Currently opt-in in Bitcoin Core since 0.20, asmap is a change to how Bitcoin Core ensures that it connects to a diverse set of outbound peers beyond just netgroup bucketing. It considers IP-to-ASN mappings to ensure diversity instead. ASNs (Autonomous System Numbers) are a scarcer resource than netgroups, and the mapping passed to Bitcoin Core by default map the gateway ASNs (bottlenecks) for IPs which have sole control over what a node with that IP would see (since P2P connections are currently not encrypted). Essentially, it's an effective mitigation against the Erebus attack, an eclipse attack where the attacker spoofs a part of the internet.

Background reading

Relevant issues and PRs

asmap Tools

  • sipa/asmap - Bottleneck finding, asmap generation and compresion. Most development happening on nextgen branch.
  • asmap-rs - A Rust tool to read and parse RIS raw data from RIPE NCC and generate a file of AS mappings. (sipa/asmap is moving forward as the preferred tool).

How do we make this default in Bitcoin Core?

  • Analyse historic RIPE RIS data to get a general idea of how much these mappings change on a monthly basis. This will advise how often the asmap updates need to happen. Current speculation is that asmaps will be useful for a duration on the order of years, so generating a new one per release would be reasonable.
  • We need a comparison tool to compare two encoded asmaps to determine how much they have changed. We may also want to compare the original non-encoded files too for auditability.
  • We need to have a way to combine/sanitise the RIPE RIS data with another source that is more trusted (involves individuals collecting their own views of the internet).
  • We need a consensus on what the steps for maintainers will be to update the asmap.