- Author(s): @PaulVMo
- Start Date: 2021-10-26
- Category: Technical
- Original HIP PR: helium#306
- Tracking Issue: helium#313
- Approved Version: 31409fe
Increase the DKG penalty amount from the current value of 1.0 to 5.0. The DKG penalty is assessed when a validator fails to complete the distributed key generation (DKG) that is required for a consensus group election to complete. A failed election is reattempted 5 blocks later with another set of validators. Repeated election failures are bad for the network and bad for the validator pool. The current DKG failure penalty amount is not significant enough to drive change in behavior for misconfigured and poor performing validators. By tuning the penalty amount in this way, failed validators should be more quickly removed from eligibility for consensus group and hopefully provide greater incentive for operators to improve the performance of their validators.
At the end of every epoch (every 30 blocks), an election takes place to select the next consensus group. Approximately one quarter of the existing group is selected to be removed from the group and then validators from among the validator pool are selected as replacements.
For an election to be successful, the resulting selected group must complete distributed key generation (DKG) within 5 blocks. The resulting key is required for the consensus protocol to work. A DKG failure occurs when one or more members of the selected group are not reachable by the others or otherwise fail to complete their portion of the DKG. 100% of the group must complete DKG. The members that cause the DKG to fail are assessed with the DKG penalty.
DKG penalties add to a validator’s overall penalty score. Validators with a higher penalty are less likely to be selected to join consensus group and to stay in consensus group once in.
If an election has not succeeded including the DKG, a new election is reattempted every 5 blocks until one succeeds. Failed elections cause epochs (the time between consensus groups) to run long of their nominal target of 30 blocks. This is bad for the network as well as for validators specifically.
First, from a network perspective, long epoch times lead to less frequent rotation of validators in and out of the consensus group. In general, frequent rotation is good for the network as it creates less predictability of which validators will be in consensus at a given time and thus provides greater security. Also, from a performance perspective, long epochs lead to longer reward block times. Rewards are calculated once per epoch at the end of the epoch. When there are more blocks in an epoch, there are more transactions to be processed in the reward block. This leads to more time to compute the rewards. Some rewards blocks can take up to 10 minutes. During this time, other transactions are not being processed leading to delays on the network.
Second, from validators’ perspective, failed elections lead to fewer rewards for the validator pool which will decrease the incentive to run a validator. In general, more validators are better because it increases the barrier to a single, malicious actor controlling a large portion of the consensus groups as well as great reliability. However, rewards for validators in the consensus group are capped at 35 blocks (which is the 30 block interval plus one 5 block election interval). If the election fails on the first try, then validators stop being rewarded until an election succeeds. This can be seen in the spikes above 35 blocks in the graph of epoch length (source: DeWi ETL).
This HIP impacts both the validator group as well as all network participants and users in general. By tuning the validator penalties, operators will be encouraged to properly configure their validators and keep them online which is beneficial for the network.
Feedback on this tuning of the penalty amounts was solicited in the #validator Discord channel.
Implementation is done by processing of a var transaction to update the value of the dkg_penalty
chain var to 5.0. No code changes are needed for this value.
Several factors justify a higher dkg failure penalty. The most significant is looking at the harm caused to others (aka lost rewards for the validator pool) vs. oneself when failing DKG.
Based on current reward rates, a single DKG failure causes the consensus pool to miss out on 5 blocks worth of rewards or 17.36 HNT (6% of 2.5 million HNT / 43,200 blocks per month * 5 blocks). For the validator that failed to get elected, their expected rewards that they are missing out on had they been elected is ~10 HNT based on current variables (~2.5 HNT per validator per epoch multiplied by an average of tenure of 4 epochs per election).
It is reasonable to assume that the pain felt by the failing validator should at least match the pain felt by the other validators. This means that not only should the failing validator miss out on the 10 HNT expected had they been elected on that attempt, but also on average should miss one more election such that the missed opportunity to them is >17.36 HNT. With the average validator penalty currently at 4.4, adding a DKG penalty of 1.0 to their score only decreases relative odds of being elected by 18% (i.e. a score of 5.4 vs. 4.4 is 82% as likely to be elected). This is far short of reducing the odds of election such that the failing validator on average will miss out on another entire election. Setting the DKG penalty to 5.0 will mean that the failed validator will then be about half as likely to be elected after receiving the penalty.
Lastly, one can look at the DKG penalty relative to the tenure penalty which is 0.5. Based on the average validator staying in consensus groups for 4 epochs upon election, then the total tenure penalty expected for an election is 2.0 (4 rounds * 0.5 tenure penalty / round). However, based on the logic above, the penalty for causing an election failure should be at least twice that, or at least 4.0 which is more closely aligned to the proposed value of 5.0 than the existing 1.0.
One drawback to increasing the penalty is that it can be seen as unfair in some cases since not all DKG failures are currently explainable. Based on validator operator feedback on Discord, well configured and adequately performing validators have experienced DKG failure penalties without sufficient information (such as logging) as to why the failure occurred.
However, given the fact that all penalties decay to zero over time (currently approximately 2 weeks’ time) and the average time between elections for any individual validator is 5 to 6 days, the risk should be seen as acceptable in order to improve the performance of the network as a whole.
The rationale for this change is explained above in the motivation. The objective of this change is to adjust the value of an existing penalty to improvement the performance of validators and the blockchain.
As this solution leverages tuning of the existing penalty system, alternatives were deprioritized. The authors felt it was prudent to first attempt to tune the existing system rather than suggest a new one, especially as this will be the first adjustment to penalties since validators went live on mainnet.
It should be noted that several novel ideas to improve the system were suggested by the community. One such that should be reserved for future iterations, if warranted, is the idea of a progressive penalty for repeat DKG failures by the same validator to help ensure they are more quickly deselected from the consensus group.
No unresolved questions.
This will be deployed as var transaction which requires no code changes or other impacts. All
validators and other node types can already accept this value for the dkg_penalty chain var.
Success of this change will result in fewer epochs that are exceed 35 blocks and fewer repeat failures in the same election cycle when there is a failure.