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

[Hal Murray]

> 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? 

Yes.  NTP assumes the network delays are symmetric.  Half of any extra delay 
in one direction will turn into a clock offset error.

The classic way to screwup NTP is to do a big download that saturates one side 
of your link.

ntpd keeps a FIFO of the last 8 packets and only uses a new response if it is 
the best of the 8.   That filters out short bursts of traffic but a long 
download can fill the FIFO with long delays.


> Ping times of around 8.3 seconds (yes, seconds).

What is the minimum ping time?

With slow links, you have to be very very careful not to shoot yourself in the 
foot.  If the round trip time is 2 seconds but your retransmit timer is only 1 
second, the pattern goes like this:
  0: send #1
  1: retransmit #1
  2: recv #1, send #2
  3: retransmit #2
  4: recv #1, retransmit #2
  5: retransmit #2
  6: recv #2, send #3
  NB: There are still 3 retransmitted copies of #2 in the pipeline.  (and 
that's assuming that the 2 second round trip time is less than 2 ticks of the 
1 second retransmit timer)
Basically, the link gets full of retransmitted packets.  It gets worse if 
there is any other traffic on the link.
 
--------

Magnus Danielson said:
> ADEV and TDEV is intended to handle random noise, ...

Network delays are often far from random.  All it takes to really screw things 
up is a download with enough bandwidth to saturate the link.

I usually start with a graph of round trip time on Y and time of day on X.  
Your eye/brain is really good at analyzing that sort of picture.


Is there any *DEV that works well with missing samples?  Occasional packets 
get lost and I'd like to see what happens if I discard wild samples.  Is there 
anything better than generating fake data by interpolating?


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