anchor.diff:                  # Set the default gateway.
                   ${optionalString (cfg.defaultGateway != "") ''
                     # FIXME: get rid of "|| true" (necessary to make it idempotent).
  -                  ip route add default via "${cfg.defaultGateway}" || true
  +                  ip route add default via "${cfg.defaultGateway}" ${windowSize} || true
                   ''}
   
                   # Turn on forwarding if any interface has enabled proxy_arp.
  
positive.code: let
  
    cfg = config.networking;
    interfaces = attrValues cfg.interfaces;
    hasVirtuals = any (i: i.virtual) interfaces;
  
  
negative.code:       example = "131.211.84.1";
        description = ''
          The default gateway.  It can be left empty if it is auto-detected through DHCP.
        '';
      };
  
      networking.defaultGatewayWindowSize = mkOption {
        default = "";
        example = "524288";
        description = ''
          The window size of the default gateway. It limits maximal data bursts that TCP peers
          are allowed to send to us.
        '';
      };
  
  