Linux kernel tcp overview

static const struct file_operations socket_file_ops = {
        .owner =        THIS_MODULE,
        .llseek =       no_llseek,
        .read_iter =    sock_read_iter,
        .write_iter =   sock_write_iter,
        .poll =         sock_poll,
        .unlocked_ioctl = sock_ioctl,
#ifdef CONFIG_COMPAT
        .compat_ioctl = compat_sock_ioctl,
#endif
        .mmap =         sock_mmap,
        .release =      sock_close,
        .fasync =       sock_fasync,
        .sendpage =     sock_sendpage,
        .splice_write = generic_splice_sendpage,
        .splice_read =  sock_splice_read,
};

int __sock_create(struct net *net, int family, int type, int protocol,
			 struct socket **res, int kern)
{
	int err;
	struct socket *sock;
	const struct net_proto_family *pf;
...
	pf = rcu_dereference(net_families[family]);
	err = -EAFNOSUPPORT;
	if (!pf)
		goto out_release;
...
	err = pf->create(net, sock, protocol, kern);
	if (err < 0)
		goto out_module_put;
...

/**
 *	sock_register - add a socket protocol handler
 *	@ops: description of protocol
 *
 *	This function is called by a protocol handler that wants to
 *	advertise its address family, and have it linked into the
 *	socket interface. The value ops->family corresponds to the
 *	socket system call protocol family.
 */
int sock_register(const struct net_proto_family *ops)
{
	int err;

	if (ops->family >= NPROTO) {
		pr_crit("protocol %d >= NPROTO(%d)\n", ops->family, NPROTO);
		return -ENOBUFS;
	}

	spin_lock(&net_family_lock);
	if (rcu_dereference_protected(net_families[ops->family],
				      lockdep_is_held(&net_family_lock)))
		err = -EEXIST;
	else {
		rcu_assign_pointer(net_families[ops->family], ops);
		err = 0;
	}
	spin_unlock(&net_family_lock);

	pr_info("NET: Registered protocol family %d\n", ops->family);
	return err;
}
EXPORT_SYMBOL(sock_register);

static int __init inet_init(void)
{
	struct inet_protosw *q;
	struct list_head *r;
	int rc = -EINVAL;

	sock_skb_cb_check_size(sizeof(struct inet_skb_parm));

	rc = proto_register(&tcp_prot, 1);
	if (rc)
		goto out;

	rc = proto_register(&udp_prot, 1);
	if (rc)
		goto out_unregister_tcp_proto;

	rc = proto_register(&raw_prot, 1);
	if (rc)
		goto out_unregister_udp_proto;

	rc = proto_register(&ping_prot, 1);
	if (rc)
		goto out_unregister_raw_proto;

	/*
	 *	Tell SOCKET that we are alive...
	 */

	(void)sock_register(&inet_family_ops);

#ifdef CONFIG_SYSCTL
	ip_static_sysctl_init();
#endif

	/*
	 *	Add all the base protocols.
	 */

	if (inet_add_protocol(&icmp_protocol, IPPROTO_ICMP) < 0)
		pr_crit("%s: Cannot add ICMP protocol\n", __func__);
	if (inet_add_protocol(&udp_protocol, IPPROTO_UDP) < 0)
		pr_crit("%s: Cannot add UDP protocol\n", __func__);
	if (inet_add_protocol(&tcp_protocol, IPPROTO_TCP) < 0)
		pr_crit("%s: Cannot add TCP protocol\n", __func__);
#ifdef CONFIG_IP_MULTICAST
	if (inet_add_protocol(&igmp_protocol, IPPROTO_IGMP) < 0)
		pr_crit("%s: Cannot add IGMP protocol\n", __func__);
#endif

	/* Register the socket-side information for inet_create. */
	for (r = &inetsw[0]; r < &inetsw[SOCK_MAX]; ++r)
		INIT_LIST_HEAD(r);

	for (q = inetsw_array; q < &inetsw_array[INETSW_ARRAY_LEN]; ++q)
		inet_register_protosw(q);

	/*
	 *	Set the ARP module up
	 */

	arp_init();

	/*
	 *	Set the IP module up
	 */

	ip_init();

	/* Setup TCP slab cache for open requests. */
	tcp_init();

	/* Setup UDP memory threshold */
	udp_init();

	/* Add UDP-Lite (RFC 3828) */
	udplite4_register();

	raw_init();

	ping_init();

	/*
	 *	Set the ICMP layer up
	 */

	if (icmp_init() < 0)
		panic("Failed to create the ICMP control socket.\n");

	/*
	 *	Initialise the multicast router
	 */
#if defined(CONFIG_IP_MROUTE)
	if (ip_mr_init())
		pr_crit("%s: Cannot init ipv4 mroute\n", __func__);
#endif

	if (init_inet_pernet_ops())
		pr_crit("%s: Cannot init ipv4 inet pernet ops\n", __func__);
	/*
	 *	Initialise per-cpu ipv4 mibs
	 */

	if (init_ipv4_mibs())
		pr_crit("%s: Cannot init ipv4 mibs\n", __func__);

	ipv4_proc_init();

	ipfrag_init();

	dev_add_pack(&ip_packet_type);

	ip_tunnel_core_init();

	rc = 0;
out:
	return rc;
out_unregister_raw_proto:
	proto_unregister(&raw_prot);
out_unregister_udp_proto:
	proto_unregister(&udp_prot);
out_unregister_tcp_proto:
	proto_unregister(&tcp_prot);
	goto out;
}

fs_initcall(inet_init);