RPL Routing header having Next Header,Hdr Ext Len,Routing Type,Segments Left,RPL InstanceID and so on.An IP header is header information at the beginning of an Internet Protocol Packet.An IP Packet is the smallest message entity exchanged via the Internet Protocol across an IP network. IP Packets consist of a header for addressing and routing, and a payload for user data.
Open file at below location : contiki-ng/os/net/ipv6/sicslowpan.c
Here,the function implementing header length of each packet in RPL protocol.