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

feedback asking sources to reduce their rates. The TCP congestion window is now
large and is increasing. Hence, it will send data at rate greater than the source's
sending rate. The le transfer data is bottlenecked by the ABR source rate and not
by the TCP congestion window size. In this state, we say that the TCP sources are
rate-limited. Observe that UBR cannot rate-limit TCP sources and would need to
bu er the entire TCP load inside the network.
The ABR queues at the switches start increasing when the TCP idle times are
not su cient to clear the queues built up during the TCP active times. The queues
may increase until the ABR source rates converge to optimum values. Once the TCP
sources are rate-limited and the rates converge to optimum values, the lengths of the
ABR queues at the switch will start decreasing. The queues now move over to the
source end-system (outside the ATM network). Several proprietary techniques can be
used to control the TCP queues at the edge of the ATM network [59], and we cover
some of the possibilities later in this chapter.
The remaining part of the chapter is organized as follows. We rst examine
the performance of TCP under lossy conditions in section 8.4. We then study the
interaction of the TCP and ABR congestion control algorithms and the justi cation
and assumptions for bu er requirements for zero loss in section 8.14. Next, we lookat
the e ect of variation in capacity (VBR backgrounds) on the bu er requirements. We
discuss switch algorithm issues and our solutions to handle variation in demand and
capacity in section 8.16. We then develop a model of multiplexed MPEG-2 sources
over VBR, and study its e ect on TCP sources running over ABR in section 8.17.
Finally, we look at related work (an extension of this dissertation work), where the
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