Practical limitations of NTP time transfer

The Network Time Protocol (NTP) is commonly utilized to synchronize computer clocks in packet-switched, wide area networks (WANs) such as the public Internet. The delay asymmetry in WANs, often due to inconsistent routing and/or bandwidth saturation, is usually the dominant source of error. It typically limits NTP time transfer uncertainty to about one millisecond. This paper discusses the uncertainty of NTP time transfer when network asymmetry is largely eliminated. We performed NTP measurements over a local area network (LAN) when both the server and client are referenced to a common clock. Three variations of a LAN are tested, including a direct connection between the server and client with an Ethernet crossover cable. The elimination of network asymmetry reveals other uncertainty sources that serve as practical limitations for NTP time transfer, including client instability, asymmetry in network interface cards, and server instability.