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Replication & Trust

How to achieve trust ?

The PoCo offers a consensus mechanism that can certify the likelihood of a result to be valid. This consensus relies on the scoring of workers and the replication of a task's execution to combine the score of the workers that come up with the same result. This consensus is largely based on Sarmenta's work [Sarmenta2002] with specific tunning of the scoring function [Trust2018].

Contribution credibility

Each worker's contribution has an associated credibility. This credibility derives from the worker's history score. As described in [Trust2018], a worker score is a positive integer that is incremented for each valid and verified contribution. In case of bad contribution, a worker loses one third of it's score.

This credibility can be expressed as a likelihood percentage but also as a weight value that can be used to detect consensus without resorting to floating point arithmetics. Again, this is described in [Trust2018].

Requiring a trust level

Based on [Sarmenta2002] describing the way to combine workers' contribution and to evaluate a result's likelihood, a user requesting an execution can configure the level of trust he wants the PoCo to enforce. This means that a user can disable this consensus mechanism or use it to impose a certain quality of service.

The trust level corresponds to a minimum correctness likelihood that a result must achieve to be valid. For example, a trust level of 0% means any contribution would be accepted, regardless of the score of the worker proposing it. On the other hand a trust level of 99.99% means a result will only be accepted if the contribution towards it result shows a probability higher than 99.99% that it is valid.

The trust level is expressed, on-chain, by a integer trust such that threshold = 1 - 1 / trust.

Trust PoCo enforced confidence threshold
0 0%
1 0%
2 50%
100 99%
10000 99.99%
1000000 99.9999%

Limitations

A limitation of this consensus mechanism is that it requires the application to be replicable. Non deterministic applications, of long running jobs such as webservers, do not meet this requierement out of the box. If possible, the application developper can provide a deterministic result in the /iexec/consensus.iexec file. Otherwize, the user can still run its application on the iExec platform but would have to disable the PoCo's consensus layer and rather rely on hardware security.

References

[Sarmenta2002](1, 2) Luis F.G.Sarmenta. Sabotage-tolerance mechanisms for volunteer computing systems. 2002. Future Generation Computer Systems, 18(4), 561–572
[Trust2018](1, 2, 3) Trust management in the Proof of Contribution protocol. 2018. Technical report. PDF