Connections using NetScaler RDP proxy feature to backend RDP servers freeze within few seconds after the launch.

From nstrace captured we see Zero window occurring between client and RDP listener IP as shown below:

Currently there is no workaround for this issue. Also the issue is ONLY caused when SSO is disabled for RDP proxy. However, if user wants to use RDP-Proxy with SSO enabled, this issue will not be seen.

In NetScaler configuration if you have a traffic policy (as below) which is bound to Vserver or AAA group level then it disables SSO for RDP Proxy:
add vpn trafficAction disable-sso-action http -SSO OFF -kcdAccount NONE
add vpn trafficPolicy disable-sso-policy "REQ.HTTP.URL CONTAINS rdpproxy" disable-sso-action

This issue is fixed in 11.0-65.x+ builds.

Problem Cause

This issue is caused due to TCP retransmissions not occurring on the server side connection between NetScaler and RDP server when there is packet loss.