[ntp:questions] Re: Known asymmetry (DSL) compensation
Richard B. Gilbert
rgilbert88 at comcast.net
Mon Feb 14 20:48:21 UTC 2005
John Pettitt wrote:
> 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?
> 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
> 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?
I have seen the same sort of thing but, at least in my case I doubt that
it is due to network asymmetry on my end. I see consistant systematic
offsets on a couple of the better servers I use. One is usually +1.5
milliseconds and another is usually -2 milliseconds with respect to
GPS. There are periods of the day when a graph shows this clearly; at
other times there is sufficient noise or "jitter" that you would have to
"fit" or "smooth" the curve to see it.
If a facility is added to compensate for these errors, it should be on a
server by server basis since different servers can clearly show bias in
different directions at the same time.
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