Review Videos for Lecture 16: Captchas + Networking Background

• Summer 2020 Slides
• Playlist (length: 53:08)

Intro to Networking

Protocols

Local Area Networks (LANs)

Wide Area Networks

MAC Addresses

The Internet

(True/False) In the diagram where Host A passes a message through 3 routers to communicate with Host D, Host A and Host D are on the same local network.

False. Because the message is being passed through routers, they are on different local networks.

IP Addresses

(True/False) A machine can have both a MAC address and an IP address at the same time.

True. The MAC address is used when someone else on the local network wants to send a message to the machine. The IP address is used when someone else on the global network wants to send a message to the machine.

Layers

Can a single message make multiple hops across different technologies (wireless/wired) as it travels across the global network?

Yes. The choice of physical layer (layer 1) or link layer (layer 2) technology doesn’t affect the protocols at the global/inter-network layer (layer 3).

Packets

Headers with Multiple Layers

Which part(s) of the packet does the IP protocol see as the header? Which part(s) does the IP protocol see as the payload? Which part(s) does the IP not see at all, and why?

The IP protocol sees the IP header as the header.

The IP protocol sees the transport layer header and the application data (the higher layers) as the payload.

The IP protocol doesn’t see the link layer header (the lower layer), because by the time it receives this packet, the link layer protocol has already interpreted the link layer header and passed on the rest of the packet (without the link layer header) to the IP protocol.

Dumb Network

Which of the three layers we've seen so far are implemented hop-by-hop, and which are implemented end-to-end?

The physical and link layers (layers 1 and 2) are implemented hop-by-hop. The internet layer (layer 3) is implemented end-to-end, since it combines many hops to send the message to the destination.

Best Effort Delivery

Optional Bonus

This optional bonus video shows how all the networking protocols interact when you visit a website. (Note: It includes some protocols we haven’t covered yet.)