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More on the GET Request

HTTP/1.0 permits the GET request (and other HTTP request types, see later) to additionally send a series of optional Request Headers along with the request. For example, here's a typical request to ironbark, snarfed from the local network: 
GET /index.html HTTP/1.0
User-Agent: Mozilla/3.0 (X11; IRIX 5.3 IP12)
Host: ironbark.bendigo.latrobe.edu.au
Accept: image/gif, image/jpeg, */*
Referer: http://ironbark.bendigo.latrobe.edu.au/index.html
The request headers are terminated with a blank line -- hence the need for two newlines, as seen in the first slide of today's lecture. It's also possible for the request to contain a "message body", just like a response message -- we defer discussion of this until later in the unit.
 
Perhaps the most interesting optional request header is "If-modified-since:", which takes an HTTP standard GMT time/date string as its value. If the requested page has not, in fact, been modified in the specified period, it won't be returned -- instead, a "304 Not Modified" response is sent. This is called a Conditional-GET and is very useful in support of caching, of which more later.
 

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