Event Serialization With Round, Event Descriptors, and Multiple Other Parents

[edward-swirldslabs]

This HIP will introduce multiple changes to an Event's serialized form.

1. Add the round number during which the event was created to the event meta data.

• Needed to enable dynamic address books.
• Storing the round number during which the event was created is necessary to validate the event's signature against the correct address book. Like the event's generation, the round number will also be used to determine when an event becomes ancient.

2. Store parent linkages as EventDescriptors.

• An EventDescriptor is a 4-tuple of (Hash, Creator NodeId, Generation, and Round). An EventDescriptor will be serialized per parent.
• This organizes the parent meta data into contiguous pieces.

3. Allow for multiple other parents

• Enables improvement to the consensus algorithm.
• Currently when a node creates an event there is 1 self-parent event and 1 parent from another node. This proposal changes only the serialization scheme and is necessary before we can proceed to experiment with improved algorithms. Events will continue to have 1 other parent in the new serialization scheme until algorithmic improvements have been fully implemented and tested.

4. Unhashed meta data is reduced to just the Event's signature.

• Items 1-3 above are all part of the content of the event that is hashed and signed.
• Since the other parent's node id will now be in an EventDescriptor which is being hashed, the only data remaining that is not hashed is the signature of the event.

The current implementation of this proposal is contained in the following PR: hashgraph/hedera-services#9344