[ntp:questions] Leap bits, when to set?

Tapio Sokura oh2kku at iki.fi
Thu Dec 22 08:02:09 UTC 2005

There has been some discussion on the NTP pool mailing list about leap 
bits, specifically when to set them. Reading RFCs 1305 and 2030 leads me 
to believe that the correct behavior is to set the leap bit not earlier 
than 24 hours before the actual leap second occurs. I.e. the bit should 
be set on Dec 31st sometime after 00:00:00 UTC and cleared immediately 
after the leap second is over.

If the above is the correct way to time it, there seems to be a number 
of NTP servers that are out of spec as they already have the leap bit 
set. Can someone who knows the protocol and its history better shed some 
light on this? Have the leap bits been set more than 24h before the leap 
second insertion in previous years, when leap seconds have been inserted?

I just did a quick poll (Dec 22nd around 08:00 UTC) on the startum one 
and two time servers listed at the NTP wiki. Of 112 startum one servers 
that responded, 7 had the leap bit set and 2 reported clock 
unsynchronized. Of the 173 startum two servers that responded, 36 had 
the leap bit set.


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