[DO NOT MERGE] Erlay: bandwidth-efficient transaction relay protocol (Full implementation) #35591

Erlay Project Tracking: #30249 Conceptual Discussion: #34542

This is a full implementation of Erlay. Its purpose is to check the integrity and correctness of the implementation against changes/additions that may originate from the review process and/or rebases on top of newer functionality.

This is not to be merged. Functionality will be spread across multiple smaller PRs to ease the review process.

Approach

This approach uses Erlay as a fallback mechanism for transaction propagation. Instead of mixing fanout and reconciliation into a single connection type, the current approach leaves the existing connections as they are, and adds additional low-bandwidth connections to be used in case the node is being eclipsed. This connections should have minimal cost under normal circumstances, and only undergo real traffic in case other 8 full-outbound connections are being captured.

outbound-full-reconciliation connections

For now, we are adding 4 additional reconciliation-only connections to the node while we test it's impact on real node running the approach. Further analysis may be needed to pick a meaningful value for this. The number of inbound connections should also be scaled based on how many connections we are adding.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35591.

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Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35587 (Remove boost as a unit test runner by rustaceanrob)
• #35561 (net: move some CNodeState fields to Peer by Crypt-iQ)
• #35522 (refactor: Extract per-message helpers from SendMessages() (move-only) by pablomartin4btc)
• #35511 (RFC: consensus: Make CAmount a class by hodlinator)
• #35502 (refactor: extract per-message helpers from ProcessMessage (move-only) by w0xlt)
• #34824 (net: encapsulate TxRelay state and replace recursive mutexes by w0xlt)
• #34565 (refactor: extract BlockDownloadManager from PeerManagerImpl by w0xlt)
• #28690 (build: Introduce internal kernel library by sedited)
• #28463 (p2p: Increase inbound capacity for block-relay only connections by mzumsande)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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LLM Linter (✨ experimental)

Possible typos and grammar issues:

• src/node/txreconciliation_impl.cpp: // Schedules the a reconciliation request -> // Schedules a reconciliation request [“the a” is a typo]
• test/functional/p2p_reqtxrcncl.py: self.log.info('REQTXRCNCL sent to an peer 2') -> ...to a peer 2 [“an peer” is misspelled]
• test/functional/p2p_txrecon_responder.py: # Most of them will be pick for reconciliation. -> ...will be picked... [“pick” should be “picked”]
• test/functional/p2p_txrecon_responder.py: # ...the phase moves to EXT_REQUESTED, an a subsequent requests are not allowed -> ...and subsequent requests are not allowed [“an a” is a typo]

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• self.generate_txs(self.wallet, 0, 0, 2) in test/functional/p2p_txrecon_initiator.py
• self.generate_txs(self.wallet, 0, 10, 0) in test/functional/p2p_txrecon_initiator.py
• self.test_reconciliation_initiator_no_extension(20, 15, 0) in test/functional/p2p_txrecon_initiator.py
• self.generate_txs(self.wallet, 0, 1, 0) in test/functional/p2p_txrecon_responder.py

^{2026-06-24 16:06:16}

Rebased on master.

Opening this so it can be tested against CI and to make sure nothing obvious is missing. Next step will be testing with real nodes (most likely in Warnet).

Things to consider/add:

• This is currently using sendtxrcncl as a way, for outbound nodes, to signal they want to establish a full-reconciliation connection, plus adding a limit on the number of full-reconcilition inbounds that will be accepted by a node. This is not the originally intended way of using sendtxrcncl. It may be worth considering an approach based on BIP-434.
• The extension phase was designed so peers with an ongoing reconciliation that had underpredicted the sketch capacity could still reconcile without having to directly fallback to fanout. Given reconciliation is now used as fallback, and the extra bandwidth may not be as problematic, it may be worth considering overshooting the initial sketch capacity and getting rid of the extension phase.
• Fuzz tests are missing

Fixed some typos and addressed some of the linter suggestions

6554707...e63db1a

Fixed several bugs and added a new test for receiving reconciliation messages when reconciliation is not enabled.

e63db1a...35bcd04

Addressed some issues pointed out by corecheck.

35bcd04...280d603