Microsoft pushes the Remote Desktop Protocol envelope

Microsoft's Remote Desktop Protocol wasn't up to snuff until the company acquired Calista Technologies.

This is the second section in our E-book on virtual desktop infrastructures. Check out part one, VDI: The big picture, to learn about desktop virtualization vendors.

Competitive forces have always driven Microsoft to improve the performance of Remote Desktop Protocol (RDP), which at times has trailed behind remote display protocols from other virtual desktop infrastructure (VDI) vendors. In 2008, Microsoft acquired Calista Technologies, a pioneering virtualization company, to improve the performance of RDP and add features to it.

The Calista acquisition gave Microsoft several independent but interoperable technologies that enhance the end-user experience and have been incorporated into Microsoft's RDP. One of the most important capabilities is the virtual graphics processing unit (GPU), which allows a VDI host to support many users and a full range of client devices with advanced graphics. A virtual GPU shares (virtualizes) a single GPU on a server between multiple virtual machines (VMs), eliminating many of the problems that other technologies encounter.

The virtual GPU is a major step forward for VDI. Previous options for rich graphics on the remote client relied on CPU rendering, which is less efficient, cannot handle rich 3-D graphics and uses CPU cycles that would otherwise be available only to the application workload. It also dedicated a virtual GPU for each VM, which is very expensive and not scalable.

Microsoft's implementation of Calista's virtual GPU technology has major implications for centralized computing scenarios, like VDI and Terminal Services, as well as other virtual machine applications.

The Calista acquisition also brought host-side rendering under Microsoft's RDP umbrella. Previously, RDP used graphics drawing commands to make graphics remote. In such primitive methods, lines were drawn or pixels were populated via the host. Results were then transmitted over the network to the client system, which would render the commands and convert the primitive images to pixels on the screen. This process occupied a lot of bandwidth, required significant local and remote processing, and affected the responsiveness of applications.

Calista's host-side rendering, when combined with selective primitive remoting, overcomes much of these performance-sapping problems for an optimized end user. The key to Calista's host-side rendering is its "smart capture" mechanism, which monitors which parts of the screen have changed and need to be forwarded to the client.

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Host-side rendering and smart capturing significantly increase the range of client devices that can be used to support a full Windows desktop experience. The technology is well suited for thin clients, lightweight clients and even so-called zero-client technology.

Rounding out RDP's expected improvements is Calista's compression technology. Efficient compression is critical because technologies that improve the end-user experience -- such as host-side rendering -- often increase bandwidth requirements. Calista uses rich media redirection, hints, select primitive remoting, advanced caching and other techniques to improve the end-user experience but at the cost of increased bandwidth requirements. Such requirements could limit scalability, which no enterprise deploying a VDI solution wants to lose.

To reduce demands on bandwidth, Calista developed an advanced compression scheme specifically designed to make the Windows desktop remote. The coder/decoder (codec) is optimized for low-latency, high-frequency data environments and can run on the CPU. More importantly, large parts of the codec can also run on the GPU. This allows some of the required processing to use untapped reserves of the processing power commonly found on GPUs, reducing CPU overhead. This processing method should lead to improved performance and increased compression, especially when dealing with large bitmap images.

For implementations that rely on thin-client computing resources that often lack powerful CPUs/GPUs, Calista enables scalability with a reference ASIC implementation of the codec. This allows hardware partners to develop complementary devices. Although ASIC-enabled devices are less expensive than fat clients, they still offer a user experience that's comparable to a robust CPU/GPU combo on a non-thin-client implementation.

Incorporating Calista's technology into Microsoft RDP can dramatically improve end-user experiences, especially for 3-D and delivery of Microsoft-based multimedia applications. Virtual desktop deployments, server-hosted virtualized desktops or applications using Microsoft Terminal Services will also benefit.

The combination of RDP and Calista technologies is expected to reduce network bandwidth requirements to remotely display rich media content, allowing remote workers to receive a modern Windows desktop experience without dedicated hardware. That full desktop experience will become increasingly important as multimedia and 3-D graphics are become standard in business and consumer environments.

To flourish in the VDI market, Microsoft must ensure that users have access to a rich remote experience for server-hosted virtual desktops and applications. Adding Calista's technology should help pave the way for the adoption of Windows desktops across the enterprise on a variety of client hardware platforms -- from netbooks and thin-client terminals to PC-based software clients.

Frank Ohlhorst is an IT journalist who has also served as a network administrator and applications programmer before forming his own computer consulting firm.

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