title: What variable is sent in every PayPal IPN?
body: <p>I've been trying to base my IPN off <code>txn_type</code>, then <code>payment_status</code> but I've been getting exceptions since not every IPN has those. What variable can I base my IPN off of?</p>
  
  <p><code>txn_type</code> was not sent in refund IPNs.</p>