alt Hacker NewsTCP has an "urgent data" feature that might have been used for this kind of thing, used for Ctrl-C in telnet, etc. It can be used to bypass any pending send buffer and received by the server ahead of any unread data.
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ZiiS • today at 11:14 AM
Unfortunately the can be many buffers between you and the server which "urgent data" doesn't skip by design. (the were also lots of implementation problems)
Fun fact: Oracle implements cancellation this way.
The downside is that sometimes connections are proxied in ways that lose these unusual packets. Looking at you, Docker...