China Telecom China Telecom CERNET Center/Tsinghua University Verizon Inc. Swisscom For the IPv6 transition, IPv6-only is considered the final stage where only IPv6 protocol is used for transport while maintaining global reachability for both IPv6 and IPv4 services. This document introduces a framework for a multi-domain IPv6-only underlay network from the perspective of network providers. In particular, it proposes stateless address mapping as the basis for enabling IPv4 service data transmission in a 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, and discusses the security considerations. This framework is not intended to replace existing IPv6-only technologies, but rather to leverage or remain compatible with them.