eth_type_trans返回协议疑惑

tc1989tc

__be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
{
        struct ethhdr *eth;
        unsigned char *rawp;

        skb->dev = dev;
        skb_reset_mac_header(skb);
        skb_pull(skb, ETH_HLEN);
        eth = eth_hdr(skb);

        if (is_multicast_ether_addr(eth->h_dest)) {
                if (!compare_ether_addr(eth->h_dest, dev->broadcast))
                        skb->pkt_type = PACKET_BROADCAST;
                else
                        skb->pkt_type = PACKET_MULTICAST;
        }

        /*
         *      This ALLMULTI check should be redundant by 1.4
         *      so don't forget to remove it.
         *
         *      Seems, you forgot to remove it. All silly devices
         *      seems to set IFF_PROMISC.
         */

        else if (1 /*dev->flags&IFF_PROMISC */ ) {
                if (unlikely(compare_ether_addr(eth->h_dest, dev->dev_addr)))
                        skb->pkt_type = PACKET_OTHERHOST;
        }

        if (ntohs(eth->h_proto) >= 1536)
                return eth->h_proto;

        rawp = skb->data;

        /*
         *      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.
         */
        if (*(unsigned short *)rawp == 0xFFFF)
                return htons(ETH_P_802_3);

        /*
         *      Real 802.2 LLC
         */
        return htons(ETH_P_802_2);
}
这个函数的返回链路层协议类型，咋么以太网802.3的判断依据是skb->data的前两个自己必须是0xFFFF
通过wiresherk抓包工具 发现以太网数据包的数据段并没有0xFFFF字段啊   

卖萌犯法

http://blog.csdn.net/magina3/article/details/7323265

自己看吧