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.

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 a Mininet instance with three switches (s1, s2, s3)
      configured in a triangle, each connected to one host (h1, h2,
      and h3).
    • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, and 10.0.3.3.

  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 to h2:

    ./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.
    Then, to stop mininet:

    make stop

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

    make clean

The message was not received 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 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.p4info 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, your message from
h1 should be delivered to h2.

Food for thought

The "test suite" for your solution---sending a message from h1 to
h2---is not very robust. What else should you test to be confident
of your implementation?

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 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 /tmp/p4s.<switch-name>.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! In the next exercise we
will build on top of this and add support for a basic tunneling
protocol: basic_tunnel!