[ovs-discuss] MTU considerations for OVN

[Ryan Moats]

"discuss" <discuss-bounces at openvswitch.org> wrote on 05/04/2016 06:09:04
PM:

> From: Jesse Gross <jesse at kernel.org>
> To: Matt Kassawara <mkassawara at gmail.com>
> Cc: discuss <discuss at openvswitch.org>
> Date: 05/04/2016 06:09 PM
> Subject: Re: [ovs-discuss] MTU considerations for OVN
> Sent by: "discuss" <discuss-bounces at openvswitch.org>
>
> On Tue, May 3, 2016 at 3:50 PM, Matt Kassawara <mkassawara at gmail.com>
wrote:
> > Jesse,
> >
> > I'm resurrecting this thread after a fairly lengthy discussion of MTU
with
> > Ben at the recent OpenStack summit. Have you given the topic any
further
> > thought toward implementation in a reasonable way? Can you elaborate on
the
> > architectural limitations? At the moment, the OpenStack implementation
of
> > OVN doesn't use DPDK.
>
> The issue that I alluded to before is that when OVS (and by extension
> OVN) does L3 processing the packets aren't traversing the Linux IP
> stack and so the usual MTU checks don't apply. Instead OVS just does a
> single combined lookup for all flow processing and then applies some
> actions like set SMAC/DMAC and decrement TTL. Not only is there no
> code to check the outgoing MTU but there's no obvious outgoing device
> to fetch the desired MTU from.

I'm not 100% sure why this would be an issue - IIRC (based on my scanning
the code)
when a packet is going to be outputed, it looks like the MTU of the
physical device
is checked and a fragmentation decision made.  Isn't that good enough for
our
purposes?

Ryan Moats
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