# The First Application

## Whetting Your Appetite

If you want to manage the network gears (switches, routers, etc) at your way, you need to write your Ryu application. Your application tells Ryu how you want to manage the gears. Then Ryu configures the gears by using OpenFlow protocol, etc.

Writing Ryu application is easy. It’s just Python scripts.

## Start Writing

We show a Ryu application that make OpenFlow switches work as a dumb layer 2 switch.

Open a text editor creating a new file with the following content:

    from ryu.base import app_manager

    class L2Switch(app_manager.RyuApp):
        def __init__(self, *args, **kwargs):
            super(L2Switch, self).__init__(*args, **kwargs)

Ryu application is just a Python script so you can save the file with any name, extensions, and any place you want. Let’s name the file ‘l2.py’ at your home directory.

This application does nothing useful yet, however it’s a complete Ryu application. In fact, you can run this Ryu application:

    % ryu-manager ~/l2.py
    loading app /Users/fujita/l2.py
    instantiating app /Users/fujita/l2.py

All you have to do is defining needs a new subclass of RyuApp to run your Python script as a Ryu application.

Next let’s add the functionality of sending a received packet to all the ports.

    from ryu.base import app_manager
    from ryu.controller import ofp_event
    from ryu.controller.handler import MAIN_DISPATCHER
    from ryu.controller.handler import set_ev_cls

    class L2Switch(app_manager.RyuApp):
        def __init__(self, *args, **kwargs):
            super(L2Switch, self).__init__(*args, **kwargs)

        @set_ev_cls(ofp_event.EventOFPPacketIn, MAIN_DISPATCHER)
        def packet_in_handler(self, ev):
            msg = ev.msg
            dp = msg.datapath
            ofp = dp.ofproto
            ofp_parser = dp.ofproto_parser

            actions = [ofp_parser.OFPActionOutput(ofp.OFPP_FLOOD)]
            out = ofp_parser.OFPPacketOut(
                datapath=dp, buffer_id=msg.buffer_id, in_port=msg.in_port,
                actions=actions)
            dp.send_msg(out)

A new method ‘packet_in_handler’ is added to L2Switch class. This is called when Ryu receives an OpenFlow packet_in message. The trick is ‘set_ev_cls’ decorator. This decorator tells Ryu when the decorated function should be called.

The first argument of the decorator indicates an event that makes function called. As you expect easily, every time Ryu gets a packet_in message, this function is called.

The second argument indicates the state of the switch. Probably, you want to ignore packet_in messages before the negotiation between Ryu and the switch finishes. Using ‘MAIN_DISPATCHER’ as the second argument means this function is called only after the negotiation completes.

Next let’s look at the first half of the ‘packet_in_handler’ function.

- ev.msg is an object that represents a packet_in data structure.
- msg.dp is an object that represents a datapath (switch).
- dp.ofproto and dp.ofproto_parser are objects that represent the OpenFlow protocol that Ryu and the switch negotiated.

Ready for the second half.

- OFPActionOutput class is used with a packet_out message to specify a switch port that you want to send the packet out of. This application need a switch to send out of all the ports so OFPP_FLOOD constant is used.
- OFPPacketOut class is used to build a packet_out message.
- If you call Datapath class’s send_msg method with a OpenFlow message class object, Ryu builds and send the on-wire data format to the switch.

Here, you finished implementing your first Ryu application. You are ready to run this Ryu application that does something useful.

A dumb l2 switch is too dumb? You want to implement a learning l2 switch? Move to the next step. You can learn from the existing Ryu applications at ryu/app directory and integrated tests directory.