HDX MediaStream Flash Redirection - Network Latency Performance Issues

Description

Citrix SD-WAN, formerly NetScaler SD-WAN

HDX MediaStream Flash Redirection in XenDesktop 4 is intended for LAN-connected users and might perform poorly if the latency threshold is increased more than its default value of 30 ms, especially with Flash pages that contain large amounts of active content.

Symptoms include the following:

Choppy video
Slow load times
Poor performance

Background

These symptoms might be manifest for Flash applications and websites that perform heavy scripting, or if the SmartRendering latency threshold for Flash Redirection is increased more than its default value of 30 ms (roundtrip latency). This limitation is fundamental to separate the Adobe Flash Player from the browser. No HDX MediaStream Flash Redirection implementation can overcome the limitation for sites that use heavy scripting.

Resolution

In general, HDX automatically selects the method of Flash content delivery (Flash Redirection or CTX124516 - How to Optimize HDX MediaStream Server-Rendered Video) that provides the best results.

During initialization of the ICA session, the HDX components measure and evaluate the network latency between the user device and the Citrix server. If network latency is greater than the SmartRendering latency threshold (default 30 ms), then the Adobe Flash content is rendered on the server. If it is within the set threshold, then the content is streamed to the client device through HDX Flash Redirection.

Note: Add the sites that exhibit poor performance because of a very chatty scripting to the Per-URL blacklist GPO ADM HDX MediaStream Flash Redirection administrative template. This forces such sites to be server-rendered, thereby providing the best user experience.

Problem Cause

The primary goal of HDX MediaStream Flash Redirection is to increase server scalability by offloading Adobe Flash rendering for LAN-connected users from the server to the user device. The basic design of the feature is that of ActiveX remote. The Adobe Flash Player is an ActiveX plug-in to Internet Explorer. With HDX MediaStream, the Flash Player executes on the user device. Since the ActiveX interface is generally implemented in process or at least on a single machine, it does not attempt to control the number of method invocations. Since many of these COM method invocations must be remote by HDX MediaStream Flash Redirection, the redirection approach would result in poor performance on a network connection with significant latency.

Note that any network protocol performs poorly with a high enough latency. The difference with HDX MediaStream Flash Redirection is, much control cannot be exercised over the protocol design (because essentially remote ActiveX calls are done). In such a scenario, HDX MediaStream Flash Redirection makes significant and widespread optimizations to short-circuit ActiveX calls wherever possible. However, the optimizations in the current version of the feature (XenDesktop 4 and XenApp 6) are intended for network connections with a maximum of 30 ms roundtrip latency. Therefore, 30 ms is the default value of the SmartRendering latency threshold for the feature. Refer to CTX124190 - How to Deploy and Configure HDX MediaStream for Flash for more information.

Some Flash applications and websites script each other in a very chatty way and perform poorly over HDX MediaStream Flash Redirection. In addition, some Flash-based webpages contain many instances of the Flash Player. Since Flash Player instances on a webpage typically load in serial, such webpages might perform poorly with Flash Redirection even with modest latency.