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Citrix's remoting protocol for XenApp, HDX, creates a local client experience on a remotely connected device. But...
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HDX connectivity issues can happen, and diagnosing them can be complex. Here's what to do if you find yourself troubleshooting XenApp and HDX:
HDX was originally called ICA. An HDX connection onXenApp relies on a client -- Citrix Receiver -- and a server, the HDX Listener. HDX needs a transport protocol to get it back and forth between the client and the server; it uses TCP/IP. The listener resides on the XenApp Server.
By default, the server listens for HDX connections on TCP port 1494. One of the first troubleshooting steps to test is communication on this port. Using telnet from inside the organization's network on this port, you can perform an "ICA ping." If HDX is working, the telnet will echo back the text "ICA" repeatedly.
One variable in the HDX communication path is a XenApp feature called Session Reliability. Session Reliability handles situations when the client loses connectivity for a brief time. When this condition happens and Session Reliability is enabled, the Citrix Receiver freezes the display of the application or desktop session to stop it from appearing disconnected. A disconnection without Session Reliability requires the user to reconnect and re-enter credentials. Session Reliability "parks" the session, and once connectivity is restored, the display unfreezes and normal operation resumes.
Session Reliability listens by default on TCP port 2598 and forwards to HDX port 1494 via Common Gateway Protocol. The potential breakdown can happen on that communication. You might be troubleshooting XenApp error logs such as "Unable to connect to the CGP tunnel destination (127.0.0.1:1494)." Temporarily disabling Session Reliability in XenApp policy or farm settings can rule it out as a source of an issue.
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