The MD5 hash that is stored in the transaction is used to look up and delete the transaction from the hash table. When a transaction enters a terminated state, the transaction management layer destroys the transaction if the reference count drops to zero. If the application registered a callback function during initialization, that callback function notifies the application when the transaction changes state. If the application needs to use the transaction in the future, it must hold a reference to the transaction by using the sip_hold_trans() function to prevent the stack from destroying the transaction. After using the transaction the application must release the reference by using the sip_release_trans() function.
The four terminated states for transactions are listed below:
Client invite terminated
Client non-invite terminated
Server invite terminated
Server non-invite terminated