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Introduction

The P4 program flowcache.p4 is a very simple "toy" P4 program to demonstrate how a P4 program can "punt" packets to a P4Runtime controller using the PacketIn mechanism, and the controller can respond to receiving such packets by adding entries to a table.

There are many useful things that this demonstration does not do, e.g. it does not keep statistics of forwarded packets, or number of table entries added. It does not use the idle timeout mechanism to detect when entries added to the table flow_cache have been unmatched for a long time, and notify the controller to delete those stale entries.

Instructions for use

Set up some environment variables needed for the commands below to succeed. This needs to be done in each terminal window you create:

export P4GUIDE="$HOME/p4-guide"
export PYPKG_TESTLIB="$P4GUIDE/testlib"

Replace the example file path above in the first line with the path to the directory p4-guide that is your copy of the p4-guide repository, if it is different than the example.

Compile the P4 program with the command:

make compile

Start the BMv2 software switch with:

sudo $P4GUIDE/bin/veth_setup.sh
make runswitch

Leave the switch running in the terminal where you started it, and use separate terminal windows for the commands below.

Load the compiled P4 program into the switch with:

make loadp4prog

Run the controller:

make runcontroller

Leave the controller running in the terminal where you started it, and use separate terminal windows for the commands below.

The P4 program and controller are up and running now.

All instructions below are completely optional, simply shown as one way to exercise the system.

Send a packet from an interactive Python session using the Scapy library:

$ sudo PATH=$PATH VIRTUAL_ENV=$VIRTUAL_ENV python3
Python 3.8.10 (default, Nov 22 2023, 10:22:35) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Inside that interactive Python session:

from scapy.all import *
pkt1=Ether()/IP()
pkt2=Ether()/IP(src='10.0.1.1',dst='10.0.2.2')/TCP()
sendp(pkt1,iface='veth2')

Within 5 seconds you should see a message like this in the terminal where the controller is running, indicating that the controller got a PacketIn message from the switch, and reacted by adding a table entry to the P4 table named flow_cache:

2024-03-11 16:54:01,948 - root - INFO - For flow (SA=127.0.0.1, DA=127.0.0.1, proto=0) added table entry to send packets to port 1 with new DSCP 5

After that, any packet sent with the same IPv4 source address, destination address, and protocol should match that table entry, and be forwarded to the port number given in the controller message.

To try with a packet with a different value for those 3 fields:

sendp(pkt2,iface='veth4')

You can use Ctrl-C in the terminal where the controller is running to quit it. You may need to type Ctrl-C twice to actually terminate it. Similarly for the terminal where the switch is running.