1. Packets and Making a Reliable Internet
Lesson 1-11
AP Computer Science Principals

2. Objectives Students will be able to:
Explain why protocols are necessary to overcome the underlying unreliability of the Internet.
Develop a protocol for reliable communication on the Internet.
Justify the need for acknowledgements and packet numbering in TCP.

3. Journal
Your friend sent you a message on the Internet, but you never received it. Based on what you already know about routers and the physical Internet, list what reasons might explain this fact.

4. Packets
When we communicate on the Internet, we are not just sending short text messages. We also use the Internet to exchange documents, videos, music, and scientific data, and these files can easily grow to enormous size.
This would not be a problem if the Internet were perfectly reliable, but in reality, errors sometimes occur.
Lines can be cut, routers can be overwhelmed with traffic, and interference with electric or radio signals can cause messages to become corrupted.
The response to this problem is to split large messages into smaller pieces of information called “packets.”

5. Packets Splitting up a message into packets provides many benefits.
If a faster route opens up halfway through transmitting a large file, it is easy to reroute later packets in the transmission through that route.
Having smaller messages to balance also makes it easy to rebalance the entire network of routers, providing more efficient usage of the entire Internet

6. Packets
Splitting up a message into smaller chunks doesn’t solve all the problems of unreliability on the Internet, and it may even add some.
Packets can still be dropped or arrive out of order.
Today’s challenge is to develop a protocol to make a reliable Internet, in spite of the challenges this new system poses.

7. Activity Guide
The version of the Internet Simulator we will be using today has been structured to simulate the unreliability of the Internet. You’ll notice a few changes:
You will only be allowed to send packets containing 8 characters of text; anything larger will be dropped (to simulate the fact that large messages are much more likely to have errors).
Every message has a small chance of being dropped on each “hop” it makes between routers.
You may construct a message with multiple packets prior to sending it.

8. Activity Guide In your groups:
Group members need to connect to different routers!
Start a conversation with your group members.
It is easy to follow the conversation?
What issues do you see with this version of the simulator.

9. Try Sending This Sentence
The transformation of a caterpillar into a butterfly is one of the great wonders of nature.

10. Activity Guide – Develop Your Protocol
You will turn in just one activity guide per group this time.
Develop a protocol that will allow you to overcome the unreliability of the network so that a message can be sent and both sender and receiver can be confident the full message was received.
Guidelines for Protocol:
All communication can only be done through the Internet Simulator.
The message sent will be at least 60 characters long and might be entirely random (i.e. there’s no way to use human intuition to reconstruct the message). The message is not known beforehand.
Each of you needs to create your own message and send it someone else.
The sender and receiver must be confident the full message was successfully transmitted and reconstructed.

11. Test is Out Pick a person to go across the room and login.
I will give one person a message. Send the message to the person across the room and have the person write it down on the back of their activity guide.

12. Test is Out
Look at the original message and the received message. Don’t correct it. I am not grading you on correctness!
Answer these questions on the back of the paper (add a sheet if you need to):
Did your protocol work?
If not, where did it go wrong?
If you were going to improve it what would you do?

13. Videos The Internet: Packets, Routing, and Reliability
There and Back Again- A Packet's Tale. How Does the Internet Work-

14. Stage 11
Complete Stage 11