A Definition of a Protocol for asynchronous File Transfer for the Internet
--------------------------------------------------------------------------

		    and a UNIX Reference Implementation
		    -----------------------------------



			      Ulli Horlacher

			      Allmandring 30

		    Rechenzentrum Universität Stuttgart

		       framstag@rus.uni-stuttgart.de


Abstract
--------

SAFT (Simple Asynchronous File Transfer) is a proposed new Internet
protocol for sending files and messages asynchronously.  This allows the
sender to transfer files without to log on to the receiving site. You
simply tell the sendfile program a file name and where to send it:
"sendfile your_file user@somedomain" (Of course there are options).

The package includes: A sendfile client (which sends files), a sendmsg
client (which sends messages), a receive client (which copies files from
the local sendfile spool to the recipient's current directory) and a
sendfiled server (which receives files and messages and stores them in the
local sendfile spool).


This pre-RFC is partially influenced by RFCftp. 

The meanings of "should" ... are defined in RFC...

Also considered were RFCtelnet RFCutf7, RFC1440, ..., see also 


Rationale:
----------

More information about asynchronous file transfer and comparison with 
---------------------------------------------------------------------
existing services
-----------------

With asynchronous file transfer, files are transmitted from a sender to
a recipient, without the latter having to take an active part.  Among
familiar Internet services, e-mail is an asynchronous service, while ftp 
represents a synchronous service.

Asynchronous file transfer effectively has not existed until now on the 
Internet.  If a user A wanted to send a file to a user B, he has been 
forced to use the following less than ideal procedures:

- ftp [13] to the recipient's account

  To do this, A must know the password of B's account. If A and B are not
  identical, this method is out of question for obvious security reasons.
  Even if A and B are the same person, doing a transfer this way requires 
  the password to be sent unencrypted via the Internet, which will be
  readable by the "bad guys".

- ftp via anonymous ftp

  To do this, A must "put" the file to the anonymous ftp server.  Then he 
  must inform B via e-mail, that there is a file to pick up.  This can
  only be done if the ftp server allows anonymous write access, in which
  case received files can be read or modified by other anonymous users prior
  to pickup.  Also, using this method requires most files to be transferred 
  twice. 

- sending via e-mail

  To do this, A must send B the file as an e-mail.  However, according to
  RFC 822 an e-mail may only contain characters from the NVT-ASCII
  character set (a printable subset of the regular 7 bit ASCII character
  set).  Thus the file transfer is restricted to English text documents
  ("Foreign" language texts contain 8 or even 16 bit wide characters, like
  German umlauts). To send more interesting documents, you have to encode
  the file appropriately, so that it will contain only NVT-ASCII
  characters during transmission.  For encoding you can use uuencode or
  MIME [16], but these are complicated to use, and do not support all file
  attributes.  They also inevitably enlarge the file size, which isn't
  helpful, since many mailing systems limit e-mails to as little as 100
  kilobytes. In addition e-mail has no resend option: an interrupted
  transfer has to be begin at byte #1, again.

File Transfer in Bitnet
-----------------------

In Bitnet there is an asynchronous file transfer service, which was the model
for our new Internet service.  We are making improvements though.

If you look closely at the Bitnet services, you will find that they are
all based on asynchronous file transfers; however, Bitnet allows only file
names to contain 8 Bytes, with another 8 Bytes for file name extensions
(IBM-internal restrictions).  Records must be not longer that 80 Bytes and
the character set is EBCDIC or 7 bit ASCII.


The SIFT/UFT Protocol
---------------------

There is an "experimental" Internet protocol for asynchronous file sending.  
However, SIFT/UFT (Sender-Initiated/Unsolicited File Transfer) protocol, RFC
1440 [15] has serious problems and inconsistencies.

The deficiencies of RFC 1440 are:

- the character set of the protocol is not defined

- the character sets of the files are not defined

- only VM file types are supported

- the date format is not defined

- a string "EOF" in the file terminates the transfer

- the return codes from the server are not defined

- there aren't many SIFT/UFT servers on the Internet


The SAFT Protocol
-----------------

The protocol we propose is named Simple Asynchronous File Transfer, 
or SAFT.

Essential attributes are:

- Independence

  SAFT should be available on all operating systems in the Internet and 
  not be bound to a particular operating system.

- Simplicity

  SAFT should be an easily comprehensible protocol on an ASCII basis 
  which can be debugged via telnet to the server port.

- Extensibility

  There should not be limits on later extension. A bad example perhaps
  is the 7 bit limitation of smtp / RFC 822.

Sending short asynchronous messages has been added to SAFT as a by-product. 
Such messages are defined as one line text strings, which normally would 
be written to the recipient's terminal.  An example use might be, 
"Frams, The file I sent you called sendfile_tricks is a .dvi file."

SAFT is a client/server protocol.  The SAFT client (typically as a user
program) sends files or messages via Internet to a SAFT server which
accepts them and delivers them to a local recipient, or saves them in a
special spool area.  The one line messages, however, will not be spooled
but will either be immediately displayed or dismissed. Recipients can pick
up the received files when convenient with the SAFT receive client.  This
works similarly to Internet mail, so users will be immediately comfortable
with it.  Actually, the receive client and the spool mechanism are not
part of the SAFT protocol but are mentioned here as an example how to deal
with incoming files. SAFT only defines the pure transfer protocol.

SAFT supports the following file attributes:

- File name in Unicode [19] of any length


- Time stamp

  Specification by ISO-8601 [7] (UTC full date & time)


- File type binary

  Byte stream without any format


- File type source

  File consists of lines of any length with CR/LF (ASCII 13, ASCII 10) as
  an end of line (EOL) mark


- File type text

  Like file type source but the sub-attribute CHARSET (see below) is evaluated


- File type MIME

  File is a MIME message as described in [16]


- Name of the character set

  Specification by RFC 1345 [14]


- Operating system specific attributes


These attributes can be freely introduced by the author of the first SAFT
implementation for a specific operating system, but should be announced
to the maintainer of the SAFT protocol (see author's address at the front
page of this document).  Compatibility is principally guaranteed only among
client and server of the same operating system, of course.

SAFT can transfer files in compressed mode using the gzip algorithm.  This
does not represent a file attribute but a transfer attribute.  This
happens transparently for the sender and the recipient, so they don't have
to deal with it. The compression has been introduced to save net
bandwidth. As a rule, the bottle neck of a file transfer is the capacity
of the network and not the performance of the local CPU.

SAFT uses tcp as transport layer and tcp port 487, which has been
registered by the IANA [21]. The SAFT client connects to this port at the
host of the SAFT server.

The client/server communication is divided into two parts: the actual
communication protocol and the file which has to be transfered as a
structureless "data-stream" (stream of octetts = bytes of 8 bit). This is
the only true restriction of SAFT: the smallest transfer unit is an octet
and machines with other byte configurations are not supported. But,
generally such machines belong to history.

The communication protocol conforms to NVT (network virtual telnet) [13],
using 7 bit ASCII without any control codes and CR/LF (ASCII 13, ASCII 10)
as EOL (end of line) mark.  HT (ASCII 9) is valid, too, but one should
avoid it.

A command from the client consists of a single text line, which contains a
command token and on demand one or more parameters, each separated with a
whitespace.  A whitespace is a non-null string of SPACE (ASCII 32) or HT
(ASCII 9) in any order.  If possible a whitespace should be a single SPACE.

The following commands are defined:

- FROM <sender> [<real name>]

  Sender login name and optionally real name.


- TO <recipient>		

  Recipient login name.


- FILE <name>	

  Name of the file which is to be transferred.


- DATE <date>		

  Time stamp of the file in UTC ISO-8601 format (YYYY:MM:DD hh:mm:ss).


- TYPE BINARY|SOURCE|MIME|TEXT[=<charset> [COMPRESSED[=GZIP]|CRYPTED[=PGP]]

  File type and transfer encoding. So far, for compressing only the gzip
  algorithm is allowed and for encrypting only pgp. Therefore these
  keywords are optional.

  <charset> is the name of the character set of a text file as defined by
  RFC 1345 (&charset entry). Alias names are not allowed. If not specified,
  ISO_8859-1:1987 is assumed.


- SIGN <signature>

  A digital signature corresponding to FILE. So far, only pgp armor
  signatures are supported.


- ATTR <attribute-string>	

  Operating system specific file attribute extension (depends on the
  implementation).


- MSG <message>		

  A one line text message, which shall be written directly onto the
  recipient's terminal.


- DEL		

  The file which has been transferred before will be deleted.
  

- RESEND		

  After a preceding link failure the file will be sent again.

  The first string (string delimiter is a whitespace) in the reply from
  the server contains the number of bytes which have already been
  transferred: <transmitted>


- SIZE <size> <size uncompressed>	

  Size of the file in bytes.  The first parameter is the number of bytes
  which really have to be transferred; the second parameter is the file
  size after decompressing.  The last one is for information purposes for a
  receive client.


- DATA		

  After this command <size> - <transmitted> bytes of the file are sent as
  a contiguous stream of octets.


- QUIT		

  End of session.


The command tokens may be written in upper or lower case or even in mixed
case. FROM, from or FrOm are equal.  If possible the command tokens should
be written in upper case.

<sender>, <real name>, <recipient>, <name> and <message> are strings
encoded with UTF-7 [20].  If possible one should only use NVT-ASCII or ISO
Latin-1 characters [14].  UTF-7 defines a reversible encoding of Unicode
strings to strings of the mbase64 character set, which itself is a subset
of NVT-ASCII.  Unicode is *the* 16 bit character set which will be the
successor of all current 8 bit character sets. For more details see [14].

To transfer a file, at least the commands FROM, TO, FILE and SIZE have to
be specified.  DATA then starts the actual transfer.  The other commands are
optional.  In general, the order of the commands does not matter.  Exceptions
from this rule are ( Format: <command> : <commands which precede> ):

- MSG :     FROM, TO 

- DEL :	    FROM, TO, FILE

- DATA :    FROM, TO, FILE, SIZE 

- RESEND :  FROM, TO, FILE, SIZE, DATE 


On every command from the client the server responds with a so called
"reply-message", which has the following format (notation is in EBNF):

reply-message	=	{reply-line} reply-end

reply-line	=	reply-code "-" text

reply-end	=	reply-code " " text

reply-code	=	digit digit digit 

digit	=	"0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

text	=	char {char} CR LF

char	=	<one character from the NVT-ASCII character set>

CR is ASCII 13, LF is ASCII 10, all other terminal-symbols are ASCII characters.
text should be encoded in ASCII or UTF-7. ASCII should be preferred.

The first digit of the reply-codedetermines the category of the reply-message:

- 2 stands for: command successfully executed 

- 3 stands for: more data/information is needed

- 4 stands for: a fatal error has occurred and the connection will be terminated

- 5 stands for: other error, which can be corrected with further commands


The following "reply-messages" are defined:

- 200 Command ok..

- 201 File has been correctly received.

- 202 Command not implemented, superfluous at this site.

- 205 Non-ASCII character in command line ignored.

- 214 <help-text>

- 220 <hostname> SAFT server (sendfiled <version> on <OS>) ready.

- 221 Goodbye.

- 230 <number> Bytes already received.

- 302 Header ok, send data.

- 410 Spool directory does not exist.

- 411 Can't create user spool directory.

- 412 Can't write to user spool directory.

- 415 TCP error: received too few data.

- 421 Service not available.

- 451 Requested action aborted: local error in processing.

- 452 Insufficient storage space.

- 453 Insufficient system resources.

- 500 Syntax error, command unrecognized.

- 501 Syntax error in parameters or arguments.

- 502 Command not implemented.

- 503 Bad sequence of commands.

- 504 Command not implemented for that parameter.

- 505 Missing argument.

- 510 This SAFT-server can only receive messages. Send files to saft://xx/yy

- 511 This SAFT-server can only receive files.

- 520 User unknown.

- 521 User is not allowed to receive files or messages.

- 522 User cannot receive messages.

- 523 You are not allowed to send to this user.

- 530 User cannot receive messages.

- 531 This file has been already received.

- 599 Unknown error.


Only the 3 digit reply-codes are reserved, the texts behind can be changed
at your pleasure as long as they conform to the meaning of the message.
Exceptions are the texts of the reply codes 220 and 230: 
220 must contain the ASCII-string "SAFT" and 230 must follow one space 
and the number of bytes which have already been transferred as first string.

number = digit {digit}


Examples
--------

Examples of SAFT sessions using a direct telnet connection to the server
port:


> telnet linux saft

Trying 129.69.58.50...

Connected to linux.rus.uni-stuttgart.de.

Escape character is '^]'.

220 linux.rus.uni-stuttgart.de SAFT server (sendfiled 1.4 on Linux) ready.

FROM gaga

200 Command ok.

TO framstag

200 Command ok.

FILE blubb

200 Command ok.

SIZE 5 5

200 Command ok.

DATA

302 Header ok, send data.

ABC

201 File has been correctly received.

QUIT

221 Goodbye.

Connection closed by foreign host.

> telnet linux saft

Trying 129.69.58.50...

Connected to linux.rus.uni-stuttgart.de.

Escape character is '^]'.

220 linux.rus.uni-stuttgart.de SAFT server (sendfiled 1.4 on Linux) ready.

HELP

214-The following commands are recognized:

214-  FROM <sender> [<real name>]

214-  TO <recipient>

214-  FILE <name>

214-  SIZE <size to transfer> <size uncompressed>

214-  TYPE BINARY|SOURCE|TEXT [COMPRESSED|CRYPTED]

214-  SIGN <pgp signature>

214-  DATE <ISO-8601 date string>

214-  CHARSET <RFC-1345 character set name>

214-  ATTR TAR|EXE|NONE

214-  MSG <message>

214-  DEL

214-  RESEND

214-  DATA

214-  QUIT

214-All argument strings have to be UTF-7 encoded.

214 You must specify at least FROM, TO, FILE, SIZE and DATA to send a file.

FROM gaga

200 Command ok.

TO dengibtsnicht

520 User unknown.

TO framstag

200 Command ok.

MSG huhu!

530 User cannot receive messages.

TYPE TEXT

200 Command ok.

FILE x1

200 Command ok.

SIZE 6 6

200 Command ok.

abcd

500 Syntax error, command unrecognized.

DATA

302 Header ok, send data.

abcd

201 File has been correctly received.

FILE x2

200 Command ok.

SIZE 3 3

200 Command ok.

SIZE 5 5

200 Command ok.

DATA

302 Header ok, send data.

123

201 File has been correctly received.

QUIT

221 Goodbye.

Connection closed by foreign host.


(Note the difference between the number of bytes in the SIZE and DATA commands.
Telnet transfers a line with CR LF as EOL mark.  Such bytes count, too.)



Information and literature list
===============================

[1] Andrew Tanenbaum: Computer Networks

[2] Bettina Reimer, Paul Müller: Kommunikationssysteme auf der Basis des
    ISO-Referenzmodells

[3] Kernighan, Ritchie: Programmieren in C

[4] Jürgen Gulbins: UNIX

[5] W. R. Stevens: Advanced Programming in the UNIX Environment

[6] W. R. Stevens: UNIX Network Programming

[7] ISO-8601 - International Time and Date Representing

[8] C-FAQ-list in news.answers

[9] Umlaute-FAQ in de.comp.standards

[10] internationalization/programming-faq in news.answers

[11] mail/mime-faq in news.answers

[12] http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-v10-spec-00.txt

[13] RFC 859 - ftp 

[14] RFC 1345 - Character Mnemonics & Character Sets

[15] RFC 1440 - SIFT/UFT: Sender-Initiated/Unsolicited File Transfer

[16] RFC 2045-2049 - MIME

[18] RFC 1543 - Instructions to RFC Authors

[19] RFC 1641 - Using Unicode with MIME

[20] RFC 1642 - UTF-7

[21] RFC 1700 - Assigned Numbers


Still missing from this document:

- rationale section

- programmer's documentation of the programs of the sendfile package 

- a nice postscript version

You can find that which is missing in the German version, doku.ps
I'm translating the missing parts as fast as I can.
