0. Socket Address Structures
Chapter 03.
Socket Address Structures

1. Socket address structures Byte ordering IP address conversion
Goal
Socket address structures
Byte ordering
IP address conversion
Domain name conversion

2. Def. of socket address structures (1/6)
Sockets use the socket address structure to pass and to receive addresses
Several types depending on address family
example) TCP/IP  SOCKADDR_IN, IrDA  SOCKADDR_IRDA
Basic type is SOCKADDR structure

3. Def. of socket address structures (2/6)
SOCKADDR structure
sa_family
16-bit integer value identifying the protocol family being used
e.g.) TCP/IP  AF_INET
sa_data
Address information used in the protocol family
e.g.) TCP/IP  IP address and port number
struct sockaddr {
u_short sa_family;
char sa_data[14]; };
typedef struct sockaddr SOCKADDR;

4. Def. of socket address structures (3/6)
SOCKADDR_IN structure
sin_addr
32-bit in_addr structure
struct sockaddr_in {
short sin_family; // AF_INET
u_short sin_port; // port number
struct in_addr sin_addr; // IP address
char sin_zero[8]; // always 0 };
typedef struct sockaddr_in SOCKADDR_IN;

5. Def. of socket address structures (4/6)
IN_ADDR structure
struct in_addr {
union {
struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b;
struct { u_short s_w1,s_w2; } S_un_w;
u_long S_addr;
} S_un;
#define s_addr S_un.S_addr };
typedef struct in_addr IN_ADDR;

6. Def. of socket address structures (5/6)
comparison of socket address structures
SOCKADDR{}
SOCKADDR_IN{}
SOCKADDR_IRDA{}
sa_family
(2)
sin_family
(2)
irdaAddressFamily
(2)
sa_data
(14)
sin_port
(2)
irdaDeviceID
(4)
sin_addr
(4)
irdaServiceName
(25)
sin_zero
(8)

7. Def. of socket address structures (6/6)
Use of socket address structure
example1)
Example 2)
SOCKADDR_IN addr1;
// initialize socket address structure
...
f((SOCKADDR *)&addr1, ...);
SOCKADDR_IN addr2;
g((SOCKADDR *)&addr2, ...);
// use socket address structure
...

8. Byte ordering function calls (1/6)
Two ways to store data in their memories
big-endian: the high-order octet is stored in the lowest memory location
little-endian: the high-order octet is stored in the highest memory location
0x12
0x34
0x56
0x78
Big-endian
Little-endian
0x1000
0x1001
0x1002
0x1003
  

9. Byte ordering function calls (2/6)
What if byte ordering is not considered in the below?
End system
router
data
IP address
Port number ?
(a)
(b)
(c)

10. Byte ordering function calls (3/6)
Cases that byte ordering must be considered in network applications
Address information for protocol
(a) IP address  big-endian
(b) port number  big-endian
Data that applications send and receive
(c) big-endian or little-endian
Ref.
 network byte ordering
: big-endian
 host byte ordering
: byte ordering that system uses

11. Byte ordering function calls (4/6)
Byte ordering functions(unix compatible)
Byte ordering functions(Winsock extended)
u_short htons (u_short hostshort); // host-to-network-short
u_long htonl (u_long hostlong); // host-to-network-long
u_short ntohs (u_short netshort); // network-to-host-short
u_long ntohl (u_long netlong); // network-to-host-long
int WSAHtons (SOCKET s, u_short hostshort, u_short* lpnetshort);
int WSAHtonl (SOCKET s, u_long hostlong, u_long* lpnetlong);
int WSANtohs (SOCKET s, u_short netshort, u_short* lphostshort);
int WSANtohl (SOCKET s, u_long netlong, u_long* lphostlong);

12. Byte ordering function calls (5/6)
Use of byte ordering functions
Data to pass
Use data in application
hton*( )
ntoh*( )
Data from
socket call
Socket system call
Socket system call

13. Byte ordering function calls (6/6)
Byte ordering of SOCKADDR_IN structure
sin_family
(2)
sin_port
sin_addr
(4)
sin_zero
(8)
SOCKADDR_IN{}
Host byte ordering
Network byte ordering

14. Byte ordering function calls (6/6)
ByteOrder.cpp

15. Function calls to convert IP address (1/4)
Example.

16. Function calls to convert IP address(2/4)
IP address coversion function calls
Converts an IP address in ASCII dotted-decimal format to 32-bit binary format (network byte ordering)
Converts 32-bit binary format (network byte ordering) to an IP address in ASCII dotted-decimal format
unsigned long inet_addr (const char* cp);
char* inet_ntoa (struct in_addr in); // network-to-ascii

17. Function calls to convert IP address(3/4)
Use ① of byte ordering and IP address conversion function calls
Case that address information is passed to socket system call after socket address structure initialization (usually sender side)
f( ) is socket system call
// initialize socket address structure
SOCKADDR_IN addr;
ZeroMemory(&addr, sizeof(addr)); // initialize to 0
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(" ");
addr.sin_port = htons(9010);
// socket system call
f((SOCKADDR *)&addr, ...);

18. Function calls to convert IP address (4/4)
Use ② of byte ordering and IP address conversion function calls
Case that application prints address information after socket system call (usually receiver side)
g( ) is socket system call
// declaration of socket address structure
SOCKADDR_IN addr;
// socket system call
g((SOCKADDR *)&addr, ...);
// print IP address and port number
printf("IP address=%s, port number=%d\n",
inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));

19. Byte ordering function calls (6/6)
IPAddr.cpp

20. coversion between domain name and IP address (1/6)
Example: Domain name  IP address

21. coversion between domain name and IP address (2/6)
Domain name  IP address
/* domain name  IP address(network byte ordering) */
struct hostent* gethostbyname (
const char* name // domain name );
/* address(network byte ordering)  domain name */
struct hostent* gethostbyaddr (
const char* addr, // network byte ordered IP address
int len, // length of IP address(e.g.: 4)
int type // address family(e.g.: AF_INET)

22. coversion between domain name and IP address(3/6)
hostent struct
struct hostent {
char * h_name; // official name of host
char ** h_aliases; // alias list
short h_addrtype; // host address type
short h_length; // length of address
char ** h_addr_list; // list of addresses
#define h_addr h_addr_list[0] // address, for backward compatibility };
typedef struct hostent HOSTENT;

23. coversion between domain name and IP address(4/6)
hostent struct (cont’d)
HOSTENT{}
h_name
official domain name\0
h_aliases
alias #1\0
h_addrtype
AF_INET
alias #2\0
h_length 4
NULL
h_addr_list
IN_ADDR{}
IP address #1
IP address #2
NULL
h_length = 4

24. coversion between domain name and IP address(5/6)
User defined function ①
// domain name -> IP address
BOOL GetIPAddr(char *name, IN_ADDR *addr) {
HOSTENT *ptr = gethostbyname(name);
if(ptr == NULL){
err_display("gethostbyname()");
return FALSE; }
memcpy(addr, ptr->h_addr, ptr->h_length);
return TRUE;

25. coversion between domain name and IP address (6/6)
User defined function ②
// IP address -> domain name
BOOL GetDomainName(IN_ADDR addr, char *name) {
HOSTENT *ptr = gethostbyaddr((char *)&addr,
sizeof(addr), AF_INET);
if(ptr == NULL){
err_display("gethostbyaddr()");
return FALSE; }
strcpy(name, ptr->h_name);
return TRUE;

26. Byte ordering function calls (6/6)
INameResolution.cpp