Congestion Control for High Bandwidth -Delay Product Networks

The authors portray at first the problem that can face TCP in the future Internet. As the delay-bandwidth product increases, TCP becomes oscillatory and prone to instability regardless of the queue scheme, inefficient and unfair. Thus they developed explicit Control Protocol XCP by which the router notifies the sender about the congestion and the degree of the congestion. Also XCP implies the decoupling of the efficiency control from the fairness control, distinguishing error losses from congestion losses and detection of misbehaving sources. It remains stable, efficient and fair as the delay-bandwidth product and number of sources increase (i.e. independent of the environment). The efficiency here involves high utilization, small queue and almost no drop.
The new in that congestion control are

• Don't depend on binary signal as a feedback to indicate the congestion state because this signal has to reflect the degree of congestion.


• The reaction of the source should be adjusted depended on the delay of the feedback


• The aggregate traffic should be independent from the number of flows while the fairness in bandwidth allocation depends on the number of flows. This leads to decouple the efficiency control EC from the fairness control FC. EC uses a MIMD law, while FC uses AIMD law.

With XCP, the packet carries a congestion header to inform the router about the flow state (congestion window and round trip time), and feedback from the router to the receiver. To compute the feedback the router uses an efficiency controller and a fairness controller over the average round trip time to prevent the queue from building up to the point at which a packet has to be dropped. Each router on the path to the receiver compares the aggregate input traffic rate with the link bandwidth to find positive or negative feedback proportional on the link bandwidth and queue size. Then the router tries to compute the feedback for the individual flow using AIMD law. When the feedback reaches the receiver, the congestion header is copied from the data packet to its acknowledgment.