Question on Loosely Synchronized Router Clocks

Stephen Sprunk stephen at
Tue Sep 18 19:40:16 UTC 2007

Thus spake "Xin Liu" <smilerliu at>
> 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 
on it.

> 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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