参考一下博客：
http://yaoyang.blog.51cto.com/7657153/1269713
http://blog.csdn.net/eric_liufeng/article/details/10475583
网络驱动接收到报文后，会初始化skb->protocol字段，再通过netif_rx(skb);传输给3层协议，3层协议根据skb->protocol字段决定使用哪个协议实例处理。
某一mac协议类型的全部网络设备都是使用相同的xx_type_trans()函数来获取protocol的：以太网设备驱动调用eth_type_trans(), FDDI网络设备驱动调用fddi_type_trans().
/**
* eth_type_trans - determine the packet's protocol ID.
* @skb: received socket data
* @dev: receiving network device
*
* The rule here is that we
* assume 802.3 if the type field is short enough to be a length.
* This is normal practice and works for any 'now in use' protocol.
*/
__be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
{
unsigned short _service_access_point;
const unsigned short *sap;
const struct ethhdr *eth;
skb->dev = dev;
skb_reset_mac_header(skb);
//获取第二层数据包的报头
skb_pull_inline(skb, ETH_HLEN);
eth = eth_hdr(skb);
/*
* 在skb->pkt_type中得到识别和注册
* PACKET_BROADCAST：广播包
* PACKET_MULTICAST：2层组播地址
* PACKET_HOST：发送给本机的报文
* PACKET_OTHERHOST: 发送给其他主机的报文，这里主机应该处于混杂模式中
*/
if (unlikely(is_multicast_ether_addr(eth->h_dest))) {
if (ether_addr_equal_64bits(eth->h_dest, dev->broadcast))
skb->pkt_type = PACKET_BROADCAST;
else
skb->pkt_type = PACKET_MULTICAST;
}
else if (unlikely(!ether_addr_equal_64bits(eth->h_dest,
dev->dev_addr)))
skb->pkt_type = PACKET_OTHERHOST;
/*
* Some variants of DSA tagging don't have an ethertype field
* at all, so we check here whether one of those tagging
* variants has been configured on the receiving interface,
* and if so, set skb->protocol without looking at the packet.
*/
if (unlikely(netdev_uses_dsa(dev)))
return htons(ETH_P_XDSA);
/*如果长度大于1536（最大帧长度ETH_P_802_3_MIN），那么他是802.3兼容以太网卡，协议字段在eth->h_proto中标识*/
if (likely(ntohs(eth->h_proto) >= ETH_P_802_3_MIN))
return eth->h_proto;
/*
* This is a magic hack to spot IPX packets. Older Novell breaks
* the protocol design and runs IPX over 802.3 without an 802.2 LLC
* layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
* won't work for fault tolerant netware but does for the rest.
*/
sap = skb_header_pointer(skb, 0, sizeof(*sap), &_service_access_point);
if (sap && *sap == 0xFFFF)
return htons(ETH_P_802_3);
/*
* Real 802.2 LLC
* 802.2的协议头返回后，被当做三层报文处理，
* 协议处理被散列在ptype_base中，协议处理函数为p8022_rcv()
* 对应的三层协议可以通过 register_8022_client() 将自己的处理函数注册到p8022_rcv()中
* 如果 register_8022_client(0xAA, snap_rcv)就将SNAP的三层处理函数注册进去了
*/
return htons(ETH_P_802_2);
}