[ovs-dev] Path MTU discovery on GRE interfaces

Matthias May matthias.may at westermo.com
Fri Jun 25 15:20:44 UTC 2021

On 23/06/2021 19:06, Ben Pfaff wrote:
> [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.

OK i guessed as much.
I was thinking about what address to use when there is none available.
Maye the least breaking thing would be to just use as source of the ICMP unreachable the destination to which the frames
are addressed.


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