22    ICP_OP_DENIED
      23    ICP_OP_HIT_OBJ

   ICP_OP_INVALID
      A place holder to detect zero-filled or malformed messages.  A
      cache must never intentionally send an ICP_OP_INVALID message.
      ICP_OP_ERR should be used instead.

   ICP_OP_QUERY
      A query message.  NOTE this opcode has a different payload format
      than most of the others.  First is the requester's IPv4 address,
      followed by a URL.  The Requester Host Address is not that of the
      cache generating the ICP message, but rather the address of the
      caches's client that originated the request.  The Requester Host
      Address is often zero filled.  An ICP message with an all-zero
      Requester Host Address address should be taken as one where the
      requester address is not specified; it does not indicate a valid
      IPv4 address.








 
RFC 2186                          ICP                     September 1997


      ICP_OP_QUERY payload format:

        0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Requester Host Address                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      /                       Null-Terminated URL                     /
      /                                                               /
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      In response to an ICP_OP_QUERY, the recipient must return one of:
      ICP_OP_HIT, ICP_OP_MISS, ICP_OP_ERR, ICP_OP_MISS_NOFETCH,
      ICP_OP_DENIED, or ICP_OP_HIT_OBJ.

   ICP_OP_SECHO
      Similar to ICP_OP_QUERY, but for use in simulating a query to an
      origin server.  When ICP is used to select the closest neighbor,
      the origin server can be included in the algorithm by bouncing an
      ICP_OP_SECHO message off it's echo port.  The payload is simply
      the null-terminated URL.

      NOTE: the echo server will not interpret the data (i.e. we could
      send it anything).  This opcode is used to tell the difference
      between a legitimate query or response, random garbage, and an
      echo response.

   ICP_OP_DECHO
      Similar to ICP_OP_QUERY, but for use in simulating a query to a
      cache which does not use ICP.  When ICP is used to choose the
      closest neighbor, a non-ICP cache can be included in the algorithm
      by bouncing an ICP_OP_DECHO message off it's echo port.  The
      payload is simply the null-terminated URL.

      NOTE: one problem with this approach is that while a system's echo
      port may be functioning perfectly, the cache software may not be
      running at all.

   One of the following six ICP opcodes are sent in response to an
   ICP_OP_QUERY message.  Unless otherwise noted, the payload must be
   the null-terminated URL string.  Both the URL string and the Request
   Number field must be exactly the same as from the ICP_OP_QUERY
   message.







 
RFC 2186                          ICP                     September 1997


   ICP_OP_HIT
      An ICP_OP_HIT response indicates that the requested URL exists in
      this cache and that the requester is allowed to retrieve it.

   ICP_OP_MISS
      An ICP_OP_MISS response indicates that the requested URL does not
      exist in this cache.  The querying cache may still choose to fetch
      the URL from the replying cache.

   ICP_OP_ERR
      An ICP_OP_ERR response indicates some kind of error in parsing or
      handling the query message (e.g. invalid URL).

   ICP_OP_MISS_NOFETCH