The Request-URI is a SIP URL as described in Section 2 or a general
URI. It indicates the user or service to which this request is being
addressed. Unlike the To field, the Request-URI MAY be re-written by
proxies.

When used as a Request-URI, a SIP-URL MUST NOT contain the
transport-param, maddr-param, ttl-param, or headers elements. A
server that receives a SIP-URL with these elements removes them
before further processing.

     Typically, the UAC sets the Request-URI and To to the same
     SIP URL, presumed to remain unchanged over long time
     periods. However, if the UAC has cached a more direct path
     to the callee, e.g., from the Contact header field of a
     response to a previous request, the To would still contain
     the long-term, "public" address, while the Request-URI
     would be set to the cached address.

Proxy and redirect servers MAY use the information in the Request-URI
and request header fields to handle the request and possibly rewrite
the Request-URI. For example, a request addressed to the generic
address sip:sales@acme.com is proxied to the particular person, e.g.,
sip:bob@ny.acme.com , with the To field remaining as
sip:sales@acme.com.  At ny.acme.com , Bob then designates Alice as
the temporary substitute.