RFC 783

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.

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.

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

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

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)

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

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.

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.

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     Data (DATA)
            4     Acknowledgment (ACK)
            5     Error (ERROR)

The TFTP header of a packet contains the opcode associated with that

packet.

Figure 5-1: RRQ/WRQ packet

            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 5-1. The file name is a sequence of bytes in netascii

terminated by a zero byte. The mode field contains the string

"netascii", "octet", or "mail" (or any comibnation of upper and lower

case, such as "NETASCII", NetAscii", etc.) 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. Octet mode is used

to transfer a file that is in the 8-bit format of the machine from which

the file is being transferred. It is assumed that each type of machine

has a single 8-bit format that is more common, and that that format is

chosen. For example, on a DEC-20, a 36 bit machine, this is four 8-bit

bytes to a word with four bits of breakage. If a host receives a octet

file and then returns it, the returned file must be identical to the

original. 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. The mail recipient string should be of the form "username" or

"username@hostname". If the second form is used, it allows the option

of mail forwarding by a relay computer.

The discussion above assumes that both the sender and recipient are

operating in the same mode, but there is no reason that this has to be

the case. For example, one might build a storage server. There is no

reason that such a machine needs to translate netascii into its own form

of text. Rather, the sender might send files in netascii, but the

storage server might simply store them without translation in 8-bit

format. Another such situation is a problem that currently exists on

DEC-20 systems. Neither netascii nor octet accesses all the bits in a

word. One might create a special mode for such a machine which read all

the bits in a word, but in which the receiver stored the information in

8-bit format. When such a file is retrieved from the storage site, it

must be restored to its original form to be useful, so the reverse mode

must also be implemented. The user site will have to remember some

information to achieve this. In both of these examples, the request

packets would specify octet mode to the foreign host, but the local host

would be in some other mode. No such machine or application specific

modes have been specified in TFTP, but one would be compatible with this

specification.

It is also possible to define other modes for cooperating pairs of

hosts, although this must be done with care. There is no requirement

that any other hosts implement these. There is no central authority

that will define these modes or assign them names.

Figure 5-2: DATA packet

                   2 bytes     2 bytes      n bytes
                   ----------------------------------
                  | Opcode |   Block #  |   Data     |
                   ----------------------------------

Data is actually transferred in DATA packets depicted in Figure 5-2.

DATA packets (opcode = 3) 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 end of the transfer. (See the section on Normal

Termination for details.)

All packets other than those used for termination are acknowledged

individually unless a timeout occurs. 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

Figure 5-3: ACK packet

                         2 bytes     2 bytes
                         ---------------------
                        | Opcode |   Block #  |
                         ---------------------

ACK packets are acknowledged by DATA or ERROR packets. Figure 5-3

depicts an ACK packet; the opcode is 4. 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.

Figure 5-4: ERROR packet

               2 bytes     2 bytes      string    1 byte
               -----------------------------------------
              | Opcode |  ErrorCode |   ErrMsg   |   0  |
               -----------------------------------------

An ERROR packet (opcode 5) takes the form depicted in Figure 5-4. 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.

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.

I. Appendix

Order of Headers

2 bytes

 ----------------------------------------------------------

 ----------------------------------------------------------

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  |
       ----------------------------------------

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.

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.

Internet User Datagram Header [2]

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.

_______________

   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.

References

  [1]     USA  Standard  Code  for  Information Interchange, USASI X3.4-

1968.

  [2]     Postel, Jon., "User Datagram  Protocol,"  RFC768,  August  28,

1980.

  [3]     "Telnet Protocol Specification," RFC764, June, 1980.