TCP IP Illustrated

322 of the host requirements rec and section 119 of

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Unformatted text preview: apter 4 4.1 Issuing an rsh command establishes a TCP connection with the other host. Doing that causes IP datagrams to be exchanged between the two hosts. This requires the ARP cache on the other host to have an entry for our host. Therefore, even if the ARP cache was empty before we executed the rsh command, it's guaranteed to have an entry for our host when the rsh server executes the arp command. 4.2 Make sure that your host does not have an entry in its ARP cache for some other host on its Ethernet, say foo. Make sure foo sends a gratuitous ARP request when it bootstraps, perhaps running tcpdump on another host when foo bootstraps. Then shut down the host foo and enter an incorrect entry into the ARP cache on your system for foo, using the arp command and being sure to specify the temp option. Bootstrap foo and when it's up, look at your host's ARP cache entry for it to see whether the incorrect entry has been corrected. 4.3 Read Section of the Host Requirements REC and Section 11.9 of this text. 4.4 Assuming that a completed ARP entry existed for the server on the client when the server was taken down, if we continually try to contact the (down) server, the ARP timeout gets extended for another 20 minutes. When the server finally reboots with a new hardware address, if it doesn't issue a gratuitous ARP, the old, invalid ARP entry will still exist on the client. We won't be able to contact the server at its new hardware address until we either manually delete the ARP cache entry or stop trying to contact it for 20 minutes. Chapter 5 file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_d.htm (2 of 20) [12/09/2001 14.48.03] Appendix D: Solutions to Selected Exercises 5.1 A separate frame type is not an absolute requirement, since the op field in Figure 4.3 has a different value for all four operations (ARP request, ARP reply, RARP request, and RARP reply). But the implementation of an RARP server, separate from the kernel's ARP...
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