It is often useful to learn the path that packets take through the Internet. This is especially important for dealing with certain denial-of-service attacks, where the source IP is forged. There are other uses as well, including path characterization and detection of asymmetric routes. There are existing tools, such as traceroute, but these generally provide the forward path, not the reverse. We propose an ICMP Traceback message to help solve this problem. When forwarding packets, routers can, with a low probability, generate a Traceback message that is sent along to the destination. With enough Traceback messages from enough routers along the path, the traffic source and path can be determined. The output of this group will be a standards-track RFC describing the format of such a message, along with guidelines for generation and interpretation of such messages. Major issues to be addressed include end user privacy, the desire of some ISPs to keep their network structure proprietary, authentication of these messages, and the structure of any necessary PKI.