Grasp a beginner tcpdump caught, write their own server packages above, I have a question is at the back of the length is 54, on behalf of the package 54 bytes long, the back of the 2 is the size of the data, the 54-2-20 (IP header byte) - 20 (TCP header byte)=12 bytes, this 12 bytes is what ah, excuse me each big giant!