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RFC 0913

Simple File Transfer Protocol

Pages: 15

ToP   noToC   RFC0913 - Page 1
Network Working Group                                     Mark K. Lottor
Request for Comments: 913                                            MIT
                                                          September 1984

                     Simple File Transfer Protocol


   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.


   SFTP is a simple file transfer protocol.  It fills the need of people
   wanting a protocol that is more useful than TFTP but easier to
   implement (and less powerful) than FTP.  SFTP supports user access
   control, file transfers, directory listing, directory changing, file
   renaming and deleting.

   SFTP can be implemented with any reliable 8-bit byte stream oriented
   protocol, this document describes its TCP specification.  SFTP uses
   only one TCP connection; whereas TFTP implements a connection over
   UDP, and FTP uses two TCP connections (one using the TELNET


   SFTP is used by opening a TCP connection to the remote hosts' SFTP
   port (115 decimal).  You then send SFTP commands and wait for
   replies.  SFTP commands sent to the remote server are always 4 ASCII
   letters (of any case) followed by a space, the argument(s), and a
   <NULL>.  The argument can sometimes be null in which case the command
   is just 4 characters followed by <NULL>.  Replies from the server are
   always a response character followed immediately by an ASCII message
   string terminated by a <NULL>.  A reply can also be just a response
   character and a <NULL>.

      <command> : = <cmd> [<SPACE> <args>] <NULL>

      <cmd> : =  USER ! ACCT ! PASS ! TYPE ! LIST ! CDIR
                 KILL ! NAME ! DONE ! RETR ! STOR

      <response> : = <response-code> [<message>] <NULL>

      <response-code> : =  + | - |   | !

      <message> can contain <CRLF>

   Commands that can be sent to the server are listed below.  The server
ToP   noToC   RFC0913 - Page 2
   replies to each command with one of the possible response codes
   listed under each message.  Along with the response, the server
   should optionally return a message explaining the error in more
   detail.  Example message texts are listed but do not have to be
   followed.  All characters used in messages are ASCII 7-bit with the
   high-order bit zero, in an 8 bit field.

   The response codes and their meanings:

      +  Success.

      -  Error.

         An error occurred while processing your command.


         The number-sign is followed immediately by ASCII digits
         representing a decimal number.

      !  Logged in.

         You have sent enough information to be able to log yourself in.
         This is also used to mean you have sent enough information to
         connect to a directory.

   To use SFTP you first open a connection to the remote SFTP server.
   The server replies by sending either a positive or negative greeting,
   such as:

      +MIT-XX SFTP Service

         (the first word should be the host name)

      -MIT-XX Out to Lunch
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   If the server send back a '-' response it will also close the
   connection, otherwise you must now send a USER command.

      USER user-id

         Your userid on the remote system.

         The reply to this command will be one of:

            !<user-id> logged in

               Meaning you don't need an account or password or you
               specified a user-id not needing them.

            +User-id valid, send account and password

            -Invalid user-id, try again

         If the remote system does not have user-id's then you should
         send an identification such as your personal name or host name
         as the argument, and the remote system would reply with '+'.

      ACCT account

         The account you want to use (usually used for billing) on the
         remote system.

         Valid replies are:

            ! Account valid, logged-in

               Account was ok or not needed. Skip the password.

            +Account valid, send password

               Account ok or not needed. Send your password next.

            -Invalid account, try again
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      PASS password

         Your password on the remote system.

         Valid replies are:

            ! Logged in

               Password is ok and you can begin file transfers.

            +Send account

               Password ok but you haven't specified the account.

            -Wrong password, try again
ToP   noToC   RFC0913 - Page 5
   You cannot specify any of the following commands until you receive a
   '!' response from the remote system.

      TYPE { A | B | C }

         The mapping of the stored file to the transmission byte stream
         is controlled by the type.  The default is binary if the type
         is not specified.

         A - ASCII

            The ASCII bytes are taken from the file in the source
            system, transmitted over the connection, and stored in the
            file in the destination system.

            The data is the 7-bit ASCII codes, transmitted in the
            low-order 7 bits of 8-bit bytes.  The high-order bit of the
            transmission byte must be zero, and need not be stored in
            the file.

            The data is "NETASCII" and is to follow the same rules as
            data sent on Telnet connections.  The key requirement here
            is that the local end of line is to be converted to the pair
            of ASCII characters CR and LF when transmitted on the

            For example, TOPS-20 machines have 36-bit words.  On TOPS-20
            machines, The standard way of labeling the bits is 0 through
            35 from high-order to low-order.  On TOPS-20 the normal way
            of storing ASCII data is to use 5 7-bit bytes per word.  In
            ASCII mode, the bytes transmitted would be [0-6], [7-13],
            [14-20], [21-27], [28-34], (bit 35 would not be
            transmitted), each of these 7-bit quantities would be
            transmitted as the low-order 7 bits of an 8-bit byte (with
            the high-order bit zero).

            For example, one disk page of a TOPS-20 file is 512 36-bit
            words.  But using only 35 bits per word for 7-bit bytes, a
            page is 17920 bits or 2560 bytes.
ToP   noToC   RFC0913 - Page 6
         B - BINARY

            The 8-bit bytes are taken from the file in the source
            system, transmitted over the connection, and stored in the
            file in the destination system.

            The data is in 8-bit units.  In systems with word sizes
            which are not a multiple of 8, some bits of the word will
            not be transmitted.

            For example, TOPS-20 machines have 36-bit words.  In binary
            mode, the bytes transmitted would be [0-7], [8-15], [16-23],
            [24-31], (bits 32-35 would not be transmitted).

            For example, one disk page of a TOPS-20 file is 512 36-bit
            words.  But using only 32 bits per word for 8-bit bytes, a
            page is 16384 bits or 2048 bytes.

         C - CONTINUOUS

            The bits are taken from the file in the source system
            continuously, ignoring word boundaries, and sent over the
            connection packed into 8-bit bytes.  The destination system
            stores the bits received into the file continuously,
            ignoring word boundaries.

            For systems on machines with a word size that is a multiple
            of 8 bits, the implementation of binary and continuous modes
            should be identical.

            For example, TOPS-20 machines have 36-bit words.  In
            continuous mode, the bytes transmitted would be [first word,
            bits 0-7], [first word, bits 8-15], [first word, bits
            16-23], [first word, bits 24-31], [first word, bits 32-35 +
            second word, bits 0-3], [second word, bits 4-11], [second
            word, bits 12-19], [second word, bits 20-27], [second word,
            bits 28-35], then the pattern repeats.

            For example, one disk page of a TOPS-20 file is 512 36-bit
            words.  This is 18432 bits or 2304 8-bit bytes.

         Replies are:

            +Using { Ascii | Binary | Continuous } mode

            -Type not valid
ToP   noToC   RFC0913 - Page 7
      LIST { F | V } directory-path

         A null directory-path will return the current connected
         directory listing.

         F specifies a standard formatted directory listing.

            An error reply should be a '-' followed by the error message
            from the remote systems directory command.  A directory
            listing is a '+' followed immediately by the current
            directory path specification and a <CRLF>.  Following the
            directory path is a single line for each file in the
            directory.  Each line is just the file name followed by
            <CRLF>.  The listing is terminated with a <NULL> after the
            last <CRLF>.

         V specifies a verbose directory listing.

            An error returns '-' as above.  A verbose directory listing
            is a '+' followed immediately by the current directory path
            specification and a <CRLF>.  It is then followed by one line
            per file in the directory (a line ending in <CRLF>).  The
            line returned for each file can be of any format.  Possible
            information to return would be the file name, size,
            protection, last write date, and name of last writer.
ToP   noToC   RFC0913 - Page 8
      CDIR new-directory

         This will change the current working directory on the remote
         host to the argument passed.

         Replies are:

            !Changed working dir to <new-directory>

            -Can't connect to directory because: (reason)

            +directory ok, send account/password

         If the server replies with '+' you should then send an ACCT or
         PASS command.  The server will wait for ACCT or PASS commands
         until it returns a '-' or '!' response.

            Replies to ACCT could be:

               !Changed working dir to <new-directory>

               +account ok, send password

               -invalid account

            Replies to PASS could be:

               !Changed working dir to <new-directory>

               +password ok, send account

               -invalid password

      KILL file-spec

         This will delete the file from the remote system.

         Replies are:

            +<file-spec> deleted

            -Not deleted because (reason)
ToP   noToC   RFC0913 - Page 9
      NAME old-file-spec

         Renames the old-file-spec to be new-file-spec on the remote


            +File exists

            -Can't find <old-file-spec>

               NAME command is aborted, don't send TOBE.

         If you receive a '+' you then send:

            TOBE new-file-spec

         The server replies with:

            +<old-file-spec> renamed to <new-file-spec>

            -File wasn't renamed because (reason)


         Tells the remote system you are done.

         The remote system replies:

            +(the message may be charge/accounting info)

         and then both systems close the connection.
ToP   noToC   RFC0913 - Page 10
      RETR file-spec

         Requests that the remote system send the specified file.

         Receiving a '-' from the server should abort the RETR command
         and the server will wait for another command.

         The reply from the remote system is:

             <number-of-bytes-that-will-be-sent> (as ascii digits)

            -File doesn't exist

         You then reply to the remote system with:

            SEND (ok, waiting for file)

               The file is then sent as a stream of exactly the number
               of 8-bit bytes specified.  When all bytes are received
               control passes back to you (the remote system is waiting
               for the next command).  If you don't receive a byte
               within a reasonable amount of time you should abort the
               file transfer by closing the connection.

            STOP (You don't have enough space to store file)

               Replies could be:

                  +ok, RETR aborted

         You are then ready to send another command to the remote host.
ToP   noToC   RFC0913 - Page 11
      STOR { NEW | OLD | APP } file-spec

         Tells the remote system to receive the following file and save
         it under that name.

         Receiving a '-' should abort the STOR command sequence and the
         server should wait for the next command.

         NEW specifies it should create a new generation of the file and
         not delete the existing one.

            Replies could be:

               +File exists, will create new generation of file

               +File does not exist, will create new file

               -File exists, but system doesn't support generations

         OLD specifies it should write over the existing file, if any,
         or else create a new file with the specified name.

            Replies could be:

               +Will write over old file

               +Will create new file

               (OLD should always return a '+')

         APP specifies that what you send should be appended to the file
         on the remote site.  If the file doesn't exist it will be

            Replies could be:

               +Will append to file

               +Will create file

               (APP should always return a '+')
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         You then send:

            SIZE <number-of-bytes-in-file> (as ASCII digits)

               where number-of-bytes-in-file

                  is the exact number of 8-bit bytes you will be

         The remote system replies:

            +ok, waiting for file

               You then send the file as exactly the number of bytes
               specified above.

               When you are done the remote system should reply:

                  +Saved <file-spec>

                  -Couldn't save because (reason)

            -Not enough room, don't send it

               This aborts the STOR sequence, the server is waiting for
               your next command.

         You are then ready to send another command to the remote host.
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   An example file transfer.  'S' is the sender, the user process.  'R'
   is the reply from the remote server.  Remember all server replies are
   terminated with <NULL>.  If the reply is more than one line each line
   ends with a <CRLF>.

      R: (listening for connection)
      S: (opens connection to R)
      R: +MIT-XX SFTP Service
      S: USER MKL
      R: +MKL ok, send password
      S: PASS foo
      R: ! MKL logged in
      S: LIST F PS: <MKL>
      R: +PS: <MKL>
      S: LIST V
      R: +PS: <MKL>
         Small.File  1        69(7)  P775240  2-Aug-84 20:08  MKL
         Large.File  100  255999(8)  P770000  9-Dec-84 06:04  MKL
      R:  69
      S: SEND
      R: This is a small file, the file is sent without
         a terminating null.
      S: DONE
      R: +MIT-XX closing connection
ToP   noToC   RFC0913 - Page 14

   Mark Lotter receives full credit for all the good ideas in this memo.
   As RFC editor, i have made an number of format changes, a few wording
   changes, and one or two technical changes (mostly in the TYPEs).  I
   accept full responsibility for any flaws i may have introduced.

   A draft form of this memo was circulated for comments.  I will
   attempt to list the issues raised and summarize the pros and cons,
   and resolution for each.

      ASCII Commands vs Binary Operation Codes

         The ASCII command style is easier to debug, the extra
         programming cost in minimal, the extra transmission cost is

         Binary operation codes are more efficient, and a few days of
         debugging should not out weigh years of use.

         Resolution:  I have kept the ASCII Commands.

      Additional Modes

         Pro:  For some machines you can't send all the bits in a word
         using this protocol.  There should be some additional mode to
         allow it.

         Con:  Forget it, this is supposed to be SIMPLE file transfer.
         If you need those complex modes use real FTP.

         Resolution:  I have added the Continuous mode.
ToP   noToC   RFC0913 - Page 15
      CRLF Conversion

         Pro:  In ASCII type, convert the local end of line indicator to
         CRLF on the way out of the host and onto the network.

         Con:  If you require that you have to look at the bytes as you
         send them, otherwise you can just send them.  Most of the time
         both sides will have the same end of line convention anyway.
         If someone needs a conversion it can be done with a TECO macro

         Resolution:  I have required CRLF conversion in ASCII type.  If
         you have the same kind of machines and the same end of line
         convention you can avoid the extra cost of conversion by using
         the binary or continuous type.

      TCP Urgent

         Pro:  Use TCP Urgent to abort a transfer, instead of aborting
         the connection.  Then one could retry the file, or try a
         different file without having to login again.

         Con:  That would couple SFTP to TCP too much.  SFTP is supposed
         to be able to be work over any reliable 8-bit data stream.

         Resolution:  I have not made use of TCP Urgent.

      Random Access

         Pro:  Wouldn't it be nice if (WIBNIF) SFTP had a way of
         accessing parts of a file?

         Con:  Forget it, this is supposed to be SIMPLE file transfer.
         If you need random access use real FTP (oops, real FTP doesn't
         have random access either -- invent another protocol?).

         Resolution:  I have not made any provision for Random Access.

   -- jon postel.