# Implementing ECN ## Introduction The objective of this tutorial is to extend basic L3 forwarding with an implementation of Explict 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: ```bash 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..`). * The control plane programs the P4 tables in each switch based on `sx-commands.txt` 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: ```bash mininet> xterm h1 h11 h2 h22 ``` 3. In `h2`'s XTerm, start the server that captures packets: ```bash ./receive.py ``` 4. in `h22`'s XTerm, start the iperf UDP server: ```bash iperf -s -u ``` 5. In `h1`'s XTerm, send one packet per second to `h2` using send.py say for 30 seconds: ```bash ./send.py 10.0.2.2 "P4 is cool" 30 ``` The message "P4 is cool" should be received in `h2`'s xterm, 6. In `h11`'s XTerm, start iperf client sending for 15 seconds ```bash iperf -c 10.0.2.22 -t 15 -u ``` 7. At `h2`, the `ipv4.tos` field (DiffServ+ECN) is always 1 8. 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 verfiication 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` ```bash ./receive.py > h2.log ``` and just print the `tos` values `grep tos build/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-commands.txt` files that `make` tries to install using the BMv2 CLI. In this case, `make` will report these errors to `stderr`. 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 `build/logs/.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/-.pcap` also contains the pcap of packets on each interface. Use `tcpdump -r -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 `p4app.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: ```bash make stop ``` ## Next Steps Congratulations, your implementation works! Move on to the next exercise: [Multi-Hop Route Inspection](../mri), which identifies which link is the source of congestion.