Created basic protocol description

doc/protocol.txt

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+Terms
+=====
+
+Parts between < and > are mandatory. They will never contain white space and
+thus can be mentioned as words.
+
+Parts between [ and ] are optional. It can be one word, several words, or even
+missing.
+
+White space can be a space or Tab character. New line and carriage return are
+not considered as white space.
+
+<MID> is a message ID. It can contain alphanumeric characters (must match the
+regular expression [a-zA-Z0-9]+), except for server messages which are not
+generated in response to a client request, which begins with a dot (.).
+
+<RESPONSE CODE> is similar to HTTP Status codes. The first digit depends on
+the generator module. 0 is for the messages generated by RPGServ core; 1 is
+for messages generated by RPG System modules. Other numbers are currently
+reserved for future use. The second digit is the message level: 0 are
+informational, or success messages; 1 are warnings, which are recoverable,
+while 2 is for errors. 3 indicates server-side system failure, and is always
+followed by a disconnect. 8-9 are two-level debug messages. 4-7 are currently
+reserved for future use.
+
+Client side
+===========
+
+<MID> <MESSAGE SIZE> <COMMAND> [ARGUMENTS]
+
+MESSAGE SIZE contains the length of the whole message. Although white space
+characters are ignored during message parsing, MESSAGE SIZE must include the
+length of these characters, either.
+
+COMMAND is the command itself.
+
+ARGUMENTS are the (optional) command arguments. Arguments are case sensitive.
+Each word in ARGUMENTS is a separate argument, white space and newline is
+ignored between them, except the argument is wrapped between two " or '
+characters. These two have identical meaning, but can not be swapped. To put a
+plain wrapper character in a wrapped text, prefix it with a backslash.
+
+Server side
+===========
+
+<MID> <RESPONSE CODE> <PIECE-NR> <PIECE-SIZE> <MESSAGE-CHUNK>
+
+PIECE-NR is the packet number for the given message. Every message is divided
+into packets based on the server side settings. Messages that fit in one
+packet are sent with this field set to 0. Longer messages, or messages that
+are generated over time and pieces can be sent meanwhile are numbered from 1.
+The last piece of message for a given MID is always marked as LAST in the
+PIECE-NR field.
+
+PIECE-SIZE is the size of the current piece in bytes.
+
+MESSAGE-CHUNK contains the message itself, or the chunk transmitted in this
+packet. The message or message chunk doesn't have a trailing character, like a
+newline.