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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.


 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 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]

Sollins [Page 1] RFC 1350 TFTP Revision 2 July 1992

 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
 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

Sollins [Page 2] RFC 1350 TFTP Revision 2 July 1992

 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.

Sollins [Page 3] RFC 1350 TFTP Revision 2 July 1992

  1. ————————————————–

| Local Medium | Internet | Datagram | TFTP |

  1. ————————————————–
                    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

Sollins [Page 4] RFC 1350 TFTP Revision 2 July 1992

    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 packet.

Sollins [Page 5] RFC 1350 TFTP Revision 2 July 1992

          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

Sollins [Page 6] RFC 1350 TFTP Revision 2 July 1992

 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.

Sollins [Page 7] RFC 1350 TFTP Revision 2 July 1992

             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.

Sollins [Page 8] RFC 1350 TFTP Revision 2 July 1992

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.

Sollins [Page 9] RFC 1350 TFTP Revision 2 July 1992

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.)
  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.

Sollins [Page 10] RFC 1350 TFTP Revision 2 July 1992


 [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

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

Sollins [Page 11]

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