How to log free space in the TCP receiver window?

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Hi, I want to monitor the free space in the receiver window and the field "receiver window" on wireshark shows the size of the receiver window. As the data is queued in the window until its passed to the application layer, to my understanding, the instantaneous receiver window (filled up window size or remaining size) should be varying depending on the throughput of the connection. Is there a tool or syscall in Linux to monitor the changes to the receiver window in live?

I experimented with varying receiver window (small size), but the wireshark shows a constant window size.

Thanks in advance!