Integration of Containers with OVN and OpenStack

Isolation between containers is weaker than isolation between VMs, so some environments deploy containers for different tenants in separate VMs as an additional security measure. This document describes creation of containers inside VMs and how they can be made part of the logical networks securely. The created logical network can include VMs, containers and physical machines as endpoints. To better understand the proposed integration of containers with OVN and OpenStack, this document describes the end to end workflow with an example.

A OpenStack tenant creates a VM (say VM-A) with a single network interface that belongs to a management logical network. The VM is meant to host containers. OpenStack Nova chooses the hypervisor on which VM-A is created.
A Neutron port may have been created in advance and passed in to Nova with the request to create a new VM. If not, Nova will issue a request to Neutron to create a new port. The ID of the logical port from Neutron will also be used as the vif-id for the virtual network interface (VIF) of VM-A.
When VM-A is created on a hypervisor, its VIF gets added to the Open vSwitch integration bridge. This creates a row in the Interface table of the Open_vSwitch database. As explained in the integration guide, the vif-id associated with the VM network interface gets added in the external_ids:iface-id column of the newly created row in the Interface table.
Since VM-A belongs to a logical network, it gets an IP address. This IP address is used to spawn containers (either manually or through container orchestration systems) inside that VM and to monitor the health of the created containers.
The vif-id associated with the VM’s network interface can be obtained by making a call to Neutron using tenant credentials.
This flow assumes a component called a “container network plugin”. If you take Docker as an example for containers, you could envision the plugin to be either a wrapper around Docker or a feature of Docker itself that understands how to perform part of this workflow to get a container connected to a logical network managed by Neutron. The rest of the flow refers to this logical component that does not yet exist as the “container network plugin”.
All the calls to Neutron will need tenant credentials. These calls can either be made from inside the tenant VM as part of a container network plugin or from outside the tenant VM (if the tenant is not comfortable using temporary Keystone tokens from inside the tenant VMs). For simplicity, this document explains the work flow using the former method.
The container hosting VM will need Open vSwitch installed in it. The only work for Open vSwitch inside the VM is to tag network traffic coming from containers.
When a container needs to be created inside the VM with a container network interface that is expected to be attached to a particular logical switch, the network plugin in that VM chooses any unused VLAN (This VLAN tag only needs to be unique inside that VM. This limits the number of container interfaces to 4096 inside a single VM). This VLAN tag is stripped out in the hypervisor by OVN and is only useful as a context (or metadata) for OVN.
The container network plugin then makes a call to Neutron to create a logical port. In addition to all the inputs that a call to create a port in Neutron that are currently needed, it sends the vif-id and the VLAN tag as inputs.
Neutron in turn will verify that the vif-id belongs to the tenant in question and then uses the OVN specific plugin to create a new row in the Logical_Switch_Port table of the OVN Northbound Database. Neutron responds back with an IP address and MAC address for that network interface. So Neutron becomes the IPAM system and provides unique IP and MAC addresses across VMs and containers in the same logical network.
The Neutron API call above to create a logical port for the container could add a relatively significant amount of time to container creation. However, an optimization is possible here. Logical ports could be created in advance and reused by the container system doing container orchestration. Additional Neutron API calls would only be needed if the port needs to be attached to a different logical network.
When a container is eventually deleted, the network plugin in that VM may make a call to Neutron to delete that port. Neutron in turn will delete the entry in the Logical_Switch_Port table of the OVN Northbound Database.

As an example, consider Docker containers. Since Docker currently does not have a network plugin feature, this example uses a hypothetical wrapper around Docker to make calls to Neutron.

Create a Logical switch:
```
$ ovn-docker --cred=cca86bd13a564ac2a63ddf14bf45d37f create network LS1
```
The above command will make a call to Neutron with the credentials to create a logical switch. The above is optional if the logical switch has already been created from outside the VM.

List networks available to the tenant:

$ ovn-docker --cred=cca86bd13a564ac2a63ddf14bf45d37f list networks

Create a container and attach a interface to the previously created switch as a logical port:
```
$ ovn-docker --cred=cca86bd13a564ac2a63ddf14bf45d37f --vif-id=$VIF_ID \
    --network=LS1 run -d --net=none ubuntu:14.04 /bin/sh -c \
    "while true; do echo hello world; sleep 1; done"
```
The above command will make a call to Neutron with all the inputs it currently needs to create a logical port. In addition, it passes the $VIF_ID and a unused VLAN. Neutron will add that information in OVN and return back a MAC address and IP address for that interface. ovn-docker will then create a veth pair, insert one end inside the container as ‘eth0’ and the other end as a port of a local OVS bridge as an access port of the chosen VLAN.