Asynchronous Protocol API's

Every protocol implmentation needs to define a class implementing the following interfaces: All errors are to be translated into HTTP error codes (the richest set of the currently understood protocols).

Pseudo-protocols like "mailto:" that don't return data can return an error code 204 ("request accepted, no return document").

Protocols like "hdl:" that map into other protocols can return an error code 301 ("document moved"); the request is then transparently retried by the higher layer