Unable to download files which are greater than 20 MB through NetScaler. 

The TCP Windows Size depreciates (observed in network trace).

NetScaler software version 11.0 supports TLS 1.1/1.2 protocol negotiation for connections between a NetScaler and backend servers. The support is available on MPX, SDX, and MPX-FIPS appliances. When TLS 1.1/1.2 is negotiated for backend server connections, some failures have recently been reported from the field for large data transfers, such as an email with large attachments or cases involving long-lived TCP connections. The issue is present in builds 55.x, 62.x and 63.x of NetScaler software version 11.0. 

This issue does not affect TLS 1.1/1.2 negotiated for front-end connections (between a client and NetScaler. 

The problem may be alleviated by disabling TLS 1.1/1.2 negotiation for backend connections using the following command line examples: 
set ssl service <service_name> -tls11 di -tls12 di 
set ssl servicegroup <servicegroup_name> -tls11 di -tls12 di