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Implementing a Control Plane using P4Runtime

Introduction

In this exercise, we will be using P4Runtime to send flow entries to the switch instead of using the switch's CLI. We will be building on the same P4 program that you used in the basic_tunnel exercise. The P4 program has been renamed to advanced_tunnel.p4 and has been augmented with two counters (ingressTunnelCounter, egressTunnelCounter) and two new actions (myTunnel_ingress, myTunnel_egress).

You will use the starter program, mycontroller.py, and a few helper libraries in the p4runtime_lib directory to create the table entries necessary to tunnel traffic between host 1 and 2.

Spoiler alert: There is a reference solution in the solution sub-directory. Feel free to compare your implementation to the reference.

Step 1: Run the (incomplete) starter code

The starter code for this assignment is in a file called mycontroller.py, and it will install only some of the rules that you need to tunnel traffic between two hosts.

Let's first compile the new P4 program, start the network, use mycontroller.py to install a few rules, and look at the ingressTunnelCounter to see that things are working as expected.

  1. In your shell, run:

    make

    This will:

    • compile advanced_tunnel.p4,
    • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle, each connected to one host (h1, h2, h3), and
    • assign IPs of 10.0.1.1, 10.0.2.2, 10.0.3.3 to the respective hosts.

  2. You should now see a Mininet command prompt. Start a ping between h1 and h2:

    mininet> h1 ping h2

    Because there are no rules on the switches, you should not receive any replies yet. You should leave the ping running in this shell.

  3. Open another shell and run the starter code:

    cd ~/tutorials/exercises/p4runtime
./mycontroller.py

    This will install the advanced_tunnel.p4 program on the switches and push the tunnel ingress rules. The program prints the tunnel ingress and egress counters every 2 seconds. You should see the ingress tunnel counter for s1 increasing:

     s1 ingressTunnelCounter 100: 2 packets

    The other counters should remain at zero.

  4. Press Ctrl-C to the second shell to stop mycontroller.py

Each switch is currently mapping traffic into tunnels based on the destination IP address. Your job is to write the rules that forward the traffic between the switches based on the tunnel ID.

Potential Issues

If you see the following error message when running mycontroller.py, then the gRPC server is not running on one or more switches.

p4@p4:~/tutorials/exercises/p4runtime$ ./mycontroller.py
...
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with (StatusCode.UNAVAILABLE, Connect Failed)>

You can check to see which of gRPC ports are listening on the machine by running:

sudo netstat -lpnt

The easiest solution is to enter Ctrl-D or exit in the mininet> prompt, and re-run make.

A note about the control plane

A P4 program defines a packet-processing pipeline, but the rules within each table are inserted by the control plane. In this case, mycontroller.py implements our control plane, instead of installing static table entries like we have in the previous exercises.

Important: A P4 program also defines the interface between the switch pipeline and control plane. This interface is defined in the advanced_tunnel.p4info file. The table entries that you build in mycontroller.py refer to specific tables, keys, and actions by name, and we use a P4Info helper to convert the names into the IDs that are required for P4Runtime. Any changes in the P4 program that add or rename tables, keys, or actions will need to be reflected in your table entries.

Step 2: Implement Tunnel Forwarding

The mycontroller.py file is a basic controller plane that does the following:

  1. Establishes a gRPC connection to the switches for the P4Runtime service.
  2. Pushes the P4 program to each switch.
  3. Writes tunnel ingress and tunnel egress rules for two tunnels between h1 and h2.
  4. Reads tunnel ingress and egress counters every 2 seconds.

It also contains comments marked with TODO which indicate the functionality that you need to implement.

Your job will be to write the tunnel transit rule in the writeTunnelRules function that will match on tunnel ID and forward packets to the next hop.

topology

In this exercise, you will be interacting with some of the classes and methods in the p4runtime_lib directory. Here is a summary of each of the files in the directory:

  • helper.py
    • Contains the P4InfoHelper class which is used to parse the p4info files.
    • Provides translation methods from entity name to and from ID number.
    • Builds P4 program-dependent sections of P4Runtime table entries.
  • switch.py
    • Contains the SwitchConnection class which grabs the gRPC client stub, and establishes connections to the switches.
    • Provides helper methods that construct the P4Runtime protocol buffer messages and makes the P4Runtime gRPC service calls.
  • bmv2.py
    • Contains Bmv2SwitchConnection which extends SwitchConnections and provides the BMv2-specific device payload to load the P4 program.
  • convert.py
    • Provides convenience methods to encode and decode from friendly strings and numbers to the byte strings required for the protocol buffer messages.
    • Used by helper.py

Step 3: Run your solution

Follow the instructions from Step 1. If your Mininet network is still running, you will just need to run the following in your second shell:

./my_controller.py

You should start to see ICMP replies in your Mininet prompt, and you should start to see the values for all counters start to increment.

Extra Credit and Food for Thought

You might notice that the rules that are printed by mycontroller.py contain the entity IDs rather than the table names. You can use the P4Info helper to translate these IDs into entry names.

Also, you may want to think about the following:

  • What assumptions about the topology are baked into your implementation? How would you need to change it for a more realistic network?

  • Why are the byte counters different between the ingress and egress counters?

  • What is the TTL in the ICMP replies? Why is it the value that it is? Hint: The default TTL is 64 for packets sent by the hosts.

If you are interested, you can find the protocol buffer and gRPC definitions here:

Cleaning up Mininet

If the Mininet shell crashes, it may leave a Mininet instance running in the background. Use the following command to clean up:

make clean

Running the reference solution

To run the reference solution, you should run the following command from the ~/tutorials/P4D2_2017_Fall/exercises/p4runtime directory:

solution/my_controller.py

Next Steps

Congratulations, your implementation works! Move onto the next assignment mri!

Relevant Documentation

The documentation for P4_16 and P4Runtime is available here

All excercises in this repository use the v1model architecture, the documentation for which is available at:

  1. The BMv2 Simple Switch target document accessible here talks mainly about the v1model architecture.
  2. The include file v1model.p4 has extensive comments and can be accessed here.