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Date: Fri, 20 Sep 2013 19:26:26 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130920172626.GA14589@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] Qemu-KVM VETH
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To: Tim Epkes <tim.epkes@gmail.com>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On Fri, Sep 20, 2013 at 11:48:50AM -0400, Tim Epkes wrote:
> The problem aI face is that a bridge in the middle ( using taps) breaks
> LLDP ( L2 discovery protocol) and should break ISIS as well.  Sockets
> aren't bad, but if for some reason I take the listener VM down and bring
> back up, then I have to bring down all connector VMs, which chains on
> itself if there is a lot of connectivity defined.  When UDP was available
> that wasn't an issue.

I just checked linux.git but this patch has not been applied (although
it's trivial if you're willing to rebuild your kernel from source):
http://comments.gmane.org/gmane.linux.network/208908

It sounds like improving net/socket.c might be the right place to look.

> Anyway, that is how I came to VETHs.  I am aware that you can use UDP
> multicast, but performance past one hop is extremely poor ( resulting in 3
> of 5 pings to be lost.  Thanks

Unfortunately the veth driver does not hand Ethernet frames to/from
userspace.  We really need something tap-like where userspace can
inject/extract packets.

Stefan