<p>Ouroboros claims to be &lt;50% Byzantine fault tolerant with regards to persistence. I was wondering why is it that some other PoS systems settle for 33% (e.g. Casper), when 50% is provably achievable?</p>
<p>In the Ouroboros Classic paper, as referenced in this talk by Peter Gazi (<a href="https://www.youtube.com/watch?v=PoNaw-Mtxgo" rel="nofollow noreferrer">https://www.youtube.com/watch?v=PoNaw-Mtxgo</a>), it seems like all that is needed to achieve &lt;50% fault tolerance is simply to have a sequence of random slot leaders create new blocks. That's all the forkability analysis and persistence/liveness conclusion with probabilistic finality seems to depend on (probability of forkability falls exponentially with length of the characteristic string with p = (1-e)/2). So why are some PoS chains out there still 33% BFT? Don't they all pretty much follow a slot leader system like Ouroboros, and thus should also have 50% BFT by this theorem?
I understand that in the Classic paper they assume synchrony. But this assumption is relaxed in Ouroboros Praos, with the same 50% BFT.</p>
<p>Does it have something to do with absolute finality vs probabilistic finality? I.e. does Casper achieve absolute finality, and in doing so have to sacrifice the fault tolerance somehow? Is there another factor?</p>
<p>Thank you!</p>

<ouroboros>
Can someone who is familiar with Ouroboros explain its fault tolerance threshold?
# Can someone who is familiar with Ouroboros explain its fault tolerance threshold?

(Asked by: username_0 on 2021-08-29)

Ouroboros claims to be <50% Byzantine fault tolerant with regards to persistence. I was wondering why is it that some other PoS systems settle for 33% (e.g. Casper), when 50% is provably achievable?

In the Ouroboros Classic paper, as referenced in this talk by Peter Gazi (<https://www.youtube.com/watch?v=PoNaw-Mtxgo>), it seems like all that is needed to achieve <50% fault tolerance is simply to have a sequence of random slot leaders create new blocks. That's all the forkability analysis and persistence/liveness conclusion with probabilistic finality seems to depend on (probability of forkability falls exponentially with length of the characteristic string with p = (1-e)/2). So why are some PoS chains out there still 33% BFT? Don't they all pretty much follow a slot leader system like Ouroboros, and thus should also have 50% BFT by this theorem?
I understand that in the Classic paper they assume synchrony. But this assumption is relaxed in Ouroboros Praos, with the same 50% BFT.

Does it have something to do with absolute finality vs probabilistic finality? I.e. does Casper achieve absolute finality, and in doing so have to sacrifice the fault tolerance somehow? Is there another factor?

Thank you!

**Tags:** ouroboros

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## 0 answers

