back port of net plugin changes from #10390

plugins/net_plugin/net_plugin.cpp

@@ -1801,7 +1801,7 @@ namespace eosio {
       // sync need checks; (lib == last irreversible block)
       //
       // 0. my head block id == peer head id means we are all caught up block wise
-      // 1. my head block num < peer lib - start sync locally
+      // 1. my head block num < peer lib - send handshake (if not sent in handle_message) and wait for receipt of notice message to start syncing
       // 2. my lib > peer head num - send an last_irr_catch_up notice if not the first generation
       //
       // 3  my head block num < peer head block num - update sync state and send a catchup request
@@ -1828,6 +1828,9 @@ namespace eosio {
                   ("ep", c->peer_name())("lib", msg.last_irreversible_block_num)("head", msg.head_num)
                   ("id", msg.head_id.str().substr(8,16)) );
          c->syncing = false;
+         if (c->sent_handshake_count > 0) {
+            c->send_handshake(true);
+         }
          return;
       }
       if (lib_num > msg.head_num ) {