Identify a visit via session id (no replies)

Hi Piwik Community,

Currently I'm evaluating different Web Analytics Solutions for an online portal. Every user has to login and must allow cookies.

The question is:
Will Piwik create it's own session ids to identify a visit?
Or will Piwik use the user-agent / IP-adress combination to identify a visit?
...or can Piwik use exisiting session IDs which were created by the portal?

It would also be interesting to know if other Web Analytics systems handle this differently

Best Regards