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Re: [Ntp] Leap second

"Seaman, Robert Lewis - (rseaman)" <rseaman@email.arizona.edu> Fri, 24 January 2020 15:17 UTC

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From: "Seaman, Robert Lewis - (rseaman)" <rseaman@email.arizona.edu>
To: NTP WG <ntp@ietf.org>
Thread-Topic: [Ntp] Leap second
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Date: Fri, 24 Jan 2020 15:17:09 +0000
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Subject: Re: [Ntp] Leap second
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Just a reminder that UTC does not have repeated seconds, whatever POSIX says, or NTP implements. Seconds are interposed (or elided) as needed. UTC second number 61 in that minute is distinct from second number 60. Or second number 59 of one minute is immediately followed by second number 1 of the next. If you want different behavior, define or adopt a different time scale.

Rob Seaman
Lunar and Planetary Laboratory
University of Arizona
--

On 1/24/20, 7:48 AM, "ntp on behalf of Tony Finch" <ntp-bounces@ietf.org on behalf of dot@dotat.at> wrote:

    Hal Murray <hmurray@megapathdsl.net> wrote:
    > Warner Losh said:
    > > NTP timestamp repeats the last second of the day. Once without the leap bit
    > > set and then again with it set.
    >
    > Which bit is that?  I don't know of a single "leap bit" in the header.
    >
    > There is a 2 bit header field with: normal (no leap), and add+del (leap
    > pending) states.  The 4th state is used for non-in-sync.  Assuming you meant
    > "bits" rather than bit, then the on-off is backwards.  I expect it to be
    > sending leap-pending for most of the day, then send normal (no leap-pending)
    > for the repeated last second of the day.
    
    My understanding is that the leap indicator bits are not synchronized with
    the leap second, so they can't be used to disambiguate the repeated
    second.
    
    https://mailarchive.ietf.org/arch/msg/ntp/5KD48S2BFAMemNx99zQSB83hOJ8
    
    This isn't specified in the NTP RFC, but I believe the way it works in
    practice is that ntpd tells the kernel about the leap indicator bits using
    ntp_adjtime(STA_INS), then continues with the same leap indicator
    bits through the leap second until a subsequent ntp_adjtime() returns
    TIME_WAIT, at which point the leap indicator bits are cleared. When this
    happens is related to ntpd's polling interval, and not coupled to the leap
    second at all.
    
    Tony.
    -- 
    f.anthony.n.finch  <dot@dotat.at>  http://dotat.at/
    democracy, participation, and the co-operative principle
    
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