title: Recognize changes in files
body: <p>my app checks a website for some files with simple <code>NSURLConnection</code> Method. Now I want to recognize if one of the files has changed without downloading the file and compare it.</p>
  
  <p>I thought about md5 checksums but how can I do this without wasting traffic downloading the whole file.</p>
  
  <p>Do you have any ideas for this?</p>