[time-nuts] Re: how much is my router influencing time-keeping over the network

[folkert]

> > For fun I'm developing a router for HAM packet networks. What it does is
> > route AX.25 packets between radios and tunnels (it can also bridge- and
> > filter them). 
> 
> > What I would like to measure now is, how bad does it influence time-keeping
> > when syncing time takes place over a network. I could of course just setup
> > tcp/ip and let two ntp instances sync over it and then calculate an allan
> > deviation plot. 
> 
> After an exchange of NTP packets, the client has 4 time stamps.
>   The time the request packet left the client
>   The time the request packet arrived at the server
>   The time the reply packet left the server
>   The time the reply packet arrived at the client
> 
> There are 3 unknowns:
>   Transit time client to server
>   Transit time server to client
>   Clock offset between client and server
> 
> With the 4 time stamps, you can setup 2 equations.  You need one more.
> NTP assumes the transit times are equal.
> 
> If you have good clocks at both ends, you can assume the clocks are equal and 
> compute transit times in each direction.

Indeed, but I also would like measure how good the path between the two
points is. I mean: if the network randomly delays packets (or so), then
that would influence the syncing I think?

In any case, I've done a little experiment with that: I invoke a ping
100k times and then write down the time when the reply was received.
I've explained it further at: https://vanheusden.com/time/ping-test/

> I would like to see some graphs of network transit times over radio links.  
> How noisy is yor radio link?  It will be interesting to see if if you can get 
> a decent ADEV graph.

I'm currently working on that. The two radios are close to each other, with
signal levels of -57dB and SNR of 10.5. Ping times of around 8.3 seconds
(yes, seconds).

> Timings will depend a lot on network traffic.  It would be neat if you can get 
> data under both light load and heavy load.  Do you know about bufferbloat?  ...

I do know about bufferbloat yes.