The original remote execution schemes were telnet and rsh, but there is no reason at all for using either of those if you can possibly use ssh instead.
From Tcl, it's easiest to drive ssh as a subprocess, possibly with the assistance of Expect though you can do without it with the right options and the setting up of appropriate keypairs (a topic outside the scope of thise page).
Someone frequently asks a question like, "How do use I Tcl to access a program on a different machine?" These words turn out to cover a VERY wide territory, much of it with no particular relation to Tcl. An answer that satisfies the original questioner might involve: