From my understanding nodes are able to communicate with each other to send and receive information such as newer block info etc...
I would like to understand what those informations are, and why can nodes cannot make RPC call to other nodes.
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Eth client's will use Devp2p protocol. Devp2p nodes communicate by sending messages using RLPx, an encrypted and authenticated transport protocol. Peers are free to advertise and accept connections on any TCP ports they wish, however, a default port on which the connection may be listened and made will be 30303.
Devp2p nodes communicate in terms of packets. RLPx provides facilities to send and receive packets. For more information about RLPx, refer to the protocol specification.
Peers will share current version of ledger, and newly updated blocks headers, Block info (merkle tree), (state's, storage),bootstrap nodes, new nodes, and status's
Please check below link for more details: