tcp/ip 协议栈版本：lwip-1.4.0 参考阅读：《嵌入式网络那些事》--传输控制协议  章节 TCP初始编程--常用部分函数解析连接：

• 原型: struct tcp_pcb * tcp_new (void)
• 原型: err_t  tcp_bind(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port)
  
• #define tcp_listen(pcb)  tcp_listen_with_backlog(pcb, TCP_DEFAULT_LISTEN_BACKLOG)
  

简介几个概念： 滑动窗口：
TCP的滑动窗口主要有两个作用，

• 提供TCP的可靠性
• 提供TCP的流控特性
  

参考： TCP 滑动窗口（发送窗口和接收窗口）

TCP报文结构： 这里关注两个标志：SYN和FIN 连接建立/关闭
正文： 控制块连接  对于客户端程序来说，需要执行主动打开操作，就是向服务器端发送一个SYN握手报文段，通过tcp_connect函数实现。 控制快的四条链表： /* The TCP PCB lists.--四条链表来连接处于不同状态下的控制块 */ /** List of all TCP PCBs bound but not yet (connected || listening) --新创建的且绑定了本地端口号，但还没有连接或者侦听的PCBs*/ struct tcp_pcb *tcp_bound_pcbs; /** List of all TCP PCBs in LISTEN state --进入LISTEN侦听状态（被动连接）的*/ union tcp_listen_pcbs_t tcp_listen_pcbs; /** List of all TCP PCBs that are in a state in which--连接所有处于其他状态(另外三个链表之外的)的控制块:稳定状态/活跃的链表 * they accept or send data. */ struct tcp_pcb *tcp_active_pcbs; /** List of all TCP PCBs in TIME-WAIT state ---处于TIME-WAIT状态的控制块链表*/ struct tcp_pcb *tcp_tw_pcbs;一个本地全局数组，指向上诉四条链表指针的指针： 以此减少代码量 /** An array with all (non-temporary) PCB lists, mainly used for smaller code size */ struct tcp_pcb ** const tcp_pcb_lists[] = {&tcp_listen_pcbs.pcbs, &tcp_bound_pcbs, &tcp_active_pcbs, &tcp_tw_pcbs};
相关宏： #define NUM_TCP_PCB_LISTS 4 //TCP控制块链表类型数目 #define NUM_TCP_PCB_LISTS_NO_TIME_WAIT 3 //TCP控制块链表类型数目，不带TIME_WAIT那类链表
源码+注释如下： /** * Connects to another host. The function given as the "connected" -连接到另外一个主机。 * argument will be called when the connection has been established.-当连接已经建立时，函数的参数"connected"参数将被调用 *主动向服务器发送一个SYN握手报文 * @param pcb the tcp_pcb used to establish the connection * @param ipaddr the remote ip address to connect to * @param port the remote tcp port to connect to * @param connected callback function to call when connected (or on error) * @return ERR_VAL if invalid arguments are given * ERR_OK if connect request has been sent * other err_t values if connect request couldn't be sent */ err_t tcp_connect(struct tcp_pcb *pcb, ip_addr_t *ipaddr, u16_t port, tcp_connected_fn connected) { err_t ret; u32_t iss; u16_t old_local_port; LWIP_ERROR("tcp_connect: can only connect from state CLOSED", pcb->state == CLOSED, return ERR_ISCONN); LWIP_DEBUGF(TCP_DEBUG, ("tcp_connect to port %"U16_F" ", port)); if (ipaddr != NULL) { pcb->remote_ip = *ipaddr;//对端ip初始化 } else { return ERR_VAL; } pcb->remote_port = port;//对端端口号 /* check if we have a route to the remote host --检查是否有一个路由/途径到远程主机*/ if (ip_addr_isany(&(pcb->local_ip))) {//本地ip是 无效或者广播? /* no local IP address set, yet. 还没有设置本地IP地址。*/ struct netif *netif = ip_route(&(pcb->remote_ip));//尝试找到一个能够到达目的地址的本地ip if (netif == NULL) {//没有就算啦 /* Don't even try to send a SYN packet if we have no route since that will fail. */ return ERR_RTE; } /* Use the netif's IP address as local address. */ ip_addr_copy(pcb->local_ip, netif->ip_addr);//给pcb一个可用的本地ip } old_local_port = pcb->local_port; if (pcb->local_port == 0) {//端口无效，给他找一个 pcb->local_port = tcp_new_port(); } #if SO_REUSE //这部分详情度娘搜索：SO_REUSEADDR 关键字 if ((pcb->so_options & SOF_REUSEADDR) != 0) { /* Since SOF_REUSEADDR allows reusing a local address, we have to make sure==地址可重用 now that the 5-tuple is unique. */ struct tcp_pcb *cpcb; int i; /* Don't check listen- and bound-PCBs, check active- and TIME-WAIT PCBs. 仅需要遍历检查tcp_active_pcbs 和 tcp_tw_pcbs链表*/ for (i = 2; i < NUM_TCP_PCB_LISTS; i++) { for(cpcb = *tcp_pcb_lists[i]; cpcb != NULL; cpcb = cpcb->next) { if ((cpcb->local_port == pcb->local_port) && (cpcb->remote_port == port) && ip_addr_cmp(&cpcb->local_ip, &pcb->local_ip) && ip_addr_cmp(&cpcb->remote_ip, ipaddr)) { /* linux returns EISCONN here, but ERR_USE should be OK for us */ return ERR_USE; } } } } #endif /* SO_REUSE */ iss = tcp_next_iss();//初始化 序列号 pcb->rcv_nxt = 0; //设置发送窗口的各个字段 pcb->snd_nxt = iss; pcb->lastack = iss - 1; pcb->snd_lbb = iss - 1; pcb->rcv_wnd = TCP_WND;//接收窗口的字段 pcb->rcv_ann_wnd = TCP_WND; pcb->rcv_ann_right_edge = pcb->rcv_nxt; pcb->snd_wnd = TCP_WND; /* As initial send MSS, we use TCP_MSS but limit it to 536. The send MSS is updated when an MSS option is received. */ pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS; #if TCP_CALCULATE_EFF_SEND_MSS //根据需要设置有效发送mss字段 pcb->mss = tcp_eff_send_mss(pcb->mss, ipaddr); #endif /* TCP_CALCULATE_EFF_SEND_MSS */ pcb->cwnd = 1; pcb->ssthresh = pcb->mss * 10; #if LWIP_CALLBACK_API pcb->connected = connected;//注册connected回调函数 #else /* LWIP_CALLBACK_API */ LWIP_UNUSED_ARG(connected); #endif /* LWIP_CALLBACK_API */ /* Send a SYN together with the MSS option. 拼接带着MSS选项的SYN标志请求 */ ret = tcp_enqueue_flags(pcb, TCP_SYN); if (ret == ERR_OK) { /* SYN segment was enqueued, changed the pcbs state now */ pcb->state = SYN_SENT;// pcb按规则入队，状态置为SYN_SENT。此处放入tcp_active_pcbs if (old_local_port != 0) { TCP_RMV(&tcp_bound_pcbs, pcb);//状态更新，pcb移出相应链表 } TCP_REG(&tcp_active_pcbs, pcb); snmp_inc_tcpactiveopens();//未知 tcp_output(pcb);//将pcb上连接的报文发送出去 } return ret; }关于tcp_enqueue_flags、tcp_output函数，再议。。。