Merge #7974: More comments on the design of AttemptToEvictConnection.

d90351f More comments on the design of AttemptToEvictConnection. (Gregory Maxwell)

src/net.cpp

@@ -877,6 +877,14 @@ class CompareNetGroupKeyed
     }
 };
 
+/** 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 them than honest peers.
+ */
 static bool AttemptToEvictConnection(bool fPreferNewConnection) {
     std::vector<CNodeRef> vEvictionCandidates;
     {
@@ -905,15 +913,15 @@ static bool AttemptToEvictConnection(bool fPreferNewConnection) {
 
     if (vEvictionCandidates.empty()) return false;
 
-    // Protect the 8 nodes with the best ping times.
+    // Protect the 8 nodes with the lowest minimum ping time.
     // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
     std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), ReverseCompareNodeMinPingTime);
     vEvictionCandidates.erase(vEvictionCandidates.end() - std::min(8, static_cast<int>(vEvictionCandidates.size())), vEvictionCandidates.end());
 
     if (vEvictionCandidates.empty()) return false;
 
     // Protect the half of the remaining nodes which have been connected the longest.
-    // This replicates the existing implicit behavior.
+    // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
     std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), ReverseCompareNodeTimeConnected);
     vEvictionCandidates.erase(vEvictionCandidates.end() - static_cast<int>(vEvictionCandidates.size() / 2), vEvictionCandidates.end());
 
@@ -941,6 +949,7 @@ static bool AttemptToEvictConnection(bool fPreferNewConnection) {
     vEvictionCandidates = mapAddrCounts[naMostConnections];
 
     // Do not disconnect peers if there is only one unprotected connection from their network group.
+    // This step excessively favors netgroup diversity, and should be removed once more protective criteria are established.
     if (vEvictionCandidates.size() <= 1)
         // unless we prefer the new connection (for whitelisted peers)
         if (!fPreferNewConnection)