Simple File Transfer Protocol
RFC 913

Network Working Group Mark K. Lottor
Request for Comments: 913 MIT
 September 1984

 Simple File Transfer Protocol

STATUS OF THIS MEMO

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

INTRODUCTION

 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
 protocol).

THE PROTOCOL

 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

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RFC 913 September 1984
Simple File Transfer Protocol

 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.

 Number.

 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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RFC 913 September 1984
Simple File Transfer Protocol

 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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RFC 913 September 1984
Simple File Transfer Protocol

 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

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RFC 913 September 1984
Simple File Transfer Protocol

 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
 connection.

 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.

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RFC 913 September 1984
Simple File Transfer Protocol

 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

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RFC 913 September 1984
Simple File Transfer Protocol

 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.

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RFC 913 September 1984
Simple File Transfer Protocol

 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)

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RFC 913 September 1984
Simple File Transfer Protocol

 NAME old-file-spec

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

 Replies:

 +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)

 DONE

 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.

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RFC 913 September 1984
Simple File Transfer Protocol

 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.

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RFC 913 September 1984
Simple File Transfer Protocol

 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
 created.

 Replies could be:

 +Will append to file

 +Will create file

 (APP should always return a '+')

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RFC 913 September 1984
Simple File Transfer Protocol

 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
 sending.

 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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RFC 913 September 1984
Simple File Transfer Protocol

AN EXAMPLE

 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>
 Small.File
 Large.File
 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
 S: RETR SMALL.FILE
 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

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RFC 913 September 1984
Simple File Transfer Protocol

EDITORS NOTE

 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
 trivial.

 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.

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RFC 913 September 1984
Simple File Transfer Protocol

 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
 separately.

 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.

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