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Four key components that make Citrix HDX tick

Citrix HDX is a group of remote display technologies, rather than a single protocol, and IT can get a better handle on its XenApp and XenDesktop deployments by taking a look under the hood.

Many times when IT talks about Citrix, the conversation is broadly connected to its HDX remote display technologies, so it's worth digging down into what Citrix HDX actually consists of.

Sometimes people interchangeably use the term ICA, which is the base remote display protocol that HDX is built upon and harkens back to Citrix's earliest days. ICA isn't the only term that's associated with Citrix HDX, though, and I want to spend a little time going over some of the other terms you might hear: ThinWire, ThinWire Plus, Framehawk and H.264. This may sound like propeller-headed, engineer-type stuff, but understanding the nitty-gritty of what goes into HDX can help you get a grip on your Citrix deployment.

This article is meant to be an introduction to some of the technologies that make up Citrix HDX, and it should give you some background on how Citrix delivers its XenApp and XenDesktop platforms.


ThinWire is the term for the legacy version of the HDX/ICA protocol, sometimes called ThinWire Legacy or ThinWire Compatibility. In the 1990s, Microsoft created Graphics Device Interface (GDI) to handle low-level graphics display commands. Citrix figured out how to remotely deliver GDI commands, which it referred to as GDI Remoting, and used that as the foundation of the protocol. Citrix then licensed the technology back to Microsoft -- where it then became Remote Desktop Protocol -- while also using ThinWire to create its own ICA protocol.

ThinWire consumes the least bandwidth of all the protocols that Citrix offers, and it has 20 years of development behind it. Citrix still supports ThinWire, but due to limitations we'll get into in the next section, IT can only use it for Windows 7 and older operating systems.

ThinWire Plus

Understanding the nitty-gritty of what goes into HDX can help you get a grip on your Citrix deployment.

ThinWire Plus is the name for the next generation of the ThinWire protocol, and it's intended for use with Windows 8, Windows 8.1 and Windows 10. With the release of Windows 8, Microsoft got rid of GDI, replacing it with a new application programming interface (API) called Direct2D, which is more aligned to what the corporate IT world needs today. After all, GDI and its successor, GDI+, were nearly 20 years old. The problem was that GDI was Citrix's bread and butter, and the new API was incompatible with the original ThinWire.

ThinWire Plus made its debut in XenApp/XenDesktop 7.6 Feature Pack 3. It shares a name with its older sibling, but ThinWire Plus is quite different from a networking perspective. It can deliver better virtual desktop performance, but it also consumes more network bandwidth.


Organizations have widely adopted H.264 as the video codec of choice for both quality and bandwidth consumption. H.264 is natively supported by software such as web browsers, but also by graphics processing units and dedicated H.264 decoding and encoding hardware. Even single-board computers such as Raspberry Pi have built-in H.264 decoders.

Citrix and VMware have both capitalized by making H.264 versions of their protocols that ease remote desktop and application delivery to any hardware. The Raspberry Pi is a great example. Ordinarily, it wouldn't have enough horsepower to run a useable remote desktop, but because it has that built-in H.264 decoder, it is now a capable thin client.

The challenge is that H.264 requires nearly double the bandwidth of a typical ThinWire Plus remote desktop session. User performance isn't affected, but IT needs to keep its bandwidth consumption in mind. There's also a slight uptick in CPU usage, but hardware-based H.264 can be helpful in mitigating both CPU and network consumption.


Citrix acquired Framehawk in 2014. Framehawk started off doing something like app refactoring -- transforming Windows applications to deliver them remotely to mobile devices. Framehawk was a bit ahead of its time, but what was really valuable was that its protocol was extremely resilient in high-latency, low-bandwidth scenarios. This aspect was, in part, because key personnel got their start working with NASA, where high latency and low bandwidth is the name of the game.

Citrix acquired Framehawk for this technology, and incorporated it into XenApp/XenDesktop 7.6 Feature Pack 2. Framehawk serves as another option for delivering remote desktops and applications, but I would reserve it for tricky network situations where the other protocol options won't work. If you have high latency or low bandwidth, you might give it a look.

These are just some of the technologies that make up Citrix HDX, but it's good to at least get this basic background, because the more you know, the more of HDX's capabilities you'll be able to use.

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