Postel’s Prescription

What Does Postel’s Prescription Mean?

Postel’s prescription is a commonly cited idea in IT, particularly in the design of the internet, that discusses how to design protocols and specifications for systems. It is named for American computer scientist Jon Postel.


Postel’s prescription states that systems should be “liberal in what they accept, and conservative in what they send.”

Techopedia Explains Postel’s Prescription

Postel’s prescription has also been called the “IETF maxim” (describing the workings of the Internet Engineering Task Force) and the “internet engineering principle,” as well as the “liberal/conservative rule” of network implementations. The idea is that systems should be able to handle a diversity of input, also supplying hard and fast rules for processes.

In today’s internet world, Postel’s prescription has actually generated some amount of controversy. Critics of the idea suggest that while Postel’s prescription may have been good for the early days of the internet, where a certain amount of fault tolerance was needed, the idea of accepting diverse inputs may have harmed some processes of maintaining standards and specifications over the long term. This criticism suggests that now, with such heterogenous use of internet protocols in such a huge and inherently chaotic system, there should be less tolerance of different types of “buggy script.” Some also suggest that fans of Postel should pay more attention to his other contributions to internet engineering than to a saying that some believe has been taken somewhat out of context. However, in many senses, Postel’s prescription has been a guiding philosophy for internet development.


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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.