contiki ipv6测试问题

I'm using simple_udp_sendto to communicate between nodes. A border
router was initialized with a prefix to start the RPL mesh netwrok.

I'm sending large files (ELF modules, 1~2 KB length) divided in chunks
(about 30 - 40 chunks of 64B each) using this function, however, some
time after I got this error:

tcpip_ipv6_output: no route found, using default route

And then communication between nodes is broken in only one direction.

The "server" node can receive messages from the "client" asking for
chunks, but not in the other way.