Unit 4 Lab 1: Reliable Communication, Page 2

EU 6.1 The Internet is a network of autonomous systems.

We partly cover LO 6.1.1 in FYTD #1. --MF, 1/17/18

LO 6.1.1 Explain the abstractions in the Internet and how the Internet functions. [P3]
EU 6.2 Characteristics of the Internet influence the systems built on it.

We partly cover LO 6.2.1 in FYTD #1. --MF, 1/17/18

LO 6.2.1 Explain characteristics of the Internet and the systems built on it. [P5]
EK 6.2.1A The Internet and the systems built on it are hierarchical and redundant.
EK 6.2.1D Routing on the Internet is fault tolerant and redundant.
EK 6.2.2A Hierarchy and redundancy help systems scale.
EK 6.2.2B The redundancy of routing (i.e., more than one way to route data) between two points on the Internet increases the reliability of the Internet and helps it scale to more devices and more people.

Given the enormous amount of data traveling around, the Internet needs to be reliable. We have achieved this by building many redundant connections into the physical systems of the Internet. Wherever information is going, there is more than one way to get there, so that if part of the Internet fails, the rest remains connected even if the failed part is in the usual path from one place to another. This increases the Internet's fault tolerance (ability to work around problems). And it also helps the Internet scale (expand) to more devices and people.

Describe what's going on in this animation.

A redundant system is one that has back-up elements in case one part fails.

Fault tolerance is the ability of a system to work around problems.

Scalability is the ability of the Internet to keep working as it grows.
Work through the following questions.

In this model of a network, what is the minimum number of nodes (connection points) that can stop working before the sender and the receiver can't communicate? (Other than the sender or the receiver themselves, of course.)

There are no nodes that are vital to the system. Pick any node to stop working, and you can still find another path.

Correct! If the node with 6 connections goes down and also either of the two to its left, the sender and receiver can't communicate.

Try to find a smaller number of nodes that can stop working and still break communication.

In the same model network, what is the maximum number of nodes that can fail and still let Sender and Receiver communicate?

If all 10 nodes fail, the sender and receiver can't communicate.

If 9 nodes fail, the sender and receiver can't communicate.

Correct! If the 4 nodes on the right and also the 4 nodes on the left all fail, the remaining 2 nodes in the middle will still allow the sender and receiver to communicate.

Try to find a higher number of nodes that can stop working and still ensure communication.

Network Redundancy