Is one-way delay really half the round trip?
Is one-way delay really half the round trip?
Posted Dec 15, 2024 8:09 UTC (Sun) by marcH (subscriber, #57642)Parent article: Providing precise time over the network
> Both mechanisms take the same basic approach, however: they assume that the network delay between a device and the reference clock is symmetrical (which is usually a safe assumption for wired networks),
Veritasium has a nice video where they explain the universe could be asymmetrical and the speed of light faster one way than the other. No one believes that but... we can't prove it! It's a fun watch.
More realistically, it's not hard to imagine that a path through a switch could be not perfectly symmetrical, for instance because the "upstream" and downstream ports are not exactly the same or some other implementation detail.
Posted Dec 15, 2024 13:47 UTC (Sun)
by mathstuf (subscriber, #69389)
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I've seen switches with prioritized ports (typically for the WAN gateway). The other tiers were "gaming" (presumably latency-tuned) and "everything else".
Posted Dec 17, 2024 12:51 UTC (Tue)
by wallnerw (subscriber, #131634)
[Link]
As symmetrical delays are a basic assumption of measurements in PTP, such asymmetric paths cannot be synchronized, as the PTP participants don't "see" this asymmetry. A PTP device might report that it is synchronized with an offset of 0ns to the reference clock, but actually measuring the offset with an oscilloscope will show that it is off by half of that "invisible" path asymmetry.
Posted Dec 18, 2024 14:55 UTC (Wed)
by nim-nim (subscriber, #34454)
[Link]
Is one-way delay really half the round trip?
Is one-way delay really half the round trip?
Is one-way delay really half the round trip?