max 1400 bytes RF Data in Rx IPV4 API frame but TCP window size used is 1460

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The window size is 1460 bytes in TCP packet sent out by Xbee although max 1400 bytes RF Data is allowed in Rx IPV4 API frame.
The window size of 1460 tells my PC application to send max 1460 bytes to Xbee in streaming mode if PC application have more than 1400 bytes to send.
So, if my PC application send 1460 bytes to Xbee, nothing is received from RX IPV4 API frame. If my PC application send 300 bytes to Xbee, 300 bytes is received from RX IPV4 API frame.
Any solution?