so that it can be displayed in another format such in HTML [17]. Different XSL formats can 

then be used to display the same data in different ways, for different uses. This provides the 

ability to render the same information differently for different situations of for different 

devices. 

2.5.2.3 Using XML to describe a SLA 

Increasingly XML is emerging, as the de facto standard for describing information content 

and the area of SLAs is no exception. What are the advantages of using XML to describe a 

SLA? First of all, once a DTD is defined, it allows the creation of SLAs in XML and both 

computers and humans can easily understand XML due to its structured form. It presents the 

possibility for linking and merging multiple components of SLAs to define a higher level 

SLA. XML is particularly suitable as a transport encoding format and the fact that SLAs 

often need to be transported between different systems provides another advantage in using 

it. It also supports different views of the same data through the use of XSL [16]. This is 

useful in the fact that some items of a SLA might not be that interesting to a customer but 

would be interesting to the Service Provider.  

2.5.3 HTTP Proxies and Caching 

HTTP proxies are an integral part of the Web today. A proxy is defined as an intermediary 

program that acts as both a server and a client for the purpose of making requests on behalf 

of other clients. Requests are serviced internally or by passing them on, with possible 

translation, to other servers. A proxy must implement both the client and server 

requirements. A "transparent proxy" is a proxy that does not modify the request or response 

beyond what is required for proxy authentication and identification. A "non transparent 

proxy" is a proxy that modifies the request or response in order to provide some added 

service to the user agent, such as group annotation services, media type transformation, 

protocol reduction, or anonymity filtering. Except where either transparent or non 

transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of 

proxies [15].