About HTTP Headers
HTTP headers allow the client and the server to pass additional information with the request or the response. An HTTP header consists of its case-insensitive name followed by a colon ‘:’, then by its value (without line breaks). Leading white space before the value is ignored.
Custom proprietary headers can be added using the ‘X-‘ prefix, but this convention was deprecated in June 2012, because of the inconveniences it caused when non-standard fields became standard in RFC 6648; others are listed in an IANA registry, whose original content was defined in RFC 4229. IANA also maintains a registry of proposed new HTTP message headers.