Module checkpoint 

Certified checkpoints and state transfer (W5; docs/rhdl_ubft-checkpoints.md).

Problem. A replica that falls behind cannot catch up: ubft_new has no checkpoints (N18); upstream throws “state transfer not implemented” (U12), spins on a tail gap (U09), and accepts a checkpoint equal to its own without binding the certificate to the content (UE-C20).

Idea.

• At every C = 2^CS decided instances the application reports the digest a of its state; the replica signs CHECKPOINT(low, a) (role 5, share context with view 0, instance low >> CS, digest a) and sends the share to every replica. q verified shares certify exactly (low, a).
• A replica behind a checkpoint it never computed takes a peer’s offer (low, a) with its shares (forwarded into the same collector), and adopts it only once every share verifies here.
• Behind a certified checkpoint, it requests the state at low, hashes the K chunks as they arrive (a chain, one SHA-512 block each), writes them to the application’s shadow copy, and commits only if the digest equals a; otherwise it aborts and asks the next peer. Then consensus jumps to low.
• Every index, length and peer id from the network is range-checked before use; every wait has a deadline.

Modules

phase

Phases of the catch-up.

Structs

AppDigest

The application’s digest at a checkpoint.

Chunk

One state chunk from a peer.

CkConfig

Configuration.

CkDigest

A chain digest answer.

CkDigestReq

Chain digest request (a 128-byte block; the job adds the pad block).

CkIn

Inputs.

CkKey

Open a checkpoint-collector key (0, low >> CS, digest), or give it our own share.

CkOut

Outputs (holding registers).

CkSignReq

Sign request.

CkState

State.

Offer

A peer’s offer of a certified checkpoint (its shares follow on the collector’s share input).

ShadowCommit

Shadow commit or discard.

ShadowWrite

Shadow write.

StateReq

State request.

Constants

CHUNK

Words per state chunk (64 bytes).

ROLE_CHECKPOINT

Role byte of a checkpoint share.

STA_DOMAIN

"RUB2-STA" as a little-endian word.

Functions

ck_step

One cycle of checkpointing and catch-up. K chunks make a state; CS is log2 of the checkpoint interval.

state_block

One link of the state chain: "RUB2-STA" ‖ session ‖ low ‖ j ‖ h_j ‖ chunk_j (128 bytes, big-endian SHA-512 words).