Signature-free broadcast-based intrusion tolerance: never decide a Byzantine value

• Authors: Achour Mostéfaoui;Michel Raynal
• Venue: OPODIS'10 Proceedings of the 14th international conference on Principles of distributed systems
• Year: 2010

Providing application processes with strong agreement guarantees despite failures is a fundamental problem of fault-tolerant distributed computing. Correct processes have not to be "polluted" by the erroneous behavior of faulty processes. This paper considers the consensus agreement problem in a setting where some processes can behave arbitrarily (Byzantine behavior). In such a context it is possible that Byzantine processes collude to direct the correct processes to decide on a "bad" value (a value proposed only by faulty processes). The paper has several contributions. It presents a family of consensus algorithms in which no bad value is ever decided by correct processes. These processes always decide a value they have proposed (and this is always the case when they all propose the same value) or a default value ⊥. These algorithms are called intrusion-free consensus algorithms. To that end, each consensus algorithm is based on an appropriate underlying broadcast algorithm. One of these abstractions, called validated broadcast is new and allows the design of a resilienceoptimal consensus algorithm (i.e., it copes with up to t n/3 faulty processes where n is the total number of processes). All proposed consensus algorithms assume the underlying system is enriched with additional computational power provided by a binary Byzantine consensus algorithm. The paper presents also a resilience-optimal randomized binary consensus algorithm based on the validated broadcast abstraction. An important feature of all these algorithms lies in the fact that they are signature-free (and hence particularly efficient).