[3] W3CLIST にも掲載されています。

[1] tcp://host.dom:21

Appendix 1 - Values http://www.rebol.com/docs/core23/rebolcore-16.html#section-2.13.2

[2]

FFmpeg Protocols Documentation (2017-01-22 02:22:36 +09:00) https://ffmpeg.org/ffmpeg-protocols.html#tcp

Transmission Control Protocol.
The required syntax for a TCP url is:
tcp://hostname:port[?options]
options contains a list of &-separated options of the form key=val.