It is a pointer to ICMP header. To know about how to attach the variable in control message headers in rpl using contiki cooja simulator.
Step 1: Open file at location:
Home/user/contiki/core/net/ip /uip.h
struct uip_icmp_hdr – to add variables in icmp header
Step 2: To store and access variable
Location: contiki/core/net/rpl/rpl-icmp6.c
UIP_ICMP_BUF->values = node location (any value)