Linux Networking (II)

Today's Lecture

• Linux Networking: Output Chain

Multiplexing

• Socket has many "family"
  • PF_INET, ...
• And many types
  • SOCK_STREAM, SOCK_DGRAM, SOCK_RAW
• Each family can have many protocols:
  • PF_INET: IPPROTO_TCP, IPPROTO_UDP, ...

How is a Socket Created?

• sys_socket() calls sock_create()
• sock_create() calls:

  	sock = sock_alloc()
  	sock->type  = type;
  	net_families[family]->create(sock, protocol)
  

How is a net_family Defined?

• int sock_register(ops)

  	net_families[ops->family]=ops;
  

• Called for each "network protocol family"
• Example: IPv4 (net/ipv4/af_inet.c)

IPv4 INET Family

static int __init inet_init(void)
	...
	(void) sock_register(&inet_family_ops);
	...

struct net_proto_family inet_family_ops = {
        family: PF_INET,
        create: inet_create
};

Protocols in INET Family

• Registrating

  static struct inet_protosw inetsw_array[] = { ... }
  static int __init inet_init(void)
  	... 
  	for each q in inetsw_array[]
                inet_register_protosw(q);
  

• Socket creating

  int inet_create(struct socket *sock, int protocol)
  
  	/* Look for the requested type/protocol pair. */
  	sock->ops = ... ->ops; sk->prot = ... ->prot;
  

Example with UDP

Socket Send (and Receive)

• sys_send() calls sock_sendmsg()
• sock_sendmsg() calls
  • sock->ops->sendmsg(sock, msg, size, &scm)
• Under PF_INET:
  • sock->ops->sendmsg ==> inet_sendmsg()
• inet_sendmsg() calls
  • sk->prot->sendmsg(sk, msg, size);
• Under IPPROTO_UDP:
  • sk->prot->sendmsg ==> udp_sendmsg()

Protocol Layer

• udp_sendmsg()
  • Build the UDP header, etc.
  • Calls ip_build_xmit() to output the packet

IP Output

• Fast path: ip_build_xmit()
• Allocate skb (sk_buff)
• Fill in the IP header
• Get the fragment from user space
• Send out with NF hooks
  • New in 2.4 kernel
  • Older kernels: send to device directly
  • NF: provides kernel hooks for packet manipulation, such as firewall, QoS, etc.

Next Lecture

• Input Chain