Service Discovery

The key difference between FSDLP and prior solutions is its decentralized nature. Rather than a single entity federating communications between aircraft, air traffic units, and virtual airline operations centers, the responsibility is shared between all participants.

To facilitate this, a method for service discovery is necessary, as an airborne client must know which provider clients are available and what capabilities they posess. A pre-determined port range of 60860—60867 is used to make port scanning easier for airborne clients.

Note

The port range above only allows for up to eight clients on a single machine. We believe this to be a sufficient quantity, as there are typically at most two at any given moment: a network and a virtual airline. A tigher range reduces the amount of work that clients must perform in order to complete a scan.

Identification

The service discovery flow boils down to requesting identification details from a potential provider client on one of the pre-defined ports, and using that information to determine where to route downlink messages.

Identification is performed over HTTP with the /id endpoint, over which provider clients will provide a JSON payload matching the schema provided below.

Schema

{
    "protocol": "fsdlp",
    "version": 1,
    "provider": {
        "name": "",
        "type": "",
        "features": []
    }
}

1. The name field shall be a brief identifier of the network, virtual airline, or other kind of provider. This information may be shown in virtual cockpits when prompting pilots to establish a data link connection.
2. The type field shall be either network or other to indicate the kind of provider client. This determines which client has priority when using a feature that is supported by multiple clients, as networks will have priority in such cases (learn more).
3. The features field shall be an array with with all features supported by the provider client. A list of features can be found on the TODO page. A provider client providing an empty features array may not receive any requests.

Provider Clients

Provider clients shall bind to local address 127.0.0.1 on any available port in the range 60860—60867, inclusive. Once bound, the following identification flow must be satisfied:

1. Provider client receives HTTP GET request on /id:

   GET /id HTTP/1.1

2. Provider client responds with the below JSON payload, for example

   {
       "protocol": "fsdlp",
       "version": 1,
       "provider": {
           "name": "VASTSIM",
           "type": "network",
           "features": ["a", "b", "c"]
       }
   }

Note

While not necessarily a focus of this protocol, this method does allow provider clients to discover other providers. This could potentially be useful for automatically detecting if a pilot is flying on a network and/or for a virtual airline.

Airborne Clients

TODO