Description
This article describes how to troubleshoot TCP windows scaling on a FortiGate.
On a FortiGate unit where the Windows Optimization feature is disabled, the TCP Windowing is a parameter configured and negotiated exclusively between a Host device and a Server application. A FortiGate unit does not influence its formation, control, or manipulate how it behaves.
Scope
FortiGate.
Solution
To show the client that the FortiGate does not change, affect, or block the TCP Windowing Scaling, a TCP test can be used to verify its status on both ends of the connection and on the FortiGate:
On the Server application side, run the 'iperf' command:
#iperf -s
Run the same command on the Host device:
iperf -c <server @ IP> -i -t 30
On the FortiGate unit run a "sniffer packet capture" on the LAN and WAN interfaces to confirm the TCP windows size used between the client and the Server:
diag sniffer packet any 'host <client-IP> and host <server-IP>' 6 0 a
Related article:
Technical Note: FortiGate - Generate CSR via CLI when Subject Alternative Name field is long
