Function SelectNodeToEvict

src/node/eviction.cpp:178–240

}

[[nodiscard]] std::optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates)
{
    // Protect connections with certain characteristics

    ProtectNoBanConnections(vEvictionCandidates);

    ProtectOutboundConnections(vEvictionCandidates);

    // Deterministically select 4 peers to protect by netgroup.
    // An attacker cannot predict which netgroups will be protected
    EraseLastKElements(vEvictionCandidates, CompareNetGroupKeyed, 4);
    // Protect the 8 nodes with the lowest minimum ping time.
    // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
    EraseLastKElements(vEvictionCandidates, ReverseCompareNodeMinPingTime, 8);
    // Protect 4 nodes that most recently sent us novel transactions accepted into our mempool.
    // An attacker cannot manipulate this metric without performing useful work.
    EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
    // Protect up to 8 non-tx-relay peers that have sent us novel blocks.
    EraseLastKElements(vEvictionCandidates, CompareNodeBlockRelayOnlyTime, 8,
                       [](const NodeEvictionCandidate& n) { return !n.m_relay_txs && n.fRelevantServices; });

    // Protect 4 nodes that most recently sent us novel blocks.
    // An attacker cannot manipulate this metric without performing useful work.
    EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);

    // Protect some of the remaining eviction candidates by ratios of desirable
    // or disadvantaged characteristics.
    ProtectEvictionCandidatesByRatio(vEvictionCandidates);

    if (vEvictionCandidates.empty()) return std::nullopt;

    // If any remaining peers are preferred for eviction consider only them.
    // This happens after the other preferences since if a peer is really the best by other criteria (esp relaying blocks)
    //  then we probably don't want to evict it no matter what.
    if (std::any_of(vEvictionCandidates.begin(),vEvictionCandidates.end(),[](NodeEvictionCandidate const &n){return n.prefer_evict;})) {
        vEvictionCandidates.erase(std::remove_if(vEvictionCandidates.begin(),vEvictionCandidates.end(),
                                  [](NodeEvictionCandidate const &n){return !n.prefer_evict;}),vEvictionCandidates.end());
    }

    // Identify the network group with the most connections and youngest member.
    // (vEvictionCandidates is already sorted by reverse connect time)
    uint64_t naMostConnections;
    unsigned int nMostConnections = 0;
    NodeClock::time_point nMostConnectionsTime{NodeClock::epoch};
    std::map<uint64_t, std::vector<NodeEvictionCandidate> > mapNetGroupNodes;
    for (const NodeEvictionCandidate &node : vEvictionCandidates) {
        std::vector<NodeEvictionCandidate> &group = mapNetGroupNodes[node.nKeyedNetGroup];
        group.push_back(node);
        const auto grouptime{group[0].m_connected};

        if (group.size() > nMostConnections || (group.size() == nMostConnections && grouptime > nMostConnectionsTime)) {
            nMostConnections = group.size();
            nMostConnectionsTime = grouptime;
            naMostConnections = node.nKeyedNetGroup;
        }
    }

    // Reduce to the network group with the most connections