[LEAPSECS] Another vote on UTC, facebook

[Judah Levine]

There are troubling aspects about the facebook method:

1. The time *accuracy* of a client system that uses a public network
connection to any time server will be limited by the asymmetry in the
inbound-outbound network delays. The end-point application cannot
improve on this, and hardware time-stamping by the client will not help.
(A client with a local primary reference, such as a local GPS receiver,
is a different matter.)

2. The time *stability* of a client system that uses a public network
connection can be improved by hardware time-stamps and by rather
complicated statistics. (There are a lot of my papers on the NIST web
page at tf.nist.gov that discuss this question.)

It is important to distinguish between these two parameters.

3. The time to synchronize the clock on a client system following a cold
start is often faster with Chrony, but this is a trade-off against
increased sensitivity to false-tickers (servers transmitting the wrong
time with no indication). The shorter synchronization time is likely to
be particularly troublesome on networks with unstable delays. Your
mileage may vary.

4. The facebook method of applying the smear *after* the leap second is
the most troubling aspect of the process. It is not consistent with the
definition of UTC or with the other smear methods that apply the smear
before the leap second. The question of whether smear methods are "good
enough" has no absolute answer. They are very definitely *NOT* good
enough to satisfy the European rules for time-stamp accuracy of
financial transactions, and these rules are likely to be adopted by the
regulators in the US in the foreseeable future. A client system that
sends requests to servers with different smearing algorithms will be
really confused in the vicinity of a leap second. In the worst case, the
client may reject all of the replies as requiring a time-step that
exceeds some maximum threshold, such as 128 milliseconds.

5. If your application requires time stamps that are legally traceable
to national or international standards or if it depends on international
time coordination, then you should seek competent advice. Your mileage
is guaranteed to vary.

Judah Levine
Time and Frequency Division
NIST Boulder