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

of managing bu ers, queueing, scheduling and drop policies with the complexity of
implementing a good ABR feedback scheme.
In section 8.13 we rst noted that TCP can achieve maximum throughput over
ATM if switches provide enough bu ering to avoid loss. The study of TCP dynamics
over the ABR service showed that initially when the network is underloaded or the
TCP sources are starting to send new data, they are limited by their congestion win-
dow sizes (window-limited), rather than by the network-assigned rate (rate-limited).
When cell losses occur, TCP loses throughput due to two reasons - a) a single cell
loss results in a whole TCP packet to be lost, and, b) TCP loses time due to its large
timer granularity. Intelligent drop policies (like avoiding drop of \End of Message
(EOM)" cells, and \Early Packet Discard (EPD)" can help improve throughput). A
large number of TCP sources can increase the total thoughput because each window
size is small and the e ect of timeout and the slow start procedure is reduced. We
also saw that the ATM layer \Cell Loss Ratio (CLR)" metric is not a good indicator
of TCP throughput loss. Further, we saw that the switch bu ers should not be di-
mensioned based on the ABR Source parameter \Transient Bu er Exposure (TBE)".
Bu er dimensioning should be based upon the performance of the switch algorithm
(for ABR), and the round trip time.
In section 8.15.6, we summarized the derivation and simulation of switch bu er
requirements for maximum throughput of TCP over ABR. The factors a ecting the
bu er requirements are round trip time, switch algorithm parameters, feedback delay,
presence of VBR tra c, or two-way TCP tra c. For a switch algorithm like ERICA,
the bu er requirements are about 3 RT T Link bandwidth. The derivation is valid
for in nite applications (like le transfer) running over TCP. Though the queueing
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