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Trivial File Transfer Protocol (TFTP) is a file transfer protocol notable for its simplicity. It is generally used for automated transfer of configuration or boot files between machines in a local environment. Compared to FTP, TFTP is extremely limited, providing no authentication, and is rarely used interactively by a user.
Due to its simple design, TFTP can be implemented using a very small amount of memory. It is therefore useful for booting computers such as routers which may not have any data storage devices. It is an element of the Preboot Execution Environment (PXE) network boot protocol, where it is implemented in the firmware ROM / NVRAM of the host's network card.
It is also used to transfer small amounts of data between hosts on a network, such as IP phone firmware or operating system images when a remote X Window System terminal or any other thin client boots from a network host or server. The initial stages of some network based installation systems (such as Solaris Jumpstart, Red Hat Kickstart, Symantec Ghost and Windows NT's Remote Installation Services) use TFTP to load a basic kernel that performs the actual installation. It was used for saving router configurations on Cisco routers, but was later augmented by other protocols.
TFTP was first defined in 1980 by IEN 133. Since 1992, it has been defined by RFC 1350. There have been some extensions to the TFTP protocol documented in later RFCs (see the section on Extensions, below). TFTP is based in part on the earlier protocol EFTP, which was part of the PUP protocol suite. TFTP support appeared first as part of 4.3 BSD.
Due to the lack of security, it is dangerous to use it over the Internet. Thus, TFTP is generally only used on private, local networks.
TFTP is a simple protocol for transferring files, implemented on top of the User Datagram Protocol (UDP) using port number 69. TFTP was designed to be small and easy to implement, and therefore it lacks most of the features of a regular FTP. TFTP only reads and writes files from or to a remote server. It cannot list directories, and has no provisions for user authentication.
In TFTP, communication is initiated by the client issuing a request to read or write a file on the server. If the server grants the request, the connection is opened and the file is sent in fixed length blocks of 512 bytes. Each data packet contains one block of data, and must be acknowledged by an acknowledgment packet before the next packet can be sent. A data packet of less than 512 bytes signals termination of a transfer. If a packet gets lost in the network, the intended recipient will timeout and may retransmit their last packet (which may be data or an acknowledgment), thus causing the sender of the lost packet to retransmit that lost packet. The sender has to keep just one packet on hand for retransmission, since the lock step acknowledgment guarantees that all older packets have been received. Notice that both machines involved in a transfer are considered senders and receivers. One sends data and receives acknowledgments, the other sends acknowledgments and receives data.
TFTP typically uses UDP as its transport protocol, but it is not a requirement. Data transfer is initiated on port 69, but the data transfer ports are chosen independently by the sender and receiver during initialization of the connection. The ports are chosen at random according to the parameters of the networking stack, typically from the range of ephemeral ports.
TFTP defines three modes of transfer: netascii, octet, and mail.
Netascii is a modified form of ASCII, defined in RFC 764. It consists of an 8-bit extension of the 7-bit ASCII character space from 0x20 to 0x7F (the printable characters and the space) and eight of the control characters. The allowed control characters include the null (0x00), the line feed (LF, 0x0A), and the carriage return (CR, 0x0D). Netascii also requires that the end of line marker on a host be translated to the character pair CR LF for transmission, and that any CR must be followed by either a LF or the null.
Octet allows for the transfer of arbitrary 8-bit bytes, with the received file identical to the sent file. More correctly, if a host receives an octet file and then returns it, the returned file must be identical to the original.
Mail transfer mode uses Netascii transfer, but the file is sent to an email recipient by specifying that recipient's email address as the file name. RFC 1350 declared this mode of transfer obsolete.
No security or authentication is provided by the protocol specification. Unix implementations often restrict file transfers to a single configured directory, and only to read from files with world readability, and only write to already existing files that have world writeability.