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Getting Started: Web Playground

The playground is the fastest way to try 4ward — edit, compile, and trace P4 programs in the browser.

4ward Web Playground

Start the server

Clone and build if you haven't already:

git clone https://github.com/smolkaj/4ward.git && cd 4ward

Then start the playground:

bazel run //web:playground

The server prints:

4ward Playground
  Web UI:   http://localhost:8080
  gRPC:     localhost:9559

Your browser opens automatically. If it doesn't, navigate to http://localhost:8080.

Load an example

Click the example selector dropdown (top of the editor) and choose basic_table. You'll see a short v1model program with one table:

table port_table {
    key = { hdr.ethernet.etherType : exact; }
    actions = { forward; drop; NoAction; }
    default_action = drop();
}

Click Compile & Load. The status bar turns green and shows the pipeline summary (tables, actions). The control-flow graph appears at the bottom, showing the ingress pipeline's table and control flow.

Install a table entry

Switch to the Tables tab on the right panel.

  1. Select table port_table.
  2. Set the match field hdr.ethernet.etherType to 0x0800 (IPv4).
  3. Select action forward, set port to 1.
  4. Click Add Entry.

The entry appears in the installed entries list below.

Send a packet

Switch to the Packets tab.

  1. Click the IPv4 preset — it fills the payload with a valid Ethernet + IPv4 frame.
  2. Set Ingress port to 0.
  3. Click Send Packet.

The output panel shows one packet exiting on port 1 with decoded header fields.

Read the trace

Switch to the Trace tab to see every decision the simulator made:

  1. Parser — start → accept after extracting the Ethernet header.
  2. Ingress — port_table: hit → forward with the matched entry details.
  3. Action — forward(port=1) executed.
  4. Deparser — Ethernet header emitted (14 bytes).
  5. Output — packet exits on port 1.

Use the arrow keys (← →) to step through events one at a time. Each step highlights the corresponding P4 source line in the editor and the active node in the control-flow graph. Press Escape to reset.

Try a miss

Send an ARP packet (click the ARP preset) without installing a matching entry. The trace shows port_table: miss → drop and the packet is dropped.

What's next