rfc1350 - The TFTP Protocol (Revision 2)
Description: The TFTP Protocol (Revision 2)
Date: July 1992

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# RFC1350

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    Network Working Group                                         K. Sollins
    Request For Comments: 1350                                           MIT
    STD: 33                                                        July 1992
    Obsoletes: RFC 783

                         THE TFTP PROTOCOL (REVISION 2)

    Status of this Memo

       This RFC specifies an IAB standards track protocol for the Internet
       community, and requests discussion and suggestions for improvements.
       Please refer to the current edition of the "IAB Official Protocol
       Standards" for the standardization state and status of this protocol.
       Distribution of this memo is unlimited.

    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.

    Acknowlegements

       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 USC-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.

       The May, 1992 revision to fix the "Sorcerer's Apprentice" protocol
       bug [4] and other minor document problems was done by Noel Chiappa.

       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 exclude 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.

       Three modes of transfer are currently supported: netascii (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.); octet (This replaces the "binary" mode
       of previous versions of this document.) raw 8 bit bytes; mail,
       netascii characters sent to a user rather than a file.  (The mail
       mode is obsolete and should not be implemented or used.)  Additional
       modes can be defined by pairs of cooperating hosts.

       Reference [4] (section 4.2) should be consulted for further valuable
       directives and suggestions on TFTP.

    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 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 (UDP).  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.

              ---------------------------------------------------
             |  Local Medium  |  Internet  |  Datagram  |  TFTP  |
              ---------------------------------------------------

                          Figure 3-1: Order of Headers

    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.

                2 bytes     string    1 byte     string   1 byte
                ------------------------------------------------
               | Opcode |  Filename  |   0  |    Mode    |   0  |
                ------------------------------------------------

                           Figure 5-1: RRQ/WRQ packet

       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 combination 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.

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

                            Figure 5-2: DATA packet

       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 duplicate ACK's and those used for
       termination are acknowledged unless a timeout occurs [4].  Sending a
       DATA packet is an acknowledgment for the first ACK packet of the
       previous DATA packet. The WRQ and DATA packets are acknowledged by
       ACK or ERROR packets, while RRQ

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

                             Figure 5-3: ACK packet

       and 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.

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

                            Figure 5-4: ERROR packet

       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]

       (This has been included only for convenience.  TFTP need not be
       implemented on top of the Internet User Datagram Protocol.)

         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 UDP packet, including UDP header.

       Checksum        Reference 2 describes rules for computing checksum.
                       (The implementor of this should be sure that the
                       correct algorithm is used here.)
                       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.

    References

       [1]  USA Standard Code for Information Interchange, USASI X3.4-1968.

       [2]  Postel, J., "User Datagram  Protocol," RFC 768, USC/Information
            Sciences Institute, 28 August 1980.

       [3]  Postel, J., "Telnet Protocol Specification," RFC 764,
            USC/Information Sciences Institute, June, 1980.

       [4]  Braden, R., Editor, "Requirements for Internet Hosts --
            Application and Support", RFC 1123, USC/Information Sciences
            Institute, October 1989.

    Security Considerations

       Since TFTP includes no login or access control mechanisms, care must
       be taken in the rights granted to a TFTP server process so as not to
       violate the security of the server hosts file system.  TFTP is often
       installed with controls such that only files that have public read
       access are available via TFTP and writing files via TFTP is
       disallowed.

    Author's Address

       Karen R. Sollins
       Massachusetts Institute of Technology
       Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139-1986

       Phone: (617) 253-6006

       EMail: SOLLINS@LCS.MIT.EDU

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