1、ethernetif_init()初始化底层接口

err_t
ethernetif_init(struct netif *netif)
{
  struct ethernetif *ethernetif;

  ethernetif 是一个结构体，用来描述底层硬件设备，该结构体唯一不可或缺的是MAC地址，它是LWIP用于相应ARP查询的核心数据。其他如果没有特殊需要，可以不添加其他成员数据。
    
  ethernetif = mem_malloc(sizeof(struct ethernetif));
  if (ethernetif == NULL) {
    LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_init: out of memory\n"));
    return ERR_MEM;
  }

#if LWIP_NETIF_HOSTNAME
  /* Initialize interface hostname */
  netif->hostname = "lwip";
#endif /* LWIP_NETIF_HOSTNAME */

  /*
   * Initialize the snmp variables and counters inside the struct netif.
   * The last argument should be replaced with your link speed, in units
   * of bits per second.
   */
  NETIF_INIT_SNMP(netif, snmp_ifType_ethernet_csmacd, ???);

  netif->state = ethernetif;
  netif->name[0] = IFNAME0;
  netif->name[1] = IFNAME1;
  /* We directly use etharp_output() here to save a function call.
   * You can instead declare your own function an call etharp_output()
   * from it if you have to do some checks before sending (e.g. if link
   * is available...) */
  netif->output = ethernetif_output;

向LwIP注册发送函数。注意，这个函数并不是真正的发送函数，它是通过调用下一句完成信息包发送的。
  netif->linkoutput = low_level_output;
向LwIP注册链路层发送函数，该函数完成实际的信息包发送。这个函数需要结合实际的硬件情况亲自编写实现，LwIP仅提供了一个参考结构。
  ethernetif->ethaddr = (struct eth_addr *)&(netif->hwaddr[0]);
ethernetif->ethaddr指针指向netif中保存的网卡MAC地址 。
  /* initialize the hardware */
  low_level_init(netif);

网卡初始化，建立稳定的物理连接链路并建立接收线程（见图5.4.1中的第(3)、(4)步）。这个函数直接与底层硬件打交道，LwIP仅能为其提供一个参考结构，需要我们结合实际硬件情况重新设计实现。

  return ERR_OK;
}

 

2、low_level_output()――链路层发送函数

static err_t
low_level_output(struct netif *netif, struct pbuf *p)
{
  struct ethernetif *ethernetif = netif->state;
  struct pbuf *q;

  initiate transfer();
  
#if ETH_PAD_SIZE
  pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif

  for(q = p; q != NULL; q = q->next) {
    /* Send the data from the pbuf to the interface, one pbuf at a
       time. The size of the data in each pbuf is kept in the ->len
       variable. */
    send data from(q->payload, q->len);
  }

  signal that packet should be sent();

#if ETH_PAD_SIZE
  pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif
  发送数据包
  LINK_STATS_INC(link.xmit);

  return ERR_OK;
}

3、low_level_init()――网卡初始化函数

static void
low_level_init(struct netif *netif)
{
  struct ethernetif *ethernetif = netif->state;
  
  /* set MAC hardware address length */
  netif->hwaddr_len = ETHARP_HWADDR_LEN;

设置网卡MAC地址的长度。这个长度由LwIP定义的宏NETIF_MAX_HWADDR_LEN指定，长度值为6，单位为字节

  /* set MAC hardware address */
  netif->hwaddr[0] = ;
  ...
  netif->hwaddr[5] = ;

设定MAC地址

  /* maximum transfer unit */
  netif->mtu = 1500;
 设定mtu值 
  /* device capabilities */
  /* don't set NETIF_FLAG_ETHARP if this device is not an ethernet one */
  netif->flags = NETIF_FLAG_BROADCAST | NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;

允许处理网络广播通信，ARP。
 
  /* Do whatever else is needed to initialize interface. */  
}

4、ethernetif_input()――实现接收线程

static void
ethernetif_input(struct netif *netif)
{
  struct ethernetif *ethernetif;
  struct eth_hdr *ethhdr;
  struct pbuf *p;

  ethernetif = netif->state;

获取当前MAC地址

  /* move received packet into a new pbuf */
  p = low_level_input(netif);

读取EMAC的接收缓冲区
  /* no packet could be read, silently ignore this */
  if (p == NULL) return;
  /* points to packet payload, which starts with an Ethernet header */
  ethhdr = p->payload;

获得以太网帧头

  switch (htons(ethhdr->type)) {
  /* IP or ARP packet? */
  case ETHTYPE_IP:
  case ETHTYPE_ARP:
#if PPPOE_SUPPORT
  /* PPPoE packet? */
  case ETHTYPE_PPPOEDISC:
  case ETHTYPE_PPPOE:
#endif /* PPPOE_SUPPORT */
    /* full packet send to tcpip_thread to process */
    if (netif->input(p, netif)!=ERR_OK)
     { LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_input: IP input error\n"));
       pbuf_free(p);
       p = NULL;
     }
    break;

  default:
    pbuf_free(p);
    p = NULL;
    break;
  }

根据以太网帧头携带的上层协议类型值传递数据。以太网帧格式定义：

目的MAC地址 源MAC地址           类型/长度                  数据                     校验

   6字节                  6字节               2字节                   46-1500字节              4字节

                                                 ip:0x0800

                                                ARP:0x0806

最大帧长1518字节 最小字节64字节

}