Names of variables, fields, and macros beginning with two underscores are reserved for internal use by tcng macros. Users should never use such names for fields or variables defined in their own configurations.
In rare cases, items with such names are part of a published interface (e.g. the macro __trTCM_green), and may then be used with due caution.
In order to avoid conflicts with parameter names or other reserved words of the tcng language, field names should be composed of the protocol name, an underscore, and the name of the actual header field, e.g. ip_proto or tcp_sport.