accuracy achieved is directly dependent on the time taken to achieve it.
Fortunately, the frequency of measurements can be quite low and almost
always non-intrusive to normal net operations.
Implementation Model
In what may be the most common client/server model a client sends an NTP
message to one or more servers and processes the replies as received.
The server interchanges addresses and ports, overwrites certain fields
in the message, recalculates the checksum and returns the message
immediately. Information included in the NTP message allows the client
to determine the server time with respect to local time and adjust the
local clock accordingly. In addition, the message includes information
to calculate the expected timekeeping accuracy and reliability, as well
as select the best from possibly several servers.
While the client/server model may suffice for use on local nets
involving a public server and perhaps many workstation clients, the full
generality of NTP requires distributed participation of a number of
client/servers or peers arranged in a dynamically reconfigurable,
hierarchically distributed configuration. It also requires sophisticated
algorithms for association management, data manipulation and local-clock
control. Throughout the remainder of this document the term host refers
to an instantiation of the protocol on a local processor, while the term
peer refers to the instantiation of the protocol on a remote processor
connected by a network path.
Figure 1<$&fig1> shows an implementation model for a host including
three processes sharing a partitioned data base, with a partition
dedicated to each peer, and interconnected by a message-passing system.
The transmit process, driven by independent timers for each peer,
collects information in the data base and sends NTP messages to the
peers. Each message contains the local timestamp when the message is
sent, together with previously received timestamps and other information
necessary to determine the hierarchy and manage the association. The
message transmission rate is determined by the accuracy required of the
local clock, as well as the accuracies of its peers.
The receive process receives NTP messages and perhaps messages in other
protocols, as well as information from directly connected radio clocks.
When an NTP message is received, the offset between the peer clock and
the local clock is computed and incorporated into the data base along
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