basic_tunnel

Implementing Basic Tunneling

Introduction

In this exercise, we will add support for a basic tunneling protocol to the IP router that you completed in the previous assignment. To do so, we will define a new header type to encapsulate the IP packet and modify the switch to perform routing using our new header.

The new header type will contain a protocol ID, which indicates the type of packet being encapsulated, along with a destination ID to be used for routing.

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 starter code for this assignment is in a file called basic_tunnel.p4 and is simply the solution to the IP router from the previous exercise.

Let's first compile this code to and send a packet between two end hosts to ensure that the IP routing is working as expected.

1. In your shell, run:

   make run

   This will:

   • compile basic_tunnel.p4, and
   • start a Mininet instance with three switches (s1, s2, s3) configured in a triangle, each connected to one host (h1, h2, and h3).
   • The hosts are assigned IPs of 10.0.1.1, 10.0.2.2, and 10.0.3.3.

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 to h2:

   ./send.py 10.0.2.2 "P4 is cool"

   The packet should be received at h2. If you examine the received packet you should see that is consists of an Ethernet header, an IP header, a TCP header, and the message. If you change the destination IP address (e.g. try to send to 10.0.3.3) then the message should not be received by h2.

5. Type exit or Ctrl-D to leave each xterm and the Mininet command line.

Each switch is forwarding based on the destination IP address. Your job is to change the switch functionality so that they instead decide the destination port using our new tunnel header.

A note about the control plane

A P4 program defines a packet-processing pipeline, but the rules within each table are inserted 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.

For this exercise, we have already added the necessary static control plane entries. As part of bringing up the Mininet instance, the make run command will install packet-processing rules in the tables of each switch. These are defined in the sX-runtime.json files, where X corresponds to the switch number.

Important: We use P4Runtime to install the control plane rules. The content of files sX-runtime.json refer to specific names of tables, keys, and actions, as defined in the P4Info file produced by the compiler (look for the file build/basic.p4info after executing make run). Any changes in the P4 program that add or rename tables, keys, or actions will need to be reflected in these sX-runtime.json files.

Step 2: Implement Basic Tunneling

The basic_tunnel.p4 file contains an implementation of a basic IP router. It also contains comments marked with TODO which indicate the functionality that you need to implement. A complete implementation of the basic_tunnel.p4 switch will be able to forward based on the contents of a custom encapsulation header as well as perform normal IP forwarding if the encapsulation header does not exist in the packet.

Your job will be to do the following:

1. NOTE: A new header type has been added called myTunnel_t that contains two 16-bit fields: proto_id and dst_id.
2. NOTE: The myTunnel_t header has been added to the headers struct.
3. TODO: Update the parser to extract either the myTunnel header or ipv4 header based on the etherType field in the Ethernet header. The etherType corresponding to the myTunnel header is 0x1212. The parser should also extract the ipv4 header after the myTunnel header if proto_id == TYPE_IPV4 (i.e. 0x0800).
4. TODO: Define a new action called myTunnel_forward that simply sets the egress port (i.e. egress_spec field of the standard_metadata bus) to the port number provided by the control plane.
5. TODO: Define a new table called myTunnel_exact that perfoms an exact match on the dst_id field of the myTunnel header. This table should invoke either the myTunnel_forward action if the there is a match in the table and it should invoke the drop action otherwise.
6. TODO: Update the apply statement in the MyIngress control block to apply your newly defined myTunnel_exact table if the myTunnel header is valid. Otherwise, invoke the ipv4_lpm table if the ipv4 header is valid.
7. TODO: Update the deparser to emit the ethernet, then myTunnel, then ipv4 headers. Remember that the deparser will only emit a header if it is valid. A header's implicit validity bit is set by the parser upon extraction. So there is no need to check header validity here.
8. TODO: Add static rules for your newly defined table so that the switches will forward correctly for each possible value of dst_id. See the diagram below for the topology's port configuration as well as how we will assign IDs to hosts. For this step you will need to add your forwarding rules to the sX-runtime.json files.

Step 3: Run your solution

Follow the instructions from Step 1. This time when you send a packet from h1 to h2 try using the following command to send a packet that uses our new myTunnel header.

./send.py 10.0.2.2 "P4 is cool" --dst_id 2

You should see a packet arrive at h2 which contains the MyTunnel header. Also note that changing the destination IP address will not prevent the packet from arriving at h2. This is because the switch is no longer using the IP header for routing when the MyTunnel header is in the packet.

Python Scapy does not natively support the myTunnel header type so we have provided a file called myTunnel_header.py which adds support to Scapy for our new custom header. Feel free to inspect this file if you are interested in learning how to do this.

Food for thought

To make this tunneling exercise a bit more interesting (and realistic) how might you change the P4 code to have the switches add the myTunnel header to an IP packet upon ingress to the network and then remove the myTunnel header as the packet leaves to the network to an end host?

Hints:

• The ingress switch will need to map the destination IP address to the corresponding dst_id for the myTunnel header. Also, remember to set the validity bit for the myTunnel header so that it can be emitted by the deparser.
• The egress switch will need to remove the myTunnel header from the packet after looking up the appropriate output port using the dst_id field.

Troubleshooting

There are several problems that might manifest as you develop your program:

1. basic_tunnel.p4 might fail to compile. In this case, make run will report the error emitted from the compiler and halt.

2. basic_tunnel.p4 might compile but fail to support the control plane rules in the sX-runtime.json files that make run tries to install using the P4Runtime. In this case, make run will report errors if control plane rules cannot be installed. Use these error messages to fix your basic_tunnel.p4 implementation or forwarding rules.

3. basic_tunnel.p4 might compile, and the control plane rules might be installed, but the switch might not process packets in the desired way. The /tmp/p4s.<switch-name>.log files contain detailed logs that 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:

make stop

Next Steps

Congratulations, your implementation works! Move onto the next assignment p4runtime!