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Command: assign-id
  Assign new ID to client, or fetch current ID

Purpose: assigning ID to clients so server can respond to that client

Compulsivity: manditory (core infrastructure)

More documentation: doc/messages

Reference implementation: mds-server

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Command: intercept
  Sign up for reception of message

Optional header: Stop
  Stop reception of messages if `yes`

Optional header: Priority
  Signed 64-bit integer of reception priority (reversed of order)

Optional header: Modifying
  Send message asynchronously and await modification if `yes`

Optional header: Length
  Length of the message

Message: list of headers and header–value-pairs that qualifies
         a message for reception, all messages qualifies if this
	 list is empty

Purpose: filter received message for clients and servers
Purpose: assigned interception order for modification of messages

Compulsivity: manditory (core infrastructure)

More documentation: doc/messages

Reference implementation: mds-server

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Command: echo
  Echo back a message

Required header: Client ID
  Your ID, provided by `ID assignment`
  in response to `Command: assign-id`
  
Optional header: Length
  Length of the message

Message: message to echo

Purpose: debugging and testing
Purpose: network heartbeat

Compulsivity: recommended for network enabled servers

Reference implementation: mds-echo

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Command: register
  Register availability of a command for which you implement a service

Required header: Client ID
  Your ID, provided by `ID assignment`
  in response to `Command: assign-id`
  
Conditionally required header: Length
  Required if: `Action: list`
  Length of the message

Optional header: Action
  Remove availability for registry if `remove`.
  Wait until listed commands are available if `wait`,
    however if a protocol becomes unavailable during this
    wait period it will still be counted as available for
    this wait action.
  Send a list of availability commands if `list`.

Message: List of values for the header `Command` that you implement

Purpose: Identify supported display server operations
Purpose: Initialisation process synchronisation

Compulsivity: highly recommended (infrastructure), programs may
	      stall a bit from time to time without it, or at
	      initialisation depending on the program's implementation

Reference implementation: mds-registry

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Command: reregister
  Request that all servers resends `Command: register` with either
  `Action: add` or without the `Action` header (does the same thing)

Purpose: Rebuild registry created with `Command: register` if the
	 registry server crashes

Compulsivity: highly recommended (infrastructure), programs may
	      think a protocol is not supported of the registry
	      server crashes if you do not implement this in your
	      server

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