What are the disadvantages of treating this event as an error discarding the packet and sending an ICMP message?

Answer 1

The IPv6 specification allows routers to ignore Flow Labels and also

allows for the possibility that IPv6 datagrams may carry flow setup

information in their options. Unknown Flow Labels may also occur if a

router crashes and loses its state. During a recovery period, the

router will receive datagrams with Flow Labels it does not know, but

this is arguably not an error, but rather a part of the recovery period.

Finally, if the controversial suggestion that each TCP connection be

assigned a separate Flow Label is adopted, it may be necessary to

manage Flow Labels using an LRU cache (to avoid Flow Label cache

overflow in routers), in which case an active but infrequently used

flow's state may have been intentionally discarded. In any case, it is

clear that treating this situation as an error and, say dropping the

datagram and sending an ICMP message, is inappropriate. Indeed, it

seems likely that in most cases, simply forwarding the datagram as

one would a datagram with a zero Flow Label would give better

service to the flow than dropping the datagram.