netdev - Re: [PATCH net-next v3 0/4] net: ipv6: Improve user experience with multipath routes

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Message-ID: <d987faff-ced4-8791-8bdb-ba8a66aec6f5@cumulusnetworks.com>
Date:   Mon, 30 Jan 2017 09:12:05 -0700
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Roopa Prabhu <roopa@...ulusnetworks.com>
Cc:     netdev@...r.kernel.org, nicolas.dichtel@...nd.com
Subject: Re: [PATCH net-next v3 0/4] net: ipv6: Improve user experience with
 multipath routes

On 1/30/17 8:49 AM, Roopa Prabhu wrote:
>> Single next hop delete will be around because IPv6 allows it -- and because IPv4 needs to support it.
>>
> understand single next hop delete for ipv6 will be around..and my only point was to leave it around but not optimize for that case.
> I don't think we should support single nexthop delete in ipv4 (I have not seen a requirement for that)... ipv4 is good as it is right now.
> the additional complexity is not needed.
> 

IPv4 has a known bug -- delete a virtual interface in a multihop route and the entire route is deleted, including the nexthops for other devices. This does not happen for IPv6.

Simple example of that bug:

ip li add dummy1 type dummy
ip li add dummy2 type dummy
ip addr add dev dummy1 10.11.1.1/28
ip li set dummy1 up
ip addr add dev dummy2 10.11.2.1/28
ip li set dummy2 up
ip ro add 1.1.1.0/24 nexthop via 10.11.1.2 nexthop via 10.11.2.2
ip li del dummy2

--> the entire multipath route has been deleted.

And, fixing this bug enables work to make IPv4 append to be sane -- appending a route should modify an existing route by adding the nexthop, not adding a new route that I believe can never actually be hit.

Both cases mean modifying an IPv4 route -- adding or removing nexthops -- a capability that IPv6 allows so fixing this means closing another difference between the stacks.