RDP or ICA

In the article "Speed Kills," December 1998, John Enck compared the performance of Microsoft's RDP (Windows NT Server 4.0, Terminal Server Edition's native protocol) and Citrix's Independent Computing Architecture. ICA is the more mature protocol, which Citrix developed in its WinFrame products. Citrix has packaged ICA in a Terminal Server add-on product called MetaFrame. The question John Enck attempted to answer in December was, "Which protocol is faster?"

Now that Terminal Server has been available for a while, and more people have had the chance to test it in their environments, deciding between ICA and RDP is simpler. In this article, I'll look at how the two protocols compete with and complement each other. After running both protocols for the past 9 months, I've concluded that each performs at an adequate level in most situations. I'll discuss performance differences, but I'll emphasize what I consider to be more important considerations—features and cost. (To learn about a third display protocol that UNIX users who connect to Terminal Server can use, see the sidebar "The X-11 Protocol," page 90.)

Does Speed Still Kill?
Although Citrix intended ICA for low-bandwidth applications, John Enck pointed out that RDP also works well in a low-bandwidth environment. Either protocol's speed depends on what you ask the protocol to do—RDP is sometimes significantly faster and sometimes significantly slower than ICA. Determining factors in a speed comparison include whether you require a full-screen draw and redraw or a partial-screen draw and redraw. Other factors include the applications you run (e.g., Internet Explorer—IE—is screen-redraw dependent, whereas a small Microsoft Word text document is not), whether you share limited bandwidth with other users (RDP is a streaming protocol that uses bandwidth when you are connected, regardless of task, whereas ICA sends packets only when the mouse or keyboard is in use), the client you run from and how busy the client is, and the kind of traffic that shares your bandwidth segment. One other significant factor is that ICA dynamically determines its compression ratio according to server performance parameters: If the server has few available processing cycles, ICA compresses less; if more processing cycles are available, ICA compresses more to better optimize bandwidth.

Although I stated that RDP and ICA performance is adequate in most cases, I don't mean improvement isn't desirable. In fact, a servercentric approach becomes more viable and less expensive each time a protocol speed advancement occurs. Every performance improvement translates to more computers fitting on less bandwidth, which lowers the per-connection cost, especially for people who use a servercentric solution over a WAN (a significant percentage of Terminal Server customers use Terminal Server to reduce administration costs at remote sites).

In late February, Citrix released MetaFrame 1.8. One of its features is faster ICA performance based on SpeedScreen 2 technology. SpeedScreen 2 builds on intelligent agent technology, which Citrix introduced in MetaFrame 1.0, and reduces the transmission of frequently redrawn screens. In comparison with MetaFrame 1.0, version 1.8 reduces bandwidth consumption by an average of 25 to 30 percent and reduces the number of transmitted total packets by as much as 60 percent, significantly improving measured speed on restricted bandwidth connections. SpeedScreen 2 facilitates consistent performance regardless of network connection by reducing latency and improving the feel of the server-based application. This technology should help ICA leap ahead of RDP in low-bandwidth speed. But Microsoft won't be far behind with improved performance in RDP, if you believe the rumors of enhancements packaged in Windows 2000 Server (Win2K Server). To read more about improvements to Terminal Server in Win2K Server, see Christa Anderson, "Terminal Server Grows Up," page 103.

Is There More to Life than Speed?
No matter how fast a protocol is, you shouldn't base your decision to use a protocol solely on speed. Although RDP is a capable protocol, it doesn't yet support features that MetaFrame and the ICA protocol support, as you can see in Table 1. How much time passes before Microsoft begins to develop some of these features in RDP is a matter for speculation, and some third-party vendors have already begun such development work. For example, if you purchase a ThinSTAR 300 thin client from Network Computing Devices (NCD), you can also buy an add-on that lets Terminal Server send sound to the ThinSTAR terminal via RDP. The add-on includes a load-balancing feature. Both NCD add-on features work only with the ThinSTAR 300 client, whereas MetaFrame offers the same features independent of client platform (you can use any thin client, PC, or Macintosh computer as long as the machine supports sound).

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