream. In any case, senders and receivers
SHOULD still adhere to the maximum RTCP bandwidth but make sure
that they are capable of transmitting at least regularly scheduled
RTCP packets. Senders SHOULD carefully consider how to adjust
their transmission bandwidth when encountering strange reporting
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behavior; they MUST NOT increase their transmission bandwidth even
if ignoring suspicious feedback.
Attacks using false RTCP packets (Regular as well as Early ones)
can be avoided by authenticating all RTCP messages. This can be
achieved by using the AVPF profile together with the Secure RTP
profile as defined in [22]; as a prerequisite, an appropriate
combination of those two profiles (an "SAVPF") is being specified
[21]. Note that, when employing group authentication (as opposed
to source authentication), the aforementioned attacks may be
carried out by malicious or malfunctioning group members in
possession of the right keying material.
9 IANA Considerations
The following contact information shall be used for all
registrations included here:
Contact: Joerg Ott
mailto:jo@acm.org
tel:+49-421-201-7028
The feedback profile as an extension to the profile for audio-
visual conferences with minimal control needs to be registered for
the Session Description Protocol (specifically the type "proto"):
"RTP/AVPF".
SDP Protocol ("proto"):
Name: RTP/AVPF
Long form: Extended RTP Profile with RTCP-based Feedback
Type of name: proto
Type of attribute: Media level only
Purpose: RFC XXXX
Reference: RFC XXXX
SDP Attribute ("att-field"):
Attribute name: rtcp-fb
Long form: RTCP Feedback parameter
Type of name: att-field
Type of attribute: Media level only
Subject to charset: No
Purpose: RFC XXXX
Reference: RFC XXXX
Values: See this document and registrations below
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A new registry needs to be set up for the "rtcp-fb" attribute, with
the following registrations created initially: "ack", "nack", "trr-
int", and "app" as defined in this document.
Initial value registration for the attribute "rtcp-fb"
Value name: ack
Long name: Positive acknowledgement
Reference: RFC XXXX.
Value name: nack
Long name: Negative Acknowledgement
Reference: RFC XXXX.
Value name: trr-int
Long name: Minimal receiver report interval
Reference: RFC XXXX.
Value name: app
Long name: Application-defined paramater
Reference: RFC XXXX.
Further entries may be registered on a first-come first-serve
basis. Each new registration needs to indicate the parameter name
and the syntax of possible additional arguments. For each new
registration, it is mandatory that a permanent, stable, and
publicly accessible document exists that specifies the semantics of
the registered parameter, the syntax and semantics of its
parameters as well as corresponding feedback packet formats (if
needed). The general registration procedures of [3] apply.
For use with both "ack" and "nack", a joint sub-registry needs to
be set up that initially registers the following values:
Initial value registration for the attribute values "ack" and
"nack":
Value name: sli
Long name: Slice Loss Indication
Usable with: nack
Reference: RFC XXXX.
Value name: pli
Long name: Picture Loss Indication
Usable with: nack
Reference: RFC XXXX.
Value name: rpsi
Long name: Reference Picture Selection Indication
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Usable with: ack, nack
Reference: RFC XXXX.
Value name: app
Long name: Application layer feedback
Usable with: ack, nack
Reference: RFC XXXX.
Further entries may be registered on a first-come first-serve
basis. Each registrations needs to indicate the parameter name,
the syntax of possible additional arguments, and whether the
parameter is applicable to "ack" or "nack" feedback or both or some
different "rtcp-fb" attribute parameter. For each new
registration, it is mandatory that a permanent, stable, and
publicly accessible document exists that specifies the semantics of
the registered parameter, the syntax and semantics of its
parameters as well as corresponding feedback packet formats (if
needed). The general registration procedures of [3] apply.
Two RTCP Control Packet Types: for the class of transport layer FB
messages ("RTPFB") and for the class of payload-specific FB
messages ("PSFB"). Section 6 suggests RTPFB=205 and PSFB=206 to be
added to the RTCP registry.
RTP RTCP Control Packet types (PT):
Name: RTPFB
Long name: Generic RTP Feedback
Value: 205
Reference: RFC XXXX.
Name: PSFB
Long name: Payload-specific
Value: 206
Reference: RFC XXXX.
As AVPF defines additional RTCP payload types, the corresponding
"reserved" RTP payload type space (72--76, as defined in [2]),
needs to be expanded accordingly.
A new sub-registry needs to be set up for the FMT values for both
the RTPFB payload type and the PSFB payload type, with the
following registrations created initially:
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Within the RTPFB range, the following two format (FMT) values are
initially registered:
Name: Generic NACK
Long name: Generic negative acknowledgement
Value: 1
Reference: RFC XXXX.
Name: Extension
Long name: Reserved for future extensions
Value: 31
Reference: RFC XXXX.
Within the PSFB range, the following five format (FMT) values are
initially registered:
Name: PLI
Long name: Picture Loss Indication
Value: 1
Reference: RFC XXXX.
Name: SLI
Long name: Slice Loss Indication
Value: 2
Reference: RFC XXXX.
Name: RPSI
Long name: Reference Picture Selection Indication
Value: 3
Reference: RFC XXXX.
Name: AFB
Long name: Application Layer Feedback
Value: 15
Reference: RFC XXXX.
Name: Extension
Long name: Reserved for future extensions.
Value: 31
Reference: RFC XXXX.
Further entries may be registered following the "Specification
Required" rules as defined in RFC 2434 [10]. Each registration
needs to indicate the FMT value, if there is a specific FB message
to go into the FCI field, and whether or not multiple FB messages
may be stacked in a single FCI field. For each new registration,
it is mandatory that a permanent, stable, and publicly accessible
document exists that specifies the semantics of the registered
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parameter as well as the syntax and semantics of the associated FB
message (if any). The general registration procedures of [3]
apply.
NOTE TO THE RFC EDITOR: Please replace all occurrences of RFC XXXX by
the RFC number assigned to this document.
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. The authors
would also like to particularly thank Magnus Westerlund for his
review and his valuable suggestions, Shigeru Fukunaga for the
contributions on for FB message formats and semantics.
We would also like to thank Andreas Buesching and people at
Panasonic for their simulations and the first independent
implementations of the feedback profile.
11 Authors' Addresses
Joerg Ott {sip,mailto}:jo@tzi.org
Uni Bremen TZI
MZH 5180
Bibliothekstr. 1
D-28359 Bremen
Germany
Stephan Wenger stewe@stewe.org
TU Berlin
Sekr. FR 6-3
Franklinstr. 28-29
D-10587 Berlin
Germany
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Noriyuki Sato sato652@oki.com
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
Carsten Burmeister burmeister@panasonic.de
Panasonic European Laboratories GmbH
Jose Rey rey@panasonic.de
Panasonic European Laboratories GmbH
Monzastr. 4c, 63225 Langen, Germany
Tel. +49-(0)6103-766-134
Fax. +49-(0)6103-766-166
12 Bibliography
12.1 Normative references
[1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP
- A Transport Protocol for Real-time Applications," RFC 3550
(STD0064), July 2003.
[2] H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control," RFC 3551 (STD0065), July
2003.
[3] M. Handley, V. Jacobson, and Colin Perkins, "SDP: Session
Description Protocol", Internet Draft draft-ietf-mmusic-sdp-
new-18.txt, June 2004.
[4] S. Casner, "SDP Bandwidth Modifiers for RTCP Bandwidth", RFC
3556, July 2003.
[5] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119, March 1997.
[6] T. Turletti and C. Huitema, "RTP Payload Format for H.261
Video Streams, RFC 2032, October 1996.
[7] G. Camarillo, J. Holler, G. Eriksson, H. Schulzrinne,
"Grouping of media lines in SDP," RFC 3388, December 2002.
[8] M. Handley, J. Padhye, S. Floyd, J. Widmer, "TCP friendly Rate
Control (TFRC): Protocol Specification," RFC 3448, January
2003.
[9] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
SDP," RFC 3264, June 2002.
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[10] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs," RFC 2434, October 1998.
12.2 Informative References
[11] C. Perkins and O. Hodson, "Options for Repair of Streaming
Media," RFC 2354, June 1998.
[12] J. Rosenberg and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction,", RFC 2733, December 1999.
[13] 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.
[14] 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.
[15] B. Girod, N. Faerber, "Feedback-based error control for mobile
video transmission," Proceedings IEEE, Vol. 87, No. 10, pp.
1707 - 1723, October, 1999.
[16] ISO/IEC 14496-2:2001/Amd.1:2002, "Information technology -
Coding of audio-visual objects - Part2: Visual", 2001.
[17] ITU-T Recommendation H.263, "Video Coding for Low Bit Rate
Communication," November 2000.
[18] H. Schulzrinne and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals," RFC 2833, May 2000.
[19] E. Kohler, M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)," Internet Draft draft-ietf-dccp-spec-
076.txt, Work in Progress, February July 2004.
[20] M. Handley, S. Floyd, J. Padhye, and J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol Specification," RFC 3448,
January 2003.
[21] J. Ott and E. Carrara, "Extended Secure RTP Profile for RTCP-
based Feedback (RTP/SAVPF)," Internet Draft draft-ietf-avt-
profile-savpf-01.txt, Work in Progress, July 2004.
[22] M. Baugher, D. McGrew, M. Naslund, E. Carrarra, and K.
Norrman, "The Secure Real-Time Transport Protocol," RFC 3711,
March 2004.
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[23] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, and H. Kimata,
"RTP Payload Format for MPEG-4 Audio/Visual Streams," RFC
3016, November 2000.
13 Disclaimer of Validity
The IETF takes no position regarding the validity or scope of any
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to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
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it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
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14 Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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 MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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15 Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ott et al. Expires February 2005 [Page 52]
