[ntp:questions] Re: Known asymmetry (DSL) compensation

Jan Ceuleers janspam.ceuleers at computer.org
Tue Feb 15 08:23:22 UTC 2005


On Mon, 14 Feb 2005 18:25:14 GMT, John Pettitt <jpp at cloudview.com>
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?

The problem with defining a fixed "correction" for this is the
following:

What you want to correct is the difference in latency in the upstream
vs. downstream directions. The trouble is that this cannot be readily
calculated from any known and constant values, such as the upstream
and downstream bitrates.

Whereas you would expect that the uplink (from the CPE to the DSLAM)
would have higher latency due to the lower bitrate, upstream traffic
deeper in the network (DSLAM-BRAS and BRAS-Internet) generally uses
less-loaded links than those in the downstream direction. It is
impossible to know whether the latter (more than) compensates for the
former.

Moreover, the upstream latency between CPE and DSLAM is very dependent
on the offered upstream traffic (in other words: whether upstream
packets sit in a queue in the CPE before being forwarded onto the
line, or whether upstream traffic is low enough to be able to be
forwarded immediately).

FWIW, my ntp.conf contains tinker huffpuff 7200.

Jan




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