netdev - Re: [PATCH v3] net: sysctl for RA default route MTU

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Message-Id: <20150331.160538.1808487957641759931.davem@davemloft.net>
Date:	Tue, 31 Mar 2015 16:05:38 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	klamm@...dex-team.ru
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	hannes@...essinduktion.org
Subject: Re: [PATCH v3] net: sysctl for RA default route MTU

From: Roman Gushchin <klamm@...dex-team.ru>
Date: Mon, 30 Mar 2015 15:30:57 +0300

> This patch introduces new ipv6 sysctl: ra_default_route_mtu.
> If it's set (> 0), it defines per-route MTU for any new default route
> received by RA.
> 
> This sysctl will help in the following configuration: we want to use
> jumbo-frames for internal networks and default ethernet frames for
> default route. Per-route MTU can only lower per-link MTU, so link MTU
> should be set to ~9000 (statically or via RA).
> 
> Due to dynamic nature of RA, setting MTU for default route will require
> userspace agent, that will monitor changes of default route
> and (re)configure it. Not simple. The suggested sysctl solves this
> problem.
> 
> Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
> Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

I don't like this change at all.  The way I see things you already
have the mechanisms necessary to do this.

You obviously control the entity providing the default routes and
these RA messages, therefore you absolutely can configure it to
provide an appropriate MTU value in those RA messages.

Problem solved, and no kernel changes necessary.

I am warning you ahead of time that I will have a very low tolerance
for replies to this email containing stories explaining why this is
"difficult" to do.  The fact is that the mechanism is there and if you
have designed things at your site in a way such that the mechanism
designed for this has become less useful, that isn't my problem.

I'm not adding facilities that duplicated existing methods that
already exist to accomplish this task.
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