# Out-of-Band messaging¶

OOB, or Out-Of-Band, means sending data between Evennia and the user’s client without the user prompting it or necessarily being aware that it’s being passed. Common uses would be to update client health-bars, handle client button-presses or to display certain tagged text in a different window pane.

If you haven’t, you should be familiar with the Messagepath, which describes how a message enters and leaves Evennia and how along the way, all messages are converted to a generic format called a commandtuple:

(commandname, (args), {kwargs})


## Sending and receiving an OOB message¶

Sending is simple. You just use the normal msg method of the object whose session you want to send to.

    caller.msg(commandname=((args, ...), {key:value, ...}))


The keyword becomes the command-name part of the commandtuple and the value its args and kwargs parts. You can also send multiple messages of different commandnames at the same time.

A special case is the text call. It’s so common that it’s the default of the msg method. So these are equivalent:

    caller.msg("Hello")
caller.msg(text="Hello")


You don’t have to specify the full commandtuple definition. So for example, if your particular command only needs kwargs, you can skip the (args) part. Like in the text case you can skip writing the tuple if there is only one arg … and so on - the input is pretty flexible. If there are no args at all you need to give the empty tuple msg(cmdname=(,) (giving None would mean a single argument None).

### Which command-names can I send?¶

This depends on the client and protocol. If you use the Evennia webclient, you can modify it to have it support whatever command-names you like.

Many third-party MUD clients support a range of OOB protocols listed below. If a client does not support a particular OOB instruction/command, Evennia will just send the text command to them and quietly drop all other OOB instructions.

Note that a given message may go to multiple clients with different capabilities. So unless you turn off telnet completely and only rely on the webclient, you should never rely on non-text OOB messages always reaching all targets.

### Which command-names can I receive¶

This is decided by which Inputfuncs you define. You can extend Evennia’s default as you like, but adding your own functions in a module pointed to by settings.INPUT_FUNC_MODULES.

## Supported OOB protocols¶

Evennia supports clients using one of the following protocols:

### Telnet¶

By default telnet (and telnet+SSL) supports only the plain text outputcommand. Evennia detects if the Client supports one of two MUD-specific OOB extensions to the standard telnet protocol - GMCP or MSDP. Evennia supports both simultaneously and will switch to the protocol the client uses. If the client supports both, GMCP will be used.

Note that for Telnet, text has a special status as the “in-band” operation. So the text outputcommand sends the text argument directly over the wire, without going through the OOB translations described below.

#### Telnet + GMCP¶

GMCP, the Generic Mud Communication Protocol sends data on the form cmdname + JSONdata. Here the cmdname is expected to be on the form “Package.Subpackage”. There could also be additional Sub-sub packages etc. The names of these ‘packages’ and ‘subpackages’ are not that well standardized beyond what individual MUDs or companies have chosen to go with over the years. You can decide on your own package names, but here are what others are using:

Evennia will translate underscores to . and capitalize to fit the specification. So the outputcommand foo_bar will become a GMCP command-name Foo.Bar. A GMCP command “Foo.Bar” will be come foo_bar. To send a GMCP command that turns into an Evennia inputcommand without an underscore, use the Core package. So Core.Cmdname becomes just cmdname in Evennia and vice versa.

On the wire, the commandtuple

("cmdname", ("arg",), {})


will be sent over the wire as this GMCP telnet instruction

IAC SB GMCP "cmdname" "arg" IAC SE


where all the capitalized words are telnet character constants specified in ]evennia/server/portal/telnet_oob. These are parsed/added by the protocol and we don’t include these in the listings below.

commandtuple

GMCP-Command

(cmd_name, (), {})

Cmd.Name

(cmd_name, (arg,), {})

Cmd.Name arg

(cmd_na_me, (args,...),{})

Cmd.Na.Me [arg, arg...]

(cmd_name, (), {kwargs})

Cmd.Name {kwargs}

(cmdname, (arg,), {kwargs})

Core.Cmdname [[args],{kwargs}]

Since Evennia already supplies default Inputfuncs that don’t match the names expected by the most common GMCP implementations we have a few hard-coded mappings for those:

GMCP command name

commandtuple command name

"Core.Hello"

"client_options"

"Core.Supports.Get"

"client_options"

"Core.Commands.Get"

"get_inputfuncs"

"Char.Value.Get"

"get_value"

"Char.Repeat.Update"

"repeat"

"Char.Monitor.Update"

"monitor"

#### Telnet + MSDP¶

MSDP, the Mud Server Data Protocol, is a competing standard to GMCP. The MSDP protocol page specifies a range of “recommended” available MSDP command names. Evennia does not support those - since MSDP doesn’t specify a special format for its command names (like GMCP does) the client can and should just call the internal Evennia inputfunc by its actual name.

MSDP uses Telnet character constants to package various structured data over the wire. MSDP supports strings, arrays (lists) and tables (dicts). These are used to define the cmdname, args and kwargs needed. When sending MSDP for ("cmdname", ("arg",), {}) the resulting MSDP instruction will look like this:

IAC SB MSDP VAR cmdname VAL arg IAC SE


The various available MSDP constants like VAR (variable), VAL (value), ARRAYOPEN/ARRAYCLOSE and TABLEOPEN/TABLECLOSE are specified in evennia/server/portal/telnet_oob.

commandtuple

MSDP instruction

(cmdname, (), {})

VAR cmdname VAL

(cmdname, (arg,), {})

VAR cmdname VAL arg

(cmdname, (arg,...),{})

VAR cmdname VAL ARRAYOPEN VAL arg VAL arg ... ARRAYCLOSE

(cmdname, (), {kwargs})

VAR cmdname VAL TABLEOPEN VAR key VAL val ... TABLECLOSE

(cmdname, (args,...), {kwargs})

VAR cmdname VAL ARRAYOPEN VAL arg VAL arg ... ARRAYCLOSE VAR cmdname VAL TABLEOPEN VAR key VAL val ... TABLECLOSE

Observe that VAR ... VAL always identifies cmdnames, so if there are multiple arrays/dicts tagged with the same cmdname they will be appended to the args, kwargs of that inputfunc. Vice-versa, a different VAR ... VAL (outside a table) will come out as a second, different command input.

### SSH¶

SSH only supports the text input/outputcommand.

### Web client¶

Our web client uses pure JSON structures for all its communication, including text. This maps directly to the Evennia internal output/inputcommand, including eventual empty args/kwargs.

commandtuple

Evennia Webclient JSON

(cmdname, (), {})

["cmdname", [], {}]

(cmdname, (arg,), {})

["cmdname", [arg], {}]

(cmdname, (arg,...),{})

["cmdname", [arg, ...], {})

(cmdname, (), {kwargs})

["cmdname", [], {kwargs})

(cmdname, (arg,...), {kwargs})

["cmdname", [arg, ...], {kwargs})

Since JSON is native to Javascript, this becomes very easy for the webclient to handle.