See issue #6079 as the roll-up for node upgrades.

This component of the node upgrade mechanism should:

• query the versions of the nodes
• determine what version nodes should be
• determine the subset of the nodes to upgrade
• determine which version those nodes should be upgraded to
• pass the results to the rest of the upgrade mechanism and continue

Blocked by #4855:

• a concrete definition of a node spec: a version tuple of (kubelet, kube-proxy, docker, kernel/OS)
• the ability to query the versions of the nodes

References:

• The most relevant discussion regarding the node version spec and the allowed upgrade path is Cluster versioning #4855.
• Adding node components version to the API was added in Add node components version to API #5951.