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RFC 783 - TFTP Protocol (revision 2)

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Network Working Group                                      K. R. Sollins
Request for Comments: 783                                            MIT
                                                              June, 1981
Updates: IEN 133

                     THE TFTP PROTOCOL (REVISION 2)


  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.

   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
   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


   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


          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


                       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



  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
|  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  |


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] 


 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.
   The  implementor of this should be sure that the correct algorithm is
used here.



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


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


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



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