When starting TLS over HTTP/1.1 the

"application_layer_protocol_negotiation (16)": {
  [http/1.1]
}

ALPN extension must be present in SSL ClientHello handshake message.

When starting TLS over HTTP/1.1 with upgrade request to HTTP2 the

"application_layer_protocol_negotiation (16)": {
  [h2, http/1.1]
}

ALPN extension must be present in SSL ClientHello handshake message.

In the past Undertow client was either sending no application_layer_protocol_negotiation ALPN extension when doing SSL over HTTP 1.1 or

was sending just h2 value in application_layer_protocol_negotiation ALPN extension when doing SSL over HTTP 1.1 with HTTP2 upgrade request.

This was causing SSL handshaking issues with servers / proxies that were configured to operate on HTTP 1.1 only because SSL protocol on client side

was complaining when server responded with "application_layer_protocol_negotiation (16)": {
    [http/1.1]
  }

on client handshake message that was containing just h2 value in application_layer_protocol_negotiation ALPN extension header.