Thanks Hari Kumar! Hari pointed out a solution using 'when' rule field under 'XML validation' may be it could be used to determine whether URL is up or not. If URL is not accessible then don't proceed further with the XML validation; fail sooner. From the exception stack trace, It can found out whether when rule called before the URL access or not. If when rule is called before accessing the URL then this suggestion can be used as local change. Downside is that extra check for the validity of URL for each call.
your idea sounds doable. The extra check for each call is something I wouldn't like too much. As there is no such setting on the rule form level I was hoping to learn about a setting in one of the configuration xml files.
If there is no further suggestion we might try to implement your proposed solution. Additionally I'll open an SR and reference it here.
It is reasonable to expect PRPC to timeout in a reasonable amount of time while attempting to get a schema document. I suspect we should be able to implement a default timeout of a few minutes and then provide a dynamic system setting for tweaking it.
Are you sure the schema-providing server NEVER responds? I remember reading that some (popular public) sites explicitly delay the response when a DTD or a schema is requested (because of otherwise excessive traffic and to encourage HTTP caching practices).