This statement reads the TCP packet header from the specified socket.
This option is only available for the W5300 which includes a packet header with the packet size when align is set to 0.
TCP packets start with a 2 byte size header.
After you have read the TCP header, you can use TCPDATASIZE to read the number of bytes available in the packet.
TCPDATASIZE is a word variable you need to dimension yourself.
Socket is a constant or variable in the range from 0-7.