Occurrences

Paper

../papers/03-03-03/03-03-03.html

Date of Presentation

Wednesday, 22 May

Time of Presentation

11.00

Presentation Level

Technical

Abstract

The ASN.1 group of ISO/IEC/ITU-T has produced a new standard that specifies an XML encoding of messages that can be defined in the ASN.1 language. For the writers and implementors of protocols that use ASN.1, the newly standardized XML Encoding Rules are an alternative to the traditional binary encodings of ASN.1 types. This along with enhancements that have been made to the language itself makes ASN.1 a complete schema language for XML and a viable alternative to both W3C XML Schema and RELAX NG. ASN.1 is a mature and stable notation for defining protocol messages and is widely deployed in the telecommunications networks and used in many protocols in the areas of cellular telephony, security, videoconferencing, air transportation, messaging, directory, and more. A key aspect of ASN.1 is the clean separation between the 'abstract syntax' (or 'message description') and the 'transfer syntax' (or 'encoding') of data structures and messages. The ASN.1 notation defines a set of simple data types, more complex set of types such as choices, structures and arrays, and subtype constraints for restricting the values of all types. These types are 'abstract', in the sense that no particular representation (encoding) of the data is normally stated or implied in the type definitions themselves. The greatest benefit of the separation between the abstract syntax and the encoding is that the applications that implement protocols that use ASN.1 don't need to be concerned with how the data is encoded on the line, and therefore can concentrate on the data itself, its relationships and semantics. Any existing ASN.1 message description can be encoded either in a compact binary form, or in XML by using the XML Encoding Rules. In addition to this, what makes ASN.1 different from the other XML schema languages is the higher level of abstraction in the description of the structure of the XML instances, which is reflected in the application interfaces. In this paper, several examples are given and four main scenarios are envisioned: 1) A set of networked systems implementing an ASN.1-based protocol and exchanging binary-encoded messages is shown. Some machines produce additional XML encodings of the same data for use locally. The XML messages can be produced for diagnostics, monitoring or other purposes. 2) A system producing XML documents conforming to an XML Schema specification is shown. An ASN.1 specification equivalent to the XML Schema specification can be written, and an ASN.1-based system implemented to process and re-package the data. This makes it possible to receive XML messages from the XML-based system, then decode and re-encode them into a more-compact binary encoding. 3) A protocol specified both in ASN.1 and (compatibly) in XML Schema and allowing use of both binary and XML encodings is shown. This grants much freedom to implementors, who can decide whether to write the application by using an XML Schema toolkit or an ASN.1 toolkit. 4) An XML-based protocol specified in ASN.1 as the primary XML schema language is shown. This provides users with a simple data model, a simple API and excellent tool support.