Question on Loosely Synchronized Router Clocks
stephen at sprunk.org
Tue Sep 18 19:40:16 UTC 2007
Thus spake "Xin Liu" <smilerliu at gmail.com>
> Sorry for the confusion. Let me clarify.
> We are interested in a number of questions:
> 1. Can we assume loosely synchronized router clocks in the
> Internet, or we have to make absolutely no assumption about
> router clocks at all?
That assumption is _generally_ true, but not often enough that you can rely
> 2. If the router clocks are indeed loosely synchronized, what is
> the granularity we can assume? Particularly, we are interested in
> whether we can assume router clocks are synchronized within
> 10 minutes.
My experience is they'll either be within a few seconds or off by several
days to years. There's not much middle ground.
> 3. It's always possible that a router's clock goes wrong. In
> practice, how often does this happen?
It's unlikely to "go wrong" to any noticeable degree _if it was ever correct
in the first place_. However, many people do not bother setting the clocks
at all (which will often result in a clock that's off by a decade or more),
or intentionally set them to be wrong. A lot of folks had to set their
clocks back a few years around Y2k, for instance.
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking
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