What is HDX?

Citrix HDX is a suite of proprietary technologies that delivers a high-definition experience to users of virtual desktops and virtual applications. A central component of the Citrix Virtual Apps and Desktops family of offerings (part of Citrix Workspace), HDX ensures users have a consistent experience across a wide array of devices and networks. Built on top of the Independent Computing Architecture (ICA) remoting protocol, it’s used by global enterprises to support millions of users.

HDX is designed around three technical principles: intelligent redirection, adaptive compression, and data de-duplication. Applied in different combinations, they are designed to:

  • Optimize the user experience and IT control
  • Decrease bandwidth consumption
  • Increase user density per hosting server

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What are the three technical principles behind HDX technology?

1. Intelligent redirection

Intelligent redirection examines screen activity, application commands, endpoint devices, and network and server capabilities to instantly determine how and where to render an application or desktop activity. Rendering can occur on either the endpoint device or hosting server.

Client redirection leverages processing power on endpoints, if available, to free up the server to handle other processes or more users. Device redirection intercepts peripherals such as webcams, printers, scanners, digital pens, and 3D mice at a local level to allow users to interact with these devices in sessions.

2. Adaptive compression

Adaptive compression is a core intellectual property of the ICA protocol which allows rich multimedia displays to be delivered on thin network connections. HDX first evaluates variables such as the type of input, device, and display (text, video, voice, multimedia). Next, it chooses the optimal compression codec and the best proportion of CPU or GPU utilization. It then intelligently adapts based on each unique user or session.

3. De-duplication of network traffic

De-duplication of network traffic reduces the aggregate data sent between client and server by taking advantage of repeated patterns in commonly accessed data such as bitmap graphics, documents, print jobs, and streamed media. Caching these patterns allows only the changes to be transmitted across the network, eliminating duplicate traffic. HDX also supports multicasting of multimedia streams, where a single transmission from the source is viewed by multiple “subscribers” at one location, rather than a one-to-one connection for each user.

What is HDX 3D Pro?

HDX 3D Pro is a specialized technology that pioneered the use of graphics processors (GPUs) in Citrix Virtual Apps and Desktops for visually demanding workloads such as CAD/CAM. Today, there are virtualization-aware GPU cards available, which provide hardware acceleration to multiple virtual machines (VMs) using a single physical GPU. Originally designed for high-end 3D professional graphics, the technology is now available to optimize the performance of business graphics apps that leverage GPU acceleration, such as web browsers and Microsoft Office apps.

How HDX optimizes voice, video, and multimedia performance

Users expect app and desktop delivery solutions to have near real-time audio and multimedia characteristics—similar to those they receive from local app and desktop implementations. HDX technologies deliver bandwidth-efficient multimedia and video through the use of policies and templates—and they do it with exceptional display optimization.

HDX Adaptive Transport ensures that all functions within user sessions perform well, including graphics, even as network characteristics change. This display optimization is accomplished through Citrix’s Enlightened Data Transport (EDT) Protocol which makes intelligent, reliable use of UDP for optimum performance. Adaptive Transport allows sessions to seamlessly switch between EDT/UDP and TCP as needed without any configuration required. The end result is a highly responsive session for a wide range of client endpoints.

Citrix display optimizations make use of both H.264 and H.265 codecs for superior image quality with minimal bandwidth consumption. Encoding using these standards can happen for the entire session, which is ideal for full-screen motion, or selectively to certain regions with motion as required. Both server and client GPU hardware are leveraged where available for stunning visuals that also conserve resources. These optimizations are especially notable in more intensive use cases, such as:

Unified communications

Web conferences require real-time audio and, increasingly, high-fidelity video and screen-sharing as well. HDX ensures audio traffic always receives the highest network priority and includes packet tagging for Quality of Service (QoS), on a dedicated virtual channel within the ICA protocol for RTP/UDP audio. This minimizes latency for jitter-free audio quality.

Client-side webcam compression for video communication reduces bandwidth requirements by an order of magnitude, ensuring a native user experience even on mobile networks or at very remote locations.

For instances where the app is centrally hosted in a virtual environment, Citrix has joint solutions with major unified communication vendors including Microsoft, Cisco, Zoom, and Avaya to enable peer to-peer communication between endpoints. Real-time optimization policies prevent latency due to “hairpinning” so audio and video can move from endpoint to endpoint without traveling all the way back to the central host in the datacenter. The result is a great user experience and higher user density on the host server.


YouTube video rendering, along with other types of intensive web-based content, can be offloaded to the local endpoint with Browser Content Redirection. Fully configurable via policies for URL control, fetching behavior and more, this feature dramatically reduces server resource consumption and host-to-client network traffic while delivering a truly local experience—all within the context of a web browser running in a virtual desktop.

Multicast video support permits a few, hundreds, or even thousands of users to view video content that may be rendered and transmitted just once. This produces savings in bandwidth and datacenter resources for live video events, news channels, and training programs delivered to branch offices.

Multimedia redirection has a huge impact on reducing CPU utilization of the host servers, allowing more users to be supported per server for greater scalability. Flash redirection and Windows Media redirection offload the playback rendering of rich content to compatible client devices, using server-rendered multimedia only as a fallback option.

4K resolution and multi-monitors

In many industries such as healthcare, manufacturing, and finance, there’s often a need for high-resolution displays on more than one monitor. For these scenarios, underlying graphics hardware capabilities along with HDX technologies provide crisp imagery, smooth video, and clear text.


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How HDX accelerates printing and scanning

Citrix supports both local and network printer types. Locally-defined printers on endpoint clients are redirected into sessions as required. Network printers may be assigned to user sessions based on Active Directory group membership, user physical location, or other criteria. Admins can leverage the Citrix Universal Print Drivers and Citrix Universal Print Server for simplicity and stability along with native printer functionality.

Citrix developed these print driver and server technologies from the ground up. They provide many of the features needed for daily printing while taking into consideration the amount of bandwidth a print job requires. Using a single driver to manage hundreds or thousands of printers greatly simplifies management of print drivers in the Citrix Workspace or Citrix Virtual Apps and Desktops environment.

With the Universal Print Server, the print engine is moved to a Windows print server, which prevents the need to install device-specific drivers in the server or desktop image. Firewall-friendly printing protocols provide greater bandwidth efficiency than native Windows protocols and extend printing to non-Windows devices. The latest Universal Print Server also provides improved scalability, greater OS-support, and performance monitoring.

Although scanners can be supported through basic USB redirection, the Citrix implementation permits additional intelligence between the endpoint and the host. Local scanner termination allows the user to interact with the peripheral at native USB speeds. Once the scan is completed, ICA compresses the scanned image and sends it to the host server; thus, the only content that travels over the network is a compressed image.

In addition, HDX supports the largest variety of peripheral devices, with plug-and-play connectivity for webcams, music players, audio recorders, and specialty peripherals. USB redirection to cloud makes these peripherals available within sessions hosted hundreds of miles away from the point of use. HDX technologies optimize USB traffic over common wide area network conditions, to ensure real-time feedback and full-feature support. Support for digital pens, electronic signature pads, and drawing tablets in cloud-hosted apps is an important requirement in retail, finance, design, CAD, and other such markets. HDX meets these needs with a wide array of peripherals and solutions certified for Citrix environments through the Citrix Ready Program.

How HDX ensures high-quality service and reliability

Virtual app and desktop traffic competes with everything else on a network. To meet stringent service level agreements (SLAs), network administrators need tools that help prioritize and optimize application and desktop traffic to deliver a great user experience every day. HDX technologies ensure the highest quality of service and reliability through a combination of ICA traffic prioritization, branch office caching, and ICA protocol optimizations.

HDX traffic prioritization capabilities include:

  • Prioritizing virtual app and desktop traffic based on usage type by segmenting the communication channel into five independent streams through standard QoS routing techniques. Virtual desktop traffic can be segmented into interactive, dedicated real-time, background, bulk and RTP/ UDP voice channels.
  • Identifying application types, thereby enabling network administrators to prioritize web and client-server applications alongside virtual app and desktop traffic. This allows admins to maintain QoS while making the best possible use of available network bandwidth.
  • Adaptive caching technology that orchestrates with Citrix Virtual Apps and Desktops and Citrix Workspace to disable the native ICA compression used for optimizing single-user sessions. This technology then optimizes delivery across multiple user sessions by locally caching and de-duplicating transmission of commonly accessed data, including bitmap graphics, files, print jobs, and streamed media.

ICA protocol optimizations include:

  • Adaptive TCP flow control to accelerate the flow of all TCP-based traffic, including ICA, by sensing and responding to high network latency and packet loss. The result is significantly higher network throughput and performance than with standard TCP implementations.
  • Adaptive traffic compression that uses a highly tuned engine to compress ICA traffic based on its characteristics, as well as infrastructure capabilities and network conditions.
  • Adaptive protocol acceleration which performs intelligent acceleration of ICA traffic while sensing and responding to network and traffic conditions.                                                                           

When it comes to administration, pre-defined HDX policy templates based on use cases make it easy to ensure users receive the best possible experience for their connection scenario while also meeting IT objectives. Templates for high server scalability, bandwidth-constrained WAN users, and security and control are included. These templates may be customized as required and applied to sessions using a variety of policy filters.

Citrix solutions for HDX

HDX technologies are the foundation of the Citrix DaaS and VDI platform. They allow companies to deliver a superior virtual app and desktop experience—one that rivals that of a local PC even when using multimedia, USB peripherals, and 3D graphics apps over low-bandwidth, high-latency networks. Best of all, Citrix HDX allows IT to support “anytime, anywhere” work while maintaining a high level of security and control over corporate data.

Get an up-close look at Citrix HDX technology—baked into every edition of Citrix Virtual Apps and Desktops