AP CSP Day 50: Internet Protocols | Cycle 2
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The layered model of internet protocols assigns each layer a specific responsibility: the application layer (HTTP/HTTPS/DNS) handles user-facing services, the transport layer (TCP/UDP) manages reliable or fast delivery, the internet layer (IP) handles addressing and routing, and the link layer handles physical transmission. Each layer communicates only with the layers directly above and below it, which means application developers can build on TCP/IP without knowing the physical medium. AP CSP Cycle 2 protocol questions ask students to match a described communication problem to the correct protocol layer and justify why that layer is responsible.
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Protocol Layers: Matching Problems to Layers
The Layered Model
Internet protocols are organized in layers where each layer provides services to the layer above it and uses services from the layer below. This separation means application developers can use TCP/IP without knowing whether the underlying physical medium is fiber optic, WiFi, or copper cable.
Layer Responsibilities
Application layer (HTTP, DNS, HTTPS): user-facing services. Transport layer (TCP, UDP): end-to-end delivery reliability. Internet layer (IP): addressing and routing across networks. Link layer: physical transmission on a single network segment.
Practice Question
A user types a URL into their browser. Which of the following correctly describes the sequence of events that occurs?
I. DNS converts the domain name into an IP address.
II. A TCP connection is established with the web server.
III. The server sends the requested page data back in packets.
The correct sequence is: (1) DNS lookup translates the domain name to an IP address so the browser knows where to connect. (2) TCP establishes a reliable connection with the server at that IP address. (3) The server sends the requested HTML data back through the established connection.
D) A TCP connection cannot be established without first knowing the server's IP address (which DNS provides). C) Data cannot be sent before a connection exists. B) These steps are sequential and depend on each other — they cannot happen simultaneously.
Students may think TCP establishes a connection before DNS resolves the domain, or they may not recognize the dependency chain: domain name → IP address → connection → data transfer.
The Internet communication sequence follows a logical dependency: you need an address (DNS) before you can connect (TCP), and you need a connection before you can transfer data.
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