A reference architecture

There are two major plugin types within the Neutron architecture:

The ML2 plugin relies on different types of drivers to determine the types of networks to implement and the mechanisms used to implement them. Type drivers describe different types of network supported by Neutron, including flat, VLAN, VXLAN, GRE and local. Mechanism drivers are used to implement the described networks in software or on physical hardware.

Third-party vendors have implemented support for their respective network technologies by developing their own plugins that implement the Neutron API and extend network services. Vendors including Cisco, Arista, Brocade, Radware, F5, and VMware have created plugins that allow Neutron to interface with OpenFlow controllers, load balancers, switches, and other physical and virtual network hardware. While third-party drivers are outside the scope of this book, we will cover some of the common type and mechanism drivers in Chapter 5, Switching.

The Neutron server is the centralized controller of the network and is responsible for providing an API to users and storing information about the network in the database. However, the actual commands to implement the network are executed on the compute and network nodes by agents that run on those nodes. Neutron agents receive messages and instructions from the Neutron server on the message bus and execute the changes accordingly.

The Dynamic Host Configuration Protocol (DHCP) is a protocol used for dynamically distributing network configuration parameters, such as IP addresses and routes, to network interfaces. Many cloud instances require the use of DHCP to acquire their IP address and other network information. Neutron is capable of providing DHCP services to all networks created in the cloud, and it uses a DHCP agent to manage those services. In a reference implementation, a Neutron DHCP agent runs on one or more infrastructure nodes and spawns a dnsmasq process for each network where DHCP is enabled.

OpenStack provides metadata services, which enable users to retrieve information about their instances that can then be used to configure or manage the running instance. Metadata includes information such as the hostname, fixed and floating IPs, and public SSH keys. In addition to metadata, users can access user data and scripts that are provided during the launching of an instance and are executed during the boot process.

The Neutron metadata agent proxies requests from instances to the Nova metadata API, and it is accessible to instances via http://169.254.169.254/metadata.

The Neutron plugin agents are services that run on compute and network nodes and are responsible for configuring and implementing the virtual network on the local node. Plugin agents listen for messages from the Neutron server and construct the local network based on information in those messages. An example of how the agents work together with the Neutron server to build the virtual network can be observed in the following diagram:

In the preceding diagram, the following actions take place among various Neutron components: