rich1.0npub14ca6vjq25eaema6zxke2g8hjc6ztkf5ej698f5fhyg98zqq85zsqu0m739https://nostr.ae/npub14ca6vjq25eaema6zxke2g8hjc6ztkf5ej698f5fhyg98zqq85zsqu0m739njumphttps://nostr.ae📅 Original date posted:2022-09-23 📝 Original message:Hi everyone, I'm writing to propose a very simple set of mempool/transaction relay policies intended to aid L2/contract protocols. I realized that the previously proposed Package Mempool Accept package RBF [1] had a few remaining problems after digging into the RBF logic more [2]. This additional set of policies solves them without requiring a huge RBF overhaul. I've written an implementation (and docs) for Bitcoin Core: https://github.com/bitcoin/bitcoin/pull/25038 (You may notice that this proposal incorporates feedback on the PR - thanks Suhas Daftuar, Gregory Sanders, Bastien Teinturier, Anthony Towns, and others.) If you are interested in using package RBF/relay to bump presigned transactions, I think you may be interested in reviewing this proposal. This should solve Rule 3 pinning and perhaps allow us to get rid of CPFP carve-out (yay!). I'm keen to hear if people find the 1-anchor-output, 1000vB child limit too restrictive. Also, if you find a pinning attack or something that makes it unusable for you, I would really really like to know. Note that transactions with nVersion=3 ("V3 transactions") are currently non-standard in Bitcoin Core. That means **anything that was standard before this policy change would still be standard afterwards.** If you don't want your transactions to be subject to these rules, just continue whatever you're doing and don't use nVersion=3. AFAICT this shouldn't break anything, but let me know if this would be disruptive for you? **New Policies:** This includes: - a set of additional policy rules applying to V3 transactions - modifications to package RBF rules **V3 transactions:** Existing standardness rules apply to V3 (e.g. min/max tx weight, standard output types, cleanstack, etc.). The following additional rules apply to V3: 1. A V3 transaction can be replaced, even if it does not signal BIP125 replaceability. (It must also meet the other RBF rules around fees, etc. for replacement to happen). 2. Any descendant of an unconfirmed V3 transaction must also be V3. *Rationale*: Combined with Rule 1, this gives us the property of "inherited" replaceability signaling when descendants of unconfirmed transactions are created. Additionally, checking whether a transaction signals replaceability this way does not require mempool traversal, and does not change based on what transactions are mined. It also makes subsequent rules about descendant limits much easier to check. *Note*: The descendant of a *confirmed* V3 transaction does not need to be V3. 3. An unconfirmed V3 transaction cannot have more than 1 descendant. *Rationale*: (Upper bound) the larger the descendant limit, the more transactions may need to be replaced. This is a problematic pinning attack, i.e., a malicious counterparty prevents the transaction from being replaced by adding many descendant transactions that aren't fee-bumping. (Lower bound) at least 1 descendant is required to allow CPFP of the presigned transaction. The contract protocol can create presigned transactions paying 0 fees and 1 output for attaching a CPFP at broadcast time ("anchor output"). Without package RBF, multiple anchor outputs would be required to allow each counterparty to fee-bump any presigned transaction. With package RBF, since the presigned transactions can replace each other, 1 anchor output is sufficient. 4. A V3 transaction that has an unconfirmed V3 ancestor cannot be larger than 1000 virtual bytes. *Rationale*: (Upper bound) the larger the descendant size limit, the more vbytes may need to be replaced. With default limits, if the child is e.g. 100,000vB, that might be an additional 100,000sats (at 1sat/vbyte) or more, depending on the feerate. (Lower bound) the smaller this limit, the fewer UTXOs a child may use to fund this fee-bump. For example, only allowing the V3 child to have 2 inputs would require L2 protocols to manage a wallet with high-value UTXOs and make batched fee-bumping impossible. However, as the fee-bumping child only needs to fund fees (as opposed to payments), just a few UTXOs should suffice. With a limit of 1000 virtual bytes, depending on the output types, the child can have 6-15 UTXOs, which should be enough to fund a fee-bump without requiring a carefully-managed UTXO pool. With 1000 virtual bytes as the descendant limit, the cost to replace a V3 transaction has much lower variance. *Rationale*: This makes the rule very easily "tacked on" to existing logic for policy and wallets. A transaction may be up to 100KvB on its own (`MAX_STANDARD_TX_WEIGHT`) and 101KvB with descendants (`DEFAULT_DESCENDANT_SIZE_LIMIT_KVB`). If an existing V3 transaction in the mempool is 100KvB, its descendant can only be 1000vB, even if the policy is 10KvB. **Package RBF modifications:** 1. The rule around unconfirmed inputs was originally "A package may include new unconfirmed inputs, but the ancestor feerate of the child must be at least as high as the ancestor feerates of every transaction being replaced." The package may still include new unconfirmed inputs. However, the new rule is modified to be "The minimum between package feerate and ancestor feerate of the child is not lower than the individual feerates of all directly conflicting transactions and the ancestor feerates of all original transactions." *Rationale*: We are attempting to ensure that the replacement transactions are not less incentive-compatible to mine. However, a package/transaction's ancestor feerate is not perfectly representative of its incentive compatibility; it may overestimate (some subset of the ancestors could be included by itself if it has other high-feerate descendants or are themselves higher feerate than this package/transaction). Instead, we use the minimum between the package feerate and ancestor feerate of the child as a more conservative value than what was proposed originally. 2. A new rule is added, requiring that all package transactions with mempool conflicts to be V3. This also means the "sponsoring" child transaction must be V3. *Note*: Combined with the V3 rules, this means the package must be a child-with-parents package. Since package validation is only attempted if the transactions do not pay sufficient fees to be accepted on their own, this effectively means that only V3 transactions can pay to replace their ancestors' conflicts, and only V3 transactions' replacements may be paid for by a descendant. *Rationale*: The fee-related rules are economically rational for ancestor packages, but not necessarily other types of packages. A child-with-parents package is a type of ancestor package. It may be fine to allow any ancestor package, but it's more difficult to account for all of the possibilities. For example, it gets much harder to see that we're applying the descendant limits correctly if the package has a gnarly, many-generation, non-tree shape. I'm also not sure if this policy is 100% incentive-compatible if the sponsor is not a direct descendant of the sponsee. Please see doc/policy/version3_transactions.md and doc/policy/packages.md in the PR for the full set of rules. **Intended usage for LN:** Commitment transactions should be V3 and have 1 anchor output. They can be signed with 0 fees (or 1sat/vbyte) once package relay is deployed on a significant portion of the network. If the commitment tx must be broadcast, determine the desired feerate at broadcast time and spend the anchor output in a high feerate transaction. I'm going to call the broadcasted commitment tx "the parent" and the attached fee-bumping tx "the child." - This child must be V3. - This child must be at most 1000vB. Note this restricts the number of inputs you can use to fund the fee bump. Depending on the output types, this is around 6-15. - One child may fund fees for multiple commitment tx ("batched fee-bumping"). - To do a second fee-bump to add more fees, replace the *child* with a higher-feerate tx. Do not try to attach a grandchild. Otherwise, never try to spend from an unconfirmed V3 transaction. The descendant limits for V3 transactions are very restrictive. **Expected Questions:** "Does this fix Rule 3 Pinning?" Yes. The V3 descendant limit restricts both you and your counterparty. Assuming nodes adopted this policy, you may reasonably assume that you only need to replace the commitment transaction + up to 1000vB. "Only 1 anchor output? What if I need to bump counterparty's commitment tx in mempool?" You won't need to fee-bump a counterparty's commitment tx using CPFP. You would just package RBF it by attaching a high-feerate child to your commitment tx. "Is this a privacy issue, i.e. doesn't it allow fingerprinting LN transactions based on nVersion?" Indeed it may be unrealistic to assume V3 transactions will be in widespread use outside of L2. IIUC, unilateral closes are already obvious LN transactions because of the HTLC inputs. For e.g. cooperative closes and opens, I think it makes sense to continue using V2. So, unless I'm missing something, this shouldn't make it worse. "So a V3 transaction that doesn't signal BIP125 replaceability is replaceable? Is that a backward compatibility issue?" Yes it's replaceable. It's not an issue AFAICT because, under previous policy, the V3 transaction wouldn't have been in the mempool in the first place. "Can a V2 transaction replace a V3 transaction and vice versa?" Yes, otherwise someone can use V3 transactions to censor V2 transactions spending shared inputs. Note if the original V3 transaction has an unconfirmed V3 parent, this would violate the "inherited V3" rule and would be rejected. Thanks for reading! Feedback and review would be much appreciated. [1]: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-September/019464.html [2]: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019817.html Best, Gloria -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20220923/9b52c0c4/attachment.html>