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reconbot/so you want to send a message.md

Created December 16, 2024 19:51
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so you want to send a message.md

So you want to send a message

Setup

  • Index cards and black markers
  • Two whiteboards
  • Get a volunteer (eventually 3)
  • Networking is divided into 7 layers

Layer 1 Physical

The physical layer is responsible for the actual physical connections between devices. They deal in bits and move bits from one thing (lets call it a node) to another. This is your ethernet cable and your hubs. A hub takes all input and brings it to all outputs.

  • Our physical layer will be these index cards, these sharpies and this whiteboard. We’re going to call it ethernet!
  • Ethernet is a “shared medium” where someone says something and everybody hears it.
  • draw a square
  • Write a message on this index card and hold it to the board
  • Wait you have nobody to talk to, 2nd volunteer!
  • 2nd: you write something and hold it on the board
  • First rule, if two people say something at the same time, nobody sees it and you tear it up! That’s a collision!
  • You got a few problems here. If one node keeps going on and on, never stopping, nobody else can talk!
  • When you say something nobody knows who your talking to!
  • Both of you send the word “GitHub” one letter at a time.
    • If you detect a collision, tear it up and resend your letter! Tear up the letter in a collision.
    • First person to send it wins!
  • What would happen if we had 10 people trying to play this game? Would anyone be able to play?

Layer 2 Data Link

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. This brings in a new pice of hardware, the simple switch!

  • Framing - The bits are broken into Frames, they are a collection of bits with an address. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
  • Hardware address - Every node now has to have an Address - a MAC address (means media access control address but nobody cares)
    • Every network card has a mac address from their manufacturer. They will only pay attention to frames with their mac address on them.
    • 48 bits and manufacturer prefixes means no two cards anywhere have the same address. LOL that’s not true at all.
  • Lets write our messages like this SOURCE|DESTINATION|message
  • Our first problem is we don't know anyone's address
    • address discovery
  • I’m going to give you each a mac address but I want you to hide it.
    • AC / DC
  • Now both of you send the message "HI" to each other
  • You can’t, you don't know their address!
    • FF is broadcast to everyone!
  • Now send hi to each other
    • YAY!
  • If you sent “GitHub” again would it go better? Would 10 people be able to do it? NO!
    • This hasn't helped us at all!
  • So we add a new thing, a switch - It slices our network into different "collision domains", a collision domain is that shared medium. It likes to give frames only to ports where the destination address lives.
    • It listens for frames and builds a table of which port has what mac addresses.
    • If it gets an unknown destination address it just sends it everywhere
    • This way most of the time only the intended nodes see the message! Now 10 people can play and it works fine.
  • Draw a bunch of squares
  • Send "HI" again
    • I'll build a table of who's sending what, and give it to the right person
  • Adds latency of course, but totally worth it.
  • If we had 10 people this would still work. If we have thousands… I’d fall over. I can't keep track of that many MAC addresses. So what do we do?

BONUS

VLANS - virtual local area networking - It's an extra tag you can add to a frame that identifies a virtual network. Devices set to that network will get those frames, devices not set to that network will ignore those frames.

  • If I write SOURCE|DESTINATION in blue, anyone set to red will ignore it
  • If I write it in Red anyone set to blue will ignore it

Layer 3 - Network layer

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. This is IP or internet protocol. And this introduces Routers.

  • We now take a frame and stick an source and destination IP inside of it - we call that a packet!
  • Lets write that as From|TO
    • MAC|MAC|IP|IP|MESSAGE
  • You now have IP addresses too, 20.5 and 20.7
  • P1 wants to send a message to 20.7
    • What do you need to know to do that? IP and Mac Address
    • You don't know their mac address, so you have to do something dumb to find it out.
    • You got to yell!
    • "WHO HAS 20.7!"
    • "I DO!"
    • And now you can talk with IP addresses.
  • So how does this scale to thousands?
    • We divide our nodes into networks!
  • Now instead of a switch we'll have two routers, send p2 over to a different whiteboard.
    • They have mac address DA BF
  • We now have someone on this network over here and the other person over there
  • Give p2 a new address of 30.7
  • You want to send each other "hello"
  • How?
  • Subnet mask and default gateway.
    • Your gateway is 20.1
    • Your subnet mask is 255.0
    • AND the bits together to find out your network address and your host address
    • If your target IP isn't on your network, you need to send the packet to your default gateway
  • So put the MAC address of your gateway
  • And the IP of your target.
  • Send a "HI"
  • It gets the frame with the packet for someone else. it knows that targets network is over here, so it changes the frame's address to that router.
    • change the mac address
  • That router gets that frame and packet, changes the mac addresses again to the right computer
    • (WHO HAS XXX!?)
    • change it again
  • and they get it!

You can all sit down.

Layer 4 - Transport This is where TCP/IP and UDP live. They worry about retransmission for reliability, prioritization and other junk Layer 5 - who cares Layer 6 - who cares Layer 7 - application layer

  • This is where your HTTP, SMTP, FTP, BitTorrent live

And now you know how to talk to each other on computers.

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