s6-networking: the s6-taiclock program

The `s6-taiclock` program

s6-taiclock is a client for the TAICLOCK protocol. It connects to a TAICLOCK server, computes an estimated discrepancy between the local clock time and the absolute time given by the server, and outputs it on stdout.

Interface

     s6-taiclock [ -f ] [ -v verbosity ] [ -r roundtrips ] [ -t triptimeout ] [ -h throttle ] [ -T totaltimeout ] [ -e errmax ] [ -p port ] ipaddress | s6-clockview

s6-taiclock exchanges TAICLOCK messages with a server such as s6-taiclockd listening on ipaddress, UDP port 4014. ipaddress can be IPv4 or IPv6.
It computes the mean difference between the absolute time given by the system clock and the one given by the server.
It prints the difference to stdout in a format understood by s6-clockadd and s6-clockview. It then exits 0.

Options

-f : force. Normally, s6-taiclock exits 111 if it cannot compute a time with a smaller uncertainty than errmax. If this option is set, it will output a time difference and exit 0 even if the error is too big.
-v verbosity : be more or less verbose. By default, verbosity is 1. 0 means only print fatal error messages; 2 means print advanced warnings.
-r roundtrips : perform roundtrips exchanges with the server. By default, roundtrips is 10. A lower value yields a higher time uncertainty; a higher value puts more load on the server.
-t triptimeout : if a TAICLOCK exchange with the server takes more than triptimeout milliseconds, abort this exchange and move on to the next one. By default, triptimeout is 2000.
-h throttle : wait throttle milliseconds between exchanges with the server. A lower value gets the final result earlier, but exerts more load on the server. A higher value puts a lighter load on the server, but delays the computation. By default, throttle is 0. It is recommended to set it to a reasonable nonzero value when increasing roundtrips.
-T totaltimeout : if the whole operation takes more than totaltimeout milliseconds, abort and exit 1. By default, totaltimeout is 10000.
-e errmax : accept a maximum time uncertainty of errmax milliseconds. By default, errmax is 100.
-p port : contact a server on port port. By default, port is 4014.

Notes

On the usage of NTP vs. TAICLOCK

TAICLOCK is not as generic or failproof as NTP. It is not as resistant to network latency. It has been designed to broadcast time on a local area network, whereas NTP has been designed to broadcast time over the whole Internet.
TAICLOCK will produce faster results on a LAN; moreover, the point of TAICLOCK is to broadcast TAI instead of UTC, so it is more accurate around a leap second.
The Internet is much more reliable latency-wise today than it was when dialout connections and broken routing protocols were the norm. So it is possible to use TAICLOCK across a WAN if the accuracy expectations are not too strict.
TAICLOCK is much easier to implement. The s6-sntpclock client binary code (statically linked on a i386) is 50% bigger than the s6-taiclock client. Also, the s6-taiclockd server is extremely small (close to 50% smaller than the client), whereas NTP servers, even SNTP servers, are behemoths requiring a project of their own.

Related work