Version: 2017.3 (switch to 2017.4)
Host Migration
Setting up Unity Multiplayer
Other Versions

Using the Transport Layer API

In addition to the High Level networking API that we provide - which provides easy-to-use systems for managing your players, networked GameObjects, and other common requirements - we also give access to a lower level API called the “Transport Layer”. This provides you with the ability to build your own networking systems at a lower level, which can be useful if you have more specific or advanced requirements for your game’s networking.

The Transport Layer is a thin layer working on top of the operating system’s sockets-based networking. It’s capable of sending and receiving messages represented as arrays of bytes, and offers a number of different “quality of service” options to suit different scenarios. It is focused on flexibility and performance, and exposes an API within the UnityEngine.Networking.NetworkTransport class.

The Transport Layer supports base services for network communication. These base services include:

  • Establishing Connections
  • Communicating using a variety of “quality of services”
  • Flow control
  • Base statistics
  • Additional services like communication via relay server or local discovery

The Transport Layer can use two protocols: UDP for generic communications, and WebSockets for WebGL. To use the Transport Layer directly, the typical workflow would be as follows:

  1. Initialize the Network Transport Layer
  2. Configure network topology
  3. Create Host
  4. Start communication (handling connections and sending/receiving messages)
  5. Shut down library after use.

Initializing the Network Transport Layer

When initializing the Network Transport Layer, you can choose between the default initialization, with no arguments, or you can provide parameters which control the overall behaviour of the network layer, such as the maximum packet size and the thread timout limit.

    // Initializing the Transport Layer with no arguments (default settings)
    // An example of initializing the Transport Layer with custom settings
    GlobalConfig gConfig = new GlobalConfig();
    gConfig.MaxPacketSize = 500;

In the 2nd example above, the Transport Layer is initialized with a custom “MaxPacketSize” value specified of 500. Custom Init values should only be used if you have an unusual networking environment and are familiar with the specific settings you need. As a rule of thumb, if you are developing a typical multiplayer game designed to be played across the internet, the default Init() settings with no arguments should be appropriate.


The next step is configuration of connection between peers. You may want to define several communication channels, each with a different quality of service level specified to suit the specific types of messages that you want to send, and their relative importance within your game.

    ConnectionConfig config = new ConnectionConfig();
    int myReiliableChannelId  = config.AddChannel(QosType.Reliable);
    int myUnreliableChannelId = config.AddChannel(QosType.Unreliable);

In the example above, we define two communication channels with different quality of service values. “QosType.Reliable” will deliver message and assure that the message is delivered, while “QosType.Unreliable” will send message without any assurance, but will do this faster.

It’s also possible to specify configuration settings specifically for each connection, by adjusting properties on the ConnectionConfig object. However, when making a connection from one client to another, the settings should be the same for both connected peers or the connection will fail with a CRCMismatch error.


The final step of network configuration is topology definition. Network topology defines how many connections allowed and what connection configuration will used:

HostTopology topology = new HostTopology(config, 10);

Here we created a topology that allows up to 10 connections, each of them are configured by the parameters defined in previous step.

Host creating

As all preliminary steps are done, we can create the host (open socket):

int hostId = NetworkTransport.AddHost(topology, 8888);

Here we add a new host on port 8888 and any ip addresses. This host will support up to 10 connections, and each connection will have parameters as we defined in config object.


When the host is created, we can start our communication. To do this we send different commands to the host and check its status. There are 3 main commands that we can send:

connectionId = NetworkTransport.Connect(hostId, "", 8888, 0, out error);
NetworkTransport.Disconnect(hostId, connectionId, out error);
NetworkTransport.Send(hostId, connectionId, myReiliableChannelId, buffer, bufferLength,  out error);
  1. The first command will send a connect request to the peer with ip “” and port 8888. It will return the id assigned to this connection.
  2. The second will send a disconnect request,
  3. The third will send a message to the connection with the id that equals the connectionId, using the reliable channel with the id that equals myReiliableChannelId. The message should be stored in buffer[] and the length of the message should be defined by bufferLength.

For checking the host status you can use two functions:

NetworkTransport.Receive(out recHostId, out connectionId, out channelId, recBuffer, bufferSize, out dataSize, out error);
NetworkTransport.ReceiveFromHost(recHostId, out connectionId, out channelId, recBuffer, bufferSize, out dataSize, out error);

Both of them returns an event. The first function returns events from any host (and return host id via recHostId). The second form checks the host with id recHostId. You can use any of these functions inside the Update() method:

void Update()
    int recHostId; 
    int connectionId; 
    int channelId; 
    byte[] recBuffer = new byte[1024]; 
    int bufferSize = 1024;
    int dataSize;
    byte error;
    NetworkEventType recData = NetworkTransport.Receive(out recHostId, out connectionId, out channelId, recBuffer, bufferSize, out dataSize, out error);
    switch (recData)
        case NetworkEventType.Nothing:         //1
        case NetworkEventType.ConnectEvent:    //2
        case NetworkEventType.DataEvent:       //3
        case NetworkEventType.DisconnectEvent: //4
  • Point 1: No interesting events have been returned.
  • Point 2: Connection event come in. It can be new connection, or it can be response on previous connect command:
myConnectionId = NetworkTransport.Connect(hostId, "", 8888, 0, out error);
NetworkEventType recData = NetworkTransport.Receive(out recHostId, out connectionId, out channelId, recBuffer, bufferSize, out dataSize, out error);
switch (recData)
    case NetworkEventType.ConnectEvent: 
        if(myConnectionId == connectionId)
            //my active connect request approved
            //somebody else connect to me
  • Point 3: Data received. In this case recHostId will define a host, connectionId will define a connection, channelId will define a channel, and dataSize will define the size of the received data. If recBuffer is big enough to contain data, data will be copied in the buffer. If not, error will contain MessageToLong error and you will need to reallocate the buffer and call this function again.
  • Point 4: Disconnection signal come in. It can be a signal that established connection has been disconnected or that your connect request failed.
myConnectionId = NetworkTransport.Connect(hostId, "", 8888, 0, out error);
NetworkEventType recData = NetworkTransport.Receive(out recHostId, out connectionId, out channelId, recBuffer, bufferSize, out dataSize, out error);
switch (recData)
    case NetworkEventType. DisconnectEvent: 
        if(myConnectionId == connectionId)
            //cannot connect by some reason see error
            //one of the established connection has been disconnected

WebGL support

WebSocket on client is supported. For the client side, all the steps described above (including topology and configuration) should be the same. Web clients can connect to the server only, where the server is a standalone player (Win, Mac or Linux only). On the server you should call

NetworkTransport.AddWebsocketHost(topology, port, ip);

Where ip address is listening address, you can pass null as ip address in this case host will listen all network interfaces. Server can support only one Websocket Host and in the same time, it can handle generic hosts too:

NetworkTransport.AddWebsocketHost(topology, 8887, null);
NetworkTransport.AddHost(topology, 8888);

Will open tcp socket handling web socket protocol on port 8887 and udp socket handling genetic protocol on port 8888.

Did you find this page useful? Please give it a rating:

Host Migration
Setting up Unity Multiplayer
Copyright © 2023 Unity Technologies
优美缔软件(上海)有限公司 版权所有
"Unity"、Unity 徽标及其他 Unity 商标是 Unity Technologies 或其附属机构在美国及其他地区的商标或注册商标。其他名称或品牌是其各自所有者的商标。