Networking / Beginners

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Extensible Markup Language

The eXtensible Markup Language is a subset of the Standard Generalized Markup Language (SGML) specified in ISO 8879. XML defines data objects known as XML documents and the rules by which applications access these objects. XML documents look very much like Hypertext Markup Language (HTML) documents (e.g., Web pages), but XML document specifications include strict definitions of the data type in each fi eld of an object. This makes XML documents applicable to database formats, whereas HTML documents are more suited for text management. Thus, while presentation instructions (such as "center this text and print it in Arial 12 point") are part of SGML, they are not relevant to XML but are very important in HTML.

In effect, XML provides encoding rules for commands that are used to transfer and update data objects. The syntax of these commands can be precisely specified and can be automatically parsed by a simple text-based application. Just as in HTML, formatting and control are managed using text tags that delimit the data, but unlike in HTML, the semantics of a tag is not global, but is specific to a given XML document. The data themselves are presented as strings of bytes with each string enclosed by a pair of tags known as a single XML element. ISO 8879 defines how tags are used to enclose XML elements and what the meaning of the tags is (i.e., how the tags cause the receiving application to operate on the data in the XML element).

The collection of tags in an XML document is referred to as the markup data. The markup data not only give instructions on the interpretation of individual data elements, but define how the elements are associated, and also describe the purpose of the entire document and its applicability.

XML is developed by the World Wide Web Consortium (W3C) based on SGML. SGML was standardized in the mid-1980s and work on XML started in 1996, reaching its first standard (or Recommendation) in 1998. As such, XML is neither a communications protocol, nor tied to use within the Internet, but its applicability and increasing popularity as a configuration and management tool for Internet devices makes it worthy of further examination.