hi , question function of net_rx(struct net_device *dev)

[lx]

hi :
     the source code of net_rx(struct net_device *dev) is:
http://lxr.oss.org.cn/source/drivers/net/ethernet/cirrus/cs89x0.c#L670

I can't understand codes about malloc buffer.
##########################################
684         /* Malloc up new buffer. */
685         skb = netdev_alloc_skb(dev, length + 2);
686         if (skb == NULL) {
687                 dev->stats.rx_dropped++;
688                 return;
689         }
690         skb_reserve(skb, 2);    /* longword align L3 header */
##########################################

How to explain 'length + 2' in line of 685? Why add 2 ?

Thank you.