3.1.5 Message Processing Events and Sequencing Rules
The protocol has two classes of messages, protocol initialization messages, and data transmission messages. Protocol initialization messages are always sent from the RDP server session to the RDP client when the session is first established or has reconnected. The client responds with the appropriate data transmission message if it has any cached data. The client MUST never send data transmission messages to the server except in response to a protocol initialization message.
After the protocol initialization message has been received, the server MUST send a data transmission message to the client every time the data is changed in the RDP session. When the client receives a data transmission message, it MUST discard any old cached copy of the data and cache the current data.