Allen's Hierarchy of Conversational Agents

James Allen heads a research group at the University of Rochester.
He wrote the standard NLU text almost 20 years ago.
For the last 10 or 15 years, his large group has been working on this problem. Their best system is probably the TRAINS system.
By their own admission, they've come no where close to solving it.

Allen has a hierarchy of types of system

Technique Used	Example Task	Dialogue Phenomena Handled
Finite-state Script	Long-distance dialing	User answers questions
Frame-based	Getting train arrival and departure information	User asks questions, simple clarifications by system
Sets of Contexts	Travel booking agent	Shifts between predetermined topics
Plan-based Models	Kitchen design consultant	Dynamically generated topic structures, collaborative negotiation subdialogues
Agent-based Models	Disaster relief management	Different modalities (e.g. planned world and actual world)