3 Network Working Group J. Postel
4 Request for Comments: 959 J. Reynolds
6 Obsoletes RFC: 765 (IEN 149) October 1985
8 FILE TRANSFER PROTOCOL (FTP)
13 This memo is the official specification of the File Transfer
14 Protocol (FTP). Distribution of this memo is unlimited.
16 The following new optional commands are included in this edition of
19 CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
20 (Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
21 (Print Directory), and SYST (System).
23 Note that this specification is compatible with the previous edition.
27 The objectives of FTP are 1) to promote sharing of files (computer
28 programs and/or data), 2) to encourage indirect or implicit (via
29 programs) use of remote computers, 3) to shield a user from
30 variations in file storage systems among hosts, and 4) to transfer
31 data reliably and efficiently. FTP, though usable directly by a user
32 at a terminal, is designed mainly for use by programs.
34 The attempt in this specification is to satisfy the diverse needs of
35 users of maxi-hosts, mini-hosts, personal workstations, and TACs,
36 with a simple, and easily implemented protocol design.
38 This paper assumes knowledge of the Transmission Control Protocol
39 (TCP) [2] and the Telnet Protocol [3]. These documents are contained
40 in the ARPA-Internet protocol handbook [1].
44 In this section, the history, the terminology, and the FTP model are
45 discussed. The terms defined in this section are only those that
46 have special significance in FTP. Some of the terminology is very
47 specific to the FTP model; some readers may wish to turn to the
48 section on the FTP model while reviewing the terminology.
56 Postel & Reynolds [Page 1]
61 File Transfer Protocol
66 FTP has had a long evolution over the years. Appendix III is a
67 chronological compilation of Request for Comments documents
68 relating to FTP. These include the first proposed file transfer
69 mechanisms in 1971 that were developed for implementation on hosts
70 at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
72 RFC 172 provided a user-level oriented protocol for file transfer
73 between host computers (including terminal IMPs). A revision of
74 this as RFC 265, restated FTP for additional review, while RFC 281
75 suggested further changes. The use of a "Set Data Type"
76 transaction was proposed in RFC 294 in January 1982.
78 RFC 354 obsoleted RFCs 264 and 265. The File Transfer Protocol
79 was now defined as a protocol for file transfer between HOSTs on
80 the ARPANET, with the primary function of FTP defined as
81 transfering files efficiently and reliably among hosts and
82 allowing the convenient use of remote file storage capabilities.
83 RFC 385 further commented on errors, emphasis points, and
84 additions to the protocol, while RFC 414 provided a status report
85 on the working server and user FTPs. RFC 430, issued in 1973,
86 (among other RFCs too numerous to mention) presented further
87 comments on FTP. Finally, an "official" FTP document was
90 By July 1973, considerable changes from the last versions of FTP
91 were made, but the general structure remained the same. RFC 542
92 was published as a new "official" specification to reflect these
93 changes. However, many implementations based on the older
94 specification were not updated.
96 In 1974, RFCs 607 and 614 continued comments on FTP. RFC 624
97 proposed further design changes and minor modifications. In 1975,
98 RFC 686 entitled, "Leaving Well Enough Alone", discussed the
99 differences between all of the early and later versions of FTP.
100 RFC 691 presented a minor revision of RFC 686, regarding the
101 subject of print files.
103 Motivated by the transition from the NCP to the TCP as the
104 underlying protocol, a phoenix was born out of all of the above
105 efforts in RFC 765 as the specification of FTP for use on TCP.
107 This current edition of the FTP specification is intended to
108 correct some minor documentation errors, to improve the
109 explanation of some protocol features, and to add some new
113 Postel & Reynolds [Page 2]
118 File Transfer Protocol
121 In particular, the following new optional commands are included in
122 this edition of the specification:
124 CDUP - Change to Parent Directory
126 SMNT - Structure Mount
130 RMD - Remove Directory
134 PWD - Print Directory
138 This specification is compatible with the previous edition. A
139 program implemented in conformance to the previous specification
140 should automatically be in conformance to this specification.
146 The ASCII character set is as defined in the ARPA-Internet
147 Protocol Handbook. In FTP, ASCII characters are defined to be
148 the lower half of an eight-bit code set (i.e., the most
149 significant bit is zero).
153 Access controls define users' access privileges to the use of a
154 system, and to the files in that system. Access controls are
155 necessary to prevent unauthorized or accidental use of files.
156 It is the prerogative of a server-FTP process to invoke access
161 There are two byte sizes of interest in FTP: the logical byte
162 size of the file, and the transfer byte size used for the
163 transmission of the data. The transfer byte size is always 8
164 bits. The transfer byte size is not necessarily the byte size
165 in which data is to be stored in a system, nor the logical byte
166 size for interpretation of the structure of the data.
170 Postel & Reynolds [Page 3]
175 File Transfer Protocol
180 The communication path between the USER-PI and SERVER-PI for
181 the exchange of commands and replies. This connection follows
186 A full duplex connection over which data is transferred, in a
187 specified mode and type. The data transferred may be a part of
188 a file, an entire file or a number of files. The path may be
189 between a server-DTP and a user-DTP, or between two
194 The passive data transfer process "listens" on the data port
195 for a connection from the active transfer process in order to
196 open the data connection.
200 The data transfer process establishes and manages the data
201 connection. The DTP can be passive or active.
205 The end-of-line sequence defines the separation of printing
206 lines. The sequence is Carriage Return, followed by Line Feed.
210 The end-of-file condition that defines the end of a file being
215 The end-of-record condition that defines the end of a record
220 A procedure that allows a user to recover from certain errors
221 such as failure of either host system or transfer process. In
222 FTP, error recovery may involve restarting a file transfer at a
227 Postel & Reynolds [Page 4]
232 File Transfer Protocol
237 A set of commands that comprise the control information flowing
238 from the user-FTP to the server-FTP process.
242 An ordered set of computer data (including programs), of
243 arbitrary length, uniquely identified by a pathname.
247 The mode in which data is to be transferred via the data
248 connection. The mode defines the data format during transfer
249 including EOR and EOF. The transfer modes defined in FTP are
250 described in the Section on Transmission Modes.
254 The Network Virtual Terminal as defined in the Telnet Protocol.
258 The Network Virtual File System. A concept which defines a
259 standard network file system with standard commands and
260 pathname conventions.
264 A file may be structured as a set of independent parts called
265 pages. FTP supports the transmission of discontinuous files as
266 independent indexed pages.
270 Pathname is defined to be the character string which must be
271 input to a file system by a user in order to identify a file.
272 Pathname normally contains device and/or directory names, and
273 file name specification. FTP does not yet specify a standard
274 pathname convention. Each user must follow the file naming
275 conventions of the file systems involved in the transfer.
279 The protocol interpreter. The user and server sides of the
280 protocol have distinct roles implemented in a user-PI and a
284 Postel & Reynolds [Page 5]
289 File Transfer Protocol
294 A sequential file may be structured as a number of contiguous
295 parts called records. Record structures are supported by FTP
296 but a file need not have record structure.
300 A reply is an acknowledgment (positive or negative) sent from
301 server to user via the control connection in response to FTP
302 commands. The general form of a reply is a completion code
303 (including error codes) followed by a text string. The codes
304 are for use by programs and the text is usually intended for
309 The data transfer process, in its normal "active" state,
310 establishes the data connection with the "listening" data port.
311 It sets up parameters for transfer and storage, and transfers
312 data on command from its PI. The DTP can be placed in a
313 "passive" state to listen for, rather than initiate a
314 connection on the data port.
318 A process or set of processes which perform the function of
319 file transfer in cooperation with a user-FTP process and,
320 possibly, another server. The functions consist of a protocol
321 interpreter (PI) and a data transfer process (DTP).
325 The server protocol interpreter "listens" on Port L for a
326 connection from a user-PI and establishes a control
327 communication connection. It receives standard FTP commands
328 from the user-PI, sends replies, and governs the server-DTP.
332 The data representation type used for data transfer and
333 storage. Type implies certain transformations between the time
334 of data storage and data transfer. The representation types
335 defined in FTP are described in the Section on Establishing
341 Postel & Reynolds [Page 6]
346 File Transfer Protocol
351 A person or a process on behalf of a person wishing to obtain
352 file transfer service. The human user may interact directly
353 with a server-FTP process, but use of a user-FTP process is
354 preferred since the protocol design is weighted towards
359 The data transfer process "listens" on the data port for a
360 connection from a server-FTP process. If two servers are
361 transferring data between them, the user-DTP is inactive.
365 A set of functions including a protocol interpreter, a data
366 transfer process and a user interface which together perform
367 the function of file transfer in cooperation with one or more
368 server-FTP processes. The user interface allows a local
369 language to be used in the command-reply dialogue with the
374 The user protocol interpreter initiates the control connection
375 from its port U to the server-FTP process, initiates FTP
376 commands, and governs the user-DTP if that process is part of
398 Postel & Reynolds [Page 7]
403 File Transfer Protocol
408 With the above definitions in mind, the following model (shown in
409 Figure 1) may be diagrammed for an FTP service.
414 ||Interface|<--->| User |
415 |\----^----/| --------
417 |/------\| FTP Commands |/----V----\|
418 ||Server|<---------------->| User ||
419 || PI || FTP Replies || PI ||
420 |\--^---/| |\----^----/|
422 -------- |/--V---\| Data |/----V----\| --------
423 | File |<--->|Server|<---------------->| User |<--->| File |
424 |System| || DTP || Connection || DTP || |System|
425 -------- |\------/| |\---------/| --------
426 ---------- -------------
430 NOTES: 1. The data connection may be used in either direction.
431 2. The data connection need not exist all of the time.
433 Figure 1 Model for FTP Use
435 In the model described in Figure 1, the user-protocol interpreter
436 initiates the control connection. The control connection follows
437 the Telnet protocol. At the initiation of the user, standard FTP
438 commands are generated by the user-PI and transmitted to the
439 server process via the control connection. (The user may
440 establish a direct control connection to the server-FTP, from a
441 TAC terminal for example, and generate standard FTP commands
442 independently, bypassing the user-FTP process.) Standard replies
443 are sent from the server-PI to the user-PI over the control
444 connection in response to the commands.
446 The FTP commands specify the parameters for the data connection
447 (data port, transfer mode, representation type, and structure) and
448 the nature of file system operation (store, retrieve, append,
449 delete, etc.). The user-DTP or its designate should "listen" on
450 the specified data port, and the server initiate the data
451 connection and data transfer in accordance with the specified
452 parameters. It should be noted that the data port need not be in
455 Postel & Reynolds [Page 8]
460 File Transfer Protocol
463 the same host that initiates the FTP commands via the control
464 connection, but the user or the user-FTP process must ensure a
465 "listen" on the specified data port. It ought to also be noted
466 that the data connection may be used for simultaneous sending and
469 In another situation a user might wish to transfer files between
470 two hosts, neither of which is a local host. The user sets up
471 control connections to the two servers and then arranges for a
472 data connection between them. In this manner, control information
473 is passed to the user-PI but data is transferred between the
474 server data transfer processes. Following is a model of this
475 server-server interaction.
478 Control ------------ Control
479 ---------->| User-FTP |<-----------
483 -------------- --------------
484 | Server-FTP | Data Connection | Server-FTP |
485 | "A" |<---------------------->| "B" |
486 -------------- Port (A) Port (B) --------------
491 The protocol requires that the control connections be open while
492 data transfer is in progress. It is the responsibility of the
493 user to request the closing of the control connections when
494 finished using the FTP service, while it is the server who takes
495 the action. The server may abort data transfer if the control
496 connections are closed without command.
498 The Relationship between FTP and Telnet:
500 The FTP uses the Telnet protocol on the control connection.
501 This can be achieved in two ways: first, the user-PI or the
502 server-PI may implement the rules of the Telnet Protocol
503 directly in their own procedures; or, second, the user-PI or
504 the server-PI may make use of the existing Telnet module in the
507 Ease of implementaion, sharing code, and modular programming
508 argue for the second approach. Efficiency and independence
512 Postel & Reynolds [Page 9]
517 File Transfer Protocol
520 argue for the first approach. In practice, FTP relies on very
521 little of the Telnet Protocol, so the first approach does not
522 necessarily involve a large amount of code.
524 3. DATA TRANSFER FUNCTIONS
526 Files are transferred only via the data connection. The control
527 connection is used for the transfer of commands, which describe the
528 functions to be performed, and the replies to these commands (see the
529 Section on FTP Replies). Several commands are concerned with the
530 transfer of data between hosts. These data transfer commands include
531 the MODE command which specify how the bits of the data are to be
532 transmitted, and the STRUcture and TYPE commands, which are used to
533 define the way in which the data are to be represented. The
534 transmission and representation are basically independent but the
535 "Stream" transmission mode is dependent on the file structure
536 attribute and if "Compressed" transmission mode is used, the nature
537 of the filler byte depends on the representation type.
539 3.1. DATA REPRESENTATION AND STORAGE
541 Data is transferred from a storage device in the sending host to a
542 storage device in the receiving host. Often it is necessary to
543 perform certain transformations on the data because data storage
544 representations in the two systems are different. For example,
545 NVT-ASCII has different data storage representations in different
546 systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
547 ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
548 store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
549 as four 9-bit characters in a 36-bit word. It is desirable to
550 convert characters into the standard NVT-ASCII representation when
551 transmitting text between dissimilar systems. The sending and
552 receiving sites would have to perform the necessary
553 transformations between the standard representation and their
554 internal representations.
556 A different problem in representation arises when transmitting
557 binary data (not character codes) between host systems with
558 different word lengths. It is not always clear how the sender
559 should send data, and the receiver store it. For example, when
560 transmitting 32-bit bytes from a 32-bit word-length system to a
561 36-bit word-length system, it may be desirable (for reasons of
562 efficiency and usefulness) to store the 32-bit bytes
563 right-justified in a 36-bit word in the latter system. In any
564 case, the user should have the option of specifying data
565 representation and transformation functions. It should be noted
569 Postel & Reynolds [Page 10]
574 File Transfer Protocol
577 that FTP provides for very limited data type representations.
578 Transformations desired beyond this limited capability should be
579 performed by the user directly.
583 Data representations are handled in FTP by a user specifying a
584 representation type. This type may implicitly (as in ASCII or
585 EBCDIC) or explicitly (as in Local byte) define a byte size for
586 interpretation which is referred to as the "logical byte size."
587 Note that this has nothing to do with the byte size used for
588 transmission over the data connection, called the "transfer
589 byte size", and the two should not be confused. For example,
590 NVT-ASCII has a logical byte size of 8 bits. If the type is
591 Local byte, then the TYPE command has an obligatory second
592 parameter specifying the logical byte size. The transfer byte
593 size is always 8 bits.
597 This is the default type and must be accepted by all FTP
598 implementations. It is intended primarily for the transfer
599 of text files, except when both hosts would find the EBCDIC
600 type more convenient.
602 The sender converts the data from an internal character
603 representation to the standard 8-bit NVT-ASCII
604 representation (see the Telnet specification). The receiver
605 will convert the data from the standard form to his own
608 In accordance with the NVT standard, the <CRLF> sequence
609 should be used where necessary to denote the end of a line
610 of text. (See the discussion of file structure at the end
611 of the Section on Data Representation and Storage.)
613 Using the standard NVT-ASCII representation means that data
614 must be interpreted as 8-bit bytes.
616 The Format parameter for ASCII and EBCDIC types is discussed
626 Postel & Reynolds [Page 11]
631 File Transfer Protocol
636 This type is intended for efficient transfer between hosts
637 which use EBCDIC for their internal character
640 For transmission, the data are represented as 8-bit EBCDIC
641 characters. The character code is the only difference
642 between the functional specifications of EBCDIC and ASCII
645 End-of-line (as opposed to end-of-record--see the discussion
646 of structure) will probably be rarely used with EBCDIC type
647 for purposes of denoting structure, but where it is
648 necessary the <NL> character should be used.
652 The data are sent as contiguous bits which, for transfer,
653 are packed into the 8-bit transfer bytes. The receiving
654 site must store the data as contiguous bits. The structure
655 of the storage system might necessitate the padding of the
656 file (or of each record, for a record-structured file) to
657 some convenient boundary (byte, word or block). This
658 padding, which must be all zeros, may occur only at the end
659 of the file (or at the end of each record) and there must be
660 a way of identifying the padding bits so that they may be
661 stripped off if the file is retrieved. The padding
662 transformation should be well publicized to enable a user to
663 process a file at the storage site.
665 Image type is intended for the efficient storage and
666 retrieval of files and for the transfer of binary data. It
667 is recommended that this type be accepted by all FTP
672 The data is transferred in logical bytes of the size
673 specified by the obligatory second parameter, Byte size.
674 The value of Byte size must be a decimal integer; there is
675 no default value. The logical byte size is not necessarily
676 the same as the transfer byte size. If there is a
677 difference in byte sizes, then the logical bytes should be
678 packed contiguously, disregarding transfer byte boundaries
679 and with any necessary padding at the end.
683 Postel & Reynolds [Page 12]
688 File Transfer Protocol
691 When the data reaches the receiving host, it will be
692 transformed in a manner dependent on the logical byte size
693 and the particular host. This transformation must be
694 invertible (i.e., an identical file can be retrieved if the
695 same parameters are used) and should be well publicized by
696 the FTP implementors.
698 For example, a user sending 36-bit floating-point numbers to
699 a host with a 32-bit word could send that data as Local byte
700 with a logical byte size of 36. The receiving host would
701 then be expected to store the logical bytes so that they
702 could be easily manipulated; in this example putting the
703 36-bit logical bytes into 64-bit double words should
706 In another example, a pair of hosts with a 36-bit word size
707 may send data to one another in words by using TYPE L 36.
708 The data would be sent in the 8-bit transmission bytes
709 packed so that 9 transmission bytes carried two host words.
711 3.1.1.5. FORMAT CONTROL
713 The types ASCII and EBCDIC also take a second (optional)
714 parameter; this is to indicate what kind of vertical format
715 control, if any, is associated with a file. The following
716 data representation types are defined in FTP:
718 A character file may be transferred to a host for one of
719 three purposes: for printing, for storage and later
720 retrieval, or for processing. If a file is sent for
721 printing, the receiving host must know how the vertical
722 format control is represented. In the second case, it must
723 be possible to store a file at a host and then retrieve it
724 later in exactly the same form. Finally, it should be
725 possible to move a file from one host to another and process
726 the file at the second host without undue trouble. A single
727 ASCII or EBCDIC format does not satisfy all these
728 conditions. Therefore, these types have a second parameter
729 specifying one of the following three formats:
733 This is the default format to be used if the second
734 (format) parameter is omitted. Non-print format must be
735 accepted by all FTP implementations.
740 Postel & Reynolds [Page 13]
745 File Transfer Protocol
748 The file need contain no vertical format information. If
749 it is passed to a printer process, this process may
750 assume standard values for spacing and margins.
752 Normally, this format will be used with files destined
753 for processing or just storage.
755 3.1.1.5.2. TELNET FORMAT CONTROLS
757 The file contains ASCII/EBCDIC vertical format controls
758 (i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
759 process will interpret appropriately. <CRLF>, in exactly
760 this sequence, also denotes end-of-line.
762 3.1.1.5.2. CARRIAGE CONTROL (ASA)
764 The file contains ASA (FORTRAN) vertical format control
765 characters. (See RFC 740 Appendix C; and Communications
766 of the ACM, Vol. 7, No. 10, p. 606, October 1964.) In a
767 line or a record formatted according to the ASA Standard,
768 the first character is not to be printed. Instead, it
769 should be used to determine the vertical movement of the
770 paper which should take place before the rest of the
773 The ASA Standard specifies the following control
776 Character Vertical Spacing
778 blank Move paper up one line
779 0 Move paper up two lines
780 1 Move paper to top of next page
781 + No movement, i.e., overprint
783 Clearly there must be some way for a printer process to
784 distinguish the end of the structural entity. If a file
785 has record structure (see below) this is no problem;
786 records will be explicitly marked during transfer and
787 storage. If the file has no record structure, the <CRLF>
788 end-of-line sequence is used to separate printing lines,
789 but these format effectors are overridden by the ASA
797 Postel & Reynolds [Page 14]
802 File Transfer Protocol
805 3.1.2. DATA STRUCTURES
807 In addition to different representation types, FTP allows the
808 structure of a file to be specified. Three file structures are
811 file-structure, where there is no internal structure and
812 the file is considered to be a
813 continuous sequence of data bytes,
815 record-structure, where the file is made up of sequential
818 and page-structure, where the file is made up of independent
821 File-structure is the default to be assumed if the STRUcture
822 command has not been used but both file and record structures
823 must be accepted for "text" files (i.e., files with TYPE ASCII
824 or EBCDIC) by all FTP implementations. The structure of a file
825 will affect both the transfer mode of a file (see the Section
826 on Transmission Modes) and the interpretation and storage of
829 The "natural" structure of a file will depend on which host
830 stores the file. A source-code file will usually be stored on
831 an IBM Mainframe in fixed length records but on a DEC TOPS-20
832 as a stream of characters partitioned into lines, for example
833 by <CRLF>. If the transfer of files between such disparate
834 sites is to be useful, there must be some way for one site to
835 recognize the other's assumptions about the file.
837 With some sites being naturally file-oriented and others
838 naturally record-oriented there may be problems if a file with
839 one structure is sent to a host oriented to the other. If a
840 text file is sent with record-structure to a host which is file
841 oriented, then that host should apply an internal
842 transformation to the file based on the record structure.
843 Obviously, this transformation should be useful, but it must
844 also be invertible so that an identical file may be retrieved
845 using record structure.
847 In the case of a file being sent with file-structure to a
848 record-oriented host, there exists the question of what
849 criteria the host should use to divide the file into records
850 which can be processed locally. If this division is necessary,
851 the FTP implementation should use the end-of-line sequence,
854 Postel & Reynolds [Page 15]
859 File Transfer Protocol
862 <CRLF> for ASCII, or <NL> for EBCDIC text files, as the
863 delimiter. If an FTP implementation adopts this technique, it
864 must be prepared to reverse the transformation if the file is
865 retrieved with file-structure.
867 3.1.2.1. FILE STRUCTURE
869 File structure is the default to be assumed if the STRUcture
870 command has not been used.
872 In file-structure there is no internal structure and the
873 file is considered to be a continuous sequence of data
876 3.1.2.2. RECORD STRUCTURE
878 Record structures must be accepted for "text" files (i.e.,
879 files with TYPE ASCII or EBCDIC) by all FTP implementations.
881 In record-structure the file is made up of sequential
884 3.1.2.3. PAGE STRUCTURE
886 To transmit files that are discontinuous, FTP defines a page
887 structure. Files of this type are sometimes known as
888 "random access files" or even as "holey files". In these
889 files there is sometimes other information associated with
890 the file as a whole (e.g., a file descriptor), or with a
891 section of the file (e.g., page access controls), or both.
892 In FTP, the sections of the file are called pages.
894 To provide for various page sizes and associated
895 information, each page is sent with a page header. The page
896 header has the following defined fields:
900 The number of logical bytes in the page header
901 including this byte. The minimum header length is 4.
905 The logical page number of this section of the file.
906 This is not the transmission sequence number of this
907 page, but the index used to identify this page of the
911 Postel & Reynolds [Page 16]
916 File Transfer Protocol
921 The number of logical bytes in the page data. The
922 minimum data length is 0.
926 The type of page this is. The following page types
931 This is used to indicate the end of a paged
932 structured transmission. The header length must
933 be 4, and the data length must be 0.
937 This is the normal type for simple paged files
938 with no page level associated control
939 information. The header length must be 4.
943 This type is used to transmit the descriptive
944 information for the file as a whole.
946 3 = Access Controlled Page
948 This type includes an additional header field
949 for paged files with page level access control
950 information. The header length must be 5.
954 Further header fields may be used to supply per page
955 control information, for example, per page access
958 All fields are one logical byte in length. The logical byte
959 size is specified by the TYPE command. See Appendix I for
960 further details and a specific case at the page structure.
962 A note of caution about parameters: a file must be stored and
963 retrieved with the same parameters if the retrieved version is to
968 Postel & Reynolds [Page 17]
973 File Transfer Protocol
976 be identical to the version originally transmitted. Conversely,
977 FTP implementations must return a file identical to the original
978 if the parameters used to store and retrieve a file are the same.
980 3.2. ESTABLISHING DATA CONNECTIONS
982 The mechanics of transferring data consists of setting up the data
983 connection to the appropriate ports and choosing the parameters
984 for transfer. Both the user and the server-DTPs have a default
985 data port. The user-process default data port is the same as the
986 control connection port (i.e., U). The server-process default
987 data port is the port adjacent to the control connection port
990 The transfer byte size is 8-bit bytes. This byte size is relevant
991 only for the actual transfer of the data; it has no bearing on
992 representation of the data within a host's file system.
994 The passive data transfer process (this may be a user-DTP or a
995 second server-DTP) shall "listen" on the data port prior to
996 sending a transfer request command. The FTP request command
997 determines the direction of the data transfer. The server, upon
998 receiving the transfer request, will initiate the data connection
999 to the port. When the connection is established, the data
1000 transfer begins between DTP's, and the server-PI sends a
1001 confirming reply to the user-PI.
1003 Every FTP implementation must support the use of the default data
1004 ports, and only the USER-PI can initiate a change to non-default
1007 It is possible for the user to specify an alternate data port by
1008 use of the PORT command. The user may want a file dumped on a TAC
1009 line printer or retrieved from a third party host. In the latter
1010 case, the user-PI sets up control connections with both
1011 server-PI's. One server is then told (by an FTP command) to
1012 "listen" for a connection which the other will initiate. The
1013 user-PI sends one server-PI a PORT command indicating the data
1014 port of the other. Finally, both are sent the appropriate
1015 transfer commands. The exact sequence of commands and replies
1016 sent between the user-controller and the servers is defined in the
1017 Section on FTP Replies.
1019 In general, it is the server's responsibility to maintain the data
1020 connection--to initiate it and to close it. The exception to this
1025 Postel & Reynolds [Page 18]
1029 RFC 959 October 1985
1030 File Transfer Protocol
1033 is when the user-DTP is sending the data in a transfer mode that
1034 requires the connection to be closed to indicate EOF. The server
1035 MUST close the data connection under the following conditions:
1037 1. The server has completed sending data in a transfer mode
1038 that requires a close to indicate EOF.
1040 2. The server receives an ABORT command from the user.
1042 3. The port specification is changed by a command from the
1045 4. The control connection is closed legally or otherwise.
1047 5. An irrecoverable error condition occurs.
1049 Otherwise the close is a server option, the exercise of which the
1050 server must indicate to the user-process by either a 250 or 226
1053 3.3. DATA CONNECTION MANAGEMENT
1055 Default Data Connection Ports: All FTP implementations must
1056 support use of the default data connection ports, and only the
1057 User-PI may initiate the use of non-default ports.
1059 Negotiating Non-Default Data Ports: The User-PI may specify a
1060 non-default user side data port with the PORT command. The
1061 User-PI may request the server side to identify a non-default
1062 server side data port with the PASV command. Since a connection
1063 is defined by the pair of addresses, either of these actions is
1064 enough to get a different data connection, still it is permitted
1065 to do both commands to use new ports on both ends of the data
1068 Reuse of the Data Connection: When using the stream mode of data
1069 transfer the end of the file must be indicated by closing the
1070 connection. This causes a problem if multiple files are to be
1071 transfered in the session, due to need for TCP to hold the
1072 connection record for a time out period to guarantee the reliable
1073 communication. Thus the connection can not be reopened at once.
1075 There are two solutions to this problem. The first is to
1076 negotiate a non-default port. The second is to use another
1079 A comment on transfer modes. The stream transfer mode is
1082 Postel & Reynolds [Page 19]
1086 RFC 959 October 1985
1087 File Transfer Protocol
1090 inherently unreliable, since one can not determine if the
1091 connection closed prematurely or not. The other transfer modes
1092 (Block, Compressed) do not close the connection to indicate the
1093 end of file. They have enough FTP encoding that the data
1094 connection can be parsed to determine the end of the file.
1095 Thus using these modes one can leave the data connection open
1096 for multiple file transfers.
1098 3.4. TRANSMISSION MODES
1100 The next consideration in transferring data is choosing the
1101 appropriate transmission mode. There are three modes: one which
1102 formats the data and allows for restart procedures; one which also
1103 compresses the data for efficient transfer; and one which passes
1104 the data with little or no processing. In this last case the mode
1105 interacts with the structure attribute to determine the type of
1106 processing. In the compressed mode, the representation type
1107 determines the filler byte.
1109 All data transfers must be completed with an end-of-file (EOF)
1110 which may be explicitly stated or implied by the closing of the
1111 data connection. For files with record structure, all the
1112 end-of-record markers (EOR) are explicit, including the final one.
1113 For files transmitted in page structure a "last-page" page type is
1116 NOTE: In the rest of this section, byte means "transfer byte"
1117 except where explicitly stated otherwise.
1119 For the purpose of standardized transfer, the sending host will
1120 translate its internal end of line or end of record denotation
1121 into the representation prescribed by the transfer mode and file
1122 structure, and the receiving host will perform the inverse
1123 translation to its internal denotation. An IBM Mainframe record
1124 count field may not be recognized at another host, so the
1125 end-of-record information may be transferred as a two byte control
1126 code in Stream mode or as a flagged bit in a Block or Compressed
1127 mode descriptor. End-of-line in an ASCII or EBCDIC file with no
1128 record structure should be indicated by <CRLF> or <NL>,
1129 respectively. Since these transformations imply extra work for
1130 some systems, identical systems transferring non-record structured
1131 text files might wish to use a binary representation and stream
1132 mode for the transfer.
1139 Postel & Reynolds [Page 20]
1143 RFC 959 October 1985
1144 File Transfer Protocol
1147 The following transmission modes are defined in FTP:
1151 The data is transmitted as a stream of bytes. There is no
1152 restriction on the representation type used; record structures
1155 In a record structured file EOR and EOF will each be indicated
1156 by a two-byte control code. The first byte of the control code
1157 will be all ones, the escape character. The second byte will
1158 have the low order bit on and zeros elsewhere for EOR and the
1159 second low order bit on for EOF; that is, the byte will have
1160 value 1 for EOR and value 2 for EOF. EOR and EOF may be
1161 indicated together on the last byte transmitted by turning both
1162 low order bits on (i.e., the value 3). If a byte of all ones
1163 was intended to be sent as data, it should be repeated in the
1164 second byte of the control code.
1166 If the structure is a file structure, the EOF is indicated by
1167 the sending host closing the data connection and all bytes are
1172 The file is transmitted as a series of data blocks preceded by
1173 one or more header bytes. The header bytes contain a count
1174 field, and descriptor code. The count field indicates the
1175 total length of the data block in bytes, thus marking the
1176 beginning of the next data block (there are no filler bits).
1177 The descriptor code defines: last block in the file (EOF) last
1178 block in the record (EOR), restart marker (see the Section on
1179 Error Recovery and Restart) or suspect data (i.e., the data
1180 being transferred is suspected of errors and is not reliable).
1181 This last code is NOT intended for error control within FTP.
1182 It is motivated by the desire of sites exchanging certain types
1183 of data (e.g., seismic or weather data) to send and receive all
1184 the data despite local errors (such as "magnetic tape read
1185 errors"), but to indicate in the transmission that certain
1186 portions are suspect). Record structures are allowed in this
1187 mode, and any representation type may be used.
1189 The header consists of the three bytes. Of the 24 bits of
1190 header information, the 16 low order bits shall represent byte
1191 count, and the 8 high order bits shall represent descriptor
1192 codes as shown below.
1196 Postel & Reynolds [Page 21]
1200 RFC 959 October 1985
1201 File Transfer Protocol
1206 +----------------+----------------+----------------+
1207 | Descriptor | Byte Count |
1208 | 8 bits | 16 bits |
1209 +----------------+----------------+----------------+
1212 The descriptor codes are indicated by bit flags in the
1213 descriptor byte. Four codes have been assigned, where each
1214 code number is the decimal value of the corresponding bit in
1219 128 End of data block is EOR
1220 64 End of data block is EOF
1221 32 Suspected errors in data block
1222 16 Data block is a restart marker
1224 With this encoding, more than one descriptor coded condition
1225 may exist for a particular block. As many bits as necessary
1228 The restart marker is embedded in the data stream as an
1229 integral number of 8-bit bytes representing printable
1230 characters in the language being used over the control
1231 connection (e.g., default--NVT-ASCII). <SP> (Space, in the
1232 appropriate language) must not be used WITHIN a restart marker.
1234 For example, to transmit a six-character marker, the following
1237 +--------+--------+--------+
1238 |Descrptr| Byte count |
1240 +--------+--------+--------+
1242 +--------+--------+--------+
1243 | Marker | Marker | Marker |
1244 | 8 bits | 8 bits | 8 bits |
1245 +--------+--------+--------+
1247 +--------+--------+--------+
1248 | Marker | Marker | Marker |
1249 | 8 bits | 8 bits | 8 bits |
1250 +--------+--------+--------+
1253 Postel & Reynolds [Page 22]
1257 RFC 959 October 1985
1258 File Transfer Protocol
1261 3.4.3. COMPRESSED MODE
1263 There are three kinds of information to be sent: regular data,
1264 sent in a byte string; compressed data, consisting of
1265 replications or filler; and control information, sent in a
1266 two-byte escape sequence. If n>0 bytes (up to 127) of regular
1267 data are sent, these n bytes are preceded by a byte with the
1268 left-most bit set to 0 and the right-most 7 bits containing the
1274 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
1275 |0| n | | d(1) | ... | d(n) |
1276 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
1281 String of n data bytes d(1),..., d(n)
1282 Count n must be positive.
1284 To compress a string of n replications of the data byte d, the
1285 following 2 bytes are sent:
1290 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
1292 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
1294 A string of n filler bytes can be compressed into a single
1295 byte, where the filler byte varies with the representation
1296 type. If the type is ASCII or EBCDIC the filler byte is <SP>
1297 (Space, ASCII code 32, EBCDIC code 64). If the type is Image
1298 or Local byte the filler is a zero byte.
1307 The escape sequence is a double byte, the first of which is the
1310 Postel & Reynolds [Page 23]
1314 RFC 959 October 1985
1315 File Transfer Protocol
1318 escape byte (all zeros) and the second of which contains
1319 descriptor codes as defined in Block mode. The descriptor
1320 codes have the same meaning as in Block mode and apply to the
1321 succeeding string of bytes.
1323 Compressed mode is useful for obtaining increased bandwidth on
1324 very large network transmissions at a little extra CPU cost.
1325 It can be most effectively used to reduce the size of printer
1326 files such as those generated by RJE hosts.
1328 3.5. ERROR RECOVERY AND RESTART
1330 There is no provision for detecting bits lost or scrambled in data
1331 transfer; this level of error control is handled by the TCP.
1332 However, a restart procedure is provided to protect users from
1333 gross system failures (including failures of a host, an
1334 FTP-process, or the underlying network).
1336 The restart procedure is defined only for the block and compressed
1337 modes of data transfer. It requires the sender of data to insert
1338 a special marker code in the data stream with some marker
1339 information. The marker information has meaning only to the
1340 sender, but must consist of printable characters in the default or
1341 negotiated language of the control connection (ASCII or EBCDIC).
1342 The marker could represent a bit-count, a record-count, or any
1343 other information by which a system may identify a data
1344 checkpoint. The receiver of data, if it implements the restart
1345 procedure, would then mark the corresponding position of this
1346 marker in the receiving system, and return this information to the
1349 In the event of a system failure, the user can restart the data
1350 transfer by identifying the marker point with the FTP restart
1351 procedure. The following example illustrates the use of the
1354 The sender of the data inserts an appropriate marker block in the
1355 data stream at a convenient point. The receiving host marks the
1356 corresponding data point in its file system and conveys the last
1357 known sender and receiver marker information to the user, either
1358 directly or over the control connection in a 110 reply (depending
1359 on who is the sender). In the event of a system failure, the user
1360 or controller process restarts the server at the last server
1361 marker by sending a restart command with server's marker code as
1362 its argument. The restart command is transmitted over the control
1367 Postel & Reynolds [Page 24]
1371 RFC 959 October 1985
1372 File Transfer Protocol
1375 connection and is immediately followed by the command (such as
1376 RETR, STOR or LIST) which was being executed when the system
1379 4. FILE TRANSFER FUNCTIONS
1381 The communication channel from the user-PI to the server-PI is
1382 established as a TCP connection from the user to the standard server
1383 port. The user protocol interpreter is responsible for sending FTP
1384 commands and interpreting the replies received; the server-PI
1385 interprets commands, sends replies and directs its DTP to set up the
1386 data connection and transfer the data. If the second party to the
1387 data transfer (the passive transfer process) is the user-DTP, then it
1388 is governed through the internal protocol of the user-FTP host; if it
1389 is a second server-DTP, then it is governed by its PI on command from
1390 the user-PI. The FTP replies are discussed in the next section. In
1391 the description of a few of the commands in this section, it is
1392 helpful to be explicit about the possible replies.
1396 4.1.1. ACCESS CONTROL COMMANDS
1398 The following commands specify access control identifiers
1399 (command codes are shown in parentheses).
1403 The argument field is a Telnet string identifying the user.
1404 The user identification is that which is required by the
1405 server for access to its file system. This command will
1406 normally be the first command transmitted by the user after
1407 the control connections are made (some servers may require
1408 this). Additional identification information in the form of
1409 a password and/or an account command may also be required by
1410 some servers. Servers may allow a new USER command to be
1411 entered at any point in order to change the access control
1412 and/or accounting information. This has the effect of
1413 flushing any user, password, and account information already
1414 supplied and beginning the login sequence again. All
1415 transfer parameters are unchanged and any file transfer in
1416 progress is completed under the old access control
1424 Postel & Reynolds [Page 25]
1428 RFC 959 October 1985
1429 File Transfer Protocol
1434 The argument field is a Telnet string specifying the user's
1435 password. This command must be immediately preceded by the
1436 user name command, and, for some sites, completes the user's
1437 identification for access control. Since password
1438 information is quite sensitive, it is desirable in general
1439 to "mask" it or suppress typeout. It appears that the
1440 server has no foolproof way to achieve this. It is
1441 therefore the responsibility of the user-FTP process to hide
1442 the sensitive password information.
1446 The argument field is a Telnet string identifying the user's
1447 account. The command is not necessarily related to the USER
1448 command, as some sites may require an account for login and
1449 others only for specific access, such as storing files. In
1450 the latter case the command may arrive at any time.
1452 There are reply codes to differentiate these cases for the
1453 automation: when account information is required for login,
1454 the response to a successful PASSword command is reply code
1455 332. On the other hand, if account information is NOT
1456 required for login, the reply to a successful PASSword
1457 command is 230; and if the account information is needed for
1458 a command issued later in the dialogue, the server should
1459 return a 332 or 532 reply depending on whether it stores
1460 (pending receipt of the ACCounT command) or discards the
1461 command, respectively.
1463 CHANGE WORKING DIRECTORY (CWD)
1465 This command allows the user to work with a different
1466 directory or dataset for file storage or retrieval without
1467 altering his login or accounting information. Transfer
1468 parameters are similarly unchanged. The argument is a
1469 pathname specifying a directory or other system dependent
1470 file group designator.
1472 CHANGE TO PARENT DIRECTORY (CDUP)
1474 This command is a special case of CWD, and is included to
1475 simplify the implementation of programs for transferring
1476 directory trees between operating systems having different
1481 Postel & Reynolds [Page 26]
1485 RFC 959 October 1985
1486 File Transfer Protocol
1489 syntaxes for naming the parent directory. The reply codes
1490 shall be identical to the reply codes of CWD. See
1491 Appendix II for further details.
1493 STRUCTURE MOUNT (SMNT)
1495 This command allows the user to mount a different file
1496 system data structure without altering his login or
1497 accounting information. Transfer parameters are similarly
1498 unchanged. The argument is a pathname specifying a
1499 directory or other system dependent file group designator.
1503 This command terminates a USER, flushing all I/O and account
1504 information, except to allow any transfer in progress to be
1505 completed. All parameters are reset to the default settings
1506 and the control connection is left open. This is identical
1507 to the state in which a user finds himself immediately after
1508 the control connection is opened. A USER command may be
1513 This command terminates a USER and if file transfer is not
1514 in progress, the server closes the control connection. If
1515 file transfer is in progress, the connection will remain
1516 open for result response and the server will then close it.
1517 If the user-process is transferring files for several USERs
1518 but does not wish to close and then reopen connections for
1519 each, then the REIN command should be used instead of QUIT.
1521 An unexpected close on the control connection will cause the
1522 server to take the effective action of an abort (ABOR) and a
1525 4.1.2. TRANSFER PARAMETER COMMANDS
1527 All data transfer parameters have default values, and the
1528 commands specifying data transfer parameters are required only
1529 if the default parameter values are to be changed. The default
1530 value is the last specified value, or if no value has been
1531 specified, the standard default value is as stated here. This
1532 implies that the server must "remember" the applicable default
1533 values. The commands may be in any order except that they must
1534 precede the FTP service request. The following commands
1535 specify data transfer parameters:
1538 Postel & Reynolds [Page 27]
1542 RFC 959 October 1985
1543 File Transfer Protocol
1548 The argument is a HOST-PORT specification for the data port
1549 to be used in data connection. There are defaults for both
1550 the user and server data ports, and under normal
1551 circumstances this command and its reply are not needed. If
1552 this command is used, the argument is the concatenation of a
1553 32-bit internet host address and a 16-bit TCP port address.
1554 This address information is broken into 8-bit fields and the
1555 value of each field is transmitted as a decimal number (in
1556 character string representation). The fields are separated
1557 by commas. A port command would be:
1559 PORT h1,h2,h3,h4,p1,p2
1561 where h1 is the high order 8 bits of the internet host
1566 This command requests the server-DTP to "listen" on a data
1567 port (which is not its default data port) and to wait for a
1568 connection rather than initiate one upon receipt of a
1569 transfer command. The response to this command includes the
1570 host and port address this server is listening on.
1572 REPRESENTATION TYPE (TYPE)
1574 The argument specifies the representation type as described
1575 in the Section on Data Representation and Storage. Several
1576 types take a second parameter. The first parameter is
1577 denoted by a single Telnet character, as is the second
1578 Format parameter for ASCII and EBCDIC; the second parameter
1579 for local byte is a decimal integer to indicate Bytesize.
1580 The parameters are separated by a <SP> (Space, ASCII code
1583 The following codes are assigned for type:
1586 A - ASCII | | N - Non-print
1587 |-><-| T - Telnet format effectors
1588 E - EBCDIC| | C - Carriage Control (ASA)
1592 L <byte size> - Local byte Byte size
1595 Postel & Reynolds [Page 28]
1599 RFC 959 October 1985
1600 File Transfer Protocol
1603 The default representation type is ASCII Non-print. If the
1604 Format parameter is changed, and later just the first
1605 argument is changed, Format then returns to the Non-print
1608 FILE STRUCTURE (STRU)
1610 The argument is a single Telnet character code specifying
1611 file structure described in the Section on Data
1612 Representation and Storage.
1614 The following codes are assigned for structure:
1616 F - File (no record structure)
1617 R - Record structure
1620 The default structure is File.
1622 TRANSFER MODE (MODE)
1624 The argument is a single Telnet character code specifying
1625 the data transfer modes described in the Section on
1628 The following codes are assigned for transfer modes:
1634 The default transfer mode is Stream.
1636 4.1.3. FTP SERVICE COMMANDS
1638 The FTP service commands define the file transfer or the file
1639 system function requested by the user. The argument of an FTP
1640 service command will normally be a pathname. The syntax of
1641 pathnames must conform to server site conventions (with
1642 standard defaults applicable), and the language conventions of
1643 the control connection. The suggested default handling is to
1644 use the last specified device, directory or file name, or the
1645 standard default defined for local users. The commands may be
1646 in any order except that a "rename from" command must be
1647 followed by a "rename to" command and the restart command must
1648 be followed by the interrupted service command (e.g., STOR or
1649 RETR). The data, when transferred in response to FTP service
1652 Postel & Reynolds [Page 29]
1656 RFC 959 October 1985
1657 File Transfer Protocol
1660 commands, shall always be sent over the data connection, except
1661 for certain informative replies. The following commands
1662 specify FTP service requests:
1666 This command causes the server-DTP to transfer a copy of the
1667 file, specified in the pathname, to the server- or user-DTP
1668 at the other end of the data connection. The status and
1669 contents of the file at the server site shall be unaffected.
1673 This command causes the server-DTP to accept the data
1674 transferred via the data connection and to store the data as
1675 a file at the server site. If the file specified in the
1676 pathname exists at the server site, then its contents shall
1677 be replaced by the data being transferred. A new file is
1678 created at the server site if the file specified in the
1679 pathname does not already exist.
1683 This command behaves like STOR except that the resultant
1684 file is to be created in the current directory under a name
1685 unique to that directory. The 250 Transfer Started response
1686 must include the name generated.
1688 APPEND (with create) (APPE)
1690 This command causes the server-DTP to accept the data
1691 transferred via the data connection and to store the data in
1692 a file at the server site. If the file specified in the
1693 pathname exists at the server site, then the data shall be
1694 appended to that file; otherwise the file specified in the
1695 pathname shall be created at the server site.
1699 This command may be required by some servers to reserve
1700 sufficient storage to accommodate the new file to be
1701 transferred. The argument shall be a decimal integer
1702 representing the number of bytes (using the logical byte
1703 size) of storage to be reserved for the file. For files
1704 sent with record or page structure a maximum record or page
1705 size (in logical bytes) might also be necessary; this is
1706 indicated by a decimal integer in a second argument field of
1709 Postel & Reynolds [Page 30]
1713 RFC 959 October 1985
1714 File Transfer Protocol
1717 the command. This second argument is optional, but when
1718 present should be separated from the first by the three
1719 Telnet characters <SP> R <SP>. This command shall be
1720 followed by a STORe or APPEnd command. The ALLO command
1721 should be treated as a NOOP (no operation) by those servers
1722 which do not require that the maximum size of the file be
1723 declared beforehand, and those servers interested in only
1724 the maximum record or page size should accept a dummy value
1725 in the first argument and ignore it.
1729 The argument field represents the server marker at which
1730 file transfer is to be restarted. This command does not
1731 cause file transfer but skips over the file to the specified
1732 data checkpoint. This command shall be immediately followed
1733 by the appropriate FTP service command which shall cause
1734 file transfer to resume.
1738 This command specifies the old pathname of the file which is
1739 to be renamed. This command must be immediately followed by
1740 a "rename to" command specifying the new file pathname.
1744 This command specifies the new pathname of the file
1745 specified in the immediately preceding "rename from"
1746 command. Together the two commands cause a file to be
1751 This command tells the server to abort the previous FTP
1752 service command and any associated transfer of data. The
1753 abort command may require "special action", as discussed in
1754 the Section on FTP Commands, to force recognition by the
1755 server. No action is to be taken if the previous command
1756 has been completed (including data transfer). The control
1757 connection is not to be closed by the server, but the data
1758 connection must be closed.
1760 There are two cases for the server upon receipt of this
1761 command: (1) the FTP service command was already completed,
1762 or (2) the FTP service command is still in progress.
1766 Postel & Reynolds [Page 31]
1770 RFC 959 October 1985
1771 File Transfer Protocol
1774 In the first case, the server closes the data connection
1775 (if it is open) and responds with a 226 reply, indicating
1776 that the abort command was successfully processed.
1778 In the second case, the server aborts the FTP service in
1779 progress and closes the data connection, returning a 426
1780 reply to indicate that the service request terminated
1781 abnormally. The server then sends a 226 reply,
1782 indicating that the abort command was successfully
1787 This command causes the file specified in the pathname to be
1788 deleted at the server site. If an extra level of protection
1789 is desired (such as the query, "Do you really wish to
1790 delete?"), it should be provided by the user-FTP process.
1792 REMOVE DIRECTORY (RMD)
1794 This command causes the directory specified in the pathname
1795 to be removed as a directory (if the pathname is absolute)
1796 or as a subdirectory of the current working directory (if
1797 the pathname is relative). See Appendix II.
1799 MAKE DIRECTORY (MKD)
1801 This command causes the directory specified in the pathname
1802 to be created as a directory (if the pathname is absolute)
1803 or as a subdirectory of the current working directory (if
1804 the pathname is relative). See Appendix II.
1806 PRINT WORKING DIRECTORY (PWD)
1808 This command causes the name of the current working
1809 directory to be returned in the reply. See Appendix II.
1813 This command causes a list to be sent from the server to the
1814 passive DTP. If the pathname specifies a directory or other
1815 group of files, the server should transfer a list of files
1816 in the specified directory. If the pathname specifies a
1817 file then the server should send current information on the
1818 file. A null argument implies the user's current working or
1819 default directory. The data transfer is over the data
1820 connection in type ASCII or type EBCDIC. (The user must
1823 Postel & Reynolds [Page 32]
1827 RFC 959 October 1985
1828 File Transfer Protocol
1831 ensure that the TYPE is appropriately ASCII or EBCDIC).
1832 Since the information on a file may vary widely from system
1833 to system, this information may be hard to use automatically
1834 in a program, but may be quite useful to a human user.
1838 This command causes a directory listing to be sent from
1839 server to user site. The pathname should specify a
1840 directory or other system-specific file group descriptor; a
1841 null argument implies the current directory. The server
1842 will return a stream of names of files and no other
1843 information. The data will be transferred in ASCII or
1844 EBCDIC type over the data connection as valid pathname
1845 strings separated by <CRLF> or <NL>. (Again the user must
1846 ensure that the TYPE is correct.) This command is intended
1847 to return information that can be used by a program to
1848 further process the files automatically. For example, in
1849 the implementation of a "multiple get" function.
1851 SITE PARAMETERS (SITE)
1853 This command is used by the server to provide services
1854 specific to his system that are essential to file transfer
1855 but not sufficiently universal to be included as commands in
1856 the protocol. The nature of these services and the
1857 specification of their syntax can be stated in a reply to
1858 the HELP SITE command.
1862 This command is used to find out the type of operating
1863 system at the server. The reply shall have as its first
1864 word one of the system names listed in the current version
1865 of the Assigned Numbers document [4].
1869 This command shall cause a status response to be sent over
1870 the control connection in the form of a reply. The command
1871 may be sent during a file transfer (along with the Telnet IP
1872 and Synch signals--see the Section on FTP Commands) in which
1873 case the server will respond with the status of the
1874 operation in progress, or it may be sent between file
1875 transfers. In the latter case, the command may have an
1876 argument field. If the argument is a pathname, the command
1877 is analogous to the "list" command except that data shall be
1880 Postel & Reynolds [Page 33]
1884 RFC 959 October 1985
1885 File Transfer Protocol
1888 transferred over the control connection. If a partial
1889 pathname is given, the server may respond with a list of
1890 file names or attributes associated with that specification.
1891 If no argument is given, the server should return general
1892 status information about the server FTP process. This
1893 should include current values of all transfer parameters and
1894 the status of connections.
1898 This command shall cause the server to send helpful
1899 information regarding its implementation status over the
1900 control connection to the user. The command may take an
1901 argument (e.g., any command name) and return more specific
1902 information as a response. The reply is type 211 or 214.
1903 It is suggested that HELP be allowed before entering a USER
1904 command. The server may use this reply to specify
1905 site-dependent parameters, e.g., in response to HELP SITE.
1909 This command does not affect any parameters or previously
1910 entered commands. It specifies no action other than that the
1911 server send an OK reply.
1913 The File Transfer Protocol follows the specifications of the Telnet
1914 protocol for all communications over the control connection. Since
1915 the language used for Telnet communication may be a negotiated
1916 option, all references in the next two sections will be to the
1917 "Telnet language" and the corresponding "Telnet end-of-line code".
1918 Currently, one may take these to mean NVT-ASCII and <CRLF>. No other
1919 specifications of the Telnet protocol will be cited.
1921 FTP commands are "Telnet strings" terminated by the "Telnet end of
1922 line code". The command codes themselves are alphabetic characters
1923 terminated by the character <SP> (Space) if parameters follow and
1924 Telnet-EOL otherwise. The command codes and the semantics of
1925 commands are described in this section; the detailed syntax of
1926 commands is specified in the Section on Commands, the reply sequences
1927 are discussed in the Section on Sequencing of Commands and Replies,
1928 and scenarios illustrating the use of commands are provided in the
1929 Section on Typical FTP Scenarios.
1931 FTP commands may be partitioned as those specifying access-control
1932 identifiers, data transfer parameters, or FTP service requests.
1933 Certain commands (such as ABOR, STAT, QUIT) may be sent over the
1934 control connection while a data transfer is in progress. Some
1937 Postel & Reynolds [Page 34]
1941 RFC 959 October 1985
1942 File Transfer Protocol
1945 servers may not be able to monitor the control and data connections
1946 simultaneously, in which case some special action will be necessary
1947 to get the server's attention. The following ordered format is
1948 tentatively recommended:
1950 1. User system inserts the Telnet "Interrupt Process" (IP) signal
1951 in the Telnet stream.
1953 2. User system sends the Telnet "Synch" signal.
1955 3. User system inserts the command (e.g., ABOR) in the Telnet
1958 4. Server PI, after receiving "IP", scans the Telnet stream for
1959 EXACTLY ONE FTP command.
1961 (For other servers this may not be necessary but the actions listed
1962 above should have no unusual effect.)
1966 Replies to File Transfer Protocol commands are devised to ensure
1967 the synchronization of requests and actions in the process of file
1968 transfer, and to guarantee that the user process always knows the
1969 state of the Server. Every command must generate at least one
1970 reply, although there may be more than one; in the latter case,
1971 the multiple replies must be easily distinguished. In addition,
1972 some commands occur in sequential groups, such as USER, PASS and
1973 ACCT, or RNFR and RNTO. The replies show the existence of an
1974 intermediate state if all preceding commands have been successful.
1975 A failure at any point in the sequence necessitates the repetition
1976 of the entire sequence from the beginning.
1978 The details of the command-reply sequence are made explicit in
1979 a set of state diagrams below.
1981 An FTP reply consists of a three digit number (transmitted as
1982 three alphanumeric characters) followed by some text. The number
1983 is intended for use by automata to determine what state to enter
1984 next; the text is intended for the human user. It is intended
1985 that the three digits contain enough encoded information that the
1986 user-process (the User-PI) will not need to examine the text and
1987 may either discard it or pass it on to the user, as appropriate.
1988 In particular, the text may be server-dependent, so there are
1989 likely to be varying texts for each reply code.
1991 A reply is defined to contain the 3-digit code, followed by Space
1994 Postel & Reynolds [Page 35]
1998 RFC 959 October 1985
1999 File Transfer Protocol
2002 <SP>, followed by one line of text (where some maximum line length
2003 has been specified), and terminated by the Telnet end-of-line
2004 code. There will be cases however, where the text is longer than
2005 a single line. In these cases the complete text must be bracketed
2006 so the User-process knows when it may stop reading the reply (i.e.
2007 stop processing input on the control connection) and go do other
2008 things. This requires a special format on the first line to
2009 indicate that more than one line is coming, and another on the
2010 last line to designate it as the last. At least one of these must
2011 contain the appropriate reply code to indicate the state of the
2012 transaction. To satisfy all factions, it was decided that both
2013 the first and last line codes should be the same.
2015 Thus the format for multi-line replies is that the first line
2016 will begin with the exact required reply code, followed
2017 immediately by a Hyphen, "-" (also known as Minus), followed by
2018 text. The last line will begin with the same code, followed
2019 immediately by Space <SP>, optionally some text, and the Telnet
2025 234 A line beginning with numbers
2028 The user-process then simply needs to search for the second
2029 occurrence of the same reply code, followed by <SP> (Space), at
2030 the beginning of a line, and ignore all intermediary lines. If
2031 an intermediary line begins with a 3-digit number, the Server
2032 must pad the front to avoid confusion.
2034 This scheme allows standard system routines to be used for
2035 reply information (such as for the STAT reply), with
2036 "artificial" first and last lines tacked on. In rare cases
2037 where these routines are able to generate three digits and a
2038 Space at the beginning of any line, the beginning of each
2039 text line should be offset by some neutral text, like Space.
2041 This scheme assumes that multi-line replies may not be nested.
2043 The three digits of the reply each have a special significance.
2044 This is intended to allow a range of very simple to very
2045 sophisticated responses by the user-process. The first digit
2046 denotes whether the response is good, bad or incomplete.
2047 (Referring to the state diagram), an unsophisticated user-process
2048 will be able to determine its next action (proceed as planned,
2051 Postel & Reynolds [Page 36]
2055 RFC 959 October 1985
2056 File Transfer Protocol
2059 redo, retrench, etc.) by simply examining this first digit. A
2060 user-process that wants to know approximately what kind of error
2061 occurred (e.g. file system error, command syntax error) may
2062 examine the second digit, reserving the third digit for the finest
2063 gradation of information (e.g., RNTO command without a preceding
2066 There are five values for the first digit of the reply code:
2068 1yz Positive Preliminary reply
2070 The requested action is being initiated; expect another
2071 reply before proceeding with a new command. (The
2072 user-process sending another command before the
2073 completion reply would be in violation of protocol; but
2074 server-FTP processes should queue any commands that
2075 arrive while a preceding command is in progress.) This
2076 type of reply can be used to indicate that the command
2077 was accepted and the user-process may now pay attention
2078 to the data connections, for implementations where
2079 simultaneous monitoring is difficult. The server-FTP
2080 process may send at most, one 1yz reply per command.
2082 2yz Positive Completion reply
2084 The requested action has been successfully completed. A
2085 new request may be initiated.
2087 3yz Positive Intermediate reply
2089 The command has been accepted, but the requested action
2090 is being held in abeyance, pending receipt of further
2091 information. The user should send another command
2092 specifying this information. This reply is used in
2093 command sequence groups.
2095 4yz Transient Negative Completion reply
2097 The command was not accepted and the requested action did
2098 not take place, but the error condition is temporary and
2099 the action may be requested again. The user should
2100 return to the beginning of the command sequence, if any.
2101 It is difficult to assign a meaning to "transient",
2102 particularly when two distinct sites (Server- and
2103 User-processes) have to agree on the interpretation.
2104 Each reply in the 4yz category might have a slightly
2105 different time value, but the intent is that the
2108 Postel & Reynolds [Page 37]
2112 RFC 959 October 1985
2113 File Transfer Protocol
2116 user-process is encouraged to try again. A rule of thumb
2117 in determining if a reply fits into the 4yz or the 5yz
2118 (Permanent Negative) category is that replies are 4yz if
2119 the commands can be repeated without any change in
2120 command form or in properties of the User or Server
2121 (e.g., the command is spelled the same with the same
2122 arguments used; the user does not change his file access
2123 or user name; the server does not put up a new
2126 5yz Permanent Negative Completion reply
2128 The command was not accepted and the requested action did
2129 not take place. The User-process is discouraged from
2130 repeating the exact request (in the same sequence). Even
2131 some "permanent" error conditions can be corrected, so
2132 the human user may want to direct his User-process to
2133 reinitiate the command sequence by direct action at some
2134 point in the future (e.g., after the spelling has been
2135 changed, or the user has altered his directory status.)
2137 The following function groupings are encoded in the second
2140 x0z Syntax - These replies refer to syntax errors,
2141 syntactically correct commands that don't fit any
2142 functional category, unimplemented or superfluous
2145 x1z Information - These are replies to requests for
2146 information, such as status or help.
2148 x2z Connections - Replies referring to the control and
2151 x3z Authentication and accounting - Replies for the login
2152 process and accounting procedures.
2154 x4z Unspecified as yet.
2156 x5z File system - These replies indicate the status of the
2157 Server file system vis-a-vis the requested transfer or
2158 other file system action.
2160 The third digit gives a finer gradation of meaning in each of
2161 the function categories, specified by the second digit. The
2162 list of replies below will illustrate this. Note that the text
2165 Postel & Reynolds [Page 38]
2169 RFC 959 October 1985
2170 File Transfer Protocol
2173 associated with each reply is recommended, rather than
2174 mandatory, and may even change according to the command with
2175 which it is associated. The reply codes, on the other hand,
2176 must strictly follow the specifications in the last section;
2177 that is, Server implementations should not invent new codes for
2178 situations that are only slightly different from the ones
2179 described here, but rather should adapt codes already defined.
2181 A command such as TYPE or ALLO whose successful execution
2182 does not offer the user-process any new information will
2183 cause a 200 reply to be returned. If the command is not
2184 implemented by a particular Server-FTP process because it
2185 has no relevance to that computer system, for example ALLO
2186 at a TOPS20 site, a Positive Completion reply is still
2187 desired so that the simple User-process knows it can proceed
2188 with its course of action. A 202 reply is used in this case
2189 with, for example, the reply text: "No storage allocation
2190 necessary." If, on the other hand, the command requests a
2191 non-site-specific action and is unimplemented, the response
2192 is 502. A refinement of that is the 504 reply for a command
2193 that is implemented, but that requests an unimplemented
2196 4.2.1 Reply Codes by Function Groups
2199 500 Syntax error, command unrecognized.
2200 This may include errors such as command line too long.
2201 501 Syntax error in parameters or arguments.
2202 202 Command not implemented, superfluous at this site.
2203 502 Command not implemented.
2204 503 Bad sequence of commands.
2205 504 Command not implemented for that parameter.
2222 Postel & Reynolds [Page 39]
2226 RFC 959 October 1985
2227 File Transfer Protocol
2230 110 Restart marker reply.
2231 In this case, the text is exact and not left to the
2232 particular implementation; it must read:
2234 Where yyyy is User-process data stream marker, and mmmm
2235 server's equivalent marker (note the spaces between markers
2237 211 System status, or system help reply.
2238 212 Directory status.
2241 On how to use the server or the meaning of a particular
2242 non-standard command. This reply is useful only to the
2244 215 NAME system type.
2245 Where NAME is an official system name from the list in the
2246 Assigned Numbers document.
2248 120 Service ready in nnn minutes.
2249 220 Service ready for new user.
2250 221 Service closing control connection.
2251 Logged out if appropriate.
2252 421 Service not available, closing control connection.
2253 This may be a reply to any command if the service knows it
2255 125 Data connection already open; transfer starting.
2256 225 Data connection open; no transfer in progress.
2257 425 Can't open data connection.
2258 226 Closing data connection.
2259 Requested file action successful (for example, file
2260 transfer or file abort).
2261 426 Connection closed; transfer aborted.
2262 227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
2264 230 User logged in, proceed.
2266 331 User name okay, need password.
2267 332 Need account for login.
2268 532 Need account for storing files.
2279 Postel & Reynolds [Page 40]
2283 RFC 959 October 1985
2284 File Transfer Protocol
2287 150 File status okay; about to open data connection.
2288 250 Requested file action okay, completed.
2289 257 "PATHNAME" created.
2290 350 Requested file action pending further information.
2291 450 Requested file action not taken.
2292 File unavailable (e.g., file busy).
2293 550 Requested action not taken.
2294 File unavailable (e.g., file not found, no access).
2295 451 Requested action aborted. Local error in processing.
2296 551 Requested action aborted. Page type unknown.
2297 452 Requested action not taken.
2298 Insufficient storage space in system.
2299 552 Requested file action aborted.
2300 Exceeded storage allocation (for current directory or
2302 553 Requested action not taken.
2303 File name not allowed.
2306 4.2.2 Numeric Order List of Reply Codes
2308 110 Restart marker reply.
2309 In this case, the text is exact and not left to the
2310 particular implementation; it must read:
2312 Where yyyy is User-process data stream marker, and mmmm
2313 server's equivalent marker (note the spaces between markers
2315 120 Service ready in nnn minutes.
2316 125 Data connection already open; transfer starting.
2317 150 File status okay; about to open data connection.
2336 Postel & Reynolds [Page 41]
2340 RFC 959 October 1985
2341 File Transfer Protocol
2345 202 Command not implemented, superfluous at this site.
2346 211 System status, or system help reply.
2347 212 Directory status.
2350 On how to use the server or the meaning of a particular
2351 non-standard command. This reply is useful only to the
2353 215 NAME system type.
2354 Where NAME is an official system name from the list in the
2355 Assigned Numbers document.
2356 220 Service ready for new user.
2357 221 Service closing control connection.
2358 Logged out if appropriate.
2359 225 Data connection open; no transfer in progress.
2360 226 Closing data connection.
2361 Requested file action successful (for example, file
2362 transfer or file abort).
2363 227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
2364 230 User logged in, proceed.
2365 250 Requested file action okay, completed.
2366 257 "PATHNAME" created.
2368 331 User name okay, need password.
2369 332 Need account for login.
2370 350 Requested file action pending further information.
2372 421 Service not available, closing control connection.
2373 This may be a reply to any command if the service knows it
2375 425 Can't open data connection.
2376 426 Connection closed; transfer aborted.
2377 450 Requested file action not taken.
2378 File unavailable (e.g., file busy).
2379 451 Requested action aborted: local error in processing.
2380 452 Requested action not taken.
2381 Insufficient storage space in system.
2393 Postel & Reynolds [Page 42]
2397 RFC 959 October 1985
2398 File Transfer Protocol
2401 500 Syntax error, command unrecognized.
2402 This may include errors such as command line too long.
2403 501 Syntax error in parameters or arguments.
2404 502 Command not implemented.
2405 503 Bad sequence of commands.
2406 504 Command not implemented for that parameter.
2408 532 Need account for storing files.
2409 550 Requested action not taken.
2410 File unavailable (e.g., file not found, no access).
2411 551 Requested action aborted: page type unknown.
2412 552 Requested file action aborted.
2413 Exceeded storage allocation (for current directory or
2415 553 Requested action not taken.
2416 File name not allowed.
2419 5. DECLARATIVE SPECIFICATIONS
2421 5.1. MINIMUM IMPLEMENTATION
2423 In order to make FTP workable without needless error messages, the
2424 following minimum implementation is required for all servers:
2426 TYPE - ASCII Non-print
2428 STRUCTURE - File, Record
2429 COMMANDS - USER, QUIT, PORT,
2431 for the default values
2435 The default values for transfer parameters are:
2437 TYPE - ASCII Non-print
2441 All hosts must accept the above as the standard defaults.
2450 Postel & Reynolds [Page 43]
2454 RFC 959 October 1985
2455 File Transfer Protocol
2460 The server protocol interpreter shall "listen" on Port L. The
2461 user or user protocol interpreter shall initiate the full-duplex
2462 control connection. Server- and user- processes should follow the
2463 conventions of the Telnet protocol as specified in the
2464 ARPA-Internet Protocol Handbook [1]. Servers are under no
2465 obligation to provide for editing of command lines and may require
2466 that it be done in the user host. The control connection shall be
2467 closed by the server at the user's request after all transfers and
2468 replies are completed.
2470 The user-DTP must "listen" on the specified data port; this may be
2471 the default user port (U) or a port specified in the PORT command.
2472 The server shall initiate the data connection from his own default
2473 data port (L-1) using the specified user data port. The direction
2474 of the transfer and the port used will be determined by the FTP
2477 Note that all FTP implementation must support data transfer using
2478 the default port, and that only the USER-PI may initiate the use
2479 of non-default ports.
2481 When data is to be transferred between two servers, A and B (refer
2482 to Figure 2), the user-PI, C, sets up control connections with
2483 both server-PI's. One of the servers, say A, is then sent a PASV
2484 command telling him to "listen" on his data port rather than
2485 initiate a connection when he receives a transfer service command.
2486 When the user-PI receives an acknowledgment to the PASV command,
2487 which includes the identity of the host and port being listened
2488 on, the user-PI then sends A's port, a, to B in a PORT command; a
2489 reply is returned. The user-PI may then send the corresponding
2490 service commands to A and B. Server B initiates the connection
2491 and the transfer proceeds. The command-reply sequence is listed
2492 below where the messages are vertically synchronous but
2493 horizontally asynchronous:
2507 Postel & Reynolds [Page 44]
2511 RFC 959 October 1985
2512 File Transfer Protocol
2515 User-PI - Server A User-PI - Server B
2516 ------------------ ------------------
2518 C->A : Connect C->B : Connect
2520 A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
2521 C->B : PORT A1,A2,A3,A4,a1,a2
2523 C->A : STOR C->B : RETR
2524 B->A : Connect to HOST-A, PORT-a
2528 The data connection shall be closed by the server under the
2529 conditions described in the Section on Establishing Data
2530 Connections. If the data connection is to be closed following a
2531 data transfer where closing the connection is not required to
2532 indicate the end-of-file, the server must do so immediately.
2533 Waiting until after a new transfer command is not permitted
2534 because the user-process will have already tested the data
2535 connection to see if it needs to do a "listen"; (remember that the
2536 user must "listen" on a closed data port BEFORE sending the
2537 transfer request). To prevent a race condition here, the server
2538 sends a reply (226) after closing the data connection (or if the
2539 connection is left open, a "file transfer completed" reply (250)
2540 and the user-PI should wait for one of these replies before
2541 issuing a new transfer command).
2543 Any time either the user or server see that the connection is
2544 being closed by the other side, it should promptly read any
2545 remaining data queued on the connection and issue the close on its
2550 The commands are Telnet character strings transmitted over the
2551 control connections as described in the Section on FTP Commands.
2552 The command functions and semantics are described in the Section
2553 on Access Control Commands, Transfer Parameter Commands, FTP
2554 Service Commands, and Miscellaneous Commands. The command syntax
2557 The commands begin with a command code followed by an argument
2558 field. The command codes are four or fewer alphabetic characters.
2559 Upper and lower case alphabetic characters are to be treated
2560 identically. Thus, any of the following may represent the
2564 Postel & Reynolds [Page 45]
2568 RFC 959 October 1985
2569 File Transfer Protocol
2572 RETR Retr retr ReTr rETr
2574 This also applies to any symbols representing parameter values,
2575 such as A or a for ASCII TYPE. The command codes and the argument
2576 fields are separated by one or more spaces.
2578 The argument field consists of a variable length character string
2579 ending with the character sequence <CRLF> (Carriage Return, Line
2580 Feed) for NVT-ASCII representation; for other negotiated languages
2581 a different end of line character might be used. It should be
2582 noted that the server is to take no action until the end of line
2585 The syntax is specified below in NVT-ASCII. All characters in the
2586 argument field are ASCII characters including any ASCII
2587 represented decimal integers. Square brackets denote an optional
2588 argument field. If the option is not taken, the appropriate
2621 Postel & Reynolds [Page 46]
2625 RFC 959 October 1985
2626 File Transfer Protocol
2631 The following are the FTP commands:
2633 USER <SP> <username> <CRLF>
2634 PASS <SP> <password> <CRLF>
2635 ACCT <SP> <account-information> <CRLF>
2636 CWD <SP> <pathname> <CRLF>
2638 SMNT <SP> <pathname> <CRLF>
2641 PORT <SP> <host-port> <CRLF>
2643 TYPE <SP> <type-code> <CRLF>
2644 STRU <SP> <structure-code> <CRLF>
2645 MODE <SP> <mode-code> <CRLF>
2646 RETR <SP> <pathname> <CRLF>
2647 STOR <SP> <pathname> <CRLF>
2649 APPE <SP> <pathname> <CRLF>
2650 ALLO <SP> <decimal-integer>
2651 [<SP> R <SP> <decimal-integer>] <CRLF>
2652 REST <SP> <marker> <CRLF>
2653 RNFR <SP> <pathname> <CRLF>
2654 RNTO <SP> <pathname> <CRLF>
2656 DELE <SP> <pathname> <CRLF>
2657 RMD <SP> <pathname> <CRLF>
2658 MKD <SP> <pathname> <CRLF>
2660 LIST [<SP> <pathname>] <CRLF>
2661 NLST [<SP> <pathname>] <CRLF>
2662 SITE <SP> <string> <CRLF>
2664 STAT [<SP> <pathname>] <CRLF>
2665 HELP [<SP> <string>] <CRLF>
2678 Postel & Reynolds [Page 47]
2682 RFC 959 October 1985
2683 File Transfer Protocol
2686 5.3.2. FTP COMMAND ARGUMENTS
2688 The syntax of the above argument fields (using BNF notation
2689 where applicable) is:
2691 <username> ::= <string>
2692 <password> ::= <string>
2693 <account-information> ::= <string>
2694 <string> ::= <char> | <char><string>
2695 <char> ::= any of the 128 ASCII characters except <CR> and
2697 <marker> ::= <pr-string>
2698 <pr-string> ::= <pr-char> | <pr-char><pr-string>
2699 <pr-char> ::= printable characters, any
2700 ASCII code 33 through 126
2701 <byte-size> ::= <number>
2702 <host-port> ::= <host-number>,<port-number>
2703 <host-number> ::= <number>,<number>,<number>,<number>
2704 <port-number> ::= <number>,<number>
2705 <number> ::= any decimal integer 1 through 255
2706 <form-code> ::= N | T | C
2707 <type-code> ::= A [<sp> <form-code>]
2708 | E [<sp> <form-code>]
2710 | L <sp> <byte-size>
2711 <structure-code> ::= F | R | P
2712 <mode-code> ::= S | B | C
2713 <pathname> ::= <string>
2714 <decimal-integer> ::= any decimal integer
2735 Postel & Reynolds [Page 48]
2739 RFC 959 October 1985
2740 File Transfer Protocol
2743 5.4. SEQUENCING OF COMMANDS AND REPLIES
2745 The communication between the user and server is intended to be an
2746 alternating dialogue. As such, the user issues an FTP command and
2747 the server responds with a prompt primary reply. The user should
2748 wait for this initial primary success or failure response before
2749 sending further commands.
2751 Certain commands require a second reply for which the user should
2752 also wait. These replies may, for example, report on the progress
2753 or completion of file transfer or the closing of the data
2754 connection. They are secondary replies to file transfer commands.
2756 One important group of informational replies is the connection
2757 greetings. Under normal circumstances, a server will send a 220
2758 reply, "awaiting input", when the connection is completed. The
2759 user should wait for this greeting message before sending any
2760 commands. If the server is unable to accept input right away, a
2761 120 "expected delay" reply should be sent immediately and a 220
2762 reply when ready. The user will then know not to hang up if there
2767 Sometimes "the system" spontaneously has a message to be sent
2768 to a user (usually all users). For example, "System going down
2769 in 15 minutes". There is no provision in FTP for such
2770 spontaneous information to be sent from the server to the user.
2771 It is recommended that such information be queued in the
2772 server-PI and delivered to the user-PI in the next reply
2773 (possibly making it a multi-line reply).
2775 The table below lists alternative success and failure replies for
2776 each command. These must be strictly adhered to; a server may
2777 substitute text in the replies, but the meaning and action implied
2778 by the code numbers and by the specific command reply sequence
2781 Command-Reply Sequences
2783 In this section, the command-reply sequence is presented. Each
2784 command is listed with its possible replies; command groups are
2785 listed together. Preliminary replies are listed first (with
2786 their succeeding replies indented and under them), then
2787 positive and negative completion, and finally intermediary
2792 Postel & Reynolds [Page 49]
2796 RFC 959 October 1985
2797 File Transfer Protocol
2800 replies with the remaining commands from the sequence
2801 following. This listing forms the basis for the state
2802 diagrams, which will be presented separately.
2804 Connection Establishment
2828 500, 501, 502, 421, 530, 550
2831 500, 501, 502, 421, 530, 550
2834 500, 501, 502, 421, 530, 550
2849 Postel & Reynolds [Page 50]
2853 RFC 959 October 1985
2854 File Transfer Protocol
2863 500, 501, 502, 421, 530
2866 500, 501, 504, 421, 530
2869 500, 501, 504, 421, 530
2872 500, 501, 504, 421, 530
2873 File action commands
2877 500, 501, 504, 421, 530
2879 500, 501, 502, 421, 530
2885 425, 426, 451, 551, 552
2892 425, 426, 451, 551, 552
2906 Postel & Reynolds [Page 51]
2910 RFC 959 October 1985
2911 File Transfer Protocol
2919 500, 501, 502, 421, 530
2925 500, 501, 502, 421, 530
2930 425, 426, 451, 551, 552
2931 532, 450, 550, 452, 553
2932 500, 501, 502, 421, 530
2935 500, 501, 502, 421, 530
2940 500, 501, 502, 503, 421, 530
2944 500, 501, 502, 421, 530
2947 500, 501, 502, 421, 530, 550
2950 500, 501, 502, 421, 530, 550
2953 500, 501, 502, 421, 550
2963 Postel & Reynolds [Page 52]
2967 RFC 959 October 1985
2968 File Transfer Protocol
2971 Informational commands
2978 500, 501, 502, 421, 530
2982 Miscellaneous commands
3020 Postel & Reynolds [Page 53]
3024 RFC 959 October 1985
3025 File Transfer Protocol
3030 Here we present state diagrams for a very simple minded FTP
3031 implementation. Only the first digit of the reply codes is used.
3032 There is one state diagram for each group of FTP commands or command
3035 The command groupings were determined by constructing a model for
3036 each command then collecting together the commands with structurally
3039 For each command or command sequence there are three possible
3040 outcomes: success (S), failure (F), and error (E). In the state
3041 diagrams below we use the symbol B for "begin", and the symbol W for
3044 We first present the diagram that represents the largest group of FTP
3052 +---+ cmd +---+ 2 +---+
3053 | B |---------->| W |---------->| S |
3061 This diagram models the commands:
3063 ABOR, ALLO, DELE, CWD, CDUP, SMNT, HELP, MODE, NOOP, PASV,
3064 QUIT, SITE, PORT, SYST, STAT, RMD, MKD, PWD, STRU, and TYPE.
3077 Postel & Reynolds [Page 54]
3081 RFC 959 October 1985
3082 File Transfer Protocol
3085 The other large group of commands is represented by a very similar
3093 +---+ cmd +---+ 2 +---+
3094 | B |---------->| W |---------->| S |
3095 +---+ --->+---+ +---+
3098 | 1 | ----------->| F |
3102 This diagram models the commands:
3104 APPE, LIST, NLST, REIN, RETR, STOR, and STOU.
3106 Note that this second model could also be used to represent the first
3107 group of commands, the only difference being that in the first group
3108 the 100 series replies are unexpected and therefore treated as error,
3109 while the second group expects (some may require) 100 series replies.
3110 Remember that at most, one 100 series reply is allowed per command.
3112 The remaining diagrams model command sequences, perhaps the simplest
3113 of these is the rename sequence:
3116 +---+ RNFR +---+ 1,2 +---+
3117 | B |---------->| W |---------->| E |
3118 +---+ +---+ -->+---+
3121 -------------- ------ |
3123 | ------------->| S |
3128 +---+ RNTO +---+ 4,5 ----->+---+
3129 | |---------->| W |---------->| F |
3134 Postel & Reynolds [Page 55]
3138 RFC 959 October 1985
3139 File Transfer Protocol
3142 The next diagram is a simple model of the Restart command:
3145 +---+ REST +---+ 1,2 +---+
3146 | B |---------->| W |---------->| E |
3147 +---+ +---+ -->+---+
3150 -------------- ------ |
3152 | ------------->| S |
3157 +---+ cmd +---+ 4,5 ----->+---+
3158 | |---------->| W |---------->| F |
3159 +---+ -->+---+ +---+
3165 Where "cmd" is APPE, STOR, or RETR.
3167 We note that the above three models are similar. The Restart differs
3168 from the Rename two only in the treatment of 100 series replies at
3169 the second stage, while the second group expects (some may require)
3170 100 series replies. Remember that at most, one 100 series reply is
3171 allowed per command.
3191 Postel & Reynolds [Page 56]
3195 RFC 959 October 1985
3196 File Transfer Protocol
3199 The most complicated diagram is for the Login sequence:
3203 +---+ USER +---+------------->+---+
3204 | B |---------->| W | 2 ---->| E |
3205 +---+ +---+------ | -->+---+
3208 -------------- ----- | | |
3214 +---+ PASS +---+ 2 | ------>+---+
3215 | |---------->| W |------------->| S |
3216 +---+ +---+ ---------->+---+
3219 -------------- -------- |
3225 +---+ ACCT +---+-- | ----->+---+
3226 | |---------->| W | 4,5 -------->| F |
3227 +---+ +---+------------->+---+
3248 Postel & Reynolds [Page 57]
3252 RFC 959 October 1985
3253 File Transfer Protocol
3256 Finally, we present a generalized diagram that could be used to model
3257 the command and reply interchange:
3260 ------------------------------------
3264 | +---+ cmd +---+ 2 +---+ |
3265 -->| |------->| |---------->| | |
3266 | | | W | | S |-----|
3267 -->| | -->| |----- | | |
3268 | +---+ | +---+ 4,5 | +---+ |
3270 | | | 1| |3 | +---+ |
3272 | | ---- | ---->| F |-----
3305 Postel & Reynolds [Page 58]
3309 RFC 959 October 1985
3310 File Transfer Protocol
3313 7. TYPICAL FTP SCENARIO
3315 User at host U wanting to transfer files to/from host S:
3317 In general, the user will communicate to the server via a mediating
3318 user-FTP process. The following may be a typical scenario. The
3319 user-FTP prompts are shown in parentheses, '---->' represents
3320 commands from host U to host S, and '<----' represents replies from
3323 LOCAL COMMANDS BY USER ACTION INVOLVED
3325 ftp (host) multics<CR> Connect to host S, port L,
3326 establishing control connections.
3327 <---- 220 Service ready <CRLF>.
3328 username Doe <CR> USER Doe<CRLF>---->
3329 <---- 331 User name ok,
3330 need password<CRLF>.
3331 password mumble <CR> PASS mumble<CRLF>---->
3332 <---- 230 User logged in<CRLF>.
3333 retrieve (local type) ASCII<CR>
3334 (local pathname) test 1 <CR> User-FTP opens local file in ASCII.
3335 (for. pathname) test.pl1<CR> RETR test.pl1<CRLF> ---->
3336 <---- 150 File status okay;
3339 Server makes data connection
3342 <---- 226 Closing data connection,
3343 file transfer successful<CRLF>.
3344 type Image<CR> TYPE I<CRLF> ---->
3345 <---- 200 Command OK<CRLF>
3346 store (local type) image<CR>
3347 (local pathname) file dump<CR> User-FTP opens local file in Image.
3348 (for.pathname) >udd>cn>fd<CR> STOR >udd>cn>fd<CRLF> ---->
3349 <---- 550 Access denied<CRLF>
3350 terminate QUIT <CRLF> ---->
3354 8. CONNECTION ESTABLISHMENT
3356 The FTP control connection is established via TCP between the user
3357 process port U and the server process port L. This protocol is
3358 assigned the service port 21 (25 octal), that is L=21.
3362 Postel & Reynolds [Page 59]
3366 RFC 959 October 1985
3367 File Transfer Protocol
3370 APPENDIX I - PAGE STRUCTURE
3372 The need for FTP to support page structure derives principally from
3373 the need to support efficient transmission of files between TOPS-20
3374 systems, particularly the files used by NLS.
3376 The file system of TOPS-20 is based on the concept of pages. The
3377 operating system is most efficient at manipulating files as pages.
3378 The operating system provides an interface to the file system so that
3379 many applications view files as sequential streams of characters.
3380 However, a few applications use the underlying page structures
3381 directly, and some of these create holey files.
3383 A TOPS-20 disk file consists of four things: a pathname, a page
3384 table, a (possibly empty) set of pages, and a set of attributes.
3386 The pathname is specified in the RETR or STOR command. It includes
3387 the directory name, file name, file name extension, and generation
3390 The page table contains up to 2**18 entries. Each entry may be
3391 EMPTY, or may point to a page. If it is not empty, there are also
3392 some page-specific access bits; not all pages of a file need have the
3393 same access protection.
3395 A page is a contiguous set of 512 words of 36 bits each.
3397 The attributes of the file, in the File Descriptor Block (FDB),
3398 contain such things as creation time, write time, read time, writer's
3399 byte-size, end-of-file pointer, count of reads and writes, backup
3400 system tape numbers, etc.
3402 Note that there is NO requirement that entries in the page table be
3403 contiguous. There may be empty page table slots between occupied
3404 ones. Also, the end of file pointer is simply a number. There is no
3405 requirement that it in fact point at the "last" datum in the file.
3406 Ordinary sequential I/O calls in TOPS-20 will cause the end of file
3407 pointer to be left after the last datum written, but other operations
3408 may cause it not to be so, if a particular programming system so
3411 In fact, in both of these special cases, "holey" files and
3412 end-of-file pointers NOT at the end of the file, occur with NLS data
3419 Postel & Reynolds [Page 60]
3423 RFC 959 October 1985
3424 File Transfer Protocol
3427 The TOPS-20 paged files can be sent with the FTP transfer parameters:
3428 TYPE L 36, STRU P, and MODE S (in fact, any mode could be used).
3430 Each page of information has a header. Each header field, which is a
3431 logical byte, is a TOPS-20 word, since the TYPE is L 36.
3433 The header fields are:
3435 Word 0: Header Length.
3437 The header length is 5.
3441 If the data is a disk file page, this is the number of that
3442 page in the file's page map. Empty pages (holes) in the file
3443 are simply not sent. Note that a hole is NOT the same as a
3446 Word 2: Data Length.
3448 The number of data words in this page, following the header.
3449 Thus, the total length of the transmission unit is the Header
3450 Length plus the Data Length.
3454 A code for what type of chunk this is. A data page is type 3,
3455 the FDB page is type 2.
3457 Word 4: Page Access Control.
3459 The access bits associated with the page in the file's page
3460 map. (This full word quantity is put into AC2 of an SPACS by
3461 the program reading from net to disk.)
3463 After the header are Data Length data words. Data Length is
3464 currently either 512 for a data page or 31 for an FDB. Trailing
3465 zeros in a disk file page may be discarded, making Data Length less
3466 than 512 in that case.
3476 Postel & Reynolds [Page 61]
3480 RFC 959 October 1985
3481 File Transfer Protocol
3484 APPENDIX II - DIRECTORY COMMANDS
3486 Since UNIX has a tree-like directory structure in which directories
3487 are as easy to manipulate as ordinary files, it is useful to expand
3488 the FTP servers on these machines to include commands which deal with
3489 the creation of directories. Since there are other hosts on the
3490 ARPA-Internet which have tree-like directories (including TOPS-20 and
3491 Multics), these commands are as general as possible.
3493 Four directory commands have been added to FTP:
3497 Make a directory with the name "pathname".
3501 Remove the directory with the name "pathname".
3505 Print the current working directory name.
3509 Change to the parent of the current working directory.
3511 The "pathname" argument should be created (removed) as a
3512 subdirectory of the current working directory, unless the "pathname"
3513 string contains sufficient information to specify otherwise to the
3514 server, e.g., "pathname" is an absolute pathname (in UNIX and
3515 Multics), or pathname is something like "<abso.lute.path>" to
3520 The CDUP command is a special case of CWD, and is included to
3521 simplify the implementation of programs for transferring directory
3522 trees between operating systems having different syntaxes for
3523 naming the parent directory. The reply codes for CDUP be
3524 identical to the reply codes of CWD.
3526 The reply codes for RMD be identical to the reply codes for its
3527 file analogue, DELE.
3529 The reply codes for MKD, however, are a bit more complicated. A
3530 freshly created directory will probably be the object of a future
3533 Postel & Reynolds [Page 62]
3537 RFC 959 October 1985
3538 File Transfer Protocol
3541 CWD command. Unfortunately, the argument to MKD may not always be
3542 a suitable argument for CWD. This is the case, for example, when
3543 a TOPS-20 subdirectory is created by giving just the subdirectory
3544 name. That is, with a TOPS-20 server FTP, the command sequence
3549 will fail. The new directory may only be referred to by its
3550 "absolute" name; e.g., if the MKD command above were issued while
3551 connected to the directory <DFRANKLIN>, the new subdirectory
3552 could only be referred to by the name <DFRANKLIN.MYDIR>.
3554 Even on UNIX and Multics, however, the argument given to MKD may
3555 not be suitable. If it is a "relative" pathname (i.e., a pathname
3556 which is interpreted relative to the current directory), the user
3557 would need to be in the same current directory in order to reach
3558 the subdirectory. Depending on the application, this may be
3559 inconvenient. It is not very robust in any case.
3561 To solve these problems, upon successful completion of an MKD
3562 command, the server should return a line of the form:
3564 257<space>"<directory-name>"<space><commentary>
3566 That is, the server will tell the user what string to use when
3567 referring to the created directory. The directory name can
3568 contain any character; embedded double-quotes should be escaped by
3569 double-quotes (the "quote-doubling" convention).
3571 For example, a user connects to the directory /usr/dm, and creates
3572 a subdirectory, named pathname:
3575 200 directory changed to /usr/dm
3577 257 "/usr/dm/pathname" directory created
3579 An example with an embedded double quote:
3582 257 "/usr/dm/foo""bar" directory created
3584 200 directory changed to /usr/dm/foo"bar
3590 Postel & Reynolds [Page 63]
3594 RFC 959 October 1985
3595 File Transfer Protocol
3598 The prior existence of a subdirectory with the same name is an
3599 error, and the server must return an "access denied" error reply
3603 200 directory changed to /usr/dm
3605 521-"/usr/dm/pathname" directory already exists;
3606 521 taking no action.
3608 The failure replies for MKD are analogous to its file creating
3609 cousin, STOR. Also, an "access denied" return is given if a file
3610 name with the same name as the subdirectory will conflict with the
3611 creation of the subdirectory (this is a problem on UNIX, but
3612 shouldn't be one on TOPS-20).
3614 Essentially because the PWD command returns the same type of
3615 information as the successful MKD command, the successful PWD
3616 command uses the 257 reply code as well.
3620 Because these commands will be most useful in transferring
3621 subtrees from one machine to another, carefully observe that the
3622 argument to MKD is to be interpreted as a sub-directory of the
3623 current working directory, unless it contains enough information
3624 for the destination host to tell otherwise. A hypothetical
3625 example of its use in the TOPS-20 world:
3628 200 Working directory changed
3630 257 "<some.where.overrainbow>" directory created
3632 431 No such directory
3633 CWD <some.where.overrainbow>
3634 200 Working directory changed
3637 200 Working directory changed to <some.where>
3639 257 "<unambiguous>" directory created
3642 Note that the first example results in a subdirectory of the
3643 connected directory. In contrast, the argument in the second
3644 example contains enough information for TOPS-20 to tell that the
3647 Postel & Reynolds [Page 64]
3651 RFC 959 October 1985
3652 File Transfer Protocol
3655 <unambiguous> directory is a top-level directory. Note also that
3656 in the first example the user "violated" the protocol by
3657 attempting to access the freshly created directory with a name
3658 other than the one returned by TOPS-20. Problems could have
3659 resulted in this case had there been an <overrainbow> directory;
3660 this is an ambiguity inherent in some TOPS-20 implementations.
3661 Similar considerations apply to the RMD command. The point is
3662 this: except where to do so would violate a host's conventions for
3663 denoting relative versus absolute pathnames, the host should treat
3664 the operands of the MKD and RMD commands as subdirectories. The
3665 257 reply to the MKD command must always contain the absolute
3666 pathname of the created directory.
3704 Postel & Reynolds [Page 65]
3708 RFC 959 October 1985
3709 File Transfer Protocol
3712 APPENDIX III - RFCs on FTP
3714 Bhushan, Abhay, "A File Transfer Protocol", RFC 114 (NIC 5823),
3715 MIT-Project MAC, 16 April 1971.
3717 Harslem, Eric, and John Heafner, "Comments on RFC 114 (A File
3718 Transfer Protocol)", RFC 141 (NIC 6726), RAND, 29 April 1971.
3720 Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 172
3721 (NIC 6794), MIT-Project MAC, 23 June 1971.
3723 Braden, Bob, "Comments on DTP and FTP Proposals", RFC 238 (NIC 7663),
3724 UCLA/CCN, 29 September 1971.
3726 Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 265
3727 (NIC 7813), MIT-Project MAC, 17 November 1971.
3729 McKenzie, Alex, "A Suggested Addition to File Transfer Protocol",
3730 RFC 281 (NIC 8163), BBN, 8 December 1971.
3732 Bhushan, Abhay, "The Use of "Set Data Type" Transaction in File
3733 Transfer Protocol", RFC 294 (NIC 8304), MIT-Project MAC,
3736 Bhushan, Abhay, "The File Transfer Protocol", RFC 354 (NIC 10596),
3737 MIT-Project MAC, 8 July 1972.
3739 Bhushan, Abhay, "Comments on the File Transfer Protocol (RFC 354)",
3740 RFC 385 (NIC 11357), MIT-Project MAC, 18 August 1972.
3742 Hicks, Greg, "User FTP Documentation", RFC 412 (NIC 12404), Utah,
3745 Bhushan, Abhay, "File Transfer Protocol (FTP) Status and Further
3746 Comments", RFC 414 (NIC 12406), MIT-Project MAC, 20 November 1972.
3748 Braden, Bob, "Comments on File Transfer Protocol", RFC 430
3749 (NIC 13299), UCLA/CCN, 7 February 1973.
3751 Thomas, Bob, and Bob Clements, "FTP Server-Server Interaction",
3752 RFC 438 (NIC 13770), BBN, 15 January 1973.
3754 Braden, Bob, "Print Files in FTP", RFC 448 (NIC 13299), UCLA/CCN,
3757 McKenzie, Alex, "File Transfer Protocol", RFC 454 (NIC 14333), BBN,
3761 Postel & Reynolds [Page 66]
3765 RFC 959 October 1985
3766 File Transfer Protocol
3769 Bressler, Bob, and Bob Thomas, "Mail Retrieval via FTP", RFC 458
3770 (NIC 14378), BBN-NET and BBN-TENEX, 20 February 1973.
3772 Neigus, Nancy, "File Transfer Protocol", RFC 542 (NIC 17759), BBN,
3775 Krilanovich, Mark, and George Gregg, "Comments on the File Transfer
3776 Protocol", RFC 607 (NIC 21255), UCSB, 7 January 1974.
3778 Pogran, Ken, and Nancy Neigus, "Response to RFC 607 - Comments on the
3779 File Transfer Protocol", RFC 614 (NIC 21530), BBN, 28 January 1974.
3781 Krilanovich, Mark, George Gregg, Wayne Hathaway, and Jim White,
3782 "Comments on the File Transfer Protocol", RFC 624 (NIC 22054), UCSB,
3783 Ames Research Center, SRI-ARC, 28 February 1974.
3785 Bhushan, Abhay, "FTP Comments and Response to RFC 430", RFC 463
3786 (NIC 14573), MIT-DMCG, 21 February 1973.
3788 Braden, Bob, "FTP Data Compression", RFC 468 (NIC 14742), UCLA/CCN,
3791 Bhushan, Abhay, "FTP and Network Mail System", RFC 475 (NIC 14919),
3792 MIT-DMCG, 6 March 1973.
3794 Bressler, Bob, and Bob Thomas "FTP Server-Server Interaction - II",
3795 RFC 478 (NIC 14947), BBN-NET and BBN-TENEX, 26 March 1973.
3797 White, Jim, "Use of FTP by the NIC Journal", RFC 479 (NIC 14948),
3798 SRI-ARC, 8 March 1973.
3800 White, Jim, "Host-Dependent FTP Parameters", RFC 480 (NIC 14949),
3801 SRI-ARC, 8 March 1973.
3803 Padlipsky, Mike, "An FTP Command-Naming Problem", RFC 506
3804 (NIC 16157), MIT-Multics, 26 June 1973.
3806 Day, John, "Memo to FTP Group (Proposal for File Access Protocol)",
3807 RFC 520 (NIC 16819), Illinois, 25 June 1973.
3809 Merryman, Robert, "The UCSD-CC Server-FTP Facility", RFC 532
3810 (NIC 17451), UCSD-CC, 22 June 1973.
3812 Braden, Bob, "TENEX FTP Problem", RFC 571 (NIC 18974), UCLA/CCN,
3818 Postel & Reynolds [Page 67]
3822 RFC 959 October 1985
3823 File Transfer Protocol
3826 McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
3827 Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
3830 Sussman, Julie, "FTP Error Code Usage for More Reliable Mail
3831 Service", RFC 630 (NIC 30237), BBN, 10 April 1974.
3833 Postel, Jon, "Revised FTP Reply Codes", RFC 640 (NIC 30843),
3834 UCLA/NMC, 5 June 1974.
3836 Harvey, Brian, "Leaving Well Enough Alone", RFC 686 (NIC 32481),
3839 Harvey, Brian, "One More Try on the FTP", RFC 691 (NIC 32700), SU-AI,
3842 Lieb, J., "CWD Command of FTP", RFC 697 (NIC 32963), 14 July 1975.
3844 Harrenstien, Ken, "FTP Extension: XSEN", RFC 737 (NIC 42217), SRI-KL,
3847 Harrenstien, Ken, "FTP Extension: XRSQ/XRCP", RFC 743 (NIC 42758),
3848 SRI-KL, 30 December 1977.
3850 Lebling, P. David, "Survey of FTP Mail and MLFL", RFC 751, MIT,
3853 Postel, Jon, "File Transfer Protocol Specification", RFC 765, ISI,
3856 Mankins, David, Dan Franklin, and Buzz Owen, "Directory Oriented FTP
3857 Commands", RFC 776, BBN, December 1980.
3859 Padlipsky, Michael, "FTP Unique-Named Store Command", RFC 949, MITRE,
3875 Postel & Reynolds [Page 68]
3879 RFC 959 October 1985
3880 File Transfer Protocol
3885 [1] Feinler, Elizabeth, "Internet Protocol Transition Workbook",
3886 Network Information Center, SRI International, March 1982.
3888 [2] Postel, Jon, "Transmission Control Protocol - DARPA Internet
3889 Program Protocol Specification", RFC 793, DARPA, September 1981.
3891 [3] Postel, Jon, and Joyce Reynolds, "Telnet Protocol
3892 Specification", RFC 854, ISI, May 1983.
3894 [4] Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC 943,
3932 Postel & Reynolds [Page 69]
projects / oftp / blob