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P4D2_2017_Fall/exercises/ecn/README.md
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P4D2_2017_Fall/exercises/ecn/README.md
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# Implementing ECN
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## Introduction
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The objective of this tutorial is to extend basic L3 forwarding with
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an implementation of Explict Congestion Notification (ECN).
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ECN allows end-to-end notification of network congestion without
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dropping packets. If an end-host supports ECN, it puts the value of 1
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or 2 in the `ipv4.ecn` field. For such packets, each switch may
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change the value to 3 if the queue size is larger than a threshold.
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The receiver copies the value to sender, and the sender can lower the
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rate.
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As before, we have already defined the control plane rules for
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routing, so you only need to implement the data plane logic of your P4
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program.
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> **Spoiler alert:** There is a reference solution in the `solution`
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> sub-directory. Feel free to compare your implementation to the reference.
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## Step 1: Run the (incomplete) starter code
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The directory with this README also contains a skeleton P4 program,
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`ecn.p4`, which initially implements L3 forwarding. Your job (in the
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next step) will be to extend it to properly append set the ECN bits
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Before that, let's compile the incomplete `ecn.p4` and bring up a
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network in Mininet to test its behavior.
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1. In your shell, run:
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```bash
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./run.sh
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```
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This will:
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* compile `ecn.p4`, and
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* start a Mininet instance with three switches (`s1`, `s2`, `s3`) configured
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in a triangle. There are 5 hosts. `h1` and `h11` are connected to `s1`.
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`h2` and `h22` are connected to `s2` and `h3` is connected to `s3`.
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* The hosts are assigned IPs of `10.0.1.1`, `10.0.2.2`, etc
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(`10.0.<Switchid>.<hostID>`).
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* The control plane programs the P4 tables in each switch based on
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`sx-commands.txt`
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2. We want to send a low rate traffic from `h1` to `h2` and a high
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rate iperf traffic from `h11` to `h22`. The link between `s1` and
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`s2` is common between the flows and is a bottleneck because we
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reduced its bandwidth to 512kbps in p4app.json. Therefore, if we
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capture packets at `h2`, we should see the right ECN value.
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3. You should now see a Mininet command prompt. Open four terminals
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for `h1`, `h11`, `h2`, `h22`, respectively:
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```bash
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mininet> xterm h1 h11 h2 h22
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```
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3. In `h2`'s XTerm, start the server that captures packets:
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```bash
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./receive.py
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```
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4. in `h22`'s XTerm, start the iperf UDP server:
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```bash
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iperf -s -u
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```
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5. In `h1`'s XTerm, send one packet per second to `h2` using send.py
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say for 30 seconds:
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```bash
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./send.py 10.0.2.2 "P4 is cool" 30
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```
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The message "P4 is cool" should be received in `h2`'s xterm,
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6. In `h11`'s XTerm, start iperf client sending for 15 seconds
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```bash
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iperf -c 10.0.2.22 -t 15 -u
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```
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7. At `h2`, the `ipv4.tos` field (DiffServ+ECN) is always 1
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8. type `exit` to close each XTerm window
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Your job is to extend the code in `ecn.p4` to implement the ECN logic
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for setting the ECN flag.
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## Step 2: Implement ECN
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The `ecn.p4` file contains a skeleton P4 program with key pieces of
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logic replaced by `TODO` comments. These should guide your
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implementation---replace each `TODO` with logic implementing the
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missing piece.
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First we have to change the ipv4_t header by splitting the TOS field
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into DiffServ and ECN fields. Remember to update the checksum block
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accordingly. Then, in the egress control block we must compare the
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queue length with ECN_THRESHOLD. If the queue length is larger than
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the threshold, the ECN flag will be set. Note that this logic should
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happen only if the end-host declared supporting ECN by setting the
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original ECN to 1 or 2.
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A complete `ecn.p4` will contain the following components:
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1. Header type definitions for Ethernet (`ethernet_t`) and IPv4 (`ipv4_t`).
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2. Parsers for Ethernet, IPv4,
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3. An action to drop a packet, using `mark_to_drop()`.
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4. An action (called `ipv4_forward`), which will:
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1. Set the egress port for the next hop.
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2. Update the ethernet destination address with the address of
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the next hop.
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3. Update the ethernet source address with the address of the switch.
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4. Decrement the TTL.
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5. An egress control block that checks the ECN and
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`standard_metadata.enq_qdepth` and sets the ipv4.ecn.
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6. A deparser that selects the order in which fields inserted into the outgoing
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packet.
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7. A `package` instantiation supplied with the parser, control,
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checksum verfiication and recomputation and deparser.
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## Step 3: Run your solution
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Follow the instructions from Step 1. This time, when your message from
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`h1` is delivered to `h2`, you should see `tos` values change from 1
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to 3 as the queue builds up. `tos` may change back to 1 when iperf
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finishes and the queue depletes.
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To easily track the `tos` values you may want to redirect the output
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of `h2` to a file by running the following for `h2`
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```bash
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./receive.py > h2.log
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```
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and just print the `tos` values `grep tos build/h2.log` in a separate window
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```
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x3
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tos = 0x3
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tos = 0x3
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tos = 0x3
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tos = 0x3
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tos = 0x3
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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tos = 0x1
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```
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### Food for thought
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How can we let the user configure the threshold?
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### Troubleshooting
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There are several ways that problems might manifest:
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1. `ecn.p4` fails to compile. In this case, `run.sh` will report the
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error emitted from the compiler and stop.
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2. `ecn.p4` compiles but does not support the control plane rules in
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the `sX-commands.txt` files that `run.sh` tries to install using
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the BMv2 CLI. In this case, `run.sh` will report these errors to
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`stderr`. Use these error messages to fix your `ecn.p4`
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implementation.
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3. `ecn.p4` compiles, and the control plane rules are installed, but
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the switch does not process packets in the desired way. The
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`build/logs/<switch-name>.log` files contain trace messages
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describing how each switch processes each packet. The output is
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detailed and can help pinpoint logic errors in your implementation.
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The `build/<switch-name>-<interface-name>.pcap` also contains the
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pcap of packets on each interface. Use `tcpdump -r <filename> -xxx`
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to print the hexdump of the packets.
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4. `ecn.p4` compiles and all rules are installed. Packets go through
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and the logs show that the queue length was not high enough to set
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the ECN bit. Then either lower the threshold in the p4 code or
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reduce the link bandwidth in `p4app.json`
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#### Cleaning up Mininet
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In the latter two cases above, `run.sh` may leave a Mininet instance
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running in the background. Use the following command to clean up
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these instances:
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```bash
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mn -c
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```
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## Next Steps
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Congratulations, your implementation works! Move on to the next
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exercise: [Multi-Hop Route Inspection](../mri), which identifies which
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link is the source of congestion.
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