①SYN=1,seq=x ②SYN=1,ACK=0,seq=x,ack=y ③ACK=1,seq=x+1,ack=y+1
①SYN=1,seq=x ②SYN=1,ACK=1,seq=y+1,ack=x ③ACK=1,seq=x+1,ack=y+1
①SYN=1,seq=x ②SYN=1,ACK=1,seq=y,ack=x+1 ③ACK=1,seq=x+1,ack=y+1
①SYN=1,seq=x ②SYN=1,ACK=1,seq=y,ack=x+1 ③ACK=1,seq=x,ack=y+1
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