North American Network Operators Group

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Re: Question on Loosely Synchronized Router Clocks

  • From: Stephen Sprunk
  • Date: Wed Sep 19 00:51:58 2007


Thus spake "Xin Liu" <[email protected]>
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.


S

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