Network Working Group K. R. Sollins
Request for Comments: 783 MIT
June, 1981
Updates: IEN 133
THE TFTP PROTOCOL (REVISION 2)
Summary
TFTP is a very simple protocol used to transfer files. It is from
this that its name comes, Trivial File Transfer Protocol or TFTP. Each
nonterminal packet is acknowledged separately. This document describes
the protocol and its types of packets. The document also explains the
reasons behind some of the design decisions.
ACKNOWLEDGEMENTS
The protocol was originally designed by Noel Chiappa, and was
redesigned by him, Bob Baldwin and Dave Clark, with comments from Steve
Szymanski. The current revision of the document includes modifications
stemming from discussions with and suggestions from Larry Allen, Noel
Chiappa, Dave Clark, Geoff Cooper, Mike Greenwald, Liza Martin, David
Reed, Craig Milo Rogers (of UCS-ISI), Kathy Yellick, and the author.
The acknowledgement and retransmission scheme was inspired by TCP, and
the error mechanism was suggested by PARC's EFTP abort message.
This research was supported by the Advanced Research Projects Agency of
the Department of Defense and was monitored by the Office of Naval
Research under contract number N00014-75-C-0661.
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1. Purpose
TFTP is a simple protocol to transfer files, and therefore was named
the Trivial File Transfer Protocol or TFTP. It has been implemented on
top of the Internet User Datagram protocol (UDP or Datagram) [2] so it
may be used to move files between machines on different networks
implementing UDP. (This should not exlude the possibility of
implementing TFTP on top of other datagram protocols.) It is designed
to be small and easy to implement. Therefore, it lacks most of the
features of a regular FTP. The only thing it can do is read and write
files (or mail) from/to a remote server. It cannot list directories,
and currently has no provisions for user authentication. In common with
other Internet protocols, it passes 8 bit bytes of data.
1 2
Three modes of transfer are currently supported: netascii ; octet ,
raw 8 bit bytes; mail, netascii characters sent to a user rather than a
file. Additional modes can be defined by pairs of cooperating hosts.
_______________
1
This is ascii as defined in "USA Standard Code for Information
Interchange" [1] with the modifications specified in "Telnet Protocol
Specification" [3]. Note that it is 8 bit ascii. The term "netascii"
will be used throughout this document to mean this particular version of
ascii.
2
This replaces the "binary" mode of previous versions of this
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2. Overview of the Protocol
Any transsfer begins with a request to read or write a file, which also
serves to request a connection. 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 his 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.
Most errors cause termination of the connection. An error is
signalled by sending an error packet. This packet is not acknowledged,
and not retransmitted (i.e., a TFTP server or user may terminate after
sending an error message), so the other end of the connection may not
get it. Therefore timeouts are used to detect such a termination when
the error packet has been lost. Errors are caused by three types of
events: not being able to satisfy the request (e.g., file not found,
access violation, or no such user), receiving a packet which cannot be
explained by a delay or duplication in the network (e.g. an incorrectly
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formed packet), and losing access to a necessary resource (e.g., disk
full or access denied during a transfer).
TFTP recognizes only one error condition that does not cause
termination, the source port of a received packet being incorrect. In
this case, an error packet is sent to the originating host.
This protocol is very restrictive, in order to simplify
implementation. For example, the fixed length blocks make allocation
straight forward, and the lock step acknowledgement provides flow
control and eliminates the need to reorder incoming data packets.
3. Relation to other Protocols
As mentioned TFTP is designed to be implemented on top of the Datagram
protocol. Since Datagram is implemented on the Internet protocol,
packets will have an Internet header, a Datagram header, and a TFTP
header. Additionally, the packets may have a header (LNI, ARPA header,
etc.) to allow them through the local transport medium. As shown in
Figure 3-1, the order of the contents of a packet will be: local medium
header, if used, Internet header, Datagram header, TFTP header, followed
by the remainder of the TFTP packet. (This may or may not be data
depending on the type of packet as specified in the TFTP header.) TFTP
does not specify any of the values in the Internet header. On the other
hand, the source and destination port fields of the Datagram header (its
format is given in the appendix) are used by TFTP and the length field
reflects the size of the TFTP packet. The transfer identifiers (TID's)
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used by TFTP are passed to the Datagram layer to be used as ports;
therefore they must be between 0 and 65,535. The initialization of
TID's is discussed in the section on initial connection protocol.
The TFTP header consists of a 2 byte opcode field which indicates the
packet's type (e.g., DATA, ERROR, etc.) These opcodes and the formats
of the various types of packets are discussed further in the section on
TFTP packets.
Figure 3-1: Order of Headers
---------------------------------------------------
| Local Medium | Internet | Datagram | TFTP |
---------------------------------------------------
4. Initial Connection Protocol
A transfer is established by sending a request (WRQ to write onto a
foreign file system, or RRQ to read from it), and receiving a positive
reply, an acknowledgment packet for write, or the first data packet for
read. In general an acknowledgment packet will contain the block number
of the data packet being acknowledged. Each data packet has associated
with it a block number; block numbers are consecutive and begin with
one. Since the positive response to a write request is an
acknowledgment packet, in this special case the block number will be
zero. (Normally, since an acknowledgment packet is acknowledging a data
packet, the acknowledgment packet will contain the block number of the
data packet being acknowledged.) If the reply is an error packet, then
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the request has been denied.
In order to create a connection, each end of the connection chooses a
TID for itself, to be used for the duration of that connection. The
TID's chosen for a connection should be randomly chosen, so that the
probability that the same number is chosen twice in immediate succession
is very low. Every packet has associated with it the two TID's of the
ends of the connection, the source TID and the destination TID. These
TID's are handed to the supporting UDP (or other datagram protocol) as
the source and destination ports. A requesting host chooses its source
TID as described above, and sends its initial request to the known TID
69 decimal (105 octal) on the serving host. The response to the
request, under normal operation, uses a TID chosen by the server as its
source TID and the TID chosen for the previous message by the requestor
as its destination TID. The two chosen TID's are then used for the
remainder of the transfer.
As an example, the following shows the steps used to establish a
connection to write a file. Note that WRQ, ACK, and DATA are the names
of the write request, acknowledgment, and data types of packets
respectively. The appendix contains a similar example for reading a
file.
1. Host A sends a "WRQ" to host B with source= A's TID,
destination= 69.
2. Host B sends a "ACK" (with block number= 0) to host A with
source= B's TID, destination= A's TID.
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At this point the connection has been established and the first data
packet can be sent by Host A with a sequence number of 1. In the next
step, and in all succeeding steps, the hosts should make sure that the
source TID matches the value that was agreed on in steps 1 and 2. If a
source TID does not match, the packet should be discarded as erroneously
sent from somewhere else. An error packet should be sent to the source
of the incorrect packet, while not disturbing the transfer.
This can be done only if the TFTP in fact receives a packet with an
incorrect TID. If the supporting protocols do not allow it, this
particular error condition will not arise.
The following example demonstrates a correct operation of the protocol
in which the above situation can occur. Host A sends a request to host
B. Somewhere in the network, the request packet is duplicated, and as a
result two acknowledgments are returned to host A, with different TID's
chosen on host B in response to the two requests. When the first
response arrives, host A continues the connection. When the second
response to the request arrives, it should be rejected, but there is no
reason to terminate the first connection. Therefore, if different TID's
are chosen for the two connections on host B and host A checks the
source TID's of the messages it receives, the first connection can be
maintained while the second is rejected by returning an error packet.
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5. TFTP Packets
TFTP supports five types of packets, all of which have been mentioned
An ERROR packet (opcode 5) takes the form depicted in Figure 5-4. An
ERROR packet can be the acknowledgment of any other type of packet. The
error code is an integer indicating the nature of the error. A table of
values and meanings is given in the appendix. (Note that several error
codes have been added to this version of this document.) The error
message is intended for human consumption, and should be in netascii.
Like all other strings, it is terminated with a zero byte.
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6. Normal Termination
The end of a transfer is marked by a DATA packet that contains between
0 and 511 bytes of data (i.e. Datagram length < 516). This packet is
acknowledged by an ACK packet like all other DATA packets. The host
acknowledging the final DATA packet may terminate its side of the
connection on sending the final ACK. On the other hand, dallying is
encouraged. This means that the host sending the final ACK will wait
for a while before terminating in order to retransmit the final ACK if
it has been lost. The acknowledger will know that the ACK has been lost
if it receives the final DATA packet again. The host sending the last
DATA must retransmit it until the packet is acknowledged or the sending
host times out. If the response is an ACK, the transmission was
completed successfully. If the sender of the data times out and is not
prepared to retransmit any more, the transfer may still have been
completed successfully, after which the acknowledger or network may have
experienced a problem. It is also possible in this case that the
transfer was unsuccessful. In any case, the connection has been closed.
7. Premature Termination
If a request can not be granted, or some error occurs during the
transfer, then an ERROR packet (opcode 5) is sent. This is only a
courtesy since it will not be retransmitted or acknowledged, so it may
never be received. Timeouts must also be used to detect errors.
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I. Appendix
Order of Headers
2 bytes
----------------------------------------------------------
| Local Medium | Internet | Datagram | TFTP Opcode |
----------------------------------------------------------
1. Host A sends a "RRQ" to host B with source= A's TID,
destination= 69.
2. Host B sends a "DATA" (with block number= 1) to host A with
source= B's TID, destination= A's TID.
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Error Codes
Value Meaning
0 Not defined, see error message (if any).
1 File not found.
2 Access violation.
3 Disk full or allocation exceeded.
4 Illegal TFTP operation.
5 Unknown transfer ID.
6 File already exists.
7 No such user.
Dest. Port Picked by destination machine (69 for RRQ or WRQ).
Length Number of bytes in packet after Datagram header.
4
Checksum Reference 2 describes rules for computing checksum.
Field contains zero if unused.
Note: TFTP passes transfer identifiers (TID's) to the Internet User
Datagram protocol to be used as the source and destination ports.
_______________
3
This has been included only for convenience. TFTP need not be
implemented on top of the Internet User Datagram Protocol.
4
The implementor of this should be sure that the correct algorithm is
used here.
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References
[1] USA Standard Code for Information Interchange, USASI X3.4-
1968.
[2] Postel, Jon., "User Datagram Protocol," RFC768, August 28,
