Open Stack

MTU has been an ongoing issue in Neutron for _years_.

It's such a hassle, that most people just throw up their hands and set
their physical infrastructure to jumbo frames. We even document it.

http://docs.openstack.org/juno/install-guide/install/apt-debian/content/neutron-network-node.html

> Ideally, you can prevent these problems by enabling jumbo frames on
> the physical network that contains your tenant virtual networks. Jumbo
> frames support MTUs up to approximately 9000 bytes which negates the
> impact of GRE overhead on virtual networks.

We've pushed this onto operators and deployers. There's a lot of
code in provisioning projects to handle MTUs.

http://codesearch.openstack.org/?q=MTU&i=nope&files=&repos=

We have mentions to it in our architecture design guide

http://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/arch-design/source/network-focus-architecture.rst#n150

I want to get Neutron to the point where it starts discovering this
information and automatically configuring, in the optimistic cases. I
understand that it can be complex and have corner cases, but the issue
we have today is that it is broken in some multinode jobs, even Neutron
developers are configuring it correctly.

I also had this discussion on the DevStack side in https://review.openstack.org/#/c/112523/
where basically, sure we can fix it in DevStack and at the gate, but it
doesn't fix the problem for anyone who isn't using DevStack to deploy
their cloud.

Today we have a ton of MTU configuration options sprinkled throghout the
L3 agent, dhcp agent, l2 agents, and at least one API extension to the
REST API for handling MTUs.

So yeah, a lot of knobs and not a lot of documentation on how to make
this thing work correctly. I'd like to try and simplify.


Further reading:

http://techbackground.blogspot.co.uk/2013/06/path-mtu-discovery-and-gre.html

http://lists.openstack.org/pipermail/openstack/2013-October/001778.html

https://ask.openstack.org/en/question/6140/quantum-neutron-gre-slow-performance/

https://ask.openstack.org/en/question/12499/forcing-mtu-to-1400-via-etcneutrondnsmasq-neutronconf-per-daniels/

http://blog.systemathic.ch/2015/03/05/openstack-mtu-pitfalls-with-tunnels/

https://twitter.com/search?q=openstack%20neutron%20MTU

-- 
Sean M. Collins