Lab 6. 6LowPAN and RPL

Introduction and goals

Edge routers are routers that can be found on the edge of a network, routing the traffic of that network to a second external network. Its function, in short, is to connect one network to another.

In this lab, we will see how to build a simulation using an edge router in Contiki. More specifically, we will see how a Contiki edge router can be used to route traffic between an RPL network (a Contiki sensor network with routing protocol RPL over IPv6) and an external IPv4 network, following the diagram below:

The goal of the lab is to provide an overview of how to deploy both an RPL network with Contiki in the Cooja simulator, as well as making it interact with a second real external network using the tool tunslip.

Installation of software requirements

The basic installation of Contiki (in its version 2.7) is in the /home/ubuntu/contiki directory of your virtual machine.

Before you begin, you will need to install some support software:

sudo apt install -y openjdk-8-jdk openjdk-8-jre

Next, be sure to select Java version 8 for a correct functioning of the Cooja compilation process, run the following command and make sure that the option /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java is selected:

ubuntu@ubuntu2004:~/contiki/tools/cooja$ sudo update-alternatives --config java

Finally, you will need to install the compiler that will allow you to generate the images for the simulated nodes:

sudo apt install gcc-msp430 gdb-msp430

Contiki Code

In the development of the lab assignment, we will use the following files, all located in the examples/ipv6/rpl-border-router subdirectory of the Contiki installation path:

• border_router.c: code that we will use for the edge router.
• udp-client.c orudp_server.c (in the examples/ipv6/rpl-udp directory): that will we will use for the nodes in our RPL network (its functionality is not relevant for the moment).
• slip-bridge.c: contains the callback functions to process a SLIP connection request.
• httpd-simple.c: it contains a simple web server that will allow us check the routing tables of the edge router.

Nodes that implement the udp-client.c or border_router.c code will form a DAG, with the edge router configured as root. The edge router will receive the network prefix via an SLIP connection (Serial Line Interface Protocol) and will communicate it to the rest of the nodes of the RPL network so that they can build their corresponding global IPv6 addresses.

The following code snippets, taken from the edge router, show the point where it waits for the network prefix. Once received, the edge router is configured as the root of the DAG and sends the prefix to all other nodes of the network:

/* Request prefix until it has been received */ 
 while(!prefix_set) { 
   etimer_set(&et, CLOCK_SECOND); 
   request_prefix(); 
   PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&et)); 
 } 


 dag = rpl_set_root(RPL_DEFAULT_INSTANCE,(uip_ip6addr_t *)dag_id); 
 if(dag != NULL) { 
   rpl_set_prefix(dag, &prefix, 64); 
   PRINTF("created a new RPL dag\n"); 
 }

By default, the edge router hosts a simple web page that will allows us to check the status of its routing table. This page will be displayed by introducing in a web browser of the virtual machine the IPv6 address of the edge router. The use or not of this page is controlled with the macro WEBSERVER in the http-simple.c file:

PROCESS(border_router_process, "Border router process");
#if WEBSERVER==0
/* No webserver */
AUTOSTART_PROCESSES(&border_router_process);
#elif WEBSERVER>1
/* Use an external webserver application */
#include "webserver-nogui.h"
AUTOSTART_PROCESSES(&border_router_process,&webserver_nogui_process); 

Code compilation

The code for the edge router can be found in the path examples/ipv6/rpl-border-router. In a terminal, use the following command to perform the compilation:

cd examples/ipv6/rpl-border-router
make TARGET=z1

Once executed, a file called border-router.z1 will be created, which will be used to program the mote (simulated device) for the edge router in the cooja simulator.

To demonstrate the functionality of the edge router, we will create a network of nodes with the edge router as root. For the rest of the nodes, we will use UDP client code, implemented in the file udp-client.c. Prepare images for these motes as follows:

cd examples/ipv6/rpl-udp
make TARGET=z1

As before, you will obtain a file named udp-client.z1, that will that shall be used to program the rest of the motes of the DAG.

Simulation with Cooja

After compiling the images, it is time to configure the complete simulation in Cooja. Start the simulator using the following command:

cd tools/cooja
ant run

Once cooja starts follow these steps to create a new simulation:

1. Select the File->New Simulation option. Then the UDGM option and enter the name of the simulation. Press Create.
2. In the Motes menu, selectAdd New Motes->Create new motes and select Z1 as the type of mote.
3. Find the location of the image for the border router (examples/ipv6/rpl-border-router) and select the file rpl-border-router.z1. Click on Create and add one mote of this type.
4. Repeat steps 2 and 3, but this time with the UDP client image that you created earlier. Add four or five motes of this type (selecting four instead one in step 3) and manually distribute them throughout the simulation, so that not all of them can be directly reached from the edge router.

Select the View menu options as shown in the figure. This will allow you to easily create your topology (you can temporarily add the IP address as well, although it might overload the figure with too much information):

Next, create a bridge between the simulated RPL network in Cooja and the local machine by This can be done by selecting Tools and Serial Socket (SERVER) on the edge router mote (identify it with its numeric value). If every thing went well you will get a message like the one in the figure below (note that the message indicates Listening on port 60001):

Then start the simulation (Start button).

The tunslip tool

As we have said, an edge router acts as a link to connect a network to another network. In this example, the edge router is used to route data between the RPL network and an external network. So far, we have only created the RPL network, so we need to simulate a scenario where this RPL network connects to an external network. To do this, we use the tunslip utility provided with Contiki. It will create a bridge between the RPL network and the local machine.

The code tunslip6.c is located in thetools directory of the contiki installation path, and it can be compiled with the following command:

make tunslip6

Then we can establish a connection between the RPL network and the local machine by running the following command:

sudo ./tunslip6 -a 127.0.0.1 aaaa::1/64

If the execution was correct, you will see an output in the terminal similar to this one:

ubuntu@ubuntu2004:~/contiki/tools$ sudo ./tunslip6 -a 127.0.0.1 aaaa::1/64
slip connected to ``127.0.0.1:60001''
opened tun device ``/dev/tun0''
ifconfig tun0 inet `hostname` mtu 1500 up
ifconfig tun0 add aaaa::1/64
ifconfig tun0 add fe80::0:0:0:1/64
ifconfig tun0

tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500
        inet 127.0.1.1  netmask 255.255.255.255  destination 127.0.1.1
        inet6 aaaa::1  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::1  prefixlen 64  scopeid 0x20<link>
        inet6 fe80::ace4:dadf:8e12:be05  prefixlen 64  scopeid 0x20<link>
        unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 500  (UNSPEC)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

*** Address:aaaa::1 => aaaa:0000:0000:0000
Got configuration message of type P
Setting prefix aaaa::
Server IPv6 addresses:
 aaaa::c30c:0:0:1
 fe80::c30c:0:0:1

The program has created a bridge interface tun0 with IPv4 127.0.1.1, and has sent, via serial, a configuration message to the edge router indicating the desired IPv6 prefix for the RPL network nodes (aaaa). The last two lines are part of the output of the edge router, and indicate their IPv6 addresses after the reception of the prefix.

Note that in the Cooja window a message with the text Client connected: /127.0.0.1 has appeared.

Simulation analysis

It is possible to verify the address of the edge router through a command ping from your virtual machine:

ubuntu@ubuntu2004:~/contiki/tools$ ping aaaa::c30c:0:0:1
PING aaaa::c30c:0:0:1(aaaa::c30c:0:0:1) 56 data bytes
64 bytes from aaaa::c30c:0:0:1: icmp_seq=1 ttl=64 time=21.5 ms
64 bytes from aaaa::c30c:0:0:1: icmp_seq=2 ttl=64 time=7.44 ms
64 bytes from aaaa::c30c:0:0:1: icmp_seq=3 ttl=64 time=8.57 ms
64 bytes from aaaa::c30c:0:0:1: icmp_seq=4 ttl=64 time=62.7 ms
64 bytes from aaaa::c30c:0:0:1: icmp_seq=5 ttl=64 time=15.2 ms
--- aaaa::c30c:0:0:1 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4015ms
rtt min/avg/max/mdev = 7.442/23.066/62.661/20.427 ms

So as with any other node in the RPL network, for instance with node 4:

ubuntu@ubuntu2004:~/contiki/tools$ ping aaaa::c30c:0:0:4
PING aaaa::c30c:0:0:4(aaaa::c30c:0:0:4) 56 data bytes
64 bytes from aaaa::c30c:0:0:4: icmp_seq=1 ttl=62 time=116 ms
64 bytes from aaaa::c30c:0:0:4: icmp_seq=2 ttl=62 time=106 ms
64 bytes from aaaa::c30c:0:0:4: icmp_seq=3 ttl=62 time=108 ms
64 bytes from aaaa::c30c:0:0:4: icmp_seq=4 ttl=62 time=111 ms
64 bytes from aaaa::c30c:0:0:4: icmp_seq=5 ttl=62 time=79.0 ms
^C
--- aaaa::c30c:0:0:4 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4016ms
rtt min/avg/max/mdev = 79.002/104.028/115.794/12.937 ms

The address of each node can be obtained by filtering the log screen based on the destination node (mote) ID.

In addition you can open a web browser on the virtual machine and browse the IP address of the edge router to observe its routing table: