Which of the following can cause errors in NTP synchronization?

• A. NTP server clock drift
• B. Route flapping causing instability
• C. DNS resolution delays
• D. Asymmetric routes and network congestion

Answer: (D)

NTP relies on measuring two things from a time source: how long a packet takes to travel to the source and back, and the offset between the source’s clock and the local clock. For the offset to be accurate, the path the packets take in both directions should be fairly similar in time. When routes are asymmetric or a link is congested, the forward and reverse delays no longer match and jitter increases. This makes the calculated offset unreliable, so the local clock can drift away from the correct time and synchronization errors occur. In other words, differing delays in the two directions and noisy network conditions directly corrupt NTP’s time estimate.

Clock drift on the time source is something NTP is designed to handle by adjusting the local clock, so it doesn’t inherently cause persistent errors. DNS resolution delays can slow initial contact with a time server but don’t affect the accuracy of the time once the source is reached. Route instability can disrupt reachability, but the root cause of mis-synchronization is the mismatch of forward and reverse delays caused by asymmetric routes and congestion.