Port 91: MIT Dover Spooler — A Print Queue for a Printer That No Longer Exists

What Port 91 Does

Port 91 is registered with IANA as mit-dov, the MIT Dover Spooler. It was assigned for both TCP and UDP. Its job was to accept print jobs over the network and spool them to a Xerox Dover laser printer at MIT.

Nobody runs this protocol today. The printer it served has been gone for decades. But the port assignment remains in the IANA registry, exactly where it was placed in the early 1980s.

The Xerox Dover

The Xerox Dover was one of the first laser printers ever built.¹ Developed by Gary Starkweather at Xerox PARC in the late 1970s, the Dover was constructed by taking Xerox 7000 reduction copiers, stripping out the document-copying optics, and replacing them with a Raster Output Scanner, a laser unit that could paint pages from digital data. The original copying optics were returned to Xerox's copier division for credit.²

About 35 Dovers were ever manufactured.³ They printed at roughly one page per second on ordinary paper, driven by a dedicated Xerox Alto computer running a spooler program called Spruce. Files in Press format (a distant ancestor of PostScript) were sent to the spooler over Ethernet using the PUP EFTP protocol.

A Dover was installed at the MIT AI Lab around 1982, shared between the AI Lab and the Laboratory for Computer Science. It was connected to the ITS KA10 MIT-AI system and became the primary printer at Technology Square.⁴

This was a time when a laser printer was exotic, expensive, and shared across entire departments. If you wanted to print, your job went into a queue. That queue needed a network port.

The Port Assignment

The contact person listed for port 91 in RFC 1700 is Eliot Moss.⁵ J. Eliot B. Moss was an MIT student from 1975 to 1981, earning his bachelor's, master's, and doctoral degrees in computer science there.⁶ His time at MIT overlapped precisely with the Dover era.

Moss went on to become a professor at the University of Massachusetts Amherst and co-invented transactional memory with Maurice Herlihy, one of the foundational concepts in concurrent programming.⁷ But before all that, he was at MIT, and there was a printer that needed a port.

The ports surrounding 91 in the registry tell a story about this era:

These were individual people at individual institutions, registering ports for specific machines and specific needs. Port assignment was personal in the early Internet.

A Printer That Changed Everything (Indirectly)

The Dover sits at the edge of one of the most consequential moments in software history. Around 1980, Richard Stallman at the MIT AI Lab had been modifying the software for their Xerox Graphics Printer (the Dover's predecessor) so it would notify users when their print jobs finished and alert everyone if the printer jammed.⁸ When a newer Xerox printer arrived and Xerox refused to share its source code, Stallman couldn't add the same features. The frustration from that refusal became a catalyst for the Free Software movement and eventually the GNU Project.

Port 91's printer wasn't the printer that sparked free software. But it lived in the same lab, served the same people, and carried the same assumption: that the machines belong to the community that uses them.

The Dover's Legacy

The Dover was a prototype. Xerox's first commercial laser printer, the 9700, printed 120 pages per minute and became one of Xerox's best-selling products, generating billions in revenue.⁹ Every laser printer you've ever used descends from the line of machines that began with Gary Starkweather's work at PARC.

Port 91 is a fossil record of the moment when network printing was being invented, when a handful of research labs had machines that could turn bits into marks on paper at the speed of light, and when that capability was rare enough that a single printer could justify a well-known port.

How to Check What's Listening on Port 91

No legitimate service uses port 91 today. If something is listening on this port, investigate it.

Linux:

sudo ss -tlnp | grep :91
sudo lsof -i :91

macOS:

sudo lsof -i :91

Windows:

netstat -an | findstr :91