Question on Loosely Synchronized Router Clocks

Kevin Oberman oberman at
Mon Sep 17 21:28:45 UTC 2007

> Date: Mon, 17 Sep 2007 14:03:33 -0700
> From: "Xin Liu" <smilerliu at>
> Sender: owner-nanog at
> Dear Nanogers,
> We are a bunch of academic researchers interested in Internet
> security. We notice that some research papers require that BGP router
> clocks be globally synchronized to a 5-minute granularity. If a
> router's clock is off by more than 5 minutes, it cannot forward
> packets, but there's no other side effect. From an operational point
> of view, do you think it is a practical requirement? If not, what are
> the potential problems that prevent router clocks from being loosely
> synchronized? If you consider 5-minute too small, what do you consider
> as a practical clock skew requirement for BGP routers on the Internet?
> We'd appreciate your input very much. It will help us understand
> what's practical and what's not in our work.  Please reply to us
> directly.

What papers? I can assure you that BGP has no such requirement. I
suspect sBGP and SoBGP might have such a requirement, but that's not
"real world". 

I had a router that lost it's NTP servers and was off by about 20
minutes. The only obvious problem was the timestamps in syslog. (That's
what alarmed to cause us to notice and fix it.)
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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