• To: <costello@m...>,<xml-dev@l...>
• Subject: RE: RE: Are people really using Identity constraints specified in XML schema?
• From: "Michael Kay" <michael.h.kay@n...>
• Date: Thu, 19 Aug 2004 10:07:05 +0100
• In-reply-to: <76ce88c7.88c776ce@m...>
• Thread-index: AcSFeabZYuElR95ORZOsp78KkY2BSwATkjEA

> 
> I remember reading a long time ago a posting by David 
> Megginson.  Paraphrasing, David said, "when dealing with XML 
> you are working down at the bare metal".  
> 
> Other technologies work down at the bare metal, such as 
> TCP/IP.  Perhaps there are lessons to be learned there?  
> Certain TCP/IP packets are rejected as bad and the other 
> packets are accepted and passed up to other layers, where 
> those layers perform additional constraint checking.
> 

TCP is 4 layers above the bare metal, IP is 5 layers above, XML is 6 layers
above, and a specific XML vocabulary (as described by a schema) is 7 layers
above. Every layer in the protocol stack needs to check that its own rules
are satisfied.

Michael Kay 

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