Using the IPv6 Flow Label for Path Consistency: A Large-Scale Measurement Study

RFC 6437 specifies the usage of an IPv6 3-tuple (flow label, source and destination address fields) for flow classification in routers. Is this the only flow classification approach in use with IPv6 when the flow label is not zero? If so, end hosts could benefit from knowing that multiple flows between the same host pair traverse the same network bottleneck (for example, single-path congestion control coupling mechanisms could then reduce loss and delay). This paper attempts to answer this question with an investigation of whether path consistency is achievable via the IPv6 3-tuple in the Internet. We employ a novel method that extends Paris traceroute to control the path behavior solely using IPv6 network-layer header data, and conduct a large-scale measurement study from globally stationed vantage points to examine i) the extent to which the flow label is delivered to its destination, and ii) how it influences the behavior of load-balancing algorithms. Our results suggest that the flow label always passes through the network unaltered, but the 3-tuple is not a reliable flow identifier in the current Internet.