Flow Labels

A source can use the 24-bit flow label field in the IPv6 header to label those packets for which it requests special handling by the IPv6 routers, such as non-default quality of service or real-time service. This aspect of IPv6 is still experimental and subject to change as the requirements for flow support in the Internet become clearer. Hosts or routers that do not support the functions of the flow label field are required to set the field to zero when originating a packet, forward the field unchanged when forwarding a packet, and ignore the field when receiving a packet.

What Is a Flow?

A flow is a sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers. The nature of that special handling might be conveyed to the routers by a control protocol, such as a resource reservation protocol, or by information within the flow's packets themselves, for example, in a hop-by-hop option.

Active flows from a source to a destination can be multiple and can contain traffic that is not associated with any flow. The combination of a source address and a non-zero flow label uniquely identifies a flow. Packets that do not belong to a flow carry a flow label of zero.

The flow's source node assigns a flow label to a flow. New flow labels must be chosen (pseudo-)randomly and uniformly from the range 1 to FFFFFF hex. This random allocation makes any set of bits within the flow label field suitable for use as a hash key by routers for looking up the state associated with the flow.

Packets Belonging to the Same Flow

All packets belonging to the same flow must be sent with the same source address, same destination address, and same non-zero flow label. If any of those packets includes a hop-by-hop options header, then they must all be originated with the same hop-by-hop options header contents (excluding the next header field of the hop-by-hop options header). If any of those packets includes a routing header, then they must all be originated with the same contents in all extension headers up to and including the routing header (excluding the next header field in the routing header). The routers or destinations are permitted, but not required, to verify that these conditions are satisfied. If a violation is detected, it should be reported to the source by an ICMP parameter problem message, Code 0, pointing to the high-order octet of the flow label field (that is, offset 1 within the IPv6 packet).

Routers are free to opportunistically set up the flow-handling state for any flow, even when no explicit flow establishment information has been provided to them from a control protocol, a hop-by-hop option, or other means. For example, upon receiving a packet from a particular source with an unknown, non-zero flow label, a router can process its IPv6 header and any necessary extension headers as if the flow label were zero. That processing would include determining the next-hop interface, and possibly other actions, such as updating a hop-by-hop option, advancing the pointer and addresses in a routing header, or deciding how to queue the packet based on its Priority field. The router can then choose to remember the results of those processing steps and cache that information, using the source address plus the flow label as the cache key. Subsequent packets with the same source address and flow label can then be handled by referring to the cached information rather than examining all those fields that, according to the requirements of the previous paragraph, can be assumed unchanged from the first packet seen in the flow.