1. 1 Chapter 1 Web Components (Introduction) Web Protocols and Practice

2. 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. 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. 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. 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. 6 Semantic Components of the Web Web Protocols and Practice INTRODUCTION  Three main semantic components of the Web are:  A naming infrastructure (URI)URI  A document language (HTML)HTML  A message exchange protocol (HTTP)HTTP

7. 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. 8 HTML (Hypertext Markup Language) Web Protocols and Practice INTRODUCTION  HTML provides a standard representation for hypertext documents in ASCII format.

9. 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. 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. 11 Table 1.1. Common Web terms DefinitionTerm 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. 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. 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. 14 Table 1.2. Terminology related to Web resources and HTTP messages DefinitionTerm 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. 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. 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. 17 Table 1.3. Terminology related to the software components of the Web DefinitionTerm 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. 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. 19 Table 1.4. Terminology related to the Internet and its protocols of the Web DefinitionTerm 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. 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. 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. 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. 23 Standardization Web Protocols and Practice INTRODUCTION  World Wide Web Consortium (W3C) was founded in 1994 to encourage the growth of Web.  The W3C 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. 24 Table 1.5. Terminology related to Internet protocol standards DefinitionTerm 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. 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. 26 Table 1.6. Terminology related to Web traffic and performance DefinitionTerm 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. 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. 28 Table 1.7. Terminology related to Web catching and multimedia streaming DefinitionTerm 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