[ntp:questions] Garmin 18 LVC: whether to fudge

Jan Ceuleers janspam.ceuleers at skynet.be
Wed Feb 11 20:13:03 UTC 2009

Brian Utterback wrote:
> Decreased bandwidth means increased latency. The two are related.

Only indirectly so.

There are at least two components to the higher latency on the ADSL 
uplink as compared to the downlink. A minor component is the fact that 
the lower bitrate means that equal-sized packets are in flight longer on 
the uplink than they are on the downlink.

The main component is however that there is a transmit queue in the CPE 
that packets take a while to get through before actually being sent up 
the link, particularly under load. So this component is not only 
dominant, it is also variable with upstream load.

One way to get around that is to set up multiple transmit queues for 
different flows, perhaps based on TOS marking. This way, NTP packets, 
when suitably marked, can be made to bypass the transmit queue for 
best-effort traffic.

Cheers, Jan

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