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.

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.