What is the IPv6 Flow Label field for?

The 20-bit Flow Label tags all packets belonging to a single data flow between two applications so routers can give them consistent, special handling (e.g. for QoS or a fixed path).

A "flow" is a stream of related packets — say one video call or one TCP session. By stamping them all with the same flow label, the source tells the network "treat these as one unit":

• Routers can apply the same QoS/priority decision to the whole flow without re-inspecting upper-layer headers
• Can be used to pin a flow to a particular path, useful for delay-sensitive audio/video
• Combined with the Traffic Class field, this gives IPv6 its built-in Quality-of-Service capability

Tip: The Flow Label means a router can recognise "this belongs to the call I'm already prioritising" by reading one IPv6 field, instead of digging into TCP/UDP port numbers.

Go deeper:

• IPv6 packet — Flow Label (Wikipedia) — what the 20-bit flow label tags and how routers use it for consistent handling.