Understanding Proxy Server Components

This area will attempt to better define to the components of the
architecture that we have used, but may not have defined.

ISAPI Filter

The ISAPI Filter interface is one of the components of the web proxy
service. The interface provides an extension that the Web server calls
whenever it receives an HTTP request.

An ISAPI Filter is called for every request, regardless of the identity of
the resource requested in the URL. An ISAPI filter can monitor, log, modify,
redirect and authenticate all requests that are received by the Web server.
The Web service can call an ISAPI filter DLL's entry point at various times
in the processing of a request or response. The Proxy Server ISAPI filter is
contained in the w3proxy.dll file. This filter examines each request to
determine if the request is a standard HTTP request or not.

ISAPI Application

The ISAPI Application is the second of the two web proxy components. ISAPI
applications can create dynamic HTML and integrate the web with other
service applications like databases.

Unlike ISAPI Filters, an ISAPI Application is invoked for a request only if
the request references that specific application. An ISAPI Application does
not initiate a new process for every request. The ISAPI Application is also
contained in the w3proxy.dll file.

Proxy Servers Caching Mechanism

Microsoft Proxy Server handles caching in two different ways, Passive and Active caching.

* Passive Caching: Passive caching is the basic mode of caching. Proxy
Server interposes itself between a client and an internal or external Web
site and then intercepts client requests. Before forwarding the request on
to the Web server, Proxy Server checks to see if it can satisfy the request
from its cache. Normally, in passive caching, Proxy Server places a copy of
retrieved objects in the cache and associates a TTL (time-to-live) with that
object. During this TTL, all requests for that object are satisfied from the
cache. When the TTL is expired, the next client request for that object will
prompt Proxy Server to retrieve a fresh copy from the web. If the disk space
for the cache is too full to hold new data, Proxy Server removes older
objects from the cache using a formula based on age, popularity, and size.

* Active Caching: Active Caching works with passive caching to optimize the
client performance by increasing the likelihood that a popular will be
available in cache, and up to date. Active caching changes the passive
caching mechanism by having the Proxy Server automatically generate requests
for a set of objects. The objects that are chosen are based on popularity,
TTL, and Server Load.

Windows Sockets

Windows Sockets is the mechanism for communication between applications
running on the same computer or those running on different computers which
are connected to a LAN or WAN. Windows Sockets defines a set of standard
API's that an application uses to communicate with one or more other
applications, usually across a network. Windows Sockets supports initiating
an outbound connection, accepting inbound connections, sending and receiving
data on those connections, and terminating a session.

Windows socket is a port of the Berkeley Sockets API that existed on Unix,
with extensions for integration into the Win16 and Win32 application
environments. Windows Sockets also includes support for other transports
such as IPX/SPX and NetBEUI.

Windows Sockets supports point-to-point connection-oriented communications
and point-to-point or multipoint connectionless communications when using
TCP/IP. Windows Socket communication channels are represented by data
structures called sockets. A socket is identified by an address and a port,
for example;

131.107.2.200:80

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