RTPS - what is it?
RTPS (Real Time Publish Subscribe) is a spec that describes a pub/sub based messaging system and wire protocol. It can run on any transport capable of delivering datagrams (delimited, uncorrupted packets), so UDP is a good fit.
An RTPS system consists of a network of participants (there is no master node). A participant is a grouping of topic publications and subscriptions - it usually corresponds one to one with a process.