1. Please take out the homework - viewing sheet fro the movie

2. Lesson 11: Packets and Making a Reliable Internet
Unit One Lesson 11

3. 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.
Wires are cut
Interference on a radio channel
Router malfunctions or cannot keep up with traffic being directed to it

4. activity simulates issues of packets traveling on the internet with a paper-passing activity:
Make a simple drawing, or write a handwritten message on a piece of paper.
“You have just made a message that you are going to send to someone else on the other side of the room.  However, we’re going to imagine that it’s too big to send all at once.  So tear it into a few pieces (4 is fine)”
Look across the room to find a partner - they can point at or make eye contact with someone.  (Students in the middle should find partners who aren’t at the middle table)
Once each student has a partner, tell your partner your IP address.
Students should write their partner’s IP address on each piece of paper that makes up their drawing, clearly indicating that it is going to that address.  For example “To: 4.2”.
Lets play!

5. Rules:
Goal: You are going to try to send your drawing to your partner across the room, and also to receive and re-construct your partner’s drawing.
No moving - You may not move from your current position.
Only one piece at a time - You may hand one (and only one) piece of paper at a time to any person who is within arm’s reach. You can’t pass a handful of paper to anyone.
Forward messages to destination - If you receive a piece of paper that is not for you, you should hand it to someone who can get it closer to its intended destination.
Collect all your pieces and reconstruct the drawing - You are looking for pieces of paper with your address on it, and once you have them all to reconstruct the drawing your partner sent you.
Start and stop on my command - When I say “GO” everyone start passing pieces of paper.  And stop when I call “STOP”

6. The good the bad and the ugly?
What worked?
What didn’t?
How could you write this protocol better?

7. Video: The Internet: Packets, Routing, and Reliability
tu.be
Video: The Internet: Packets, Routing, and Reliability

8. 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.
What to Look For
In student solutions, you should primarily be looking for:
Some way to confirm receipt of a packet / message
Some way to define the ordering of packets

9. Then you would send back a message like:
A Simple Solution
Sender: The simplest solution is to develop something akin to TCP, in which you use part of the message to indicate the the packet number and how many there are total. For example:
Message 1: 1/4 Hi
Message 2: 2/4 Th
Message 3: 3/4 er
Meesage 4: 4/4 e!
Receiver: should know how many messages there are and can request a re-send of any that are missing. For example if you received messages 1, 2 and 4. You could send a message back, something like one of these:
req: 3 or need 3 or send 3
The way the real TCP works is this: every once in a while TCP sends back a “cumulative acknowledgement” of how many in-order packets it has received. For example, in an 8-packet message if you received packets:
1 2 _ _ 8
Then you would send back a message like:
ack 2 or maybe ack 3
This would indicate to the sender that next packet you need is packet 3. The sender would send 3, and once you got it, your next message would be:
ack 6 or ack 7
Because you received packets 4, 5 and 6 already.

10. The challenges we encountered in today’s activity very closely mirror those that exist on the actual Internet.
The response was the development of a protocol called the Transmission Control Protocol, or more simply, TCP.
TCP divides larger messages into smaller packets which have ordering information added to their header.
When a packet arrives at a destination computer, an acknowledgement is sent to the sender, letting them know they don’t need to resend that packet.
Once all the packets have arrived, the ordering information in the headers of the packets allows them to be reordered to create the original message.

11. Code.org – answer the questions