draft-ietf-rmcat-cc-requirements-04.txt   draft-ietf-rmcat-cc-requirements-05.txt 
Network Working Group R. Jesup Network Working Group R. Jesup
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Informational April 19, 2014 Intended status: Informational July 4, 2014
Expires: October 21, 2014 Expires: January 5, 2015
Congestion Control Requirements For RMCAT Congestion Control Requirements For RMCAT
draft-ietf-rmcat-cc-requirements-04 draft-ietf-rmcat-cc-requirements-05
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
transfers like Web pages. Due to an increasing amount of RTP-based transfers like Web pages. Due to an increasing amount of RTP-based
real-time media traffic on the Internet (e.g. with the introduction real-time media traffic on the Internet (e.g. with the introduction
of WebRTC[I-D.ietf-rtcweb-overview]), it is especially important to of WebRTC[I-D.ietf-rtcweb-overview]), it is especially important to
ensure that this kind of traffic is congestion controlled. ensure that this kind of traffic is congestion controlled.
This document attempts to describe a set of requirements that can be This document describes a set of requirements that can be used to
used to evaluate other congestion control mechanisms in order to evaluate other congestion control mechanisms in order to figure out
figure out their fitness for this purpose, and in particular to their fitness for this purpose, and in particular to provide a set of
provide a set of possible requirements for proposals coming out of possible requirements for realtime media congestion avoidance
the RMCAT Working Group. technique.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
The terms are presented in many cases using lowercase for
readability.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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 October 21, 2014.
This Internet-Draft will expire on January 5, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8 6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8 6.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
Most of today's TCP congestion control schemes were developed with a Most of today's TCP congestion control schemes were developed with a
focus on an use of the Internet for reliable bulk transfer of non- focus on an use of the Internet for reliable bulk transfer of non-
time-critical data, such as transfer of large files. They have also time-critical data, such as transfer of large files. They have also
been used successfully to govern the reliable transfer of smaller been used successfully to govern the reliable transfer of smaller
chunks of data in as short a time as possible, such as when fetching chunks of data in as short a time as possible, such as when fetching
Web pages. Web pages.
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A. It should provide this as-low-as-possible-delay transit even A. It should provide this as-low-as-possible-delay transit even
when faced with intermediate bottlenecks and competing when faced with intermediate bottlenecks and competing
flows. Competing flows may limit what's possible to flows. Competing flows may limit what's possible to
achieve. achieve.
B. It should handle routing changes which may alter or remove B. It should handle routing changes which may alter or remove
bottlenecks or change the bandwidth available, and react bottlenecks or change the bandwidth available, and react
quickly, especially if there is a reduction in available quickly, especially if there is a reduction in available
bandwidth or increase in bottleneck delay. bandwidth or increase in bottleneck delay.
C. It should handle interface changes (WiFi to 3G data, etc) C. It should handle interface changes (WLAN to 3G data, etc)
which may radically change the bandwidth available or which may radically change the bandwidth available or
bottlenecks, and react quickly, especially if there is a bottlenecks, and react quickly, especially if there is a
reduction in available bandwidth or increase in bottleneck reduction in available bandwidth or increase in bottleneck
delay. It is assumed that an interface change can generate delay. It is assumed that an interface change can generate
a notification to the algorithm. a notification to the algorithm.
D. The offered load may be less than the available bandwidth at D. The offered load may be less than the available bandwidth at
any given moment, and may vary dramatically over time, any given moment, and may vary dramatically over time,
including dropping to no load and then resuming a high load, including dropping to no load and then resuming a high load,
such as in a mute operation. The reaction time between a such as in a mute operation. The reaction time between a
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bottleneck router or link, but also clear quickly, and bottleneck router or link, but also clear quickly, and
should recover quickly when the burst ends. This is should recover quickly when the burst ends. This is
inherently at odds with the need to react quickly-enough to inherently at odds with the need to react quickly-enough to
avoid queue buildup. avoid queue buildup.
F. Similarly periodic bursty flows such as MPEG DASH F. Similarly periodic bursty flows such as MPEG DASH
[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 compete in bursts with the algorithm, and may not be
adaptive within a burst. They are often layered on top of adaptive within a burst. They are often layered on top of
TCP. The algorithm must avoid too much delay buildup during TCP. The algorithm must avoid too much delay buildup during
those bursts, and quickly recover. Note that this traffic those bursts, and quickly recover. Note that this competing
may on an access link, or may cause a shift in the location traffic may on a shared access link, or the traffic burst
of the bottleneck for the duration of the 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 realtime flows 2. The algorithm must be fair to other flows, both realtime flows
(such as other instances of itself), and TCP flows, both long- (such as other instances of itself), and TCP flows, both long-
lived and bursts such as the traffic generated by a typical web lived and bursts such as the traffic generated by a typical web
browsing session. Note that 'fair' is a rather hard-to-define browsing session. Note that 'fair' is a rather hard-to-define
term. It should be self-fair with itself, giving roughly equal term. It should be fair with itself, giving roughly equal
bandwidth to multiple flows with similar RTTs, and if possible bandwidth to multiple flows with similar RTTs, and if possible
to multiple flows with different RTTs. 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 flows to that bottleneck, and must allow new flows to ramp
up to a useful share of the bottleneck bandwidth quickly. up to a useful share of the bottleneck bandwidth quickly.
Note that relative RTTs may affect the rate new flows can Note that relative RTTs may affect the rate new flows can
ramp up to a reasonable share. ramp up to a 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. An algorithm may be more successful at avoiding starvation A. An algorithm may be more successful at avoiding starvation
from short-lived TCP long-lived/saturating TCP flows. from short-lived TCP than long-lived/saturating TCP flows.
B. In order to avoid starvation other goals may need to be B. In order to avoid starvation other goals may need to be
compromised (such as delay). compromised (such as delay).
4. The algorithm should quickly adapt to initial network conditions 4. The algorithm should quickly adapt to initial network conditions
at the start of a flow. This should occur both if the initial at the start of a flow. This should occur both if the initial
bandwidth is above or below the bottleneck bandwidth. bandwidth is above or below the bottleneck bandwidth.
A. The startup adaptation may be faster than adaptation later A. The startup adaptation may be faster than adaptation later
in a flow. It should allow for both slow-start operation in a flow. It should allow for both slow-start operation
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B. Alternative methods to help startup like probing during B. Alternative methods to help startup like probing during
setup with dummy data may be useful in some applications; in setup with dummy data may be useful in some applications; in
some cases there will be a considerable gap in time between some cases there will be a considerable gap in time between
flow creation and the initial flow of data. flow creation and the initial flow of data.
C. A flow may need to change adaptation rates due to network C. A flow may need to change adaptation rates due to network
conditions or changes in the provided flows (such as un- conditions or changes in the provided flows (such as un-
muting or sending data after a gap). muting or sending data after a gap).
5. It should be stable if the RTP streams are halted or 5. It should be stable if the RTP streams are halted or
discontinuous (VAD/DTX). discontinuous (Voice Activity Detection/Discontinuous
Transmission).
A. After a resumption of RTP data it may adapt more quickly A. After a resumption of RTP data it may adapt more quickly
(similar to the start of a flow), and previous bandwidth (similar to the start of a flow), and previous bandwidth
estimates may need to be aged or thrown away. estimates may need to be aged or thrown away.
6. The algorithm should where possible merge information across 6. The algorithm should where possible merge information across
multiple RTP streams between the same endpoints, whether or not multiple RTP streams between the same endpoints, whether or not
they're multiplexed on the same ports, in order to allow they're multiplexed on the same ports, in order to allow
congestion control of the set of streams together instead of as congestion control of the set of streams together instead of as
multiple independent streams. This allows better overall multiple independent streams. This allows better overall
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they should be utilized if possible. they should be utilized if possible.
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 RTCP; one alternative backchannel typically should be done via RTCP; one alternative
would be to include it instead in a reverse RTP channel using would be to include it instead in a reverse RTP channel using
header extensions. 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. feedback must be used.
B. Note that in some cases, backchannel messages may be delayed B. Note that in some cases, backchannel messages may be delayed
until the RTCP channel can be allocated enough bandwidth, until the RTCP channel can be allocated enough bandwidth,
even under AVPF rules. This may also imply negotiating a even under AVPF rules. This may also imply negotiating a
higher maximum percentage for RTCP data or allowing RMCAT higher maximum percentage for RTCP data or allowing RMCAT
solutions to violate or modify the rules specified for AVPF. solutions to violate or modify the rules specified for AVPF.
C. Bandwidth for the feedback messages should be minimized C. Bandwidth for the feedback messages should be minimized
(such as via RFC 5506 [RFC5506]to allow RTCP without SR/RR) (such as via RFC 5506 [RFC5506]to allow RTCP without Sender
Reports/Receiver Reports)
D. Header extensions would avoid the RTCP timing rules issues, D. Header extensions would avoid the RTCP timing rules issues,
and allow the application to allocate bandwidth as needed and allow the application to allocate bandwidth as needed
for the congestion algorithm. for the congestion algorithm.
E. Backchannel data should be minimized to avoid taking too E. Backchannel data should be minimized to avoid taking too
much reverse-channel bandwidth (since this will often be much reverse-channel bandwidth (since this will often be
used in a bidirectional set of flows). In areas of used in a bidirectional set of flows). In areas of
stability, backchannel data may be sent more infrequently so stability, backchannel data may be sent more infrequently so
long as algorithm stability and fairness are maintained. long as algorithm stability and fairness are maintained.
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equilibrium after a change, backchannel feedback may be more equilibrium after a change, backchannel feedback may be more
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 at a 9. Flows managed by this algorithm and flows competing against at a
bottleneck may have different DSCP[RFC5865] markings depending bottleneck may have different DSCP[RFC5865] markings depending
on the type of traffic, or may be subject to flow-based QoS. A on the type of traffic, or may be subject to flow-based QoS. A
particular bottleneck or section of the network path may or may particular bottleneck or section of the network path may or may
not honor DSCP markings. The algorithm SHOULD attempt to not honor DSCP markings. The algorithm should attempt to
leverage DSCP markings when they're available. leverage DSCP markings when they're available.
A. In WebRTC, a division of packets into 4 classes is A. In WebRTC, a division of packets into 4 classes is
envisioned in order of priority: faster-than-audio, audio, envisioned in order of priority: faster-than-audio, audio,
video, best-effort, and bulk-transfer. Typically the flows video, best-effort, and bulk-transfer. Typically the flows
managed by this algorithm would be audio or video in that managed by this algorithm would be audio or video in that
heirarchy, and feedback flows would be faster-than-audio. heirarchy, and feedback flows would be faster-than-audio.
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
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the flow can fake congestion signals, unless they are passed on a the flow can fake congestion signals, unless they are passed on a
tamper-proof path. Since some possible algorithms depend on the tamper-proof path. Since some possible algorithms depend on the
timing of packet arrival, even a traditional protected channel does timing of packet arrival, even a traditional protected channel does
not fully mitigate such attacks. not fully mitigate such attacks.
An attack that reduces bandwidth is not necessarily significant, An attack that reduces bandwidth is not necessarily significant,
since an on-path attacker could break the connection by discarding since an on-path attacker could break the connection by discarding
all packets. Attacks that increase the percieved available bandwidth all packets. Attacks that increase the percieved available bandwidth
are concievable, and need to be evaluated. are concievable, and need to be evaluated.
Algorithm designers SHOULD consider the possibility of malicious on- Algorithm designers should consider the possibility of malicious on-
path attackers. path attackers.
5. Acknowledgements 5. 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.
6. References 6. References
6.1. Normative References 6.1. Normative References
[I-D.ietf-rtcweb-overview] [I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower- Alvestrand, H., "Overview: Real Time Protocols for
based Applications", draft-ietf-rtcweb-overview-09 (work Browser-based Applications", draft-ietf-rtcweb-overview-10
in progress), February 2014. (work in progress), June 2014.
[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.
[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.
[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, February 2008. (RTP/SAVPF)", RFC 5124, February 2008.
6.2. Informative References 6.2. Informative References
[I-D.ietf-rtcweb-data-channel] [I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-08 (work in Channels", draft-ietf-rtcweb-data-channel-10 (work in
progress), April 2014. progress), June 2014.
[I-D.welzl-rmcat-coupled-cc] [I-D.welzl-rmcat-coupled-cc]
Welzl, M., Islam, S., and S. Gjessing, "Coupled congestion Welzl, M., Islam, S., and S. Gjessing, "Coupled congestion
control for RTP media", draft-welzl-rmcat-coupled-cc-02 control for RTP media", draft-welzl-rmcat-coupled-cc-03
(work in progress), October 2013. (work in progress), May 2014.
[MPEG_DASH] [MPEG_DASH]
"Dynamic adaptive streaming over HTTP (DASH) -- Part 1: "Dynamic adaptive streaming over HTTP (DASH) -- Part 1:
Media presentation description and segment formats", April Media presentation description and segment formats", April
2012. 2012.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001. 3168, September 2001.
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