[bitcoin-dev] Extension block proposal by Jeffrey et al

Recently there has been some discussion of an apparent work-in-progress extension block proposal by Christopher Jeffrey, Joseph Poon, Fedor Indutny, and Steven Pair. Since this hasn't been formally posted on the ML yet, perhaps it is still in pre-draft stages and not quite ready for review, but in light of public interest, I think it is appropriate to open it to discussion, and toward this end, I have reviewed the current revision. For reference, the WIP proposal itself is here: https://github.com/tothemoon-org/extension-blocks ==Overall analysis & comparison== This is a relatively complicated proposal, creating a lot of additional technical debt and complexity in comparison to both BIP 141 and hardforks. It offers no actual benefits beyond BIP 141 or hardforks, so seems irrational to consider at face value. In fact, it fits much better the inaccurate criticisms made by segwit detractors against BIP 141. That being said, this proposal is very interesting in construction and is for the most part technically sound. While ill-fit to merely making blocks larger, it may be an ideal fit for fundamentally different block designs such as Rootstock and MimbleWimble in absence of decentralised non-integrated sidechains (extension blocks are fundamentally sidechains tied into Bitcoin directly). ==Fundamental problem== Extension blocks are a risk of creating two classes of "full nodes": those which verify the full block (and are therefore truly full nodes), and those which only verify the "base" block. However, because the extension is consensus-critical, the latter are in fact not full nodes at all, and are left insecure like pseudo-SPV (not even real SPV) nodes. This technical nature is of course true of a softfork as well, but softforks are intentionally designed such that all nodes are capable of trivially upgrading, and there is no expectation for anyone to run with pre-softfork rules. In general, hardforks can provide the same benefits of an extension block, but without the false expectation and pointless complexity. ==Other problems & questions== > These outpoints may not be spent inside the mempool (they must be redeemed from the next resolution txid in reality). This breaks the ability to spend unconfirmed funds in the same block (as is required for CPFP). The extension block's transaction count is not cryptographically committed-to anywhere. (This is an outstanding bug in Bitcoin today, but impractical to exploit in practice; however, exploiting it in an extension block may not be as impractical, and it should be fixed given the opportunity.) > The merkle root is to be calculated as a merkle tree with all extension block txids and wtxids as the leaves. This needs to elaborate how the merkle tree is constructed. Are all the txids followed by all the wtxids (tx hashes)? Are they alternated? Are txid and wtxid trees built independently and merged at the tip? > Output script code aside from witness programs, p2pkh or p2sh is considered invalid in extension blocks. Why? This prevents extblock users from sending to bare multisig or other various possible destinations. (While static address forms do not exist for other types, they can all be used by the payment protocol.) Additionally, this forbids datacarrier (OP_RETURN), and forces spam to create unprovably-unspendable UTXOs. Is that intentional? > The maximum extension size should be intentionally high. This has the same "attacks can do more damage than ordinary benefit" issue as BIP141, but even more extreme since it is planned to be used for future size increases. > Witness key hash v0 shall be worth 1 point, multiplied by a factor of 8. What is a "point"? What does it mean multiplied by a factor of 8? Why not just say "8 points"? > Witness script hash v0 shall be worth the number of accurately counted sigops in the redeem script, multiplied by a factor of 8. Please define "accurately counted" here. Is this using BIP16 static counting, or accurately counting sigops during execution? > To reduce the chance of having redeem scripts which simply allow for garbage data in the witness vector, every 73 bytes in the serialized witness vector is worth 1 additional point. Is the size rounded up or down? If down, 72-byte scripts will carry 0 points...) ==Trivial & process== BIPs must be in MediaWiki format, not Markdown. They should be submitted for discussion to the bitcoin-dev mailing list, not social media and news. > Layer: Consensus (soft-fork) Extension blocks are more of a hard-fork IMO. > License: Public Domain BIPs may not be "public domain" due to non-recognition in some jurisdictions. Can you agree on one or more of these? https://github.com/bitcoin/bips/blob/master/bip-0002.mediawiki#Recommended_licenses > ## Abstract > > This specification defines a method of increasing bitcoin transaction throughput without altering any existing consensus rules. This is inaccurate. Even softforks alter consensus rules. > ## Motivation > > Bitcoin retargetting ensures that the time in between mined blocks will be roughly 10 minutes. It is not possible to change this rule. There has been great debate regarding other ways of increasing transaction throughput, with no proposed consensus-layer solutions that have proven themselves to be particularly safe. Block time seems entirely unrelated to this spec. Motivation is unclear. > Extension blocks leverage several features of BIP141, BIP143, and BIP144 for transaction opt-in, serialization, verification, and network services, and as such, extension block activation entails BIP141 activation. As stated in the next paragraph, the rules in BIP 141 are fundamentally incompatible with this one, so saying BIP 141 is activated is confusingly incorrect. > This specification should be considered an extension and modification to these BIPs. Extension blocks are _not_ compatible with BIP141 in its current form, and will require a few minor additional rules. Extension blocks should be compatible with BIP 141, there doesn’t appear to be any justification for not making them compatible. > This specification prescribes a way of fooling non-upgraded nodes into believing the existing UTXO set is still behaving as they would expect. The UTXO set behaves fundamentally different to old nodes with this proposal, albeit in a mostly compatible manner. > Note that canonical blocks containing entering outputs MUST contain an extension block commitment (all zeroes if nothing is present in the extension block). Please explain why in Rationale. > Coinbase outputs MUST NOT contain witness programs, as they cannot be sweeped by the resolution transaction due to previously existing consensus rules. Seems like an annoying technical debt. I wonder if it can be avoided. > The genesis resolution transaction MAY also include a 1-100 byte pushdata in the first input script, allowing the miner of the genesis resolution to add a special message. The pushdata MUST be castable to a true boolean. Why? Unlike the coinbase, this seems to create additional technical debt with no apparent purpose. Better to just have a consensus rule every input must be null. > The resolution transaction's version MUST be set to the uint32 max (`2^32 - 1`). Transaction versions are signed, so I assume this is actually simply -1. (While signed transaction versions seemed silly to me, using it for special cases like this actually makes sense.) > ### Exiting the extension block Should specify that spending such an exit must use the resolution txid, not the extblock's txid. > On the policy layer, transaction fees may be calculated by transaction cost as well as additional size/legacy-sigops added to the canonical block due to entering or exiting outputs. BIPs should not specify policy at all. Perhaps prefix "For the avoidance of doubt:" to be clear that miners may perform any fee logic they like. > Transactions within the extended transaction vector MAY include a witness vector using BIP141 transaction serialization. Since extblock transactions are all required to be segwit, why wouldn't this be mandatory? > - BIP141's nested P2SH feature is no longer available, and no longer a consensus rule. Note this makes adoption slower: wallets cannot use the extblock until the economy has updated to support segwit-native addresses. > To reduce the chance of having redeem scripts which simply allow for garbage data in the witness vector, every 73 bytes in the serialized witness vector is worth 1 additional point. Please explain why 73 bytes in Rationale. > This leaves room for 7 future soft-fork upgrades to relax DoS limits. How so? Please explain. > A consensus dust threshold is now enforced within the extension block. Why? > If the second highest transaction version bit (30th bit) is set to to `1` within an extension block transaction, an extra 700-bytes is reserved on the transaction space used up in the block. Why wouldn't users set this on all transactions? > `default_witness_commitment` has been renamed to `default_extension_commitment` and includes the extension block commitment script. `default_witness_commitment` was never part of the GBT spec. At least describe what this new key is. > - Deployment name: `extblk` (appears as `!extblk` in GBT). Should be just `extblk` if backward compatibility is supported (and `!extblk` when not). > The "deactivation" deployment's start time... What about timeout? None? To continue the extension block, must it be deactivated and reactivated in parallel?