One reason this is so difficult relates to the problem
statement in target-uri in that
RFC 3261 doesn't differentiate the mechanism by which the new
(target) Request-URI is selected. Another issue is that some
of the terminology in
RFC 3261 is overloaded - e.g., "forwarding" refers both to a
Proxy
which does not have responsibility for the domain of the
request-URI
in the incoming request, thus the proxy just "forwards" the
request to
the next hop AND "forwarding" is used to describe the process
whereby
the outgoing request is built and "forwarded" to the next hop
at which
point the proxy does not know how the new request-uri was
selected.
RFC 4244 has attempted to clarify the terms and attempts to
use "forward"
in the context of the former situation and "retarget" for the
case whereby
a proxy is responsible for the domain and thus can use a
number of
mechanism to select the new target for the request - e.g., a
REGISTRAR,
configured data, etc.