Introduction to Computational Thinking - Lecture Notes

Lecture Topic: The World Wide Web

Client-Server Model

• Definition: Organizes services across a network.
  • Servers: Provide services over a network.
  • Clients: Utilize services provided by servers.
• Server Details:
  • Listen at reserved TCP or UDP ports.
  • May handle multiple TCP connections simultaneously.
  • Often called "daemons," typically named ending in 'd' (e.g., httpd).
  • Can be a group of processes or threads.
• Clients:
  • Initiate connections to servers.
  • Assigned ephemeral ports by the operating system.
  • Generally simpler than servers; security burden lies mostly on servers.
• Components Needed:
  • Communication protocol (e.g., HTTP for web services).
  • Server programs (e.g., web server).
  • Client programs (e.g., web browser).
  • Communication channels (TCP or UDP).

Cloud Computing

• Definition: Use of data storage and computational resources provided by internet servers.
• Benefits:
  • Reduced personnel and capital investment.
  • Shared infrastructure among various clients.
  • Access to software (SaaS), virtual computers (PaaS), and infrastructure (IaaS).
• Drawbacks:
  • Dependency on external privacy and security.
  • Reduced management control and computational speed.

Internet Layering Model

• Layers:
  • Hardware Layer: Connection of hosts to physical networks.
  • Network Interface Layer: Connectionless and uses frames.
  • Internet Layer: Connectionless using IP datagrams.
  • Transport Layer: Connection-oriented (TCP) or connectionless (UDP).
  • Application Layer: Protocols like HTTP for web services.
• Addressing:
  • Physical addresses at the network interface.
  • IP addressing at the internet layer.
  • Protocol ports at the transport layer.
• Main Protocols:
  • ARP: Matches physical and IP addresses.
  • IP/ICMP: Main internet protocol.
  • TCP/UDP: Transport protocols; TCP offers reliability.

Domain Name System (DNS)

• Purpose: Provides names for hosts and collections of hosts, mapping host names to IP addresses and vice versa.
• Managed by: Set of name servers using TCP/UDP for communication.
• Structure:
  • Domain names are sequences of labels separated by dots.
  • Subdomains are suffixes of domain names.
• Types of Domains:
  • Top-level domains (TLD): Generic, country code, and infrastructure (e.g., ARPA).
  • Original TLDs: COM, NET, ORG, EDU, GOV, MIL.
  • Expanded to over 1500 TLDs.
  • Country Code TLDs: 312 codes (e.g., CA for Canada, DE for Germany).

Key Question

• The internet is both connectionless and connection-oriented based on the layer (e.g., internet and transport layers).

Next Topic

• Discussion on the origin of the World Wide Web in the next lecture.