The document discusses web technologies including three-way and four-way handshaking for establishing client-server connections and contrasting non-persistent and persistent HTTP connections. It also covers web caching techniques to improve quality of service, the role of proxy servers in managing traffic, and cache consistency mechanisms for maintaining updated content. Additionally, it addresses the limitations of proxy servers regarding dynamically generated content and frequent updates.

1. WEB TECHNOLOGY UNIT I
Part E

2.  Three-way handshaking:
 The actual message exchange, TCP establishes a logical
connection between the client and server.
Example:
 Once the connection is established, the client can send a
request to the server.
 The web server receives the request and responds with a
response message
 Four-way handshaking:
 HTTP/0.9 and HTTP/1.0 specifications, the server terminations
the connection immediately, after the response is sent.

4. NON-PERSISTENT CONNECTIONS:
 If the client wants to send another request, it must
follow same set of steps.
 It means every request is considered as new
request.
 It means the connection doesn’t exist (persist) after
a response is sent.

6. PERSISTENT CONNECTIONS:
 This mechanism is that it consumes significant
bandwidth due to connection establishment and
connect release procedure every time the client
wants to send a request to the server.

8. PERSISTENT CONNECTIONS:

9. IMPROVEMENT:
 HTTP/1.1 is byte serving, i.e., a server may send
just the requested portion of the documents instead
of sending the entire content.

17. WEB CACHING:
 It is an important technique to improve Quality of Service
(QoS) of the web server.
 Caching can reduce network latency experienced by
clients.
 Example:
 Web pages can be loaded more quickly in the browser.
 Caching can also conserve bandwidth on network and
increasing scalability and availability of web pages with
the help of an HTTP proxy cache.
 Web resources that are cached remain accessible even
if source of the resources or an intermediate network link
goes down.

18. PROXY:
 It is an application program or a computer system
that behaves like an intermediary between servers
and clients looking for services from those servers.
 Many kinds of proxy.
 Each one is designed to perform some specific
tasks.

20. WEB PROXY:
 It is one between web clients and web servers and
provides web services on behalf of web servers.

22. PROXY SERVER:
 They are especially important on relatively large networks
such as institutional networks, corporate intranets, and ISP
networks.
 They are application layer and provide more granularity in
controlling traffic.
 Connection sharing
 Traffic filtering and fire walling
 Caching
Need:
 Data privacy is critical and as a consequence.

25. CONTENT FILTERING:
 It is used in crucial commercial and non-commercial
sectors such as corporate, education, or digital library
environment, etc.
 If requesting client is validated by the filter, it is allowed
to get the service.
 Filtering Technique:
 URL or DNS blacklist
 MIME filtering
 Content keyword filtering
 URL regexp filtering, etc.
 Example:
 It can divide available bandwidth equitably to all
computers.

26. ARCHITECTURE:
 A web proxy may be placed in user’s local computer or
at various other points between clients and target web
servers.

27. CACHE CONSISTENCY:
 It mechanisms ensure that cached copies of web
pages are eventually updated to reflect changes in
the original web pages.
 Two cache consistency mechanisms in use for
HTTP proxies:
 Pull method
 Push method

28. PULL METHOD:
 Each web page is assigned a time-to-store field, which
indicates the time of storing web page in the cache.
 An expiry time of one or two days.
 Expiry time determines how long a web page should remain
valid in the cache.
 Example:
 The page is referred, and page is found in the cache, the web
server checks whether the web page has expired or not.
 If so, a fresh copy is obtained (pulled) from the original server
and forwarded to the client.

29. PUSH METHOD:
 Web server is assigned the responsibility of making all
cached copies consistent with the server copy.
 Whenever a web page is modified, the web server
sends an update request in every web proxy that
cached this web page.
 Advantage:
 The cached pages are modified only when the original
copies are modified.
 Disadvantage:
 Web server has to update all cached copies that are
stored different client’s caches.
 This increases the load on the web server.

30. DRAWBACKS:
 Proxy servers aren’t useful in the case of refreshed
pages.
 Web pages, on some sites, use HTML META tags to
expire quickly so proxy server must reload these
expired pages in the cache.
 Poor performance:
 The pages whose contents are changed very frequently
such as news sites, web blogs, etc.
 Generated by server-side scripting such as Perl, PHP,
ASP, JSP can’t be cache because contents of web
pages depend on user input as well as information form
a server like database server.

31.  Proxy servers can only cache home pages from
websites that are generally static.
 Generate contents of subsequent pages
dynamically, proxy servers can’t help much in this
regard.