NetworkDesign: Difference between revisions

The idea behind arianne network protocol is to use a single stream of UDP packets to comunicate server and clients, to allow the different kind of actions we create different types of messages that are then interpreted at the opposite peer.

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.

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 )

Each message is contained in one or more UDP Packets, and the UDP Packet format is as follows: <br>

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.

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 Message Factory ( "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.

*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.

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.

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

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.

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 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

!!Message S2C Action ACK

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.

*A type that can be DELTA or TOTAL

*A string indicating the name of the zone the perception is related to.

*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.

The TransferREQ message is a message sent from the Server to the Client to notify the Client about the request of the server to send content useful for the game where play will happen.

The message is composed of: An array of TransferContent objects containing all the name of each resource, its timestamp and if the resource is cacheable or not.

=Versioning and port numbering=