"CoAP ping" is acknowleged

[svenschwermer]

IsPing correctly identifying empty confirmable messages over unreliable transport as "CoAP pings" (RFC 7252).

go-coap/message/message.go

Lines 53 to 58 in 71407da

	func (r *Message) IsPing(isTCP bool) bool {
	if isTCP {
	return r.Code == codes.Ping
	}
	return r.Code == codes.Empty && r.Type == Confirmable && len(r.Token) == 0 && len(r.Options) == 0 && len(r.Payload) == 0
	}

However, the response is an acknowledgment which is incorrect. It should be a Reset Message (RST).

go-coap/udp/client/conn.go

Lines 645 to 658 in 71407da

	func (cc *Conn) sendPong(w *responsewriter.ResponseWriter[*Conn], r *pool.Message) {
	if err := w.SetResponse(codes.Empty, message.TextPlain, nil); err != nil {
	cc.errors(fmt.Errorf("cannot send pong response: %w", err))
	}
	if r.Type() == message.Confirmable {
	w.Message().SetType(message.Acknowledgement)
	w.Message().SetMessageID(r.MessageID())
	} else {
	if w.Message().Type() != message.Reset {
	w.Message().SetType(message.NonConfirmable)
	}
	w.Message().SetMessageID(cc.GetMessageID())
	}
	}

Quoting RFC 7252:

Provoking a Reset message (e.g., by sending an Empty Confirmable message) is also useful as an inexpensive check of the liveness of an endpoint ("CoAP ping").

and

"*" means that the combination is not used in normal operation but only to elicit a Reset message ("CoAP ping").

where * refers to an empty confirmable message.