This week’s newsletter summarizes a discussion about lowering the default minimum transaction relay feerate in Bitcoin Core and other nodes. Also included are our regular sections with the summary of a Bitcoin Core PR Review Club, announcements of new releases and release candidates, and descriptions of notable changes to popular Bitcoin infrastructure projects.


  • Lowering the default minimum transaction relay feerate: Bitcoin Core only relays individual unconfirmed transactions that pay a feerate of at least one satoshi per vbyte (1 sat/vbyte). If a node’s mempool fills with transactions paying at least 1 sat/vbyte, then a higher feerate will need to be paid. Transactions paying a lower feerate can still be included in blocks by miners and those blocks will be relayed. Other node software implements similar policies.

    Lowering the default minimum feerate has been discussed in the past (see Newsletter #3) but hasn’t been merged into Bitcoin Core. The topic saw renewed discussion in the past couple weeks:

    • Individual change effectiveness: it was debated by several people how effective it was for individual node operators to change their policies.

    • Past failures: it was mentioned that the previous attempt to lower the default feerate was hampered by the lower rate also reducing the cost of several minor denial-of-service (DoS) attacks.

    • Alternative relay criteria: it was suggested that transactions violating certain default criteria (such as the default minimum feerate) could instead fulfill some separate criteria that make DoS attacks costly—for example, if a modest amount of hashcash-style proof of work committed to the transaction to relay.

    The discussion did not reach a clear conclusion as of this writing.

Bitcoin Core PR Review Club

In this monthly section, we summarize a recent Bitcoin Core PR Review Club meeting, highlighting some of the important questions and answers. Click on a question below to see a summary of the answer from the meeting.

Decouple validation cache initialization from ArgsManager is a PR by Carl Dong that separates node configuration logic from the initialization of signature and script caches. It is part of the libbitcoinkernel project.

  • What does the ArgsManager do? Why or why not should it belong in src/kernel versus src/node?

    ArgsManager is a global data structure for handling configuration options (bitcoin.conf and command line arguments). While the consensus engine may contain parameterizable values (namely, the sizes of caches), a node does not need this data structure to stay in consensus. Rather, as Bitcoin Core-specific functionality, code handling these configuration options belongs in src/node

  • What are the validation caches? Why would they belong in src/kernel versus src/node?

    When a new block arrives, the most computationally expensive part of validation is script (i.e. signature) validation for its transactions. Since nodes keeping a mempool will have usually seen and validated these transactions already, block validation performance is significantly increased by caching (successful) script and signature verifications. These caches are logically part of the consensus engine because consensus-critical block validation code needs them. As such, these caches belong in src/kernel

  • What does it mean for something to be consensus-critical if it isn’t a consensus rule? Does src/consensus contain all the consensus-critical code?

    Participants agreed that signature verification enforces consensus rules, while caching doesn’t. However, if the caching code contains a bug that results in storing invalid signatures, the node would no longer be enforcing consensus rules. As such, signature caching is considered consensus-critical. Consensus code doesn’t actually live in src/kernel or src/consensus yet; much of the consensus rules and consensus-critical code lives in validation.cpp

  • What tools do you use for “code archeology” to understand the background of why a value exists?

    Participants listed several commands and tools including git blame, git log, entering the commit hash into the pull requests page, using the Github Blame button when viewing a file, and using the Github search bar. 

  • This PR changes the type of signature_cache_bytes and script_execution_cache_bytes from int64_t to size_t. What is the difference between int64_t, uint64_t, and size_t, and why should a size_t hold these values?

    The int64_t and uint64_t types are 64-bits (signed and unsigned, respectively) across all platforms and compilers. The size_t type is an unsigned integer guaranteed to be able to hold the length (in bytes) of any object in memory; its size depends on the system. As such, size_t is exactly suited for these variables holding the cache sizes as a number of bytes. 

Releases and release candidates

New releases and release candidates for popular Bitcoin infrastructure projects. Please consider upgrading to new releases or helping to test release candidates.

Notable code and documentation changes

Notable changes this week in Bitcoin Core, Core Lightning, Eclair, LDK, LND, libsecp256k1, Hardware Wallet Interface (HWI), Rust Bitcoin, BTCPay Server, BDK, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.

  • Bitcoin Core #25610 sets by default the startup option -walletrbf and uses by default the replaceable option for the RPCs createrawtransaction and createpsbt. Transactions created via the GUI were already opt-in RBF by default. This follows the update mentioned in Newsletter #208, enabling node operators to switch their node’s transaction replacement behavior from the default opt-in RBF (BIP125) to full RBF. RPC opt-in by default was proposed in 2017 in Bitcoin Core #9527 when the primary objections were the novelty at the time, the inability to bump transactions and the GUI not having functionality to disable RBF—all of which have since been addressed.

  • Bitcoin Core #24584 amends coin selection to prefer input sets composed of a single output type. This addresses scenarios in which mixed-type input sets reveal the change output of preceding transactions. This follows a related privacy improvement to always match the change type to a recipient output (see Newsletter #181).

  • Core Lightning #5071 adds a bookkeeper plugin that provides an accounting record of movements of bitcoins by the node running the plugin, including the ability to track the amount spent on fees. The merged PR includes several new RPC commands.

  • BDK #645 adds a way to specify which taproot spend paths to sign for. Previously, BDK would sign for the keypath spend if it was able, plus sign for any scriptpath leaves it had the keys for.

  • BOLTs #911 adds the ability for an LN node to announce a DNS hostname that resolves to its IP address. Previous discussion about this idea was mentioned in Newsletter #167.