wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Clock Synchronization in Distributed Systems 18-7
18.9 Network Time Protocol Strata
A basic principle in NTP is to assign to every clock a so-called stratum. This hierarchically organized
system of clock strata defines the accuracy of a computer’s clock or more precisely the distance to a reference
clock. Within NTP, device strata are identified with an integer value starting at stratum-0. Like
illustrated in Figure 18.5, these strata are defined as follows:
• Stratum-0 devices have highly accurate clocks such as cesium or rubidium time normals. It
has to be noted that devices with this stratum are never connected directly to a network—
time and frequency transfer happens via serial interfaces or transmission of defined electrical
pulses.
• Stratum-1 nodes are always directly connected to devices with stratum-0. The usual way is to have
these nodes act as time servers for stratum-2 clients.
• Stratum-2 nodes communicate via NTP with stratum-1 servers. It is possible for these devices to
peer with other stratum-2 nodes in order to obtain a more robust time source for all other devices
in the so-called peer group. Again, in the usual case, these devices will act as servers for stratum-3
clocks.
• Stratum-3 is the class for the next hierarchical layer in NTP. These nodes perform basically the
same tasks as NTP devices in the layer below, with the difference that time is obtained from
stratum-2 devices. In general, NTP, depending on the version, allows strata higher than stratum-3
(up to 256 depending on the version). Nevertheless, future developments will most likely restrict
the NTP strata to 8–16.
A study by Minar in 1999 [Minar1999] revealed that at the time of the study, at least 175,000 hosts use
NTP in the Internet. Among those are more then 300 stratum-1 servers and 20,000 and 80,000 stratum-2
and stratum-3 servers, respectively.
Stratum-0
Direct link
e.g., RS232
Stratum-1
NTP link
Stratum-2 peer group
for increased accuracy
Stratum-2
Stratum-3
FIGURE 18.5
Example for an NTP network with node strata (NTPStratums.pdf).
© 2011 by Taylor and Francis Group, LLC