Independent Computing Architecture

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Independent Computing Architecture ( ICA ), like Microsoft's RDP, is a proprietary protocol for a terminal server / application service providing system that was developed by Citrix Systems . The protocol defines a specification for transferring data between server and clients, but is not tied to any particular platform.

The ICA-compliant application products include the Citrix XenApp editions. These enable normal Windows programs to run on a suitable Windows server (or, more uncommonly: Unix derivative programs on a suitable [e.g. Solaris] Unix derivative server) and with every supported client access these applications. The client platform does not need to run under Windows. There are clients for Macintosh and Unix , for example .

ICA is broadly similar to window servers such as the X Window System , but has a broader range of uses. However, it also ensures the retransmission of user inputs and, to a limited extent (e.g. access to local drives, voice input via microphone, ...) also to end device outputs from the client to the server as well as a variety of means for the server to transmit audiovisual output from the running application to the client.

One of the main challenges for such an architecture is performance. A graphics-intensive application (which is the case with most applications presented via a GUI ) places high demands on compression and optimization so that the application can be used by the client. This is especially true if the application has to be made available over a slow network connection. ICA uses very little network bandwidth, 10-20 kbps for a typical session. As a result of this and through performance-enhancing protocol extensions (called SpeedScreen up to XenApp 5, HDX from XenApp 6.0 & XenDesktop 5.0), in contrast to RDP, it can also be used with narrow-band network connections (e.g. ISDN or GPRS ).

Since the client does not need to be a Windows platform, it does not have to and cannot have the Windows graphics libraries locally available. In this case, the server must transfer the pure image data and all other data (e.g. files from USB sticks) via the relatively slow network connection.