In order to be routed, a fragmented packet must be reassembled at
every hop of a multihop link where lower layer fragmentation occurs.
Considering that the IPv6 minimum MTU is 1280 bytes and that an an
802.15.4 frame can have a payload limited to 74 bytes in the worst
case, a packet might end up fragmented into as many as 18 fragments
at the 6LoWPAN shim layer. If a single one of those fragments is
lost in transmission, all fragments must be resent, further
contributing to the congestion that might have caused the initial
packet loss. This draft introduces a simple protocol to forward and
recover individual fragments that might be lost over multiple hops
between 6LoWPAN endpoints.