IE Types 32768..65535 can be used by custom implementation and the Enterprise-ID must be set to the IANA SMI Network Management Private Enterprise Codes of the issuing party. Very similar to GTP-C, PFCP uses UDP. Port 8805 is reserved. For reliability, a similar re-transmission strategy as for GTP-C is employed, lost messages being sent N1-times at T1-intervals. Transactions are identified by the 3-byte long Sequence Number, the IP address and port of the communication peer. The protocol includes an own Heart-beat Request/Response model, which allows monitoring the availability of communication peers and detecting restarts (by use of a Recovery-Timestamp Information Element). For User-Plane packet exchanges between the Control and User Plane functional elements, GTP-U for the Sx-u interface, or alternatively a simpler UDP or Ethernet encapsulation for the N4-u interface (to be confirmed, as standards are still incomplete).