Microsoft KB Archive/196462

= How RDP Transmits Data in Both LAN and WAN Environments =

Article ID: 196462

Article Last Modified on 2/23/2007

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APPLIES TO


 * Microsoft Windows NT Server 4.0, Terminal Server Edition Service Pack 4

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This article was previously published under Q196462



SUMMARY
RDP uses cached glyphs and bitmaps for transmission across WAN and LAN connections and, therefore, does not transfer data in a recognizable form.



MORE INFORMATION
RDP uses a highly efficient encoding algorithm that represent the vast majority of Windows graphics operations with a small amount of network traffic. For example, most dialog boxes consist of text items, a series of gray, white, and so on rectangles, and light and dark shaded lines for a 3-D effect. These are not transmitted as comparative deltas from the previous screen, but are encoded as pattern blts and cached glyphs. For instance, we can draw the gray dialog box background color with "Draw color X at x1,y1,x2,x2". Field compression is always enabled such that a second draw with only a change in the x1,y1 position would only send an even smaller packet with a single bit representing the fields that have not changed, and the minimum possible number of bits to represent the delta change in the coordinates. Bulk compression is applied over the final packets (when enabled) for an even further improvement.

RDP text is displayed by using glyph caching. Almost immediately, the client builds up the required set of glyphs, and the server needs only transmit a short hash value to display the text. Bitmap caching works similarly. The light and dark gray lines are similar to the patblt in that field compression is highly effective. Standard Windows clipping applies such that the RDP driver only receives output commands for the areas that are actually visible.

RDP works the same over LAN or WAN, thus the description above holds for both. In other words, performance of the protocol does not depend on whether RDP is running over LAN or WAN; rather, it depends on what the application is displaying. Making a statement about a completely unknown WAN configuration is not possible, as each configuration is different. There is a check box for "Low Speed Connection" that can be enabled at the client side when making a server connection. That check box simply enables further compression on the final bitstream; however, it does not change the fundamental RDP algorithm described above.

Keywords: kbinfo KB196462

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