rfc166.txt
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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
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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"
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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}
0
operator ::= + | - | * | /
primary ::= identifier | L(identifier) |
V(identifier) | INTEGER
256
literal ::= literaltype "{CHARACTER} "
0
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}
0
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
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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.
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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.
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