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Implementing Basic Forwarding

Introduction

The objective of this exercise is to write a P4 program that implements basic forwarding. To keep things simple, we will just implement forwarding for IPv4.

With IPv4 forwarding, the switch must perform the following actions for every packet: (i) update the source and destination MAC addresses, (ii) decrement the time-to-live (TTL) in the IP header, and (iii) forward the packet out the appropriate port.

Your switch will have a single table, which the control plane will populate with static rules. Each rule will map an IP address to the MAC address and output port for the next hop. 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 following topology for this exercise. It is a single pod of a fat-tree topology and henceforth referred to as pod-topo: pod-topo

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 desribes 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, basic.p4, which initially drops all packets. Your job will be to extend this skeleton program to properly forward IPv4 packets.

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

  1. In your shell, run:

    make run

    This will:

    • compile basic.p4, and
    • start the pod-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 basic.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.txt 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 L3 forwarding

The basic.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 basic.p4 will contain the following components:

  1. Header type definitions for Ethernet (ethernet_t) and IPv4 (ipv4_t).
  2. TODO: Parsers for Ethernet and IPv4 that populate ethernet_t and ipv4_t fields.
  3. An action to drop a packet, using mark_to_drop().
  4. TODO: An action (called ipv4_forward) that:
    1. Sets the egress port for the next hop.
    2. Updates the ethernet destination address with the address of the next hop.
    3. Updates the ethernet source address with the address of the switch.
    4. Decrements the TTL.
  5. TODO: A control that:
    1. Defines a table that will read an IPv4 destination address, and invoke either drop or ipv4_forward.
    2. An apply block that applies the table.
  6. TODO: A deparser that selects the order in which fields inserted into the outgoing packet.
  7. A package instantiation supplied with the parser, control, and deparser.

    In general, a package also requires instances of checksum verification and recomputation controls. These are not necessary for this tutorial and are replaced with instantiations of empty controls.

Step 3: Run your solution

Follow the instructions from Step 1. This time, you should be able to sucessfully ping between any two hosts in the topology.

Food for thought

The "test suite" for your solution---sending pings between hosts in the topology---is not very robust. What else should you test to be confident that you implementation is correct?

Although the Python scapy library is outside the scope of this tutorial, it can be used to generate packets for testing. The send.py file shows how to use it.

Other questions to consider:

  • How would you enhance your program to respond to ARP requests?
  • How would you enhance your program to support traceroute?
  • How would you enhance your program to support next hops?
  • Is this program enough to replace a router? What's missing?

Troubleshooting

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

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

  2. basic.p4 might compile but fail to support the control plane rules in the s1-runtime.json through s3-runtime.json files 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 basic.p4 implementation.

  3. basic.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

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.