netdev - Re: Netlink, route monitoring, RTM_DELROUTE not issued for ppp peer address. Bug or feature?

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Date:	Tue, 17 Apr 2012 10:24:53 +0200
From:	David Lamparter <equinox@...c24.net>
To:	Stian Skjelstad <stian@...ia.no>
Cc:	Denys Fedoryshchenko <denys@...p.net.lb>,
	Stephen Hemminger <shemminger@...tta.com>,
	eric.dumazet@...il.com, stephen.hemminger@...tta.com,
	netdev@...r.kernel.org
Subject: Re: Netlink, route monitoring, RTM_DELROUTE not issued for ppp peer
 address. Bug or feature?

On Mon, Apr 16, 2012 at 10:21:46PM +0200, Stian Skjelstad wrote:
> > It looks like "correct" behavior should not harm existing programs that
> > much (just there will be netlink message to delete route that are
> > already deleted by program, e.g. quagga), but looking to code,
> > implementing this feature will make significant overhead and in some
> > cases it can be harmful.
> > For example if full view BGP interface dropped, it is hundreds of
> > thousands netlink messages. So seems better i will listen to
> > address/link changes also and keep routing table in memory.
> > So better seems to keep as is.

Yes, this concern is quite real, I'd very much like to avoid processing
400000 DELROUTE messages that add exactly 0 additional meaning since it
can be inferred from the interface going down.

> If I understand correct, the "missing" RTM_DELROUTE messages actually
> comes from something similar to "ip route flush dev eth0".
> 
> If the amount of traffic is a worry, couldn't there be a RTM_FLUSHROUTE or
> a special RTM_DELROUTE message that tells that all routes for a given
> device is removed?

The interface-down message has exactly these semantics. I don't know
if/where this is documented, but adding a new message seems superfluous
to me, especially at this point where compatibility is a concern.
(The ip route flush command deletes routes individually btw.)

-David

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