NTP time offset calculation

wxdata/include/scwx/types/ntp_types.hpp

@@ -48,8 +48,7 @@ struct NtpPacket
    std::uint32_t rxTm_s; // Received time-stamp seconds.
    std::uint32_t rxTm_f; // Received time-stamp fraction of a second.
 
-   std::uint32_t txTm_s; // The most important field the client cares about.
+   std::uint32_t txTm_s; // Transmit time-stamp seconds.
-                         // Transmit time-stamp seconds.
    std::uint32_t txTm_f; // Transmit time-stamp fraction of a second.
 
    static NtpPacket Parse(const std::span<std::uint8_t> data);

wxdata/source/scwx/network/ntp_client.cpp

@@ -6,6 +6,7 @@
 #include <boost/asio/ip/udp.hpp>
 #include <boost/asio/thread_pool.hpp>
 #include <boost/asio/use_future.hpp>
+#include <fmt/chrono.h>
 
 namespace scwx::network
 {
@@ -15,15 +16,86 @@ static const auto        logger_    = scwx::util::Logger::Create(logPrefix_);
 
 static constexpr std::size_t kReceiveBufferSize_ {48u};
 
+class NtpTimestamp
+{
+public:
+   // NTP epoch: January 1, 1900
+   // Unix epoch: January 1, 1970
+   // Difference = 70 years = 2,208,988,800 seconds
+   static constexpr std::uint32_t kNtpToUnixOffset_ = 2208988800UL;
+
+   // NTP fractional part represents 1/2^32 of a second
+   static constexpr std::uint64_t kFractionalMultiplier_ = 0x100000000ULL;
+
+   static constexpr std::uint64_t _1e9 = 1000000000ULL;
+
+   std::uint32_t seconds_ {0};
+   std::uint32_t fraction_ {0};
+
+   explicit NtpTimestamp() = default;
+   explicit NtpTimestamp(std::uint32_t seconds, std::uint32_t fraction) :
+       seconds_ {seconds}, fraction_ {fraction}
+   {
+   }
+   ~NtpTimestamp() = default;
+
+   NtpTimestamp(const NtpTimestamp&)            = default;
+   NtpTimestamp& operator=(const NtpTimestamp&) = default;
+   NtpTimestamp(NtpTimestamp&&)                 = default;
+   NtpTimestamp& operator=(NtpTimestamp&&)      = default;
+
+   template<typename Clock = std::chrono::system_clock>
+   std::chrono::time_point<Clock> ToTimePoint() const
+   {
+      // Convert NTP seconds to Unix seconds
+      // Don't cast to a larger type to account for rollover, and this should
+      // work until 2106
+      const std::uint32_t unixSeconds = seconds_ - kNtpToUnixOffset_;
+
+      // Convert NTP fraction to nanoseconds
+      const auto nanoseconds =
+         static_cast<std::uint64_t>(fraction_) * _1e9 / kFractionalMultiplier_;
+
+      return std::chrono::time_point<Clock>(
+         std::chrono::duration_cast<typename Clock::duration>(
+            std::chrono::seconds {unixSeconds} +
+            std::chrono::nanoseconds {nanoseconds}));
+   }
+
+   template<typename Clock = std::chrono::system_clock>
+   static NtpTimestamp FromTimePoint(std::chrono::time_point<Clock> timePoint)
+   {
+      // Convert to duration since Unix epoch
+      const auto unixDuration = timePoint.time_since_epoch();
+
+      // Extract seconds and nanoseconds
+      const auto unixSeconds =
+         std::chrono::duration_cast<std::chrono::seconds>(unixDuration);
+      const auto nanoseconds =
+         std::chrono::duration_cast<std::chrono::nanoseconds>(unixDuration -
+                                                              unixSeconds);
+
+      // Convert Unix seconds to NTP seconds
+      const auto ntpSeconds =
+         static_cast<std::uint32_t>(unixSeconds.count() + kNtpToUnixOffset_);
+
+      // Convert nanoseconds to NTP fractional seconds
+      const auto ntpFraction = static_cast<std::uint32_t>(
+         nanoseconds.count() * kFractionalMultiplier_ / _1e9);
+
+      return NtpTimestamp(ntpSeconds, ntpFraction);
+   }
+};
+
 class NtpClient::Impl
 {
 public:
    explicit Impl();
    ~Impl();
-   Impl(const Impl&)             = delete;
+   Impl(const Impl&)            = delete;
-   Impl& operator=(const Impl&)  = delete;
+   Impl& operator=(const Impl&) = delete;
-   Impl(const Impl&&)            = delete;
+   Impl(Impl&&)                 = delete;
-   Impl& operator=(const Impl&&) = delete;
+   Impl& operator=(Impl&&)      = delete;
 
    void Open(std::string_view host, std::string_view service);
    void Poll();
@@ -31,14 +103,22 @@ public:
 
    boost::asio::thread_pool threadPool_ {2u};
 
+   bool enabled_;
+
    types::ntp::NtpPacket transmitPacket_ {};
 
    boost::asio::ip::udp::socket                  socket_;
    std::optional<boost::asio::ip::udp::endpoint> serverEndpoint_ {};
    std::array<std::uint8_t, kReceiveBufferSize_> receiveBuffer_ {};
 
-   std::vector<std::string> serverList_ {
+   std::chrono::system_clock::duration timeOffset_ {};
-      "time.nist.gov", "ntp.pool.org", "time.windows.com"};
+
+   std::vector<std::string> serverList_ {"time.nist.gov",
+                                         "time.cloudflare.com",
+                                         "ntp.pool.org",
+                                         "time.aws.com",
+                                         "time.windows.com",
+                                         "time.apple.com"};
 };
 
 NtpClient::NtpClient() : p(std::make_unique<Impl>()) {}
@@ -59,6 +139,18 @@ void NtpClient::Poll()
 
 NtpClient::Impl::Impl() : socket_ {threadPool_}
 {
+   using namespace std::chrono_literals;
+
+   const auto now =
+      std::chrono::floor<std::chrono::days>(std::chrono::system_clock::now());
+
+   // The NTP timestamp will overflow in 2036. Overflow is handled in such a way
+   // that should work until 2106. Additional handling for subsequent eras is
+   // required.
+   static constexpr auto kMaxYear_ = 2106y;
+
+   enabled_ = now < kMaxYear_ / 1 / 1;
+
    transmitPacket_.fields.vn   = 3; // Version
    transmitPacket_.fields.mode = 3; // Client (3)
 }
@@ -95,10 +187,15 @@ void NtpClient::Impl::Poll()
 
    try
    {
+      const auto originTimestamp =
+         NtpTimestamp::FromTimePoint(std::chrono::system_clock::now());
+      transmitPacket_.txTm_s = ntohl(originTimestamp.seconds_);
+      transmitPacket_.txTm_f = ntohl(originTimestamp.fraction_);
+
       std::size_t transmitPacketSize = sizeof(transmitPacket_);
       // Send NTP request
       socket_.send_to(boost::asio::buffer(&transmitPacket_, transmitPacketSize),
-         *serverEndpoint_);
+                      *serverEndpoint_);
 
       // Receive NTP response
       auto future =
@@ -131,8 +228,55 @@ void NtpClient::Impl::ReceivePacket(std::size_t length)
 {
    if (length >= sizeof(types::ntp::NtpPacket))
    {
-      auto packet = types::ntp::NtpPacket::Parse(receiveBuffer_);
+      const auto destinationTime = std::chrono::system_clock::now();
-      (void) packet;
+
+      const auto packet = types::ntp::NtpPacket::Parse(receiveBuffer_);
+
+      if (packet.stratum == 0)
+      {
+         const std::uint32_t refId = ntohl(packet.refId);
+         const std::string   kod =
+            std::string(reinterpret_cast<const char*>(&refId), 4);
+
+         logger_->warn("KoD packet received: {}", kod);
+
+         if (kod == "DENY" || kod == "RSTR")
+         {
+            // TODO
+            // The client MUST demobilize any associations to that server and
+            // stop sending packets to that server
+         }
+         else if (kod == "RATE")
+         {
+            // TODO
+            // The client MUST immediately reduce its polling interval to that
+            // server and continue to reduce it each time it receives a RATE
+            // kiss code
+         }
+      }
+      else
+      {
+         const auto originTimestamp =
+            NtpTimestamp(packet.origTm_s, packet.origTm_f);
+         const auto receiveTimestamp =
+            NtpTimestamp(packet.rxTm_s, packet.rxTm_f);
+         const auto transmitTimestamp =
+            NtpTimestamp(packet.txTm_s, packet.txTm_f);
+
+         const auto originTime   = originTimestamp.ToTimePoint();
+         const auto receiveTime  = receiveTimestamp.ToTimePoint();
+         const auto transmitTime = transmitTimestamp.ToTimePoint();
+
+         const auto& t0 = originTime;
+         const auto& t1 = receiveTime;
+         const auto& t2 = transmitTime;
+         const auto& t3 = destinationTime;
+
+         // Update time offset
+         timeOffset_ = ((t1 - t0) + (t2 - t3)) / 2;
+
+         logger_->debug("Time offset updated: {:%jd %T}", timeOffset_);
+      }
    }
    else
    {

wxdata/source/scwx/types/ntp_types.cpp

@@ -20,15 +20,6 @@ NtpPacket NtpPacket::Parse(const std::span<std::uint8_t> data)
 
    packet = *reinterpret_cast<const NtpPacket*>(data.data());
 
-   // Detect Kiss-o'-Death (KoD) packet
-   if (packet.stratum == 0)
-   {
-      // TODO
-      std::string kissCode =
-         std::string(reinterpret_cast<char*>(&packet.refId), 4);
-      (void) kissCode;
-   }
-
    packet.rootDelay      = ntohl(packet.rootDelay);
    packet.rootDispersion = ntohl(packet.rootDispersion);
    packet.refId          = ntohl(packet.refId);