TitleRevised FTP reply codes
AuthorJ. Postel
DateJune 1974
Format:TXT, HTML

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Revised FTP Reply Codes

                                                           Jon Postel
                                                            19 JUN 75

                          Revised FTP Reply Codes                          1

    This document describes a revised set of reply codes for the File
    Transfer Protocol.                                                     2

    The aim of this revision is to satisfy the goal of using reply
    codes to enable the command issuing process to easily determine
    the outcome of each command. The user protocol interpreter should
    be able to determine the success or failure of a command by
    examining the first digit of the reply code.                           3

    An important change in the sequencing of commands and replies
    which may not be obvious in the following documents concerns the
    establishment of the data connection.                                  4

       In the previous FTP specifications when an actual transfer
       command (STOR, RETR, APPE, LIST, NLIST, MLFL) was issued the
       preliminary reply was sent after the data connection was
       established. This presented a problem for some user protocol
       interpreters which had difficulty monitoring two connections
       asynchronously.                                                    4a

       The current specification is that the preliminary reply to the
       actual transfer commands indicates that the file can be
       transferred and either the connection was previously
       established or an attempt is about to be made to establish the
       data connection.                                                   4b

    This reply code revision is a modification of the protocol in
    described in RFC 542, that is to say that the protocol
    implementation associated with socket number 21 (decimal) is the
    protocol specified by the combination of RFC 542 and this RFC.         5

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
                                                         Nancy Neigus
                                                           Ken Pogran
                                                           Jon Postel
                                                            19 JUN 75

                     A New Schema for FTP Reply Codes                      7

    Replies to File Transfer Protocol commands were devised to ensure
    the synchronization of requests and actions in the process of
    file transfer, and to guarantee that the user process always
    knows the state of the Server. Every command must generate at
    least one reply, although there may be more than one; in the
    latter case, the multiple replies must be easily distinguished.
    In addition, some commands occur in sequential groups, such as
    USER, PASS and ACCT, or RNFR and RNTO.  The replies show the
    existence of an intermediate state if all preceding commands have
    been successful.  A failure at any point in the sequence
    necessitates the repetition of the entire sequence from the
    beginning.                                                             8

       Details of the command-reply sequence will be made explicit in
       a state diagram.                                                   8a

    An FTP reply consists of a three digit number (transmitted as
    three alphanumeric characters) followed by some text.  The number
    is intended for use by automata to determine what state to enter
    next; the text is intended for the human user.  It is intended
    that the three digits contain enough encoded information that the
    user-process (the User-PI described in RFC 542) will not need to
    examine the text and may either discard it or pass it on to the
    user, as appropriate.  In particular, the text may be
    server-dependent, so there are likely to be varying texts for
    each reply code.                                                       9

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [3]

    indicate the state of the transaction.  To satisfy all factions
    it was decided that both the first and last line codes should be
    the same.                                                             10

       Thus the format for multi-line replies is that the first line
       will begin with the exact required reply code, followed
       immediately by a Hyphen, "-" (also known as Minus), followed
       by text.  The last line will begin with the same code,
       followed immediately by Space <SP>, optionally some text, and
       TELNET <eol>.                                                     10a

          For example:
                               123-First line
                               Second line
                                 234 A line beginning with numbers
                               123 The last line                        10a1

       The user-process then simply needs to search for the second
       occurrence of the same reply code, followed by <SP> (Space),
       at the beginning of a line, and ignore all intermediary lines.
       If an intermediary line begins with a 3-digit number, the
       Server must pad the front to avoid confusion.                     10b

          This scheme allows standard system routines to be used for
          reply information (such as for the STAT reply), with
          "artificial" first and last lines tacked on.  In the rare
          cases where these routines are able to generate three
          digits and a Space at the beginning of any line, the
          beginning of each text line should be offset by some
          neutral text, like Space.                                     10b1

       This scheme assumes that multi-line replies may not be nested.
       We  have found that, in general, nesting of replies will not
       occur, except for random system messages (called spontaneous
       replies in the previous FTP incarnations) which may interrupt
       another reply.  Spontaneous replies are no longer defined;
       system messages (i.e. those not processed by the FTP server)
       will NOT carry reply codes and may occur anywhere in the
       command-reply sequence.  They may be ignored by the
       User-process as they are only information for the human user.     10c

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [4]

    occurred (e.g. file system error, command syntax error) may
    examine the second digit, reserving the third digit for the
    finest gradation of information (e.g. RNTO command without a
    preceding RNFR.)                                                      11

       There are four values for the first digit of the reply code:      11a

       1yz   Positive Preliminary reply                                  11b

          The requested action is being initiated; expect another
          reply before proceeding with a new command.  (The
          user-process sending another command before the  completion
          reply would be in violation of protocol; but server-FTP
          processes should queue any commands that arrive while a
          preceeding command is in progress.)  This type of reply can
          be used to indicate that the command was accepted and the
          user-process may now pay attention to the data connections,
          for implementations where simultaneous monitoring is
          difficult.                                                    11b1

       2yz   Positive Completion reply                                   11c

          The requested action has been successfully completed.  A
          new request may be initiated.                                 11c1

       3yz   Positive Intermediate reply                                 11d

          The command has been accepted, but the requested action is
          being held in abeyance, pending receipt of further
          information.  The user should send another command
          specifying this information.  This reply is used in command
          sequence groups.                                              11d1

       4yz   Transient Negative Completion reply                         11e

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [5]

          arguments used; the user does not change his file access or
          user name; the server does not put up a new
          implementation.)                                              11e1

       5yz   Permanent Negative Completion reply                         11f

          The command was not accepted and the requested action did
          not take place.  The User-process is discouraged from
          repeating the exact request (in the same sequence).  Even
          some "permanent" error conditions can be corrected, so the
          human user may want to direct his User-process to
          reinitiate the command sequence by direct action at some
          point in the future (e.g. after the spelling has been
          changed, or the user has altered his directory status.)       11f1

       The following function groupings are encoded in the second
       digit:                                                            11g

          x0z   Syntax - These replies refer to syntax errors,
                  syntactically correct  commands that don't fit any
                  functional category, unimplemented or superfluous
                  commands.                                             11g1

          x1z   Information -  These are replies to requests for
                  information, such as status or help.                  11g2

          x2z   Connections - Replies referring to the TELNET and
                  data connections.                                     11g3

          x3z   Authentication and accounting - Replies for the logon
                  process and accounting procedures.                    11g4

          x4z   Unspecified as yet                                      11g5

          x5z   File system - These replies indicate the status of
                  the Server file system vis-a-vis the requested
                  transfer or other file system action.                 11g6

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [6]

       If additional codes are found to be necessary, the details
       should be submitted to the FTP committee, through Jon Postel.     11h

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [7]

          125  Data connection already open; transfer starting          11k5
          225  Data connection open; no transfer in progress            11k6
          425  Can't open data connection                               11k7
          226  Closing data connection; requested file action
                  successful (for example, file transfer or file
                  abort.)                                               11k8
          426  Connection trouble, closed; transfer aborted.            11k9
          227  Entering [passive, active] mode                         11k10
          230  User logged on, proceed                                  11l1
          530  Not logged in                                            11l2
          331  User name okay, need password                            11l3
          332  Need account for login                                   11l4
          532  Need account for storing files                           11l5
          150  File status okay; about to open data connection.         11m1
          250  Requested file action okay, completed.                   11m2
          350  Requested file action pending further information        11m3
          450  Requested file action not taken: file unavailable
                  (e.g. file not found, no access)                      11m4
          550  Requested action not taken:  file unavailable (e.g.
                  file busy)                                            11m5
          451  Requested action aborted: local error in processing      11m6
          452  Requested action not taken:  insufficient storage
                  space in system                                       11m7
          552  Requested file action aborted:  exceeded storage
                  allocation (for current directory or dataset)         11m8
          553  Requested action not taken: file name not allowed        11m9
          354  Start mail input; end with <CR><LF>.<CR><LF>            11m10

                          Command-Reply Sequences                         12

     In this section, the command-reply sequence is presented.  Each
    command is listed with its possible replies; command groups are
    listed together.  Preliminary replies are listed first (with
    their succeeding replies under them), then positive and negative
    completion, and finally intermediary replies with the remaining
    commands from the sequence following.  This listing forms the
    basis for the state diagrams, which will be presented separately.     13

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [8]

       Logon                                                             13b

          USER                                                          13b1
             230                                                       13b1a
             530                                                       13b1b
             500, 501, 421                                             13b1c
             331, 332                                                  13b1d
          PASS                                                          13b2
             230                                                       13b2a
             202                                                       13b2b
             530                                                       13b2c
             500, 501, 503, 421                                        13b2d
             332                                                       13b2e
          ACCT                                                          13b3
             230                                                       13b3a
             202                                                       13b3b
             530                                                       13b3c
             500, 501, 503, 421                                        13b3d

       Logoff                                                            13c

          QUIT                                                          13c1
             221                                                       13c1a
             500                                                       13c1b
          REIN                                                          13c2
             120                                                       13c2a
                220                                                   13c2a1
             220                                                       13c2b
             421                                                       13c2c
             500, 502                                                  13c2d

       Transfer parameters                                               13d

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [9]

       File action commands                                              13e

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                      FTP Reply Codes  [10]

             500, 501, 502, 503, 421, 530                              13e9c
          DELE                                                         13e10
             250                                                      13e10a
             450, 550                                                 13e10b
             500, 501, 502, 421, 530                                  13e10c
          ABOR                                                         13e11
             225, 226                                                 13e11a
             500, 501, 502, 421                                       13e11b
          MAIL                                                         13e12
             354                                                      13e12a
                250                                                  13e12a1
                451, 552                                             13e12a2
             450, 550, 452, 553                                       13e12b
             500, 501, 502, 421, 530                                  13e12c

       Informational commands                                            13f

          STAT                                                          13f1
             211, 212, 213                                             13f1a
             450                                                       13f1b
             500, 501, 502, 421, 530                                   13f1c
          HELP                                                          13f2
             211, 214                                                  13f2a
             500, 501, 502, 421                                        13f2b

       Miscellaneous commands                                            13g

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
                                                           Jon Postel
                                                            19 JUN 75

                            FTP State Diagrams                            14

    Here we present state diagrams for a very simple minded FTP
    implementation. Only the first digit of the reply codes is used.
    There is one state diagram for each group of FTP commands or
    command sequences.                                                    15

    The command groupings were determined by constructing a model for
    each command then collecting together the commands with
    structurally identical models.                                        16

    For each command or command sequence there are three possible
    outcomes: success (S), failure (F), and error (E). In the state
    diagrams below we use the symbol B for "begin", and the symbol W
    for "wait for reply".                                                 17

    We first present the diagram that represents the largest group of
    FTP commands:                                                         18

                                1,3    +---+
                           ----------->! E !
                          !            +---+
       +---+    cmd    +---+    2      +---+
       ! B !---------->! W !---------->! S !
       +---+           +---+           +---+
                          !     4,5    +---+
                           ----------->! F !

       This diagram models the commands:                                 18b

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Postel                                    FTP State Diagrams [12]

    The other large group of commands is represented by a very
    similar diagram:                                                      19

                                3      +---+
                           ----------->! E !
                          !            +---+
       +---+    cmd    +---+    2      +---+
       ! B !---------->! W !---------->! S !
       +---+       --->+---+           +---+
                  !     ! !
                  !     ! !     4,5    +---+
                  !  1  !  ----------->! F !
                   -----               +---+

       This diagram models the commands:                                 19b

          APPE, (ICP), LIST, MLFL, NLST, REIN, RETR, STOR.              19b1

    Note that this second model could also be used to represent the
    first group of commands, the only difference being that in the
    first group the 100 series replies are unexpected and therefore
    treated as error, while the second group expects (some may
    require) 100 series replies.                                          20

    The remaining diagrams model command sequences, perhaps the
    simplest of these is the rename sequence:                             21

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Postel                                    FTP State Diagrams [13]

    A very similar diagram models the Mail command:                       22

       +---+   MAIL    +---+    1,2    +---+
       ! B !---------->! W !---------->! E !
       +---+           +---+        -->+---+
                        ! !        !
                 3      ! ! 4,5    !
          --------------  ------   !
         !                      !  !   +---+
         !               ------------->! S !
         !              !   1,3 !  !   +---+
         !             2!  --------
         !              ! !     !
         V              ! !     !
       +---+   text    +---+ 4,5 ----->+---+
       !   !---------->! W !---------->! F !
       +---+           +---+           +---+

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Postel                                    FTP State Diagrams [14]

    The next diagram is a simple model of the Restart command:            23

       +---+   REST    +---+    1,2    +---+
       ! B !---------->! W !---------->! E !
       +---+           +---+        -->+---+
                        ! !        !
                 3      ! ! 4,5    !
          --------------  ------   !
         !                      !  !   +---+
         !               ------------->! S !
         !              !   3   !  !   +---+
         !             2!  --------
         !              ! !     !
         V              ! !     !
       +---+   cmd     +---+ 4,5 ----->+---+
       !   !---------->! W !---------->! F !
       +---+        -->+---+           +---+
                   !      !
                   !  1   !

          Where "cmd" is APPE, STOR, RETR, or MLFL.                     23a1

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Postel                                    FTP State Diagrams [15]

    The most complicated diagram is for the Logon sequence:               25

    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Postel                                    FTP State Diagrams [16]

    Finally we present a generalized diagram that could be used to
    model the command and reply interchange:                              26