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

that if not controlled, the TCP load increases at most linearly after the bot-
tleneck is loaded, even though the sources are in their exponential rise phase.
In other words, the load may change in the following pattern: cycle 0, load =
0.25� cycle 1, load = 0.5� cycle 2, load = 1, cycle 3, load = 2, cycle 4, load =
3, and so on. Note that the load change from 0.25 to 1 was exponential, and
then became linear because of the bottleneck becoming fully loaded. However,
even the linear load increase can result in unbounded switch queues unless the
sources are controlled. The switch algorithm is therefore the key which decides
how much queues build up before the sources rates stabilize. The above state-
ments assume that the TCP windows increase smoothly and not in bursts (i.e.,
TCP acknowledgements are not bunched together on receipt at the sources).
With ERICA, once the link is fully loaded, the measurements (input rate, num-
ber of active source etc) can be made more reliably. There is also a continuous
ow of BRM cells in the reverse direction, which can carry the rate feedback to
the sources. This results in accurate feedback to sources. The sources are asked
to reduce their rates, and the e ect is seen at the switch within one feedback
delay. Recall that the feedback delay is de ned as the sum of the delay from the
switch to the source and from the source to the same switch. Feedback delay
is always less than a round-trip time. In the worst case, the feedback delay is
equal to the round trip time.
From our observation that the overload increases at most by a factor of two
every round-trip time, the maximum overload with ERICA in the rst RTT
after the link is fully loaded is two. The typical convergence time of ERICA
(time to reach the steady state) is about two round trip times (one round trip
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