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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

To Do

Outline

Test & prototype the idea in two different scenarios:

  • LAN, 1 NTP server
  • WAN, multiple NTP servers

Tasks

  • 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...)

Resources

People & Credits

  • Dave Mills (for NTP)
  • Thomas Gleixner (for current timekeeping.c)
  • Pavel Pisa (guidance to tglx's timekeeping.c)
  • Jan Pacner (ptpd port to QNX, started ntpd vs. ptpd comparisons)
  • Marek Peca (noPLL NTP idefix, clock ensembling algorithms)

[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)