static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,

                      const struct tcphdr *th)

 {

     if (th->syn) {

         /* We see SYN without ACK. It is attempt of

          * simultaneous connect with crossed SYNs.

          * Particularly, it can be connect to self.

          */

         tcp_set_state(sk, TCP_SYN_RECV);

         if (tp->rx_opt.saw_tstamp) {

             tp->rx_opt.tstamp_ok = ;

             tcp_store_ts_recent(tp);

             tp->tcp_header_len =

                 sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;

         } else {

             tp->tcp_header_len = sizeof(struct tcphdr);

         }

         tp->rcv_nxt = TCP_SKB_CB(skb)->seq + ;

         tp->copied_seq = tp->rcv_nxt;

         tp->rcv_wup = TCP_SKB_CB(skb)->seq + ;

         /* RFC1323: The window in SYN & SYN/ACK segments is

          * never scaled.

          */

         tp->snd_wnd    = ntohs(th->window);

         tp->snd_wl1    = TCP_SKB_CB(skb)->seq;

         tp->max_window = tp->snd_wnd;

         tcp_ecn_rcv_syn(tp, th);

         tcp_mtup_init(sk);

         tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);

         tcp_initialize_rcv_mss(sk);

         tcp_send_synack(sk);

 }

 int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)

 {

         /* step 5: check the ACK field */

     acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |

                       FLAG_UPDATE_TS_RECENT) > ;

     switch (sk->sk_state) {

     case TCP_SYN_RECV:

         /* ack处理失败 */

         if (!acceptable)

             return ;

         /* RTT */

         if (!tp->srtt_us)

             tcp_synack_rtt_meas(sk, req);

         /* Once we leave TCP_SYN_RECV, we no longer need req

          * so release it.

          */

         if (req) {

             inet_csk(sk)->icsk_retransmits = ;

             reqsk_fastopen_remove(sk, req, false);

         } else {

             /* Make sure socket is routed, for correct metrics. */

             /* 检查重建路由 */

             icsk->icsk_af_ops->rebuild_header(sk);

             /* 初始化拥塞邋控制 */

             tcp_init_congestion_control(sk);

             /* 路径mtu发现初始化 */

             tcp_mtup_init(sk);

             /* 用户待读取数据初始化 */

             tp->copied_seq = tp->rcv_nxt;

             /* 调整接收发送缓存以及窗口等 */

             tcp_init_buffer_space(sk);

         }

         smp_mb();

         /* 连接更新为已连接状态 */

         tcp_set_state(sk, TCP_ESTABLISHED);

         sk->sk_state_change(sk);

                 /* 省略了一些代码 */

 }