China Telecom China Telecom CERNET Center/Tsinghua University Indirection Network Inc. Alibaba Cloud This document defines MP-BGP extension and the procedures for IPv4 service delivery in multi-domain IPv6-only underlay networks. It defines a new TLV in the BGP Tunnel Encapsulation attribute, used in conjunction with a specific AFI/SAFI combination for advertising IPv4-over-IPv6 mapping rules. The behaviors of each type of network (IPv4 and IPv6) are also illustrated. In addition, this document provides the deployment and operation considerations when the extension is deployed.