A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFCs 7230, 7231, 7232, 7233, 7234, and 7235.
Non-standard header fields were conventionally marked by prefixing the field name with X- but this convention was deprecated in June 2012 because of the inconveniences it caused when non-standard fields became standard [REF].