1 chapter 1 web components (introduction) web protocols and practice

1

Chapter 1

Web Components (Introduction)

Web Protocols and Practice

2

Topics


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


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


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


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


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)


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)


INTRODUCTION

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

9

HTTP (Hypertext Transfer Protocol)


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)


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

TermDefinitionWWW/Web

Hypertext

Internet

Web pageWeb siteBrowser

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 URICollection of related Web pagesApplication for requesting and displaying Web resources


INTRODUCTION

12

Content on the Web


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


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

TermDefinitionResourceMessageSender/receiver

HeaderEntity

Network data object or service identified by a URIBasic unit of communication in HTTPComponent responsible for sending/ receiving a messageControl portion of a messageInformation transferred in the body of a message


INTRODUCTION

15

Software Components


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


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

TermDefinitionUser agentWeb client Web Server

Origin Server

Intermediary

Proxy

Cookie

Client program that initiates a request (e.g., a browser)Program that sends an HTTP request to a Web serverProgram that receives an HTTP request from a Web client and transmits a responseServer where the requested resource resides or is createdWeb 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 serverState information passed between the user agent and the origin server


INTRODUCTION

18

Underlying Network


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

TermDefinitionHostPacketIP

IP addressHostnameDNS

TCP

Connection

Computer or machine connected to the networkBasic unit of communication in the InternetInternet Protocol, a protocol that coordinates the Delivery of individual packets between hosts32-bit numerical address identifying an Internet hostCase-insensitive string identifying an Internet hostDomain 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 connectionLogical communication channel between two hosts


INTRODUCTION

20

Standardization


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


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


INTRODUCTION

A standards document proceeds through three stages:

Proposed StandardDraft 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


INTRODUCTION

World Wide Web Consortium (W3C) was founded in 1994 to encourage the growth of Web.

The W3C works on» The 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

TermDefinitionIETF

Working Group

Internet Draft

RFC

Internet Engineering Task force, an open community contributing to the evolution of the InternetIETF group chartered to work on a particular standards specificationInformal version of a standards documents reflecting work in progressRequest for comments, an official document related to Internet standards


INTRODUCTION

25

Web Traffic and Performance


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

TermDefinitionLatency

User-perceived latency BandwidthWorkloadLog

Time between the initiation of an action and the first Indication of a responseTime between a user action and the initial display of the contentAmount of traffic that can be carried per unit timeInputs received by a Web component over timeRecord of transactions performed by a Web component


INTRODUCTION

27

Web Applications


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

TermDefinitionCache

Cache coherency

Replication

Content distribution

Audio/video streamStreaming

Media player

Store of messages used to reduce user- perceived latency and load on the network and serverMechanism to lower the possibility of returning out-of-date messages from the cacheDuplication of resources on multiple origin serversDelivery of resources on behalf of an origin server

Sequence of audio samples or video framesOverlap of the server transmission and client playback of audio/video dataHelper application for playing multimedia streams


INTRODUCTION

1 chapter 1 web components (introduction) web protocols and practice

Documents

webweb protocols

performanceweb protocols

practicetopicsweb protocols

http messagesweb protocols

common web terms web

web resources