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Re: IPv6 Anycast has been killed by LINUX patch in 2016 - who cares?

Christian Huitema <huitema@huitema.net> Mon, 09 August 2021 19:40 UTC

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To: Warren Kumari <warren@kumari.net>, Tom Herbert <tom@herbertland.com>
Cc: Theodore Ts'o <tytso@mit.edu>, Phillip Hallam-Baker <phill@hallambaker.com>, 6man WG <ipv6@ietf.org>, IETF discussion list <ietf@ietf.org>
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From: Christian Huitema <huitema@huitema.net>
Subject: Re: IPv6 Anycast has been killed by LINUX patch in 2016 - who cares?
Message-ID: <0154c564-fd1c-3b5c-b95c-7fab5d6dab8e@huitema.net>
Date: Mon, 09 Aug 2021 12:40:29 -0700
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On 8/9/2021 11:54 AM, Warren Kumari wrote:

> There are dire proclamations that IPv6 TCP anycast cannot work.... And yet
> there are a bunch of existing implementations where it is clearly working,
> and people have built their business models around it, showing that it is
> working fine.

Or something in between. Whether TCP anycast works or not depends on how 
long one needs the TCP connection to last. If the connection lasts long 
enough, route flaps end up happening, and then the TCP connection 
breaks. How long that lasts also depends on how long the route is. If 
the anycast destination is just one AS hop away from the TCP source, 
then you can expect BGP paths to be fairly stable. If the anycast target 
is multiple ASes away, the frequency of route flaps is going to increase 
markedly.

For big CDN deployments, or for the edge servers of big tech companies, 
there are many servers and the AS paths are quite short. TCP sessions 
will mostly work. On the other hand, if you start a deployment with just 
a couple servers, say one per continent, the AS paths will be long and 
the rate of TCP failure will be high.

-- Christian Huitema

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