public async Task <NtpMessage> SendAsync()
{
var timeServerEndPoint = GetIPEndPoint(this.Server, NtpPort);
NtpMessage responsePacket;
Timestamp transmitTimestamp;
using (var updClient = new UdpClient(this.Port))
{
updClient.Connect(timeServerEndPoint);
transmitTimestamp = new Timestamp(DateTime.UtcNow);
var ntpPacket = new NtpMessage(NtpVersion, Mode.Client, transmitTimestamp);
var ntpRequestPayload = ntpPacket.GetBytes();
await updClient.SendAsync(ntpRequestPayload, ntpRequestPayload.Length);
// TODO Add timeout
var response = await updClient.ReceiveAsync();
responsePacket = new NtpMessage(response.Buffer, DateTime.UtcNow);
}
// Validate the response
if (!ResponseIsValid(responsePacket, transmitTimestamp))
{
throw new InvalidOperationException("The response was not valid.");
}
return(responsePacket);
}
private static bool ResponseIsValid(NtpMessage response, Timestamp transmitTimestamp)
{
if (response.Stratum == Stratum.Unspecified)
{
return(false);
}
if (response.OriginateTimestamp.Seconds != transmitTimestamp.Seconds)
{
return(false);
}
/*
* https://tools.ietf.org/html/rfc4330
* "4. The server reply should be discarded if any of the LI, Stratum,
* or Transmit Timestamp fields is 0 or the Mode field is not 4
* (unicast) or 5 (broadcast)."
*
* LI should not be 3?
*/
//if (response.LeapIndicator == LeapIndicator.NoWarning)
//{
// return false;
//}
if (response.Mode != Mode.Server)
{
return(false);
}
if (response.TransmitTimestamp.Seconds == 0)
{
return(false);
}
if (response.RootDelay.Seconds >= 1)
{
return(false);
}
if (response.RootDispersion.Seconds >= 1)
{
return(false);
}
return(true);
}