Open Stack

Hello.

We've hit badly changes in behaviour of OVS when we switched from 3.08 
to 3.13 kernel. When runs on  3.11 or above, OVS starts to use kernel 
GRE services. And they copy DNF (do not fragment) flag from encapsulated 
packet to GRE packet. And this mess up all things, because ICMP messages 
about dropped GRE never reach neither source nor destination of 
underlying TCP.

We've fixed problems with MTU by using option for DHCP for dnsmasq. This 
lower MTU inside instances. But there are routers (router namespaces) 
and they are still using 1500 bytes MTU.

I feel like this can cause problems with some types of traffic, when 
client (outside of openstack) sending DNF packets to instance (via 
floating) and that packet is silently dropped.

1) Is those concerns have any real life implication? TCP should take in 
account MTU on server and works smoothly, but other protocols?
2) Is there any way to lower MTU inside router namespace?

Thanks.

P.S. Jumbo frames is not an option due reasons outside of our reach.