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Re: [Xen-devel] Ethernet MTU

To: "Sylvain Coutant" <sco@xxxxxxxxxx>
Subject: Re: [Xen-devel] Ethernet MTU
From: "Molle Bestefich" <molle.bestefich@xxxxxxxxx>
Date: Wed, 16 Aug 2006 13:36:02 +0200
Cc: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Sylvain Coutant wrote:
> 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

That's wrong.  Counted by the number of people on xen-users having
problems with VLANs and MTUs, it is very much a problem.

> 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.

Really?  If you pass off 100kB of TCP data to the IP layer, will it
get split in correctly sized IP packets, but the fragment flag won't
be set?

Sounds odd, and I think you're wrong.
But I'd like to know for sure.

(The discussion is relevant either way, since a Xen domain is often
used as a router.)

Spamming xen-devel with non-Xen related network education and RFCs
was not my intent,

You cross-posted with xen-users.  I think the information is relevant
for a couple of people there, at least.

I was just wondering about jumbo frames support into Xen networking layer

Fair nuff, hope someone can answer that?

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