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                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759Nancy J. Neigus                            See Also:  RFCs 354, 454, 495Bolt Beranek and Newman, Inc.Cambridge, Mass.              File Transfer Protocol for the ARPA Network                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759                                PREFACEThis document is the result of several months discussion via RFC(relevant numbers are 430, 448, 454, 463, 468, 478, 480), followed by ameeting of the FTP committee at BBN on March 16, followed by furthercommunication among committee members.  There are a considerable numberof changes for the last "official" version, see RFCs 354, 385, but thegross structure remains the same.  The places to look for differencesare (1) in the definitions pf types and modes, (2) in the specificationof the data connection and data sockets, (3) in the command-replysequences, (4) in the functions dependent on the TELNET protocol (FTPhas been altered to correspond to the new TELNET spec).  The model hasbeen clarified and enlarged to allow inter-server file transfer, andseveral new commands have been added to accommodate more specialized (orsite-specific) functions.  It is my belief that this new specificiationreflects the views expressed by the committee at the above-mentionedmeeting and in subsequent conversations.The large number of incompatibilities would complicate a phasedimplementation schedule, such as is in effect for the TELNET protocol.Therefore we have assigned a new socket, decimal 21, as a temporarylogger socket for the new version and a change-over date of 1 February1974.  Until that date the old (354, 385) version of FTP will beavailable on Socket 3 and the new version (attached) should beimplemented on Socket 21.  On 1 February the new version will shift toSocket 3 and the old disappear from view.The File Transfer protocol should be considered stable at least untilFebruary, though one should feel free to propose further changes viaRFC.  (Implementation of new commands on an experimental basis isencouraged and should also be reported by RFC.)  In addition, members ofthe FTP committee may be contacted directly about changes.  Based onattendance at the March 16 meeting, they are:   Abhay Bhushan MIT-DMCG   Bob Braden UCLA-CCN   Bob Bressler BBN-NET   Bob Clements BBN-TENEX   John Day ILL-ANTS   Peter Deutsch PARC-MAXC   Wayne Hathaway AMES-67   Mike Kudlick SRI-ARC   Alex McKenzie BBN-NET   Bob Merryman UCSD-CC   Nancy Neigus BBN-NET   Mike Padlipsky MIT-Multics   Jim Pepin USC-44   Ken Pogran MIT-Multics   Jon Postel UCLA-NMC                                   1                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   Milton Reese FNWC   Brad Reussow HARV-10   Marc Seriff MIT-DMCG   Ed Taft HARV-10   Bob Thomas BBN-TENEX   Ric Werme CMU-10   Jim White SRI-ARCI would especially like to thank Bob Braden, Ken Pogran, Wayne Hathaway,Jon Postel, Ed Taft and Alex McKenzie for their help in preparing thisdocument.NJN/jm                                   2                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759                         FILE TRANSFER PROTOCOLINTRODUCTION   The File Transfer Protocol (FTP) is a protocol for file transfer   between Hosts (including Terminal Interface Message Processors   (TIPs)) on the ARPA Computer Network (ARPANET).  The primary function   of FTP is to transfer files efficiently and reliably among Hosts and   to allow the convenient use of remote file storage capabilities.   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, TIPs, and the Datacomputer, with a   simple, and easily implemented protocol design.   This paper assumes knowledge of the following protocols described in   NIC #7104:      The Host-Host Protocol      The Initial Connection Protocol      The TELNET ProtocolDISCUSSION   In this section, the terminology and the FTP model are discussed.   The terms defined in this section are only those that have special   significance in FTP.   TERMINOLOGY      ASCII         The USASCII character set as defined in NIC #7104.  In FTP,         ASCII characters are defined to be the lower half of an         eight-bit code set (i.e., the most significant bit is zero).      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.                                   3                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         It is the prerogative of a server-FTP process to provide access         controls.      byte size         The byte size specified for the transfer of data.  The data         connection is opened with this byte size.  The data connection         byte size is not necessarily the byte size in which data is to         be stored in a system, nor the logical byte size for         interpretation of the structure of the data.      data connection         A simplex connection over which data is transferred, in a         specified byte size, 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 socket         The passive data transfer process "listens" on the data socket         for an RFC from the active transfer process (server) in order         to open the data connection.  The server has fixed data         sockets; the passive process may or may not.      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.                                   4                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      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.      NVT         The Network Virtual Terminal as defined in the ARPANET TELNET         Protocol.      NVFS         The Network Virtual File System.  A concept which defines a         standard network file system with standard commands and         pathname conventions.  FTP only partially embraces the NVFS         concept at this time.      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 he wishes to use.      record         A sequential file may be structured as a number of contiguous         parts called records.  Record structures are supported by FTP         but a file need not have record structure.      reply         A reply is an acknowledgment (positive or negative) sent from         server to user via the TELNET connections 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.                                   5                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      server-DTP         The data transfer process, in its normal "active" state,         establishes the data connection by RFC to the "listening" data         socket, sets up parameters for transfer and storage, and         tranfers data on command from its PI.  The DTP can be placed in         a "passive" state to listen for, rather than initiate, an RFC         on the data socket.      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 protocol interpreter "listens" on Socket 3 for an ICP from         a user-PI and establishes a TELNET communication connection.         It receives standard FTP commands from the user-PI, sends         replies, and governs the server-DTP.      TELNET connections         The full-duplex communication path between a user-PI and a         server-PI.  The TELNET connections are established via the         standard ARPANET Initial Connection Protocol (ICP).      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 human being or a process on behalf of a human being 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.                                   6                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      user-DTP         The data transfer process "listens" on the data socket for an         RFC 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 protocol interpreter initiates the ICP to the server-FTP         process, initiates FTP commands, and governs the user-DTP if         that process is part of the file transfer.   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 !                                         !\----:----/!    --------               ----------                !     V     !               !/------\!  FTP Commands  !/---------\!               !!Server!<-----------------!   User  !!               !!  PI  !----------------->!    PI   !!               !\--:---/!   FTP Replies  !\----:----/!               !   V    !                !     V     !   --------    !/------\!      Data      !/---------\!    --------   ! File !<--->!Server!<---------------->!  User   !<--->! File !   !System!    !! DTP  !!   Connections  !!   DTP   !!    !System!   --------    !\------/!                !\---------/!    --------               ----------                -------------               Server-FTP                   User-FTP   NOTES: 1. The data connection may be in either direction.          2. The data connection need not exist all of the time.                     Figure 1  Model for FTP Use                                   7                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   In the model described in Figure 1, the user-protocol interpreter   initiates the TELNET connections. At the initiation of the user,   standard FTP commands are generated by the user-PI and transmitted to   the server process via the TELNET connections.  (The user may   establish a direct TELNET connection to the server-FTP, from a TIP   terminal for example, and generate standard FTP commands himself,   by-passing the user-FTP process.) Standard replies are sent from the   server-PI to the user-PI over the TELNET connections in response to   the commands.   The FTP commands specify the parameters for the data connection (data   socket, byte size, 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 socket, and the server initiate the data connection   and data transfer in accordance with the specified parameters.  It   should be noted that the data socket need not be in the same Host   that initiates the FTP commands via the TELNET connections, but the   user or his user-FTP process must ensure a "listen" on the specified   data socket.  It should also be noted that two data connections, one   for send and the other for receive, may exist simultaneously.   In another situation a user might wish to transfer files between two   Hosts, neither of which is his local Host. He sets up TELNET   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 he server data   transfer processes.  Following is a model of this server-server   interaction.                 TELNET     ------------    TELNET                 -----------! User-FTP !------------                 ! -------->! User-PI  !<--------- !                 ! !        !   "C"    !         ! !                 V !        ------------         ! V         --------------                        --------------         ! Server-FTP !   Data Connection      ! Server-FTP !         !    "A"     !<-----------------------!    "B"     !         -------------- Socket(A)    Socket(B) --------------                             Figure 2                                   8                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   The protocol requires that the TELNET connections be open while data   transfer is in progress.  It is the responsibility of the user to   request the closing of the TELNET connections when finished using the   FTP service, while it is the server who takes the action.  The server   may abort data transfer if the TELNET connections are closed without   command.DATA TRANSFER FUNCTIONS   Files are transferred only via the data connection(s).  The TELNET   connection is used for the transfer of commands, which describe the   functions to be performed, and the replies to these commands (see the   Section on FTP Replies).  Several commands are concerned with the   transfer of data between Hosts.  These data transfer commands include   the BYTE, MODE, and SOCKet commands which specify how the bits of the   data are to be transmitted, and the STRUcture and TYPE commands,   which are used to define the way in which the data are to be   represented. The transmission and representation are basically   independent but "Stream" transmission mode is dependent on the file   structure attribute and if "Compressed" transmission mode is used the   nature of the filler byte depends on the representation type.   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 diffeent   systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII   characters, left-justified in a 36-bit word. 360's store NVT-ASCII as   8-bit EBCDIC codes. Multics stores NVT-ASCII as four 9-bit characters   in a 36-bit word.  It may be 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                                   9                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   performed by the user directly or via the use of the Data   Reconfiguration Sevice (DRS, RFC #138, NIC #6715).  Additonal   representation types may be defined later if there is a demonstrable   need.   Data representations are handled in FTP by a user specifying a   representation type.  This type may implicitly (as in ASCII or   EBCDIC) or explicitly (as in Local byte) define a byte size for   interpretation which is referred to as the "logical byte size."  This   has nothing to do with the byte size used for transmission over the   data connection(s) (called the "transfer byte size") and the two   should not be confused.  For example, NVT-ASCII has a logical byte   size of 8 bits but an ASCII file might be transferred using a   transfer byte size of 32.  If the type is Local byte, then the TYPE   command has an obligatory second parameter specifying the logical   byte size.   The types ASCII and EBCDIC also take a second (optional) parameter;   this is to indicate what kind of vertical format control, if any, is   associated with a file.  The following data representation types are   defined in FTP:      ASCII Format         This is the default type and must be accepted by all FTP         implementations.  It is intended primarily for the transfer of         text files, except when both Hosts would find the EBCDIC type         more convenient.         The sender converts the data from his internal character         representation to the standard 8-bit NVT-ASCII representation         (see the TELNET specification).  The receiver will convert the         data from the standard form to his own internal form.         In accordance with the NVT standard, the <CRLF> sequence should         be used, where necessary, to denote the end of a line of text.         (See the discussion of file structure at the end of the Section         on Data Representation and Storage).         Using the standard NVT-ASCII representation means that data         must be interpreted as 8-bit bytes.  If the BYTE command (see         the Section on Transfer Parameter Commands) specifies a         transfer byte size different from 8 bits, the 8-bit ASCII         characters should be packed contiguously without regard for         transfer byte boundaries.         The Format parameter for ASCII and EBCDIC types is discussed         below.                                   10                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      EBCDIC Format         This type is intended for efficient transfer between Hosts         which use EBCDIC for their internal character representation.         For transmission the data are represented as 8-bit EBCDIC         characters.  The character code is the only difference between         the functional specifications of EBCDIC and ASCII types.         End-of-line (as opposed to end-of-record--see the discussion of         structure) will probably be rarely used with EBCDIC type for         purposes of denoting structure, but where it is necessary the         <NL> character should be used.   A character file may be transferred to a Host for one of three   purposes: for printing, for storage and later retrieval, or for   processing.  If a file is sent for printing, the receiving Host must   know how the vertical format control is represented.  In the second   case, it must be possible to store a file at a Host and then retrieve   it later in exactly the same form.  Finally, it ought to be possible   to move a file from one Host to another and process the file at the   second Host without undue trouble.  A single ASCII or EBCDIC format   does not satisfy all these conditions and so these types have a   second parameter specifying one of the following three formats:      Non-print         This is the default format to be used if the second (format)         parameter is omitted.  Non-print format must be accepted by all         FTP implementations.         The file need contain no vertical format information.  If it is         passed to a printer process, this process may assume standard         values for spacing and margins.         Normally, this format will be used with files destined for         processing or just storage.      TELNET Format Controls         The file contains ASCII/EBCDIC vertical format controls (i.e.,         <CR>, <LF>, <NL>, <VT>, <FF>) which the printer process will         interpret appropriately.  <CRLF>, in exactly this sequence,         also denotes end-of-line.      Carriage Control (ASA)         The file contains ASA (FORTRAN) vertical format control         characters.  (See NWG/RFC #189Appendix C and Communications of                                   11                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         the ACM, Vol. 7, No. 10, 606 (Oct. 1964)).  In a line or a         record, formatted according to the ASA Standard, the first         character is not to be printed.  Instead it should be used to         determine the vertical movement of the paper which should take         place before the rest of the record is printed.  The ASA         Standard specifies the following control characters:            Character     Vertical Spacing            blank         Move paper up one line            0             Move paper up two lines            1             Move paper to top of next page            +             No movement, i.e., overprint         Clearly there must be some way for a printer process to         distinguish the end of the structural entity.  If a file has         record structure (see below) this is no problem; records will         be explicitly marked during transfer and storage.  If the file         has no record structure, the <CRLF> end-of-line sequence is         used to separate printing lines, but these format effectors are         overridden by the ASA controls.      Image         The data are sent as contiguous bits which, for transfer, are         packed into transfer bytes of the size specified in the BYTE         command.  The receiving site must store the data as contiguous         bits.  The structure of the storage system might necessitate         the padding of the file (or of each record, for a         record-structured file) to some convenient boundary (byte, word         or block).  This padding, which must be all zeroes, may occur         only at the end of the file (or at the end of each record) and         there must be a way of identifying the padding bits so that         they may be stripped off if the file is retrieved.  The padding         transformation should be well publicized to enable a user to         process a file at the storage site.         Image type is intended for the efficient storage and retrieval         of files and for the transfer of binary data.  It is         recommended that this type be accepted by all FTP         implementations.      Local byte Byte size         The data is transferred in logical bytes of the size specified         by the obligatory second parameter, Byte size.  The value of         Byte size must be a decimal integer; there is no default value.         The logical byte size is not necessarily the same as the         transfer byte size.  If there is a difference in byte sizes,                                   12                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         then the logical bytes should be packed contiguously,         disregarding transfer byte boundaries and with any necessary         padding at the end.         When the data reaches the receiving Host it will be transformed         in a manner dependent on the logical byte size and the         particular Host.  This transformation must be invertible (that         is an identical file can be retrieved if the same parameters         are used) and should be well publicized by the FTP         implementors.         This type is intended for the transfer of structured data.  For         example, a user sending 36-bit floating-point numbers to a Host         with a 32-bit word could send his data as Local byte with a         logical byte size of 36.  The receiving Host would then be         expected to store the logical bytes so that they could be         easily manipulated; in this example putting the 36-bit logical         bytes into 64-bit double words should suffice.   A note of caution about parameters:  a file must be stored and   retrieved with the same parameters if the retrieved version is to be   identical to the version originally transmitted.  Conversely, FTP   implementations must return a file identical to the original if the   parameters used to store and retrieve a file are the same.   In addition to different representation types, FTP allows the   structure of a file to be specified.  Currently two file structures   are recognized in FTP: file-structure, where there is no internal   structure, and record-structure, where the file is made up of   records.  File-structure is the default, to be assumed if the   STRUcture command has not been used but both structures must be   accepted for "text" files (i.e., files with TYPE ASCII or EBCDIC) by   all FTP implementations.  The structure of a file will affect both   the transfer mode of a file (see the Section on Transmission Modes)   and the interpretation and storage of the file.   The "natural" structure of a file will depend on which Host stores   the file.  A source-code file will usually be stored on an IBM 360 in   fixed length records but on a PDP-10 as a stream of characters   partitioned into lines, for example by <CRLF>.  If the transfer of   files between such disparate sites is to be useful, there must be   some way for one site to recognize the other's assumptions about the   file.   With some sites being naturally file-oriented and others naturally   record-oriented there may be problems if a file with one structure is   sent to a Host oriented to the other.  If a text file is sent with   record-structure to a Host which is file oriented, then that Host   should apply an internal transformation to the file based on the                                   13                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   record structure.  Obviously this transformation should be useful but   it must also be invertible so that an identical file may be   retreieved using record structure.   In the case of a file being sent with file-structure to a   record-oriented Host, there exists the question of what criteria the   Host should use to divide the file into records which can be   processed locally.  If this division is necessary the FTP   implementation should use the end-of-line sequence, <CRLF> for ASCII,   or <NL> for EBCDIC text files, as the delimiter.  If an FTP   implementation adopts this technique, it must be prepared to reverse   the transformation if the file is retrieved with file-structure.   ESTABLISHING DATA CONNECTIONS   The mechanics of transferring data consists of setting up the data   connection to the appropriate sockets and choosing the parameters for   transfer--byte size and mode. Both the user and the server-DTPs have   default data sockets; these are the two sockets (for send and   receive) immediately following the standard ICP TELNET socket ,i.e.,   (U+4) and (U+5) for the user-process and (S+2), (S+3) for the server.   The use of default sockets will ensure the security of the data   transfer, without requiring the socket information to be explicitly   exchanged.   The byte size for the data connection is specified by the BYTE   command, or, if left unspecified, defaults to 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 protocol does not require servers to accept all possible byte   sizes. Since the use of various byte sizes is intended for efficiency   of transfer, servers may implement only those sizes for which their   data transfer is efficient including the default byte size of 8 bits.   The passive data transfer process (this may be a user-DTP or a second   server-DTP) shall "listen" on the data socket prior to sending a   transfer request command.  The FTP request command determines the   direction of the data transfer and thus which data socket (odd or   even) is to be used in establishing the connection.  The server, upon   receiving the transfer request, will initiate the data connection by   RFC to the appropriate socket using the specified (or default) byte   size.  When the connection is opened, the data transfer begins   between DTP's, and the server-PI sends a confirming reply to the   user-PI.   It is possible for the user to specify an alternate data socket by   use of the SOCK command.  He might want a file dumped on a TIP line   printer or retrieved from a third party Host.  In the latter case the   user-PI sets up TELNET connections with both server-PI's and sends                                   14                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   each a SOCK command indicating the fixed data sockets of the other.   One server is then told (by an FTP command) to "listen" for an RFC   which the other will initiate and 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 the RFC's and the closes.  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 socket or byte size specification is changed by a command         from the user.      4. The TELNET connections are closed legally or otherwise.      5. An irrecoverable error condition occurs.   Otherwise the close is a server option, the exercise of which he must   indicate to the user-process by an appropriate reply.   TRANSMISSION MODES   The next consideration in transferring data is choosing the   appropriate transmission mode.  There are three modes: one which   formats the data and allows for restart procedures; one which also   compresses the data for efficient transfer; and one which passes the   data with little or no processing.  In this last case the mode   interacts with the structure attribute to determine the type of   processing.  In the compressed mode the representation type   determines the filler byte.   All data transfers must be completed with an end-of-file (EOF) which   may be explicitly stated or implied by the closing of the data   connection.  For files with record structure, all the end-of-record   markers (EOR) are explicit, including the final one.   Note:  In the rest of this section, byte means "transfer byte" except   where explicitly stated otherwise.                                   15                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   The following transmission modes are defined in FTP:      Stream         The data is transmitted as a stream of bytes.  There is no         restriction on the representation type used; record structures         are allowed, in which case the transfer byte size must be at         least 3 bits!         In a record structured file EOR and EOF will each be indicated         by a two-byte control code of whatever byte size is used for         the transfer.  The first byte of the control code will be all         ones, the escape character.  The second byte will have the low         order bit on and zeroes elsewhere for EOR and the second low         order bit on for EOF; that is, the byte will have value 1 for         EOR and value 2 for EOF.  EOR and EOF may be indicated together         on the last byte transmitted by turning both low order bits on,         i.e., the value 3.  If a byte of all ones was intended to be         sent as data, it should be repeated in the second byte of the         control code.         If the file does not have record structure, the EOF is         indicated by the sending Host closing the data connection and         all bytes are data bytes.   For the purpose of standardized transfer, the sending Host will   translate his internal end of line or end of record denotation into   the representation prescribed by the transfer mode and file   structure, and the receiving Host will perform the inverse   translation to his internal denotation.  An IBM 360 record count   field may not be recognized at another Host, so the end of record   information may be transferred as a two byte control code in Stream   mode or as a flagged bit in a Block or Compressed mode descriptor.   End of line in an ASCII or EBCDIC file with no record structure   should be indicated by <CRLF> or <NL>, respectively.  Since these   transformations imply extra work for some systems, identical systems   transferring non-record structured text files might wish to use a   binary representation and stream mode for the transfer.      Block         The file is transmitted as a series of data blocks preceded by         one or more header bytes.  The header bytes contain a count         field, and descriptor code.  The count field indicates the         total length of the data block in bytes, thus marking the         beginning of the next data block (there are no filler bits).         The descriptor code defines:  last block in the file (EOF) last         block in the record (EOR), restart marker (see the Section on         Error Recovery and Restart) or suspect data (i.e., the data                                   16                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         being transferred is suspected of errors and is not reliable).         This last code is NOT intended for error control within FTP.         It is motivated by the desire of sites exchanging certain types         of data (e.g., seismic or weather data) to send and receive all         the data despite local errors (such as "magnetic tape read         errors"), but to indicate in the transmission that certain         portions are suspect).  Record structures are allowed in this         mode, and any representation type may be used.  There is no         restriction on the transfer byte size.         The header consists of the smallest integral number of bytes         whose length is greater than or equal to 24 bits.  Only the         LEAST significant 24 bits (right-justified) of header shall         have information; the remaining most significant bits are         "don't care" bits.  Of the 24 bits of header information, the         16 low order bits shall represent byte count, and the 8 high         order bits shall represent descriptor codes as shown below.         Integral number of bytes greater than or equal to 24 bits         --------------------------------------------------------         !    Don't care    !    Descriptor    !    Byte Count  !         !  0 to 231 bits   !      8 bits      !     16 bits    !         --------------------------------------------------------         The descriptor codes are indicated by bit flags in the         descriptor byte.  Four codes have been assigned, where each         code number is the decimal value of the corresponding bit in         the byte.            Code     Meaning             128     End of data block is EOR              64     End of data block is EOF              32     Suspected errors in data block              16     Data block is a restart marker         With this encoding more than one descriptor coded condition may         exist for a particular block.  As many bits as necessary may be         flagged.         The restart marker is embedded in the data stream as an         integral number of 8-bit bytes representing printable         characters in the language being used over the TELNET         connection (e.g., default--NVT-ASCII).  These marker bytes are         right-justified in the smallest integral number of transfer         bytes greater than or equal to 8 bits.  For example, if the                                   17                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         byte size is 7 bits, the restart marker byte would be one byte         right-justified per two 7-bit bytes as shown below:                 Two 7-bit bytes            -------------------------            !        !  Marker Char !            !        !     8 bits   !            -------------------------         If the transfer byte size is 16 or more bits, the maximum         possible number of complete marker bytes should be packed,         right-justified, into each transfer byte.  The restart marker         should begin in the first marker byte.  If there are any unused         marker bytes, these should be filled with the character <SP>         (Space, in the appropriate language).  <SP> must not be used         WITHIN a restart marker.  For example, to transmit a         six-character marker with a 36-bit transfer byte size, the         following three 36-bit bytes would be sent:            ------------------------------------------            ! Don't care !Descriptor! Byte count = 2 !            !   12 bits  ! code = 16!                !            ------------------------------------------            ------------------------------------------            !    ! Marker ! Marker ! Marker ! Marker !            !    ! 8 bits ! 8 bits ! 8 bits ! 8 bits !            ------------------------------------------            ------------------------------------------            !    ! Marker ! Marker ! Space  ! Space  !            !    ! 8 bits ! 8 bits ! 8 bits ! 8 bits !            ------------------------------------------      Compressed         The file is transmitted as series of bytes of the size         specified by the BYTE command.  There are three kinds of         information to be sent:  regular data, sent in a byte string;         compressed data, consisting of replications or filler; and         control information, sent in a two-byte escape sequence.  If         the byte size is B bits and n>0 bytes of regular data are sent,         these n bytes are preceded by a byte with the left-most bit set         to 0 and the right-most B-1 bits containing the number n.                                   18                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759                        1  B-1    B        B                       -------  ------   ------         Byte string:  !0! n !  !d(1)!...!d(n)!                       -------  ------   ------                                ^             ^                                !---n bytes---!                                    of data            String of n data bytes d(1),..., d(n)            Count n must be positive         To compress a string of n replications of the data byte d, the         following 2 bytes are sent:                              2     B-2       B                           ---------------  ------         Replicated Byte:  ! 1 0 !   n   !  ! d  !                           ---------------  ------         A string of n filler bytes can be compressed into a single         byte, where the filler byte varies with the representation         type.  If the type is ASCII or EBCDIC the filler byte is <SP>         (Space, ASCII code 32., EBCDIC code 64).  If the transfer byte         size is not 8, the expanded byte string should be filled with         8-bit <SP> characters in the manner described in the definition         of ASCII representation type (see the Section on Data         Representation and Storage).  If the type is Image or Local         byte the filler is a zero byte.                            2     B-2                         ---------------         Filler String:  ! 1 1 !   n   !                         ---------------         The escape sequence is a double byte, the first of which is the         escape byte (all zeroes) and the second of which contains         descriptor codes as defined in Block mode.  This implies that         the byte size must be at least 8 bits, which is not much of a         restriction for efficiency in this mode.  The descriptor codes         have the same meaning as in Block mode and apply to the         succeeding string of bytes.         Compressed mode is useful for obtaining increased bandwidth on         very large network transmissions at a little extra CPU cost.         It is most efficient when the byte size chosen is that of the         word size of the transmitting Host, and can be most effectively         used to reduce the size of printer files such as those         generated by RJE Hosts.                                   19                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   ERROR RECOVERY AND RESTART   There is no provision for detecting bits lost or scrambled in data   transfer.  This issue is perhaps handled best at the NCP level where   it benefits most users.  However, a restart procedure is provided to   protect users from gross system failures (including failures of a   Host, an FTP-process, or the IMP subnet).   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 TELNET connection.  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 recieving 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 TELNET connection in a 251 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 restrart command is transmitted over the TELNET connection and is   immediately followed by the command (such as RETR, STOR or LIST)   which was being executed when the system failure occurred.FILE TRANSFER FUNCTIONS   The communication channel from the user-PI to the server-PI is   established by ICP from the user to a standard server socket.  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.                                   20                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   FTP COMMANDS   The File Transfer Protocol follows the specifications of the TELNET   protocol for all communications over the TELNET connection - see NIC   #7104.  Since, in the future, the language used for TELNET   communication may be a negotiated option, all references in the next   two sections will be to the "TELNET language" and the corresponding   "TELNET end of line code".  Currently one may take these to mean   NVT-ASCII and <CRLF>.  No other specifications of the TELNET protocol   will be cited.   FTP commands are "TELNET strings" terminated by the "TELNET end of   line code".  The command codes themselves are alphabetic characters   terminated by the character <SP> (Space) if parameters follow and   TELNET-EOL otherwise.  The command codes and the semantics of   commands are described in this section; the detailed syntax of   commands is specified in the Section on Commands, the reply sequences   are discussed in the Section on Sequencing of Commands and Replies,   and scenarios illustrating the use of commands are provided in the   Section on Typical FTP Scenarios.   FTP commands may be partitioned as those specifying access-control   identifiers, data transfer parameters, or FTP service requests.   Certain commands (such as ABOR, STAT, BYE) may be sent over the   TELNET connections while a data transfer is in progress.  Some   servers may not be able to monitor the TELNET and data connections   simultaneously, in which case some special action will be necessary   to get the server's attention.  The exact form of the "special   action" is related to decisions currently under review by the TELNET   committee; but the following ordered format is tentatively   recommended:      1. User system inserts the TELNET "Interrupt Process" (IP) signal         in the TELNET stream.      2. User system sends the TELNET "Synch" signal      3. User system inserts the command (e.g., ABOR) in the TELNET         stream.      4. Server PI,, after receiving "IP", scans the TELNET stream for         EXACTLY ONE FTP command.   (For other servers this may not be necessary but the actions listed   above should have no unusual effect.)                                   21                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   ACCESS CONTROL COMMANDS   The following commands specify access control identifiers (command   codes are shown in parentheses).      USER NAME (USER)         The argument field is a TELNET string identifying the user.         The user identification is that which is required by the server         for access to its file system.  This command will normally be         the first command transmitted by the user after the TELNET         connections are made (some servers may require this).         Additional identification information in the form of a password         and/or an account command may also be required by some servers.         Servers may allow a new USER command to be entered at any point         in order to change the access control and/or accounting         information.  This has the effect of flushing any user,         password, and account information already supplied and         beginning the login sequence again.  All transfer parameters         are unchanged and any file transfer in progress is completed         under the old acccount.      PASSWORD (PASS)         The argument field is a TELNET string identifying the user's         password.  This command must be immediately preceded by the         user name command, and, for some sites, completes the user's         identification for access control.  Since password information         is quite sensitive, it is desirable in general to "mask" it or         suppress typeout.  It appears that the server has no foolproof         way to achieve this.  It is therefore the responsibility of the         user-FTP process to hide the sensitive password information.      ACCOUNT (ACCT)         The argument field is a TELNET string identifying the user's         account.  The command is not necessarily related to the USER         command, as some sites may require an account for login and         others only for specific access, such as storing files.  In the         latter case the command may arrive at any time.  There are two         reply codes to differentiate these cases for the automaton:         when account information is required for login, the response to         a successful PASSword command is reply code 331; then if a         command other than ACCounT is sent, the server may remember it         and return a 331 reply, prepared to act on the command after         the account information is received; or he may flush the         command and return a 433 reply asking for the account.  On the         other hand, if account information is NOT required for login,         the reply to a successful PASSword command is 230; and if the                                   22                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         information is needed for a command issued later in the         dialogue, the server should return a 331 or 433 reply depending         on whether he stores (pending receipt of the ACCounT command)         or discards the command, respectively.      REINITIALIZE (REIN)         This command terminates a USER, flushing all I/O and account         information, except to allow any transfer in progress to be         completed.  All parameters are reset to the default settings         and the TELNET connection is left open.  This is identical to         the state in which a user finds himself immediately after the         ICP is completed and the TELNET connections are opened.  A USER         command may be expected to follow.      LOGOUT (BYE)         This command terminates a USER and if file transfer is not in         progress, the server closes the TELNET connection.  If file         transfer is in progress, the connection will remain open for         result response and the server will then close it.  If the         user-process is transferring files for several USERs but does         not wish to close and then reopen connections for each, then         the REIN command should be used instead of BYE.         An unexpected close on the TELNET connection will cause the         server to take the effective action of an abort (ABOR) and a         logout (BYE).   TRANSFER PARAMETER COMMANDS   All data transfer parameters have default values, and the commands   specifying data transfer parameters are required only if the default   parameter values are to be changed.  The default value is the last   specified value, or if no value has been specified, the standard   default value as stated here.  This implies that the server must   "remember" the applicable default values.  The commands may be in any   order except that they must precede the FTP service request.  The   following commands specify data transfer parameters.      BYTE SIZE (BYTE)         The argument is a decimal integer (1 through 255) specifying         the byte size for the data connection.  The default byte size         is 8 bits.  A server may reject certain byte sizes that he has         not implemented.                                   23                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      DATA SOCKET (SOCK)         The argument is a HOST-SOCKET specification for the data socket         to be used in data connection.  There may be two data sockets,         one for transfer from the "active" DTP to the "passive" DTP and         one for "passive" to "active".  An odd socket number defines a         send socket and an even socket number defines a receive socket.         The default HOST is the user Host to which TELNET connections         are made.  The default data sockets are (U+4) and (U+5) where U         is the socket number used in the TELNET ICP and the TELNET         connections are on sockets (U+2) and (U+3).  The server has         fixed data sockets (S+2) and (S+3) as well, and under normal         circimstances this command and its reply are not needed.      PASSIVE (PASV)         This command requests the server-DTP to "listen" on both of his         data sockets and to wait for an RFC to arrive for one socket         rather than initiate one upon receipt of a transfer command.         It is assumed the server has already received a SOCK command to         indicate the foreign socket from which the RFC will arrive to         ensure the security of the transfer.      REPRESENTATION TYPE (TYPE)         The argument specifies the representation type as described in         the Section on Data Representation and Storage.  Several types         take a second parameter.  The first parameter is denoted by a         single TELNET character, as is the second Format parameter for         ASCII and EBCDIC; the second parameter for local byte is a         decimal integer to indicate Bytesize.  The parameters are         separated by a <SP> (Space, ASCII code 32.).  The following         codes are assigned for type:                      \    /            A - ASCII !    ! N - Non-print                      !-><-! T - TELNET format effectors            E - EBCDIC!    ! C - Carriage Control (ASA)                      /    \            I - Image            L # - Local byte Bytesize         The default representation type is ASCII Non-print.  If the         Format parameter is changed, and later just the first argument         is changed, Format then returns to the Non-print default.                                   24                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      FILE STRUCTURE (STRU)         The argument is a single TELNET character code specifying file         structure described in the Section on Data Representation and         Storage.  The following codes are assigned for structure:            F - File (no record structure)            R - Record structure         The default structure is File  (i.e., no records).      TRANSFER MODE (MODE)         The argument is a single TELNET character code specifying the         data transfer modes described in the Section on Transmission         Modes.  The following codes are assigned for transfer modes:            S - Stream            B - Block            C - Compressed         The default transfer mode is Stream.   FTP SERVICE COMMANDS   The FTP service commands define the file transfer or the file system   function requested by the user.  The argument of an FTP service   command will normally be a pathname.  The syntax of pathnames must   conform to server site conventions (with standard defaults   applicable), and the language conventions of the TELNET connection.   The suggested default handling is to use the last specified device,   directory or file name, or the standard default defined for local   users.  The commands may be in any order except that a "rename from"   command must be followed by a "rename to" command and the restart   command must be followed by the interrupted service command.  The   data, when transferred in response to FTP service commands, shall   always be sent over the data connection, except for certain   informative replies.  The following commands specify FTP service   requests:      RETRIEVE (RETR)         This command causes the server-DTP to transfer a copy of the         file, specified in the pathname, to the server- or user-DTP at         the other end of the data connection.  The status and contents         of the file at the server site shall be unaffected.                                   25                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      STORE (STOR)         This command causes the server-DTP to accept the data         transferred via the data connection and to store the data as a         file at the server site.  If the file specified in the pathname         exists at the server site then its contents shall be replaced         by the data being transferred.  A new file is created at the         server site if the file specified in the pathname does not         already exist.      APPEND (with create) (APPE)         This command causes the server-DTP to accept the data         transferred via the data connection and to store the data in a         file at the server site.  If the file specified in the pathname         exists at the server site, then the data shall be appended to         that file; otherwise the file specified in the pathname shall         be created at the server site.      ALLOCATE (ALLO)         This command may be required by some servers to reserve         sufficient storage to accommodate the new file to be         transferred.  The argument shall be a decimal integer         representing the number of bytes (using the logical byte size)         of storage to be reserved for the file.  For files sent with         record structure a maximum record size (in logical bytes) might         also be necessary; this is indicated by a decimal integer in a         second argument field of the command.  This second argument is         optional, but when present should be separated from the first         by the three TELNET characters <SP> R <SP>.  This command shall         be followed by a STORe or APPEnd command.  The ALLO command         should be treated as a NOOP (no operation) by those servers         which do not require that the maximum size of the file be         declared beforehand, and those servers interested in only the         maximum record size should accept a dummy value in the first         argument and ignore it.      RESTART (REST)         The argument field represents the server marker at which file         transfer is to be restarted.  This command does not cause file         transfer but "spaces" over the file to the specified data         checkpoint.  This command shall be immediately followed by the         appropriate FTP service command which shall cause file transfer         to resume.                                   26                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      RENAME FROM (RNFR)         This command specifies the file which is to be renamed.  This         command must be immediately followed by a "rename to" command         specifying the new file pathname.      RENAME TO (RNTO)         This command specifies the new pathname of the file specified         in the immediately preceding "rename from" command.  Together         the two commands cause a file to be renamed.      ABORT (ABOR)         This command indicates to the server to abort the previous FTP         service command and any associated transfer of data.  The abort         command may require "special action", as discussed in the         Section on FTP Commands, to force recognition by the server.         No action is to be taken if the previous command has been         completed (including data transfer).  The TELNET connections         are not to be closed by the server, but the data connection         must be closed.  An appropriate reply should be sent by the         server in all cases.      DELETE (DELE)         This command causes the file specified in the pathname to be         deleted at the server site.  If an extra level of protection is         desired (such as the query, "DO you really wish to delete?"),         it should be provided by the user-FTP process.      LIST (LIST)         This command causes a list to be sent from the server to the         passive DTP.  If the pathname specifies a directory, the server         should transfer a list of files in the specified directory.  If         the pathname specifies a file then the server should send         current information on the file.  A null argument implies the         user's current working or default directory.  The data transfer         is over the data connection in type ASCII or type EBCDIC.  (The         user must ensure that the TYPE is appropriately ASCII or         EBCDIC).      NAME-LIST (NLST)         This command causes a directory listing to be sent from server         to user site.  The pathname should specify a directory or other         system-specific file group descriptor; a null argument implies         the current directory.  The server will return a stream of                                   27                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         names of files and no other information.  The data will be         transferred in ASCII or EBCDIC type over the data connection as         valid pathname strings separated by <CRLF> or <NL>.  (Again the         user must ensure that the TYPE is correct.)      SITE PARAMETERS (SITE)         This command is used by the server to provide services specific         to his system that are essential to file transfer but not         sufficiently universal to be included as commands in the         protocol.  The nature of these services and the specification         of their syntax can be stated in a reply to the HELP SITE         command.      STATUS (STAT)         This command shall cause a status response to be sent over the         TELNET connection in the form of a reply.  The command may be         sent during a file transfer (along with the TELNET IP and Synch         signals--see the Section on FTP Commands) in which case the         server will respond with the status of the operation in         progress, or it may be sent between file transfers.  In the         latter case the command may have an argument field.  If the         argument is a pathname, the command is analogous to the "list"         command except that data shall be trasferred over the TELNET         connection.  If a partial pathname is given, the server may         respond with a list of file names or attributes associated with         that specification.  If no argument is given, the server should         return general status information about the server FTP process.         This should include current values of all transfer parameters         and the status of connections.      HELP (HELP)         This command shall cause the server to send helpful information         regarding its implementation status over the TELNET connection         to the user.  The command may take an argument (e.g., any         command name) and return more specific information as a         response.  The reply is type Oxx, general system status.  It is         suggested that HELP be allowed before entering a USER command.         The server may use this reply to specify site-dependent         parameters, e.g., in response to HELP SITE.      NOOP (NOOP)         This command does not affect any parameters or previously         entered commands. It specifies no action other than that the         server send a 200 reply.                                   28                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   MISCELLANEOUS COMMANDS   There are several functions that utilize the services of file   transfer but go beyond it in scope.  These are the Mail and Remote   Job Entry functions.  It is suggested that these become auxiliary   protocols that can assume recognition of file transfer commands on   the part of the server, i.e., they may depend on the core of FTP   commands.  The command sets specific to Mail and RJE will be given in   separate documents.   Commands that are closely related to file transfer but not proven   essential to the protocol may be implemented by servers on an   experimental basis.  The command name should begin with an X and may   be listed in the HELP command.  The official command set is   expandable from these experiments; all experimental commands or   proposals for expanding the official command set should be announced   via RFC.  An example of a current experimental command is:      Change Working Directory (XCWD)         This command allows the user to work with a different directory         or dataset for file storage or retrieval without altering his         login or accounting information.  Transfer parameters are         similarly unchanged.  The argument is a pathname specifying a         directory or other system dependent file group designator.   FTP REPLIES   The server sends FTP replies over the TELNET connection in response   to user FTP commands.  The FTP replies constitute the acknowledgment   or completion code (including errors).  The FTP-server replies are   formatted for human or program interpretation.  Single line replies   consist of a leading three-digit numeric code followed by a space,   followed by a one-line text explanation of the code.  For replies   that contain several lines of text, the first line will have a   leading three-digit numeric code followed immediately by the   character "-" (Hyphen, ASCII code 45), and possibly some text.  All   succeeding continuation lines except the last are constrained NOT to   begin with three digits; the last line must repeat the numeric code   of the first line and be followed immediately by a space.  For   example:      100-First Line      Continuation Line      Another Line      100 Last Line   It is possible to nest (but not overlap) a reply withiin a multi-line                                   29                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   reply.  The same format for matched number-coded first and last lines   holds.   The numeric codes are assigned by groups and for ease of   interpretation by programs in a manner consistent with other   protocols such as the RJE protocol.  The three digits of the code are   to be interpreted as follows:      1. The first digit specifies type of response as indicated below:         0xx  These replies are purely informative and constitute              neither a positive nor a negative acknowledgment.         1xx  Informative replies to status inquiries.  These constitute              a positive acknowledgment to the status command.         2xx  Positive acknowledgment of previous command or other              successful action.         3xx  Incomplete information.  Activity cannot proceed without              further specification and input.         4xx  Unsuccessful reply.  The request is correctly specified              but the server is unsuccessful in correctly fulfilling it.         5xx  Incorrect or illegal command.  The command or its              parameters were invalid or incomplete from a syntactic              viewpoint, or the command is inconsistent with a previous              command.  The command in question has been completely              ignored.         6xx-9xx  Reserved for future expansion.      2. The second digit specifies the general category to which the         response refers:         x00-x29  General purpose replies, not assignable to other              categories.         x3x  Primary access.  Informative replies to the "log-on"              attempt.         x4x  Secondary access.  The primary server is commenting on its              ability to access a secondary service.         x5x  FTP results.         x6x  RJE results.                                   30                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759         x7x  Mail Portocol results.         x8x-x9x  Reserved for future expansion.      3. The final digit specifies a particular message type.  Since the         code is designed for an automaton process to interpret, it is         not necessary for every variation of a reply to have a unique         number.  Only the basic meaning of replies need have unique         numbers.  The text of a reply can explain the specific reason         for that reply to a human user.   Each TELNET line delimited by a numeric code and the TELNET EOL (or   group of text lines bounded by coded lines) that is sent by the   server is intended to be a complete reply message. It should be noted   that the text of replies is intended for a human user. Only the reply   codes and in some instances the first line of text are intended for   programs.   The assigned reply codes relating to FTP are:   000  Announcing FTP.   010  Message from system operator.   020  Exected delay.   030  Server availability information.   050  FTP commentary or user information.   100  System status reply.   110  System busy doing...   150  File status reply.   151  Directory listing reply.   200  Last command received correctly.   201  An ABORT has terminated activity, as requested.   202  Abort request ignored, no activity in progress.   230  User is "logged in".  May proceed.   231  User is "logged out".  Service terminated.   232  Logout command noted, will complete when transfer done.   233  User is "logged out".  Parameters reinitialized.   250  FTP file transfer started correctly.   251  FTP Restart-marker reply.      Text is:  MARK yyyy = mmmm         where 'yyyy' is user's data stream marker (yours)         and mmmm is server's equivalent marker (mine)      (Note the spaces between the markers and '=').   252  FTP transfer completed correctly.   253  Rename completed.   254  Delete completed.   257  Closing the data connection, transfer completed.   300  Connection greeting message, awaiting input.   301  Current command incomplete (no <CRLF> for long time).   330  Enter password.                                   31                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   331  Enter account (if account required as part of login sequence).   332  Login first, please.   400  This service not implemented.   401  This service not accepting users now, goodbye.   402  Command not implemented for requested value or action.   430  Log-on time or tries exceeded, goodbye.   431  Log-on unsuccessful.  User and/or password invalid.   432  User not valid for this service.   433  Cannot transfer files without valid account.  Enter account and        resend command.   434  Log-out forced by operator action.  Phone site.   435  Log-out forced by system problem.   436  Service shutting down, goodbye.   450  FTP:  File not found.   451  FTP:  File access denied to you.   452  FTP:  File transfer incomplete, data connection closed.   453  FTP:  File transfer incomplete, insufficient storage space.   454  FTP:  Cannot connect to your data socket.   455  FTP:  File system error not covered by other reply codes.   456  FTP:  Name duplication; rename failed.   457  FTP:  Transfer parameters in error.   500  Last command line completely unrecognized.   501  Syntax of last command is incorrect.   502  Last command incomplete, parameters missing.   503  Last command invalid (ignored), illegal parameter combination.   504  Last command invalid, action not possible at this time.   505  Last command conflicts illegally with previous command(s).   506  Last command not implemented by the server.   507  Catchall error reply.   550  Bad pathname specification (e.g., syntax error).                                   32                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759DECLARATIVE SPECIFICATIONS   MINIMUM IMPLEMENTATION   In order to make FTP workable without needless error messages, the   following minimum implementation is required for servers:      TYPE - ASCII Non-print      MODE - Stream      STRUCTURE - File                  Record      BYTE - 8      COMMANDS - USER, BYE, SOCK,                 TYPE, BYTE, MODE, STRU,                   for the default values                 RETR, STOR,                 NOOP.   The initial default values for transfer parameters are:      TYPE - ASCII Non-print      BYTE - 8      MODE - Stream      STRU - File   All Hosts must accept the above as the standard defaults.   CONNECTIONS   The server protocol interpreter shall "listen" on Socket 3.  The user   or user protocol interpreter shall initiate the full-duplex TELNET   connections performing the ARPANET standard initial connection   protocol (ICP) to server Socket 3.  Server- and user- processes   should follow the conventions of the TELNET protocol as specified in   NIC #7104.  Servers are under no obligation to provide for editing of   command lines and may specify that it be done in the user Host.  The   TELNET connections 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 sockets (send and/or   receive); these may be the default user sockets (U+4) and (U+5) or a   socket specified in the SOCK command.  The server shall initiate the   data connection from his own fixed sockets (S+2) and (S+3) using the   specified user data socket and byte size (default - 8 bits).  The                                   33                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   direction of the transfer and the sockets used will be determined by   the FTP service command.   When data is to be transferred between two servers, A and B (refer to   Figure 2), the user-PI, C, sets up TELNET connections with both   server-PI's.  He then sends A's fixed sockets, S(A), to B in a SOCK   command and B's to A; replies are returned.  One of the servers, say   A, is then sent a PASV command telling him to "listen" on his data   sockets rather than initiate an RFC when he receives a transfer   service command.  When the user-PI receives an acknowledgment to the   PASV command, he may send (in either order) the corresponding service   commands to A and B.  Server B initiates the RFC 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 : ICP                        C->B : ICP      C->A : SOCK HOST-B, SKT-S(B)      C->B : SOCK HOST-A, SKT-S(A)      A->C : 200 Okay                   B->C : 200 Okay      C->A : PASV      A->C : 200 Okay      C->A : STOR                       C->B : RETR   The data connection shall be closed by the server under the   conditions described in the Section on Establishing Data Connections.   If the server wishes to close the connection after a transfer where   it is not required, he should do so immediately after the file   transfer is completed.  He should not wait until after a new transfer   command is received because the user-process will have already tested   the data connection to see if it needs to do a "listen"; (recall that   the user must "listen" on a closed data socket BEFORE sending the   transfer request).  To prevent a race condition here, the server   sends a secondary reply (257) after closing the data connection (or   if the connection is left open, a "file transfer completed" reply   (252) and the user-PI should wait for one of these replies before   issuing a new transfer command.   COMMANDS   The commands are TELNET character string transmitted over the TELNET   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.                                   34                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   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 <CRLF> (Carriage Return, Linefeed)   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.   The following are all the currently defined FTP commmands:      USER <SP> <username> <CRLF>      PASS <SP> <password> <CRLF>      ACCT <SP> <acctno> <CRLF>      REIN <CRLF>      BYE <CRLF>      BYTE <SP> <byte size> <CRLF>      SOCK <SP> <Host-socket> <CRLF>      PASV <CRLF>      TYPE <SP> <type code> <CRLF>      STRU <SP> <structure code> <CRLF>      MODE <SP> <mode code> <CRLF>      RETR <SP> <pathname> <CRLF>      STOR <SP> <pathname> <CRLF>      APPE <SP> <pathname> <CRLF>      ALLO <SP> <decimal integer> [<SP> R <SP> <decimal integer>] <CRLF>      REST <SP> <marker> <CRLF>      RNFR <SP> <pathname> <CRLF>      RNTO <SP> <pathname> <CRLF>      ABOR <CRLF>      DELE <SP> <pathname> <CRLF>      LIST [<SP> <pathname>] <CRLF>      NLST  [<SP> <pathname>] <CRLF>      SITE <SP> <string> <CRLF>      STAT [<SP> <pathname>] <CRLF>      HELP [<SP> <string>] <CRLF>                                   35                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      NOOP <CRLF>   The syntax of the above argument fields (using BNF notation where   applicable ) is:      <username> ::= <string>      <password> ::= <string>      <acctno> ::= <string>      <string> ::= <char>|<char><string>      <char> ::= any of the 128 ASCII characters except <CR> and <LF>      <marker> ::= <pr string>      <pr string> ::= <pr char>|<pr char><pr string>      <pr char> ::= any ASCII code 33. through 126., printable         characters      <byte size> ::= any decimal integer 1 through 255      <Host-socket> ::= <socket>|<Host number>, <socket>      <Host-number> ::= a decimal integer specifying an ARPANET Host.      <socket> ::= decimal integer between 0 and (2**32)-1      <form code> ::= N|T|C      <type code> ::= A[<SP> <form code>]|E [SP> <form code>]|I|      L <SP> <byte size>      <structure code> ::= F|R      <mode code> ::= S|B|C      <pathname> ::= <string>   SEQUENCING OF COMMANDS AND REPLIES   The communication between the user and server is intended to be an   alternating dialogue.  As such, the user issues an FTP command and   the server responds with a prompt primary reply.  The user should   wait for this initial primary success or failure response before   sending further commands.   Certain commands require a second reply for which the user should   also wait.  These replies may, for example, report on the progress or   completion of file transfer or the closing of the data connection.   They are secondary replies to file transfer commands.   The third class of replies are informational and spontaneous replies   which may arrive at any time.  The user-PI should be prepared to   receive them.  These replies are listed below as sponteneous.   One important group of spontaneous replies is the connection   greetings.  Under normal circumstances, a server will send a 300   reply, "awaiting input", when the ICP is completed.  The user should   wait for this greeting message before sending any commands.  If the   server is unable to accept input right away, he should send a 000   "announcing FTP" or a 020 "expected delay" reply immediately and a                                   36                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   300 reply when ready.  The user will then know not to hang up if   there is a delay.   The table below lists alternative success and failure replies for   each command.  These must be strictly adhered to; a server may   substitute text in the replies, but the meaning and action implied by   the code numbers and by the specific command reply sequence cannot be   altered.                   COMMAND-REPLY CORRESPONDENCE TABLE   COMMAND             SUCCESS         FAILURE   USER                230,330         430-432,500-505,507   PASS                230,330         430-432,500-507   ACCT                230             430-432,500-507   REIN                232,233         401,436,500-507      Secondary Reply  300   BYE                 231,232         500-505,507   BYTE                200,331         402,500-505,507   SOCK                200,331         500-505,507   PASV                200,331         500-507   TYPE                200,331         402,500-505,507   STRU                200,331         500-505,507   MODE                200,331         402,500-505,507   RETR                250             402,433,450,451,454,455,457,                                         500-505,507,550      Secondary Reply  252,257         452   STOR                250             402,433,451,454,455,457,                                         500-505,507,550      Secondary Reply  252,257         452,453   APPE                250             402,433,451,454,455,457,500-507,                                         550      Secondary Reply  252,257         452,453   ALLO                200,331         402,500-507   REST                200,331         500-507   RNFR                200             402,433,450,451,455,500-507,550   RNTO                253             402,433,450,451,455,456,500-507,                                         550   ABOR                201,202,331     500-507   DELE                254             402,433,450,451,455,500-507,550   LIST                250             402,433,450,451,454,455,457,                                         500-507,550      Secondary Reply  252,257         452   NLST                250             402,433,450,451,454,455,457,                                         500-507,550      Secondary Reply  252,257         452   SITE                200,331         402,500-507                                   37                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759   STAT                100,110,        450,451,455,500-507,550                         150,151,331   HELP                030,050         500-507   NOOP                200             500-505,507   Spontaneous Replies 000,010,020,    400,401,434-436                         300,301,251,255   TYPICAL FTP SCENARIOS   TIP User wanting to transfer file from Host X to local printer:      1. TIP user opens TELNET connections by ICP to Host X socket 3.      2. The following commands and replies are exchanged:         TIP                       HOST X         <---------- 300 Awaiting input <CRLF>         USER username <CRLF> ---------->         <---------- 330 Enter Password <CRLF>         PASS password <CRLF> ---------->         <---------- 230 User logged in <CRLF>         SOCK 65538 <CRLF> ---------->         <---------- 200 Commmand received OK<CRLF>         RETR this.file <CRLF> ---------->            (Host X initiates data connection to TIP socket 65538,             i.e., PORT 1 receive)         <---------- 250 File transfer started <CRLF>         <---------- 252 File transfer completed <CRLF>         BYE<CRLF>         ---------->         <---------- 231 User logged out <CRLF>      3. Host X closes the TELNET and data connections.         Note: The TIP user should be in line mode.   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.                                   38                                                  File Transfer Protocol                                                         (Aug. 12, 1973)RFC 542 NIC 17759      LOCAL COMMANDS BY USER              ACTION INVOLVED      ftp (host) multics<CR>         ICP to Host S, socket 3,                                     establishing TELNET connections                                     <---- 330 Awaiting input <CRLF>      username Doe <CR>              USER Doe<CRLF>---->                                     <---- 330 password<CRLF>      password mumble <CR>           PASS mumble<CRLF>---->                                     <---- 230 Doe logged in.<CRLF>      retrieve (local type) ASCII<CR>      (local pathname) test 1 <CR>   User-FTP opens local file in ASCII.      (for.pathname) testp11<CR>     RETR test.p11<CRLF> ---->                                     Server makes data connection to      (U+4)                                     <---- 250 File transfer starts      <CRLF>                                     <---- 252 File transfer      complete<CRLF>      type Image<CR>                 TYPE I<CRLF> ---->                                     <---- 200 Command OK<CRLF>      byte 36<CR>                    BYTE 36<CR>LF ---->                                     <---- 200 Command OK<CRLF>      store (local type) image<CR>      (local pathname) file dump<CR> User-FTP opens local file in Image.      (for.pathname) >udd>cn>fd<CR>  STOR >udd>cn>fd<CRLF> ---->                                     <---- 451 Access denied<CRLF>      terminate                      BYE <CRLF> ---->                                     Server closes all connections.                                   39

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