Trivial File Transfer Protocol (TFTP) is a simple
lockstep File Transfer Protocol
The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...
which allows a
client
Client(s) or The Client may refer to:
* Client (business)
* Client (computing), hardware or software that accesses a remote service on another computer
* Customer or client, a recipient of goods or services in return for monetary or other valuable ...
to get a file from or put a file onto a remote
host. One of its primary uses is in the early stages of nodes booting from a
local area network. TFTP has been used for this application because it is very simple to implement.
TFTP was first standardized in 1981 and the current specification for the protocol can be found in .
Overview
Due to its simple design, TFTP can be easily implemented by code with a small
memory footprint
Memory footprint refers to the amount of main memory that a program uses or references while running.
The word footprint generally refers to the extent of physical dimensions that an object occupies, giving a sense of its size. In computing, the ...
. It is therefore the protocol of choice for the initial stages of any
network booting strategy like
BOOTP
The Bootstrap Protocol (BOOTP) is a computer networking protocol used in
Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951.
While some par ...
,
PXE,
BSDP, etc., when targeting from highly resourced computers to very low resourced
Single-board computers (SBC) and
System on a Chip (SoC). It is also used to transfer
firmware images and configuration files to network appliances like
routers,
firewalls,
IP phones, etc. Today, TFTP is virtually unused for Internet transfers.
TFTP's design was influenced from the earlier protocol
EFTP
EFTP was a very simple file transfer protocol developed as part of the PARC Universal Packet protocol suite at Xerox PARC in the late 1970s. It was the inspiration for the Trivial File Transfer Protocol (TFTP) in the TCP/IP suite.
As with it ...
, which was part of the
PARC Universal Packet protocol suite. TFTP was first defined in 1980 by
IEN 133.
In June 1981 The TFTP Protocol (Revision 2) was published as RFC 783 and later updated in July 1992 by RFC 1350 which fixed among other things the
Sorcerer's Apprentice Syndrome
Sorcerer's Apprentice Syndrome (SAS) is a network protocol flaw in the original versions of TFTP. It was named after Goethe's 1797 poem " Der Zauberlehrling" (popularized by the " Sorcerer's Apprentice" segment of the 1940 animated film ''Fantasi ...
. In March 1995 the TFTP Option Extension RFC 1782 updated later in May 1998 by RFC 2347, defined the option negotiation mechanism which establishes the framework for file transfer options to be negotiated prior to the transfer using a mechanism which is consistent with TFTP's original specification.
TFTP is a simple protocol for transferring files, implemented on top of the
UDP/IP protocols using
well-known port
This is a list of TCP and UDP port numbers used by protocols for operation of network applications.
The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) only need one port for duplex, bidirectional traffic. They usually u ...
number 69. TFTP was designed to be small and easy to implement, and therefore it lacks most of the advanced features offered by more robust file transfer protocols. TFTP only reads and writes files from or to a remote server. It cannot list, delete, or rename files or directories and it has no provisions for user authentication.
Today TFTP is generally only used on
local area networks (LAN).
Details
In TFTP, a transfer is initiated by the client issuing a request to read or write a particular file on the server. The request can optionally include a set of negotiated transfer parameters proposed by the client under the terms specified by RFC 2347. If the server grants the request, the file is sent in fixed length blocks of 512 bytes by default or the number specified in the blocksize negotiated option defined by RFC 2348. Each block of transferred data, which is usually carried within a single IP packet in order to avoid IP fragmentation, must be acknowledged by an acknowledgment packet before the next block can be sent. A data packet of less than 512 bytes or the agreed blocksize option 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 correctly received. Notice that both devices involved in a transfer are considered senders and receivers. One sends data and receives acknowledgments, the other sends acknowledgments and receives data.
TFTP defines three modes of transfer: netascii, octet, and mail.
# Netascii is a modified form of
ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
, 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 raw 8-bit bytes, with the received file resulting byte-per-byte identical to the one sent. 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.
TFTP uses
UDP as its
transport protocol. A transfer request is always initiated targeting port 69, but the data transfer ports are chosen independently by the sender and receiver during the transfer initialization. The ports are chosen at random according to the parameters of the networking stack, typically from the range of
ephemeral ports.
# The initiating host A sends an RRQ (read request) or WRQ (write request) packet to host S at port number 69, containing the filename, transfer mode, and optionally any negotiated option under the terms of RFC 2347.
# S replies with an option ACK if options were used, and an ACK (acknowledgement) packet to WRQ and directly with a DATA packet to RRQ. Packet is sent from a randomly allocated
ephemeral port, and all future packets to host S should be directed to this port.
# The source host sends numbered DATA packets to the destination host, all but the last containing a full-sized block of data (512 bytes default). The destination host replies with numbered ACK packets for all DATA packets.
# The final DATA packet must contain less than a full-sized block of data to signal that it is the last. If the size of the transferred file is an exact multiple of the block-size, the source sends a final DATA packet containing 0 bytes of data.
# Receiver responds to each DATA with associated numbered ACK. Sender responds to the first received ACK of a block with DATA of the next block.
# If an ACK is not eventually received, a retransmit timer re-sends DATA packet.
TFTP has always been associated to network booting. One of the first attempts in this regard was the Bootstrap Loading using TFTP standard RFC 906, published in 1984, which established the 1981 published Trivial File Transfer Protocol standard RFC 783 to be used as the standard file transfer protocol for bootstrap loading. It was followed shortly after by the
Bootstrap Protocol standard RFC 951 (BOOTP), published in 1985, which allowed a disk-less client machine to discover its own IP address, the address of a TFTP server, and the name of a
Network Bootstrap Program (NBP) to be TFTP transferred, loaded into memory, and executed.
Dynamic Host Configuration Protocol standard RFC 2131 (DHCP) published in 1997 improved BOOTP capabilities. Finally, the
Preboot Execution Environment (PXE) version 2.0 was released in December 1998, and the update 2.1 was made public in September 1999 counting on TFTP as its file transfer protocol.
Intel has recently decided to widely support PXE within the new
UEFI specification extending the TFTP support to all EFI/UEFI environments.
The original protocol has a transfer file size limit of 512 bytes/block x 65535 blocks = 32 MB. In 1998 this limit was extended to 65535 bytes/block x 65535 blocks = 4 GB by TFTP Blocksize Option RFC 2348. If the defined blocksize produces an IP packet size that exceeds the minimum
MTU at any point of the network path, IP fragmentation and reassembly will occur not only adding more overhead but also leading to total transfer failure when the minimalist
IP stack
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...
implementation in a host's
BOOTP
The Bootstrap Protocol (BOOTP) is a computer networking protocol used in
Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951.
While some par ...
or
PXE ROM does not (or fails to properly) implement IP fragmentation and reassembly. If TFTP packets should be kept within the standard Ethernet MTU (1500), the blocksize value is calculated as 1500 minus headers of TFTP (4 bytes), UDP (8 bytes) and IP (20 bytes) = 1468 bytes/block, this gives a limit of 1468 bytes/block x 65535 blocks = 92 MB. Today most servers and clients support block number roll-over (block counter going back to 0 or 1
after 65535) which gives an essentially unlimited transfer file size.
Since TFTP utilizes UDP, it has to supply its own transport and session support. Each file transferred via TFTP constitutes an independent exchange. Classically, this transfer is performed in lock-step, with only one
packet
Packet may refer to:
* A small container or pouch
** Packet (container), a small single use container
** Cigarette packet
** Sugar packet
* Network packet, a formatted unit of data carried by a packet-mode computer network
* Packet radio, a form ...
(either a block of data, or an 'acknowledgement') alternatively in flight on the network at any time. Due to this single data block strategy instead of sending a larger amount of uninterrupted data blocks before pausing the transfer to wait for the corresponding acknowledge (windowing), TFTP provides low
throughput especially over high
latency links. Microsoft introduced windowed TFTP in Windows 2008 as part of their Windows Deployment Services (WDS), in January 2015 TFTP Windowsize Option RFC 7440 was published. This substantially improves performance for things like
PXE booting without the IP fragmentation side effect sometimes observed on Blocksize Option RFC 2348
Security considerations
TFTP includes no login or access control mechanisms. Care must be taken when using TFTP for file transfers where authentication, access control, confidentiality, or integrity checking are needed. Note that those security services could be supplied above or below the layer at which TFTP runs. Care must also be taken in the rights granted to a TFTP server process so as not to violate the security of the server's file system. TFTP is often installed with controls such that only files that have public read access are available via TFTP. Also listing, deleting, renaming, and writing files via TFTP are typically disallowed. TFTP file transfers are not recommended where the inherent protocol limitations could raise insurmountable liability concerns.
[RFC 7440, page 7.]
IETF standards documentation
See also
*
Simple File Transfer Protocol
References
{{reflist
Networking standards
Network protocols
Network file transfer protocols
Network booting