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

networks like ATM can have complete knowledge about a ow based upon mea-
surement, and can give authoritative feedback.
The steady state itself exhibits oscillatory behavior in terms of queue length and
rate allocations. The reason for the behavior is that the control is based upon
the queue length, a highly variable quantity in rate-based control. Further,
when the queue length is zero, or beyond a threshold, the schemes essentially
\guess" the allocations due to the unreliability ofthe metric.
The technique requires several parameters to force convergence. The system is
also quite sensitive to the parameter settings.
The bu er requirement at switches is large, and increases linearly with the
number of connections.
The \beat-down" fairness problem needs to be solved in every implementation.
This adds to the complexity atswitches.
A theme we gather from the above observations is that the bit-based feedback
and the technique of using queue length as a congestion indicator is a legacy from
the window-based control schemes for low-speed, connectionless networks. The adap-
tation to rate-based, high-speed, connection-oriented networks like ATM has some
advantages in terms of simplicity, and hence cost-e ectiveness. But, the performance
in the Wide Area scenario leaves a lot to be desired. This led to the introduction of
explicit rate feedback schemes.
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