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

[Warren Kumari <[e-mail]>]

On Mon, Aug 9, 2021 at 3:40 PM Christian Huitema <[e-mail]>
wrote:

> 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.


Yup. In many cases the traffic to/from CDN is also HTTP/similar, and the
application layer either expects to have short lived connections, and / or
handles retries gracefully.
If the path changes while my browser is fetching
https://www.example.com/favicon.ico, no-harm, no-foul -- my browser gets
the RST and tries again.


> 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.
>

Yup, or if I were, for example, doing SSH over TCP over IPv6 over Anycast
IP (for some weird convoluted reason :-)). But, that would be silly, and I
wouldn't try and do it in v4 either...

W

>
> -- Christian Huitema
>
>
>

-- 
The computing scientist’s main challenge is not to get confused by the
complexities of his own making.
  -- E. W. Dijkstra