Ntp server

[acaliaro]

Hi @rdavisau , how are you. Some times ago you have helped me writing this code

    public static class NtpClient
    {
        public static async Task<DateTime> GetNetworkTimeAsync()
        {

            Debug.WriteLine ("Entro in GetNTP " + DateTime.Now);
            //default Windows time server
            const string ntpServer = "time.windows.com";
            //const string ntpServer = "pool.ntp.org";
            //const string ntpServer = "time.nist.gov";

            // NTP message size - 16 bytes of the digest (RFC 2030)
            var ntpData = new byte[48];

            //Setting the Leap Indicator, Version Number and Mode values
            ntpData[0] = 0x1B; //LI = 0 (no warning), VN = 3 (IPv4 only), Mode = 3 (Client Mode)

            var tcs = new TaskCompletionSource<byte[]>();

            using (var socket = new UdpSocketReceiver())
            {
                socket.MessageReceived += async (sender, args) =>
                {
                    await socket.StopListeningAsync();
                    tcs.SetResult(args.ByteData);
                };

                Debug.WriteLine ("StartListening " + DateTime.Now);

                await socket.StartListeningAsync(); // any free port >1000 will do 

                Debug.WriteLine ("SendTo " + DateTime.Now);
                await socket.SendToAsync (ntpData, ntpServer, 123).ContinueWith(_=> Task.FromResult(true)).TimeoutAfter(TimeSpan.FromSeconds(3)); //.TimeoutAfter(TimeSpan.FromSeconds(3));
                Debug.WriteLine ("SendTo conclusa");

                ntpData = await tcs.Task.TimeoutAfter(TimeSpan.FromSeconds(8));
            }

            //Offset to get to the "Transmit Timestamp" field (time at which the reply 
            //departed the server for the client, in 64-bit timestamp format."
            const byte serverReplyTime = 40;

            //Get the seconds part
            ulong intPart = BitConverter.ToUInt32(ntpData, serverReplyTime);

            //Get the seconds fraction
            ulong fractPart = BitConverter.ToUInt32(ntpData, serverReplyTime + 4);

            //Convert From big-endian to little-endian
            intPart = SwapEndianness(intPart);
            fractPart = SwapEndianness(fractPart);

            var milliseconds = (intPart * 1000) + ((fractPart * 1000) / 0x100000000L);

            //**UTC** time
            var networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);

            Debug.WriteLine ("Esco da GetNTP " + DateTime.Now + " - " + networkDateTime + " - LOCALTIME: " + networkDateTime.ToLocalTime() + " - UTC: " + networkDateTime.ToUniversalTime());
            return networkDateTime.ToLocalTime();
        }

        // stackoverflow.com/a/3294698/162671
        static uint SwapEndianness(ulong x)
        {
            return (uint)(((x & 0x000000ff) << 24) +
                ((x & 0x0000ff00) << 8) +
                ((x & 0x00ff0000) >> 8) +
                ((x & 0xff000000) >> 24));
        }
    }

Now I have a problem. I am using this code in XF PCL project. I have a wifi tablet that is connected to my home wifi. This code finish always with Timeout. If I activate my iPhone as hotspot and connect Android Tablet to iPhone hotspot, the same code start to works... There is a reason for this?

Thanks and sorry for my English. I hope you have understand...

[acaliaro]

@rdavisau have you some news about this problem? I am using this code in an app but It often returns a timeout

[rdavisau]

Hi Alessandro, sorry for the delay in getting back to you. It's hard to guess what could be the issue! Let's see:

• does the code work on your iPhone, if it is connected to your Wifi?
• if you replace "time.windows.com" with the ip address (for example, I get 23.102.23.44 when looking up time.windows.com), does it work then?

Let me know what happens in those cases and maybe it will hint at something.. :shipit:

[acaliaro]

@rdavisau thanks for your answer. 1- there are not so much differences between wifi on or wifi off. 2- using ip address it seems to work well...very well.. I create an IPA and send it to my client so he will start some tests. Thanks!

[acaliaro]

by the way, after some other tests: 1: 3G works well than WiFi 2: ip address works well than "name coded" address

So, by now, 3G + Ip Address is the best solution

[acaliaro]

Hi @rdavisau I have left my iphone active this night and I have found it in "timeout" calling NTP. It has Wifi on. Now I have closed wifi and I try to leave my iphone without wifi so I can verify it if works well (by now... the answer if yes). I think there is something when it is connected with wifi...

[rdavisau]

Interesting - I suggested the IP address approach because I thought that the need to perform a DNS lookup on time.windows.com might be a factor. It has a TTL of 60 minutes though, so the lookup would only need to be performed once an hour.

But does the android wifi time out every time? It never succeeds?

[acaliaro]

I have only tried wit iOS. I try to develope an app that create a thread that every 20 seconds calls ntp and add the result (timeout or good) to a list or a counter so we can do some simple tests

[acaliaro]

Hi @rdavisau I am trying to create this "test app" but I have this problem...

https://www.dropbox.com/s/yt1m7oxkosfz66l/Screenshot%202016-04-21%2018.40.30.png?dl=0

[acaliaro]

ok Ryan, understand the problem...

[acaliaro]

So Ryan, I have done this little project

TestNtp

Every 10 seconds, Ntp server is called and a listview is filled with "good" or "bad" answer.

In this moment (iOS Iphone with Wifi ON) I have 60 OK and 15 BAD (25% bad). I continue the test. Thanks again for you help

[rdavisau]

Ok, I had a quick look at your repro - I don't think it will make any difference in this case but I would change your OnStart and OnResume like this:

protected override async void OnStart ()
{
    // Handle when your app starts
    await timer ();
}

(mark method async and await the call to timer()) In your case you are catching exceptions so I shouldn't matter but it may save you some problems later on.

Onto your results - UDP protocol is not guaranteed to deliver but to me 25% seems like a very high failure rate - especially if you are only sending requests every 10 seconds. Does the error rate increase or stay the same if you send every 5 seconds?

[acaliaro]

Hi Ryan. One night of test with a call every 10 seconds with Wifi ON and ip Address.. Result:

ok = 1884 bad = 1118

bad > 50%

Now I start test with wifi off

[acaliaro]

Without wifi

Ok 77 Bad 102

I think there is a problem.

@rdavisau have you some suggestions? Tia Alessandro

[rdavisau]

I think you should try to do a 'control' experiment against a service that is not time.windows.com. I wonder if there is a rate limit that you are hitting? Is it possible that after some number of requests from a single IP the NTP stops responding for a while?

[acaliaro]

So polling on various IP?

[rdavisau]

Maybe - I just wonder if time.windows.com is simply not responding sometimes. 50% is a very high timeout rate (I think).