Flow-Label Trends in IPv6 Traffic: A 9-Year Analysis of a Dataset Collected in Japan

RFC 6437 introduces the 20-bit flow label field in the IPv6 header and recommends it to be used alongside source and destination IP addresses for efficient flow classification. A recent paper from Google authors explains how they have begun to use this field in their IPv6 network to select an alternative path, thereby preventing congestion or outages. However, there is no prior research investigating whether connections make use of this field on the Internet. In this paper, we present a comprehensive analysis of the 9-year MAWI dataset (2016-2024), the Internet traces gathered from a large backbone link in Japan, and examine whether the IPv6 flow label is indeed employed on the Internet. Our findings reveal an increasing upward trend in the adoption of the flow label field. In particular, this trend correlates with the use of QUIC, whose specification—different from TCP—explicitly repeats the recommendation to utilize the flow label.