TCP IP Illustrated

2521333 icmp host unreachable from gateway sun

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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 ( ICMP Host Unreachable from gateway sun ( ^? type interrupt key to stop Figure 9.2 shows the tcpdump output for this example, run on the host bsdi. 1 2 3 4 svr4 > gemini: icmp: echo request 0.0 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 this IP address is not connected to the Internet PING 56 data bytes ICMP Host Unreachable from gateway ( for icmp from sun ( 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.

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