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

u ered at the end-system, and not inside the ATM network. This is a sig-
ni cant performance beni t considering the fact that without rate-control the
bu er requirement may bevery large [82].
7. E ect of VBR backgrounds: The presence of higher priority background
tra c implies that the ABR capacity isvariable. There are two implications of
the variation in capacity: a) the e ect on the rate of TCP acks and the window
growth, and, b) the e ect on the switch rate allocation algorithm (ERICA).
Part a): The e ect of VBR background on the TCP \ack clock" (i.e., the rate
of TCP acknowledgements (ACKs) is described below.
If VBR comes on during zero ABR load, it does not a ect the TCP ACK
clock because there are no ABR cells in the pipe. This is the case when
VBR comes on during the idle periods of TCP, especially in the initial
startup phase of TCP.
If VBR comes on during low load (initial load less than unity), such that
the resulting total load is less than unity, it increases the TCP packet
transmission time. Increased packet transmission time means that the
inter-ACK time is increased (called \ACK expansion"). Increase in inter-
ACK time slows down the window increase since the \ACK clock" runs
slower now. Slower window increases imply reduced ABR load in the next
cycle, irrespective of whether VBR is o or on.
Speci cally, if VBR goes o , the inter-ACK spacing starts decreasing by
half every round trip (due to TCP window doubling every round trip).
Since we have assumed that the system was in low load previously, this
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