IPv6 Flow Label Specification
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S. Amante, B. Carpenter, S. Jiang, J. Rajahalme · 2011-11
Abstract
RFC 6437 specifies the IPv6 Flow Label field, a 20-bit value in the IPv6 header that allows sources to label sequences of packets as belonging to the same flow. Routers can use the flow label for load balancing and quality-of-service treatment without inspecting higher-layer headers.
Why This RFC Matters
The 20-bit Flow Label field in the IPv6 header was present from the beginning but lacked clear semantics for years. RFC 6437 finally gave it a practical definition: a pseudo-random value set by the source and consistent for the lifetime of a flow, enabling routers to perform equal-cost multipath (ECMP) load balancing on a per-flow basis without deep packet inspection. This matters enormously in modern data centers where traffic hashing across multiple links is critical for performance. The flow label also enables stateless traffic classification in hardware, making it one of IPv6's practical advantages over IPv4 in high-throughput network environments.