PacketHandshake

Singleton implementing handshake over UDP.

The point of the handshake itself is to confirm two-way connection between two parties - i.e. both parties can receive message from the other and receive acknowledgement from the other that their messages have arrived.

This is an important step before establishing connection for actual game play, as this lets both the client's and server's routers ( if any ) know that traffic is expected and should be let through.

NAT punchthrough

Most players are behind a router. Routers are directly connected to the internet, and protect machines behind them from unwanted traffic.

When routers receive packets from an unknown source, those packets are rejected and don't reach the player's device. When data was sent to that address first, routers see traffic as a reply and allow incoming data.

To take an example, if a random PC starts sending traffic your way, the router will reject it. If you send data to the host behind godotengine.org, your router will allow incoming traffic from it. Otherwise, you wouldn't be able to open the website in your browser, as the incoming HTTP response would be rejected.

This can be used to our advantage. If both players start sending traffic towards eachother, eventually the routers will assume it's a response to some request and allow the traffic.

This is a very simplified description of how routers work. NAT punchthrough does not always work. For further reading, see Network address translation.

Handshake process

To confirm two-way connectivity, a string is sent back and forth, encoding the player's knowledge about the connection:

The Read flag is set once we have received data from the other player.

The Write flag is set once we send data to the other player. Since data is always sent, this flag is always set.

The Duplex flag is set when we have received data from the other player knowing that they have also received data from us. This means that data flows both ways.

The handshake process is successful when both players have the Duplex flag set and both players know that the other player has the Duplex flag set.

Each flag is encoded as its specific character or a hyphen. The encoded string is prepended with a dollar sign. For example:

• $rw- means that we have sent and received data from the other player
• $rwx means that the read, write, and duplex flags are all set

Here's the handshake process illustrated:

Handshake over PacketPeer

To run the handshake over raw UDP, call PacketHandshake.over_packet_peer. The specified PacketPeer will be used to send data until two-way connectivity is confirmed or the timeout is reached. Between every packet sent, it takes a short pause.

Handshake over ENetConnection

If the game is already running, the handshake must be done over the already active connection. For this case, use PacketHandshake.over_enet.

This connection can't be used to receive custom packets, only to send them. So the target address will be spammed with traffic confirming two-way connectivity until timeout. Handshake will always be considered successful.

If the connectivity exists, players will simply connect. Otherwise, connectivity will fail as expected, regardless of the handshake results.