The congestion control for multimedia applications (Voice over IP, video on demand) is an open issue. We have evaluated the congestion control strategies employed by leading multimedia applications such as Skype for VoIP applications and RealNetworks video on demand applications. We have found that both applications don't employ an efficient congestion control scheme.
The TCP/IP stack has been extremely successful for reliable delivery of best-effort, time insensitive elastic type data traffic. Nowadays, the Internet is rapidly evolving to become an equally efficient platform for multimedia content delivery. Key examples of this evolution are, to name few, YouTube, Skype Audio/Video, IPTV, P2P video distribution such as Coolstreaming or Joost. While YouTube streams videos using the Transmission Control Protocol (TCP), applications that are time-sensitive such as Skype VoIP or Video Conferencing employ the UDP because they can tolerate small loss percentages but not delays due to TCP recovery of losses via retransmissions. Since the UDP does not implement congestion control, these applications must implement those functionalities at the application layer in order to avoid congestion and preserve network stability. In this paper we investigate Skype Video in order to discover at what extent this application is able to throttle its sending rate to match the unpredictable Internet bandwidth while preserving resource for co-existing best-effort TCP traffic.
Skype is the most popular VoIP application with over 250 million userbase spread all over the world. It is important to study how skype reacts to packet losses in order to infer if a huge amount of skype calls can result in a congestion collapse.
The figure shows the sending rate, the loss rate and the available bandwidth. It can be noticed that Skype adapts its sending rate when the available bandwidth decreases but this adaptation takes 40s, thus leading to high packet loss rates.
For the before mentioned reason Skype is not able to cope with sudden bandwidth variations as it can be seen in the next figure.
Skype's response to bandwidth variation is sluggish and leads to unfriendliness with respect to TCP flows.
The Figure above shows that TCP connection suffers a large number of timeouts.
Two Skype calls have been placed flowing in the same bottleneck in order to investigate if Skype's congestion control is able to guarantee fairness.
We investigated the end-to-end quality of service (QoS) that is provided by the Apple Darwin Streaming Server and the Quick-Time client player in the presence of time-varying available bandwidth and multiple concurrent streaming sessions.
More can be found in the paper: An Experimental Investigation of the End-to-End QoS of the Apple Darwin Streaming Server (pdf).
We have evaluated how Helix Player behaves when available bandwidth reductions take place in order to find out how it reacts to congestion episodes. The figure below shows how the throughput of an helix connection experiences up to 30% of packet losses when another helix flow enters the link at 30s and exits at 90s.
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