[ovs-discuss] OVN - MTU path discovery

[Miguel Angel Ajo Pelayo]

We didn’t understand why a MTU missmatch in one direction worked (N/S),
but in other direction (S/N) didn’t work… and we found that that it’s
actually
working (at least for TCP, via MSS negotiation), we had a missconfiguration
In one of the physical interfaces.

So, in the case of TCP we are fine. TCP is smart enough to negotiate
properly.

Other protocols like ICMP with the DF flag, or UDP… would not get the ICMP
that notifies the sender about the MTU miss-match.

I suspect that the most common cases are covered, and that it’s not worth
pursuing what I was asking for at least with a high priority, but I’d like
to hear
opinions.


Best regards,
Miguel Ángel.

On 3 August 2018 at 08:11:01, Miguel Angel Ajo Pelayo (majopela at redhat.com)
wrote:

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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