ntp - NTP如何计算它的下一次轮询间隔

Question

我尝试阅读 NTPv4 RFC 4905。我似乎无法理解它计算下一个轮询间隔以向其对等方发送 NTP 请求数据包的依据是什么。 ppoll 和 hpoll 在查找轮询时间中的作用是什么？另外，MINPOLL 和 MAXPOLL 之间的轮询间隔如何变化？

Answer 1

可以找到关于轮询间隔的很好的解释 here以下是摘录。

NTP uses an intricate heuristic algorithm to automatically control the poll interval for maximum accuracy consistent with minimum network overhead. The algorithm measures the incidental offset and jitter to determine the best poll interval. When ntpd starts, the interval is the default minimum 64 s. Under normal conditions when the clock discipline has stabilized, the interval increases in steps to the default maximum 1024 s. In addition, should a server become unreachable after some time, the interval increases in steps to the maximum in order to reduce network overhead. Additional information about the algorithm is on the Poll Program page.

The default poll interval range is suitable for most conditions, but can be changed using options on the Server Commands and Options and Miscellaneous Options pages. However, when using maximum intervals much larger than the default, the residual clock frequency error must be small enough for the discipline loop to capture and correct. The capture range is 500 PPM with a 64-s interval decreasing by a factor of two for each interval doubling. At a 36-hr interval, for example, the capture range is only 0.24 PPM.

In the NTPv4 specification and reference implementation, the poll interval is expressed in log2 units, properly called the poll exponent. It is constrained by the lower limit minpoll and upper limit maxpoll options of the server command. The limits default to 6 (64 s) and 10 (1024 s), respectively, which are appropriate for the vast majority of cases.

hpoll :这是当前用于主机的轮询间隔。每次无法访问服务器时，该值都会递增。