HP Labs Technical Reports

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An Improved SSCOP-Like Scheme for Avoiding Unnecessary Retransmissions and Achieving Ideal Throughput

Cohen, Reuven



Abstract: SSCOP is a new data link control protocol designed for ATM networks. To achieve high throughput, SSCOP uses the selective repeat retransmission policy, enforced using the exchange of POLL and STAT control frames between the sender and the receiver. To avoid unnecessary retransmission of information frames (I-frames), SSCOP uses the "checkpoint concept" where every POLL has a sequence number and every outstanding I-frame is associated in the sender buffers with the sequence number of the POLL sent before the last transmission of that I-frame. Using these sequence numbers, the sender knows to ignore unnecessary retransmission requests. The paper proposes an improved scheme for avoiding unnecessary retransmission of I-frames. The new scheme significantly reduces the memory needed for storing control information of the protocol at the sender interface. Compared with the SSCOP, the only potential drawback of the proposed scheme is that under certain circumstances the retransmission of lost I-frames may be delayed. However, the paper analyzes the sequence of events that must take place in such a case, and concludes that the probability for such a sequence is negligible and that the new protocol performs as well as the SSCOP.

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