binkp
A protocol for moving FidoNet mail over TCP/IP instead of phone lines.
FidoNet was built for modems: nodes called each other at night and swapped packets using protocols designed to survive line noise and dropped carriers. Once the internet took over, those protocols were needlessly chatty on an error-free link. binkp arrived in the mid-1990s alongside the binkd mailer by Dima Maloff, and leans on the fact that TCP already guarantees delivery.
The protocol is specified in FTS-1026 and listens on port 24554 by default. It sends multiple files per session, resumes interrupted transfers and supports password-protected sessions. Nearly all remaining netmail and echomail travels this way today.