1. Chapter 07.
Socket Options

2. Understanding and practice of various socket options
Goal
Understanding and practice of various socket options

3. Socket programming model
Overview (1/7)
Socket programming model
application
TCP IP
UDP
IrDA
Bluetooth
...
socket
Socket code
Protocol
implementation
code

4. Two types of socket option
Overview (2/7)
socket options
Changing the basic operation of socket call
Two types of socket option
① handling in socket code
socket option is analyzed and proceeded in socket code
② handling in protocol implementation code
socket option is analyzed and proceeded in protocol implementation code

5. Socket option setting Overview (3/7) int setsockopt ( SOCKET s,
int level,
int optname,
const char* optval,
int optlen
) ;
success: 0, fail: SOCKET_ERROR

6. Socket option getting Overview (4/7) int getsockopt ( SOCKET s,
int level,
int optname,
char* optval,
int* optlen
) ;
success: 0, fail: SOCKET_ERROR

7. Socket option - SOL_SOCKET
Overview (5/7)
Socket option - SOL_SOCKET
optname
SO_BROADCAST
SO_DONTROUTE
SO_KEEPALIVE
SO_LINGER
SO_SNDBUF
SO_RCVBUF
SO_SNDTIMEO
SO_RCVTIMEO
SO_REUSEADDR
optval type
BOOL
struct linger{}
int
get
set 
description
permit sending broadcast datagram
No routing table reference in data transfer
Checking connection status periodically
Changing closesocket() property
Setting socket send/receive buffer size
Setting timeout for send(), recv() and so on
Reuse of Local address(IP address, port number)

8. Socket option - IPPROTO_IP
Overview (6/7)
Socket option - IPPROTO_IP
optname
IP_HDRINCL
IP_TTL
IP_MULTICAST_IF
IP_MULTICAST_TTL
IP_MULTICAST_LOOP
IP_ADD_MEMBERSHIP
IP_DROP_MEMBERSHIP
optval type
BOOL
int
IN_ADDR{}
struct ip_mreq{}
get
set 
description
Including IP header in data sending
Changing IP packet TTL(time-to-live)
Setting interface to send multicast packet
Changing TTL of multicast packet
Setting of loop-back of multicast packet
Join and leave multicast group

9. Socket option - IPPROTO_TCP
Overview (7/7)
Socket option - IPPROTO_TCP
optname
optval type
get
set
description
TCP_NODELAY
BOOL 
Stop Nagle algorithm in TCP

10. It is supported only for datagram sockets. Its default value is off.
SO_BROADCAST option
Enables or disables the ability of a process to send broadcast messages.
It is supported only for datagram sockets.
Its default value is off.

11. SO_DONTROUTE option
usage
used by routing daemon to make sure a packet goes through a certain interface
Code example
BOOL optval = TRUE;
if(setsockopt(listen_sock, SOL_SOCKET, SO_DONTROUTE,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

12. SO_KEEPALIVE option
If set, this option is used for detecting peer host crashes.
If this option is set and no data has been exchanged for 2 hours, then TCP sends keepalive probe to the peer. Cases:
Peer responds with ACK. Another probe will be sent only after 2 hours of inactivity.
Peer responds with RST (has crashed and rebooted). Error is set to ECONNRESET and the socket is closed.
No response. 8 more probes are sent after which the socket’s pending error is set to ETIMEDOUT and the socket is closed.
Code example
BOOL optval = TRUE;
if(setsockopt(listen_sock, SOL_SOCKET, SO_KEEPALIVE,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

13. l_onoff=0 : defualt behaviour
SO_LINGER option
This option specifies how the close function operates for connection-oriented protocols
struct linger {
int l_onoff; /* Linger active */
int l_linger; /* How long to linger for */ };
l_onoff=0 : defualt behaviour
l_onoff=1, l_linger=val : close returns only after val seconds, if socket is blocking
l_onoff=1, l_linger=0 : RST sent on close

14. SO_SNDBUF, SO_RCVBUF option
usage
send and receive buffer size
Code example
int optval;
int optlen = sizeof(optval);
if(getsockopt(listen_sock, SOL_SOCKET, SO_RCVBUF,
(char *)&optval, &optlen) == SOCKET_ERROR) err_quit("getsockopt()");
printf(“receive buffer size = %d bytes \n", optval);
optval *= 2;
if(setsockopt(listen_sock, SOL_SOCKET, SO_RCVBUF,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) err_quit("setsockopt()");

15. SO_SNDTIMEO, SO_RCVTIMEO option
Usage
Setting timeout for send(), recv(), sendto(), recvfrom(), etc
Code example
int optval = 3000;
if(setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

16. SO_REUSEADDR option
It allows a listening server to restart and bind its well known port even if previously established connections exist.
It allows multiple instances of the same server to be started on the same port, as long as each instance binds a different local IP address.
It allows a single process to bind the same port to multiple sockets, as long as each bind specifies a different local IP address.
It allows completely duplicate bindings only for UDP sockets (broadcasting and multicasting).

17. Multicast address Features
multicasting (1/3)
Multicast address
Features
Join and leave a multicast group , all group members are equal
A member have to join a multicast group to receive multicast data
A member don’t have to join a multicast group to send multicast data
28 bit
Multicast group ID 1

18. Multicast data transfer (1)
multicasting (2/3)
Multicast data transfer (1) A B
Multicast group A B
Multicast group

19. Multicast data transfer (2)
multicasting (3/3)
Multicast data transfer (2)
MulticastSender.cpp, Multicastreceiver.cpp A B C
Multicast group

20. Socket options setsockopt() getsockopt() API support IP_MULTICAST_LOOP
yes
IP_MULTICAST_TTL
IP_MULTICAST_IF
IP_ADD_MEMBERSHIP no
IP_DROP_MEMBERSHIP
Vinayak Jagtap, IP Broadcasting and Multicasting

21. IP_MULTICAST_IF option (1/2)
Usage
Specify the interface for outgoing multicast datagrams sent on this socket
Code example
IN_ADDR localaddr;
localaddr.s_addr = inet_addr(" ");
if(setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF,
(char *)&localaddr, sizeof(localaddr)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

22. IP_MULTICAST_IF option (2/2)
Option setting result
application

23. IP_MULTICAST_TTL option
Usage
Set the IPv4 TTL parameter (if not specified, default=1)
Code example
// multicast TTL setting
int ttl = 2;
retval = setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL,
(char *)&ttl, sizeof(ttl));
if(retval == SOCKET_ERROR) err_quit("setsockopt()");

24. IP_MULTICAST_LOOP option
Usage
Enable or disable local loopback (default is enabled)
Code example
BOOL optval = FALSE; // no receiving the data sent
if(setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

25. IP_ADD_MEMBERSHIP, IP_DROP_MEMBERSHIP option (1/3)
Usage
Join/leave a multicast group on a specified local interface
Option value
#include <ws2tcpip.h>
struct ip_mreq {
struct in_addr imr_multiaddr; /* IP multicast address of group */
struct in_addr imr_interface; /* local IP address of interface */ };

26. IP_ADD_MEMBERSHIP, IP_DROP_MEMBERSHIP option (2/3)
Code example
struct ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr(" ");
mreq.imr_interface.s_addr = inet_addr(" ");
if(setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
(char *)&mreq, sizeof(mreq)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }

27. IP_ADD_MEMBERSHIP, IP_DROP_MEMBERSHIP option (3/3)
Option setting result
application

28. TCP_NODELAY option (1/2)
usage
Used to enable/disable Nagle’s algorithm in TCP
Nagle algorithm
① send the data when the size of the sending data becomes to MSS(maximum segment size)
② wait for the ACK of the previous data when the sending data size is less that. Once ACK is arrived, data is sent to peer side
BOOL optval = TRUE; // Stop Nagle algorithm
if(setsockopt(sock, IPPROTO_TCP, TCP_NODELAY,
(char *)&optval, sizeof(optval)) == SOCKET_ERROR) {
err_quit("setsockopt()"); }