client can send multiple requests over the same connection for those resources. It also allows 

a client to make requests without having to wait for each response before it makes the next 

request, allowing a single TCP connection to be used much more efficiently. Technologies 

for SLAs and QoS Monitoring 

2.5.2 XML 

One of the primary focuses of this dissertation is to investigate a method of describing how 

an HTTP delivered service should perform. XML, the Extensible Markup Language, is 

emerging as a key technology in describing the structure of data. However, XML is not a 

language as its name suggests, it's actually a metalanguage or a language used to describe 

other languages [16]. It allows the creation of customised (Extensible) markups so unlike 

HyperText Markup Language (HTML) where the markup is limited and can only describe 

one type of document, XML allows the creation of customised markups. One key feature of 

XML in comparison to HTML is XML tells what data signifies rather than how to display it. 

XML is based on the SGML (Standard Generalized Markup Language), which is the 

international standard for defining descriptions of the structure and content of different types 

of electronic document. However XML has been designed so that is simpler to use that 

SGML with the aim of promoting the use of SGML on the Web.  

2.5.2.1 DTDs 

Document Type Definition  (DTDs) specifies the tags that can be included in an XML 

document, and the valid arrangements of those tags. The DTD specification is actually part 

of the XML specification, rather than a separate entity. However a DTD it optional and 

XML documents can be created without one. DTDs help avoid creating invalid XML 

structures and allow the verification of the XML structure of a particular document. 

2.5.2.2 XSL 

As stated already, XML specifies how to identify data, not how to display it. HTML, on the 

other hand, tells us how something should be displayed without identifying what's 

displayed. Extensible Stylesheet Language (XSL) specifies what to convert an XML tag into 

&