What is Horizontal Relay-Routed Message Passing (HRMP)?

Horizontal Relay-Routed Message Passing (HRMP) is a solution built for the Kusama and Polkadot networks that helps parachains communicate and share data with each other. In May 2022, HRMP was replaced by the more advanced Cross-Chain Message Passing (XCMP) technology. HRMP was more computationally demanding than XCMP because it stored all messages in Polkadot's Relay Chain. HRMP and XCMP are both designed to help facilitate cross-chain communication and the transfer of assets and other data from one blockchain protocol to another.