16.4 Route Information Preprocessing

   The proxy MUST inspect the Request-URI of the request.  If the
   Request-URI of the request contains a value this proxy previously
   placed into a Record-Route header field (see Section 16.6 item 4),
   the proxy MUST replace the Request-URI in the request with the last
   value from the Route header field, and remove that value from the
   Route header field.  The proxy MUST then proceed as if it received
   this modified request.

      This will only happen when the element sending the request to the
      proxy (which may have been an endpoint) is a strict router.  This
      rewrite on receive is necessary to enable backwards compatibility
      with those elements.  It also allows elements following this
      specification to preserve the Request-URI through strict-routing
      proxies (see Section 12.2.1.1).

      This requirement does not obligate a proxy to keep state in order
      to detect URIs it previously placed in Record-Route header fields.
      Instead, a proxy need only place enough information in those URIs
      to recognize them as values it provided when they later appear.

   If the Request-URI contains a maddr parameter, the proxy MUST check
   to see if its value is in the set of addresses or domains the proxy
   is configured to be responsible for.  If the Request-URI has a maddr
   parameter with a value the proxy is responsible for, and the request
   was received using the port and transport indicated (explicitly or by
   default) in the Request-URI, the proxy MUST strip the maddr and any
   non-default port or transport parameter and continue processing as if
   those values had not been present in the request.

      A request may arrive with a maddr matching the proxy, but on a
      port or transport different from that indicated in the URI.  Such
      a request needs to be forwarded to the proxy using the indicated
      port and transport.

   If the first value in the Route header field indicates this proxy,
   the proxy MUST remove that value from the request.

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