The constraint really belongs at the target, not at the reference
Added test for the server for every request on a direct reference
1. Itís the owner of the target, not the owner of the reference, who cares about the secrecy of the targetís location.
But putting a constraint at the target would mean:
A header for use with PUT, MKCOL, MKREF
An ineffective attempt to control creation of references on other servers
But, if the location is really secret, the only person who could create a reference to it is its owner. So there is some use to providing an option at reference creation time to hide the location of the target. In the real world there will be people other than the owner who know of the existence of the location and may create references to it.
2. Itís more complexity for the server. In general, the server is required to return Ref-Target with every response on a direct reference. But if we have an option to hide the location, the server must check before every response whether to return Ref-Target or not.