SIMD-0218: Intermediate vote credits

Author: AshwinSekar

proposals/0218-intermediate-vote-credits.md

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+---
+simd: '0218'
+title: Intermediate vote credits
+authors:
+  - Ashwin Sekar
+category: Standard
+type: Core
+status: Review
+created: 2025-01-04
+feature: (fill in with feature tracking issues once accepted)
+extends: SIMD-0033 Timely Vote Credits
+---
+
+## Summary
+
+This feature aims to build upon the effort of timely vote credits in order
+to improve the vote credit algorithm.
+
+Credits are awarded for a block voted on by a validator that ultimately gets
+finalized. For consensus purposes a vote for a block is implicitely a vote for
+all its ancestor blocks as well. We propose a mechanism in which all intermediate
+blocks between the previous root and the new root on a fork are awarded credits.
+
+## Motivation
+
+Although greatly improved via TVC, the credit accounting algorithm is still lacking
+in measuring the contribution of validators to consensus. There still exist
+gaps whereby validators can gained increased credits through modifications that
+do not have any consensus benefits. Running unaudited code poses a risk to the network.
+
+Since there is no consensus benefits to these modifications, by aligning the credit
+algorithm to more accurately reflect consensus contriubtion, we hope to reduce the
+potential danger of these modifications.
+
+Similarly, honest validators are not fully rewarded for all of their contributions.
+This SIMD aims to take a step in reducing the gap and making the credit algorithm
+more representative of consensus contributions.
+
+## New Terminology
+
+N/A
+
+## Detailed Design
+
+Validators earn credits based on the latency of their votes on blocks that
+eventually get rooted. Occasionally validators will have gaps in voting due to
+forking, safety checks or other factors outside of their control.
+
+Consider this example where a validator missed out on voting for blocks 103 and
+104 as it was momentarily tricked by a minor fork occuring. It otherwise submitted
+latency 2 votes:
+
+```
+[100] - [101] - [103] - [104] - [105]
+ v 2     v 2                     v 2
+            \ - [102]
+                 v
+```
+
+Assuming this top fork gets finalized, the validator will receive 3 * 16 = 48 credits.
+Although the validator did not vote on 103 or 104, its timely vote on 105 conveyed
+its vote on these intermediate blocks implicitely. Thus we propose that the
+intermediate blocks should be considered as having been voted on in the block in
+which the vote for 105 landed for the purpose of accounting:
+
+```
+[100] - [101] - [103] - [104] - [105]
+ v 2     v 2     i 4     i 3     v 2
+            \ - [102]
+                 v
+```
+
+With this accounting, the validator earns 77 credits.
+As the vote for 105 landed in 107, we treat any intermediate block as also having
+a vote landed in 107. In fact this is the approach many "backfill" mods take,
+by stuffing all missed slots in the first vote after a gap.
+
+This backfilling is an unecessary transmission of information which also increases
+lockout with no basis. Instead we modify the credit algorithm to recognize the vote
+after a gap as a vote for the intermediate ancestors as well.
+
+Note that this does not remove the competitive advantage of being a high performing
+validator. If for example the same validator did not vote on the minor fork:
+
+```
+[100] - [101] - [103] - [104] - [105]
+ v 2     v 2     v 2     v 2     v 2
+            \ - [102]
+
+```
+
+It would earn 80 credits.
+
+More formally when the feature flag `enable_ivc` is activated the vote program must:
+
+* Collect all ancestors `A` between the previous root and the new root as reported
+  via the `SlotHashes` sysvar when rooting a lockout `r` with latency `l`
+* Assign each ancestor `a` in `A` latency of `l + (r - a)`
+* Then award credits for each slot in `A` and `r` using the scheme in SIMD-0033
+
+## Alternatives Considered
+
+N/A
+
+## Impact
+
+This proposal will more fairly assign credits to consensus contributions and remove
+the unfair advantage of modifications that do not contribute any consensus value.
+
+## Security Considerations
+
+N/A
+
+## Backwards Compatibility
+
+The feature is not backwards compatible