rfc8836v2.txt   rfc8836.txt 
Internet Engineering Task Force (IETF) R. Jesup Internet Engineering Task Force (IETF) R. Jesup
Request for Comments: 8836 Mozilla Request for Comments: 8836 Mozilla
Category: Informational Z. Sarker, Ed. Category: Informational Z. Sarker, Ed.
ISSN: 2070-1721 Ericsson ISSN: 2070-1721 Ericsson AB
November 2020 January 2021
Congestion Control Requirements for Interactive Real-Time Media Congestion Control Requirements for Interactive Real-Time Media
Abstract Abstract
Congestion control is needed for all data transported across the Congestion control is needed for all data transported across the
Internet, in order to promote fair usage and prevent congestion Internet, in order to promote fair usage and prevent congestion
collapse. The requirements for interactive, point-to-point real-time collapse. The requirements for interactive, point-to-point real-time
multimedia, which needs low-delay, semi-reliable data delivery, are multimedia, which needs low-delay, semi-reliable data delivery, are
different from the requirements for bulk transfer like FTP or bursty different from the requirements for bulk transfer like FTP or bursty
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Internet Engineering Steering Group (IESG). Not all documents Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841. Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8836. https://www.rfc-editor.org/info/rfc8836.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction 1. Introduction
1.1. Requirements Language
2. Requirements 2. Requirements
3. Deficiencies of Existing Mechanisms 3. Deficiencies of Existing Mechanisms
4. IANA Considerations 4. IANA Considerations
5. Security Considerations 5. Security Considerations
6. References 6. References
6.1. Normative References 6.1. Normative References
6.2. Informative References 6.2. Informative References
Acknowledgements Acknowledgements
Authors' Addresses Authors' Addresses
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mechanism operate within the constraints defined by these circuit mechanism operate within the constraints defined by these circuit
breakers when a circuit breaker is present and that it should not breakers when a circuit breaker is present and that it should not
cause congestion collapse when a circuit breaker is not implemented. cause congestion collapse when a circuit breaker is not implemented.
Given that this use case is the focus of this document, use cases Given that this use case is the focus of this document, use cases
involving non-interactive media such as video streaming and those involving non-interactive media such as video streaming and those
using multicast/broadcast-type technologies, are out of scope. using multicast/broadcast-type technologies, are out of scope.
The terminology defined in [RFC8825] is used in this memo. The terminology defined in [RFC8825] is used in this memo.
1.1. Requirements Language
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 BCP 14 [RFC2119].
2. Requirements 2. Requirements
1. The congestion control algorithm must attempt to provide as-low- 1. The congestion control algorithm MUST attempt to provide as-low-
as-possible-delay transit for interactive real-time traffic as-possible-delay transit for interactive real-time traffic
while still providing a useful amount of bandwidth. There may while still providing a useful amount of bandwidth. There may
be lower limits on the amount of bandwidth that is useful, but be lower limits on the amount of bandwidth that is useful, but
this is largely application specific, and the application may be this is largely application specific, and the application may be
able to modify or remove flows in order to allow some useful able to modify or remove flows in order to allow some useful
flows to get enough bandwidth. For example, although there flows to get enough bandwidth. For example, although there
might not be enough bandwidth for low-latency video+audio, there might not be enough bandwidth for low-latency video+audio, there
could be enough for audio only. could be enough for audio only.
a. Jitter (variation in the bitrate over short timescales) is a. Jitter (variation in the bitrate over short timescales) is
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[MPEG_DASH] or proprietary media streaming algorithms may [MPEG_DASH] or proprietary media streaming algorithms may
compete in bursts with the algorithm and may not be adaptive compete in bursts with the algorithm and may not be adaptive
within a burst. They are often layered on top of TCP but within a burst. They are often layered on top of TCP but
use TCP in a bursty manner that can interact poorly with use TCP in a bursty manner that can interact poorly with
competing flows during the bursts. The algorithm must not competing flows during the bursts. The algorithm must not
increase the already existing delay buildup during those increase the already existing delay buildup during those
bursts. Note that this competing traffic may be on a shared bursts. Note that this competing traffic may be on a shared
access link, or the traffic burst may cause a shift in the access link, or the traffic burst may cause a shift in the
location of the bottleneck for the duration of the burst. location of the bottleneck for the duration of the burst.
2. The algorithm must be fair to other flows, both real-time flows 2. The algorithm MUST be fair to other flows, both real-time flows
(such as other instances of itself) and TCP flows, both long- (such as other instances of itself) and TCP flows, both long-
lived flows and bursts such as the traffic generated by a lived flows and bursts such as the traffic generated by a
typical web-browsing session. Note that "fair" is a rather typical web-browsing session. Note that "fair" is a rather
hard-to-define term. It should be fair with itself, giving a hard-to-define term. It SHOULD be fair with itself, giving a
fair share of the bandwidth to multiple flows with similar RTTs, fair share of the bandwidth to multiple flows with similar RTTs,
and if possible to multiple flows with different RTTs. and if possible to multiple flows with different RTTs.
a. Existing flows at a bottleneck must also be fair to new a. Existing flows at a bottleneck must also be fair to new
flows to that bottleneck and must allow new flows to ramp up flows to that bottleneck and must allow new flows to ramp up
to a useful share of the bottleneck bandwidth as quickly as to a useful share of the bottleneck bandwidth as quickly as
possible. A useful share will depend on the media types possible. A useful share will depend on the media types
involved, total bandwidth available, and the user-experience involved, total bandwidth available, and the user-experience
requirements of a particular service. Note that relative requirements of a particular service. Note that relative
RTTs may affect the rate at which new flows can ramp up to a RTTs may affect the rate at which new flows can ramp up to a
reasonable share. reasonable share.
3. The algorithm should not starve competing TCP flows and should, 3. The algorithm SHOULD NOT starve competing TCP flows and SHOULD,
as best as possible, avoid starvation by TCP flows. as best as possible, avoid starvation by TCP flows.
a. The congestion control should prioritize achieving a useful a. The congestion control should prioritize achieving a useful
share of the bandwidth depending on the media types and share of the bandwidth depending on the media types and
total available bandwidth over achieving as-low-as-possible total available bandwidth over achieving as-low-as-possible
transit delay, when these two requirements are in conflict. transit delay, when these two requirements are in conflict.
4. The algorithm should adapt as quickly as possible to initial 4. The algorithm SHOULD adapt as quickly as possible to initial
network conditions at the start of a flow. This should occur network conditions at the start of a flow. This SHOULD occur
whether the initial bandwidth is above or below the bottleneck whether the initial bandwidth is above or below the bottleneck
bandwidth. bandwidth.
a. The algorithm should allow different modes of adaptation; a. The algorithm should allow different modes of adaptation;
for example, the startup adaptation may be faster than for example, the startup adaptation may be faster than
adaptation later in a flow. It should allow for both slow- adaptation later in a flow. It should allow for both slow-
start operation (adapt up) and history-based startup (start start operation (adapt up) and history-based startup (start
at a point expected to be at or below channel bandwidth from at a point expected to be at or below channel bandwidth from
historical information, which may need to adapt down quickly historical information, which may need to adapt down quickly
if the initial guess is wrong). Starting too low and/or if the initial guess is wrong). Starting too low and/or
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high above the available bandwidth causes other problems for high above the available bandwidth causes other problems for
user experience, so there's a tension here. Alternative user experience, so there's a tension here. Alternative
methods to help startup, such as probing during setup with methods to help startup, such as probing during setup with
dummy data, may be useful in some applications; in some dummy data, may be useful in some applications; in some
cases, there will be a considerable gap in time between flow cases, there will be a considerable gap in time between flow
creation and the initial flow of data. Again, a flow may creation and the initial flow of data. Again, a flow may
need to change adaptation rates due to network conditions or need to change adaptation rates due to network conditions or
changes in the provided flows (such as unmuting or sending changes in the provided flows (such as unmuting or sending
data after a gap). data after a gap).
5. The algorithm should be stable if the RTP streams are halted or 5. The algorithm SHOULD be stable if the RTP streams are halted or
discontinuous (for example, when using Voice Activity discontinuous (for example, when using Voice Activity
Detection). Detection).
a. After stream resumption, the algorithm should attempt to a. After stream resumption, the algorithm should attempt to
rapidly regain its previous share of the bandwidth; the rapidly regain its previous share of the bandwidth; the
aggressiveness with which this is done will decay with the aggressiveness with which this is done will decay with the
length of the pause. length of the pause.
6. Where possible, the algorithm should merge information across 6. Where possible, the algorithm SHOULD merge information across
multiple RTP streams sent between two endpoints when those RTP multiple RTP streams sent between two endpoints when those RTP
streams share a common bottleneck, whether or not those streams streams share a common bottleneck, whether or not those streams
are multiplexed onto the same ports. This will allow congestion are multiplexed onto the same ports. This will allow congestion
control of the set of streams together instead of as multiple control of the set of streams together instead of as multiple
independent streams. It will also allow better overall independent streams. It will also allow better overall
bandwidth management, faster response to changing conditions, bandwidth management, faster response to changing conditions,
and fairer sharing of bandwidth with other network users. and fairer sharing of bandwidth with other network users.
a. The algorithm should also share information and adaptation a. The algorithm should also share information and adaptation
with other non-RTP flows between the same endpoints, such as with other non-RTP flows between the same endpoints, such as
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coordinating their bandwidth use and congestion control, the coordinating their bandwidth use and congestion control, the
algorithm should allow the application to control the algorithm should allow the application to control the
relative split of available bandwidth. The most correlated relative split of available bandwidth. The most correlated
bandwidth usage would be with other flows on the same bandwidth usage would be with other flows on the same
5-tuple, but there may be use in coordinating measurement 5-tuple, but there may be use in coordinating measurement
and control of the local link(s). Use of information about and control of the local link(s). Use of information about
previous flows, especially on the same 5-tuple, may be previous flows, especially on the same 5-tuple, may be
useful input to the algorithm, especially regarding startup useful input to the algorithm, especially regarding startup
performance of a new flow. performance of a new flow.
7. The algorithm should not require any special support from 7. The algorithm SHOULD NOT require any special support from
network elements to be able to convey congestion-related network elements to be able to convey congestion-related
information. As much as possible, it should leverage available information. As much as possible, it SHOULD leverage available
information about the incoming flow to provide feedback to the information about the incoming flow to provide feedback to the
sender. Examples of this information are the packet arrival sender. Examples of this information are the packet arrival
times, acknowledgements and feedback, packet timestamps, packet times, acknowledgements and feedback, packet timestamps, packet
losses, and Explicit Congestion Notification (ECN) [RFC3168]; losses, and Explicit Congestion Notification (ECN) [RFC3168];
all of these can provide information about the state of the path all of these can provide information about the state of the path
and any bottlenecks. However, the use of available information and any bottlenecks. However, the use of available information
is algorithm dependent. is algorithm dependent.
a. Extra information could be added to the packets to provide a. Extra information could be added to the packets to provide
more detailed information on actual send times (as opposed more detailed information on actual send times (as opposed
to sampling times), but such information should not be to sampling times), but such information should not be
required. required.
8. Since the assumption here is a set of RTP streams, the 8. Since the assumption here is a set of RTP streams, the
backchannel typically should be done via the RTP Control backchannel typically SHOULD be done via the RTP Control
Protocol (RTCP) [RFC3550]; instead, one alternative would be to Protocol (RTCP) [RFC3550]; instead, one alternative would be to
include it in a reverse-RTP channel using header extensions. include it in a reverse-RTP channel using header extensions.
a. In order to react sufficiently quickly when using RTCP for a a. In order to react sufficiently quickly when using RTCP for a
backchannel, an RTP profile such as RTP/AVPF [RFC4585] or backchannel, an RTP profile such as RTP/AVPF [RFC4585] or
RTP/SAVPF [RFC5124] that allows sufficiently frequent RTP/SAVPF [RFC5124] that allows sufficiently frequent
feedback must be used. Note that in some cases, backchannel feedback must be used. Note that in some cases, backchannel
messages may be delayed until the RTCP channel can be messages may be delayed until the RTCP channel can be
allocated enough bandwidth, even under AVPF rules. This may allocated enough bandwidth, even under AVPF rules. This may
also imply negotiating a higher maximum percentage for RTCP also imply negotiating a higher maximum percentage for RTCP
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frequent and use more reverse-channel bandwidth. This is an frequent and use more reverse-channel bandwidth. This is an
area with considerable flexibility of design, and different area with considerable flexibility of design, and different
approaches to backchannel messages and frequency are approaches to backchannel messages and frequency are
expected to be evaluated. expected to be evaluated.
9. Flows managed by this algorithm and flows competing against each 9. Flows managed by this algorithm and flows competing against each
other at a bottleneck may have different Differentiated Services other at a bottleneck may have different Differentiated Services
Code Point (DSCP) [RFC5865] markings depending on the type of Code Point (DSCP) [RFC5865] markings depending on the type of
traffic or may be subject to flow-based QoS. A particular traffic or may be subject to flow-based QoS. A particular
bottleneck or section of the network path may or may not honor bottleneck or section of the network path may or may not honor
DSCP markings. The algorithm should attempt to leverage DSCP DSCP markings. The algorithm SHOULD attempt to leverage DSCP
markings when they're available. markings when they're available.
10. The algorithm should sense the unexpected lack of backchannel 10. The algorithm SHOULD sense the unexpected lack of backchannel
information as a possible indication of a channel-overuse information as a possible indication of a channel-overuse
problem and react accordingly to avoid burst events causing a problem and react accordingly to avoid burst events causing a
congestion collapse. congestion collapse.
11. The algorithm should be stable and maintain low delay when faced 11. The algorithm SHOULD be stable and maintain low delay when faced
with Active Queue Management (AQM) algorithms. Also note that with Active Queue Management (AQM) algorithms. Also note that
these algorithms may apply across multiple queues in the these algorithms may apply across multiple queues in the
bottleneck or to a single queue. bottleneck or to a single queue.
3. Deficiencies of Existing Mechanisms 3. Deficiencies of Existing Mechanisms
Among the existing congestion control mechanisms, TCP Friendly Rate Among the existing congestion control mechanisms, TCP Friendly Rate
Control (TFRC) [RFC5348] is the one that claims to be suitable for Control (TFRC) [RFC5348] is the one that claims to be suitable for
real-time interactive media. TFRC is an equation-based congestion real-time interactive media. TFRC is an equation-based congestion
control mechanism that provides a reasonably fair share of bandwidth control mechanism that provides a reasonably fair share of bandwidth
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are conceivable and need to be evaluated. Such attacks could result are conceivable and need to be evaluated. Such attacks could result
in starvation of competing flows and permit amplification attacks. in starvation of competing flows and permit amplification attacks.
Algorithm designers should consider the possibility of malicious on- Algorithm designers should consider the possibility of malicious on-
path attackers. path attackers.
6. References 6. References
6.1. Normative References 6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[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, Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006, DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>. <https://www.rfc-editor.org/info/rfc4585>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, <https://www.rfc-editor.org/info/rfc5124>. 2008, <https://www.rfc-editor.org/info/rfc5124>.
[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for [RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825, Browser-Based Applications", RFC 8825,
DOI 10.17487/RFC8825, November 2020, DOI 10.17487/RFC8825, January 2021,
<https://www.rfc-editor.org/info/rfc8825>. <https://www.rfc-editor.org/info/rfc8825>.
6.2. Informative References 6.2. Informative References
[CH09] Choi, S. and M. Handley, "Designing TCP-Friendly Window- [CH09] Choi, S. and M. Handley, "Designing TCP-Friendly Window-
based Congestion Control for Real-time Multimedia based Congestion Control for Real-time Multimedia
Applications", Proceedings of PFLDNeT, May 2009. Applications", Proceedings of PFLDNeT, May 2009.
[MPEG_DASH] [MPEG_DASH]
ISO, "Information Technology -- Dynamic adaptive streaming ISO, "Information Technology -- Dynamic adaptive streaming
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Interactive Real-Time Communication", RFC 7295, Interactive Real-Time Communication", RFC 7295,
DOI 10.17487/RFC7295, July 2014, DOI 10.17487/RFC7295, July 2014,
<https://www.rfc-editor.org/info/rfc7295>. <https://www.rfc-editor.org/info/rfc7295>.
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control: [RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083, Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017, DOI 10.17487/RFC8083, March 2017,
<https://www.rfc-editor.org/info/rfc8083>. <https://www.rfc-editor.org/info/rfc8083>.
[RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data [RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
Channels", RFC 8831, DOI 10.17487/RFC8831, November 2020, Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
<https://www.rfc-editor.org/info/rfc8831>. <https://www.rfc-editor.org/info/rfc8831>.
Acknowledgements Acknowledgements
This document is the result of discussions in various fora of the This document is the result of discussions in various fora of the
WebRTC effort, in particular on the <rtp-congestion@alvestrand.no> WebRTC effort, in particular on the <rtp-congestion@alvestrand.no>
mailing list. Many people contributed their thoughts to this. mailing list. Many people contributed their thoughts to this.
Authors' Addresses Authors' Addresses
Randell Jesup Randell Jesup
Mozilla Mozilla
United States of America United States of America
Email: randell-ietf@jesup.org Email: randell-ietf@jesup.org
Zaheduzzaman Sarker (editor) Zaheduzzaman Sarker (editor)
Ericsson Ericsson AB
Torshamnsgatan 23
SE-164 83 Stockholm
Sweden Sweden
Phone: +46 10 717 37 43
Email: zaheduzzaman.sarker@ericsson.com Email: zaheduzzaman.sarker@ericsson.com
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