China Telecom China Telecom CERNET Center/Tsinghua University Verizon Inc Swisscom For the IPv6 transition, IPv6-only is considered as the final stage, where only IPv6 protocol is used for transport while maintaining global reachability for both IPv6 and IPv4 services. This document illustrates a framework of multi-domain IPv6-only underlay network from an operator's perspective. In particular, it proposes stateless address mapping as the base for enabling IPv4 service data transmission in an multi-domain IPv6-only environment(i.e.,IPv4-as- a-Service). It describes the methodology of stateless IPv4/IPv6 mapping, illustrates the behaviors of network devices, analyzes the options of IPv6 mapping prefix allocation, examines the utilization of SRv6, and discusses the security considerations.