I wanted to know why standard HTTP/1.1 header fields define Cache-Control as directives ? and other header fields not marked as same.
I went though the all standard HTTP/1.1 header fields [https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html] definition it only apply with Cache-Control general-header field except Pragma general-header field where I also does not have idea.
Well for a start RFC 2616 has been superseded by RFC 7234
At a guess I would say they are “directives” as they MUST be obeyed, as opposed to other headers which do not define any specific behaviour.
The original HTTP/1.1 RFC 2616 has this to say, over which I have highlighted some relevant phrases:
13.1.3 Cache-control Mechanisms
The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches. In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header for this purpose.
The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms.
