概述
在主动关闭方发送了FIN之后,进入FIN_WAIT_1状态,在此状态收到了ACK,则进入FIN_WAIT_2状态,而FIN_WAIT_2后续要做的工作是等待接收对端发过来的FIN包,并且发送ACK,进而进入到TIME_WAIT状态;本文主要关注从FIN_WAIT_1进入FIN_WAIT_2状态,以及在FIN_WAIT_2状态来包或者定时器触发后的处理流程;
进入FIN_WAIT_2
tcp_rcv_state_process函数中对于ack的处理步骤中,假如连接处于FIN_WAIT_1,且数据均已经被确认完,则进入TIME_WAIT_2状态;如果无需在该状态等待(linger2<0),或者收到了乱序数据段,则直接关闭连接;如果需要等待,则需要判断等待时间与TIMEWAIT时间的大小关系,若>TIMEWAIT_LEN,则添加TIME_WAIT_2定时器,否则直接进入TIME_WAIT接管(其子状态仍然是FIN_WAIT_2),接管之后会添加TIME_WAIT定时器;
另,tcp_close函数调用时,如果当前状态是FIN_WAIT_2也会用相似方式进入TIME_WAIT接管,不再单独介绍;
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 caseTCP_FIN_WAIT1: { 9 inttmo; 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 gotodiscard; 91 } 92 break; 93 } 94 }
状态转换触发
FIN_WAIT_2状态的走向有以下几个流程触发点,(1)TIME_WAIT_2定时器未超时时间内,收到数据段触发; (2)TIME_WAIT_2定时器超时触发; (3)TIME_WAIT定时器未超时时间内,收到数据段触发; (4)TIME_WAIT定时器超时触发;
(1)TIME_WAIT_2定时器未超时时间内,收到数据段触发,如果设置FIN标记,则直接进入TIME_WAIT状态;
在函数tcp_rcv_state_process处理数据段的过程中,FIN_WAIT_2状态最终会调用tcp_data_queue来处理数据段;
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 caseTCP_CLOSE_WAIT: 6 caseTCP_CLOSING: 7 caseTCP_LAST_ACK: 8 if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) 9 break; 10 caseTCP_FIN_WAIT1: 11 caseTCP_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 caseTCP_ESTABLISHED: 26 tcp_data_queue(sk, skb); 27 queued = 1; 28 break; 29 } 30 }
tcp_data_queue在处理数据段的时候,有对FIN标记的检查,如果有该标记,则进入tcp_fin函数;
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 }
tcp_fin函数中,如果此时连接状态为FIN_WAIT_2,则发送ACK,并且直接进入TIME_WAIT状态;在tcp_time_wait函数处理中,会删除当前控制块,所以FIN_WAIT_2定时器也就不存在了;
1 void tcp_fin(struct sock *sk) 2 { 3 /*... */ 4 switch (sk->sk_state) { 5 caseTCP_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 }
(2)TIME_WAIT_2定时器超时触发,如果linger2<0,或者等待时间<=TIMEWAIT_LEN,直接发送reset关闭连接;如果linger2>=0,且等待时间>TIMEWAIT_LEN,则进入TIME_WAIT接管;
1 static void tcp_keepalive_timer (unsigned longdata) 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 gotodeath; 22 } 23 /*...*/ 24 }
(3)TIME_WAIT定时器未超时时间内,收到数据段触发,若收到合法的FIN,则进入真正的TIME_WAIT状态;
tcp_v4_rcv收入数据段过程中,会对TIME_WAIT状态做特别处理,而对于TIME_WAIT子状态的处理在函数tcp_timewait_state_process中;
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 gotodiscard_it; 8 } 9 10 /*校验和错误 */ 11 if(tcp_checksum_complete(skb)) { 12 inet_twsk_put(inet_twsk(sk)); 13 gotocsum_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 caseTCP_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 gotoprocess; 39 } 40 /*Fall through to ACK */ 41 } 42 43 /*发送ack */ 44 caseTCP_TW_ACK: 45 tcp_v4_timewait_ack(sk, skb); 46 break; 47 /*发送rst */ 48 caseTCP_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 gotodiscard_it; 53 /*成功*/ 54 caseTCP_TW_SUCCESS:; 55 } 56 gotodiscard_it; 57 }
tcp_timewait_state_process函数处理流程中,如果TIME_WAIT的子状态为FIN_WAIT_2,并且收到了合法的FIN之后,会进入真正的TIME_WAIT状态,即子状态也为TIME_WAIT,并且设置TIME_WAIT定时器;
1 enumtcp_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 returntcp_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 gotokill; 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 returnTCP_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 returnTCP_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 returnTCP_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 returnTCP_TW_ACK; 62 } 63 /*...*/ 64 }
(4)TIME_WAIT定时器超时触发,定时器超时,将tw控制块从ehash和bhash中删除,在收到数据段会发送reset;
定时器超时会进入到tw_timer_handler处理函数,该函数在统计信息之后,调用inet_twsk_kill;
1 static void tw_timer_handler(unsigned longdata) 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 }
inet_twsk_kill从ehash和bhash中把tw控制块删除,并且释放之;
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 }