While testing, if TIP 601 did anything on the channel system, I have made two tests with TCL 8.6.12 and TCL 9.0 on Windows, compiled with MS-VC6 and VS2015 for 32 bit.

When sending a character not contained in the current encoding, the replacement character "?" is sent. On 9.0, for my perception, nothing is sent and sending stops.

I started two tclsh shells with the following pasted into the console:

<verbatim>
set serverhandle [socket -server connect 30000]
proc connect {handle args} {
	if {[catch {
		fconfigure $handle -encoding iso8859-1
		puts $handle A\u2022B"
		close $handle
	} res]} {
		puts stderr "Write error: $res"
	}
}
vwait forever
</verbatim>

and the client:

<verbatim>
set clienthandle [socket localhost 30000]
fconfigure $clienthandle -encoding iso8859-1 -eofchar ""
puts "Read data: '[get $clienthandle]'"
close $clienthandle
</verbatim>

TCL 8.6.12 returns on the client side:
<verbatim>
Read data: 'A?B"'
</verbatim>

and on the TCL 9.0 client side:
<verbatim>
Read data: 'A'
</verbatim>

I think, on 9.0, the replacement "?" for the not representable character \u2022 (a centered dot) is not sent.

I am not sure, what the correct result would be. I am a bit lost in the discussion.

Take care,
Harald