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Unformatted text preview: ts own IP address. This is usually done when the interface is configured at bootstrap time.
In our internet, if we bootstrap the host bsdi and run tcpdump on the host sun, we see the packet shown in
Figure 4.7. 1 0.0 0:0:c0:6f:2d:40 ff:ff:ff:ff:ff:ff arp 60:
arp who has 22.214.171.124 tell 126.96.36.199
Figure 4.7 Example of gratuitous ARP. (We specified the -n flag for tcpdump to print numeric dotted-decimal addresses, instead of hostnames.) In
terms of the fields in the ARP request, the sender's protocol address and the target's protocol address are identical:
188.8.131.52 for host bsdi. Also, the source address in the Ethernet header, 0:0:c0:6f:2d:40 as shown by
tcpdump, equals the sender's hardware address (from Figure 4.4). Gratuitous ARP provides two features. 1. It lets a host determine if another host is already configured with the same IP address. The host bsdi is
not expecting a reply to this request. But if a reply is received, the error message "duplicate IP address
sent from Ethernet address: a:b:c:d:e:f" is logged on the console. This is a warning to the system
administrator that one of the systems is misconfigured. 2. If the host sending the gratuitous ARP has just changed its hardware address (perhaps the host was shut
down, the interface card replaced, and then the host was rebooted), this packet causes any other host on
the cable that has an entry in its cache for the old hardware address to update its ARP cache entry
accordingly. A little known fact of the ARP protocol [Plummer 1982] is that if a host receives an ARP
request from an IP address that is already in the receiver's cache, then that cache entry is updated with the
sender's hardware address (e.g., Ethernet address) from the ARP request. This is done for any ARP
request received by the host. (Recall that ARP requests are broadcast, so this is done by all hosts on the
network each time an ARP request is sent.)
[Bhide, Einozahy, and Morgan 1991] describe an application that can use this feature of ARP to allow a
backup file server to take over fro...
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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