netdev - Re: ipv6: a question about ECMP

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Date:	Thu, 7 Nov 2013 13:32:15 -0500
From:	sowmini varadhan <sowmini05@...il.com>
To:	Duan Jiong <duanj.fnst@...fujitsu.com>,
	David Miller <davem@...emloft.net>, nicolas.dichtel@...nd.com,
	netdev@...r.kernel.org
Subject: Re: ipv6: a question about ECMP

On Thu, Nov 7, 2013 at 7:16 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> Hi Duan!
>
> On Thu, Nov 07, 2013 at 06:33:20PM +0800, Duan Jiong wrote:
>>   After reading the ip6_pol_route(), i have a question about ECMP. Why we call
>> the rt6_multipath_select() after calling rt6_select()?
>>   In my opinion, the route returned by rt6_select() has a highest score, but the route
>> returned by rt6_multipath_select() may has a lower score than the former, because the
>> ECMP don't take the route preference into consideration. That means that the kernel will
>> choose a less-desirable route.
>
> ECMP routes only differ in the gateway the specify, so I doubt there will be
> any change in the score they woud receive. rt6_multipath_select does merly
> make sure we don't select the same route again and again.

 rt6_multipath_select() -> rt6_socre_route() seems to require that the
interface *must* matchi, which is consistent with your assertion above that
"ECMP routes differ in gw only".

But for IPv6, the gw addr is a a link-local, which is only required to be
unique on the link. Thus, e.g.,  you can have fe80::1 as the gw on both eth0 and
eth1.

What is the assumption around "cost" for ECMP here- are we assuming some
form of link bundling (Section 6 of rfc 2991) here? or is the "multiple parallel
links" case handled somewhere else, that I am missing?

--Sowmini

>
> Please note, the rt6_info's siblings fields were added for the solely purpose
> of ECMP and the insertion only updates the siblings list if the above criteria
> did hold. They make sure the routes lookup up do differ on each lookup, so it
> does actually do multipath and does not depend on the order the routes where
> inserted.
>
> Hope that helps,
>
>   Hannes
>
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