The File Transfer Protocol (FTP) is one of the chief and most frequently used protocols on the Internet. It is generally integrated into the browsers. FTP facilitates the enormous bulk of file transfers across the Internet. FTP is used to exchange and manipulate files over a Transmission Control Protocol (TCP) computer network, such as the internet. Initially FTP was running on hosts inside MIT. Later it was adapted to Arpanet. FTP was designed to allow users to copy files between two different host computers. These separate hosts could potentially be running different operating systems, using different file storage systems, and using different character sets. FTP works using two different TCP connections. The first connection is called a control connection, and the second is called the data connection. The control connection is established at the beginning of the FTP session and is maintained for the duration of the session. Commands issued by the client, and replies originating from server are exchanged along this connection. The data connection on the other hand is transitory and brought up and torn down as needed. This data connection is used to transfer files and directory listings to and from the client at the client's request. Once a user has logged in to the FTP server, the client establishes how the data is to be transported between the two hosts. There are two possible ways of establishing a data connection: Active, and passively-initiated connections. In the active technique, the client first issues a PORT command to the server. This indicates to the server which port the client wishes to receive the data on. The server acknowledges this PORT command. Then, it opens a data connection from port on the server to the previously established ephemeral port on the client. Any data requested by the client is then exchanged on this TCP connection. When the transfer is complete, the server closes the data connection by transmitting a FIN packet and the TCP connection is torn down. The next time the client requests data, a new data connection is built and the whole process repeats. The second way of creating a data connection is called a passively-initiated data transfer. Instead of issuing a PORT command, the client can issue the PASV command. Many FTP servers are set up to disseminate information freely to the public. Often these servers allow anonymous logins. Though usernames are essentially irrelevant in this case, the actual identification takes place in the same manner. Occasionally FTP servers will do a reverse DNS lookup on the IP to record the server you connected from. FTP doesn't account for host verification, data authentication, or data protection. With basic FTP, the server doesn't really have any means of verifying authenticity the clients. Another potential problem is the very act of getting FTP through a firewall. Regulating data transmissions with FTP can be very tricky, particularly if the data is being actively transmitted. With active data transmissions the server makes inbound connections to unknown ports on machines behind the firewall.
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