[ovs-dev] Path MTU discovery on GRE interfaces

Ben Pfaff blp at ovn.org
Wed Jun 23 17:06:33 UTC 2021

[updating Jesse's email address]

On Wed, Jun 23, 2021 at 04:48:29PM +0200, Matthias May via dev wrote:
> I'm currently fighting with issues where TCP/UDP frames that are larger than the MTU of a GRE tunnel are dropped.
> I'm aware of the whys and how to work around the issue, but while looking for solutions i stumbled over the fact that:
> * [1] added PMTUD support to OVS
> * [2] disabled/removed with v1.9.0 respectively v1.10.0 the feature
> Even after some significant time looking through the history i haven't found a reason why this was removed, just that it
> was removed.
> I started some preliminary work to add PMTUD support to OVS (again), but the fact that it was removed 8 years ago seems
> to me like a red flag to not do it (again).
> Could someone fluent with the OVS history from 8 years ago shed some light on why PMTUD support was dropped?
> Any pointers to a thread on this topic?

It was a layering violation.  This caused problems like, for example,
not having a good IP address to send the "frag needed" message from.

Jesse may remember more.

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