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The design of both Windows 9x and Windows NT (before NT4) was based on the assumption that the OS/kernel is driving a single local graphical console session (this is visible today: Windows Server 2022’s “Core” installation mode still fires-up a desktop environment with a mouse pointer and floating windows rather than Linux’ text-mode-only environment); for example, huge parts of NT3.1/3.5 (especially USER32, GDI, etc) assumed exclusive ownership of graphics hardware.

While NT3.1 was a true “multiuser, multitasking” OS, I understand that largely concerns security and (Correct me if I'm wrong) that it wasn’t until NT4 that processes were grouped into "sessions" - along with NT4's own Terminal Server SKU which came about from MS licensing Citrix’ IP back from them.

So how did Citrix-for-Windows NT 3.1/3.5 work? How was it able to support multiple concurrent desktop sessions?

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    Source-level modification of NT, as far as I understand it.
    – dave
    Commented Nov 27, 2023 at 13:45
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    MultiWin (and ICA) is/was the core technology that made it work. Microsoft licensed MultiWin from Citrix to make Terminal Services in NT4. Prior to that, MultiWin (from Ed Iacobucci founder of Citrus/Citrix) was integrated into (a licensed(? - I can't quite remember) copy of) NT3.5(.1) to make WinFrame, which was sold by Citrix. Here's a fairly decent summary of all the various terms, if you follow the links. Disclaimer: I worked for Citrix from the NT3.5 period onwards.
    – Greenonline
    Commented Nov 27, 2023 at 22:28
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    Actually, IIRC, Microsoft happily licensed MultiWin for NT3.x, and then "reverse-engineered" (or something like that) MultiWin to make Terminal Services for themselves for NT4, to cut Citrix out of the loop, so Citrix then produced MetaFrame to "add value" to NT4 in order to stay in the game - something like that, memory is hazy. Later, when Longhorn was announced there was another worry about it being a threat to Citrix's marketplace - Again, I can't quite remember what feature that was exactly. I could jog my memory if I dug out and looked up some old files/emails.
    – Greenonline
    Commented Nov 27, 2023 at 23:04
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    For WinFrame, Citrix had the licensed the NT 3.x source code bundled with the MultiWin (and added the possibility of more concurrent users than just standard NT 3.x (which might have been just two?), I think). Should I put all that into an answer, or is it too historical and doesn't answer the question..?
    – Greenonline
    Commented Nov 27, 2023 at 23:11
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    @Greenonline Windows 2000 and XP had RDP servers too - and XP had fast-user-switching; concurrent sessions have always been disabled in consumer editions of Windows even through to today - excepting Media Center Extenders and “loopback sessions” in Windows 8 which were added for the “Device Simulator” in VS2012.
    – Dai
    Commented Nov 29, 2023 at 7:41

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There wasn't one.

Citrix WinFrame began with NT 3.5 and 3.51, I believe.

There is some info on how it works on ToastyTech: http://toastytech.com/guis/remotecitrix.html

The earlier version ran on OS/2 1.2: https://winworldpc.com/product/citrix-multiuser/10

There's some of the history here: https://www.basvankaam.com/inside-citrix-chapter-nineteen-the-one-with-the-ica-hdx-protocol/

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