Postel's law

networkingprogramming

'Be conservative in what you do, be liberal in what you accept from others' - the robustness principle from the TCP specification.

Jon Postel wrote the maxim into the 1980 TCP specification, and it shaped a generation of interoperable protocol implementations. Decades later the field partially recanted: tolerating malformed input lets errors and attacks hide, so modern guidance (including RFC 9413) argues for stricter validation. The law endures as the axis of one of protocol engineering's longest-running debates.

Also known as: robustness principle, be liberal in what you accept

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