TCP IP Illustrated

Well watch this using udp chapter 11 one rule of udp

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Unformatted text preview: We can force a port unreachable using the TFTP client. (We describe TFTP in Chapter 15.) The well-known UDP port for the TFTP server to be reading from is 69. But most TFTP client programs allow us to specify a different port using the connect command. We use this to specify a port of 8888: bsdi % tftp tftp> connect svr4 8888 tftp> get temp.foo Transfer timed out. tftp> quit specify the hostname and port number try to fetch a file about 15 seconds later The connect command saves the name of the host to contact and the port number on that host, for when we later issue the get command. After typing the get command a UDP datagram is sent to port 8888 on host svr4. Figure 6.8 shows the tcpdump output for the exchange of packets that takes place. Before the UDP datagram can be sent to svr4 an ARP request is sent to determine its hardware address (line 1). The ARP reply (line 2) is returned and then the UDP datagram is sent (line 3). (We have left the ARP request-reply in this tcpdump output to remind us that this exchange may be required before the first IP datagram is sent from one host to the other. In future output we'll delete this exchange if it's not relevant to the topic being discussed.) 1 2 3 4 0.0 arp who-has svr4 tell bsdi 0.002050 (0.0020) arp reply svr4 is-at 0:0:c0:c2:9b:26 0.002723 (0.0007) bsdi.2924 > svr4.8888: udp 20 0.006399 (0.0037) svr4 > bsdi: icmp: svr4 udp port 8888 file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (10 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol 5 6 7 8 9 10 11 12 5.000776 (4.9944) bsdi. 2924 > svr4.8888: udp 20 5.004304 (0.0035) svr4 > bsdi: icmp: svr4 udp port 10.000887 (4.9966) bsdi. 2924 > svr4.8888: udp 20 10.004416 svr4 > bsdi: icmp: svr4 udp port (0.0035) 15.001014 bsdi.2924 > svr4.8888: udp 20 (4.9966) 15.004574 svr4 > bsdi: icmp: svr4 udp port (0.0036) 20.001177 (4.9966) bsdi.2924 > svr4.8888: udp 2...
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