[ntp:questions] Asymmetric Network and Delay Questions.

Brad Knowles brad at stop.mail-abuse.org
Thu Jul 14 08:46:11 UTC 2005


At 5:01 PM -0500 2005-07-13, Matt Kinard wrote:

>  I'm working on developing an NTP implementation that would possibly
>  include an attempt to run NTP over a highly asymmetric WAN.

	Ouch.  Bad news.

>  I've read that NTP v3 does not handle network assymmetry very well I was
>  wondering if anyone could provide any details and/or specifics regarding
>  these issues.

	No version of NTP handles asymmetry well.  One of the fundamental 
underlying principles of NTP is that any delays are largely 
symmetrical -- the algorithms are complex, but basically the client 
measures the round-trip delay and adds half that value to the 
returned time stamp from the server, and takes the resulting value to 
be the "true time" that it should sync to.

	If there is significant asymmetry, you can pretty much throw out 
all the NTP algorithms, because you are violating one of the most 
basic assumptions.

>  My managers aren't going to be happy when I my reply to "Why can't we do
>  NTP over this WAN?"  is "'Cause I said so...".   Any information would
>  be greatly appreciated.

	Well, you could take a look at 
<http://www.eecis.udel.edu/~mills/database/papers/survey5.pdf>.

	Googling for "ntp asymmetry 'network time protocol'", several 
other useful papers show up within the top five or ten links.  I 
would encourage you to check them out.

-- 
Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.



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