1. FSU CIS 5930 Internet Protocols
Concept of sockets
Reading: Chapter 26
Fall 2004
FSU CIS 5930 Internet Protocols

2. FSU CIS 5930 Internet Protocols
Sockets
Common interface network programming
Underneath, there are lots of protocol specific sockets (protocol families)
PF_INET (for Internet protocol family)
PF_PACKET (for directly accessing network device)
PF_NETLINK (not for transporting data, but rather for configuring Linux network kernel)
Fall 2004
FSU CIS 5930 Internet Protocols

3. FSU CIS 5930 Internet Protocols
BSD sockets
BSD Sockets
PF_INET sockets
PF_PACKET
PF_NETLINK …
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
NETLINK_ROUTE
NETLINK_SKIP
NETLINK_USERSOCK
NETLINK_FIREWALL
NETLINK_ARPD
NETLINK_ROUTE6
NETLINK_IP6_FW
NETLINK_DNRTMSG
NETLINK_TAPBASE
TCP
UDP IP
Network device
Fall 2004
FSU CIS 5930 Internet Protocols

4. FSU CIS 5930 Internet Protocols
sys_socketcall()
Linux kernel has only one socket-related system call
sys_socketcall(int call, unsigned long *args)
Parameter “call” specifies the desired call (function)
SYS_SOCKET, SYS_BIND, SYS_CONNECT …
sys_socketcall dispatch processing to different functions based on “call”
Fall 2004
FSU CIS 5930 Internet Protocols

5. FSU CIS 5930 Internet Protocols
Socket structure
Struct socket {
socket_state state;
unsigned long flags;
struct proto_ops *ops;
struct inode *inode;
struct fasync_struct *fasync_list;
struct file *file;
struct sock *sk;
wait_queue_head_t wait;
short type;
unsigned char passcred }
Fall 2004
FSU CIS 5930 Internet Protocols

6. What happens when you call socket()
In socket programming library
socket() sys_socketcall(SYS_SOCKET, )
In sys_socketcall():
case SYS_SOCKET:
sys_socket()
Create/initialize socket structure (sock_create)
Allocate file descriptor (sock_map_fd())
Fall 2004
FSU CIS 5930 Internet Protocols

7. sock_create/sock_map_fd
Some sanity checking
Allocating socket structure/inode (sock_alloc())
Calling net_faimilies[family]->create() for family specific handling
sock_alloc()
Reserving an inode, allocating/initializing socket structure
Sock_map_fd()
Allocating file descriptor, filling in the file entry
Fall 2004
FSU CIS 5930 Internet Protocols

8. Protocol specific create()
Filling other fields of socket structure
In particular, struct proto_ops *ops;
Protocol specific function processing, such bind, connect, etc.
Example of processing flow: sendto() socket call
sys_socketcall(SYS_SENDTO, )
sys_sendto()
sock_sendmsg()
sock->ops->sendmsg()
Fall 2004
FSU CIS 5930 Internet Protocols

9. Protocol specific sockets
Central structure of all protocol specific sockets
struct sock
PF_INET
PF_PACKET
PF_NETLINK
Fall 2004
FSU CIS 5930 Internet Protocols

10. FSU CIS 5930 Internet Protocols
PF_INET sockets
inet_create()
Initializing sock structure
Filling in protocol specific ops
Filling in transport protocol specific functions, if desirable.
Fall 2004
FSU CIS 5930 Internet Protocols

11. FSU CIS 5930 Internet Protocols
PF_PACKET
By passing transport and network layers
Packet_sendmsg()
It will directly access the network device
dev_queue_xmit()
Similarly, packets passed to application directly
packet_rcv() is installed for incoming packets
Fall 2004
FSU CIS 5930 Internet Protocols