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Blockchains

Blockchains. Lecture 7. Broadcast. Best-effort broadcast or simply broadcast (BEB) Unreliable Reliable broadcast (RB) Reliable We are interested. Unreliable Broadcast. broadcast(m). p 1. deliver(p 1 ,m). p 2. p 3. deliver(p 1 ,m). p 3. Reliable Broadcast Abstractions.

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Blockchains

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  1. Blockchains Lecture 7

  2. Broadcast • Best-effort broadcast or simply broadcast (BEB) • Unreliable • Reliable broadcast (RB) • Reliable • We are interested

  3. Unreliable Broadcast broadcast(m) p1 deliver(p1,m) p2 p3 deliver(p1,m) p3

  4. Reliable Broadcast Abstractions • Best-effort broadcast • Guarantees reliability only if sender is correct • Reliable broadcast • Guarantees reliability independent of whether sender is correct

  5. BEB Example No • Is this allowed? broadcast(m) p1 deliver(p1,m) p2 p3 deliver(p1,m) p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  6. BEB Example (2) Yes • Is this allowed? broadcast(m) p1 deliver(p1,m) p2 p3 deliver(p1,m) p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  7. Reliable Broadcast • BEB gives no guarantees if sender crashes • Strengthen to give guarantees if sender crashes Ali Ghodsi, alig(at)cs.berkeley.edu

  8. Reliable Broadcast A designated replica (the broadcaster) broadcasts a message to all replicas, with the following guarantees: • (Validity) If the broadcaster is correct, then every correct replica delivers the broadcaster’s message. • (Consistency) If two correct servers deliver two messages B and B′ then B = B′. • (Totality) If any correct replica delivers a broadcast message B, then all correct replicas deliver B.

  9. RB Example Yes • Is this allowed? broadcast(m) p1 p2 p3 p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  10. RB Example Yes • Is this allowed? broadcast(m) p1 p2 p3 deliver(p1,m) p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  11. RB Example No • Is this allowed? broadcast(m) p1 p2 p3 deliver(p1,m) p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  12. RB Example No • Is this allowed? broadcast(m) p1 deliver(p1,m) p2 p3 p3 Ali Ghodsi, alig(at)cs.berkeley.edu

  13. How to achieve Byzantine Reliable Broadcast

  14. Reliable Broadcast [Bracha, Information and Computation2018] • Bracha’s protocol tolerates f faulty replicas out of n = 3f+1.

  15. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] B p0 p1 p2 p3 “Initiate”

  16. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] B p0 p1 p2 p3 “Initiate” “Echo”

  17. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] B p0 p1 p2 p3 “Initiate” “Echo”

  18. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] Communication complexity is O(n2|B|) B p0 p1 p2 p3 “Initiate” “Ready” “Echo”

  19. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] The algorithm implements Byzantine reliable broadcast if n > 3t.

  20. Bracha’s Reliable Broadcast [Bracha, Information and Computation2018] The algorithm implements Byzantine reliable broadcast if n > 3t.

  21. Proof? (Discussion Time!) B p0 p1 p2 p3 “Initiate” “Ready” “Echo” (Validity) If the broadcaster is correct, then every correct replica delivers the broadcaster’s message. (Easy!) (Consistency) If two correct servers deliver two messages B and B′ then B = B′. (Note that n > 3t) (Totality) If any correct replica delivers a broadcast message B, then all correct replicas deliver B. (Hint: Use Amplification!)

  22. Applications?

  23. Reliable Broadcast • One building primitive for one particular BFT (aka atomic broadcast, permissioned blockchain) • Weaker than BFT • BFT implies reliable broadcast

  24. Acknowledgement • Some slides from Dr. Ali Ghodsi with some adaptations.

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