Unit 1 Lesson 13

Basic understanding of what HTTP is HTTP like DNS is an ASCII-text based protocol - it’s just two computers sending text messages to each other. What makes it a protocol are the rules of the “conversation” the two machines are having. In the case of HTTP, it is a call-and-response protocol for a client/server relationship, where a client requests a web page or other content (image, sound, video, etc.) from a server. The server looks for it and sends it back.

HTTP as a “high level” protocol that sits on top of all the other protocols and internet systems we’ve learned about in the course. That text message conversation between the computers is being broken up into TCP/IP packets, and all the data gets sent as bits over wires and airwaves, taking different paths, and it gets interpreted reassembled at the end.

We often talk about how the Internet works in “layers” and this is a perfect example of abstraction on the Internet, as one layer makes use of the functionality provided by the layer below it, without worrying about the details of how this functionality is achieved And HTTP doesn’t have to worry about anything other than the text protocol of HTTP works. The network software and devices on your and others’ computers handle looking up addresses, breaking down data, packeting, routing, transmission and interpretation and reassembly. It’s really amazing.

System constructed with layers of abstraction Each layer only needs to concern itself with its specific job, and then hands it off to another layer. At the top layer, for example, DNS just thinks is “speaking DNS” to some other computer - the DNS protocol does not even need to know how the other layers work It just relies on them doing their jobs.

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Vocabulary Abstraction: Reducing information and detail to focus on essential characteristics. It is typically possible to look at a system at many levels of abstraction, depending on how much detail is necessary to approach the challenge at hand. Server: A computer that awaits and responds to requests for data Example: a DNS server awaits and responds to requests for URLs to be translated to IP addresses. Client: A computer that requests data stored on a server Example: When you type an address into your browser, your computer is the client and it sends the request to the DNS server.

Abstraction on the Internet Abstraction plays a key role in the relationship between the layers of the Internet, as higher levels make use of the functionality provided by lower levels, without worrying about how they function. The Physical Internet: copper wire, fiber optic cable, radio waves, etc. IP: Assigns IP addresses to computers and routes packets of data to the correct address DNS: Translates a URL into an IP address. Note that a DNS request still is conducted over TCP/IP TCP: Breaks larger messages into multiple packets and ensures all packets are received and ordered correctly. Higher level protocols use lower levels without needing to understand precisely how they work Lower level protocols guarantee they will function without stating specifically how this will be accomplished

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Wrap-up What surprised you about the HTTP traffic you observed? What does it mean to say that high-level layers of the Internet use low-level layers “abstractly”? What other examples of abstraction have we seen in this course? Hint: Unit 1 is basically all about abstraction.

Vocab DNS - The service that translates URLs to IP addresses. HTTP - HyperText Transfer Protocol - the protocol used for transmitting web pages over the Internet IP Address - A number assigned to any item that is connected to the Internet. TCP - Transmission Control Protocol - provides reliable, ordered, and error-checked delivery of a stream of packets on the internet. TCP is tightly linked with IP and usually seen as TCP/IP in writing. URL - An easy-to-remember address for calling a web page (like