By this method the system clock never runs backwards. These sources can be radio clocks, GPS satellite timing receivers, or even atomic clocks. Stratum 2 servers might be campus servers that obtain time from several primary servers over the Internet in order to relay time to their own local clients. The stratum 2 servers may be configured to peer with each other, comparing clocks and coming to a consensus on a synchronized time value. NTP operates well over the unpredictable paths of the packet-switched Internet, because NTP makes robust estimates of three key variables in the relationship between a client host and a timeserver.
Network delay is an estimate of how long it takes to get packets across the network.
Dispersion of packet exchanges measures the repeatability of network delay times. A large dispersion affects estimation of the third variable, which is the correction that should be applied to a client's clock to bring it into agreement with its servers. When an NTP client sends a request for time to a server, the client timestamps the packet with the time of departure. When the client receives the packet in return, a fourth timestamp is written. From these timestamps the values of delay, offset, and dispersion are found.