Reliable Transport Protocol
Reliable Transport Protocol (RTP) is a Cisco proprietary protocol used for communication between EIGRP routers. It is a Transport Layer protocol that reliably allows EIGRP routers to exchange their various types of EIGRP packets.
Reliable Transport Protocol is not to be confused with Real-time Transport Protocol which uses the same acronym.
See also: EIGRP - RTP's reliable message exchange process.
Links:
https://forum.networklessons.com/t/eigrp-packets-explained/910/30?u=lagapides