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Unformatted text preview: The difference is the received timestamp minus the
originate timestamp. Figure 6.7 shows the relationship between these values. Figure 6.7 Relationship between values printed by our icmptime program.
If we believe the RTT and assume that one-half of the RTT is for the request, and the other
half for the reply, then the sender's clock needs to be adjusted by difference minus onehalf the RTT, to have the same time as the host being queried. In the preceding example,
the clock on bsdi was 7 and 8 ms behind the clock on sun.
Since the timestamp values are the number of milliseconds past midnight, UTC, they should
always be less than 86,400,000 (24 x 60 x 60 x 1000). These examples were run just before
4:00 P.M. in a time zone that is 7 hours behind UTC, so the values being greater than
82,800,000 (2300 hours) makes sense.
If we run this program several times to the host bsdi we see that the final digit in the
receive and transmit timestamp is always 0. This is because the software release (Version
0.9.4) only provides a 10-ms clock. (We describe this in Appendix B.)
If we run the program twice to the host svr4 we see that the low-order three digits of the
SVR4 timestamp are always 0:
sun % icmptime svr4
orig = 83588210, recv = 83588000, xmit = 83588000, rtt = 4
difference = -210 ms file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (7 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol sun % icmptime svr4
orig = 83591547, recv = 83591000, xmit = 83591000, rtt = 4
difference = -547 ms
For some reason SVR4 doesn't provide any millisecond resolution using the ICMP timestamp. This imprecision makes the calculated differences useless for subsecond
If we try two other hosts on the 140.252.1 subnet, the results show that one clock differs
from sun's by 3.7 seconds, and the other by nearly 75 seconds:
sun % icmptime gemini
orig = 83601883, recv = 83598140, xmit = 83598140, rtt = 247
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