Except in broadcast mode, an NTP association is formed when two
peers exchange messages and one or both of them create and
maintain an instantiation of the protocol machine, called an
association. The association can operate in one of five modes as
indicated by the host-mode variable (peer.mode): symmetric
active, symmetric passive, client, server and broadcast, which
are defined as follows: Symmetric Active (1): A host operating
in this mode sends periodic messages regardless of the
reachability state or stratum of its peer. By operating in this
mode the host announces its willingness to synchronize and be
synchronized by the peer. Symmetric Passive (2): This type of
association is ordinarily created upon arrival of a message from
a peer operating in the symmetric active mode and persists only
as long as the peer is reachable and operating at a stratum
level less than or equal to the host; otherwise, the association
is dissolved. However, the association will always persist until
at least one message has bee