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<?xml version="1.0" encoding="ISO-8859-1" standalone="no"?>
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Postel's Prescription</title><link rel="stylesheet" href="../../jargon.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.61.0"/><link rel="home" href="../index.html" title="The Jargon File"/><link rel="up" href="../P.html" title="P"/><link rel="previous" href="postcardware.html" title="postcardware"/><link rel="next" href="posting.html" title="posting"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Postel's Prescription</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="postcardware.html">Prev</a><EFBFBD></td><th width="60%" align="center">P</th><td width="20%" align="right"><EFBFBD><a accesskey="n" href="posting.html">Next</a></td></tr></table><hr/></div><dt><a id="Postels-Prescription"/><dt xmlns="" id="Postels-Prescription"><b>Postel's Prescription</b></dt></dt><dd><p>[proposed] Several of the key Internet <a href="../R/RFC.html"><i class="glossterm">RFC</i></a>s,
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especially 1122 and 791 contain a piece of advice due to Jon Postel,
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which is most often stated as:</p><div class="blockquote"><blockquote class="blockquote"><p>“<span class="quote">Be liberal in what you accept, and conservative in what you
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send.</span>”</p></blockquote></div><p>That is, a well-engineered implementation of any of the
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Internet protocols should be willing to deal with marginal
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and imperfectly-formed inputs, but should not assume that the
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program on the other end (that is, the program dealing with the
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well-engineered implementation's output) will be anything other
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than rigid and inflexible, and perhaps even incomplete or downright
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buggy.</p><p>This property is valuable because a network of programs adhering
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to it will be much more robust in the presence of any uncertainties
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in the protocol specifications, or any individual implementor's
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failure to understand those specifications perfectly. Though the
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policy does tend to accommodate broken implementations it is held
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to more important to get the communication flowing than to
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immediately (but terminally) diagnose the broken implementations
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at the expense of the people trying to use them.</p><p>The principle is a well-known one in the design of programs
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that handle Internet wire protocols, especially network
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relays and servers, and it is regularly applied by extension
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in any situation where two or more separately-implemented
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pieces of software are supposed to interoperate even though the
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various implementors have never talked to each other and have
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absolutely nothing whatsoever in common other than having
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all read the same protocol specification. The principle travels
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under several different names, including “<span class="quote">the Internet credo</span>”,
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“<span class="quote">the IETF maxim</span>”, “<span class="quote">the Internet Engineering
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Principle</span>”, and
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“<span class="quote">the liberal/conservative rule</span>”; the [proposed] term
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“<span class="quote">Postel' Prescription</span>” is a tribute to its inventor, the first
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RFC editor
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and (until his untimely death) probably the single most respected
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individual in the Internet engineering community.</p></dd><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="postcardware.html">Prev</a><EFBFBD></td><td width="20%" align="center"><a accesskey="u" href="../P.html">Up</a></td><td width="40%" align="right"><EFBFBD><a accesskey="n" href="posting.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">postcardware<EFBFBD></td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"><EFBFBD>posting</td></tr></table></div></body></html>
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