software integration pitfalls when using iframe

There is a portal and now I would like to add a completely stand alone web application to that portal. One approach would be to simply put the whole thing in an iframe. But besides showing the web app in the portal its user/login management needs to be integrated, so that a portal user can log in on the portal and when he navigates to the web app, he sh开发者_开发技巧all already be logged in on the web app too. What are the basic steps in approaching this? And what problems come to your mind?

thanks for your thoughts, teebee