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gen_switch.md

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The gen_switch behaviour

The gen_switch is a behaviour that defines set of callbacks that one has to provide in order to implement the switch. Currently there are two implementations of gen_switch behaviour:

  • linc_us3 which is a purely userspace, Erlang implementation of an OFP 1.2 switch
  • linc_us4 which is a purely userspace, Erlang implementation of an OFP 1.3 switch

One can think of various other backends to implement, like:

  • hardware based switch,
  • userspace implementation in other programming language (like C),
  • interfacing with openvswitch.ko Linux kernel module.

Callbacks

gen_switch consists of the following callbacks:

  • start/2
  • stop/1
  • handle_message/2

Specs for the callbacks are in the gen_switch.erl file.

The start function takes a single argument of backend-specific type and returns {ok, State} tuple. The stop function takes State as an argument, it's return value is ignored.

The handle_message callback is called when a packet is received. It takes two arguments – state and packet and, on success, return either {ok, NewState} or {ok, Reply, NewState} depending on if the packet is one that requires response or not. In case of failure the callback can either return {error, OfpError, NewState} tuple, or throw #ofp_error{} record. In the latter case the state stays unchanged. In both cases the error is delivered to the sender.