RFC 121 (rfc121) - Page 2 of 13

Network on-line operators

RFC 121                Network On-Line Operators              April 1971


   In all operations involving the On-Line System Network operators,
   there are certain conventions concerning the format of the data sent
   and received.  The data is grouped in 'messages' consisting of three
   fields, op code, length, an text, in that order.  The op code is one
   byte in length and is a code which indicates how the text field is to
   be interpreted.  The length field is two bytes long, and gives the
   length, in bits, of the text field, which contains the actual data.
   (The op code and length fields together are termed a header.)

   The following op codes are presently defined:

     op code                        meaning

       0          This op code is a NOP.  No text field exists, and the
                  contents of the length field are unpredictable. (This
                  op code is used mainly as a delimiter.)

       1          The text field contains EBCDIC characters, one
                  character per byte.  The On-Line System operators
                  consider the characters as intended for display as
                  soon as the text field has been received.

       2          The text contains codes for keypushes, one byte per
                  key.  The On-Line operators consider the keys as
                  intended for execution as soon as the text field has
                  been received.

       3          The same as for an op code of 2, except that the On-
                  Line operators consider that the execution of the keys
                  will be delayed until all data for that receive
                  operation has been received.

   The standard format of data sent or received by the On-Line System
   operators is a string of messages, with the last message indicated by
   a header with a NOP op code, called a trailer.  These conventions are
   the default situation; any of them may be overridden by appropriate
   programming.  Following is a description of each operator, its
   function and key sequence.

   (1) OPEN

      By invoking this operator, the user requests that the specified
      local socket be removed from the closed state and thus be prepared
      to participate in data transfer.  There are three distinct
      operations that can be performed by this operator and these are
      described below:





Krilanovich