Improve docs about feeler connections

Author: naumenkogs

src/net.cpp

@@ -1784,7 +1784,7 @@ void CConnman::SetTryNewOutboundPeer(bool flag)
 
 // Return the number of peers we have over our outbound connection limit
 // Exclude peers that are marked for disconnect, or are going to be
-// disconnected soon (eg one-shots and feelers)
+// disconnected soon (eg ADDR_FETCH and FEELER)
 // Also exclude peers that haven't finished initial connection handshake yet
 // (so that we don't decide we're over our desired connection limit, and then
 // evict some peer that has finished the handshake)

src/net.h

@@ -143,10 +143,19 @@ enum class ConnectionType {
     MANUAL,
 
     /**
-     * Feeler connections are short lived connections used to increase the
-     * number of connectable addresses in our AddrMan. Approximately every
-     * FEELER_INTERVAL, we attempt to connect to a random address from the new
-     * table. If successful, we add it to the tried table.
+     * Feeler connections are short-lived connections made to check that a node
+     * is alive. They can be useful for:
+     * - test-before-evict: if one of the peers is considered for eviction from
+     *   our AddrMan because another peer is mapped to the same slot in the tried table,
+     *   evict only if this longer-known peer is offline.
+     * - move node addresses from New to Tried table, so that we have more
+     *   connectable addresses in our AddrMan.
+     * Note that in the literature ("Eclipse Attacks on Bitcoin’s Peer-to-Peer Network")
+     * only the latter feature is referred to as "feeler connections",
+     * although in our codebase feeler connections encompass test-before-evict as well.
+     * We make these connections approximately every FEELER_INTERVAL:
+     * first we resolve previously found collisions if they exist (test-before-evict),
+     * otherwise connect to a node from the new table.
      */
     FEELER,