Replicated Byte: 2 6 8 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |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 type is Image or Local byte the filler is a zero byte. Filler String: 2 6 +-+-+-+-+-+-+-+-+ |1 1| n | +-+-+-+-+-+-+-+-+ The escape sequence is a double byte, the first of which is the escape byte (all zeros) and the second of which contains descriptor codes as defined in Block 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 can be most effectively used to reduce the size of printer files such as those generated by RJE Hosts. ERROR RECOVERY AND RESTART There is no provision for detecting bits lost or scrambled in data transfer; this level of error control is handled by the TCP. However, a restart procedure is provided to protect users from gross system failures (including failures of a Host, an FTP-process, or the underlying network). The restart procedure is defined only for the block and compressed modes of data transfer. It requires the sender of data to insert a special marker code in the data stream with some marker information. The marker information has meaning only to the sender, but must consist of printable characters in the default or negotiated language of the TELNET connection (ASCII or EBCDIC). The marker could represent a bit-count, a record-count, or any
other information by which a system may identify a data checkpoint. The receiver of data, if it implements the restart procedure, would then mark the corresponding position of this marker in the receiving system, and return this information to the user. In the event of a system failure, the user can restart the data transfer by identifying the marker point with the FTP restart procedure. The following example illustrates the use of the restart procedure. The sender of the data inserts an appropriate marker block in the data stream at a convenient point. The receiving Host marks the corresponding data point in its file system and conveys the last known sender and receiver marker information to the user, either directly or over the TELNET connection in a 110 reply (depending on who is the sender). In the event of a system failure, the user or controller process restarts the server at the last server marker by sending a restart command with server's marker code as its argument. The restart command is transmitted over the 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 a TCP connection from the user to a standard server port. The user protocol interpreter is responsible for sending FTP commands and interpreting the replies received; the server-PI interprets commands, sends replies and directs its DTP to set up the data connection and transfer the data. If the second party to the data transfer (the passive transfer process) is the user-DTP then it is governed through the internal protocol of the user-FTP Host; if it is a second server-DTP then it is governed by its PI on command from the user-PI. The FTP replies are discussed in the next section. In the description of a few of the commands in this section it is helpful to be explicit about the possible replies. FTP COMMANDS 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 account. 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 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 332. On the other hand, if account information is NOT required for login, the reply to a successful PASSword command is 230; and if the account information is needed for a command issued later in the dialogue, the server should
return a 332 or 532 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 TELNET connection is opened. A USER command may be expected to follow. LOGOUT (QUIT) 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 QUIT. An unexpected close on the TELNET connection will cause the server to take the effective action of an abort (ABOR) and a logout (QUIT). 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. DATA PORT (PORT) The argument is a HOST-PORT specification for the data port to be used in data connection. There defaults for both the user and server data ports, and under normal circumstances this command and its reply are not needed. If this command
is used the argument is the concatenation of a 32-bit internet host address and a 16-bit TCP port address. This address information is broken into 8-bit fields and the value of each field is transmitted as a decimal number (in character string representation). The fields are separated by commas. A port command would be: PORT h1,h2,h3,h4,p1,p2 where, h1 is the high order 8 bits of the internet host address. PASSIVE (PASV) This command requests the server-DTP to "listen" on a data port (which is not its default data port) and to wait for a connection rather than initiate one upon receipt of a transfer command. The response to this command includes the host and port address this server is listening on. 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 <byte size> - Local byte Byte size 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.
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 P - Page structure The default structure is File. 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. 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. MAIL FILE (MLFL) The intent of this command is to enable a user at the user site to mail data (in form of a file) to another user at the server site. It should be noted that the files to be mailed are transmitted via the data connection in ASCII or EBCDIC type. (It is the user's responsibility to ensure that the type is correct.) These files should be inserted into the destination user's mailbox by the server in accordance with serving Host mail conventions. The mail may be marked as sent from the particular user HOST and the user specified by the 'USER' command. The argument field may contain a Host system ident, or it may be empty. If the argument field is empty or blank (one or more spaces), then the mail is destined for a printer or other designated place for general delivery site mail.
MAIL (MAIL) This command allows a user to send mail that is NOT in a file over the TELNET connection. The argument field may contain system ident, or it may be empty. The ident is defined as above for the MLFL command. After the 'MAIL' command is received, the server is to treat the following lines as text of the mail sent by the user. The mail text is to be terminated by a line containing only a single period, that is, the character sequence "CRLF.CRLF". It is suggested that a modest volume of mail service should be free; i.e., it may be entered before a USER command. MAIL SEND TO TERMINAL (MSND) This command is like the MAIL command, except that the data is displayed on the addressed user's terminal, if such access is currently allowed, otherwise an error is returned. MAIL SEND TO TERMINAL OR MAILBOX (MSOM) This command is like the MAIL command, except that the data is displayed on the addressed user's terminal, if such access is currently allowed, otherwise the data is placed in the user's mailbox. MAIL SEND TO TERMINAL AND MAILBOX (MSAM) This command is like the MAIL command, except that the data is displayed on the addressed user's terminal, if such access is currently allowed, and, in any case, the data is placed in the user's mailbox. MAIL RECIPIENT SCHEME QUESTION (MRSQ) This FTP command is used to select a scheme for the transmission of mail to several users at the same host. The schemes are to list the recipients first, or to send the mail first. MAIL RECIPIENT (MRCP) This command is used to identify the individual recipients of the mail in the transmission of mail for multiple users at one host.
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 or page structure a maximum record or page 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 or page 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. 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 tells 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 connection is not to be closed by the server, but the data connection must be closed. There are two cases for the server upon receipt of this command: (1) the FTP service command was already completed, or (2) the FTP service command is still in progress. In the first case, the server closes the data connection (if it is open) and responds with a 226 reply, indicating that the abort command was successfully processed. In the second case, the server aborts the FTP service in progress and closes the data connection, returning a 426 reply to indicate that the service request terminated in abnormally. The server then sends a 226 reply, indicating that the abort command was successfully processed. 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. CHANGE WORKING DIRECTORY (CWD) 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. 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 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 transferred 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 211 or 214. 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 an OK reply. The File Transfer Protocol follows the specifications of the TELNET protocol for all communications over the TELNET connection. Since, 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, QUIT) may be sent over the TELNET connection 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 undefined; 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.) FTP REPLIES Replies to File Transfer Protocol commands are devised to ensure the synchronization of requests and actions in the process of file transfer, and to guarantee that the user process always knows the state of the Server. Every command must generate at least one reply, although there may be more than one; in the latter case, the multiple replies must be easily distinguished. In addition, some commands occur in sequential groups, such as USER, PASS and ACCT, or RNFR and RNTO. The replies show the existence of an intermediate state if all preceding commands have been successful. A failure at any point in the sequence necessitates the repetition of the entire sequence from the beginning. The details of the command-reply sequence are made explicit in a set of state diagrams below. An FTP reply consists of a three digit number (transmitted as three alphanumeric characters) followed by some text. The number is intended for use by automata to determine what state to enter next; the text is intended for the human user. It is intended that the three digits contain enough encoded information that the user-process (the User-PI) will not need to examine the text and may either discard it or pass it on to the user, as appropriate. In particular, the text may be server-dependent, so there are likely to be varying texts for each reply code. Formally, a reply is defined to contain the 3-digit code, followed by Space <SP>, followed by one line of text (where some maximum line length has been specified), and terminated by the TELNET end-of-line code. There will be cases, however, where the text is longer than a single line. In these cases the complete text must
be bracketed so the User-process knows when it may stop reading the reply (i.e. stop processing input on the TELNET connection) and go do other things. This requires a special format on the first line to indicate that more than one line is coming, and another on the last line to designate it as the last. At least one of these must contain the appropriate reply code to indicate the state of the transaction. To satisfy all factions it was decided that both the first and last line codes should be the same. Thus the format for multi-line replies is that the first line will begin with the exact required reply code, followed immediately by a Hyphen, "-" (also known as Minus), followed by text. The last line will begin with the same code, followed immediately by Space <SP>, optionally some text, and the TELNET end-of-line code. For example: 123-First line Second line 234 A line beginning with numbers 123 The last line The user-process then simply needs to search for the second occurrence of the same reply code, followed by <SP> (Space), at the beginning of a line, and ignore all intermediary lines. If an intermediary line begins with a 3-digit number, the Server must pad the front to avoid confusion. This scheme allows standard system routines to be used for reply information (such as for the STAT reply), with "artificial" first and last lines tacked on. In the rare cases where these routines are able to generate three digits and a Space at the beginning of any line, the beginning of each text line should be offset by some neutral text, like Space. This scheme assumes that multi-line replies may not be nested. We have found that, in general, nesting of replies will not occur, except for random system messages (also called spontaneous replies) which may interrupt another reply. System messages (i.e. those not processed by the FTP server) will NOT carry reply codes and may occur anywhere in the command-reply sequence. They may be ignored by the User-process as they are only information for the human user.
The three digits of the reply each have a special significance. This is intended to allow a range of very simple to very sophisticated response by the user-process. The first digit denotes whether the response is good, bad or incomplete. (Referring to the state diagram) an unsophisticated user-process will be able to determine its next action (proceed as planned, redo, retrench, etc.) by simply examining this first digit. A user-process that wants to know approximately what kind of error occurred (e.g. file system error, command syntax error) may examine the second digit, reserving the third digit for the finest gradation of information (e.g. RNTO command without a preceding RNFR.) There are five values for the first digit of the reply code: 1yz Positive Preliminary reply The requested action is being initiated; expect another reply before proceeding with a new command. (The user-process sending another command before the completion reply would be in violation of protocol; but server-FTP processes should queue any commands that arrive while a preceding command is in progress.) This type of reply can be used to indicate that the command was accepted and the user-process may now pay attention to the data connections, for implementations where simultaneous monitoring is difficult. 2yz Positive Completion reply The requested action has been successfully completed. A new request may be initiated. 3yz Positive Intermediate reply The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information. The user should send another command specifying this information. This reply is used in command sequence groups. 4yz Transient Negative Completion reply The command was not accepted and the requested action did not take place, but the error condition is temporary and the action may be requested again. The user should
return to the beginning of the command sequence, if any. It is difficult to assign a meaning to "transient", particularly when two distinct sites (Server and User-processes) have to agree on the interpretation. Each reply in the 4yz category might have a slightly different time value, but the intent is that the user-process is encouraged to try again. A rule of thumb in determining if a reply fits into the 4yz or the 5yz (Permanent Negative) category is that replies are 4yz if the commands can be repeated without any change in command form or in properties of the User or Server (e.g. the command is spelled the same with the same arguments used; the user does not change his file access or user name; the server does not put up a new implementation.) 5yz Permanent Negative Completion reply The command was not accepted and the requested action did not take place. The User-process is discouraged from repeating the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct his User-process to reinitiate the command sequence by direct action at some point in the future (e.g. after the spelling has been changed, or the user has altered his directory status.) The following function groupings are encoded in the second digit: x0z Syntax - These replies refer to syntax errors, syntactically correct commands that don't fit any functional category, unimplemented or superfluous commands. x1z Information - These are replies to requests for information, such as status or help. x2z Connections - Replies referring to the TELNET and data connections. x3z Authentication and accounting - Replies for the login process and accounting procedures. x4z Unspecified as yet
x5z File system - These replies indicate the status of the Server file system vis-a-vis the requested transfer or other file system action. The third digit gives a finer gradation of meaning in each of the function categories, specified by the second digit. The list of replies below will illustrate this. Note that the text associated with each reply is recommended, rather than mandatory, and may even change according to the command with which it is associated. The reply codes, on the other hand, must strictly follow the specifications in the last section; that is, Server implementations should not invent new codes for situations that are only slightly different from the ones described here, but rather should adapt codes already defined. A command such as TYPE or ALLO whose successful execution does not offer the user-process any new information will cause a 200 reply to be returned. If the command is not implemented by a particular Server-FTP process because it has no relevance to that computer system, for example ALLO at a TOPS20 site, a Positive Completion reply is still desired so that the simple User-process knows it can proceed with its course of action. A 202 reply is used in this case with, for example, the reply text: "No storage allocation necessary." If, on the other hand, the command requests a non-site-specific action and is unimplemented, the response is 502. A refinement of that is the 504 reply for a command that IS implemented, but that requests an unimplemented parameter. Reply Codes by Function Groups 200 Command okay 500 Syntax error, command unrecognized [This may include errors such as command line too long.] 501 Syntax error in parameters or arguments 202 Command not implemented, superfluous at this site. 502 Command not implemented 503 Bad sequence of commands 504 Command not implemented for that parameter 110 Restart marker reply.
In this case the text is exact and not left to the particular implementation; it must read: MARK yyyy = mmmm where yyyy is User-process data stream marker, and mmmm server's equivalent marker. (note the spaces between markers and "=".) 119 Terminal not available, will try mailbox. 211 System status, or system help reply 212 Directory status 213 File status 214 Help message (on how to use the server or the meaning of a particular non-standard command. This reply is useful only to the human user.) 215 <scheme> is the preferred scheme. 120 Service ready in nnn minutes 220 Service ready for new user 221 Service closing TELNET connection (logged out if appropriate) 421 Service not available, closing TELNET connection. This may be a reply to any command if the service knows it must shut down.] 125 Data connection already open; transfer starting 225 Data connection open; no transfer in progress 425 Can't open data connection 226 Closing data connection; requested file action successful (for example, file transfer or file abort.) 426 Connection closed; transfer aborted. 227 Entering Passive Mode. h1,h2,h3,h4,p1,p2 230 User logged in, proceed 530 Not logged in 331 User name okay, need password 332 Need account for login 532 Need account for storing files 150 File status okay; about to open data connection. 151 User not local; Will forward to <user>@<host>. 152 User Unknown; Mail will be forwarded by the operator. 250 Requested file action okay, completed. 350 Requested file action pending further information 450 Requested file action not taken: file unavailable (e.g. file busy) 550 Requested action not taken:
file unavailable (e.g. file not found, no access) 451 Requested action aborted: local error in processing 551 Requested action aborted: page type unknown 452 Requested action not taken: insufficient storage space in system 552 Requested file action aborted: exceeded storage allocation (for current directory or dataset) 553 Requested action not taken: file name not allowed 354 Start mail input; end with <CR><LF>.<CR><LF> Numeric Order List of Reply Codes 110 Restart marker reply. In this case the text is exact and not left to the particular implementation; it must read: MARK yyyy = mmmm where yyyy is User-process data stream marker, and mmmm server's equivalent marker. (note the spaces between markers and "=".) 119 Terminal not available, will try mailbox. 120 Service ready in nnn minutes 125 Data connection already open; transfer starting 150 File status okay; about to open data connection. 151 User not local; Will forward to <user>@<host>. 152 User Unknown; Mail will be forwarded by the operator. 200 Command okay 202 Command not implemented, superfluous at this site. 211 System status, or system help reply 212 Directory status 213 File status 214 Help message (on how to use the server or the meaning of a particular non-standard command. This reply is useful only to the human user.) 215 <scheme> is the preferred scheme. 220 Service ready for new user 221 Service closing TELNET connection (logged out if appropriate) 225 Data connection open; no transfer in progress 226 Closing data connection; requested file action successful (for example, file transfer or file abort.) 227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
230 User logged in, proceed 250 Requested file action okay, completed. 331 User name okay, need password 332 Need account for login 350 Requested file action pending further information 354 Start mail input; end with <CR><LF>.<CR><LF> 421 Service not available, closing TELNET connection. This may be a reply to any command if the service knows it must shut down.] 425 Can't open data connection 426 Connection closed; transfer aborted. 450 Requested file action not taken: file unavailable (e.g. file busy) 451 Requested action aborted: local error in processing 452 Requested action not taken: insufficient storage space in system 500 Syntax error, command unrecognized [This may include errors such as command line too long.] 501 Syntax error in parameters or arguments 502 Command not implemented 503 Bad sequence of commands 504 Command not implemented for that parameter 530 Not logged in 532 Need account for storing files 550 Requested action not taken: file unavailable (e.g. file not found, no access) 551 Requested action aborted: page type unknown 552 Requested file action aborted: exceeded storage allocation (for current directory or dataset) 553 Requested action not taken: file name not allowed
DECLARATIVE SPECIFICATIONS MINIMUM IMPLEMENTATION In order to make FTP workable without needless error messages, the following minimum implementation is required for all servers: TYPE - ASCII Non-print MODE - Stream STRUCTURE - File, Record COMMANDS - USER, QUIT, PORT, TYPE, MODE, STRU, for the default values RETR, STOR, NOOP. The default values for transfer parameters are: TYPE - ASCII Non-print MODE - Stream STRU - File All Hosts must accept the above as the standard defaults. CONNECTIONS The server protocol interpreter shall "listen" on Port L. The user or user protocol interpreter shall initiate the full-duplex TELNET connection. Server- and user- processes should follow the conventions of the TELNET protocol as specified in the ARPA Internet Protocol Handbook. 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 connection shall be closed by the server at the user's request after all transfers and replies are completed. The user-DTP must "listen" on the specified data port; this may be the default user port (U) or a port specified in the PORT command. The server shall initiate the data connection from his own default data port (L-1) using the specified user data port. The direction of the transfer and the port used will be determined by the FTP service command.
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. One of the servers, say A, is then sent a PASV command telling him to "listen" on his data port rather than initiate a connection when he receives a transfer service command. When the user-PI receives an acknowledgment to the PASV command, which includes the identity of the host and port being listened on, the user-PI then sends A's port, a, to B in a PORT command; a reply is returned. The user-PI may then send the corresponding service commands to A and B. Server B initiates the connection and the transfer proceeds. The command-reply sequence is listed below where the messages are vertically synchronous but horizontally asynchronous: User-PI - Server A User-PI - Server B ------------------ ------------------ C->A : Connect C->B : Connect C->A : PASV A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2 C->B : PORT A1,A2,A3,A4,a1,a2 B->C : 200 Okay C->A : STOR C->B : RETR B->A : Connect to HOST-A, PORT-a 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 port BEFORE sending the transfer request). To prevent a race condition here, the server sends a reply (226) after closing the data connection (or if the connection is left open, a "file transfer completed" reply (250) and the user-PI should wait for one of these replies before issuing a new transfer command.
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. 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 the FTP commands: USER <SP> <username> <CRLF> PASS <SP> <password> <CRLF> ACCT <SP> <account information> <CRLF> REIN <CRLF> QUIT <CRLF> PORT <SP> <Host-port> <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> MLFL [<SP> <ident>] <CRLF> MAIL [<SP> <ident>] <CRLF> MSND [<SP> <ident>] <CRLF> MSOM [<SP> <ident>] <CRLF> MSAM [<SP> <ident>] <CRLF> MRSQ [<SP> <scheme>] <CRLF> MRCP <SP> <ident> <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> CWD <SP> <pathname> <CRLF> LIST [<SP> <pathname>] <CRLF> NLST [<SP> <pathname>] <CRLF> SITE <SP> <string> <CRLF> STAT [<SP> <pathname>] <CRLF> HELP [<SP> <string>] <CRLF> NOOP <CRLF>