Traffic Management for the Available Bit Rate (ABR) Service in ...

to equalize rates for fairness, and one round trip to bring the load to unity). See
the performance evaluation of ERICA in chapter 6 for further details. Assuming
that the rate allocations do not result in the increase of the total load, the link
is overloaded by a factor of two during the convergence period.
The queue growth during this period is 2 RT T Link Bandwidth (the rate of
queue growth is at most Link Bandwidth, since the maximum overload factor
is two). However, in general, the switch algorithm takes a few round trips
and a few feedback delays before it converges. The feedback delay becomes a
factor because many switch algorithms give feedback as the RM cell travels in
the reverse direction (rather than in the forward direction), which results in a
sources responding faster to feedback. Also note that we have used an explicit
rate scheme (ERICA) which results in faster convergence and hence smaller
bu er requirements. A binary (EFCI) feedback scheme or an ER-scheme which
is sensitive to variation may require larger bu ers.
After this convergence phase, the source rates are controlled. In other words, the
sources transition from a "window-limited" state to a "rate-limited" state. This
is a stable state. The switch queues built up during the convergence phase are
now drained out and the system reaches its \congestion avoidance" operating
point of \high utilization and low queueing delay."
Note that even if a new VC carrying TCP tra c starts transmitting data, the
additional load is small. This is because the TCP source sends only one segment
(due to the slow start algorithm which starts with a window of one) irrespective
of the Initial Cell Rate (ICR) of the VC. Note that switches have to provide
additional bu ering for sources which may carry non-TCP sources and may
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