[ntp:hackers] smearing the leap second

Martin Burnicki martin.burnicki at burnicki.net
Fri Jul 10 14:09:19 UTC 2015

Mike S wrote:
> On 7/9/2015 5:48 PM, Harlan Stenn wrote:
>> We still see huge numbers of people running very old versions of NTP.
>> How do you propose to get lots of devices to get upgraded kernels in
>> order to properly handle a leap second?
> That's not NTP's problem to solve. It's a device issue.

But it helps quite a number of users/admins a lot. If you don't want it 
then don't use it.

>> If they *do* properly handle the leap second, how do you get lots of
>> instances of *appliations* to properly handle what looks to them like a
>> backward time step?
> Ditto, except it is an NTP issue since it only looks like a backward
> time step because the canonical implementation of NTP doesn't follow its
> own RFC, and doesn't use a monotonic timescale. If NTP did it right,
> there wouldn't be any issue.

Nope. On standard installations ntpd just passes the leap second warning 
down to the OS kernel, so the kernel can handle the leap second at the 
right point in time. However, *how* it is handled depends on the kernel.

Dave Mills has made a *proposal* how this could be implemented in a 
kernel so that the time increases monotonically even when a leap second 
is inserted. Unfortunately most *kernels* just step the time back, so if 
the kernel maintainers implement it in a different way than proposed by 
Dave Mills, please don't blame ntpd.


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