This part of ISO/IEC 17811 provides the specification of Reliable Message Delivery Protocol (RMDP), which is an application-layer protocol used to provide uniform and reliable message delivery among devices regardless of the underlying network protocols or interfaces. The network security is out of scope in this part of ISO/IEC 17811. However, the security services might be necessary according to applications of RMDP. RMDP might suffer from many network-specific threats. To countermeasure those threats, some security mechanism can be deployed.