1. HW5 What’s a quorum? How can we use the concept of quorum?
A fraction of nodes in the system
How can we use the concept of quorum?
If we know a node collects votes from a quorum of nodes, all the correct nodes will reach a consensus in the end.
Why can we use the concept of quorum to ensure correctness?
There is at least ONE CORRECT node in common of two quorums. This correct node can ensure that it does not vote for inconsistent results.
Why there is at least one correct node is in common is different for crash quorum and byzantine quorum. Please refer to the slides for more detailed explanation.

2. HW5 Can a faulty node stop progress of the system?
If the node is the primary, yes. But the faulty behavior can be detected eventually.
If the node is a backup, no.
This is liveness
Can a faulty node cause an inconsistency?
No, no matter which role the node is.
This is safety of the system

3. HW5
What’s the difference between permissionless and permissioned blockchains?
Permissionless: everyone can participate
Permissioned: everyone knows the identity of each other but no one needs to trust anyone.

4. HW5
Why and how do permissionless blockchains solve the same problem of BFT?
The same problem: linearizability/total order of client requests (transactions)
BFT
There is a single leader everyone agrees on
Everyone actively participates in the agreement
Permissionless
Use PoW to select a leader (rotating leader)