The BasicMessage protocol describes a stateless, easy to support user message protocol. It has a single message type used to communicate.
This protocol is defined here: https://github.com/hyperledger/aries-rfcs/tree/main/features/0095-basic-message
Note: as pointed in the abovementioned link, the version of this protocol is 1.0 and was defined initially in Hyperledger Aries using DIDComm v1 headers/decorators. Readers should be aware that there’s a new 2.0 version of this protocol that also complies with DIDComm v2 headers. Version 2.0 can be found here