Binary Message Forms in Computer
RFC 31

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Network Working Group
Request for Comments: 31

               BINARY MESSAGE FORMS IN COMPUTER NETWORKS

                             Daniel Bobrow
                       Bolt, Beranek, and Newman
                        Cambridge, Massachusetts

                         William R. Sutherland
                         MIT Lincoln Laboratory
                        Lexington, Massachusetts

                             February 1968

                                                                [Page 1]
RFC 31                    Binary Message Forms             February 1968

                   MESSAGE FORMS IN COMPUTER NETWORKS

INTRODUCTION

     Network communication between computers is becoming increasingly
   important.  However, the variety of installations working in the area
   probably precludes standardization of the content and form of inter-
   computer messages.  There is some hope, however, that a standard way
   of defining and describing message forms can be developed and used to
   facilitate communication between computers.  Just as ALGOL serves as
   a standard vehicle for describing numerous algorithms, and BNF serves
   as a standard for describing language syntax, a message description
   language would be useful as a standard vehicle for defining message
   formats.
     Considerable progress has been made at the low level of message
   handling protocol and one can expect the ASCII protocols to be used.
   The discussion which follows assumes that the mechanics of exchanging
   messages, check sums, repeat requests, etc., have been worked out.
   The topic of concern is how to describe the content and intent of a
   binary message body when the network header and trailer details have
   been stripped off.
     Most attempts at describing the content of binary messages
   jump immediately into a consideration of the bit codings to be used.
   Long, thin rectangles are drawn to represent the binary bit stream;
   this stream is sliced up into boxes, and tables generally describe
   the bit options for each box.  A better approach would be to provide
   a symbolic method for describing messages.  The symbolism, by
   avoiding immediate references to specific bit details, should help
   one's understanding of the message content and the alternatives
   available in the message body.  When the basic form of the binary
   message body is clear, the coding details of the actual bit fields
   can be shown.

                                                                [Page 2]
RFC 31                    Binary Message Forms             February 1968

     Describing a binary message body is not much different from
   describing a text body or language.  Text assumes fixed bit fields
   each containing one character.  Standard language description methods
   (BNF) then show how the characters can be concatenated and what
   interpretation should be placed on character groups.  Binary message
   descriptions require the additional capacity of defining various size
   fields in the message and the interpretation to be placed on the bits
   contained in the field.
     A message description is initially intended as a reference standard
   to be written down on paper and made available to new users of a
   computer network.  From this standard, the new user can discover the
   kind and form of the binary data being exchanged over the network.
   Once this is known, the programs necessary for using the network
   facilities can be created.  Later on, in an established network, one
   can envision the promulgation of standards for newly developed binary
   formats via the exchange of ASCII text messages over the network
   itself instead of on paper through the mail.  Still farther into the
   future, the text of a binary format standard could be used as input
   to compiler-like programs which automatically create data translation
   programs for converting one binary format to another.  Right now,
   though, some kind of binary data description method, however trivial,
   is desperately needed.

                                                                [Page 3]
RFC 31                    Binary Message Forms             February 1968

               A SUGGESTED BINARY FORMAT DESCRIPTION METHOD

     The basic component of a binary message is a simple field
   consisting of a consecutive number of bits in the message.  Binary
   messages consist of concatenated fields.  A format description for a
   binary message will consist of a title and four declarative sections.

     1) Symbolic names are declared for all the different kinds of
        fields found in the binary format being defined.
     2) Symbolic names are declared for commonly used values of
        particular fields.
     3) The legal ways of concatenating fields are indicated.
     4) The number of bits in each field and any special considerations