lwip socket探秘之accept

  1 /**
  2 * The initial input processing of TCP. It verifies the TCP header, demultiplexes
  3 * the segment between the PCBs and passes it on to tcp_process(), which implements
  4 * the TCP finite state machine. This function is called by the IP layer (in
  5 * ip_input()).
  6 *
  7 * @param p received TCP segment to process (p->payload pointing to the IP header)
  8 * @param inp network interface on which this segment was received
  9 */
 10 void
 11 tcp_input(struct pbuf *p, struct netif *inp)
 12 {
 13   struct tcp_pcb *pcb, *prev;
 14   struct tcp_pcb_listen *lpcb;
 15   u8_t hdrlen;
 16   err_t err;
 17 
 18   PERF_START;
 19 
 20   TCP_STATS_INC(tcp.recv);
 21   snmp_inc_tcpinsegs();
 22 
 23   iphdr = p->payload; // 得到了IP header
 24   tcphdr = (struct tcp_hdr *)((u8_t *)p->payload + IPH_HL(iphdr) * 4); // 得到了TCP header
 25 
 26 ............
 27 
 28   /* Convert fields in TCP header to host byte order. */
 29   tcphdr->src = ntohs(tcphdr->src); // 把tcp header的一些内容从网络字节序转成主机字节序。可以猜测ip header已经在ip层被转过了。
 30   tcphdr->dest = ntohs(tcphdr->dest);
 31   seqno = tcphdr->seqno = ntohl(tcphdr->seqno);
 32   ackno = tcphdr->ackno = ntohl(tcphdr->ackno);
 33   tcphdr->wnd = ntohs(tcphdr->wnd);
 34 
 35 ..................
 36 
 37 //下面出现了3个pcb链表，分别是tcp_active_pcbs、tcp_tw_pcbs和tcp_listen_pcbs。
 38   for(pcb = tcp_active_pcbs; pcb != NULL; pcb = pcb->next) {
 39     LWIP_ASSERT("tcp_input: active pcb->state != CLOSED", pcb->state != CLOSED);
 40     LWIP_ASSERT("tcp_input: active pcb->state != TIME-WAIT", pcb->state != TIME_WAIT);
 41     LWIP_ASSERT("tcp_input: active pcb->state != LISTEN", pcb->state != LISTEN);
 42     if (pcb->remote_port == tcphdr->src &&
 43        pcb->local_port == tcphdr->dest &&
 44        ip_addr_cmp(&(pcb->remote_ip), &(iphdr->src)) &&
 45        ip_addr_cmp(&(pcb->local_ip), &(iphdr->dest))) {
 46 
 47       /* Move this PCB to the front of the list so that subsequent
 48          lookups will be faster (we exploit locality in TCP segment
 49          arrivals). */
 50       LWIP_ASSERT("tcp_input: pcb->next != pcb (before cache)", pcb->next != pcb);
 51       if (prev != NULL) {
 52         prev->next = pcb->next;
 53         pcb->next = tcp_active_pcbs;
 54         tcp_active_pcbs = pcb;
 55       }
 56       LWIP_ASSERT("tcp_input: pcb->next != pcb (after cache)", pcb->next != pcb);
 57       break;
 58     }
 59     prev = pcb;
 60   }
 61 
 62   if (pcb == NULL) {
 63     /* If it did not go to an active connection, we check the connections
 64        in the TIME-WAIT state. */
 65     for(pcb = tcp_tw_pcbs; pcb != NULL; pcb = pcb->next) {
 66       LWIP_ASSERT("tcp_input: TIME-WAIT pcb->state == TIME-WAIT", pcb->state == TIME_WAIT);
 67       if (pcb->remote_port == tcphdr->src &&
 68          pcb->local_port == tcphdr->dest &&
 69          ip_addr_cmp(&(pcb->remote_ip), &(iphdr->src)) &&
 70          ip_addr_cmp(&(pcb->local_ip), &(iphdr->dest))) {
 71         /* We don‘t really care enough to move this PCB to the front
 72            of the list since we are not very likely to receive that
 73            many segments for connections in TIME-WAIT. */
 74         LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_input: packed for TIME_WAITing connection.\n"));
 75         tcp_timewait_input(pcb);
 76         pbuf_free(p);
 77         return;
 78       }
 79     }
 80 
 81 //我们暂时只关心tcp_listen_pcbs这个链表。这个链表包含了所有处在listen状态的pcb。一个listen状态的pcb是什么时候把自己注册进tcp_listen_pcbs这个链表的？这个待会讲到。
 82   /* Finally, if we still did not get a match, we check all PCBs that
 83      are LISTENing for incoming connections. */
 84     prev = NULL;
 85     for(lpcb = tcp_listen_pcbs.listen_pcbs; lpcb != NULL; lpcb = lpcb->next) {
 86       if ((ip_addr_isany(&(lpcb->local_ip)) || // 如果pcb->local_ip是0或者NULL，意味着接收任何ip的连接请求
 87         ip_addr_cmp(&(lpcb->local_ip), &(iphdr->dest))) &&  // 或者pcb->local_ip和接收到的tcp segment的ip地址一致，并且pcb->local_port和接收到的tcp segment的port号也一致，认为找到了之前注册的、并且是remote client发过来的这个tcp segment的目标的pcb。
 88         lpcb->local_port == tcphdr->dest) {
 89         /* Move this PCB to the front of the list so that subsequent
 90            lookups will be faster (we exploit locality in TCP segment
 91            arrivals). */
 92         if (prev != NULL) {
 93           ((struct tcp_pcb_listen *)prev)->next = lpcb->next;
 94                 /* our successor is the remainder of the listening list */
 95           lpcb->next = tcp_listen_pcbs.listen_pcbs;
 96                 /* put this listening pcb at the head of the listening list */
 97           tcp_listen_pcbs.listen_pcbs = lpcb;
 98         }
 99      
100         LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_input: packed for LISTENing connection.\n"));
101         tcp_listen_input(lpcb); // 得到server正在listen状态的pcb后，用刚收到的remote tcp segment更新pcb，马上会分析这个函数
102         pbuf_free(p);
103         return;
104       }
105       prev = (struct tcp_pcb *)lpcb;
106     }
107   }