ISRO | ISRO CS 2014 | Question 58

Assume the following information:
Original timestamp value = 46
Receive timestamp value = 59
Transmit timestamp value = 60
Timestamp at arrival of packet = 69
Which of the following statements is correct?
(A) Receive clock should go back by 3 milliseconds
(B) Transmit and Receive clocks are synchronized
(C) Transmit clock should go back by 3 milliseconds
(D) Receive clock should go ahead by 1 milliseconds


Answer: (A)

Explanation: This question is given from the topic network layer ICMP.

Note that TS is stand for Timestamp

assume us assume that there is a one side as sender and another side as receiver.

Original Timestamp : Time at which sender sent packet acknowledgement to its own clock and Receiving Timestamp at Time at which receiver receives that packet acknowledgement to its own clock.

Now, Transmit Timestamp is the Time at which receiver sent a reply of that packet acknowledgement to its own clock.

Returned Timestamp is the Time at which sender of receives the reply acknowledgement to its own clock.

Now,

=> Sending time = Receiving time – Original time

And we know that, 
=> Receiving time = Return time – Transmit time

And value is given that Receive timestamp value = 59

Transmit timestamp value = 60 

Timestamp at arrival of packet = 69 

Therefore

=> Receive timestamp value = Sending Time + Receiving time 

=> One way time should be RTT2 => 10ms

Hence .

=> Sender sent the packet at 46 and receiver should have received it at 56, but it is receiving it at 59, i.e., it is 3 ms ahead.
So it should set its clock 3 ms behind to sync with sender.

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