proxy/http/README.via

      [u<client-stuff> c<cache-lookup-stuff> s<server-stuff> f<cache-fill-stuff> p<proxy-stuff>] e<error-codes>

      client stuff
         E    error in request
         I    IMS
         N    no-cache
         C    cookie
         S    simple request (not conditional)

      cache lookup stuff
         M    miss
         A    in cache, not acceptable
         S    in cache, stale
         H    in cache, fresh
         R    in cache, fresh RAM hit

      server stuff
         E    error in response
         N    not-modified
         S    served

      cache fill stuff
         D    cached copy deleted
         W    written into cache (new copy)
         U    updated cache

      proxy stuff
         N    not-modified
         S    served
         R    origin server revalidated

      error codes
         N    no error
         A    authorization failure
         C    connection to server failed
         D    dns failure
         F    request forbidden
         H    header syntax unacceptable
         M    moved temporarily
         R    cache read error
         S    server related error
         T    connection timed out

      For example:

      [u lH o  f  pS eN]     cache hit
      [u lM oS fF pS eN]     cache miss
      [uN l oS f  pS eN]     no-cache origin server fetch

     -------------------------------------------------------

      Then there are the detailed via codes which follow a ":"

      t<tunneling information>c<cache type><cache lookup result>p<parent proxy conn info>s<server conn info>

      tunneling information
         F    tunneling due to a header field (such as presence of If-Range header)
         M    tunneling due to a method (e.g. CONNECT)
         O    tunneling because cache is turned off
         U    tunneling because of url (url suggests dynamic content)

      cache type
         C    cache
         L    cluster (not used)

      cache lookup result
         N    conditional hit (client sent conditional, fresh in cache, returned 304)
         H    cache hit
         I    conditional miss (client sent conditional, fresh in cache, returned 412)
         M    cache miss (url not in cache)
         S    cache hit, but expired
         C    cache hit, but config forces revalidate
         U    cache hit, but client forces revalidate (e.g. Pragma: no-cache)
         D    cache hit, but method forces revalidated (e.g. not anonymous)

      pp conn info
         S    connection opened successfully
         F    connection open failed

      server conn info
         S    connection opened successfully
         F    connection open failed