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:
The numbers in diagram correspond to the following steps, which implement the IPN message authentication protocol:
PayPal sends your IPN listener a message that notifies you of the event
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
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
After successfully completing the protocol, your back-office or administrative process vets the contents of the message and responds appropriately. For example, the IPN messages may trigger you to print shipping labels for items whose payments have cleared, investigate disputes and chargebacks, store information in an accounting system, and so on.
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