Plausible Liveness on Casper FFG

Question

This is from the Casper FFG document on Arxiv.

1. Is a higher than b? If so how is b the target checkpoint if it is lower?
2. If a' is a descendant of a (which I take means a previous checkpoint) how does h(a') = h(b) + 1 look like?

Answer 1

Rode this a few weeks ago. Let's try to explain it.

Supermajority links are the way in which the checkpoints are related more efficiently, as you can see on the photo provided, on figure C, supermajority links let us verify the blocks/chain so much faster because let us make "safe assumptions"

• A is the highest justified checkpoint.
• Target checkpoint B is the highest on everyone is betting now (more than 2/3 as the protocol requires.
• Then, if any A'(descendant of the highest justified checkpoint) which is obviously justified because it's a descendant of a justified checkpoint, has a greater height than the target checkpoint B, we can assume, we can go directly to look at A checkpoint. And to don't loose this time evaluating the checkpoint B, is why the supermajority links are implemented.

EDIT Add the graph where Account safety and Plausible Liveness are demonstrated:

The paper's description of the picture and site where apply the example.

Here you have the definitions as the paper/Article says:

Well, hope it helps!