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Re: Application of Postel's Law to XML (as Re: Trust andcontro

• From: Liam R E Quin <liam@w3.org>
• To: Amelia A Lewis <amyzing@talsever.com>
• Date: Mon, 04 Feb 2013 13:28:39 -0500

On Mon, 2013-02-04 at 12:09 -0500, Amelia A Lewis wrote:
> On Mon, 4 Feb 2013 16:57:33 +0000, Costello, Roger L. wrote:
> > I want to fully understand what it means to apply Postel's Law to XML.
> 
> So far as I can see, XML's insistence on well-formedness is a direct 
> contradiction to Postel's Law. It mandates that receivers be rigorous. 

If you take it that way, most Web servers contravene it - they are not
strict in what they put out, being willing to serve up content as
text/html that is not valid, or application/xml that is no well-formed.

The maxim came from the development of low-level protocols and should
not really be applied as a "best practice" to higher layers. When you
have something like TCP/IP or even FTP, a certain amount of tolerance
does help, but the software was never expected to handle total garbage
and make the best of it that it could.  An example for TCP/IP would be
accepting "trailing packet headers", a variant used because the DEC VAX
at the time wasn't fast enough to analyze network packets in real time
to know whether it should save them. It's a known and documented variant
and it made sense to support it widely.  But TCP/IP systems still reject
packets with bad checksums or incorrect length fields, and demand
retransmission.

Liam

-- 
Liam Quin - XML Activity Lead, W3C, http://www.w3.org/People/Quin/
Pictures from old books: http://fromoldbooks.org/
Ankh: irc.sorcery.net irc.gnome.org freenode/#xml

• References:
  • Application of Postel's Law to XML (as Re: Trust and control )
    • From: "Costello, Roger L." <costello@mitre.org>
  • Re: Application of Postel's Law to XML (as Re: Trust andcontrol )
    • From: Amelia A Lewis <amyzing@talsever.com>