Implementing L3 Forwarding

Introduction

The objective of this tutorial is to implement basic L3 forwarding. To keep the exercise small, 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, ipv4_forward.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 ip4v_forward.p4 and bring up a switch in Mininet to test its behavior.

In your shell, run:
```
./run.sh
```
This will:
- compile ip4v_forward.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 10.0.1.10, 10.0.2.10, etc.
You should now see a Mininet command prompt. Open two terminals for h1 and h2, respectively:
```
mininet> xterm h1 h2
```
Each host includes a small Python-based messaging client and server. In h2's xterm, start the server:
```
./receive.py
```
In h1's xterm, send a message from the client:
```
./send.py 10.0.2.10 "P4 is cool"
```
The message will not be received.
Type exit to leave each xterm and the Mininet command line.

The message was not received because each switch is programmed with ip4v_forward.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 run.sh 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: Implement L3 forwarding

The ipv4_forward.p4 file contains a skeleton P4 program with key pieces of logic replaced by TODO comments. These should guide your implementation---replace each TODO with logic implementing the missing piece.

A complete ipv4_forward.p4 will contain the following components:

Header type definitions for Ethernet (ethernet_t) and IPv4 (ipv4_t).
TODO: Parsers for Ethernet and IPv4 that populate ethernet_t and ipv4_t fields.
An action to drop a packet, using mark_to_drop().
TODO: An action (called ipv4_forward), which will:
1. Set the egress port for the next hop.
2. Update the ethernet destination address with the address of the next hop.
3. Update the ethernet source address with the address of the switch.
4. Decrement the TTL.
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.
A deparser that selects the order in which fields inserted into the outgoing packet.
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.

Troubleshooting

There are several ways that problems might manifest:

ipv4_forward.p4 fails to compile. In this case, run.sh will report the error emitted from the compiler and stop.
ipv4_forward.p4 compiles but does not support the control plane rules in the sX-commands.txt files that run.sh tries to install using the BMv2 CLI. In this case, run.sh will report these errors to stderr. Use these error messages to fix your ipv4_forward.p4 implementation.
ipv4_forward.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, run.sh 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 basic network telemetry Multi-Hop Route Inspection.

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