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

TCP smoothes out its window increase by using small steps on the receipt of
every new ack from the destination. If the ow of acks is smooth, the window
increases are smooth. As a result, TCP data cannot load the network in sudden
bursts. However, when the tra c ows in both directions on TCP connections,
the behavior is more complex. This is because the acknowledgements may be
received in bursts, rather than being spaced out over time. As a result, the
TCP window increases by amounts larger than one MSS, and the source sends
data in larger bursts.
In the worst case, all the acknowledgements for the previous cycle are received
at once, i.e., acknowledgements for D = 1 RTT Link bandwidth bytes of
data is received at once. This results in the 2D bytes of data (D due to data
using the portion of the windows acknowledged, and D due to data using the
expansion of the window by the slow-start exponential increase policy) sent in
the current cycle. Assuming that there is bandwidth available to transmit this
data upto the bottleneck, the worst case queue is 2D. Observe that the total
average load measured over an RTT is still twice the Link bandwidth, however
there is an extra instantaneous queue of D =1 RTT Link bandwidth. This
is because the entire queue builds up within a cell transmission time. In the
case when TCP load is smooth, two cells are being input for every cell drained
from the queue.
Observe that once the sources are rate-limited, the TCP burstiness due to ack
compression is hidden from the ATM network because the excess cells are
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