Interaction between TCP flow control and ABR rate control

Abstract

This study deals with TCP/IP traffic using the available bit rate (ABR) service and focuses particularly on the interaction between the TCP flow control and the ABR explicit rate control, when zero cell loss is provided by the ATM layer. The TCP retransmission mechanism is based on timeout of acknowledgements, which means that if the ABR service introduces too much delay variation, which may be the case when fast reaction rate control algorithms are used, false retransmissions will occur. Apart from, obviously, being a waste of bandwidth, a false retransmission actually causes a transient rate increase when the TCP slow start mechanism is mistakenly invoked, which may cause additional false retransmissions. Simulations of two ABR algorithms and TCP implementations have been performed to evaluate both the transient steady state behaviour as well as the average throughput when on-off VBR traffic was added. Based on the simulations, it can be concluded that the TCP performance improves if the TCP packet emission rates are closely adjusted to the rate offered by the ABR service, which also allows a small granularity of the retransmission timer. Moreover, of the two evaluated ABR algorithms, the one providing the fastest rate reactions gives the overall highest throughput compared to the one with more restrained rate change behaviour.