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.