Designing my OS

When I design and code my operating system, I usually document what I do in a couple markdown files. I thought maybe some people would find it usefull to see them so I am posting one here. This one is about the networking architecture of my OS. Note that this is really the product of my imagination so it might not be (or should I say it definitely isn't) the best way of doing things. It's just the way I imagined it and it actually works.

Of course, I can't pretend this is only the product of my imagination. I did get a lot of inspiration from many sources online. But I tend to stay away (as much as I can) from the linux source code. Since I think Linux is such a great implementation, it would be too easy to assimilate a lot of its concepts while looking at it. Because once you look at it, you realize that what you're looking at is probably the best way to do things. Take sockets for example. Once you are used to using sockets with open/send/recv/close, how else would you do it? So I ended up doing that because it's what I know. But I have no idea how linux works behind the socket implementation, so I tried to design it the way I think it works. I'm not looking at conceiving the best thing, I'm looking at picking my brain and create something that works the way I imagined it. Maybe like art instead of science.

There are flaws and still open questions but it does work. I don't have any code to post here because I am thinking on either posting the whole code on my website or on github. I have to sort that out.

Network card drivers

The netcard abstration layer is contained in netcard.c.

When the OS boots, net_init() is called. This function iterates through the PCI bus's devices to find all devices that match one of the OS's netcard drivers. For each netcard found, a struct NetworkCard is instanciated. The structure contains several function pointers that are initialized with the matching driver's functions. For example, NetworkCard::receive would be set to &rtl8139_receive if the netcard was a rtl8139. Each of these NetworkCard instances will be refered to as the "netcard implementation"

Only the rtl8139 driver is implemented right now. You can view the source code I have posted a while ago in Realtek 8139 network card driver

When a netcard IRQ occurs, a softIRQ is scheduled. (I know, softIRQ is a linux concept. But I actually thought about it before knowing it existed in linux. I didn't have a name for it so I found that Linux had a great name so I stole it. But it doesn't really work like linux's softIRQ. Linux has a way better way of doing it than my OS). When the softIRQ handler is invoked, it calls net_process(). net_process() iterates through all netcard that was discovered during boot. It then checks if data is available and forwards the the data up to the TCP/IP stack if the data is an IP packet, or to the ICMP stack if the packet is of the ICMP type.

TCP/IP stack

Receiving data

The ICMP handler responds immediately to ping requests. Therefore the ICMP response is sent from the softIRQ thread. This allows consistent RTT. The IP handler forwards the message to the TCP or UDP handlers. The UDP handler is not implemented yet. The TCP handler forwards segments to active sockets by finding the socket listening to the port and ip of the message. This is done by by finding the socket instance within a hash list. The message is added in the socket's queue. The user thread is then responsible for dequeuing those messages.

Sending data

The netcard abstraction's net_send() function locks the netcard implementation's send spinlock. This way, only one thread can send to one given netcard at the same time.

net_send() takes an interface index as destination parameter. ip_send takes an ip address as destination parameter. ip_send invokes ip_routing_route() to determine on what netcard to send the message based the destination address.

net_send() will send 1 frame of maximum MTU size. It returns 0 if sending failed or size of frame if sending suceeded. Frames are guaranteed to be sent in full or not at all but never partly.

ip_send() will send a packet of maximum 65536 bytes. It will do ip fragmentation in order to send frames of appropriate size.

tcp_send() will send 1 segment of maximum 64k. This means that the underlying ip_send() can end up calling net_send several times.

IP routing

IP routing is a whole subject itself. There are many algorithms and many ways to do this. It's actually the center of a very big industry. Basically, when a packet needs to be sent out, we know destination IP to which we want to send. The OS needs to know out of with netcard (if there are more than one) to send the packet and it also needs to know the source IP address to put in the packet. Normally, there would be one IP per netcard. Linux allows you to setup multiple IP for one physical netcard, so this would be done by creating virtual netcards. So the rule would still hold: 1 IP per netcard. My OS does not support virtual netcards, so only one IP per physical netcards.

This is all done with a routing table. By default, the routing table will consist of an entry for each IP configured IP for the host. Those route would be something like "anything within my own LAN must be forwarded out on my netcard". So if netcard0 has IP 192.168.1.88/24 and netcard1 has IP 192.168.2.25/24 then the routes would be:

• 192.168.1.0/24 -> netcard with IP 192.168.1.88 (netcard0)
• 192.168.2.0/24 -> netcard with IP 192.168.2.25 (netcard1)

A route can either say:

• if destination IP matches my subnet, then route out of interface X
• if destination IP matches my subnet, then route out to GW with IP address X

To match a rule, the rule's netmask is applied to the destination IP and the rule's IP. These masked IPs are subnets. If both subnets match, then the rule matches. A rule of 0.0.0.0/0 would be a default route and would guarantee to always match. It would be the last resort route.

Usually, routing out of an interface is done when we expect a device connected on that interface (through a switch probably) to be able to recognize that destination IP. When routing to a GW IP, it is because we expect a specific device to know where to find that destination IP. This would be a router. A default route would be configured like that. If an outgoing packet's destination IP address matches the default route, it means it couldn't match any of the other routes. So we want to send that packet to a gateway. We expect the gateway to be able to route the packet because our own routing table can't.

So the destination subnet is verified in order to find a route for that subnet. The subnet is the IP address logically AND'ed with the netmask.

Then a route for the default gateway must be added. This will tell: "Anything not matched in the LAN IPs must be sent to this IP". So something like: 0.0.0.0/24 -> 192.168.2.1. So if something needs to be sent to 192.168.5.6 then that route would be matched. The OS would then compare the subnet (using the netmask) of that gateway with the IPs configured on the host. It would find that this IP (of the gateway) is on the same LAN than 192.168.2.25 so netcard1 will be chosen as the outgoing netcard and the destination MAC address of the gateway will be used. So it is clear that using a gateway that reside on a subnet that doesn't match any of the host's IPs would be an invalid configuration.

Frame forwarding

At the layer 2, devices don't know about IP addresses. They only know about MAC addresses. Only devices connected together on the same layer 2 network (physically connected with switches) can talk to each other. When a packet is ready to be sent out, its MAC address might not be known yet. The OS will discover it by sending an ARP request: "who has IP 192.168.2.1?". Then, that device will answer "Me, and my MAC is xx:xx:xx:xx:xx:xx". So that destination MAC address will be used. When sending a packet to an external network, that is behind a router, such as the example of 192.168.5.6 earlier, the destination MAC of the gateway will be used. So if we send a packet to a device (by selecting its MAC as the destination) but the destination IP does not match that device's IP, it means we are expecting that device to be able to do layer 3 routing. Such a device would generally be a router. But it could also be any workstation on the LAN. Those workstations could be able to route. Linux does it.

Blocking operations

All socket operations such as connect, accept, send, receive and close are non-blocking. This implies that the lower-level layer operations are non-blocking also. There are some exceptions to this, but only because there are design flaws that need to be addressed.

Locking

At any time, only two threads can access the tcp/ip stack: The softIRQ thread, for processing incomming frames, and the owner of the socket. If multiple threads want to share socket usage, they will have to implement their own locking mechanism. The tcp/ip stack will only guarantee thread safety between the two threads mentioned above. Sockets should be used by one thread only in a user application. Two different sockets can be used by two different threads at the same time though.

net_send() locking

net_send() is used by the softIRQ context and user threads. Since the softIRQ context has high priority, it means that if a thead is preempted while it was holding the netcard's send spinlock, and then the softIRQ attempts to request the lock, a deadlock might occur. On uni-CPU systems, a deadlock would occur because the softIRQ will never give CPU time to other threads until it has completed its work. This could also happen on systems where the number of netcard is greater than or equal to the number of CPUs.

To solve this problem, the spinlock will disable interrupts.

• Spinlock will prevent another CPU from accessing the send function
• The thread would not be preempted on the local CPU so there is no chances that a softIRQ would deadlock (since softIRQ are prioritized over that thread, it could continue to run and never give time to the thread to release the lock).
• On a single-CPU system, the interrupt cleared and spinlock would be redundant but would not cause a problem

Socket locking

No locking is currently done at the socket level. The following is a list of problems that would arise, and the associated solution

Accepting incomming connections

A socket is created using create_socket(). Then the socket is placed in listening mode with listen(). listen() will set the source ip/port of the socket as per parameters of the function and destination ip/port to 0. A backlog buffer is created with size=(sizeof(socket*)*backlogCount).

When a segment is processed by tcp_process (in softirq context), a socket will try to be matched. If no socket is found then tcp_process() will try to find a listening socket that uses the same source ip/port (matching the received segment's destination fields) and with destination ip/port set to 0. if a socket is found, then we know that this segment is for an incomming connection.

The listening socket will only process SYN segments and will ignore any other segments. When processing a SYN segment, it will create a new socket with the same source ip/port and with destination ip/port matching the source of the incomming segment. The state will be set to CONNECTING. The new pending socket will be saved in the listening socket's backlog. The new socket will stay in the backlog until it gets picked up by accept(). accept() will then move the socket to the main list. The socket created in the backlog is only temporary. accept() will create a new socket instance based on that socket so that the new instance will reside in the accepting process's heap.

When the accept function is called, it will go through the backlog of the listening socket and will finish creating the socket. It will clone the socket and create the receive buffer and send the syn/ack. The socket will stay in "CONNECTING" state until it receives the ack of the syn/ack accept will move the socket from the backlog to the main list.

Other articles about my OS

Process Context ID and the TLB
Thread management in my hobby OS
Enabling Multi-Processors in my hobby OS
Block caching and writeback
Memory Paging
Stack frame and the red zone (x86_64)
AVX/SSE and context switching
Realtek 8139 network card driver

Introduction

In the past couple of months, I've taken interest in the popular ESP8266 wifi enable uC. I've also wanted for a long time to start designing PCBs, so I decided to mix both interests and build an irrigation controller. I am no expert in PCB design nor any electronics for that matter. I'm a software guy. So this whole PCB design is done in a very amateur way and some of the experts out there might find many flaws in my design. But the important thing is that it does work as expected and most of all: I had a lot of fun and learned a lot of things while doing this. That project may look like a failure to some, but it is definitely a victory for me since I ended up learning things. And next time, I'll learn even more until I get good at it. That's the process I went through to learn about software design. I have to fail once before taking on the theory.

Overview

An irrigation controller is a pretty simple device. It's just a couple of relays, a microcontroller and a power supply. For my project, I decided to use 4 relays (to control 4 valves). The uC I chose is a esp8266, more specifically, the ESP-03 board. This brings wifi capabilities to my project. So the esp8266 connects to my network and with a multicast message, it advertises its presence. My New Home Automation system then picks it up and connects to it. From that point, it can control all 4 valves.

Part list

• My custom built PCB
• esp-03
• 4 G5LE-1A4 relays
• 1 LM2596
• capacitors (see diagram for values)
• resistors (see diagram for values)
• diodes (see diagram for values)
• uf.l SMD connector
• 2.4ghz adhesive antenna
• 4 MPS2907A transistors (PNP)

Power supply

The ESP8266 needs a 3.3 VDC input and my valves need 24VAC. So I decided to supply my board with 24VAC, supply the valves (through the relays) directly and convert to 3.3vdc before the ESP8266 input. I had very limited experience in designing a power supply before this project. I still do. But I figured that using a diode and a capacitor, I could easily convert VAC to VDC. I then used a switched regulator to convert the high VDC down to 3.3v. I used the LM2596T regulator because I already had some pre-built power supplies that had that on it and found that they were working pretty well. The choice of capacitor to use was confusing. Looking online, I found many people using different values than what the datasheet suggests. So I experimented with a breadboard until I found a combination that worked (I know....). So bascially: 24VAC gets converted to something like 33VDC after going through 1 diode and a capacitor to smooth it out. Then Through a LM2596 to step down to 3.3V.

Antenna

I wasn't expecting to use an external antenna at first. But when trying my board for the first time, I saw that it couldn't connect to wifi. When I designed the PCB, I added a trace to a SMD uf.l connector. But at that time, I had no connector nor any antenna on hand. So I was wondering if that trace could be the problem. So I cut it. But it still wasn't working. So I finally bought a connector and an antenna, soldered a wire on the board because the trace had been cut. And then everything worked out great. So yeah, you need an external antenna. The small ceramic antenna on the esp-03 won't do the job. Possibly because I put it too close to the power supply on my board? Or because I have no ground plane? I have no idea, that's something I'm gonna have to investigate.

Transistors

The choice of PNP transistors was because the ESP8266 GPIO are set high on boot. So I wanted my relays to stay opened during that time. I keep all GPIOs high when valves must be shut off, and pull the GPIO down when a valve needs to be opened. I did a lot of searching about where to put the load (my relay) in the circuit. Do I put it after the collector of before the emitter? Apperently it's better to put it after the collector. I still don't understand why though. So I still have some reading to do. But it works.

Firmware

I've already talked about the basics of the ESP8266 in my Led strip controller using ESP8266. article. It is using my custom made protocole "DWN protocol" to speak with my home automation software. So after my home automation software is connected to the irrigation controller, it sends 2-bytes commands to activate a valve or to turn it off. The firmware takes care of a couple of things like:

• Hard limit of 30 minutes "on" time for every valve. In case you forget to shut it off. If longer period is needed, then the controlling software will need to take care of it.
• Only 1 valve at a time can be opened. If the controlling software opens a valve, the irrigation controller will shut off any other opened valves.
• No valve sequence support. For example: V1 on for 10min, then V3 for 5min and then V2 for 1min. This needs to be done by the controlling software.

The sources:

• Firmware source code
• Eagle schema
• Eagle board file

Flaws

So far the biggest flaw I found is that I mixed up the RX/TX pins of the FTDI header. In eagleCAD, the part I was using for the header had to have its RX connected to the uC's RX pin. I didn't know that, I thought I had to reverse them. If I had looked at the FTDI pinout, I would have noticed it. Lesson learned. But it's not a big deal, I made an adapter that reverses both pins.

Also, like I said earlier, the wifi doesn't work without an external antenna. No big deal, I can use an external antenna. But I added that SMD footprint like 5 min before sending to the fab. I'm pretty glad I did that. The lesson learned here: anticipate for errors: put extra stuff in case you need it. That's obviously not good thinking for production but for a hobbyist like me, it's a good thing to do. Noticed the "spare" capacitor foot print on the board? hehehe.

Questions?

I've seen a lot of amateur projects like mine on sites like Hackaday and a lot of people always make the same comments so I figured I'd answer those questions immediately.

• Why did you do it this way instead of that way? Because I felt like doing it that way.
• Why not use this and that instead? Because that's not what I felt like playing with.
• Isn't this a fire hazard? I don't know! Is it?!?!?! Please tell me at youjustbuiltadeathtrapyoulunatic@dumaisnet.ca
• Why not use an Insteon EZFlora? I did. But I wanted to build an irrigation controller. So now my EZFlora is useless

I recently installed an RGB led string under my kitchen cabinet. I wanted to way to control it using any web browser on a cell phone, tablet or desktop computer. There are many ways I could have achieved this but this time, I wanted to do it all by myself and design my first PCB. This is how I did it.

ESP8266

The ESP8266 is small cheap wifi microcontroller. It is a 32bit microcontroller with and external 1024kb or flash memory. There are tons of information out there about this chip so I won't dig too much into the basics.

When I discovered this board, I realized the potential it could bring to my home automation projects. I can now have multiple devices that talk to my central software (New Home Automation system) wirelessly.

Firmwares

The ESP8266 usually comes with a pre-loaded "AT" firmware. If it isn't, you can download it for free and upload it, using an FTDI cable, to your chip. The AT firmware is a simple firmware that lets you send "AT" style commands, like in the old days with modems, to control the chip. There's an AT command to connect to an access point, another to start a TCP server, another to send a packet, etc.

A lot of people use the AT firmware. This allows you to connect a another microcontroller to the ESP8266 using the UART pins and send commands to the device. But you can also write your own firmware and take advantage of the the 1024kb of flash on the chip. You have a few GPIOs and 2 UARTs. You don't need an external microcontroller.

Environment

To upload a firmware, whether it is the AT firmware or your own, you will need the flash tool. To build your firmware, you will need a complete toolchain. The esptool can be found at https://github.com/themadinventor/esptool/. Once you have downloaded the file, just copy it somewhere like in /usr/bin so that it is in your exec path. To upload a firmware, you must obviously obtain one or build one. The firmware is separated in a couple of file. If you are uploading the AT firmware, this should work (change /dev/ttyUSB8 for the serial port where your FTDI cable is attached):

Programming

Keep in mind that the chip must be reset while GPIO0 is pulled low in order to enter the bootloader to accept a firmware update. To create your own firmware, you will need to download the toolchain. The following should get you going:

Now add /opt/xtensa-lx106-elf/bin in your PATH. Next, you will need a Makefile. I've created the following:

Again, remember to reset the chip with GPIO0 pulled low before invoking 'make install'. Here is a basic source file to build a firmware

Notice how the entry function is called user_init() instead of main(int argc, char** argv). This is because you are not running your firmware bare-metal on the board. There is an OS running in there and it will invoke user_init(). This is an event-based system. You cannot make an infinite loop like you would on another microcontroller. Instead, you let the OS run and hook your app to some events with callback functions. There is a way to create a timer and give the OS a pointer to your timer callback function. You can also give the OS a function pointer to a message handler and post messages to your app in order to receive messages in your message handler. If your code takes too much time inside a callback, the OS watchdog will reset your chip. So you cannot make infinit loops nor complex functions that do massive computations. You have to build your architecture around a single-threaded event-based system. Kindof like a javascript application.

The SDK, included in the toolchain, is pretty easy to use. For examle, to connect to a wireless access point, all you need to do is:

If you want to create an infinite loop, as I said previously, you cannot do this within a callback since it will block the os thread and the watchdog will reset your board. Instead, you need to post your self a message so that the OS will call your function everytime. For example:

The Board

I'm not going to show my board diagram, because frankly, I'm a little ashamed of the work I did. This is my first PCB design and I'm still learning basic electronics. Bascially, I made a PCB with 3 mosfets, a step-down voltage regulator (12V to 3.3V) and a ESP-03 chip.

PWM

The chip supports software PWM for all GPIOs but I never was able to make it work properly. So I ended up writing my own software PWM. It's pretty straight forward, I just setup a timer and toggle GPIOs according to a duty cycle set by the user application. You can get my source code here:

• pat_pwm.c
• pat_pwm.h

DWN protocol

For each of my new Wifi devices to talk to my home automation server, I had to invent a new protocol. I could have used MQTT but I felt like my idea was even more simple. I created what I call the DWN protocol (DHAS Wifi Nodes).

The protocol is very simple. The wifi node broadcasts a JSON message containing its name and unique ID every 5 seconds on a multicast address at port 242. Whenever my home automation server sees such an advertisement, it will detect the source IP address of the broadcasting device and connect to the device's TCP server listening on port 242. Once the wifi device has a TCP client connected, it continue to send advertisements every 5 seconds and also sends heartbeats to the connected client. The DWN protocol does not define the format of payloads being exchanged between the home automation server and the wifi device. The DWN protocol is just a protocol that allows wifi nodes discovery.

Using such a system, my home automation can detect any wifi devices that gets turned on in my house and it will connect to it. Since the devices broadcast their name (ie: "IRRIGATION_CONTROLLER"), the server loads the appropriate "driver" or "module" for that device and starts communicating with them. This allows me to have a plug-and-play architecture. If I want to add a second led controller device, I just build one, turn it on and the home automation server automatically detects it and starts using it.

Future projects

This was my led controller board. I am planning on making a sprinkler controller with a esp-03 board with 4 relays, using the DWM protocol with my home automation server. I will also make a small device with a ESP-01 board with 2 input GPIOs to monitor my alarm system's relays. I currently monitor my alarm system's relay with an arduino setup as an ACM device plugged in the USB of my computer. I'd rather go wifi now. I would also like to build a device that would stay connected in my car. Onces I park my car in the garage, because of the DWN protocol, my home automation server will know when my car is at home and when it isn't. So as you can see, these esp8266 chips can be used for many things in the home automation world.

openvswitch

I want to create two VM on two separate hosts (one per host) that reside on the same network. I also want the VMs to reside in a different network than the host's network. To do this with openvswitch is really easy.

• Make sure openvswitch and tun/tap are compiled in kernel
• install openvswitch tools
• create bridge on each VMs
• create masquerading rules to share internet connection with VM network

Installing openvswitch

Openvswitch is already part of the kernel. It needs to be enabled when compiling the kernel. Then the tools need to be installed. On slackware, it was just a matter of ./configure, make, make install.

Creating a bridge with GRE tunnel

To have both VMs communicate with each other, we can build a virtual switch that spans accross the two hosts. This is done with a bridge containing a GRE tunnel. Creating a bridge on each VMs is done as follow. Unless noted otherwise, these commands should be re-executed after every reboot

Then repeat the same on the other host

Now, it is possible to start a VM on each host using their respective TAP7 interface. Each vm is assigned an IP address in the same subnet but different than the one of the hosts. In my case I used 192.168.2.2 and 192.168.2.9. Because a GRE interface is present in the bridges, both VM can communicate with each other now.

Access to the internet

Since the VMs reside on subnet 192.168.2.0 but my internet gateway is at 192.168.1.1, there is no way for my VMs to communicate with the outside world. So the first thing that needs to be done, is to assign an ip address to the bridge of one of the host. This will be the gateway. So obviously, only one is needed. I chose to put it on host2. So the "br1" on host1 still has no ip address assigned to it. On host2 I did:

Then, masquerading needs to be done. This is not absolutely required. But if nat is not used, then the gateway of the host's network must know about the VM network and have a route to redirect traffic to the 192.168.2.1 gateway. In my case, I chose to use NAT. The following commands did the trick:

# Enable forwarding. So that linux will see that # traffic on br1 that wants to go out on a network known by another # interface, eth0 in my case, will be forwarded on that interface. echo 1 > /proc/sys/net/ipv4/ip_forward # Create IP masquering rules. Assuming that eth0 is the host's interface that can # communicate with the internet gateway (or is the gateway). Packets destined # for external traffic will leave out eth0. So make a postrouting rule # that will masquerade those outgoing packets. This will allow other networks # to reach the VM network without knowing anything about it. iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Of course, the default route, within the VMs, should point to 192.168.2.1. At this point VMs should be able to connect to this website

Additional notes

• port of type "patch" can be used to "connect a patch cable" between 2 vswitches. This would be used to connect two vswitches together as long as they reside on the same host. To connect two vswitches that reside on different hosts, a port of type GRE would be needed.
• To get additional information on what type of interface can be created (patch, internal, gre, etc.): http://openvswitch.org/ovs-vswitchd.conf.db.5.pdf
• In the example above, the ovsdb-server was started with --remote=ptcp:6640. This means that it is possible to control the vswitch from any other host using "ovs-vsctl --db=tcp:192.168.1.2:6640 ". This is great when a virtual switch spans across several host because all host bridges can be controlled from one single point.

Openflow

The best description of OpenFlow and SDN I found was this: OpenFlow is a protocol which lets you offload the control plane of all the switches to a central controller and lets a central software define the behavior of the network (also called Software Defined Networks). To me, that description was the one that opened the doors to SDN in a time where all of those terminologies was confusing to me.

Flows

Before going any further, let's first define what a flow is. An open flow switch makes forwarding/routing decisions based on its flow tables. Each flow represent a "rule" that must be matched and the action that must be taken if the rule does match. Flows are added by a separate piece of software to the switch. The switch does not "build" its own flows. These must be explicitely added to the switch by the switch administrator when first configuring the switch or dynamically by an SDN controller. If using an SDN controller, when a frame/packet/segment does not match any flows, it will be sent to the SDN controller in order to find a flow that matches.

The flows contain a "match" part. It defines fields of frame/packet/segment must be matched so that the flow applies. If the flow does apply, then the "action" part is used. Matches can be specify that a flow must match a source MAC address, a destination TCP port, or any other L2/3/4 fields. Combinations of fields can be matched and wildcards can be used.

The flows also contain an "action" part. Actions are very flexible. I won't detail all the possible actions here. An action could be to drop the packet, forward the frame, redirect to another flow table (internal to the switch) etc.

Using openflow with openvswitch

The man page for ovs-ofctl contains a section that details the flow syntax. Here is an example on how to add a flow that would drop any TCP segment with destination port 80

This one would rewrite any segment going towards TCP port 80 to go to tcp port 25. This is basically port forwarding. Note that two flows need to be written. Once the server on port 25 sends a reply, it needs to be modified to use port 80 since the client is expecting to be communicating on port 80.

This will add flows to the bridge on the local host. This could be done when configuring the switch initially. Obviously, more complex (and more useful) flows could be written, but this is just to get a hang of it.

It could also be useful to have the switch's flow tables be populated dynamically by some other "brain". Such a brain would be an "opendlow controller". The controller can reside on a separate host.

I downloaded pox, an openflow controller, on one of my host. I am running it on a different host than the one that is hosting the vswitch. vswitch is running on 192.168.1.2 and pox is running on 192.168.1.3

And then, on the vswitch host, I tell the bridge who the controller is

Using "forwarding.l2_learning" with pox creates a very basic flow that makes the switch act like a normal layer2 switch. You could write other modules for pox, but that is beyond the scope of this example. At least, now you know how to have a remote controller generate the flows dynamically. You could download another software to do it or even write your own.

By the way, using an openflow controller, we could drop the iptables usage from the above section and implement NAT using openflow only.

What this means

I wanted to learn what this was all about and I'm glad I did. I don't fully understand how this can be very usefull though. I do see the power that this brings to networking infrastructures. But for me, running a couple of computers in my basement with a relatively simple setup, it is not that usefull. I can't list a lot of uses-cases where this would change the world. I'm sure that one day, I'm going to stumble across a problem and say "Hey! That would be super-easy to do with openflow" and I'm gonna be glad I learned the basics of it. And now, 2017, I do fully understand the benefits of it because I work for a major SDN solution manufacturer :)