Trivial File Transfer Protocol (TFTP) is a very simple protocol used to transfer files over the UDP transport protocol. It is commonly used for bootstrapping diskless systems (normally workstations or routers).

The protocol follows a very simple procedure. The client sends a request to read (RRQ) or write (WRQ) a file to the server's UDP/69 port. If the server grants the request a connection is opened and the file server starts sending the file in fixed length blocks of 512 bytes. TFTP transports data in netascii encoding format (ASCII text with each line terminated by the 2-character sequence of a carriage return followed by a linefeed called CR/LF) or octet (data as 8-bit bytes with no interpretation) which is set by the mode indicator at the end of the RRQ/WRQ message. The DATA packet also contains a block number which is used later for acknowledgment. Every packet sent must be acknowledged by the receiver, which guarantees that the previous packet has been received. If a packet is lost the receiver sends a request after a timeout. The server keeps just one packet in store for retransmission until the acknowledgment arrives. A packet shorter than 512 bytes indicates the end of the transmission.

Most errors cause termination of the transfer process and are signaled by the sending of an error packet. This is neither acknowledged nor retransmitted. If an error occurred, then an ERROR packet is sent. If a network error occurred then even the ERROR packet might get lost, therefore timeout is also used to detect errors.

Normal transmission termination is started by a packet smaller than 512 bytes. The packet is acknowledged by a normal ACK packet like all the previous packet. Then the host sends the final ACK and waits for a while before it terminates the transmission. If the final ACK is not acknowledged or the the connection timed out the final ACK packet is retransmitted.