QUESTION

(a) Describe what is meant by Clock Skew and Clock Drift among clocks in a distributed system.

(b) A clock is reading 08:25:46.0 (hr:min:sec) when it is discovered to be 6 seconds fast. Describe why it is undesirable to set it back to the right time at that point and show (numerically) how it should be adjusted so as to be correct after 12 seconds has elapsed.

(c) An NTP server B receives server A?s message at 10:44:22.380 bearing a timestamp 10:44:13.230 and replies to it. A receives the message at 10:44:49.500, bearing B?s timestamp 10:44:40.300. Estimate the offset between B and A and the accuracy of the estimate.

 (d) A client attempts to synchronize with a time server. It records the round-trip times and timestamps returned by the server in the table below.

Round-trip (ms)                              Time (hr:min:sec)

32                                                        08:32:33.540

35                                                        08:32:35.600

30                                                        08:32:38.300

(i) Which of these times should it use to set its clock?

(ii) To what time it should be set?

(iii) Estimate the accuracy of the setting with respect to the server's clock. If it is known that the time between sending and receiving a message in the system concerned is at least 10 ms, do your answers change?