[Bridge] bridge netfilter question

[raj at roinet.com (Rajashekhar Mansanpally)]

Hi,
I found this block of code in br_dev_queue_xmit() @
br_forward.c, after applying 'netfilter' patch for
2.4.21 kernel

Can someone explain what this block of code is doin?

#ifdef CONFIG_NETFILTER
        if (skb->nf_bridge)
                memcpy(skb->data - 16,
skb->nf_bridge->hh, 16);
#endif

1. What is 16 bytes here...? Ethernet hdr is just 14
bytes
2. Why the ethernet hdr is being overwritten with
nf_bridge->hh? what is there in nf_bridge->hh? and
when is nf_bridge is actually assigned this content.

-Raj