Merge "Neutron OVS agent - Dont Fragment flag"

specs/juno/neutron-ovs-agent-df-gre-vxlan.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Neutron OVS agent - Dont Fragment flag
+==========================================
+
+https://blueprints.launchpad.net/neutron/+spec/neutron-ovs-agent-df-gre-vxlan
+
+Overlay network introduce an additionnal overhead. Depends on the underlying
+protocol and physical transport the 'real' MTU may have strong constraints.
+However, from an instance, or a cloud user point of view, this should not be
+a problem. And the Ethernet MTU inside VM should be, when possible, at least
+1500 bytes.
+
+Thus when using overlay protocol -here GRE and VXLAN- that use IP, it is
+possible under certain conditions to leverage the Dont Fragment -DF- bit. Once
+that bit will be set to 0, it will allow encapsulated packet to be split by
+the IP stack of the hypervisor. The main goal is to allow 1500 bytes MTU
+overlayed network to cross 1500 byte MTU physical network.
+
+
+Problem description
+===================
+
+The problem is to span overlayed network on physical network with a smaller (or
+equal MTU). As said before, the main usecase is to carry 1500 bytes MTU virtual
+network over 1500 bytes MTU physical network. It's mainly required on older
+network part. Once the network will be upgraded, the MTU on physical adapter
+will be raised, and no fragementation will happen anymore.
+
+
+Proposed change
+===============
+
+Set the options:df_default option in OVS when creating VXLAN and GRE tunnels.
+
+Alternatives
+------------
+
+Use iptables with -j DF --clear somewehere.
+
+Data model impact
+-----------------
+
+None.
+
+REST API impact
+---------------
+
+None.
+
+Security impact
+---------------
+
+None.
+
+Notifications impact
+--------------------
+
+None.
+
+Other end user impact
+---------------------
+
+None.
+
+Performance Impact
+------------------
+
+IP fragmentation could impact the network efficiency and causes some
+additional load on network nodes
+
+Other deployer impact
+---------------------
+
+It will ease some deployments by being able to span virtual networks over
+MTU-limited physical network.
+
+Developer impact
+----------------
+
+None.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  pierre-rognant
+
+Work Items
+----------
+
+The work concern only the OVS agent. No impact on other neutron component.
+
+Dependencies
+============
+
+None.
+
+Testing
+=======
+
+Ensure that the default behaviour remains unchanged.
+
+
+Documentation Impact
+====================
+
+Add the new 'dont_fragment' flag in the documentation.
+
+
+References
+==========
+
+OVS reference documentation:
+* http://openvswitch.org/ovs-vswitchd.conf.db.5.pdf (p. 24)