OSPF: time to get rid of the totally not so stubby legacy

▼ Recently, I was looking through some networking certification material. A very large part of it was about OSPF. That's fair, OSPF is probably the most widely used routing protocol in IP networks. But the poor students were submitted to a relentless sequence of increasingly baroquely named features: stub areas, not-so-stubby-areas, totally stubby areas, culminating in totally not-so-stubby areas.

Can we please get rid of some of that legacy? And if not from the standard documents or the router implementations, then at least from the certification requirements and training materials?

Shortest path first, but not so fast

The Open Shortest Path First routing protocol (OSPF, Internet Standard 54) was first defined in RFC 1131 in 1989. So in internet time, OSPF is truly ancient. The base OSPFv2 specification is over 200 pages, with additional extensions in separate documents spanning the early 1990s to the late 2010s.

OSPF is powered by Edsgar Dijkstra's shortest path first algorithm. SPF is a relatively efficient algorithm for finding the shortest path between two places, in the real world or in a network. Still, in a large network there's a lot of paths to check until you can be sure you've found the shortest one. The problem here is that for a network that's 10 times larger, SPF needs 60 times as long to run. So if a router in a network with, say, 100 routers, needs a second to do its SPF calculations after an update, in a network with 1000 routers that takes a minute, and in a network with 10,000 routers an hour.

So in order to make OSPF useful in large networks, you can split your network into different areas. The SPF calculations are then contained to the routers within each area. So rather than calculate SPF over a 10,000-router network, you could have 100 areas with 100 routers each. Then routers that connect two areas would have to calculate SPF over 100 routers for two areas, so 2 seconds rather than an hour worth of SPF calculations.

But if each of those 10,000 routers still injects two, three or four address blocks into OSPF, that means the OSPF database will have something like 30,000 entries. So now updating and remembering all those address blocks becomes a bottleneck. Solution: summarize link advertisements. So if routers in area 35 advertise address blocks 10.35.1.x, 10.35.2.x, … 10.35.95.x, rather than push out all that information to all 10,000 routers throughout the network, the area border routers for area 35 simply say “10.35.x.x” to the rest of the network.

Even better: if an area only connects to the “backbone” area (area 0) and doesn't learn any routing information from other areas or from outside OSPF, it's a stub area that really doesn't even need to know anything that's happening in the rest of the network, so let's give it a default route to reach the rest of the world.

Variations on a stubby theme

Stub areas still have some OSPF routing information from other areas. We can get rid of that too, and then we have a totally stubby area.

On the other hand, maybe we want to import external routing information into OSPF even in our stub area, and then propagate that external information to other areas. This makes for a not-so-stubby area.

And who said you can't have your cake and eat it: let's make our totally stubby area not-so-stubby, and we'll have a totally not-so-stubby area, guaranteeing certification income for years to come. (See Wikipedia's page on OSPF for more details.)

Spring cleaning

As protocol designers, we're really good at adding more capabilities, more options. As network architects and engineers, we're really good at adding complexity to make our networks do something they won't do out of the box. But we can't just keep adding options and complexity without ever taking any of it away. At least not if we want to have a fighting chance at teaching our craft to the next generation so we can retire at some point.

Our routers/computers are now 1000 times as fast and have 1000 times the memory as the 68030-based routers/computers back in 1990. OSPF implementations support incremental SPF.

10,000 routers in one area will melt the network operations center long before the SPF calculations melt the router CPUs. I've personally worked on a network with 600 routers in area 0 back in 1999. SPF performance was the least of our concerns.

So I'm calling it: OSPF areas and summarization are now legacy. New and current OSPF networks should just use a flat area 0 rather than try to micromanage the information flow between areas. Students should no longer have to learn how areas work, and only be informed about the various flavors of stubbiness as an example of humorous naming that doesn't age well.

Permalink - posted 2022-05-12

First impressions: THEA500 Mini

▼ On friday, I received my preordered THEA500 Mini. This is smaller and modern version of the Commodore Amiga 500 computer from 1987. By the same company that makes a similar version of the Commodore 64.

After exploring the A500 Mini for a few days, I can't really make up my mind on whether I like it, or it's too limited. So here some first impressions, I'll probably do a more complete review later.

The hardware

The hardware looks really nice, and is quite small. The C64 Mini was right about half the size of an actual C64, but the A500 Mini is more like 44% the size of an Amiga 500. The included mouse is fine but not great, the included gamepad is a departure from the joysticks we used with the Amiga back in the day, but so far I'm not hating it and having the extra buttons is sometimes useful. You can also use the THEC64 joysticks, but a different one didn't work.

The A500 Mini is 130 euros with 25 games included, compared to the C64 Mini at 80 euros and 64 games included. Could they have made the A500 Mini a bit cheaper by leaving out the mouse? Most of the games don't use the mouse, most people will already have a USB mouse and they could of course sell an appropriate mouse separately.

The included games

Unlike the C64 Mini, which has my all-time favorite game Boulder Dash included, the A500 Mini doesn't come with any games that I really played in the past. Sure, I played Stunt Car Racer sometimes, but I wasn't any good at it. (In that regard, nothing has changed.)

The point of the A500 Mini is to play games, so the quality of the included games is important. I guess that means I'll have to give all 25 a try! However, so far I'm having some trouble figuring out how to play the games, as this is often not obvious and there is no documentation on the included games, just an overview of the button mapping on the gamepad, mouse or joystick.

WHDLoad

If you want to go beyond the included games, you can load additional ones with WHDLoad. I had quite mixed success with that: some load quickly and run without trouble, for others Amiberry fires up to set up a bunch of things and then the game never starts.

The problem with WHDLoad is that regular users can't make existing software run under it; you'll have to wait for people with the right skills to create a WHDLoad "slave". This severely limits what you can do with the A500 Mini. It would be so much better if we could also run software off of ADF disk images. (However, WHDLoad is the better option for most games.)

So at this point I'm frustrated by being limited to WHDLoad images, and then having a good number of those not work with no clues on how to solve the issue. However, with such an obvious target, it's very like the Amiga community will make sure as much Amiga software as possible will run under the A500 Mini's flavor of WHDLoad. I've already seen a few examples of this in Youtube videos, but nothing I could try myself just yet.

So more later.

Permalink - posted 2022-04-10