Web Components (ppt) - Department of Computer Science

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Nov 15, 2013 (3 years and 8 months ago)

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1







Chapter
1

Web
Components
(Introduction)

Web

Protocols

and

Practice

2

Topics

Web

Protocols

and

Practice

INTRODUCTION


Web History


Web Definition


Semantic Components of the Web


Content on the Web


Software Components


Underlying Network


Standardization


Web Traffic and Performance


Web Applications

3

Web History

Web

Protocols

and

Practice

INTRODUCTION


1945
: Vanner Bush proposed Memex which is a device
to extend human memory by providing a large scaling
indexing of text.


1965
: Hypertext: Non
-
sequential writing that presents
information as the collection of linked nodes.


1960
-
1970
: U.S. Department of Defense extended the
use of its communication infrastructure (ARPANET) for
the connected computers. In
1980
they deployed TCP/IP
that caused rapid growth in size and scope of ARPANET.


1989
: Tim Berners Lee proposed using the hypertext for
accessing the information of the computers at CERN

4

Web History

Web

Protocols

and

Practice

INTRODUCTION


During
1980
-
1990
these systems have been used widely
on Internet to access the information :


FTTP: For file transfer. It works by knowing the ftp server.


Gopher: Provided the ways for the users to search the
servers in the network.


WAIS (Wide Area Information Servers): Allowed users to
send queries to the databases around the network.


Archie: Global index of ftp servers that allowed the users to
do the search based on file name.


1992
: The first official release of the web browser.


1993
: First graphical web browser (MOSAIC)



5

Web Definition

Web

Protocols

and

Practice

INTRODUCTION


The World Wide Web, or simply the Web, is the
universe of information accessible via networked
computers.


Internet is different from web. It is a network of
computers, in which a computer may not
necessarily act as a web client or web server.


6

Semantic Components of the Web

Web

Protocols

and

Practice

INTRODUCTION


Three main semantic components of the Web
are:


A naming infrastructure (
URI
)


A document language (
HTML
)


A message exchange protocol (
HTTP
)

7

URI (Uniform Resource Identifier)

Web

Protocols

and

Practice

INTRODUCTION


Accessing and manipulating resources distributed
throughout the Web requires a way to identify them. URI
is a universal naming mechanism for identifying resource
on Web independent of its current location or value.


URI can be thought of as a pointer to a black box to
which request method can be applied to generate
different responses at different times. Request method is
a simple operation such as fetching, changing, or
deleting a resource.


for example in the high level a string such as


http:// www.foo.com/coolpic.gif

is a URI.



Later we will see how it is different from URL.

8

HTML (Hypertext Markup Language)

Web

Protocols

and

Practice

INTRODUCTION


HTML provides a standard representation for
hypertext documents in ASCII format.

9

HTTP (Hypertext Transfer Protocol)

Web

Protocols

and

Practice

INTRODUCTION


HTTP is the most common way of transferring
resources on the Web.


HTTP defines the format and meaning of
messages exchanged between web
components, such as clients and servers.


HTTP is simply a language that has specific
syntax and semantics associate with the use of
the language elements.

10

HTTP (Hypertext Transfer Protocol)

Web

Protocols

and

Practice

INTRODUCTION


HTTP is a
request
-
response

protocol


The client sends a request message and then the
server replies with the response message.


HTTP is a
stateless

protocol


clients and servers treat each message
exchange independently and are not required to
maintain any state across requests and
responses.

11

Table
1.1
.
Common Web terms



Definition

Term

World Wide Web, the universe of information accessible

via networked computers

Nonlinear writing or linking related documents for


navigation

Worldwide collection of interconnected networks using


the Internet Protocol (IP)

Document accessible on the Web via a URI

Collection of related Web pages

Application for requesting and displaying Web resources

WWW/Web


Hypertext


Internet


Web page

Web site

Browser

Web

Protocols

and

Practice

INTRODUCTION

12

Content on the Web

Web

Protocols

and

Practice

INTRODUCTION


Each resource may be available in different
formats for example:


HTML


PostScript


A resource may be:


A static file on a machine


Generated dynamically at the time of the request

13

Content on the Web

Web

Protocols

and

Practice

INTRODUCTION


Each HTTP transfer consists of two messages:



The request message


»

Sent by the client




The response message


»

Sent by the server

14

Table 1.2.
Terminology related to Web resources and HTTP messages

Definition

Term

Network data object or service identified by a URI

Basic unit of communication in HTTP

Component responsible for sending/ receiving a


message

Control portion of a message

Information transferred in the body of a message


Resource

Message

Sender/receiver


Header


Entity



Web

Protocols

and

Practice

INTRODUCTION

15

Software Components

Web

Protocols

and

Practice

INTRODUCTION


User agent


A user agent can be a Web browser that
generates requests on behalf of a user and
performs a variety of other tasks, such as
displaying Web pages and storing the user's
bookmarks
.


Proxy


A proxy is an intermediary between clients and
servers that performs a variety of functions:

»

filtering of requests to undesirable Web sites

»

Providing a degree of anonymity to clients

»

caching popular resources.

16

Software Components

Web

Protocols

and

Practice

INTRODUCTION


Server


The server may instruct the user agent to retain
state across a series of requests and responses
by storing a
cookie
. We will discuss cookies later

17

Table
1.3
.
Terminology related to the software components of the Web

Definition

Term

Client program that initiates a request (e.g., a browser)

Program that sends an HTTP request to a Web server

Program that receives an HTTP request from a Web


client and transmits a response

Server where the requested resource resides or is


created

Web component in the path between the user agent


and an origin server (e.g., a proxy, gateway)

Intermediary program that functions as a server to a


client and as a client to a server

State information passed between the user agent and


the origin server

User agent

Web client

Web Server


Origin Server


Intermediary


Proxy



Cookie



Web

Protocols

and

Practice

INTRODUCTION

18

Underlying Network

Web

Protocols

and

Practice

INTRODUCTION


A Web client identifies the Web server by the
hostname

(e.g., www.att.com), rather than an IP
address by using
Domain name system (DNS)



The two applications exchange HTTP messages


By using
Transmission Control Protocol
(TCP)

The client and the server establish a TCP
connection
.

19

Table
1.4
.
Terminology related to the Internet and its protocols of the
Web

Definition

Term

Computer or machine connected to the network

Basic unit of communication in the Internet

Internet Protocol, a protocol that coordinates the


Delivery of individual packets between hosts

32
-
bit numerical address identifying an Internet host

Case
-
insensitive string identifying an Internet host

Domain name System, a distributed infrastructure for


translating between hostnames and IP addresses

Transmission Control Protocol, a protocol that


provides the abstraction of a reliable, bidirectional


connection

Logical communication channel between two hosts

Host


Packet


IP



IP address

Hostname

DNS



TCP




Connection


Web

Protocols

and

Practice

INTRODUCTION

20

Standardization

Web

Protocols

and

Practice

INTRODUCTION

A protocol standard is needed for interoperation of
the components.


The Internet Engineering Task Force (IETF) is
an open community that deals with Internet
standardization through a series of official
publications called Request for Comments
(RFC)


Not all Internet Drafts become RFCs. RFCs are
divided into different tracks: standards, historic,
informational and Experimental

21

Standardization

Web

Protocols

and

Practice

INTRODUCTION

Standard documents have compliance
requirements of the following levels:


Any compliant implementation has to meet all
the
MUST
-
level requirements.


An implementation can be considered
conditionally compliant if it meets all the
SHOULD
-
level requirements.


The
MAY
-

level requirements are optional for an
implementation to meet.

22

Standardization

Web

Protocols

and

Practice

INTRODUCTION


A standards document proceeds through three
stages:


Proposed Standard


Draft Standard


Internet Standard


Some RFCs reflect the Best Current Practices
(
BCP
)


Standards do not last forever; they can be retired
and replaced by a superior specification.


23

Standardization

Web

Protocols

and

Practice

INTRODUCTION


World Wide Web Consortium (
W
3
C
)

was founded in
1994
to encourage the growth of Web.


The W
3
C works on

»
T
he representation of Web content, such as the
HTML language, rather than the networking aspects

»
Architectural issues

»
User
-
interface issues



Formats



Languages

»

Social issues

»

Legal and public policy matters

»

Accessibility issues to ensure that people with
disabilities are able to have access to the technology

24

Table 1.5.
Terminology related to Internet protocol standards

Definition

Term

Internet Engineering Task force, an open community


contributing to the evolution of the Internet

IETF group chartered to work on a particular


standards specification

Informal version of a standards documents reflecting


work in progress

Request for comments, an official document related


to Internet standards

IETF


Working Group


Internet Draft


RFC





Web

Protocols

and

Practice

INTRODUCTION

25

Web Traffic and Performance

Web

Protocols

and

Practice

INTRODUCTION


User expectations for quick responses have
focused attention on performance issues.


High user perceived latency can be because of
variety of factors such as:


DNS overhead


Network congestion


Load on server


Analysis of logs is a useful for knowing the
workload characteristics such as time between
the requests and size of the requests and
resource popularity, which have the important
implications on Web performance


26

Table 1.6.
Terminology related to Web traffic and performance

Definition

Term

Time between the initiation of an action and the first


Indication of a response

Time between a user action and the initial display of


the content

Amount of traffic that can be carried per unit time

Inputs received by a Web component over time

Record of transactions performed by a Web


component

Latency


User
-
perceived


latency


Bandwidth

Workload

Log


Web

Protocols

and

Practice

INTRODUCTION

27

Web Applications

Web

Protocols

and

Practice

INTRODUCTION


Important applications are:


Web caching

»
Caching moves contents closer to the user.

»
A cache can be located at



A user's browser



An origin server



A machine in the path between the user and the
origin server


Multimedia streaming

»
The client plays the samples and frames as they
arrive from the server, rather than downloading the
content in its entirety before beginning playout.

28

Table 1.7.
Terminology related to Web catching and multimedia
streaming

Definition

Term

Store of messages used to reduce user
-



perceived latency and load on the network


and server

Mechanism to lower the possibility of returning


out
-
of
-
date messages from the cache

Duplication of resources on multiple origin


servers

Delivery of resources on behalf of an origin


server


Sequence of audio samples or video frames

Overlap of the server transmission and client


playback of audio/video data

Helper application for playing multimedia


streams

Cache




Cache coherency



Replication


Content distribution



Audio/video stream

Streaming



Media player


Web

Protocols

and

Practice

INTRODUCTION