UXN NETWORKING USING MULTIPLEXED IO

Each message type consists of a single ASCII mode character, then one
string of binary data. Any byte other than NUL (\0) is valid to use as
data in these protocols.

OUTPUT   CH  MESSAGE               DESCRIPTION
listen   @   {addr}/{port}\0       listen to local {addr} on {port}
open     ^   {id}/{addr}/{port}\0  label connection to remote {addr} on {port} as {id}
close    !   {id}\0                close connection to remote {id}
send     >   {id}/{data}\0         send {data} to remote {id}

INPUT    CH  MESSAGE               DESCRIPTION
started  $   {id}/{addr}/{port}\0  note connection from {addr} and {port} labelled as {id}
ended    #   {id}\0                note end of connection labelled as {id}
receive  <   {id}/{data}\0         receive {data} from remote {id}
error    *   {msg}\0               error described by {msg}

Addresses are either DNS hostnames, IPv4 addresses, or IPv6 addresses.
URLs and URIs are not allowed (e.g. / should not appear in them).

The {id} may contain any characters other than '/' or NUL (\0).
Clients initiating a connection may choose any {id} they wish. Servers
receiving a connection will have an {id} automatically-generated. The
{id} will be unique at any given time but may be reused later after a
connection is closed. Labels are not shared between the client and the
server (e.g. a client may label a connection as "game" but the server
might see the client's connection labelled as "client-3").

The maximum size allowed for any message is 4096 bytes, including the
mode character and terminating NUL. The maximum size for any {id} is 8
bytes. The maximum size of {addr} is 253 bytes (the maximum size
allowed by DNS).

NOTE: clients should be prepared to read up to 4096 bytes but are only
allowed to send up to 4080 bytes of data in > messages. This leaves
extra room for receivers with a longer {id} than used by the sender.

Messages are interpreted by a sidecar program which handles TCP/IP
networking and maintaining connections. This protocol is not designed
to be compatible with any other networking protocol.

EXAMPLE SESSION

Here's an example transcript from a hypothetical echo service.

ACTOR   ACTION  MESSAGE                EFFECT
server  writes  @127.0.0.1/9999\0      opens port 9999 at 127.0.0.1
server  reads   $/127.0.0.1/9999\0     confirms service is listening
client  writes  ^srv/127.0.0.1/9999\0  connects to server (id=srv)
client  reads   $srv/127.0.0.1/9999\0  confirms client is connected
server  reads   $001/127.0.0.1/9999\0  notified client is connected (id=001)
client  writes  >srv/hello world\0     sends "hello world" to server
server  reads   <001/hello world\0     server reads message
server  writes  >001/hello world\0     server echoes message back
client  reads   <srv/hello world\0     client reads message
client  writes  !srv\0                 client disconnects from server
client  reads   #srv\0                 client notified of disconnect
server  reads   #001\0                 server notified of disconnect