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Unable to download large file completely from via ProxySG


Article ID: 169278


Updated On:


ProxySG Software - SGOS


The issue is unable to download large file completely from The download stopped half way (at approximately 40-50%) with 0 B/s download speed.

For example, a zip file size 120MB with following download URL stopped at ~50MB and download speed become 0 B/s.



The Origin Content Server (OCS) URL is
Resolved IP Address is  (NOTE: IP is subject to change)

Packet capture file shows following Expert Information in wireshark using display filter: (tcp.analysis.window_full || tcp.analysis.zero_window)
[TCP Window Full] - TCP window specified by the receiver is now completely full
[TCP ZeroWindow] - TCP Zero Window segment

Source: OCS (URL:, IP:
Destination: ProxySG
The frame with [TCP Window Full] indicates the Upstream OCS has completely filled the receive buffer of ProxySG.

Source: ProxySG
Destination: OCS
[TCP ZeroWindow] means ProxySG sends an ACK to the Upstream with a window size of 0. This means that the window size will remain at 0 for a specified amount of time, ProxySG unable to receive any more data at this moment, and the TCP transmission is halted until it can process the information in its receive buffer.


Resolved by increasing the TCP-IP Windows Size from default 65535 to 256000. After that able to download the zip file completely.

To manually increase the initial window size to 256K or higher:
> enable
# config t

#(config)tcp-ip window-size 256000

Note: Enable Window Scaling, as described in RFC-1323; otherwise, the TCP Window Size remains at the default even after you increase it. Rrefer to KB article 000015466 for details on RFC-1323.