161 lines
6.2 KiB
Text
161 lines
6.2 KiB
Text
charset negotiation vs mtts: why tintin++ says WILL then REJECTED
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=================================================================
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the problem
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-----------
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when connecting to a telnetlib3 server, there's a 4-second delay before the
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shell starts. the server is waiting for encoding to resolve. it never does.
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what the logs show
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------------------
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the full option negotiation between telnetlib3 and tintin++ takes about 500ms.
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everything resolves quickly - TTYPE, SGA, NAWS, NEW_ENVIRON. but CHARSET
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negotiation goes like this:
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26.220 server sends DO CHARSET
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26.332 tintin responds WILL CHARSET ("i can do charset negotiation")
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26.332 server sends SB CHARSET REQUEST UTF-8 UTF-16 LATIN1 ... US-ASCII
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26.442 tintin responds SB CHARSET REJECTED
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tintin++ said "i support charset negotiation" and then rejected a list of 16
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charsets including UTF-8 and US-ASCII. every charset the server knows about.
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the server marks encoding as unresolved, waits for connect_maxwait (4.0s),
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gives up, falls back to US-ASCII, and finally starts the shell.
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30.122 server "encoding failed after 4.00s: US-ASCII"
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30.122 server "negotiation complete after 4.00s."
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so the actual work is done in 500ms. the remaining 3.5 seconds is dead air.
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but tintin++ already told us the answer
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----------------------------------------
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in the same connection, tintin++ sent this through NEW_ENVIRON:
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CHARSET=ASCII
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CLIENT_NAME=TinTin++
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CLIENT_VERSION=2.02.61
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MTTS=2825
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TERMINAL_TYPE=tmux-256color
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and through TTYPE cycling (3 rounds):
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ttype1: TINTIN++
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ttype2: tmux-256color
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ttype3: MTTS 2825
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MTTS is the MUD Terminal Type Standard, a bitmask the MUD community created
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for advertising client capabilities. 2825 in binary:
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2825 = 2048 + 512 + 256 + 8 + 1
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bit 0 (1) = ANSI color
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bit 3 (8) = UTF-8
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bit 8 (256) = 256 colors
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bit 9 (512) = OSC color palette
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bit 11 (2048) = true color
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bit 3 is set. tintin++ explicitly advertises UTF-8 support through MTTS.
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two standards, one winner
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-------------------------
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RFC 2066 CHARSET (1997) is a proper telnet standard. it defines a full
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negotiation dance:
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1. one side sends DO CHARSET, other responds WILL CHARSET
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2. requester sends SB CHARSET REQUEST <sep> <charset-1> <sep> <charset-2> ...
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3. responder either ACCEPTED <charset> or REJECTED
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this is thorough but cumbersome. the MUD community needed something simpler.
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MTTS piggybacks on the TTYPE mechanism that already existed. instead of adding
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a new option negotiation, it uses the third round of TTYPE cycling to send a
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bitfield encoding all client capabilities in a single integer. no extra
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round-trips, no subnegotiation, works with any server that already does TTYPE.
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round 1: client name "TINTIN++"
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round 2: terminal type "tmux-256color"
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round 3: MTTS bitfield "MTTS 2825"
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the MUD world settled on MTTS. clients implement it properly. RFC 2066 CHARSET
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gets a polite WILL ("sure i know what that option is") followed by REJECTED
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("but i don't actually want to use it"). tintin++ is not alone in this - most
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MUD clients behave the same way.
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what telnetlib3 could do better
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-------------------------------
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right now telnetlib3 waits for CHARSET to resolve encoding. when it gets
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REJECTED, it has no fallback path except running out the clock. but by the time
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CHARSET is rejected (26.442), the server already has:
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- MTTS=2825 via NEW_ENVIRON (26.595) or will have it momentarily
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- MTTS 2825 via TTYPE round 3 (26.595)
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if the server checked MTTS bit 3 after CHARSET rejection, it could resolve
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encoding to UTF-8 immediately and start the shell in under 600ms instead of
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waiting the full 4 seconds.
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this would be a good upstream contribution to telnetlib3: when CHARSET is
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rejected, check MTTS flags for encoding hints before falling back to the
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timeout.
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the full annotated session
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--------------------------
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timestamps are seconds within the connection. total negotiation takes ~477ms.
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the 4-second mark is the connect_maxwait timeout.
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+0.000 server Connection from <Peer 127.0.0.1 51910>
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-- phase 1: TTYPE discovery --
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+0.000 server send IAC DO TTYPE
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+0.101 tintin recv IAC WILL TTYPE
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+0.101 server send IAC SB TTYPE SEND IAC SE (ask for ttype1)
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-- phase 2: advanced negotiation begins after WILL TTYPE --
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+0.102 server send IAC WILL SGA
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+0.102 server send IAC WILL BINARY
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+0.102 server send IAC DO NAWS
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+0.102 server send IAC DO CHARSET
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+0.212 tintin recv IAC SB TTYPE IS 'TINTIN++' (ttype1: client name)
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+0.212 server (recognizes MUD client, skips WILL ECHO)
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+0.213 server send IAC DO NEW_ENVIRON
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+0.213 server send IAC SB TTYPE SEND IAC SE (ask for ttype2)
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+0.213 tintin recv IAC DO SGA -> local_option[SGA] = True
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+0.214 tintin recv IAC DONT BINARY -> local_option[BINARY] = False
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+0.214 tintin recv IAC WILL NAWS
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+0.214 tintin recv IAC SB NAWS (cols=89, rows=56) (terminal size!)
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+0.214 tintin recv IAC WILL CHARSET
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+0.214 server send IAC WILL CHARSET (reciprocating)
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+0.214 server send IAC SB CHARSET REQUEST UTF-8 ... US-ASCII IAC SE
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-- charset rejected, but negotiation continues --
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+0.323 tintin recv IAC WILL NEW_ENVIRON
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+0.323 server send IAC SB NEW_ENVIRON SEND ...
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+0.323 tintin recv IAC SB TTYPE IS 'tmux-256color' (ttype2: terminal)
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+0.324 server send IAC SB TTYPE SEND IAC SE (ask for ttype3)
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+0.324 tintin recv IAC SB CHARSET REJECTED *** rejected ***
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+0.434 tintin recv NEW_ENVIRON IS: CHARSET=ASCII
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+0.477 tintin recv NEW_ENVIRON IS: CLIENT_NAME=TinTin++
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+0.477 tintin recv NEW_ENVIRON IS: CLIENT_VERSION=2.02.61
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+0.477 tintin recv NEW_ENVIRON IS: MTTS=2825 *** utf-8 is here ***
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+0.477 tintin recv NEW_ENVIRON IS: TERMINAL_TYPE=tmux-256color
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+0.477 tintin recv IAC SB TTYPE IS 'MTTS 2825' (ttype3: capability bits)
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-- all options resolved, but encoding still "pending" --
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+4.004 server "encoding failed after 4.00s: US-ASCII"
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+4.004 server "negotiation complete after 4.00s."
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-- shell finally starts, 3.5 seconds of dead air later --
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