TCP IP Illustrated

1333 in this example are actually sent to the loop

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Unformatted text preview: for each interface), and in both cases the request is received from the loopback interface. 6.4 ICMP Timestamp Request and Reply The ICMP timestamp request allows a system to query another for the current time. The recommended value to be returned is the number of milliseconds since midnight, Coordinated Universal Time (UTC). (Older manuals refer to UTC as Greenwich Mean Time.) The nice feature of this ICMP message is that it provides millisecond resolution, whereas some other methods for obtaining the time from another host (such as the rdate command provided by some Unix systems) provide a resolution of seconds. The drawback is that only the time since midnight is returned-the caller must know the date from some other means. Figure 6.6 shows the format of the ICMP timestamp request and reply messages. Figure 6.6 ICMP timestamp request and reply messages. The requestor fills in the originate timestamp and sends the request. The replying system fills in the receive timestamp when it receives the request, and the transmit time-stamp when it sends the reply. In actuality, however, most implementations set the latter two fields to the same value. (The reason for providing the three fields is to let the sender compute the time for the request to be sent, and separately compute the time for the reply to be sent.) We can write a simple program (named icmptime) that sends an ICMP timestamp request file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (6 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol to a host and prints the returned reply. We try it first on our small internet: sun % icmptime bsdi orig = 83573336, recv = 83573330, xmit = 83573330, rtt = 2 ms difference = -6 ms sun % icmptime bsdi orig = 83577987, recv = 83577980, xmit = 83577980, rtt = 2 ms difference = -7 ms The program prints the three timestamps in the ICMP message: the originate (orig), receive (recv), and transmit (xmit) timestamps. As we can see in this and the following examples, all the hosts set the receive and transmit timestamps to the same value. We also calculate the round-trip time (rtt), which is the time the reply is received minus the time the request was sent....
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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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