Example #3 Calculator

Example #3 - Calculator

In this example, we use the default settings of P4Pi as depicted in the following figure:

This example is based on the P4 tutorial exercise called calc. Accordingly, this P4 program implements a basic calculator using a custom protocol header written in P4. The header contains an operation to perform and two operands. When the switch receives a calculator packet header encapsulated into an Ethernet frame, it executes the operation on the operands, and encodes the result into the packet, swaps the MAC addresses in the Ethernet frame and returns the frame to the sender.

The calculator header is carried over Ethernet, using the Ethernet type 0x1234 to indicate the presence of the header. Table calculator is used to perform the proper calculation on the operands and store the result in one of the header fields. The table only contains constant entries.

P4 source code: calc.p4

Helper scripts: in the examples folder

Step 1 - Connecting to P4Pi

Connect your laptop to the wireless access point called p4pi. After that your laptop will get an IP address assigned by the DHCP service (from the default address pool 192.168.4.0/24).

Step 2 - Launching the P4 program

Start the P4 program (calc.p4) through the web interface or manually with the following commands in an SSH terminal:

ssh pi@192.168.4.1

pi@p4pi:~$ sudo -i
root@p4pi:~# echo 'calc' > /root/t4p4s-switch
root@p4pi:~# systemctl restart t4p4s.service

Step 3 - Start traffic receiver

You can run the code on your laptop connected to P4Pi as follows:

sudo python3 receive.py

The receiver script filters packets with Ethernet type 0x1234 and shows their content. The script prints both packets that have been send as well as received.

Step 4 - Send test traffic

This script provides a new prompt for typing basic expressions. If no expression has been entered, 1 + 2 will be used. After parsing the entered expression, it creates a packet with the corresponding operator and operands. Then it sends the packet on the wireless interface.

sudo python3 send.py

Using interface: wlp1s0
Payload [1 + 2]: 3 + 4
Sending packet with payload: 3 + 4
.
Sent 1 packets.
Payload [1 + 2]:
Sending packet with payload: 1 + 2
.
Sent 1 packets.
Payload [1 + 2]:

The result can be seen in the last line while additional status messages are also depicted.

Note that send.py and receive.py require the scapy dependency to be installed.

Running the test script on a laptop with Windows OS

The easiest way is to use an Anaconda installation with Python 3: Installing on Windows. The dependencies can be installed inside an Anaconda Prompt:

pip3 install scapy

When running on Windows OS, the send.py and receive.py scripts display an input prompt to specify the wireless interface name (e.g., 'Wi-Fi' in most Windows systems).

Finally, you can execute the send.py and receive.py scripts in an Anaconda Prompt as in Linux systems:

python3 send.py

python3 receive.py

Functionalities to be added during the hackathon or later

• Emulating division and multiplication by applying logarithm and exponential tables