Description of File Transfer Protocol.
The File Transfer Protocol (FTP) is being defined by the proposed
Request For Comments(RFC)959. Following below are the extracts from
it that give a perfect definition of what FTP is.
1. Introduction
The objectives of FTP are 1) to promote sharing of files (computer
programs and/or data), 2) to encourage indirect or implicit (via
programs) use of remote computers, 3) to shield a user from
variations in file storage systems among hosts, and 4) to transfer
data reliably and efficiently. FTP, though usable directly by a user
at a terminal, is designed mainly for use by programs.
The attempt in this specification is to satisfy the diverse needs of
users of maxi-hosts, mini-hosts, personal workstations, and TACs,
with a simple, and easily implemented protocol design.
This paper assumes knowledge of the Transmission Control Protocol
(TCP) [2] and the Telnet Protocol [3]. These documents are contained
in the ARPA-Internet protocol handbook [1].
2.1 History
FTP has had a long evolution over the years. Appendix III is a
chronological compilation of Request for Comments documents
relating to FTP. These include the first proposed file transfer
mechanisms in 1971 that were developed for implementation on hosts
at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
RFC 172 provided a user-level oriented protocol for file transfer
between host computers (including terminal IMPs). A revision of
this as RFC 265, restated FTP for additional review, while RFC 281
suggested further changes. The use of a "Set Data Type"
transaction was proposed in RFC 294 in January 1982.
RFC 354 obsoleted RFCs 264 and 265. The File Transfer Protocol
was now defined as a protocol for file transfer between HOSTs on
the ARPANET, with the primary function of FTP defined as
transfering files efficiently and reliably among hosts and
allowing the convenient use of remote file storage capabilities.
RFC 385 further commented on errors, emphasis points, and
additions to the protocol, while RFC 414 provided a status report
on the working server and user FTPs. RFC 430, issued in 1973,
(among other RFCs too numerous to mention) presented further
comments on FTP. Finally, an "official" FTP document was
published as RFC 454.
By July 1973, considerable changes from the last versions of FTP
were made, but the general structure remained the same. RFC 542
was published as a new "official" specification to reflect these
changes. However, many implementations based on the older
specification were not updated.
In 1974, RFCs 607 and 614 continued comments on FTP. RFC 624
proposed further design changes and minor modifications. In 1975,
RFC 686 entitled, "Leaving Well Enough Alone", discussed the
differences between all of the early and later versions of FTP.
RFC 691 presented a minor revision of RFC 686, regarding the
subject of print files.
Motivated by the transition from the NCP to the TCP as the
underlying protocol, a phoenix was born out of all of the above
efforts in RFC 765 as the specification of FTP for use on TCP.
This current edition of the FTP specification is intended to
correct some minor documentation errors, to improve the
explanation of some protocol features, and to add some new
optional commands.
2.2. TERMINOLOGY
access controls
Access controls define users' access privileges to the use of a
system, and to the files in that system. Access controls are
necessary to prevent unauthorized or accidental use of files.
It is the prerogative of a server-FTP process to invoke access
controls.
control connection
The communication path between the USER-PI and SERVER-PI for
the exchange of commands and replies. This connection follows
the Telnet Protocol.
data connection
A full duplex connection over which data is transferred, in a
specified mode and type. The data transferred may be a part of
a file, an entire file or a number of files. The path may be
between a server-DTP and a user-DTP, or between two
server-DTPs.
data port
The passive data transfer process "listens" on the data port
for a connection from the active transfer process in order to
open the data connection.
DTP
The data transfer process establishes and manages the data
connection. The DTP can be passive or active.
End-of-Line
The end-of-line sequence defines the separation of printing
lines. The sequence is Carriage Return, followed by Line Feed.
EOF
The end-of-file condition that defines the end of a file being
transferred.
EOR
The end-of-record condition that defines the end of a record
being transferred.
error recovery
A procedure that allows a user to recover from certain errors
such as failure of either host system or transfer process. In
FTP, error recovery may involve restarting a file transfer at a
given checkpoint.
FTP commands
A set of commands that comprise the control information flowing
from the user-FTP to the server-FTP process.
file
An ordered set of computer data (including programs), of
arbitrary length, uniquely identified by a pathname.
mode
The mode in which data is to be transferred via the data
connection. The mode defines the data format during transfer
including EOR and EOF. The transfer modes defined in FTP are
described in the Section on Transmission Modes.
pathname
Pathname is defined to be the character string which must be
input to a file system by a user in order to identify a file.
Pathname normally contains device and/or directory names, and
file name specification. FTP does not yet specify a standard
pathname convention. Each user must follow the file naming
conventions of the file systems involved in the transfer.
PI
The protocol interpreter. The user and server sides of the
protocol have distinct roles implemented in a user-PI and a
server-PI.
reply
A reply is an acknowledgment (positive or negative) sent from
server to user via the control connection in response to FTP
commands. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes
are for use by programs and the text is usually intended for
human users.
server-DTP
The data transfer process, in its normal "active" state,
establishes the data connection with the "listening" data port.
It sets up parameters for transfer and storage, and transfers
data on command from its PI. The DTP can be placed in a
"passive" state to listen for, rather than initiate a
connection on the data port.
server-FTP process
A process or set of processes which perform the function of
file transfer in cooperation with a user-FTP process and,
possibly, another server. The functions consist of a protocol
interpreter (PI) and a data transfer process (DTP).
server-PI
The server protocol interpreter "listens" on Port L for a
connection from a user-PI and establishes a control
communication connection. It receives standard FTP commands
from the user-PI, sends replies, and governs the server-DTP.
type
The data representation type used for data transfer and
storage. Type implies certain transformations between the time
of data storage and data transfer. The representation types
defined in FTP are described in the Section on Establishing
Data Connections.
user
A person or a process on behalf of a person wishing to obtain
file transfer service. The human user may interact directly
with a server-FTP process, but use of a user-FTP process is
preferred since the protocol design is weighted towards
automata.
user-DTP
The data transfer process "listens" on the data port for a
connection from a server-FTP process. If two servers are
transferring data between them, the user-DTP is inactive.
user-FTP process
A set of functions including a protocol interpreter, a data
transfer process and a user interface which together perform
the function of file transfer in cooperation with one or more
server-FTP processes. The user interface allows a local
language to be used in the command-reply dialogue with the
user.
user-PI
The user protocol interpreter initiates the control connection
from its port U to the server-FTP process, initiates FTP
commands, and governs the user-DTP if that process is part of
the file transfer.
2.3. THE FTP MODEL
With the above definitions in mind, the following model (shown in
Figure 1) may be diagrammed for an FTP service.
-------------
|/---------\|
|| User || --------
||Interface|<--->| User |
|\----^----/| --------
---------- | | |
|/------\| FTP Commands |/----V----\|
||Server|<---------------->| User ||
|| PI || FTP Replies || PI ||
|\--^---/| |\----^----/|
| | | | | |
-------- |/--V---\| Data |/----V----\| --------
| File |<--->|Server|<---------------->| User |<--->| File |
|System| || DTP || Connection || DTP || |System|
-------- |\------/| |\---------/| --------
---------- -------------
Server-FTP USER-FTP
NOTES: 1. The data connection may be used in either direction.
2. The data connection need not exist all of the time.
Figure 1 Model for FTP Use
In the model described in Figure 1, the user-protocol interpreter
initiates the control connection. The control connection follows
the Telnet protocol. At the initiation of the user, standard FTP
commands are generated by the user-PI and transmitted to the
server process via the control connection. (The user may
establish a direct control connection to the server-FTP, from a
TAC terminal for example, and generate standard FTP commands
independently, bypassing the user-FTP process.) Standard replies
are sent from the server-PI to the user-PI over the control
connection in response to the commands.
The FTP commands specify the parameters for the data connection
(data port, transfer mode, representation type, and structure) and
the nature of file system operation (store, retrieve, append,
delete, etc.). The user-DTP or its designate should "listen" on
the specified data port, and the server initiate the data
connection and data transfer in accordance with the specified
parameters. It should be noted that the data port need not be in
the same host that initiates the FTP commands via the control
connection, but the user or the user-FTP process must ensure a
"listen" on the specified data port. It ought to also be noted
that the data connection may be used for simultaneous sending and
receiving.
In another situation a user might wish to transfer files between
two hosts, neither of which is a local host. The user sets up
control connections to the two servers and then arranges for a
data connection between them. In this manner, control information
is passed to the user-PI but data is transferred between the
server data transfer processes. Following is a model of this
server-server interaction.
Control ------------ Control
---------->| User-FTP |<-----------
| | User-PI | |
| | "C" | |
V ------------ V
-------------- --------------
| Server-FTP | Data Connection | Server-FTP |
| "A" |<---------------------->| "B" |
-------------- Port (A) Port (B) --------------
Figure 2
The protocol requires that the control connections be open while
data transfer is in progress. It is the responsibility of the
user to request the closing of the control connections when
finished using the FTP service, while it is the server who takes
the action. The server may abort data transfer if the control
connections are closed without command.
The Relationship between FTP and Telnet:
The FTP uses the Telnet protocol on the control connection.
This can be achieved in two ways: first, the user-PI or the
server-PI may implement the rules of the Telnet Protocol
directly in their own procedures; or, second, the user-PI or
the server-PI may make use of the existing Telnet module in the
system.
Ease of implementaion, sharing code, and modular programming
argue for the second approach. Efficiency and independence
argue for the first approach. In practice, FTP relies on very
little of the Telnet Protocol, so the first approach does not
necessarily involve a large amount of code.
3.1. DATA REPRESENTATION AND STORAGE
Data is transferred from a storage device in the sending host to a
storage device in the receiving host. Often it is necessary to
perform certain transformations on the data because data storage
representations in the two systems are different. For example,
NVT-ASCII has different data storage representations in different
systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
as four 9-bit characters in a 36-bit word. It is desirable to
convert characters into the standard NVT-ASCII representation when
transmitting text between dissimilar systems. The sending and
receiving sites would have to perform the necessary
transformations between the standard representation and their
internal representations.
A different problem in representation arises when transmitting
binary data (not character codes) between host systems with
different word lengths. It is not always clear how the sender
should send data, and the receiver store it. For example, when
transmitting 32-bit bytes from a 32-bit word-length system to a
36-bit word-length system, it may be desirable (for reasons of
efficiency and usefulness) to store the 32-bit bytes
right-justified in a 36-bit word in the latter system. In any
case, the user should have the option of specifying data
representation and transformation functions. It should be noted
that FTP provides for very limited data type representations.
Transformations desired beyond this limited capability should be
performed by the user directly.
3.2. ESTABLISHING DATA CONNECTIONS
The mechanics of transferring data consists of setting up the data
connection to the appropriate ports and choosing the parameters
for transfer. Both the user and the server-DTPs have a default
data port. The user-process default data port is the same as the
control connection port (i.e., U). The server-process default
data port is the port adjacent to the control connection port
(i.e., L-1).
The transfer byte size is 8-bit bytes. This byte size is relevant
only for the actual transfer of the data; it has no bearing on
representation of the data within a host's file system.
The passive data transfer process (this may be a user-DTP or a
second server-DTP) shall "listen" on the data port prior to
sending a transfer request command. The FTP request command
determines the direction of the data transfer. The server, upon
receiving the transfer request, will initiate the data connection
to the port. When the connection is established, the data
transfer begins between DTP's, and the server-PI sends a
confirming reply to the user-PI.
Every FTP implementation must support the use of the default data
ports, and only the USER-PI can initiate a change to non-default
ports.
It is possible for the user to specify an alternate data port by
use of the PORT command. The user may want a file dumped on a TAC
line printer or retrieved from a third party host. In the latter
case, the user-PI sets up control connections with both
server-PI's. One server is then told (by an FTP command) to
"listen" for a connection which the other will initiate. The
user-PI sends one server-PI a PORT command indicating the data
port of the other. Finally, both are sent the appropriate
transfer commands. The exact sequence of commands and replies
sent between the user-controller and the servers is defined in the
Section on FTP Replies.
In general, it is the server's responsibility to maintain the data
connection--to initiate it and to close it. The exception to this
is when the user-DTP is sending the data in a transfer mode that
requires the connection to be closed to indicate EOF. The server
MUST close the data connection under the following conditions:
1. The server has completed sending data in a transfer mode
that requires a close to indicate EOF.
2. The server receives an ABORT command from the user.
3. The port specification is changed by a command from the
user.
4. The control connection is closed legally or otherwise.
5. An irrecoverable error condition occurs.
Otherwise the close is a server option, the exercise of which the
server must indicate to the user-process by either a 250 or 226
reply only.
3.3. DATA CONNECTION MANAGEMENT
Default Data Connection Ports: All FTP implementations must
support use of the default data connection ports, and only the
User-PI may initiate the use of non-default ports.
Negotiating Non-Default Data Ports: The User-PI may specify a
non-default user side data port with the PORT command. The
User-PI may request the server side to identify a non-default
server side data port with the PASV command. Since a connection
is defined by the pair of addresses, either of these actions is
enough to get a different data connection, still it is permitted
to do both commands to use new ports on both ends of the data
connection.
Reuse of the Data Connection: When using the stream mode of data
transfer the end of the file must be indicated by closing the
connection. This causes a problem if multiple files are to be
transfered in the session, due to need for TCP to hold the
connection record for a time out period to guarantee the reliable
communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to
negotiate a non-default port. The second is to use another
transfer mode.
A comment on transfer modes. The stream transfer mode is
inherently unreliable, since one can not determine if the
connection closed prematurely or not. The other transfer modes
(Block, Compressed) do not close the connection to indicate the
end of file. They have enough FTP encoding that the data
connection can be parsed to determine the end of the file.
Thus using these modes one can leave the data connection open
for multiple file transfers.
3.5. ERROR RECOVERY AND RESTART
There is no provision for detecting bits lost or scrambled in data
transfer; this level of error control is handled by the TCP.
However, a restart procedure is provided to protect users from
gross system failures (including failures of a host, an
FTP-process, or the underlying network).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert
a special marker code in the data stream with some marker
information. The marker information has meaning only to the
sender, but must consist of printable characters in the default or
negotiated language of the control connection (ASCII or EBCDIC).
The marker could represent a bit-count, a record-count, or any
other information by which a system may identify a data
checkpoint. The receiver of data, if it implements the restart
procedure, would then mark the corresponding position of this
marker in the receiving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the
restart procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the control connection in a 110 reply (depending
on who is the sender). In the event of a system failure, the user
or controller process restarts the server at the last server
marker by sending a restart command with server's marker code as
its argument. The restart command is transmitted over the control
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.
4. FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established as a TCP connection from the user to the standard server
port. The user protocol interpreter is responsible for sending FTP
commands and interpreting the replies received; the server-PI
interprets commands, sends replies and directs its DTP to set up the
data connection and transfer the data. If the second party to the
data transfer (the passive transfer process) is the user-DTP, then it
is governed through the internal protocol of the user-FTP host; if it
is a second server-DTP, then it is governed by its PI on command from
the user-PI. The FTP replies are discussed in the next section. In
the description of a few of the commands in this section, it is
helpful to be explicit about the possible replies.
4.2. FTP REPLIES
Replies to File Transfer Protocol commands are devised to ensure
the synchronization of requests and actions in the process of file
transfer, and to guarantee that the user process always knows the
state of the Server. Every command must generate at least one
reply, although there may be more than one; in the latter case,
the multiple replies must be easily distinguished. In addition,
some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an
intermediate state if all preceding commands have been successful.
A failure at any point in the sequence necessitates the repetition
of the entire sequence from the beginning.
The details of the command-reply sequence are made explicit in
a set of state diagrams below.
An FTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is intended for the human user. It is intended
that the three digits contain enough encoded information that the
user-process (the User-PI) will not need to examine the text and
may either discard it or pass it on to the user, as appropriate.
In particular, the text may be server-dependent, so there are
likely to be varying texts for each reply code.
A reply is defined to contain the 3-digit code, followed by Space
, followed by one line of text (where some maximum line length
has been specified), and terminated by the Telnet end-of-line
code. There will be cases however, where the text is longer than
a single line. In these cases the complete text must be bracketed
so the User-process knows when it may stop reading the reply (i.e.
stop processing input on the control connection) and go do other
things. This requires a special format on the first line to
indicate that more than one line is coming, and another on the
last line to designate it as the last. At least one of these must
contain the appropriate reply code to indicate the state of the
transaction. To satisfy all factions, it was decided that both
the first and last line codes should be the same.
Thus the format for multi-line replies is that the first line
will begin with the exact required reply code, followed
immediately by a Hyphen, "-" (also known as Minus), followed by
text. The last line will begin with the same code, followed
immediately by Space , optionally some text, and the Telnet
end-of-line code.
For example:
123-First line
Second line
234 A line beginning with numbers
123 The last line
The user-process then simply needs to search for the second
occurrence of the same reply code, followed by (Space), at
the beginning of a line, and ignore all intermediary lines. If
an intermediary line begins with a 3-digit number, the Server
must pad the front to avoid confusion.
This scheme allows standard system routines to be used for
reply information (such as for the STAT reply), with
"artificial" first and last lines tacked on. In rare cases
where these routines are able to generate three digits and a
Space at the beginning of any line, the beginning of each
text line should be offset by some neutral text, like Space.
This scheme assumes that multi-line replies may not be nested.
The three digits of the reply each have a special significance.
This is intended to allow a range of very simple to very
sophisticated responses by the user-process. The first digit
denotes whether the response is good, bad or incomplete.
(Referring to the state diagram), an unsophisticated user-process
will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by simply examining this first digit. A
user-process that wants to know approximately what kind of error
occurred (e.g. file system error, command syntax error) may
examine the second digit, reserving the third digit for the finest
gradation of information (e.g., RNTO command without a preceding
RNFR).
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The requested action is being initiated; expect another
reply before proceeding with a new command. (The
user-process sending another command before the
completion reply would be in violation of protocol; but
server-FTP processes should queue any commands that
arrive while a preceding command is in progress.) This
type of reply can be used to indicate that the command
was accepted and the user-process may now pay attention
to the data connections, for implementations where
simultaneous monitoring is difficult. The server-FTP
process may send at most, one 1yz reply per command.
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The user should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not take place, but the error condition is temporary and
the action may be requested again. The user should
return to the beginning of the command sequence, if any.
It is difficult to assign a meaning to "transient",
particularly when two distinct sites (Server- and
User-processes) have to agree on the interpretation.
Each reply in the 4yz category might have a slightly
different time value, but the intent is that the
user-process is encouraged to try again. A rule of thumb
in determining if a reply fits into the 4yz or the 5yz
(Permanent Negative) category is that replies are 4yz if
the commands can be repeated without any change in
command form or in properties of the User or Server
(e.g., the command is spelled the same with the same
arguments used; the user does not change his file access
or user name; the server does not put up a new
implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not take place. The User-process is discouraged from
repeating the exact request (in the same sequence). Even
some "permanent" error conditions can be corrected, so
the human user may want to direct his User-process to
reinitiate the command sequence by direct action at some
point in the future (e.g., after the spelling has been
changed, or the user has altered his directory status.)
The following function groupings are encoded in the second
digit:
x0z Syntax - These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, unimplemented or superfluous
commands.
x1z Information - These are replies to requests for
information, such as status or help.
x2z Connections - Replies referring to the control and
data connections.
x3z Authentication and accounting - Replies for the login
process and accounting procedures.
x4z Unspecified as yet.
x5z File system - These replies indicate the status of the
Server file system vis-a-vis the requested transfer or
other file system action.
The third digit gives a finer gradation of meaning in each of
the function categories, specified by the second digit. The
list of replies below will illustrate this. Note that the text
associated with each reply is recommended, rather than
mandatory, and may even change according to the command with
which it is associated. The reply codes, on the other hand,
must strictly follow the specifications in the last section;
that is, Server implementations should not invent new codes for
situations that are only slightly different from the ones
described here, but rather should adapt codes already defined.
A command such as TYPE or ALLO whose successful execution
does not offer the user-process any new information will
cause a 200 reply to be returned. If the command is not
implemented by a particular Server-FTP process because it
has no relevance to that computer system, for example ALLO
at a TOPS20 site, a Positive Completion reply is still
desired so that the simple User-process knows it can proceed
with its course of action. A 202 reply is used in this case
with, for example, the reply text: "No storage allocation
necessary." If, on the other hand, the command requests a
non-site-specific action and is unimplemented, the response
is 502. A refinement of that is the 504 reply for a command
that is implemented, but that requests an unimplemented
parameter.
5. DECLARATIVE SPECIFICATIONS
5.1. MINIMUM IMPLEMENTATION
In order to make FTP workable without needless error messages, the
following minimum implementation is required for all servers:
TYPE - ASCII Non-print
MODE - Stream
STRUCTURE - File, Record
COMMANDS - USER, QUIT, PORT,
TYPE, MODE, STRU,
for the default values
RETR, STOR,
NOOP.
The default values for transfer parameters are:
TYPE - ASCII Non-print
MODE - Stream
STRU - File
All hosts must accept the above as the standard defaults.
5.2. CONNECTIONS
The server protocol interpreter shall "listen" on Port L. The
user or user protocol interpreter shall initiate the full-duplex
control connection. Server- and user- processes should follow the
conventions of the Telnet protocol as specified in the
ARPA-Internet Protocol Handbook [1]. Servers are under no
obligation to provide for editing of command lines and may require
that it be done in the user host. The control connection shall be
closed by the server at the user's request after all transfers and
replies are completed.
The user-DTP must "listen" on the specified data port; this may be
the default user port (U) or a port specified in the PORT command.
The server shall initiate the data connection from his own default
data port (L-1) using the specified user data port. The direction
of the transfer and the port used will be determined by the FTP
service command.
Note that all FTP implementation must support data transfer using
the default port, and that only the USER-PI may initiate the use
of non-default ports.
When data is to be transferred between two servers, A and B (refer
to Figure 2), the user-PI, C, sets up control connections with
both server-PI's. One of the servers, say A, is then sent a PASV
command telling him to "listen" on his data port rather than
initiate a connection when he receives a transfer service command.
When the user-PI receives an acknowledgment to the PASV command,
which includes the identity of the host and port being listened
on, the user-PI then sends A's port, a, to B in a PORT command; a
reply is returned. The user-PI may then send the corresponding
service commands to A and B. Server B initiates the connection
and the transfer proceeds. The command-reply sequence is listed
below where the messages are vertically synchronous but
horizontally asynchronous:
User-PI - Server A User-PI - Server B
------------------ ------------------
C->A : Connect C->B : Connect
C->A : PASV
A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
C->B : PORT A1,A2,A3,A4,a1,a2
B->C : 200 Okay
C->A : STOR C->B : RETR
B->A : Connect to HOST-A, PORT-a
Figure 3
The data connection shall be closed by the server under the
conditions described in the Section on Establishing Data
Connections. If the data connection is to be closed following a
data transfer where closing the connection is not required to
indicate the end-of-file, the server must do so immediately.
Waiting until after a new transfer command is not permitted
because the user-process will have already tested the data
connection to see if it needs to do a "listen"; (remember that the
user must "listen" on a closed data port BEFORE sending the
transfer request). To prevent a race condition here, the server
sends a reply (226) after closing the data connection (or if the
connection is left open, a "file transfer completed" reply (250)
and the user-PI should wait for one of these replies before
issuing a new transfer command).
Any time either the user or server see that the connection is
being closed by the other side, it should promptly read any
remaining data queued on the connection and issue the close on its
own side.
5.3. COMMANDS
The commands are Telnet character strings transmitted over the
control connections as described in the Section on FTP Commands.
The command functions and semantics are described in the Section
on Access Control Commands, Transfer Parameter Commands, FTP
Service Commands, and Miscellaneous Commands. The command syntax
is specified here.
The commands begin with a command code followed by an argument
field. The command codes are four or fewer alphabetic characters.
Upper and lower case alphabetic characters are to be treated
identically. Thus, any of the following may represent the
retrieve command:
RETR Retr retr ReTr rETr
This also applies to any symbols representing parameter values,
such as A or a for ASCII TYPE. The command codes and the argument
fields are separated by one or more spaces.
The argument field consists of a variable length character string
ending with the character sequence (Carriage Return, Line
Feed) for NVT-ASCII representation; for other negotiated languages
a different end of line character might be used. It should be
noted that the server is to take no action until the end of line
code is received.
The syntax is specified below in NVT-ASCII. All characters in the
argument field are ASCII characters including any ASCII
represented decimal integers. Square brackets denote an optional
argument field. If the option is not taken, the appropriate
default is implied.
7. TYPICAL FTP SCENARIO
User at host U wanting to transfer files to/from host S:
In general, the user will communicate to the server via a mediating
user-FTP process. The following may be a typical scenario. The
user-FTP prompts are shown in parentheses, '---->' represents
commands from host U to host S, and '<----' represents replies from
host S to host U.
LOCAL COMMANDS BY USER ACTION INVOLVED
ftp (host) multics Connect to host S, port L,
establishing control connections.
<---- 220 Service ready .
username Doe USER Doe---->
<---- 331 User name ok,
need password.
password mumble PASS mumble---->
<---- 230 User logged in.
retrieve (local type) ASCII
(local pathname) test 1 User-FTP opens local file in ASCII.
(for. pathname) test.pl1 RETR test.pl1 ---->
<---- 150 File status okay;
about to open data
connection.
Server makes data connection
to port U.
<---- 226 Closing data connection,
file transfer successful.
type Image TYPE I ---->
<---- 200 Command OK
store (local type) image
(local pathname) file dump User-FTP opens local file in Image.
(for.pathname) >udd>cn>fd STOR >udd>cn>fd ---->
<---- 550 Access denied
terminate QUIT ---->
Server closes all
connections.
8. CONNECTION ESTABLISHMENT
The FTP control connection is established via TCP between the user
process port U and the server process port L. This protocol is
assigned the service port 21 (25 octal), that is L=21.
REFERENCES
[1] Feinler, Elizabeth, "Internet Protocol Transition Workbook",
Network Information Center, SRI International, March 1982.
[2] Postel, Jon, "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981.
[3] Postel, Jon, and Joyce Reynolds, "Telnet Protocol
Specification", RFC 854, ISI, May 1983.
[4] Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC 943,
ISI, April 1985.
romm@empire.tau.ac.il
Last modified: Tue Oct 15 03:10:14 1996
