11

infrastructure such as a firewall, and should be equipped with support for databases,

transaction management, communication, legacy data, and other functionalities [79].

After receiving a client's request, an application server parses and converts it to a query.

Then, it sends the generated query to a database and gets back the response from

the database. Finally, it converts the response into an HTML based document and

sends it back to the client. The application server provides important functionalities for

on line business such as online billing, banking and inventory management. Therefore,

majority of the contents here are generated dynamically and require an adequate security

mechanism.

The backend database layer houses the most confidential and secure data. The

main communication characteristic of the database layer is frequent disk access through

the (SAN) [80].

2.3

User Level Communication

One architectural feature that has made System Area Networks (SANs) attrac 

tive for high performance computing is user level communication. User level communi 

cation approaches such as U Net [36], Fast Messages (FM) [60], Active Messages [37]

and SHRIMP [16] allow direct access to the network interface, without involving the

OS kernel. A generic communication layer, called Virtual Interface Architecture (VIA)

[30, 19], was introduced considering the importance of user level communication. Figure

2.3 depicts the VIA architecture. VIA defines a set of functions and associated semantics

for providing such communication and has been implemented in commercial platforms

[80, 57]. Virtual Interface (VI) is a logical channel provided to an application by the VIA