Chapter 8: NetKAT Repeater

So far, we’ve seen how to implement OpenFlow controllers using the Ox platform. Most of the controllers we built followed a two-step recipe:

• Write a packet_in handler that implements the desired packet-processing functionality.

• Use flow_mod messages to configure the switch flow tables to implement the same functionality efficiently.

In the next few chapters, we will explore a completely different approach: express policies using a high-level, domain-specific programming language, and let a compiler and run-time system handle the details related to configuring switch flow tables (as well as sending requests for statistics, accumulating replies, etc.)

You will place these files in netkat-tutorial-solutions.

$ cd netkat-tutorial-solutions

Example 1: A Repeater (Redux)

Solution

In the OxRepeater chapter, we wrote an efficient repeater that installs forwarding rules in the switch flow table. Recall that a repeater simply outputs packets out all ports, except the one that the packet came in on. Suppose that the topology consists of a single switch with four ports, numbered 1 through 4:

The following program implements a repeater in NetKAT:

open Frenetic_NetKAT
open Core.Std
open Async.Std

(* a simple repeater *)
let%nk repeater =
  {| if port = 1 then port := 2 + port := 3 + port := 4
     else if port = 2 then port := 1 + port := 3 + port := 4
     else if port = 3 then port := 1 + port := 2 + port := 4
     else if port = 4 then port := 1 + port := 2 + port := 3
     else drop
  |}

let _ =
  let module Controller = Frenetic_NetKAT_Controller.Make (Frenetic_OpenFlow0x01_Plugin) in
  Controller.start 6633;
  Deferred.don't_wait_for (Controller.update repeater);
  never_returns (Scheduler.go ());

The main part of this code uses a ppx extension of OCaml syntax, let%nk repeater = {| ... |} to switch into NetKAT syntax. The embedded NetKAT program uses a cascade of nested conditionals (if ... then ... else ...) to match packets on each port (port = 1) and forward them out on all other ports (port := 2 + port := 3 + port := 4) except the one the packet came in on. The last line starts a controller that configures the switch with a static NetKAT policy.

Run the Example

To run the repeater, type the code above into a file Repeater1.ml within the netkat-tutorial-solutions directory. Then compile and start the repeater controller using the following commands.

$ ./netkat-build Repeater.d.byte
$ ./Repeater.d.byte

Next, in a separate terminal, start up mininet.

$ sudo mn --controller=remote --topo=single,4 --mac --arp

Test the Example

At the mininet prompt, test your repeater program by pinging h2 from h1:

mininet> h1 ping -c 1 h2

You should see output similar to the following:

PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
64 bytes from 10.0.0.2: icmp_req=1 ttl=64 time=0.216 ms

--- 10.0.0.2 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.216/0.216/0.216/0.000 ms

Try pinging h1 from h2 as well.

Example 2: Referring to OCaml variables

Solution

In many programs it is useful to refer to an OCaml variable. We can do this by writing $x, where x is the name of an OCaml variable. As an example, here is an equivalent, but more concise version of the repeater that uses list-processing functions to build up the NetKAT policy. This program would be easier to maintain, for example, if we wanted to implement repeaters on switches with different numbers of ports.

open Frenetic_NetKAT
open Core.Std
open Async.Std

(* a simple repeater *)
let all_ports : int32 list = [1l; 2l; 3l; 4l]
let%nk drop = {| drop |}

let flood (n : int32) : policy =
  List.fold_left
    all_ports
    ~f: (fun pol m ->
      let%nk flood = {| $pol + port:= $m |} in
      if n = m then pol else flood)
    ~init: drop

let repeater : policy =
  List.fold_right
    all_ports 
    ~f: (fun m pol ->
      let p = flood m in
      let%nk repeat = {| if port = $m then $p else $pol |} in
      repeat)
    ~init: drop

let _ =
  let module Controller = Frenetic_NetKAT_Controller.Make (Frenetic_OpenFlow0x01_Plugin) in
  Controller.start 6633;
  Deferred.don't_wait_for (Controller.update repeater);
  never_returns (Scheduler.go ());

NetKAT Reference Manual

The complete NetKAT language is described here.