Subject: Re: ipv6: a question about ECMP

From: Nicolas Dichtel <[email-masked]>
Date: Fri, 08 Nov 2013 11:09:59 +0100

Le 07/11/2013 19:32, sowmini varadhan a écrit :
> On Thu, Nov 7, 2013 at 7:16 AM, Hannes Frederic Sowa
> <[email-masked]> 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".
In fact, ECMP routes have the same metric/weight and destination but not the
same next hop (ie gw + oif).

>
> 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.
Yes, oif can be different.
Note that gw can also be a global address.

>
> 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?
rt6_score_route() is called to check requested oif (see 52bd4c0c1551 "ipv6: fix 
ecmp lookup when oif is specified").

Regards,
Nicolas

>
> --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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