• Introduction to Sockets
• Types of Sockets
  • Stream Sockets
  • Datagram Sockets
  • Raw Sockets
• Socket Address Family
  • AF_INET Address Family
  • AF_INET6 Address Family
  • AF_UNIX Address Family

Introduction to Sockets

• Sockets are the “virtual” endpoints of any kind of network communications done between 2 hosts over in a network.
• Sockets are uniquely identified by:
  • An internet Address
  • An end-to-end protocol(TCP or UDP protocol)
  • A port number
• One major difference between the socket and other inter-process communication is that socket allows communication between the process on different system unlike other which allows communication in a single system.
• For example, when we connect to http://www.google.com in our web browser, it opens a socket on port 80 and connects to google.com to fetch the page and show it to us.

Diag-1: Sockets

Types of Socket:
There are three types of sockets:

• Stream Socket
• Datagram Socket
• Raw Socket

Stream Socket:
These are connection-oriented sockets, which use Transport Control Protocol. Each packet has a sequence number and transmits data reliably and in order with the support of error and flow control.

Datagram Socket:
These are connectionless socket, which uses UDP. Each packet sent or received on a datagram socket is individually addressed. Order and reliability are not guaranteed with datagram sockets.

Raw Socket:
It allows direct sending and receiving of IP packets without any protocol-specific transport layer formatting that is packet bypasses Layer 4. One of the common examples of raw sockets is Wireshark(packet Analyzer).

Socket Address Family or Domain:
The address family specifies the format of address structure to be used on socket API
There is three important address family:

• AF_INET Address Family
• AF_INET6 Address Family
• AF_UNIX Address Family

AF_INET Address Family:

• This is the most common address family which is used for IPv4 internet addressing.
• Addresses for AF_INET sockets are IP address (such as 130.99.128.1) or in its 32–bit form (X’82638001′) and port numbers.
• The AF_INET address family uses the sockaddr_in structure.

struct sockaddr_in {
short int sin_family;
unsigned short int sin_port;
struct in_addr  sin_addr;
};

struct in_addr {
  unsigned long int s_addr;
};

AF_INET6 Address Family:

• This is used for Internet Protocol version 6(IPv6).
• AF_INET6 address family uses a 128 bit address.
• It uses sockaddr_in6 structure.

AF_UNIX Address Family:

• This address family is used for Unix Domain Sockets(UDS).
• It provides inter-process communication between processes that run on the same system.
• The address is actually a path name to an entry in the file system, hence also known as file system sockets.
• Sockets with the address family AF_UNIX use the sockaddr_un address structure

Relevant Posts:

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

Categories: Operating system (OS)