  In this tutorial paper, we look into the evolution and prospect of network
architecture and propose a novel conceptual architecture for the 6th generation
(6G) networks. The proposed architecture has two key elements, i.e., holistic
network virtualization and pervasive artificial intelligence (AI). The holistic
network virtualization consists of network slicing and digital twin, from the
aspects of service provision and service demand, respectively, to incorporate
service-centric and user-centric networking. The pervasive network intelligence
integrates AI into future networks from the perspectives of networking for AI
and AI for networking, respectively. Building on holistic network
virtualization and pervasive network intelligence, the proposed architecture
can facilitate three types of interplay, i.e., the interplay between digital
twin and network slicing paradigms, between model-driven and data-driven
methods for network management, and between virtualization and AI, to maximize
the flexibility, scalability, adaptivity, and intelligence for 6G networks. We
also identify challenges and open issues related to the proposed architecture.
By providing our vision, we aim to inspire further discussions and developments
on the potential architecture of 6G.
