calc

Implementing a P4 Calculator

Introduction

The objective of this tutorial is to implement a basic calculator using a custom protocol header written in P4. The header will contain an operation to perform and two operands. When a switch receives a calculator packet header, it will execute the operation on the operands, and return the result to the sender.

Step 1: Run the (incomplete) starter code

The directory with this README also contains a skeleton P4 program, calc.p4, which initially drops all packets. Your job will be to extend it to properly implement the calculator logic.

As a first step, compile the incomplete calc.p4 and bring up a switch in Mininet to test its behavior.

1. In your shell, run:

   make

   This will:

   • compile calc.p4, and

   • start a Mininet instance with one switches (s1) connected to two hosts (h1, h2).

   • The hosts are assigned IPs of 10.0.1.1 and 10.0.1.2.

2. We've written a small Python-based driver program that will allow you to test your calculator. You can run the driver program directly from the Mininet command prompt:

mininet> h1 python calc.py 
> 

3. The driver program will provide a new prompt, at which you can type basic expressions. The test harness will parse your expression, and prepare a packet with the corresponding operator and operands. It will then send a packet to the switch for evaluation. When the switch returns the result of the computation, the test program will print the result. However, because the calculator program is not implemented, you should see an error message.

> 1+1
Didn't receive response
>

Step 2: Implement Calculator

To implement the calculator, you will need to define a custom calculator header, and implement the switch logic to parse header, perform the requested operation, write the result in the header, and return the packet to the sender.

We will use the following header format:

         0                1                  2              3
  +----------------+----------------+----------------+---------------+
  |      P         |       4        |     Version    |     Op        |
  +----------------+----------------+----------------+---------------+
  |                              Operand A                           |
  +----------------+----------------+----------------+---------------+
  |                              Operand B                           |
  +----------------+----------------+----------------+---------------+
  |                              Result                              |
  +----------------+----------------+----------------+---------------+

• P is an ASCII Letter 'P' (0x50)
• 4 is an ASCII Letter '4' (0x34)
• Version is currently 0.1 (0x01)
• Op is an operation to Perform:
• '+' (0x2b) Result = OperandA + OperandB
• '-' (0x2d) Result = OperandA - OperandB
• '&' (0x26) Result = OperandA & OperandB
• '|' (0x7c) Result = OperandA | OperandB
• '^' (0x5e) Result = OperandA ^ OperandB

We will assume that the calculator header is carried over Ethernet, and we will use the Ethernet type 0x1234 to indicate the presence of the header.

Given what you have learned so far, your task is to implement the P4 calculator program. There is no control plane logic, so you need only worry about the data plane implementation.

A working calculator implementation will parse the custom headers, execute the mathematical operation, write the result in the result field, and return the packet to the sender.

Step 3: Run your solution

Follow the instructions from Step 1. This time, you should see the correct result:

> 1+1
2
>

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.