netdev - Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

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Message-ID: <20141204101707.GB17122@redhat.com>
Date:	Thu, 4 Dec 2014 12:17:07 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Jesse Gross <jesse@...ira.com>, Thomas Graf <tgraf@...g.ch>,
	"Du, Fan" <fan.du@...el.com>, Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"fw@...len.de" <fw@...len.de>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	Pravin Shelar <pshelar@...ira.com>
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

On Wed, Dec 03, 2014 at 10:56:02AM -0800, Rick Jones wrote:
> Trying to "fake-out" an ICMP message to paper-over "devices" in the "middle"
> of same Layer2 network having different MTUs from the ends goes back to at
> least the days when people started joining FDDI networks to Ethernet
> networks with bridges rather than routers.  Perhaps back even further.  It
> had problems them (including not all traffic being IP), I doubt today would
> be any different.
> 
> All devices in a Layer2 network must have the same MTU. (*)
> 
> The only exception to that which does not lead one down a rathole is that
> you can have the MTU at the "edges" of the Layer 2 network be smaller than
> the MTU in the "middle."  And then only if the middle "never" tries to talk
> itself to the edges directly.
> 
> rick jones
> 
> (*) Or at least any communicating device must be kept ignorant of what one
> does to have a smaller MTU in there somewhere.

What I've been pointing out is that just because something is within a VM,
we can't assume it's an edge.

-- 
MST
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