The document discusses the architecture of web browsers. It describes a reference architecture with 8 subsystems: user interface, browser engine, rendering engine, networking, JavaScript interpreter, XML parser, display backend, and data persistence. It then discusses specific architectures for Mozilla and other browsers. Key aspects covered include session and navigation control, caching, and modeling approaches for sessions, caching, and secure pages. Overall the document provides an overview of common elements in web browser architecture and differences between browser implementations.

1. Sub topic : Web Browser Architecture
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2. Objectives
After completion of this period you would be able
to understand about
• Reference web browser
• Reference web browser architecture
• Various elements
• Different browsers and their architectures
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3. Web Browser Architecture
• The architecture of different browsers are different
• But they are similar in some respects
• Therefore a generic or reference architecture is
explained here, followed by specific architectures

4. A Reference Architecture to Web Browsers
Fig.1 4

5. A Reference Architecture to Web Browsers
• The reference architecture shown in the Figure 1
comprises eight major subsystems. They are
explained as follows
• User Interface
– The User Interface subsystem is the layer between
the user and the Browser Engine
– It provides features such as toolbars, visual page-
load progress, smart download handling,
preferences, and printing
– It may be integrated with the desktop environment
to provide browser session management or
communication with other desktop applications

6. The Reference Architecture
• Browser Engine
– It is an embeddable component that provides a
high-level interface to the Rendering Engine
– It loads a given URI and supports primitive
browsing actions such as forward, back, and reload
– It provides hooks for viewing various aspects of the
browsing session such as current page load
progress and JavaScript alerts
– It also allows the querying and manipulation of
Rendering Engine settings

7. The Reference Architecture contd…
• Rendering Engine
– The Rendering Engine subsystem produces a visual
representation for a given URI
– It is capable of displaying HTML and Extensible
Markup Language (XML) documents, optionally
styled with CSS, as well as embedded content such
as images
– It calculates the exact page layout and may use
“reflow” algorithms to incrementally adjust the
position of elements on the page
– This subsystem also includes the HTML parser

8. The Reference Architecture contd…
• Networking subsystem
– The Networking subsystem implements file transfer
protocols such as HTTP and FTP
– It translates between different character sets, and
resolves MIME media types for files
– It may implement a cache of recently retrieved
resources

9. The Reference Architecture contd…
• JavaScript Interpreter
– The JavaScript Interpreter evaluates JavaScript (also
known as ECMAScript) code, which may be
embedded in web pages
– JavaScript is an object-oriented scripting language
developed by Netscape
– Certain Java- Script functionality, such as the opening
of pop-up windows, may be disabled by the Browser
Engine or Rendering Engine for security purposes

10. The Reference Architecture contd…
• XML Parser
– The XML Parser subsystem parses XML documents into
a Document Object Model (DOM) tree
– This is one of the most reusable subsystems in the
architecture
– In fact, almost all browser implementations leverage an
existing XML Parser rather than creating their own from
scratch

11. The Reference Architecture contd…
• Display Backend subsystem
– The Display Backend subsystem provides drawing and
windowing primitives, a set of user interface widgets, and
a set of fonts
– It may be tied closely with the operating system

12. The Reference Architecture contd…
• Data Persistence subsystem
– The Data Persistence subsystem stores various data
associated with the browsing session on disk
– This may be high-level data such as bookmarks or toolbar
settings, or it may be low-level data such as cookies,
security certificates, or cache

13. Other Web Browser Architectures
Presentation Module
User Interface Object Model History stack
URL HTML Parser Web Browser
Network Interface
Browser
HTTP Module Cache Management Cache
Request Response Internet
Web Server 1 Web Server 2
Web Server 2 …… Web Server N
Fig.9 13

14. Web Local Cache
• cache settings (inside browser)
examples
automatically: all cacheable web pages are valid
per session: in current session, all cacheable web
pages are valid
• HTTP cache controls (associated with received web
pages)
a web page
cacheable
valid period
uncacheable: its expire time as the same time it is
created
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15. Modeling of Caching Settings
• assume given setting is automatically
• a web page is cacheable: if the cacheable setting is
included in the header part of the HTTP message
containing the web page
• For HTTP cache control, no model of expire time
if a page is cacheable, the page is always fresh
• each page associated with an attribute for cacheability
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16. Session Control
• most web applications need to maintain communication
sessions with their client browsers, and monitor each
client's individual status and activities
• example: an online banking system should maintain a
communication session with a specific user during the
time the user has logged in (and not yet logged out)
• HTTP is stateless, no functionality on session control
• cookie technique -- a solution
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17. Modeling of Session Control
• only sessions for authentications
• no consideration of relationship between cookie and
dynamic content/link
• assume it is given whether a page is secure or not
secure page: should always be accessed with
authentication session open
all entry pages are by nature not secure
• two special actions SignIn and SignOut: session will
remain open for the consecutive accesses to secure pages
until SignOut performed
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18. Web Browser - Architecture
Fig.10 18

19. Web Browser - Architecture
Fig.11 19

20. Web Browser – Architecture : Mozilla
Fig.12 20

21. Web Browser - Architecture
Fig.13 21

22. Summary
In this class, you have learnt about
• Web Browser Introduction
• Web Browser Architecture
• Session, Navigation Control
• Different Browser Examples
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23. Frequently Asked Questions
1. Explain in detail about Web browser architecture
2. List few famous Web browsers
3 List few applications of Web browsers
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