the actual tcp send window is not increased further

edit

Hi,

I am handling an issue that the smb file transfer over tcp does not fill the 1Gbit pipe between the Windows file server and a macOS 10.14.5 client. They are on different subnet so it can be any intermediate devices that is blocking something.

The weird thing I am seeing is that the sender was stuck at sending 8 * SMSS and no more. According to the Ack, the receiver window is still large enough to accept more incoming data, but the actual send window didn't increase to more than 8 * SMSS followed by each of the RTT (Ack). There is no window full reported either.

I am wondering what can be the possible reason that the sending speed was stuck at 8 * SMSS? I am no saying 8 is low, but it was just not as good as, i don't know, 14 * SMSS?

thanks!