Instant Confirmation Rafael Pass and Elaine Shi Cornell Tech & - - PowerPoint PPT Presentation

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Instant Confirmation Rafael Pass and Elaine Shi Cornell Tech & - - PowerPoint PPT Presentation

Dec 03, 2022 124 likes •631 views

Thunderella: Blockchains with Optimistic Instant Confirmation Rafael Pass and Elaine Shi Cornell Tech & Cornell University State-machine replication ( a.k.a. linearly ordered log, consensus, blockchain) State-machine replication ( a.k.a.

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Thunderella: Blockchains with Optimistic Instant Confirmation

Rafael Pass and Elaine Shi

Cornell Tech & Cornell University

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State-machine replication

(a.k.a. linearly ordered log, consensus, blockchain)

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State-machine replication

(a.k.a. linearly ordered log, consensus, blockchain)

Consistency:

honest nodes agree on log

Liveness:

TXs are incorporated soon

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Classical

(e.g. PBFT, Paxos)

Blockchains

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Classical

(e.g. PBFT, Paxos)

Fast (most of the time) Complex Asynch

Blockchains

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Classical

(e.g. PBFT, Paxos)

Fast (most of the time) Complex Asynch

Blockchains

(PoW and non-PoW)

Slow Sync Simple Robust

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Thunderella

As simple and robust as a blockchain Confirm in 2 actual network rounds in the “optimistic case” Fall back to blockchain when things “go bad”

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Classical

(e.g. PBFT, Paxos)

Let’s start with this

Roadmap

Blockchains

(PoW and non-PoW)

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Classical

(e.g. PBFT, Paxos)

Blockchains

(PoW and non-PoW)

Roadmap

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Leader proposes transaction

(Seq, )

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Everyone “ack’s”

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Confirm on upon “enough” acks

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Ex: Assume ⅔n+1 honest; wait for ⅔n+1 acks

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⅔n+1 ⅔n+1

“Y”

Must intersect at an honest node

Assume ⅔n+1 honest

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⅔n+1 ⅔n+1

“Y”

Must intersect at an honest node

Assume 1/3n malicious

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⅔n+1 ⅔n+1

“Y”

Must intersect at an honest node

Thus X = Y

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Assume ⅔ honest and online

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Assume ⅔ honest and online

Consistency Liveness

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Assume ⅔ honest and online

Consistency Liveness Consistency No liveness

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How do we achieve liveness?

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How do we achieve liveness?

You don’t want to know …

[PBFT, Paxos...]

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Anatomy of classical consensus

Simple normal path Complicated recovery path

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Classical

(e.g. PBFT, Paxos)

Blockchains

(PoW and non-PoW)

Roadmap

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blockchain

Thunderella

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Thunderella

for

permissionless Thunderella

for

permissioned

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Thunderella

for

permissionless

For concreteness, we’ll focus on this

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blockchain miners Leader/ ”accelerator” Committee

(recent miners/ stakeholders)

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3/4 fraction honest and online

“Optimistic” mode: Instant confirmation

honest and

  • nline

majority honest

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majority honest majority honest

(but need not be online)

But, still SECURE as long as:

majority Arbitrary deviation!

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¾n+1 “X” ¾n+1 “Y”

Must intersect at an honest node

Assume ½n+1 honest

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Ack Propose (seq, )

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Propose (seq, ) Ack

¾ acks:

notarized

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¾ voted:

notarized Confirm maximal “lucky” sequence

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Confirm maximal “lucky” sequence

¾ voted:

notarized

No liveness when

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blockchain collects evidence

  • f
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blockchain collects evidence

  • f

Now enter slow mode

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What evidence do we collect?

Need: faulty nodes cannot implicate honest leader

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Miners “tell blockchain” everything they know

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What evidence do we collect?

k blocks

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What evidence do we collect?

k blocks

has not appeared in a lucky sequence

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blockchain collects evidence

  • f

Now enter slow mode

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Now enter slow mode

Nodes have different logs when entering slow mode

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Now enter slow mode

Need: agree on log before entering slow mode

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Grace period: k blocks

  • Stop optimistic output
  • Share knowledge
  • All knowledge → blockchain

Stop “acking” new transactions Tell others what you know Miners tell blockchain what they know

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Summary

k blocks

Slow mode has not appeared in a lucky sequence Grace period

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3/4 fraction honest and online

“Optimistic” mode: Instant confirmation

honest and

  • nline

majority honest

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majority honest majority honest

(but need not be online)

But, still SECURE as long as:

majority Arbitrary deviation!

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Thank you.