We mentioned earlier that a transport protocol can have simultaneous connections to a computer- yet the receiving computer still knows where each data packet should be sent. This is accomplished through ports and sockets.

A port is simply an internal address that acts as a pathway to control data flow. Since we need each port to be specific to a certain application, there are thousands of ports for use. If you are using the internet, for instance, data is being routed through TCP port 80. This port is called the HTTP port. Ports that have specific purposes (such as the HTTP port) are also known as well-known ports. (And in case you were wondering, there are over 65,535 ports to experiment with.)

 So now we know two things. First, the IP address is used to route the data to a specific computer. Next, the port number is used to tell the receiving computer what kind of application should handle it. But to actually accomplish both of these tasks, we use what is called a socket. A socket is simply an address formed by using the IP address and then tacking on the port number at the end.

So what good is a socket? For one thing, multiplexing and demultiplexing is made possible. When you are running multiple programs that are communicating with other computers, you are making use of both of these technologies.

Let’s look at a practical example. You are running a telnet server in which multiple computers are connected to. Each computer uses a socket address to tell the server which computer and which port the data is coming from. If each computer broadcasted at the same time, there may be a jam at the transport layer. We use multiplexing in this case to combine all incoming data into one stream.

Don’t worry- UDP is a lot simpler as compared to the TCP counterpart. This is due to the lack of error checking UDP contains. Although UDP is famously known for not having error checking, there are slight UDP functionalities that can act as basic error checking procedures.

It’s important to know that the UDP error checking functionalities are quite basic. For instance, the UDP protocol contains functionality for a checksum so that we may check the integrity of data. This field can be turned off, however, so that a faster connection can be had. Also important to note is that UDP uses a fake header (known as a pseudo-header) that contains a destination address, so that we can determine whether or not the packets were sent to the right place. If they are indeed sent to the wrong place, a simple ICMP message may notify the source machine that the source is unreachable.

Keep in mind that even if errors are found within a UDP connection, data will not be retransmitted. So how much different is a UDP datagram from a TCP segment?