850f8d8210
This blueprint spec provides information about a new option for the OVS agent. It introduces a dont_fragment option that allow OVS tunnel -GRE and VXLAN- to be fragmented at the IP layer. It will permit to carry virtual network with a bigger (or equal) MTU than the underlying physical network. Change-Id: I5a490469f270bbcd014f3f8fb55cf024ff7f3e00
2.6 KiB
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)