1、Basic WWW Technologies,2.1 Web Documents. 2.2 Resource Identifiers: URI, URL, and URN. 2.3 Protocols. 2.4 Log Files. 2.5 Search Engines.,2,What Is the World Wide Web?,The world wide web (web) is a network of information resources. The web relies on three mechanisms to make these resources readily av
2、ailable to the widest possible audience: 1. A uniform naming scheme for locating resources on the web (e.g., URIs). 2. Protocols, for access to named resources over the web (e.g., HTTP). 3. Hypertext, for easy navigation among resources (e.g., HTML).,3,Internet vs. Web,Internet: Internet is a more g
3、eneral term Includes physical aspect of underlying networks and mechanisms such as email, FTP, HTTP Web: Associated with information stored on the Internet Refers to a broader class of networks, i.e. Web of English Literature Both Internet and web are networks,4,Essential Components of WWW,Resources
4、: Conceptual mappings to concrete or abstract entities, which do not change in the short term ex: ICS website (web pages and other kinds of files) Resource identifiers (hyperlinks): Strings of characters represent generalized addresses that may contain instructions for accessing the identified resou
5、rce http:/www.ics.uci.edu is used to identify the ICS homepage Transfer protocols: Conventions that regulate the communication between a browser (web user agent) and a server,5,Standard Generalized Markup Language (SGML),Based on GML (generalized markup language), developed by IBM in the 1960s An in
6、ternational standard (ISO 8879:1986) defines how descriptive markup should be embedded in a document Gave birth to the extensible markup language (XML), W3C recommendation in 1998,6,SGML Components,SGML documents have three parts: Declaration: specifies which characters and delimiters may appear in
7、the application DTD/ style sheet: defines the syntax of markup constructs Document instance: actual text (with the tag) of the documents More info could be found: http:/www.W3.Org/markup/SGML,7,DTD Example One, ELEMENT is a keyword that introduces a new element type unordered list (UL) The two hyphe
8、ns indicate that both the start tag and the end tag for this element type are required Any text between the two tags is treated as a list item (LI),8,DTD Example Two, The element type being declared is IMG The hyphen and the following “O“ indicate that the end tag can be omitted Together with the co
9、ntent model “EMPTY“, this is strengthened to the rule that the end tag must be omitted. (no closing tag),9,HTML Background,HTML was originally developed by Tim Berners-Lee while at CERN, and popularized by the Mosaic browser developed at NCSA. The Web depends on Web page authors and vendors sharing
10、the same conventions for HTML. This has motivated joint work on specifications for HTML. HTML standards are organized by W3C : http:/www.w3.org/MarkUp/,10,HTML Functionalities,HTML gives authors the means to: Publish online documents with headings, text, tables, lists, photos, etc Include spread-she
11、ets, video clips, sound clips, and other applications directly in their documents Link information via hypertext links, at the click of a button Design forms for conducting transactions with remote services, for use in searching for information, making reservations, ordering products, etc,11,HTML Ve
12、rsions,HTML 4.01 is a revision of the HTML 4.0 Recommendation first released on 18th December 1997. HTML 4.01 Specification:http:/www.w3.org/TR/1999/REC-html401-19991224/html40.txt HTML 4.0 was first released as a W3C Recommendation on 18 December 1997 HTML 3.2 was W3Cs first Recommendation for HTML
13、 which represented the consensus on HTML features for 1996 HTML 2.0 (RFC 1866) was developed by the IETFs HTML Working Group, which set the standard for core HTML features based upon current practice in 1994.,12,Sample Webpage,13,Sample Webpage HTML Structure, The title of the webpage Body of the we
14、bpage ,14,HTML Structure,An HTML document is divided into a head section (here, between and ) and a body (here, between and ) The title of the document appears in the head (along with other information about the document) The content of the document appears in the body. The body in this example cont
15、ains just one paragraph, marked up with ,15,HTML Hyperlink,alumni A link is a connection from one Web resource to another It has two ends, called anchors, and a direction Starts at the “source“ anchor and points to the “destination“ anchor, which may be any Web resource (e.g., an image, a video clip
16、, a sound bite, a program, an HTML document),16,Resource Identifiers,URI: Uniform Resource Identifiers URL: Uniform Resource Locators URN: Uniform Resource Names,17,Introduction to URIs,Every resource available on the Web has an address that may be encoded by a URI URIs typically consist of three pi
17、eces: The naming scheme of the mechanism used to access the resource. (HTTP, FTP) The name of the machine hosting the resource The name of the resource itself, given as a path,18,URI Example,http:/www.w3.org/TR There is a document available via the HTTP protocol Residing on the machines hosting www.
18、w3.org Accessible via the path “/TR“,19,Protocols,Describe how messages are encoded and exchanged Different Layering Architectures ISO OSI 7-Layer Architecture TCP/IP 4-Layer Architecture,20,ISO OSI Layering Architecture,21,ISOs Design Principles,A layer should be created where a different level of
19、abstraction is needed Each layer should perform a well-defined function The layer boundaries should be chosen to minimize information flow across the interfaces The number of layers should be large enough that distinct functions need not be thrown together in the same layer, and small enough that th
20、e architecture does not become unwieldy,22,TCP/IP Layering Architecture,23,TCP/IP Layering Architecture,A simplified model, provides the end-to-end reliable connection The network layer Hosts drop packages into this layer, layer routes towards destination Only promise “Try my best” The transport lay
21、er Reliable byte-oriented stream,24,Hypertext Transfer Protocol (HTTP),A connection-oriented protocol (TCP) used to carry WWW traffic between a browser and a server One of the transport layer protocol supported by Internet HTTP communication is established via a TCP connection and server port 80,25,
22、GET Method in HTTP,26,Domain Name System,DNS (domain name service): mapping from domain names to IP address IPv4: IPv4 was initially deployed January 1st. 1983 and is still the most commonly used version. 32 bit address, a string of 4 decimal numbers separated by dot, range from 0.0.0.0 to 255.255.2
23、55.255. IPv6: Revision of IPv4 with 128 bit address,27,Top Level Domains (TLD),Top level domain names, .com, .edu, .gov and ISO 3166 country codes There are three types of top-level domains: Generic domains were created for use by the Internet public Country code domains were created to be used by i
24、ndividual country The .arpa domain Address and Routing Parameter Area domain is designated to be used exclusively for Internet-infrastructure purposes,28,Registrars,Domain names ending with .aero, .biz, .com, .coop, .info, .museum, .name, .net, .org, or .pro can be registered through many different
25、companies (known as “registrars“) that compete with one another InterNIC at http:/ Registrars Directory: http:/ Log Files,Server Transfer Log: transactions between a browser and server are logged IP address, the time of the request Method of the request (GET, HEAD, POST) Status code, a response from
26、 the server Size in byte of the transaction Referrer Log: where the request originated Agent Log: browser software making the request (spider) Error Log: request resulted in errors (404),30,Server Log Analysis,Most and least visited web pages Entry and exit pages Referrals from other sites or search
27、 engines What are the searched keywords How many clicks/page views a page received Error reports, like broken links,31,Server Log Analysis,32,Search Engines,According to Pew Internet Project Report (2002), search engines are the most popular way to locate information online About 33 million U.S. Int
28、ernet users query on search engines on a typical day. More than 80% have used search engines Search Engines are measured by coverage and recency,33,Coverage,Overlap analysis used for estimating the size of the indexable web W: set of webpages Wa, Wb: pages crawled by two independent engines a and b
29、P(Wa), P(Wb): probabilities that a page was crawled by a or b P(Wa)=|Wa| / |W| P(Wb)=|Wb| / |W|,34,Overlap Analysis,P(Wa Wb| Wb) = P(Wa Wb)/ P(Wb) = |Wa Wb| / |Wb| If a and b are independent:P(Wa Wb) = P(Wa)*P(Wb) P(Wa Wb| Wb) = P(Wa)*P(Wb)/P(Wb)= |Wa| * |Wb| / |Wb| = |Wa| / |W|=P(Wa),35,Overlap Ana
30、lysis,Using |W| = |Wa|/ P(Wa), the researchers found: Web had at least 320 million pages in 1997 60% of web was covered by six major engines Maximum coverage of a single engine was 1/3 of the web,36,How to Improve the Coverage?,Meta-search engine: dispatch the user query to several engines at same t
31、ime, collect and merge the results into one list to the user. Any suggestions?,37,Web Crawler,A crawler is a program that picks up a page and follows all the links on that page Crawler = Spider Types of crawler: Breadth First Depth First,38,Breadth First Crawlers,Use breadth-first search (BFS) algor
32、ithm Get all links from the starting page, and add them to a queue Pick the 1st link from the queue, get all links on the page and add to the queue Repeat above step till queue is empty,39,Breadth First Crawlers,40,Depth First Crawlers,Use depth first search (DFS) algorithm Get the 1st link not visited from the start page Visit link and get 1st non-visited link Repeat above step till no no-visited links Go to next non-visited link in the previous level and repeat 2nd step,41,Depth First Crawlers,
