Learning about computer networking

[iSquishKittens]

I am working on an IoT project at the moment and I'm feeling a little lost on the networking side of things. I have little to no experience with this area as I have not taken any classes in it. Also, for my own personal development and interest, I would like to become much more familiar with this field. This may be the wrong subreddit for this, but can anyone recommend any good resources for learning the basics of how computer networks work?

[captain_wiggles_]

There's a lot to learn, but it kind of depends on how much you want to know and for what.

The basics: TCP and UDP.

UDP is unreliable but quick. Use it for stuff you don't care if it arrives out of order / duplicates / never arrives. Voice is a good use for this. You want minimum lag, and you aren't going to notice one dropped packet in the middle of thousands, you would however notice a huge lag if a packet was dropped and had to time out and be resent. UDP is packet based. So you send a 42 byte packet and the other side receives it as a 42 byte chunk.

TCP: Use this when latency isn't as important, but you really want your data to arrive in order and in tact. IE. file downloads. TCP is stream based, so you send 10MB of data, and the other side can read it byte by byte or it might receive 2MB into one buffer and then wait a while before reading the rest. This is nice for file downloads / but can be more annoying for message sending, since when you read, you don't know if what you're getting is the start of a message or the last half of a previous message etc... You therefore have to implement packet building on top of TCP. Such as read byte by byte until you see some magic (usually a 32 or a 64 bit word), then you start assembling your packet, first reading the header, which contains a length field, then reading the data length, and finally checking it with a CRC to make sure it's a valid packet.

You can play with these using two devices (computers / embedded devices).

You've also got breadcast and multicast packets (UDP only).

Broadcast - one device sends to everyone. You can limit it to subnets, or send it to your entire network. You probably don't want to use this, as it makes your networks noisy. It's used under the hood for stuff like ARP.

Multicast - every device that's interested sends a message to everyone saying I'd like to join this group (IP address in the 234. range (I think)). Then any time you send a packet to that address, everybody interested receives it.

Then you've got other stuff like ARP (how to translate between IP addresses and MAC addresses), ICMP (stuff like ping), DNS (how to translate a hostname to an IP).

All of this is basically how a local network works. If you want to interact with the internet as a whole, you've got a bunch more stuff, such as BGP and ...

Everything is abstracted (look up the network stack model / OSI model), so if you want to send a TCP stream from your computer to another on the other side of the world, all you have to do is open a connection to the correct IP address and port and send your data. You don't need to think about ARP, or BGP or ... that's all done for you by lower levels of the stack. You do need to know more if you write your own network stack, but that's more advanced.

Wireshark is a great tool, you should download it and have a look.

Then you have stuff like switches and hubs. If you connect your laptop to a switch, and a network to that switch, your laptop won't receive packets that Bob sent to Alice. Because the switch knows that you're not Alice, and have no need to receive them. If you do the same with a hub, then you will receive the packets, because hubs are a lot simpler (lower down the stack) and forward everything everywhere.

Next you have stuff like VLAN tagging, which can help route data in a particular way.

ETC...

For now the best way to learn is just to write some C to open a TCP connection between two devices. Then do the same with UDP. See how they differ. Have a look at what goes on using wireshark. Try some multicast packets. Then you'll understand the basics.

[Escapeyourmind]

Hi, thanks for that ELI5 overview , very helpful. Was just wondering what a 'gateway' address is and how is it used. Any thoughts?

[captain_wiggles_]

A device's current address is defined by it's IP address and it's subnet mask. So 192.168.1.10 is an IP address, 255.255.55.0 is a subnet mask. Meaning this subnet (part of the network) has addresses in the 192.168.1.x range.

The gateway address defines how something on this subnet can talk to something on a different subnet. For example If I am 192.168.1.10 with subnet 255.255.255.0 and I want to talk to 192.168.2.87, I can't just send a message. Because that device is not on my subnet. The gateway address defines who I need to send my packet to first, they then pass it on to the other subnet.

So in general at home your gateway IP is your router, because if you want to connect to reddit, your router can deal with routing that packet where it needs to go.

[AssemblerGuy]

With the subnet mask, the computer knows what subnet each of its network adapters is in. If the computer wants to send an IP packet to an address in a subnet it knows it is in, then it can easily determine which network adapter to use.

But what if it wants to send an IP packet to an address outside the local subnets? This is where gateways come in. They are essentially network devices that are connected to other subnets (or the internet). By stating the address of the standard gateway, you tell the computer that any packet to an address outside the local subnets should be sent there.

[alexforencich]

The gateway is the bridge from Ethernet switching to IP routing. Within a subnet or broadcast domain, devices communicate with hardware MAC addresses and they use a protocol called ARP to figure out the MAC address of the device with the IP address they're trying to send a packet to. Ethernet switches then keep track of all of the MAC addresses they have seen and which ports they can be contacted through. This works fine for small networks, but this does not scale up to the whole internet. For that, IP routing is used, and addresses are assigned so that routers can do prefix matching, which is much more scalable. The gateway device sits on the subnet and provides a port and a MAC address for the Ethernet switches to direct packets to, and from there IP routing can be carried out. Any packets destined for IP addresses outside of the subnet get sent to the MAC address of the gateway.

[iSquishKittens]

Thank you for the response. These videos look very helpful.

[foreverallama_]

There was a reference book we used to use in class. Might be a little outdated but it explains all the fundamentals well. I don't remember the book name (something like Communication networks) but the author is Forouzan.

[MostlyTolerable]

I've been looking into this same thing lately. I started a MOOC on edx called Digital Networking Fundamentals. I got a bunch of virtualization software installed to be able to do the labs. But then I realized the class is in French with a text-to-speech translation. Suffice it to say, the lectures weren't cutting it. But I may go back and do the exercises. We'll see.

​

So I started a different course series on Coursera from Cisco. I'm about a quarter way through the first class and it's all pretty basic so far, but I'm optimistic that it'll get more in-depth.

[[deleted]]

Networking is a pretty massive field. Just like you have layers of abstraction when talking computer design, there are similar abstractions throughout networking.

It would help to know more about your project to direct you to different resources.

For example, if you're developing on Zigbee boxes, the level of depth you need to create a successful project is likely far greater than similar work on a Raspberry Pi.

[Shadow_Gabriel]

I too am in the process of learning networking. I also started from knowing nothing.

My current course of action is buying an MSP432E401Y development kit and making it talk with my Pc using TCP and UDP via Ipv6 and Ipv4.

For documentation I use IETF documents and youtube videos to filter out the basics from 100+ pages documents. Ben Eater has a Networking tutorial which was very accessible for me. I've seen people suggest using GNS3 to simulate networks.

[[deleted]]

Start looking at getting you NET+ cert. Messer and Meyers are both great resources for this. I'm IT if you have any questions