Mark Friedenbach says "Bitcoin’s Block Size Can Be Increased Without Hard Fork".
Can someone ELI5 what are Forward Blocks?
From this week's Bitcoin Optech
One well-known method for soft forking an increase in the Bitcoin block size is extension blocks—a data structure that’s invisible to nodes that haven’t upgraded to the soft fork and so is not subject to their historic limits on block size. By itself, this is an undesirable method for increasing block size because preventing old nodes from seeing the transactions in the extension block also prevents them from being able to enforce any other consensus rules on those transactions—such as rules that prevent a malicious user from spending other users’ bitcoins or from creating more bitcoins than allowed by the 21 million bitcoin subsidy schedule.
However, one doesn’t need to increase block size to increase the amount of data that can be added to the block chain per minute—it’s also possible to increase capacity by increasing the frequency of blocks (reducing the average time between blocks). A method for gaming Bitcoin’s difficulty adjustment algorithm—called a time-warp attack—is well-known among experts and has been used successfully in demonstration attacks against Bitcoin’s testnet and real attacks against altcoins. (Note: although Bitcoin is technically vulnerable to this attack, it’d be a slow attack that would give the userbase a significant amount of time to respond.) By itself, increasing block frequency is also an undesirable method for increasing capacity because shorter block intervals increase the effectiveness of miners with large amounts of hashrate and so is likely to increase mining centralization.1
Perhaps disproving the saying that “two wrongs don’t make a right,” this talk describes a novel way of combining extension blocks and the time-warp attack to allow both upgraded nodes and old nodes to gain the same capacity increase and see all the same transactions for validation while simultaneously slightly reducing mining centralization risk. Upgraded nodes would validate one or more extension blocks (called “forward blocks”) that provide additional block space with a centralization-reducing 15 minute average interval, but the upgraded nodes would also restrict the time stamps in legacy blocks to ensure a permanent (but limited) time warp attack increased the frequency of legacy blocks enough to allow them to include the same transactions that previously appeared in the forward blocks.
Apparently, Mark Freidenbach refers to this work on Freicoin as the origins of ‘forward blocks’.