Method AttemptToEvictConnection

src/net.cpp:1000–1072

Try to find a connection to evict when the node is full. * Extreme care must be taken to avoid opening the node to attacker * triggered network partitioning. * The strategy used here is to protect a small number of peers * for each of several distinct characteristics which are difficult * to forge. In order to partition a node the attacker must be * simultaneously better at all of

 *   simultaneously better at all of them than honest peers.
 */
bool CConnman::AttemptToEvictConnection()
{
    std::vector<NodeEvictionCandidate> vEvictionCandidates;
    {
        LOCK(cs_vNodes);

        for (const CNode* node : vNodes) {
            if (node->fWhitelisted)
                continue;
            if (!node->fInbound)
                continue;
            if (node->fDisconnect)
                continue;
            NodeEvictionCandidate candidate = {node->GetId(), node->nTimeConnected, node->nMinPingUsecTime,
                                               node->nLastBlockTime, node->nLastTXTime,
                                               HasAllDesirableServiceFlags(node->nServices),
                                               node->fRelayTxes, node->pfilter != nullptr, node->addr, node->nKeyedNetGroup};
            vEvictionCandidates.push_back(candidate);
        }
    }

    // Protect connections with certain characteristics

    // 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 transactions.
    // An attacker cannot manipulate this metric without performing useful work.
    EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
    // Protect 4 nodes that most recently sent us blocks.
    // An attacker cannot manipulate this metric without performing useful work.
    EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);
    // Protect the half of the remaining nodes which have been connected the longest.
    // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
    EraseLastKElements(vEvictionCandidates, ReverseCompareNodeTimeConnected, vEvictionCandidates.size() / 2);

    if (vEvictionCandidates.empty()) return false;

    // 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;
    int64_t nMostConnectionsTime = 0;
    std::map<uint64_t, std::vector<NodeEvictionCandidate> > mapNetGroupNodes;
    for (const NodeEvictionCandidate &node : vEvictionCandidates) {
        std::vector<NodeEvictionCandidate> &group = mapNetGroupNodes[node.nKeyedNetGroup];
        group.push_back(node);
        int64_t grouptime = group[0].nTimeConnected;

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