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It is important that computers on a network have a common idea of time. Time servers are computers with special software on them. They either use an atomic clock, or they ask other computers on the network about the time. They then make this time available to all the other computers in the same network. Common protocols used to do this are the Daytime Protocol and the Network Time Protocol (NTP).
NTP calls the quality of a time server its stratum. Stratum can be anything form 0 to 255 and lower is better. It is rare to see a network with a source worse than stratum 5.
- Stratum 0 time sources are the actual clocks that track the time, usually an atomic clock or GPS receiver.
- Stratum 1 time sources are what most people mean when they talk about a time server. They get their timing information from a master clock and send it out to the network.
- Stratum 2 time sources are computers that get their clock information from a stratum 1 time server. They use NTP to talk to each other as well as their stratum 1 servers so they can have the best time information possible.
Every level that is added of time servers getting information from a source close to the master clock and passing it farther away from the master clock increases the stratum level.