The source path isolation engine (SPIE), developed at BBN, provides accurate tracing of single IP packets through a network. The SPIE system calculates several small hash values for each packet as it traverses a router, then stores these values in a data structure called a Bloom filter. Given a packet and an approximate time that packet was in the network, the SPIE system queries routers along the potential reverse path; a packet was "seen" at a router if the Bloom filter has stored the packet's hash values. The SPIE system has been proven to work effectively and efficiently for IP version 4, largely because an IPv4 packet has sufficient entropy in the first 28 bytes to allow the hash functions to "uniquely" identify each packet. We extend the traceback architecture to IP version 6, with the interesting result that the packet structure of IPv6 does not provide as much entropy in the field values as IPv4.