charset negotiation vs mtts: why tintin++ says WILL then REJECTED
=================================================================

the problem
-----------

telnetlib3 defaults to connect_maxwait=4.0, waiting for CHARSET encoding
negotiation to resolve. MUD clients like tintin++ reject CHARSET but advertise
encoding support through MTTS instead. result: 4 seconds of dead air on every
connection.

the fix (our side)
------------------

pass connect_maxwait=0.5 to create_server(). the actual option negotiation
finishes in ~500ms. CHARSET gets rejected almost immediately but telnetlib3
doesn't have a fallback path — it just waits for the timeout. 0.5s is plenty.

    server = await telnetlib3.create_server(
        host="127.0.0.1", port=PORT, shell=shell, connect_maxwait=0.5
    )

the upstream fix (telnetlib3)
-----------------------------

when CHARSET is rejected, telnetlib3 could check MTTS bit 3 for UTF-8 support
and resolve encoding immediately instead of running out the clock. this would
be a good contribution. the data is already there:

    MTTS 2825 → bit 3 set → client supports UTF-8

what happens on the wire
------------------------

telnetlib3 sends DO CHARSET. tintin++ responds WILL CHARSET ("i know what that
is") then immediately REJECTED ("but i don't want to use it"). this is normal
for MUD clients — they prefer MTTS.

    +0.000  server  send DO CHARSET
    +0.100  client  recv WILL CHARSET
    +0.100  server  send SB CHARSET REQUEST UTF-8 ... US-ASCII
    +0.200  client  recv SB CHARSET REJECTED

meanwhile, in the same negotiation, tintin++ already told us via MTTS:

    ttype3: MTTS 2825 = 0b101100001001
        bit 0  (1)    = ANSI
        bit 1  (2)    = VT100
        bit 2  (4)    = UTF-8
        bit 3  (8)    = 256 COLORS      [SET]
        bit 4  (16)   = MOUSE TRACKING
        bit 5  (32)   = OSC COLOR PALETTE
        bit 6  (64)   = SCREEN READER
        bit 7  (128)  = PROXY
        bit 8  (256)  = TRUECOLOR       [SET]
        bit 9  (512)  = MNES            [SET]
        bit 10 (1024) = MSLP
        bit 11 (2048) = SSL             [SET]

    for MTTS 2825: bits 0, 3, 8, 9, 11 are set

NOTE: The original version of this doc had incorrect MTTS bit mappings (copied
from an unreliable source). This was corrected on 2026-02-07 to match the
actual MTTS spec from tintin.mudhalla.net/protocols/mtts/. The wrong values
caused a bug in caps.py that misinterpreted client capabilities.

two protocols, one answer. MTTS (via TTYPE round 3) is what the MUD ecosystem
uses. RFC 2066 CHARSET is technically correct but practically ignored.

will echo and mud clients
-------------------------

related discovery: telnetlib3 intentionally skips WILL ECHO for detected MUD
clients. tintin++ is in the MUD_TERMINALS fingerprint list. MUD clients
interpret WILL ECHO as "password mode" and mask input, so the server avoids
sending it. the client handles its own local echo.

this means will_echo is always False for tintin++, and that's correct. don't
try to fight it or wait for it.

see also
--------

- MTTS spec: https://tintin.mudhalla.net/protocols/mtts/
- RFC 2066: CHARSET option
- telnetlib3 source: server.py _negotiate_echo(), fingerprinting.py _is_maybe_mud()
