skb headroom and tailroom

Hi, I have few basic question regarding headroom and tailroom in skb. 1. At time of skb allocation sk->sk_prot->max_header value is 272 . If we add tcp header+ip header+mac header outcome will always less than 272 .Then why allocate more ? Is it for future case ? 2. Why space left in headroom and tailroom ? Regards Varun