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Network Working Group                                         K. SollinsRequest For Comments: 1350                                           MITSTD: 33                                                        July 1992Obsoletes: RFC783THE 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]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   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 areSollins                                                         [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          ---------------------------------------------------         |  Local Medium  |  Internet  |  Datagram  |  TFTP  |          ---------------------------------------------------                      Figure 3-1: Order of Headers4. 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.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   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.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 ofSollins                                                         [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 1992I. AppendixOrder 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 1992Error 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.Sollins                                                        [Page 10]

RFC 1350                    TFTP Revision 2                    July 1992References   [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.EDUSollins                                                        [Page 11]