draft-ietf-avtcore-rtp-circuit-breakers-08.txt   draft-ietf-avtcore-rtp-circuit-breakers-09.txt 
AVTCORE Working Group C. S. Perkins AVTCORE Working Group C. S. Perkins
Internet-Draft University of Glasgow Internet-Draft University of Glasgow
Updates: 3550 (if approved) V. Singh Updates: 3550 (if approved) V. Singh
Intended status: Standards Track Aalto University Intended status: Standards Track Aalto University
Expires: June 07, 2015 December 04, 2014 Expires: September 07, 2015 March 06, 2015
Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions
draft-ietf-avtcore-rtp-circuit-breakers-08 draft-ietf-avtcore-rtp-circuit-breakers-09
Abstract Abstract
The Real-time Transport Protocol (RTP) is widely used in telephony, The Real-time Transport Protocol (RTP) is widely used in telephony,
video conferencing, and telepresence applications. Such applications video conferencing, and telepresence applications. Such applications
are often run on best-effort UDP/IP networks. If congestion control are often run on best-effort UDP/IP networks. If congestion control
is not implemented in the applications, then network congestion will is not implemented in the applications, then network congestion will
deteriorate the user's multimedia experience. This document does not deteriorate the user's multimedia experience. This document does not
propose a congestion control algorithm; instead, it defines a minimal propose a congestion control algorithm; instead, it defines a minimal
set of RTP "circuit-breakers". Circuit-breakers are conditions under set of RTP "circuit-breakers". Circuit-breakers are conditions under
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 07, 2015. This Internet-Draft will expire on September 07, 2015.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. RTP Circuit Breakers for Systems Using the RTP/AVP Profile . 6 4. RTP Circuit Breakers for Systems Using the RTP/AVP Profile . 6
4.1. RTP/AVP Circuit Breaker #1: Media Timeout . . . . . . . . 8 4.1. RTP/AVP Circuit Breaker #1: Media Timeout . . . . . . . . 8
4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout . . . . . . . . 8 4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout . . . . . . . . 8
4.3. RTP/AVP Circuit Breaker #3: Congestion . . . . . . . . . 9 4.3. RTP/AVP Circuit Breaker #3: Congestion . . . . . . . . . 10
4.4. RTP/AVP Circuit Breaker #4: Media Usability . . . . . . . 13 4.4. RTP/AVP Circuit Breaker #4: Media Usability . . . . . . . 13
4.5. Ceasing Transmission . . . . . . . . . . . . . . . . . . 14 4.5. Choice of Circuit Breaker Interval . . . . . . . . . . . 14
5. RTP Circuit Breakers for Systems Using the RTP/AVPF Profile . 14 4.6. Ceasing Transmission . . . . . . . . . . . . . . . . . . 15
6. Impact of RTCP Extended Reports (XR) . . . . . . . . . . . . 15 5. RTP Circuit Breakers and the RTP/AVPF and RTP/SAVPF Profiles 16
7. Impact of RTCP Reporting Groups . . . . . . . . . . . . . . . 15 6. Impact of RTCP Extended Reports (XR) . . . . . . . . . . . . 17
8. Impact of Explicit Congestion Notification (ECN) . . . . . . 16 7. Impact of RTCP Reporting Groups . . . . . . . . . . . . . . . 17
9. Impact of Bundled Media and Layered Coding . . . . . . . . . 16 8. Impact of Explicit Congestion Notification (ECN) . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 9. Impact of Bundled Media and Layered Coding . . . . . . . . . 18
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 17 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
14.1. Normative References . . . . . . . . . . . . . . . . . . 18 13.1. Normative References . . . . . . . . . . . . . . . . . . 19
14.2. Informative References . . . . . . . . . . . . . . . . . 18 13.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
The Real-time Transport Protocol (RTP) [RFC3550] is widely used in The Real-time Transport Protocol (RTP) [RFC3550] is widely used in
voice-over-IP, video teleconferencing, and telepresence systems. voice-over-IP, video teleconferencing, and telepresence systems.
Many of these systems run over best-effort UDP/IP networks, and can Many of these systems run over best-effort UDP/IP networks, and can
suffer from packet loss and increased latency if network congestion suffer from packet loss and increased latency if network congestion
occurs. Designing effective RTP congestion control algorithms, to occurs. Designing effective RTP congestion control algorithms, to
adapt the transmission of RTP-based media to match the available adapt the transmission of RTP-based media to match the available
network capacity, while also maintaining the user experience, is a network capacity, while also maintaining the user experience, is a
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4.1. RTP/AVP Circuit Breaker #1: Media Timeout 4.1. RTP/AVP Circuit Breaker #1: Media Timeout
If RTP data packets are being sent, but the RTCP SR or RR packets If RTP data packets are being sent, but the RTCP SR or RR packets
reporting on that SSRC indicate a non-increasing extended highest reporting on that SSRC indicate a non-increasing extended highest
sequence number received, this is an indication that those RTP data sequence number received, this is an indication that those RTP data
packets are not reaching the receiver. This could be a short-term packets are not reaching the receiver. This could be a short-term
issue affecting only a few packets, perhaps caused by a slow-to-open issue affecting only a few packets, perhaps caused by a slow-to-open
firewall or a transient connectivity problem, but if the issue firewall or a transient connectivity problem, but if the issue
persists, it is a sign of a more ongoing and significant problem. persists, it is a sign of a more ongoing and significant problem.
Accordingly, if a sender of RTP data packets receives three or more Accordingly, if a sender of RTP data packets receives CB_INTERVAL or
consecutive RTCP SR or RR packets from the same receiver, and those more consecutive RTCP SR or RR packets from the same receiver (see
packets correspond to its transmission and have a non-increasing Section 4.5), and those packets correspond to its transmission and
extended highest sequence number received field, then that sender have a non-increasing extended highest sequence number received
SHOULD cease transmission (see Section 4.5). The extended highest field, then that sender SHOULD cease transmission (see Section 4.6).
sequence number received field is non-increasing if the sender The extended highest sequence number received field is non-increasing
receives at least three consecutive RTCP SR or RR packets that report if the sender receives at least CB_INTERVAL consecutive RTCP SR or RR
the same value for this field, but it has sent RTP data packets that packets that report the same value for this field, but it has sent
would have caused an increase in the reported value if they had RTP data packets, at a rate of at least one per RTT, that would have
reached the receiver. caused an increase in the reported value if they had reached the
receiver.
The reason for waiting for three or more consecutive RTCP packets The rationale for waiting for CB_INTERVAL or more consecutive RTCP
with a non-increasing extended highest sequence number is to give packets with a non-increasing extended highest sequence number is to
enough time for transient reception problems to resolve themselves, give enough time for transient reception problems to resolve
but to stop problem flows quickly enough to avoid causing serious themselves, but to stop problem flows quickly enough to avoid causing
ongoing network congestion. A single RTCP report showing no serious ongoing network congestion. A single RTCP report showing no
reception could be caused by a transient fault, and so will not cease reception could be caused by a transient fault, and so will not cease
transmission. Waiting for more than three consecutive RTCP reports transmission. Waiting for more than CB_INTERVAL consecutive RTCP
before stopping a flow might avoid some false positives, but could reports before stopping a flow might avoid some false positives, but
lead to problematic flows running for a long time period (potentially could lead to problematic flows running for a long time period
tens of seconds, depending on the RTCP reporting interval) before (potentially tens of seconds, depending on the RTCP reporting
being cut off. Equally, an application that sends few packets when interval) before being cut off. Equally, an application that sends
the packet loss rate is high runs the risk that the media timeout few packets when the packet loss rate is high runs the risk that the
circuit breaker triggers inadvertently. The chosen timeout interval media timeout circuit breaker triggers inadvertently. The chosen
is a trade-off between these extremes. timeout interval is a trade-off between these extremes.
4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout The rationale for enforcing a minimum sending rate below which the
media timeout circuit breaker will not trigger is to avoid spurious
circuit breaker triggers when the number of packets sent per RTCP
reporting interval is small (e.g., a telephony application sends only
two RTP comfort noise packets during a five second RTCP reporting
interval, and both are lost; this is 100% packet loss, but it seems
extreme to terminate the RTP session). The one packet per RTT bound
derives from [RFC5405].
4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout
In addition to media timeouts, as were discussed in Section 4.1, an In addition to media timeouts, as were discussed in Section 4.1, an
RTP session has the possibility of an RTCP timeout. This can occur RTP session has the possibility of an RTCP timeout. This can occur
when RTP data packets are being sent, but there are no RTCP reports when RTP data packets are being sent, but there are no RTCP reports
returned from the receiver. This is either due to a failure of the returned from the receiver. This is either due to a failure of the
receiver to send RTCP reports, or a failure of the return path that receiver to send RTCP reports, or a failure of the return path that
is preventing those RTCP reporting from being delivered. In either is preventing those RTCP reporting from being delivered. In either
case, it is not safe to continue transmission, since the sender has case, it is not safe to continue transmission, since the sender has
no way of knowing if it is causing congestion. Accordingly, an RTP no way of knowing if it is causing congestion. Accordingly, an RTP
sender that has not received any RTCP SR or RTCP RR packets reporting sender that has not received any RTCP SR or RTCP RR packets reporting
on the SSRC it is using for three or more of its RTCP reporting on the SSRC it is using for three or more of its RTCP reporting
intervals SHOULD cease transmission (see Section 4.5). When intervals SHOULD cease transmission (see Section 4.6). When
calculating the timeout, the deterministic RTCP reporting interval, calculating the timeout, the deterministic RTCP reporting interval,
Td, without the randomization factor, and with a fixed minimum Td, without the randomization factor, and using the fixed minimum
interval Tmin=5 seconds) SHOULD be used. The rationale for this interval of Tmin=5 seconds, MUST be used. The rationale for this
choice of timeout is as described in Section 6.2 of RFC 3550 choice of timeout is as described in Section 6.2 of [RFC3550] ("so
[RFC3550]. that implementations which do not use the reduced value for
transmitting RTCP packets are not timed out by other participants
prematurely"), as updated by Section 6.1.4 of
[I-D.ietf-avtcore-rtp-multi-stream] to account for the use of the RTP
/AVPF profile [RFC4585] or the RTP/SAVPF profile [RFC5124].
To reduce the risk of premature timeout, implementations SHOULD NOT
configure the RTCP bandwidth such that Td is larger than 5 seconds.
Similarly, implementations that use the RTP/AVPF profile [RFC4585] or
the RTP/SAVPF profile [RFC5124] SHOULD NOT configure T_rr_interval to
values larger than 4 seconds (the reduced limit for T_rr_interval
follows Section 6.1.3 of [I-D.ietf-avtcore-rtp-multi-stream]).
The choice of three RTCP reporting intervals as the timeout is made The choice of three RTCP reporting intervals as the timeout is made
following Section 6.3.5 of RFC 3550 [RFC3550]. This specifies that following Section 6.3.5 of RFC 3550 [RFC3550]. This specifies that
participants in an RTP session will timeout and remove an RTP sender participants in an RTP session will timeout and remove an RTP sender
from the list of active RTP senders if no RTP data packets have been from the list of active RTP senders if no RTP data packets have been
received from that RTP sender within the last two RTCP reporting received from that RTP sender within the last two RTCP reporting
intervals. Using a timeout of three RTCP reporting intervals is intervals. Using a timeout of three RTCP reporting intervals is
therefore large enough that the other participants will have timed therefore large enough that the other participants will have timed
out the sender if a network problem stops the data packets it is out the sender if a network problem stops the data packets it is
sending from reaching the receivers, even allowing for loss of some sending from reaching the receivers, even allowing for loss of some
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(TCP typically treats the loss of multiple packets within a single (TCP typically treats the loss of multiple packets within a single
RTT as one loss event, but RTCP RR packets report the overall RTT as one loss event, but RTCP RR packets report the overall
fraction of packets lost, and does not report when the packet losses fraction of packets lost, and does not report when the packet losses
occurred). Using the loss fraction in place of the loss event rate occurred). Using the loss fraction in place of the loss event rate
can overestimate the loss. We believe that this overestimate will can overestimate the loss. We believe that this overestimate will
not be significant, given that we are only interested in order of not be significant, given that we are only interested in order of
magnitude comparison ([Floyd] section 3.2.1 shows that the difference magnitude comparison ([Floyd] section 3.2.1 shows that the difference
is small for steady-state conditions and random loss, but using the is small for steady-state conditions and random loss, but using the
loss fraction is more conservative in the case of bursty loss). loss fraction is more conservative in the case of bursty loss).
The congestion circuit breaker is therefore: when a sender receives The congestion circuit breaker is therefore: when a sender that is
an RTCP SR or RR packet that contains a report block for an SSRC it transmitting more than one RTP packet per RTT receives an RTCP SR or
is using, the sender MUST check the fraction lost field in the report RR packet that contains a report block for an SSRC it is using, the
block to determine if there is a non-zero packet loss rate. If the sender MUST record the value of the fraction lost field in the report
fraction lost field is zero, then continue sending as normal. If the block and the time since the last report block was received for that
fraction lost is greater than zero, then estimate the TCP throughput SSRC. If more than CB_INTERVAL (see Section 4.5) report blocks have
that would be achieved over the path using the chosen TCP throughput been received for that SSRC, the sender MUST calculate the average
equation and the measured values of the round-trip time, R, the loss fraction lost over the last CB_INTERVAL reporting intervals, and then
event rate, p (as approximated by the fraction lost), and the packet estimate the TCP throughput that would be achieved over the path
size, s. Compare this with the actual sending rate. If the actual using the chosen TCP throughput equation and the measured values of
sending rate has been more than ten times the TCP throughput estimate the round-trip time, R, the loss event rate, p (as approximated by
for three (or more) consecutive RTCP reporting intervals, then the the average fraction lost), and the packet size, s. This estimate of
congestion circuit breaker is triggered. the TCP throughput is then compared with the actual sending rate. If
the actual sending rate is more than ten times the TCP throughput
estimate, then the congestion circuit breaker is triggered.
The average fraction lost is calculated based on the sum, over the
last CB_INTERVAL reporting intervals, of the fraction lost in each
reporting interval multiplied by the duration of the corresponding
reporting interval, divided by the total duration of the last
CB_INTERVAL reporting intervals.
The rationale for enforcing a minimum sending rate below which the
congestion circuit breaker will not trigger is to avoid spurious
circuit breaker triggers when the number of packets sent per RTCP
reporting interval is small, and hence the fraction lost samples are
subject to measurement artefacts. The one packet per RTT bound
derives from [RFC5405].
When the congestion circuit breaker is triggered, the sender SHOULD When the congestion circuit breaker is triggered, the sender SHOULD
cease transmission (see Section 4.5). However, if the sender is able cease transmission (see Section 4.6). However, if the sender is able
to reduce its sending rate by a factor of (approximately) ten, then to reduce its sending rate by a factor of (approximately) ten, then
it MAY first reduce its sending rate by this factor (or some larger it MAY first reduce its sending rate by this factor (or some larger
amount) to see if that resolves the congestion. If the sending rate amount) to see if that resolves the congestion. If the sending rate
is reduced in this way and the congestion circuit breaker triggers is reduced in this way and the congestion circuit breaker triggers
again after the next three RTCP reporting intervals, the sender MUST again after the next CB_INTERVAL RTCP reporting intervals, the sender
then cease transmission. An example of such a rate reduction might MUST then cease transmission. An example of such a rate reduction
be a video conferencing system that backs off to sending audio only, might be a video conferencing system that backs off to sending audio
before completely dropping the call. If such a reduction in sending only, before completely dropping the call. If such a reduction in
rate resolves the congestion problem, the sender MAY gradually sending rate resolves the congestion problem, the sender MAY
increase the rate at which it sends data after a reasonable amount of gradually increase the rate at which it sends data after a reasonable
time has passed, provided it takes care not to cause the problem to amount of time has passed, provided it takes care not to cause the
recur ("reasonable" is intentionally not defined here). problem to recur ("reasonable" is intentionally not defined here).
The congestion circuit breaker depends on the fraction of RTP data
packets lost in a reporting interval. If the number of packets sent
in the reporting interval is too low, this statistic loses meaning,
and it is possible that a sampling error can give the appearance of
high packet loss rates. Following the guidelines in [RFC5405], an
RTP sender that sends not more than one RTP packet per RTT MAY ignore
a single trigger of the congestion circuit breaker, on the basis that
the packet loss rate estimate is unreliable with so few samples.
However, if the congestion circuit breaker triggers again after the
following three RTCP reporting intervals (i.e., if there have been
six or more consecutive RTCP reporting intervals where the actual
sending rate is more than ten times the estimated sending rate
derived from the TCP throughput equation), then the sender SHOULD
cease transmission (see Section 4.5).
The RTCP reporting interval of the media sender does not affect how The RTCP reporting interval of the media sender does not affect how
quickly congestion circuit breaker can trigger. The timing is based quickly congestion circuit breaker can trigger. The timing is based
on the RTCP reporting interval of the receiver that generates the SR/ on the RTCP reporting interval of the receiver that generates the SR/
RR packets from which the loss rate and RTT estimate are derived RR packets from which the loss rate and RTT estimate are derived
(note that RTCP requires all participants in a session to have (note that RTCP requires all participants in a session to have
similar reporting intervals, else the participant timeout rules in similar reporting intervals, else the participant timeout rules in
[RFC3550] will not work, so this interval is likely similar to that [RFC3550] will not work, so this interval is likely similar to that
of the sender). If the incoming RTCP SR or RR packets are using a of the sender). If the incoming RTCP SR or RR packets are using a
reduced minimum RTCP reporting interval (as specified in Section 6.2 reduced minimum RTCP reporting interval (as specified in Section 6.2
of RFC 3550 [RFC3550] or the RTP/AVPF profile [RFC4585]), then that of RFC 3550 [RFC3550] or the RTP/AVPF profile [RFC4585]), then that
reduced RTCP reporting interval is used when determining if the reduced RTCP reporting interval is used when determining if the
circuit breaker is triggered. circuit breaker is triggered.
As in Section 4.1 and Section 4.2, we use three reporting intervals As in Section 4.1 and Section 4.2, we use CB_INTERVAL reporting
to avoid triggering the circuit breaker on transient failures. This intervals to avoid triggering the circuit breaker on transient
circuit breaker is a worst-case condition, and congestion control failures. This circuit breaker is a worst-case condition, and
needs to be performed to keep well within this bound. It is expected congestion control needs to be performed to keep well within this
that the circuit breaker will only be triggered if the usual bound. It is expected that the circuit breaker will only be
congestion control fails for some reason. triggered if the usual congestion control fails for some reason.
If there are more media streams that can be reported in a single RTCP If there are more media streams that can be reported in a single RTCP
SR or RR packet, or if the size of a complete RTCP SR or RR packet SR or RR packet, or if the size of a complete RTCP SR or RR packet
exceeds the network MTU, then the receiver will report on a subset of exceeds the network MTU, then the receiver will report on a subset of
sources in each reporting interval, with the subsets selected round- sources in each reporting interval, with the subsets selected round-
robin across multiple intervals so that all sources are eventually robin across multiple intervals so that all sources are eventually
reported [RFC3550]. When generating such round-robin RTCP reports, reported [RFC3550]. When generating such round-robin RTCP reports,
priority SHOULD be given to reports on sources that have high packet priority SHOULD be given to reports on sources that have high packet
loss rates, to ensure that senders are aware of network congestion loss rates, to ensure that senders are aware of network congestion
they are causing (this is an update to [RFC3550]). they are causing (this is an update to [RFC3550]).
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it is unusable at the receiver is both wasteful of resources, and of it is unusable at the receiver is both wasteful of resources, and of
no benefit to the user of the application. It also is highly likely no benefit to the user of the application. It also is highly likely
to be congesting the network, and disrupting other applications. As to be congesting the network, and disrupting other applications. As
such, the congestion circuit breaker will almost certainly trigger to such, the congestion circuit breaker will almost certainly trigger to
stop flows where the media would be unusable due to high packet loss stop flows where the media would be unusable due to high packet loss
or latency. However, in pathological scenarios where the congestion or latency. However, in pathological scenarios where the congestion
circuit breaker does not stop the flow, it is desirable that the RTP circuit breaker does not stop the flow, it is desirable that the RTP
application cease sending useless traffic. The role of the media application cease sending useless traffic. The role of the media
usability circuit breaker is to protect the network in such cases. usability circuit breaker is to protect the network in such cases.
4.5. Ceasing Transmission 4.5. Choice of Circuit Breaker Interval
The CB_INTERVAL parameter determines the number of consecutive RTCP
reporting intervals that need to suffer congestion before the media
timeout circuit breaker (see Section 4.1) or the congestion circuit
breaker (see Section 4.3) triggers. It determines the sensitivity
and responsiveness of these circuit breakers.
The CB_INTERVAL parameter is set to min(floor(3+(2.5/Td)), 30) RTCP
reporting intervals, where Td is the deterministic calculated RTCP
interval described in section 6.3.1 of [RFC3550]. This expression
gives an CB_INTERVAL that varies as follows:
Td | CB_INTERVAL | Time to trigger
--------------+------------------------------+-----------------
0.016 seconds | 30 RTCP reporting intervals | 0.48 seconds
0.033 seconds | 30 RTCP reporting intervals | 0.99 seconds
0.1 seconds | 28 RTCP reporting intervals | 2.8 seconds
0.5 seconds | 8 RTCP reporting intervals | 4.0 seconds
1.0 seconds | 5 RTCP reporting intervals | 5.5 seconds
2.0 seconds | 4 RTCP reporting intervals | 8.5 seconds
5.0 seconds | 5 RTCP reporting intervals | 17.5 seconds
10.0 seconds | 3 RTCP reporting intervals | 32.5 seconds
If the RTP/AVPF profile [RFC4585] or the RTP/SAVPF [RFC5124] is used,
and the T_rr_interval parameter is used to reduce the frequency of
regular RTCP reports, then the value Td in the above expression for
the CB_INTERVAL parameter MUST be replaced by T_rr_interval.
The CB_INTERVAL parameter is calculated on joining the session, and
recalculated on receipt of each RTCP packet, after checking whether
the media timeout circuit breaker or the congestion circuit breaker
has been triggered.
To ensure a timely response to persistent congestion, implementations
SHOULD NOT configure the RTCP bandwidth such that Td is larger than 5
seconds. Similarly, implementations that use the RTP/AVPF profile
[RFC4585] or the RTP/SAVPF profile [RFC5124] SHOULD NOT configure
T_rr_interval to values larger than 4 seconds (the reduced limit for
T_rr_interval follows Section 6.1.3 of
[I-D.ietf-avtcore-rtp-multi-stream]).
Rationale: If the CB_INTERVAL was always set to the same number of
RTCP reporting intervals, this would cause higher rate RTP sessions
to trigger the RTP circuit breaker after a shorter time interval than
lower rate sessions, because the RTCP reporting interval scales based
on the RTP session bandwidth. This is felt to penalise high rate RTP
sessions too aggressively. Conversely, scaling CB_INTERVAL according
to the inverse of the RTCP reporting interval, so the RTP circuit
breaker triggers after a constant time interval, doesn't sufficiently
protect the network from congestion caused by high-rate flows. The
chosen expression for CB_INTERVAL seeks a balance between these two
extremes. It causes higher rate RTP sessions subject to persistent
congestion to trigger the RTP circuit breaker after a shorter time
interval than do lower rate RTP sessions, while also making the RTP
circuit breaker for such sessions less sensitive by requiring the
congestion to persist for longer numbers of RTCP reporting intervals.
4.6. Ceasing Transmission
What it means to cease transmission depends on the application, but What it means to cease transmission depends on the application, but
the intention is that the application will stop sending RTP data the intention is that the application will stop sending RTP data
packets to a particular destination 3-tuple (transport protocol, packets to a particular destination 3-tuple (transport protocol,
destination port, IP address), until the user makes an explicit destination port, IP address), until the user makes an explicit
attempt to restart the call. It is important that a human user is attempt to restart the call. It is important that a human user is
involved in the decision to try to restart the call, since that user involved in the decision to try to restart the call, since that user
will eventually give up if the calls repeatedly trigger the circuit will eventually give up if the calls repeatedly trigger the circuit
breaker. This will help avoid problems with automatic redial systems breaker. This will help avoid problems with automatic redial systems
from congesting the network. Accordingly, RTP flows halted by the from congesting the network. Accordingly, RTP flows halted by the
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It is recognised that the RTP implementation in some systems might It is recognised that the RTP implementation in some systems might
not be able to determine if a call set-up request was initiated by a not be able to determine if a call set-up request was initiated by a
human user, or automatically by some scripted higher-level component human user, or automatically by some scripted higher-level component
of the system. These implementations SHOULD rate limit attempts to of the system. These implementations SHOULD rate limit attempts to
restart a call to the same destination 3-tuple as used by a previous restart a call to the same destination 3-tuple as used by a previous
call that was recently halted by the circuit breaker. The chosen call that was recently halted by the circuit breaker. The chosen
rate limit ought to not exceed the rate at which an annoyed human rate limit ought to not exceed the rate at which an annoyed human
caller might redial a misbehaving phone. caller might redial a misbehaving phone.
5. RTP Circuit Breakers for Systems Using the RTP/AVPF Profile 5. RTP Circuit Breakers and the RTP/AVPF and RTP/SAVPF Profiles
Use of the Extended RTP Profile for RTCP-based Feedback (RTP/AVPF) Use of the Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)
[RFC4585] allows receivers to send early RTCP reports in some cases, [RFC4585] allows receivers to send early RTCP reports in some cases,
to inform the sender about particular events in the media stream. to inform the sender about particular events in the media stream.
There are several use cases for such early RTCP reports, including There are several use cases for such early RTCP reports, including
providing rapid feedback to a sender about the onset of congestion. providing rapid feedback to a sender about the onset of congestion.
The RTP/SAVPF Profile [RFC5124] is a secure variant of the RTP/AVPF
profile, that is treated the same in the context of the RTP circuit
breaker. These feedback profiles are often used with non-compound
RTCP reports [RFC5506] to reduce the reporting overhead.
Receiving rapid feedback about congestion events potentially allows Receiving rapid feedback about congestion events potentially allows
congestion control algorithms to be more responsive, and to better congestion control algorithms to be more responsive, and to better
adapt the media transmission to the limitations of the network. It adapt the media transmission to the limitations of the network. It
is expected that many RTP congestion control algorithms will adopt is expected that many RTP congestion control algorithms will adopt
the RTP/AVPF profile for this reason, defining new transport layer the RTP/AVPF profile or the RTP/SAVPF profile for this reason,
feedback reports that suit their requirements. Since these reports defining new transport layer feedback reports that suit their
are not yet defined, and likely very specific to the details of the requirements. Since these reports are not yet defined, and likely
congestion control algorithm chosen, they cannot be used as part of very specific to the details of the congestion control algorithm
the generic RTP circuit breaker. chosen, they cannot be used as part of the generic RTP circuit
breaker.
Reduced-size RTCP reports sent under the RTP/AVPF early feedback Reduced-size RTCP reports sent under the RTP/AVPF early feedback
rules that do not contain an RTCP SR or RR packet MUST be ignored by rules that do not contain an RTCP SR or RR packet MUST be ignored by
the congestion circuit breaker (they do not contain the information the congestion circuit breaker (they do not contain the information
needed by the congestion circuit breaker algorithm), but MUST be needed by the congestion circuit breaker algorithm), but MUST be
counted as received packets for the RTCP timeout circuit breaker. counted as received packets for the RTCP timeout circuit breaker.
Reduced-size RTCP reports sent under the RTP/AVPF early feedback Reduced-size RTCP reports sent under the RTP/AVPF early feedback
rules that contain RTCP SR or RR packets MUST be processed by the rules that contain RTCP SR or RR packets MUST be processed by the
congestion circuit breaker as if they were sent as regular RTCP congestion circuit breaker as if they were sent as regular RTCP
reports, and counted towards the circuit breaker conditions specified reports, and counted towards the circuit breaker conditions specified
in Section 4 of this memo. This will potentially make the RTP in Section 4 of this memo. This will potentially make the RTP
circuit breaker fire earlier than it would if the RTP/AVPF profile circuit breaker trigger earlier than it would if the RTP/AVPF profile
was not used. was not used.
When using ECN with RTP (see Section 8), early RTCP feedback packets When using ECN with RTP (see Section 8), early RTCP feedback packets
can contain ECN feedback reports. The count of ECN-CE marked packets can contain ECN feedback reports. The count of ECN-CE marked packets
contained in those ECN feedback reports is counted towards the number contained in those ECN feedback reports is counted towards the number
of lost packets reported if the ECN Feedback Report report is sent in of lost packets reported if the ECN Feedback Report report is sent in
an compound RTCP packet along with an RTCP SR/RR report packet. an compound RTCP packet along with an RTCP SR/RR report packet.
Reports of ECN-CE packets sent as reduced-size RTCP ECN feedback Reports of ECN-CE packets sent as reduced-size RTCP ECN feedback
packets without an RTCP SR/RR packet MUST be ignored. packets without an RTCP SR/RR packet MUST be ignored.
These rules are intended to allow the use of low-overhead RTP/AVPF These rules are intended to allow the use of low-overhead RTP/AVPF
feedback for generic NACK messages without triggering the RTP circuit feedback for generic NACK messages without triggering the RTP circuit
breaker. This is expected to make such feedback suitable for RTP breaker. This is expected to make such feedback suitable for RTP
congestion control algorithms that need to quickly report loss events congestion control algorithms that need to quickly report loss events
in between regular RTCP reports. The reaction to reduced-size RTCP in between regular RTCP reports. The reaction to reduced-size RTCP
SR/RR packets is to allow such algorithms to send feedback that can SR/RR packets is to allow such algorithms to send feedback that can
trigger the circuit breaker, when desired. trigger the circuit breaker, when desired.
The RTP/AVPF and RTP/SAVPF profiles include the T_rr_interval
parameter that can be used to adjust the regular RTCP reporting
interval. The use of the T_rr_interval parameter changes the
behaviour of the RTP circuit breaker, as described in Section 4.
6. Impact of RTCP Extended Reports (XR) 6. Impact of RTCP Extended Reports (XR)
RTCP Extended Report (XR) blocks provide additional reception quality RTCP Extended Report (XR) blocks provide additional reception quality
metrics, but do not change the RTCP timing rules. Some of the RTCP metrics, but do not change the RTCP timing rules. Some of the RTCP
XR blocks provide information that might be useful for congestion XR blocks provide information that might be useful for congestion
control purposes, others provided non-congestion-related metrics. control purposes, others provided non-congestion-related metrics.
With the exception of RTCP XR ECN Summary Reports (see Section 8), With the exception of RTCP XR ECN Summary Reports (see Section 8),
the presence of RTCP XR blocks in a compound RTCP packet does not the presence of RTCP XR blocks in a compound RTCP packet does not
affect the RTP circuit breaker algorithm. For consistency and ease affect the RTP circuit breaker algorithm. For consistency and ease
of implementation, only the reception report blocks contained in RTCP of implementation, only the reception report blocks contained in RTCP
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authenticated; authentication options are discussed in [RFC7201]. authenticated; authentication options are discussed in [RFC7201].
Timely operation of the RTP circuit breaker depends on the choice of Timely operation of the RTP circuit breaker depends on the choice of
RTCP reporting interval. If the receiver has a reporting interval RTCP reporting interval. If the receiver has a reporting interval
that is overly long, then the responsiveness of the circuit breaker that is overly long, then the responsiveness of the circuit breaker
decreases. In the limit, the RTP circuit breaker can be disabled for decreases. In the limit, the RTP circuit breaker can be disabled for
all practical purposes by configuring an RTCP reporting interval that all practical purposes by configuring an RTCP reporting interval that
is many minutes duration. This issue is not specific to the circuit is many minutes duration. This issue is not specific to the circuit
breaker: long RTCP reporting intervals also prevent reception quality breaker: long RTCP reporting intervals also prevent reception quality
reports, feedback messages, codec control messages, etc., from being reports, feedback messages, codec control messages, etc., from being
used. Implementations SHOULD impose an upper limit on the RTCP used. Implementations are expected to impose an upper limit on the
reporting interval they are willing to negotiate (based on the RTCP reporting interval they are willing to negotiate (based on the
session bandwidth and RTCP bandwidth fraction) when using the RTP session bandwidth and RTCP bandwidth fraction) when using the RTP
circuit breaker. An upper limit on the reporting interval on the circuit breaker, as discussed in Section 4.5.
order of 10 seconds is a reasonable bound.
11. IANA Considerations 11. IANA Considerations
There are no actions for IANA. There are no actions for IANA.
12. Open Issues 12. Acknowledgements
o Should the number of RTCP reporting intervals needed to trigger
the media timeout and congestion circuit breakers scale with the
duration of the RTCP reporting interval, so the circuit breaker
triggers after a fixed duration, rather than after a fixed number
of reporting intervals?
13. Acknowledgements
The authors would like to thank Bernard Aboba, Harald Alvestrand, The authors would like to thank Bernard Aboba, Harald Alvestrand,
Gorry Fairhurst, Kevin Gross, Cullen Jennings, Randell Jesup, Gorry Fairhurst, Nazila Fough, Kevin Gross, Cullen Jennings, Randell
Jonathan Lennox, Matt Mathis, Stephen McQuistin, Eric Rescorla, Jesup, Jonathan Lennox, Matt Mathis, Stephen McQuistin, Eric
Abheek Saha, and Fabio Verdicchio, for their valuable feedback. Rescorla, Abheek Saha, Fabio Verdicchio, and Magnus Westerlund for
their valuable feedback.
14. References 13. References
14.1. Normative References 13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP [RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC Friendly Rate Control (TFRC): Protocol Specification", RFC
3448, January 2003. 3448, January 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
skipping to change at page 18, line 31 skipping to change at page 20, line 18
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003. 2003.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control "Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006. 2006.
14.2. Informative References 13.2. Informative References
[Floyd] Floyd, S., Handley, M., Padhye, J., and J. Widmer, [Floyd] Floyd, S., Handley, M., Padhye, J., and J. Widmer,
"Equation-Based Congestion Control for Unicast "Equation-Based Congestion Control for Unicast
Applications", Proceedings of the ACM SIGCOMM conference, Applications", Proceedings of the ACM SIGCOMM conference,
2000, DOI 10.1145/347059.347397, August 2000. 2000, DOI 10.1145/347059.347397, August 2000.
[I-D.ietf-avtcore-rtp-multi-stream-optimisation] [I-D.ietf-avtcore-rtp-multi-stream-optimisation]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins, Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session: "Sending Multiple Media Streams in a Single RTP Session:
Grouping RTCP Reception Statistics and Other Feedback", Grouping RTCP Reception Statistics and Other Feedback",
draft-ietf-avtcore-rtp-multi-stream-optimisation-04 (work draft-ietf-avtcore-rtp-multi-stream-optimisation-05 (work
in progress), August 2014. in progress), February 2015.
[I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-06 (work in progress),
October 2014.
[Mathis] Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The [Mathis] Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The
macroscopic behavior of the TCP congestion avoidance macroscopic behavior of the TCP congestion avoidance
algorithm", ACM SIGCOMM Computer Communication Review algorithm", ACM SIGCOMM Computer Communication Review
27(3), DOI 10.1145/263932.264023, July 1997. 27(3), DOI 10.1145/263932.264023, July 1997.
[Padhye] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose, [Padhye] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
"Modeling TCP Throughput: A Simple Model and its Empirical "Modeling TCP Throughput: A Simple Model and its Empirical
Validation", Proceedings of the ACM SIGCOMM conference, Validation", Proceedings of the ACM SIGCOMM conference,
1998, DOI 10.1145/285237.285291, August 1998. 1998, DOI 10.1145/285237.285291, August 1998.
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