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Comment by ndnxhs | Hacker News Reader

What is the point of the OSI layers. It feels like some academic come up with them and everyone has been going through mental gymnastics trying to work out how to fit the real world on to it. The proposed new model doesn't seem to make much sense either. How is IP integrity?

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OSI wasn't just a layering model--it was a protocol suite. The thing is, the OSI stack lost to the TCP/IP stack. That's why it seems tricky to figure out how the "real world" fits in to it--TCP/IP networks literally don't fit.

OSI actually developed an entire set of protocols for each of the layers that adhered to the principles of their design. (Or at least they intended to, I don't know if they actually got around to tackling all of the layers). It is an entire networking stack that competes with the TCP/IP model, which was very much not designed in such a way, and thus does not mesh very well with it.

The OSI protocol stack never gained much traction. Some of them were repackaged on top of TCP/IP--X.500 became adapted into LDAP, and SSL certificates are derived from X.509. X.400 is the OSI competitor to SMTP, and still sees some niche usage (just not for email). There may be some other ones floating about in terms of the telephone network infrastructure.

Do you know what the X stood for?

X is just the letter prefix identifying the series of ITU-T Recommendations related to “Data networks, open system communications and security.”

You can see a list of all 25 series (“W” is not assigned) and download most of the documents (including Rec. ITU-T A.12, Identification and layout of ITU-T Recommendations, which defines the letters) on the ITU’s website: https://www.itu.int/pub/T-REC

Many more people who have never heard of the ITU will certainly be familiar with the H series ("audiovisual and multimedia systems"), of which the video codec H.264 is well known.

Some large IP networks still use IS-IS for routing (IS-IS is OSI's version of OSPF, and runs directly on top of ethernet rather than being encapsulated IP, and is seen as easier to deploy).

For a time, in the late 1990s, some regional ISPs effective ran over X.25 --- they leased shared "backbones" running Frame Relay (which is OSI stack); this is sort of like the phone company runs a backbone that you and some of your competitors buy and run IP over for your ISP customers. There are probably still some offices and retail chains that use frame and thus the X.25 stack.

My handle (twitter, email, whatever) comes from X.25. If you were a kid growing up on the mean streets of dial-up Internet in the 1990s, X.25 was a big deal. :)

To quote the ISO standard 7498 in which the OSI reference model is defined:

"This reference model provides a common basis for the coordination of standards development for the purpose of systems interconnection while allowing existing standards to be placed into perspective within the overall reference model."

It always made sense to me that the OSI model should be used to guide the development of internet standards. I have always doubted the second half of that statement, about understanding existing standards from an OSI perspective and I believe that is the mental gymnastics of which you speak.

Despite the mental gymnastics, out in the real world, me and my co-workers still find ourselves using OSI vocabulary to reason and talk about the networks we're working on.

If I'm told that some random black box is a "layer 3 switch" I already have a basic understanding of what it does and how to use it, even if I'm not familiar with the vendor. That's value for money right there, in my opinion.

Having some unified concept of them is nice for trouble shooting and communicating concepts; especially in network design. Helps you understand what depends on what and in some cases what certain protocols are limited by (see: broadcast domains). I personally prefer the 5 layer TCP/IP model to the 7 layer OSI model. To me the Application, Presentation, and Session layers blur into one.

It is relatively useful for understanding intentions and make sure that people are talking about the same thing, but overall I agree.

Layers 1-4 map nicely to our current system, and the academic existence of layer 5 allows us to lament its lack in the real world :)

The OSI Layers originated out of Telcos/PTTs. They more resemble a model of a Global Communications network akin to Videotex[1] than to how things work over the Internet. This becomes apparent when one looks into the history of Videotex and the structural makeup of Telcos/PTTs. Take a look here:

https://en.wikipedia.org/wiki/TM_Forum#OSI/Network_Managemen...

And here:

https://books.google.com/books?id=k9sFgIM-z6UC&lpg=PT152&ots...

[1] https://en.wikipedia.org/wiki/Videotex

The point of the OSI model is to distribute responsibilities so that people can develop protocols that work together. Otherwise you get almost entirely proprietary protocols where every application using the network has to have 10 different configurations changes to get it to talk to another machine.

Divide and Conquer. They provide abstraction layers that you can individually reason about, individually implement, independently exchange... In the real world, no abstraction is perfect: There will always be "leaks" between the abstractions, optimizations that partially dissolve them etc., but it's a good principle and allows vastly more complexity than without.

The problem with the ISO/OSI model is less how to fit real network protocols into it, it's more that it actually was proposed as a model for real network implementations, but TCP/IP, which does not fit well into the model, grew and displaced most other protocols and became the de facto standard for networking. This happened for multiple complex reasons that are not all technical.

A great thing about the OSI layers, even if what we use doesn't exactly map to them, is that it provides a common pattern for a good way to approach things. Even if a protocol doesn't map perfectly to it, it's a really solid way to build up the abstractions for end-to-end communication.

I've recently been working with LoRaWAN at a low level, and it's a great example of a standard that could have used the OSI layer to make it suck less for implementers. Lots of the encryption and checksum pieces mix up which layer they're working on. Building a nice clean layer cake for LoRa is a disaster, and I've resigned myself to just having a single big state machine that takes care of everything.

OSI model is very useful mental model for troubleshooting networking issues. Having a frame-work for where in the stack a protocol lies allows you to verify and eliminate lower-level protocols as a culprit.

The OSI model is a poor model for a world built on the 4 layer TCP/IP model.

Not all the world is TCP/IP.

If you ever spend any time with more than your standard wifi + NAT connection understanding L2 vs L3 vs L4 becomes very handy.

The TCP/IP model includes L2 vs L3 vs L4 (i.e. it's not limited to modeling just the TCP/IP portion it is named after), it doesn't include the rest of the cruft the OSI model does nor any expectations on how those layers were supposed to tie together (which never happened the way OSI laid things out anyways).

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