6.2 EIGRP Technology
6.2.3 Reliable transport protocol
One of the more important new features of EIGRP is its capability to provide guaranteed, reliable delivery of its various packets. Other protocols eschew reliable delivery and rely on other mechanisms such as the passing of time to determine whether a packet needs to be retransmitted. Unfortunately, the fundamental flaw in such an approach is that it aggravates the convergence process. The longer it takes a network to converge, the greater the opportunity for disrupting service across the network. EIGRP was given a new protocol, the Reliable Transport Protocol (RTP), to provide reliable delivery of its own packets.

RTP is a transport-layer protocol that correlates to the functionality identified by Layer 4 of the Open System Interconnection (OSI) reference model. RTP is a private innovation of Cisco Systems, however, and is not an open standard. IP uses two similar transport protocols: TCP and User Datagram Protocol (UDP). In fact, RTP embodies some of the functionality of both TCP and UDP. TCP provides reliable delivery of IP datagrams and can resequence datagrams received out of order. UDP provides a more efficient, but unreliable, delivery of IP datagrams. RTP can support both reliable and unreliable delivery of datagrams. Instead of creating a new transport protocol, the designers of EIGRP could have used TCP or UDP as the transports for EIGRP messages. However, this would have made EIGRP distinctly IP specific. The designers' goal was a truly protocol-independent routing protocol that could easily be extended to support any new routing protocol, such as IPv6, that may be developed in the future. Consequently, a new transport protocol was needed: RTP. RTP was specifically developed to meet these requirements.

RTP is used to transport all EIGRP message types through a network. However, not every EIGRP packet requires reliable delivery! Some functions, such as the exchanging of hello packets, just don't warrant the overhead of acknowledging receipt. RTP can deliver hello packets (and other packet types) in an unreliable manner.

RTP can also support both multicasting and unicasting. Multicast packets are delivered to multiple, specific destinations simultaneously using a group address. Unicast packets are explicitly addressed to a single destination. RTP can even support both multicast and unicast transmissions simultaneously for different peers.