Design at a Glance
The PoW+TFL consensus protocol is logically an extension of the Zcash consensus rules to introduce trailing finality. This is achieved by compartmentalizing the top-level PoW+TFL protocol into two consensus subprotocols, one embodying most of the current consensus logic of Zcash and another the TFL. These protocols interact through a hybrid construction, which specifies how the protocols interact, and what changes from "off-the-shelf" behavior, if any, need to be imposed on the subprotocols. Each of these components (the two subprotocols and the hybrid construction) are somewhat modular: different subprotocols or hybrid constructions may be combined (with some modification) to produce a candidate PoW+TFL protocol.
TODO: Add a protocol component diagram to "Design at a Glance" #122
Hybrid Construction
The hybrid construction is a major design component of the full consensus protocol which specifies how the subprotocols integrate. So far we have considered three candidates:
- The implied/loosely defined hybrid construction presented at Zcon4.
- The Snap-and-Chat from the Ebb-and-Flow paper.
- The Crosslink construction.
Currently we believe Crosslink is the best candidate, due to security considerations.
Subprotocols
The PoW+TFL hybrid consensus consists of two interacting subprotocols:
- PoW Subprotocol: this subprotocol is very similar to NU5 consensus. It is a design goal of the TFL design to minimize changes to this subprotocol. Note: the shorthand "PoW" is potentially misleading, because this subprotocol is also responsible for the bulk of all supply and transaction semantic consensus rules.
- PoS Subprotocol: this is a new subprotocol which provides trailing finality via a finalizing PoS protocol.
Note that the hybrid construction may require modification to the "off-the-shelf" versions of these subprotocols. In particular Crosslink requires each protocol to refer to the state of the other to provide objective validity.