"discuss" <discuss-boun...@openvswitch.org> wrote on 05/04/2016 06:09:04 PM:
> From: Jesse Gross <je...@kernel.org> > To: Matt Kassawara <mkassaw...@gmail.com> > Cc: discuss <discuss@openvswitch.org> > Date: 05/04/2016 06:09 PM > Subject: Re: [ovs-discuss] MTU considerations for OVN > Sent by: "discuss" <discuss-boun...@openvswitch.org> > > On Tue, May 3, 2016 at 3:50 PM, Matt Kassawara <mkassaw...@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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