p4tutorials-turkce/workshop_05_2016/ecmp/README.md

Implementing ECMP on top of simple_router.p4

Introduction

simple_router.p4 is a very simple P4 program which does L3 routing. All the P4 code can be found in the [p4src/simple_router.p4] (p4src/simple_router.p4) file. In this exercise we will try to build ECMP on top of the starter code. We will be assuming the following network topology:

             --------------------------------- nhop-0 10.0.1.1
             |                                        00:04:00:00:00:00
          1 - 00:aa:bb:00:00:00
             |
-------- 3--sw
             |
          2 - 00:aa:bb:00:00:01
             |
             --------------------------------- nhop-1 10.0.2.1
                                                      00:04:00:00:00:01

Note that we do not assign IPv4 addresses to the 3 switch interfaces, we do not need to for this exercise. We will be sending test packets on interface 3 of the switch. These packets will have destination IP 10.0.0.1. We will assume that both nhop-0 and nhop-1 have a path to 10.0.0.1, which is the final destination of our test packets.

Running the starter code

Before starting make sure that you run sudo ./veth_setup.sh to create the veth pairs required for the demo.

To compile and run the starter code, simply use ./run_demo.sh. The [run_demo.sh] (run_demo.sh) script will run the P4 compiler (for bmv2), start the switch and populate the tables using the CLI commands from [commands.txt] (commands.txt).

When the switch is running, you can send test packets with sudo ./run_test.py. Note that this script will take a few seconds to complete. The test sends a few hundred identical TCP packets through the switch, in bursts, on port 3. If you take a look at the P4 code and at commands.txt, you will see that each TCP packet is forwarded out of port 1; since we do not have ECMP working yet.

What you need to do

1. In this exercise, you need to update the provided [P4 program] (p4src/simple_router.p4) to perform ECMP. When you are done, each incoming TCP test packet should be forwarded to either port 1 or port 2, based on the result of a crc16 hash computation performed on the TCP 5-tuple (ipv4.srcAddr, ipv4.dstAddr, ipv4.protocol, tcp.srcPort, tcp.dstPort). You will need to refer to the [P4 spec] (http://p4.org/wp-content/uploads/2015/04/p4-latest.pdf) to familiarize yourself with the P4 constructs you will need.

2. Once you are done with the P4 code, you will need to update [commands.txt] (commands.txt) to configure your new tables.

3. After that you can run the above test again. Once again, you will observe that all packets go to the same egress port. Don't panic :)! This is because all packets are identical and therefore are forwarded in the same way, If you add --random-dport when running sudo ./run_test.py, you should observe an even distribution for the ports. This option assigns a random destination port to each test TCP packet (the 5-tuple is different, so the hash is likely to be different).

Hints and directions

1. You can easily check the syntax of your P4 program with p4-validate <path to simple_router.p4>.

2. There are 2 major ways of implementing ECMP on top of simple_router.p4. The first one requires 2 tables and the use of the modify_field_with_hash_based_offset primitive. The second one uses a single table with an action profile. You can read about modify_field_with_hash_based_offset and action profiles in the [P4 spec] (http://p4.org/wp-content/uploads/2015/04/p4-latest.pdf).

3. If you choose to use the first way (with 2 tables), your first table will match on the destination IP address and be in charge of computing an index (using modify_field_with_hash_based_offset), while the second table will match on this computed index to obtain the outgoing interface. This is a high level view of what needs to be implemented in P4.

T1
IP_prefix_1 ---> "random" index in [0, 1] using modify_field_with_hash_based_offset
IP_prefix_2 ---> "random" index in [2, 4] ...
...

T2
index(0)    ---> nhop A
index(1)    ---> nhop B
index(2)    ---> nhop C
index(3)    ---> nhop D
index(4)    ---> nhop E

Remember that T1 and T2's entries will come from your modified commands.txt.