Chris's Wiki :: blog/web/ConditionalGETAndPartialMatches
In HTTP, a conditional GET is a GET request that potentially can be replied with a HTTP '304 Not Modified' status; this is quite useful for polling relatively unchanging resources like syndication feeds (although syndication feed readers don't always do so well at it). Generally speaking, there are two potential validators for conditional GET requests; the If-None-Match header, validated against the ETag of the reply, and the If-Modified-Since header, validated against the Last-Modified of the reply. A HTTP client can remember and use either or both of your ETag and your Last-Modified values (assuming you provide both).
When a HTTP client sends both If-Modified-Since and If-None-Match, the fully correct, specifications compliant validation is to require both to match. This makes intuitive sense; both your ETag and your Last-Modified values are part of the state of what you're replying with, and if one doesn't match, the client has a different view of the URL's state than you do so you shouldn't claim it's 'not modified' from their state. Instead you should return the entire response so that they can update their view of your Last-Modified state.
In practice, two things potentially get in the way. First, it's common for syndication feed readers and other things to treat the 'If-Modified-Since' value they provide as a timestamp, not as an opaque string that echoes back your previous Last-Modified. Programs will put in what's probably some default time value, they'll use timestamps from internal events, and various other fun things. By contrast, your ETag value is opaque and has no meaning for programs to interpret, guess at, and make up; if a HTTP client sends an ETag, it's very likely to be one you provided (although this isn't certain). Second, it's not unusual for your ETag to be a much stronger validator than your Last-Modified; for example, your ETag may be a cryptographic hash of the contents and will definitely change if they do, while your Last-Modified is an imperfect approximation and may not change even if the content does.
