4 ms·

OSI layers 1, 2, 3, 4, and 7 are very real. Roughly, e.g.: 1:Fiber, 2:Ethernet, 3:Routing, 4:TCP/IP, and 7:HTTP.

by quesera 6y ago

3np 6y ago
Isn’t IP commonly considered L3?
- quesera 6y ago
  Yep, and TCP is L4. So TCP/IP is L4/L3. The usual criticism of the OSI model is that there are grey areas and dependencies, where a lower layers bleed into higher layers (e.g. L2 switching which can work a lot like routing in modern hardware), and higher layers that are tightly bound to lower layers, making the distinctions unclear. The purist in me wants to agree, but I think the model is too useful to disregard casually. The value comes from the abstraction, and like Newtonian physics, it is a great model -- until it isn't.
lmm 6y ago
Layers 2 and 3 are not real - most networks today merge their functions in the same devices. (In the OSI model switches are illegal, you can only have hubs and routers). 4 and 7 are ceasing to be real - HTTP2 is both a session protocol and an application protocol, and that's before we even get into things like DoH.
- quesera 6y ago
  You're not wrong, but you are speaking from the perspective of a single implementation stack, i.e. modern Ethernet and the modern Internet. The model is still solid, even if the most common implementation has melted a bit.
  - lmm 6y ago
    I disagree. The model was supposed to be universal but it fell apart as soon as people started doing anything slightly unanticipated with it. It's simultaneously both overengineered and fragile, not providing enough insight to justify its complexity - rather like OSI itself. There's a reason their protocol suite was a failure in the real world.
    - quesera 6y ago
      That's fair and I agree that the OSI model is not useful for a precise understanding of deployed networking technology. I do think it is useful as an architectural device or a conceptual design goal -- i.e. a model to model your models on. :) But I also concede that part of its teaching value is that it is a failure in practice. It was a formalization of the ad hoc (successful!) design strategies of early networking. I see echoes of it everywhere, most obviously in the Linux kernel, and I think it's valuable for that.

OSI layers 1, 2, 3, 4, and 7 are very real. Roughly, e.g.: 1:Fiber, 2:Ethernet, 3:Routing, 4:TCP/IP, and 7:HTTP.

3np 6y ago

Isn’t IP commonly considered L3?

quesera 6y ago
Yep, and TCP is L4. So TCP/IP is L4/L3. The usual criticism of the OSI model is that there are grey areas and dependencies, where a lower layers bleed into higher layers (e.g. L2 switching which can work a lot like routing in modern hardware), and higher layers that are tightly bound to lower layers, making the distinctions unclear. The purist in me wants to agree, but I think the model is too useful to disregard casually. The value comes from the abstraction, and like Newtonian physics, it is a great model -- until it isn't.

lmm 6y ago

Layers 2 and 3 are not real - most networks today merge their functions in the same devices. (In the OSI model switches are illegal, you can only have hubs and routers). 4 and 7 are ceasing to be real - HTTP2 is both a session protocol and an application protocol, and that's before we even get into things like DoH.

quesera 6y ago
You're not wrong, but you are speaking from the perspective of a single implementation stack, i.e. modern Ethernet and the modern Internet. The model is still solid, even if the most common implementation has melted a bit.
- lmm 6y ago
  I disagree. The model was supposed to be universal but it fell apart as soon as people started doing anything slightly unanticipated with it. It's simultaneously both overengineered and fragile, not providing enough insight to justify its complexity - rather like OSI itself. There's a reason their protocol suite was a failure in the real world.
  - quesera 6y ago
    That's fair and I agree that the OSI model is not useful for a precise understanding of deployed networking technology. I do think it is useful as an architectural device or a conceptual design goal -- i.e. a model to model your models on. :) But I also concede that part of its teaching value is that it is a failure in practice. It was a formalization of the ad hoc (successful!) design strategies of early networking. I see echoes of it everywhere, most obviously in the Linux kernel, and I think it's valuable for that.