• Byte Order and Network order
• Significance of Byte ordering
• Little-endian and Big-endian
• Network Program
  • A program to convert Ip address and port number to network order
  • A program to check endianess of a system

Byte Order:

• Byte Ordering is an attribute of the processor which defines how data are arranged or ordered.
• Byte Ordering is important when data is transferred across a network or between system using different Ordering.

Network Order:
Network Byte Order refers to how bytes are arranged when sending data over a network.
Host Byte Order:
This refers to how bytes are arranged on a computer system.

Significance of Byte Ordering:

• A byte is of 8 bits and has a limited range of 256 values.
• When the value is beyond this range, it has to be stored in multiple bytes.
• A number such as 258 in hexadecimal format is 0x0102 and required minimum 2 bytes.
• The order in which these two bytes are stored in memory can be different depending upon the processor.
• Some processor stores the values in lower memory address followed by higher memory address.

Example:
Decimal Values : 258
Hex : 0x0102

• Lower memory address followed by Higher memory : 0x01 followed by 0x02
• Higher memory address followed by Lower memory : 0x02 followed by 0x01

Thus if Byte Ordering is not appropriate according to the processor, the 0x0102 can be wrongly interpreted as 0x0201 which is 513 in decimal system.

Little-Endian and Big-Endian:
There is two common Ordering System:

• Little Endian and
• Big Endian

Little Endian:

• In this least-significant(Low-order) byte is stored at a lower address.
• Example: Intel Processor

Big Endian:

• In this the most significant(High-order) byte is stored at lower address.
• Example: IBM PowerPC or Motorola PowerPC Processor

Some Processor support both ordering and therefore Bi-Endian and are configurable. MostRISC architectures(SPARC, MIPS) are configurable now.

Diag-1: Little-endian and Big-endian

Comparing Little-Endian and Big-Endian Ordering
Host and Network Byte Order API:

• For the correct interpretation of values across the system or a network, the network program must convert their internal representation(Host Byte Order) to Network Byte order(Big-Endian).
• A host system may be little-endian but when sending data to a network, it must convert data into big-endian.
• Likewise, a little-endian machine must first convert network data into little-endian before processing.
• This can be done with a few systems call API defined in netinet/in.h are(for 32 bit long and 16-bit short)
  • unsigned long int htonl ( unsigned long int hostlong);
  • unsigned long int ntohl (unsigned long int netlong);
  • unsigned short int htons (unsigned short int hostshort);
  • unsigned short int ntohs (unsigned short int netshort);

• Two important parameters in socket programming is Port number(16 bits) and IP address(32 bits), these two must be ordered correctly.
• For port which is 16 bits, htons() and ntohs() can be used and for IP address of 32 bits htonl() and ntohl() can be used.

Diag-2: htonl-to-ntohl

Illustrating the use of htonl and ntohl function
In the AF_INET domain, the address is specified using a structure called sockaddr_in, defined in netinet/in.h, which contains at least three members.

struct sockaddr_in 
{
    short int family;                         /*AF_INET */
    unsigned short int sin_port;              /* Port Number */
    struct in_addr sin_addr;                  /* Internet Address */
};

struct in_addr
{
   unsigned long int s_addr;
};

Program to convert the Port and IP Address into Network Order(Big-Endian)

#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>

int main()
{
    struct sockaddr_in addr;
    unsigned short int port = 1234;
    unsigned long int ip = 2110443574;
    addr.sin_port= (htons (port));
    addr.sin_addr.s_addr = htonl(ip);
    /* Host Order(Little-Endian) */
    printf("\n Port in Host Order(Little-Endian) Hex: %x Decimal: 
           %d\n",port, port);
    printf("\n IP in Host Order(Little-Endian) Hex: %lx Decimal: 
           %lu\n",ip, ip);
    /* Network Order(Big-Endian) */
    printf("\n Port in Network Order(Big-Endian) Hex: %x Decimal: %d 
           \n",addr.sin_port,addr.sin_port);
    printf("\n IP address in Network Order(Big-Endian) Hex: %x Decimal: 
           %d \n",addr.sin_addr.s_addr,addr.sin_addr.s_addr);
    return 0;
}  
 

Output:

Port in Host Order(Little-Endian) Hex: 4d2 Decimal: 1234                                                                      
IP in Host Order(Little-Endian) Hex: 7dcad036 Decimal: 2110443574                                                             
Port in Network Order(Big-Endian) Hex: d204 Decimal: 53764                                                                    
IP address in Network Order(Big-Endian) Hex: 36d0ca7d Decimal:919652989                                                                

Program to check the Endiness of the system:

#include <stdio.h>
int main ()
{
  unsigned int x = 0x0102;
  char *c = (char*) &x;
 
  printf ("\n Value at c is: 0x%x\n", *c);
  if (*c == 0x02)
     printf ("Underlying architecture is little endian. \n")
  else
     printf ("Underlying architecture is big endian. \n");
  return 0;
}

Relevant Posts:

• Sockets Introduction
• Socket APIs
• TCP Socket Program
• UDP Socket Program
• Socket Options
• Multi-client server support- select() API
• Host and Network Byte ordering

Categories: Operating system (OS)