Consensus justification

all.md

@@ -1,12 +1,13 @@
 ## All SIPS
 
-| SIP #                                 | Title                       | Status |
-|---------------------------------------|-----------------------------|--------|
-| [1](./sips/dkg.md)                    | DKG                         | open-for-discussion  |
-| [2](./sips/msg_struct_encoding.md)    | Message struct and encoding | open-for-discussion  |
-| [3](./sips/qbft_sync.md)              | QBFT Sync                   | open-for-discussion  |
-| [4](./sips/change_operator.md)        | Change operators set        | open-for-discussion  |
-| [5](./sips/ecies_share_encryption.md) | ECIES Share Encryption      | open-for-discussion  |
-| [6](./sips/constant_qbft_timeout.md)  | Constant QBFT timeout       | open-for-discussion  |
-| [7](./sips/fork_support.md)           | Fork Support                | open-for-discussion  |
-| [8](./sips/pre_consensus_livness.md)                    | Pre-Consensus livness fix   | open-for-discussion  |
+| SIP #                                   | Title                       | Status              |
+|-----------------------------------------|-----------------------------|---------------------|
+| [1](./sips/dkg.md)                      | DKG                         | open-for-discussion |
+| [2](./sips/msg_struct_encoding.md)      | Message struct and encoding | open-for-discussion |
+| [3](./sips/qbft_sync.md)                | QBFT Sync                   | open-for-discussion |
+| [4](./sips/change_operator.md)          | Change operators set        | open-for-discussion |
+| [5](./sips/ecies_share_encryption.md)   | ECIES Share Encryption      | open-for-discussion |
+| [6](./sips/constant_qbft_timeout.md)    | Constant QBFT timeout       | open-for-discussion |
+| [7](./sips/fork_support.md)             | Fork Support                | open-for-discussion |
+| [8](./sips/pre_consensus_livness.md)    | Pre-Consensus livness fix   | open-for-discussion |
+| [11](./sips/consensus_justification.md) | Consensus justification     | open-for-discussion |

core.md

@@ -1,12 +1,13 @@
 ## Core
 
-| SIP #                              | Title                       | Status |
-|------------------------------------|-----------------------------|--------|
-| [1](./sips/dkg.md)                 | DKG                         | open-for-discussion  |
-| [2](./sips/msg_struct_encoding.md) | Message struct and encoding | open-for-discussion  |
-| [3](./sips/qbft_sync.md)           | QBFT Sync | open-for-discussion  |
-| [4](./sips/change_operator.md)     | Change operators set | open-for-discussion  |
-| [5](./sips/ecies_share_encryption.md) | ECIES Share Encryption      | open-for-discussion  |
-| [6](./sips/constant_qbft_timeout.md)  | Constant QBFT timeout      | open-for-discussion  |
-| [7](./sips/fork_support.md)           | Fork Support                | open-for-discussion  |
-| [8](./sips/pre_consensus_livness.md)                    | Pre-Consensus livness fix   | open-for-discussion  |
+| SIP #                                   | Title                       | Status              |
+|-----------------------------------------|-----------------------------|---------------------|
+| [1](./sips/dkg.md)                      | DKG                         | open-for-discussion |
+| [2](./sips/msg_struct_encoding.md)      | Message struct and encoding | open-for-discussion |
+| [3](./sips/qbft_sync.md)                | QBFT Sync                   | open-for-discussion |
+| [4](./sips/change_operator.md)          | Change operators set        | open-for-discussion |
+| [5](./sips/ecies_share_encryption.md)   | ECIES Share Encryption      | open-for-discussion |
+| [6](./sips/constant_qbft_timeout.md)    | Constant QBFT timeout       | open-for-discussion |
+| [7](./sips/fork_support.md)             | Fork Support                | open-for-discussion |
+| [8](./sips/pre_consensus_livness.md)    | Pre-Consensus livness fix   | open-for-discussion |
+| [11](./sips/consensus_justification.md) | Consensus justification     | open-for-discussion |

sips/consensus_justification.md

@@ -0,0 +1,87 @@
+|     Author     |          Title          | Category |       Status        |    Date    |
+| -------------- | ----------------------- | -------- | ------------------- | ---------- |
+| Matheus Franco | Consensus Justification | Core     | open-for-discussion | 2024-03-08 |
+
+## Summary
+
+Drop decided messages and include a consensus justification nested in post-consensus messages.
+
+## Motivation
+
+Decided messages represent $16$% of the exchanged messages in the network and also significantly increase the thresholds of allowed messages in message validation. Furthermore, it's not a necessary part of the protocol and the speed-up that it provides can be provided by a simpler justification in the post-consensus message.
+
+## Rationale
+
+As it's done with the pre-consensus justification, that is included in the *Proposal* message, we propose including a consensus justification (an aggregated commit message) in the post-consensus message instead of sending decided messages.
+
+First of all, the decided messages are useful to allow nodes to start the post-consensus phase if they are "behind" in the protocol execution due to not receiving messages. However, a consensus justification inside a post-consensus message allows a node to accomplish the same result while progressing even further due to the post-consensus signature. Moreover, this proposal aligns the design decisions of the (pre-consensus, consensus) and (consensus, post-consensus) inter-phases.
+
+## Improvements
+
+This proposal reduces the number of messages exchanged in the network by $16$%.
+
+## Spec change
+
+### Message structure
+
+**Adding a justification in post-consensus messages**
+
+Within a `SignedPartialSignatureMessage.PartialSignatureMessages`, all `PartialSignatureMessage` objects refer to the same `QBFT Instance`. Thus, we can simply add the justification to the `SignedPartialSignatureMessage` as follows:
+
+```go
+type SignedPartialSignatureMessage struct {
+	Message   PartialSignatureMessages
+	Signature Signature `ssz-size:"96"`
+	Signer    OperatorID
+    Justification *SignedMessage // Aggregated commit message (or []*SignedMessage if we ever use another asymmetric scheme that doesn't allow aggregation)
+}
+```
+
+It could even be added to the `PartialSignatureMessages` types. The difference is that this would *hold* the `SignedPartialSignatureMessage.Signature` also to the consensus justification.
+
+### New post-consensus logic
+
+To implement the justification logic, once a post-consensus message is received, if the consensus has not been decided yet, we ask the consensus module to process a justification in a similar way that a decided message is processed (except y the feature of holding a state and broadcasting new decided messages). If the processing is successful, we update the runner state and continue with the current post-consensus logic.
+
+```go
+func (b *BaseRunner) basePostConsensusMsgProcessing(runner Runner, signedMsg *types.SignedPartialSignatureMessage) (bool, [][32]byte, error) {
+
+	if !b.hasRunningDuty() {
+		// do nothing
+	}
+
+	// If has not decided yet, process the justification
+	if b.State.DecidedValue == nil && b.State != nil && b.State.RunningInstance != nil {
+		// Process
+		decidedMsg, err := b.QBFTController.ProcessConsensusJustification(signedMsg.Justification)
+		if err != nil {
+			// return error
+		}
+
+		// Decode
+		decidedValue = &types.ConsensusData{}
+		// ...
+
+		// Set state
+		b.highestDecidedSlot = decidedValue.Duty.Slot
+		b.State.DecidedValue = decidedValue
+	}
+	// Follow-up with the current post-consensus processing
+}
+```
+
+The justification may be large and there's no need for it in pre-consensus messages. Thus, we also recommend adding a check in the pre-consensus processing to assert that there's no consensus justification. This is useful as a part of a protection against buffer DoS attacks.
+
+```go
+func (b *BaseRunner) basePreConsensusMsgProcessing(runner Runner, signedMsg *types.SignedPartialSignatureMessage) (bool, [][32]byte, error) {
+	if signedMsg.Justification != nil {
+		// Return error
+	}
+}
+```
+
+
+## Drawbacks
+
+- The post-consensus messages would be bigger due to the aggregated commit message.
+- Exporters use decided messages to compute statistics of the network. Therefore, the proposal implies not only a protocol change but also changes in the exporter implementation.