This is a Bitcoin Relay. It uses 1 + 1/n slots per header relayed (n is currently 4), and 2 slots to externalize useful information (best chain tip and best shared ancestor of latest reorg).
At present, only a Solidity implementation is available, but we intend to add more implementations soon :)
How does it work?
The core idea behind the relay is to minimize storage costs by increasing
calldata costs. Rather than storing headers, the relay stores the
hashPrevBlockfield of each header and the height of every nth header. Should
the relay need to reference information in old headers (like the difficulty),
the header data is passed to the relay again, and validated against known
hashPrevBlock links. This allows the relay to check that newly submitted
blocks are valid extensions of existing blocks, without storing all past header
As opposed to other relays, we separate the function of the relay into two
categories: "learning about new blocks" and "following the best chain tip."
Users may add new blocks in groups of at least 5 by calling
addHeaders If the
block slice includes a difficulty retarget, users are required to call
addHeadersWithRetarget, which performs additional validation. The relay does
not update its tip unless it is specifically requested to do so by a user. The
user must call
markNewHeaviest with the new heaviest, the old heaviest
header, and the digest of their most recent common ancestor (which may be the
old heaviest header.
As part of the process, the relay externalizes the most recent common ancestor, which is to say, the heaviest header that both old and new heaviest tip confirm. This is a metric of "subjective finality" for that block. During normal operation without reorgs it lags behind the tip by 5 blocks. During reorgs, it is the shared base of the competing branches (and as such may move backwards!). This indicates that competing sets of miners both viewed it as subjectively finalized. As such, it is a reasonable source of finalization information for relay-consuming smart contracts.
This model provides large gas savings compared to previous relay designs (TODO: benchmarking). It also gets especially attractive if EIP2028 activates, reducing calldata gas costs.