  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.
