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Bitcoin Optech Newsletter #63
This week’s newsletter describes a demo implementation of eltoo and several discussions related to it, requests comments on limiting the normal number of LN gossip updates to one per day, and provides our longest-to-date list of notable changes to popular Bitcoin infrastructure projects.
None this week.
● Eltoo sample implementation and discussion: on the Lightning-Dev mailing list, Richard Myers posted a link to a sample implementation of an eltoo payment flow between nodes on a custom signet. Eltoo is a proposal to replace LN’s current LN-penalty enforcement layer with a new layer named LN-eltoo. LN-penalty prevents counterparty theft by giving nodes the ability to financially penalize a counterparty that attempts to publish an old channel state onchain. LN-eltoo accomplishes the same goal by giving the later states the ability to spend funds from earlier states within a certain period of time—eliminating the need for a penalty, simplifying many aspects of the protocol, and reducing the complexity of many proposed protocol enhancements. Myers’s sample implementation works by using the Bitcoin Core functional test framework to simulate payments in an eltoo payment channel.
This led to a discussion (1, 2) of whether using miniscript would help make LN “more future-proof and extensible than directly using Bitcoin Script.”
It also led to eltoo co-author Christian Decker writing a summary of why he thinks eltoo is especially valuable in providing a clean separation of protocol layers. For example, by making state changes in eltoo similar to state changes in Bitcoin, this would allow tools and contract protocols built for regular Bitcoin transactions (state changes) to be easily reused within payment channels.
● Request for comments on limiting LN gossip updates to once per day: Rusty Russell posted to the Lightning-Dev mailing list his plan to limit the number of gossip updates C-Lightning will accept to one per day normally. By his calculations based on the current network characteristics, this should limit the amount of bandwidth used for gossip updates to about 12 MB per day, making it easier to use LN on slow devices (like single-board computers) and low-bandwidth network connections. He requests feedback from anyone who thinks that will cause a problem for users of any current implementation.
Notable code and documentation changes
Notable changes this week in Bitcoin Core, LND, C-Lightning, Eclair, libsecp256k1, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.
● Bitcoin Core #16185 updates the
gettransactionRPC with a new parameter
decode. If set to True, a
decodedfield will be added to the RPC output containing a version of the transaction decoded into JSON fields (the same format used when requesting verbose output with the
● Bitcoin Core #15759 increases the number of outbound connections the node will make from 8 to 10. The two new connections will only be used to announce and relay new blocks; they won’t announce or relay unconfirmed transactions or
addrpeer-discovery messages. Only announcing blocks minimizes the bandwidth and memory overhead of the new connections and makes it much more difficult for an adversary to map the connections between nodes. An adversary that can map all of a node’s connections can attack that node, either by identifying which transactions originate from that node (a privacy leak) or by isolating the node from the rest of the network (potentially making theft of funds from the node possible). For details on how an adversary could map the network topology using transaction relay, see the TxProbe paper.
● Bitcoin Core #15450 allows users to create new wallets for multiwallet mode from the GUI, completing a set of GUI actions that also allows users to load and unload wallets. The action opens a dialog that lets the user name the wallet and set various wallet options.
● Bitcoin Core #16421 follows up PR#15681 (described in Newsletter #56) by allowing a carve-out transaction to be replaced using RBF. Carve-out transactions are allowed to slightly exceed Bitcoin Core’s limits on transaction size and number of ancestors. When carve-out was added, the exception to those rules was not applied to transaction replacements, so nodes would accept carve-outs but not RBF fee bumps of them. With this PR, RBF fee bumping of the carve-out is now possible, making it an even more useful tool for fee management of settlement transactions in two-party LN payment channels.
● LND #3401 caps the amount of onchain fee that a node will propose paying in a channel update transaction (commitment transaction), limiting it to 50% of the node’s current in-channel balance (the 50% default is adjustable). In theory, the channel can still be used after this—which is why it isn’t closed—although the node may not have enough funds to initiate a spend, possibly making receiving its only option unless onchain feerates drop. The LN protocol only allows the node that opens a channel to propose new commitment transactions with feerate changes, so this change only applies to channel initiators.
● LND #3390 separates tracking of HTLCs from invoices. Previously, each invoice was only meant to be associated with one HTLC, so the details were the same. More recent innovations such as Atomic Multipath Payments (AMP) will allow the same invoice to be paid incrementally by multiple HTLCs, so this change makes it possible to independently track either individual HTLCs or the overall invoice. The change also improves tracking of existing HTLCs that were paid more than once and simplifies the logic for the hold invoices described in Newsletter #38.
● C-Lightning #3025 updates the
listfundsRPC with a
blockheightfield for confirmed transactions indicating the height of the block containing them.
● C-Lightning #2938 delays reprocessing of incoming HTLCs at startup until after all plugins have been loaded. This prevents plugin hooks from being called before their plugin has loaded.
● C-Lightning #3004 removes all the features deprecated in C-Lightning 0.7.0, including:
listpaymentsRPC as well as the deprecated
descriptionparameter or output field in
waitsendpay. (See Newsletter #36)
The deprecated older style of colon-delimited short channel identifier (
Block:Transaction_index:Output_index). Instead, use the standardized BOLT7 format delimited using an x (
● C-Lightning #2924 abstracts C-Lightning’s database handling code and SQL queries so that it can be adapted to handle other backends besides the default sqlite3. Future PRs are expected to “add concrete implementations of [other database] drivers.”
● C-Lightning #2964 updates the
txprepareRPC to allow it to pay multiple outputs in the same transaction.
● Libsecp256k1 #337 makes it easy to configure the size of a pre-computed table used by the library to speed up signature generation. This can save about 32 KB of memory, which is a significant amount on some embedded devices that use libsecp256k1.
● BOLTs #656 adds a feature bits specification to BOLT11, allowing payments to indicate which features they support or require. This is planned to be used for several new features still under development.