Postel's law As defined by Jon Postel in RFC 793 https://tools.ietf.org/html/rfc793 TCP implementations will follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others. #harmful Postel's law considered harmful Being conservative in what you do disincentivizes people from being liberal in what they accept. + Being liberal in what you accept disincentivizes people from being conservative in what they do. This is not my idea, Martin Thompson wrote https://tools.ietf.org/html/draft-iab-protocol-maintenance-04 A flaw can become entrenched as a de facto standard. Any implementation of the protocol is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applying the robustness principle, and a product of a natural reluctance to avoid fatal error conditions. Ensuring interoperability in this environment is often referred to as aiming to be "bug for bug compatible". Postel's Law again https://blog.dshr.org/2017/02/postels-law-again.html Postel's Law has 2 parts http://essaysfromexodus.scripting.com/postelsLaw Robustness Principle Reconsidered https://queue.acm.org/detail.cfm?id=1999945