TCP 之 FIN_WAIT_2状态处理流程

 1 int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 2 {
 3     /* step 5: check the ACK field */
 4     acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
 5                       FLAG_UPDATE_TS_RECENT) > 0;
 6 
 7     switch (sk->sk_state) {
 8     case TCP_FIN_WAIT1: {
 9         int tmo;
10 
11         /* If we enter the TCP_FIN_WAIT1 state and we are a
12          * Fast Open socket and this is the first acceptable
13          * ACK we have received, this would have acknowledged
14          * our SYNACK so stop the SYNACK timer.
15          */
16         if (req) {
17             /* Return RST if ack_seq is invalid.
18              * Note that RFC793 only says to generate a
19              * DUPACK for it but for TCP Fast Open it seems
20              * better to treat this case like TCP_SYN_RECV
21              * above.
22              */
23             if (!acceptable)
24                 return 1;
25             /* We no longer need the request sock. */
26             reqsk_fastopen_remove(sk, req, false);
27             tcp_rearm_rto(sk);
28         }
29 
30         /* 发送数据未确认完毕 */
31         if (tp->snd_una != tp->write_seq)
32             break;
33 
34         /* 进入FIN_WAIT_2状态 */
35         tcp_set_state(sk, TCP_FIN_WAIT2);
36 
37         /* 关闭发送端 */
38         sk->sk_shutdown |= SEND_SHUTDOWN;
39 
40         /* 路由缓存确认 */
41         sk_dst_confirm(sk);
42 
43         /* 套接口不是DEAD状态，状态发生变化，唤醒等待进程 */
44         if (!sock_flag(sk, SOCK_DEAD)) {
45             /* Wake up lingering close() */
46             sk->sk_state_change(sk);
47             break;
48         }
49 
50         /* linger2<0，无需在FIN_WAIT_2等待 */
51         if (tp->linger2 < 0) {
52             /* 关闭连接 */
53             tcp_done(sk);
54             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
55             return 1;
56         }
57 
58         /* 收到期望序号以后的数据段(data, fin) */
59         if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
60             after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
61             /* Receive out of order FIN after close() */
62             if (tp->syn_fastopen && th->fin)
63                 tcp_fastopen_active_disable(sk);
64             /* 关闭连接 */
65             tcp_done(sk);
66             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
67             return 1;
68         }
69 
70         /* 获取FIN_WAIT_2等待时间 */
71         tmo = tcp_fin_time(sk);
72 
73         /* > TIMEWAIT_LEN，加入FIN_WAIT_2定时器 */
74         if (tmo > TCP_TIMEWAIT_LEN) {
75             inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
76         } 
77         /* 有fin?? 或者 被用户进程锁定，加入FIN_WAIT_2定时器 */
78         else if (th->fin || sock_owned_by_user(sk)) {
79             /* Bad case. We could lose such FIN otherwise.
80              * It is not a big problem, but it looks confusing
81              * and not so rare event. We still can lose it now,
82              * if it spins in bh_lock_sock(), but it is really
83              * marginal case.
84              */
85             inet_csk_reset_keepalive_timer(sk, tmo);
86         } 
87         /* 正常等待时间< TIMEWAIT_LEN，进入TIMEWAIT接管状态 */
88         else {
89             tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
90             goto discard;
91         }
92         break;
93     }
94 }

 1 int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 2 {
 3         /* step 7: process the segment text */
 4     switch (sk->sk_state) {
 5     case TCP_CLOSE_WAIT:
 6     case TCP_CLOSING:
 7     case TCP_LAST_ACK:
 8         if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
 9             break;
10     case TCP_FIN_WAIT1:
11     case TCP_FIN_WAIT2:
12         /* RFC 793 says to queue data in these states,
13          * RFC 1122 says we MUST send a reset.
14          * BSD 4.4 also does reset.
15          */
16         if (sk->sk_shutdown & RCV_SHUTDOWN) {
17             if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
18                 after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
19                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
20                 tcp_reset(sk);
21                 return 1;
22             }
23         }
24         /* Fall through */
25     case TCP_ESTABLISHED:
26         tcp_data_queue(sk, skb);
27         queued = 1;
28         break;
29     }
30 }

1 static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
2 {
3     /* ... */
4     if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
5         tcp_fin(sk);
6     /* ... */
7 }

 1 void tcp_fin(struct sock *sk)
 2 {
 3         /* ... */
 4     switch (sk->sk_state) {
 5     case TCP_FIN_WAIT2:
 6         /* Received a FIN -- send ACK and enter TIME_WAIT. */
 7         tcp_send_ack(sk);
 8         tcp_time_wait(sk, TCP_TIME_WAIT, 0);
 9         break;
10     }
11         /* ... */
12 }

 1 static void tcp_keepalive_timer (unsigned long data)
 2 {
 3         /*...*/
 4     /* 处于fin_wait2且socket即将销毁，用作FIN_WAIT_2定时器 */
 5     if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
 6 
 7         /* 停留在FIN_WAIT_2的停留时间>=0 */
 8         if (tp->linger2 >= 0) {
 9             /* 获取时间差值 */
10             const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;
11 
12             /* 差值>0，等待时间>TIME_WAIT时间，则进入TIME_WAIT状态 */
13             if (tmo > 0) {
14                 tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
15                 goto out;
16             }
17         }
18 
19         /* 发送rst */
20         tcp_send_active_reset(sk, GFP_ATOMIC);
21         goto death;
22     }
23         /*...*/
24 }

 1 int tcp_v4_rcv(struct sk_buff *skb)
 2 {
 3 /*...*/
 4 do_time_wait:
 5     if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
 6         inet_twsk_put(inet_twsk(sk));
 7         goto discard_it;
 8     }
 9 
10     /* 校验和错误 */
11     if (tcp_checksum_complete(skb)) {
12         inet_twsk_put(inet_twsk(sk));
13         goto csum_error;
14     }
15 
16     /* TIME_WAIT入包处理 */
17     switch (tcp_timewait_state_process(inet_twsk(sk), skb, th)) {
18 
19     /* 收到syn */
20     case TCP_TW_SYN: {
21         /* 查找监听控制块 */
22         struct sock *sk2 = inet_lookup_listener(dev_net(skb->dev),
23                             &tcp_hashinfo, skb,
24                             __tcp_hdrlen(th),
25                             iph->saddr, th->source,
26                             iph->daddr, th->dest,
27                             inet_iif(skb));
28 
29         /* 找到 */
30         if (sk2) {
31             /* 删除tw控制块 */
32             inet_twsk_deschedule_put(inet_twsk(sk));
33             /* 记录监听控制块 */
34             sk = sk2;
35             refcounted = false;
36 
37             /* 进行新请求的处理 */
38             goto process;
39         }
40         /* Fall through to ACK */
41     }
42 
43     /* 发送ack */
44     case TCP_TW_ACK:
45         tcp_v4_timewait_ack(sk, skb);
46         break;
47     /* 发送rst */
48     case TCP_TW_RST:
49         tcp_v4_send_reset(sk, skb);
50         /* 删除tw控制块 */
51         inet_twsk_deschedule_put(inet_twsk(sk));
52         goto discard_it;
53     /* 成功*/
54     case TCP_TW_SUCCESS:;
55     }
56     goto discard_it;
57 }

 1 enum tcp_tw_status
 2 tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,
 3                const struct tcphdr *th)
 4 {
 5         /*...*/
 6     /* 子状态是FIN_WAIT2 */
 7     if (tw->tw_substate == TCP_FIN_WAIT2) {
 8         /* Just repeat all the checks of tcp_rcv_state_process() */
 9 
10         /* Out of window, send ACK */
11         /* 序号回绕或者数据超出窗口范围，发送ack */
12         if (paws_reject ||
13             !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
14                    tcptw->tw_rcv_nxt,
15                    tcptw->tw_rcv_nxt + tcptw->tw_rcv_wnd))
16             return tcp_timewait_check_oow_rate_limit(
17                 tw, skb, LINUX_MIB_TCPACKSKIPPEDFINWAIT2);
18 
19         /* rst，则停止调度，销毁tw控制块 */
20         if (th->rst)
21             goto kill;
22 
23         /* syn && 序号>= 期望接收序号??? */
24         if (th->syn && !before(TCP_SKB_CB(skb)->seq, tcptw->tw_rcv_nxt))
25             return TCP_TW_RST;
26 
27         /* Dup ACK? */
28         /* 非ack || 以前的ack || 新的无数据ack */
29         if (!th->ack ||
30             !after(TCP_SKB_CB(skb)->end_seq, tcptw->tw_rcv_nxt) ||
31             TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
32             inet_twsk_put(tw);
33             return TCP_TW_SUCCESS;
34         }
35 
36         /* New data or FIN. If new data arrive after half-duplex close,
37          * reset.
38          */
39         /* 不是fin，或者fin有数据 */
40         if (!th->fin ||
41             TCP_SKB_CB(skb)->end_seq != tcptw->tw_rcv_nxt + 1)
42             return TCP_TW_RST;
43 
44         /* FIN arrived, enter true time-wait state. */
45         /* fin包，进入真正的TIME_WAIT */
46         tw->tw_substate      = TCP_TIME_WAIT;
47 
48         /* 设置下一次接收序号 */
49         tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
50 
51         /* 设置时间戳 */
52         if (tmp_opt.saw_tstamp) {
53             tcptw->tw_ts_recent_stamp = get_seconds();
54             tcptw->tw_ts_recent      = tmp_opt.rcv_tsval;
55         }
56 
57         /*重新设置tw定时器 */
58         inet_twsk_reschedule(tw, TCP_TIMEWAIT_LEN);
59 
60         /* 发送ack */
61         return TCP_TW_ACK;
62     }
63         /*...*/
64 }

 1 static void tw_timer_handler(unsigned long data)
 2 {
 3     struct inet_timewait_sock *tw = (struct inet_timewait_sock *)data;
 4 
 5     if (tw->tw_kill)
 6         __NET_INC_STATS(twsk_net(tw), LINUX_MIB_TIMEWAITKILLED);
 7     else
 8         __NET_INC_STATS(twsk_net(tw), LINUX_MIB_TIMEWAITED);
 9     inet_twsk_kill(tw);
10 }

 1 static void inet_twsk_kill(struct inet_timewait_sock *tw)
 2 {
 3     struct inet_hashinfo *hashinfo = tw->tw_dr->hashinfo;
 4     spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
 5     struct inet_bind_hashbucket *bhead;
 6 
 7     spin_lock(lock);
 8     sk_nulls_del_node_init_rcu((struct sock *)tw);
 9     spin_unlock(lock);
10 
11     /* Disassociate with bind bucket. */
12     bhead = &hashinfo->bhash[inet_bhashfn(twsk_net(tw), tw->tw_num,
13             hashinfo->bhash_size)];
14 
15     spin_lock(&bhead->lock);
16     inet_twsk_bind_unhash(tw, hashinfo);
17     spin_unlock(&bhead->lock);
18 
19     atomic_dec(&tw->tw_dr->tw_count);
20     inet_twsk_put(tw);
21 }