Network Working Group H. Tschofenig
Internet-Draft L. Eggert
Intended status: Informational Z. Sarker
Expires: April 18, 2013 October 15, 2012
Report from the IAB/IRTF Workshop on Congestion Control for Interactive
Real-Time Communication
draft-tschofenig-cc-workshop-report-00.txt
Abstract
This document provides a summary of the IAB/IRTF Workshop on
'Congestion Control for Interactive Real-Time Communication', which
took place in Vancouver, Canada, on July 28, 2012. The main goal of
the workshop was to foster a discussion on congestion control
mechanisms for interactive real-time communication. This report
summarizes the discussions and lists the conclusions and
recommendations to the Internet Engineering Task Force (IETF)
community. The views and positions in this report are those of the
workshop participants and do not necessarily reflect the views and
positions of the authors, the Internet Architecture Board (IAB) or
the Internet Research Task Force (IRTF).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 18, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. History and Current Status . . . . . . . . . . . . . . . . . . 5
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Changes to Network Infrastructure . . . . . . . . . . . . 6
3.2. Avoiding Self-Inflicted Queuing . . . . . . . . . . . . . 7
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Program Committee . . . . . . . . . . . . . . . . . . 14
Appendix B. Workshop Material . . . . . . . . . . . . . . . . . . 15
Appendix C. Accepted Position Papers . . . . . . . . . . . . . . 16
Appendix D. Workshop Participants . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Any application that sends significant amounts of data over the
Internet is expected to implement reasonable congestion control
behavior. Although the specific mechanisms depend on the application
or protocol, the motivations for congestion control are well
understood and documented in RFC 2914 [RFC2914] and RFC 5405
[RFC5405]. The goals are:
1. Preventing congestion collapse.
2. Allowing multiple flows to share the network fairly.
The Internet has been used for interactive real-time communication
for decades, most of which is being transmitted using RTP over UDP,
often over provisioned capacity and/or using only rudimentary
congestion control mechanisms.
The development and upcoming widespread deployment of web-based real-
time media communication - where RTP is used to and from web browsers
to transmit audio, video and data - will likely result in substantial
new Internet traffic. Due to the projected volume of this traffic,
as well as the fact that it is more likely to use unprovisioned
capacity, it is essential that it is transmitted with robust and
effective congestion control mechanisms.
Designing congestion control mechanisms that perform well under a
wide variety of traffic mixes and over network paths with widely
varying characteristics is not easy. Prevention of congestion
collapse can be achieved through "circuit breaker" mechanisms, but
for media flows that are supposed to coexist with a user's other
ongoing communication sessions, a congestion control mechanism that
shares capacity fairly in the presence of a mix of TCP, UDP and other
protocol flows is needed.
Many additional complications arise. Here are some examples:
1. Real-time interactive media sessions require low latencies,
whereas streaming media can use large play-out buffers.
2. RTCP feedback about an RTP session typically arrives much less
frequently than, for example, TCP ACKs for a given TCP
connection. The RTP/RTCP control loop can lead to a longer
reaction time.
3. Media codecs can usually only adjust their output rates in a much
more coarse-grained fashion than, for example, TCP, and user
experience suffers if encoding rates are switched too frequently.
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Codecs typically have a minimum sending rate as well.
4. Some bits of an encoded media stream are more important than
others. For example, losing or dropping an I-frame of a video
stream is more problematic than dropping a P-frame.
5. Ramping up the transmission rate can be problematic. Simply
increasing the output rate of the codec without knowledge whether
the network path can sustain transmission at the increased rate
runs the danger of incurring a significant amount of packet loss
that can cause playback artifacts.
6. A congestion control scheme for interactive media needs to handle
bundles of interrelated flows (audio, video, data), in a way that
accommodates the preferences of the application in the event of
congestion.
7. The desire to provide a congestion control mechanism that can be
efficiently implemented inside an application (e.g., a web server
and a browser) imposes additional restrictions.
8. There are explicit congestion signals (such as ECN), and there
are indirect indications of congestion (such as packet delay and
loss). However, congestion is not the only source of these
indirect signals. Care must be taken to account for each of
these signals, particularly if various applications react on the
same signals.
9. Network elements and end device operating systems often create
buffers to better support TCP-based applications. These buffers
introduce additional communication delay, which harms the small
delay budget available for real-time applications.
Note that this document is a report on the proceedings of the
workshop. The views and positions documented in this report are
those of the workshop participants and do not necessarily reflect the
views of the authors.
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2. History and Current Status
The currently deployed RTP-based RTC systems use congestion control
schemes that are ad hoc at best. As the Internet moves towards an
RTCWeb standard for interoperable audio/video conferencing, it needs
a standardized and most importantly effective mechanism for
congestion control.
The most immediate need is for a control mechanism that balances a
real-time flow fairly among other competing traffic. For instance,
under congested conditions, it would be inappropriate to use far more
bandwidth than flows from other applications are using, particularly
lasting gross over-utilization of the network. Simple "circuit
breaker" -style control mechanisms can prevent congestion collapse by
disabling a circuit that far exceeds a fair share and making the
circuit remain disabled for some time. But does not in any way
enable concurrent flows to share available network capacity sensibly.
Therefore, in addition to circuit breakers, more complete methods for
media congestion control are needed. The focus of the workshop was
on discussing ideas for these more complete congestion control
methods.Note that the primary focus of the workshop is RTCWeb-style
audio/video flows. The assumed underlying flows (in most attendees
minds) were digital audio/video data encapsulated in RTP over UDP
over IP. Most of the concepts presented at the workshop could be
used with different media types over different transports, but this
was the generally assumed baseline for discussions.
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3. Recommendations
The participants suggested to explore two complementary solution
tracks, as described in the two sub-sections below.
3.1. Changes to Network Infrastructure
Like for all other traffic on the network, better data plane
infrastructure improves the perceived quality of the best-effort
service the Internet provides for RTCWeb flows. The IETF has already
developed several technologies that would be of immediate usefulness,
if they were deployed. The workshop participants expressed the hope
that due to the volume and importance of RTCWeb traffic, some of
these technologies might finally see widespread use.
The first and by far most important improvement is traffic
segregation, i.e., the ability to use different queues for different
traffic types. Specifically jitter/delay sensitive and throughput
maximizing protocols need to be in different queues. It is not
possible for a single queue/AQM to be optimal for both.
Explicit Congestion Notification (ECN) allows routers along the end-
to-end path to signal the onset of congestion and allows applications
to respond early, avoiding losses and keep queue sizes short and
therefore end-to-end delay low. ECN is implemented on some end
system stacks and routers, but frequently not enabled.
Different mechanisms have been developed to facilitate traffic
segregation. Differentiated services are one possibility in this
space if applications start to mark outgoing traffic appropriately
and routers would segregate traffic accordingly, browsers could more
directly control the relative importance of their various flows, and
avoid self-competition. Compared to ECN, however, DiffServ is far
more difficult to deploy meaningfully end-to-end, especially given
that DSCPs have no defined end-to-end meaning and packets can be re-
marked. Quality-of-service (QoS) signaling together with resource
reservation facilities would enable a fine-grained and flexible way
to indicate resource needs to network elements but it is also by far
the most heavyweight proposal, and unlikely to be viable in the
global Internet.
However, any change to network infrastructure will take time
particularly if the interest of the involved stakeholders is not
aligned (as it is the case for network operators and over-the-top
application providers). It is therefore imperative that RTCWeb
congestion control provides adequate improvement in the absence of
any of the aforementioned schemes.
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3.2. Avoiding Self-Inflicted Queuing
This approach tries to ensure that the network does not get congested
by focusing on the traffic sent by a single host independent of any
changes to networks, as mentioned in the earlier chapter. A single
congestion manager within the end host or the browser could already
help to coordinate various congestion control activities and to
ensure a more coordinated approach between different applications,
and different flows.
The following activities were suggested during the workshop.
Reacting to all Congestion Signals:
To initiate the congestion control process it is important to
detect congestion in the communication path. Congestion in the
comminication can be detected using either an explicit mechanism
or an implicit mechanism. The explicit mechanism involves direct
congestion signaling usually from the congested network node, such
as ECN. The ECN bits are set in the IP header by the network node
before it starts to drop the packets and gives end hosts to react
to the congestion. In case of implicite mechanism, a particular
characteristics or event in the network traffic can be inferred as
a congestion signal. For example, packet loss events or observed
delay increase or jitter in the communication path can be taken as
an implicit congestion signals. These measurements can also be
made available in a variety of different protocols, such as RTCP
reports or transport protocols. Communication endpoints can
trigger congestion control on the event of these phenomena. It is
also possible to correlate these events to detect congestion in
the communication path. The implicit and explicit signals can
come in any sequence and at any time during the ongoing media
session. It is recommended to react to all the possible implicit
congestion signals, ideally from all applications at the end host,
to avoid self- inflicted queues.
Delay- and Loss-based Algorithms:
The main goal of designing a congestion control algorithm for
real-time conversational media is to achieve low latency. An
explicit signaling based congestion control algorithm perhaps is
the best fit for this purpose as the network nodes are directly
involved in the process. However, in case of absence of ECN
support in the end-to-end communication path delay based
algorithms are needed where the increased delay in the
communication path is taken into account as implicit congestion
signal. While it is difficult to design algorithms based on delay
since many wireless networks show a wide delay variation even in a
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non-congested network. The workshop participants recommended that
more research should be done to better understand non-congestion
related delay variation in the network. General consensus among
the workshop participants was that when latency-based techniques
were competing against loss-based techniques, the loss-based
techniques got all the bandwidth.
Algorithm Evaluation:
The Internet consists of heterogeneous networks, which also
includes misconfigured, and unmanaged network nodes. Bandwidth
and latency varies a lot. Not only this, different services
deployed using RTP/UDP have different requirements in terms of
media quality. A congestion control algorithm needs to perform
well not only in simulators but also in today's Internet. To
achieve this it is recommended to test the algorithms with real-
world loss and delay figures to be sure that the desired audio/
video rates are attainable using the proposed algorithms for the
desired services.
Media Characteristics in Focus:
Interactive real-time voice and video data is inherently variable.
Usually the content of the media and service requirements dictate
the media coding. The codec may be bursty and not all frames are
equally important (e.g., I-frames are more important than
P-frames). Thus, codecs have limited room for adaptation.
Congestion control for audio and video codecs is therefore
different to congestion control applied for bulk transfers. The
latter are allows buffering of media irrespective to their content
and more fine-grained control for a congestion control algorithm.
In the workshop these limitations were brought up and the workshop
participants recommended that a congestion controller needs to be
aware of these constraints. However, further investigation is
needed to decide what information needs to be exchanged between a
codec and the congestion manager.
Startup Behaviour:
The startup media quality is very important for real-time
interactive application and for user-perceived application
performance. The startup behavior of these is also different to
other traffic. By nature the real-time interactive communication
applications want to provide a smooth user experience and maintain
best media quality to ease the interaction. While it may be
desirable from a user experience point of view to immediately
start streaming video with high-definition quality and audio of a
wideband codec this will have impacts on the bandwidth of the
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already ongoing flows. As such, it would be ideal to start slow
enough to avoid excessive congestion to other flows but fast
enough to offer a good user experience. The sweetspot, however,
yet has to be found.
In addition, there were some discussions on how to make RTCWeb-style
flows fair to other RTCWeb-style flows, other TCP based flows, LEDBAT
flows, and other flows. This is a much more difficult problem than
simply making RTCWeb flows avoid congestion. Changing the way TCP is
used in browsers or other HTTP-based applications would already help,
for example, by avoid opening too many concurrent TCP connections,
and improved interworking with video streaming and file sharing
software. The work on HTTP 2.0 with SPDY is already the step in the
right direction since SPDY makes makes use of a more aggressive form
of multiplexing instead of opening a larger number of TCP
connections.
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4. Acknowledgments
We would like to thank the participants and the paper authors of the
position papers for their input.
Additionally, we would like to thank the following persons for their
review comments: Michael Welzl, John Leslie, Mirja Kuehlewind, Matt
Mathis, Mary Barnes
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5. IANA Considerations
This memo includes no request to IANA.
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6. Security Considerations
While two position papers focused on security congestion control
security itself was not on the agenda of the workshop. As such,
nothing beyond the material contained in those position papers can be
reported.
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7. References
7.1. Normative References
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
7.2. Informative References
[I-D.ietf-avtcore-rtp-circuit-breakers]
Perkins, C. and V. Singh, "RTP Congestion Control: Circuit
Breakers for Unicast Sessions",
draft-ietf-avtcore-rtp-circuit-breakers-00 (work in
progress), October 2012.
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Appendix A. Program Committee
This workshop was organized by Harald Alvestrand, Bernard Aboba, Mary
Barnes, Gonzalo Camarillo, Spencer Dawkins, Lars Eggert, Matthew
Ford, Randell Jesup, Cullen Jennings, Jon Peterson, Robert Sparks,
and Hannes Tschofenig.
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Appendix B. Workshop Material
o Main Workshop Page:
http://www.iab.org/activities/workshops/cc-workshop/
o Position Papers:
http://www.iab.org/activities/workshops/cc-workshop/papers/
o Slides:
http://www.iab.org/activities/workshops/cc-workshop/slides/
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Appendix C. Accepted Position Papers
1. "One control to rule them all" by Michael Welzl
2. "Congestion Avoidance Through Deterministic" by Pier Luca
Montessoro, Riccardo Bernardini, Franco Blanchini, Daniele
Casagrande, Mirko Loghi, and Stefan Wieser
3. "Congestion Control in Real Time Media - Context" by Harald
Alvestrand
4. "Improving the Interactive Real-Time Video Communication with
Network Provided Congestion Notification" by ANM Zaheduzzaman
Sarker, Ingemar Johansson
5. "Multiparty Requirements in Congestion Control for Real-Time
Interactive Media" by Magnus Westerlund
6. "On Fairness, Delay and Signaling of Different Approaches to
Real-time Congestion Control" by Stefan Holmer
7. "RTP Congestion Control and RTCWeb Application Feedback" by Ted
Hardie
8. "Issues with Using Packet Delays and Inter-arrival Times for
Inference of Internet Congestion" by Wesley M. Eddy
9. "Impact of TCP on Interactive Real-Time Communication" by Ilpo
Jarvinen, Binoy Chemmagate, Laila Daniel, Aaron Yi Ding, Markku
Kojo, and Markus Isomaki
10. "Security Concerns For RTCWeb Congestion Control" by Dan York
11. "Vendors Considered Harmfull" by Cullen Jennings, Suhas
Nandakumar, and Hein Phan
12. "Network-Assisted Dynamic Adaptation" by Xiaoqing Zhu and Rong
Pan
13. "Congestion Control for Interactive Real-Time Applications" by
Sanjeev Mehrotra, and Jin Li
14. "There is No Magic Transport Wand" by John Leslie
15. "Towards Adaptive Congestion Management for Interactive Real-
Time Communications" by Dirk Kutscher, and Miriam Kuehlewind
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16. "Enlarge the pre-congestion spectrum usage?" by Xavier Marjou,
and Emile Stephan
17. "Congestion control for users who don't have first-class
internet access" by Maire Reavy
18. "Realtime Congestion Challenges" by Randell Jesup
19. "Congestion Control for Interactive Media: Control Loops & APIs"
by Varun Singh, Joerg Ott, and Colin Perkins
20. "Some Notes on Threat Modelling Congestion Management" by Eric
Rescorla
21. "Timely Detection of Lost Packets" by Ali C. Begen
22. "Congestion Control Considerations for Data Channels" by Michael
Tuexen
23. "Position paper on CC for Interactive RT" by Matt Mathis
24. "Overall Considerations for Congestion Control" by M. Zanaty, B.
VerSteeg, B. Christensen, D. Benham, A. Romanow
25. "Fairness Considerations for Congestion Control" by Mo Zanaty
26. "Media is not Data: The Meaning of Fairness for Competing
Multimedia Flows" by Timothy B. Terriberry
27. "Thoughts on Real-Time Congestion Control" by Murari Sridharan
28. "Congestion Control for Interactive Real-Time Flows on Today's
Internet" by Keith Winstein, Anirudh Sivaraman, and Hari
Balakrishnan
29. "Congestion Control Principles for CoAP" by Carsten Bormann,
Klaus Hartke
30. "Erasure Coding and Congestion Control for Interactive Real-Time
Communication" by Pierre-Ugo Tournoux, Tuan Tran Thai, Emmanuel
Lochin, Jerome Lacan, Vincent Roca
31. "Video Conferencing Specific Considerations for RTP Congestion
Control" by Stephen Botzko and Mary Barnes
32. "The Internet is Broken, and How to Fix It" by Jim Gettys
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33. "Deployment Considerations for Congestion Control in Real-Time
Interactive Media Systems" by Jari Arkko
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Appendix D. Workshop Participants
We would like to thank the following workshop participants for
attending the workshop:
o Mat Ford
o Bernard Aboba
o Alissa Cooper
o Mary Barnes
o Lars Eggert
o Harald Alvestrand
o Gonzalo Camarillo
o Robert Sparks
o Cullen Jennings
o Dirk Kutscher
o Carsten Bormann
o Michael Welzl
o Magnus Westerlund
o Colin Perkins
o Murari Sridharan
o Klaus Hartke
o Pier Luca Montessoro
o Xavier Marjou
o Vincent Roca
o Wes Eddy
o Ali C. Begen
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o Mo Zanaty
o Jin Li
o Dave Thaler
o Bob Briscoe
o Barry Leiba
o Jari Arkko
o Stewart Bryant
o Martin Stiemerling
o Russ Housley
o Marc Blanchet
o Zaheduzzaman Sarker
o Xiaoqing Zhu
o Randell Jesup
o Eric Rescorla
o Suhas Nandakumar
o Hannes Tschofenig
o Bill VerSteeg
o Sean Turner
o Keith Winstein
o Jon Peterson
o Maire Reavy
o Michael Tuexen
o Stefan Holmer
o Joerg Ott
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o Timothy Terriberry
o Benoit Claise
o Ted Hardie
o Stephen Botzko
o Matt Mathis
o David Benham
o Jim Gettys
o Spencer Dawkins
o Sanjeev Mehrotra
o Adrian Farrel
o Greg White
o Markku Kojo
We also had remote participants, namely
o Emmanuel Lochin
o Mark Handley
o Anirudh Sivaraman
o John Leslie
o Varun Singh
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Authors' Addresses
Hannes Tschofenig
Linnoitustie 6
Espoo, 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Lars Eggert
Sonnenallee 1
Kirchheim, 85551
Germany
Phone: +49 151 12055791
Email: lars@netapp.com
URI: http://eggert.org/
Zaheduzzaman Sarker
Lulea, SE-971 28
Sweden
Phone: +46 10 717 37 43
Email: zaheduzzaman.sarker@ericsson.com
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