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This is hilarious. I happen to be preparing for a test centered around the OSI model and I don’t get why it’s set this way. Every few paragraphs the text says s

This is hilarious. I happen to be preparing for a test centered around the OSI model and I don’t get why it’s set this way. Every few paragraphs the text says something like “remember that nowadays things a bit fuzzier and don’t fit within one layer” well then why are we even using this model?

db48x 3y ago
As models go it’s pretty good, and no one has made a better one. Seriously. The layers are all useful concepts to know about and discuss, even if in some cases it is most practical to blur the lines or outright combine two of them into one protocol. You will at least know that you are doing so, and you will know to point out _why_ you are. But nobody has ever determined that it is _always_ best to combine two of the layers (which would really amount to proving that the two are in fact the same thing). But since nobody can prove that it is _never_ a good idea either, it remains a model and not a theorem with a proof. Edit: but I will also say that any test or certification which focuses too much on the OSI model is also suspect. I don’t know what test you’re about to take, but plenty of people will sell you certifications that in the end are meaningless.
- zamadatix 3y ago
  If one is going to argue for a layers based model the OSI model is still a particularly poor choice vs something with at least relevant layers such as the 4 layer model commonly referred from TCP/IP. It's been the superior model to reference for decades.
brookst 3y ago
Because all models are wrong, but some models are useful. OSI is a useful model, especially at lower layers, because it covers the abstractions from physical to network to internet to protocols, and that is a useful way to think about things.
- robertgraham 3y ago
  I discuss this point several times. I claim the model is not useful, specifically because the layering abstraction for hte lower layers is a misconception rather than the truth. For example, I describe how the OSI Model claims that layer #2 and #3 describe different functionality in the same network stack. I claim the opposite, how they describe roughly the same functionality in differnet networks. Namely, both Ethernet and the Internet forward packets based upon addresses. The difference is that Ethernet does this locally while the Internet does this locally. Otherwise, the theoretical concept of packets, forwarding, and addresses are the same.
  - brookst 3y ago
    The key concept in routing is hierarchy, which Ethernet does not have. It’s an extra dimension that changes the way the protocol and applications work at that layer. You can say a cube and a square are the same thing, but I don’t think that’s particularly useful pragmatically. Ditto for L2 and L3; if you abstract out the difference, they are indeed the same. But that’s not useful. Now, you could say that time has brought evolutions / optimizations that blur layer boundaries, like VLANs at layer 2. But that doesn’t make the model less useful.
    - fanf2 3y ago
      In my experience working alongside network engineers, they use “layer 2” as a synonym for Ethernet, “layer 3” as a synonym for IP (v4 and v6), “layer 4” as a synonym for TCP and UDP, and “layer 7” as a synonym for application protocols. They don’t talk about the other layers. It’s just jargon, without any deeper meaning.
      - brookst 3y ago
        Well yes, those are the network layers, so network engineers think about them the most. Admittedly layers 5 and 6 are kind of blurry, but so what? Doesn’t mean the model isn’t useful, just that it isn’t a perfect map=territory.
hughesjj 3y ago
You might want to read this for a bit of a primer https://apenwarr.ca/log/20170810 https://apenwarr.ca/log/20170810 > The point of all this is that ethernet and IP were getting further and further intertwined. They're nowadays almost inseparable. It's hard to imagine a network interface (except ppp0) without a 48-bit MAC address, and it's hard to imagine that network interface working without an IP address. You write your IP routing table using IP addresses, but of course you know you're lying when you name the router by IP address; you're just indirectly saying that you want to route via a MAC address. And you have ARP, which gets bridged but not really, and DHCP, which is an IP packet but is really an ethernet protocol, and so on.
technion 3y ago
Yep, for every person who says they haven't touched osi in a decade, there's someone studying the topic either at a school or for a vendor certification. And for every "the bottom layers are worthwhile", there's a network engineer applying for a job with some leetcode like tool asking them about layer 5.

db48x 3y ago

As models go it’s pretty good, and no one has made a better one. Seriously. The layers are all useful concepts to know about and discuss, even if in some cases it is most practical to blur the lines or outright combine two of them into one protocol. You will at least know that you are doing so, and you will know to point out _why_ you are. But nobody has ever determined that it is _always_ best to combine two of the layers (which would really amount to proving that the two are in fact the same thing). But since nobody can prove that it is _never_ a good idea either, it remains a model and not a theorem with a proof. Edit: but I will also say that any test or certification which focuses too much on the OSI model is also suspect. I don’t know what test you’re about to take, but plenty of people will sell you certifications that in the end are meaningless.

zamadatix 3y ago
If one is going to argue for a layers based model the OSI model is still a particularly poor choice vs something with at least relevant layers such as the 4 layer model commonly referred from TCP/IP. It's been the superior model to reference for decades.

brookst 3y ago

Because all models are wrong, but some models are useful. OSI is a useful model, especially at lower layers, because it covers the abstractions from physical to network to internet to protocols, and that is a useful way to think about things.

robertgraham 3y ago
I discuss this point several times. I claim the model is not useful, specifically because the layering abstraction for hte lower layers is a misconception rather than the truth. For example, I describe how the OSI Model claims that layer #2 and #3 describe different functionality in the same network stack. I claim the opposite, how they describe roughly the same functionality in differnet networks. Namely, both Ethernet and the Internet forward packets based upon addresses. The difference is that Ethernet does this locally while the Internet does this locally. Otherwise, the theoretical concept of packets, forwarding, and addresses are the same.
- brookst 3y ago
  The key concept in routing is hierarchy, which Ethernet does not have. It’s an extra dimension that changes the way the protocol and applications work at that layer. You can say a cube and a square are the same thing, but I don’t think that’s particularly useful pragmatically. Ditto for L2 and L3; if you abstract out the difference, they are indeed the same. But that’s not useful. Now, you could say that time has brought evolutions / optimizations that blur layer boundaries, like VLANs at layer 2. But that doesn’t make the model less useful.
  - fanf2 3y ago
    In my experience working alongside network engineers, they use “layer 2” as a synonym for Ethernet, “layer 3” as a synonym for IP (v4 and v6), “layer 4” as a synonym for TCP and UDP, and “layer 7” as a synonym for application protocols. They don’t talk about the other layers. It’s just jargon, without any deeper meaning.
    - brookst 3y ago
      Well yes, those are the network layers, so network engineers think about them the most. Admittedly layers 5 and 6 are kind of blurry, but so what? Doesn’t mean the model isn’t useful, just that it isn’t a perfect map=territory.

hughesjj 3y ago

You might want to read this for a bit of a primer https://apenwarr.ca/log/20170810 https://apenwarr.ca/log/20170810 > The point of all this is that ethernet and IP were getting further and further intertwined. They're nowadays almost inseparable. It's hard to imagine a network interface (except ppp0) without a 48-bit MAC address, and it's hard to imagine that network interface working without an IP address. You write your IP routing table using IP addresses, but of course you know you're lying when you name the router by IP address; you're just indirectly saying that you want to route via a MAC address. And you have ARP, which gets bridged but not really, and DHCP, which is an IP packet but is really an ethernet protocol, and so on.

technion 3y ago

Yep, for every person who says they haven't touched osi in a decade, there's someone studying the topic either at a school or for a vendor certification. And for every "the bottom layers are worthwhile", there's a network engineer applying for a job with some leetcode like tool asking them about layer 5.