Building Network Topologies (with OpenFlow)

This is a rough overview of the steps one needs to take in order to simulate point-to-point links between nodes sharing a single switch (e.g. within the same broadcast domain), using OpenFlow-controlled nodes as a means to move traffic from one VLAN to another. The steps described here are incomplete; Things will be updated as methods are refined/improved.

Background

VLANs are good for breaking up broadcast domains. If each node is placed on separate VLANs and given a choice of a few "gateways" out of its VLAN, it has no choice but to communicate through the gateway(s). How the gateway nodes moves packets/frames from one VLAN to another depends on which network layer(s) are involved.

Prerequisites

This page assumes that you have a setup similar to ​SB9, as well as a node with a working install of NetFPGA drivers or OpenvSwitch, depending on how links are being set up. For the OpenFlow methods, you also need a OpenFlow controller that allows you to push flows to your software defined switch. You should have access to the switch that the nodes are sharing as well, since you need to slice it into VLANs. The following links describe setup and use of theses components (internal links):

• ​OpenVswitch - A software-defined virtual switch with OpenFlow support, no special hardware required.
• ​NetFPGA - FPGA-based network device with OpenFlow support
• ​Quanta LB9A - The shared medium switch. In this page this switch will be used in XorPlus (normal) mode.
• As for the OpenFlow controller, there is a ​collection to choose from.

The system used here is Ubuntu10.10 (kernel: 2.6.35-30-generic). Command syntax will change depending on your distro.

First and foremost, the shared switch should be split into several VLANs according to your topology. Two interconnected nodes should be on the same VLAN e.g. the switch-ports connected to them should be associated with the same VLAN ID. Nodes connected to more than one element should sit on a trunked port open to all VLANs the node should associate with.

Contents

We first describe some base "sanity-test" setups that do not involve any OpenFlow elements. These are:

I Basic Methods

1.1 Kernel IP routing (Layer 3)
1.2 Linux Bridge (Layer 2)

Then we describe the (ongoing) process of topology setup using OpenFlow-related elements, such as:

II OpenFlow Methods

2.1 OpenvSwitch
2.2 NetFPGA OpenFlow switch

Extras - With Mininet

OpenFlow is rather layer-agnostic, defining traffic rules based on a combination of any of the 12 packet header fields that may be used for matching under the OpenFlow standard. These fields correspond to layers 1~4.

All save the very last method requires that the network node is VLAN aware. Before moving on to Section I we will quickly describe how to add VLAN awareness to the system.

1. Install and load VLAN module:

    apt-get install vlan
    modprobe 8021q
   

2. Add VLAN interfaces using vconfig:

    vconfig add eth0 111
    vconfig add eth0 222
   

   This creates two virtual LAN interfaces, eth0.111 and eth0.222. The module can be made to load at boot time by appending '8021q' to the list, /etc/modules.

I Basic Methods

These two methods should work on any *nix machine, so they can serve as "sanity checks" for the system you are using as the network node.

1.1 Kernel IP routing

Kernel IP routing is the simplest, in that no extra packages are required if you have multiple Ethernet ports on your node.

1.1.1 Network node setup

1. This setup assumes a 1-to-1 mapping of VLANs to subnets. Choose IP blocks, one fore each VLAN. For example, if you have two clients connected across your node, you need two IP blocks, one for each VLAN:
   • VLAN 111: 192.168.1.0/24, gateway 192.168.1.13
   • VLAN 222: 192.168.2.0/24, gateway 192.168.2.23

The gateway IPs chosen above will be the IP addresses assigned to the VLAN interfaces you have set up earlier on your network node.

2. Bring up VLAN interfaces with the IP addresses/blocks you have chosen:

    ifconfig eth0 0.0.0.0 up
    ifconfig eth0.111 inet 192.168.1.23 broadcast 192.168.1.255 netmask 0xffffff00 up
    ifconfig eth0.222 inet 192.168.2.23 broadcast 192.168.2.255 netmask 0xffffff00 up
   

   This configuration can be made permanent by modifying /etc/network/interfaces:

   auto eth0.111
   iface eth0.111 inet static
       address 192.168.1.13
       netmask 255.255.255.0
       vlan-raw-device eth0
   
   auto eth0.222
   iface eth0.222 inet static
       address 192.168.2.23
       netmask 255.255.255.0
       vlan-raw-device eth0
   

3. Enable routing on network node

    route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.13
    route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.23
    echo 1 >  /proc/sys/net/ipv4/ip_forward
   

   The last line in the above block is equivalent to running the command:

   sysctl -w net.ipv4.ip_forward=1
   

   The ip_forward flag resets itself after reboot. To make it permanent, add

   sysctl net.ipv4.ip_forward=1
   

   to /etc/sysctl.conf.

1.1.2 End node setup

Unless you have set up DHCP, you must manually assign an IP address and default gateway to each node. The former should be consistent with the subnet associated with the VLAN to which the end host belongs. For example, the following host is connected to a switch port associated with VLAN 222, so it is assigned an address from the 192.168.2.0/24 block:

 ifconfig eth0 inet 192.168.2.4

Then you must add reachability information to the node's routing table e.g. the IP addresses that it must send data to in order to have it reach remote subnets. Since there is only one other subnet in this example, a single entry specifying the destination subnet (192.168.1.0/24 - VLAN 111) and the gateway IP in/out of the current node's subnet is added:

 route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.2.23

Do this for each remote subnet that the node should be able to communicate with. Once all of the nodes are configured, you should be able to ping end-to-end.

1.2 Linux Bridge

In terms of implementation, this is probably the simplest method.

A bridge will ignore VLAN tags, so if you have two VLAN interfaces e.g. eth0.111 and 222 sitting on a trunk, the packets will come in tagged. An intermediate abstraction will strip the tag from the packet (at br0), and the packet will get tagged as appropriate on the outbound. Unlike kernel IP forwrding, bridging works purely at Layer 2, hence you do not need to worry about IP addressing.

The first three steps refer to the network node.

1. Configure and bring VLANS up as before, sans IP addresses

2. Install bridge-utils:

    apt-get install bridge-utils
   

3. Create bridge interface, add ports:

    brctl addbr br0
    brctl addif br0 eth0.111
    brctl addif br0 eth0.222
   

4. Make sure all interfaces (br0, eth0.*) are up, as they may not come up automatically.
5. Set all hosts on the bridged VLANs to the same IP block.

The Linux Foundation keeps a page that may be useful for various troubleshooting: ​http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge

II OpenFlow Methods

This section assumes that you have all of the !OpenFLow components (e.g. OVS, NetFPGA drivers) set up and working, and that you have several choices of controller. The controller used primarily in this section is the Big Switch Networks (BSN) controller.

2.1 OpenvSwitch

OpenvSwitch (OVS) is a user-space software defined switch with OpenFlow support, complete with its own implementation of a controller. It can, and is assumed to be, built as a kernel module throughout this page.

2.1.1 Initialization

OVS has three main components that must be initialized:

• openvswitch_mod.ko, the OVS kernel module
• ovsdb, the database containing configurations
• ovs-vswitchd, the OVS switch daemon

The latter configures itself using the data provided by the former; ovs-vsctl is used to modify the contents of the database in order to configure the OVS switch.

1. Load openVswitch kernel module

    cd datapath/linux/
    insmod openvswitch_mod.ko
   

Note, OVS and Linux bridging may not be used at the same time. This step will fail if the bridge module (bridge.ko) is loaded. You may need to reboot the node in order to unload bridge.ko.
If this is the first time OVS is being run, make am openvswitch directory in /usr/local/etc/ and run ovsdb-tool to create the database file:

 mkdir -p /usr/local/etc/openvswitch
 ovsdb-tool create /usr/local/etc/openvswitch/conf.db vswitchd/vswitch.ovsschema

2. Start ovs-db:

    ovsdb/ovsdb-server --remote=punix:/usr/local/var/run/openvswitch/db.sock \
           --remote=db:Open_vSwitch,manager_options \
           --pidfile --detach
   

3. Initialize the database:

    utilities/ovs-vsctl --no-wait init
   

the --no-wait allows the database to be initialized before ovs-vswitchd is invoked.

4. Start ovs-vswitchd:

    vswitchd/ovs-vswitchd unix:/usr/local/var/run/openvswitch/db.sock --pidfile --detach
   

The 'unix:…db.sock' specifies that the process attach to the socket opened by ovsdb.

2.1.2 Configuring OVS

the following only needs to be done once, in the initial configurations.

1. Add ports:

    ovs-vsctl add-br br0
    ovs-vsctl add-port br0 eth0.111
    ovs-vsctl add-port br0 eth0.222
   

By the time the 'add-port' commands are used, you should not be able to ping across the two VLANS, even with correct route table entries and packet forwarding enabled in the kernel. Here, br0 is a virtual interface similar to tap0 in the bridge. There should be one virtual interface per virtual switch to be instantiated. By default, ports added to the switch are trunked. Using the option tag=VLAN ID makes the interfaces behave as access ports for the VLAN ID specified:

 ovs-vsctl add-port br0 eth0.111 tag=111
 ovs-vsctl add-port br0 eth0.222 tag=222

However, this is unrelated to what needs to happen here so we will not explore its uses any further (for now).

2. If it has not been done already, initialize the OpenFlow controller. The procedures for this step differ according to the controller in use, and are discussed in the pages for each respective controller.
   

A sanity check for this step is to test your virtual switch with the OVS built-in controller, ovs-controller, which may be initialized on the same node running OVS:

ovs-controller -v ptcp:6633

When ovs-controller is used, the controller IP is, unsurprisingly, 127.0.0.1.

3. Point ovs-vswitchd to the OpenFlow controller.

   ovs-vsctl set-controller br0 tcp:172.16.0.14:6633
   

In this example, the OVS process is pointed to a BSN controller (kvm-big) on 172.16.0.14, listening on port 6633¹. With a properly initialized and configured database, ovs-vswitchd will spit out a bunch of messages as it attempts to connect to the controller. Its output should look something similar to this:

root@node1-4:/opt/openvswitch-1.2.2# vswitchd/ovs-vswitchd unix:/usr/local/var/run/openvswitch/db.sock --pidfile --detach
Nov 07 17:37:02|00001|reconnect|INFO|unix:/usr/local/var/run/openvswitch/db.sock: connecting...
Nov 07 17:37:02|00002|reconnect|INFO|unix:/usr/local/var/run/openvswitch/db.sock: connected
Nov 07 17:37:02|00003|bridge|INFO|created port br0 on bridge br0
Nov 07 17:37:02|00004|bridge|INFO|created port eth0.101 on bridge br0
Nov 07 17:37:02|00005|bridge|INFO|created port eth0.102 on bridge br0
Nov 07 17:37:02|00006|ofproto|INFO|using datapath ID 0000002320b91d13
Nov 07 17:37:02|00007|ofproto|INFO|datapath ID changed to 000002972599b1ca
Nov 07 17:37:02|00008|rconn|INFO|br0<->tcp:172.16.0.14:6633: connecting...

The OpenvSwitch OpenFlow switch should be functional as soon as it finds and connects to the controller. As you can see above, a DPID is chosen at random; if a random DPID does not suit your needs, a DPID may be specified manually using ovs-vsctl:

ovs-vsctl set bridge <mybr> other-config:datapath-id=<datapathid>

Where <datapathid> is a 16-digit hex value. For our network node, this becomes:

ovs-vsctl set bridge br0 other-config:datapath-id=0000009900113300

2.2 NetFPGA OpenFlow switch

This method is probably the most involved and difficult to get right, although in theory would be the best since you would get the programmatic flexibility of the OVS switch and the speed of a hardware-implemented device.

Assuming that you already have NetFPGA drivers installed, no special configurations are needed for the NetFPGA, save the OpenFlow-switch bitfile. The typical flow rules also do not apply to the NetFPGA e.g. default flow modules on NOX that work with OVS will break under the NetFPGA. Therefore extra flows must be added to compensate.

The easiest manner to control the NetFPGA is via the BSN controller. The following two flow entries needed to be added to the controller in order for the client-nodes to be able to ping each other across the NetFPGA.

  flow-entry vlan111-ip
    active True
    src-mac 00:15:17:d6:da:4a
    vlan-id 111
    actions set-vlan-id=222,output=all
  flow-entry vlan222-ip
    active True
    src-mac 00:15:17:d6:ce:20
    vlan-id 222
    actions set-vlan-id=111,output=all	  

This set of flows basically implements VLAN stitching based on source MAC address. Unlike in the Linux bridge, one cannot see the VLAN-stripped packets on the virtual interface (tap0 on the NFPGA, br0 on bridge); they will already have the proper tag, since the processing is probably occurring in the FPGA and not in the kernel.

III Morals of the story

For quick setup of a network toppology using nodes sharing a medium, point-to-point links should be defined at as low a layer as possible. The next best thing (that is even better because of its flexibility) to actually going in and connecting up the topology using cables is to carve up the shared switch into VLANs. This lets you restrict the broadcast domain however you want, without hard-wiring everything.

As for OpenFlow switching, OVS nodes controlled by a BSN controller is the flexible, least-hassle choice for this task.

Extras

​Mininet, a SDN prototyping platform, is a great tool for proof-of-concept SDN experiments where you don't have physical SDN networks to experiment with, or setup of actual hardware is too cumbersome. The following setup was built with two VMs on external3:

• VM1(10.50.250.2): 3-switch, 3-host Mininet topology pointed to external controller in VM2:

Topology:

h4   h5   h6
 |    |    | 
s1---s2---s3 

Mininet config:

# mn --topo=linear,3 --controller=remote --ip=10.50.250.17 --port=6633

• VM2(10.50.250.17): Flowvisor on port 6633, with a slice to point two switches to Floodlight instance on 6634

FlowVisor configuration:

# fvctl addFlowSpace 00:00:00:00:00:00:00:02 any "Slice:fl-1=7"
# fvctl addFlowSpace 00:00:00:00:00:00:00:02 1000 any "Slice:fl-1=7"
# fvctl addFlowSpace 00:00:00:00:00:00:00:01 1000 any "Slice:fl-1=7"

Floodlight config alteration (in src/main/resources/config.properties):

net.floodlightcontroller.core.FloodlightProvider.openflowport = 6634

^{1. This specific example requires a bit of network reconfiguration and comes with substantial risk of disconnecting your node from the network if done carelessly.}