Mark “Murch” Erhardt and Mike Schmidt are joined by Johan Torås Halseth and Abubakar Ismail to discuss Newsletter #276.
The Bitcoin Optech Podcast and transcription content is licensed Creative Commons CC BY-SA 2.0
Bitcoin Core PR Review Club
Releases and release candidates
Notable code and documentation changes
Mike Schmidt: Welcome everyone to Bitcoin Optech Newsletter #276 Recap on Twitter Spaces. Today we’re going to be discussing the mailing list and looking for a new host for the Bitcoin-Dev and potentially Lightning-Dev mailing lists, HTLC aggregation with covenants, a Bitcoin Core PR Review Club that covers performance improvements around fee estimation, the Bitcoin Core 26.0rc2, and more. I’m Mike Schmidt, I’m a contributor at Optech and also Executive Director at Brink, funding open-source Bitcoin developers. Murch?
Mark Erhardt: Hi, I’m Murch, I’m an engineer at Chaincode Labs and a moderator on Bitcoin Stack Exchange.
Mike Schmidt: Johan?
Johan Torås Halseth: Hi, I’m Johan, I’m an open-source engineer at NYDIG, working on Bitcoin and Lightning primarily.
Mike Schmidt: Abubakar?
Abubakar Sadiq Ismail: Hi everyone, I’m Abubakar Sadiq Ismail, Bitcoin co-Contributor, supported by Btrust Builders.
Mailing list hosting
Mike Schmidt: Thank you both for joining us this week. We’re going to run through the newsletter sequentially here, starting with the topic, Mailing list hosting. So, there was a post to the mailing list about the mailing list, and as folks may know, when visiting the archives, you’ll see that the Linux Foundation has been kind enough to host the mailing list, as well as web archives of the mailing list discussion. And I think there’s been some effort on their part to not do that hosting for open-source projects any longer. I think even as early as 2017, there’s been some discussion about them wanting to not do that any longer, and now it’s actually come to fruition that they’ve sort of given the, “End of year, we’re going to shut down the mailing list functionality”. So, archives will continue to be hosted at the Linux Foundation, including the existing URLs, which is nice because it would be a nightmare for all of those reference discussions to either have their URLs changed or have broken links for all of that. So, that’s great, but the mailing list functionality, the emailing functionality, is going to be going away.
So, the post was made outlining the situation and some options about how to move forward. I thought in reading the posts to the mailing list about hosting mailing list discussions moving forward, it’s evident just how conscientious the Bitcoin Development participants are. If you read the full writeup, there’s just a bunch of great consideration around making sure people can have backups in a future state, making sure that there’s accessibility for everybody, making sure that there’s privacy options and that censorship is minimized in these considerations. Murch, I don’t know if you dug into this topic or have been following some of the IRC discussion around this?
Mark Erhardt: Not too much, but maybe just to provide some context on why the mailing list is important. While many projects have their own communication channels to work on tickets and PRs and like to debate the roadmap of their own project, this is basically the only place that is institutionalized to announce new releases, well not even new releases, but just work on BIPs and things that concern the entire Bitcoin ecosystem. So for example, in the BIP process, one of the required steps is an announcement to the mailing list in order to make sure that all the affected parties have seen the BIP and can chime in. So, the Bitcoin-Developer mailing list sort of has this announcement function to make sure that all of the basically unorganized interested parties have a central way of learning about what’s going on.
Mike Schmidt: Yeah, that’s good context. And I would recommend it’s not a technical writeup, but I would recommend folks look at this post in its entirety. It goes into a lot of different considerations. You may think, “Oh, just run your own mailing list software and email server”. There’s just a lot more that goes into this, and I think it’s interesting to read through that. So, I’d recommend folks do that. As we’ve noted in previous newsletters, unrelated to this mailing list hosting issue, that a bunch of the discussions around new Bitcoin proposals or Bitcoin technology has been happening on the Delving Bitcoin web forum. And so, that is actually mentioned I think in this writeup as well, but it seems like some of the discussion’s already sort of peeling off into some different medium. And we know that Optech and Dave, who sources the material for the news section, is looking at particular parts of the Delving Bitcoin Forum to pull out news items for Bitcoin Optech. Anything else to add, Murch? Okay, thumbs up.
HTLC aggregation with covenants
Next news item from the newsletter is HTLC aggregation with covenants. Johan posted to the mailing list an idea about using covenants to aggregate multiple Hash Time Locked Contracts (HTLCs) into a single output. Luckily, we have Johan here, author of the post and idea, and Johan, maybe to start, what does aggregating multiple HTLCs into a single output actually mean? And then maybe we can get into the mechanics of it.
Johan Torås Halseth: Yeah. So, thanks for reading through that long post I posted. Hopefully, it had some good ideas that we could use going forward when we design new covenants or changes to Bitcoin Script. So, the main idea about doing HTLC aggregation is that we can, in LN today, every time you get an HTLC, I won’t go into details of what an HTLC is, but I could do that if people want, then you actually have to create a new output on the commitment transaction. And the problem here is that that’s expensive and for small payments that doesn’t really work, especially in a high fee environment.
So, the idea about HTLC aggregation is that instead of adding a new output to the commitment every time there’s a new HTLC, you just increase the value of a single HTLC output on the commitment. And that has some real good benefits, especially it gets much cheaper. The commitment won’t increase in size, and that also simplifies protocol development a lot because you don’t have to account for this commitment having a larger size, based on the number of HTLCs that are going through the node. So, that’s the main benefit, it’s the cost reduction of having just a single output in contrast to a lot of outputs on the commitment. And as a result from this as well, you also make certain types of attacks harder to do, especially the one called channel jamming with slots, since the point of doing channel jamming, like what’s called slot jamming, is that you can increase the commitment transaction size so much that a node needs to stop forwarding HTLCs because the commitment gets too large. And that wouldn’t be possible with a change like this.
Mike Schmidt: Maybe a quick tangent, what are other existing proposals to do HTLC aggregation, if any?
Johan Torås Halseth: I don’t think there has been really. We haven’t really been looking at – so that was kind of the point of this post as well, to look a little bit into the future and assuming if we get a covenant enabled on Bitcoin at some point, what kind of changes would we do to LN in order to take advantage of this new functionality. So, I think most future LN spec development has been around ANYPREVOUT and eltoo, which is a great change and simplifies a lot of the protocol in LN as well. But we haven’t really looked at what does this mean for HTLCs and the HTLC outputs on the commitment.
Mike Schmidt: So, we went over the benefits, both in terms of channel jamming and other, but I’m curious about the mechanics. You mentioned that there is some sort of a covenant required here. Does your proposal outline a particular covenant, or just the properties of a covenant that would enable this type of functionality?
Johan Torås Halseth: Yes, so I’ve implemented a proof of concept of this and for that, I’ve been using OP_CHECKCONTRACTVERIFY, which is part of the MATT proposal by Salvatore, and he’s also here, I see. But I think you could do this with many of the covenant proposals, as long as you have a recursive covenant. Because the idea here is that you have some data in the input, you peel off some HTLC of some value from the input, and then you send the rest to the same covenant in the output. So, as long as you have a recursive covenant, I think you should be able to do this. OP_CHECKCONTRACTVERIFY makes it very simple because it has this notion of having some embedded data in the input, but I’m pretty sure you could do it with something like CHECKSIGFROMSTACK or OP_TXHASH as well.
Mike Schmidt: I see in the writeup, and you’ve mentioned it just now, that you do have a demo/proof of concept doing this. So, if folks are interested in playing around with this, they can jump in and play with that; is that right?
Johan Torås Halseth: Yeah, that’s right. It should be linked in the mailing list post.
Mike Schmidt: So, one downside, maybe not a downside, but I guess the fact that there’s a prerequisite on this covenant is, maybe it could be seen as a downside. But assuming that we had this type of covenant implemented, what would be the considerations of other potential downsides or things to keep in mind or limitations if this were in place?
Johan Torås Halseth: I think it’s sort of like a strictly beneficial change in terms of LN. It’s also like a local change, so you wouldn’t need a network-wide upgrade on LN to start using this. You only need to agree with your channel peer that you move to this kind of HTLC format. Other than that, it’s obviously making a protocol change on LN as well as on Bitcoin. It takes a lot of time, a lot of testing. So, assuming we had a covenant tomorrow, I wouldn’t start implementing and rolling this out right away. There’s also so many more improvements we can make to LN if we have a new opcode and a covenant. So, I think it would take some time to actually flesh out the various ways we could upgrade LN if that was the case.
Mike Schmidt: So, we linked to the Lightning-Dev mailing list post that you had, and I don’t see that there’s been any responses there. Perhaps you’ve gotten feedback offline or elsewhere about the idea, and if so, maybe you can outline what that feedback’s been?
Johan Torås Halseth: Yes, I’ve gotten some feedback, and it’s mainly about the idea in general, “If you have this, then that would be beneficial for LN”. I think I haven’t gotten any feedback on the actual technical details of implementing it this way. I am sorry, the post got pretty long, because it outlined different use cases of this change. So, I think it takes some time to for people to grok it perhaps. And my idea was also to just get people thinking about what we can do in LN if we had covenants.
Mike Schmidt: Great. Murch, do you have any questions or comments?
Mark Erhardt: Yeah. So, I’ve looked a little bit, very briefly, at this mailing list post, and one thing that isn’t quite clear to me yet is how do you go back from the single HTLC output to executing or not executing each HTLC in the according direction, depending on what happened upstream or downstream? So, clearly each HTLC depends on whether the payment went through upstream or didn’t, and then has to be folded either to one side or the other. So, if you had folded everything together into that single HTLC, do you fan it back out to resolve the HTLCs; or how do you envision that?
Johan Torås Halseth: No, so you wouldn’t have to fan out at any point. You only consume the aggregated HTLC output, then you take whatever value you claim into your own output, and then you send the rest back to a recursive aggregated HTLC output. So, how that would work from an offered or received HTLC point is that you have two or more scriptpaths in the taproot output. So, if you’re in the timeout case, you do one of the scriptpaths; if you’re in the offered case, you do another one of the scriptpaths; and then you send it back to the same, in my example, you don’t modify the tapscript tree at all, you just kind of empty out the HTLCs you claimed, and then you send the rest of it back to the same tapscript tree.
Mark Erhardt: So, basically, you’re just peeling off the ones that you can settle already, and the remainder goes back into the caching state where they can be peeled off later once they resolve?
Johan Torås Halseth: Exactly. So, the way I did this, I encoded in a merkle tree, and the merkle tree also includes the value of the HTLC. So, in order to claim an HTLC, we can enforce in the script that you set the value of this HTLC to zero in the new output. So, that means that you cannot claim it anymore, or you could, but it’s a zero-value HTLC. So, that wouldn’t be a point in doing that.
Mike Schmidt: Johan, any calls to action for the audience, other than parsing through our writeup and your longer write-up on the mailing list and potentially playing around with the prototype?
Johan Torås Halseth: Yeah, I think playing around with it, just look at the idea. And also, one thing it seems like all these kinds of covenant proposals would need at some point is a way to do 64-bit arithmetics. So, that’s also something that will be great to start looking at again if we want to enable that on Bitcoin, because that seems like a no-brainer to me.
Mike Schmidt: Well, thanks, Johan, for joining us. You’re welcome to stay on as we go through the rest of the newsletter. If you have other things to do, you can drop.
Johan Torås Halseth: Sure. Thanks for having me.
Fee Estimator updates from Validation Interface/CScheduler thread
Mike Schmidt: Moving out of the news section and on to our monthly segment that highlights a Bitcoin Core PR review club session. This month we highlighted, “Fee Estimator updates from Validation Interface/CScheduler thread”. And the PR author, as well as the host for this PR Review Club, is here this week to chat with us. Abubakar, at a high level this PR modifies the way the transaction fee estimator data is updated. Maybe as a place to start, what was wrong with the way that the fee estimator data was updated previously?
Abubakar Sadiq Ismail: Thank you very much, Mike. So previously, the fee estimator updates are from the mempool. So, the fee estimator is a member of the mempool, so whenever a new block arrives, we update the fee estimator directly from the mempool which slows down block processing, and in turn also delays block relay to other peers. So, we will always want to finish block processing fast. And also, it will be inefficient to add other complex steps during the fee estimator update previously. So, this PR is intended to enable the fee estimator to listen to validation interface notifications to process new transactions that are added and removed from the mempool.
Mike Schmidt: So, it sounds like at the time that a block is being received and processed, that there was also, at the same time, some fee estimation code that was running that was potentially slowing down the processing and then propagation there of that block; is that right? And so what you’ve done is separated out that fee estimation, or at least the updating of the data around fee estimation, to not be blocking the processing of that block; do I have that right?
Abubakar Sadiq Ismail: Yes, previously it is updating synchronously whenever we are processing new blocks.
Mike Schmidt: How did you come across this as an issue that you could potentially work on? I’m curious, I know it’s a non-technical question, but I just was curious.
Abubakar Sadiq Ismail: Yeah, it’s basically there was attempt to the solution previously. So, it is up for grabs for everyone to work on. So, it seems like a nice issue that I think needs to be solved. So, I think BlueMatt is the author that worked on it previously. But previously, it is not only attempting to update the fee estimator from validation interface, it is also splitting the validation interface into two, whereby we have validation interface and mempool interface. What the PR intends to do is to have all notifications from validation events coming from the validation interface, and all notifications that are from mempool events will come from mempool interface. There are also some refactors and other changes. Previously, it’s a pretty huge PR, but this PR is only focusing on updating the fee estimator from validation interface notifications.
Mike Schmidt: Okay, so this original PR, this one from 2017 from BlueMatt, #11775, did something similar, but it maybe was a bit more ambitious and didn’t get merged due to the other things that were in there. So, you took the motivation behind that PR with respect to fee estimator updates and spun it off into its own PR.
Abubakar Sadiq Ismail: Yes.
Mike Schmidt: Okay, great. We highlighted, I think, nine different questions from the PR Review Club, and usually we don’t jump into all of those, because they’re usually somewhat technical and harder to follow on audio. But if folks are interested in the high level that you’ve given here, Abubakar, I would encourage them to jump into that Q&A that we have in the newsletter, as well as the transcripts for this particular PR Review Club meeting. But of those questions that were asked, and maybe even the broader discussion that was had in the PR Review Club, are there interesting pieces that you think would be worthy of highlighting for the audience, Abubakar?
Abubakar Sadiq Ismail: Yeah, I think the interesting thing that we discussed about it is the benefits of having CTxMempool as an internal component of the mempool, which means that CTxMempool will have access to all the mempool transaction data. In the future whereby we want to add other fee estimator features, it will be much easier to do that because it will have access to CTxMempool entry. But with this PR, there is a new struct that I created, which is NewMempoolTransactionInfo, whereby we only add the transaction data that the fee estimator needs and add it to the validation interface with the creation callback that the fee estimator will update from. So, that’s one of the, I think, drawbacks, but the fee estimator does not need all that information. It only needs the base fee, the fee size, and the time that the transaction was added to the mempool.
Mike Schmidt: Murch, obviously you do a lot with fees and fee estimations and feerates. I’m not sure if you have comments or questions on this particular change or some of the context behind it.
Mark Erhardt: Well, it does sound great to me that it would be removed from the block validation chain and sort of asynchronously attached to the process, so that fee estimation no longer has to be finished before we forward a block. In the broader context, of course, the latency by which each new block is received by all the nodes affects directly the mining revenue of smaller miners and also the chance of just getting tied or conflicting chain tips. So, making block validation and block forwarding as fast as possible should be generally one of our goals to make mining as fair as possible. So, I haven’t reviewed this PR directly, but what Abubakar is trying to achieve here makes a lot of sense to me.
Mike Schmidt: Abubakar, on that note, are there any metrics that you’ve come up with about how much faster or what potential impacts there would be that are quantifiable with regards to this PR?
Abubakar Sadiq Ismail: No, I have not run any benchmarks yet, but there are some comments in the PR that are asking for benchmarks. But I think high level, this is beneficial and people really want it in, even without the benchmark. But yeah, it will be interesting to see how this PR will improve locally.
Mike Schmidt: From your perspective, well actually maybe for the audience, as of this recording this PR is still open and feedback is being provided and changes are being made accordingly. Abubakar, what would you say is any holdup to getting this in? Is there any particular contention or things that needs to be updated, or is it just a matter of addressing some smaller review comments?
Abubakar Sadiq Ismail: Yeah, I think it’s review comments. It has gone through various rounds of reviews, has some ACKs from I think TheCharlatan and only TheCharlatan ACKs the PR, but it was invalidated after a review from Gloria, so it’s currently on review and I will appreciate any comments or feedback on the work so far.
Mike Schmidt: Excellent. Before we move on, Abubakar, anything else that you think people should be aware of around this PR? Otherwise, we’ll move on in the newsletter.
Abubakar Sadiq Ismail: No, I think it’s fine. Everyone who is interested should go to the GitHub repo.
Mike Schmidt: Thanks for joining us, Abubakar, you’re welcome to stay on or if you need to drop, we understand. Next section from the newsletter is Releases and release candidates. We have three this week.
Bitcoin Core 26.0rc2
The first one is Bitcoin Core 26.0rc2. I’m going to take the opportunity now to plug our podcast from #274, specifically the podcast and not the newsletter, because Murch went through a thorough tour of the 26.0 changes and some of the release notes, so I would advise folks if you’re curious what is this new release, what’s in there, what are the high-level new features and changes, check out the audio or the written transcription of his verbal overview as a good starting place. We also note in the newsletter this week that if you’re curious about how to test this RC as an end user, that the Bitcoin Core PR Review Club that we just went through with Abubakar, there’s going to be a session on that dedicated to testing this release candidate on November 15, so just in less than a week.
So, this is something that I think is approachable regardless of your technical skills. You can jump in and go through how to think about testing and mechanics of actually executing testing as an end user and providing feedback. Murch, any commentary on the RC2 binaries being out and the Review Club dedicated to testing?
Mark Erhardt: Yes, I just wanted to point out that currently the link appears to be going to the wrong page and we will fix that shortly. But there is a fairly extensive RC Testing Guide already, and it goes into some detail on now currently nine different new features that could be tested. For example, it walks you through how to install the RC in the first place, then how to connect to a BIP324 node that enables v2 transport and some tests around that. There’s basically for the nine different topics a few small steps on how you can test those. This is just a starting point; if you want to test more you can obviously just play around with these things more, and we will fix the link in the newsletter shortly.
Mike Schmidt: Which link is it; the one to the wiki that’s incorrect?
Mark Erhardt: Yeah, it seems to be going to an empty page and be missing the words, “Release Candidate” before testing.
Mike Schmidt: Yeah, I see that Max Edwards made some substantial changes to the RC Testing Guide in the last two hours, and so that would be a great link for folks as well. I suspect we’ll be covering this Testing Guide maybe a bit more. Hopefully we can get Max on maybe for next week or shortly thereafter to go through some of this in person. I know we’ve done that previously with some of the testing guides, so looking forward to that.
Core Lightning 23.11rc1
Next release is to Core Lightning (CLN). It’s actually an RC for 23.11. It’s the RC1, codenamed Bitcoin Orange Paper. There’s a couple of things that I wanted to highlight it just at a high level without being too much of a spoiler alert for this release. This release makes a bunch of additions and changes to the JSON RPCs. Too many to go through here, so check out the release notes for this RC for details if you’re using CLN, and a couple notable fixes that are included in the release. I believe we talked about one or both of these, but one is a fix for peer disconnects, and another is related to stuck HTLCs when using splicing. So, we would obviously encourage folks who are running CLN to check out those JSON RPC changes and bug fixes as well, and provide feedback accordingly. Murch?
Mark Erhardt: Yeah, I just glanced at the release notes earlier, and two things that jumped out at me was that the configuration has changed in this release. Wumbo channels are now default in this CLN release; and another one that seemed potentially useful to our listeners is, there is now a recover JSON RPC command, and it will restart an unused lightningd node with the –recover flag. So, I don’t know exactly what the recover startup option will entail, but if you need that, you might be happy to hear that it is in this current release.
Mike Schmidt: And probably related to this release is the fact that we also have five CLN PRs that we’ll be covering later in the newsletter as well, including that wumbo/large channel configuration change.
Last release this week is LND 0.17.1-beta.rc1. I saw three notable things from this release, one is a reduction in CPU usage due to the new mempool scanning logic; second one is enhancements to the CPFP logic, specifically around anchor outputs; and the last one is a bug fix for the new taproot channel type that they’ve implemented, that in some situations could have caused a channel to show as inactive in some situations. So again, call to any LND node runners to run this if this is important to your operations, if you’re a business, and provide feedback accordingly.
Mark Erhardt: Yeah, it’s funny how when the mempool is above 160 sat/vbyte, suddenly all these fixes for bumping come out.
Mike Schmidt: Yeah, necessity is the mother of the invention, right? Moving on to Notable code changes. As I mentioned, there was five from CLN and so it’s obviously nearing CLN release season.
Core Lightning #6824
First one is Core Lightning #6824, and this PR is based on a proposed change to the Lightning protocol, specifically BOLT2, which covers peer protocol and channel management. And that BOLT2 proposed change is part of the dual funding protocol PR that is currently open, and so there’s a specific commit related to this BOLT2 change. And the BOLT PR notes that there are cases where you can’t reconcile states if there’s a disconnection that occurs during the signing process. So, in order to mitigate that potential condition, additional state needs to be stored such that if a disconnect does happen, peers can reconnect and continue the signing process. So, all that is from the related BOLT PR.
This particular CLN PR reworks how it reconnects, and how that works for dual-funding channel opens, and it wants to be in line with the recommendation from the BOLTs PR, which I think was made by t-bast. Murch, did you get a chance to dig into any of this?
Mark Erhardt: Sorry, no, I have not. But well, you know, how do I put this? Any complex system grows from a very simple kernel idea, and I think that with dual funding, it seems to be exactly that process once again. It’s a very simple idea, “Hey, what if both people can add funds?” And then it turns out that there are just all these little things that crop up over time as the idea develops further, and implementations work on trying to make that fit into their previous processes, all these little kinks crop up and you get all these little improvements. And eventually, you get a much more complex solution that does address the concern comprehensively, but you can’t just jump in and immediately drop in a working solution for something like a protocol like this. Anyway, I’m rambling; go on!
Core Lightning #6783
Mike Schmidt: Next PR is Core Lightning #6783, which deprecates the large channels configuration option, which is also known historically in some circles as wumbo channels. Now, large channels are the new default for all users. And some background on this is that originally the spec limited channel sizes to, I believe, 16 million satoshis. That was early on in Lightning’s iteration and wanted to prevent loss-of-fund situations with larger amounts of funds. And I guess to your point earlier about things maturing, now that option is essentially enabled. It’s the default enable to use large channels. So, I guess that’s sort of somewhat a milestone in the Lightning world to say that they feel that it’s confident enough to use larger channels.
Mark Erhardt: It’s also kind of funny that back when wumbo was first introduced and limited max channels capacity to one-sixth of a bitcoin, that was only a few hundred bucks. And just due to how the overall exchange rate has evolved, that probably was able to carry demand for a much longer time just because, well, a sixth of a bitcoin has a lot more value transfer possibility now than it used to be.
Core Lightning #6780
Mike Schmidt: Next PR is Core Lightning as well, #6780. The PR is titled, “Anchors multi utxo”. And what we wrote in the newsletter is that, “This improves support for fee bumping onchain transactions associated with anchor outputs”. And before we jumped on, I tapped Murch on the shoulder to see if he could dig into this one, since I was not grokking it myself. So, Murch, I’m not sure if you were able to grok this?
Mark Erhardt: I stared at it a little bit and it seems to me that this change introduces the capability for bumping anchor outputs with transactions that have multiple inputs, and it seems that previously CLN would ever only use a single additional input. And, well, in cases that you need more funds or your UTXOs are too fragmented and especially as the fees are currently exploding upwards again, it seems useful to be able to have multiple inputs on your anchor output transaction.
So, when you close a channel, one of the big issues is that you are using a commitment transaction that you may have negotiated many weeks in advance, and the feerate environment might be completely different at the time that you’re closing the channel versus when you negotiated the commitment transaction. So, the anchor outputs, of course, are a mechanism that enables you to attach another transaction that is unencumbered, because the two local and two remote outputs are, of course, encumbered so that the person that closes the channel cannot immediately spend the funds, and the other person has a chance to publish their justice transaction.
So, in order to make that fair, I think on anchor output closing transactions, both the remote and local output are locked, and either can use the anchor output to bump the closing transaction. And yeah, so this just improves on the mechanics of using anchor outputs in CLN and allows for more flexibility by adding more inputs.
Core Lightning #6773
Mike Schmidt: Next Core Lightning PR is #6773, and this PR enhances backup functionality by allowing users to verify their already existing backups. And instead of a new RPC to do that verification, the decode RPC, which existed previously, now verifies that the contents of a backup file are valid and contain the latest information to perform a full recovery, related to the full recovery feature that Murch outlined in the CLN release. And there’s two pieces of information that are verified during this backup, the emergency recover and also peer storage data pieces of data stores are validated and made sure that they’re both up to date.
Core Lightning #6734
Last Core Lightning PR, #6734. So, we need to dig into this one. So, we did the writeup as, it updates the listfunds RPC in CLN to provide information around CPFP fee bumping mutual close transaction. But looking at the PR this morning, that doesn’t seem to be what Core Lightning #6734 does. The title of that PR is, “CPFP for mutual close”. So, I’m going to call this an Optech review failure and blame myself for either not catching this mismatch, or alternatively not understanding how this PR pertains to the writeup that we put in the newsletter. Murch, I know you took a look at this as well. I don’t know if you concur with that or maybe we need to get back to the folks on this.
Mark Erhardt: I didn’t.
Mike Schmidt: Okay, it’s close enough that it seems correct, but it doesn’t really seem like it affects the listfunds RPC. So, our bad on this one, guys!
Last PR this week is to the Eclair repository, #2761. And quoting t-bast from the PR, “Splicing and dual funding introduce a new scenario that could not happen before, where the channel initiator, the one paying the fees for the commitment transaction, can end up below the channel reserve, or just above the channel reserve, but any additional HTLCs would make it go below the reserve”. And in that case, it means that most of the channel’s funds are on the non-initiator side of the channel so that, “We should allow HTLCs from the non-initiator to the initiator to move funds towards the initiator”, and that would result in a more balanced channel where both sides meet the channel reserve requirement. And Eclair is allowing the ability to slightly dip into that channel reserve if that’s the case, and they limit that to at most five pending HTLCs to limit the exposure.
This change that was implemented in Eclair was actually proposed for the LN spec, but that recommendation was closed because the other LN implementations didn’t like that approach and preferred an alternate approach, which was tracking the previous channel reserve until the new channel reserve was met. And so, because the other implementations did it this other way, t-bast said he didn’t like that approach that the other implementations were taking, because it was awkward to track, and he preferred this option still, and that it achieved similar benefits by just allowing a few extra HTLCs. It’s a little bit of discrepancy in how to handle this scenario in the LN implementations. And t-bast went on his own, it sounds like. Murch?
Mark Erhardt: Yeah, I was staring at this a little bit, and I was wondering how a splice could cause the channel initiator to drop below the channel reserve. And I think the reason this is happening is, the channel reserve is required to be 1% of the channel capacity. And when you add more funds to the remote side, the overall capacity is increasing, but the initiator’s balance is not increasing, right? So, when the initiator was already close to the channel reserve on the old amount and the overall capacity increases, it now may drop below that 1% of the new capacity by no fault of their own. And so now, almost all the funds are on the non-initiator side, and in order to move them over, you couldn’t add an HTLC because that would increase the channel reserve requirement, because you basically lock in more funds for the fees because the transaction is getting bigger, and now your channel reserve on the initiator side is even smaller compared to the overall capacity.
So, that just as context for why this is an issue, and if any LN people find that I’m misrepresenting that, please leave us feedback in the comments on Twitter.
Mike Schmidt: Well, thanks to our special guests this week, Johan and Abubakar, for joining us and explaining what they’ve been working on. Thanks always to my co-host, Murch, and thanks to you all for joining us this week. Murch, any parting words?
Mark Erhardt: I will see you in three weeks. I’m on vacation, bye!
Mike Schmidt: Oh yeah, that’s right, we won’t have Murch for the next few weeks. So, I think Dave may be joining us, Dave Harding, so that will be great. And hopefully we can get by without you, and hopefully you enjoy your time off, Murch. Cheers.