What does v blocking node set mean?

Question

I have just started with stellar. I am not able to understand v blocking node set and how do they block progress of a particular node.

"An FBA protocol can guarantee liveness to a node 𝑣 only if 𝐐(𝑣) contains at least one quorum slice comprising only correct nodes. A set 𝐵 of failed nodes can violate this property if 𝐵 contains at least one member of each of 𝑣’s slices. We term such a set 𝐵 𝑣-blocking, because it has the power to block progress by 𝑣." The above lines are from the white paper.

Example Three quorum slices: Q(v1) = {v1,v2,v3}

Q(v4) = {v1,v4,v5}

Q(V6) = {v2,v4,v6}

Does node set v2,v4,v6 become blocking node set for node v1?and how do they block progress of v1?

Please help me understand this concept.

Answer 1

A v-blocking set is a set of node that intersects with any quorum: you can think of that set as nodes that must be included (but not at the same time) in order to reach quorum.

A simple example is that if you require 75% majority, any set that makes 30% of the population would be v-blocking as you cannot have any 75% majority without gaining support from members in that fixed 30% (as at least 5% would be required from that group to reach 75%).