Abbay Bhushan, MIT
Bob Braden, UCLA
Will Crowther, BBN
Eric Narslem, Rand
John Heafner, Rand
Alex McKenzie, BBH
John Melvin, SRI
Bob Sundberg, Harvard
Dick Watson, SRI
Jim White, UOSB
THE FILE TRANSFER PROTOCOL
This Paper is a revision of RF 172, Mic 6794. The changes to RFC 172 are given below. The protocol is then restated for your ocnvenience.
CHANGES TO RFC 172
1) Two new file transfer requests have been added. These are
2) The op code assignements in control transactions have been
changed to include the above requests.
3) Two new error codes indicating 'incorrect or missing indentifier' and 'file already exists' have been added. New error code assignements reflect this change.
4) Editorial changes to clarify specifications.
I. INTRODUCTION
The file transfer protocol (FTP) is a userlevel procotocol for file transfer between host computers (including terminal IMPs), on the ARPA computer network (ARPANET). The primary function of FTP is to facilitate transfer of files between hosts and to allow convenient use of storage and file handling capabilities of remote hosts. FTP uses the Data Transfer Protocol described in RFC 264 to achieve transfer of data. This paper assumes knowledge of RFC 264. The objectives of FTP are to promote sharing of files (computer programs and/or data) encourage implicit (without explicit login) use of computers, and shield the user from variations in file and storage systems of different hosts. These objetives are achieved by specifying a standard file transfer socket and initial connection protocol for implicit use, and using standard conventions for file transfer and related operations.
II. DISCUSSION
A file is considered here to be an ordered set of arbitrary
length, consisting of computer data (including programs). Files are
uniquely identified in a system by their pathnames. A pathname is
(loosely) defined to be the data string which must be input to the
file system by a network user in order to identify a file. Pathname
usually contains device and/or directory names, and file name. FTP
specifications provide standard file system commands, but do not
provide standard naming convention at this time. Each user must follow
the naming convention of the file system be wishing to use. FTP may be
extended later to include standard conventions of pathname structures.
A file may or may not have access control associated with it The
access controls designate users access privileges. In absence of
access controls, files cannot be protected from accidental or
unauthorized usage. It is the prerogative of a serving file system to
provide protection, and selective access. FTP provides identifier and
password mechanisms for exchange of access control information. it
should however ve noted, that for file sharing, it is necessary that a
user be allowed (subject to access controls) to access files not
created by him.
FTP does not restrict the nature of information in files. For
example, a file could contain ASCII text, binary data, computer
program, or any other information. A provision for indicating data
structure (type and byte size) exists in FTP to aid in parsing,
interpretation, and storage of data.
To facilitate impliict usage, a serving file transfer process my
be a disowned "demon" process which "listens" to an agreed-upon
socket, and follows the standard initial connection protocol for
establishing a fill-duplex connection. It should be noted that FTP my
also be used directly by logging into a remote host, and arranging for
file transfer over specific sockets.
FTP is readily extendable, in that additional commands and data
types may be defined by those agreeing to implement them.
Implementation of a subset of commands is specifically permitted, and
an initial subset for implementation is recommended. (*)The protocol
may also be extended to enable remote execution of programs, but no
standard procedure is suggested.
For transferring data, FTP uses the data transfer protocol
specified in RFC 264. As the data transfer protool does not specify
the manner in which it is to be used by FTP, implementation may vary
at different host sites. Hosts not wishing to separate data transfer
and file transfer functions, should take particular care in conforming
to the data transfer protocol specifications of RFC 264.
It should be noted that FTP specifications do not require
knowledge of transfer modes used by data transfer protocol. However,
as file transfer protocol requires the transfer of more than a single
control transaction over the same connection, it is essential that
hosts be able to send control transactions in either 'transparent
block' (type B9) or 'descriptor and counts' (type BA) modes. (Type BS,
the indefinite bit stream mode is not suitable as it limits transfer
to single transactions.).
The use of data transfer aborts (type B6) is neither required, nor
defined in FTP. FTP has its own error terminate wich may be used to
abort a file transfer request. FTP also does not define to structure
of files, and there are no conventions on the use of group, record and
unit separators. (*)A file separator however, indicates the end of a
file.
It is strongly recommended that default options be provided in
implementation to facilitate use of file transfer service. For
example, the main file directora on disk, a pool directory, user
directory of diretory last accessed could serve as standard pathname
defaults. Default mechanisms are convenient, as the user doesn't have
to specify the complete pathname each time ve wishes to use the file
transfer service. No standard default procedures are specified by FTP.
--------------------------------
(*)
This initial subset represents control functions necessary for
basic file transfer and "mail" operations, and some elementary file
manipulation operations. There is no attempt to provide a data
management or complete file management cpability.
(*)
It is possible that wi may, at a later date, assign meaning to
these information separators within FTP.
III. SPECIFICATIONS
1. Data Transfer
-
FTP uses the Data Transfer Protocol (described in RFC 264) for transferring data and/or control transaction. Both data and control transactions are communicated over the same connection.
2. Data Transactions
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Data transactions represent the data contained in a file. There is no data type or byte size information contained in data transactions. The structure of data communicated via control transactions. A file may be transferred as one or more data transactions. The protocol neither specifies nor impose any limitations on the structure (record, group, etc) or length of file. Such limitations may however be imposed
by a serving host. the end of a file may be indicated by a file separator (as defined in data transfer protocol). In the special case of indefinite bit-stream transfer mode (Type B0), the end of file is indicated by closing connection. In particular, a serving or usin host should not send the ETX, or other end of file character, unless such a character is part of the data in file (i.e. not provided by system).
3. Control Transactions
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The control transactions may be typified as requests, identifiers, and terminates. A request fulfillment sequence begins with a request and ends with receipt of data (followed by end-of-File) or a terminate. The user side initiates the connections as well as the request. The server side "listens" and complies with the request.
3A. Op Codes
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The first information (i.e., not descriptor) byte or control transactions indicates the control function. This byte is referred to as "opcode". A standard set of opcodes are defined below. The operations are discussed in Section 2B.2.
Implementation of a workable subset (*) of opcodes is specifically permitted. Additional standard opcodes may be assigned later. Opcodes hex 5A (octal 100) through hex FF (octal 377) are for experimental use.
Op Code Operation
Hex Octal
00 000 Set data type identifier
01 001 Retrieve Request
02 002 Create request (write file; error ir
file already exits)
03 003 Store request (write file; replace
if file already exists)
04 004 Append request (add to existing file;
error if file does not exist)
05 005 Append_with_create request (add to
file; create if file does not exist)
06 006 Delete request (delete file)
07 007 Rename_from request (change file name)
08 010 Rename_to request (the new file name)
09 011 List request (list information)
0A 012 Username identifier (for access control)
0B 013 Password identifier (for access control)
0C 014 Error of unsuccessful terminate
0D 015 Acknowledge or successful terminate
0E 016
through through Reserved for standard assignment
4F 077
5A 100
through through Assigned for experimental use
FF 377
------------------------------------
(*)
A workable subset is any request, plus terminates. Indentifiers
may be required in addition for usin "protected" file systems.
3B. Syntax and Semantics
3B.1 Data Types
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The 'set data type' control transactions indentifies the structure of data (data type and byte size) is succeeding data transactions. The 'set data type' transaction shall contain two more bytes in addition to the opcode byte. The first of these bytes shall convey a data type or code information and the second byte may convey the data byte size, where applicable. this information may be used to define the manner in which data is to be parsed, interpreted, reconfigured or stored. Set data type need be sent only when structure of data is changed from the preceding.
Although, a number of data types are defined, specific implementations may handle only limited data types or completely ignore the data type and byte size descriptors. Even if a host process does not "recognize" a data type, it must accept data (i.e., there is no such thing as data type error.) These descriptors are provided only for convenience, and it es not essential that they be used. The standard default is to assume nothing about the information and treat it as a bit stream (binary data, byte size 1)(*)whose interpretation is left to a higher level process, or the user.
The following data type codes are currently assigned. Where a byte size is not implicit in data type, it may be provided by the second byte.
-----------------------------------
(*)
It is, however, possible that this bit stream is treated like ASCII
characters in specific instances such as transmitting a file to a line
printer.
Code Implicit Data Type
Hex Octal Byte Size
00 000 1 Bit stream (standard default)
01 001 none Binary data bytes
02 002 8 Network ASCII characters
03 003 8 EBCDIC characters
04 004 36 DEC-packed ASCII (five 7-bit
characters, 36th bit 1 or 0)
05 005 8 Decimal numbers, net. ASCII
06 006 8 Octal numbers, net. ASCII
07 007 8 Hexadecimal numbers, net. ASCII
08 010
through through Reserved for standard assignemt
4f 077
5A 100
through through Assigned for experimental use
FF 377
3B.2 Requests and Identifiers
-
Retrieve, create, append, append_with_create, delete, rename_from, and rename_to requests must contain a pathname specifying a file, following the opcode in the information field. In the list request a pathname may or may not follow the opcode. If present, the pathname may specify either a file or a directory.
A file pathname must uniquely identify a file in the serving host. The syntax of pathnames and identifying information shall conform to serving host conventions, except that standard network ASCII (7-bit ASCII right justified in 8-bit) field with most signifcant bit as zero) shall be used.
The store request has a 4-byte (32 bits) 'allocate size' field followed by a pathname specifying a file. 'Allocate size' indicates the number of bits of storage to be allocated to the file. An allocate size of zero indicates that server should use his default.
Retrieve request achieves the transfer of a copy of file specified in pathname, from serving to using host. the status and contents of file in serving host should be unaffected.
Create request causes a file to be created at the serving host as specified in pathname, A copy of the file is transferred from the using to the serving host. If the file specified in pathname already exists at the serving host, an error terminate should be sent by the server.Store request achieves the transfer of copy of file from using to serving host. If file specified in pathname exists on serving hosts, then its contents shall be replaced by the contents of the file being transferred. A new file is created at the serving host if the file specified in pathname does not exist.
Append request achieves the transfer of data from using to serving host. The transferred data is appended to file specified in pathname, at serving host. If the specified file does not exist at serving host, an error terminate should be sent by the server.
Append with create request achieves the transfer of data from using to serving host. If file specified is pathname exists at serving host, then the transferred data is appended to that file, otherwise the file specified in pathname is created at the serving host.
Rename from and rename to requests cause the name of the file specified in pathname of rename_from to be changed to the name specified in pathname of rename_to. A rename_from request must always be followed by a rename_to request.
Delete request causes file specified in pathname to be deleted from the serving host. If an extra level of protection is desired such as the query "Do you really wish to delete this file?", it is to be a local implementation option in the using system. Such queries should not be transmitted over network connections.
List request causes a list to be sent from the serving to using host. If there is no pathname of if pathname is a directory, the server should send a file directory list. If the pathname specifies a file then server should send current information on the file.
Username and password identifiers contain the respective identifying information. Normally, the information will be supplied by the user of the file transfer service. These identifiers will normally be sent at the start of connetion for access control.
3B.3 Error and Acknowledge Terminates
-
The error transactions may have an error code indicated by the second information byte. Transmission of an ASCII error message in subsequent bytes is permitted with all error codes, except that with Hex '0A' error code, ASCII text is required. The errors here relate to file transfer functions only. Data synchronization and related errors in data transfer are to be handled at the DTP level. The following error codes are currently defined:
Error Code (2nd descriptor byte) Meaning Hex Octal 00 000 Error condition indicated by computer system (external to protocol) 01 001 Name syntay error 02 002 Access control violation 03 003 Abort (by user) 04 004 Allocate size too big 05 005 Allocate size overflow 06 006 Improper order for transactions 07 007 Opcode not implemented 08 010 File search failed 09 011 Incorrect or missing identifier 0A 012 Error described in text message (ASCII characters follow code) 0B 013 File already exists (in create request)At present, no completion codes are defined for acknowledge, It is assumed that acknowledge refers to the current request being fulfilled.
4. Order of transactions
4A. A certain order of transactions must be maintained in
-
fulfilling file transfer requests. The exact sequence in wich transactions occur depends on the type of request, as described in action 4B. The fullfillment of a request may be aborted anytime by either host, as explained in section 4C.
4B. Identifier transactions (set data type, username, and
password) may be sent by user at any time. The usual order
would be a username transaction followed by a password
transaction at the start of the connection. No acknowledge
is required, or permitted. The identifiers are to be used
for default handling, and access control.
-
Retrieve and list requests cause cause the transfer of file from server to user. After a complete file has been transferred, the server should indicate end-of-file (by sending CLS or file separator) to complete the request fulfillment sequence, as shown below.
Retrieve / List requests -----------------------------> User < File -- Data> Server <----------------------------- End of file indication <-----------------------------Store, create, append, and append_with_create requests cause the transfer of file from user to server. After a complete file has been transferred, the user should send an end-of-file indication. The receipt of the file must be acknowledged by the server, as shown below.
Create / Store / Append / Append_with_create requests -----------------------------> User <File --- Data> Server -----------------------------> End of file indication -----------------------------> Acknowledge <-----------------------------Rename_from request must be followed by a rename_to request. The request must be acknowledged as shown below.
User Rename_from request Server -----------------------------> Rename_ro request -----------------------------> Acknowledge <-----------------------------The delete request requires the server to acknowledge it, as shown below.
User Delete Server -----------------------------> Acknowledge <-----------------------------Error transactions my be sent by either host at any time, and these terminate the current request fulfillment sequence.
4C. Aborts. Eithe host may abort a request fulfillment sequence
-
at any time by sending an error terminate, or by closing the connection (NCP to transmit a CCLS for the connection). CLS is a more drastic type of abort and shall be used when there is a catastrophic failure, or when abort is desired in the middle of a long transaction. The abort indicates to the receiving host that sender of abort wishes to terminate request fulfillment and is now ready to initiate ar fulfill new requests. When CLS is used to abort, the using host will he responsible for reopening connection. The file transfer abort described here is different form data transfer abort which is sent only by the sender of data. The use of the data transfer is not defined in this protocol.
5. Initial Connection, CLS, and Access Control
5A. Socket 3 is the standard preassigned socket number on which
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the cooperating file transfer process at the serving host should "listen". (*)The connection establishment will be in accordance with the standard initial connection protocol, (*)establishing a full-duplex connection.
5B. The connection will be broken by trading a CLS between the
-
NCP's for each of the two connections. Normally, the user will initiate CLS.
CLS may also be used by either user or server, to abort a transation in the middle. If CLS is received in the middle of transaction, the current request fulfillment sequence will be aborted. The using host will then reopen connection.
5C. It is recommended that identifier (user name and password)
-
transactions be sent by user to server, at the start, as this would facilitate default handline and access control for the entire duration of connection. Some service sites may require the indentifier transactions. The identifier transactions do not require or permit an acknowledge, and the user can proceed directly with requests. If the identifier information is incorrect or not received, the server may send an error transaction indicating access control, violation, upon subsequent requests.
--------------------------------- (*) Socket 1 has been assigned to logger, socket 3 seems a reasonable choice for File Transfer. (*) RFC 165, or any subsequent standard applicable in initial connection to loggers.