Tcp Tutorial-13

Application Layer

Till now you have learnt how data is broken down into smaller chunks, and transferred to the destination, where the chunks are rearranged. But there is yet another aspect to a successful data transfer process, which we have not discussed yet: The Application Protocols and the Application Layer itself. A host which receives datagrams has many applications or services (daemons) running which are ready to establish a TCP connection and accept a message. Datagrams travelling on the Internet must know which application they have to establish connection with, which application they have to send the message to. A typical web server will have the FTP daemon, the HTTP daemon, the POP daemon, and the SMTP daemon running. Wouldn't the datagrams get confused as to which daemon to send the message to.

For the datagrams to know which computer to send the message to, we have IP addresses. The datagram

the Destination. A TCP address is actually fully described by 4 numbers; The IP address of the Source and

Destination and the TCP Port Numbers of each end to which data is to be sent. These numbers are found in the TCP Header.

To make it simpler to understand I have included an excerpt from the Net Tools Chapter:

What is all the hype about socket programming? What exactly are sockets?

TCP\IP or Transmission Control Protocol\ Internet Protocol is the language or the protocol used by computers to communicate with each other over the Internet. Say a computer whose IP address is 99.99.99.99 wants to communicate with another machine whose IP address is 98.98.98.98 then would will happen?

The machine whose IP is 99.99.99.99 sends a packet addressed to another machine whose IP is 98.98.98.98. When 98.98.98.98 receives the packet then it verifies that it got the message by sending a signal back to 99.99.99.99.But say the person who is using 99.99.99.99 wants to have simultaneously more than one connections to 98.98.98.98.....then what will happen? Say 99.99.99.99 wants to connect to the FTP daemon and download a file by FTP and at the same time it wants to connect to 98.98.98.98's website i.e. The HTTP daemon. Then 98.98.98.98. will have 2 connects with 99.99.99.99 simultaneously. Now how can 98.98.98.98.distinguish between the two connections...how does 98.98.98.98. know which is for the FTP daemon and which for the HTTP daemon? If there was no way to distinguish between the two connections then they would both get mixed up and there would be a lot of chaos with the message meant for the HTTP daemon going to the FTP daemon. To avoid such confusion we have ports. At each port a particular service or daemon is running by default. So now that the 99.99.99.99 computers knows which port to connect to, to download a FTP file and which port to connect to, to download the web page,