// Server

    // Middleware function, which will be called for each request.
    func loggingMiddleware(next mux.Handler) mux.Handler {
        return mux.HandlerFunc(func(w mux.ResponseWriter, r *mux.Message) {
            log.Printf("ClientAddress %v, %v\n", w.Conn().RemoteAddr(), r.String())
            next.ServeCOAP(w, r)
        })
    }

    // See /examples/simple/server/main.go
    func handleA(w mux.ResponseWriter, req *mux.Message) {
        err := w.SetResponse(codes.GET, message.TextPlain, bytes.NewReader([]byte("hello world")))
        if err != nil {
            log.Printf("cannot set response: %v", err)
        }
    }

    func main() {
        r := mux.NewRouter()
        r.Use(loggingMiddleware)
        r.Handle("/a", mux.HandlerFunc(handleA))
        r.Handle("/b", mux.HandlerFunc(handleB))

        log.Fatal(coap.ListenAndServe("udp", ":5688", r))


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

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

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

    // Client
    // See /examples/simpler/client/main.go
    func main() {
        co, err := udp.Dial("localhost:5688")

        // for tcp
        // co, err := tcp.Dial("localhost:5688")

        // for tcp-tls
        // co, err := tcp.Dial("localhost:5688", tcp.WithTLS(&tls.Config{...}))

        // for dtls
        // co, err := dtls.Dial("localhost:5688", &dtls.Config{...}))

        if err != nil {
            log.Fatalf("Error dialing: %v", err)
        }
        ctx, cancel := context.WithTimeout(context.Background(), time.Second)
        defer cancel()
        resp, err := co.Get(ctx, "/a")
        if err != nil {
            log.Fatalf("Cannot get response: %v", err)
            return
        }
        log.Printf("Response: %+v", resp)
    }