    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.

                                    2

    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

                                     3

    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

                                     4

    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)

                                     5

    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

                                     6

    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.

                                     7

    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.

                                     8

    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

                                     9

    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

                                     10

    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

                                     11

                             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.

                                     12

    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.

                                     13

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

                                     14

    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.

                                     15

    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.

                                   16

                                     3
    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.

                                                                       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.

                                     17

    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.

                                     18