5. Sending a Response to a Client

   A response is sent to the host in the sent-by parameter of the Via
   header field.  If the topmost Via header field contains the parameter
   comp=sigcomp, the response SHOULD be compressed.  Otherwise, the
   response MUST NOT be compressed.

   In order to avoid asymmetric compression (i.e., two SIP entities
   exchanging compressed requests in one direction and uncompressed
   requests in the other direction) proxies need to rewrite their
   Record-Route entries in the responses.  A proxy performing Record-
   Route inspects the Record-Route header field in the response and the
   Contact header field in the request that triggered this response (see
   example in Section 9).  It looks for the URI of the next upstream
   (closer to the user agent client) hop in the route set.  If this URI
   contains the parameter comp=sigcomp, the proxy SHOULD add
   comp=sigcomp to its entry in the Record-Route header field.  If this
   URI does not contain the parameter comp=sigcomp, the proxy SHOULD
   remove comp=sigcomp (if it is present) from its entry in the Record-
   Route header field.

   The same way, a user agent server SHOULD add comp=sigcomp to the
   Contact header field of the response if the URI of the next upstream
   hop in the route set contained the parameter comp=sigcomp.