[ntp:questions] Known asymmetry (DSL) compensation

John Pettitt jpp at cloudview.com
Mon Feb 14 18:25:14 UTC 2005


I know it's not possible to know or compensate for all asymmetry in the 
packets between two machines.  However where there is a fixed, known, 
asymmetry - like on a DSL line shouldn't it be possible to calculate a 
fudge factor?

Example:

I have two FreeBSD boxes connected to the same GPS 18 LVC - they keep 
time within 35 microseconds of each other but they are off with respect 
to the rest of the net by about a 1200 microseconds.    The connection 
from my local LAN to the Internet is a DSL line that runs at 6000kbps 
downstream and 608 kbps upstream.   The calculated difference is packet 
transmit time between upstream and downstream is almost 900 
microseconds.  I'd like to be able to bias out that known asymmetry.

If somebody who understands ntp internals  is willing to answer my 
questions (off list) I'm up for taking a shot and implementing this in a 
way that allows a bias to be specified and applied on a per net basis - 
anybody up for it?

John



More information about the questions mailing list