Network Working Group A. DeSchon
Request for Comments: 1068 R. Braden
August 1988 Background File Transfer Program (BFTP)
Status of This Memo
This memo describes an Internet background file transfer service that
is built upon the third-party transfer model of FTP. No new
protocols are involved. The purpose of this memo is to stimulate
discussion on new Internet service modes. Distribution of this memo
For a variety of reasons, file transfer in the Internet has generally
been implemented as an interactive or "foreground" service. That is,
a user runs the appropriate local FTP user interface program as an
interactive command and requests a file transfer to occur in real
time. If the transfer should fail to complete for any reason, the
user must reissue the transfer request. Foreground file transfer is
relatively simple to implement -- no subtleties of queuing or stable
storage -- and in the early days of networking it provided excellent
service, because the Internet/ARPANET was lightly loaded and
More recently, the Internet has become increasingly subject to
congestion and long delays, particularly during times of peak usage.
In addition, as more of the world becomes interconnected, planned and
unplanned outages of hosts, gateways, and networks sometimes make it
difficult for users to successfully transfer files in foreground.
Performing file transfer asynchronously (i.e., in "background"),
provides a solution to some of these problems, by eliminating the
requirement for a human user to be directly involved at the time that
a file transfer takes place. A background file transfer service
requires two components: a user interface program to collect the
parameters describing the required transfer(s), and a file transfer
control (FTC) daemon to carry them out.
Background file transfer has a number of potential advantages for a
o No Waiting
The user can request a large transfer and ignore it until a
notification message arrives through some common channel (e.g.,
o End-to-end Reliability
The FTC daemon can try a transfer repeatedly until it either
succeeds or fails permanently. This provides reliable end-to-
end delivery of a file, in spite of the source or destination
host being down or poor Internet connectivity during some time
o Multiple File Delivery
In order for background file transfer to be accepted in the
Internet, it may have to include some "value-added" services.
One such service would be an implementation of a multiple file
transfer capability for all hosts. Such a facility is suggested
in RFC-959 (see the description of "NLST") and implemented in
some User-FTP programs.
o Deferred Delivery
The user may wish to defer a large transfer until an off-peak
period. This may become important when parts of the Internet
adopt accounting and traffic-based cost-recovery mechanisms.
There is a serious human-engineering problem with background file
transfer: if the user makes a mistake in entering parameters, this
mistake may not become apparent until much later. This can be the
cause of severe user frustration. To avoid this problem, the user
interface program ought to verify the correctness of as many of the
parameters as possible when they are entered. Of course, such
foreground verification of parameters is not possible if the remote
host to which the parameters apply is currently unreachable.
To explore the usefulness of background file transfer in the present
Internet, we have implemented a file-mover service which we call the
Background File Transfer Program or BFTP.
Section 2 describes BFTP and Section 3 presents our experience and
conclusions. The appendices contain detailed information about the
user interface language for BFTP, a description of the program
organization, and sample execution scripts.
2. Background File Transfer Program
2.1 General Model
In the present BFTP design, its user interface program and its FTC
daemon program must execute on the same host, which we call the
BFTP control host.
Through the user interface program, a BFTP user will supply all of
the parameters needed to transfer a file from source host S to
destination host D, where S and D may be different from the BFTP
control host. These parameters include:
o S and D host names,
o login names and passwords on S and D hosts, and
o S and D file names (and optionally, directories).
The user may also specify a number of optional control parameters:
* Source file disposition -- Copy, move (i.e., copy and
delete), or simply delete the source file. The default is
* Destination file operation -- Create/Replace, append to, or
create a unique destination file. The default is
* FTP Parameters -- Explicitly set any of the FTP type, mode,
or structure parameters at S and D hosts.
* Multiple Transfers -- Enable "wildcard" matching to perform
* Start Time -- Set the time of day for the first attempt of
the transfer. The default is "now" (i.e., make the first
attempt as soon as the request has been queued for the FTC
Finally, the user specifies a mailbox to which a completion
notification message will be sent, and "submits" the request to
the FTC daemon queue. The user can then exit the BFTP user
If the transfer should fail permanently, the FTC daemon will send
a notification message to the user's mailbox. In the event of a
temporary failure (e.g., a broken TCP connection), the FTC daemon
will log the failure and retry the transfer after some timeout
period. The retry cycles will be repeated until the transfer
succeeds or until some maximum number of tries specified has been
reached. In either case, a notification message will then be sent
to the user's mailbox.
The user can check on the progress of the transfer by reentering
the BFTP user interface program, supplying a key that was defined
with the request, and displaying the current status of the
request. The user may then cancel the request or leave it in the
The BFTP program includes a server-Telnet module, so it can be
executed as a remotely-accessible service that can be reached via
a Telnet connection to the BFTP well-known port (152). This
allows a user on any Internet host to perform background file
transfers without running BFTP locally, but instead opening a
Telnet connection to port 152 on a BFTP service host. Of course,
a user can also run the local BFTP user interface program directly
on any host that supports it and for which the user has login
The next section discusses how BFTP uses standard FTP servers to
perform the transfers, while the following section covers the user
interface of BFTP.
2.2 File Transfer Mechanics for BFTP
The BFTP makes use of the "third party" or "Server-Server" model
incorporated in the Internet File Transfer Protocol [RFC-959].
Thus, the FTC daemon opens FTP control connections to the existing
FTP servers on source host S and destination host D and instructs
them to transfer the desired file(s) from S to D. The S and D
hosts may be any two Internet hosts supporting FTP servers (but at
least one of them must support the FTP "PASV" command). This
approach allows the implementation of a background file transfer
capability for the entire Internet at a very low cost.
Figure 1 illustrates the BFTP model of operation. Note that the
BFTP control host is not necessarily the same as S or D. Figure 2
illustrates the FTP command interchange used in a typical Server-
Server file transfer operation; this may be compared with the
User-Server FTP scenario illustrated in Section 7 of RFC-959.
Since BFTP may be asked to transfer files between any two hosts in
the Internet, it must support all the file types and transfer
modes that are defined in RFC-959, not just a subset implemented
by particular hosts.
BFTP supports the transfer of a set of files in a single request,
using the standard technique:
(1) Send an NLST command to the source host S, specifying a
pathname containing "wildcard" characters. The reply will
contain a list of matching source file names.
(2) Execute a separate transfer operation for each file in this
list. The destination file name in each case is assumed to
be the same as the source file name; this requires that these
names be compatible with the naming conventions of D.
It will typically be necessary to specify working directories for
the transfers at S and D, so the file names will be simple,
unstructured names on each system.
This approach depends upon the wildcard matching capability of the
source host S. A more general implementation would acquire a
complete list of the file names from the source host and do the
matching in the FTC daemon, for example using a regular-expression
matcher. Another useful extension would be a general pattern-
matching file name transformation capability (e.g., like the one
included in the 4.3BSD version of FTP) to generate appropriate
destination pathnames for multiple requests.
BFTP currently utilizes the following Server-FTP commands [RFC-
959]: USER, PASS, ACCT, PASV, PORT, RETR, STOR, STOU, CWD, NLST,
MODE, STRU, TYPE, and QUIT.
The FTC daemon attempts to work around FTP servers that fail to
support certain commands. For example, if a server does not
support the optional command "CWD", the FTC daemon will attempt to
construct a complete path name using the source directory name and
the source file name. However, it is necessary that at least one
of the two hosts support the FTP passive (PASV) command. While
many FTP server implementations support do this command, some (in
particular, the 4.2BSD FTP) do not. The PASV command was
officially listed as being optional in RFC-959.
2.3 Reliable Delivery
The reliable delivery function of BFTP is analogous to reliable
delivery in a transport protocol like TCP. Both depend upon
repeated delivery attempts until success is achieved, and in both
cases the choice of the retry interval requires some care to
balance overhead against unresponsiveness.
Humans are impatient, but even their impatience has a limit. If
the file cannot be transferred "soon", a human will turn to
another project; typically, there is a tendency for the transfer
to become less urgent the longer the wait. The FTC daemon of BFTP
therefore starts each transfer request with a very short retry
interval -- e.g., 10 minutes -- and then doubles this interval for
successive retries, until a maximum interval -- e.g., 4 hours --
is reached. This is essentially the exponential backoff algorithm
of the Ethernet, which is also used by transport protocols such as
TCP, although BFTP and TCP have quite different rationales for the
We must also define the meaning of reliable transmission for a
multiple-transfer request. For example, the set of files selected
by wildcard characters in a pathname is not well defined; the set
may change while the request is pending, as files are created and
deleted. Furthermore, it is unreasonable to regard the entire
multiple transfer as a single atomic operation. Suppose that
transferring a set of files fails part way through; for an atomic
operation, the files which had been successfully transferred would
have to be deleted pending the next retry of the entire set. This
would be ridiculously inefficient and may be impossible (since the
communication path may be broken when it is time to issue the
BFTP addresses these issues in the following manner:
* For a multiple file operation, the FTC daemon saves the file
name list returned by the first successful NLST command in
the request queue entry. This name list determines the set
of source files for the transfer; there can be no later
additions to the set.
* The FTC daemon maintains a transfer status pointer. On each
retry cycle, it tries to transfer only those files that have
not already been successfully transferred.
* The request is complete when all the individual file
transfers have been successful, a permanent failure has
occured, or when the retry limit is reached.
* The notification message to the user lists the status of each
of the multiple files.
2.4 BFTP User Interface
The purpose of BFTP is to simplify the file transfer process and
to place the burden of reliability on the BFTP control host. We
have attempted to provide a "user friendly" command interface to
BFTP, similar in flavor to the user interface of the TOPS-20
operating system. This interface provides extensive prompting,
defaulting, and help facilities for every command.
For a list of all BFTP commands, the user may enter "?<Return>" at
the main BFTP prompt ("BFTP>"). Entering "help<Return>" and
"explain<Return>" will provide increasing levels of explanatory
material. To obtain information on a particular command, "help
<command name><Return>" may be entered. The 'quit' or 'exit'
command will exit from BFTP. Command and subcommand names may be
abbreviated to the shortest unique sequence for that context;
alternatively, a partial name can be automatically completed by
The normal procedure for a BFTP user is to set up a set of
parameters defining the desired transfer and then submit the
request to the FTC daemon. To give the user the maximum
flexibility, BFTP supports three modes of submission:
o Background Operation
To request a reliable background file transfer, the user will
issue the BFTP 'submit' command to the FTC daemon.
o Foreground Verification, Background Operation
The BFTP 'verify' command may be used to ascertain that file
transfer parameters are valid. It causes BFTP to connect to
the FTP servers on both the source and the destination hosts
(if possible), log into both, verify the FTP parameters, and
verify that the specified source file is present.
Once the 'verify' command has successfully completed, the
user can issue the 'submit' command to schedule the actual
o Foreground Operation
The BFTP 'transfer' command will perform the specified
third-party transfer in foreground mode. This is illustrated
by the dotted path bypassing the queue in Figure 1.
The easiest way to set up the parameters is to issue the 'prompt'
command, which will prompt the user for all of the basic
parameters required for most transfers. Certain unusual
parameters must be set with the 'set' command (see Appendix B for
When entering any parameter, the following control characters may
? will display help text for the parameter, indicating its
meaning, the choices, and the default, and then reprompt for
<ESC> will display the default value (or the last value set) for
this parameter. The user can accept this default by entering
<Return>, or else erase it with Control-W and enter a
different value for the parameter, followed by <Return> to
accept the entered value.
will erase the value typed or displayed for current
will accept the value displayed for this parameter, and
continue to the next parameter, if any. If the user has not
typed a value or used <ESC> to display the default, <Return>
will display the default and then accept it.
It is important to provide a means for a user to obtain status
information about an earlier request or even to cancel an earlier
request. However, these functions, especially cancellation, must
be controlled by some user authentication. We did not want to
build a user authentication database with each BFTP instance or
require login to BFTP itself, and there is no Internet-wide user
authentication mechanism. We adopted the following weak
authentication mechanism as a compromise:
* When the 'submit' command is issued, it prompts the user for
a character string called a "keyword", which recorded with
* This keyword can be entered later as the argument to a 'find'
command, which will display the status of all requests with
* Similarly, the keyword may be used to cancel the
If two different users happen to choose the same keywords, of
course, this scheme will not protect each other's requests from
accidental or malicious cancellation. However, a notification
message will be sent at the time that a cancellation occurs.
To make a series of similar requests, the user needs only to
change the individual parameters that differ from the preceding
request and then issue a new 'submit' command, for each request.
There are commands for individually setting each of the parameters
that 'prompt' sets -- and 'time' -- to provide a shortcut for BFTP
experts. A simpler but lengthier procedure is to use the 'prompt'
command to run through the current set of parameters, reentering
the parameters that must change and using the sequence
<ESC><return> to retain the previous value for each of the others.
The same procedures may be used to correct a mistake made in
entering a particular parameter.
The current settings of all the BFTP parameters can be displayed
at any time with the 'status' command, while the 'clear' command
will return all parameters to their initial values. Finally, the
'request' command allows the user to save the current set of
parameters in a file or to restore the parameters from a
There is also a window-based BFTP user interface for use on a Sun
Workstation, described in Appendix A. The complete list of BFTP
commands is presented in Appendix B.
3. Experience and Conclusions
BFTP has been available to users at ISI for some months. Users have
reported a number of advantages of using BFTP:
(a) Some users prefer the prompting style of BFTP to the user
interface of the foreground FTP they normally use.
(b) The BFTP "verify" command allows the user to verify that host
names, passwords, and filenames are correct without having to
wait for the entire transfer to take place.
(c) Since results are returned through the mail system, a transfer
can occur without tying up a terminal line, a phone line, or
even a window.
BFTP must be able to communicate with a variety of Server-FTP
implementations, and we have observed much variation in the commands
supported, error handling, and the timing in these servers. Some of
the problems we have encountered are:
(1) Some systems (e.g., 4.2BSD) do not support the PASV command.
(2) 4.2/3BSD systems return a non-standard response to the NLST
command. Instead of returning a list of complete path-names,
they use an ad hoc format consisting of a directory name
followed by a list of files.
(3) 4.2/3BSD systems may return a "permanent negative completion
reply" (a 5xx FTP reply code) as a result of a communications
failure such as a broken TCP connection. According to RFC-959,
the appropriate response is a "transient negative completion
reply" (a 4xx FTP reply code), which would inform the BFTP that
the transfer should be retried.
(4) A number of servers return badly formatted responses. An
example of this is the 4.2/3BSD response to an NLST command for
a non-existent file name: an error string which is not preceded
by a numerical response code.
To diagnose problems that do occur, we have found it very useful to
have a complete record of the interchange between the FTC daemon and
the two FTP servers. This record is saved and is currently always
included in the notification message mailed to the user (see Appendix
D for an example). As we get more experience with this program, some
of the details of the transfer may be omitted from this log.
The use of library routines shared between modules makes it
relatively easy to implement additional user interface programs. We
are currently experimenting with a window version of BFTP, the
"bftptool", which runs in the SunView environment, and is described
in Appendix A. Some additional interfaces that might be useful are:
o A command line interface for use in shell scripts and
o A more general library interface which would make it easy to
invoke BFTP from a variety of programs.
o Additional full-screen form based interfaces, for example a tool
running in X-Window system environment.
Lastly, BFTP would benefit from the resolution of the following open
o There currently exist no provisions for Internet-wide user
authentication. In the BFTP context, this means that passwords
required for a file transfer must be present in BFTP request
files. The security of these passwords is subject to the
limitations of the file system security on the BFTP control
host. Anonymous file transfer provides a partial solution, but
a more general, long term solution is needed.
o Better mechanisms are needed to cope with the diversity of real
file systems in the Internet.
For example, an extension could be made to the FTP protocol to
allow the daemon to learn the delimiter conventions of each host
file system. This could allow a more flexible and powerful
multiple-file facility in BFTP. This could include the
automatic transfer of directory subtrees, for example.
[RFC-959] Postel, J., and J. Reynolds, "File Transfer Protocol
(FTP)", RFC-959, USC/Information Sciences Institute,
Appendix A -- BFTP Implementation Structure
BFTP has been implemented on both a Sun workstation running Sun OS
3.4 (based on 4.2BSD) and a VAX running 4.3BSD. The program modules
are: the local user interface programs "bftp", the Internet server
program "bftpd", and the FTC daemon "fts". BFTP makes use of the
"at" command, a UNIX batch job facility, to submit requests and
execute the daemon. An additional user interface program, the
"bftptool", is available for Sun OS 3.4, and runs in the SunView
BFTP keeps its state in a set of control files: request files,
command files, and message files. These files are stored in the home
directory specified for the environment of the process running
"bftp". If a user is running "bftp" directly, this will typically be
the user's home directory. In the case where a user has made a
Telnet connection to the well-known port 152 on a BFTP service host,
"bftp" is started by "bftpd" (or "inetd", indirectly). As a result,
the control files will be owned by the user-id under which "inetd"
was started, normally "root", and stored in the top level directory
"/". Note, however, that under BFTP all user files are written by
the FTP servers, which are presumed to enforce the operating systems'
access control conventions. Hence, BFTP does not constitute a system
A.1 User Interface Program
The BFTP user interface program "bftp" may be run directly via a
UNIX shell. Once the program has been started, the prompt "BFTP>"
will appear and commands may be entered. These commands are
described in detail in Appendix B.
A.2 Tool-Style User Interface Program
The BFTP user interface program "bftptool" may be started from a
shell window in the SunView environment on a Sun workstation. The
BFTP commands may be selected via the left mouse button. The
various file transfer parameters appear in a form-style interface;
defaults and multiple-choice style parameter values can be filled
in via menus. An advantage of this form-style interface program
is that it is possible to view all of the file transfer parameters
simultaneously, providing the user with a sense for which
parameter values might be mutually exclusive.
Help information can be displayed in a text subwindow by
positioning the on-screen mouse pointer over a command or a
parameter, and clicking the center mouse button. (No standard
mechanism for displaying help information is currently included in
the SunView package.)
The commands used in the "bftptool" are for the most part very
similar to the commands described in Appendix B. Request
submittal and the execution of the FTC daemon are identical for
the "bftp" and the "bftptool" interface programs.
A.3 Internet Server
The Internet server program "bftpd" can be invoked by opening a
Telnet connection to a well-known port, and does not require
login. The "bftpd" program runs under "inetd", the standard
BSD4.x well-known port dispatcher. When a SYN arrives for the
BFTP well-known port, "bftpd" opens the TCP connection and
performs Telnet negotiations. It then passes control to the user
interface "bftp" which allows the user to enter file transfer
A.4 BFTP Server Daemon
The BFTP file transfer control daemon program is named "fts" (for
"File Transfer Service"). This module contains code to actually
cause a single file transfer operation using the FTP server-server
model as shown in Figures 1 and 2. It is invoked with the command
"fts <request-file>". The <request-file> contains the necessary
parameters for the file transfer, in ASCII format, separated by
linefeeds. Such a request file may be created by the user
interface program, "bftp".
As a byproduct of the development of BFTP, "fts" represents a
server-server FTP driver that can be run independent of the "bftp"
program. Parameters used in the file transfer are read from a
request file, which is created and accessed via library routines
which can be shared between modules. This could be used to
perform FTP's under program control.
Appendix B: BFTP Command Summary
B.1 Special Editing Characters
In the "bftp" program, the special editing characters for command
words, subcommands, and parameter fields are as follows:
<return> Accept current command/field.
<escape> Complete current command/field, or display default.
<space> Complete and delimit current command.
<delete> Erase last character.
control-L Refresh screen.
control-R Refresh line.
control-U Erase line.
control-W Erase current token.
? List legal options.
B.2 BFTP Commands
The remainder of Appendix B consists of a list of the BFTP
commands. Each command should be followed by a carriage-return.
In the description of the syntax for each command, square brackets
"" are used to indicate a ssubcommand, or a list of possible
subcommands, which are separated by the "|" character. Angle
brackets "<>" are used to indicate a description of a parameter
where the choices would be too numerous to list, for example
B.2.1 Clear Command
Return all parameters to their default values.
B.2.2 Destination Commands
Set the destination directory.
ddir <directory name>
Set the destination file name.
dfile <file name>
Set the destination host, user, and password.
dhost <host name/number> <login> <password>
B.2.3 Explain Command
Display a short explanation of how to use BFTP.
B.2.4 Find Command
Find and display a previous request.
BFTP will prompt for the request id, which is printed when the
request is first submitted. An example of a request id is
"bftp583101774". BFTP also prompts for the request keyword, which
was determined by the user when the request was first submitted.
If no keyword was specified, a <CR> should be typed. If no
request id is entered, BFTP will display all requests which
contain a matching keyword.
RequestID (optional): <bftp-request-id>
After BFTP has displayed a summary of a matching request, it asks
whether the request is to be changed, or canceled.
Do you wish to change this request? [yes | no]
Do you wish to cancel this request? [yes | no]
If the user indicates that the request is to be changed, BFTP will
read in the parameters and cancel the existing request. At this
point the user may make any desired changes and use the "submit"
command to requeue the request. At this point a new request id
will be assigned and displayed.
Although this may happen extremely rarely, if at all, it is
possible that a system crash (or the interruption of the BFTP
program) at a particularly inopportune moment may leave a request
which is not queued. When the "find" command locates such a
request, it displays the warning:
Your request is NOT currently queued.
If this happens, the request may be read in and resubmitted using
the following procedure:
Your request is NOT currently queued.
Do you wish to change this request? yes
(BFTP displays the parameters that have been read in.)
Previous request canceled.
Use the 'submit' command to submit a new request.
B.2.5 Help Command
Print local help information.
B.2.6 Quit Command
Clear parameters and exit the BFTP program.
B.2.7 Prompt Command
Prompt for commonly-used parameters.
The following are the parameters that BFTP prompts for:
copy/move/delete: [copy | move | delete]
[local] <byte size>
(see "set type" for additional information)
Host: <host name/number>
Dir: <directory including a delimiter, e.g., "/" or ">">
(either an absolute path, or relative to the login)
File: <file name>
Host: <host name/number>
File: <file name>
Once the prompting has been completed, the current values of all
parameters will be displayed. Parameters not mentioned in the
prompting will be initialized with default values, and may be
changed via the "set" commands.
B.2.8 Request Commands
The request commands enable the user to save a set of BFTP
parameters in a "request-file" for future use. Subcommands are
provided to to list all available request-files, or to read,
write, or delete a request-file. All request-files are stored in
the user's home directory. Therefore, this facility is not
available when the user is accessing BFTP by telneting to port
Delete request file "bftp-save.name".
request delete <name>
List all bftp-save files.
Read a request file in as the current request.
request load <name>
Save the current request in a file named "bftp-save.name".
request store <name>
B.2.9 Set Commands
The "set" commands have complex subcommand structures and are used
to set many of the less commonly used FTP parameters. The
subcommands of "set" are as follows:
Set the account for the source/destination login.
set account [source | destination] <account string>
Set to true to append to destination file.
set append [true | false]
The source file will be copied to the destination file name.
The source file will be deleted after the file has been moved or
Set the mailbox to which the results will be returned. The
mailbox should be in standard internet format, for example:
set mailbox <mailbox string>
Set the FTP transfer mode.
set mode [stream | block | compress]
The source file will be deleted after it has been copied.
Set to true to transfer multiple files.
set multiple [true | false]
Set the port for the source/destination FTP connection.
set port [source | destination] <port number>
Set the FTP structure.
set structure [file | record | page]
Set the FTP type and format / byte size parameters. Note that a
normal text file is usually "ascii", and a "binary" file is often
the same as an "image" file.
set type [ascii|ebcdic] [nonprint|telnet|carriage-control]
set type [image]
set type [local] <byte size>
Set to true if the STOU command is to be used. If the STOU
command is supported by the destination host, the file will be
stored into a file having a unique file name.
set unique [true | false]
Set to true to display full FTP conversations for "verify" and
set verbose [true | false]
B.2.10 Source Commands
Set the source directory.
sdir <directory name>
Set the source file name.
sfile <file name>
Set the source host, user, and password.
shost <host name/number> <login> <password>
B.2.11 Status Command
Display the current parameter values.
B.2.12 Submit Command
Submit the current request for background FTP.
BFTP prompts for the following information:
StartTime: <date and/or time>
ReturnMailbox: <internet mailbox>
RequestKeyword: <made-up keyword>
B.2.13 Time Command
Set the start time, the starting retry interval, and the maximum
number of tries.
time <date and/or time> <minutes between tries>
<maximum number of tries>
B.2.14 Transfer Command
Perform the current request in the foreground.