Network Working Group
Internet Engineering Task Force (IETF) R. Jesup
Request for Comments: 8836 Mozilla
Category: Informational Z. Sarker, Ed.
Expires: March 14, 2019
ISSN: 2070-1721 Ericsson
September 10, 2018
Congestion Control Requirements for Interactive Real-Time Media
Congestion control is needed for all data transported across the
Internet, in order to promote fair usage and prevent congestion
collapse. The requirements for interactive, point-to-point real-time
multimedia, which needs low-delay, semi-reliable data delivery, are
different from the requirements for bulk transfer like FTP or bursty
transfers like Web web pages. Due to an increasing amount of RTP-based
real-time media traffic on the Internet (e.g. (e.g., with the introduction
of the Web Real-Time Communication (WebRTC)), it is especially
important to ensure that this kind of traffic is congestion
This document describes a set of requirements that can be used to
evaluate other congestion control mechanisms in order to figure out
their fitness for this purpose, and in particular to provide a set of
possible requirements for a real-time media congestion avoidance
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The terms are presented in many cases using lowercase for
Status of This Memo
This Internet-Draft document is not an Internet Standards Track specification; it is
published for informational purposes.
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provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft the IETF community. It has
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Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of six months this document, any errata,
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This Internet-Draft will expire on March 14, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Deficiencies of existing mechanisms . . . . . . . . . . . . . 8 Existing Mechanisms
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
Most of today's TCP congestion control schemes were developed with a
focus on an a use of the Internet for reliable bulk transfer of non-
time-critical data, such as transfer of large files. They have also
been used successfully to govern the reliable transfer of smaller
chunks of data in as short a time as possible, such as when fetching
These algorithms have also been used for transfer of media streams
that are viewed in a non-interactive manner, such as "streaming"
video, where having the data ready when the viewer wants it is
important, but the exact timing of the delivery is not.
When doing handling real-time interactive media, the requirements are
different. One needs to provide the data continuously, within a very
limited time window (no more delay than 100s hundreds of milliseconds end-to-end
to-end). In addition, the sources of data may be able to adapt the
amount of data that needs sending within fairly wide margins margins, but
they can be rate limited by the application- application -- even not always have having
data to send, and send. They may tolerate some amount of packet loss, but
since the data is generated in real-time, real time, sending "future" data is
impossible, and since it's consumed in real-time, real time, data delivered late
is commonly useless.
While the requirements for real-time interactive media differ from
the requirements for the other flow types, these other flow types
will be present in the network. The congestion control algorithm for
real-time interactive media must work properly when these other flow
types are present as cross traffic on the network.
One particular protocol portfolio being developed for this use case
is WebRTC [I-D.ietf-rtcweb-overview], [RFC8825], where one envisions sending multiple flows using
the Real-time Transport Protocol (RTP) [RFC3550] between two peers,
in conjunction with data flows, all at the same time, without having
special arrangements with the intervening service providers. As RTP
does not provide any congestion control
mechanism; mechanism, a set of circuit
breakers, such as
[I-D.ietf-avtcore-rtp-circuit-breakers], those described in [RFC8083], are required to
protect the network from excessive congestion caused by the non-congestion
congestion-controlled flows. When the real-time interactive media is
congestion controlled, it is recommended that the congestion control
operates operate within the constraints defined by these circuit
breakers when a circuit breaker is present and that it should not
cause congestion collapse when a circuit breaker is not implemented.
Given that this use case is the focus of this document, use cases
involving non-interactive media such as video streaming, streaming and use
using multicast/broadcast-type technologies, are out of scope.
The terminology defined in [I-D.ietf-rtcweb-overview] [RFC8825] is used in this memo.
1. The congestion control algorithm must attempt to provide as-low-
as-possible-delay transit for interactive real-time traffic
while still providing a useful amount of bandwidth. There may
be lower limits on the amount of bandwidth that is useful, but
this is largely application-specific application specific, and the application may be
able to modify or remove flows in order to allow some useful
flows to get enough bandwidth. (Example: For example, although there
might not be enough bandwidth for low-latency video+audio, but there
could be enough for audio-only.)
A. audio only.
a. Jitter (variation in the bitrate over short time scales)
also timescales) is
also relevant, though moderate amounts of jitter will be
absorbed by jitter buffers. Transit delay should be
considered to track the short-term maximums of delay delay,
b. The algorithm should provide this as-low-as-possible-delay
transit and minimize self-induced latency even when faced
with intermediate bottlenecks and competing flows.
Competing flows may limit what's possible to achieve.
c. The algorithm should be resilience resilient to the effects of the events,
such as routing changes, which may alter or remove
bottlenecks or change the bandwidth available available, especially if
there is a reduction in available bandwidth or increase in
observed delay. It is expected that the mechanism reacts
the such events to avoid delay buildup. In the
context of this memo, a 'quick' "quick" reaction is on the order of
a few RTTs, subject to the constraints of the media codec,
but is likely within a second. Reaction on the next RTT is
explicitly not required, since many codecs cannot adapt
their sending rate that quickly, but equally a response
cannot be arbitrarily delayed.
d. The algorithm should react quickly to handle both local and
remote interface changes (WLAN (e.g., WLAN to 3G data, etc) which data) that may
radically change the bandwidth available or bottlenecks,
especially if there is a reduction in available bandwidth or
an increase in bottleneck delay. It is assumed that an
interface change can generate a notification to the
e. The real-time interactive media applications can be rate
limited. This means the offered loads can be less than the
available bandwidth at any given moment, moment and may vary
dramatically over time, including dropping to no load and
then resuming a high load, such as in a mute/unmute
operation. Hence, the algorithm must be designed to handle
such behavior from a media source or application. Note that
the reaction time between a change in the bandwidth
available from the algorithm and a change in the offered
load is variable, and it may be different when increasing
f. The algorithm requires is required to avoid building up queues when
competing with short-term bursts of traffic (for example,
traffic generated by web-browsing) web browsing), which can quickly
saturate a local-bottleneck router or link, link but also clear
quickly. The algorithm should also react quickly to regain
its previous share of the bandwidth when the local- local
bottleneck or link is cleared.
g. Similarly, periodic bursty flows such as MPEG DASH
[MPEG_DASH] or proprietary media streaming algorithms may
compete in bursts with the algorithm, algorithm and may not be adaptive
within a burst. They are often layered on top of TCP but
use TCP in a bursty manner that can interact poorly with
competing flows during the bursts. The algorithm must not
increase the already existing delay buildup during those
bursts. Note that this competing traffic may be on a shared
access link, or the traffic burst may cause a shift in the
location of the bottleneck for the duration of the burst.
2. The algorithm must be fair to other flows, both real-time flows
(such as other instances of itself), itself) and TCP flows, both long-
lived flows and bursts such as the traffic generated by a
browsing web-browsing session. Note that 'fair' "fair" is a rather
hard-to-define term. It should be fair with itself, giving a
fair share of the bandwidth to multiple flows with similar RTTs,
and if possible to multiple flows with different RTTs.
a. Existing flows at a bottleneck must also be fair to new
flows to that bottleneck, bottleneck and must allow new flows to ramp up
to a useful share of the bottleneck bandwidth as quickly as
possible. A useful share will depend on the media types
involved, total bandwidth available available, and the user experience user-experience
requirements of a particular service. Note that relative
RTTs may affect the rate at which new flows can ramp up to a
3. The algorithm should not starve competing TCP flows, flows and should should,
as best as possible possible, avoid starvation by TCP flows.
a. The congestion control should prioritise prioritize achieving a useful
share of the bandwidth depending on the media types and
total available bandwidth over achieving as low as possible as-low-as-possible
transit delay, when these two requirements are in conflict.
4. The algorithm should adapt as quickly as possible adapt to initial
network conditions at the start of a flow. This should occur
whether the initial bandwidth is above or below the bottleneck
a. The algorithm should allow different modes of adaptation adaptation;
example,the example, the startup adaptation may be faster than
adaptation later in a flow. It should allow for both slow-start slow-
start operation (adapt up) and history-based startup (start
at a point expected to be at or below channel bandwidth from
historical information, which may need to adapt down quickly
if the initial guess is wrong). Starting too low and/or
adapting up too slowly can cause a critical point in a
personal communication to be poor ("Hello!"). Starting
over-bandwidth causes other problems for user experience, so
there's a tension here. Alternative methods to help startup
startup, such as probing during setup with dummy data data, may
be useful in some applications; in some cases cases, there will be
a considerable gap in time between flow creation and the
initial flow of data. Again, A a flow may need to change
adaptation rates due to network conditions or changes in the
provided flows (such as un-muting unmuting or sending data after a
5. The algorithm should be stable if the RTP streams are halted or
discontinuous (for example - example, when using Voice Activity
a. After stream resumption, the algorithm should attempt to
rapidly regain its previous share of the bandwidth; the
aggressiveness with which this is done will decay with the
length of the pause.
6. The Where possible, the algorithm should where possible merge information across
multiple RTP streams sent between two endpoints, endpoints when those RTP
streams share a common bottleneck, whether or not those streams
are multiplexed onto the same ports, in order to ports. This will allow congestion
control of the set of streams together instead of as multiple
independent streams. This allows It will also allow better overall
bandwidth management, faster response to changing conditions,
and fairer sharing of bandwidth with other network users.
a. The algorithm should also share information and adaptation
with other non-RTP flows between the same endpoints, such as
a WebRTC DataChannel [I-D.ietf-rtcweb-data-channel], data channel [RFC8831], when possible.
b. When there are multiple streams across the same 5-tuple
coordinating their bandwidth use and congestion control, the
algorithm should allow the application to control the
relative split of available bandwidth. The most correlated
bandwidth usage would be with other flows on the same
5-tuple, but there may be use in coordinating measurement
and control of the local link(s). Use of information about
previous flows, especially on the same 5-tuple, may be
useful input to the algorithm, especially to regarding startup
performance of a new flow.
7. The algorithm should not require any special support from
network elements to convey congestion related congestion-related information to be
functional. As much as possible, it should leverage available
information about the incoming flow to provide feedback to the
sender. Examples of this information are the packet arrival
times, acknowledgements and feedback, packet timestamps, and packet
losses, and Explicit Congestion Notification (ECN) [RFC3168];
all of these can provide information about the state of the path
and any bottlenecks. However, the use of available information
is algorithm dependent.
a. Extra information could be added to the packets to provide
more detailed information on actual send times (as opposed
to sampling times), but such information should not be
8. Since the assumption here is a set of RTP streams, the
backchannel typically should be done via RTCP[RFC3550]; the RTP Control
Protocol (RTCP) [RFC3550]; instead, one alternative would be to
include it instead in a reverse RTP reverse-RTP channel using header extensions.
a. In order to react sufficiently quickly when using RTCP for a
backchannel, an RTP profile such as RTP/AVPF [RFC4585] or
RTP/SAVPF [RFC5124] that allows sufficiently frequent
feedback must be used. Note that in some cases, backchannel
messages may be delayed until the RTCP channel can be
allocated enough bandwidth, even under AVPF rules. This may
also imply negotiating a higher maximum percentage for RTCP
data or allowing solutions to violate or modify the rules
specified for AVPF.
b. Bandwidth for the feedback messages should be minimized
using techniques such as via RFC 5506 [RFC5506]to those in [RFC5506], to allow RTCP
C. Sender/Receiver Reports.
c. Backchannel data should be minimized to avoid taking too
much reverse-channel bandwidth (since this will often be
used in a bidirectional set of flows). In areas of
stability, backchannel data may be sent more infrequently so
long as algorithm stability and fairness are maintained.
When the channel is unstable or has not yet reached
equilibrium after a change, backchannel feedback may be more
frequent and use more reverse-channel bandwidth. This is an
area with considerable flexibility of design, and different
approaches to backchannel messages and frequency are
expected to be evaluated.
9. Flows managed by this algorithm and flows competing against each
other at a bottleneck may have different DSCP[RFC5865] Differentiated Services
Code Point (DSCP) [RFC5865] markings depending on the type of traffic,
traffic or may be subject to flow-based QoS. A particular
bottleneck or section of the network path may or may not honor
DSCP markings. The algorithm should attempt to leverage DSCP
markings when they're available.
a. In WebRTC, a division of packets into 4 classes is
envisioned in order of priority: faster-than-audio, audio,
video, best-effort, and bulk-transfer. Typically Typically, the flows
managed by this algorithm would be audio or video in that
hierarchy, and feedback flows would be faster-than-audio.
10. The algorithm should sense the unexpected lack of backchannel
information as a possible indication of a channel overuse channel-overuse
problem and react accordingly to avoid burst events causing a
11. The algorithm should be stable and maintain low-delay low delay when faced
with Active Queue Management (AQM) algorithms. Also note that
these algorithms may apply across multiple queues in the
bottleneck or to a single queue queue.
3. Deficiencies of existing mechanisms Existing Mechanisms
Among the existing congestion control mechanisms mechanisms, TCP Friendly Rate
Control (TFRC) [RFC5348] is the one which that claims to be suitable for
real-time interactive media. TFRC is, is an equation based, equation-based congestion
control mechanism which that provides a reasonably fair share of the bandwidth
when competing with TCP flows and offers much lower throughput
variations than TCP. This is achieved by a slower response to the
available bandwidth change than TCP. TFRC is designed to perform
best with applications which has that have a fixed packet size and does do not have
a fixed period between sending packets.
TFRC operates on detecting detects loss events and reacts to congestion-caused loss caused by
reducing its sending rate. It allows applications to increase the
sending rate until loss is observed in the flows. As it
is noted in IAB/IRTF IAB/
IRTF report [RFC7295] [RFC7295], large buffers are available in the network elements
elements, which introduces introduce additional delay in the
communication, it communication. It
becomes important to take all possible congestion indications into considerations.
consideration. Looking at the current Internet deployment, TFRC's
only consideration of loss events as congestion indication can be
considered as biggest lacking.
A typical real-time interactive communication includes live encoded live-encoded
audio and video flow(s). In such a communication scenario scenario, an audio
source typically needs a fixed interval between packets, packets and needs to
vary their segment size instead of their packet rate in response to
congestion and sends smaller packets, a packets. A variant of TFRC , TFRC, Small-
Packet TFRC (TFRC-SP) [RFC4828] [RFC4828], addresses the issues related to such
kind of sources ; a sources. A video source generally varies video frame sizes,
can produce large frames which that need to be further fragmented to fit
into path Maximum Transmission Unit (MTU) size, and have has an almost
fixed interval between producing frames under a certain frame rate, rate.
TFRC is known to be less optimal when using with such video sources.
There are also some mismatches between TFRC's design assumptions and
how the media sources in a typical real-time interactive application
work. TFRC is design designed to maintain a smooth sending rate however rate; however,
media sources can change rates in steps for both rate increase and
rate decrease. TFRC can operate in two modes - modes: i) Bytes bytes per second
and ii) packets per second, where typical real-time interactive media
sources operates operate on bit per second. There are also limitations on how
quickly the media sources can adapt to specific sending rates.
Modern video encoders can operate on in a mode where in which they can vary
the output bitrate a lot depending on the way there they are configured,
the current scene it is encoding they are encoding, and more. Therefore, it is
possible that the video source does will not always output at a bitrate they are
allowed to. an allowable
bitrate. TFRC tries to raise increase its sending rate when transmitting
at the maximum allowed rate rate, and it increases only twice the current
transmission rate hence rate; hence, it may create issues when the video source sources
vary their bitrates.
Moreover, there are a number of studies on TFRC which shows it's
limitations which includes that show its
limitations, including TFRC's unfairness on to low statistically
multiplexed links, oscillatory behavior, performance issue issues in highly
dynamic loss rates conditions loss-rate conditions, and more [CH09].
Looking at all these deficiencies deficiencies, it can be concluded that the
requirements of for a congestion control mechanism for real-time
interactive media cannot be met by TFRC as defined in the standard.
4. IANA Considerations
This document makes has no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC. IANA actions.
5. Security Considerations
An attacker with the ability to delete, delay delay, or insert messages in
into the flow can fake congestion signals, unless they are passed on
a tamper-proof path. Since some possible algorithms depend on the
timing of packet arrival, even a traditional traditional, protected channel does
not fully mitigate such attacks.
An attack that reduces bandwidth is not necessarily significant,
since an on-path attacker could break the connection by discarding
all packets. Attacks that increase the perceived available bandwidth
are conceivable, conceivable and need to be evaluated. Such attacks could result
in starvation of competing flows and permit amplification attacks.
Algorithm designers should consider the possibility of malicious on-
This document is the result of discussions in various fora of the
WebRTC effort, in particular on the email@example.com
mailing list. Many people contributed their thoughts to this.
6.1. Normative References
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-19
(work in progress), November 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825,
DOI 10.17487/RFC8825, June 2020,
6.2. Informative References
[CH09] Choi, S. and M. Handley, "Designing TCP-Friendly Window-
based Congestion Control for Real-time Multimedia
Applications", PFLDNeT 2009 Workshop , Proceedings of PFLDNeT, May 2009.
Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", draft-ietf-
avtcore-rtp-circuit-breakers-18 (work in progress), August
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-13 (work in
progress), January 2015.
ISO, "Information Technology -- Dynamic adaptive streaming
over HTTP (DASH) -- Part 1: Media presentation description
and segment formats", April
2012. ISO/IEC 23009-1:2019, December 2019,
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
(TFRC): The Small-Packet (SP) Variant", RFC 4828,
DOI 10.17487/RFC4828, April 2007,
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
[RFC7295] Tschofenig, H., Eggert, L., and Z. Sarker, "Report from
the IAB/IRTF Workshop on Congestion Control for
Interactive Real-Time Communication", RFC 7295,
DOI 10.17487/RFC7295, July 2014,
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
[RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
Channels", RFC 8831, DOI 10.17487/RFC8831, June 2020,
This document is the result of discussions in various fora of the
WebRTC effort, in particular on the <firstname.lastname@example.org>
mailing list. Many people contributed their thoughts to this.
United States of America
Zaheduzzaman Sarker (editor)