Java EE (Java Enterprise Edition) General Discussion

I was reading the topic of request headers. As you must be knowing there are many and we can set status code + Response headers for transfering response to user according to the header information we got from user. But one thing I couldnt understand is there are very few headers which by default there in the request object as I got them printed. Then if we want to use some header like Authentication header and I know its not in the request as I got null in return when i tried to get the headervalue, what should be done.

Similatrly for if-modified-since header we can specify the setheader in response as well as we can have the method getLastModified but these are for the response and first of all If I make a login page I dont get this header but lets say if this header is there then who will set the header value ? is it browser ??