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.