The Protocol: Timestamps 4 really important ones Timestamp Name ID When Generated ------------------------------------------------------------ Originate Timestamp T1 time request sent by client Receive Timestamp T2 time request received by server Transmit Timestamp T3 time reply sent by server Destination Timestamp T4 time reply received by client Local clock offset is now easy to calculate t = ((T2 - T1) + (T3 - T4)) / 2