[ntp:questions] Time going backwards

Unruh unruh-spam at physics.ubc.ca
Thu Aug 14 04:15:19 UTC 2008

jbartas at speakeasy.net (John Bartas) writes:

>Hi Folks,

>I have some questions about when (if ever) NTP will set time backward. 
>I'll be using it on Embedded Linux for time stamping video frames.

It won't, unless you tell it to at when it first starts up.

>I've found several mentions in RFC 1305 that this should not happen, and 
>also in section "Step Phase Adjustments" mention that it can jump 

>Is there a simple description anywhere if what variables affect this, 
>and how? I'd like to know when it's going to jump backwards on Linux - 
>what the rules are and what control I have over it.

It will not under operation except possibly on startup. Otherwise it will
change the time by changing the clock frequency by at most 500PPM. Since to
go backwards it has to go at -1000000PPM or less, ntp will not go

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