TCP IP Illustrated

If we try two other hosts on the 1402521 subnet the

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Unformatted text preview: ce = -3743 ms sun % icmptime aix orig = 83606768, recv = 83532183, xmit = 83532183, rtt = 253 ms difference = -74585 ms Another interesting example is to the router gateway (a Cisco router). It shows that when a system returns a nonstandard timestamp value (something other than milliseconds past midnight, UTC), it is supposed to turn on the high-order bit of the 32-bit timestamp. Our program detects this, and prints the receive and transmit timestamps in angle brackets (after turning off the high-order bit). Also, we can't calculate the difference between the originate and receive timestamps, since they're not the same units. sun % icmptime gateway orig = 83620811, recv = <4871036>, xmit = <4871036>, rtt = 220 ms sun % icmptime gateway orig = 83641007, recv = <4891232>, xmit = <4891232>, rtt = 213 ms If we run our program to this host a few times it becomes obvious that the values do contain millisecond resolution and do count the number of milliseconds past some starting point, but the starting point is not midnight, UTC. (It could be a counter that's incremented every millisecond since the router was bootstrapped, for example.) As a final example we'll compare sun's clock with a system whose clock is known to be accurate-an NTP stratum 1 server. (We say more about NTP, the Network Time Protocol, below.) file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (8 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol sun % icmptime orig = 83662791, recv = 83662919, xmit = 83662919, rtt = 359 ms difference = 128 ms sun % icmptime orig = 83670425, recv = 83670559, xmit = 83670559, rtt = 345 ms difference = 134 ms If we calculate the difference minus one-half the RTT, this output indicates that the clock on sun is between 38.5 and 51.5 ms fast. Alternatives There are other ways to obtain the time and date. 1. We described the daytime service and time service in Section 1.12. The former returns the current time and date in a human readable form, a li...
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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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