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.

The new Terminal Services client software included in Windows Server 2008 (Remote Desktop Connection, or RDC) uses RDP 6.0, and many 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 640 x 480 up to 1,600 x 1,200. 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.
Here's what's new in Remote Desktop Connection 6.0:

Network level authentication (NLA) and server authentication
NLA is a new way for the RDC client to authenticate the user, client machine, and server against one another, thus removing the authentication transaction from the RDP process. Server authentication uses Transport Layer Security, or TLS, to match a server's true identity against the one it's projecting. This way clients can be sure that they're indeed talking to a real server and not a malconfigured, "owned" machine that may be posing as the real server in order to receive sensitive data.

Display improvements
You'll find that now, RDP sessions can support a maximum resolution of 4,096 x 2,048 with additional support for widescreen monitor scenarios. You can also span a session across multiple monitors if you have the hardware installed, and on all of these sessions you can get 32-bit full color depth and ClearType font smoothing.

Display data prioritization
This allows RDP to give more priority to data used to draw your RDP display during bandwidth-intensive operations like transferring large files or printing a big document, eliminating the herky-jerky user experience found in previous versions of RDP while carrying out these operations. By default, 70% of available bandwidth is used for display data and 30% for the remainder of session data. You can change this in the Registry at HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\TermDD; the two keys are FlowControlDisplayBandwidth for the display data and FlowControlChannelBandwidth for everything else.

Desktop experience and composition
This feature allows users to get the look and feel of a regular Windows Vista host, including various desktop themes and access to Windows Media Player, that were unavailable under previous versions of RDP and Terminal Services. You can access this feature and enable it by using the Add Feature selection in Server Manager; the correct entry to select is "Desktop Experience." This is entirely server-based, so no client configuration is necessary.

Plug and Play Device Redirection Framework
This feature allows you to redirect PnP device interaction from the local RDP client to the server-based session, so the user sees the same seamless user interface for these devices regardless of whether they run locally or remotely. PnP devices in a remote session are limited in scope so that they are only accessible to that session.

Terminal Services EasyPrint
TS EasyPrint removes the need to install printer drivers on the TS host in the vast majority of cases by taking advantage of the new XPS print path that was introduced in Windows Vista and Windows Server 2008, acting as a proxy and redirecting all calls for the user interface to the print driver installed on the client. Users printing from within a session will see printing progress as they expect and can even adjust printer properties as necessary.

Single Sign-On
New to Windows Server 2008, users that are logged on to a domain can gain access to a domain-joined Terminal Server machine without needing to enter credentials a second time. This feature, however, only works with the Windows Vista-Windows Server 2008 client-server duo.
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Source of Information : OReilly Windows Server 2008 The Definitive Guide

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