The OSI Deprogrammer

By on 20 Jun 2024

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Back in December, George Michaelson posted an item on the APNIC Blog titled ‘That OSI model refuses to die‘, in reaction to Robert Graham’s ‘OSI Deprogrammer‘ published in September. I had discussed the OSI Deprogrammer on Lobsters, and George’s blog post prompted me to respond, but I didn’t get around to it until now.

The main reason that Open Systems Interconnection (OSI) remains relevant is that Cisco certifications require network engineers to learn it. This makes OSI part of the common technical vocabulary and basically unavoidable, even though (as Rob Graham correctly argues) it’s deeply unsuitable.

It would be a lot better if the OSI reference model were treated as a model of OSI in particular, not a model of networking in general, as Jesse Crawford argued in 2021. OSI ought to be taught as an example alongside similar reference models of protocol stacks that are actually in use.

One of OSI’s big problems is how it enshrines layering as the architectural pattern, but other patterns are at least as important:

  • The hourglass narrow waist pattern, where a protocol stack provides a simple abstraction and only really cares about how things work on one side of the waist.

For instance, IP is a narrow waist and the Internet Protocol (IP) stack only really cares about the layers above it. Ethernet’s addressing and framing are another narrow waist, where IEEE 802 only really cares about the layers below.

  • Recursive layering of entire protocol stacks. This occurs when tunnelling, for example, MPLS or IPSEC. It works in concert with narrow waists that allow protocol stacks to be plugged together.

Tunnelling starkly highlights what nonsense OSI’s fixed layers are, leading to things like network engineers talking about ‘Layer 2.5’ when talking about tunnelling protocols that present Ethernet’s narrow waist at their endpoints.

Speaking of Ethernet, it’s very poorly served by the OSI model. Ethernet actually has three layers:

Then there’s Wi-Fi, which looks like Ethernet from the IP’s point of view but is even more complicated. And almost everything non-Ethernet has gone away or been adapted to look more like Ethernet…

So, OSI has too few lower layers to model Ethernet accurately. By contrast, it has too many upper layers: The OSI session and presentation layers don’t correspond to anything in the Internet stack. I think Rob Graham said that they came from IBM SNA and were related to terminal-related things like simplex or block-mode, and character set translation. Jack Haverty said something similar on the Internet History mailing list in 2019. The closest the ARPANET / Internet protocols get is Telnet’s feature negotiation; a lot of the problem solved by the OSI presentation layer is defined away by the Internet’s ASCII-only network virtual terminal. Graham also said that when people assign Internet functions to layers 5 and 6, they do so based on the names not based on how the OSI describes what they do.

One of the things that struck me when reading Mike Padlipsky’s Elements of Networking Style is the amount of argumentation that was directed at terminal handling back then. I guess in that light it’s not entirely surprising that OSI would dedicate two entire layers to the problem.

Padlipsky also drew the ARPANET layering as a fan instead of a skyscraper, with intermediate layers shared by some but not all higher-level protocols, for example. The Network Virtual Terminal (NVT) used by Telnet, FTP, and SMTP. I expect if he were drawing the diagram later there might be layers for 822 headers, MIME, and SASL — though they are more like design patterns than layers since they get used rather differently by SMTP, NNTP, and HTTP. The notion of pick-and-mix protocol modules seems more useful than fixed layering.

Anyway, if I could magically fix the terminology, I would prefer network engineers to talk about specific protocols (such as Ethernet or MPLS) instead of bogusly labelling them as layers (2, 2.5). If they happen to be working with a more diverse environment than usual (hello DOCSIS) then it would be better to talk about sub-IP protocol stacks. But that’s awkwardly sesquipedalian so I can’t see it catching on.

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One Comment

  1. fred baker

    Its a hard choice is OSI or ITIL with worst import from Europe. Remember that a number of companies tired to implement OSI products because of government mandates and all went out of business. I remember yelling at IBM with a bunch of other large customers to not implement OSI and put the money into TCP, they did not and sold their networking business to cisco. OSI should have been left on the side of the road 20 years ago


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