In the below diagram, PC1 is setup as the Remote desktop client and PC2 is setup as the Remote desktop server. PC1 initiates the connection to the remote desktop server. The communication is captured with wireshark, which is displayed below.
The capture screen shot shows the initial connection which the remote desktop client initiates. It can be observed that the client initiates TCP Syn connection to the destination port number (Dst port in the capture) 3389.
Remote Desktop Servers work on TCP port 3389.