netif的linkoutput和output

LwIP的注释说的很清楚：

  /** This function is called by the network device driver   *  to pass a packet up the TCP/IP stack. */  netif_input_fn input;  /** This function is called by the IP module when it wants   *  to send a packet on the interface. This function typically   *  first resolves the hardware address, then sends the packet. */  netif_output_fn output;  /** This function is called by the ARP module when it wants   *  to send a packet on the interface. This function outputs   *  the pbuf as-is on the link medium. */  netif_linkoutput_fn linkoutput;

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对于以太网接口的网卡来说：
linkoutput实际指向low_level_output，由ARP层调用，参数是：netif，ETH包
output实际指向etharp_output，由IP层调用，参数是：netif，IP包，目的IP地址
它们功能上的区别是：linkoutput是发送ETH包，output是发送IP包。
linkoutput是在已知目的MAC地址时调用，而output则相反。
output底层也是调用了linkoutput来发送组装好的ETH包的

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对于SLIP（Serial Line Internet Protocol，串行线路网际协议）的网卡，它的output是slipif_output。
对于PPPoE（Point to Point Protocol over Ethernet）的网卡，它的output是pppifOutput。
这两种网络接口都用不到IP地址，不需要ARP，不需要linkoutput，直接在output内发送数据包

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/** * Add a network interface to the list of lwIP netifs. * * @param netif a pre-allocated netif structure * @param ipaddr IP address for the new netif * @param netmask network mask for the new netif * @param gw default gateway IP address for the new netif * @param state opaque data passed to the new netif * @param init callback function that initializes the interface * @param input callback function that is called to pass * ingress packets up in the protocol layer stack. * * @return netif, or NULL if failed. */struct netif *netif_add(struct netif *netif, ip_addr_t *ipaddr, ip_addr_t *netmask,  ip_addr_t *gw, void *state, netif_init_fn init, netif_input_fn input)

netif_add函数有input，但是没有output和linkoutput，为什么呢？
因为linkoutput不是必须的，像SLIP和PPPoE就没有，所以它们是在init函数中被赋的值。