introduction-to-route-0.md

Introduction to Route 0

This lesson is an introduction to Route 0, some basic networking commands on Linux, and Wireshark.

Topology

First, let's look at the topology that we will be using for this lesson, the one_rtr topology. The network is very simple. It consists of three nodes, but only one of them, R1, is a router, hence the name of the topology. The other two are end-hosts. A host is not necessarily a different device to a router, but it has a very different role in the network. A host will only have one outgoing link and it will not forward IP packets which means that it can only be the source or destination of IP communication. The convention in Route 0 is to name routers with a name that starts with the letter R and hosts with a name starting with h.

You can launch the network by running

sudo python route0.py --topology one_rtr --scenario basic

This command instructs the driver script route0.py to start a network with the one_rtr topology running the basic scenario. The basic scenario is special and simply means to run the network and set up all the interface addresses and default routes.

Once the CLI prompt appears let us inspect Mininet's representation of the network by running

net

in the command prompt. The output tells us about all the nodes in the network and the connections between them. We can see that R1's R1-eth1 interface is connected to h1_1's h1_1-eth1 interface and R1-eth2 is connected to h1_2's h1_2-eth1 interface. You can visualise the network by copy-pasting the output into this web tool though this is of limited usefulness for small networks such as this.

Basic IP commands

Let us now inspect the network using some basic Linux commands. The three main commands we will use to investigate the state on the nodes are ip addr, ip route, and ping. To run any of these commands on a particular node, you need to prefix it with the node's name in the Mininet CLI. For example, to see all the interfaces and their addresses on R1 you would run

R1 ip addr

There is also an older obsolete command ifconfig which is still commonly used. However, all information available through ifconfig is available through the ip commands.

ip addr

This command lists all addresses assigned to the interfaces on the given device. This includes the Ethernet address as well as all IPv4 and IPv6 addresses. For the purposes of these lessons we are only interested in the IPv4 addresses which are displayed as either x.x.x.x or x.x.x.x/y.

The first thing to notice when running this command (especially on R1) is that there may be multiple IP addresses assigned to a single device. This is because IP addresses are bound to network interfaces not devices. Furthermore, it is also possible to assign multiple IP addresses to a single interface. You will notice that the lo interface on R1 actually has two IP addresses.

ip route

The ip route command is used to list all the routes installed on a particular node. The basic format of a route you will see at the moment is of the form x.x.x.x/y dev if-name which means that in order to reach x.x.x.x/y you must go via the network connected to the interface if-name.

The host nodes have a default route installed which looks like default via z.z.z.z which means that the node should route all traffic it doesn't have a more specific route for via z.z.z.z. The IP address z.z.z.z should then resolve to an interface via a routing entry like the one in the previous paragraph. If it doesn't, the packet will fail to be forwarded.

Another basic format of a route is x.x.x.x/y via z.z.z.z which says that to reach the IP network x.x.x.x/y you must go via the address z.z.z.z. This works similarly to the default route, but is more specific. Note that you won't see such routes in this network setup, because the network is too simple.

ping

The ping command sends a special IP packet to the specified destination to verify connectivity with that end-host. Connectivity is verified if a response to the ping request is received. Try sending a ping from h1_1 to an IP address on h1_2 by running

h1_1 ping 10.2.0.1

The address 10.2.0.1 is the IPv4 address assigned to the interface h1_2-eth1 on h1_2. You can check this using the ip addr command on h1_2. The ping command will keep pinging the specified destination every second. To stop press Ctrl+C. Now try pinging the other way. The intermediate node R1 knows how to forward the traffic between the two hosts, because it is directly connected to both of them.

Wireshark

Before moving on to the next lesson it would be good to introduce a particularly useful tool in studying networks, Wireshark, by using it to look at pings from h1_1 to h1_2. Wireshark is a tool that lets you capture and inspect packets sent and received over any interface on a device. Furthermore, it is able to present them in a human readable form rather than simply dumping the binary representation directly from the wire.

Start by running the command to trigger h1_1 to send pings to h1_2. Now open a new terminal window and navigate to the route0 directory. We will use the attach.py helper script to run Wireshark on R1 and h1_2. Let's start with R1 by running

sudo python attach.py --node R1 --cmd wireshark

When the Wireshark window opens you can dismiss all the Lua errors if you get any. First, we need to select which interface we would like to inspect the packets on. Let's start with R1-eth1 as that's the interface that is connected to h1_2, the source of the packets. You can either double-click on the interface name or select the appropriate button on the menu bar in the top-left corner.

Once the packet capture window opens notice how the ping packets appear every second as a request/reply pair. Look at the source and destination IP addresses as well. Note how the originating node has filled out the source address with the address of its interface h1_1-eth1 and how the reply has the addresses flipped around. Have a look around and inspect the contents if you wish, but we won't go into any detail on the format of the ping packets.

Now let's look at the packet capture on the other interface on R1. You can do this by stopping the current capture, finding the capture options button and starting a capture on R1-eth2. The packets on this interface look identical which is expected. The R1 router has forwarded the request packet from R1-eth1 to R1-eth2 and vice-versa for the reply packet.

You can also inspect the capture on h1_2, but since this is a different node you will have to close the Wireshark window and run the attach.py command on the host node.

Leaving Route 0

To exit the Mininet CLI and return to the shell just run the exit command. This will shut down all the nodes and protocols that are running.

Conclusion

In this lesson you learned how to start Route 0 experiments and learned how to inspect your network using basic Linux commands and Wireshark. You will find these tools will come in handy at all times whenever dealing with networks.