ntpd is the reference implementation of NTP. The current version is 4.1.2.
The manual page seems to say that when
iburst is used, the interval between retries to an unresponsive NTP server is 16 seconds.
When the server is unreachable and at each poll interval, send a burst of eight packets instead of the usual one. As long as the server is unreachable, the spacing between packets is about 16s to allow a modem call to complete. Once the server is reachable, the spacing between packets is about 2s. This is designed to speed the initial synchronization acquisition with the server command and s addresses and when ntpd is started with the
But the official NTP "handbook" says the retry interval will be raised to 64 seconds (assuming default settings):
iburstoption the number of packets in the burst is six, which is the number normally needed to synchronize the clock; for the
burstoption, 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
burstare enabled, the first packet of the burst is sent, but the remaining packets sent only when the reply to the first packet is received. If no reply has been received after a timeout set by the
minpolloption, 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.
Should I understand the interval between retries will actually be 64 seconds when using
iburst, unless the
minpoll option is also adjusted?