AN INCREMENTAL SPECIFICATION OF THE SLIDING-WINDOW PROTOCOL

The sliding-window protocol is specified using the notation of Communicating Sequential Processes and its partial correctness is proved using the trace semantics. First the stop-and-wait protocol is defined; its correctness, that it forms a 1-place buffer, is almost evident. Next the alternating-bit protocol is defined and described in terms of the stop-and-wait protocol, and its correctness deduced from that of the stop-and-wait protocol. Finally the sliding-window protocol is described in terms of the alternating-bit protocol and its correctness deduced accordingly. The paper has two thrusts: that modularity of a specification helps to structure proofs about it (in this case, proofs that the protocols implement buffers); and that refinement in CSP leads to structured, correct implementation in occam. In support of the latter point the appendix contains a refinement and implementation of the protocols in occam 2.