Sniffing traffic can be a very powerful tool when debugging your P4 program. Basic things like verifying if traffic crosses certain path, or if header fields look like expected can be easily achieved by just observing traffic. For that, there is a wide range of tools that can be used.

The simple_switch provides an option to save all the traffic that crosses its interfaces in a .pcap file. To enable pcap logging when starting your switch, use the --pcap=<output_dir> command line option. For Example:

sudo simple_switch -i 0@<iface0> -i 1@<iface1> --pcap=<output_dir> <path to JSON file>

Packet logging will create several files using the following naming: <sw_name>-<intf_num>_<in|out>.pcap.

If you enable .pcap files in the p4app.json configuration file or in the network configuration script, switches will be started with the --pcap option and use as output dir ./pcap.

Another option is to observe the traffic as it flows. For that you can use tools like TShark and its GUI version Wireshark, which are already installed in the VM. To capture traffic with TShark run:

sudo tshark -i <interface_name>

Similarly, and if you prefer, you can use tcpdump (also already installed in the VM). To capture traffic with tcpdump run (shows link-layer information and does not resolve addresses):

sudo tcpdump -l -enn -i <interface_name>

To enable logging, make sure that you enabled --with-nanomsg flag to configure before compiling bmv2.

To enable console logging when starting your switch, use the --log-console command line option. For example:

sudo simple_switch -i 0@<iface0> -i 1@<iface1> --log-console <path to JSON file>

This will print all the messages in the terminal. Since this is not the most convenient, you can always redirect it to a log file:

sudo simple_switch -i 0@<iface0> -i 1@<iface1> --log-console <path to JSON file> >/path_to_file/sw.log

If you enable logging in the p4app.json configuration file, switches will automatically write all the console logging into a file in the ./log directory and with <sw_name>.log.

If logging is enabled, and you use the simple_switch_CLI when starting the topology (with P4-Utils) the output of the cli will be also logged in the log folder under the name <sw_name>_cli_output.log

To enable event logging when starting your switch, use the --nanolog command line option. For example, to use the ipc address ipc:///tmp/bm-log.ipc:

sudo ./simple_switch -i 0@<iface0> -i 1@<iface1> --nanolog ipc:///tmp/bm-log.ipc <path to JSON file>

Use tools/nanomsg_client.py as follows when the switch is running:

sudo ./nanomsg_client.py [--thrift-port <port>]

The script will display events of significance (table hits / misses, parser transitions, ...) for each packet.

When using P4-Utils to create the topology, each switch will automatically get assigned with a Thrift port. There are several ways to find the mapping between switch and port, but the easiest is to check the print messages displayed by p4run. Try to find a line that looks like:

s1 -> Thrift port: 9090

To enable the debugger, make sure that you passed the --enable-debugger flag to configure. You will also need to use the --debugger command line flag when starting the switch.

Use tools/p4dbg.py as follows when the switch is running to attach the debugger to the switch:

sudo ./p4dbg.py [--thrift-port <port>]

You can find a P4 debugger user guide in the BMv2 documentation.

Some times you do not want to use the logging system or debugger, or basically they are disabled. Yet, you could still get some insights on what the code does by just modifying a header field depending on which part of the code gets executed and check that value when the packet leaves the switch. Of course you can do something more sophisticated, and use several fields, read the value of a register and save it in the header, and so on.

We already have an example covering this. Basically the idea is to use P4 tables and do an exact match to all the fields you want to track. Every time the table is executed, if the BMv2 debugging is enabled, the switch will write the values of each field that was used to match the table entry in the switch log file. See the example for more information.

P4, and all the tools around are quite new. Several times things just do not work because there is a bug in the compiler or in the software switch implementation.

1. First of all, check what the P4-16 specification says about that. The specification is quite generic and not always will be able to give you a direct answer about what should be the expected behaviour of some 'Action', since the answer can be completely related to what a specific switch implementation does (in our case the BMv2 Simple Switch).

2. Check the p4-org mailing list. Probably you are not the first one having this problem, and someone already asked what you need in the mailing list. A good trick to get only results from the mailing list when googling them is to write your query as follows : site:http://lists.p4.org/ <query>. If you type in google site:http://lists.p4.org/ "simple_switch" you will get all the threads where the word simple_switch appears.

3. Check the GitHub issues section of BMv2 and P4C repositories. By default the searching bar adds a is:issue is:open filter, you can just remove it and write a keyword to find information about the issue and how to solve it.

4. If you do not find the solution anywhere, you can write in the mailing list yourself, or open an issue in GitHub.