ecn

Explicit Congestion Notification

+--+      +--+     ++-+
|h1+------+s1+-----+h3+
+--+      +-++     +--+
            |
            |
          +-++
          |h2|
          +-++

Introduction

The link between s1 and h3 has a bandwidth of 0.5 Mbps and acts as a bottleneck for the traffic coming from the other hosts towards h3. If Explicit Congestion Notification is enabled for traffic going through the switch (ECN field of the IPv4 packet, i.e. the two least significant bits of the TOS field, must be set to 1 or 2) If the output queue builds up, the P4 switch will tag packets with the ECN flag 3.

How to run

Run the topology:

sudo p4run

or

sudo python network.py

Monitor the traffic going from s1 to h3.

sudo tcpdump -i s1-eth3 -Q out -v

Then generate a UDP flow (or multiple) from h1 towards h3.

mininet> task h1 0 0 recv_udp_flow --dport 5051
mininet> task h1 0 30 send_udp_flow --dst 10.1.3.2 --sport 5000 --dport 5051 --tos 1 --rate 10M

Observe that when congestion is built packet will start to have the ECN flag set to 3.