User Guide

IPN Guide June, 2009 13
Introducing IPN
IPN Protocol and Architecture
The numbers in diagram correspond to the following steps:
1. Your back-office or administrative process invokes a PayPal API operation; for example, it
could invoke the RefundTransaction API operation when your employee issues a
refund.
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 implements the IPN authentication protocol (steps 2, 3, and 4 in this
diagram). After successfully completing the protocol, your listener or back-office or
administrative process vets the contents of the message and responds appropriately. For
example, your system can notify the customer of the refund by email.
IPN Messages Generated by PayPal
Some IPN messages generated by PayPal are not directly associated with a web flow. PayPal
generates an IPN message when external events arise that might affect a transaction, such as
disputes, chargebacks, echeck clearing, and various recurring payment and subscription
events.
In this case, events that trigger IPN messages are not directly related to actions on your
website. The following diagram shows the steps your listener must take: