User Guide

IPN Guide June, 2009 11
Introducing IPN
IPN Protocol and Architecture
The numbers in diagram correspond to the following steps:
1. The button action initiates a payment that completes on PayPal
2. PayPal sends your IPN listener a message that notifies you of the event
3. Your listener sends the complete unaltered message back to PayPal; the message must
contain the same fields in the same order and be encoded in the same way as the original
message
4. PayPal sends a single word back, which is either VERIFIED if the message originated with
PayPal or INVALID if there is any discrepancy with what was originally sent
Your IPN listener must implement the IPN authentication protocol (steps 2, 3, and 4 in this
diagram). After successfully completing the protocol, your back-office or administrative
process vets the contents of the message and responds appropriately. For example, if the
payment status for the transaction is “Completed,” your system can print a packing list or
email a password to your customer for downloading digital media.
IPN Messages Generated by PayPal APIs
PayPal generates an IPN message when you invoke an API operation, such as
DoExpressCheckoutPayment of DoDirectPayment during checkout. You can use this
notification to kick-off order fulfillment, enable digital media downloads, store information in
a customer relationship management (CRM) or accounting system, and so on.
The following diagram shows both the web flow and the IPN message authentication protocol: