Efficient encoding of application layer PDU's for fieldbus networks
ACM SIGCOMM Computer Communication Review
High-performance ASN.1 compiler
Computer Communications
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CCITT Draft Recommendation X.409 [1] defines a manner of encoding information for communication among computer systems. This scheme includes not only an encoding syntax--rules by which communicated bits and octets are assigned meaning--but also a language for describing the form and content of communicated information in a human-readable notation. In this notation one may define the form of a protocol data unit (PDU), specifying how the PDU is composed of individual fields of integer values, bit flags, characters, etc. One may also use the notation to define specific instances of PDUs: a PDU containing the integer value 3, the bit flag "on" and the character "A", for example. From the notational description of the PDU one may directly infer its encoded representation by applying the rules of the encoding syntax. As a language for documenting the encoding of standard protocols, the notation is intended primarily for human consumption. Being a formal language, however, it is also well suited to automated processing. This paper describes some software tools developed at BBN for processing the notation. The principle purpose of the tools is to permit rapid construction of PDU encoding and decoding software from notational descriptions of PDUs. The next section of this paper contains a brief overview of the X.409 notation and syntax for those not already familiar with the Draft Recommendation. The following section describes the tools and their method of operation. Some notes about the implementation and its performance complete the paper.