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Congestion control

TCP is used for reliable transport of data in the Internet. We previously study connection management and TCP mechanisms. In the following exercises, we will get interest in a other fundamental behavior of TCP: the congestion control.

Congestion detection

TCP was designed at the end of the 70's. Several congestion control algorithms have been added since, mainly following the work of Van Jacobson published in 1988. They continue to evolve in different TCP variants. The exercises proposed in the following are founded on the last versions: RFC 5681 of September 2009.

1  For TCP, which phenomenon indicates congestion in the network?

2  What's going on inside a router to generate this phenomenon?

3  For TCP, this phenomenon can infer congestion. But it can also occur when there is no congestion in the network. In which other cases in which case such a phenomenon may occur?

4  If this phenomenon does not always indicate congestion, why is TCP based on this inference? Why don't we use an approach where the router notify explicitly the congestion by sending a message to the sender?

Congestion control algorithms

For congestion control, TCP uses a threshold that indicates the flow rate above which congestion may occur. This threshold is expressed by the parameter ssthresh (in bytes). To get the flow rate threshold, ssthresh is divided by the RTT (Round Trip Time). The flow rate can vary from below and above the threshold ssthresh/RTT. The issuer maintains an other parameter, cwnd (Size of the congestion window), which indicates the maximum number of bytes it can send before receiving an acknowledgment. When cwnd > ssthresh, the sender take care particularly to not cause congestion.

1  Suppose ssthresh is at 5000 bytes, cwnd is at 6000 bytes, and segment size is 500 bytes. The sender sends twelve segments of 500 bytes in one RTT period, and receives twelve acknowledgements (one for each segments). What happens to the values ?ssthresh and cwnd? How these values changes are called?

2  Suppose ssthresh is still at 5000~bytes, cwnd is now at 14,000~bytes, the sender sends 14.000/500 = 28 segments, and that the sender receives a congestion notification before receiving the first acknowledgement. What happens to the ssthresh and cwnd values ? How these values changes are called?

3  We have seen how increases and decreases cwnd depending on the absence or presence of indicators of congestion. How do we call this algorithm? On what principle is based this algorithm?

4  At startup, and after having received a congestion notification, the value of cwnd is smaller than the value of ssthresh. Describe how cwnd increase when it is lower thanssthresh, depending on the following example. Suppose ssthresh equal to 3000 bytes and cwnd equal to 500 bytes, the size of a segment. The transmitter has several segments ready to be sent. How many segments sends the issuer during the first RTT period? If it receives acknowledgments for all segments, what becomes the value ofcwnd? How many segments sends the issuer during the second RTT period? If it receives acknowledgments for all segments, what becomes the cwnd value? In general, how evoluate the size of cwnd?

5  How is called the period during which cwnd is smaller than ssthresh?


What happens to the value ssthresh if the sender receives a congestion notification while cwnd is smaller ssthresh that?

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