IEN 133
The TFTP Protocol
January 29, 1980
Karen R. Sollins
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 original version of this document was written by Bob
Baldwin. The current version of the document includes modifications
suggested by Noel Chiappa, Dave Clark, Liza Martin and the author. The
acknowledgement and retransmission scheme was inspired by TCP, and the
error mechanism was suggested by PARC's EFTP abort message.
1: Purpose
TFTP is a simple protocol to transfer files, and therefore was
named the Trivial File Transfer Protocol or TFTP. It is built 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. 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.
Three modes of transfer are currently supported: netascii (1);
binary, 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.
2: Overview of the Protocol
Any transfer 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
_______________
(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.
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acknowledged by an acknowledgment packet before the next packet can be
sent. A 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, or
access violation), receiving a packet which cannot be explained by a
delay or duplication in the network (e.g. an incorrectly formed packet),
and losing access to a necessary resource (e.g., disc full, or source
file truncated during transfer).
TFTP recognizes only one type of error that does not cause
termination, the source port of a received packet being incorrect. In
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this case an error packet is sent to the originating host. See the
section on the Initial Connection Protocol for more details.
This protocol is very restrictive, but that makes it easier to
implement. For example, the fixed length blocks make allocation
straight forward, and the lock step acknowledgement provides flow
control and eliminates the need to reassemble files.
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 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.
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)
used by TFTP are passed to the Datagram layer to be used as ports.
Therefore for they must be between 0 and 65,535. The initialization of
TID's is discussed in the section on initial connection protocol.
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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 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
the request is denied for the reason stated in the error packet.
In order to create a connection, TID's to be used for the duration
of the connection are chosen by the two ends of that connection. The
TID's chosen for a connection should be randomly chosen, so that the
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probability that the same number is chosen twice in immediate succession
is very low. Every packet has associated with it two TID's, the source
TID and the destination TID. A requesting host chooses its source TID
as described above, and sends its initial request to the known TID 69
(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.
3. Host A sends a "DATA" (with block number= 1) to host B with
source= A's TID, destination= B's TID.
4. Host B sends a "ACK" (with block number= 1) to host A with
source= B's TID, destination= A's TID.
In step three, and in all succeeding steps, the hosts should make
sure that the source TID matches the value that was agreed on in step 2.
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If it doesn't match, an error packet should be sent to the originator,
but the connection should not be aborted. The following example
demonstates the problem this and the randomly chosen TID's are trying to
solve.
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 repsonse 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 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.
5: TFTP Packets
TFTP supports five types of packets, all of which have been
mentioned above:
opcode operation
1 Read request (RRQ)
2 Write request (WRQ)
3 Acknowledgment (ACK)
4 Data (DATA)
5 Error (ERROR)
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The TFTP header of a packet contains the opcode associated with that
packet.
Figure 2. RRQ/WRQ
2 bytes string 1 byte string 1 byte
------------------------------------------------
| Opcode | Filename | 0 | Mode | 0 |
------------------------------------------------
RRQ and WRQ packets (opcodes 1 and 2 respectively) have the format
shown in Figure 2. The file name is a sequence of bytes in netascii
terminated by a zero byte. The mode field contains the string
"netascii", "binary", or "mail" in netascii indicating the three modes
defined in the protocol. A host which receives netascii mode data must
translate the data to its own format. Presumably, every host will
translate its character set to and from netascii. Binary mode allows
the two communicating hosts to impose their own interpretation on the
data being transmitted; between similar machines binary mode can be
used to avoid the conversion overhead. If a host receives a binary file
and then returns it, the returned file must be identical to the file it
received. Mail mode uses the name of a mail recipient in place of a
file and must begin with a WRQ. Otherwise it is identical to netascii
mode.
Figure 3. DATA
2 bytes 2 bytes n bytes
----------------------------------
| Opcode | Block # | Data |
----------------------------------
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Data is actually transferred in DATA packets depicted in Figure 3. DATA
packets (opcode = 4) have a block number and data field. The block
numbers on data packets begin with one and increase by one for each new
block of data. This restriction allows the program to use a single
number to discriminate between new packets and duplicates. The data
field is from zero to 512 bytes long. If it is 512 bytes long, the
block is not the last block of data; if it is from zero to 511 bytes
long, it signals the last data packet. (See the section on Normal
Termination for details.)
Figure 4. ACK packet
2 bytes 2 bytes
---------------------
| Opcode | Block # |
---------------------
All packets other than those used for termination are acknowledged
individually. Sending a DATA packet is an acknowledgment for the ACK
packet of the previous DATA packet. The WRQ and DATA packets are
acknowledged by ACK or ERROR packets, while RRQ and ACK packets are
acknowledged by DATA or ERROR packets. Figure 4 depicts an ACK packet;
the opcode is 3. The block number in an ACK echoes the block number of
the DATA packet being acknowledged. A WRQ is acknowledged with an ACK
packet having a block number of zero.
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Figure 5. ERROR packet
2 bytes 2 bytes string 1 byte
-----------------------------------------
| Opcode | ErrorCode | ErrMsg | 0 |
-----------------------------------------
An ERROR packet (opcode 5) takes the form depicted in Figure 5. An
ERROR packet can be the acknowledgment of any other type of packet. The
error code is a small integer indicating the nature of the error. A
table of its values and meanings is given in the appendix. The error
message is intended for human consumption, and should be in netascii.
Like all other strings, it is terminated with a zero byte.
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 final ACK packet is never retransmitted; the host acknowledging the
final DATA packet may terminate its side of the connection on sending
the final ACK. On the other hand, 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 it is an ERROR (unknown transfer ID), or 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 may have experienced a problem. It is also possible in
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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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APPENDIX
Order of Headers
2 bytes
----------------------------------------------------------
| Local Medium | Internet | Datagram | TFTP Opcode |
----------------------------------------------------------
TFTP Formats
Type Op # Format without header
____________________________________
2 bytes string 1 byte string 1 byte
-----------------------------------------------
RRQ/ | 01/02 | Filename | 0 | Mode | 0 |
WRQ -----------------------------------------------
2 bytes 2 bytes n bytes
---------------------------------
DATA | 03 | Block # | Data |
---------------------------------
2 bytes 2 bytes
-------------------
ACK | 04 | Block # |
--------------------
2 bytes 2 bytes string 1 byte
----------------------------------------
ERROR | 05 | ErrorCode | ErrMsg | 0 |
----------------------------------------
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Initial Connection Protocol for reading a file
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.
3. Host A sends an "ACK" (with block number= 1) to host B with
source= A's TID, destination= B'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 Disc full or allocation exceeded.
4 Illegal TFTP operation.
5 Unknown transfer ID.
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Internet User Datagram Header
Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Values of Fields
Source Port Picked by originator of packet.
Dest. Port Picked by destination machine (69 for RRQ or WRQ).
Length Number of bytes in packet after Datagram header.
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.
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References
1. USA Standard Code for Information Interchange,
USASI X3.4-1968.
2. Postel, Jon., "User Datagram Protocol," IEN 88, May 2,
1979.
3. "Telnet Protocol Specification," RFC552, NIC 18639,
August, 1973.