netdev - Re: [PATCH] vlan: propogate MTU changes

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Message-Id: <20081007.160553.78745069.davem@davemloft.net>
Date:	Tue, 07 Oct 2008 16:05:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	kaber@...sh.net
Cc:	rick.jones2@...com, shemminger@...tta.com, netdev@...r.kernel.org
Subject: Re: [PATCH] vlan: propogate MTU changes

From: Patrick McHardy <kaber@...sh.net>
Date: Tue, 07 Oct 2008 01:33:47 +0200

> Rick Jones wrote:
> > Patrick McHardy wrote:
> >> Rick Jones wrote:
> >>
> >>> Patrick McHardy wrote:
> >>>
> >>>> Agreed. But the question when to do automatic adjustments remains.
> >>>
> >>>
> >>> A matter of interpretation of the principle of least surprise right? Which is less surprising - that a VLAN's MTU drops to match that of the physical interface or that some traffic on the VLAN stops when the physical interface's MTU drops?
> >>
> >>
> >> The traffic actually shouldn't stop since the MTU isn't enforced by
> >> the lower layers and also usually not by the driver. So I feel unable
> >> to make a policy decision when both views don't seem unreasonable.
> >> Especially given the fact that the "more suprising" behaviour so far
> >> has been our default.
> > Does changing the MTU on a physical interface not change the size frame the NIC itself will be willing to accept?
> 
> IIRC a lot of the simpler ones just use the default eth_setup change_mtu
> callback and the ones that have their one (just had a very brief look at
> sky2, tg3 and e1000) only seem to use it indirectly for enabling jumbo
> frame support and (e1000) memory allocation.
> 
> So I guess what we should do in case of the MTU depends on what we can
> expect from the majority of hardware. If its just some older drivers
> which can be reasonably expected to handle larger frames we should cap
> at the maximum of the real device and maybe introduce the "desired
> mtu" you suggested. It would be useful if people more familiar with
> the drivers and hardware than me could comment on this.

Since there is no agreement on exactly what we should be doing, I'm
tossing this from my patch queue.

I will say, however, that our current behavior isn't so horrible. :)

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