From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp2.osuosl.org (smtp2.osuosl.org [140.211.166.133]) by lists.linuxfoundation.org (Postfix) with ESMTP id CCB4BC000F for ; Mon, 20 Sep 2021 09:19:53 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp2.osuosl.org (Postfix) with ESMTP id B18C940179 for ; Mon, 20 Sep 2021 09:19:53 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -1.899 X-Spam-Level: X-Spam-Status: No, score=-1.899 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no Authentication-Results: smtp2.osuosl.org (amavisd-new); dkim=pass (2048-bit key) header.d=acinq-fr.20210112.gappssmtp.com Received: from smtp2.osuosl.org ([127.0.0.1]) by localhost (smtp2.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 0FJRyz2Y9PhV for ; Mon, 20 Sep 2021 09:19:51 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.8.0 Received: from mail-qk1-x734.google.com (mail-qk1-x734.google.com [IPv6:2607:f8b0:4864:20::734]) by smtp2.osuosl.org (Postfix) with ESMTPS id B5DC740171 for ; Mon, 20 Sep 2021 09:19:50 +0000 (UTC) Received: by mail-qk1-x734.google.com with SMTP id ay33so40193055qkb.10 for ; Mon, 20 Sep 2021 02:19:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=acinq-fr.20210112.gappssmtp.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to; bh=/dIimZdZIL83Y7+wsdFawEajF37NsdombJQKe7L9tbo=; b=ZzDLC6bYaISgttqdn9uXro2W0mMHvw3R5CcziqhPzB6WCsoHeYCYHmYYiM6B2bGRCD Cu+Kz/LcTH7CoCp5FjLpFgheKjLNUxRDYTfWD0rrG8RrP16bSU3MWViypJgW20Uw/OEp hc7rJ2HjeTp81j6JtNVRe5wjOwfqBdhHzdTSp7NQP4W6kOemAE1ZmHEKmVAmoQzhkQjv lFr9q1LUPFePyZc6K2lDqeU/tfhxsg6w9s7y2IAXnlnalRmmwJGJ6saCruPd0uD8Awsv CHMaR/f9EWRuKF1vUTdvZ5g28naK1cod86OjGmkyz17m00FkHC3tUkrzCB1NDiqMeodi uPcQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to; bh=/dIimZdZIL83Y7+wsdFawEajF37NsdombJQKe7L9tbo=; b=vkQaxbxGYXLO5KdKIgsYup+4GFAc4JrVccfMPtGekGiyjsYQWwemByfRzgxKLXM+uk ItCvZjVsJ1/yvpx7pdUat6nQ8a+RVtJ1WpZLDNgTHDDHeN3+3ztbpaX/9N9U5gaVKqbs 7qOPFA25CyPbzLYFyyCtxdykPKimCNI3b2YiYwtUB+ywN4ayWpc7h1NRHKRoomzCTkOX ljmJTJA4rH4FiexEaKzZiCW3jkquGcspekr4lPuL2mDsliGGMPIFP332XfW+RukFccaI YnRtEBbi6glONACgOlqeWnr39Gqz5JDAQ/1Lx2NYCKa4y/8gfrFXoi5ssJ4kwnNyGRIm 8Dkg== X-Gm-Message-State: AOAM5326y+A3rb4AmBdpZU1kBenjrUQ2LddnZgp8hrCYuiYQgadV5/aA OS6PS9QTPuPkKf+yxhQKj5ydXRi4Q2sOz/MMf9HXLdfOrP/pfw== X-Google-Smtp-Source: ABdhPJyqmNkSxa23tJFcwaAYFiqr2qunj5qXLYFkZgDotjLLsthryW8ZiK9G/x94FYqvq/l/8EpUl8rKaPrvvTA8+mk= X-Received: by 2002:a25:5956:: with SMTP id n83mr29434069ybb.194.1632129589351; Mon, 20 Sep 2021 02:19:49 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Bastien TEINTURIER Date: Mon, 20 Sep 2021 11:19:38 +0200 Message-ID: To: Gloria Zhao , Bitcoin Protocol Discussion Content-Type: multipart/alternative; boundary="00000000000066a92105cc69c870" X-Mailman-Approved-At: Mon, 20 Sep 2021 09:47:57 +0000 Subject: Re: [bitcoin-dev] Proposal: Package Mempool Accept and Package RBF X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 20 Sep 2021 09:19:53 -0000 --00000000000066a92105cc69c870 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi Gloria, Thanks for this detailed post! The illustrations you provided are very useful for this kind of graph topology problems. The rules you lay out for package RBF look good to me at first glance as there are some subtle improvements compared to BIP 125. > 1. A package cannot exceed `MAX_PACKAGE_COUNT=3D25` count and > `MAX_PACKAGE_SIZE=3D101KvB` total size [8] I have a question regarding this rule, as your example 2C could be concerning for LN (unless I didn't understand it correctly). This also touches on the package RBF rule 5 ("The package cannot replace more than 100 mempool transactions.") In your example we have a parent transaction A already in the mempool and an unrelated child B. We submit a package C + D where C spends another of A's inputs. You're highlighting that this package may be rejected because of the unrelated transaction(s) B. The way I see this, an attacker can abuse this rule to ensure transaction A stays pinned in the mempool without confirming by broadcasting a set of child transactions that reach these limits and pay low fees (where A would be a commit tx in LN). We had to create the CPFP carve-out rule explicitly to work around this limitation, and I think it would be necessary for package RBF as well, because in such cases we do want to be able to submit a package A + C where C pays high fees to speed up A's confirmation, regardless of unrelated unconfirmed children of A... We could submit only C to benefit from the existing CPFP carve-out rule, but that wouldn't work if our local mempool doesn't have A yet, but other remote mempools do. Is my concern justified? Is this something that we should dig into a bit deeper? Thanks, Bastien Le jeu. 16 sept. 2021 =C3=A0 09:55, Gloria Zhao via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit : > Hi there, > > I'm writing to propose a set of mempool policy changes to enable package > validation (in preparation for package relay) in Bitcoin Core. These woul= d > not > be consensus or P2P protocol changes. However, since mempool policy > significantly affects transaction propagation, I believe this is relevant > for > the mailing list. > > My proposal enables packages consisting of multiple parents and 1 child. > If you > develop software that relies on specific transaction relay assumptions > and/or > are interested in using package relay in the future, I'm very interested > to hear > your feedback on the utility or restrictiveness of these package policies > for > your use cases. > > A draft implementation of this proposal can be found in [Bitcoin Core > PR#22290][1]. > > An illustrated version of this post can be found at > https://gist.github.com/glozow/dc4e9d5c5b14ade7cdfac40f43adb18a. > I have also linked the images below. > > ## Background > > Feel free to skip this section if you are already familiar with mempool > policy > and package relay terminology. > > ### Terminology Clarifications > > * Package =3D an ordered list of related transactions, representable by a > Directed > Acyclic Graph. > * Package Feerate =3D the total modified fees divided by the total virtua= l > size of > all transactions in the package. > - Modified fees =3D a transaction's base fees + fee delta applied by = the > user > with `prioritisetransaction`. As such, we expect this to vary acros= s > mempools. > - Virtual Size =3D the maximum of virtual sizes calculated using [BIP= 141 > virtual size][2] and sigop weight. [Implemented here in Bitcoin > Core][3]. > - Note that feerate is not necessarily based on the base fees and > serialized > size. > > * Fee-Bumping =3D user/wallet actions that take advantage of miner > incentives to > boost a transaction's candidacy for inclusion in a block, including > Child Pays > for Parent (CPFP) and [BIP125][12] Replace-by-Fee (RBF). Our intention in > mempool policy is to recognize when the new transaction is more economica= l > to > mine than the original one(s) but not open DoS vectors, so there are some > limitations. > > ### Policy > > The purpose of the mempool is to store the best (to be most > incentive-compatible > with miners, highest feerate) candidates for inclusion in a block. Miners > use > the mempool to build block templates. The mempool is also useful as a > cache for > boosting block relay and validation performance, aiding transaction relay= , > and > generating feerate estimations. > > Ideally, all consensus-valid transactions paying reasonable fees should > make it > to miners through normal transaction relay, without any special > connectivity or > relationships with miners. On the other hand, nodes do not have unlimited > resources, and a P2P network designed to let any honest node broadcast > their > transactions also exposes the transaction validation engine to DoS attack= s > from > malicious peers. > > As such, for unconfirmed transactions we are considering for our mempool, > we > apply a set of validation rules in addition to consensus, primarily to > protect > us from resource exhaustion and aid our efforts to keep the highest fee > transactions. We call this mempool _policy_: a set of (configurable, > node-specific) rules that transactions must abide by in order to be > accepted > into our mempool. Transaction "Standardness" rules and mempool > restrictions such > as "too-long-mempool-chain" are both examples of policy. > > ### Package Relay and Package Mempool Accept > > In transaction relay, we currently consider transactions one at a time fo= r > submission to the mempool. This creates a limitation in the node's abilit= y > to > determine which transactions have the highest feerates, since we cannot > take > into account descendants (i.e. cannot use CPFP) until all the transaction= s > are > in the mempool. Similarly, we cannot use a transaction's descendants when > considering it for RBF. When an individual transaction does not meet the > mempool > minimum feerate and the user isn't able to create a replacement transacti= on > directly, it will not be accepted by mempools. > > This limitation presents a security issue for applications and users > relying on > time-sensitive transactions. For example, Lightning and other protocols > create > UTXOs with multiple spending paths, where one counterparty's spending pat= h > opens > up after a timelock, and users are protected from cheating scenarios as > long as > they redeem on-chain in time. A key security assumption is that all > parties' > transactions will propagate and confirm in a timely manner. This > assumption can > be broken if fee-bumping does not work as intended. > > The end goal for Package Relay is to consider multiple transactions at th= e > same > time, e.g. a transaction with its high-fee child. This may help us better > determine whether transactions should be accepted to our mempool, > especially if > they don't meet fee requirements individually or are better RBF candidate= s > as a > package. A combination of changes to mempool validation logic, policy, an= d > transaction relay allows us to better propagate the transactions with the > highest package feerates to miners, and makes fee-bumping tools more > powerful > for users. > > The "relay" part of Package Relay suggests P2P messaging changes, but a > large > part of the changes are in the mempool's package validation logic. We cal= l > this > *Package Mempool Accept*. > > ### Previous Work > > * Given that mempool validation is DoS-sensitive and complex, it would be > dangerous to haphazardly tack on package validation logic. Many efforts > have > been made to make mempool validation less opaque (see [#16400][4], > [#21062][5], > [#22675][6], [#22796][7]). > * [#20833][8] Added basic capabilities for package validation, test > accepts only > (no submission to mempool). > * [#21800][9] Implemented package ancestor/descendant limit checks for > arbitrary > packages. Still test accepts only. > * Previous package relay proposals (see [#16401][10], [#19621][11]). > > ### Existing Package Rules > > These are in master as introduced in [#20833][8] and [#21800][9]. I'll > consider > them as "given" in the rest of this document, though they can be changed, > since > package validation is test-accept only right now. > > 1. A package cannot exceed `MAX_PACKAGE_COUNT=3D25` count and > `MAX_PACKAGE_SIZE=3D101KvB` total size [8] > > *Rationale*: This is already enforced as mempool ancestor/descendant > limits. > Presumably, transactions in a package are all related, so exceeding this > limit > would mean that the package can either be split up or it wouldn't pass th= is > mempool policy. > > 2. Packages must be topologically sorted: if any dependencies exist betwe= en > transactions, parents must appear somewhere before children. [8] > > 3. A package cannot have conflicting transactions, i.e. none of them can > spend > the same inputs. This also means there cannot be duplicate transactions. > [8] > > 4. When packages are evaluated against ancestor/descendant limits in a te= st > accept, the union of all of their descendants and ancestors is considered= . > This > is essentially a "worst case" heuristic where every transaction in the > package > is treated as each other's ancestor and descendant. [8] > Packages for which ancestor/descendant limits are accurately captured by > this > heuristic: [19] > > There are also limitations such as the fact that CPFP carve out is not > applied > to package transactions. #20833 also disables RBF in package validation; > this > proposal overrides that to allow packages to use RBF. > > ## Proposed Changes > > The next step in the Package Mempool Accept project is to implement > submission > to mempool, initially through RPC only. This allows us to test the > submission > logic before exposing it on P2P. > > ### Summary > > - Packages may contain already-in-mempool transactions. > - Packages are 2 generations, Multi-Parent-1-Child. > - Fee-related checks use the package feerate. This means that wallets can > create a package that utilizes CPFP. > - Parents are allowed to RBF mempool transactions with a set of rules > similar > to BIP125. This enables a combination of CPFP and RBF, where a > transaction's descendant fees pay for replacing mempool conflicts. > > There is a draft implementation in [#22290][1]. It is WIP, but feedback i= s > always welcome. > > ### Details > > #### Packages May Contain Already-in-Mempool Transactions > > A package may contain transactions that are already in the mempool. We > remove > ("deduplicate") those transactions from the package for the purposes of > package > mempool acceptance. If a package is empty after deduplication, we do > nothing. > > *Rationale*: Mempools vary across the network. It's possible for a parent > to be > accepted to the mempool of a peer on its own due to differences in policy > and > fee market fluctuations. We should not reject or penalize the entire > package for > an individual transaction as that could be a censorship vector. > > #### Packages Are Multi-Parent-1-Child > > Only packages of a specific topology are permitted. Namely, a package is > exactly > 1 child with all of its unconfirmed parents. After deduplication, the > package > may be exactly the same, empty, 1 child, 1 child with just some of its > unconfirmed parents, etc. Note that it's possible for the parents to be > indirect > descendants/ancestors of one another, or for parent and child to share a > parent, > so we cannot make any other topology assumptions. > > *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple paren= ts > makes it possible to fee-bump a batch of transactions. Restricting > packages to a > defined topology is also easier to reason about and simplifies the > validation > logic greatly. Multi-parent-1-child allows us to think of the package as > one big > transaction, where: > > - Inputs =3D all the inputs of parents + inputs of the child that come fr= om > confirmed UTXOs > - Outputs =3D all the outputs of the child + all outputs of the parents t= hat > aren't spent by other transactions in the package > > Examples of packages that follow this rule (variations of example A show > some > possibilities after deduplication): ![image][15] > > #### Fee-Related Checks Use Package Feerate > > Package Feerate =3D the total modified fees divided by the total virtual > size of > all transactions in the package. > > To meet the two feerate requirements of a mempool, i.e., the pre-configur= ed > minimum relay feerate (`minRelayTxFee`) and dynamic mempool minimum > feerate, the > total package feerate is used instead of the individual feerate. The > individual > transactions are allowed to be below feerate requirements if the package > meets > the feerate requirements. For example, the parent(s) in the package can > have 0 > fees but be paid for by the child. > > *Rationale*: This can be thought of as "CPFP within a package," solving t= he > issue of a parent not meeting minimum fees on its own. This allows L2 > applications to adjust their fees at broadcast time instead of > overshooting or > risking getting stuck/pinned. > > We use the package feerate of the package *after deduplication*. > > *Rationale*: It would be incorrect to use the fees of transactions that > are > already in the mempool, as we do not want a transaction's fees to be > double-counted for both its individual RBF and package RBF. > > Examples F and G [14] show the same package, but P1 is submitted > individually before > the package in example G. In example F, we can see that the 300vB package > pays > an additional 200sat in fees, which is not enough to pay for its own > bandwidth > (BIP125#4). In example G, we can see that P1 pays enough to replace M1, b= ut > using P1's fees again during package submission would make it look like a > 300sat > increase for a 200vB package. Even including its fees and size would not = be > sufficient in this example, since the 300sat looks like enough for the > 300vB > package. The calculcation after deduplication is 100sat increase for a > package > of size 200vB, which correctly fails BIP125#4. Assume all transactions > have a > size of 100vB. > > #### Package RBF > > If a package meets feerate requirements as a package, the parents in the > transaction are allowed to replace-by-fee mempool transactions. The child > cannot > replace mempool transactions. Multiple transactions can replace the same > transaction, but in order to be valid, none of the transactions can try t= o > replace an ancestor of another transaction in the same package (which > would thus > make its inputs unavailable). > > *Rationale*: Even if we are using package feerate, a package will not > propagate > as intended if RBF still requires each individual transaction to meet the > feerate requirements. > > We use a set of rules slightly modified from BIP125 as follows: > > ##### Signaling (Rule #1) > > All mempool transactions to be replaced must signal replaceability. > > *Rationale*: Package RBF signaling logic should be the same for package > RBF and > single transaction acceptance. This would be updated if single transactio= n > validation moves to full RBF. > > ##### New Unconfirmed Inputs (Rule #2) > > 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. This is contrary to BIP125#2, which states "The replaceme= nt > transaction may only include an unconfirmed input if that input was > included in > one of the original transactions. (An unconfirmed input spends an output > from a > currently-unconfirmed transaction.)" > > *Rationale*: The purpose of BIP125#2 is to ensure that the replacement > transaction has a higher ancestor score than the original transaction(s) > (see > [comment][13]). Example H [16] shows how adding a new unconfirmed input > can lower the > ancestor score of the replacement transaction. P1 is trying to replace M1= , > and > spends an unconfirmed output of M2. P1 pays 800sat, M1 pays 600sat, and M= 2 > pays > 100sat. Assume all transactions have a size of 100vB. While, in isolation= , > P1 > looks like a better mining candidate than M1, it must be mined with M2, s= o > its > ancestor feerate is actually 4.5sat/vB. This is lower than M1's ancestor > feerate, which is 6sat/vB. > > In package RBF, the rule analogous to BIP125#2 would be "none of the > transactions in the package can spend new unconfirmed inputs." Example J > [17] shows > why, if any of the package transactions have ancestors, package feerate i= s > no > longer accurate. Even though M2 and M3 are not ancestors of P1 (which is > the > replacement transaction in an RBF), we're actually interested in the enti= re > package. A miner should mine M1 which is 5sat/vB instead of M2, M3, P1, > P2, and > P3, which is only 4sat/vB. The Package RBF rule cannot be loosened to onl= y > allow > the child to have new unconfirmed inputs, either, because it can still > cause us > to overestimate the package's ancestor score. > > However, enforcing a rule analogous to BIP125#2 would not only make > Package RBF > less useful, but would also break Package RBF for packages with parents > already > in the mempool: if a package parent has already been submitted, it would > look > like the child is spending a "new" unconfirmed input. In example K [18], > we're > looking to replace M1 with the entire package including P1, P2, and P3. W= e > must > consider the case where one of the parents is already in the mempool (in > this > case, P2), which means we must allow P3 to have new unconfirmed inputs. > However, > M2 lowers the ancestor score of P3 to 4.3sat/vB, so we should not replace > M1 > with this package. > > Thus, the package RBF rule regarding new unconfirmed inputs is less stric= t > than > BIP125#2. However, we still achieve the same goal of requiring the > replacement > transactions to have a ancestor score at least as high as the original > ones. As > a result, the entire package is required to be a higher feerate mining > candidate > than each of the replaced transactions. > > Another note: the [comment][13] above the BIP125#2 code in the original R= BF > implementation suggests that the rule was intended to be temporary. > > ##### Absolute Fee (Rule #3) > > The package must increase the absolute fee of the mempool, i.e. the total > fees > of the package must be higher than the absolute fees of the mempool > transactions > it replaces. Combined with the CPFP rule above, this differs from BIP125 > Rule #3 > - an individual transaction in the package may have lower fees than the > transaction(s) it is replacing. In fact, it may have 0 fees, and the > child > pays for RBF. > > ##### Feerate (Rule #4) > > The package must pay for its own bandwidth; the package feerate must be > higher > than the replaced transactions by at least minimum relay feerate > (`incrementalRelayFee`). Combined with the CPFP rule above, this differs > from > BIP125 Rule #4 - an individual transaction in the package can have a lowe= r > feerate than the transaction(s) it is replacing. In fact, it may have 0 > fees, > and the child pays for RBF. > > ##### Total Number of Replaced Transactions (Rule #5) > > The package cannot replace more than 100 mempool transactions. This is > identical > to BIP125 Rule #5. > > ### Expected FAQs > > 1. Is it possible for only some of the package to make it into the mempoo= l? > > Yes, it is. However, since we evict transactions from the mempool by > descendant score and the package child is supposed to be sponsoring the > fees of > its parents, the most common scenario would be all-or-nothing. This is > incentive-compatible. In fact, to be conservative, package validation > should > begin by trying to submit all of the transactions individually, and only > use the > package mempool acceptance logic if the parents fail due to low feerate. > > 2. Should we allow packages to contain already-confirmed transactions? > > No, for practical reasons. In mempool validation, we actually aren't > able to > tell with 100% confidence if we are looking at a transaction that has > already > confirmed, because we look up inputs using a UTXO set. If we have > historical > block data, it's possible to look for it, but this is inefficient, not > always > possible for pruning nodes, and unnecessary because we're not going to do > anything with the transaction anyway. As such, we already have the > expectation > that transaction relay is somewhat "stateful" i.e. nobody should be > relaying > transactions that have already been confirmed. Similarly, we shouldn't be > relaying packages that contain already-confirmed transactions. > > [1]: https://github.com/bitcoin/bitcoin/pull/22290 > [2]: > https://github.com/bitcoin/bips/blob/1f0b563738199ca60d32b4ba779797fc97d0= 40fe/bip-0141.mediawiki#transaction-size-calculations > [3]: > https://github.com/bitcoin/bitcoin/blob/94f83534e4b771944af7d9ed0f40746f3= 92eb75e/src/policy/policy.cpp#L282 > [4]: https://github.com/bitcoin/bitcoin/pull/16400 > [5]: https://github.com/bitcoin/bitcoin/pull/21062 > [6]: https://github.com/bitcoin/bitcoin/pull/22675 > [7]: https://github.com/bitcoin/bitcoin/pull/22796 > [8]: https://github.com/bitcoin/bitcoin/pull/20833 > [9]: https://github.com/bitcoin/bitcoin/pull/21800 > [10]: https://github.com/bitcoin/bitcoin/pull/16401 > [11]: https://github.com/bitcoin/bitcoin/pull/19621 > [12]: https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki > [13]: > https://github.com/bitcoin/bitcoin/pull/6871/files#diff-34d21af3c614ea3ce= e120df276c9c4ae95053830d7f1d3deaf009a4625409ad2R1101-R1104 > [14]: > https://user-images.githubusercontent.com/25183001/133567078-075a971c-061= 9-4339-9168-b41fd2b90c28.png > [15]: > https://user-images.githubusercontent.com/25183001/132856734-fc17da75-f87= 5-44bb-b954-cb7a1725cc0d.png > [16]: > https://user-images.githubusercontent.com/25183001/133567347-a3e2e4a8-ae9= c-49f8-abb9-81e8e0aba224.png > [17]: > https://user-images.githubusercontent.com/25183001/133567370-21566d0e-36c= 8-4831-b1a8-706634540af3.png > [18]: > https://user-images.githubusercontent.com/25183001/133567444-bfff1142-439= f-4547-800a-2ba2b0242bcb.png > [19]: > https://user-images.githubusercontent.com/25183001/133456219-0bb447cb-dcb= 4-4a31-b9c1-7d86205b68bc.png > [20]: > https://user-images.githubusercontent.com/25183001/132857787-7b7c6f56-af9= 6-44c8-8d78-983719888c19.png > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > --00000000000066a92105cc69c870 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi Gloria,

Thanks for this detailed post!

Th= e illustrations you provided are very useful for this kind of graph
topo= logy problems.

The rules you lay out for package RBF look good to me= at first glance
as there are some subtle improvements compared to BIP 1= 25.

> 1. A package cannot exceed `MAX_PACKAGE_COUNT=3D25` count a= nd
> `MAX_PACKAGE_SIZE=3D101KvB` total size [8]

I have a quest= ion regarding this rule, as your example 2C could be
concerning for LN (= unless I didn't understand it correctly).

This also touches on t= he package RBF rule 5 ("The package cannot
replace more than 100 me= mpool transactions.")

In your example we have a parent transact= ion A already in the mempool
and an unrelated child B. We submit a packa= ge C + D where C spends
another of A's inputs. You're highlighti= ng that this package may be
rejected because of the unrelated transactio= n(s) B.

The way I see this, an attacker can abuse this rule to ensur= e
transaction A stays pinned in the mempool without confirming by
bro= adcasting a set of child transactions that reach these limits
and pay lo= w fees (where A would be a commit tx in LN).

We had to create the CP= FP carve-out rule explicitly to work around
this limitation, and I think= it would be necessary for package RBF
as well, because in such cases we= do want to be able to submit a
package A + C where C pays high fees to = speed up A's confirmation,
regardless of unrelated unconfirmed child= ren of A...

We could submit only C to benefit from the existing CPFP= carve-out
rule, but that wouldn't work if our local mempool doesn&#= 39;t have A yet,
but other remote mempools do.

Is my concern just= ified? Is this something that we should dig into a
bit deeper?

Th= anks,
Bastien

Le=C2=A0jeu. 16 sept. 2021 =C3=A0=C2=A009:55, Gloria Zhao v= ia bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit=C2=A0:
Hi ther= e,

I'm writing to propose a set of mempool policy changes to ena= ble package
validation (in preparation for package relay) in Bitcoin Cor= e. These would not
be consensus or P2P protocol changes. However, since = mempool policy
significantly affects transaction propagation, I believe = this is relevant for
the mailing list.

My proposal enables packag= es consisting of multiple parents and 1 child. If you
develop software t= hat relies on specific transaction relay assumptions and/or
are interest= ed in using package relay in the future, I'm very interested to hearyour feedback on the utility or restrictiveness of these package policies = for
your use cases.

A draft implementation of this proposal can b= e found in [Bitcoin Core
PR#22290][1].

An illustrated version of = this post can be found at
I have also linked= the images below.

## Background

Feel free to skip this sec= tion if you are already familiar with mempool policy
and package relay t= erminology.

### Terminology Clarifications

* Package =3D an o= rdered list of related transactions, representable by a Directed
=C2=A0 = Acyclic Graph.
* Package Feerate =3D the total modified fees divided by = the total virtual size of
=C2=A0 all transactions in the package.
=C2= =A0 =C2=A0 - Modified fees =3D a transaction's base fees + fee delta ap= plied by the user
=C2=A0 =C2=A0 =C2=A0 with `prioritisetransaction`. As = such, we expect this to vary across
mempools.
=C2=A0 =C2=A0 - Virtual= Size =3D the maximum of virtual sizes calculated using [BIP141
=C2=A0 = =C2=A0 =C2=A0 virtual size][2] and sigop weight. [Implemented here in Bitco= in Core][3].
=C2=A0 =C2=A0 - Note that feerate is not necessarily based = on the base fees and serialized
=C2=A0 =C2=A0 =C2=A0 size.

* Fee-= Bumping =3D user/wallet actions that take advantage of miner incentives to<= br>=C2=A0 boost a transaction's candidacy for inclusion in a block, inc= luding Child Pays
for Parent (CPFP) and [BIP125][12] Replace-by-Fee (RBF= ). Our intention in
mempool policy is to recognize when the new transact= ion is more economical to
mine than the original one(s) but not open DoS= vectors, so there are some
limitations.

### Policy

The pu= rpose of the mempool is to store the best (to be most incentive-compatible<= br>with miners, highest feerate) candidates for inclusion in a block. Miner= s use
the mempool to build block templates. The mempool is also useful a= s a cache for
boosting block relay and validation performance, aiding tr= ansaction relay, and
generating feerate estimations.

Ideally, all= consensus-valid transactions paying reasonable fees should make it
to m= iners through normal transaction relay, without any special connectivity or=
relationships with miners. On the other hand, nodes do not have unlimit= ed
resources, and a P2P network designed to let any honest node broadcas= t their
transactions also exposes the transaction validation engine to D= oS attacks from
malicious peers.

As such, for unconfirmed transac= tions we are considering for our mempool, we
apply a set of validation r= ules in addition to consensus, primarily to protect
us from resource exh= austion and aid our efforts to keep the highest fee
transactions. We cal= l this mempool _policy_: a set of (configurable,
node-specific) rules th= at transactions must abide by in order to be accepted
into our mempool. = Transaction "Standardness" rules and mempool restrictions suchas "too-long-mempool-chain" are both examples of policy.

= ### Package Relay and Package Mempool Accept

In transaction relay, w= e currently consider transactions one at a time for
submission to the me= mpool. This creates a limitation in the node's ability to
determine = which transactions have the highest feerates, since we cannot take
into = account descendants (i.e. cannot use CPFP) until all the transactions arein the mempool. Similarly, we cannot use a transaction's descendants = when
considering it for RBF. When an individual transaction does not mee= t the mempool
minimum feerate and the user isn't able to create a re= placement transaction
directly, it will not be accepted by mempools.
=
This limitation presents a security issue for applications and users re= lying on
time-sensitive transactions. For example, Lightning and other p= rotocols create
UTXOs with multiple spending paths, where one counterpar= ty's spending path opens
up after a timelock, and users are protecte= d from cheating scenarios as long as
they redeem on-chain in time. A key= security assumption is that all parties'
transactions will propagat= e and confirm in a timely manner. This assumption can
be broken if fee-b= umping does not work as intended.

The end goal for Package Relay is = to consider multiple transactions at the same
time, e.g. a transaction w= ith its high-fee child. This may help us better
determine whether transa= ctions should be accepted to our mempool, especially if
they don't m= eet fee requirements individually or are better RBF candidates as a
pack= age. A combination of changes to mempool validation logic, policy, and
t= ransaction relay allows us to better propagate the transactions with thehighest package feerates to miners, and makes fee-bumping tools more power= ful
for users.

The "relay" part of Package Relay sugges= ts P2P messaging changes, but a large
part of the changes are in the mem= pool's package validation logic. We call this
*Package Mempool Accep= t*.

### Previous Work

* Given that mempool validation is DoS-= sensitive and complex, it would be
=C2=A0 dangerous to haphazardly tack = on package validation logic. Many efforts have
been made to make mempool= validation less opaque (see [#16400][4], [#21062][5],
[#22675][6], [#22= 796][7]).
* [#20833][8] Added basic capabilities for package validation,= test accepts only
=C2=A0 (no submission to mempool).
* [#21800][9] I= mplemented package ancestor/descendant limit checks for arbitrary
=C2=A0= packages. Still test accepts only.
* Previous package relay proposals (= see [#16401][10], [#19621][11]).

### Existing Package Rules

T= hese are in master as introduced in [#20833][8] and [#21800][9]. I'll c= onsider
them as "given" in the rest of this document, though t= hey can be changed, since
package validation is test-accept only right n= ow.

1. A package cannot exceed `MAX_PACKAGE_COUNT=3D25` count and`MAX_PACKAGE_SIZE=3D101KvB` total size [8]

=C2=A0 =C2=A0*Rationale*= : This is already enforced as mempool ancestor/descendant limits.
Presum= ably, transactions in a package are all related, so exceeding this limitwould mean that the package can either be split up or it wouldn't pass= this
mempool policy.

2. Packages must be topologically sorted: i= f any dependencies exist between
transactions, parents must appear somew= here before children. [8]

3. A package cannot have conflicting trans= actions, i.e. none of them can spend
the same inputs. This also mea= ns there cannot be duplicate transactions. [8]

4. When = packages are evaluated against ancestor/descendant limits in a test
acce= pt, the union of all of their descendants and ancestors is considered. This=
is essentially a "worst case" heuristic where every transacti= on in the package
is treated as each other's ancestor and descendant= . [8]
Packages for which ancestor/descendant limits are accurately capt= ured by this
heuristic: [19]

There are also limitations su= ch as the fact that CPFP carve out is not applied
to package transaction= s. #20833 also disables RBF in package validation; this
proposal overrid= es that to allow packages to use RBF.

## Proposed Changes

The= next step in the Package Mempool Accept project is to implement submission=
to mempool, initially through RPC only. This allows us to test the subm= ission
logic before exposing it on P2P.

### Summary

- Pack= ages may contain already-in-mempool transactions.
- Packages are 2 gener= ations, Multi-Parent-1-Child.
- Fee-related checks use the package feera= te. This means that wallets can
create a package that utilizes CPFP.
= - Parents are allowed to RBF mempool transactions with a set of rules simil= ar
=C2=A0 to BIP125. This enables a combination of CPFP and RBF, where a=
transaction's descendant fees pay for replacing mempool conflicts.<= br>
There is a draft implementation in [#22290][1]. It is WIP, but feedb= ack is
always welcome.

### Details

#### Packages May Conta= in Already-in-Mempool Transactions

A package may contain transaction= s that are already in the mempool. We remove
("deduplicate") t= hose transactions from the package for the purposes of package
mempool a= cceptance. If a package is empty after deduplication, we do nothing.
*Rationale*: Mempools vary across the network. It's possible for a par= ent to be
accepted to the mempool of a peer on its own due to difference= s in policy and
fee market fluctuations. We should not reject or penaliz= e the entire package for
an individual transaction as that could be a ce= nsorship vector.

#### Packages Are Multi-Parent-1-Child

Only = packages of a specific topology are permitted. Namely, a package is exactly=
1 child with all of its unconfirmed parents. After deduplication, the p= ackage
may be exactly the same, empty, 1 child, 1 child with just some o= f its
unconfirmed parents, etc. Note that it's possible for the pare= nts to be indirect
descendants/ancestors of one another, or for parent a= nd child to share a parent,
so we cannot make any other topology assumpt= ions.

*Rationale*: This allows for fee-bumping by CPFP. Allowing mul= tiple parents
makes it possible to fee-bump a batch of transactions. Res= tricting packages to a
defined topology is also easier to reason about a= nd simplifies the validation
logic greatly. Multi-parent-1-child allows = us to think of the package as one big
transaction, where:

- Input= s =3D all the inputs of parents + inputs of the child that come from
=C2= =A0 confirmed UTXOs
- Outputs =3D all the outputs of the child + all out= puts of the parents that
=C2=A0 aren't spent by other transactions i= n the package

Examples of packages that follow this rule (variations= of example A show some
possibilities after deduplication): ![image][15]=

#### Fee-Related Checks Use Package Feerate

Package Feerate = =3D the total modified fees divided by the total virtual size of
all tra= nsactions in the package.

To meet the two feerate requirements of a = mempool, i.e., the pre-configured
minimum relay feerate (`minRelayTxFee`= ) and dynamic mempool minimum feerate, the
total package feerate is used= instead of the individual feerate. The individual
transactions are allo= wed to be below feerate requirements if the package meets
the feerate re= quirements. For example, the parent(s) in the package can have 0
fees bu= t be paid for by the child.

*Rationale*: This can be thought of as &= quot;CPFP within a package," solving the
issue of a parent not meet= ing minimum fees on its own. This allows L2
applications to adjust their= fees at broadcast time instead of overshooting or
risking getting stuck= /pinned.

We use the package feerate of the package *after deduplicat= ion*.

*Rationale*: =C2=A0It would be incorrect to use the fees of tr= ansactions that are
already in the mempool, as we do not want a transact= ion's fees to be
double-counted for both its individual RBF and pack= age RBF.

Examples F and G [14] show the same package, but P1 is subm= itted individually before
the package in example G. In example F, we can= see that the 300vB package pays
an additional 200sat in fees, which is = not enough to pay for its own bandwidth
(BIP125#4). In example G, we can= see that P1 pays enough to replace M1, but
using P1's fees again du= ring package submission would make it look like a 300sat
increase for a = 200vB package. Even including its fees and size would not be
sufficient = in this example, since the 300sat looks like enough for the 300vB
packag= e. The calculcation after deduplication is 100sat increase for a packageof size 200vB, which correctly fails BIP125#4. Assume all transactions hav= e a
size of 100vB.

#### Package RBF

If a package meets fee= rate requirements as a package, the parents in the
transaction are allow= ed to replace-by-fee mempool transactions. The child cannot
replace memp= ool transactions. Multiple transactions can replace the same
transaction= , but in order to be valid, none of the transactions can try to
replace = an ancestor of another transaction in the same package (which would thusmake its inputs unavailable).

*Rationale*: Even if we are using pac= kage feerate, a package will not propagate
as intended if RBF still requ= ires each individual transaction to meet the
feerate requirements.
We use a set of rules slightly modified from BIP125 as follows:

##= ### Signaling (Rule #1)

All mempool transactions to be replaced must= signal replaceability.

*Rationale*: Package RBF signaling logic sho= uld be the same for package RBF and
single transaction acceptance. This = would be updated if single transaction
validation moves to full RBF.
=
##### New Unconfirmed Inputs (Rule #2)

A package may include new= unconfirmed inputs, but the ancestor feerate of the
child must be at le= ast as high as the ancestor feerates of every transaction
being replaced= . This is contrary to BIP125#2, which states "The replacement
trans= action may only include an unconfirmed input if that input was included in<= br>one of the original transactions. (An unconfirmed input spends an output= from a
currently-unconfirmed transaction.)"

*Rationale*: Th= e purpose of BIP125#2 is to ensure that the replacement
transaction has = a higher ancestor score than the original transaction(s) (see
[comment][= 13]). Example H [16] shows how adding a new unconfirmed input can lower the=
ancestor score of the replacement transaction. P1 is trying to replace = M1, and
spends an unconfirmed output of M2. P1 pays 800sat, M1 pays 600s= at, and M2 pays
100sat. Assume all transactions have a size of 100vB. Wh= ile, in isolation, P1
looks like a better mining candidate than M1, it m= ust be mined with M2, so its
ancestor feerate is actually 4.5sat/vB.=C2= =A0 This is lower than M1's ancestor
feerate, which is 6sat/vB.
<= br>In package RBF, the rule analogous to BIP125#2 would be "none of th= e
transactions in the package can spend new unconfirmed inputs." Ex= ample J [17] shows
why, if any of the package transactions have ancestor= s, package feerate is no
longer accurate. Even though M2 and M3 are not = ancestors of P1 (which is the
replacement transaction in an RBF), we'= ;re actually interested in the entire
package. A miner should mine M1 wh= ich is 5sat/vB instead of M2, M3, P1, P2, and
P3, which is only 4sat/vB.= The Package RBF rule cannot be loosened to only allow
the child to have= new unconfirmed inputs, either, because it can still cause us
to overes= timate the package's ancestor score.

However, enforcing a rule a= nalogous to BIP125#2 would not only make Package RBF
less useful, but wo= uld also break Package RBF for packages with parents already
in the memp= ool: if a package parent has already been submitted, it would look
like = the child is spending a "new" unconfirmed input. In example K [18= ], we're
looking to replace M1 with the entire package including P1,= P2, and P3. We must
consider the case where one of the parents is alrea= dy in the mempool (in this
case, P2), which means we must allow P3 to ha= ve new unconfirmed inputs. However,
M2 lowers the ancestor score of P3 t= o 4.3sat/vB, so we should not replace M1
with this package.

Thus,= the package RBF rule regarding new unconfirmed inputs is less strict than<= br>BIP125#2. However, we still achieve the same goal of requiring the repla= cement
transactions to have a ancestor score at least as high as the ori= ginal ones. As
a result, the entire package is required to be a higher f= eerate mining candidate
than each of the replaced transactions.

A= nother note: the [comment][13] above the BIP125#2 code in the original RBF<= br>implementation suggests that the rule was intended to be temporary.
<= br>##### Absolute Fee (Rule #3)

The package must increase the absolu= te fee of the mempool, i.e. the total fees
of the package must be higher= than the absolute fees of the mempool transactions
it replaces. Combine= d with the CPFP rule above, this differs from BIP125 Rule #3
- an indivi= dual transaction in the package may have lower fees than the
=C2=A0 tran= saction(s) it is replacing. In fact, it may have 0 fees, and the child
p= ays for RBF.

##### Feerate (Rule #4)

The package must pay for= its own bandwidth; the package feerate must be higher
than the replaced= transactions by at least minimum relay feerate
(`incrementalRelayFee`).= Combined with the CPFP rule above, this differs from
BIP125 Rule #4 - a= n individual transaction in the package can have a lower
feerate than th= e transaction(s) it is replacing. In fact, it may have 0 fees,
and the c= hild pays for RBF.

##### Total Number of Replaced Transactions (Rule= #5)

The package cannot replace more than 100 mempool transactions. = This is identical
to BIP125 Rule #5.

### Expected FAQs

1. = Is it possible for only some of the package to make it into the mempool?
=C2=A0 =C2=A0Yes, it is. However, since we evict transactions from the= mempool by
descendant score and the package child is supposed to be spo= nsoring the fees of
its parents, the most common scenario would be all-o= r-nothing. This is
incentive-compatible. In fact, to be conservative, pa= ckage validation should
begin by trying to submit all of the transaction= s individually, and only use the
package mempool acceptance logic if the= parents fail due to low feerate.

2. Should we allow packages to con= tain already-confirmed transactions?

=C2=A0 =C2=A0 No, for practical= reasons. In mempool validation, we actually aren't able to
tell wit= h 100% confidence if we are looking at a transaction that has already
co= nfirmed, because we look up inputs using a UTXO set. If we have historical<= br>block data, it's possible to look for it, but this is inefficient, n= ot always
possible for pruning nodes, and unnecessary because we're = not going to do
anything with the transaction anyway. As such, we alread= y have the expectation
that transaction relay is somewhat "stateful= " i.e. nobody should be relaying
transactions that have already bee= n confirmed. Similarly, we shouldn't be
relaying packages that conta= in already-confirmed transactions.

[1]: https://github.com/bitcoin/= bitcoin/pull/22290
[2]: https://github.com/bitcoin/bips/blob/= 1f0b563738199ca60d32b4ba779797fc97d040fe/bip-0141.mediawiki#transaction-siz= e-calculations
[3]: https://github.com/bitcoin/bitcoin/blob/94f83534e4b771944af7= d9ed0f40746f392eb75e/src/policy/policy.cpp#L282
[4]: https://github= .com/bitcoin/bitcoin/pull/16400
[5]: https://github.com/bitcoin/bit= coin/pull/21062
[6]: https://github.com/bitcoin/bitcoin/pull/22675<= /a>
[7]: https://github.com/bitcoin/bitcoin/pull/22796
[8]: htt= ps://github.com/bitcoin/bitcoin/pull/20833
[9]: https://github.com/= bitcoin/bitcoin/pull/21800
[10]: https://github.com/bitcoin/bitcoin= /pull/16401
[11]: https://github.com/bitcoin/bitcoin/pull/19621=
[12]: https://github.com/bitcoin/bips/blob/master/bip= -0125.mediawiki
[13]: https://github.com/bitcoin/bitcoin= /pull/6871/files#diff-34d21af3c614ea3cee120df276c9c4ae95053830d7f1d3deaf009= a4625409ad2R1101-R1104
[14]: https://user-images.githubusercontent.com/25183001/133567= 078-075a971c-0619-4339-9168-b41fd2b90c28.png
[15]: https://user-images.githubuserconte= nt.com/25183001/132856734-fc17da75-f875-44bb-b954-cb7a1725cc0d.png
[= 16]: https://user-= images.githubusercontent.com/25183001/133567347-a3e2e4a8-ae9c-49f8-abb9-81e= 8e0aba224.png
[17]: https://user-images.githubusercontent.com/25183001/133567370-21566= d0e-36c8-4831-b1a8-706634540af3.png
[18]: https://user-images.githubusercontent.com/25= 183001/133567444-bfff1142-439f-4547-800a-2ba2b0242bcb.png
[19]: https://user-images.gi= thubusercontent.com/25183001/133456219-0bb447cb-dcb4-4a31-b9c1-7d86205b68bc= .png
[20]: = https://user-images.githubusercontent.com/25183001/132857787-7b7c6f56-af96-= 44c8-8d78-983719888c19.png
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