<p>For a detailed answer, please read <a href="https://lamport.azurewebsites.net/pubs/byz.pdf" rel="nofollow noreferrer">Lamport <em>et al.</em> paper</a>.</p>
<p>For a short answer, I'll explain it downwards. To coming readers, please note that we are discussing the <a href="https://pmg.csail.mit.edu/papers/osdi99.pdf" rel="nofollow noreferrer">PBFT paper</a>.</p>
<p>Assume that we have only three processes (named <em>p0</em>, <em>p1</em> and <em>p2</em>) and a client <em>c</em>, where <em>p2</em> is a faulty process. That said, we have <strong>2F+1</strong> processes.</p>
<p>In the Prepare phase, <em>p1</em> receives two votes one from <em>p0</em> (an honest vote) and one from <em>p2</em> (a faulty vote). Which decision should <em>p1</em> write into the database? You can simply notice that no-one (neither <em>p0</em> nor <em>p2</em>) is overweighing, which leads to an inconsistency. That's why we need <strong>3F+1</strong> (and accordingly, each process requires <strong>2F+1</strong> replies) processes to achieve consistency.</p>
<p>Roughly speaking: <strong>XF+1</strong> replicas means <strong>XF</strong> replies.</p>

CC BY-SA 4.0
2021-12-06T10:28:53.170Z
2021-12-06T10:40:51.817Z
2021-12-06T10:40:51.817Z
Mohamad-Jaafar NEHME
<p>I started reading PBFT and I could not understand: why do we need 2F prepare certificates?</p>
<p>The client needs only F + 1 replies, not 2F, so why can't the same rule be applicable in the prepare phase of the protocol?</p>
<p>Same question in the commit phase, but this time it is even weirder: we need at least 2F + 1, again why not only F + 1, I just know that the quorum intersection must be more than 2F + 1 but we only need F + 1 replies from that quorum intersection as far as I know.</p>

<blockchain><consensus><algorithms><hyperledger>
Why do we need 2F prepare certificates instead of just F+1 in PBFT?
ezio
# Why do we need 2F prepare certificates instead of just F+1 in PBFT?

(Asked by: username_0 on 2020-07-19)

I started reading PBFT and I could not understand: why do we need 2F prepare certificates?

The client needs only F + 1 replies, not 2F, so why can't the same rule be applicable in the prepare phase of the protocol?

Same question in the commit phase, but this time it is even weirder: we need at least 2F + 1, again why not only F + 1, I just know that the quorum intersection must be more than 2F + 1 but we only need F + 1 replies from that quorum intersection as far as I know.

**Tags:** blockchain, consensus, algorithms, hyperledger


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## 1 answers

### Answer by username_1 on 2021-12-06. Score: 1

For a detailed answer, please read [Lamport *et al.* paper](https://lamport.azurewebsites.net/pubs/byz.pdf).

For a short answer, I'll explain it downwards. To coming readers, please note that we are discussing the [PBFT paper](https://pmg.csail.mit.edu/papers/osdi99.pdf).

Assume that we have only three processes (named *p0*, *p1* and *p2*) and a client *c*, where *p2* is a faulty process. That said, we have **2F+1** processes.

In the Prepare phase, *p1* receives two votes one from *p0* (an honest vote) and one from *p2* (a faulty vote). Which decision should *p1* write into the database? You can simply notice that no-one (neither *p0* nor *p2*) is overweighing, which leads to an inconsistency. That's why we need **3F+1** (and accordingly, each process requires **2F+1** replies) processes to achieve consistency.

Roughly speaking: **XF+1** replicas means **XF** replies.


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