The world in which IPv6 was a good design

The world in which IPv6 was a good design

Last November I went to an IETF meeting for the first time. The IETF is an interesting place; it seems to be about 1/3 maintenance grunt work, 1/3 extending existing stuff, and 1/3 blue sky insanity. I attended mostly because I wanted to see how people would react to TCP BBR, which was being presented there for the first time. (Answer: mostly positively, but with suspicion. It kinda seemed too good to be true.)

Anyway, the IETF meetings contain lots and lots of presentations about IPv6, the thing that was supposed to replace IPv4, which is what the Internet runs on. (Some would say IPv4 is already being replaced; some would say it has already happened.) Along with those presentations about IPv6, there were lots of people who think it's great, the greatest thing ever, and they're pretty sure it will finally catch on Any Day Now, and IPv4 is just a giant pile of hacks that really needs to die so that the Internet can be elegant again.

I thought this would be a great chance to really try to figure out what was going on. Why is IPv6 such a complicated mess compared to IPv4? Wouldn't it be better if it had just been IPv4 with more address bits? But it's not, oh goodness, is it ever not. So I started asking around. Here's what I found.

Buses ruined everything

Once upon a time, there was the telephone network, which used physical circuit switching. Essentially, that meant moving connectors around so that your phone connection was literally just a very long wire ("OSI layer 1"). A "leased line" was a very long wire that you leased from the phone company. You would put bits in one end of the wire, and they'd come out the other end, a fixed amount of time later. You didn't need addresses because there was exactly one machine at each end.

Eventually the phone company optimized that a bit. Time-division multiplexing (TDM) and "virtual circuit switching" was born. The phone company could transparently take the bits at a slower bit rate from multiple lines, group them together with multiplexers and demultiplexers, and let them pass through the middle of the phone system using fewer wires than before. Making that work was a little complicated, but as far as we modem users were concerned, you still put bits in one end and they came out the other end. No addresses needed.

The Internet (not called the Internet at the time) was built on top of these circuits. You had a bunch of wires that you could put bits into and have them come out the other side. If one computer had two or three interfaces, then it could, if given the right instructions, forward bits from one line to another, and you could do something a lot more efficient than a separate line between each pair of computers. And so IP addresses ("layer 3"), subnets, and routing were born. Even then, with these point-to-point links, you didn't need MAC addresses, because once a packet went into the wire, there was only one place it could come out. You used IP addresses to decide where it should go after that.

Meanwhile, LANs got invented as an alternative. If you wanted to connect computers (or terminals and a mainframe) together at your local site, it was pretty inconvenient to need multiple interfaces, one for each wire to each satellite computer, arranged in a star configuration. To save on electronics, people wanted to have a "bus" network (also known as a "broadcast domain," a name that will be important later) where multiple stations could just be plugged into a single wire, and talk to any other station plugged into the same wire. These were not the same people as the ones building the Internet, so they didn't use IP addresses for this. They all invented their own scheme ("layer 2").

One of the early local bus networks was arcnet, which is dear to my heart (I wrote the first Linux arcnet driver and arcnet poetry way back in the 1990s, long after arcnet was obsolete). Arcnet layer 2 addresses were very simplistic: just 8 bits, set by jumpers or DIP switches on the back of the network card. As the network owner, it was your job to configure the addresses and make sure you didn't have any duplicates, or all heck would ensue. This was kind of a pain, but arcnet networks were usually pretty small, so it was only kind of a pain.

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