p4tutorials-turkce/P4D2_2017_Fall/exercises/scrambler

Implementing basic forwarding with scrambled addresses

Introduction

In this exercise, you will extend your solution to the basic forwarding exercise with a new twist: switches will invert the bits representing Ethernet and IPv4 address. Hence, in our triangle topology, the packets in the interior of the network will have unintelligble addresses.

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 directory with this README also contains a skeleton P4 program, scrambler.p4, which initially drops all packets. Your job (in the next step) will be to extend it to properly forward IPv4 packets.

Before that, let's compile the incomplete scrambler.p4 and bring up a switch in Mininet to test its behavior.

1. In your shell, run:

   make
   

   This will:

   • compile scrambler.p4, and
   • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle, each connected to one host (h1, h2, h3).
   • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, etc.

2. You should now see a Mininet command prompt. Open two terminals for h1 and h2, respectively:

   mininet> xterm h1 h2
   

3. Each host includes a small Python-based messaging client and server. In h2's xterm, start the server:

   ./receive.py
   

4. In h1's xterm, send a message from the client:

   ./send.py 10.0.2.2 "P4 is cool"
   

   The message will not be received.

5. Type exit to leave each xterm and the Mininet command line.

The message was not received because each switch is programmed with scrambler.p4, which drops all packets on arrival. Your job is to extend this file.

A note about the control plane

P4 programs define a packet-processing pipeline, but the rules governing packet processing are inserted into the pipeline by the control plane. When a rule matches a packet, its action is invoked with parameters supplied by the control plane as part of the rule.

In this exercise, the control plane logic has already been implemented. As part of bringing up the Mininet instance, the make script will install packet-processing rules in the tables of each switch. These are defined in the sX-commands.txt files, where X corresponds to the switch number.

Important: A P4 program also defines the interface between the switch pipeline and control plane. The sX-commands.txt files contain lists of commands for the BMv2 switch API. These commands refer to specific tables, keys, and actions by name, and any changes in the P4 program that add or rename tables, keys, or actions will need to be reflected in these command files.

Step 2: Extend the basic forwarding solution to flip bits

The scrambler.p4 file contains a skeleton P4 program in which one of the actions has a TODO comment. These should guide your implementation---replace the TODO with logic implementing the missing piece.

A complete scrambler.p4 will add an action flip() that inverts the bits in the Ethernet and IPv4 headers.

Step 3: Run your solution

Follow the instructions from Step 1. This time, your message from h1 should be delivered to h2.

Troubleshooting

There are several issues that might arise when developing your solution:

1. scrambler.p4 fails to compile. In this case, make will report the error emitted from the compiler and stop.

2. scrambler.p4 compiles but does not support the control plane rules in the sX-commands.txt files that make tries to install using the BMv2 CLI. In this case, make will report these errors to stderr. Use these error messages to fix your scrambler.p4 implementation.

3. scrambler.p4 compiles, and the control plane rules are installed, but the switch does not process packets in the desired way. The build/logs/<switch-name>.log files contain trace messages describing how each switch processes each packet. The output is detailed and can help pinpoint logic errors in your implementation.

Cleaning up Mininet

In the latter two cases above, make may leave a Mininet instance running in the background. Use the following command to clean up these instances:

mn -c

Next Steps

Congratulations, your implementation works! Move on to the next exercise: implementing Explicit Congestion Notification.