MTU and frame size filtering inaccuracy

[Stephen Hemminger]

On Wed, 2 Mar 2022 17:50:13 +0100
Morten Brørup <mb at smartsharesystems.com> wrote:

> Again, Postel's Law makes perfect sense on a server.
> 
> On a network appliance, I would prefer discarding oversize packets at ingress, rather than process them and discard them at egress.
> 
> I do admit that my viewpoint is quite academic, and mostly relevant for L2 bridging appliances, where fragmentation is not available. It is probably the best approach for DPDK to align the API with what common NIC hardware supports. In other words, I agree with Stephen. :-)

The HW should try (if it can) and filter overly large frames, but it is still the responsibility
of router and bridge software to look at MTU of the egress port and take action.
For L2 bridge, that means silently dropping and for L3 that is responding with ICMP.

The DPDK examples don't do this, and they should.