[ovs-discuss] OVN - MTU path discovery

[Miguel Angel Ajo Pelayo]

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.

But I guess, that we still need the kernel changes to match on
those “big packets”.


On 27 July 2018 at 23:35:58, Ben Pfaff (blp at ovn.org) wrote:

On Thu, Jul 12, 2018 at 04:03:33PM +0200, Miguel Angel Ajo Pelayo wrote:
> Is there any way to match packet_size > X on a flow?
>
> How could we implement this?

OVS doesn't currently have a way to do that. Adding such a feature
would require kernel changes.

You mentioned ICMP at one point. It would be pretty easy to implement
such a feature specifically for ICMP to logical router IP addresses in
OVN, because we could just slow-path such traffic to ovn-controller
(maybe we already do?) and check the packet size there. I don't know
whether there's value in that.
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