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mininet> dpctl dump-flows -O OpenFlow13
*** s1 ----------------------------------------------------------------
OFPST_FLOW reply (OF1.3) (xid=0x2):
cookie=0x0, duration=106.605s, table=0, n_packets=6, n_bytes=420, priority=0
actions=CONTROLLER:65509
mininet>

So this is the flow in an SDN switch (s1) after initially starting the controller and switch, but what is the prupose of this flow exactly? I don't quite understand it

2

It is the default flow that sends all packets to the controller. The controller can then install new flows, or process the packets via PACKET_OUT messages.

I believe that if you use the -m or --more argument it will show you that it is actually a flow that matches all fields wildcarded. This is omitted by default.

0

If you are familiar with the concept of forwarding table in conventional networking. 'Flows' has a similar role in software defined networks.

The switch under use appears to be OVS. OVS is a software switch with SDN capabilities. Thus you can instruct the switch what action to perform in a given situation. For instance:

Check destination mac address ; if mac address = aa:bb:cc:dd:ee:ff ; output interface is 1

This is an example of adding a flow to the switch.

In your show command the n_packets and n_bytes denote how many packets/bytes are effected by this flow.

you can think of flow as a rule based on which the OVS will forward packets.

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