          Rivendell Interprocess Communication Protocol

This defines the IP protocol used for communication between different
modules of Rivendell and the 'ripcd' daemon.

Connection to ripcd is by means of a TCP SOCK_STREAM connection to TCP
port 5006.  The format of a message is as follows:

     <cmd>[<arg>][...]!

where:

<cmd> is a two-letter code indicating the nature of the message.
<arg> is zero or more arguments.
! is an ASCII character 0x21.


UNPRIVILEDGED COMMANDS -- no authentication is required to execute these.

DC! -- DROP CONNECTION
End the session and drop the TCP connection.

PW <passwd>! -- SEND PASSWORD
Send a password to authenticate the connection.  Ripcd will respond
with 'PW +!' or 'PW -!', indicating the success or failure of the
authentication.


PRIVILEDGED - a connection must be authenticated before these can be
executed.

RU! -- Request User
Request the LOGIN_NAME of the user currently logged in.
Ripcd will respond with RU <user>!.

SU <user>! -- SET USER
Log <user> in.  Ripcd will respond with RU <user>!

MS <ip-addr> <echo> <rml-cmd>!
Send an RML command to the specified destination.

ME <ip-addr> <echo> <rml-echo>!
Send an RML echo to the specified destination.

GC <matrix>!
Return the current GPI line cart number for matrix <matrix>.
Return for each line:
  GC <matrix> <gpi-line> <cart-num>!
  Indicates a change in a GPI line cart number (receive only).

GD <matrix>!
Return the current GPO line cart number for matrix <matrix>.
Return for each line:
  GD <matrix> <gpo-line> <cart-num>!
  Indicates a change in a GPO line cart number (receive only).

GI <matrix>!
Return the current GPI line states for matrix <matrix>.
Return for each line:
  GI <matrix> <gpi-line> <gpi-state> <mask-state>!
  Indicates a change in a GPI line status (receive only).

GO <matrix>!
Return the current GPO line states for matrix <matrix>.
Return for each line:
  GO <matrix> <gpo-line> <gpo-state> <mask-state>!
  Indicates a change in a GPO line status (receive only).

GM <matrix>!
Return the current GPI line mask states for matrix <matrix>.
Return for each line:
  GM <matrix> <gpi-line> <state>!
  Indicates a change in a GPI line mask status (receive only).

GN <matrix>!
Return the current GPO line mask states for matrix <matrix>.
Return for each line:
  GN <matrix> <gpo-line> <state>!
  Indicates a change in a GPO line mask status (receive only).

RG!
Reload the GPI table.

TA!
Request the current state of the On Air flag.
Ripcd will respond with TA 0|1!, with 0 = Off and 1 = On.
