In 1981, Jon Postel formulated the Robustness Principle, also known as Postel's Law, as a fundamental implementation guideline for the then-new TCP. T

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In 1981, Jon Postel formulated the Robustness Principle, also known as Postel's Law, as a fundamental implementation guideline for the then-new TCP. The intent of the Robustness Principle was to maximize interoperability between network service implementations, particularly in the face of ambiguous or incomplete specifications. If every implementation of some service that generates some piece of protocol did so using the most conservative interpretation of the specification and every implementation that accepted that piece of protocol interpreted it using the most generous interpretation, then the chance that the two services would be able to talk with each other would be maximized. Experience with the Arpanet had shown that getting independently developed implementations to interoperate was difficult, and since the Internet was expected to be much larger than the Arpanet, the old ad hoc methods needed to be enhanced.

Although the Robustness Principle was specifically described for implementations of TCP, it was quickly accepted as a good proposition for implementing network protocols in general. Some have applied it to the design of APIs and even programming language design. It's simple, easy to understand, and intuitively obvious. But is it correct?