Performance and Correctness of the ATM ABR Rate Control Scheme

We study both correctness and performance of the source/destination protocol of the Available Bit Rate (ABR) service in Asynchronous Transfer Mode (ATM) networks. Although the basic source/destination protocol for congestion management is relatively simple, the protocol specification has to cope with several “real-world” cases such as failures and delayed/lost feedback which may introduce complexity. Rigorous proofs of the correct functioning of the protocol based on a formal specification is necessary. We use a formal Extended Finite State Machine (EFSM) model to show that the ABR source/destination protocol is free of live-locks, so that under all conditions both Resource Management (RM) and data cells will be transmitted. We also show that the network options of Explicit Forward Congestion Indication (EFCI) and Explicit Rate (ER) interoperate correctly. While ensuring the correct functioning of the protocol, it is essential that pathological situations do not result in abysmal performance, which is another form of incorrect operation. We use the understanding of the informal English description of the source/destination behavior and of our EFSM model to derive conditions that ensure that the source transmission rate is stable in the presence of delayed or lost feedback RM cells, especially under the operation of a source rule that requires the reduction of the source rate under these conditions. We arrive at bounds on the number of consecutive RM cell losses tolerated while the rate remains stable. It is also of vital importance that the delay for feedback of network state is well understood. There are two components of this delay contributed by the source/destination protocol: the source delay for sending out the probe Forward RM cells and the destination delay to turn-around the Backward RM cell. We provide a worst-case analysis of the delay in turning around RM cells at the destination station and the worst-case inter-departure time of Forward RM cells from the source.