6. Double Record-Routing

   Although proxies usually add zero or one Record-Route entries to a
   particular request, some proxies add two of them to avoid Record-
   Route rewriting.  A typical example of double Record-Routing is a SIP
   proxy that acts as a firewall between two networks.  Depending on
   which network a request comes from, it will be received on a
   different interface by the proxy.  The proxy adds one Record-Route
   entry for one interface and a second one for the other interface.
   This way, the proxy does not need to rewrite the Record-Route header
   field on the response.

   Proxies that receive compressed messages from one side of the dialog
   (e.g., upstream) and uncompressed messages from the other side (e.g.,
   downstream) MAY use the mechanism described above.

   If a proxy detects that the next-hop proxy for a request is the proxy
   itself and that the request will not be sent through the network, the
   proxy MAY choose not to compress the request even if the URI contains
   the comp=sigcomp parameter.