rt_clk_sync

Hard Real-Time world-wide clock synchronization utilizing new better algorithms than PTP and NTP uses.

Objectives

"NTP without PLL" -- continue the ideas of [1], [2]
- better stability (no feedback with obscured interoperation between digital quasi-VCO inside of OS which nobody understands, and the ad-hoc tuned PI-controller within ntpd in userspace
- possibly better performance
  - provide means to tune system to particular process (computer clock) & measurement (network) noises by means of their absolute variances as well as temporal characteristics
  - according to control theory laws, optimally tuned estimator will perform same or better than any possible feedback system for the NTP task
establish better NTP-like userspace program <-> OS interface
- provide more clear i/f based on current time offset & drift estimates input into OS's timekeeping core; no P, I controller parameters, no ad-hoc quasi-VCOs; the Linux' adjtimex() is an example of devil's evil
- try Linux as the reference implementation
- join efforts with PTPd guys, they probably need exactly the same
allow simultaneous locking to several NTP servers
- AFAIK impossible with current ntpd client, correct me if wrong
implement optimal linear estimator for clock processes, allowing to run with no absolute master at all [3], [4], [5]

In longer term:

adaptive estimation of both clock and network parameters (autotuning, or continuous adaptation)
revise/redesign application interface to allow e.g. delay/latency measurement between two hosts (us and some other) on demand or reliable reporting of current/min/max accuracy (delay/latency) which all other connected stations have
statistics/logging of estimated accuracy (for audits, revisions, etc.)

The ultimate goal (megalomanic):

release the Internet-wide network of computers and their clocks, managed as a non-hierarhical cloud of clocks of very different stochastical properties (from GPSes and highly stable atomic standards to ordinary PC's crystals); apply clock ensembling algorithms to perform this large-scale time fusion

Test & prototype the idea in two different scenarios:

write clk_sync_observer, preferably in Linux' user-space
- should run besides classical, non-patched ntpd
- should monitor bidirectional NTP network datagrams, parse them and record their timestamps wrt. non-disciplined local clock
- record evolution of disciplined ("ntp") time wrt. local clock
provide means to perform/record simultaneous NTP queries to serveral NTP servers from fixed set
define reference PC or "PC-like" HW platform
- Ethernet card
- some reasonable, but not unrealistically good local clock (i.e. no undeterministic, unbeatable errors, but on the other hand no unfairly good oscillators, common consumer/industrial HW only)
- a means to output OS' clock synchronous marks, at least (at worst) using GPIO
make a simple and portable FPGA & Rb clock reference measurement kit (for this purpose, 10ns is fairly good, so no fancy interpolation techniques from VZLU's TDC are needed)
perform measurements
- WAN scenario
- LAN scenario
evaluate acquired data
- perform offline simulations of proposed algorithms
- compare against bare NTP
patch Linux' timekeeping.c
- wipe out PI/quasi-VCO, replace with offset&drift estimates
  - for compatibility reason, current PI/VCO mess may perhaps become an upper shell on top of the offset/drift i/f
write a NTP client-side userspace replacement, using algorithms and new OS i/f as described
- first, only single-server mode
- later, add multiple-servers fusion
- (much later: adaptive filtering...)

[1] J.Levine: Synchronizing computer clocks using Kalman filters. 43rd PTTI Meeting, 2011.

[2] J.Ridoux, D.Veitch: Ten Microseconds Over LAN, for Free. IEEE ISPCS, 2007.

[3] K.R.Brown: The Theory of the GPS Composite Clock. Proceedings of ION GPS-91, 1991.

[4] C.A.Greenhall: A Review of Reduced Kalman Filters for Clock Ensembles. IEEE UFFC 2012.

[5] M.Peca, V.Michalek, M.Vacek: Clock Composition by Wiener Filtering Illustrated on Two Atomic Clocks. EFTF 2013 (accepted)