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Traceroutable Network

Introduction

In this exercise, we will extend our P4 router with an additional feature: responding to traceroute packets, i.e. packets where the IPv4 TTL (time to live) value is equal to 1. If a router receives such a packet, it generates an ICMP Time Exceeded message and sends it back to the original sender of the expired packet. Thus, our objective is to make the traceroute or similar tools work within our network.

In brief, traceroute works as follows: it sends several packets starting with TTL=1 and it increases the TTL by 1 for each consequent packet. As you already know, each internet router decreases the IP TTL by 1 and sends back ICMP messages when it reaches 0. By sending packets with an increasing TTL, traceroute is able to trace the path packets take from the source to a given destination.

For a detailed description of how traceroute works, have a look at the Wikipedia article. For more information about ICMP (i.e. the protocol that is used for the replies to expired traceroute packets), look at this Wikipedia article or its RFC.

Before starting

Before you start this exercise, update p4-utils, some new features and bugs were fixed during the last week:

cd ~/p4-tools/p4-utils
git pull

Further, you need the traceroute tool for this exercise (it might be installed already). Install it by running this command in a terminal:

sudo apt-get install traceroute

What is already provided

For this exercise we provide you with the following files:

  • p4app.json: describes the topology we want to create with the help of mininet and p4-utils package.
  • network.py: a Python scripts that initializes the topology using Mininet and P4-Utils. One can use indifferently network.py or p4app.json to start the network.
  • p4src/ecmp.p4: we will use the solution of the 03-ECMP exercise as starting point.
  • send.py: a small python script to generate multiple packets with different tcp port.
  • routing-controller.py: routing controller skeleton. The controller uses global topology information and the simple switch thrift_API to populate the routing tables.
  • topology_generator.py: python script that automatically generates p4app configuration files. It allows you to generate 3 types of topologies: linear, circular, and random (with a node number and degree). Run it with -h option to see the command line parameters.
  • network_generator.py: Python script that automatically generates network.py scripts. It allows you to generate 3 types of topologies: linear, circular, and random (with a node number and degree). Run it with -h option to see the command line parameters.
  • traceroute.py: python script that implements a simple version of traceroute using tcp probes. It can be used to script traceroute tests.

Notes about p4app.json

For this exercise we will use the l3 assignment strategy. Unlike the mixed strategy where hosts connected to the same switch formed a subnetwork and each switch formed a different domain. In the l3 assignment, we consider switches to only work at the layer 3, meaning that each interface must belong to a different subnetwork. If you use the namings hY and sX (e.g h1, h2, s1, s2...), the IP assignment will go as follows:

  1. Host IPs: 10.x.y.2, where x is the id of the gateway switch, and y is the host id.
  2. Switch ports directly connected to a host: 10.x.y.1, where x is the id of the gateway switch, and y is the host id.
  3. Switch to Switch interfaces: 20.sw1.sw2.<1,2>. Where sw1 is the id of the first switch (following the order in the p4app link definition), sw2 is the id of the second switch. The last byte is 1 for sw1's interface and 2 for sw2's interface.

Note that it is the second time we assign IP addresses to a switch. However, it is very important to note that actually p4-utils will not assign those IPs to the switches, but it will save them so they can be virtually used for some switch functionality (we will see what this means later).

You can find all the documentation about p4app.json in the p4-utils documentation.

Understanding traceroute

Before starting with the implementation, we will have a look at the packets that traceroute sends and receives. To do this, open a terminal and type sudo wireshark & to open Wireshark. Then, select the interface eth0 and start capturing traffic. You will see a lot of packets being sent and received, to remove them you can apply the following filter at the filter bar: ip.ttl < 10 || icmp. Now run the following command in the terminal:

sudo traceroute -n -q 1 -f 1 -m 1 -T ethz.ch

This will send a single traceroute (TCP) packet with TTL=1 towards ethz.ch. In Wireshark, you should see a TCP packet as well as the corresponding ICMP reply. Take a close look at these to packets to understand how a router builds these ICMP replies. You can use the ICMP replied by ethz.ch as a reference. For example, you can see how the IP header changes, what the ICMP header and body contain, etc.

Implementing the traceroutable switch

Take the solution from the previous exercise (both p4 code and routing-controller.py). To implement our traceroutable switch we will have to make two additions: first we will have to add a new table to the p4 program that maps egress_ports to the IP address of the switch for that port. You will also have to extend the controller with a small function that populates this table for each switch. Second we will have to extend the p4 program so the switch replies to packets with TTL=1 with ICMP packets to the original sender.

Adding the output port to IP table

Before we implement the generation of ICMP replies to expired packets in P4, we will have to add a new table to the code we already have from the previous exercise. This table will be used during the ICMP reply process to set the ipv4 source address for the packets that the switch generates. This IP address will then be used by the traceroute tool to identify which router (and interface) is the packet coming from.

You have to do tho things:

  1. Define a table called icmp_ingress_port that matches to the packet's ingress_port (the ingress port will be used as egress port for the ICMP replies). This table should call an action (set_src_icmp_ip) that takes an IP address as a parameter and sets it to the hdr.ipv4_icmp source IP address. You will in the next section what this ipv4_icmp header is.

  2. Implement the set_icmp_ingress_port_table function in the routing-controller.py. In particular, we want to assign each interface of the switch a separate IP address. For this you will have to fill the icmp_ingress_port table with the mapping from ingress port to the IP address associated to that respective port. To get information about the port to IP you will have to use the self.topo object. For more information see the documentation. Hint: there is a function to get all the interfaces a node has, with that you should be able to get their IPs and port number.

Replying with ICMP packets

To implement the ICMP packet replying system, you should:

  1. Add the ICMP header to your headers.p4 file.

  2. Extend the headers struct with the icmp header and a second ipv4 header (you can call it ipv4_icmp). As you should have seen during your traceroute tests, the ICMP packet should contain its own ipv4 header and the ipv4 header of the expired packet. As you see from the headers and the parser (from the ECMP solution), we only consider TCP as the transport layer protocol (not UDP). Therefore, you can focus on TCP (and IP) packets in your implementation.

  3. Extend the deparser control block with the two new headers. Make sure you emit the things in the right order. For that check again how the ICMP packet looks like.

  4. We now start extending the implementation of the ingress pipeline. Here, you can first check whether the received packet's TTL value is >1. If so, you forward the packet as usual (using ECMP).

  5. If the TTL is equal to 1 (and the packet is ipv4 and tcp valid), the packet expires at this router and you need to send an answer back to the sender. (make sure the packet does not go through the normal forwarding tables). To transform the received TCP packet into an ICMP reply, you need to do the following:

    • set the additional headers (ipv4_icmp, icmp) to valid (by calling hdr.X.setValid())
    • set the egress port such that the packet is sent through the port where it arrived
    • swap the source and destination MAC addresses
    • set the source IP address to the address that belongs to the interface where the packet arrived (using the table icmp_ingress_port from above)
    • set the correct values for the ipv4_icmp and the icmp header (you may use the packets that you captured in Wireshark before as a template)
    • make sure you set the ipv4_icmp.totalLen to the right value otherwise wireshark (or other tools) will not be able to compute the checksum properly.
    • truncate the packet to 70 bytes (using truncate((bit<32>)70);) to remove remaining parts from the original packets.

  6. Since both the IP headers and the ICMP header contain checksums, you need to compute them in the MyComputeChecksum control. You can use the IP checksum from previous exercises as a template for the ipv4_icmp checksum. To compute the checksum of the icmp header you have to hash all the fields from the ICMP header together with the payload (original IP header + the first 8 bytes from the tcp header). For this checksum use the same algorithm you used before (HashAlgorithm.csum16).

Testing your solution

Once you completed your implementation, you can test the program using the traceroute.py script or the real traceroute tool. If you use the traceroute tool, bear in mind that we only implemented replies to TCP packets while the tool sends UDP packets by default. Add the -T parameter to use TCP packets.

  1. Start the topology (this will also compile and load the program).

    sudo p4run

    or

    sudo python network.py

  2. Run the controller.

    python routing-controller.py

  3. Check that you can ping:

    mininet> pingall

  4. Traceroute between two hosts:

    You can either use our own implementation (traceroute.py) in a host or the default traceroute Mininet tool:

    for sport in range(6000,6020):
    print(traceroute(dst="10.6.2.2",sport=sport, dport=80))
    mininet> h1 traceroute -n -w 0.5 -q 1 -T --sport=<src_port> --port=<dst_port> 10.6.2.2

    (to make it faster, we disable DNS lookups (-n), decrease the waiting time -w, send only one packet per TTL (-q) and a fixed source and destination port (--sport,--dport) to get the actual path of a flow despite ECMP)

Note that the second router (s2, s3, s4 or s5) will always indicate which path the flow with a given 5-tuple would take in our simple topology.

Testing with another topology

As in the previous exercise, you can test that your solution does work with other topologies with the topology_generator.py script we provided you and which generates random topologies:

python topology_generator --output_name <name.json> --topo random --n <number of switches to use> -d <average switch diamiter>

This will create a random topology with n switches that have on average d interfaces (depending on n, d might not be possible). Also each switch will have one host directly connected to it (so n hosts).

Run the random topology:

sudo p4run --config <name.json>

If you want to use Python scripts instead of JSON files, you may want to run:

python network_generator.py --output_name <name.py> --topo random --n <number of switches to use> -d <average switch diamiter>

and

sudo python <name.py>

Do the rest of the steps we did in the previous section. By running multiple traceroutes between two hosts you will be able to discover which path (or paths) are being used!

Some notes on debugging and troubleshooting

If you don't receive replies to your traceroute queries, you can use Wireshark to look at the packets (i.e. capture the host's interface). Wireshark will tell you if the packet cannot be parsed (e.g. because the structure is not correct) or if the checksums are incorrect.

We have added a small guideline in the documentation section. Use it as a reference when things do not work as expected.