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 _______________________________________________ LEAPSECS mailing list [email protected] https://pairlist6.pair.net/mailman/listinfo/leapsecs
