Terminal Services

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Terminal Services
Image:Remote desktop connection icon.PNG
Developed by Microsoft
OS Microsoft Windows, Mac OS X
License MS-EULA
Website www.microsoft.com

Terminal Services is one of the components of Microsoft Windows (both server and client versions) that allows a user to access applications and data on a remote computer over any type of network, although normally best used when dealing with either a Wide Area Network (WAN) or Local Area Network (LAN), as ease and compatibility with other types of networks may differ. Terminal Services is Microsoft's implementation of thin-client terminal server computing, where Windows applications, or even the entire desktop of the computer running terminal services, are made accessible from a remote client machine. The client can either be a fully-fledged computer, running any operating system as long as the terminal services protocol is supported, or a bare-bones machine powerful enough to support the protocol (such as Windows FLP). With terminal services, only the user interface of an application is presented at the client. Any input to it is redirected over the network to the server, where all application processing takes place.[1] This is in contrast to appstreaming systems, like Microsoft Softgrid, in which the applications, while still stored on a centralized server, are streamed to the client on-demand and then processed by the client machine.

Contents

[edit] Overview

Terminal Services was first introduced in Windows NT 4.0 Terminal Server Edition. It was significantly improved for Windows 2000 and Windows Server 2003. Both the underlying protocol as well as the service was again overhauled for Windows Vista and Windows Server 2008 [2]. Windows includes two client applications which utilize terminal services: Remote Assistance and Remote Desktop. While the former, available in all versions of Windows, allow one user to assist another user, the latter, which is available in Windows XP Professional or Media Center Edition and Windows Vista Business, Enterprise or Ultimate, allows a user to log in to a remote system and access the desktop, applications and data on the system as well as control it remotely. In the client versions of Windows, Terminal Services supports only one logged in user at a time, whereas in the server operating systems, concurrent remote sessions are allowed.

Microsoft provides the client software Remote Desktop Connection (formerly called Terminal Services Client), available for most 32-bit versions of Windows including Windows Mobile and Apple's Mac OS X, that allows a user to connect to a server running Terminal Services. On Windows, both Terminal Services client and Remote Desktop Protocol use TCP port 3389 by default, which is editable in the Windows registry. It also includes an ActiveX control to embed the functionality in other applications or even a web page.[3] A Windows CE version of the client software is also available.[1] Server versions of Windows OSs also include the Remote Desktop for Administration client (a special mode of the Remote Desktop Connection client), which allows remote connection to the traditional session 0 console of the server. In Windows Vista and later this session is reserved for services and users always log onto session >0. The server functionality is provided by the Terminal Server component, which is able to handle Remote Assistance, Remote Desktop as well as the Remote Administration clients.[1] Third-party developers have created client software for other platforms, including the open source rdesktop client for common Unix platforms.

For an enterprise, Terminal Services allows IT departments to install applications on a central server. For example, instead of deploying database or accounting software on all desktops, the applications can simply be installed on a server and remote users can log on and use them via the Internet. This centralization makes upgrading, troubleshooting, and software management much easier. As long as employees have Remote Desktop software, they will be able to use enterprise software. Terminal Services can also integrate with Windows authentication systems to prevent unauthorized users from accessing the applications or data.

Microsoft has a longstanding agreement with Citrix to facilitate sharing of technologies and patent licensing between Microsoft Terminal Services and Citrix Presentation Server (formerly Citrix MetaFrame) which has now been renamed to Citrix XenApp. In this arrangement, Citrix has access to key source code for the Windows platform enabling their developers to improve the security and performance of the Terminal Services platform. In late December, 2004 the two companies announced a five-year renewal of this arrangement to cover Windows Vista.

[edit] Architecture

The server component of Terminal Services (Terminal Server) listens on TCP port 3389 (termdd.sys. When a RDP client connects to this port, it is tagged with a unique SessionID and associated with a freshly spawned console session (Session 0, keyboard, mouse and character mode UI only). The login subsystem (winlogon.exe) and the GDI graphics subsystem is then initiated, which handles the job of authenticating the user and presenting the GUI. These executables are loaded in a new session, rather than the console session. When creating the new session, the graphics and keyboard/mouse device drivers are replaced with RDP-specific drivers: RdpDD.sys and RdpWD.sys. The RdpDD.sys is the device driver and it captures the UI rendering calls into a format that is transmittable over RDP. RdpWD.sys acts as keyboard and mouse driver; it receives keyboard and mouse input over the TCP connection and presents them as keyboard or mouse inputs. It also allows creation of virtual channels, which allow other devices, such as disc, audio, printers, and COM ports to be redirected, i.e., the channels act as replacement for these devices. The channels connect to the client over the TCP connection; as the channels are accessed for data, the client is informed of the request, which is then transferred over the TCP connection to the application. This entire procedure is done by the terminal server and the client, with the RDP protocol mediating the correct transfer, and is entirely transparent to the applications.[4] RDP communications are encrypted using 128-bit RC4 encryption. Windows Server 2003 onwards, it can use a FIPS 140 compliant encryption scheme.[1]

Once a client initiates a connection and is informed of a successful invocation of the terminal services stack at the server, it loads up the device as well as the keyboard/mouse drivers. The UI data received over RDP is decoded and rendered as UI, whereas the keyboard and mouse inputs to the Window hosting the UI is intercepted by the drivers, and transmitted over RDP to the server. It also creates the other virtual channels and sets up the redirection. RDP communication can be encrypted; using either low, medium or high encryption. With low encryption, only the channels transferring sensitive information like passwords are encrypted. With medium encryption, UI packets are encrypted as well. And with high encryption, keyboard/mouse inputs are also scrambled.[4]

[edit] Terminal Server

Terminal Server is the server component of Terminal services. It handles the job of authenticating clients, as well as making the applications available remotely. It is also entrusted with the job of restricting the clients according to the level of access they have. The Terminal Server respects the configured software restriction policies, so as to restrict the availability of certain software to only a certain group of users. The remote session information is stored in specialized directories, called Session Directory which is stored at the server. Session directories are used to store state information about a session, and can be used to resume interrupted sessions. The terminal server also has to manage these directories. Terminal Servers can be used in a cluster as well.[1]

In Windows Server 2008, it has been significantly overhauled. While logging in, if the user logged on to the local system using a Windows Server Domain account, the credentials from the same sign-on can be used to authenticate the remote session. However, this requires Windows Server 2008 to be the terminal server OS, while the client OS is limited to either Windows Server 2008 or Windows Vista. In addition, the terminal server can provide access to only a single program, rather than the entire desktop, by means of a feature named RemoteApp. Terminal Services Web Access (TS Web Access) makes a RemoteApp session invocable from the web browser. It includes the TS Web Access Web Part control which maintains the list of RemoteApps deployed on the server and keeps the list up to date. A Terminal server session can also be tunneled through a gateway via the TS Gateway service. It makes the server available over the Internet by tunneling the RDP data over a HTTPS channel. Both Windows Server 2008 and Windows Server 2003, including Windows Home Server can act as TS Gateway servers. Terminal Server can also integrate with Windows System Resource Manager to throttle resource usage of remote applications.[2]

Terminal Server is managed by the Terminal Server Manager MMC snap-in. It can be used to configure the sign in requirements,as well as to enforce a single instance of remote session. It can also be configured by using Group Policy or WMI. It is, however, not available in client versions of Windows OS, where the server is pre-configured to allow only one session and enforce the rights of the user account on the remote session, without any customization.[1]

[edit] Remote Desktop Connection

Remote Desktop Connection 6.0 client in Windows XP.
Remote Desktop Connection 6.0 client in Windows XP.

Remote Desktop Connection (RDC, also called Remote Desktop) is the client application for Terminal Services. It allows a user to remotely log in to a networked computer running the terminal services server. RDC presents the desktop interface of the remote system, as if it were accessed locally.[1] With the latest version (version 6.0), if the Desktop Experience component is plugged into the remote server, the chrome of the applications will resemble the local applications, rather than the remote one. In this scenario, the remote applications will support the Aero theme if Windows Vista is used for remoting into the server (provided the local system is running Aero).[2] The RDC client also supports rendering the UI in full 24 bit color, as well as resource redirection for printers, COM ports, disc drives, mice and keyboards. With resource redirection, remote applications are able to use the resources of the local computer. Audio is also redirected, so that any sounds generated by a remote application are played back at the client system.[1][2] In addition to regular username/password for authorizing for the remote session, RDC also supports using smart cards for authorization[1] With RDC 6.0, the resolution of a remote session can be set independently of the settings at the remote computer. In addition, a remote session can also span multiple monitors at the client system, independent of the multi-monitor settings at the server. It also prioritizes UI data as well as keyboard and mouse inputs over print jobs or file transfers so as to make the applications more responsive. It also redirects plug and play devices such as cameras, portable music players, and scanners, so that input from these devices can be used by the remote applications as well.[2] Clipboard is also redirected in the latest version, so that both remote as well as local applications share the same local clipboard, thus enabling data to be copy-and-pasted between local and remote applications.[2] RDC can also be used to connect to WMC remote sessions; however, since WMC streams does not stream video using Remote Desktop Protocol, only the applications can be viewed this way, not any media. RDC can also be used to connect to computers, which are exposed via Windows Home Server RDP Gateway over the Internet.

[edit] RemoteApp

RemoteApp (or TS RemoteApp) is a special mode of Terminal Services, available only in Remote Desktop Connection 6.1 and above (with Windows Server 2008 being the RemoteApp server), where a remote session connects to a specific application only rather than the entire Windows desktop. The RDP 6.1 client ships with Windows XP SP3, KB952155 for Windows XP SP2 users [5], Windows Vista SP1 and Windows Server 2008. The UI for the RemoteApp is rendered in a window over the local desktop, and is managed like any other window for local applications. The end result of this is that remote applications behave largely like local applications. The task of establishing the remote session, as well as redirecting local resources to the remote application, is transparent to the end user.[6] Multiple applications can be started in a single RemoteApp session, each with their own windows.[7]

A RemoteApp can be packaged either as a .rdp file or distributed via an .msi Windows Installer package. When packaged as an .rdp file (which contains the address of the RemoteApp server, authentication schemes to be used, and other settings), a RemoteApp can be launched by double clicking the file. It will invoke the Remote Desktop Connection client, which will connect to the server and render the UI. The RemoteApp can also be packaged in an Windows Installer database, installing which can register the RemoteApp in the Start Menu as well as create shortcuts to launch it. A RemoteApp can also be registered as handlers for filetypes or URIs. Opening a file registered with RemoteApp will first invoke Remote Desktop Connection, which will connect to the terminal server and the open the file. Any application, which can be accessed over Remote Desktop, can be served as a RemoteApp.[6]

[edit] Windows Desktop Sharing

Windows Vista onwards, Terminal Services also includes a multi-party desktop sharing capability known as Windows Desktop Sharing. Unlike Terminal Services, which creates a new user session for every RDP connection, Windows Desktop Sharing can host the remote session in the context of the currently logged in user without creating a new session, and make the Desktop, or a subset of it, available over Remote Desktop Protocol.[8] Windows Desktop Sharing can be used to share the entire desktop, a specific region, or a particular application.[9] Windows Desktop Sharing can also be used to share multi-monitor desktops. When sharing applications individually (rather than the entire desktop), the windows are managed (whether they are minimized or maximized) independently at the server and the client side.[9]

The functionality is only provided via a public API, which can be used by any application to provide screen sharing functionality. Windows Desktop Sharing API exposes two objects: RDPSession for the sharing session and RDPViewer for the viewer. Multiple viewer objects can be instantiated for one Session object. A viewer can either be a passive viewer, who is just able to watch the application like a screencast, or an interactive viewer, who is able to interact in real time with the remote application.[8] The RDPSession object contains all the shared applications, represented as Application objects, each with Windowobjects representing their on-screen windows. Per-application filters capture the application Windows and package them as Window objects.[10] A viewer must authenticate itself before it can connect to a sharing session. This is done by generating an Invitation using the RDPSession. It contains an authentication ticket and password. The object is serialized and sent to the viewers, who need to present the Invitation when connecting.[8][10]

Windows Desktop Sharing API is used by Windows Meeting Space for providing application sharing functionality among peers; however, the application does not expose all the features supported by the API.[9] It is also used by Remote Assistance.

[edit] See also

[edit] References

  1. ^ a b c d e f g h i Technical Overview of Terminal Services in Windows Server 2003. Retrieved on 2007-07-23.
  2. ^ a b c d e f Whats new in Terminal Services in Windows Server 2008. Retrieved on 2007-07-23.
  3. ^ Frequently Asked Questions about Remote Desktop. Retrieved on 2007-07-23.
  4. ^ a b How Terminal Services Works. Retrieved on 2007-07-23.
  5. ^ Description of the Remote Desktop Connection 6.1 client update for Terminal Services in Windows XP Service Pack 2
  6. ^ a b Terminal Services RemoteApp (TS RemoteApp). Retrieved on 2007-07-23.
  7. ^ Terminal Services RemoteApp Session Termination Logic. Retrieved on 2007-10-02.
  8. ^ a b c Windows Desktop Sharing. Retrieved on 2007-10-11.
  9. ^ a b c Windows Desktop Sharing API. Retrieved on 2007-10-11.
  10. ^ a b About Windows Desktop Sharing. Retrieved on 2007-10-11.

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