In heterogeneous networks, achieving congestion avoidance is difficult because the congestion feedback from one subnetwork may have no meaning to source on other other subnetworks. We propose using changes in round-trip delay as an implicit feedback. Using a black-box model of the network, we derive an expression for the optimal window as a function of the gradient of the delay-window curve.

The problems of selfish optimum and social optimum are also addressed. It is shown that without a careful design, it is possible to get into a race condition during heavy congestion, where each user wants more resources than others, thereby leading to a diverging congestion

It is shown that congestion avoidance using round trip delay is a promising approach. The aproach has the advantage that there is absolutely no overhead for the network itself. It is exemplified by a simple scheme. The performance of the scheme is analyzed using a simulation model. The scheme is shown to be efficient, fair, convergent and adaptive to changes in network configuration.

The scheme as described works only for networks that can ne modelled with queueing servers with constant service times. Further research is required to extend it for implementation in practical networks. Several directions for future research have beensuggested.

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Prior version as DEC Technical Report is also available.