01-Reflector

Packet Reflector

Introduction

The packet reflector will be our very first P4 exercise. The main objective of this exercise is to show you how to create simple topologies with hosts and p4 switches and how to add links between them. Then you will implement a very simple p4 program that makes switches bouncing back packets to the interface the packets came from.

First Steps

Since this is a first contact with the environment, we will provide some files that will help you solving the exercise:

• p4app.json: describes the topology we want to create with the help of Mininet and the P4-Utils package.
• network.py: a Python scripts that initializes the topology using Mininet and P4-Utils. One can use indifferently network.py or p4app.json to start the network.
• send_receive.py: script to send and receive packets.
• reflector.p4: p4 program skeleton to use as a starting point.

The provided files are enough to start a topology with one host and one switch connected between each other. However, since the p4 program contains an incomplete program, the switch will just drop the packets we send to it.

Follow these instructions to create a mininet network and run reflector.p4:

1. To create the topology described in p4app.json, you just have to call p4run, which by default will check if the file p4app.json exists in the path:

   sudo p4run

   This will call a python script that parses the configuration file, creates a virtual network of hosts and p4 switches using mininet, compile the p4 program and load it in the switch. You can find the P4-Utils documentation here.

   Otherwise, one can simply execute the Python script provided:

   sudo python network.py

   After running the network with one of the two aforementioned methods, you will get the mininet CLI prompt (you can find some documentation about the CLI here):

   

2. At this point you will have a small topology that consists of a host h1 and a p4 switch s1. You can get a terminal in h1 by either typing xterm h1 in the CLI, or by using the mx command that comes already installed in the VM:

   mx h1

3. From the h1 terminal you can run send_receive.py, a small python script that sends packets to the switch and prints if the packets get reflected. Since the switch with the initial code in reflector.p4 is not doing anything, all the packets it receives will get dropped and nothing will get reflected back to h1.

4. Close all the host-terminals and type quit to leave the mininet CLI and clean the network.

   mininet> quit
   

   Alternatives: exit or Ctrl-D

So far we have seen how to start the network and send packets to the switch. Now you have all the pieces to start writing your first p4 program and load it into the switch.

Note about the CLI

You can load a new program into the switch without restarting the entire topology using the reboot command:

mininiet> p4switch_reboot s1

Just rebooting switches is very helpful since you will be able to keep all the host terminals you opened. Check the p4-utils CLI for more information.

Implementing the Packet Reflector

To solve this exercise, you only need to fill the gaps in the reflector.p4 skeleton. The places where you are supposed to write your own code are marked with a TODO.

In the end, your program should do the following:

1. Parse the ethernet header and make a transition to MyIngress. Note that the definition of the ethernet header and the headers struct is already defined for you.

2. Swap the packet's ethernet addresses. You can use an action or simply write the code directly in the control apply.

   Hint: you can define and use local variables to swap the addresses: bit<48> tmpAddr;

3. Use the ingress_port as egress_port. The value of the ingress_port will be stored in the packet metadata, in the variable standard_metadata.ingress_port. To set a packet's output port, you need to set standard_metadata.egress_spec metadata field. For more information about the standard metadata fields read: simple switch documentation.

4. Deparse the ethernet header.

Note: The use of tables is possible but not strictly necessary for completing this exercise.

Testing your solution

If you finished the 4 steps above you can repeat the steps explained in First Steps section. This time when you send a packet you should get a reflected packet from the switch with the MAC addresses swapped:

If you get the same output you can move to the next exercise, but first clean the network.