When direct communication between devices, such as via Bluetooth, is unavailable, a communication protocol is used by an initiating communication device and/or a recipient communication device to establish a communication session via a cloud-based relay server. This communication protocol efficiently allocates (or binds) the initiating communication device, the recipient communication device, and the relay server together for the communication session. The communication protocol may include handshaking messages that specify a transport type and channel used for connections in the communication session, as well as an address of the relay server. After the communication session is confirmed, the communication protocol allows the initiating communication device and the recipient communication device to share messages even when physically separated from each other.