TCP IP Illustrated

We examine resolvers and the dns in chapter 14 for

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Unformatted text preview: the DNS in Chapter 14. For now realize that if we type a ping command, and a few seconds pass before the first line of output with the IP address is printed, this is the time required for the DNS to determine the IP address corresponding to the hostname. Figure 7.2 shows the tcpdump output for this example. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.0 0.003733 (0.0037) 0.998045 (0.9943) 1.001747 (0.0037) 1.997818 (0.9961) 2.001542 (0.0037) 2.997610 (0.9961) 3.001311 (0.0037) 3.997390 (0.9961) 4.001115 (0.0037) 4.997201 (0.9961) 5.000904 (0.0037) 5.996977 (0.9961) 6.000708 (0.0037) bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (3 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program 15 16 6.996764 (0.9961) 7.000479 (0.0037) bsdi > svr4: icmp: echo request svr4 > bsdi: icmp: echo reply Figure 7.2 ping output across a LAN. The time between sending the echo request and receiving the echo reply is always 3.7 ms. We can also see that echo requests are sent approximately 1 second apart. Often the first round-trip time is larger than the rest. This occurs if the destination's hardware address isn't in the ARP cache of the sender. As we saw in Chapter 4, sending an ARP request and getting the ARP reply can take a few milliseconds before the first echo request can be sent. The following example shows this: sun % arp -a make sun ARP cache is empty sun % ping svr4 PING svr4: 56 data bytes 64 bytes from svr4 (140.252.13,34): icmp_seq=0. time=7. ms 64 bytes from svr4 (14...
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