[time-nuts] Need Time Help

Jay Grizzard elfchief-timenuts at lupine.org
Wed Oct 5 13:52:18 EDT 2016

On 2016-10-05 10:22 , Jay Grizzard wrote:
> Because NTP normally never actually sets you clock to match a server's
> clock.   It adjusts the RATE of your clock so the it cruises on the middle
> of the pack of vetted servers.

To be nitpicky, it doesn't actually track towards the median, it tracks
towards a weighted average that takes into account the jitter and root
distance of each peer.

To pick a more meaningful nit, though, the above only happens if none of
your peers are set to 'prefer'. If you have a peer set to 'prefer' (and that
peer survives the clustering algorithm), ntp will track that peer and only
that peer. A *lot* of people have a preferred peer set, so this seems worth

(In general I find that as long as my preferred peer is stable, that I get
much better time having it set that way... otherwise, you get frequency shifts
as peers come and go from the set of selected peers. Preferring a single
stable peer gives me a more stable frequency, but I still list plenty of
other peers to allow for sanity checking of the preferred peer.)

> Also your modem is likely not causing an asymmetric delay.  That modern
> wetter is is the old phone kind or a fiber optic system only takes you to
> the fist router.  The modem likely has the same time of flight in both
> directions.

On a connection with active downloads, the modem is almost definitely
causing an asymmetric delay. Especially with bufferbloat being so
common these days, all it takes to give hugely asymmetric performance is
to download a file from a site with a faster pipe than you have.

On my cablemodem at home (55Mbit down, 10Mbit up), if I turn off congestion
management (which is a feature in the custom firmware on my router that is
NOT at all standard/common on mainstream hardware), my incoming latency
will jump to something around 1000ms(!) if I download at max bandwidth,
while my outgoing latency hangs out in the single-digit ms range.

(this is, sadly, the norm today)

On a connection that isn't downloading at the full speed of the connection,
though, your assertion is correct. It just needs that very large asterisk
attached to it. ;)


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