In order to address the cross-machine communication issue and allow greater flexibility, Docker also supports user-defined networks via network plugins. These networks exist independent of the containers themselves. In this way, containers can join the same existing networks. Through the new plugin architecture, various drivers can be provided for different network use cases such as the following:

• Swarm: In a clustered situation with Swarm, the default behavior is an overlay network, which allows you to connect multiple Docker daemons running on multiple machines. In order to coordinate across multiple hosts, all containers and daemons must all agree on the available networks and their topologies. Overlay networking introduces a significant amount of complexity with dynamic port mapping that Kubernetes avoids.

• Macvlan: Docker also provides macvlan addressing, which is most similar to the networking model that Kubernetes provides, as it assigns each Docker container a MAC address that makes it appear as a physical device on your network. Macvlan offers a more efficient network virtualization and isolation as it bypasses the Linux bridge. It is important to note that as of this book's publishing, Macvlan isn't supported in most cloud providers.

As a result of these options, Docker must manage complex port allocation on a per-machine basis for each host IP, and that information must be maintained and propagated to all other machines in the cluster. Docker users a gossip protocol to manage the forwarding and proxying of ports to other containers.