source_routing

Implementing Source Routing

Introduction

The objective of this exercise is to implement source routing. With source routing, the source host guides each switch in the network to send the packet to a specific port. The host puts a stack of output ports in the packet. In this example, we just put the stack after Ethernet header and select a special etherType to indicate that. Each switch pops an item from the stack and forwards the packet according to the specified port number.

Your switch must parse the source routing stack. Each item has a bos (bottom of stack) bit and a port number. The bos bit is 1 only for the last entry of stack. Then at ingress, it should pop an entry from the stack and set the egress port accordingly. The last hop may also revert back the etherType to TYPE_IPV4.

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

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

1. In your shell, run:

   make

   This will:

   • compile source_routing.p4, and
   • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle, each connected to one host (h1, h2, h3). Check the network topology using the net command in mininet. You can also change the topology in topology.json
   • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, etc (10.0.<Switchid>.<hostID>).

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

   mininet> xterm h1 h2

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

   ./receive.py

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

   ./send.py 10.0.2.2

5. Type a list of port numbers. say 2 3 2 2 1. This should send the packet through h1, s1, s2, s3, s1, s2, and h2. However, h2 will not receive the message.

6. Type q to exit send.py and type exit to leave each xterm and the Mininet command line.

The message was not received because each switch is programmed with source_routing.p4, which drops all packets on arrival. You can verify this by looking at logs/s1.log. Your job is to extend the P4 code so packets are delivered to their destination.

Step 2: Implement source routing

The source_routing.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 source_routing.p4 will contain the following components:

1. Header type definitions for Ethernet (ethernet_t) and IPv4 (ipv4_t) and Source Route (srcRoute_t).
2. TODO: Parsers for Ethernet and Source Route that populate ethernet and srcRoutes fields.
3. An action to drop a packet, using mark_to_drop().
4. TODO: An action (called srcRoute_nhop), which will:
   1. Set the egress port for the next hop.
   2. remove the first entry of srcRoutes
5. A control with an apply block that:
   1. checks the existence of source routes.
   2. TODO: if statement to change etherent.etherType if it is the last hop
   3. TODO: call srcRoute_nhop action
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.

Check the ttl of the IP header. Each hop decrements ttl. The port sequence 2 3 2 2 1, forces the packet to have a loop, so the ttl should be 59 at h2. Can you find the port sequence for the shortest path?

Food for thought

• Can we change the program to handle both IPv4 forwarding and source routing at the same time?
• How would you enhance your program to let the first switch add the path, so that source routing would be transparent to end-hosts?

Troubleshooting

There are several ways that problems might manifest:

1. source_routing.p4 fails to compile. In this case, make will report the error emitted from the compiler and stop.
2. source_routing.p4 compiles but switches or mininet do not start. Do you have another instance of mininet running? Did the previous run of mininet crash? if yes, check "Cleaning up Mininet" bellow.
3. source_routing.p4 compiles but the switch does not process packets in the desired way. The logs/sX.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 <switch-name>-<interface-name>_<direction>.pcap files contain pcap captures of all packets sent and received on each interface. Use tcpdump -r <filename> -xxx to print the hexdump of the packets.

Cleaning up Mininet

In the 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 Load Balance.

Relevant Documentation

The documentation for P4_16 and P4Runtime is available here

All excercises in this repository use the v1model architecture, the documentation for which is available at:

1. The BMv2 Simple Switch target document accessible here talks mainly about the v1model architecture.
2. The include file v1model.p4 has extensive comments and can be accessed here.