OSI is very widely used, most of the large ISPs use it. End consumers are just unaware of that fact. See https://en.wikipedia.org/wiki/IS-IS

I think this is a stretch, like saying the X.500 Directory System is widely used based on the fact that PKIX is technically adapting X.509 and thus your TLS certificates depend on the X.500 directory system. End users aren't just "unaware" that it's actually the X.500 system, it functionally isn't the X.500 system, PKIX mandates an "alternate" scheme for the Internet and the directory called for by X.500 has never actually existed.

Likewise then, IS-IS is the protocol that OSI standardized, but we're not using as part of an OSI system.

X.500 is widely used in the form of LDAP and Active Directory, however.

Active Directory is not based on X.500 and LDAP was directly created as an alternative to the DAP standard that is part of X.500.

While X.500 is a precursor to both of these things, and influenced both of these things, and both of these things interoperated with X.500, they are not X.500. X.500 is for all intents and purposes pretty much dead in 2024, although I did deploy an X.500 based directory service in 2012 and it's probably still alive and running.

LDAP is just "lightweight" protocol for accessing an X.500 directory service.

The semantics are the same for the accessed store, IIRC. The protocol definition explicitly talks about being used to access X.500 data stores as complementary interface to DAP (which requires lower layers of OSI stack vs LDAP's raw stream of bytes)

Yes, that is why it was invented, as I alluded to. LDAP was an alternative to DAP. DAP is part of the X.500 standard, LDAP is not. LDAP when it was first invented was built to access an X.500 directory. That is no longer a base requirement and most directories in the wild are not built on X.500. Standards mean things, just because LDAP was originally built to access X.500 doesn't mean it's part of X.500, and X.500 is a very detailed and specific standard which most directory services no longer follow.

To reiterate in a slightly different form: Ldap is an inheritor of, and successor to, DAP/X.500, but not backwards compatible and only superficially resembling it at this point.

Ironically there's probably more X.500 remnants in Active Directory than in the UMich-derived LDAP servers, thanks to its Exchange 5.5 legacy.

Would call it stretch to say it was widely used by ISPs. Some old ones may still be using integrated IS-IS as their IGP (early OSPF had scaling issues and complicated solutions for that), but that's nothing like widely using the ISO stack. But they might have used IS-IS it to route NSAP in their network at some point in time to manage ATM-era equipment, the ISP I worked still had some ctunnels for that purpose, I doubt they still have them.

It was still widely used for IPv4 and v6 in 2000s. ex telco engineer told us that two major reasons was that IS-IS was simply more efficient, and that they didn't require IP communication between routers - meaning troubleshooting and management was easier than providing p2p routes for IP-based protocols.

It's still widely used, among old ISPs because why switch when it's just as good or better than the alternative and your engineers know it best. But it's not the whole stack as implied in the first reply.

It was popular primarily because OSPF didn't scale well with hundreds to thousands of routers with the minimal CPU power even large core routers had.

Scalability was one part. Later the fact that you didn't need separate routing setup for v6 also was part of it.

But for a telco, the fact that you didn't have to setup IP connectivity in order for routers to see each other, was also a crucial ability.

OSPF has supported unnumbered links for a long time, don't need IP on the link with point to point and multicast.

Unnumbered just means you have no IP address exclusive to the link. IS-IS works without establishing IP connectivity at all, not just about tricks to handle leaky internals.

This also meant autoconfiguration or supporting non-ip protocols (like routed ethernet) is simpler, though I will admit that OSPFv3 adding default use of link local V6 for router interfaces made the gap smaller.

still good reasons to choose IS-IS for greenfield :) hell, they took IS-IS and did TRILL with it for DC (rip TRILL).

but yeah, I saw one piece of SONET crap hanging off a ctunnel that spoke CLNP back in maybe 2012 or 2013? haven't seen much of it, but still learned the ctunnel stuff back in '09 because it still could occasionally rear its head.

Worked for an old ISP in 2012 (it started when the state owned telco was privatized). Also had the ctunnel to manage old SONET crap, still had modem pools running, racks full of purple Sun machines still running, vax, hpux, as400, etc. Even the old smsc that was 5 racks of dec servers was still there, was never removed as they didn't need the space because modern equipment was using all the available power anyway (was also in a mountain so a bit of a hassle to remove).

OSI was usable as a wide area network before TCP/IP was.

[citation needed]

Personal experience.

Was developing OSI applications in 1989, you could order an X.25 circuit from your PTT and run OSI over that using the ISODE [1] toolkit.

The earliest ISP in my country didn't start until 1992.

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

TCP/IP was developed in the 1970s and adopted as the protocol standard for ARPANET (the predecessor to the Internet) in 1983.

and commercialization wasnt done until the like 1994. OSI as a successor was still proposed when the first BGP4 rfc came out.

before commercialization happened, IP was mostly the realm of government and education.

I feel like your experience was quite different to mine. I used TCP/IP in the late 80s at university and doing commercial contract work. I remember OSI existing but at the places I worked it was treated as less common.

> It's also a counterexample to the bureaucracy of standards bodies --- the standard that actually became widely used was the one that got released first.

Sounds like a cautionary tale: whatever gets released first will stick. If you make a blunder, generations will have to live with it (like IPv4).