Confirmation Rule for Ethereum PoS - Consensus - Ethereum Research


    
    
  
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      Confirmation Rule for Ethereum PoS
    


      
          
            
              
              
                Consensus
              
            
            



      

        

            

            
              
                adiasg
                
              

              


                


              
                  
                    April 28, 2023,  9:29pm
                  
                  

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              This post is opened for discussion re. the following fast confirmation rule for Ethereum proof-of-stake:

	Draft paper: confirmation-rule-draft.pdf (396.2 KB)
	Explainer blog post


This work was conducted together with Francesco D’Amato @fradamt, Roberto Saltini @saltiniroberto, Luca Zanolini @luca_zanolini, & Chenyi Zhang.


Confirmation Rule

Assumptions:

	From the current slot onwards, the votes cast by honest validators in a slot are received by all validators by the end of that slot, i.e., the network is synchronous with latency < 8 seconds.
	This proposed change to the Ethereum protocol:
	If j is the highest justified checkpoint block, and the current epoch is e, then allow a branch with leaf block b if the latest justified checkpoint in the post-state of b is either j, or from an epoch \ge e-2






Notation:

	
n is the current slot, and e is the current epoch.
	
b is a block from the current epoch e.
	There are S FFG votes from epoch e in support of c.
	
W_f is the weight of validators yet to vote in epoch e, and W_t is the total weight of all validators.
	The adversary controls \beta < \frac{1}{3}^{\textrm{rd}} fraction of the validator set.
	The adversary is willing to bear a slashing of \alpha (\leq \beta) fraction of the validator set.


A short description of the rule (please see confirmation-rule-draft.pdf (396.2 KB) or blog post for explanation):

	
p_{b}^n = \frac{\textrm{honest support for block } b}{\textrm{total honest weight}} from validators in committees from b\textrm{.parent.slot} + 1 till n.
	
\textrm{isLMDConfirmed}(b, n) is defined as p_{b'}^n > \frac{1}{2(1-\beta)} for all b' in the chain of b.
	
\textrm{isConfirmed}(b,n) if:
	the latest justified checkpoint in the post-state of b is from epoch e-1, and
	
\textrm{isLMDConfirmed}(b,n), and
	
[S - \textrm{min}(S, \alpha W_t, \beta (W_t - W_f))] + (1-\beta)W_f \ge \frac{2}{3}W_t.





If \textrm{isConfirmed}(b,n), then b is said to be confirmed and will remain in the canonical chain.

Since p_b^n cannot be observed, we define a practical safety indicator q_b^n to determine if p_b^n is in the appropriate range:

	
q_{b}^n = \frac{\textrm{support for block } b}{\textrm{total weight}} from committees in slot b\textrm{.parent.slot} + 1 till slot n

	
q_{b'}^n > \frac{1}{2} \left(1+\frac{\textrm{proposer boost weight}}{\textrm{total honest weight}}\right) + \beta for all b' in the chain of b implies \textrm{isLMDConfirmed}(b, n)




Performance

In ideal conditions, the rule would confirm a block immediately after the end of its slot.

Under typical mainnet conditions, we expect the rule to confirm most blocks within 3-4 slots (under 1 minute).

We observe the following values for q (plot generated using this prototype):



q_plot846×571 38.5 KB



The current slot is 6337565, and the latest confirmed block is at slot 6337564.


Previous Work

	
Safe head with LMD – this post is an extension of the linked work.
	Safe block confirmation rule
	High confidence single block confirmations in Casper FFG


            


            
              
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                        SubnetDAS - an intermediate DAS approach
                        


                        Super-Finality: High-Safety Confirmation for Casper
                        

            
              
                czhang-fm
                
              

              



              
                  
                    September 1, 2023,  2:13am
                  
                  

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 adiasg:


in the last equation of q_{b’}^n > … total honest weight … isLMDConfirmed(b, n)




Please change to “total weight from slot(b’.parent)+1 to n”

            


            
              
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