A test sequence generation method is proposed for testing the conformance of a protocol implementation to its specification in a remote testing system where both external synchronization and input/output operation costs are taken into consideration. The method consists of a set of transformation rules that constructs a duplexU digraph from a given finite state machine (FSM) representation of a protocol specification; and an algorithm that finds a rural postman tour in the duplexU digraph to generate a synchronizable test sequence utilizing multiple UIO sequences. If the protocol satisfies a specific property, namely, the transitions to be tested and the UIO sequences to be employed form a weakly-connected subgraph of the duplexU digraph, the proposed algorithm yields a minimum-cost test sequence. X.25 DTE and ISO Class 0 transport protocols are shown to possess this property. Otherwise, the algorithm yields a test sequence whose cost is within a bound from the cost of the minimum-cost test sequence. The bound for the test sequence generated from the Q.931 network-side protocol is shown to be the cost sum of an input/output operation pair and an external synchronization operation. >