In IPv6, middle routers don't perform packets fragmentation, but the fragmentation is performed end to end.

That is, source and destination nodes are which perform, by means of the IPv6 stack, the fragmentation of a packet and the reassemble respectively.

The fragmentation process consists in dividing into smaller packets the "fragmentable" part of the source packet, and adding to each one the no fragmentable part, that allows to the destination node the reassemble of such packet.