Creator

Luca

Created

Dec 18, 2023 12:05 PM

Background

Consensus power is currently granted at least ChainFinality (=900) blocks after a sector is onboarded onchain via ProveCommit.

This is due to PowerTableLookback: at each epoch t, leader election protocol selects SPs proportionally to their quality adjusted power at epoch t- ChainFinality

This means that on one side power activation is currently deferred by at least 900 epochs, but on the other hand sector power is retained after sector termination accordingly.

Effects on Network Power

Given collateral is given back at sector termination, in practice each sector has PowerTableLookback epochs of power which are not covered by collateral.

This aspect is not ideal, as malicious parties could buy power for a limited window of time at cheap price.

Indeed, assuming t = termination epoch, from epoch t to epoch t + PowerTableLoobback only sector storage costs need to be paid by the SP in order to effectively benefit by sector power (given sector is indeed terminated, WindowPost is needed collateral is already returned).

Mitigations

There are two fix that we can put in place in order to avoid the possibility of cheap power rental:

• Give back sector collateral PowerTableLookback epochs after termination. Note that this would translate into additional PowerTableLookback of capital cost with respect to the status quo
• Accounting power for not terminated sectors only. Note that this change would immediately translate into a loss of power for sectors of PowerTableLookback epochs, compared with the status quo.

None of the above solution is costless. Ideally, we could explore the possibility of allowing SP to opt-in, when onboarding a sector to any of the two alternatives, depending on which one fits best for them.

TODOs

Before moving forward we should

• Understand how complex it is to put in place (any of) the two fixes proposed above
• Understand if it is doable to offer opt-in possibility to any of the two fixes when onboarding a new sector

Related topics: Sample only active sectors for WinningPost

In the process of proposing a longer PowerTableLookback (from current 900 epochs to 7 days)

we acknowledged that this change would lead to an additional storage overhead for SP who want to keep the power for PowerTableLookback epochs after termination (again from current 900 epochs to 7 day). One idea which has been discussed was the following:

• At epoch e grant power looking at the power table of epoch e = PowerTableLookback ( same way as today)
• For WinningPoSt sector sampling purposes, consider only sectors which are active at epoch e.

While this would be beneficial for SP in terms of storage costs overhead (power is maintained for a window of time which has same length as sector lifetime, without the need to keep storing the sector for PowerTableLookback after termination), this improvement would make the uncollateralized sector power issue highlighted above even worse.