DECnet

~ the eulogy ~

I got online in 1998, into a network that had already finished arguing about which protocol it was going to be. That is the same year Digital Equipment Corporation stopped existing, bought by Compaq, which was itself absorbed into HP four years later. So DECnet is a thing I have only ever met as a chapter in a networking course, in the section that explains what people did before everybody agreed on TCP/IP, taught by someone who had to keep reminding the room that the agreement was not obvious at the time.

DECnet was Digital's own protocol suite, first shipped in 1975 after Phase I ran on a pair of PDP-11s the year before. It was a peer to peer architecture at a moment when the industry assumed networking meant terminals talking to a mainframe, and it turned Digital from a minicomputer company into a networking company. Phase III came in 1980, Phase IV in 1982, and Phase V in 1987 with an OSI-compatible stack bolted on for a future that never arrived.

Phase IV is the one worth remembering because of how it counted. Addresses were six bits of area and ten bits of node, which caps the entire universe at 63 areas of 1023 machines, or 64,449 nodes total. In 1982 that looked like more computers than anyone would ever connect. By 1990 Digital's own internal network, Easynet, held 54,000 of them and was pressing against the ceiling, which is a very specific kind of engineering failure: not being wrong, just being right about a number that stopped being big.

~ the dossier ~

~ what came after ~

DECnet did the work that the internet gets credited for, in the places where the internet had not arrived yet. NASA ran SPAN, the Space Physics Analysis Network, over it, and CERN and the high energy physics community ran HEPnet. Between them that was more than 17,000 nodes by 1989, which made DECnet the network science actually used while the TCP/IP internet was still mostly universities in the United States.

It also hosted the first worm anybody remembers for political reasons. In october 1989, days before the Galileo launch, a program calling itself WANK went through the DECnet machines on SPAN, rewriting login banners with an anti-nuclear message. It came in over DECnet's own trust between nodes, which was the price of the peer to peer design. Every network with a founding assumption that the other machine is a colleague eventually learns the same lesson.

~ the last line of code ~

Linux carried a DECnet implementation for decades, mostly so that a shrinking number of people could talk to VMS machines that were never going to be replaced. Christine Caulfield at Red Hat maintained it and then marked it orphaned on 18 february 2010, and after that it sat in the tree receiving nothing but the occasional janitorial fix from people who had never used it and never would.

On 31 july 2022 Stephen Hemminger posted the patch that ended it, with a note that reads like an epitaph somebody wrote without meaning to: "Decnet is an obsolete network protocol that receives more attention from kernel janitors than users. It belongs in computer protocol history museum not in Linux kernel." The removal landed on 22 august 2022 and took roughly twelve thousand lines with it. Nobody who was still running DECnet was running it on a current Linux kernel, so nothing broke. A protocol dies twice, once when people stop choosing it and once when somebody finally deletes the code, and for DECnet those two events are thirty years apart.

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