Plan for Today

• Feedback
• Paper instructions
• How the internet works (how computers communicate and are connected):
  • Abstraction
  • Protocols (rules) for communication

Course Feedback

• Thank you to everyone for filling out the course feedback
• Homeworks:
  • Goal of this class is technical literacy - Googling and reading technical articles is a big part of the homework for that reason
  • General feedback was that homeworks take about 2-5 hours per week; if you're spending longer, please come talk to us during office hours (or email us to make an appointment)
• Will expand lecture slides with some more information
• Will go over common misconceptions in homework
• Please continue to give us feedback!
• I know this is a challenging class - keep up the great work!

Last week's homework

Paper

• Information here
• Goal: demonstrate understanding of technical problems in society
• Goal: read and understand technical articles (at the level of WIRED, not research papers)
• Goal: weigh the advantages and disadvantages of new technology
• A fun series of comics that you might want to keep in mind as you write your paper

What is the Internet?

• A global system of computers connected together
• A network of networks!
• Designed to be redundant - can reach a computer through multiple paths
• Hierarchical - organized into ever smaller groups (like mail addresses)
  • IP address - each computer has its own
  • Domain name - human-readable address for computers
• Internet allows computers to send information (bytes) to each other through IP addresses

Routers

• Each router is a gateway: connects one network to another
• LAN: Local Area Network
• Home router connects your LAN to the Internet Service Provider (ISP)
• Computers are connected on the internet through a sequence of routers (some for small areas, like LANs, others for larger areas)

Client-Server Model:
Loading a website

• Ask for a webpage (with a URL)
• Your computer (client) asks another computer at that url (server) for the information (request)
• Request is sent to the server through a sequence of routers
• Server decodes the request, sends back the information (response)
• Client interprets the response

Accessing Information: URLs

• Uniform Resource Locator (URL)
• Protocol (rules for the information) (http)
• Domain name (gets converted to an IP address) via a Domain Name Server (DNS)
• Information about the file on that computer

TCP/IP Protocol

• Rules for sending information between computers
• Developed by the Department of Defense, used by everyone
• Four layers of abstraction (most to least abstract)
  • Application Protocol
  • Transmission Control Protocol
  • Internet Protocol
  • Link Layer

Application Protocol

• Standardizes how different types of data (e.g. files, emails, http) are interpreted
• Way of interpreting the bits of data sent from one computer to another
• Http response codes
  • 200s: all is good
  • 400s: client errors
  • 500s: server errors

Transmission Control Protocol (TCP)

• Separates the information into chunks (packets)
• Sends packets to receiver
• Reassembles the packets into the full request/response
• Figures out if packets are missing (checksum)
• Reorders packets
• Responsible for ensuring the information is complete and correct

Internet Protocol (IP)

• XX.XX.XX.XX (each XX is a byte) - you can Google for your IP address
• First byte is large area, and so on (hierarchical)
• Determines the path to send the packets (the next router to send to)
• Protocol for IP addresses
• "Traceroute" example
• Expanding to IPv6 (6 bytes of addresses instead of 4)
• Also includes Domain Name Servers (DNS)
  • Convert from domains to IP addresses
  • Internet Service Provider has DNS servers
  • Kept up to date as IP addresses can change

Link Layer

• Converts between binary and network signals
• Wired uses Ethernet protocol
  • Signals sent via voltages in a wire
• Wireless uses Wi-Fi protocol
  • Signals sent via amplitude modulation of radio waves (like AM radio)
• Handles communication to/from router
  • Note: packets are sent to every computer on the LAN at once
  • Computers figure out which to listen to and which to ignore

Recap

• Recurring theme: abstraction (domain names and TCP/IP protocol)
• Internet is a connection of computers through routers
• Protocols define how computers interact with each other

Announcements

• Midterm grades are released on Gradescope. You should have gotten an email with the relevant statistics.
• Midterm solutions are at the front
• Remember: the class is curved
  • 47+ is around an A range on the exam
  • 35-47 is around a B range
  • 20-35 is around a C range
• Midterm is only 25% of your grade (less than your homeworks)

Midterm: common errors

• Problem 1a: types of data includes strings, numbers, and images; not RAM or persistent storage, which are types of data storage
• Problem 4: spreadsheets