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

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Date:	Tue, 07 Oct 2008 01:02:08 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Rick Jones <rick.jones2@...com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] vlan: propogate MTU changes

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.

> If physical interface MTUs are going to be bouncing around and VLANs get 
> their MTUs changed then perhaps a VLAN needs both a desired and actual 
> MTU setting.  The VLAN's interface would then be the minimum of the 
> desired and actual MTU.  I suppose it isn't too unlike having both an 
> administrative (desired) and operational (actual) interface state.

Thats assuming that the VLAN device is actually restricted by the
ethernet device settings. I don't know if its always not the case,
but I'm pretty sure it usually isn't. Which means there's no real
need for an operational state wrt. MTUs.
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