Tsinghua University Tsinghua University Tsinghua University Zhongguancun Laboratory Due to the fact that the Internet forwards packets in accordance with the IP destination address, packet forwarding generally occurs without examination of the source address. As a result, malicious attacks have been initiated by utilizing spoofed source addresses. The inter-domain source address validation architecture represents an endeavor to enhance the Internet by employing state machines to generate consistent tags. When two end hosts at different address domains (ADs) of the IPv6 network communicate with each other, tags will be appended to the packets to identify the authenticity of the IPv6 source address. This memo focuses on the communication protocol between ACSs and AERs of the SAVA-X mechanism.