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rfc:rfc166

Network Working Group Bob Anderson Request for Comments: 166 Rand NIC 6780 Vint Cerf

                                                                  UCLA
                                                          Eric Harslem
                                                          John Haefner
                                                                  Rand
                                                            Jim Madden
                                                        U. of Illinois
                                                          Bob Metcalfe
                                                                   MIT
                                                         Arie Shoshani
                                                                   SDC
                                                             Jim White
                                                                  UCSB
                                                            David Wood
                                                                 Mitre
                                                           25 May 1971
  DATA RECONFIGURATION SERVICE -- AN IMPLEMENTATION SPECIFICATION
                               CONTENTS
   I.  INTRODUCTION ...................................  2
       Purpose of this RFC ............................  2
       Motivation .....................................  2
  II.  OVERVIEW OF THE DATA RECONFIGURATION SERVICE ...  3
       Elements of the Data Reconfiguration SERVICE ...  3
       Conceptual Network Connections .................  3
       Conception Protocols and Message Formats .......  4
       Example Connection Configurations ..............  7
 III.  THE FORM MACHINE ...............................  8
       Input/Output Streams and Forms .................  8
       Form Machine BNF Syntax ........................  8
       Alternate Specification of Form Machine Syntax .  9
       Forms .......................................... 10
       Rules .......................................... 10
       Terms .......................................... 11
         Term Format 1 ................................ 11
         Term Format 2 ................................ 11
         Term Format 3 ................................ 14
         Term Format 4 ................................ 14

Anderson, et al. [Page 1] RFC 166 Data Reconfiguration Service May 1971

         The Application of a Term .................... 14
         Restrictions and Interpretations of Term
           Functions .................................. 15
         Term and Rule Sequencing ..................... 16
  IV.  EXAMPLES ....................................... 17
       Remarks ........................................ 17
       Field Insertion ................................ 17
       Deletion ....................................... 17
       Variable Length Records ........................ 18
       String Length Computation ...................... 18
       Transposition .................................. 18
       Character Packing and Unpacking ................ 18
                           I.  INTRODUCTION

PURPOSE OF THIS RFC

 The Purpose of this RFC is to specify the Data Reconfiguration
 Service (DRS.)
 The DRS experiment involves a software mechanism to reformat Network
 data streams.  The mechanism can be adapted to numerous Network
 application programs.  We hope that the result of the experiment will
 lead to a future standard service that embodies the principles
 described in this RFC.

MOTIVATION

 Application programs require specific data I/O formats yet the
 formats are different from program to program.  We take the position
 that the Network should adapt to the individual program requirements
 rather than changing each program to comply with a standard.  This
 position doesn't preclude the use of standards that describe the
 formats of regular message contents; it is merely an interpretation
 of a standard as being a desirable mode of operation but not a
 necessary one.
 In addition to differing program requirements, a format mismatch
 problem occurs where users wish to employ many different kinds of
 consoles to attach to a single service program.  It is desirable to
 have the Network adapt to individual console configurations rather
 than requiring unique software packages for each console
 transformation.

Anderson, et al. [Page 2] RFC 166 Data Reconfiguration Service May 1971

 One approach to providing adaptation is for those sites with
 substantial computing power to offer a data reconfiguration service;
 this document is a specification of such a service.
 The envisioned modus operandi of the service is that an applications
 programmer defines _forms_ that describe data reconfigurations.  The
 service stores the forms by name.  At a later time, a user (perhaps a
 non-programmer) employs the service to accomplish a particular
 transformation of a Network data stream, simply by calling the form
 by name.
 We have attempted to provide a notation tailored to some specifically
 needed instances of data reformatting while keeping the notation and
 its underlying implementation within some utility range that is
 bounded on the lower end by a notation expressive enough to make the
 experimental service useful, and that is bounded on the upper end by
 a notation short of a general purpose programming language.
           II.  OVERVIEW OF THE DATA RECONFIGURATION SERVICE

ELEMENTS OF THE DATA RECONFIGURATION SERVICE

 An implementation of the Data Reconfiguration Service (DRS) includes
 modules for connection protocols, a handler of some requests that can
 be made of the service, a compiler and/or interpreter (called the
 Form Machine) to act on those requests, and a file storage module for
 saving and retrieving definitions of data reconfigurations (forms).
 This section describes connection protocols and requests.  The next
 section covers the Form Machine language in some detail.  File
 storage is not described in this document because it is transparent
 to the use of the service an its implementation is different at each
 DRS host.

CONCEPTUAL NETWORK CONNECTIONS

 There are three conceptual Network connections to the DRS, see Fig.
 1.
       1)  The control connection (CC) is between an originating user
           and the DRS.  Forms specifying data reconfigurations are
           defined over this connection.  The user indicates (once)
           forms to be applied to data passing over the two
           connections described below.
       2)  The user connection (UC) is between a user process and the
           DRS.

Anderson, et al. [Page 3] RFC 166 Data Reconfiguration Service May 1971

       3)  The server connection (SC) is between the DRS and the
           serving process.
 Since the goal is to adapt the Network to user and server processes,
 a minimum of requirements are imposed on the UC and SC.
    +------------+              +------+          +---------+
    | ORIGINATING|     CC       | DRS  |    SC    | SERVER  |
    | USER       |--------------|      |----------| PROCESS |
    +------------+     ^        +------+     ^    +---------+
                       |           /         |
                       |        UC/ <-----\  |
                       |         /         \ |
                       |   +-----------+    \|
       TELNET ---------+   | USER      |     +-- Simplex or Duplex
       Protocol            | PROCESS   |         Connections
       Connection          +-----------+
              Figure 1.  DRS Network Connections

CONNECTION PROTOCOLS AND MESSAGE FORMATS

 Over a control connection the dialog is directly between an
 originating user and the DRS.  Here the user is defining forms or
 assigning predefined forms to connections for reformatting.
 The user connects to the DRS via the standard initial connection
 protocol (ICP).  Rather than going through a logger, the user calls
 on a particular socket on which the DRS alway listens. (Experimental
 socket numbers will be published later.) DRS switches the user to
 another socket pair.
 Messages sent over a control connection are of the types and formats
 specified for TELNET.  (The data type code should specify ASCII --
 the default.)  Thus, a user at a terminal should be able to connect
 to a DRS via his local TELNET, for example, as shown in Fig. 2.
                          +---------+   CC  +---------+
                +---------| TELNET  |-------|   DRS   |
                |         +---------+       +---------+
    +-----------------------+
    |         USER          |
    | (TERMINAL OR PROGRAM) |
    +-----------------------+
                Figure 2. A TELNET Connection to DRS

Anderson, et al. [Page 4] RFC 166 Data Reconfiguration Service May 1971

 When a user connects to DRS he supplies a six-character user ID (UID)
 as a qualifier to guarantee the uniqueness of his form names.  He
 will initially have the following commands:
       1.  DEFFORM (form)
       2.  ENDFORM (form)
           These two commands define a form, the text of which is
           chronologically entered between them.  The form is stored
           in the DRS local file system.
       3.  PURGE (form)
           The named form, as qualified by the current UID, is purged
           from the DRS file system.
       4.  LISTNAMES (UID)
           The unqualified names of all forms assigned to UID are
           returned.
       5.  LISTFORM (form)
           The source text of a named form is returned.
       6.  DUPLEXCONNECT (user site, user receive socket, user method,
           server site, server receive socket, server method, user-
           to-server form name, server-to-user form name)
           A duplex connection is made between two processes using the
           receive sockets and the sockets one greater.  Method is
           defined below.  The forms define the transformations on
           these connections.
       7.  SIMPLEXCONNECT (user site, user socket, user method, server
           site, server socket, server method, form)
           A simplex connection is made between the two sockets as
           specified by method.
       8.  ABORT (site, receive socket)
           The reconfiguration of data is terminated by closing both
           the UC and SC specified in part in the command.
 Either one, both, or neither of the two parties specified in 6 or 7
 may be at the same host as the party issuing the request.  Sites and
 sockets specify user and server for the connection.  Method indicates

Anderson, et al. [Page 5] RFC 166 Data Reconfiguration Service May 1971

 the way in which the connection is established.
 The following rules apply to these commands:
       1)  Commands may be abbreviated to the minimum number of
           characters to identify them uniquely.
       2)  All commands should be at the start of a line.
       3)  Parameters are enclosed in parentheses and separated by
           commas.
       4)  Imbedded blanks are ignored.
       5)  The parameters are:
           form name        1-6 characters
           UID              1-6 characters
           Site             1-2 characters specifying
                                the hexadecimal host number
           Socket           1-8 characters specifying the
                                hexadecimal socket number
           Method           A single character
       6)  Method has the following values:
           C      The site/socket is already connected
                  to the DRS as a dummy control connection
                  (should not be the real control connection).
           I      Connect via the standard ICP (does not
                  apply to SIMPLEXCONNECT).
           D      Connect directly via STR, RTS.
           The DRS will make at least the following minimal
           responses to the user:
           1)  A positive or negative acknowledgement after
               each line (CR/LF)
           2)  If a form fails or terminates
           TERMINATE, ASCII Host # as hex, ASCII Socket # as hex,
                       ASCII Return Code as decimal
           thus identifying at least one end of the connection.

Anderson, et al. [Page 6] RFC 166 Data Reconfiguration Service May 1971

EXAMPLE CONNECTION CONFIGURATIONS

 There are basically two modes of DRS operation: 1) the user wishes to
 establish a DRS UC/SC connection(s) between the programs and 2) the
 user wants to establish the same connection(s) where he (his
 terminal) is at the end of the UC or the SC.  The latter case is
 appropriate when the user wishes to interact from his terminal with
 the serving process (e.g., a logger).
 In the first case (Fig. 1, where the originating user is either a
 terminal or a program) the user issues the appropriate CONNECT
 command.  The UC/SC can be simplex or duplex.
 The second case has two possible configurations, shown in Figs. 3 and
 4.
 +-------+    +--------+   CC    +-----+        +----+
 |       |----|        |---------|     |   SC   |    |
 | USER  |    | TELNET |   UC    | DRS |--------| SP |
 |       |----|        |---------|     |        |    |
 +-------+    +--------+         +-----+        +----+
          Figure 3.  Use of Dummy Control Connection
              +---------+
 +------+    /| USER    |   CC   +-----+
 |      |---/ | SIDE    |--------|     |   SC   +----+
 | USER |     +---------+   UC   | DRS |--------| SP |
 |      |---\ | SERVING |--------|     |        +----+
 +------+    \| SIDE    |        +-----+
              +---------+
          Figure 4.  Use of Server TELNET
 In Fig. 3 the user instructs his TELNET to make two duplex
 connections to DRS.  One is used for control information (the CC) and
 the other is a dummy.  When he issues the CONNECT he references the
 dummy duplex connection (UC) using the "already connected" option.
 In Fig. 4 the user has his TELNET (user side) call the DRS.  When he
 issues the CONNECT the DRS calls the TELNET (server side) which
 accepts the call on behalf of the console.  This distinction is known
 only to the user since to the DRS the configuration Fig. 4 appears
 identical to that in Fig. 1.  Two points should be noted:
      1)  TELNET protocol is needed only to define forms and direct
          connections.  It is not required for the using and serving

Anderson, et al. [Page 7] RFC 166 Data Reconfiguration Service May 1971

          processes.
      2)  The using and serving processes need only a minimum of
          modification for Network use, i.e., an NCP interface.
                        III.  THE FORM MACHINE

INPUT/OUTPUT STREAMS AND FORMS

 This section describes the syntax and semantics of forms that specify
 the data reconfigurations.  The Form Machine gets an input stream,
 reformats the input stream according to a form describing the
 reconfiguration, and emits the reformatted data as an output stream.
 In reading this section it will be helpful to envision the
 application of a form to the data stream as depicted in Fig. 5.  An
 input stream pointer identifies the position of data (in the input
 stream) that is being analyzed at any given time by a part of the
 form.  Likewise, an output stream pointer locates data being emitted
 in the output stream.
     /\/\                                                  /\/\
^    |  |                     FORM                         |  |   ^
|    |  |                -----------------                 |  |   |
|    |  |            +-  -----------------  -+             |  |   |
|    |  |            |   CURRENT PART OF     |             |  |   |

INPUT | |⇐ CURRENT < —————– > CURRENT ⇒ | | OUTPUT STREAM | | POINTER | FORM BEING APPLIED | POINTER | | STREAM

     |  |            +-  -----------------  -+             |  |
     |  |                -----------------                 |  |
     |  |                -----------------                 |  |
     |  |                -----------------                 |  |
     \/\/                                                  \/\/
            Figure 5.  Application of Form to Data Streams

Anderson, et al. [Page 8] RFC 166 Data Reconfiguration Service May 1971

FORM MACHINE BNF SYNTAX

 form           ::=  rule | rule form
 rule           ;;=  label  inputstream  outputstream ;
 label          ::=  INTEGER | <null>
 inputstream    ::=  terms | <null>
 terms          ::=  term | terms , term
 outputstream   ::=  : terms | <null>
 term           ::=  identifier | identifier  descriptor |
                     descriptor | comparator
 identifier     ::=  an alpha character followed by 0 to 3
                     alphanumerics
 descriptor     ::=  (replicationexpression , datatype ,
                     valueexpression , lengthexpression  control)
 comparator     ::=  (value  connective  value  control)  |
                     (identifier  *<=*  control)
 replicationexpression  ::=  # | arithmeticexpression | <null>
 datatype       ::=  B | O | X | E | A
 valueexpression  ::=  value | <null>
 lengthexpression  ::=      arithmeticexpression | <null>
 connective     ::=  .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.
 value          ::=  literal | arithmeticexpression
 arithmeticexpression  ::=  primary | primary operator
                            arithmeticexpression
 primary        ::=  identifier | L(identifier) | V(identifier) |
                     INTEGER
 operator       ::=  + | - | * | /
 literal        ::=  literaltype "string"

Anderson, et al. [Page 9] RFC 166 Data Reconfiguration Service May 1971

 literaltype    ::=  B | O | X | E | A
 string         ::=  from 0 to 256 characters
 control        ::=  :  options | <null>
 options        ::=  S(where) | F(where) | U(where) |
                     S(where) , F(where) |
                     F(where) , S(where)
 where          ::=  arithmeticexpression | R(arithmeticexpression)

ALTERNATE SPECIFICATION OF FORM MACHINE SYNTAX

                                 infinity

form ::= {rule}

                                 1
                                    1         1          1

rule ::= {INTEGER} {terms} {:terms} ;

                                    0         0          0
                                       infinity

terms ::= term {,term}

                                       0
                                                    1

term ::= identifier | {identifier} descriptor

                                                    0
                           | comparator
                                                  1

descriptor ::= ({arithmeticexpression} , datatype ,

                                                  0
                                  1                     1          1
                           {value} ,  {lengthexpression}  {:options}
                                  0                     0          0
                                                               1

comparator ::= (value connective value {:options} ) |

                                                               0
                                                            1
                           (identifier .<=. value {:options} )
                                                            0

connective ::= .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.

lengthexpression ::= # | arithmeticexpression

datatype ::= B | O | X | E | A

value ::= literal | arithmeticexpression

Anderson, et al. [Page 10] RFC 166 Data Reconfiguration Service May 1971

                                                       infinity

arithmeticexpression ::= primary {operator primary}

operator ::= + | - | * | /

primary ::= identifier | L(identifier) |

                           V(identifier) | INTEGER
                                                    256

literal ::= literaltype "{CHARACTER} "

literaltype ::= B | O | X | A | E

                                               1

options ::= S(where) {,F(where)} |

                                               0
                                               1
                           F(where) {,S(where)}  | U(where)
                                               0

where ::= arithmeticexpression |

                           R(arithmeticexpression)
                                                   3

identifier ::= ALPHABETIC {ALPHAMERIC}

FORMS

 A form is an ordered set of rules.
       form ::=  rule | rule form
 The current rule is applied to the current position of the input
 stream.  If the (input stream part of a) rule fails to correctly
 describe the contents of the current input then another rule is made
 current and applied to the current position of the input stream.  The
 next rule to be made current is either explicitly specified by the
 current term in the current rule or it is the next sequential rule by
 default.  Flow of control is more fully described under TERM AND RULE
 SEQUENCING.
 If the (input stream part of a) rule succeeds in correctly describing
 the current input stream, then some data may be emitted at the
 current position in the output stream according to the rule.  The
 input and output stream pointers are advanced over the described and
 emitted data, respectively, and the next rule is applied to the now
 current position of the input stream.
 Application of the form is terminated when an explicit return
 (R(arithmeticexpression)) is encountered in a rule.  The user and

Anderson, et al. [Page 11] RFC 166 Data Reconfiguration Service May 1971

 server connections are closed and the return code
 (arithmeticexpression) is sent to the originating user.

RULES

 A rule is a replacement, comparison, and/or an assignment operation
 of the form shown below.
       rule ::= label  inputstream  outputstream
 A label is the name of a rule and it exists so that the rule may be
 referenced elsewhere in the form for explicit rule transfer of
 control.  Labels are of the form below.
       label ::=  INTEGER | <null>
 The optional integer labels are in the range 0 >= INTEGER >= 9999.
 The rules need not be labeled in ascending numerical order.

TERMS

 The inputstream (describing the input stream to be matched) and the
 outputstream (describing data to be emitted in the output stream)
 consist of zero or more terms and are of the form shown below.
       inputstream   ::=  terms | <null>
       outputstream  ::=  :terms | <null>
       terms         ::=  term | terms , term
 Terms are of one of four formats as indicated below.
       term ::=  identifier | identifier  descriptor |
                 descriptor | comparator

Term Format 1

 The first term format is shown below.
       identifier
 The identifier is a symbolic reference to a previously identified
 term (term format 2) in the form.  It takes on the same attributes
 (value, length, type) as the term by that name.  Term format 1 is
 normally used to emit data in the output stream.
 Identifiers are formed by an alpha character followed by 0 to 3
 alphanumeric characters.

Anderson, et al. [Page 12] RFC 166 Data Reconfiguration Service May 1971

Term Format 2

 The second term format is shown below.
       identifier descriptor
 Term format 2 is generally used as an input stream term but can be
 used as an output stream term.
 A descriptor is defined as shown below.
       descriptor ::= (replicationexpression, datatype,
                      valueexpression, lengthexpression
                      control)
 The identifier is the symbolic name of the term in the usual
 programming language sense.  It takes on the type, length, value, and
 replication attributes of the term and it may be referenced elsewhere
 in the form.
 The replication expression, if specified, causes the unit value of
 the term to be generated the number of times indicated by the value
 of the replication expression.  The unit value of the term (quantity
 to be replicated) is determined from the data type, value expression,
 and length expression attributes.  The data type defines the kind of
 data being specified.  The value expression specifies a nominal value
 that is augmented by the other term attributes.  The length
 expression determines the unit length of the term.  (See the IBM SRL
 Form C28-6514 for a similar interpretation of the pseudo instruction,
 defined constant, after which the descriptor was modeled.)
 The replication expression is defined below.
       replicationexpression ::= # | arithmeticexpression | <null>
       arithmeticexpression ::= primary | primary operator
                                arithmeticexpression
       operator ::= + | - | * | /
       primary ::= identifier | L(identifier) | V(identifier) |
                   INTEGER
 The replication expression is a repeat function applied to the
 combined data type value, and length expressions.  It expresses the
 number of times that the nominal value is to be repeated.
 The terminal symbol # means an arbitrary replication factor.  It must
 be explicitly terminated by a match or non-match to the input stream.
 This termination may result from the same or the following term.

Anderson, et al. [Page 13] RFC 166 Data Reconfiguration Service May 1971

 A null replication expression has the value of one.  Arithmetic
 expressions are evaluated from left-to-right with no precedence.
 The L(identifier) is a length operator that generates a 32-bit binary
 integer corresponding to the length of the term named.  The
 V(identifier) is a value operator that generates a 32-bit binary
 integer corresponding to the value of the term named.  (See
 Restrictions and Interpretations of Term Functions.)  The value
 operator is intended to convert character strings to their numerical
 correspondents.
 The data type is defined below.
           datatype ::= B | O | X | E | A
 The data type describes the kind of data that the term represents.
 (It is expected that additional data types, such as floating point
 and user-defined types, will be added as needed.)
      Data Type         Meaning              Unit Length
          B             Bit string              1 bit
          O             Bit string              3 bits
          X             Bit string              4 bits
          E             EBCDIC character        8 bits
          A             Network ASCII character 8 bits
 The value expression is defined below.
          valueexpression ::= value | <null>
          value ::= literal | arithmeticexpression
          literal ::= literaltype "string"
          literaltype ::= B | O | X | E | A
 The value expression is the nominal value of a term expressed in the
 format indicated by the data type.  It is repeated according to the
 replication expression.
 A null value expression in the input stream defaults to the data
 present in the input stream.  The data must comply with the datatype
 attribute, however.
 A null value expression generates padding according to Restrictions
 and Interpretations of Term Functions.
 The length expression is defined below.
       lengthexpression ::= arithmeticexpression | <null>

Anderson, et al. [Page 14] RFC 166 Data Reconfiguration Service May 1971

 The length expression states the length of the field containing the
 value expression.
 If the length expression is less than or equal to zero, the term
 succeeds but the appropriate stream pointer is not advanced.
 Positive lengths cause the appropriate stream pointer to be advanced
 if the term otherwise succeeds.
 Control is defined under TERM AND RULE SEQUENCING.

Term Format 3

 Term format 3 is shown below.
       descriptor
 It is identical to term format 2 with the omission of the identifier.
 Term format 3 is generally used in the output stream.  It is used in
 the input stream where input data is to be passed over but not
 retained for emission or later reference.

Term Format 4

 The fourth term format is shown below.
       comparator    ::= (value connective value control) |
                         (identifier *<=* value  control)
       value         ::= literal | arithmeticexpression
       literal       ::= literaltype "string"
       literaltype   ::= B | O | X | E | A
       string        ::= from 0 to 256 characters
       connective    ::= .LE. | .LT. | .GE. | .GT. | .EQ. | .NE.
 The fourth term format is used for assignment and comparison.
 The assignment operator *<=* assigns the value to the identifier.
 The connectives have their usual meaning.  Values to be compared must
 have the same type and length attributes or an error condition arises
 and the form fails.

The Application of a Term

 The elements of a term are applied by the following sequence of
 steps.
       1.  The data type, value expression, and length expression
           together specify a unit value, call it x.

Anderson, et al. [Page 15] RFC 166 Data Reconfiguration Service May 1971

       2.  The replication expression specifies the number of times x
           is to be repeated.  The value of the concatenated xs
           becomes y of length L.
       3.  If the term is an input stream term then the value of y of
           length L is tested with the input value beginning at the
           current input pointer position.
       4.  If the input value satisfies the constraints of y over
           length L then the input value of length L becomes the value
           of the term.
 In an output stream term, the procedure is the same except that the
 source of input is the value of the term(s) named in the value
 expression and the data is emitted in the output stream.
 The above procedure is modified to include a one term look-ahead
 where replicated values are of indefinite length because of the
 arbitrary symbol, #.

Restrictions and Interpretations of Term Functions

 1.    Terms having indefinite lengths because their values are
       repeated according to the # symbol, must be separated by some
       type-specific data such as a literal.  (A literal isn't
       specifically required, however.  An arbitrary number of ASCII
       characters could be terminated by a non-ASCII character.)
 2.    Truncation and padding is as follows:
       a)  Character to character (A <-> E) conversion is left-
           justified and truncated or padded on the right with blanks.
       b)  Character to numeric and numeric to numeric conversions are
           right-justified and truncated or padded on the left with
           zeros.
       c)  Numeric to character conversions is right-justified and
           left-padded with blanks.
 3.    The following are ignored in a form definition over the control
       connection.
       a)  TELNET control characters.
       b)  Blanks except within quotes.
       c)  /* string */ is treated as comments except within quotes.
 4.    The following defaults prevail where the term part is omitted.
       a)  The replication expression defaults to one.
       b)  # in an output stream term defaults to one.
       c)  The value expression of an input stream term defaults to

Anderson, et al. [Page 16] RFC 166 Data Reconfiguration Service May 1971

           the value found in the input stream, but the input stream
           must conform to the data type and length expression.  The
           value expression of an output stream term defaults to
           padding only.
       e)  The length expression defaults to the size of the quantity
           determined by the data type and value expression.
       f)  Control defaults to the next sequential term if a term is
           successfully applied; else control defaults to the next
           sequential rule.  If _where_ evaluates to an undefined
           _label_ the form fails.
 5.    Arithmetic expressions are evaluated left-to-right with no
       precedence.
 6.    The following limits prevail.
       a)  Binary lengths are <= 32 bits
       b)  Character strings are <= 256 8-bit characters
       c)  Identifier names are <= 4 characters
       d)  Maximum number of identifiers is <= 256
       e)  Label integers are >= 0 and <= 9999
 7.    Value and length operators product 32-bit binary integers.  The
       value operator is currently intended for converting A or E type
       decimal character strings to their binary correspondents.  For
       example, the value of E'12' would be 0......01100.  The value
       of E'AB' would cause the form to fail.

TERM AND RULE SEQUENCING

 Sequencing may be explicitly controlled by including control in a
 term.
      control ::=  :options | <null>
      options ::=  S(where) | F(where) | U(where)
                   S(where) , F(where) |
                   F(where) , S(where)
      where   ::=  arithmeticexpression | R(arithmeticexpression)
 S, F, and U denote success, fail, and unconditional transfers,
 respectively.  _Where_ evaluates to a _rule_ label, thus transfer can
 be effected from within a rule (at the end of a term) to the
 beginning of another rule.  R means terminate the form and return the
 evaluated expression to the initiator over the control connection (if
 still open).
 If terms are not explicitly sequenced, the following defaults
 prevail.

Anderson, et al. [Page 17] RFC 166 Data Reconfiguration Service May 1971

      1)  When a term fails go to the next sequential rule.
      2)  When a term succeeds go to the next sequential
          term within the rule.
      3)  At the end of a rule, go to the next sequential
          rule.
 Note in the following example, the correlation between transfer of
 control and movement of the input pointer.
      1   XYZ(,B,,8:S(2),F(3)) : XYZ ;
      2   . . . . . . .
      3   . . . . . . .
 The value of XYZ will never be emitted in the output stream since
 control is transferred out of the rule upon either success or
 failure.  If the term succeeds, the 8 bits of input will be assigned
 as the value of XYZ and rule 2 will then be applied to the same input
 stream data.  That is, since the complete left hand side of rule 1
 was not successfully applied, the input stream pointer is not
 advanced.
                             IV.  EXAMPLES

REMARKS

 The following examples (forms and also single rules) are simple
 representative uses of the Form Machine.  The examples are expressed
 in a term-per-line format only to aid the explanation.  Typically, a
 single rule might be written as a single line.

FIELD INSERTION

 To insert a field, separate the input into the two terms to allow the
 inserted field between them.  For example, to do line numbering for a
 121 character/line printer with a leading carriage control character,
 use the following form.
 (NUMB*<=*1);       /*initialize line number counter to one*/
 1 CC(,E,,1:F(R(99))),  /*pick up control character and save
                        as CC*/
                        /*return a code of 99 upon exhaustion*/
 LINE(,E,,121 : F(R(98)))  /*save text as LINE*/
 :CC,               /*emit control character*/
 (,E,NUMB,2),       /*emit counter in first two columns*/
 (,E,E".",1),       /*emit period after line number*/
 (,E,LINE,117),     /*emit text, truncated in 117 byte field*/
 (NUMB*<=*NUMB+1:U(1));   /*increment line counter and go to
                            rule one*/;;

Anderson, et al. [Page 18] RFC 166 Data Reconfiguration Service May 1971

DELETION

 Data to be deleted should be isolated as separate terms on the left,
 so they may be omitted (by not emitting them) on the right.
 (,B,,8),           /*isolate 8 bits to ignore*/
 SAVE(,A,,10)       /*extract 10 ASCII characters from
                      input stream*/
 :(,E,SAVE,);       /*emit the characters in SAVE as EBCDIC
                      characters whose length defaults to the
                      length of SAVE, i.e., 10, and advance to
                      the next rule*/
 In the above example, if either input stream term fails,
 the next sequential rule is applied.

VARIABLE LENGTH RECORDS

 Some devices, terminals and programs generate variable
 length records.  The following rule picks up variable length
 EBCDIC records and translates them to ASCII.
 CHAR(#,E,,1),      /*pick up all (an arbitrary number of)
                      EBCDIC characters in the input stream*/
 (,X,X"FF",2)       /*followed by a hexadecimal literal,
                      FF (terminal signal)*/
 :(,A,CHAR,),       /*emit them as ASCII*/
 (,X,X"25",2);      /*emit an ASCII carriage return*/

STRING LENGTH COMPUTATION

 It is often necessary to prefix a length field to an arbitrarily long
 character string.  The following rule prefixes an EBCDIC string with
 a one-byte length field.
 Q(#,E,,1),         /*pick up all EBCDIC characters*/
 TS(,X,X"FF",2)     /*followed by a hexadecimal literal, FF*/
 :(,B,L(Q)+2,8),    /*emit the length of the characters
                      plus the length of the literal plus
                      the length of the count field itself,
                      in an 8-bit field*/
 Q,                 /*emit the characters*/
 TS,                /*emit the terminal*/

Anderson, et al. [Page 19] RFC 166 Data Reconfiguration Service May 1971

TRANSPOSITION

 It is often desirable to reorder fields, such as the following
 example.
 Q(,E,,20), R(,E,,10) , S(,E,,15), T(,E,,5) : R, T, S, Q ;
 The terms are emitted in a different order.

CHARACTER PACKING AND UNPACKING

 In systems such as HASP, repeated sequences of characters are packed
 into a count followed by the character, for more efficient storage
 and transmission.  The first form packs multiple characters and the
 second unpacks them.
 /*form to pack EBCDIC streams*/
 /*returns 99 if OK, input exhausted*/
 /*returns 98 if illegal EBCDIC*/
 /*look for terminal signal FF which is not a legal EBCDIC*/
 /*duplication count must be 0-254*/
 1 (,X,X"FF",2 : S(R(99))) ;
 /*pick up an EBCDIC char/*
 CHAR(,E,,1) ;
 /*get identical EBCDIC chars/*
 LEN(#,E,CHAR,1)
 /*emit the count and the char/*
 : (,B,L(LEN)+1,8), CHAR, (:U(1));
 /*end of form*/;;
 /*form to unpack EBCDIC streams*/
 /*look for terminal*/
 1 (,X,X"FF",2 : S(R(99))) ;
 /*emit character the number of times indicated*/
 /*by the count, in a field the length indicated*/
 /*by the counter contents*/
 CNT(,B,,8), CHAR(,E,,1) : (CNT,E,CHAR,1:U(1));
 /*failure of form*/
 (:U(R(98))) ;;
     [ This RFC was put into machine readable form for entry ]
      [ into the online RFC archives by Simone Demmel 03/98 ]

Anderson, et al. [Page 20]

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