// Server
	// See /examples/simple/server/main.go
	func handleA(w coap.ResponseWriter, req *coap.Request) {
		log.Printf("Got message in handleA: path=%q: %#v from %v", req.Msg.Path(), req.Msg, req.Client.RemoteAddr())
		w.SetContentFormat(coap.TextPlain)
		log.Printf("Transmitting from A")
		ctx, cancel := context.WithTimeout(req.Ctx, time.Second)
		defer cancel()
		if _, err := w.WriteWithContext(ctx, []byte("hello world")); err != nil {
			log.Printf("Cannot send response: %v", err)
		}
	}

	func main() {
		mux := coap.NewServeMux()
		mux.Handle("/a", coap.HandlerFunc(handleA))

		log.Fatal(coap.ListenAndServe("udp", ":5688", mux))
		
		// for tcp
		// log.Fatal(coap.ListenAndServe("tcp", ":5688",  mux))

		// for tcp-tls
		// log.Fatal(coap.ListenAndServeTLS("tcp-tls", ":5688", &tls.Config{...}, mux))

		// for udp-dtls
		// log.Fatal(coap.ListenAndServeDTLS("udp-dtls", ":5688", &dtls.Config{...}, mux))
	}

	// Client
	// See /examples/simpler/client/main.go
	func main() {
		co, err := coap.Dial("udp", "localhost:5688")
		
		// for tcp
		// co, err := coap.Dial("tcp", "localhost:5688")
		
		// for tcp-tls
		// co, err := coap.DialTLS("tcp-tls", localhost:5688", &tls.Config{...})

		// for udp-dtls
		// co, err := coap.DialDTLS("udp-dtls", "localhost:5688", &dtls.Config{...}, mux))

		if err != nil {
			log.Fatalf("Error dialing: %v", err)
		}

		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
		defer cancel()
		resp, err := co.GetWithContext(ctx, path)


		if err != nil {
			log.Fatalf("Error sending request: %v", err)
		}

		log.Printf("Response payload: %v", resp.Payload())
	}