C# SNTP Client based on android.net.SntpClient

SntpClient.cs

/*
 * Derived from https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/net/SntpClient.java
 * 
 * Copyright (C) 2016 BravoTango86
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

public class SntpClient {
    public string DefaultHostName { get; set; } = "time1.google.com";
    public int DefaultPort { get; set; } = 123;
    public int DefaultTimeout { get; set; } = 1000;
    public bool Debug { get; set; } = true;
    private const int REFERENCE_TIME_OFFSET = 16;
    private const int ORIGINATE_TIME_OFFSET = 24;
    private const int RECEIVE_TIME_OFFSET = 32;
    private const int TRANSMIT_TIME_OFFSET = 40;
    private const int NTP_PACKET_SIZE = 48;
    private const int NTP_PORT = 123;
    private const int NTP_MODE_CLIENT = 3;
    private const int NTP_MODE_SERVER = 4;
    private const int NTP_MODE_BROADCAST = 5;
    private const int NTP_VERSION = 3;
    private const int NTP_LEAP_NOSYNC = 3;
    private const int NTP_STRATUM_DEATH = 0;
    private const int NTP_STRATUM_MAX = 15;
    // Number of seconds between Jan 1, 1900 and Jan 1, 1970
    // 70 years plus 17 leap days
    private const long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;
    // system time computed from NTP server response
    public long NTPTime { get; private set; }
    // value of SystemClock.elapsedRealtime() corresponding to mNtpTime
    public long NTPTimeReference { get; private set; }
    public long NTPOffset { get; private set; }
    // round trip time in milliseconds
    public long RoundTripTime { get; private set; }

    private class InvalidServerReplyException : Exception {
        public InvalidServerReplyException(string message) : base(message) {
        }
    }

    public async Task<bool> RequestTime(IPEndPoint endPoint, int? timeout = null) => await DoRequestTime(endPoint: endPoint, timeout: timeout);

    public async Task<bool> RequestTime(string hostName, int? port = 123, int? timeout = null) => await DoRequestTime(hostName: hostName, port: port, timeout: timeout);

    public async Task<bool> RequestTime(int? timeout = null) => await RequestTime(DefaultHostName, timeout);

    private async Task<bool> DoRequestTime(IPEndPoint endPoint = null, string hostName = null, int? port = null, int? timeout = null) {
        UdpClient client = null;
        try {
            if (endPoint == null && string.IsNullOrEmpty(hostName)) throw new ArgumentException("No destination specified");
            using (client = new UdpClient()) {
                byte[] buffer = new byte[NTP_PACKET_SIZE];
                // set mode = 3 (client) and version = 3
                // mode is in low 3 bits of first byte
                // version is in bits 3-5 of first byte
                buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);
                // get current time and write it to the request packet
                long requestTime = DateTimeOffset.Now.ToUnixTimeMilliseconds();
                long requestTicks = Environment.TickCount;
                writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);
                if (endPoint != null) await client.SendAsync(buffer, buffer.Length, endPoint);
                else await client.SendAsync(buffer, buffer.Length, hostName, port ?? 123);
                // No point in using this, won't timeout
                // client.Client.ReceiveTimeout = timeout ?? DefaultTimeout;
                // buffer = (await client.ReceiveAsync()).Buffer;
                // Messy, not sure how well this works but waiting perpetually for data is hardly efficient...
                Task<UdpReceiveResult> Receiver = client.ReceiveAsync();
                if (Task.WaitAny(Task.Delay(timeout ?? DefaultTimeout), Receiver) == 0) {
                    if (Debug) Console.WriteLine("Timed out on receive");
                    return false;
                } else buffer = Receiver.Result.Buffer;
                long responseTicks = Environment.TickCount;
                long responseTime = requestTime + (responseTicks - requestTicks);
                // extract the results
                byte leap = (byte)((buffer[0] >> 6) & 0x3);
                byte mode = (byte)(buffer[0] & 0x7);
                int stratum = (int)(buffer[1] & 0xff);
                long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);
                long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);
                long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);
                /* do sanity check according to RFC */
                // TODO: validate originateTime == requestTime.
                checkValidServerReply(leap, mode, stratum, transmitTime);
                long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime);
                // receiveTime = originateTime + transit + skew
                // responseTime = transmitTime + transit - skew
                // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
                //             = ((originateTime + transit + skew - originateTime) +
                //                (transmitTime - (transmitTime + transit - skew)))/2
                //             = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
                //             = (transit + skew - transit + skew)/2
                //             = (2 * skew)/2 = skew
                long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime)) / 2;
                if (Debug) Console.WriteLine("round trip: {0}ms clock offset: {1}ms", roundTripTime, clockOffset);
                // save our results - use the times on this side of the network latency
                // (response rather than request time)
                NTPTime = responseTime + clockOffset;
                NTPTimeReference = responseTicks;
                NTPOffset = clockOffset;
                RoundTripTime = roundTripTime;
            }
        } catch (Exception e) {
            if (Debug) Console.WriteLine("request time failed: {0}", e);
            return false;
        }
        return true;
    }

    /// <summary>
    /// Provides the current <see cref="NTPTime"/> as a DateTimeOffset relative to local time or UTC
    /// </summary>
    /// <param name="local">Uses local TimeZoneInfo if true else defaults to UTC</param>
    public DateTimeOffset GetDateTimeOffset(bool local = false) =>
        TimeZoneInfo.ConvertTime(DateTimeOffset.FromUnixTimeMilliseconds(NTPTime), local ? TimeZoneInfo.Local : TimeZoneInfo.Utc);

    /// <summary>
    /// Provides the current NTPOffset as a TimeSpan 
    /// </summary>
    public TimeSpan GetOffset() => TimeSpan.FromMilliseconds(NTPOffset);

    private static void checkValidServerReply(byte leap, byte mode, int stratum, long transmitTime) {
        if (leap == NTP_LEAP_NOSYNC) {
            throw new InvalidServerReplyException("unsynchronized server");
        }
        if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) {
            throw new InvalidServerReplyException("untrusted mode: " + mode);
        }
        if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) {
            throw new InvalidServerReplyException("untrusted stratum: " + stratum);
        }
        if (transmitTime == 0) {
            throw new InvalidServerReplyException("zero transmitTime");
        }
    }

    /**
     * Reads an unsigned 32 bit big endian number from the given offset in the buffer.
     */
    private long read32(byte[] buffer, int offset) {
        byte b0 = buffer[offset];
        byte b1 = buffer[offset + 1];
        byte b2 = buffer[offset + 2];
        byte b3 = buffer[offset + 3];
        // convert signed bytes to unsigned values
        int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0);
        int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1);
        int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2);
        int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3);
        return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3;
    }

    /**
     * Reads the NTP time stamp at the given offset in the buffer and returns
     * it as a system time (milliseconds since January 1, 1970).
     */
    private long readTimeStamp(byte[] buffer, int offset) {
        long seconds = read32(buffer, offset);
        long fraction = read32(buffer, offset + 4);
        // Special case: zero means zero.
        if (seconds == 0 && fraction == 0) {
            return 0;
        }
        return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);
    }

    /**
     * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp
     * at the given offset in the buffer.
     */
    private void writeTimeStamp(byte[] buffer, int offset, long time) {
        // Special case: zero means zero.
        if (time == 0) {
            //Arrays.fill(buffer, offset, offset + 8, (byte)0x00);
            Buffer.BlockCopy(new byte[8], 0, buffer, offset, 8);
            return;
        }
        long seconds = time / 1000L;
        long milliseconds = time - seconds * 1000L;
        seconds += OFFSET_1900_TO_1970;
        // write seconds in big endian format
        buffer[offset++] = (byte)(seconds >> 24);
        buffer[offset++] = (byte)(seconds >> 16);
        buffer[offset++] = (byte)(seconds >> 8);
        buffer[offset++] = (byte)(seconds >> 0);
        long fraction = milliseconds * 0x100000000L / 1000L;
        // write fraction in big endian format
        buffer[offset++] = (byte)(fraction >> 24);
        buffer[offset++] = (byte)(fraction >> 16);
        buffer[offset++] = (byte)(fraction >> 8);
        // low order bits should be random data
        buffer[offset++] = (byte)(new Random().Next(255));
    }

}