General Description

The Multiple Transports feature allows apps connected to SDL Core to start another connection over a different transport for certain services. For example, an app connected over Bluetooth can use WiFi as a Secondary Transport for video streaming. This guide will give an overview of the process which is used to establish a Secondary Transport connection. See SDL-0141 - Supporting Simultaneous Multiple Transports for more details on the original feature proposal.

Implementation

After the proxy is connected to Core, it initiates another connection over a different transport.
Core tells the proxy which transport can be used as Secondary Transport.
The services that are allowed on the Secondary Transport are specified by Core.

Note

RPC and Hybrid services only run on the Primary Transport

There are three protocol control frames which are used in the implementation of Multiple Transports.

StartService ACK

Payload Example:

{
    ...
    "audioServiceTransports" : [1, 2],
    "videoServiceTransports" : [1, 2],
    "secondaryTransports" : ["TCP_WIFI"]
}

Core responds to the proxy's StartService request with additional parameters audioServiceTransports, videoServiceTransports and secondaryTransports.

The secondaryTransports parameter contains an array of the allowed Secondary Transports for the current Primary Transport.
audioServiceTransports and videoServiceTransports describe which services are allowed to run on which transports (Primary=1, Secondary=2, or both). The proxy uses this information and starts services only on allowed transports.
This response is constructed by Core using the configurations defined in the SDL INI file, described in this guide.
Since RPC and Hybrid services always run on Primary Transport, only Video and Audio services are configurable.

TransportEventUpdate

Payload Example:

{
    "tcpIpAddress" : "192.168.1.1",
    "tcpPort" : 12345
}

Core sends a TransportEventUpdate notification to the proxy to provide additional information required to connect over the TCP transport when it is available.

If the tcpIpAddress field is empty, the Secondary Transport is unavailable and the proxy will not send a RegisterSecondaryTransport request.

RegisterSecondaryTransport

Using the information in the StartService ACK and TransportEventUpdate frames, the proxy sends a RegisterSecondaryTransport request over the Secondary Transport with the same session ID as the Primary Transport.