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64 bytes from 220.127.116.11: icmp_seq-0 ttl=-251 time-650 ms
RR: bsdi (18.104.22.168)
gateway (22.214.171.124) why is this router used?
64 bytes from aix: icmp_seq=l ttl=251 time=610 ms (same route)
64 bytes from aix: icmp_seq=2 ttl=251 time"600 ms (same route)
-- aix ping statistics - file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (10 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program 4 packets transmitted, 3 packets received, 25% packet loss
round-trip min/avg/max = 600/620/650 ms
We could have run this example from the host bsdi. We chose to run it from slip to see all
nine IP addresses in the RR list used.
The puzzle in this output is why the outgoing datagram (the ICMP echo request) went directly
from netb to aix, but the return (the ICMP echo reply) went from aix, through the router
gateway, before going to netb. What we're seeing here is a feature of IP routing that we
describe below. Figure 7.6 shows the path of the datagrams. Figure 7.6 ping with record route, showing IP routing feature.
The problem is that aix does not know to send IP datagrams destined for the subnet 140.252.13
to netb. Instead, aix has a default entry in its routing table that tells it to send all datagrams to
the router gateway if it doesn't have a particular route for the destination. The router
gateway has more routing knowledge than any of the hosts on the 140.252.1 subnet. (There
are more than 150 hosts on this Ethernet and instead of running a routing daemon on every one,
each has a "default" entry that points to the router gateway.)
An unanswered question here is why doesn't gateway send an ICMP redirect (Section 9.5) to
aix to update its routing table? For some reason (perhaps that the datagram generating the
redirect is an ICMP echo request message) the redirect is not genera...
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- Spring '12