netdev - Re: [PATCH net-next v3] net: ipv4: add support for ECMP hash policy choice

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Date:   Tue, 14 Mar 2017 14:10:22 -0700
From:   Roopa Prabhu <roopa@...ulusnetworks.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Miller <davem@...emloft.net>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        David Ahern <dsa@...ulusnetworks.com>, jkbs@...hat.com,
        Eric Dumazet <edumazet@...gle.com>,
        Peter Christensen <pch@...bogen.com>
Subject: Re: [PATCH net-next v3] net: ipv4: add support for ECMP hash policy choice

On Tue, Mar 14, 2017 at 1:25 PM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Tue, 14 Mar 2017 11:48:37 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
>
>> From: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> Date: Tue, 14 Mar 2017 17:58:46 +0200
>>
>> > On 14/03/17 17:55, Stephen Hemminger wrote:
>> >> On Tue, 14 Mar 2017 17:36:15 +0200
>> >> Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
>> >>
>> >>> This patch adds support for ECMP hash policy choice via a new sysctl
>> >>> called fib_multipath_hash_policy and also adds support for L4 hashes.
>> >>> The current values for fib_multipath_hash_policy are:
>> >>>  0 - layer 3 (default)
>> >>>  1 - layer 4
>> >>> If there's an skb hash already set and it matches the chosen policy then it
>> >>> will be used instead of being calculated (currently only for L4).
>> >>> In L3 mode we always calculate the hash due to the ICMP error special
>> >>> case, the flow dissector's field consistentification should handle the
>> >>> address order thus we can remove the address reversals.
>> >>>
>> >>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>> >>
>> >> It is good to see ECMP come back from the grave.
>> >> Linux used to support it long ago but was abandoned after it was unstable
>> >> and removed from iproute2 in 2012.
>> >>
>> >> The old API was through route attributes which makes more sense than
>> >> doing it with sysctl. It makes more sense to use netlink instead.
>> >> Therefore please go back and do something like the old API rather than doing it through
>> >> sysctl.
>> >>
>> >
>> > That's what my initial version did, but this was discussed during NetConf in Seville
>> > and it was decided that it's best to make a global sysctl, thus the change.
>>
>> Correct, we discussed this, and we all agreed to only have a sysctl for now.
>
> Why? If you are going to have private discussions please post the rationale
> in public.

Stephen, is there any reason to have a per ecmp route multipath algo
selection ?.
All platforms have a global multipath selection algo. I also don't see
routing daemons ready or willing to specify a per ecmp route multipath
selection algo attribute.