NTP

  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

The roundtrip delay d and system clock offset t are defined as:

  d = (T4 - T1) - (T3 - T2)     t = ((T2 - T1) + (T3 - T4)) / 2.