IPN Overview :

• PayPal Instant Payment Notification is a call back system that will get initiated once a tranction is completed(eg: When ExpressCheckout completed successfully).
• you will receive the transaction related ipn variables on your call back url that you have specified in your request.
• You have to send this ipn variable back to PayPal system for verification, Upon verification PayPal will send a response string "VERIFIED" or "INVALID".
• PayPal will continuously resend this ipn, if a wrong ipn is sent.

IPN How to run?

• Ipn Listener script is provided for different language.
• Deploy IPN Listener script in Cloud environment or you can expose your server port using any third party LocalTunneling software , so that you can receive PayPal IPN call back.
• Make a PayPal api call (eg: DoDirect Payment request), setting the IpnNotificationUrl field of api request class to the url of deployed IPNListener script.
• You will receive ipn call back from PayPal.

IPN Reference :

• You can refer IPN guide at [https://www.x.com/developers/paypal/documentation-tools/ipn/integration-guide/IPNIntro]