In the ideal situation webstart startup logic should behave as follows:

If all of the following is true:
  a) initial JNLP file is local (even if it is in browser cache yet)
  b) all required JNLP files contain <update check="background">
  c) all required resources are present in the cache

Then startup of webstart application should not require any network requests.

Background thread should check for updates and install updates to the cache.