Understanding Linux Network Internals

Selecting the IP Header's ID Field

The main function for the initialization of the IP packet ID is _ _ip_select_ident. This function can be called both directly and indirectly via ip_select_ident or ip_select_ident_more. Both of these wrapper functions differentiate between packets that can and cannot be fragmented (based on the MF flag). Two cases are defined:

Packets cannot be fragmented (DF=1)

This case was added to handle a bug found with some Windows systems' IP stacks.^[*] The ID is extracted indirectly from the sock data structures (inet_sk(sk)->sk), where it is incremented each time the wrapper accesses it. This ensures that the IP ID changes at every transmission.

Packets can be fragmented (DF=0)

ip_select_ident takes care of the ID.

ip_select_ident_more, which is used by TCP (see ip_queue_xmit), receives one more input parameter (more) that is used in those cases where the device supports TCP offloading.

Let's go back to _ _ip_select_ident:

    void _ _ip_select_ident(struct iphdr *iph, struct dst_entry *dst, int more)
    {
        struct rtable *rt = (struct rtable *) dst;
     
        if (rt) {
            if (rt->peer == NULL)
                rt_bind_peer(rt, 1);
            if (rt->peer) {
                iph->id = htons(inet_getid(rt->peer, more));
                return;
            }
        } else
            printk(KERN_DEBUG "rt_bind_peer(0) @%p\n",
                   _ _builtin_return_address(0));
     
            ip_select_fb_ident(iph);
    }

We saw in the section "Long-Living IP Peer Information" that for each IP peer there is an inet_peer data structure that keeps, among other things, a counter that can be used to set the ...