[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Sat Mar 17 11:41:37 UTC 2012


unruh wrote:
>> Would it be worth it to recruit an electrical or systems engineer who
>> claimed to know something about filtering data to take a serious look at
>> NTPD's data filtering approach?  There has to be some reason that there is a
>
> David Mills claims to know about filtering data. Not that I always agree
> with him, but he is not stupid.

That must be the understatement of the day...

> And it sounds like you have assymetric delays. Note that most ISPs
> deliver very different rates for up vs down, and that may well come with
> assymetric delays. (eg 600Kb/s, vs 30Mb/s for my cable access)

Ouch!

That is 1:50 which means that you must be very lucky indeed to ever get 
those 30 Mbit down:

The most efficient protocol for bulk transfers is still FTP, if you get 
zero packet drops FTP can run with one ACK packet (about 64 bytes) for 
every pair of received data packets (2x1514 max).

The ratio between those two numbers is just over 1:50, so in order to 
ever get nearly the full 30 Mbit down, you must have perfect saturation 
of the uplink just to send the ACK packets, and any background traffic, 
even something as simple as NTP of email polling will interfere.

You should never accept much more than 10:1 speed difference between 
down and up.

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"



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