Service Function Chaining demo with devstack

After a first high-level post, it is time to actually show networking-sfc in action! Based on a documentation example, we will create a simple demo, where we route some HTTP traffic through some VMs, and check the packets on them with tcpdump:

SFC demo diagram

This will be hosted on a single node devstack installation, and all VMs will use the small footprint CirrOS image, so this should run on “small” setups.

Installing the devstack environment

On your demo system (I used Centos 7), check out devstack on the Mitaka branch (remember to run devstack as a sudo-capable user, not root):

[stack@demo ~]$ git clone -b stable/mitaka

Grab my local configuration file that enables the networking-sfc plugin, rename it to local.conf in your devstack/ directory.
If you prefer to adapt your current configuration file, just make sure your devstack checkout is on the mitaka branch, and add the SFC parts:
enable_plugin networking-sfc

Then run the usual “./” command, and go grab a coffee.

Deploy the demo instances

To speed this step up, I regrouped all the following items in a script. You can check it out (at a tested revision for this demo):
[stack@demo ~]$ git clone -b sfc_mitaka_demo

The script will:

  • Configure security (disable port security, set a few things in security groups, create a SSH key pair)
  • Create source, destination systems (with a basic web server)
  • Create service VMs, configuring the network interfaces and static IP routing to forward the packets
  • Create the SFC items (port pair, port pair  group, flow classifier, port chain)

I highly recommend to read it, it is mostly straightforward and commented, and where most of the interesting commands are hidden. So have a look, before running it:
[stack@demo ~]$ ./openstack-scripts/
WARNING: setting legacy OS_TENANT_NAME to support cli tools.
Updated network: private
Created a new port:

route: SIOCADDRT: File exists
WARN: failed: route add -net "" gw ""
You can safely ignore the route errors at the end of the script (they are caused by duplicate default route on the service VMs).

Remember, from now on, to source the credentials file in your current shell before running CLI commands:
[stack@demo ~]$ source ~/devstack/openrc demo demo

We first get the IP addresses for our source and destination demo VMs:[vagrant@defiant-devstack ~]$ openstack server show source_vm -f value -c addresses; openstack server show dest_vm -f value -c addresses

private=, fd73:381c:4fa2:0:f816:3eff:fe65:12fd

Now, we look for the tap devices associated to our service VMs:
[stack@demo ~]$ neutron port-list -f table -c id -c name

| name           | id                                   |
| p1in           | 897df85a-26c3-4491-888e-8cc58f19cea1 |
| p1out          | fa838294-317d-46df-b10e-b1734dd62faf |
| p2in           | c86dafc7-bda6-4537-b806-be2282f7e11e |
| p2out          | 12e58ea8-a9ab-4d0b-9fd7-707dc6e99f20 |
| p3in           | ee14f406-e9d6-4047-812b-aa04514f50dd |
| p3out          | 2d86403b-4639-40a0-897e-68fa0c759f01 |

These devices names follow the tap<port ID first 10 digits> pattern, so for example tap897df85a-26 is the associated  for the p1in port here

See SFC in action

In this example we run a request loop from client_vm to dest_vm (remember to use the IP addresses found in the previous section):
[stack@demo ~]$ ssh cirros@
$ while true; do curl; sleep 1; done
Welcome to dest-vm
Welcome to dest-vm
Welcome to dest-vm

So we do have access to the web server! But does the packets really go through the service VMs? To confirm that, in another shell, run tcpdump on the tap interfaces:

# On the outgoing interface of VM 3
$ sudo tcpdump port 80 -i tap2d86403b-46
tcpdump: WARNING: tap2d86403b-46: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap2d86403b-46, link-type EN10MB (Ethernet), capture size 65535 bytes
11:43:20.806571 IP > Flags [S], seq 2951844356, win 14100, options [mss 1410,sackOK,TS val 5010056 ecr 0,nop,wscale 2], length 0
11:43:20.809472 IP > Flags [.], ack 3583226889, win 3525, options [nop,nop,TS val 5010057 ecr 5008744], length 0
11:43:20.809788 IP > Flags [P.], seq 0:136, ack 1, win 3525, options [nop,nop,TS val 5010057 ecr 5008744], length 136
11:43:20.812226 IP > Flags [.], ack 39, win 3525, options [nop,nop,TS val 5010057 ecr 5008744], length 0
11:43:20.817599 IP > Flags [F.], seq 136, ack 40, win 3525, options [nop,nop,TS val 5010059 ecr 5008746], length 0

Here are some other examples (skipping the tcpdump output for clarity):
# You can check other tap devices, confirming both VM 1 and VM2 get traffic
$ sudo tcpdump port 80 -i tapfa838294-31
$ sudo tcpdump port 80 -i tap12e58ea8-a9

# Now we remove the flow classifier, and check the tcpdump output
$ neutron port-chain-update --no-flow-classifier PC1
$ sudo tcpdump port 80 -i tap2d86403b-46 # Quiet time

# We restore the classifier, but remove the group for VM3, so tcpdump will only show traffic on other VMs
$ neutron port-chain-update --flow-classifier FC_demo --port-pair-group PG1 PC1
$ sudo tcpdump port 80 -i tap2d86403b-46 # No traffic
$ sudo tcpdump port 80 -i tapfa838294-31 # Packets!

# Now we remove VM1 from the first group
$ neutron port-pair-group-update PG1 --port-pair PP2
$ sudo tcpdump port 80 -i tapfa838294-31 # No more traffic
$ sudo tcpdump port 80 -i tap12e58ea8-a9 # Here it is

# Restore the chain to its initial demo status
$ neutron port-pair-group-update PG1 --port-pair PP1 --port-pair PP2
$ neutron port-chain-update --flow-classifier FC_demo --port-pair-group PG1 --port-pair-group PG2 PC1

Where to go from here

Between these examples, the commands used in the demo script, and the documentation, you should have enough material to try your own commands! So have fun experimenting with these VMs.

Note that in the meantime we released the Newton version (3.0.0), which also includes the initial OpenStackClient (OSC) interface, so I will probably update this to run on Newton and with some shiny “openstack sfc xxx” commands. I also hope to make a nicer-than-tcpdumping-around demo later on, when time permits.

1 thought on “Service Function Chaining demo with devstack”

  1. Hi Bernard Cafarelli,

    Thanks for this very great blog. It’s very helpful for me to stage and test SFC using devstack.

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