为什么 NTP 无法计算延迟时间

Question

我正在使用公共网络库中的 NTP 来同步我的 Android 应用程序的时间。我尝试使用此代码获得延迟：

public static final String TIME_SERVER = "time-a.nist.gov";

public static long getCurrentNetworkTime() throws IOException {
    NTPUDPClient timeClient = new NTPUDPClient();
    InetAddress inetAddress = InetAddress.getByName(TIME_SERVER);
    TimeInfo timeInfo = timeClient.getTime(inetAddress);
    long returnTime = timeInfo.getMessage().getTransmitTimeStamp().getTime(); // server time

    Log.d("TAG", "delay: " + timeInfo.getDelay() + " time:" + returnTime
            + " local time:" + System.currentTimeMillis());

    return returnTime;
}

但是 timeInfo.getDelay() 我得到 null。根据此方法的文档，它可能不可用：

    /**
     * Get round-trip network delay. If null then could not compute the delay.
     *
     * @return Long or null if delay not available.
     */
    public Long getDelay()
    {
        return _delay;
    }

为什么不能计算延迟？

Answer 1

我的问题通过覆盖 NTPUDPClient 解决了，复制此 class 的代码并更改 TimeInfo 中的参数以获取详细信息：

TimeInfo info = new TimeInfo(recMessage, returnTime, true);

这是 MyNTPUDPClient class，用它代替 NTPUDPClient:

public final class MyNTPUDPClient extends DatagramSocketClient {
    public static final int DEFAULT_PORT = 123;

private int _version = NtpV3Packet.VERSION_3;

public TimeInfo getTime(InetAddress host, int port) throws IOException {
    // if not connected then open to next available UDP port
    if (!isOpen()) {
        open();
    }

    NtpV3Packet message = new NtpV3Impl();
    message.setMode(NtpV3Packet.MODE_CLIENT);
    message.setVersion(_version);
    DatagramPacket sendPacket = message.getDatagramPacket();
    sendPacket.setAddress(host);
    sendPacket.setPort(port);

    NtpV3Packet recMessage = new NtpV3Impl();
    DatagramPacket receivePacket = recMessage.getDatagramPacket();

    TimeStamp now = TimeStamp.getCurrentTime();

    message.setTransmitTime(now);

    _socket_.send(sendPacket);
    _socket_.receive(receivePacket);

    long returnTime = System.currentTimeMillis();
    TimeInfo info = new TimeInfo(recMessage, returnTime, true);

    return info;
}

public TimeInfo getTime(InetAddress host) throws IOException {
    return getTime(host, NtpV3Packet.NTP_PORT);
}

public int getVersion() {
    return _version;
}

public void setVersion(int version) {
    _version = version;
}

}

为什么 NTP 无法计算延迟时间

Why NTP could not compute delay time

java

android

ntp