NetworkDesign

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!!!UDP Packet Format Last updated: 2003/08/26

The Network design is based on Messages being transmitted using UDP Packets. There are two types of streams, one is Server to Client and another one that is Client to Server.

!!UDP Client to Server communication stream Each message is self contained in a SINGLE UDP Packet. There is no mechanism for recovery of lost or repeated messages.

Each UDP Packet is composed of:

  • Protocol version ( 1 byte )
  • Type of Message ( 1 byte )
  • Client ID ( 4 bytes )
  • Rest of Message ( up to 1494 bytes )

!!UDP Server to Client communication stream Each message is contained in one or more UDP Packets, and the UDP Packet format is as follows: %%%1st UDP Packet is composed of:

  • Total number of packets (1 byte)
  • Position of this message (1 byte)
  • Signature of the message (1 byte)
  • Protocol version (1 byte)
  • Type of Message (1 byte)
  • Client ID (4 bytes)
  • Rest of Message (up to 1491 bytes)

The following UDP Packets are composed of:

  • Total number of packets (1 byte)
  • Position of this message (1 byte)
  • Signature of the message (1 byte)
  • Rest of Message (up to 1497 bytes)

The idea is to send the serialized message inside several UDP Packets and then rebuild the full message on the Client. There is no kind of error recovery, or retransmission.

Messages are sent from the Server by simply serializing them. So the message itself contains the protocol version, the type of message and the Client id.

Receiving the message is a bit more complex. First we need to determine that we are running a compatible version of the protocol. Once we have agreed that the protocol is compatible we read the type of message and we ask a ~MessageFactory ( "Java Design Patterns" ) to build an object of that type with the coresponding data. Once the message is build we simply store it in a queue of incoming messages waiting to be processed.

<verbatim> Thread Read 

 {
 receive UDP

 if(Packet[0]==PROTOCOL_VERSION)
   {
   push MessageFactory.getMessage(Packet[1])
   }
 }

Thread Write 

 {
 send UDP
 }

</verbatim>

!!!Message Types Last updated: 2003/09/11

To communicate, the Client and Server of Marauroa use a stream of UDP packets. The message system belongs to the marauroa.net package, so refer to code.

We have Client to Server, aka C2S, and Server to Client, aka S2C, messages. Most messages have two versions of themselves: the C2S and S2C versions.

Communication Model 

Client                   Server
------ 	                 ------
--onConnect:
Send C2S Login           If C2S Login is correct
                           Send S2C Login ACK
                         else
                           Send S2C Login NACK
                         end if

                         Send S2C Character List

                         Send S2C Server Info

Send Choose Character    If character is correct
                           Send Choose Character ACK
                         else
                           Send Choose Character NACK
                         Finish

--onTimer:
Send Perception ACK      Send Perception

--on Game Start:
Send Map Hash            Send Map

--onEvent:
Send Action              Send Action ACK

--onExit:
Send Logout              If Client can logout
                           Send Logout ACK
                         else
                           Send Logout NAC

Let's discuss in detail each message type.

!!Message C2S Login The login Message is sent from the Client to the Server to request the right to access to the game. The message is composed of a username and password. The Username is a string that is already registered on Server as the name of a user account, and password is a string that is associated with that account. If the username/password combination is correct then Server must send a Login ACK Message to indicate to the Client that the message has been correctly processed. On the contrary, if the username/password is wrong the Server will send a Login NACK.

!!Message S2C Login ACK The LoginACK Message is sent from the Server to the Client to indicate that the login process has been completed correctly and that it can proceed to the next step. It has no params.

!!Message S2C Login NACK The LoginNACK Message is sent from the Server to the Client to tell the Client that its login message is incorrect because either the username or password is wrong or because Server has decided to reject the login request, e.g. if the Server is full or because the IP is banned.

The message is composed of: Message indicating the resolution.

!!Message S2C Character List The ~CharacterList Message is sent from the Server to the Client to give the Client a choice of character to play with. This feature models the behavior of having several characters associated with a single account.

The message is composed of: A List of Character names

Each character name must be unique at Server level, and it is assigned when the character is created.

!!Message S2C Server Info The Server Info Message is send from the Server to the Client to tell the Client about what Server is running, and how to inform Server administrator of problems.

The message is composed of: A List of strings of the type "attribute=value" !!Message C2S Choose Character The ~ChooseCharacter Message is sent from the Client to the Server to indicate what character the user wishes to play the game with.

The message is composed of: the name of the character

The name of the character must be one of the names listed in the Character List Message or the Server will reply with a Choose Character NACK message. Once the character has been chosen and the acknoledge has been recieved the Client is ready to start the game.

The Server will reply to this message with a Choose Character ACK or a Choose Character NACK message. The Client must have already logged in before receiving this message, if not the Server will send a Choose Character NACK.

Once a character is chosen, the only way to choose a new one is by logging out and logging in again.

!!Message S2C Choose Character ACK The Choose Character ACK Message is sent from the Server to the Client to notify the Client that the character has been chosen. The message is composed of: the object.ID of the character

We need this value to track our own character.

!!Message S2C Choose Character NACK The Choose Character NACK Message is sent from the Server to the Client to notify the Client that the character has not been chosen. This also implies to the Client that it should send another ~ChooseCharacter Message.

It has no params.

!!Message C2S Logout The Logout Message is sent from the Client to the Server to indicate that it wants to finish the session. It has no params.

If the Client can logout the Server will send a Logout ACK Message and the Client can safely close the connection, but if the Server sends a Logout NACK Message, the Client should keep playing as disconnecting at that moment will leave its avatar in the game uncontrolled ( idle ).

!!Message S2C Logout ACK The Logout ACK Message is sent from the Server to the Client to notify the Client that the logout has been accepted.

It has no params.

!!Message S2C Logout NACK The Logout NACK Message is sent from the Server to the Client to tell the Client that the logout has not been accepted, possibly because something related to the RP entity, for example, combat is in progress.

It has no params.

!!Message C2S Action The Action message is sent from the Client to the Server to notify the Server of the Clients desire to execute an action. The message is simply composed of a single action.

!!Message S2C Action ACK The Action ACK is sent from the Server to the Client to identify that the action has been received in the Server. It doesn't acknowledge that the action has been accepted by the RP Manager, it just means that the action successfully arrived at the Server.

It has 1 param: action_id - is used to identify to which action you referred. In Marauroa you can command several actions per turn.

!!Message S2C Perception The Perception message is a message sent from the Server to the Client to notify the Client about changes to the objects that exist near it. The message is based on the idea explained in Delta perception document.

The message is composed of:

  • A type that can be DELTA or TOTAL
  • A time stamp value that will be just one unit bigger than the previous perception
  • A List of RPObject that contains added object
  • A List of RPObject that contains modified added object attributes
  • A List of RPObject that contains modified deleted object attributes
  • A List of RPObject that contains deleted object
  • A RPObject that describe the things the rest of players don't see about OUR own object.

Read the Delta perception algorithm to understand what it is for.

!!Message C2S Perception ACK The perception ACK message is sent from the Client to the Server. We use this message to inform the Server that the Client is still there and listening. It has no params.

!!Message C2S Map Hash (TODO)

!!Message S2C Map The Map message is a message sent from the Server to the Client to notify the Client about the content of the map where the game play will happen.

The message is composed of: An array of Byte containing all the raw data.

This message is totally game dependent.

!!!Network Manager Last updated: 2003/09/26

The Network Manager is our router that sends and receives messages to and from the network. The manager exposes the interfaces that allow:

  • Reading a message from the network
  • Sending a message to the network
  • Finalizing the manager

The read operation is a blocking type operation so we have two options, either Polling and Blocking. We choose Blocking because we don't want to waste CPU time Polling the network for messages, we just want to sleep until messages are available. Hence we create a Thread to read from the Network, let's call it Network Manager Read. To write messages to the network so we can safely code it simply as a method of Network Manager as write is an operation that is non blocking by nature.

The Network Manager opens a Socket from which it will receive all the messages from the network, and to where it will write all the messages to the network. Both write and read use the same Socket.

To encapsulate all this we create both the Read and Write methods as inner classes of Network Manager. The only difference between them being that Read extends the Thread class while Write is just a standard class. <verbatim> NetworkManager 

 {
 socket
 messages

 NetworkManagerRead isa Thread
   {
   read socket
   build message
   store in messages
   }

 NetworkManagerWrite
   {
   build UDP from message
   send socket
   }
 }

</verbatim> As you can see, messages are stored in a list when they are received, hence access to the list is synchronized.

Now lets get back to the interface as exposed to other objects. The Write method is immediate, just call it with the message to send, making sure that you have correctly filled ~SourceAddress and ClientID, and it will be sent to the Client. It is important to notice that as the transport is based on UDP there are no guarantees that the message is correctly sent. This situation is in the protocol itself. The Read method is blocking, when you call the Read method it either returns a message from the queue or if the queue is empty the thread blocks until one arrives. That is the basic idea of the Network Manager; however, the manager gets a bit more complex as a result of the need to support Server to Client messages that are bigger than one UDP package, the manager just sends the stream of packets once, and doesn't confirm if any of the messages are received. Please refer to UDP Packet Format for more info about it.


!!! Game Manager Last updated: 2003/09/26

The idea behind the Game Manager is to handle all the "business logic". This Manager decides how to reply to each individual message.

The logic is something like this: <verbatim> GameManager 

 {
 NetworkManager read Message

 switch(Message type)
   {
   case ...;
   }
 }

</verbatim>

So let's define the reply to each message. Before explaining how to process eahc message, let's clarify that the best way of modelling this system is using finite automates, (finite state machine) where, based on the input, we change the state we are currently in and produce an output. <verbatim> Process C2S Login ( STATE_BEGIN_LOGIN ) 

 Precondition: The state MUST be NULL

 Test if there is room for more players.
 if there is no more room
   {
   reply S2C Login NACK( SERVER_FULL )
   state = NULL
   }

 if check username, password in database is correct
   {
   create clientid
   add PlayerEntry
   notify database

   reply S2C Login ACK

   get characters list of the player
   reply S2C CharacterList

   state = STATE_LOGIN_COMPLETE
   }
 else
   {
   notify database

   reply S2C Login NACK( LOGIN_INCORRECT )
   state = NULL
   }

 Postcondition: The state MUST be NULL or STATE_LOGIN_COMPLETE
   and a we have created a PlayerEntry for this player with an unique correct clientid.


Process C2S ChooseCharacter ( STATE_LOGIN_COMPLETE ) 

 Precondition: The state MUST be STATE_LOGIN_COMPLETE

 if character exists in database
   {
   add character to Player's PlayerEntry
   add character to game
   reply S2C Choose Character ACK

   state = STATE_GAME_BEGIN
   }
 else
   {
   reply S2C Choose Character NACK
   state = STATE_LOGIN_COMPLETE
   }

 Postcondition: The state MUST be STATE_GAME_BEGIN and the PlayerStructure
   should be completely filled or if the character choise was wrong the state is STATE_LOGIN_COMPLETE


Process C2S Logout ( STATE_GAME_END ) 

 Precondition: The state can be anything but STATE_LOGIN_BEGIN

 if( rpEngine allows player to logout )
   {
   reply S2C Logout ACK
   state = NULL

   store character in database
   remove character from game
   delete PlayerEntry
   }
 else
   {
   reply S2C Logout NACK
   }

 Postcondition: Either the same as the input state or the state currently in

Process C2S Perception ACK 

 Precondition: The state must be STATE_LOGIN_BEGIN

 notify that the player received the perception.

 Postcondition: The state is STATE_LOGIN_BEGIN and Timestamp field in
 PlayerContainer is updated.

</verbatim>

!!!Versioning and port numbering Last updated: 2004/05/24

A problem arises when we need to have two versions of Marauroa on the same machine. This happens for example when you run a release version and a development version on the same machine.

So to avoid port collisions we will use different ports for each release. The release version will run on the standard game port bu the development version will run on the (standard game port)*10

For example: release version of gladiators runs at 3214 development version runs at 32140

To allow different servers of Marauroa to run on the same machine running different games we will assign an unique port number to each game.

gladiators runs at 3214 mapacman runs at 3215

Feel free to follow or disgregard these port rules if you wish. They may not apply to you.

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