[dns-operations] TCP anycast was Re: DNS query logging

Edward Lewis Ed.Lewis at neustar.biz
Thu Mar 17 18:44:57 UTC 2011

At 10:49 -0700 3/17/11, Rick Jones wrote:
>So, what is the frequency of "shifts" for anycast anyway?  The NTP folks
>(as expressed in posts to comp.protocols.time.ntp) seem to not be
>terribly fond of it even for UDP-based time synchronization.  Those
>folks are generally pretty detail-oriented so I'd not necessarily
>suspect they would be easily given to fear uncertainty and doubt, but
>cannot rule it out entirely.
>rick jones
>it is merely coincidental anecdote - the NTP daemon on my workstation at
>work routinely "discards" time from the internal, anycasted NTP service
>in favor of servers that are even of higher stratum.  Could be for any
>number of reasons I suppose besides anycast (perhaps OS choice on the
>servers) but it does also show the highest jitter of the five servers I
>have configured to poll for time.

At the risk of saying the obvious, NTP is more time-sensitive than 
DNS.  NTP is going to prefer to go to a server where the round trip 
time is very predictable, so it knows what time delta to add to what 
it's told.  So I'd say it's fair that NTP is very picky about network 
latency "rubber banding."  (Keep in mind - it's been a long time 
since I was into NTP though.)

There have been studies done about client "flip flopping" between 
anycast instances.  The last one I recall (2004 or so) is here


Slide 27 has something on that.

(It says this though: "DO NOT RUN Anycast with Stateful Transport")

See also this follow up


Courrent "shifts" - low enough that we don't think about it.

Edward Lewis             
NeuStar                    You can leave a voice message at +1-571-434-5468

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