Congestion occurs at a bottleneck along an Internet path; multiple flows between the same sender and receiver pairs can benefit from using only a single congestion control instance when they share the same bottleneck. These benefits include the ability to control the rate allocation between flows and reduced overall delay (multiple congestion control instances cause more queuing delay than one since each has no knowledge of the congestion episodes experienced by the others). We present a mechanism for coupling congestion control for real-time media and show its benefits by coupling multiple congestion controlled flows that share the same bottleneck.

We present ctrlTCP, a method to combine the congestion controls of multiple TCP connections. In contrast to the previous methods such as the Congestion Manager, ctrlTCP can couple all TCP flows that leave one sender, traverse a common bottleneck (e.g., a home user's thin uplink) and arrive at different destinations. Using ns-2 simulations and an implementation in the FreeBSD kernel, we show that our mechanism reduces queuing delay, packet loss, and short flow completion times while enabling precise allocation of the share of the available bandwidth between the connections according to the needs of the applications.

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Middleboxes in private networks have been known to change packets in many ways, making it hard to design protocol extensions that work for the large majority of Internet users. Addressing the need to know what such middleboxes do, we introduce a tool called fling ("flexible ping"). fling can carry out (almost) any kind of protocol dialogue between a server and a client based on a simple specification in a json and a pcap file, and identify what middleboxes do to the packets of the dialogue. This fills a gap in the state of the art, where other tools that control both ends of a path are either limited in some form or have to be updated for every new test. We present results from small-scale tests that prove the flexibility of fling, which is a prerequisite for our next step: development of a large-scale measurement platform.

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