netdev - Re: ECMP ipv6 vs ipv4

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Date:	Wed, 17 Apr 2013 11:03:21 +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 18:53, Wilco Baan Hofman a écrit :
> On Mon, 2013-04-15 at 17:51 +0200, Nicolas Dichtel wrote:
>> 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).
>
> Sure, but how do we add nexthop weights and algorithm selection (hash,
> random) to this API? I personally prefer to have the routing behaviour
> of ipv4 and ipv6 to be as similar as possible, as the basics are the
> same anyway.
You can use something like this:

$ ip -6 route add 3ffe:304:124:2306::/64 dev eth0 nexthop via 
fe80::230:1bff:feb4:dd4f weight 1
$ ip -6 route append 3ffe:304:124:2306::/64 dev eth0 nexthop via 
fe80::230:1bff:feb4:e05c weight 2

>
>>>
>>> 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.
>
> Yes it is... in my tree, but I want to extend it to also include support
> for algorithm for hash based, etc.. and to keep it as close to the
> existing APIs as possible I think the nexthop structure makes the most
> sense for this.
>
>>>
>>> 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?
The pro of the current implementation is that you can add or delete a nexthop 
withtout removing the whole route. You don't need to list again all nexthops 
each time you want to modify one.

Nicolas
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