关闭TCP连接

关闭TCP连接的面试问题三境界

1. 描述下TCP连接关闭的过程？
2. 描述下TCP连接关闭的步骤，以及每一步的状态？
3. shutdwon与close的区别？

step	packet	client state	server state	remark
0		ESTABLISHED	ESTABLISHED
1	client 向 server 发送FIN包(seq=x) server收到之前	FIN_WAIT_1	ESTABLISHED	client调用close
2	server收到FIN包，server向client发送ACK包(ack=x+1)，client收到之后	FIN_WAIT_2	CLOSE_WAIT
3	server向client发送 FIN包(seq=y)，client收到之前	FIN_WAIT_2	LAST_ACK	server 调用close
4	client向server发送 ACK包(ack=y+1)，server收到确认之后	TIME_WAIT	CLOSED

###shutdwon & close

man 2 shutdown

NAME
       shutdown - shut down part of a full-duplex connection

SYNOPSIS
       #include <sys/socket.h>

       int shutdown(int sockfd, int how);

DESCRIPTION
       The  shutdown()  call  causes  all  or part of a full-duplex connection on the socket associated with sockfd to be shut down.  If how is SHUT_RD, further receptions will be disallowed.  If how is SHUT_WR, further
       transmissions will be disallowed.  If how is SHUT_RDWR, further receptions and transmissions will be disallowed.

shutdown 可以通过how参数选择关闭sockfd读端、写端或者读写端。
close 会关闭sockfd的读写两端，不能选择性关闭。