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    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
    
    A note of thanks to those who contributed to this work: Ken
    Pogran, Mark Krilanovich, Wayne Hathway, and especially Nancy
    Neigus.                                                                6
    
    
    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
    
    Formally, a reply is defined to contain the 3-digit code,
    followed by Space <SP>, followed by one line of text (where some
    maximum line length has been specified), and terminated by the
    TELNET end-of-line code.  There will be cases, however, where the
    text is longer than a single line.  In these cases the complete
    text must be bracketed so the User-process knows when it may stop
    reading the reply (i.e. stop processing input on the TELNET
    connection) and go do other things.  This requires a special
    format on the first line to indicate that more than one line is
    coming, and another on the last line to designate it as the last.
    At least one of these must contain the appropriate reply code to
    
    
    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
    
    The three digits of the reply each have a special significance.
    This is intended to allow a range of very simple to very
    sophisticated response by the user-process.  The first digit
    denotes whether the response is good, bad or incomplete.
    (Referring to the state diagram) an unsophisticated user-process
    will be able to determine its next action (proceed as planned,
    redo, retrench, etc.) by simply examining this first digit.  A
    user-process that wants to know approximately what kind of error
    
    
    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
    
          The command was not accepted and the requested action did
          not take place, but the error condition is temporary and
          the action may be requested again.  The user should return
          to the beginning of the command sequence, if any.  It is
          difficult to assign a meaning to "transient", particularly
          when two distinct sites (Server and User-processes) have to
          agree on the interpretation.  Each reply in the 4yz
          category might have a slightly different time value, but
          the intent is that the user-process is encouraged to try
          again.  A rule of thumb in determining if a reply fits into
          the 4yz or the 5yz (Permanent Negative) category is that
          replies are 4yz if the commands can be repeated without any
          change in command form or in properties of the User or
          Server (e.g. the command is spelled the same with the same
    
    
    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
    
       The third digit gives a finer gradation of meaning in each of
       the function categories, specified by the second digit.  The
       list of replies below will illustrate this.  Note that the
       text associated with each reply is suggestive, rather than
       mandatory, and may even change according to the command with
       which it is associated.  The reply codes, on the other hand,
       should strictly follow the specifications in the last section;
       that is, Server implementations should not invent new codes
       for situations that are only slightly different from the ones
       described here, but rather should adapt codes already defined.
    
    
    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
    
          A command such as TYPE or ALLO whose successful execution
          does not offer the user-process any new information will
          cause a 200 reply to be returned.  If the command is not
          implemented by a particular Server-FTP process because it
          has no relevance to that computer system, for example ALLO
          at a TENEX site, a Positive Completion reply is still
          desired so that the simple User-process knows it can
          proceed with its course of action.  A 202 reply is used in
          this case with, for example, the reply text:  "No storage
          allocation necessary."  If, on the other hand, the command
          requests a non-site-specific action and is unimplemented,
          the response is 502.  A refinement of that is the 504 reply
          for a command that IS implemented, but that requests an
          unimplemented parameter.                                      11h1
                                                                         11i
          200  Command okay                                             11i1
          500  Syntax error, command unrecognized
                  [This may include errors such as command line too
                  long.]                                                11i2
          501  Syntax error in parameters or arguments                  11i3
          202  Command not imlemented, superfluous at this site.        11i4
          502  Command not implemented                                  11i5
          503  Bad sequence of commands                                 11i6
          504  Command not implemented for that parameter               11i7
                                                                         11j
          110  Restart marker reply.
                  In this case the text is exact and not left to the
                  particular implementation; it must read:
                               MARK yyyy = mmmm
                  where yyyy is User-process data stream marker, and
                  mmmm is Server's equivalent marker.  (note the
                  spaces between the markers and "=".)                  11j1
          211  System status, or system help reply                      11j2
          212  Directory status                                         11j3
          213  File status                                              11j4
          214  Help message (on how to use the server or the meaning
                  of a particular non-standard command.  This reply
                  is useful only to the human user.)                    11j5
                                                                         11k
          120  Service ready in nnn minutes                             11k1
          220  Service ready for new user                               11k2
          221  Service closing TELNET connection (logged off if
                  appropriate)                                          11k3
          421  Service not available, closing TELNET connection.
                  [This may be a reply to any command if the service
                  knows it must shut down.]                             11k4
    
    
    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
                                                                         11l
          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
                                                                         11m
          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
    
       ICP                                                               13a
          120                                                           13a1
             220                                                       13a1a
          220                                                           13a2
          421                                                           13a3
    
    
    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
    
          SOCK                                                          13d1
             200                                                       13d1a
             500, 501, 421, 530                                        13d1b
          PASV                                                          13d2
             227                                                       13d2a
             500, 501, 502, 421, 530                                   13d2b
          ACTV                                                          13d3
             227                                                       13d3a
             202                                                       13d3b
             500, 501, 421, 530                                        13d3c
          BYTE, MODE, TYPE, STRU                                        13d4
             200                                                       13d4a
             500, 501, 504, 421, 530                                   13d4b
    
    
    NWG/RFC# 640                               JBP NJN 5-JUN-74 16:07  30843
    Neigus                                       FTP Reply Codes  [9]
    
    
    
       File action commands                                              13e
    
          ALLO                                                          13e1
             200                                                       13e1a
             202                                                       13e1b
             500, 501, 504, 421, 530                                   13e1c
          REST                                                          13e2
             500, 501, 502, 421, 530                                   13e2a
             350                                                       13e2b
          STOR                                                          13e3
             125, 150                                                  13e3a
                (110)                                                 13e3a1
                226, 250                                              13e3a2
                425, 426, 451, 552                                    13e3a3
             532, 450, 452, 553                                        13e3b
             500, 501, 421, 530                                        13e3c
          RETR                                                          13e4
             125, 150                                                  13e4a
                (110)                                                 13e4a1
                226, 250                                              13e4a2
                425, 426, 451                                         13e4a3
             450, 550                                                  13e4b
             500, 501, 421, 530                                        13e4c
          LIST, NLST                                                    13e5
             125, 150                                                  13e5a
                226, 250                                              13e5a1
                425, 426, 451                                         13e5a2
             450                                                       13e5b
             500, 501, 502, 421, 530                                   13e5c
          APPE                                                          13e6
             125, 150                                                  13e6a
                (110)                                                 13e6a1
                226, 250                                              13e6a2
                425, 426, 451, 552                                    13e6a3
             532, 450, 550, 452, 553                                   13e6b
             500, 501, 502, 421, 530                                   13e6c
          MLFL                                                          13e7
             125, 150                                                  13e7a
                226, 250                                              13e7a1
                425, 426, 451, 552                                    13e7a2
             532, 450, 550, 452, 553                                   13e7b
             500, 501, 502, 421, 530                                   13e7c
          RNFR                                                          13e8
             450, 550                                                  13e8a
             500, 501, 502, 421, 530                                   13e8b
             350                                                       13e8c
          RNTO                                                          13e9
             250                                                       13e9a
             532, 553                                                  13e9b
    
    
    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
    
          SITE                                                          13g1
             200                                                       13g1a
             202                                                       13g1b
             500, 501, 530                                             13g1c
          NOOP                                                          13g2
             200                                                       13g2a
             500                                                       13g2b
    
    
    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 !
                                       +---+
                                                                         18a
    
    
       This diagram models the commands:                                 18b
    
    
          ABOR, ACTV, ALLO, BYTE, DELE, HELP, MODE, NOOP, PASV, QUIT,
          SITE, SOCK, STAT, STRU, TYPE.                                 18b1
    
    
    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 !
                   -----               +---+
                                                                         19a
    
    
       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
    
    
       +---+   RNFR    +---+    1,2    +---+
       ! B !---------->! W !---------->! E !
       +---+           +---+        -->+---+
                        ! !        !
                 3      ! ! 4,5    !
          --------------  ------   !
         !                      !  !   +---+
         !               ------------->! S !
         !              !   1,3 !  !   +---+
         !             2!  --------
         !              ! !     !
         V              ! !     !
       +---+   RNTO    +---+ 4,5 ----->+---+
       !   !---------->! W !---------->! F !
       +---+           +---+           +---+
                                                                         21a
    
    
    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 !
       +---+           +---+           +---+
                                                                         22a
    
    
       Note that the "text" here is a series of lines sent from the
       user to the server with no response expected until the last
       line is sent, recall that the last line must consist only of a
       single period.                                                    22b
    
    
    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   !
                    ------
                                                                         23a
    
    
          Where "cmd" is APPE, STOR, RETR, or MLFL.                     23a1
    
    We note that the above three models are similar, in fact the Mail
    diagram and the Rename diagram are structurally identical. The
    Restart differs from the other two only in the treatment of 100
    series replies at the second stage.                                   24
    
    
    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
    
    
                             1
       +---+   USER    +---+------------->+---+
       ! B !---------->! W ! 2       ---->! E !
       +---+           +---+------  !  -->+---+
                        ! !       ! ! !
                      3 ! ! 4,5   ! ! !
          --------------   -----  ! ! !
         !                      ! ! ! !
         !                      ! ! ! !
         !                 ---------  !
         !               1!     ! !   !
         V                !     ! !   !
       +---+   PASS    +---+ 2  !  ------>+---+
       !   !---------->! W !------------->! S !
       +---+           +---+   ---------->+---+
                        ! !   ! !     !
                      3 ! !4,5! !     !
          --------------   --------   !
         !                    ! !  !  !
         !                    ! !  !  !
         !                 -----------
         !             1,3!   ! !  !
         V                !  2! !  !
       +---+   ACCT    +---+--  !   ----->+---+
       !   !---------->! W ! 4,5 -------->! F !
       +---+           +---+------------->+---+
                                                                         25a
    
    
    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
    
    
                ------------------------------------
               !                                    !
       Begin   !                                    !
         !     V                                    !
         !   +---+  cmd   +---+ 2         +---+     !
          -->!   !------->!   !---------->!   !     !
             !   !        ! W !           ! S !-----!
          -->!   !     -->!   !-----      !   !     !
         !   +---+    !   +---+ 4,5 !     +---+     !
         !     !      !    ! !      !               !
         !     !      !   1! !3     !     +---+     !
         !     !      !    ! !      !     !   !     !
         !     !       ----  !       ---->! F !-----
         !     !             !            !   !
         !     !             !            +---+
          -------------------
               !
               !
               V
              End
                                                                         26a
    

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