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Unformatted text preview: cpip/tcp-ip-illustrated/ip_rout.htm (8 of 17) [12/09/2001 14.46.51] Chapter 9. IP Routing ICMP Host Unreachable from gateway sun (18.104.22.168)
ICMP Host Unreachable from gateway sun (22.214.171.124)
^? type interrupt key to stop
Figure 9.2 shows the tcpdump output for this example, run on the host bsdi.
4 svr4 > gemini: icmp: echo request
0.00 (0.00) sun > svr4: icmp: host gemini unreachable
0.99 (0.99) svr4 > gemini: icmp: echo request
0.99 (0.00) sun > svr4: icmp: host gemini unreachable
Figure 9.2 ICMP host unreachable in response to ping. When the router sun finds no route to the host gemini, it responds to the echo request
with a host unreachable.
If we bring the SLIP link to the Internet up, and try to ping an IP address that is not
connected to the Internet, we expect an error. What is interesting is to see how far the
packet gets into the Internet, before the error is returned:
sun % ping 126.96.36.199 this IP address is not connected to
PING 188.8.131.52: 56 data bytes
ICMP Host Unreachable from gateway enss142.UT.westnet.net
(184.108.40.206) for icmp from sun (220.127.116.11) to
Looking at Figure 8.5 we see that the packet made it through six routers before detecting
that the IP address was invalid. Only when it got to the border of the NSFNET backbone
was the error detected. This implies that the six routers that forwarded the packet were
doing so because of default entries, and only when it reached the NSFNET backbone did a
router have complete knowledge of every network connected to the Internet. This illustrates
that many routers can operate with just partial knowledge of the big picture.
[Ford, Rekhter, and Braun 1993] define a top-level routing domain as one that maintains
routing information to most Internet sites and does not use default routes. They note that
five of these top-level routing domains exist on the Internet: the NFSNET backbone, the
Commercial Internet Exchange (CIX), the NASA Science Internet (NSI), SprintLink, and
the European IP Backbone (EBONE). 9.4 To Forward or Not to Forw...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12