File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file from one host to another.

FTP uses two well-known TCP ports: Port 21 is used for the control connection, and port 20 is used for the data connection.

Figure shows the basic model of FTP. The client has three components: user interface, client control process, and the client data transfer process. The server has two components: the server control process and the server data transfer process.

The control connection is made between the control processes. The data connection is made between the data transfer processes.

The control connection remains connected during the entire interactive FTP session.The data connection is opened and then closed for each file transferred. It openseach time commands that involve transferring files are used, and it closes when the fileis transferred.

The two FTP connections, control and data, use different strategies and different port numbers.

Control Connection:

The control connection is created in the same way as other application programsdescribed so far. There are two steps:

1. The server issues a passive open on the well-known port 21 and waits for a client.

2. The client uses an ephemeral port and issues an active open.

Figure shows the initial connection between the server and the client.

Data Connection:

The data connection uses the well-known port 20 at the server site.The followingshows how FTP creates a data connection:

1. The client, not the server, issues a passive open using an ephemeral port. This must bedone by the client because it is the client that issues the commands for transferring files.

2. The client sends this port number to the server using the PORT command.

3. The server receives the port number and issues an active open using the wellknownport 20 and the received ephemeral port number.

The steps for creating the initial data connection are shown in Figure.

Limitations of FTP:

Transferring files with FTP is not secure.

All data is sent in clear text, including usernames and passwords.

To simply copy a file,FTP need to download the bootstrap and configuration files, but TFTP don’t need all sophistication. TFTP uses the services of UDP on the well-known port 69.