Sockets

1
Sockets

• Socket is an endpoint of communication.
• Software construct that allows applications to
  access network.
• Computer has one or more hardware network
  interface card (NIC).
• This is the gate to the external network.
• May be shared by many sockets.
• Socket Name consists of IP address and Port
  number.

2
Winsock

• Windows networking library called winsock.
• Windows Sockets
• When client socket connects to a server socket
  form an association.

3
Network Program Sketch (Client)

• Open a socket.
• Name the socket.
• Connect to server socket.
• Send and receive messages between client and
  server sockets.
• Very similar to message passing between windows.
• Close the socket.

4
Step 1 Open a Socket

• SOCKET sock
• sock socket (AF_INET, SOCK_STREAM, 0)
• SOCK_DGRAM for UDP.
• AF_INET Internet family
• SOCK_STREAM Indicates TCP protocol
• Third value ignored for TCP and UDP.

5
Step2 Naming a Socket

• Name consists of three attributes
• Protocol
• IP Address
• Port Number
• Generic Socket Address Structure
• struct sockaddr
• u_short as_family
• char sa_data14 //Value depends on address
  family

6
Socket Address Structure

• struct sock_addr is generic. For TCP/IP use
• struct sockaddr_in data structure.
• To name a socket, fill in the three fields of
  this struct.
• struct sockaddr_in
• short sin_family // always AF_INET
• u_short sin_port // in network order
• struct sin_addr // IP Address.

7
Port Number

• Port 0 1023 reserved for well known services
• HTTP 80
• FTP 23
• Users can assign numbers 1025 5000.

8
IP Address

• struct sin_addr holds 32 bit IP Address
• Access s_addr field of sin_addr.
• Example (Almost correct)
• struct sockaddr_in sa
• sa.sin_family AF_INET
• sa.sin_port 3000
• sa.sin_addr.s_addr 32.33.14.55

9
Host/Network Order

• sa.sin_port htons(3000)
• htons converts short to network byte order.
• sa.sin_addr.s_addr inet_addr( 32.33.14.55)
• inet_addr takes string (dotted notation) and
  converts is to a long in network order.

10
Client Fills in Address and Port of Server

• struct sockaddr_in sa
• sa.sin_family AF_INET
• sa.sin_port htons (3000)
• sa.sin_addr.s_addr inet_addr(32.33.14.55 )

11
Step 3 Association with Another Socket

• Client and server set up the association
  differently.
• For the client, it names the socket with the IP
  address and port number of the server.
• Client performs a connect() function call to
  connect to the server.

12
Connect

• Fill in sockaddr_in structure with IP address and
  port number of server.
• sa.sin_family AF_INET
• sa.sin_port htons (Server_Port)
• sa.sin_addr.s_addr inet_addr(xxx.xxx.xx.XX)
• int ret_val connect(sock, (struct sock_addr )
  sa, sizeof(sa))
• Success Returns 0
• Fail Returns SOCKET_ERROR

13

• connect will fail if the server is not in the
  listening state and accepts the connection.
• On success, the client has established a virtual
  TCP connection on which it can send and receive
  data reliably.

14
Step 4 Send/Receive Messages

• Must use a connected socket.
• char buf1024
• int ret_val recv(sock, buf, 1024, 0)
• Success Returns number of bytes actually
  received
• Fail Returns SOCKET_ERROR
• NOTE May not return all requested data.

15
Step 5 Close Socket

• Should call closesocket() when socket no longer
  needed.
• ret_val closesocket(sock)
• if (ret_val ! 0)
• // Some error condition. I am going to call my TA.

16
Other Program Requirements

• Applications must
• include ltwinsock2.hgt
• Link in the winsock dlls.
• Initialize the socket dlls

17
Linking with Winsock Dlls

• Go to the project tab and click on settings.
• Choose the link tab and add WS2_32.lib to the
  object/library modules.

18
Initialization

• WSAData wsadata
• int ret_val
• ret_val WSAStartup( MAKEWORD(2,0), wsadata )
• if (stat ! 0)
• I have a problem
• This initialization must precede all other socket
• functions.
• Need to call WSACleanup() when finished with
  DLLs.