CS168: Discussion 1

Intro to the Internet

Spring 2024

Agenda

• Introductions
• Terms
• Poking the Internet

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Light discussion today

Introductions

Hi, I’m Bryce! (he/him)

I’m a 5th year EECS Masters student from the Bay Area with an emphasis in computer security. Previously, I’ve been a head TA for CS 61A.

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I enjoy playing rhythm games (osu!), following (e)sports, and lots of cute stuff (Pokemon, cat videos, etc.)

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My office hours: Wednesday 2pm-3pm in Soda 347.

My email: brywong@berkeley.edu (Edstem is faster though)

About You

Show of hands survey

• Major?
• Year?
• Where you’re from?
• Why you’re taking this class?

Slides / Resources

• Section Resources: http://tinyurl.com/bryce-168-sp24
• Slides / Worksheets are also available on the main 168 website
• Will upload written notes from future sections here

Questions from Lecture

Terms

Terms

• Routers/Switches: Devices that forward packets arriving on one link to another link. We make no distinction between routers/switches at this point

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• End-host: a device attached to the network that sends or receives packets.
• Examples: mobile phone, laptop, security camera, smart fridge

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Terms

• Packets: A Bags of bits with a
• Header-- info for network and network stack to make decisions
• Body-- contains a payload. Ex. A file, image, an application header
• The network doesn’t really care about what’s in the payload.

“Go Bears”

Body

Terms

• Naming:
• Network name: which host it is
• Network address: where host is located
• When you move a server to a new building, its name does not change but its address does

Name: mywebsite.com

Addr: 142.250.72.206

Name: mywebsite.com

Addr: 208.117.236.69

Terms

• Network “Stack”: Networking SW on host.
• Replicates some router/switch functionality and adds some additional functionality before passing the body of packets to the application

Terms (cont’d)

• ISP (Internet Service Provider): A network of packet switches and links that provide network access (i.e. Comcast, ATT, Sonic)
• ASes (Autonomous Systems): Groups of routers under the same control
• Usually each ISP has one AS, but may have multiple ASes
• Routers within the same AS will have information about each other

Poking the Internet

Ping, Traceroute, Dig

• Internet is large and complex. Network engineers and researchers have built some handy tools to get some insight into what is going on inside and across the internet.
• We’re going to play around with them a little bit

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Think of this as a “tinker discussion” - you aren’t expected to know any of these concepts yet. We’ll learn about them throughout the semester.

Ping, Traceroute, Dig

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• Simple utility that lets you “poke” a website and see if it moves (spoiler: most do!)
• You say hi and see if the server says hi back
• This by itself is not super interesting
• Ping also tells you how long the reply took to come back
• This is more interesting!
• Let’s try out a few websites.

Ping, Traceroute, Dig

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Predictions?

• berkeley.edu
• google.com
• csail.mit.edu (MIT’s CS department)
• lmu.de (University of Munich)

Ping: A prediction

• We’ve pinged a couple websites and seen pretty significant differences in latency.
• Latency is the time between when a request is sent and when the response is heard.
• What about differences in latency for the same website, but in different regions?
• We’ve pinged google.com and seen its latency.
• How many times longer will it take for a ping to google.co.uk to come back?

Ping, Traceroute, Dig

• Tool to trace the route that packets take from your computer to the destination.
• Specifically lets you see the routers/switches that are forwarding your packets.

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Ping, Traceroute, Dig

Demo, pick your favorite(s):

• berkeley.edu
• google.com
• csail.mit.edu (MIT’s CS department)
• lmu.de (University of Munich)

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Now let’s visualize it online at geotraceroute.com

Traceroute: Notice anything?

• Traceroute gives us a lot more interesting feedback than ping.
• Latency to every step along the way.
• Can see a breakdown of latencies!
• Router names.
• Often have locations in them (i.e. city name)
• Can roughly trace packet path on a map!
• Weird stars
• Some routers just don’t respond ⁻\_(ツ)_/⁻

How does Traceroute work?

• Traceroute gives you the path of routers and switches your packets take.
• How?
• Takes advantage of something called a TTL in the packet IP header.
• TTL denotes how many times a packet should be forwarded before it is discarded.
• Why does this exist?
• To stop the internet from collapsing! (We’ll cover this when we get to routing)
• Sets the TTL to 1, 2, 3, etc
• When packets are dropped because of TTL expiring, most routers send back a message telling us.
• Use the source of this notification to identify the routers along the packet’s path.

Ping, Traceroute, Dig

• When humans want to go to a website, we think in terms of names
• i.e. google.com
• The internet does not think this way, it thinks in terms of addresses
• i.e. “1.2.3.4”
• It’s like the postal service
• You wouldn’t just write “To: Alice” on a letter
• You would look up Alice’s address in some directory
• Then mail the letter to her address
• Dig lets you lookup the address of a website by its name
• Command line interface to the Domain Name Service (DNS)

Ping, Traceroute, Dig

Demo

• berkeley.edu
• google.com
• csail.mit.edu (MIT’s CS department)
• lmu.de (University of Munich)

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(optionally see approximate physical location at https://www.iplocation.net/)

Dig: A breakdown

• When using the +trace option, there was a lot more information
• We could see the steps that were taken when resolving the names
• First, the ‘root’ servers were queried
• Then, the TLD (top level domain) server was queried
• After that, successive servers were asked until the IP was found
• More on how this works when we discuss DNS

Bonus: What’s your IP Address?

On a Mac, go to System Preferences -> Network -> Advanced -> TCP/IP:

Emergency Screenshots