multicast

Implementing Multicast

Introduction

The objective of this exercise is to write a P4 program that multicasts packets to a group of ports.

Upon receiving an Ethernet packet, the switch looks up the output port based on the destination MAC address. If it is a miss, the switch broadcast packets on ports belonging to a multicast group (if ingress port appears in the group, the packet will be dropped in the egress pipeline).

Your switch will have a single table, which the control plane will populate with static rules. Each rule will map an Ethernet MAC address to the output port. We have already defined the control plane rules, so you only need to implement the data plane logic of your P4 program.

We will use the star topology for this exercise. It is a single switch that connects four hosts as follow:

Our P4 program will be written for the V1Model architecture implemented on P4.org's bmv2 software switch. The architecture file for the V1Model can be found at: /usr/local/share/p4c/p4include/v1model.p4. This file describes the interfaces of the P4 programmable elements in the architecture, the supported externs, as well as the architecture's standard metadata fields. We encourage you to take a look at it.

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, multicast.p4, which initially drops all packets. Your job will be to extend this skeleton program to properly forward Ethernet packets.

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

1. In your shell, run:

   make run

   This will:

   • compile multicast.p4, and
   • start the sig-topo in Mininet and configure all switches with the appropriate P4 program + table entries, and
   • configure all hosts with the commands listed in pod-topo/topology.json

2. You should now see a Mininet command prompt. Try to ping between hosts in the topology:

   mininet> h1 ping h2
   mininet> pingall

3. Type exit to leave each xterm and the Mininet command line. Then, to stop mininet:

   make stop

   And to delete all pcaps, build files, and logs:

   make clean

The ping failed because each switch is programmed according to multicast.p4, which drops all packets on arrival. Your job is to extend this file so it forwards packets.

A note about the control plane

A P4 program defines a packet-processing pipeline, but the rules within each table are inserted 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, we have already implemented the control plane logic for you. As part of bringing up the Mininet instance, the make run command will install packet-processing rules in the tables of each switch. These are defined in the sX-runtime.json files, where X corresponds to the switch number.

Important: We use P4Runtime to install the control plane rules. The content of files sX-runtime.json refer to specific names of tables, keys, and actions, as defined in the P4Info file produced by the compiler (look for the file build/basic.p4.p4info.txtpb after executing make run). Any changes in the P4 program that add or rename tables, keys, or actions will need to be reflected in these sX-runtime.json files.

Step 2: Implement L2 Multicast

The multicast.p4 file contains a skeleton P4 program with key pieces of logic replaced by TODO comments. Your implementation should follow the structure given in this file---replace each TODO with logic implementing the missing piece.

A complete multicast.p4 will contain the following components:

1. Header type definitions for Ethernet (ethernet_t)
2. An action to drop a packet, using mark_to_drop().
3. TODO: An action (called multicast) that sends multiple copies of packets to a group of output ports.
4. TODO: Add the multicast action to the list of available actions
5. TODO: Set multicast as default action for table mac_lookup

Step 3: Run your solution

Follow the instructions from Step 1. This time, you should be able to successfully ping between h1, h2 and h3 but not h4 in the topology.

6. TODO: Add port 4 to the multicast group in file sig-topo/s1-runtime.json

Food for thought

Other questions to consider:

• How would you enhance your program to respond to ARP requests?
• How would you enhance your program to support MAC learning from the controller?

Troubleshooting

There are several problems that might manifest as you develop your program:

1. multicast.p4 might fail to compile. In this case, make run will report the error emitted from the compiler and halt.

2. multicast.p4 might compile but fail to support the control plane rules in the s1-runtime.json file that make run tries to install using P4Runtime. In this case, make run will report errors if control plane rules cannot be installed. Use these error messages to fix your multicast.p4 implementation.

3. multicast.p4 might compile, and the control plane rules might be installed, but the switch might not process packets in the desired way. The logs/sX.log files contain detailed logs that 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 run may leave a Mininet instance running in the background. Use the following command to clean up these instances:

make stop

Next Steps

Congratulations, your implementation works! Move on to Firewall.

Relevant Documentation

Documentation on the Usage of Gateway (gw) and ARP Commands in topology.json is here

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.