Rails keep adding _session_id in redirect..why?

Hi:

Thanks to Russell N. I could redirect from one rails app to another
app.
But during redirect… rails add an extra session key. Which is not in
my app at all. i.e my session name is _myapp_session_id and both of my
have the same session key as well as the session-domain set to
“.domain.com”.

But during re-direct I get an extra session key “_session_id”… why do i
get this? what does this mean and where can i find a bit more
information about it. Any hint…/help…

Cheers