How to implement single sign on in joomla?

Question

Here is the scenario:

I want to implement single sign on in joomla.

User's data should be at central database(Main site).

And there will be multiple sites(may have their own database). They might be hosted on separate domain or a sub domain of main site.

I have searched on Google and came across some extensions like JMultisite and JFusion.

But could not find how they actually work at back end.

My requirements:

1. User's database should be at main site
2. Single Sign On should be possible
3. Sites should be independent(development and deployment point of view)
4. Intersite communication should be easy.

How should I implement this in joomla?

Any possible links or suggestions will be appreciated.

Thanks