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 been sent in reply. By operating in this
mode the host announces its willingness to synchronize and be
synchronized by the peer.
Client (3): 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, usually a LAN workstation, announces
its willingness to be synchronized by, but not to synchronize the peer.
Server (4): This type of association is ordinarily created upon arrival
of a client request message and exists only in order to reply to that
request, after which the association is dissolved. By operating in this
mode the host, usually a LAN time server, announces its willingness to
synchronize, but not to be synchronized by the peer.
Broadcast (5): A host operating in this mode sends periodic messages
regardless of the reachability state or stratum of the peers. By
operating in this mode the host, usually a LAN time server operating on
a high-speed broadcast medium, announces its willingness to synchronize
all of the peers, but not to be synchronized by any of them.
A host operating in client mode occasionally sends an NTP message to a
host operating in server mode, perhaps right after rebooting and at
periodic intervals thereafter. The server responds by simply
interchanging addresses and ports, filling in the required information
and returning the message to the client. Servers need retain no state
information between client requests, while clients are free to manage
the intervals between sending NTP messages to suit local conditions. In
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