Large Files fail to download from the Backend Server if the traffic is coming from Citrix ADC.
 

Based on the Wireshark Traces, we were able to see lot of "TCP zero window "  packets from Citrix ADC SNIP to Backend Server, the above situation signifies the TCP Profile can be looked upon in such scenario to verify if the receiving buffer / Window Scaling Factor / Selective Acknowledgment can be modified.

• Window Scaling Factor under TCP Profile:
  • If the scaling is selected, change the value of scaling can be made up to max.
• Receiving TCP buffer / TCP Buffer:
  • As downloading the file is causing an issue, receive buffer can be made up to max to hold the maximum receiving packets from the backend server.
• Selective Acknowledgment :
  • SACK improves TCP throughput by only retransmitting a portion of the TCP window, depending on what segments may have be lost during transmission of course. Avoiding the need to retransmit the entire TCP window will positively increase the overall throughput capability, and thus improve the user experience / SACK.

As a result enabling the SACK, would also fine tune the download of file in such scenarios.

 

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Problem Cause

Fine Tune TCP Profile