&gt;Postel&#x27;s law states that one should be conservative in what one does, but liberal in what one accepts from others; this is great advice for life, but terrible advice for writing software.<p>This perfectly sums up what I&#x27;ve tried to say about this &quot;robustness principle&quot; for years. Fantastic.<p>SIP inherits these status code ideas, plus adds super-neato whiz-bang headers, like &quot;Retry-After&quot;, that <i>allow comments</i>. Their actual example is &quot;Retry-After: 300 (in a meeting)&quot;. It&#x27;s beyond delusional.<p>Except.... someone read the spec. They see Retry-After. They then implement it in some big iron that connects to the PSTN. Little SIP VoIP provider comes along, passes along this dumb header, Retry-After. Suddenly, big switch guy is pissed, as Verizon has disabled all his lines as &quot;Retry-After&quot; gets translated into &quot;shutdown trunk&quot; or something. Awesome. Really a round of applause for the IETF there. (You have got to know that these headers came about just sitting around thinking shit up, vs actually trying to implement telephony. That&#x27;s why SIP says it might be used &quot;to run a chess game&quot;, but has a mandatory &quot;Call-ID&quot; field. Oops.)