Unformatted text preview: A final point from the tcpdump output is that we don't see an ARP request from svr4 before it
sends its first TCP segment (line 4). While it's possible that svr4 already had an entry for bsdi
in its ARP cache, normally when a system receives an ARP request, in addition to sending the
ARP reply it also saves the requestor's hardware address and IP address in its own ARP cache.
This is on the logical assumption that if the requestor is about to send it an IP datagram, the
receiver of the datagram will probably send a reply.
ARP Request to a Nonexistent Host
What happens if the host being queried for is down or nonexistent? To see this we specify a
nonexistent Internet address-the network ID and subnet ID are that of the local Ethernet, but there
is no host with the specified host ID. From Figure 3.10 we see the host IDs 36 through 62 are
nonexistent (the host ID of 63 is the broadcast address). We'll use the host ID 36 in this example.
bsdi % date ; telnet 220.127.116.11 ; date
Sat Jan 30 06:46:33 MST 1993
telnet: Unable to connect to remote host:
Connection timed out
Sat Jan 30 06:47:49 MST 1993
bsdi % arp -a
? (18.104.22.168) at (incomplete)
Figure 4.5 shows the tcpdump output. telnet to an address
this time, not a
76 seconds after
previous date output
check the ARP cache file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (7 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol 1
3 arp who has 22.214.171.124 tell bsdi
arp who has 126.96.36.199 tell bsdi
29.509745 (24.0007) arp who has 188.8.131.52 tell bsdi
Figure 4.5 ARP requests to a nonexistent host. This time we didn't specify the -e option since we already know that the ARP requests are
What's interesting here is to see the frequency of the ARP requests: 5.5 seconds after the first
request, then again 24 seconds later. (We examine TCP's timeout and retransmission algorithms
in more detail in Chapter 21.) The total t...
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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