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

metric of congestion, using a single metric localizes the errors possible. Further,
since the end-result of all errors is additional queues, taking the queue length
as a metric is a safe method to avoid divergence, even when the demand and
capacity are variable.
The drawbacks of this approach are:
The bit-based feedback schemes were initially designed for low-speed networks.
Since a bit gives only two pieces of information (\up" or \down"), the system
may take several round trips to converge to stable values. That is, the transient
convergence period is long. During this period, the network might either be
underutilized, or queues might build up, which is a de nite concern in high-
speed networks.
The bit-based feedback was originally designed for window- ow control where
the maximum queue is simply the sum of all source windows. In rate control, if
the sum of the source rates is larger than the capacity, the queues could grow
to in nity unless the rates are changed [50, 48]. The transient convergence
period determines what this worst case queue will be. The queue can be large
(proportional to the steady state queue plus a term proportional to the transient
convergence time) when a new source starts up after the system is in the steady
state.
The bit-based feedback was originally designed for connectionless networks
where it is possible that packets from a source to a destination maytakemultiple
paths. Hence, it is not a good idea to give authoritative feedback information
based on partial knowledge. On the other hand, switches in connection-oriented
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