TCP IP Illustrated

252192 56 data bytes 64 bytes from 140252192 icmpseq 0

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Unformatted text preview: 2): 56 data bytes 64 bytes from icmp_seq-0 ttl=-251 time-650 ms RR: bsdi ( sun ( netb ( aix ( gateway ( why is this router used? netb ( sun ( bsdi ( slip ( 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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