Postel’s Law (Robustness Principle)

Jon Postel was an American computer scientist who helped to develop the basic protocols on which the Internet is built. Among other accomplishments, he is remembered today for the formulation of a robustness principle, now often referred to as Postel’s Law:

Be conservative in what you send, and liberal in what you accept.

In a nutshell, the law describes the nature of a robust system: when sending to another system, adhere as closely to a rigid standard as possible, but when receiving data from another system, allow for as much variability as possible.

The essence of the law is a mandate of individual responsibility. Each implementor is to hold himself to the highest standard (of compliance with the protocol specification), while being as forgiving as possible of the failures of others to do the same. Each is encouraged to strive for perfection in his own actions, while expecting far less from others.

Postel’s law, as originally stated, was intended to apply to program-to-program communications. But it can be applied to communications between programs and humans: for instance, when a user interface asks for your phone number, and then rejects it because you’ve entered it without dashes — or with, depending on the whims of the interface designer — then you know you’re dealing with an intolerant interface.

As human societies develop from traditional, fundamentalist levels to postmodern, pluralistic levels, Postel’s Law can also be applied to people-to-people communications: Live and let live. Turn the other cheek. Do unto others as you would have them do unto you. All of these maxims reflect the same underlying intent of Postel’s Law: follow the highest standards in your own behavior, but accept diversity in others.

Summing-up: We’re none of us computers, so we won’t follow any protocol laid out for us perfectly. At bottom line, we should all try to behave as civilly toward other people as our computer behaves toward other computers, thanks to Jon Postel.

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