tcp_process(struct tcp_pcb pcb)函数进行此插口（socket）状态的转移和处理
if ((pcb->flags & TF_RXCLOSED) == 0) {
/ Update the PCB (in)activity timer unless rx is closed (see tcp_shutdown) */
pcb->tmr = tcp_ticks;
}
如果没有关闭的话 更新此链接的保活时间
/* Do different things depending on the TCP state. */
switch (pcb->state) {
CLOSED = 0,
LISTEN = 1,
SYN_SENT = 2,
SYN_RCVD = 3,
ESTABLISHED = 4,
FIN_WAIT_1 = 5,
FIN_WAIT_2 = 6,
CLOSE_WAIT = 7,
CLOSING = 8,
LAST_ACK = 9,
TIME_WAIT = 10
如上图进行状态转换
case SYN_SENT:
/* received SYN ACK with expected sequence number? */
if ((flags & TCP_ACK) && (flags & TCP_SYN)
&& (ackno == pcb->lastack + 1)) {
pcb->state = ESTABLISHED;
主动打开 这边收到之后就已经建立连接，然后设置ack位 tcp_input函数判断此位然后发送ack包
tcp_ack_now(pcb);
}
/* received ACK? possibly a half-open connection */
else if (flags & TCP_ACK) {
这只有可能对面也在主动打开 ， 但是我们没有收到syc ，否则我们就进入 SYN_RCVD 状态了（如图）
/* send a RST to bring the other side in a non-synchronized state. */
tcp_rst(pcb, ackno, seqno + tcplen, ip_current_dest_addr(),
ip_current_src_addr(), tcphdr->dest, tcphdr->src);
}
case SYN_RCVD:
if (flags & TCP_ACK) {
/* expected ACK number? /
if (TCP_SEQ_BETWEEN(ackno, pcb->lastack + 1, pcb->snd_nxt)) {
pcb->state = ESTABLISHED;
这是被动打开 ，收到ack说明握手成功，如果此时也收到关闭进入关闭逻辑
if (recv_flags & TF_GOT_FIN) {
tcp_ack_now(pcb);
pcb->state = CLOSE_WAIT;
}
} else {
如果ack号不匹配握手失败 rst；
/ incorrect ACK number, send RST /
tcp_rst(pcb, ackno, seqno + tcplen, ip_current_dest_addr(),
ip_current_src_addr(), tcphdr->dest, tcphdr->src);
}
} else if ((flags & TCP_SYN) && (seqno == pcb->rcv_nxt - 1)) {
如果发来了重复的syn，就重新发送syn-ack
/ Looks like another copy of the SYN - retransmit our SYN-ACK */
tcp_rexmit(pcb);
}
break;
case CLOSE_WAIT:
/* FALLTHROUGH /
case ESTABLISHED:
tcp_receive(pcb);//时候据的话就会调用用户回调函数
//被动 关闭 收到fin 发 ack
if (recv_flags & TF_GOT_FIN) { / passive close */
tcp_ack_now(pcb);
pcb->state = CLOSE_WAIT;
}
break;
case FIN_WAIT_1:
tcp_receive(pcb);
if (recv_flags & TF_GOT_FIN) {//同时关闭
if ((flags & TCP_ACK) && (ackno == pcb->snd_nxt) &&
tcp_ack_now(pcb);
tcp_pcb_purge(pcb);
TCP_RMV_ACTIVE(pcb);
pcb->state = TIME_WAIT;
TCP_REG(&tcp_tw_pcbs, pcb);
} else {
tcp_ack_now(pcb);
pcb->state = CLOSING;
}
} else if ((flags & TCP_ACK) && (ackno == pcb->snd_nxt) &&
pcb->unsent == NULL) {
pcb->state = FIN_WAIT_2;
}
break;