Anycast but for egress

Bill Woodcock woody at
Wed Jul 28 15:09:31 UTC 2021

> On Jul 28, 2021, at 3:21 AM, Mark Tinka <mark at> wrote:
> On 7/28/21 01:16, Daniel Corbe wrote:
>>> This is interesting... I wonder whether Anycast will still have some failure modes and break TCP connections if routing (configuration) were to change?  I checked the PDF linked by Bill Woodcock... while the methodology is the same from 20y ago, would the data still be the same (order of magnitude)? :)
> We are Anycast'ing DNS (authoritative and recursive), NTP and TACACS+. All works well, across 11 or so countries.

I was about to say something about us having equal success over 105 or so countries, when I came to the realization that inviting quantitative comparisons of manhood with Mark is the very definition of folly.  :-)

Anyway, yeah, the folks who were scared of anycast in the 1990s were running from shadows, not basing it on experience or data.  In the real world, the number of stateful flows affected by route changes is dwarfed by those disrupted by other causes, and is immeasurably small.  And when they do crop up on the radar, it’s almost always someone’s equal-cost-multi-path gone wrong, rather than an actual shift.  So, not an issue at all in the real world, just in the imaginations of folks who thought TCP was a complex thing reserved for the specific use-cases that they’d already conceived of in the 1980s.  Took a while to get beyond their protestations, but here we are in the 21st century.  Planck's principle holds.  Science progresses one funeral at a time.


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