meter

Meter

+--+      +--+     ++-+
|h1+------+s1+-----+h2+
+--+      +-++     +--+

Introduction

This program illustrates as simply as possible how to use meters in P4 with bmv2. bmv2 uses two-rate three-color meters as described here.

For each incoming packet the m_read table is applied and the appropriate meter (based on the packet's source MAC address) is executed. Based on the observed traffic rate for this sender and the meter's configuration, executing the meter will yield one of 3 values: 0 (GREEN), 1 (YELLOW) or 2 (RED). This value will be copied to metadata field meta.meter_tag. Note that if no meter was associated to the sender's MAC address, the table will be a no-op. This table also redirects all packets - with a known source MAC address- to port 2 of the switch.

After that, the packet will go through a second table, m_filter, which can either be a no-op or drop the packet based on how the packet was tagged by the meter.

Note, that we provide the same example with direct and indirect meters. Direct meters are meters associated to table entries, while indirect meters can be addressed by index. You can have a look at both source codes to see the difference.

If you take a look at the source code for indirect counter you can see that we set the default action to drop. Also, if you take a look at the control plane commands you can see (line 2) how we set a rule for meta.meter_tag == 0. Thus, all packets for which meta.meter_tag is not 0 will be dropped.

The control plane command file also gives you the meter configuration. In this case, the first rate is 0.5 packets per second, with a burst size of 1, and the second rate is 10 packets per second, with a burst size of 1 also. Feel free to play with the numbers, but these play nicely with the demonstration below.

How to run

There are two examples, one using a direct and one using an indirect meter. You can start them using:

sudo p4run --config p4app_direct.json
sudo p4run --config p4app_indirect.json

or

sudo python network_direct.py
sudo python network_indirect.py

In the mininet CLI, you can start the demo script which periodically sends packets from the host 1 interface and listens for packets on the host 2 interface. The script takes the time interface (in seconds) as argument, e.g.:

mininet> sh ./send_and_receive.py 1

(Works for both direct and indirect meters)

If you run the script with an interval of one second, you should observe the following output:

Received one
Sent one
Sent one
Received one
Sent one
Sent one
Received one
Sent one
...

This is because we send one packet every second, while the first rate of the meter is 0.5 packets per second. The P4 program therefore drops on average one packet out of two.

If you run the script with in interval of two seconds, you will notice that no packet gets dropped anymore.