RFC 971 (rfc971) - Page 3 of 9

Survey of data representation standards

RFC 971                                                     January 1986
A Survey of Data Representation Standards


   2.2. Message Standards

      The message oriented standards, including DARPA Multimedia Mail,
      NBS CBMS, and the CCITT X.409 standards, seem to favor more
      general, highly extensible type definitions.  This may have
      something to do with the expectation that a system will include
      many different machines, programmed using many different
      programming languages.

      2.2.1. DARPA Multimedia Mail

         The DARPA Multimedia Mail system was developed for use in DoD
         Internet community.  The set of data elements used in the
         Multimedia Message Handling Facility (MMHF) is referred to as
         its "presentation transfer syntax".  The encoding of these data
         elements varies with the data type being represented. Each
         begins with a one-octet "element-code".  Some data elements are
         of a pre-determined length.  For example, the INTEGER data
         element occupies five octets, one for the element-code, and
         four which contain the "value component".  Other data elements,
         however, may vary in length.  For example, the TEXT data
         element, is made up of a one-octet element-code, a three-octet
         count of the characters to follow, and a variable number of
         octets, each containing one right-justified seven bit ASCII
         character.  The element-code and the length constitute the "tag
         component".

         A "base data element" is self contained, while a "structured
         data element" is formed using other data elements.  The LIST
         data element is used to create structures composed of other
         elements.  The tag component of a LIST is made up of a
         one-octet element-code, a three-octet count of the number of
         octets to follow, and a two-octet count of the number of
         elements that follow.  The PROPLIST data element is used to
         create a structure that consists of a set of unordered
         name-value pairs.  The tag component of a PROPLIST is made up
         of a one-octet element-code, a three-octet count of the number
         of octets to follow, and a one-octet count of the number of
         name-value pairs in the PROPLIST.  Both the LIST and the
         PROPLIST elements are followed by an ENDLIST data element.

      2.2.2. NBS Computer Based Message System

         The NBS Computer Based Message System (CBMS) standard was
         developed to specify the format of a message at the interface
         between different computer-based message systems.  Each data
         element consists of a series of "components".  The five


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