Conventional computer engineering relies on test-and-debug development processes, with the behavior of common interfaces described (at best) with prose specification documents. But prose specifications cannot be used in test-and-debug development in any automated way, and prose is a poor medium for expressing complex (and loose) specifications. The TCP/IP protocols and Sockets API are a good example of this: they play a vital role in modern communication and computation, and interoperability

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... ween implementations is essential. But what exactly they are is surprisingly obscure: their original development focused on "rough consensus and running code," augmented by prose RFC specifications that do not precisely define what it means for an implementation to be correct. Ultimately, the actual standard is the de facto one of the common implementations, including, for example, the 15 000 to 20 000 lines of the BSD implementation-optimized and multithreaded C code, time dependent, with asynchronous event handlers, intertwined with the operating system, and security critical. This article reports on work done in the Netsem project to develop lightweight mathematically rigorous techniques that can be applied to such systems: to specify their behavior precisely (but loosely enough to permit the required implementation variation) and to test whether these specifications and the implementations correspond with specifications that are executable as test oracles. We developed post hoc specifications of TCP, UDP, and the Sockets API, both of the service that they provide to applications (in terms of TCP bidirectional stream connections) and of the internal operation of the protocol (in terms of TCP segments and UDP datagrams), together with a testable abstraction function relating the two. These S.