Telnet negotiation

The game uses libtelnet for Telnet command framing and option negotiation. A new connection negotiates terminal type (TTYPE), window size (NAWS), UTF-8 character set, server status (MSSP), output compression (MCCP2), GMCP, and the RFC 1572 NEW-ENVIRON option.

TTYPE and MTTS

The server requests the Telnet Terminal-Type option and sends up to three TTYPE SEND sub-negotiations after the client agrees. This follows the MUD Terminal Type Standard (MTTS) convention: the first response names the MUD client, the second identifies its terminal emulation, and the third can be an MTTS <bitvector> capability report.

Terminal type and MTTS select the connection’s color depth. DUMB disables color, XTERM and terminal names containing 256COLOR select 256 colors, and names containing TRUECOLOR select truecolor. From an MTTS bitvector the server recognizes ANSI, 256-color, truecolor, and screen-reader capabilities. A screen-reader report suppresses styled output unless the player explicitly overrides the color setting for that session and has the persistent ANSI flag enabled. Before negotiation, the server assumes vt100 with 16-color ANSI support.

NAWS

The server requests Negotiate About Window Size (NAWS). Each valid four-byte sub-negotiation replaces the connection’s width and height with the two unsigned 16-bit values supplied by the client. The current dimensions are available to server-side features and to the @telnet diagnostic. The defaults are 80 columns by 25 rows; they are restored if the client declines or later disables NAWS.

CHARSET

The server offers the CHARSET option and requests UTF-8 as its only choice. It also accepts a client-initiated CHARSET request when UTF-8 appears among the offered values, and rejects requests that have no supported choice. CHARSET does not enable transcoding: the game uses UTF-8 for all connections, defaults to UTF-8 when negotiation is unavailable, and rejects malformed UTF-8 command input.

MSSP

After a client accepts the MUD Server Status Protocol (MSSP), the server sends one status sub-negotiation containing:

The status is a snapshot produced when MSSP is enabled on the connection; the server does not stream later changes.

MCCP2

The server offers MUD Client Compression Protocol version 2 (MCCP2) for outbound traffic. When the client accepts, the server sends the empty MCCP2 start sub-negotiation and immediately begins a zlib stream. Every subsequent server-to-client byte, including later Telnet negotiations, is inside that compressed stream. Client-to-server traffic is not compressed; MCCP3 is not supported.

GMCP

The server offers the Generic MUD Communication Protocol (GMCP). Its current package support is intentionally small: when an enabled client sends Core.Ping, with or without a payload, the server replies with a bodyless Core.Ping. Other GMCP packages are ignored. In particular, clients should not assume that negotiating the GMCP Telnet option implies support for the broader collection of standard packages.

NEW-ENVIRON

After a client accepts NEW-ENVIRON, the server sends an unqualified SEND request. Under RFC 1572 this requests the client’s default environment, including both well-known VAR and user-defined USERVAR entries. The server does not request a MUD-specific extension or interpret received variables as trusted identity or authorization data.

The environment is scoped to one live descriptor and discarded at disconnect. IS and INFO messages add, replace, or remove values. VAR and USERVAR are separate namespaces, and a defined empty value remains distinct from an absent value. Storage is limited to 64 entries, 256 bytes per name, 4096 bytes per value, and 65536 bytes total per connection. Malformed or oversized messages are rejected atomically.

C code can query a descriptor with descriptor_telnet_environment_has() and descriptor_telnet_environment_get() from mux/network/telnet_environment.h. Names and values are length-delimited so escaped protocol bytes and empty values are preserved. Lua code has equivalent functions in the built-in mux package. Wizards can inspect all negotiated state with @telnet <player>; non-printable bytes are escaped in its output. The diagnostic groups each value beneath the Telnet option that supplied it.

ECHO

Telnet ECHO is negotiated dynamically rather than during initial connection setup. The server asks the client to suppress local echo while the user enters a password, then restores client-side echo when the input no longer contains a secret. This is only display protection; password input still travels over the connection and requires a transport-security layer to be confidential on the network.

OSC 8 capabilities

OSC 8 is not a Telnet option and has no separate negotiation. At output time, the styled-text renderer checks three per-descriptor NEW-ENVIRON USERVARs: OSC_HYPERLINKS for http:, https:, and ftp: links, OSC_HYPERLINKS_SEND for send:, and OSC_HYPERLINKS_PROMPT for prompt:. Each feature is enabled independently only when its variable is present with the exact one-byte value 1. An INFO update therefore affects subsequent output immediately.

Tier 2 visual styling checks OSC_HYPERLINKS_STYLE_BASIC for base colors, font styles, and decorations, and OSC_HYPERLINKS_STYLE_STATES for interactive state overrides. Each is enabled independently only for an exact 1. The renderer uses full JSON property names and appends the percent-encoded object as the link URI’s reserved config parameter.

Tier 3 checks OSC_HYPERLINKS_TOOLTIP for custom hover text and OSC_HYPERLINKS_MENU for context menus and menu titles. Menu actions also require the corresponding web, send, or prompt capability. Unsupported actions and redundant separators are filtered before the full-name JSON configuration is emitted. Styled titles additionally require OSC_HYPERLINKS_STYLE_BASIC.

Tier 4 checks OSC_HYPERLINKS_VISIBILITY, OSC_HYPERLINKS_SPOILER, and OSC_HYPERLINKS_DISABLED independently for dynamic hide/reveal behavior, spoiler text, and permanently non-interactive links. Unsupported visibility and spoiler data is omitted. A requested disabled=true link is rendered as plain text if the disabled capability is absent, preserving its non-actionable intent.

Tier 5 checks OSC_HYPERLINKS_SELECTION for client-local radio-button and checkbox state. An exact value of 1 enables the selection JSON object; without it the primary link remains active without selection behavior. Selection callbacks add or replace the selected query item on decoded send and prompt commands. Web selection remains local to the client and does not currently alter the opened URL.

Selection is independent of OSC_HYPERLINKS_STYLE_STATES, so behavior can be enabled without custom selected/disabled visuals.

Tier 6 checks OSC_HYPERLINKS_COMPACT and OSC_HYPERLINKS_PRESETS independently. Compact support selects Mudlet’s abbreviated JSON keys. Preset support causes the configured session-scoped definitions to be sent once, immediately before the first normal output after negotiation, and subsequent links reference them by name. Clients without preset support receive merged inline configuration instead.

An advertised Tier 2 through Tier 5 configuration capability, or Tier 6 compact support, reserves the config query parameter in web links, even when a particular link adds no configuration. Existing parameters with that name are percent-encoded before output. Preset support separately reserves the preset query parameter.

OSC capability checks are independent of the player ANSI flag, negotiated color depth, and MTTS screen-reader state. Connections without a corresponding capability receive the link’s visible text as a plain fallback.