sw 1

• This lecture introduces Semantic Web technologies
• It describes how the web works

Course Information

• Lecture C01 covers hypertext, the Internet, and the Web
• The lecturer is Prof. Stefan Decker
• Slides are by Rezaul Karim, Sascha Welten, and Lars Gleim
• Exercises are mandatory, needing at least 50% points to qualify for the final exam
• Six exercises are to be be submitted online and will be auto-graded
• Exercises are based on Jupyter notebooks, but exercise 1 is an exception
• Notebooks can be accessed through RWTH Jupyter Hub at https://jupyter.rwth-aachen.de
• Exercise information can be found in the Moodle room

Creative Commons License

• The slides are derived from the "Semantic Web Technologies" lecture at RWTH Aachen University
• Slides are based on previous work by Stefan Decker, Benedikt Köhler, Andreas Harth, and Lars Gleim
• Content is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0)
• License details: http://creativecommons.org/licenses/by/4.0/

Semantic Web Context

• Semantic Web bridges databases, knowledge representation/AI, and the Internet
• It combines elements of data management, AI, and web technologies

Agenda

• The vision
• The Internet
• The World Wide Web
• Towards a Web of Data

A History of Communication

• Cave drawings were an early form of communication (10000 BC)
• Sumerian cuneiform marked the advent of writing (3200 BC)
• Gutenberg's printing press revolutionized information dissemination (1450)
• Photography (Daguerre, 1839), Phonograph (Edison, 1877), Movies (Lumiere, 1895)

Early Visionaries

• Paul Otlet: Developed hierarchical modelling for organizing knowledge.
• Vannevar Bush (1945): Envisioned the "Memex" as a device for storing and retrieving books, records, and communications.
• J.C.R. Licklider (1965): Discussed the possibility of systems that could transfer chemical formulas without human reading or key pressing.
• Doug Engelbart (1968): Created the oNLine System (NLS), featuring the mouse, word processing, data sharing, and hypertext.
• Ted Nelson (1960s): Conceived Xanadu, emphasizing the deep interconnection of everything

ARPANET and the Internet

• The ARPANET was created by John Postel, David Crocker, and Vint Cerf, with the first internet protocols being defined (1969)
• Vint Cerf and Bob Kahn are key figures in the Internet's development
• The internet Architecture has a need for Internet Standardization and governance

Internet Engineering Task Force (IETF)

• The IETF is hosted by the Internet Society and provides a forum for internet technology specifications
• Specifications are called Request for Comments (RFCs) and provide essential protocols, methods, and concepts
• It also published RFCs that serve as foundation of the World Wide Web: Uniform Resource Identifiers (URIs) and Hypertext Transfer Protocol (HTTP)
• Internet standards are specifications with significant implementation and operational experience

The Robustness Principle

• Jon Postel (1943-1998) was an RFC editor and Internet Hall of Fame Pioneer
• Postel's Robustness Principle (RFC 793) states a system should "be conservative in what you do, be liberal in what you accept from others."
• Internet systems should be strict in what they emit, and liberal in what they accept

Governance of the Internet

• The Internet Corporation for Assigned Names and Numbers (ICANN) governs the internet
• ICANN is an internationally organized, non-profit with responsibility for Internet Protocol address space allocation, protocol identifier assignment, domain name system management, and root server system management functions
• IANA (a department of ICANN) coordinates IP address allocation and administers the DNS
• DNS is hierarchically organized and maps hostnames to IP addresses

IETF Internet Standards

• Internet Protocol (IP) RFC 791 (Sept 1981) is responsible for sending packets
• Transmission Control Protocol (TCP) RFC 793 (Sept 1981) is responsible for breaking data into packets
• Domain Names RFC 1034 (Nov 1987) introduces the Domain Name System (DNS)

Network Effects

• Metcalfe's Law states that the cost of a network is proportional to the number of cards installed, while the value is proportional to the square of the number of users

World Wide Web

• Vision: the computers help people analyse and make sense of what they are doing
• Goal is to work better together.

Web Architecture

• Web architecture principles are published in "Architecture of the World Wide Web, Volume One" (W3C, 2004)
• Document covers URIs and HTTP usage within the web
• Representational State Transfer (REST) is the theoretical foundation

Resources

• Resource definition is An abstract notion for things of discourse, be they abstract or concrete, physical or virtual
• electronic documents, images, collections of other resources, chemical elements, numeric values, rooms
• "files" : Information resources
• "directories": Collection Resources
• Non-information Resources: abstract concepts

Uniform Resource Identifiers (URIs)

• URI definition is A uniform Resource Identifier (URI) is a character sequence that identified a resrouce
• URIs are names for resources, and are defined in RFC 3986 (Jan 2005; Internet Standard)

Valid URI Examples

• ftp://ftp.is.co.za/rfc/rfc1808.txt
• http://www.ietf.org/rfc/rfc2396.txt
• ldap://[2001:db8::7]/c=GB?objectClass?one
• mailto:[email protected]
• news:comp.infosystems.www.servers.unix
• tel:+1-816-555-1212
• telnet://192.0.2.16:80/
• urn:oasis:names:specification:docbook:dtd:xml:4.1.2

URI Structure

• The generic URI syntax includes the scheme, authority, path, query, and fragment with an URI syntax specification in RFC 3986
• The hierarchical part contains the authority (headed by "//") and an optional path
• URI = scheme ":" hierarchical part[ "?" query ][ "#" fragment ]
• Scheme specifies the URI type and associated protocol
• Authority includes hostname and optional port number
• Path references content and can be interpreted hierarchically
• Query contains data that does not match the tree, like variables.
• Fragment addresses the local part of a document

Protocols

• The internet provides the infrastructure for global data exchange, and information is accessed using different protocols
• HTTP (Hypertext Transfer Protocol) is foundation of the World Wide Web
• Common protocols include : FTP, SMTP, and others

HTTP (Hypertext Transfer Protocol)

• HTTP 1.1 RFCs is the communication between agents and servers via HTTP
• RFC 7230 is the protocol for message syntax and routing
• RFC 7231 defines semantics and content.
• HTTP is a stateless application-level protocol for distributed, collaborative, hypertext information systems.
• User agents pose requests and servers issue responses

HTTP interactions

• User agents include web browsers, appliances, cars, or command line programs
• Server sends a HTTP response. Trailing newlines indicates the end of message.
• Connection is terminated following the HTTP response

HTTP - Stateless interactions

• HTTP doesn't retain client information between interactions.
• Permanent authentications can be handled through: HTTP authentication, Session-IDs and Cookies

HTTP Request and Responses Structure

• Uses a start line: request for HTTP requests, and is the status line for HTTP responses
• Uses header fields: Additional information is message in key/value format. Header fields are optional
• Transferred content is displayed as the message body (a list of bytes).

HTTP Methods

• The HTTP method is part of the request line
• Used to inform what purpose is used in the request
• The "GET" method is mentioned, more methods may exist

HTTP Status Codes

• Status codes are interactions between the user agent reporting
• Each statues has a different code classification like: success, fail and redirection codes
• Informational is the 1xx
• Successful 2xx
• Redirection is 3xx
• Client Error is 4xx
• Server Error is 5xx

Selected HTTP header fields

• The header field is named Used by and Description
• Host, U, User agent specifies the hostname
• User-Agent, U, Name of user agent
• Server, S, Name of the server
• Accept, U, agent can identify media types
• Content-Type, U, S, The media type of the message body
• Location, S, Location of the requested URI
• Content- Location, S, Content is available at the specified location, not the URI

HTTP Message Body

• The message body contains a byte stream of content
• A server response is only the requested content
• The recipient can access the bytes contained in the message body on the type of content
• The processes of figuring out each messeges is called content negotiation

Selected content / MediaType

• text/html is HTML
• application/xhtml+xml XML
• application/rdf+xml RDF XML
• application/Id+json JSON-LD
• text/turtle RDF Turtle
• text/n3 Notation3

Hypertext Markup Language is used for

• HTML is Hypertext documents and to represent document structure, such as headings and paragraphs
• To further document other documets, documents can reference media or provide links
• 1995: HTML 2.0 was specified as an IETF RFC 18661
• HTML 5, initiated in 2004 by Apple, the Mozilla Foundation and Opera Software
• HTML is used to link with other documents
• Browsers are used by links can naviguate to other pages and display info

Extensible Markup Language (XML)

• XML is readable to both humans and machines
• Documents stick adhere to syntactic rules than HTML
• Attributes in XML do use URIs for identification, however, XML does not provide the capability to reference URIs as attribute values
• XML 1.0 is a W3C recommendation in 1998

World Wide Web Consortium

• An International Community where the public works together to develop web standards
• W3C aims for the worldwide availability of web access and helps security and progress.
• Tim berner Lee Is the director of W3C
• Comparable to IETF’s standardisation through RFCs, W3C publishes reports that pass different maturity levels until they are officially recommended
• Working Draft(WD), Candidate Recommendation (CR ), Proposed Recommendation (PR), W3C Recommendation (REC)

Learning goals

• Distinguish between hierarchical and network structure
• Outline basic internet, web technologies, relationship between them
• Assess the decentralisation characteristics in info and system architectures.
• Explain the Metcalfe’s Law
• Identify, name resources, information resourses apart from other resources based on textual description

Vision for data on the web

• Memex, individual stores all books communications and records
• Augmenthing human intellect through comprehension and approach with complex situation
• Making the computer help analyse to make better better