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