User Guide

Introducing IPN
IPN Protocol and Architecture
12 June, 2009 IPN Guide
The numbers in diagram correspond to the following steps:
1. The API operation initiates a payment 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 a Back-Office Procedure
PayPal generates an IPN message when you perform actions that invoke the PayPal API,
regardless of whether it is from your website or from a back-office or administrative
procedure. You can use this notification to trigger an email to your customer, store information
in a CRM or accounting system, and so on.
The following diagram shows both an administrative web flow and the IPN message
authentication protocol: