RFC 2075 (rfc2075) - Page 3 of 5
IP Echo Host Service
Alternative Format: Original Text Document
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).
Partridge Experimental