4.5 KiB
| title |
|---|
| Basic Networking |
Basic Networking
Basic Networking in C mainly involves opening sockets and communicating through them. This begs the question, what is a socket?
What is a Socket
A socket is one endpoint of a two-way communication link between two programs running on a network. An endpoint is a combination of an IP address and a port number. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to.
When a program is running on a network, it is available to access from different locations other than the local location. By different locations, I mean all the computers on the same network can access it. But, how will they? Every program registers itself with a port number. Think of port number as an apartment number in a huge apartment complex. If a letter is sent to an apartment, the apartment number tells the post office the specific apartment within the building he should go to.
But, how will it arrive at the apartment? Every apartment has their own unique address. The post office looks at that unique address and decides the destination of the letter. In this case, every computer connected to a network will have an IP address which is like an address used when sending a letter through the post office. Likewise, a computer connected to a network needs to know the IP addresses of the other computers on the same network to communicate with them. To communicate with a specific program on a specific computer the port number for that program is needed. (Think of the apartment number from our analogy.)
Basics of Socket Programming
Socket programming is a way of connecting two nodes on a network to communicate with each other. One socket(node) listens on a particular port at an IP, while other socket reaches out to the other to form a connection. Server forms the listener socket while client reaches out to the server.
Types of Sockets
There are two widely used socket types, stream sockets, and datagram sockets. Stream sockets treat communications as a continuous stream of characters, while datagram sockets have to read entire messages at once. Each uses its own communciations protocol. Stream sockets use TCP (Transmission Control Protocol), which is a reliable, stream oriented protocol, and datagram sockets use UDP (Unix Datagram Protocol), which is unreliable and message oriented.
Difference between stream and datagram sockets
1.Datagrams are unreliable, which means that if a packet of information gets lost somewhere in the Internet, the sender is not told (and of course the receiver does not know about the existence of the message). In contrast, with a stream socket, the underlying TCP protocol will detect that a message was lost because it was not acknowledged, and it will be retransmitted without the process at either end knowing about this. 2.Message boundaries are preserved in datagram sockets. If the sender sends a datagram of 100 bytes, the receiver must read all 100 bytes at once. This can be contrasted with a stream socket, where if the sender wrote a 100 byte message, the receiver could read it in two chunks of 50 bytes or 100 chunks of one byte. 3.The communication is done using special system calls sendto() and receivefrom() rather than the more generic read() and write(). 4.There is a lot less overhead associated with a datagram socket because connections do not need to be established and broken down, and packets do not need to be acknowledged. This is why datagram sockets are often used when the service to be provided is short, such as a time-of-day service.
I have Socket... Now what?
Once a socket has been created between two nodes (a.k.a hosts or computing systems), a language needs to be agreed upon so the hosts can communicate between themselves - and others. This language is often referred to as a protocol - or an agreed upon way to communicate between hosts. While there are many different protocols, the two most often used protocols are UDP (User Datagram Protocol and TCP (Transmission Control Protocol).