Let's say that we have many blockchain nodes managed by parity clients. what is the protocol used in communication between those nodes?
The Ethereum wire protocol is called ÐΞVp2p. It consists of a DHT discovery part, derived from Kademlia, and a sync part, which is based on RLP-encoded encrypted messages.
Peer-to-peer communications between nodes running Ethereum clients run using the underlying ÐΞVp2p Wire Protocol.