Share files and session details among sub domains

[eluser]max123[/eluser]
I have several related sub domains and I want to share session details and files among sub domains

eg:

http://sub_domain1/abc.com
http://sub_domain2/abc.com
http://sub_domain3/abc.com

there are 3 related web sites for above 3 sub domains. But I want to use the same header and footer views for all the sub domains.

Also once I logged in to the system from sub_domain1, and once I move to sub_domain2 or sub_domain3, still the user should be kept as an already logged user. That means no need to sign in again.

How can I do these 2 things