TCP IP Illustrated

Figure 1117 shows the tcpdump output 1 2 3 4 5 6 7 8

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Unformatted text preview: ated by IP and this presents two timing questions that we'll need to use tcpdump to answer: are the remaining fragments ready to be sent before the ARP reply is received, and if so, what does ARP do with multiple packets to a given destination when it's waiting for an ARP reply? Figure 11.17 shows the tcpdump output. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.0 0.001234 0.001941 0.002775 0.003495 0.004319 0.008772 0.009911 0.011127 0.011255 0.012562 0.013458 0.014526 0.015583 (0.0012) (0.0007) (0.0008) (0.0007) (0.0008) (0.0045) (0.0011) (0.0012) (0.0001) (0.0013) (0.0009) (0.0011) (0.0011) arp who-has svr4 tell bsdi arp who-has svr4 tell bsdi arp who-has svr4 tell bsdi arp who-has svr4 tell bsdi arp who-has svr4 tell bsdi arp who-has svr4 tell bsdi arp reply svr4 is-at 0:0:c0:c2:9b:26 arp reply svr4 is-at 0:0:c0:c2:9b:26 bsdi > svr4: (frag 10863:[email protected]) arp reply svr4 is-at 0:0:c0:c2:9b:26 arp reply svr4 is-at 0:0:c0:c2:9b:26 arp reply svr4 is-at 0:0:c0:c2:9b:26 arp reply svr4 is-at 0:0:c0:c2:9b:26 arp reply svr4 is-at 0:0:c0:c2:9b:26 Figure 11.17 Packet exchange when an 8192-byte UDP datagram is sent on an Ethernet. There are a few surprises in this output. First, six ARP requests are generated before the first ARP reply is returned. What we guess is happening is that IP generates the six fragments rapidly, and each one causes an ARP request. Next, when the first ARP reply is received (line 7) only the last fragment is sent (line 9)! It appears that the first five fragments have been discarded. Indeed, this is the normal operation of ARP. Most implementations keep only the last packet sent to a given destination while waiting file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (17 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol for an ARP reply. The Host Requirements RFC requires an implementation to prevent this type of ARP flooding (repeatedly sending an ARP request for the same IP address at a high rate). The recommended maximum rate is one per secon...
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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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