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Intro to OpenFlow Tutorial (OVS) with Ryu Controller

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Step 3. Execute Experiment

Now that the switch is up and running we are ready to start working on the controller. For this tutorial we are going to use the Ryu controller. The software is already installed in the controller host for running the Ryu controller.

3a. Login to your hosts

To start our experiment we need to ssh into all of our hosts.

To get ready for the tutorial you will need to have the following windows open:

  • one window with ssh into the controller
  • four windows with ssh into OVS
  • one window with ssh into host1
  • two windows with ssh into host2
  • one window with ssh into host3

Depending on which tool and OS you are using there is a slightly different process for logging in. If you don't know how to SSH to your reserved hosts learn how to login. Once you have logged in follow the rest of the instructions.

3b. Use a Learning Switch Controller

In this example we are going to run a very simple learning switch controller to forward traffic between host1 and host2.

  1. First start a ping from host1 to host2, which should timeout, since there is no controller running.
ping host2 -c 10
  1. We have installed the Ryu controller under /tmp/ryu on the controller host. Ryu comes with a set of example modules that you can use out of the box. One of the modules is a learning switch. Start the learning switch controller which is already available by running the following two commands:

cd /tmp/ryu
./bin/ryu-manager ryu/app/

The output should look like this:

loading app ryu/app/
loading app ryu.controller.ofp_handler
instantiating app ryu.controller.ofp_handler of OFPHandler
instantiating app ryu/app/ of SimpleSwitch
  1. In the terminal of host1, ping host2:
    [experimenter@host1 ~]$ ping host2 
    PING host2-lan1 ( 56(84) bytes of data.
    From host1-lan0 ( icmp_seq=2 Destination Host Unreachable
    From host1-lan0 ( icmp_seq=3 Destination Host Unreachable
    From host1-lan0 ( icmp_seq=4 Destination Host Unreachable
    64 bytes from host2-lan1 ( icmp_req=5 ttl=64 time=23.9 ms
    64 bytes from host2-lan1 ( icmp_req=6 ttl=64 time=0.717 ms
    64 bytes from host2-lan1 ( icmp_req=7 ttl=64 time=0.654 ms
    64 bytes from host2-lan1 ( icmp_req=8 ttl=64 time=0.723 ms
    64 bytes from host2-lan1 ( icmp_req=9 ttl=64 time=0.596 ms

Now the ping should work.

  1. Go to your controller host and take a look at the print outs. You should see that your controller installed flows based on the mac addresses of your packets.

3c. Look around your OVS switch

  1. If you are using OVS, to see the flow table entries on your OVS switch:
    sudo ovs-ofctl dump-flows br0
    You should see at least two table entries: One for ICMP Echo (icmp_type=8) messages from host1 to host2 and one for ICMP Echo Reply (icmp_type=0) messages from host2 to host1. You may also see flow entries for arp packets.
  1. To see messages go between your switch and your controller, open a new ssh window to your controller node and run tcpdump on the eth0 interface and on the tcp port that your controller is listening on usually 6633. (You can also run tcpdump on the OVS control interface if you desire.)
    sudo tcpdump -i eth0 tcp port 6633
    You will see (1) periodic keepalive messages being exchanged by the switch and the controller, (2) messages from the switch to the controller (e.g. when there is a table miss) and an ICMP Echo message in, and (3) messages from the controller to the switch (e.g. to install new flow entries).
  1. Kill your Ryu controller by pressing Ctrl-C.
  1. Notice what happens to your ping on host1.
  1. If you are using OVS, check the flow table entries on your switch:
    sudo ovs-ofctl dump-flows br0
    You will see flow table entries in the switch. The entries time to expire value is set to infinity.
  1. Delete the entries on the OVS:
    sudo ovs-ofctl del-flows br0
    Notice what happens to your ping on host1.

Soft vs Hard Timeouts

All rules on the switch have two different timeouts:

  • Soft Timeout: This determines for how long the flow will remain in the forwarding table of the switch if there are no packets received that match the specific flow. As long as packets from that flow are received the flow remains on the flow table.
  • Hard Timeout: This determines the total time that a flow will remain at the forwarding table, independent of whether packets that match the flow are received; i.e. the flow will be removed after the hard timeout expires.

Can you tell now why there were packets flowing even after you killed your controller?

3d. Download the Ryu apps

To help you get started with your controller writing, we will provide:

  • skeleton files for the controllers where you only need to complete some missing functionality
  • the solution: fully implemented controllers
  • a utility library that makes some of the Ryu messages easier to write

In the controller terminal execute:

  mkdir /tmp/ryu/ryu/ext
  cd /tmp/ryu/ryu/ext/
  sudo wget
  sudo tar xvfz ryu-intro-ctrlapps.tar.gz

Useful Tips for writing your controller

In order to make this first experience of writing a controller easier, we wrote some helpful functions that will abstract some of the particularities of Ryu away. These functions are located in /tmp/ryu/ryu/ext/, so while you write your controller consult this file for details.

Functions that are implemented include:

  • packetIsIP : Test if the packet is IP
  • packetIsARP : Test if the packet is ARP
  • packetIsRequestARP : Test if this is an ARP Request packet
  • packetIsReplyARP : Test if this is an ARP Reply packet
  • packetArpDstIp : Test what is the destination IP in an ARP packet
  • packetArpSrcIp : Test what is the source IP in an ARP packet
  • packetIsTCP : Test if a packet is TCP
  • packetDstIp : Test the destination IP of a packet
  • packetSrcIp : Test the source IP of a packet
  • packetDstTCPPort : Test the destination TCP port of a packet
  • packetSrcTCPPort : Test the source TCP port of a packet
  • createOFAction : Create one OpenFlow action
  • getFullMatch : get the full match out of a packet
  • createFlowMod : create a flow mod
  • createArpRequest : Create an Arp Request for a different destination IP
  • createArpReply : Create an Arp Reply for a different source IP

3e. Debugging your Controller

While you are developing your controller, some useful debugging tools are:

i. Print messages

Run your controller in verbose mode (add --verbose) and add print messages at various places to see what your controller is seeing.

ii. Check the status in the switch

If you are using an OVS switch, you can dump information from your switch. For example, to dump the flows:

sudo ovs-ofctl dump-flows br0

Two other useful commands show you the status of your switch:

sudo ovs-vsctl show 
sudo ovs-ofctl show br0

iii. Use Wireshark to see the OpenFlow messages

Many times it is useful to see the OpenFlow messages being exchanged between your controller and the switch. This will tell you whether the messages that are created by your controller are correct and will allow you to see the details of any errors you might be seeing from the switch. You can use wireshark on both ends of the connection, in hardware switches you have to rely only on the controller view.

The controller host and OVS has wireshark installed, including the openflow dissector. For more information on wireshark you can take a look at the wireshark wiki.

Here we have a simple case of how to use the OpenFlow dissector for wireshark.

If you are on a Linux friendly machine (this includes MACs) open a terminal and ssh to your controller machine using the -Y command line argument, i.e.

ssh -Y <username>@<controller>

Assuming that the public IP address on the controller is eth0, run wireshark by typing:

sudo wireshark -i eth0&

When the wireshark window pops up, you might still have to choose eth0 for a live capture. And you will want to use a filter to cut down on the chatter in the wireshark window. One such filter might be just seeing what shows up on port 6633. To do that type tcp.port eq 6633 in the filter window, assuming that 6633 is the port that the controller is listening on.

3f. Run a traffic duplication controller

In the above example we ran a very simple learning switch controller.
The power of OpenFlow comes from the fact that you can decide to forward the packet anyway you want based on the supported OpenFlow actions. A very simple but powerful modification you can do, is to duplicate all the traffic of the switch out a specific port. This is very useful for application and network analysis. You can imagine that at the port where you duplicate traffic you connect a device that does analysis. For this tutorial we are going to verify the duplication by doing tcpdump on two ports on the OVS switch.

  1. Use the interfaces that are connected to host2 and host3.
    • Software Switch (OVS): You should have noted down the interfaces for host2 and host3 in section 2a. If you have not noted the interfaces for host2 and host3 down (in section 2a), you can run tcpdump on OVS interfaces to figure out the interfaces for host2 and host3. This will allow you to see all traffic going out the interfaces.

To see that duplication is happening, on the ovs host, run:

sudo tcpdump -i <data_interface_name>  [data_interface to host2]
sudo tcpdump -i <data_interface_name>  [data_interface to host3]

You should see traffic from host1 to host2 showing up in the tcpdump window for host3. As a comparison, you will notice that no traffic shows up in that window when the controller is running the learning switch (at /tmp/ryu, type ./bin/ryu-manager ryu/ext/

  1. In the controller host directory /tmp/ryu/ryu/ext you should see two files:
  1. : This is the file that has instructions about how to complete the missing information. Go ahead and try to implement your first controller.
  2. : This has the actual solution. You can just run this if you don't want to bother with writing a controller.
  3. duplicate.config : in this file, you need to specify which port you want to duplicate traffic to. We will be duplicating host2 traffic and send it to host3, so put host3 port number here. We noted down port numbers in section 2a. To figure out which port maps to which interface, use can also use "sudo ovs-ofctl show br0" on ovs node. (You can use any editor to edit the file, e.g. use nano by typing "sudo nano duplicate.config")
  1. Run your newly written controller to duplicate the traffic. (update duplicate.config file with host3 port number. We noted it down in section 2a ). You can also use the solution file to duplicate traffic:
    cd /tmp/ryu
    ./bin/ryu-manager ryu/ext/

  1. To test it go to the terminal of host1 and try to ping host2:
    If your controller is working, your packets will register in both terminals running tcpdump.
  1. Stop the Ryu controller using Ctrl-C.

3g. Run a port forward Controller

Now let's do a slightly more complicated controller. OpenFlow gives you the power to overwrite fields of your packets at the switch, for example the TCP source or destination port and do port forwarding. You can have clients trying to contact a server at port 5000, and the OpenFlow switch can redirect your traffic to a service listening on port 6000.

  1. Under the /tmp/ryu/ryu/ext directory there are two files: and that are similar to the previous exercise. Both of these controllers are configured by a configuration file at ext/port_forward.config. Use to write your own port forwarding controller.
  1. To test your controller we are going to use netcat. Go to the two terminals of host2. In one terminal run:
    nc -l 5000

and in the other terminal run

nc -l 6000
  1. Now, start the simple layer 2 forwarding controller. We are doing this to see what happens with a simple controller.
    cd /tmp/ryu
    ./bin/ryu-manager ryu/ext/
  1. Go to the terminal of host1 and connect to host2 at port 5000:
    nc 5000

  1. Type something and you should see it at the terminal of host2 at port 5000.
  1. Now, stop the simple layer 2 forwarding controller by Ctrl-C.
  1. And start your port forwarding controller (if you have written your controller then use myPortForwarding in the following command):
    ./bin/ryu-manager ryu/ext/
  1. Repeat the netcat scenario described above. Now, your text should appear on the other terminal of host2 which is listening to port 6000.
  1. Stop your port forwarding controller using Ctrl-C.

3h. Run a Server Proxy Controller

As our last exercise, instead of diverting the traffic to a different server running on the same host, we will divert the traffic to a server running on a different host and on a different port.

  1. Under the /tmp/ryu/ryu/ext/ directory there are two files: and that are similar to the previous exercise. Both of these controllers are configured by the configuration file proxy.config. Use to write your own proxy controller.
  1. On the terminal of host3 run a netcat server:
    nc -l 7000
  1. On your controller host, open the /tmp/ryu/ryu/ext/ file, and edit it to implement a controller that will divert traffic destined for host2 to host3. Before you start implementing think about what are the side effects of diverting traffic to a different host.
    • Is it enough to just change the IP address?
    • Is it enough to just modify the TCP packets?

If you want to see the solution, it's available in file /tmp/ryu/ryu/ext/ file.

  1. To test your proxy controller run (if you have written your controller then use myProxy in the following command):
    cd /tmp/ryu
    ./bin/ryu-manager ryu/ext/ 
  1. Go back to the terminal of host1 and try to connect netcat to host2 port 5000
    nc 5000
  1. If your controller works correctly, you should see your text showing up on the terminal of host3.

3i. Delete your bridge

Before moving to the next step make sure you delete the bridge you have created, especially if you are using the same reservation for a different exercise:

  sudo ovs-vsctl del-br br0

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