The Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft. However, there are a number of open-source implementations of RDP, including:

• FreeRDP: FreeRDP is a free and open-source implementation of the RDP protocol. It is available for Windows, macOS, Linux, iOS, and Android. FreeRDP is a good option if you are looking for a flexible and customizable remote desktop solution.
  
• Remmina: Remmina is a free and open-source remote desktop client that supports a variety of remote desktop protocols, including RDP, VNC, and SSH. It is available for Windows, macOS, and Linux. Remmina is a good option if you are looking for a versatile remote desktop client that can be used with a variety of remote machines.
  
• xrdp: xrdp is an open-source implementation of the RDP server. It is available for Linux and BSD-based operating systems. xrdp is a good option if you are looking for a way to make your Linux or BSD machine accessible via RDP.
  

These are just a few of the many open-source implementations of RDP. The best option for you will depend on your specific needs and preferences.

It is important to note that while the RDP protocol itself is not open source, the implementations of the protocol that are available are free to use and modify. This means that you can use them to create your own remote desktop solutions or to customize the existing solutions to meet your specific needs.