Research : Congestion Control

An Evaluation of RTP Circuit Breaker Performance on LTE Networks

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Our paper on An Evaluation of RTP Circuit Breaker Performance on LTE Networks will be presented in the IEEE Infocom Workshop on Communication and Networking Techniques for Contemporary Video in Toronto, Canada, in April 2014. Real-time multimedia comprises a large, and growing, fraction of mobile data traffic. An important subset of such flows are from interactive conferencing applications using RTP on UDP/IP to reduce latency. UDP has no congestion control, and while the IETF is developing RTP-level congestion control algorithms as part of the WebRTC standards, these will take time to finalise and deploy. In the interim, we proposed an RTP circuit breaker to the IETF. This can detect and stop RTP flows that cause excessive network congestion, acting as an envelope within which a congestion control algorithm can operate. In this paper, we review the circuit breaker design, and present an initial performance evaluation on LTE networks. The algorithm is shown to be conservative in overload situations with low delay and high loss, which can occur due to AQM in LTE networks. We propose changes to the circuit breaker to better suit such networks.

Circuit Breakers for Multimedia Congestion Control

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Our paper on Circuit Breakers for Multimedia Congestion Control was presented at the 20th International Packet Video workshop in San Jose, CA, USA, last week. The abstract reads: “Real-time multimedia flows comprise a large, and increasing, fraction of the traffic on the Internet. An important subset of that traffic, primarily due to interactive applications, runs over UDP/IP, and requires applications to implement congestion control to ensure the stability of the network. The IETF is developing congestion control algorithms for such uses as part of the new WebRTC standards, but there is no standard algorithm that can be used at this time. We do not propose a congestion control algorithm. Rather, we propose a circuit breaker for RTP sessions that can detect when an application is causing excessive network congestion, and shut down the transmission. This can be used as an envelope within which congestion control algorithms can operate, providing a safety net to prevent congestion collapse. We present the RTP circuit breaker algorithm, and provide an initial performance evaluation to show that it performs as desired.”

IETF 86 — RTCP Feedback for Unicast Multimedia Congestion Control

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I submitted a draft on the use of RTCP Feedback for Unicast Multimedia Congestion Control. Congestion control requires a feedback loop, to report on reception quality, and in the case of RTP-based multimedia, this is has typically been provided by RTCP feedback. There have been some suggestions in the working group that RTCP feedback is too slow for effective congestion control. This draft provides a rough sketch of how quickly feedback can be provided by RTCP. It seems clear that per-packet feedback cannot be provided using RTCP, but per-frame feedback is very possible. If RTCP is to be used in it's current form for congestion feedback, then the algorithms should be designed to work with feedback per-frame or per-RTT, rather than per packet.

Slides from my talk at IETF 86 are available.

Can Congestion-controlled Interactive Multimedia Traffic Co-exist with TCP?

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I gave an invited talk on “Can Congestion-controlled Interactive Multimedia Traffic Co-exist with TCP?” in the Capacity Sharing Workshop at ACM CoNEXT 2012 in Nice, France, yesterday. The abstract for my talk was: “The WebRTC activity in the Internet Engineering Task Force (IETF) and World-Wide Web Consortium (W3C) is adding standards-based interactive multimedia conferencing features to web browsers. WebRTC systems are expected to see extremely wide deployment, starting in the next year. This deployment poses a challenge, since congestion control algorithms for interactive multimedia have not been standardized. The nature of interactive multimedia traffic complicates congestion control, and the interactions between WebRTC traffic, TCP flows, and the modern network worsen the problem. The region where congestion-controlled interactive multimedia traffic can feasibly co-exist with TCP is outlined, and areas where additional development seems necessary are highlighted.”

IETF 84 — RTP Media Congestion Avoidance Techniques BoF

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A birds-of-a-feather (BoF) session on RTP Media Congestion Avoidance Techniques was held at the 84th IETF meeting in Vancouver on 2 August 2012. I co-chaired this BoF with Michael Welzl from the University of Oslo. The agenda was as follows:

13:00 Introduction (Chairs)
13:05 Problem statement (Alvestrand)
13:25 Context: IAB/IRTF congestion control workshop (Welzl)
13:35 Context: buffer bloat and AQM (Gettys)
13:45 Context: competing traffic (Mathis)
13:55 Potential solutions: rrtcc (Alvestrand)
14:00 Potential solutions: dflow (O'Hanlon)
14:05 Proposed charter (Chairs)
14:15 Discussion  
14:50 Wrap-up and next steps (Chairs/ADs)
15:00 Close  

An audio recording and minutes of the BoF are available.

RTP Congestion Control: Circuit Breakers for Unicast Sessions (-01)

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We've just submitted an updated version of the RTP circuit breakers draft. This is a minor update, to address the comments from our presentations at IETF 83 in Paris earlier this year. The main change in this version is to use the simple TCP throughput model of Mathis et al in place of the more complex model used in the earlier version of the draft. There have also been various minor editorial clarifications and corrections.

IETF 83 — RTP Circuit Breakers Presentations

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I gave a presentation about the RTP Circuit Breakers draft to the IRTF ICCRG, meeting in Paris in March 2012. The presentation was generally well-received. The main conclusion from the discussion following the presentations was that the simple TCP throughput model of Mathis et al is sufficient for this purpose, and there is little need to use the more complex model of Padhye et al. An updated version of the draft will be submitted in a few weeks to address this, and other, feedback.

IETF 83 — TCP Segment Caching Presentation

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Pasi Sarolahti gave a presentation on our TCP Segment Caching work at the IRTF Internet Congestion Control Research Group (ICCRG) meeting in Paris on 27 March 2012. The presentation slides are now available.

TCP Segment Caching

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This internet-draft describes Content- and Cache-Aware TCP (CATCP). This is an extension to TCP that allows caching of TCP segments, so they can be re-used between different flows transmitting same data. When large amounts of redundant data are being simultaneously sent to multiple receivers, this can lead to significant load reductions and performance improvements. A typical use-case might be to improve the efficiency of HTTP-based streaming video services, through ubiquitous in-network caching.

  • Pasi Sarolahti, Jörg Ott, and Colin Perkins, TCP Segment Caching, Internet Engineering Task Force, March 2012, Work in progress (draft-sarolahti-irtf-catcp-00.txt).

This draft marks the start of a formal specification of the ideas previously outlined in our technical report on Poor Man's Content Centric Networking (with TCP) .

Alvaro Saurin — Congestion Control for Video-conferencing Applications

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Congratulations to Alvaro Saurin, who has completed his MSc thesis on Congestion Control for Video-conferencing Applications, looking into how well TCP-Friendly Rate Control (TFRC) works on real-world networks. The abstract reads:

In the Internet, transmission systems must deal with congestion in order to keep the stability of the network. However, the model used for congestion control determines some important properties of the traffic. The most important algorithm currently used, found in the TCP protocol, has characteristics that make it unsuitable for videoconferencing systems.

The aim of this dissertation is to provide an insight into the field of congestion control for such systems. In particular, this work examines one of the most promising alternatives available, TCP-Friendly Rate Control (TFRC), to answer the question “is TFRC suitable for interactive videoconferencing applications?”

This dissertation presents the results obtained with TFRC, focusing on some practical aspects and providing recommendations for the implementation of such a rate-based congestion control system. This work examines the scenarios where TFRC is an adequate solution, exposing the behavior that can be expected and suggesting future improvements.

The thesis also presents the experiences of integrating TFRC in the UltraGrid videoconferencing application. It shows the difficulties found, demonstrating that this integration requires an significant amount of support from the application, and questioning the suit- ability of TFRC in some situations.

This works was completed as part of the NSF-funded UltraGrid project.

New research student: Alvaro Saurin

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Welcome to Alvaro Saurin, who will be working on integration of TCP Friendly Rate Control (TFRC) with high performance interactive video, as part of the UltraGrid project.

Congestion Control for Real-Time Media

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Provision of congestion control is a significant open issue for interactive real-time networked multimedia systems. Numerous congestion control algorithms have been proposed, but they frequently conflict with the demands of interactive multimedia applications. My paper “Building Adaptive Applications: On The Need For Congestion Control” (an invited paper presented at the 17th SPIE/IS&T International Symposium on Electronic Imaging, San Jose, CA, USA, January 2005) outlines some of the issues.

My research has focussed on the effects of jitter and reordering on congestion controlled transport protocols, on TCP friendly rate control, and on the integration of real time applications with the Datagram Congestion Control Protocol (DCCP).

Effects of Jitter and Reordering

Evaluation of the effects of network timing jitter and packet reordering on multimedia applications. Design of congestion control protocols to decouple response to loss and reordering. Comparison with TCP and TCP-Friendly Rate Control.


Developing a mapping of the Real-time Transport Protocol (RTP) onto the Datagram Congestion Control Protocol (DCCP).

Developing a UDP encapsulation of DCCP: