TCP IP Illustrated

2 ms the first tcp segment is sent 07 ms after this

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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 ; date Sat Jan 30 06:46:33 MST 1993 Trying telnet: Unable to connect to remote host: Connection timed out Sat Jan 30 06:47:49 MST 1993 bsdi % arp -a ? ( at (incomplete) Figure 4.5 shows the tcpdump output. telnet to an address this time, not a hostname 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 2 3 arp who has tell bsdi 0.0 5.509069 (5.5091) arp who has tell bsdi 29.509745 (24.0007) arp who has 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 broadcast. 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.

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