ecn

Implementing ECN

Introduction

The objective of this tutorial is to extend basic L3 forwarding with an implementation of Explicit Congestion Notification (ECN).

ECN allows end-to-end notification of network congestion without dropping packets. If an end-host supports ECN, it puts the value of 1 or 2 in the ipv4.ecn field. For such packets, each switch may change the value to 3 if the queue size is larger than a threshold. The receiver copies the value to sender, and the sender can lower the rate.

As before, we have already defined the control plane rules for routing, 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, ecn.p4, which initially implements L3 forwarding. Your job (in the next step) will be to extend it to properly append set the ECN bits

Before that, let's compile the incomplete ecn.p4 and bring up a network in Mininet to test its behavior.

1. In your shell, run:

   make

   This will:

   • compile ecn.p4, and
   • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle. There are 5 hosts. h1 and h11 are connected to s1. h2 and h22 are connected to s2 and h3 is connected to s3.
   • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, etc (10.0.<Switchid>.<hostID>).
   • The control plane programs the P4 tables in each switch based on sx-runtime.json

2. We want to send a low rate traffic from h1 to h2 and a high rate iperf traffic from h11 to h22. The link between s1 and s2 is common between the flows and is a bottleneck because we reduced its bandwidth to 512kbps in topology.json. Therefore, if we capture packets at h2, we should see the right ECN value.

3. You should now see a Mininet command prompt. Open four terminals for h1, h11, h2, h22, respectively:

   mininet> xterm h1 h11 h2 h22

4. In h2's XTerm, start the server that captures packets:

   ./receive.py

5. in h22's XTerm, start the iperf UDP server:

   iperf -s -u

6. In h1's XTerm, send one packet per second to h2 using send.py say for 30 seconds:

   ./send.py 10.0.2.2 "P4 is cool" 30

   The message "P4 is cool" should be received in h2's xterm,
7. In h11's XTerm, start iperf client sending for 15 seconds

   iperf -c 10.0.2.22 -t 15 -u

8. At h2, the ipv4.tos field (DiffServ+ECN) is always 1
9. type exit to close each XTerm window

Your job is to extend the code in ecn.p4 to implement the ECN logic for setting the ECN flag.

Step 2: Implement ECN

The ecn.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.

First we have to change the ipv4_t header by splitting the TOS field into DiffServ and ECN fields. Remember to update the checksum block accordingly. Then, in the egress control block we must compare the queue length with ECN_THRESHOLD. If the queue length is larger than the threshold, the ECN flag will be set. Note that this logic should happen only if the end-host declared supporting ECN by setting the original ECN to 1 or 2.

A complete ecn.p4 will contain the following components:

1. Header type definitions for Ethernet (ethernet_t) and IPv4 (ipv4_t).
2. Parsers for Ethernet, IPv4,
3. An action to drop a packet, using mark_to_drop().
4. 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.
5. An egress control block that checks the ECN and standard_metadata.enq_qdepth and sets the ipv4.ecn.
6. A deparser that selects the order in which fields inserted into the outgoing packet.
7. A package instantiation supplied with the parser, control, checksum verification and recomputation and deparser.

Step 3: Run your solution

Follow the instructions from Step 1. This time, when your message from h1 is delivered to h2, you should see tos values change from 1 to 3 as the queue builds up. tos may change back to 1 when iperf finishes and the queue depletes.

To easily track the tos values you may want to redirect the output of h2 to a file by running the following for h2

./receive.py > h2.log

and just print the tos values grep tos h2.log in a separate window

     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x3
     tos       = 0x3
     tos       = 0x3
     tos       = 0x3
     tos       = 0x3
     tos       = 0x3
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1
     tos       = 0x1

Food for thought

How can we let the user configure the threshold?

Troubleshooting

There are several ways that problems might manifest:

1. ecn.p4 fails to compile. In this case, make will report the error emitted from the compiler and stop.
2. ecn.p4 compiles but does not support the control plane rules in the sX-runtime.json files that make tries to install using a Python controller. In this case, make will log the controller output in the logs directory. Use these error messages to fix your ecn.p4 implementation.
3. ecn.p4 compiles, and the control plane rules are installed, but the switch does not process packets in the desired way. The /tmp/p4s.<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. The build/<switch-name>-<interface-name>.pcap also contains the pcap of packets on each interface. Use tcpdump -r <filename> -xxx to print the hexdump of the packets.
4. ecn.p4 compiles and all rules are installed. Packets go through and the logs show that the queue length was not high enough to set the ECN bit. Then either lower the threshold in the p4 code or reduce the link bandwidth in topology.json

Cleaning up Mininet

In the latter two cases above, make may leave a Mininet instance running in the background. Use the following command to clean up these instances:

make stop

Next Steps

Congratulations, your implementation works! Move on to the next exercise: Multi-Hop Route Inspection, which identifies which link is the source of congestion.