The general HTTP header format contains colon-separated name - value pairs in the header field. Each of the name-value pair end with a carriage return CR and a line feed LF character sequence. Empty fields at the end of each header indicate the end of the header. General Header fields have common applicability in request and response messages. The header fields apply only to the transmitted message and do not apply on the transferred entity. Cache-control field specifies directives that have to be followed by every caching mechanism on a request and response system.
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They define the operating parameters of an HTTP transaction. The header fields are transmitted after the request line in case of a request HTTP message or the response line in case of a response HTTP message , which is the first line of a message. Header fields are colon-separated key-value pairs in clear-text string format, terminated by a carriage return CR and line feed LF character sequence. The end of the header section is indicated by an empty field line , resulting in the transmission of two consecutive CR-LF pairs. In the past, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space SP or horizontal tab HT as the first character on the next line.