10.1. The Remote Desktop Protocol The Remote Desktop Protocol (RDP) is the protocol that drives Terminal Services. RDP is based on and is an extension of the T.120 protocol family of standards. It is a multichannel-capable protocol that allows for separate virtual channels for carrying device communication and presentation data from the server, as well as encrypted client mouse and keyboard data. RDP provides a very extensible base from which to build many additional capabilities, supporting up to 64,000 separate channels for data transmission as well as provisions for multipoint transmission. Figure 10-1 shows the structure of RDP and its functionality from a high-level perspective. Figure 10-1. An overview of RDP 
The new Terminal Services client software included in Windows Server 2003 (Remote Desktop Connection, or RDC) uses RDP 5.2, and many of the local resources are available within the remote session: the client drives, smart cards, audio card, serial ports, printers (including network), and clipboard. Additionally, you can select color depth from 256 colors (8-bit) to True Color (24-bit) and resolution from 640x480 up to 1600x1200. RDP basically takes instructions from a terminal server host machine on screen images and draws them onto a client's screen, refreshing that image about 20 times every second if there's activity on the client side. (To save bandwidth, if no activity is detected on the client side, it cuts the refresh rate in half.) It then notes any keyboard and mouse activity (among other things) and relays those signals to the terminal server host machine for processing. This two-way exchange of information is wrapped into what's called a session, which consists of the programs running on the host machine and the information being sent over RDP between the terminal server and the client machine. What Happened to Citrix? When Microsoft introduced its revamped version of Terminal Services in Windows 2000, many predicted the demise of Citrix's venerable MetaFrame product. Citrix basically invented the idea of multiple sessions on one Windows host, and RDP is loosely based on Citrix's protocol, Independent Computing Architecture (ICA). MetaFrame traditionally has sat atop Terminal Services, using its basic functionality as a foundation and adding useful features on top. But many people have asked what with all the improvements to RDP, including bandwidth reduction, client-side caching, device mapping, sound redirection, and increased color depth, why would a corporation continue to buy Citrix's flagship product? A feature known as Seamless Windows one-to-many connections is available with MetaFrame, but not with raw Terminal Services. For the client, this means applications you see are displayed without having to scroll or use full-screen mode. On the server, though, it's even better: all sessions on the same server that use Seamless Windows are operating out of the same physical instance of the program, saving on hardware resources. Another hot item is application publishing, which uses a Citrix concept called Program Neighborhood to direct a user to applications via a menu of program options. The user doesn't have to know which servers run those applications with Citrix because he can just choose a program from the menu and be connected directly to it, but to use native Terminal Services and RDP, he would need to find the appropriate server and connect to it. Additional core features of MetaFrame distinguish it from native Terminal Services. And as always, Citrix releases new features in its Service Releases. Larger organizations that use Terminal Services connections extensively might want to investigate how MetaFrame might improve their efficiency. However, most organizations, especially small businesses, will find that the initially free Terminal Services components of Windows Server 2003 (you pay only for licensing and not the initial software purchase) will suffice for their needs. |
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