checksum calculation in ip packet forwarding

[cranium 2003 <cranium.2003@gmail.com>]

Hello,
1) How Router knows destinations mtu. In IP forwarding case following
function is used that checks pmtu value to decide whether outgoing
packet is required to fragment a packet or not?
static inline int ip_send(struct sk_buff *skb)
{
	if (skb->len > skb->dst->pmtu)
		return ip_fragment(skb, ip_finish_output);
	else
		return ip_finish_output(skb);
}

2) Can anybody help me how following function works in calcualting checksum?
static inline int ip_decrease_ttl(struct iphdr *iph)
{
	u32 check = iph->check;
	check += __constant_htons(0x0100);
	iph->check = check + (check>=0xFFFF);
	return --iph->ttl;
}
regards,
cranium