Payment Plugin Development

I am developing a payment gateway plugin. To process a payment, I submit a webform form to the payment processor (TouchNet Payment Systems) that contains the name, address, email address, phone number and total amount of the purchase to TouchNet by redirecting the WordPress session to the payment vendors site. When payment processing has completed, Touchnet calls back to a predefined web address with information concerning the success or failure of the transaction. Can someone point me to the documentation that shows how I receive that callback and post the results to MarketPress then continue the users WordPress session?

Thanks,
Mike Cagle