The poll process sends NTP packets at intervals determined by the clock
discipline algorithm. The process is designed to provide a sufficient
update rate to maximize accuracy while minimizing network overhead. The
process is designed to operate over a poll exponent range between 3 (8
s) and 17 (36 hr). The minimum and maximum poll exponent within this
range can be set using the minpoll and maxpoll options of the
server command, with default 6 (64 s) and 10
(1024 s), respectively.
The poll interval is managed by a heuristic algorithm developed over several years of experimentation. It depends on an exponentially weighted average of clock offset differences, called clock jitter, and a jiggle counter, which is initially set to zero. When a clock update is received and the offset exceeds the clock jitter by a factor of 4, the jiggle counter is increased by the poll exponent; otherwise, it is decreased by twice the poll exponent. If the jiggle counter is greater than an arbitrary threshold of 30, it is reset to 0 and the poll exponent is increased by 1. If the jiggle counter is less than -30, it is set to 0 and the poll exponent decreased by 1. In effect, the algorithm has a relatively slow reaction to good news, but a relatively fast reaction to bad news.
As an option of the server command, instead
of a single packet, the poll process can send a burst of several packets
at 2-s intervals. This is designed to reduce the time to synchronize the
clock at initial startup (iburst) and/or to reduce the phase noise at
the longer poll intervals (burst). The iburst option is effective
only when the server is unreachable, while the burst option is
effective only when the server is reachable. The two options are
independent of each other and both can be enabled at the same time.
For the iburst option the number of packets in the burst is six, which
is the number normally needed to synchronize the clock; for the burst
option, the number of packets in the burst is determined by the
difference between the current poll exponent and the minimum poll
exponent as a power of 2. For instance, with the default minimum poll
exponent of 6 (64 s), only one packet is sent for every poll, while the
full number of eight packets is sent at poll exponents of 9 (512 s) or
more. This insures that the average headway will never exceed the
minimum headway.
The burst options can result in increased load on the network if not
carefully designed. Both options are affected by the provisions
described on the Rate Management and the Kiss-o'-Death
Packet page. In addition, when iburst or burst are enabled, the
first packet of the burst is sent, but the remaining packets sent only
when the reply to the fist packet is received. If no reply has been
received after a timeout set by the minpoll option, the first packet
is sent again. This means that, even if a server is unreachable, the
network load is no more than at the minimum poll interval.
To further reduce the network load when a server is unreachable, an
unreach timer is incremented by 1 at each poll interval, but is set to 0
as each packet is received. If the timer exceeds the unreach threshold
set at 10, the poll exponent is incremented by 1 and the unreach timer
set to 0. This continues until the poll exponent reaches the maximum set
by the maxpoll option.
