Write a commentCitrix HDX is critical to the user experience in Virtual Apps and Desktops environments. This article highlights three areas where default settings and misunderstandings can impact performance and complicate administration: Enlightened Data Transport (EDT), outdated legacy graphics policies, and the often misconfigured HDX 3D Pro acceleration. The goal is to provide actionable insights for proactive HDX optimization.
The EDT dilemma - re-evaluation of UDP as a standard
Understanding Enlightened Data Transport (EDT)
EDT is a proprietary Citrix transport protocol built on UDP and designed for an improved user experience over "challenging long distance connections" and "unreliable networks". The Adaptive Transport mechanism that utilizes EDT is set to "Preferred" by default in Citrix policies, making EDT the primary protocol with automatic fallback to TCP.
The arguments for the standard deactivation in LAN/WAN
In typical enterprise LAN or WAN environments with latencies below 100 ms, enabling EDT by default often results in performance degradation that manifests as "black screens" or "fragmented screen display". These issues are often due to Maximum Transmission Unit (MTU) fragmentation, as UDP does not inherently handle fragmentation. This is particularly common with VPN connections, Wi-Fi access points and cellular networks, where the effective MTU can be lower than the standard 1500 bytes. EDT is "much more sensitive" to MTU issues than the classic ICA protocol over TCP.
Although EDT MTU Discovery is enabled by default and is designed to automatically determine the optimal MTU, it has limitations. It requires Session Reliability to be enabled and can fail in scenarios with asymmetric network paths or DS-Lite networks. A failed MTU discovery can result in too high a detected MTU, causing fragmentation and poor performance. The TCP fallback mechanism can also increase session startup time by up to five seconds. The default "Preferred" setting for Adaptive Transport is problematic for most enterprise networks as the EDT implementation is sensitive to MTU issues.
Table: Comparison of HDX Adaptive Transport (EDT vs. TCP) performance in common network scenarios
| Network scenario | Standard EDT behavior | Observed performance problems (EDT) | Main cause (EDT problems) | Recommended protocol for scenario |
| Typical enterprise LAN/WAN (<100ms) | Preferred, activ | Black screens, fragmented display | MTU fragmentation, UDP sensitivity | TCP |
| VPN connection | Preferred, activ | Black screens, fragmented display | MTU fragmentation, lower effective MTU | TCP |
| Long-distance WAN (>100ms) | Preferred, activ | Optimal, but MTU problems possible | MTU discovery restrictions | EDT |
| Unreliable Wi-Fi/mobile network | Preferred, activ | Black screens, fragmented display | MTU fragmentation, unstable connection | TCP |
| Connection via Citrix Gateway | TCP (often forced) | No EDT problems (as UDP ports are often closed) | UDP ports not open/EDT not activated in gateway | TCP (often forced) |
The standard for HDX connections and error detection
I often hear from customers that connections via the Citrix Gateway show no problems, and this is because either no EDT is activated in the gateway or the UDP ports 1494/2598 inbound, 443 outbound have not been opened for Citrix Gateway and therefore only TCP connections are established.
Other indicators of UDP problems can be the aforementioned "black screens" or the "fragmented screen display". If you are not sure which transport protocol is being used, you can check this via Citrix Director or via the command CtxSession.exe -v.
Given that most customers do not use transatlantic connections, the default setting for HDX Adaptive Transport should be re-evaluated. I recommend that Citrix set this to "Off" by default.
Citrix Legacy graphics policies
Purpose and pitfalls of the legacy graphics mode
The legacy graphics mode was developed for older operating systems such as Windows 7 and Windows 2008 R2. However, it is not supported or tested for modern operating systems (Windows 8, 8.1, 10, Server 2012/2012 R2/2016). The continued presence of numerous policies explicitly tied to "legacy graphics mode" (e.g. Extra color compression, Heavyweight compression, Lossy compression level, Progressive compression) causes significant confusion for administrators. Although the legacy graphics mode itself is not activated by administrators, they often configure many of the policies that depend on it because they "sound good" without recognizing their incompatibility. Despite the availability of a "Legacy settings only" filter in Web Studio, this tool is currently rarely used by on-premise administrators, making it difficult to identify and avoid these misconfigurations. This confusion is a long-standing problem. In a conversation with then HDX product manager Derek Thorslund, I expressed the need for a filter to manage these policies, but a comprehensive solution was never fully implemented.
Overview of legacy graphics policies
The following screens list policies that are either explicitly tied to the "legacy graphics mode" or were mainly relevant for older graphics rendering methods. Modern Citrix environments predominantly use Thinwire, which adapts dynamically and uses video codecs (H.264, H.265, AV1), making many of these legacy settings redundant or ineffective.
| Included legacy graphics policies | current graphics policies | |
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Citrix should accelerate the cleanup of the Policy Console by removing legacy policies and no longer provide warnings or filters for legacy policies.
HDX 3D Pro Optimize - Enable GPU acceleration
Basics of HDX 3D Pro and GPU usage
HDX 3D Pro, now known as HDX Graphics, provides a high-performance user experience for demanding graphics workloads by utilizing GPUs for hardware acceleration of DirectX and OpenGL applications. It uses adaptive H.264 or H.265-based depth compression for optimal WAN and wireless performance. Hardware encoding with H.264 and H.265 is used with NVIDIA, Intel and AMD cards that support NVENC.28 H.265 and AV1 provide superior compression and image quality. The correct installation of GPU drivers and hypervisor configuration are critical to the effective use of vGPUs.
Key guidelines for optimal HDX 3D Pro performance
The optimization of HDX 3D Pro requires a precise configuration of the Citrix guidelines:
- Use video codec for compression: For intensive 3D graphics, "For the entire screen" is recommended to configure Thinwire with full screen H.264, H.265 or AV1.
- Use hardware encoding for video codec: Enabled by default, this setting enables the use of GPU hardware acceleration to compress screen elements, which significantly improves FPS.
- Visual quality and Allow visually lossless compression: These policies are critical to achieving pixel-perfect or visually lossless graphics.
- Optimize for 3D graphics workload: Although listed as "deprecated" in version 2411, it is still mentioned in other documents, which can lead to confusion.
HDX 3D Pro "automatically uses all available bandwidth", which can have a negative impact on performance if there are large fluctuations.
The indispensable role of the Remote Display Analyzer (RDA)
The main problem, however, is that even with the Citrix guideline "Use hardware encoding for video codec" activated, the hardware is not actually active under certain conditions! This means that customers spend a lot of money on expensive graphics cards but do not use them at all! Citrix recently introduced a feature in Director that shows that Teams is HDX-optimized, but nowhere can the administrator see whether and how the vGPU is being used in the session, and this is exactly where the Remote Display Analyzer can help! The HDX display in Director in particular has been around for many years and urgently needs a new coat of paint!
The Remote Display Analyzer (RDA) is a "perfect tool" for validating and troubleshooting HDX 3D Pro configurations. RDA provides automatic detection of display mode and encoder, real-time statistics on CPU times, bandwidth and latency, and the ability to change settings live. It also detects EDT MTU size and checks NVIDIA licensing and hardware encoding usage. RDA's ability to change settings on the fly fills a critical gap in existing Citrix monitoring tools. Citrix should acquire this tool and integrate it into its product for a comprehensive solution for HDX graphics optimization.
The simple yet concise indication of whether hardware support is active or not is significant, because even with the policy "Use hardware encoding for video codec" enabled, it does not automatically mean that it is!
Conclusions and recommendations
Optimizing Citrix HDX environments requires a deep understanding of protocols and policies. The article has highlighted three key areas where current default settings and policy practices can lead to challenges:
- EDT as the default transport protocol: Enabling EDT by default is counterproductive in most LAN/WAN environments due to MTU fragmentation issues and EDT's sensitivity to network elements. Evaluate the suitability of EDT for your network segments! Set HDX Adaptive Transport to "Off".
- Legacy graphics policies: The continued presence of legacy policies that are not supported by modern operating systems creates significant confusion and leads to misconfigurations. Disable or remove all legacy graphics policies from deployments with modern operating systems. Citrix should speed up the process of removing outdated legacy graphics policies!
- HDX 3D Pro optimization: Proper use of HDX 3D Pro requires precise policy configuration and careful GPU setup, with inconsistencies in documentation adding to the challenge. Citrix should look at integrating advanced real-time monitoring and configuration validation capabilities, similar to those of the Remote Display Analyzer, directly into Citrix Director.
Finally, I would like to take this opportunity to thank the team of Remote Display Analyzer for my NFR version!
