[ovs-discuss] OVN - MTU path discovery

Miguel Angel Ajo Pelayo majopela at redhat.com
Fri Aug 3 06:11:01 UTC 2018


I’m going to capture some example traffic and try to figure out which RFCs
talk about that behaviour so we can come up with a consistent solution.
I can document it in the project.

To be honest, when I looked at it, I was expecting that the router would
fragment, and I ended up discovering that we had this path MTU discovery
mechanism in play for IPv4 .

On 2 August 2018 at 22:21:28, Ben Pfaff (blp at ovn.org) wrote:

On Thu, Aug 02, 2018 at 01:19:57PM -0700, Ben Pfaff wrote:
> On Wed, Aug 01, 2018 at 10:46:07AM -0400, Miguel Angel Ajo Pelayo wrote:
> > Hi Ben, ICMP is used as a signal from the router to tell the sender
> > “next hop has a lower mtu, please send smaller packets”, we would
> > need at least something in OVS to slow-path the “bigger than X”
packets,
> > at that point ova-controller could take care of constructing the ICMP
packet
> > and sending it to the source.
>
> Yes.
>
> > But I guess, that we still need the kernel changes to match on
> > those “big packets”.
>
> Maybe. If we only need to worry about ICMP, though, we can set up OVN
> so that it always slow-paths ICMP.

Oh, I think maybe I was just being slow. The ICMP is generated, not
processed. Never mind.
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