ignored, i.e. no reply generated.  The application may choose to
      note the anomalous behaviour in a log file.

3.  ICP Option Flags

   0x80000000  ICP_FLAG_HIT_OBJ
      This flag is set in an ICP_OP_QUERY message indicating that it is
      okay to respond with an ICP_OP_HIT_OBJ message if the object data
      will fit in the reply.

   0x40000000  ICP_FLAG_SRC_RTT
      This flag is set in an ICP_OP_QUERY message indicating that the
      requester would like the ICP reply to include the responder's
      measured RTT to the origin server.

      Upon receipt of an ICP_OP_QUERY with ICP_FLAG_SRC_RTT bit set, a
      cache should check an internal database of RTT measurements.  If
      available, the RTT value MUST be expressed as a 16-bit integer, in
      units of milliseconds.  If unavailable, the responder may either
      set the RTT value to zero, or clear the ICP_FLAG_SRC_RTT bit in
      the ICP reply.  The ICP reply MUST not be delayed while waiting
      for the RTT measurement to occur.

      This flag is set in an ICP reply message (ICP_OP_HIT, ICP_OP_MISS,
      ICP_OP_MISS_NOFETCH, or ICP_OP_HIT_OBJ) to indicate that the low
      16-bits of the Option Data field contain the measured RTT to the
      host given in the requested URL.  If ICP_FLAG_SRC_RTT is clear in
      the query then it MUST also be clear in the reply.  If
      ICP_FLAG_SRC_RTT is set in the query, then it may or may not be
      set in the reply.

4.  Security Considerations

   The security issues relating to ICP are discussed in the companion
   document, RFC2187.















 
RFC 2186                          ICP                     September 1997


5.  References

   [1] Fielding, R., et. al, "Hypertext Transfer Protocol -- HTTP/1.1",
   RFC 2068, UC Irvine, January 1997.

   [2] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
   Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
   December 1994.

   [3] Bowman M., Danzig P., Hardy D., Manber U., Schwartz M., and
   Wessels D., "The Harvest Information Discovery and Access System",
   Internet Research Task Force - Resource Discovery,
   http://harvest.transarc.com/.

   [4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National
   Laboratory for Applied Network Research,
   http://www.nlanr.net/~wessels/Papers/icp-squid.ps.gz

   [5] Wessels D., "The Squid Internet Object Cache", National
   Laboratory for Applied Network Research,
   http://squid.nlanr.net/Squid/

6.  Acknowledgments

   The authors wish to thank Paul A Vixie <paul@vix.com> for providing
   excellent feedback on this document.

7.  Authors'  Addresses

   Duane Wessels
   National Laboratory for Applied Network Research
   10100 Hopkins Drive
   La Jolla, CA 92093

   EMail: wessels@nlanr.net


   K. Claffy
   National Laboratory for Applied Network Research
   10100 Hopkins Drive
   La Jolla, CA 92093

   EMail: kc@nlanr.net