Spirent TestCenter is built on a different stack than Avalanche and is more intended for functionality than performance testing.
Spirent TestCenter ALP performance is limited by the TestCenter ALP window size capabilities.
TCP stack in the Linux kernel will determine what the window size should be according to the design of the stack and the real-time conditions it encounters; however the window size should not exceed the limit in the client profile.
The advertised Window is the capable window size that the Client or Server can accept.
Default window size on Avalanche is 32768 and is configurable and the window size will adjust if required, the maximum is generally 32768.
For Spirent TestCenter ALP the current Maximum Scaled Window size is 4062 and will adjust as required, the max window size ”Receive Window Size Limit” being configurable to 32768.
”Receive Window Size Limit” parameter is supposed to set a ceiling on how big the TCP window can get.
The TCP stack in the kernel will set the window size according to the rules it plays by, but it should not set it greater than the configured max size.
Wireshark displays not the actual window size but a scaled number that is 2 ** 5, or 32 greater the actual window size.
Wireshark will display the window size of 130 Kilobytes however the actual window size on Spirent Wireshark is 130000/32 = 4062