NTP users are strongly urged to take immediate action to ensure that their NTP daemons are not susceptible to being used in distributed denial-of-service (DDoS) attacks. Please also take this opportunity to defeat denial-of-service attacks by implementing Ingress and Egress filtering through BCP38.Computer Network Time Synchronization, The Network Time Protocol on Earth and in Space (2nd edition), by David L. Mills. In particular, the following references should be most valuable. If you find (links to) on-line versions of any of these, please add those links:
ntp-4.2.8p15was released on 23 June 2020. It addresses 1 medium-severity security issue in ntpd, and provides 13 non-security bugfixes over 4.2.8p13.
Please see the NTP Security Notice for vulnerability and mitigation details.Are you using Autokey in production? If so, please contact Harlan - he's got some questions for you.
The next step in clock algorithms is to understand what already exists. For that purpose, you should study and thoroughly understand the phase-lock-loop algorithm that was in the original recipe of NTP, and the frequency-lock algorithm that you can find in my papers and that was included in NTP starting with the first version 4, as I remember. Almost all of my papers are on line at the publications menu on https://tf.nist.gov. If you enter my name you will see a listing of all of my papers. The earliest one that is important here was published in about 1995 in the transactions on networking. That paper introduced the frequency loop principles. For background information, you could also look at my review article that appeared in the Review of Scientific Instruments in 1999. A lot of this stuff is also in David Mills' book, and there is probably lots of other stuff out there that I don't know about.