FTP and TFTP

Describe the capabilities and function of TFTP/FTP in the network

Purpose of FTP/TFTP

• FTP and TFTP are industry standard protocols used to transfer files over a network
• Both use a client-server model
  • clients can use FTP or TFTP to copy files from a server
  • clients can use FTP or TFTP to copy files to a server

• Network engineers used FTP/TFTP to upgrade operating systems of network devices such as routers, switches, etc.
  • Use FTP/TFTP to download newer version of Cisco IOS from a server then reboot the device with the new IOS image

TFTP

TFTP Functions

• Trivial File transfer protocol (TFTP) was first standardized in 1981
• Lightweight and Simple ( only allows client to copy a file to/or from a server
• Released after FTP
• No authentication
• No encryption (plain text)
• Meant for controlled environment only
• Listens on UDP port 69
• Does not no provide reliability due to UDP(connectionless)
• Re-transmission and reliability built-in feature( at least it does the job)

TFTP Reliability

• Every TFTP data message is acknowledged
  • if the client is transferring a file to the server, the server will send Ack messages
  • if the server is transferring a file to the client, the client will send Ack messages

• TFTP uses lock-step communication(in the case of a failure, retransmission of data can happen)

TFTP Connections

TFTP file transfers have three phases:

1. Connection
   • TFTP client sens a request to the server, and the server responds back initializing the connection.
2. Data transfer
   • The client and server exchange TFTP messages. One sends data and the other sends acknowledgements.
3. Connection termination
   • After the last data message has been sent, a final acknowledgement is sent to terminate the connection.

Transfer Identifier(TID)

• When the client sends the first message to the server, the destination port is UDP 69 and the source is a random ephemeral port
• The ephemeral port is called a Transfer Identifier or TID and identifies the data transfer
• The server then also selects a random TID to use as the source port when it replies, NOT 69
• When the client sends the next message, the destination port will be the server’s TID, NOT 69

FTP

FTP Functions

• File Transfer Protocol or FTP
• Standardized in 1971
• Uses TCP port 20 and 21
• Username & passwords are used for authentication
• No encryption
• FTPS is an upgrade of FTP
• SSH File Transfer Protocol (SFTP) is a new protocol with greater security
• Transfer files, navigate file directories, add/remove/list files
• FTP commands used to perform above operations on a file

FTP Control Connections

FTP uses two types of connections:

• FTP control connection
  • uses TCP port 21
  • established and used to send FTP commands and replies
• FTP data connections
  • When files or data are to be transferred, separate FTP data connections are established and terminated as needed

FTP Data Connections

Active Mode

• Default method of establishing FTP data connections is ACTIVE MODE, in which the server initiates the TCP connection

Passive Mode

• In PASSIVE MODE, the client initiates the data connection. This is often necessary when the client is behind a firewall, which could block incoming connection from the serve

FTP vs TFTP