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

[Hal Murray]

> 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.

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.

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?  ...
 


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