From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp3.osuosl.org (smtp3.osuosl.org [IPv6:2605:bc80:3010::136]) by lists.linuxfoundation.org (Postfix) with ESMTP id 00049C000B for ; Fri, 11 Feb 2022 05:27:10 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp3.osuosl.org (Postfix) with ESMTP id C61AB607B4 for ; Fri, 11 Feb 2022 05:27:10 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -2.098 X-Spam-Level: X-Spam-Status: No, score=-2.098 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, 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: smtp3.osuosl.org (amavisd-new); dkim=pass (2048-bit key) header.d=gmail.com Received: from smtp3.osuosl.org ([127.0.0.1]) by localhost (smtp3.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id HmvkyuZO7nvL for ; Fri, 11 Feb 2022 05:27:08 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.8.0 Received: from mail-yb1-xb2c.google.com (mail-yb1-xb2c.google.com [IPv6:2607:f8b0:4864:20::b2c]) by smtp3.osuosl.org (Postfix) with ESMTPS id 721426079D for ; Fri, 11 Feb 2022 05:27:08 +0000 (UTC) Received: by mail-yb1-xb2c.google.com with SMTP id y129so21772367ybe.7 for ; Thu, 10 Feb 2022 21:27:08 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to; bh=Ootu4AaqPP8i3DeFJ3b5dkifAuMLgzvP+Fm8RiA9Crk=; b=Tf5RFVAscsi/H972KgCBBQV4dDt2U9pFU0G6YCdWr20QuDEIJAt6IGoXa87BBkevEF He735J/utOsIQ1T8qQTg/LTvx1tMvamYdHe7g4ZtHXLQlaMLM0rmJ5O7+whxhlPDBOaa DL5fD6ojDt0epOfnzX2FLorpvgZLiiE/6M5UKNKq2STgg8cg49DoSoy/bmFI0cct0Dz7 QANfLQlNASHbFSeduZPWLIbst5vf3c71OZYnO5zsAd87RjX5L2+F4zRI8nMgKJ4oc6rT XhiSqZVPDhAnr2u6+atVHCv5J1Ndl+MnbYTaOHnrHdVmdC7xa6X+7++zxdIUAO5UN5A5 9cCg== 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=Ootu4AaqPP8i3DeFJ3b5dkifAuMLgzvP+Fm8RiA9Crk=; b=wo8w6jdZjWC4wWkSjRFSbDrhD+7bSQaZWa+R5BHJSX9dZh0VEYEh71xTkGvu5pye73 diq2b/R3nu1YXbM2kCj/U80mlP9POZliNzROcYOKtf/8+4naVIuufN36Dp5HjBbdsCu6 s0HOktz1DYYDpXswkd2R01ZlA1xewjWBSbWmQrV+LUx0lQku5AgmvAWeJTwZplltMUbz 7IxePWcH76DX218arx17/slfS0Gjq3qibXIM7REM1mt4wBdldFkbbxx0MVyrXKDvj5DT gDDDfcqBL4G48jYZd2ghq3uyBbqm6Z/V4WzErc5WlulmKlT8pITChYjwaPs17iE+d83a lvJg== X-Gm-Message-State: AOAM533rQnilxjm/2Oeuw3b8zqFkciZiQI82eM+gC0SpETlvAOlFokMk TicqFE0Ec+L87lfS9xUzi8Js1JE04J5hzTELsQg= X-Google-Smtp-Source: ABdhPJxSwS1pFWLO7cAjP5Y80vek+cTIePLsqLIrZJWSCe7yeakjeHJwo58658FoErrE/fe5DrkEBGJVYUhFefqe0jA= X-Received: by 2002:a81:b61b:: with SMTP id u27mr127913ywh.450.1644557226115; Thu, 10 Feb 2022 21:27:06 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Antoine Riard Date: Fri, 11 Feb 2022 00:26:53 -0500 Message-ID: To: "James O'Beirne" , Bitcoin Protocol Discussion Content-Type: multipart/alternative; boundary="00000000000046865005d7b75109" X-Mailman-Approved-At: Fri, 11 Feb 2022 09:12:53 +0000 Subject: Re: [bitcoin-dev] Thoughts on fee bumping 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: Fri, 11 Feb 2022 05:27:11 -0000 --00000000000046865005d7b75109 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi James, I fully agree on the need to reframe the conversation around mempools/fee-bumping/L2s though please see my following comments, it's far from simple! > Layering on special cases, more carve-outs, and X and Y percentage > thresholds is going to make reasoning about the mempool harder than it > already is. I think that's true with a lot of (all ?) pieces of software, there is a trend towards complexification. As new Bitcoin use-cases such as LN or vaults appear, it's not surprising to see the base layer upper interfaces changing to support the requirements. Same with kernels, at beginning, you can have a basic memory support with paging/memory rights/kernel allocators then as you start to support more platforms/devices you might have to support swaps/DMA/VMs management... That we should keep the complexity reasonably sane to enable human auditing, and maybe learn from the failures of systems engineering, that's something to muse on. > The countervailing force here ends up being spam prevention (a la min-relay-fee) > to prevent someone from consuming bandwidth and mempool space with a long series of > infinitesimal fee-bumps. I think here we should dissociate a) long-chain of transactions and b) high-number of repeated fee-bumps. For a) _if_ SIGHASH_ANYPREVOUT is deployed and Eltoo adopted as a primary update mechanism for stateful L2s, one might envision long-chain of update transactions servicing as a new pinning vector, where all the chain elements are offering a compelling feerate/fees. It might be solvable with smarter mempool logic sorting the elements from the best offer to the lower ones, though that issue would need more serious investigation. For b) if we bound with a hard constant the number of RBF attempts, we decrease the fault-tolerance of L2 transaction issuers. Some of them might connect directly to the miners because they're offering higher number of incentive-compatible RBF attempts than vanilla full-nodes. That might provoke a more or slow centralization of the transaction relay topology... > Instead of prompting a rebroadcast of the original transaction for > replacement, which contains a lot of data not new to the network, it > makes more sense to broadcast the "diff" which is the additive > contribution towards some txn's feerate. In a distributed system such as the Bitcoin p2p network, you might have transaction A and transaction B broadcast at the same time and your peer topology might fluctuate between original send and broadcast of the diff, you don't know who's seen what... You might inefficiently announce diff A on top of B and diff B on top A. We might leverage set reconciliation there a la Erlay, though likely with increased round-trips. > It's probably uncontroversial at this > point to say that even RBF itself is kind of a hack - a special > sequence number should not be necessary for post-broadcast contribution > toward feerate. I think here we should dissociate the replace-by-fee mechanism itself from the replacement signaling one. To have a functional RBF, you don't need signaling at all, just consider all received transactions as replaceable. The replacement signaling one has been historically motivated to protect the applications relying on zero-confs (with all the past polemics about the well-foundedness of such claims on other nodes' policy). > In a sane design, no structural foresight - and certainly no wasted >bytes in the form of unused anchor outputs - should be needed in order >to add to a miner's reward for confirming a given transaction. Have you heard about SIGHASH_GROUP [0] ? It would move away from the transaction to enable arbitrary bundles of input/outputs. You will have your pre-signed bundle of inputs/outputs enforcing your LN/vaults/L2 and then at broadcast time, you can attach an input/output. I think it would answer your structural foresight. > One of the practical downsides of CPFP that I haven't seen discussed in > this conversation is that it requires the transaction to pre-specify the > keys needed to sign for fee bumps. This is problematic if you're, for > example, using a vault structure that makes use of pre-signed > transactions. It's true it requires to pre-specify the fee-bumping key. Though note the fee-bumping key can be fully separated from the "vaults"/"channels" set of main keys and hosted on replicated infrastructure such as watchtowers. > The interface for end-users is very straightforward: if you want to bump > fees, specify a transaction that contributes incrementally to package > feerate for some txid. Simple. As a L2 transaction issuer you can't be sure the transaction you wish to point to is already in the mempool, or have not been replaced by your counterparty spending the same shared-utxo, either competitively or maliciously. So as a measure of caution, you should broadcast sponsor + target transactions in the same package, thus cancelling the bandwidth saving (I think). > This theoretical concession seems preferable to heaping more rules onto an already labyrinthine mempool policy that is difficult for both implementers and users to reason about practically and conceptually. I don't think a sponsor is a silver-bullet to solve all the L2-related mempool issues. It won't solve the most concerning pinning attacks, as I think the bottleneck is replace-by-fee. Neither solve the issues encumbered by the L2s by the dust limit. > If a soft-fork is the cost of cleaning up this essential process, > consideration should be given to paying it as a one-time cost. This > topic merits a separate post, but consider that in the 5 years leading > up to the 2017 SegWit drama, we averaged about a soft-fork a year. > Uncontroversial, "safe" changes to the consensus protocol shouldn't be > out of the question when significant practical benefit is plain to see. Zooming out, I think we're still early in solving those L2 issues, as the most major second-layers are still in a design or deployment phase. We might freeze our transaction propagation interface, and get short for some of the most interesting ones like channel factories and payment pools. Further, I think we're not entirely sure how the mining ecosystem is going to behave once the reward is drained and their incentives towards L2 confirmations. Still, if we think we have a correct picture of the fee-bumping/mempools issues and are sufficiently confident with the stability of L2 designs, I think the next step would be to come with quantitative modelling of each resources consumed by fee-bumping (CPU validation/bandwidth/signing interactivity for the L2s...) and then score the "next-gen" fee-bumping primitives. > I'm not out to propose soft-forks lightly, but the current complexity > in fee management feels untenable, and as evidenced by all the > discussion lately, fees are an increasingly crucial part of the system. Overall, I think that's a relevant discussion to have ecosystem-wise. Though there is a lot of context and I don't think there is a simple way forward. Maybe better to stick to an evolutionary development process with those mempool/fee-bumping issues. We might envision two-or-three steps ahead though unlikely more. Cheers, Antoine [0] SIGHASH_GROUP described here https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/019031.htm= l and roughly roughly implemented here : https://github.com/ariard/bitcoin/pull/1 Le jeu. 10 f=C3=A9vr. 2022 =C3=A0 14:48, James O'Beirne via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit : > There's been much talk about fee-bumping lately, and for good reason - > dynamic fee management is going to be a central part of bitcoin use as > the mempool fills up (lord willing) and right now fee-bumping is > fraught with difficulty and pinning peril. > > Gloria's recent post on the topic[0] was very lucid and highlights a > lot of the current issues, as well as some proposals to improve the > situation. > > As others have noted, the post was great. But throughout the course > of reading it and the ensuing discussion, I became troubled by the > increasing complexity of both the status quo and some of the > proposed remedies. > > Layering on special cases, more carve-outs, and X and Y percentage > thresholds is going to make reasoning about the mempool harder than it > already is. Special consideration for "what should be in the next > block" and/or the caching of block templates seems like an imposing > dependency, dragging in a bunch of state and infrastructure to a > question that should be solely limited to mempool feerate aggregates > and the feerate of the particular txn package a wallet is concerned > with. > > This is bad enough for protocol designers and Core developers, but > making the situation any more intractable for "end-users" and wallet > developers feels wrong. > > I thought it might be useful to step back and reframe. Here are a few > aims that are motivated chiefly by the quality of end-user experience, > constrained to obey incentive compatibility (i.e. miner reward, DoS > avoidance). Forgive the abstract dalliance for a moment; I'll talk > through concretes afterwards. > > > # Purely additive feerate bumps should never be impossible > > Any user should always be able to add to the incentive to mine any > transaction in a purely additive way. The countervailing force here > ends up being spam prevention (a la min-relay-fee) to prevent someone > from consuming bandwidth and mempool space with a long series of > infinitesimal fee-bumps. > > A fee bump, naturally, should be given the same per-byte consideration > as a normal Bitcoin transaction in terms of relay and block space, > although it would be nice to come up with a more succinct > representation. This leads to another design principle: > > > # The bandwidth and chain space consumed by a fee-bump should be minimal > > Instead of prompting a rebroadcast of the original transaction for > replacement, which contains a lot of data not new to the network, it > makes more sense to broadcast the "diff" which is the additive > contribution towards some txn's feerate. > > This dovetails with the idea that... > > > # Special transaction structure should not be required to bump fees > > In an ideal design, special structural foresight would not be needed > in order for a txn's feerate to be improved after broadcast. > > Anchor outputs specified solely for CPFP, which amount to many bytes of > wasted chainspace, are a hack. It's probably uncontroversial at this > point to say that even RBF itself is kind of a hack - a special > sequence number should not be necessary for post-broadcast contribution > toward feerate. Not to mention RBF's seemingly wasteful consumption of > bandwidth due to the rebroadcast of data the network has already seen. > > In a sane design, no structural foresight - and certainly no wasted > bytes in the form of unused anchor outputs - should be needed in order > to add to a miner's reward for confirming a given transaction. > > Planning for fee-bumps explicitly in transaction structure also often > winds up locking in which keys are required to bump fees, at odds > with the idea that... > > > # Feerate bumps should be able to come from anywhere > > One of the practical downsides of CPFP that I haven't seen discussed in > this conversation is that it requires the transaction to pre-specify the > keys needed to sign for fee bumps. This is problematic if you're, for > example, using a vault structure that makes use of pre-signed > transactions. > > What if the key you specified n the anchor outputs for a bunch of > pre-signed txns is compromised? What if you'd like to be able to > dynamically select the wallet that bumps fees? CPFP does you no favors > here. > > There is of course a tension between allowing fee bumps to come from > anywhere and the threat of pinning-like attacks. So we should venture > to remove pinning as a possibility, in line with the first design > principle I discuss. > > > --- > > Coming down to earth, the "tabula rasa" thought experiment above has led > me to favor an approach like the transaction sponsors design that Jeremy > proposed in a prior discussion back in 2020[1]. > > Transaction sponsors allow feerates to be bumped after a transaction's > broadcast, regardless of the structure of the original transaction. > No rebroadcast (wasted bandwidth) is required for the original txn data. > No wasted chainspace on only-maybe-used prophylactic anchor outputs. > > The interface for end-users is very straightforward: if you want to bump > fees, specify a transaction that contributes incrementally to package > feerate for some txid. Simple. > > In the original discussion, there were a few main objections that I noted= : > > 1. In Jeremy's original proposal, only one sponsor txn per txid is > allowed by policy. A malicious actor could execute a pinning-like > attack by specifying an only-slightly-helpful feerate sponsor that > then precludes other larger bumps. > > I think there are some ways around this shortcoming. For example: what > if, by policy, sponsor txns had additional constraints that > > - each input must be signed {SIGHASH_SINGLE,SIGHASH_NONE}|ANYONECANPAY, > - the txn must be specified RBFable, > - a replacement for the sponsor txn must raise the sponsor feerate, > including ancestors (maybe this is inherent in "is RBFable," but > I don't want to conflate absolute feerates into this). > > That way, there is still at most a single sponsor txn per txid in the > mempool, but anyone can "mix in" inputs which bump the effective > feerate of the sponsor. > > This may not be the exact solution we want, but I think it demonstrates > that the sponsors design has some flexibility and merits some thinking. > > The second objection about sponsors was > > 2. (from Suhas) sponsors break the classic invariant: "once a valid > transaction is created, it should not become invalid later on unless > the inputs are double-spent." > > This doesn't seem like a huge concern to me if you consider the txid > being sponsored as a sort of spiritual input to the sponsor. While the > theoretical objection against broadening where one has to look in a txn > to determine its dependencies is understandable, I don't see what the > practical cost here is. > > Reorg complexity seems comparable if not identical, especially if we > broaden sponsor rules to allow blocks to contain sponsor txns that are > both for txids in the same block _or_ already included in the chain. > > This theoretical concession seems preferable to heaping more rules onto > an already labyrinthine mempool policy that is difficult for both > implementers and users to reason about practically and conceptually. > > A third objection that wasn't posed, IIRC, but almost certainly would > be: > > 3. Transaction sponsors requires a soft-fork. > > Soft-forks are no fun, but I'll tell you what also isn't fun: being on > the hook to model (and sometimes implement) a dizzying potpourri of > mempool policies and special-cases. Expecting wallet implementers to > abide by a maze of rules faithfully in order to ensure txn broadcast and > fee management invites bugs for perpetuity and network behavior that is > difficult to reason about a priori. Use of CPFP in the long-term also > risks needless chain waste. > > If a soft-fork is the cost of cleaning up this essential process, > consideration should be given to paying it as a one-time cost. This > topic merits a separate post, but consider that in the 5 years leading > up to the 2017 SegWit drama, we averaged about a soft-fork a year. > Uncontroversial, "safe" changes to the consensus protocol shouldn't be > out of the question when significant practical benefit is plain to see. > > --- > > I hope this message has added some framing to the discussion on fees, > as well prompting other participants to go back and give the > transaction sponsor proposal a serious look. The sponsors interface is > about the simplest I can imagine for wallets, and it seems easy to > reason about for implementers on Core and elsewhere. > > I'm not out to propose soft-forks lightly, but the current complexity > in fee management feels untenable, and as evidenced by all the > discussion lately, fees are an increasingly crucial part of the system. > > > > [0]: > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/0198= 17.html > [1]: > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/01= 8168.html > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > --00000000000046865005d7b75109 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi James,

I fully agree on the need to reframe the = conversation around mempools/fee-bumping/L2s though please see my following= comments, it's far from simple!

> Layering on special cases,= more carve-outs, and X and Y percentage
> thresholds is going to mak= e reasoning about the mempool harder than it
> already is.

I t= hink that's true with a lot of (all ?) pieces of software, there is a t= rend towards complexification. As new Bitcoin use-cases such as LN or vault= s appear, it's not surprising to see the base layer upper interfaces ch= anging to support the requirements. Same with kernels, at beginning, you ca= n have a basic memory support with paging/memory rights/kernel allocators t= hen as you start to support more platforms/devices you might have to suppor= t swaps/DMA/VMs management...

That we should keep the complexity rea= sonably sane to enable human auditing, and maybe learn from the failures of= systems engineering, that's something to muse on.

> The coun= tervailing force here ends up being spam prevention (a la min-relay-fee)> to prevent someone from consuming bandwidth and mempool space with a = long series of
> infinitesimal fee-bumps.

I think here we shou= ld dissociate a) long-chain of transactions and b) high-number of repeated = fee-bumps.

For a) _if_ SIGHASH_ANYPREVOUT is deployed and Eltoo adop= ted as a primary update mechanism for stateful L2s, one might envision long= -chain of update transactions servicing as a new pinning vector, where all = the chain elements are offering a compelling feerate/fees. It might be solv= able with smarter mempool logic sorting the elements from the best offer to= the lower ones, though that issue would need more serious investigation.
For b) if we bound with a hard constant the number of RBF attempts, w= e decrease the fault-tolerance of L2 transaction issuers. Some of them migh= t connect directly to the miners because they're offering higher number= of incentive-compatible RBF attempts than vanilla full-nodes. That might p= rovoke a more or slow centralization of the transaction relay topology...
> Instead of prompting a rebroadcast of the original transaction f= or
> replacement, which contains a lot of data not new to the network= , it
> makes more sense to broadcast the "diff" which is th= e additive
> contribution towards some txn's feerate.

In a= distributed system such as the Bitcoin p2p network, you might have transac= tion A and transaction B=C2=A0 broadcast at the same time and your peer top= ology might fluctuate between original send and broadcast
of the diff, y= ou don't know who's seen what... You might inefficiently announce d= iff A on top of B and diff B on top A. We might leverage set reconciliation= there a la Erlay, though likely with increased round-trips.

> It= 's probably uncontroversial at this
> point to say that even RBF = itself is kind of a hack - a special
> sequence number should not be = necessary for post-broadcast contribution
> toward feerate.

I = think here we should dissociate the replace-by-fee mechanism itself from th= e replacement signaling one. To have a functional RBF, you don't need s= ignaling at all, just consider all received transactions as replaceable. Th= e replacement signaling one has been historically motivated to protect the = applications relying on zero-confs (with all the past polemics about the we= ll-foundedness of such claims on other nodes' policy).

> In a= sane design, no structural foresight - and certainly no wasted
>byte= s in the form of unused anchor outputs - should be needed in order
>t= o add to a miner's reward for confirming a given transaction.

Ha= ve you heard about SIGHASH_GROUP [0] ? It would move away from the transact= ion to enable arbitrary bundles of input/outputs. You will have your pre-si= gned bundle of inputs/outputs enforcing your LN/vaults/L2 and then at broad= cast time, you can attach an input/output. I think it would answer your str= uctural foresight.

> One of the practical downsides of CPFP that = I haven't seen discussed in
> this conversation is that it requir= es the transaction to pre-specify the
> keys needed to sign for fee b= umps. This is problematic if you're, for
> example, using a vault= structure that makes use of pre-signed
> transactions.

It'= ;s true it requires to pre-specify the fee-bumping key. Though note the fee= -bumping key can be fully separated from the "vaults"/"chann= els" set of main keys and hosted on replicated infrastructure such as = watchtowers.

> The interface for end-users is very straightforwar= d: if you want to bump
> fees, specify a transaction that contributes= incrementally to package
> feerate for some txid. Simple.

As = a L2 transaction issuer you can't be sure the transaction you wish to p= oint to is already in the mempool, or have not been replaced by your counte= rparty spending the same shared-utxo, either competitively or maliciously. = So as a measure of caution, you should broadcast sponsor + target transacti= ons in the same package, thus cancelling the bandwidth saving (I think).
> This theoretical concession seems preferable to heaping more rule= s onto
an already labyrinthine mempool policy that is difficult for both=
implementers and users to reason about practically and conceptually.
I don't think a sponsor is a silver-bullet to solve all the L2-rel= ated mempool issues. It won't solve the most concerning pinning attacks= , as I think the bottleneck is replace-by-fee. Neither solve the issues enc= umbered by the L2s by the dust limit.

> If a soft-fork is the cos= t of cleaning up this essential process,
> consideration should be gi= ven to paying it as a one-time cost. This
> topic merits a separate p= ost, but consider that in the 5 years leading
> up to the 2017 SegWit= drama, we averaged about a soft-fork a year.
> Uncontroversial, &quo= t;safe" changes to the consensus protocol shouldn't be
> out= of the question when significant practical benefit is plain to see.
Zooming out, I think we're still early in solving those L2 issues, as = the most major second-layers are still in a design or deployment phase. We = might freeze our transaction propagation interface, and get short for some = of the most interesting ones like channel factories and payment pools. Furt= her, I think we're not entirely sure how the mining ecosystem is going = to behave once the reward is drained and their incentives towards L2 confir= mations.

Still, if we think we have a correct picture of the fee-bum= ping/mempools issues and are sufficiently confident with the stability of L= 2 designs, I think the next step would be to come with quantitative modelli= ng of each resources consumed by fee-bumping (CPU validation/bandwidth/sign= ing interactivity for the L2s...) and then score the "next-gen" f= ee-bumping primitives.

> I'm not out to propose soft-forks li= ghtly, but the current complexity
> in fee management feels untenable= , and as evidenced by all the
> discussion lately, fees are an increa= singly crucial part of the system.

Overall, I think that's a rel= evant discussion to have ecosystem-wise. Though there is a lot of context a= nd I don't think there is a simple way forward. Maybe better to stick t= o an evolutionary development process with those mempool/fee-bumping issues= . We might envision two-or-three steps ahead though unlikely more.

C= heers,
Antoine

[0] SIGHASH_GROUP described here htt= ps://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/019031.html
and roughly roughly implemented here : https://github.com/ariard/bitcoin/pull/1

Le= =C2=A0jeu. 10 f=C3=A9vr. 2022 =C3=A0=C2=A014:48, James O'Beirne via bit= coin-dev <bitco= in-dev@lists.linuxfoundation.org> a =C3=A9crit=C2=A0:
There= 9;s been much talk about fee-bumping lately, and for good reason -
dynam= ic fee management is going to be a central part of bitcoin use as
the me= mpool fills up (lord willing) and right now fee-bumping is
fraught with = difficulty and pinning peril.

Gloria's recent post on the topic[= 0] was very lucid and highlights a
lot of the current issues, as well as= some proposals to improve the
situation.

As others have noted, t= he post was great. But throughout the course
of reading it and the ensui= ng discussion, I became troubled by the
increasing complexity of both th= e status quo and some of the
proposed remedies.

Layering on special cases, more carve-outs, and X and Y percentagethresholds is going to make reasoning about the mempool harder than italready is. Special consideration for "what should be in the next
= block" and/or the caching of block templates seems like an imposingdependency, dragging in a bunch of state and infrastructure to a
questi= on that should be solely limited to mempool feerate aggregates
and the f= eerate of the particular txn package a wallet is concerned
with.
This is bad enough for protocol designers and Core developers, but
maki= ng the situation any more intractable for "end-users" and wallet<= br>developers feels wrong.

I thought it might be useful to step back= and reframe. Here are a few
aims that are motivated chiefly by the qual= ity of end-user experience,
constrained to obey incentive compatibility = (i.e. miner reward, DoS
avoidance). Forgive the abstract dalliance for a= moment; I'll talk
through concretes afterwards.


# Purely= additive feerate bumps should never be impossible

Any user should a= lways be able to add to the incentive to mine any
transaction in a purel= y additive way. The countervailing force here
ends up being spam prevent= ion (a la min-relay-fee) to prevent someone
from consuming bandwidth and= mempool space with a long series of
infinitesimal fee-bumps.

A = fee bump, naturally, should be given the same per-byte consideration
as = a normal Bitcoin transaction in terms of relay and block space,
although= it would be nice to come up with a more succinct
representation. This l= eads to another design principle:


# The bandwidth and chain spac= e consumed by a fee-bump should be minimal

Instead of prompting a re= broadcast of the original transaction for
replacement, which contains a = lot of data not new to the network, it
makes more sense to broadcast the= "diff" which is the additive
contribution towards some txn= 9;s feerate.

This dovetails with the idea that...


# Spec= ial transaction structure should not be required to bump fees

In an = ideal design, special structural foresight would not be needed
in order= for a txn's feerate to be improved after broadcast.

Anchor outp= uts specified solely for CPFP, which amount to many bytes of
wasted chai= nspace, are a hack. It's probably uncontroversial at this
point to s= ay that even RBF itself is kind of a hack - a special
sequence number sh= ould not be necessary for post-broadcast contribution
toward feerate. No= t to mention RBF's seemingly wasteful consumption of
bandwidth due t= o the rebroadcast of data the network has already seen.

In a sane de= sign, no structural foresight - and certainly no wasted
bytes in the for= m of unused anchor outputs - should be needed in order
to add to a miner= 's reward for confirming a given transaction.

Planning for fee-b= umps explicitly in transaction structure also often
winds up locking in = which keys are required to bump fees, at odds
with the idea that...
<= br>
# Feerate bumps should be able to come from anywhere

One of t= he practical downsides of CPFP that I haven't seen discussed in
this= conversation is that it requires the transaction to pre-specify the
key= s needed to sign for fee bumps. This is problematic if you're, for
e= xample, using a vault structure that makes use of pre-signed
transaction= s.

What if the key you specified n the anchor outputs for a bunch o= f
pre-signed txns is compromised? What if you'd like to be able todynamically select the wallet that bumps fees? CPFP does you no favorshere.

There is of course a tension between allowing fee bumps to co= me from
anywhere and the threat of pinning-like attacks. So we should ve= nture
to remove pinning as a possibility, in line with the first design<= br>principle I discuss.


---

Coming down to earth, the &qu= ot;tabula rasa" thought experiment above has led
me to favor an app= roach like the transaction sponsors design that Jeremy
proposed in a pri= or discussion back in 2020[1].

Transaction sponsors allow feerates t= o be bumped after a transaction's
broadcast, regardless of the struc= ture of the original transaction.
No rebroadcast (wasted bandwidth) is r= equired for the original txn data.
No wasted chainspace on only-maybe-us= ed prophylactic anchor outputs.

The interface for end-users is very= straightforward: if you want to bump
fees, specify a transaction that c= ontributes incrementally to package
feerate for some txid. Simple.
In the original discussion, there were a few main objections that I noted= :

1. In Jeremy's original proposal, only one sponsor txn per txi= d is
=C2=A0 =C2=A0allowed by policy. A malicious actor could execute a p= inning-like
=C2=A0 =C2=A0attack by specifying an only-slightly-helpful = feerate sponsor that
=C2=A0 =C2=A0then precludes other larger bumps.
I think there are some ways around this shortcoming. For example: what=
if, by policy, sponsor txns had additional constraints that

=C2= =A0 - each input must be signed {SIGHASH_SINGLE,SIGHASH_NONE}|ANYONECANPAY,=
=C2=A0 - the txn must be specified RBFable,
=C2=A0 - a replacement f= or the sponsor txn must raise the sponsor feerate,
=C2=A0 =C2=A0 includ= ing ancestors (maybe this is inherent in "is RBFable," but
= =C2=A0 =C2=A0 I don't want to conflate absolute feerates into this).
That way, there is still at most a single sponsor txn per txid in the<= br>mempool, but anyone can "mix in" inputs which bump the effecti= ve
feerate of the sponsor.

This may not be the exact solution we = want, but I think it demonstrates
that the sponsors design has some flex= ibility and merits some thinking.

The second objection about sponsor= s was

2. (from Suhas) sponsors break the classic invariant: "on= ce a valid
=C2=A0 =C2=A0transaction is created, it should not become inv= alid later on unless
=C2=A0 =C2=A0the inputs are double-spent."
This doesn't seem like a huge concern to me if you consider the tx= id
being sponsored as a sort of spiritual input to the sponsor. While th= e
theoretical objection against broadening where one has to look in a tx= n
to determine its dependencies is understandable, I don't see what = the
practical cost here is.

Reorg complexity seems comparable if= not identical, especially if we
broaden sponsor rules to allow blocks t= o contain sponsor txns that are
both for txids in the same block _or_ al= ready included in the chain.

This theoretical concession seems prefe= rable to heaping more rules onto
an already labyrinthine mempool policy = that is difficult for both
implementers and users to reason about practi= cally and conceptually.

A third objection that wasn't posed, IIR= C, but almost certainly would
be:

3. Transaction sponsors require= s a soft-fork.

Soft-forks are no fun, but I'll tell you what als= o isn't fun: being on
the hook to model (and sometimes implement) a = dizzying potpourri of
mempool policies and special-cases. Expecting wall= et implementers to
abide by a maze of rules faithfully in order to ensur= e txn broadcast and
fee management invites bugs for perpetuity and netwo= rk behavior that is
difficult to reason about a priori. Use of CPFP in t= he long-term also
risks needless chain waste.

If a soft-fork is t= he cost of cleaning up this essential process,
consideration should be g= iven to paying it as a one-time cost. This
topic merits a separate post,= but consider that in the 5 years leading
up to the 2017 SegWit drama, w= e averaged about a soft-fork a year.
Uncontroversial, "safe" c= hanges to the consensus protocol shouldn't be
out of the question wh= en significant practical benefit is plain to see.

---

I hope = this message has added some framing to the discussion on fees,
as well p= rompting other participants to go back and give the
transaction sponsor = proposal a serious look. The sponsors interface is
about the simplest I = can imagine for wallets, and it seems easy to
reason about for implement= ers on Core and elsewhere. =C2=A0 =C2=A0 =C2=A0 =C2=A0

I'm not o= ut to propose soft-forks lightly, but the current complexity
in fee mana= gement feels untenable, and as evidenced by all the
discussion lately, f= ees are an increasingly crucial part of the system.



[0]: https://lists.linuxfoundation.org/piperma= il/bitcoin-dev/2022-January/019817.html
[1]: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020= -September/018168.html
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