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6.3. Network Architecture

Time Estimate: 90 minutes

6.3.1. Introduction and Goals

As we've learned in previous lessons, the Internet is a network of networks that is managed by a collection of protocols. We've already seen the role that the HTTP protocol plays in supporting the World Wide Web application. In this lesson we delve more deeply into the basic architecture and infrastructure of the Internet. We'll learn that the main Internet protocols are organized into a hierarchy of abstraction layers -- the application, transport, internet, and link layers -- each of which manages certain specific tasks required to route messages between hosts on the Internet. We'll learn about

• Internet architecture and abstraction layers
• The packet switching routing scheme
• The TCP/IP protocol

After this lesson you should have a pretty good understanding of how some of your familiar applications -- web browsing, email, smartphone apps -- are supported by the underlying Internet hardware and software. Here is a short video called A Packet's Tale to get us started on our journey.

Learning Objectives: I will learn to

• explain how data are sent through the Internet via packets
• identify and describe the benefits of fault-tolerant networks

Language Objectives: I will be able to

• discuss the benefits of packet switching and Internet abstraction layers
• use target vocabulary, such as router, IP Address, packet switching, scalability and fault tolerant while describing how data is transported across the Internet, with the support of concept definitions and vocabulary notes from this lesson

6.3.2. Learning Activities

• Slides
• |
• YouTube Video Part 1
• |
• Ping and Traceroute Activity


• YouTube Video Part 2
• |
• Packet Routing Activity

Part 1: Network Architecture and Packet Switching

This first video focuses on basic architecture of the Internet and the technique of packet switching. The Internet has been engineered to be fault-tolerant, which means it can support failures and still continue to function. Network redundancy allows having more than one path between any two connected devices in case something part of the network fails. If a particular device or connection on the Internet fails, subsequent data will be sent via a different route, if possible. Redundancy within a system often requires additional resources but can provide the benefit of fault tolerance. The redundancy of routing options between two points increases the reliability of the Internet and helps it scale to more devices and more people. The Internet was designed to be scalable. The scalability of a system is the capacity for the system to change in size and scale to meet new demands.

The video also illustrates how the ping utility can be used to test whether certain hosts are reachable on the network. And it introduces a new tool, traceroute, that can be used to trace the routes that packets take from one computer to another on the Internet.

POGIL Activity 1 - Using Ping and Traceroute to Investigate the Internet (15 minutes)

Break into POGIL teams of 4. Record your answers using this worksheet

Role	Responsibility
Facilitator	Uses the ping and trace tools online at network-tools.com.
Spokesperson	Reads out questions and reports the team's answers to the class.
Quality Control	Records the team's answers in the worksheet (File-Make a Copy to have a version you can edit).
Process Analyst	Keeps track of the teams progress and assesses its performance.

In this activity, you will use the ping and traceroute utilities at network-tools.com to measure the latency and observe trace routes to answer the following question.

Does geographical distance between the source and the destination on the network affect latency?

You will use multiple trials of ping and trace for 5 university servers around the globe (mit.edu, stanford.edu, ox.ac.uk, kyoto-u.ac.jp, usp.br) to answer the questions below.

1. Did any of the servers lose packets or time out? Some servers will block ping and trace for security reasons which are seen as time outs. Were there any surprising locations in the hops that the packet went through?
2. Did different trials have different results for the same destination? Do packets always get routed in the same way?
3. Can you guess where the network-tools server is located based on the latency data you collected? Can you confirm your guess using trace or whois (which gives you information about who owns a server)?
4. Select one route and specify all the directly-connected computing devices along the route which form a path between the sender and the receiver. How many hops are taken on this path? List each device on this path.
5. How does the number of hops in the trace affect latency (the round trip time seen in ping)?
6. (Portfolio) How does geographical distance affect latency? What are some other factors that may be affecting latency?

Part 2: Internet Abstraction Layers

Data is sent through the Internet in data streams made up of data packets. Each packet contains data and metadata used for routing the packet between the origin and the destination on the Internet, as well as for data reassembly. This video focuses on the TCP/IP model which is a set of abstraction layers with different protocols that manage the routing of messages on the Internet. Protocols such as TCP, IP, UDP, and SMTP work together in the hierarchy to support applications such as email and web browsing.

POGIL Activity 2 - TCP/IP Packet Routing (15 minutes)

In the same POGIL team of four, you will act out the layers in the TCP/IP model to encapsulate and transmit data. Each group of four should be given an IP address such as 10.1, 10.2, etc. in a local area network (LAN). Each group member should take on the role of one of the layers below:

Application Layer (protocol: SMTP)	Send: Composes a message to another student and passes it to the Transport layer. Receive: Receives and reads out messages from other students passed on from the Transport layer.
Transport Layer (protocol: TCP)	Send: Splits the message into packets, adds TCP headers to number the packets, and sends them to the Internet layer. Receive: Receives packets from the Internet layer, puts them in order, and passes them to the Application layer when all are received.
Internet Layer (protocol: IP)	Send: Uses a routing table (given in the handout) to add the destination IP address to each packet and passes them to the Link layer. Receive: Receives packets from the Link layer and checks that it’s their own group’s IP address. If it is, it passes it to the Transport layer. If it is not, it gives it back to the Link layer to give to another group.
Link Layer (protocol: Ethernet)	Send: Passes the individual packets randomly to the Link layer of other groups. Receive: Receives packets from other groups and passes them to the Internet Layer.

Follow these handouts to simulate packet routing in the TCP/IP model. Once your group has sent and received a message, discuss the following questions.

1. Packet Order. Does it matter whether the packets of a message arrive in order? Explain how this set of protocols handles that.
2. (Portfolio) Missing Packets. What should happen if a packet goes missing? Who (which layer) would handle this? What action would they have to take? And what additional information would be needed in the packet in order to handle it?
3. Corrupted Packets. Suppose there is some kind hardware glitch that corrupts one or more bits in a packet? Can this be detected? What action should be taken in this case? What additional information would be needed to handle this issue?
4. Fault-Tolerance. Complex systems, like networks, fail at unexpected times. Often multiple devices fail at the same time.  Explain how the TCP/IP model continues to function even when parts of the system fail.
   
5. (Portfolio) Security/Privacy. As the packets are being transmitted through the network, can people other than the sender and receiver read the messages? What methods can we use to protect the message?

6.3.3. Summary

In this lesson, you learned how to:

6.3.4. Still Curious

There is a wealth of good introductory and comprehensive information about the Internet and the WWW.

• As always, Wikipedia is an excellent resource as a first stop. Its pages on Internet protocol, Ping, Traceroute, and the other topics in this lesson are all accurate and accessible.
• The How Stuff Works site has a nice animated description of how routers work to direct traffic on the Internet.
• Warriors of the Net is a classic 12 minute animated video about packets traveling through the Internet.
• For a very detailed and comprehensive discussion of networking -- sort of like reading a short book -- see the CTDP's Networking Tutorial, which covers the topic in much more detail than we have done here.
• Explore this map of the underwater cables that carry 99% of international data.(2016 interactive version of map).
• Watch this video that describes how a hacker could view your data on a public network using a packet sniffer.

6.3.5. Self-Check

Here is a table of some of the technical terms discussed in this lesson. Hover over the terms to review the definitions.

host router IP address ping traceroute

packet packet switching fault-tolerant abstraction layer scalability

Q-3:

True or False: The main difference between circuit switching and packet switching is that in circuit switching data is broken up and sent in parts and in packet switching data is sent out on a continuous circuit.

• True
• If it were easy, you wouldn’t be learning anything!
• False
• That's right! Circuit switching means that there is a continuous circuit set up to send the data, as in a telephone call. Packet switching means that data is broken up into small packets and sent out in parts.

Q-4:

True or False: All routes on the internet are specified in advance and not set dynamically. For every packet of data sent over the Internet, there is only one route it can take to reach its destination.

• True
• We’re in the learning zone today. Mistakes are our friends!
• False
• That's right! Data packets sent over the Internet have different possible routes that they can take to get to their destination.

Q-5: When you send an e-mail message using Gmail in your browser or using the Gmail app __________________.

• the Gmail server puts the message into a single packet that is routed through the Internet to the recipient's email
• That's not quite right. Unless the message is very small, it would be divided into multiple packets, each of which is routed individually to the recipient's email.
• the Gmail server divides the message into packets that are routed individually over the Internet to the recipient's email
• That's right! Information that is sent over the Internet is first divided into packets and then each packet is routed individually over the Internet.
• the Gmail server divides the message into packets that are routed through a dedicated channel to the recipient's email
• That's not quite right. The packets would be routed independently, possibly using separate paths to the recipient.
• the Gmail server puts the message into a single packet that is routed through a dedicated channel to the recipient's email
• That's not quite right. Unless it is very small, the message would be divided into several packets that are routed independently to the recipient's email.

Q-6: Which of the following are benefits of redundancy in routing on the Internet where there is more than one route for packets to travel through the Internet?

• Increasing the fault-tolerance of the Internet so that packets will reach their destination even if some connections have failed.
• Increasing the ease with which the Internet can scale or grow where extra routes can be easily added.
• Allowing many different protocols.
• Although the Internet does allow different protocols, it is not a benefit of redundancy in routing.
• Avoiding loss of packets.
• Packets can still be lost on the Internet. The redundancy in routing does not help with this.

Q-7:

AP 2021 Sample Question: Which of the following best explains how data is typically assembled in packets for transmission over the Internet?

• Each packet contains data to be transmitted, along with metadata containing information used for routing the data.
• Each packet contains an encrypted version of the data to be transmitted, along with metadata containing the key needed to decrypt the data.
• Each packet contains only the metadata used to establish a direct connection so that the data can be transmitted.
• Each packet contains multiple data files bundled together, along with metadata describing how to categorize each data file.

6.3.6. Reflection: For Your Portfolio

Answer the following portfolio reflection questions as directed by your instructor. Questions are also available in this Google Doc where you may use File/Make a Copy to make your own editable copy.

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