[wpkg-users] http based download checksum?

[Paul McGrath]

Hi,
  I want to try using the http option to try and deploy to remote workers.  In the event that I 'install' or 'upgrade' is it possible to prevent a file (zip file) from being downloaded again to the user if it already exists on the target computer?
How would you code this into the 'install' line?  Do you have to use some sort of checksum tool incase the zip file has changed?

My idea is this:
My XMLs are hosted on web server and the client reads them.  The first install command download zip files to the client temp folder, extracted and then the remaining install/upgrade commands are executed.

Subsequently I have an update but only in one zip file.  I don't want to just have an 'upgrade' command as this would also be a new 'install' zip as well so I want to check the file sizes against the existing ones.
<upgrade include="install" /> -  there are multiple zip files to download, only one has changed for the 'upgrade' so only that one is downloaded, the rest stay the same so they shouldnt be downloaded

I hope you can follow my train of thought here.
Any ideas?
thanks
Paul