netdev - Re: ECMP ipv6 vs ipv4

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Date:	Mon, 15 Apr 2013 17:51:46 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Wilco Baan Hofman <wilco@...nhofman.nl>
CC:	netdev <netdev@...r.kernel.org>
Subject: Re: ECMP ipv6 vs ipv4

Le 15/04/2013 09:58, Wilco Baan Hofman a écrit :
> Hi,
>
> I'm working on a patch to implement 'nexthop weight' for multipath ipv6.
> However, the ECMPv6 implementation has a few flaws that are quite
> annoying.
>
> One of the flaws is that the netlink nexthop API is asymmetrical, you
> can add nexthops through the netlink API, but when the result is
> requested it is completely different, resulting in bird6 removing the
> route as it does not match the initial route set.
In fact, there is two ways to add ECMP routes:
$ ip -6 route add 3ffe:304:124:2306::/64 \
	nexthop via fe80::230:1bff:feb4:e05c dev eth0 \
	nexthop via fe80::230:1bff:feb4:dd4f dev eth0
or
$ ip -6 route add 3ffe:304:124:2306::/64 via fe80::230:1bff:feb4:dd4f dev
eth0
$ ip -6 route append 3ffe:304:124:2306::/64 via fe80::230:1bff:feb4:e05c dev
eth0

Note that the second way matchs what is returned by the kernel (ie one entry per 
nexthop).

>
> Another one of the flaws is that if I add nexthop weight or algorithm
> (weighted hash or weighted random) I need to add this to the main rt
> node, this seems like an inefficient memory structure, as this needs to
> be added to all the siblings as well.
Nexthop weight (rtnh->rtnh_hops) is not implemented.

>
> I propose that we have a nexthop structure to an exclusive route,
> similar what we have for IPv4, where we store the gateway, device and
> weight for all nexthops and the algorithm in the route. This would make
> the netlink API symmetrical again and fixes the n*n inefficiencies when
> adding routes (all siblings need to know about all siblings).
>
> What are your thoughts on this?
>
> Regards,
>
> Wilco Baan Hofman
>
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