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Nova-LXD VIF Design Notes
VIF plugging workflow
Nova-LXD makes use of the os-vif interface plugging library to wire LXD instances into underlying Neutron networking; however there are some subtle differences between the Nova-Libvirt driver and the Nova-LXD driver in terms of how the last mile wiring is done to the instances.
In the Nova-Libvirt driver, Libvirt is used to start the instance in a paused state, which creates the required tap device and any required wiring to bridges created in previous os-vif plugging events.
The concept of 'start-and-pause' does not exist in LXD, so the driver creates a veth pair instead, allowing the last mile wiring to be created in advance of the actual LXD container being created.
This allows Neutron to complete the underlying VIF plugging at which point it will notify Nova and the Nova-LXD driver will create the LXD container and wire the pre-created veth pair into its profile.
tap/tin veth pairs
The veth pair created to wire the LXD instance into the underlying Neutron networking uses the tap and tin prefixes; the tap named device is present on the host OS, allowing iptables based firewall rules to be applied as they are for other virt drivers, and the tin named device is passed to LXD as part of the container profile. LXD will rename this device internally within the container to an ethNN style name.
The LXD profile devices for network interfaces are created as 'physical' rather than 'bridged' network devices as the driver handles creation of the veth pair, rather than LXD (as would happen with a bridged device).
LXD profile interface naming
The name of the interfaces in each containers LXD profile maps to the devname provided by Neutron as part of VIF plugging - this will typically be of the format tapXXXXXXX. This allows for easier identification of the interface during detachment events later in instance lifecycle.
Prior versions of the nova-lxd driver did not take this approach; interface naming was not consistent depending on when the interface was attached. The legacy code used to detach interfaces based on MAC address is used as a fallback in the event that the new style device name is not found, supporting upgraders from previous versions of the driver.
Supported Interface Types
The Nova-LXD driver has been validated with:
- OpenvSwitch (ovs) hybrid bridge ports.
- OpenvSwitch (ovs) standard ports.
- Linuxbridge (bridge) ports