Simply put, the TCP receive window is how the receiver tells the sender “This is how many bytes you can send me without me ACKing any of them”.

In short, the receive window size halved after the kernel upgrade. Hence the throughput was cut in half, causing the data transfer time to double.

While it technically wasn’t the “network” to blame, this time it turned out the culprit was the software components that make up the network (i.e. the TCP implementation in the kernel).

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