Remote Desktop Protocol
Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft, which provides a user with a graphical interface to connect to another computer over a network connection. The user employs RDP client software for this purpose, while the other computer must run RDP server software.
Clients exist for most versions of Microsoft Windows (including Windows Mobile), Linux, Unix, Mac OS X, iOS, Android, and other modern operating systems. RDP servers are built into Windows operating systems; an RDP server for Linux also exists. By default, the server listens on TCP port 3389.
Microsoft currently refers to their official RDP server software as Remote Desktop Services, formerly "Terminal Services". Their official client software is currently referred to as Remote Desktop Connection, formerly "Terminal Services Client".
Every Windows version beginning with Windows XP (excluding 'Home' edition) includes an installed Remote Desktop Connection (RDC) ("Terminal Services") client (mstsc.exe) whose version is determined by that of the operating system or last applied Windows Service Pack. The Terminal Services server is supported as an official feature on Windows NT 4.0 Terminal Server Edition, Windows 2000 Server, all editions of Windows XP except Windows XP Home Edition, Windows Server 2003, Windows Home Server, on Windows Fundamentals for Legacy PCs, in Windows Vista Ultimate, Enterprise and Business editions, Windows Server 2008 and Windows Server 2008 R2 and on Windows 7 Professional and above.
Microsoft provides the client required for connecting to newer RDP versions for downlevel operating systems. Since the server improvements are not available downlevel, the features introduced with each newer RDP version only work on downlevel operating systems when connecting to a higher version RDP server from these older operating systems, and not when using the RDP server in the older operating system.
Version 4.0 
Based on the ITU-T T.128 application sharing protocol (during draft also known as "T.share") from the T.120 recommendation series, the first version of RDP (named version 4.0) was introduced by Microsoft with "Terminal Services", as a part of their product Windows NT 4.0 Server, Terminal Server Edition. The Terminal Services Edition of NT 4.0 relied on Citrix's MultiWin technology, previously provided as a part of Citrix WinFrame atop Windows NT 3.51, in order to support multiple users and login sessions simultaneously. Microsoft required Citrix to license their MultiWin technology to Microsoft in order to be allowed to continue offering their own terminal services product, then named Citrix MetaFrame, atop Windows NT 4.0. The Citrix provided DLLs included in Windows NT 4.0 Terminal Services Edition still carry a Citrix copyright rather than a Microsoft copyright. Later versions of Windows integrated the necessary support directly. The T.128 application sharing technology was acquired by Microsoft from UK software developer Data Connection Limited.
Version 5.0 
This version was introduced with Windows 2000 Server, added support for a number of features, including printing to local printers, and aimed to improve network bandwidth usage.
Version 5.1 
This version was introduced with Windows XP Professional, included support for 24-bit color and sound. The client is available for Windows 2000, Windows 9x, Windows NT 4.0. With this version, the name of the client was changed from Terminal Services Client to Remote Desktop Connection.
Version 5.2 
This version was introduced with Windows Server 2003, included support for console mode connections, a session directory, and local resource mapping. It also introduces Transport Layer Security (TLS) 1.0 for server authentication, and to encrypt terminal server communications. This version is built into Windows XP Professional x64 Edition and Windows Server 2003 x64 & x86 Editions.
Version 6.0 
This version was introduced with Windows Vista and incorporated support for Windows Presentation Foundation applications, Network Level Authentication, multi-monitor spanning and large desktop support, and support for TLS 1.0 connections.  Version 6.0 client is available for Windows XP SP2, Windows Server 2003 SP1/SP2 (x86 and x64 editions) and Windows XP Professional x64 Edition. Microsoft Remote Desktop Connection Client for Macintosh OS X is also available with support for Intel and PowerPC Mac OS versions 10.4.9 and greater.
Version 6.1 
This version was released in February 2008 and is included with Windows Server 2008, as well as with Windows Vista Service Pack 1. The client is included with Windows XP Service Pack 3. It is also installable through KB952155 for Windows XP SP2. In addition to changes related to how a remote administrator connects to the "console", this version incorporates new functionality introduced in Windows Server 2008, such as connecting remotely to individual programs and a new Terminal Services Easy Print driver, a new client-side printer redirection system that makes the client's full print capabilities available to applications running on the server, without having to install print drivers on the server.
Version 7.0 
This version was released to manufacturing in July 2009 and is included with Windows Server 2008 R2, as well as with Windows 7. With this release, the server name was also changed from Terminal Services to Remote Desktop Services. This version incorporates new functionality such as Windows Media Player redirection, bidirectional audio, true multimonitor support, Aero glass support, enhanced bitmap acceleration (which improves user experience over high latency network connections), Easy Print redirection, Language Bar docking. The RDP 7.0 client is available on Windows XP SP3 and Windows Vista SP1/SP2. RDP 6.1 client and RDP 7.0 client are not supported on Windows Server 2003 x86 and Windows Server 2003 / Windows XP Professional x64 editions. RDP 7.0 clients also do not support connecting to terminal servers running Windows 2000 Server.
Most RDP 7.0 features like Aero glass remoting, bidirectional audio, Windows Media Player redirection, true multiple monitor support, Remote Desktop Easy Print are only available in Windows 7 Enterprise or Ultimate editions.
Version 7.1 
This version appeared in Windows 7 SP1 and Server 2008 R2 SP1. It adds RemoteFX functionality.
Version 8.0 
This version appeared in Windows 8 and Windows Server 2012. The RDP 8.0 client is also available as an add-on for Windows 7 SP1 and Windows Server 2008 R2 SP1. Server 2012 can support upto rdc 6.0 .
- 32-bit color support. 8-, 15-, 16-, and 24-bit color are also supported.
- 128-bit encryption, using the RC4 encryption algorithm, as of Version 6.
- Audio Redirection allows users to run an audio program on the remote desktop and have the sound redirected to their local computer.
- File System Redirection allows users to use their local files on a remote desktop within the terminal session.
- Printer Redirection allows users to use their local printer within the terminal session as they would with a locally or network shared printer.
- Port Redirection allows applications running within the terminal session to access local serial and parallel ports directly.
- The clipboard can be shared between the remote computer and the local computer.
- Seamless Windows: Remote applications can run on a client machine that is served by a Remote Desktop connection. It is available since RDP 6.
- RemoteFX: RemoteFX provides virtualized GPU support and host side encoding and is being shipped as part of Windows Server 2008 R2 SP1.
The following features were introduced with the release of RDP 6.0 in 2006:
- Remote Programs: Application publishing with client-side file type associations.
- Terminal Services Gateway: Enables the ability to use a front-end IIS server to accept connections (over port 443) for back-end Terminal Services servers via an https connection, similar to how RPC over https allows Outlook clients to connect to a back-end Exchange 2003 server. Requires Windows Server 2008
- Network Level Authentication
- Support for remoting the Aero Glass Theme (or Composed Desktop), including ClearType font smoothing technology.
- Support for remoting of Windows Presentation Foundation applications: Compatible clients that have .NET Framework 3.0 support will be able to display full Windows Presentation Foundation effects on a local machine.
- Rewrite of device redirection to be more general-purpose, allowing a greater variety of devices to be accessed.
- Fully configurable and scriptable via Windows Management Instrumentation.
- Improved bandwidth tuning for RDP clients.
- Support for Transport Layer Security (TLS) 1.0 on both server and client ends (can be negotiated if both parties agree, but is not mandatory in a default configuration of any version of Windows).
- Multiple monitor support for allowing one session to use multiple monitors on the client (disables desktop composition)
Security issues 
RDP sessions are also susceptible to in-memory credential harvesting, which can be used to launch pass the hash attacks.
In March 2012, Microsoft released an update for a critical security vulnerability in the RDP protocol. The vulnerability allowed a Windows computer to be compromised by unauthenticated clients and computer worms.
Non-Microsoft implementations 
There are numerous non-Microsoft implementations of RDP clients and servers that implement subsets of the Microsoft functionality. For instance, the open-source command-line client rdesktop is available for Linux/Unix and Microsoft Windows operating systems. There are many GUI clients, like tsclient and KRDC, that are built on top of rdesktop. In 2009, rdesktop was forked as FreeRDP, a new project aiming at modularizing the code, addressing various issues, and implementing new features. FreeRDP comes with its own command-line-client xfreerdp, which supports Seamless Windows in RDP6. There’s also a GTK-Application named Remmina.
An open-source implementation of a Remote Desktop Protocol server on Linux is xrdp. The Windows' Remote Desktop Connection client can be used to connect to a server running xrdp. Proprietary RDP client solutions such as rdpclient are available as a stand-alone application or embedded with client hardware.
A new access paradigm, browser-based access, has enabled users to access Windows desktops and applications on any RDP hosts, such as Microsoft Remote Desktop (RDS) Session Hosts (Terminal Services) and virtual desktops. There are now solutions that run directly within a browser, for example Ericom AccessToGo which is a free RDP client.
There is also a so-called VRDP protocol used in the VirtualBox virtual machine implementation by Oracle. This protocol is compatible with all RDP clients, such as that provided with Windows but, unlike the original RDP protocol, can be configured to accept unencrypted and password unprotected connections, which may be useful in secure and trusted networks, such as home or office LANs. By default, Microsoft's RDP server refuses connections to user accounts with empty passwords (but this can be changed with the Group Policy Editor). External and guest authorization options are provided by VRDP as well. It does not matter which operating system is installed as a guest because VRDP is implemented on the virtual machine (host) level, not in the guest system. The proprietary VirtualBox Extension Pack is required.
See also 
- Comparison of remote desktop software
- Ulteo OVD
- Citrix ICA
- Desktop virtualization
- Virtual Network Computing (RFB protocol)
- Virtual private server
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- Microsoft. "Connect to another computer using Remote Desktop Connection". Retrieved 2012-12-22.
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- Using Remote Desktop Easy Print in Windows 7 and Windows Server 2008 R2
- Announcing the availability of Remote Desktop Connection 7.0 for Windows XP SP3, Windows Vista SP1, and Windows Vista SP2
- Remote Desktop Connection Client 7.0 does not support accessing Windows 2000 servers
- Aero Glass Remoting in Windows Server 2008 R2
- Remote Desktop Connection 7 for Windows 7, Windows XP & Windows Vista
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- "Configuring Terminal Servers for Server Authentication to Prevent "Man in the Middle" Attacks". Microsoft. July 12, 2008.
- "Microsoft Security Bulletin MS12-020 - Critical". Microsoft. 13 March 2012. Retrieved 16 March 2012.
- Jelle Bens: Windows 7 RDP with blank password
- Microsoft Remote Desktop Connection for Mac
- Technical Overview of Terminal Services - a more detailed feature list.
- Remote Desktop Protocol - from Microsoft's Developer Network
- Understanding the Remote Desktop Protocol - from support.microsoft.com
- MS-RDPBCGR: Remote Desktop Protocol: Basic Connectivity and Graphics Remoting Specification - from Microsoft's Developer Network