RFC 2075 (rfc2075) - Page 3 of 5

IP Echo Host Service

RFC 2075                  IP Echo Host Service              January 1997


      in the list.  (This rewrite makes the option appear consistent
      with the new source and destination IP addresses, and retains the
      source timestamp, while losing information about the path to the
      echo host).

      2. If the first entry in the timestamp option does not contain the
      IP address of the source host, the entry SHOULD be echoed back
      unchanged. The echo host SHOULD NOT appear in the timestamp
      option.  (This approach retains the entire history of the path,
      though observe that on a symmetric route, it means every router
      may appear twice in the path).

   Finally, if the IP datagram contains an ICMP packet (i.e. the IP
   protocol field value is 1), the datagram SHOULD be discarded.  The
   reason for this rule is that the most likely reason for receiving an
   ICMP datagram is that an echoed datagram has encountered a problem at
   some router in the path and the router has sent back an ICMP
   datagram.  Echoing the ICMP datagram back to the router may confuse
   the router and thus SHOULD be avoided.  (This rule simply follows the
   Internet maxim of being conservative in what we send).

   However, in some cases the ICMP datagram will have useful information
   for the source host which it would be desirable to echo.  A
   sophisticated echo host MAY choose to echo ICMP datagrams according
   to the following rules:

      1. Any ICMP datagram in which the destination address in the
      encapsulated IP header (the header within the ICMP datagram)
      matches the source address of the ICMP datagram MAY be safely
      echoed.

      2. ICMP Source Quench and ICMP Destination Unreachable with a code
      of 4 (fragmentation needed and DF set) MAY be sent to the
      *destination* of the encapsulated IP datagram if the source IP
      address of the encapsulated IP datagram is that of the echo host.
      When the ICMP message is sent on, it SHOULD be rewritten as an
      ICMP message from the echo host to the source.

      3. All other ICMP messages MUST be discarded.

   These rules were chosen to try to ensure that end-to-end ICMP
   messages are passed through, as are messages from routers which are
   fairly safe and useful (or necessary) to the end system, but that
   potentially dangerous messages such as Redirects are suppressed.
   (The ICMP Destination Unreachable with code 4 is required for MTU
   discovery under RFC-1191).





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