Tftp error codes

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]



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   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



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   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.











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          ---------------------------------------------------
         |  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.









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      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.







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            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



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   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.








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               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.








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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.














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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.






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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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