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RTP Audio - Is dedicated UDP channel monitored for dataflow after initial handshake?


Christine Vass

Question

Greetings

 

This question is regarding RTP audio sessions using XenDesktop 7.15.400.

 

Can anyone confirm what's the expected outcome in the case where:

 

1.  There's an established RTP session (dedicated UDP audio channel), and

2.  The dedicated UDP channel somehow gets blocked (such as the firewall/AV on the Workspace PC decides to block the UDP traffic)

 

I'm wanting to know if:

 

a.  Citrix will identify that the UDP channel has been blocked, and fall back to using a virtual audio channel within the ICA pipe, or

b.  Citrix will not realize the UDP channel has been blocked, and continue to send audio over the channel, not realizing the user on the Workspace side is no longer getting the audio.

 

The article https://support.citrix.com/article/CTX133024 says there’s an initial handshake over UDP before data is sent to RTP, but it doesn’t mention what happens if the RTP UDP session gets interrupted after the handshake occurs.  In our testing, if the firewall on the computer running Workspace blocks the UDP port after the RTP session is established, audio is lost on the Workspace PC, while Director still reports audio over UDP is still active, so it doesn’t appear to be falling back to using a virtual channel within the ICA.

 

From https://support.citrix.com/article/CTX133024:

"By default, the Audio virtual channel would start on a regular ICA TCP connection, and then an ICA module starts initializing corresponding ICA RTP sessions. Both VDA and Workspace app would do handshake over UDP before they start sending data over RTP. UDP handshake confirms that data can flow over UDP in each direction. If the UDP handshake fails then the Audio virtual channel would continue to use ICA TCP to send and receive the data."

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