PayPal IPN unique identifier

Question

I always assumed that txn_id sent with IPN message is unique. PayPal guidelines seem to support this idea - https://cms.paypal.com/us/cgi-bin/?cmd=_render-conte

Answer 1

Not all IPN messages contain a $_POST['txn_id'], so if you solely check for a txn_id, you may intermittently not log IPN messages where they don't contain this key.