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RE: [Xen-devel] Ethernet MTU

To: "'Molle Bestefich'" <molle.bestefich@xxxxxxxxx>, "'James Harper'" <james.harper@xxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Ethernet MTU
From: "Sylvain Coutant" <sco@xxxxxxxxxx>
Date: Wed, 16 Aug 2006 13:05:25 +0200
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> James Harper wrote:
> > 802.1q lives under IP.
> That's right.
> But the VLAN exposes a virtual interface.
> That virtual interface should automatically have an MTU size of the
> underlying interface minus 4 bytes (for the VLAN tag).

It should. It does not. It's usually not a problem as networking equipment 
handle frames up to 1520-1540 bytes without much trouble, at least equipments 
able to manage tagged frames.

> And then IP fragmentation, working on top of that virtual Ethernet
> interface, should perform fragmentation correctly.

IP fragmentation handling is for routers when they connect two networks with 
different mtu sizes. This is not the case here, the IP stack generates frames 
that are as large as the mtu makes it possible.

Spamming xen-devel with non-Xen related network education and RFCs was not my 
intent, I was just wondering about jumbo frames support into Xen networking 
layer ...



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