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Discussion 1

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CS 168, Summer 2025 @ UC Berkeley

Slides credit: Sylvia Ratnasamy, Rob Shakir, Peyrin Kao, Iuniana Oprescu

Intro to the Internet 1 🧑‍💻

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Agenda

Introductions
Logistics
Terms
Poking the Internet
Intro to Project Traceroute

Today’s a light discussion

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About Me (Jamie)

I’m a 5th semester transfer student studying CS.

I’m from San Diego. I like playing tennis, reading, and playing Valorant occasionally 🙄.

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

CS168 TA Picture Here (fun!)

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Logistics

Project 1A: Basic Traceroute

Deadline: Thursday, June 26th

Project 1B: Traceroute Error Handling

Deadline: Thursday, July 3rd

Far ahead: Midterm on July 22nd, 7-9pm (put it in your calendar)!

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Terms

Routers/Switches
End-host
Packets
Naming
Network “Stack”
ISP (Internet Service Provider)
ASes (Autonomous Systems)

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Terms

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

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

FEEL FREE TO CUSTOMIZE THESE TWO SLIDES IN YOUR OWN VERSION

Network

- Routers/Switches and links

Network Stack

- Networking SW on host. It replicates some routers/switch functionality then adds some additional network functionality before giving packet to application

Packets

- bags of bits. Consists of:

- Header w/meaningful info for network and network stack to make decisions

- Body containing a payload. Ex. A file, imagine, an application header, etc.

Routers/Switches

- devices that take a packet arriving on one of its incoming communication links and forwards said packet on one of its outgoing links. We make no distinction between routers/switches at this point because the difference is subtle

Application

- Most high level part of network connection, responsible for sending and receiving data as well as understanding the content of the data

Port:

- In routing context in the network, this refers to a physical port, which is responsible for providing a connection between routers/switches.

- In network stack and application context, this could be a logical port which is a means of which an application can connect to the network stack. (More on ports later in the semester)

Socket

- OS mechanism that connects processes to network stack via (logical) ports. More on this later..

Naming

- Network address: where host is located

- Network name: which host it is

- When you move a server to a new building, it’s name does not change but it’s address does

Internet Service Provider (ISP)

- Essentially an ISP is in itself a network of packet switches and communication links providing network access to end systems/links

Autonomous Systems (ASes)

- A group of routers that are under the same administrative control.

- Often routers in an ISP and the links that interconnect them constitute a single AS; however, some ISPs partition their network into multiple ASes

- Routers within the same AS will run the same routing algorithm and have information about each other

Routing table

- Table used by routers/switches to determine which outgoing link (port) to send a packet

Control plane

- mechanisms used to computer touting tables. i.e. concerned with determining/creating routing decisions

Data plane

- Using said tables to actually forward packets. i.e. concerned with the actual semantics/performance of forwarding packets

Circuit Switching

- method of data transfer in which an end system reserves BW along a path - sets up a circuit - for communication. No need for packets.

Packet Switching

- method of data transfer in which routers/switches service each packet they receive independently by inspecting its header

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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.

Header

“Go Bears”

Body

FEEL FREE TO CUSTOMIZE THESE TWO SLIDES IN YOUR OWN VERSION

Network

- Routers/Switches and links

Network Stack

- Networking SW on host. It replicates some routers/switch functionality then adds some additional network functionality before giving packet to application

Packets

- bags of bits. Consists of:

- Header w/meaningful info for network and network stack to make decisions

- Body containing a payload. Ex. A file, imagine, an application header, etc.

Routers/Switches

- devices that take a packet arriving on one of its incoming communication links and forwards said packet on one of its outgoing links. We make no distinction between routers/switches at this point because the difference is subtle

Application

- Most high level part of network connection, responsible for sending and receiving data as well as understanding the content of the data

Port:

- In routing context in the network, this refers to a physical port, which is responsible for providing a connection between routers/switches.

- In network stack and application context, this could be a logical port which is a means of which an application can connect to the network stack. (More on ports later in the semester)

Socket

- OS mechanism that connects processes to network stack via (logical) ports. More on this later..

Naming

- Network address: where host is located

- Network name: which host it is

- When you move a server to a new building, it’s name does not change but it’s address does

Internet Service Provider (ISP)

- Essentially an ISP is in itself a network of packet switches and communication links providing network access to end systems/links

Autonomous Systems (ASes)

- A group of routers that are under the same administrative control.

- Often routers in an ISP and the links that interconnect them constitute a single AS; however, some ISPs partition their network into multiple ASes

- Routers within the same AS will run the same routing algorithm and have information about each other

Routing table

- Table used by routers/switches to determine which outgoing link (port) to send a packet

Control plane

- mechanisms used to computer touting tables. i.e. concerned with determining/creating routing decisions

Data plane

- Using said tables to actually forward packets. i.e. concerned with the actual semantics/performance of forwarding packets

Circuit Switching

- method of data transfer in which an end system reserves BW along a path - sets up a circuit - for communication. No need for packets.

Packet Switching

- method of data transfer in which routers/switches service each packet they receive independently by inspecting its header

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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

FEEL FREE TO CUSTOMIZE THESE TWO SLIDES IN YOUR OWN VERSION

Network

- Routers/Switches and links

Network Stack

- Networking SW on host. It replicates some routers/switch functionality then adds some additional network functionality before giving packet to application

Packets

- bags of bits. Consists of:

- Header w/meaningful info for network and network stack to make decisions

- Body containing a payload. Ex. A file, imagine, an application header, etc.

Routers/Switches

- devices that take a packet arriving on one of its incoming communication links and forwards said packet on one of its outgoing links. We make no distinction between routers/switches at this point because the difference is subtle

Application

- Most high level part of network connection, responsible for sending and receiving data as well as understanding the content of the data

Port:

- In routing context in the network, this refers to a physical port, which is responsible for providing a connection between routers/switches.

- In network stack and application context, this could be a logical port which is a means of which an application can connect to the network stack. (More on ports later in the semester)

Socket

- OS mechanism that connects processes to network stack via (logical) ports. More on this later..

Naming

- Network address: where host is located

- Network name: which host it is

- When you move a server to a new building, it’s name does not change but it’s address does

Internet Service Provider (ISP)

- Essentially an ISP is in itself a network of packet switches and communication links providing network access to end systems/links

Autonomous Systems (ASes)

- A group of routers that are under the same administrative control.

- Often routers in an ISP and the links that interconnect them constitute a single AS; however, some ISPs partition their network into multiple ASes

- Routers within the same AS will run the same routing algorithm and have information about each other

Routing table

- Table used by routers/switches to determine which outgoing link (port) to send a packet

Control plane

- mechanisms used to computer touting tables. i.e. concerned with determining/creating routing decisions

Data plane

- Using said tables to actually forward packets. i.e. concerned with the actual semantics/performance of forwarding packets

Circuit Switching

- method of data transfer in which an end system reserves BW along a path - sets up a circuit - for communication. No need for packets.

Packet Switching

- method of data transfer in which routers/switches service each packet they receive independently by inspecting its header

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Terms

Network “Stack”: Networking Software on host.

Replicates some router/switch functionality and adds some additional functionality before passing the body of packets to the application

FEEL FREE TO CUSTOMIZE THESE TWO SLIDES IN YOUR OWN VERSION

Network

- Routers/Switches and links

Network Stack

- Networking SW on host. It replicates some routers/switch functionality then adds some additional network functionality before giving packet to application

Packets

- bags of bits. Consists of:

- Header w/meaningful info for network and network stack to make decisions

- Body containing a payload. Ex. A file, imagine, an application header, etc.

Routers/Switches

- devices that take a packet arriving on one of its incoming communication links and forwards said packet on one of its outgoing links. We make no distinction between routers/switches at this point because the difference is subtle

Application

- Most high level part of network connection, responsible for sending and receiving data as well as understanding the content of the data

Port:

- In routing context in the network, this refers to a physical port, which is responsible for providing a connection between routers/switches.

- In network stack and application context, this could be a logical port which is a means of which an application can connect to the network stack. (More on ports later in the semester)

Socket

- OS mechanism that connects processes to network stack via (logical) ports. More on this later..

Naming

- Network address: where host is located

- Network name: which host it is

- When you move a server to a new building, it’s name does not change but it’s address does

Internet Service Provider (ISP)

- Essentially an ISP is in itself a network of packet switches and communication links providing network access to end systems/links

Autonomous Systems (ASes)

- A group of routers that are under the same administrative control.

- Often routers in an ISP and the links that interconnect them constitute a single AS; however, some ISPs partition their network into multiple ASes

- Routers within the same AS will run the same routing algorithm and have information about each other

Routing table

- Table used by routers/switches to determine which outgoing link (port) to send a packet

Control plane

- mechanisms used to computer touting tables. i.e. concerned with determining/creating routing decisions

Data plane

- Using said tables to actually forward packets. i.e. concerned with the actual semantics/performance of forwarding packets

Circuit Switching

- method of data transfer in which an end system reserves BW along a path - sets up a circuit - for communication. No need for packets.

Packet Switching

- method of data transfer in which routers/switches service each packet they receive independently by inspecting its header

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Terms

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

Network

- Routers/Switches and links

Network Stack

- Networking SW on host. It replicates some routers/switch functionality then adds some additional network functionality before giving packet to application

Packets

- bags of bits. Consists of:

- Header w/meaningful info for network and network stack to make decisions

- Body containing a payload. Ex. A file, imagine, an application header, etc.

Routers/Switches

- devices that take a packet arriving on one of its incoming communication links and forwards said packet on one of its outgoing links. We make no distinction between routers/switches at this point because the difference is subtle

Application

- Most high level part of network connection, responsible for sending and receiving data as well as understanding the content of the data

Port:

- In routing context in the network, this refers to a physical port, which is responsible for providing a connection between routers/switches.

- In network stack and application context, this could be a logical port which is a means of which an application can connect to the network stack. (More on ports later in the semester)

Socket

- OS mechanism that connects processes to network stack via (logical) ports. More on this later..

Naming

- Network address: where host is located

- Network name: which host it is

- When you move a server to a new building, it’s name does not change but it’s address does

Internet Service Provider (ISP)

- Essentially an ISP is in itself a network of packet switches and communication links providing network access to end systems/links

Autonomous Systems (ASes)

- A group of routers that are under the same administrative control.

- Often routers in an ISP and the links that interconnect them constitute a single AS; however, some ISPs partition their network into multiple ASes

- Routers within the same AS will run the same routing algorithm and have information about each other

Routing table

- Table used by routers/switches to determine which outgoing link (port) to send a packet

Control plane

- mechanisms used to computer routing tables. i.e. concerned with determining/creating routing decisions

Data plane

- Using said tables to actually forward packets. i.e. concerned with the actual semantics/performance of forwarding packets

Circuit Switching

- method of data transfer in which an end system reserves BW along a path - sets up a circuit - for communication. No need for packets.

Packet Switching

- method of data transfer in which routers/switches service each packet they receive independently by inspecting its header

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Poking the Internet

Ping
Traceroute
Dig

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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

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.

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Ping, Traceroute, Dig

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.

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Ping, Traceroute, Dig

Predictions?

berkeley.edu
google.com
ford.com (Ford, a car company headquartered in Michigan)
csail.mit.edu (MIT’s CS department)
lmu.de (University of Munich)
unam.edu.na (University of Namibia)

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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?

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Ping, Traceroute, Dig

ping berkeley.edu -c 3

PING berkeley.edu (141.193.213.21): 56 data bytes

64 bytes from 141.193.213.21: icmp_seq=0 ttl=50 time=14.142 ms

64 bytes from 141.193.213.21: icmp_seq=1 ttl=50 time=20.024 ms

64 bytes from 141.193.213.21: icmp_seq=2 ttl=50 time=17.540 ms

--- berkeley.edu ping statistics ---

3 packets transmitted, 3 packets received, 0.0% packet loss

round-trip min/avg/max/stddev = 14.142/17.235/20.024/2.411 ms

ping csail.mit.edu -c 3

PING csail.mit.edu (128.52.132.19): 56 data bytes

64 bytes from 128.52.132.19: icmp_seq=0 ttl=32 time=108.253 ms

64 bytes from 128.52.132.19: icmp_seq=1 ttl=32 time=109.574 ms

64 bytes from 128.52.132.19: icmp_seq=2 ttl=32 time=98.613 ms

--- csail.mit.edu ping statistics ---

3 packets transmitted, 3 packets received, 0.0% packet loss

round-trip min/avg/max/stddev = 98.613/105.480/109.574/4.886 ms

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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.

1

2

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Ping, Traceroute, Dig

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.

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Ping, Traceroute, Dig

Demo, pick your favorite(s):

berkeley.edu
google.com
csail.mit.edu (MIT’s CS department)
ford.com (Ford, a car company headquartered in Michigan)
lmu.de (University of Munich)
unam.edu.na (University of Namibia)

Now let’s visualize it online at geotraceroute.com

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Ping, Traceroute, Dig

traceroute berkeley.edu

traceroute: Warning: berkeley.edu has multiple addresses; using 141.193.213.20

traceroute to berkeley.edu (141.193.213.20), 64 hops max, 40 byte packets

1 sut-mdc-sr9--ae0-558.net.berkeley.edu (10.40.232.2) 6.154 ms 10.428 ms 7.135 ms

2 sut-mdc-cr1--xe-2-1-13.net.berkeley.edu (128.32.255.96) 16.284 ms 19.354 ms 7.312 ms

3 reccev-cev-cr2--et-0-2-8.net.berkeley.edu (128.32.255.177) 5.652 ms

sut-mdc-cr2--et-0-2-8.net.berkeley.edu (128.32.255.41) 10.454 ms *

4 * * *

5 * sut-mdc-fw9--xe-0-0-1-3.net.berkeley.edu (128.32.255.143) 6.366 ms 5.512 ms

6 reccev-cev-sr1--xe-0-1-1-0-2.net.berkeley.edu (128.32.255.140) 5.309 ms 4.949 ms 5.388 ms

7 reccev-cev-sr1--lt-0-1-0-0.net.berkeley.edu (128.32.255.166) 4.160 ms 5.037 ms 4.338 ms

8 reccev-cev-cr2--et-0-0-0-3.net.berkeley.edu (128.32.255.74) 5.742 ms 9.072 ms 7.551 ms

9 sut-mdc-cr1--et-0-1-0.net.berkeley.edu (128.32.255.40) 7.412 ms

sut-mdc-cr1--et-0-1-5.net.berkeley.edu (128.32.255.176) 5.063 ms

reccev-cev-cr1--et-0-1-0.net.berkeley.edu (128.32.255.174) 6.135 ms

10 reccev-cev-br1--et-1-1-1.net.berkeley.edu (128.32.0.38) 4.994 ms 5.771 ms

reccev-cev-br1--et-1-1-0.net.berkeley.edu (128.32.0.36) 5.349 ms

11 emvl1-agg-01--ucb--100g.cenic.net (137.164.3.26) 9.055 ms 5.400 ms 5.872 ms

12 svl-agg10--emvl1-agg-01--400g--01.cenic.net (137.164.11.94) 9.312 ms 9.272 ms 8.780 ms

13 198.32.251.193 (198.32.251.193) 18.352 ms * 33.886 ms

14 172.68.188.80 (172.68.188.80) 10.743 ms

172.68.188.94 (172.68.188.94) 10.407 ms

172.68.188.80 (172.68.188.80) 26.217 ms

15 141.193.213.20 (141.193.213.20) 8.597 ms 9.852 ms 8.677 ms

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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)_/⁻

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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)

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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

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Ping, Traceroute, Dig

Demo

berkeley.edu
google.com
csail.mit.edu (MIT’s CS department)
ford.com (Ford, a car company headquartered in Michigan)
lmu.de (University of Munich)
unam.edu.na (University of Namibia)

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

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Bonus: What’s your IP Address?

On a Mac, go to System Settings -> Network -> Select “Details” on the network you’re connect to -> TCP/IP:

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Worksheet

Internet Layers
Design Philosophy (Clark Paper)
A Gentle Intro to Traceroute

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Question 1: Internet Layers

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Worksheet

Internet Layers
Design Philosophy (Clark Paper)
A Gentle Intro to Traceroute

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Question 2: Design Philosophy (Clark Paper)

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Worksheet

Internet Layers
Design Philosophy (Clark Paper)
A Gentle Intro to Traceroute

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Question 3: A Gentle Intro to Traceroute

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Question 3: A Gentle Intro to Traceroute

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Question 3: A Gentle Intro to Traceroute

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Question 3: A Gentle Intro to Traceroute

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Question 3: A Gentle Intro to Traceroute

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Questions?

Feedback Form: https://tinyurl.com/cs168-su25-disc-feedback