Section 2.7

Learning Goals

1. Students will explain how data are sent through the Internet via packets. (CSN-1.C)

2. Students will identify and describe the following protocols: IP, TCP and UDP.

3. Students will determine the benefits of fault tolerant systems.

4. Students will assess the impact that fault tolerance, redundancy and open protocols have made on the scalability of the internet.

Important Vocabulary: router, firewall, gateway, packet, open protocol, transmission control protocol (TCP), internet protocol (IP), TCP/IP, fault tolerant, redundant, scalability

Objectives and General Description

From the APCSP course description: Computer systems and networks are used to transfer data. One of the largest and most commonly used networks is the Internet. Through a series of protocols, the Internet can be used to send and receive information and ideas throughout the world.

The objective of this section is for the students to demonstrate how messages are broken into packets, routed through the Internet, arrive at the destination and are reassembled. By actively participating in a demonstration, students will develop a deeper understanding of this abstract concept. Students will be writing encrypted messages, converting these messages to data packets and routing them through redundant paths to arrive at the destination computer. Once the packets are received, students will be reassembling them and determining if the communication was successful. Throughout this process, the class will be creating protocols that enable consistent and reliable communication. The reflection discussion will highlight the packet switching process, standard protocols used, fault tolerance, redundancy, open protocols and scalability. Students will be asked to write a summary of the experience while it is fresh in their minds.

Activities

Activity 2.7.1 (Budget 55 minutes)

This activity is adapted from CS Unplugged Tablets of Stone.

1. Assemble class groups. Divide the class into thirds.

   • One-third of the students will be routers. They should be in the middle of the classroom, evenly spaced out.

   • One third of the students will be creating message packets and will be on one side of the room.

   • The remaining third of the students will be receiving packets and reassembling the messages. This group is on the opposite side of the room from the message creators.

   • GROUPS SHOULD NOT TALK TO EACH OTHER.

2. Role of the Message Packet creators

   • Assign each student a “message” that they need to have delivered to a receiving “computer” across the room. I use complex passwords so the receiving computer can’t guess the message. I also use passwords longer than 6 characters so that the students need to use more than one packet for the message. Samples of messages I tell these students who they are sending the message to so that I can be assured every receiving student should be getting a message delivered.

   • The message is written on multiple packets (slips of paper). Each packet can old the destination, the originating computer and 6 characters of the message.

   • Do not tell the students how to write the message (top to bottom, left to right, etc.). They will discover the need for this within the activity itself.

   • The students will give each packet to an available router for delivery.

3. Role of the routers

   • The goal of the router is to get and deliver a packet to its correct destination. The router can not talk to anyone and can only hold one packet at a time.

   • The router is restricted in movement and can not walk around the room. They must pass a packet to another router until the packet can be successfully delivered to its destination.

   • In addition, the teacher will randomly give out one of these directions to individual routers. They can not tell anyone what the direction is but must follow the instructions on it. This will make the efficiency of the routing system decrease.

4. Role of the recipient

   • The receiving computer should gather all required packets and reassemble them into a message. In this case, it is a password.

   • During the reassembly process, do not tell the student how to reassemble the message or how many packets they need for a full message. They will discover this throughout the process.

   • When a recipient feels that he/she has a complete message, they raise their hand or call time.

   • The teacher stops all activity and the recipient states who the message is from and then reads the password. The sending computer will verify the accuracy of the message.

Flow of the game:

1. Teacher needs to have a quick meeting with each group to explain that groups roles and rules. Start with the creator group so that they can be creating the packets while you meet with the other two groups.

2. I will often start with a basic message, like “Hello” just to get a practice round before we try the complex passwords.

3. When everyone is ready, give the harder passwords to the creator group.

4. Start the game. Resist the urge to give hints for success. When a receiving computer feels that the message has been correctly reassembled, immediately stop the game and have them read the message. It will probably be incorrect. That’s ok.

5. Lead a discussion

   • What was the message supposed to be? Why was it wrong?

   • Lead the discussion toward these ideas…

     • The recipient needs to know how many packets are coming

     • The recipient needs to know the order to reassemble the packets

   • How can we do this when the packets are limited to 6 characters?

     • Designate two of the boxes on the packet for critical information --- total number of packets and order of reassembly.

       • Example: This packet would be the 1st packet out of 4.

     • Students also need to decide how to reassemble the remaining characters --- top to bottom, left to right, etc. For example, if the students decided to read rows from top to bottom and left to right, this packet would rebuild as CAKE

     • At the end of this discussion, emphasize to the students that they have all just agreed on protocols. Since they are all aware of the protocol and how it works, this is an example of an Open Protocol.

6. Have the creators swap passwords and try the game again with the new protocols.

7. Optional: you can make one of the routers a DNS server and have messages go first to the DNS server to retrieve the IP address and then have the creator modify the message to show the IP address.

8. Once there have been a few successful message deliveries, have a concluding conversation that emphasizes:

   • Transmission Control Protocol (TCP): the rules regarding how a message is broken into packets and reassembled

   • Internet Protocol (IP): rules for addressing and movement

   • Commonly thought of together as TCP/IP

   • What is dynamic routing?

   • Fault tolerant and redundancy of routing

   • Open system: as long as we know the protocols and follow them, the system works. The rules are not proprietary or secret.

   • What is scalability? How have the combination of fault tolerance, redundancy and open protocols allowed for the scalability of the internet?

9. Have students complete the Summary of Packet Switching note sheet.

Activity 2.7.2 (Can be completed inside or outside of class)

1. Reinforcement assignment: Choose one or do both

   1. The Internet: Packets, Routing and Reliability video (6 minutes, 25 seconds) and guided notes

   2. Warriors of the Net video (13 minutes) and summary exercise

Resources

• Activity 2.7.1 Sample messages (teacher resource)

• Activity 2.7.1 Packet slip template (activity resource)

• Activity 2.7.1 Directions for routers (activity resource)

• Activity 2.7.1 Example of a completed packet slip (teacher resource)

• Activity 2.7.1 Summary of Packet Switching (student worksheet)

• Activity 2.7.2 Guided notes for Code.org Packets, Routing & Reliability (student worksheet)

• Activity 2.7.2 Guided notes for Code.org Packets...KEY (teacher resource)

• Activity 2.7.2 Summary exercise for Warriors of the Net (Student assignment)