Linux四次挥手源码分析

TCP_STATES

四次挥手过程分析【V5.8，正常流程】
1、客户端主动断开连接，状态从TCP_ESTABLISHED变为TCP_FIN_WAIT1，发送FIN包给服务端

A、状态变为TCP_FIN_WAIT1
tcp_close->tcp_close_state
->tcp_set_state(sk, new_state[TCP_ESTABLISHED])，也就是TCP_FIN_WAIT1

B、发送FIN包
tcp_close->tcp_close_state
->tcp_send_fin

2、服务端收到FIN包，状态从TCP_ESTABLISHED变为TCP_CLOSE_WAIT，并返回ACK包

A、状态变为TCP_CLOSE_WAIT
【tcp_protocol.handler】tcp_v4_rcv->tcp_v4_do_rcv->tcp_rcv_established
->tcp_data_queue
->->tcp_fin
->->->inet_csk_schedule_ack; 安排ack
->->->sk->sk_shutdown |= RCV_SHUTDOWN; 模拟了close
->->->sock_set_flag(sk, SOCK_DONE);
->->->case TCP_ESTABLISHED:
->->->tcp_set_state(sk, TCP_CLOSE_WAIT); 修改状态
->->inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;  ACS是否立即发送

B、发送ACK包
【tcp_protocol.handler】tcp_v4_rcv->tcp_v4_do_rcv->tcp_rcv_established【接上面】
->tcp_ack_snd_check->__tcp_ack_snd_check->tcp_send_ack

3、客户端收到ACK包，状态从TCP_FIN_WAIT1变为TCP_FIN_WAIT2，然后被替换为状态TCP_TIME_WAIT，子状态TCP_FIN_WAIT2

【tcp_protocol.handler】tcp_v4_rcv->tcp_v4_do_rcv->tcp_rcv_state_process
->case TCP_FIN_WAIT1:
->tcp_set_state(sk, TCP_FIN_WAIT2);
->tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
->->tw = inet_twsk_alloc(sk, tcp_death_row, state);
->->->tw->tw_state = TCP_TIME_WAIT;   
->->->tw->tw_substate = TCP_FIN_WAIT2;
->->->timer_setup(&tw->tw_timer, tw_timer_handler, TIMER_PINNED);

4、服务端状态从TCP_CLOSE_WAIT变为TCP_LAST_ACK，发送FIN包

A、状态变为TCP_LAST_ACK
tcp_close->tcp_close_state
->tcp_set_state(sk, new_state[TCP_CLOSE_WAIT])，也就是TCP_LAST_ACK

B、发送FIN包
tcp_close->tcp_close_state
->tcp_send_fin

5、客户端收到FIN包，子状态从TCP_FIN_WAIT2变为TCP_TIME_WAIT，返回ACK包

A、状态和子状态都为TCP_TIME_WAIT
【tcp_protocol.handler】tcp_v4_rcv->
->if (sk->sk_state == TCP_TIME_WAIT) goto do_time_wait;
->do_time_wait:
->tcp_timewait_state_process
->->if (tw->tw_substate == TCP_FIN_WAIT2)
->->tw->tw_substate = TCP_TIME_WAIT;
->->inet_twsk_reschedule，重新设置回调时间
->->return TCP_TW_ACK;

B、返回ACK
->case TCP_TW_ACK:
->tcp_v4_timewait_ack(sk, skb);

6、服务端收到ACK包，状态从TCP_LAST_ACK变为TCP_CLOSE

【tcp_protocol.handler】tcp_v4_rcv->tcp_v4_do_rcv->tcp_rcv_state_process
->case TCP_LAST_ACK:
->tcp_done
->->tcp_set_state(sk, TCP_CLOSE);

7、客户端超时回调

A、超时时间定义
#define TCP_TIMEWAIT_LEN (60*HZ)
#define TCP_FIN_TIMEOUT TCP_TIMEWAIT_LEN

B、超时后，回调tw_timer_handler->inet_twsk_kill，进行inet_timewait_sock清理工作

C、没有找到状态变从TCP_TIME_WAIT变为TCP_CLOSE的代码

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