netdev - Re: [PATCH net-next v2] IPv6: add option to use Subnet-Router anycast addresses as source addresses

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Message-ID: <20140102231730.GH22494@order.stressinduktion.org>
Date:	Fri, 3 Jan 2014 00:17:30 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	François-Xavier Le Bail <fx.lebail@...oo.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki Yoshifuji <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH net-next v2] IPv6: add option to use Subnet-Router anycast addresses as source addresses

On Thu, Jan 02, 2014 at 10:56:17AM -0800, François-Xavier Le Bail wrote:
> > I wouldn't actually add this knob, we can just enable that by default.
> 
> I think we need to keep by default the existing behaviour (ICMPv6 echo reply with unicast)
> in case some setups use it, and use "enable_anycast_src" to change the behaviour 
> according to RFC4942.

We try to extract the destination address and reuse it in the reply. The
other system should not notice if it communicates with a unicast or
anycast address. Am I missing something?

> > Also it should be no problem to bind() to those addresses.
>  
> > I am still unsure what to do with a bind to any or if the address must be
> > bound specifically (maybe that is worth a knob).
> 
> > In the long term I would like to see that we can add additional anycast
> > addresses via iproute:
>  
> > ip -6 a a 1234::1/64 dev eth0 anycast
>  
> > or something like that. That should be rather easy, as in just incrementing
> > the anycast counter for an interface in inet6_rtm_newaddr
> > and adding a flag to ifa_flags which gets passed down by iproute2.
>  
> > I'll do some research on if this should also get integrated with source address selection.
> 
> It can be the objects of future evolutions.
> I have to think about these.

Me, too. :) I don't know what the consequences are if we make the whole
unicast path agnostic to anycast. It could very well be a bad idea.

Thanks,

  Hannes

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