message: Justify the choice of ADDR cache lifetime
diff: @@ -2568,6 +2568,31 @@ std::vector<CAddress> CConnman::GetAddresses(Network requestor_network, size_t m
  if (m_addr_response_caches.find(requestor_network) == m_addr_response_caches.end() ||
  m_addr_response_caches[requestor_network].m_update_addr_response < current_time) {
  m_addr_response_caches[requestor_network].m_addrs_response_cache = GetAddresses(max_addresses, max_pct);
  +
  + // Choosing a proper cache lifetime is a trade-off between the privacy leak minimization
  + // and the usefulness of ADDR responses to honest users.
  + //
  + // Longer cache lifetime makes it more difficult for an attacker to scrape
  + // enough AddrMan data to maliciously infer something useful.
  + // By the time an attacker scraped enough AddrMan records, most of
  + // the records should be old enough to not leak topology info by
  + // e.g. analyzing real-time changes in timestamps.
  + //
  + // It takes only several hundred requests to scrape everything from an AddrMan containing 100,000 nodes,
  + // so ~24 hours of cache lifetime indeed makes the data less inferable by the time
  + // most of it could be scraped (considering that timestamps are updated via
  + // ADDR self-announcements and when nodes communicate).
  + // We also should be robust to those attacks which may not require scraping *full* victim's AddrMan
  + // (because even several timestamps of the same handful of nodes may leak privacy).
  + //
  + // On the other hand, longer cache lifetime makes ADDR responses
  + // outdated and less useful for an honest requestor, e.g. if most nodes
  + // in the ADDR response are no longer active.
  + //
  + // However, the churn in the network is known to be rather low. Since we consider
  + // nodes to be "terrible" (see IsTerrible()) if the timestamps are older than 30 days,
  + // max. 24 hours of "penalty" due to cache shouldn't make any meaningful difference
  + // in terms of the freshness of the response.
  m_addr_response_caches[requestor_network].m_update_addr_response = current_time + std::chrono::hours(21) + GetRandMillis(std::chrono::hours(6));
  }
  return m_addr_response_caches[requestor_network].m_addrs_response_cache;
  