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Load Balancing

In this exercise, you will implement a form of load balancing based on a simple version of Equal-Cost Multipath Forwarding. The switch you will implement will use two tables to forward packets to one of two destination hosts at random. The first table will use a hash function (applied to a 5-tuple consisting of the source and destination IP addresses, IP protocol, and source and destination TCP ports) to select one of two hosts. The second table will use the computed hash value to forward the packet to the selected host.

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, load_balance.p4, which initially drops all packets. Your job (in the next step) will be to extend it to properly forward packets.

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

  1. In your shell, run:

    make

    This will:

    • compile load_balance.p4, and
    • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle, each connected to one host (h1, h2, h3).
    • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, etc.
    • We use the IP address 10.0.0.1 to indicate traffic that should be load balanced between h2 and h3.

  2. You should now see a Mininet command prompt. Open three terminals for h1, h2 and h3, respectively:

    mininet> xterm h1 h2 h3

  3. Each host includes a small Python-based messaging client and server. In h2 and h3's XTerms, start the servers:

    ./receive.py

  4. In h1's XTerm, send a message from the client:

    ./send.py 10.0.0.1 "P4 is cool"

    The message will not be received.

  5. Type exit to leave each XTerm and the Mininet command line.

The message was not received because each switch is programmed with load_balance.p4, which drops all packets on arrival. Your job is to extend this file.

A note about the control plane

P4 programs define a packet-processing pipeline, but the rules governing packet processing are inserted into the pipeline by the control plane. When a rule matches a packet, its action is invoked with parameters supplied by the control plane as part of the rule.

In this exercise, the control plane logic has already been implemented. As part of bringing up the Mininet instance, the make script will install packet-processing rules in the tables of each switch. These are defined in the sX-runtime.json files.

Step 2: Implement Load Balancing

The load_balance.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.

A complete load_balance.p4 will contain the following components:

  1. Header type definitions for Ethernet (ethernet_t) and IPv4 (ipv4_t).
  2. Parsers for Ethernet and IPv4 that populate ethernet_t and ipv4_t fields.
  3. An action to drop a packet, using mark_to_drop().
  4. TODO: An action (called set_ecmp_select), which will:
    1. Hashes the 5-tuple specified above using the hash extern
    2. Stores the result in the meta.ecmp_select field
  5. TODO: A control that:
    1. Applies the ecmp_group table.
    2. Applies the ecmp_nhop table.
  6. A deparser that selects the order in which fields inserted into the outgoing packet.
  7. A package instantiation supplied with the parser, control, and deparser.

    In general, a package also requires instances of checksum verification and recomputation controls. These are not necessary for this tutorial and are replaced with instantiations of empty controls.

Step 3: Run your solution

Follow the instructions from Step 1. This time, your message from h1 should be delivered to h2 or h3. If you send several messages, some should be received by each server.

Troubleshooting

There are several ways that problems might manifest:

  1. load_balance.p4 fails to compile. In this case, make will report the error emitted from the compiler and stop.

  2. load_balance.p4 compiles but does not support the control plane rules in the sX-runtime.json files that make tries to install using the Python controller. In this case, make will log the controller output in the logs directory. Use the error messages to fix your load_balance.p4 implementation.

  3. load_balance.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.

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:

mn -c

Next Steps

Congratulations, your implementation works! Move on to Multi-Hop Route Inspection.