What is the difference between if-modified-since and unless-modified-since (http headers)? What web server response should we send to this? IE9 is requesting the unless-modified-since header which we are unfamiliar with.
Most of my research pushes is leading towards the if-modified-since header. My understanding is that the unless-modified-since header is IE specific.
You can go to this link: https://i.sstatic.net/nacRF.jpg to see the screen shot of the request header.
Unless-Modified-Since was proposed in a draft for the Range extension/feature of HTTP.
According to the draft, the idea was that a client can send a Range request with a Unless-Modified-Since header and the server/cache would return the range, unless the file was modified since that date, in which case the server/cache would return the entire file instead. (This might be useful for say resuming a download - if the file was not modified, it can continue, if it was it would restart)
If-Range is the standard alternative (It is the same, but will take ETags as well).
If-Modified-Since is relevant for full requests.
If-Unmodified-Since can make the request fail, instead of returning the full file (which If-Range would do) if it changed
