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blockchainfault-tolerance

How PBFT applied in block chain?

I am trying to understand how PBFT(practical byzantine fault tolerance) applied in block chain. After reading paper, I found that process for PBFT to reach a consensus is like below:

  1. A client sends a request to invoke a service operation to the primary
  2. The primary multicasts the request to the backups
  3. Replicas execute the request and send a reply to the client
  4. The client waits for f + 1 replies from different replicas with the same result; this is the result of the operation.

This is how I understand how it is applied in block chain:

  1. At first, the elected primary node wants to write transaction A to chain, it will broadcast transaction A to other nodes.
  2. Any node receives the transaction checks if the transaction legal. If the transaction is thought as legal, the node will broadcast a legal signal to all of nodes in this round of consensus.
  3. Any node that receives equal or greater than f + 1 responds will write the transaction to the its own chain.

Here are my questions:

For malfunctioned nodes, if they keep failing to write block into its chain, they will hold a different chains with healthy node. In next consensus, the existing chain will be picked up at first. How do nodes know which one is the correct chain?

In step 1, the elected node send transaction to other nodes. Does "other nodes" means all nodes in the network? How to make sure if all nodes included in the consensus because there is not a centralized agency.

Solution

  • How do nodes know which one is the correct chain?

    For tolerating Byzantine faulty nodes, It needs at least 3f+1 nodes in the network. PBFT is one of the algorithms which can tolerate Byzantine failure. So PBFT can tolerate up to f Byzantine nodes. f number of malicious nodes can be tolerated if you use PBFT. If there are f number of malicious nodes which keep failing to write block into its chain, resulting in inconsistency with correct nodes, then one can figure that the same chains from rest 2f + 1 nodes are correct. (Correct nodes always output exactly same data to the same request in same order).

    Does "other nodes" means all nodes in the network? How to make sure if all nodes included in the consensus because there is not a centralized agency.

    In PBFT setup, identities of all nodes should be established. To do that, there should be central authority to determine whether a node can join the network or not. (Important: central authority only engages in identity management, not the algorithm itself)

    Why this is needed? It's because PBFT works by voting mechanism and voting is not secure when anyone (including malicious node) can join the network. For example, a proposed value by the primary only can be recorded to all nodes in the way of state machine replication, which it means that there needs at least 2f + 1 agreed matching messages for the value to be accepted to the correct nodes.

    Without the trusted identity management, Sybil attack is possible. And this is the main reason why PBFT is not for the open blockchain which allows any node can freely join or leave the network.