0.史上最容易理解的：TCP三次握手，四次挥手
https://cloud.tencent.com/developer/news/257281
A 理解TCP/IP三次握手与四次挥手的正确姿势
https://www.cnblogs.com/lms0755/p/9053119.html
B 四次挥手过程理解
https://blog.csdn.net/qq_38950316/article/details/81087809
C TCP三次握手四次挥手详解
http://www.cnblogs.com/zmlctt/p/3690998.html