This application discloses communications methods, apparatuses, and systems. In one implementation, a method includes: performing a function of a control-plane protocol layer, wherein the control-plane protocol layer comprises a first protocol layer of the network node corresponding to a first protocol layer of a relay node, the first protocol layer is on an interface between the network node and the relay node, and the network node is a donor base station or a centralized unit of a donor base station; and sending, from the first protocol layer of the network node, a first message to the first protocol layer of the relay node, wherein the first protocol layer of the network node and the first protocol layer of the relay node are F1 application protocol layers, and the first message is a F1AP message that comprises one or more of context management information of a terminal device served by the relay node or a radio resource control message of the terminal device.