skb-6

struct rtable *rt = inet->cork.rt;
struct iphdr *iph;

skb->nh.raw = skb_push(skb, sizeof(struct iphdr));
iph = skb->nh.iph;
iph->version = 4;
iph->ihl = 5;
iph->tos = inet->tos;
iph->tot_len = htons(skb->len);
iph->frag_off = 0;
iph->id = htons(inet->id++);
iph->ttl = ip_select_ttl(inet, &rt->u.dst);
iph->protocol = sk->sk_protocol; /* IPPROTO_UDP in this case */
iph->saddr = rt->rt_src;
iph->daddr = rt->rt_dst;
ip_send_check(iph);

skb->priority = sk->sk_priority;
skb->dst = dst_clone(&rt->u.dst);

This is what a new SKB looks like after we push the IPv4 header tothe front of the SKB.

Just as above for UDP, skb_push() decrements 'skb->data' andincrements 'skb->len'. We update the 'skb->nh.raw' pointer to thebeginning of the new space, and build the IPv4 header.

This packet is basically ready to be pushed out to the device oncewe have the necessary information to build the ethernet header(from the generic neighbour layer and ARP).