Seen in Docker 1.12-rc2 logs when using Weave network (as an example of a remote driver; this issue does not appear to have anything to do with Weave):

Jun 25 20:57:53 host1 docker[15099]: time="2016-06-25T20:57:53.409535574Z" level=warning msg="Failed to set datapath keys in driver weavemesh: Unknown discovery type : 3"

This appears to be because agent.go calls DiscoverNew with type EncryptionKeysUpdate but the remote glue code errors on anything that is not NodeDiscovery.