Building Network Topologies

This page aims to describe some things-to-consider when setting up topologies for experimentation.

1. Simulating Point-to-Point Links
   
2. Topology Description Methods

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.

I. Simulating point-to-point Links

This section aims to provide 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 standard and OpenFlow-controlled nodes. In general, we want to partition the shared switch so that the nodes are isolated from each other, and then introduce relays that can move traffic between these partitions in a controlled manner. The way the traffic is relayed produces the topology. The general topology we use to describe our methods is the following:

 A-[r]-B

Where A and B are nodes 'trapped' in their partitions, and [r] is a relay node that straddles the partitioning on the shared switch. We call A and B end nodes and [r] a network node; From this logic it follows that the partition is the link (despite it actually being a logical setup on the shared switch, rather than a wire). Given that the partitions have identifiers, such as IP block addresses, nodes on the same link must share the identifier, and relays must know the identifiers of all partitions that it connects.

Contents

1.1 Some Considerations
1.2 Non-OpenFlow Methods

1.2.1 Kernel IP routing (Layer 3)
1.2.2 Linux Bridge (Layer 2)
1.2.3 Packet filters (Layers 2-4)

1.3 OpenFlow Methods

1.3.1 OpenvSwitch
1.3.2 NetFPGA OpenFlow switch
1.3.3 Prototyping - With Mininet

1.4 Summary

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.

1.1 Some Considerations

The techniques used to partition the broadcast domain will heavily depend on two things:

1. the type of experiment
2. the available interfaces on the nodes

In terms of 1., for example - we don't want to use TCP/IP-based schemes such as IP routing if we don't plan on using TCP/IP, or are planning to modify layer 3. 2. is important in that, depending on technique the number of links you can have (the node degree in terms of graphs) will be restricted to how-many-ever interfaces you have. When you only have one interface, you will want to use virtual interfaces to increase the number of links to/from your node. In turn, you may also need to modify the partitioning scheme of the shared switch.

A standard way to deploy virtual interfaces is in combination with VLANs and trunking. This is not a bad, since VLANs may be combined with other configuration schemes, are relatively simple to configure, and a good portion of networked devices understand them. Many of the examples here that require virtual interfaces will make use of this standard technique. So, to make things easier, we will quickly describe how to add virtual interfaces and VLAN awareness to a node before moving on to Section 1.1 .

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 on eth0. The module can be made to load at boot time by appending '8021q' to the list, /etc/modules.

Note, virtual interfaces are workarounds to being restricted to one physical interface. Any setup with nodes with multiple interfaces (e.g. using NetFPGAs) will not require the above configs, lest you want more interfaces than you have. For nodes with multiple physical interfaces, the steps describing 'eth0.xxx' can be replaced by the names of each unique interface. Keep in mind, however, that if the interface is connected to a switchport configured as a trunk, it must also be made VLAN aware even if it does not hold multiple virtual interfaces.

1.2 non-OpenFlow Methods

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

1.2.1 Kernel IP routing

Kernel IP routing has the least requirements, in that no extra packages are required if you have multiple Ethernet ports on your node. As its name indicates, it works strictly at layer 3. Partitioning occurs across IP blocks; you would need one block per link. It can be combined with VLANs and/or virtual interfaces if you are limited in the number of physical interfaces you have on your relay.

Network node setup

1. This setup assumes a 1-to-1 mapping of VLANs to subnets. Choose IP blocks, one for each VLAN/interface. 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.

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.

1.2.2 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

1.3 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.

1.3.1 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.

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.

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

1.3.2 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.

1.4 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, even when you are restricted to just one physical interface per node.

As for OpenFlow switching, OVS nodes (or a Mininet prototype topology) controlled by a controller supporting flow pushing (e.g. BSN or Floodllight) is the most flexible, least-hassle choice for this task.

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II. Topology Description Methods

The previous section described some methods to manually configure a link. The ultimate goal in topology configuration is to be able to do it automatically for multiple links in various configurations. This implies a process along the following lines:

 [Topology File] -> [parse] -> [configure] -> [topology in network] 

The Topology file must be able to describe not only the topology, but also various other factors that will change between experiments. This section attempts to explore some topology description formats that may be decent candidates for an automated system.

Contents

2.1 Some Considerations
2.2 Formats

2.2.1 GEXF
2.2.2 GDF

2.3 Summary

2.1 Some Considerations

The minimum requirements for a format are:

• ability to describe a graph (nodes and edges)
• ability to add descriptions to nodes and edges
• can be parsed easily

Being easy to read and write is a added bonus.

Some things that we would like to be able to describe are

• node/edge IDs
• node behavior (a switch, host, router, service…)
• edge weight (throughput, connectivity, delay…)

2.2 Some Formats

2.2.1 GEXF

link: ​http://gexf.net/format/index.html

2.2.2 GDF

link: ​https://gephi.org/users/supported-graph-formats/gdf-format/

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^{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.}