on a local IISExpress all runs fine:

I recieve:

{
    "negotiateVersion": 1,
    "connectionId": "****",
    "connectionToken": "****",
    "availableTransports": [
        {
            "transport": "WebSockets",
            "transferFormats": [
                "Text",
                "Binary"
            ]
        },
        {
            "transport": "ServerSentEvents",
            "transferFormats": [
                "Text"
            ]
        },
        {
            "transport": "LongPolling",
            "transferFormats": [
                "Text",
                "Binary"
            ]
        }
    ]
}

which is followed by:

which means CORS is fine and, it seems, wss is activated.

On the production server, after activation wss for IIS, I recieve the same json followed by:

which seems to mean:

• CORS is fine,
• can’t activate wss,
• fail for ServerSentEvents,
• OK for LongPolling.

My question is what may I do to have wss activatable on IIS ?