1. A computer readable medium for use in validating an eXtensible Markup Language (XML) message in a particular wire format, said computer readable medium containing computer-executable instructions which, when performed by a processor in a computing device, cause the computing device to: from a custom XML schema based message model having at least one wire format specific rendering option associated with an XML entity representing at least a portion of said XML message, generate an XML schema fragment for use in validating said XML message at a node that lacks said custom XML schema based message model, wherein said XML schema fragment conforms to said particular wire format, wherein said custom XML schema based message model comprises a logical model of said message that complies with a standard XML schema, and wherein said custom XML schema based message model comprises a physical model that customizes said message using a logical model extension that is unsupported by said standard XML schema, and wherein said physical model comprises said logical model extension and a wire format specific rendering option, wherein said wire format specific rendering option is a directive that specifies a different wire format for each partial component of said message.