Shaker/Communication/WatsonTcp/WatsonTcpServer.cs

namespace WatsonTcp
{
    using System;
    using System.Collections.Concurrent;
    using System.Collections.Generic;
    using System.IO;
    using System.Linq;
    using System.Net;
    using System.Net.NetworkInformation;
    using System.Net.Security;
    using System.Net.Sockets;
    using System.Runtime.InteropServices;
    using System.Security.AccessControl;
    using System.Security.Cryptography.X509Certificates;
    using System.Text;
    using System.Threading;
    using System.Threading.Tasks;

    /// <summary>
    /// Watson TCP server, with or without SSL.
    /// </summary>
    public class WatsonTcpServer : IDisposable
    {
        #region Public-Members
     
        /// <summary>
        /// Watson TCP server settings.
        /// </summary>
        public WatsonTcpServerSettings Settings
        {
            get
            {
                return _Settings;
            }
            set
            {
                if (value == null) _Settings = new WatsonTcpServerSettings();
                else _Settings = value;
            }
        }
         
        /// <summary>
        /// Watson TCP server events.
        /// </summary>
        public WatsonTcpServerEvents Events
        {
            get
            {
                return _Events;
            }
            set
            {
                if (value == null) _Events = new WatsonTcpServerEvents();
                else _Events = value;
            }
        }

        /// <summary>
        /// Watson TCP server callbacks.
        /// </summary>
        public WatsonTcpServerCallbacks Callbacks
        {
            get
            {
                return _Callbacks;
            }
            set
            {
                if (value == null) _Callbacks = new WatsonTcpServerCallbacks();
                else _Callbacks = value;
            }
        }

        /// <summary>
        /// Watson TCP statistics.
        /// </summary>
        public WatsonTcpStatistics Statistics
        {
            get
            {
                return _Statistics;
            }
        }

        /// <summary>
        /// Watson TCP keepalive settings.
        /// </summary>
        public WatsonTcpKeepaliveSettings Keepalive
        {
            get
            {
                return _Keepalive;
            }
            set
            {
                if (value == null) _Keepalive = new WatsonTcpKeepaliveSettings();
                else _Keepalive = value;
            }
        }

        /// <summary>
        /// Watson TCP server SSL configuration.
        /// </summary>
        public WatsonTcpServerSslConfiguration SslConfiguration
        {
            get
            {
                return _SslConfiguration;
            }
            set
            {
                if (value == null) _SslConfiguration = new WatsonTcpServerSslConfiguration();
                else _SslConfiguration = value;
            }
        }

        /// <summary>
        /// JSON serialization helper.
        /// </summary>
        public ISerializationHelper SerializationHelper
        {
            get
            {
                return _SerializationHelper;
            }
            set
            {
                if (value == null) throw new ArgumentNullException(nameof(SerializationHelper));
                _SerializationHelper = value;
                _MessageBuilder.SerializationHelper = value;
            }
        }

        /// <summary>
        /// Retrieve the number of current connected clients.
        /// </summary>
        public int Connections
        {
            get
            {
                return _Connections;
            }
        }

        /// <summary>
        /// Flag to indicate if Watson TCP is listening for incoming TCP connections.
        /// </summary>
        public bool IsListening
        {
            get
            {
                return _IsListening;
            }
        }

        #endregion

        #region Private-Members

        private string _Header = "[WatsonTcpServer] ";
        private WatsonMessageBuilder _MessageBuilder = new WatsonMessageBuilder();
        private WatsonTcpServerSettings _Settings = new WatsonTcpServerSettings();
        private WatsonTcpServerEvents _Events = new WatsonTcpServerEvents();
        private WatsonTcpServerCallbacks _Callbacks = new WatsonTcpServerCallbacks();
        private WatsonTcpStatistics _Statistics = new WatsonTcpStatistics();
        private WatsonTcpKeepaliveSettings _Keepalive = new WatsonTcpKeepaliveSettings();
        private WatsonTcpServerSslConfiguration _SslConfiguration = new WatsonTcpServerSslConfiguration();
        private ClientMetadataManager _ClientManager = new ClientMetadataManager();
        private ISerializationHelper _SerializationHelper = new DefaultSerializationHelper();

        private int _Connections = 0;
        private bool _IsListening = false;

        private Mode _Mode;
        private TlsVersion _TlsVersion = TlsVersion.Tls12;
        private string _ListenerIp;
        private int _ListenerPort;
        private IPAddress _ListenerIpAddress;
        private TcpListener _Listener;

        private X509Certificate2 _SslCertificate;

        private CancellationTokenSource _TokenSource = new CancellationTokenSource();
        private CancellationToken _Token;
        private Task _AcceptConnections = null;
        private Task _MonitorClients = null;

        private readonly object _SyncResponseLock = new object();
        private event EventHandler<SyncResponseReceivedEventArgs> _SyncResponseReceived;

        #endregion

        #region Constructors-and-Factories

        /// <summary>
        /// Initialize the Watson TCP server without SSL.
        /// Supply a specific IP address on which to listen.  Otherwise, use 'null' for the IP address to listen on any IP address.
        /// If you do not specify an IP address, you may have to run WatsonTcp with administrative privileges.  
        /// Call Start() afterward to start the server.
        /// </summary>
        /// <param name="listenerIp">The IP address on which the server should listen.  If null, listen on any IP address (may require administrative privileges).</param>
        /// <param name="listenerPort">The TCP port on which the server should listen.</param>
        public WatsonTcpServer(
            string listenerIp,
            int listenerPort)
        {
            if (listenerPort < 1) throw new ArgumentOutOfRangeException(nameof(listenerPort));

            _Mode = Mode.Tcp; 

             // According to the https://github.com/dotnet/WatsonTcp?tab=readme-ov-file#local-vs-external-connections
            if (string.IsNullOrEmpty(listenerIp) || listenerIp.Equals("*") || listenerIp.Equals("+") || listenerIp.Equals("0.0.0.0"))
            {
                _ListenerIpAddress = IPAddress.Any;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else if (listenerIp.Equals("localhost") || listenerIp.Equals("127.0.0.1") || listenerIp.Equals("::1"))
            {
                _ListenerIpAddress = IPAddress.Loopback;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else
            {
                _ListenerIpAddress = IPAddress.Parse(listenerIp);
                _ListenerIp = listenerIp;
            }

            _ListenerPort = listenerPort;

            SerializationHelper.InstantiateConverter(); // Unity fix
        }

        /// <summary>
        /// Initialize the Watson TCP server with SSL.  
        /// Supply a specific IP address on which to listen.  Otherwise, use 'null' for the IP address to listen on any IP address.
        /// If you do not specify an IP address, you may have to run WatsonTcp with administrative privileges.  
        /// Call Start() afterward to start the server.
        /// </summary>
        /// <param name="listenerIp">The IP address on which the server should listen.  If null, listen on any IP address (may require administrative privileges).</param>
        /// <param name="listenerPort">The TCP port on which the server should listen.</param>
        /// <param name="pfxCertFile">The file containing the SSL certificate.</param>
        /// <param name="pfxCertPass">The password for the SSL certificate.</param>
        /// <param name="tlsVersion">The TLS version required for client connections.</param>
        public WatsonTcpServer(
            string listenerIp,
            int listenerPort,
            string pfxCertFile,
            string pfxCertPass,
            TlsVersion tlsVersion = TlsVersion.Tls12)
        {
            if (listenerPort < 1) throw new ArgumentOutOfRangeException(nameof(listenerPort));
            if (String.IsNullOrEmpty(pfxCertFile)) throw new ArgumentNullException(nameof(pfxCertFile));
             
            _Mode = Mode.Ssl;
            _TlsVersion = tlsVersion;

            if (String.IsNullOrEmpty(listenerIp))
            {
                _ListenerIpAddress = IPAddress.Any;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else if (listenerIp.Equals("localhost") || listenerIp.Equals("127.0.0.1") || listenerIp.Equals("::1"))
            {
                _ListenerIpAddress = IPAddress.Loopback;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else
            {
                _ListenerIpAddress = IPAddress.Parse(listenerIp);
                _ListenerIp = listenerIp;
            }

            _SslCertificate = null; 
            if (String.IsNullOrEmpty(pfxCertPass))
            {
                _SslCertificate = new X509Certificate2(pfxCertFile);
            }
            else
            {
                _SslCertificate = new X509Certificate2(pfxCertFile, pfxCertPass);
            }

            _ListenerPort = listenerPort;

            SerializationHelper.InstantiateConverter(); // Unity fix
        }

        /// <summary>
        /// Initialize the Watson TCP server with SSL.  
        /// Supply a specific IP address on which to listen.  Otherwise, use 'null' for the IP address to listen on any IP address.
        /// If you do not specify an IP address, you may have to run WatsonTcp with administrative privileges.  
        /// Call Start() afterward to start the server.
        /// </summary>
        /// <param name="listenerIp">The IP address on which the server should listen.  If null, listen on any IP address (may require administrative privileges).</param>
        /// <param name="listenerPort">The TCP port on which the server should listen.</param>
        /// <param name="cert">The SSL certificate.</param>
        /// <param name="tlsVersion">The TLS version required for client connections.</param>
        /// <exception cref="ArgumentOutOfRangeException"></exception>
        public WatsonTcpServer(
            string listenerIp,
            int listenerPort,
            X509Certificate2 cert,
            TlsVersion tlsVersion = TlsVersion.Tls12)
        {
            if (listenerPort < 1) throw new ArgumentOutOfRangeException(nameof(listenerPort));
            if (cert == null) throw new ArgumentNullException(nameof(cert));

            _Mode = Mode.Ssl;
            _TlsVersion = tlsVersion;
            _SslCertificate = cert;

            if (String.IsNullOrEmpty(listenerIp))
            {
                _ListenerIpAddress = IPAddress.Any;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else if (listenerIp.Equals("localhost") || listenerIp.Equals("127.0.0.1") || listenerIp.Equals("::1"))
            {
                _ListenerIpAddress = IPAddress.Loopback;
                _ListenerIp = _ListenerIpAddress.ToString();
            }
            else
            {
                _ListenerIpAddress = IPAddress.Parse(listenerIp);
                _ListenerIp = listenerIp;
            }

            _ListenerPort = listenerPort;

            SerializationHelper.InstantiateConverter(); // Unity fix
        }

        #endregion

        #region Public-Methods

        /// <summary>
        /// Tear down the server and dispose of background workers.
        /// Do not reuse the object after disposal.
        /// </summary>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Start accepting connections.
        /// </summary>
        public void Start()
        {
            if (_IsListening) throw new InvalidOperationException("WatsonTcpServer is already running.");

            _ClientManager = new ClientMetadataManager();
            _TokenSource = new CancellationTokenSource();
            _Token = _TokenSource.Token;
            _Statistics = new WatsonTcpStatistics();
            _Listener = new TcpListener(_ListenerIpAddress, _ListenerPort);

            if (!_Events.IsUsingMessages && !_Events.IsUsingStreams)
                throw new InvalidOperationException("One of either 'MessageReceived' or 'StreamReceived' events must first be set.");

            if (_Mode == Mode.Tcp)
            {
                _Settings.Logger?.Invoke(Severity.Info, _Header + "starting on " + _ListenerIp + ":" + _ListenerPort);
            }
            else if (_Mode == Mode.Ssl)
            {
                _Settings.Logger?.Invoke(Severity.Info, _Header + "starting with SSL on " + _ListenerIp + ":" + _ListenerPort);
            }
            else
            {
                throw new ArgumentException("Unknown mode: " + _Mode.ToString());
            }

            _Listener.Start();
            _AcceptConnections = Task.Run(() => AcceptConnections(_Token), _Token); // sets _IsListening
            _MonitorClients = Task.Run(() => MonitorForIdleClients(_Token), _Token);
            _Events.HandleServerStarted(this, EventArgs.Empty);
        }
         
        /// <summary>
        /// Stop accepting connections.
        /// </summary>
        public void Stop()
        {
            _IsListening = false;
            _Listener.Stop();
            _TokenSource.Cancel();

            _Settings.Logger?.Invoke(Severity.Info, _Header + "stopped");
            _Events.HandleServerStopped(this, EventArgs.Empty);
        }

        #region SendAsync

        /// <summary>
        /// Send data and metadata to the specified client, asynchronously.
        /// </summary>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="data">String containing data.</param>
        /// <param name="metadata">Dictionary containing metadata.</param>
        /// <param name="start">Start position within the supplied array.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>Task with Boolean indicating if the message was sent successfully.</returns>
        public async Task<bool> SendAsync(Guid guid, string data, Dictionary<string, object> metadata = null, int start = 0, CancellationToken token = default)
        {
            byte[] bytes = Array.Empty<byte>();
            if (!String.IsNullOrEmpty(data)) bytes = Encoding.UTF8.GetBytes(data);
            return await SendAsync(guid, bytes, metadata, start, token).ConfigureAwait(false);
        }

        /// <summary>
        /// Send data and metadata to the specified client, asynchronously.
        /// </summary>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="data">Byte array containing data.</param>
        /// <param name="metadata">Dictionary containing metadata.</param>
        /// <param name="start">Start position within the supplied array.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>Task with Boolean indicating if the message was sent successfully.</returns>
        public async Task<bool> SendAsync(Guid guid, byte[] data, Dictionary<string, object> metadata = null, int start = 0, CancellationToken token = default)
        {
            if (data == null) data = Array.Empty<byte>();
            WatsonCommon.BytesToStream(data, start, out int contentLength, out Stream stream);
            return await SendAsync(guid, contentLength, stream, metadata, token).ConfigureAwait(false);
        }

        /// <summary>
        /// Send data and metadata to the specified client using a stream, asynchronously.
        /// </summary>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="contentLength">The number of bytes in the stream.</param>
        /// <param name="stream">The stream containing the data.</param>
        /// <param name="metadata">Dictionary containing metadata.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>Task with Boolean indicating if the message was sent successfully.</returns>
        public async Task<bool> SendAsync(Guid guid, long contentLength, Stream stream, Dictionary<string, object> metadata = null, CancellationToken token = default)
        {
            if (contentLength < 0) throw new ArgumentException("Content length must be zero or greater.");
            if (token == default(CancellationToken)) token = _Token;
            ClientMetadata client = _ClientManager.GetClient(guid);
            if (client == null)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "unable to find client " + guid.ToString());
                throw new KeyNotFoundException("Unable to find client " + guid.ToString() + ".");
            }

            if (stream == null) stream = new MemoryStream(Array.Empty<byte>());
            WatsonMessage msg = _MessageBuilder.ConstructNew(contentLength, stream, false, false, null, metadata);
            return await SendInternalAsync(client, msg, contentLength, stream, token).ConfigureAwait(false);
        }

        #endregion

        #region SendAndWaitAsync

        /// <summary>
        /// Send data and wait for a response for the specified number of milliseconds.  A TimeoutException will be thrown if a response is not received.
        /// </summary>
        /// <param name="timeoutMs">Number of milliseconds to wait before considering a request to be expired.</param>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="data">Data to send.</param>
        /// <param name="metadata">Metadata dictionary to attach to the message.</param>
        /// <param name="start">Start position within the supplied array.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>SyncResponse.</returns>
        public async Task<SyncResponse> SendAndWaitAsync(int timeoutMs, Guid guid, string data, Dictionary<string, object> metadata = null, int start = 0, CancellationToken token = default)
        {
            byte[] bytes = Array.Empty<byte>();
            if (!String.IsNullOrEmpty(data)) bytes = Encoding.UTF8.GetBytes(data);
            return await SendAndWaitAsync(timeoutMs, guid, bytes, metadata, start, token);
                // SendAndWaitAsync(timeoutMs, guid, bytes, metadata, token).ConfigureAwait(false);
        }

        /// <summary>
        /// Send data and wait for a response for the specified number of milliseconds.
        /// </summary>
        /// <param name="timeoutMs">Number of milliseconds to wait before considering a request to be expired.</param>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="data">Data to send.</param>
        /// <param name="metadata">Metadata dictionary to attach to the message.</param>
        /// <param name="start">Start position within the supplied array.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>SyncResponse.</returns>
        public async Task<SyncResponse> SendAndWaitAsync(int timeoutMs, Guid guid, byte[] data, Dictionary<string, object> metadata = null, int start = 0, CancellationToken token = default)
        {
            if (data == null) data = Array.Empty<byte>();
            WatsonCommon.BytesToStream(data, start, out int contentLength, out Stream stream);
            return await SendAndWaitAsync(timeoutMs, guid, contentLength, stream, metadata, token);
        }

        /// <summary>
        /// Send data and wait for a response for the specified number of milliseconds.  A TimeoutException will be thrown if a response is not received.
        /// </summary>
        /// <param name="timeoutMs">Number of milliseconds to wait before considering a request to be expired.</param>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="contentLength">The number of bytes to send from the supplied stream.</param>
        /// <param name="stream">Stream containing data.</param>
        /// <param name="metadata">Metadata dictionary to attach to the message.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        /// <returns>SyncResponse.</returns>
        public async Task<SyncResponse> SendAndWaitAsync(int timeoutMs, Guid guid, long contentLength, Stream stream, Dictionary<string, object> metadata = null, CancellationToken token = default)
        {
            if (contentLength < 0) throw new ArgumentException("Content length must be zero or greater.");
            if (timeoutMs < 1000) throw new ArgumentException("Timeout milliseconds must be 1000 or greater.");
            ClientMetadata client = _ClientManager.GetClient(guid);
            if (client == null)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "unable to find client " + guid.ToString());
                throw new KeyNotFoundException("Unable to find client " + guid.ToString() + ".");
            }
            if (stream == null) stream = new MemoryStream(Array.Empty<byte>());
            DateTime expiration = DateTime.UtcNow.AddMilliseconds(timeoutMs);
            WatsonMessage msg = _MessageBuilder.ConstructNew(contentLength, stream, true, false, expiration, metadata);
            return await SendAndWaitInternalAsync(client, msg, timeoutMs, contentLength, stream, token);
        }

        #endregion

        /// <summary>
        /// Determine whether or not the specified client is connected to the server.
        /// </summary>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <returns>Boolean indicating if the client is connected to the server.</returns>
        public bool IsClientConnected(Guid guid)
        {
            return _ClientManager.ExistsClient(guid);
        }

        /// <summary>
        /// Retrieve the client metadata associated with each connected client.
        /// </summary>
        /// <returns>An enumerable collection of client metadata.</returns>
        public IEnumerable<ClientMetadata> ListClients()
        {
            Dictionary<Guid, ClientMetadata> clients = _ClientManager.AllClients();
            if (clients != null && clients.Count > 0)
            {
                foreach (KeyValuePair<Guid, ClientMetadata> client in clients)
                {
                    yield return client.Value;
                }
            }
        }

        /// <summary>
        /// Disconnects the specified client.
        /// </summary>
        /// <param name="guid">Globally-unique identifier of the client.</param>
        /// <param name="status">Reason for the disconnect.  This is conveyed to the client.</param>
        /// <param name="sendNotice">Flag to indicate whether the client should be notified of the disconnect.  This message will not be sent until other send requests have been handled.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        public async Task DisconnectClientAsync(Guid guid, MessageStatus status = MessageStatus.Removed, bool sendNotice = true, CancellationToken token = default)
        {
            ClientMetadata client = _ClientManager.GetClient(guid);
            if (client == null)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "unable to find client " + guid.ToString());
            }
            else
            {
                if (!_ClientManager.ExistsClientTimedout(guid)) _ClientManager.AddClientKicked(guid);

                if (sendNotice)
                {
                    WatsonMessage removeMsg = new WatsonMessage();
                    removeMsg.Status = status;
                    await SendInternalAsync(client, removeMsg, 0, null, token).ConfigureAwait(false);
                }

                client.Dispose();
                _ClientManager.RemoveClient(guid);
            }
        }

        /// <summary>
        /// Disconnects all connected clients.
        /// </summary>
        /// <param name="status">Reason for the disconnect.  This is conveyed to each client.</param>
        /// <param name="sendNotice">Flag to indicate whether the client should be notified of the disconnect.  This message will not be sent until other send requests have been handled.</param>
        /// <param name="token">Cancellation token to cancel the request.</param>
        public async Task DisconnectClientsAsync(MessageStatus status = MessageStatus.Removed, bool sendNotice = true, CancellationToken token = default)
        {
            Dictionary<Guid, ClientMetadata> clients = _ClientManager.AllClients();
            if (clients != null && clients.Count > 0)
            {
                foreach (KeyValuePair<Guid, ClientMetadata> client in clients)
                {
                    await DisconnectClientAsync(client.Key, status, sendNotice, token).ConfigureAwait(false);
                }
            }
        }

        #endregion

        #region Private-Methods

        /// <summary>
        /// Tear down the server and dispose of background workers.
        /// Do not reuse the object after disposal.
        /// </summary>
        /// <param name="disposing">Indicate if resources should be disposed.</param>
        protected virtual void Dispose(bool disposing)
        { 
            if (disposing)
            {
                _Settings.Logger?.Invoke(Severity.Info, _Header + "disposing");

                if (_IsListening) Stop();

                DisconnectClientsAsync(MessageStatus.Shutdown).Wait();

                if (_Listener != null)
                {
                    if (_Listener.Server != null)
                    {
                        _Listener.Server.Close();
                        _Listener.Server.Dispose();
                    }
                }

                if (_SslCertificate != null)
                {
                    _SslCertificate.Dispose();
                }

                if (_ClientManager != null)
                {
                    _ClientManager.Dispose();
                }

                _Settings = null;
                _Events = null;
                _Callbacks = null;
                _Statistics = null;
                _Keepalive = null;
                _SslConfiguration = null;

                _ListenerIp = null;
                _ListenerIpAddress = null;
                _Listener = null;

                _SslCertificate = null;

                _TokenSource = null;

                _AcceptConnections = null;
                _MonitorClients = null;

                _IsListening = false; 
            } 
        }

        #region Connection

        private void EnableKeepalives(TcpClient client)
        {
            // issues with definitions: https://github.com/dotnet/sdk/issues/14540

            try
            {
#if NET6_0_OR_GREATER

                client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
                client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, _Keepalive.TcpKeepAliveTime);
                client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, _Keepalive.TcpKeepAliveInterval);

                // Windows 10 version 1703 or later

                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
                    && Environment.OSVersion.Version >= new Version(10, 0, 15063))
                {
                    client.Client.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, _Keepalive.TcpKeepAliveRetryCount);
                }

#elif NETFRAMEWORK

                // .NET Framework expects values in milliseconds

                byte[] keepAlive = new byte[12]; 
                Buffer.BlockCopy(BitConverter.GetBytes((uint)1), 0, keepAlive, 0, 4); 
                Buffer.BlockCopy(BitConverter.GetBytes((uint)(_Keepalive.TcpKeepAliveTime * 1000)), 0, keepAlive, 4, 4);  
                Buffer.BlockCopy(BitConverter.GetBytes((uint)(_Keepalive.TcpKeepAliveInterval * 1000)), 0, keepAlive, 8, 4);  
                client.Client.IOControl(IOControlCode.KeepAliveValues, keepAlive, null);

#elif NETSTANDARD

#endif
            }
            catch (Exception)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "keepalives not supported on this platform, disabled");
                _Keepalive.EnableTcpKeepAlives = false;
            }
        }

        private async Task AcceptConnections(CancellationToken token)
        { 
            _IsListening = true;

            while (true)
            { 
                try
                {
                    token.ThrowIfCancellationRequested();

                    #region Check-for-Maximum-Connections

                    if (!_IsListening && (_Connections >= _Settings.MaxConnections))
                    {
                        await Task.Delay(100);
                        continue;
                    }
                    else if (!_IsListening)
                    {
                        _Listener.Start();
                        _IsListening = true;
                    }

                    #endregion

                    #region Accept-and-Validate

                    TcpClient tcpClient = await _Listener.AcceptTcpClientAsync().ConfigureAwait(false);
                    tcpClient.LingerState.Enabled = false;
                    tcpClient.NoDelay = _Settings.NoDelay;

                    if (_Keepalive.EnableTcpKeepAlives) EnableKeepalives(tcpClient);

                    string clientIp = ((IPEndPoint)tcpClient.Client.RemoteEndPoint).Address.ToString();
                    if (_Settings.PermittedIPs.Count > 0 && !_Settings.PermittedIPs.Contains(clientIp))
                    {
                        _Settings.Logger?.Invoke(Severity.Info, _Header + "rejecting connection from " + clientIp + " (not permitted)");
                        tcpClient.Close();
                        continue;
                    }

                    if (_Settings.BlockedIPs.Count > 0 && _Settings.BlockedIPs.Contains(clientIp))
                    {
                        _Settings.Logger?.Invoke(Severity.Info, _Header + "rejecting connection from " + clientIp + " (blocked)");
                        tcpClient.Close();
                        continue;
                    }

                    ClientMetadata client = new ClientMetadata(tcpClient);
                    client.SendBuffer = new byte[_Settings.StreamBufferSize];

                    _ClientManager.AddClient(client.Guid, client);
                    _ClientManager.AddClientLastSeen(client.Guid);

                    CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(_Token, client.Token);

                    #endregion

                    #region Check-for-Maximum-Connections

                    Interlocked.Increment(ref _Connections);
                    if (_Connections >= _Settings.MaxConnections)
                    {
                        _Settings.Logger?.Invoke(Severity.Info, _Header + "maximum connections " + _Settings.MaxConnections + " met (currently " + _Connections + " connections), pausing");
                        _IsListening = false;
                        _Listener.Stop();
                    }

                    #endregion

                    #region Initialize-Client

                    Task unawaited = null;

                    if (_Mode == Mode.Tcp)
                    {
                        unawaited = Task.Run(() => FinalizeConnection(client, linkedCts.Token), linkedCts.Token);
                    }
                    else if (_Mode == Mode.Ssl)
                    {
                        if (_Settings.AcceptInvalidCertificates)
                        {
                            client.SslStream = new SslStream(client.NetworkStream, false, _SslConfiguration.ClientCertificateValidationCallback);
                        }
                        else
                        {
                            client.SslStream = new SslStream(client.NetworkStream, false);
                        }

                        unawaited = Task.Run(async () =>
                        {
                            bool success = await StartTls(client, linkedCts.Token).ConfigureAwait(false);
                            if (success)
                            {
                                await FinalizeConnection(client, linkedCts.Token).ConfigureAwait(false);
                            }
                            else
                            {
                                _ClientManager.RemoveClient(client.Guid);
                                _ClientManager.RemoveClientLastSeen(client.Guid);

                                client.Dispose();
                            }

                        }, linkedCts.Token);
                    }
                    else
                    {
                        throw new ArgumentException("Unknown mode: " + _Mode.ToString());
                    }

                    _Settings.Logger?.Invoke(Severity.Debug, _Header + "accepted connection from " + client.ToString());

                    #endregion
                }
                catch (TaskCanceledException)
                {
                    break;
                }
                catch (ObjectDisposedException)
                {
                    break;
                }
                catch (Exception e)
                {
                    _Settings.Logger?.Invoke(Severity.Error, _Header + "listener exception: " + e.Message);
                    _Events.HandleExceptionEncountered(this, new ExceptionEventArgs(e));
                    break;
                }
            } 
        }

        private async Task<bool> StartTls(ClientMetadata client, CancellationToken token)
        {
            try
            {
                token.ThrowIfCancellationRequested();

                await client.SslStream.AuthenticateAsServerAsync(_SslCertificate, _SslConfiguration.ClientCertificateRequired, _TlsVersion.ToSslProtocols(), !_Settings.AcceptInvalidCertificates).ConfigureAwait(false);

                if (!client.SslStream.IsEncrypted)
                {
                    _Settings.Logger?.Invoke(Severity.Error, _Header + "stream from " + client.ToString() + " not encrypted");
                    client.Dispose();
                    Interlocked.Decrement(ref _Connections);
                    return false;
                }

                if (!client.SslStream.IsAuthenticated)
                {
                    _Settings.Logger?.Invoke(Severity.Error, _Header + "stream from " + client.ToString() + " not authenticated");
                    client.Dispose();
                    Interlocked.Decrement(ref _Connections);
                    return false;
                }

                if (_Settings.MutuallyAuthenticate && !client.SslStream.IsMutuallyAuthenticated)
                {
                    _Settings.Logger?.Invoke(Severity.Error, _Header + $"mutual authentication with {client.ToString()} ({_TlsVersion}) failed");
                    client.Dispose(); 
                    Interlocked.Decrement(ref _Connections);
                    return false;
                }
            }
            catch (Exception e)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + $"disconnected during SSL/TLS establishment with {client.ToString()} ({_TlsVersion}): " + e.Message);
                _Events.HandleExceptionEncountered(this, new ExceptionEventArgs(e));

                client.Dispose(); 
                Interlocked.Decrement(ref _Connections);
                return false;
            } 

            return true;
        }

        private async Task FinalizeConnection(ClientMetadata client, CancellationToken token)
        { 
            #region Request-Authentication

            if (!String.IsNullOrEmpty(_Settings.PresharedKey))
            {
                _Settings.Logger?.Invoke(Severity.Debug, _Header + "requesting authentication material from " + client.ToString());
                _ClientManager.AddUnauthenticatedClient(client.Guid);

                byte[] data = Encoding.UTF8.GetBytes("Authentication required");
                WatsonMessage authMsg = new WatsonMessage();
                authMsg.Status = MessageStatus.AuthRequired;
                await SendInternalAsync(client, authMsg, 0, null, token).ConfigureAwait(false);
            }

            #endregion

            #region Start-Data-Receiver

            _Settings.Logger?.Invoke(Severity.Debug, _Header + "starting data receiver for " + client.ToString());
            client.DataReceiver = Task.Run(() => DataReceiver(client, token), token);

            #endregion 
        }

        private bool IsClientConnected(ClientMetadata client)
        {
            if (client != null && client.TcpClient != null)
            {
                var state = IPGlobalProperties.GetIPGlobalProperties()
                    .GetActiveTcpConnections()
                        .FirstOrDefault(x =>
                            x.LocalEndPoint.Equals(client.TcpClient.Client.LocalEndPoint)
                            && x.RemoteEndPoint.Equals(client.TcpClient.Client.RemoteEndPoint));

                if (state == default(TcpConnectionInformation)
                    || state.State == TcpState.Unknown
                    || state.State == TcpState.FinWait1
                    || state.State == TcpState.FinWait2
                    || state.State == TcpState.Closed
                    || state.State == TcpState.Closing
                    || state.State == TcpState.CloseWait)
                {
                    return false;
                }

                byte[] tmp = new byte[1];
                bool success = false;

                try
                {
                    client.WriteLock.Wait();
                    client.TcpClient.Client.Send(tmp, 0, 0);
                    success = true;
                }
                catch (SocketException se)
                {
                    if (se.NativeErrorCode.Equals(10035)) success = true;
                }
                catch (Exception)
                {
                }
                finally
                {
                    if (client != null)
                    {
                        client.WriteLock.Release();
                    }
                }

                if (success) return true;

                try
                {
                    client.WriteLock.Wait();

                    if ((client.TcpClient.Client.Poll(0, SelectMode.SelectWrite))
                        && (!client.TcpClient.Client.Poll(0, SelectMode.SelectError)))
                    {
                        byte[] buffer = new byte[1];
                        if (client.TcpClient.Client.Receive(buffer, SocketFlags.Peek) == 0)
                        {
                            return false;
                        }
                        else
                        {
                            return true;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
                catch (Exception)
                {
                    return false;
                }
                finally
                {
                    if (client != null) client.WriteLock.Release();
                }
            }
            else
            {
                return false;
            }
        }

        #endregion

        #region Read

        private async Task DataReceiver(ClientMetadata client, CancellationToken token)
        {
            while (true)
            {
                try
                {
                    token.ThrowIfCancellationRequested();

                    if (!IsClientConnected(client)) break;

                    WatsonMessage msg = await _MessageBuilder.BuildFromStream(client.DataStream);
                    if (msg == null)
                    {
                        await Task.Delay(30, token).ConfigureAwait(false);
                        continue;
                    }
                     
                    if (!String.IsNullOrEmpty(_Settings.PresharedKey))
                    {
                        if (_ClientManager.ExistsUnauthenticatedClient(client.Guid))
                        {
                            _Settings.Logger?.Invoke(Severity.Debug, _Header + "message received from unauthenticated endpoint " + client.ToString());

                            byte[] data = null;
                            WatsonMessage authMsg = null;
                            int contentLength = 0;
                            Stream authStream = null;

                            if (msg.Status == MessageStatus.AuthRequested)
                            {
                                // check preshared key
                                if (msg.PresharedKey != null && msg.PresharedKey.Length > 0)
                                {
                                    string clientPsk = Encoding.UTF8.GetString(msg.PresharedKey).Trim();
                                    if (_Settings.PresharedKey.Trim().Equals(clientPsk))
                                    {
                                        _Settings.Logger?.Invoke(Severity.Debug, _Header + "accepted authentication for " + client.ToString());
                                        _ClientManager.RemoveUnauthenticatedClient(client.Guid);
                                        _Events.HandleAuthenticationSucceeded(this, new AuthenticationSucceededEventArgs(client));

                                        data = Encoding.UTF8.GetBytes("Authentication successful");
                                        WatsonCommon.BytesToStream(data, 0, out contentLength, out authStream);
                                        authMsg = _MessageBuilder.ConstructNew(contentLength, authStream, false, false, null, null);
                                        authMsg.Status = MessageStatus.AuthSuccess;
                                        await SendInternalAsync(client, authMsg, 0, null, token).ConfigureAwait(false);
                                        continue;
                                    }
                                    else
                                    {
                                        _Settings.Logger?.Invoke(Severity.Warn, _Header + "declined authentication for " + client.ToString());
                                        await DisconnectClientAsync(client.Guid, MessageStatus.AuthFailure, false, token).ConfigureAwait(false);
                                        break;
                                    }
                                }
                            }

                            // decline and terminate
                            _Settings.Logger?.Invoke(Severity.Warn, _Header + "no authentication material for " + client.ToString());
                            await DisconnectClientAsync(client.Guid, MessageStatus.AuthFailure, false, token).ConfigureAwait(false);
                            break;
                        }
                    }

                    if (msg.Status == MessageStatus.Shutdown)
                    {
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + "client " + client.ToString() + " is disconnecting");
                        break;
                    }
                    else if (msg.Status == MessageStatus.Removed)
                    {
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + "sent disconnect notice to " + client.ToString());
                        break;
                    }
                    else if (msg.Status == MessageStatus.RegisterClient)
                    {
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + "client " + client.ToString() + " attempting to register GUID " + msg.SenderGuid.ToString());
                        _ClientManager.ReplaceGuid(client.Guid, msg.SenderGuid);
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + "updated client GUID from " + client.Guid + " to " + msg.SenderGuid);

                        client.Guid = msg.SenderGuid;
                        _Events.HandleClientConnected(this, new ConnectionEventArgs(client));
                        continue;
                    }
                     
                    if (msg.SyncRequest)
                    {
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + client.ToString() + " synchronous request received: " + msg.ConversationGuid.ToString());

                        DateTime expiration = WatsonCommon.GetExpirationTimestamp(msg);
                        byte[] msgData = await WatsonCommon.ReadMessageDataAsync(msg, _Settings.StreamBufferSize, token).ConfigureAwait(false);

                        if (DateTime.UtcNow < expiration)
                        {
                            Task unawaited = Task.Run(async () =>
                            {
                                SyncRequest syncReq = new SyncRequest(
                                    client,
                                    msg.ConversationGuid,
                                    msg.ExpirationUtc.Value,
                                    msg.Metadata,
                                    msgData);

                                SyncResponse syncResp = null;

#pragma warning disable CS0618 // Type or member is obsolete
                                if (_Callbacks.SyncRequestReceivedAsync != null)
                                {
                                    syncResp = await _Callbacks.HandleSyncRequestReceivedAsync(syncReq);
                                }
                                else if (_Callbacks.SyncRequestReceived != null)
                                {
                                    syncResp = _Callbacks.HandleSyncRequestReceived(syncReq);
                                }
#pragma warning restore CS0618 // Type or member is obsolete

                                if (syncResp != null)
                                {
                                    WatsonCommon.BytesToStream(syncResp.Data, 0, out int contentLength, out Stream stream);
                                    WatsonMessage respMsg = _MessageBuilder.ConstructNew(
                                        contentLength,
                                        stream,
                                        false,
                                        true,
                                        msg.ExpirationUtc.Value,
                                        syncResp.Metadata);

                                    respMsg.ConversationGuid = msg.ConversationGuid;
                                    await SendInternalAsync(client, respMsg, contentLength, stream, token).ConfigureAwait(false);
                                }
                            }, token);
                        }
                        else
                        { 
                            _Settings.Logger?.Invoke(Severity.Debug, _Header + "expired synchronous request received and discarded from " + client.ToString());
                        } 
                    }
                    else if (msg.SyncResponse)
                    {
                        // No need to amend message expiration; it is copied from the request, which was set by this node
                        // DateTime expiration = WatsonCommon.GetExpirationTimestamp(msg); 
                        _Settings.Logger?.Invoke(Severity.Debug, _Header + client.ToString() + " synchronous response received: " + msg.ConversationGuid.ToString());
                        byte[] msgData = await WatsonCommon.ReadMessageDataAsync(msg, _Settings.StreamBufferSize, token).ConfigureAwait(false);

                        if (DateTime.UtcNow < msg.ExpirationUtc.Value)
                        {
                            lock (_SyncResponseLock)
                            {
                                _SyncResponseReceived?.Invoke(this, new SyncResponseReceivedEventArgs(msg, msgData));
                            }
                        }
                        else
                        {
                            _Settings.Logger?.Invoke(Severity.Debug, _Header + "expired synchronous response received and discarded from " + client.ToString());
                        }
                    }
                    else
                    {
                        byte[] msgData = null; 

                        if (_Events.IsUsingMessages)
                        { 
                            msgData = await WatsonCommon.ReadMessageDataAsync(msg, _Settings.StreamBufferSize, token).ConfigureAwait(false); 
                            MessageReceivedEventArgs mr = new MessageReceivedEventArgs(client, msg.Metadata, msgData);
                            await Task.Run(() => _Events.HandleMessageReceived(this, mr), token);
                        }
                        else if (_Events.IsUsingStreams)
                        {
                            StreamReceivedEventArgs sr = null;
                            WatsonStream ws = null;

                            if (msg.ContentLength >= _Settings.MaxProxiedStreamSize)
                            {
                                ws = new WatsonStream(msg.ContentLength, msg.DataStream);
                                sr = new StreamReceivedEventArgs(client, msg.Metadata, msg.ContentLength, ws);
                                _Events.HandleStreamReceived(this, sr); 
                            }
                            else
                            {
                                MemoryStream ms = await WatsonCommon.DataStreamToMemoryStream(msg.ContentLength, msg.DataStream, _Settings.StreamBufferSize, token).ConfigureAwait(false);
                                ws = new WatsonStream(msg.ContentLength, ms); 
                                sr = new StreamReceivedEventArgs(client, msg.Metadata, msg.ContentLength, ws);
                                await Task.Run(() => _Events.HandleStreamReceived(this, sr), token);
                            } 
                        }
                        else
                        {
                            _Settings.Logger?.Invoke(Severity.Error, _Header + "event handler not set for either MessageReceived or StreamReceived");
                            break;
                        }
                    }

                    _Statistics.IncrementReceivedMessages();
                    _Statistics.AddReceivedBytes(msg.ContentLength);
                    _ClientManager.UpdateClientLastSeen(client.Guid, DateTime.UtcNow);
                }
                catch (ObjectDisposedException ode)
                {
                    _Settings?.Logger?.Invoke(Severity.Debug, _Header + "object disposed exception encountered");
                    _Events?.HandleExceptionEncountered(this, new ExceptionEventArgs(ode));
                    break;
                }
                catch (TaskCanceledException tce)
                {
                    _Settings?.Logger?.Invoke(Severity.Debug, _Header + "task canceled exception encountered");
                    _Events?.HandleExceptionEncountered(this, new ExceptionEventArgs(tce));
                    break;
                }
                catch (OperationCanceledException oce)
                {
                    _Settings?.Logger?.Invoke(Severity.Debug, _Header + "operation canceled exception encountered");
                    _Events?.HandleExceptionEncountered(this, new ExceptionEventArgs(oce));
                    break;
                }
                catch (IOException ioe)
                {
                    _Settings?.Logger?.Invoke(Severity.Debug, _Header + "IO exception encountered");
                    _Events?.HandleExceptionEncountered(this, new ExceptionEventArgs(ioe));
                    break;
                }
                catch (Exception e)
                {
                    _Settings?.Logger?.Invoke(Severity.Error, _Header + "data receiver exception for " + client.ToString() + ": " + e.Message);
                    _Events?.HandleExceptionEncountered(this, new ExceptionEventArgs(e));
                    break;
                }
            }

            if (_Settings != null && _Events != null)
            {
                DisconnectionEventArgs cd = null;
                if (_ClientManager.ExistsClientKicked(client.Guid)) cd = new DisconnectionEventArgs(client, DisconnectReason.Removed);
                else if (_ClientManager.ExistsClientTimedout(client.Guid)) cd = new DisconnectionEventArgs(client, DisconnectReason.Timeout);
                else cd = new DisconnectionEventArgs(client, DisconnectReason.Normal);
                _Events.HandleClientDisconnected(this, cd);

                _ClientManager.Remove(client.Guid);
                Interlocked.Decrement(ref _Connections);

                _Settings?.Logger?.Invoke(Severity.Debug, _Header + "client " + client.ToString() + " disconnected");
                client.Dispose();
            }
        }

        #endregion

        #region Send

        private async Task<bool> SendInternalAsync(ClientMetadata client, WatsonMessage msg, long contentLength, Stream stream, CancellationToken token)
        {
            if (client == null) throw new ArgumentNullException(nameof(client));
            if (msg == null) throw new ArgumentNullException(nameof(msg));

            if (contentLength > 0)
            {
                if (stream == null || !stream.CanRead)
                {
                    throw new ArgumentException("Cannot read from supplied stream.");
                }
            }

            if (token == default(CancellationToken))
            {
                CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(token, _Token);
                token = linkedCts.Token;
            }

            await client.WriteLock.WaitAsync(token).ConfigureAwait(false);

            try
            {
                await SendHeadersAsync(client, msg, token).ConfigureAwait(false);
                await SendDataStreamAsync(client, contentLength, stream, token).ConfigureAwait(false);

                _Statistics.IncrementSentMessages();
                _Statistics.AddSentBytes(contentLength);
                return true;
            }
            catch (TaskCanceledException)
            {
                return false;
            }
            catch (OperationCanceledException)
            {
                return false;
            }
            catch (Exception e)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "failed to write message to " + client.ToString() + ": " + e.Message);
                _Events.HandleExceptionEncountered(this, new ExceptionEventArgs(e));
                return false;
            }
            finally
            {
                if (client != null) client.WriteLock.Release();
            }
        }

        private async Task<SyncResponse> SendAndWaitInternalAsync(ClientMetadata client, WatsonMessage msg, int timeoutMs, long contentLength, Stream stream, CancellationToken token)
        {
            if (client == null) throw new ArgumentNullException(nameof(client));
            if (msg == null) throw new ArgumentNullException(nameof(msg));

            if (contentLength > 0)
            {
                if (stream == null || !stream.CanRead)
                {
                    throw new ArgumentException("Cannot read from supplied stream.");
                }
            }

            await client.WriteLock.WaitAsync();

            SyncResponse ret = null;
            AutoResetEvent responded = new AutoResetEvent(false);

            // Create a new handler specially for this Conversation.
            EventHandler<SyncResponseReceivedEventArgs> handler = (sender, e) =>
            {
                if (e.Message.ConversationGuid == msg.ConversationGuid)
                {
                    ret = new SyncResponse(e.Message.ConversationGuid, e.Message.ExpirationUtc.Value, e.Message.Metadata, e.Data);
                    responded.Set();
                }
            };

            // Subscribe                
            _SyncResponseReceived += handler;

            try
            {
                await SendHeadersAsync(client, msg, token);
                await SendDataStreamAsync(client, contentLength, stream, token);
                _Settings.Logger?.Invoke(Severity.Debug, _Header + client.ToString() + " synchronous request sent: " + msg.ConversationGuid);

                _Statistics.IncrementSentMessages();
                _Statistics.AddSentBytes(contentLength);
            }
            catch (Exception e)
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + client.ToString() + " failed to write message: " + e.Message);
                _Events.HandleExceptionEncountered(this, new ExceptionEventArgs(e));
                _SyncResponseReceived -= handler;
                throw;
            }
            finally
            {
                if (client != null) client.WriteLock.Release();
            }

            // Wait for responded.Set() to be called
            responded.WaitOne(new TimeSpan(0, 0, 0, 0, timeoutMs));

            // Unsubscribe                
            _SyncResponseReceived -= handler;

            if (ret != null)
            {
                return ret;
            }
            else
            {
                _Settings.Logger?.Invoke(Severity.Error, _Header + "synchronous response not received within the timeout window");
                throw new TimeoutException("A response to a synchronous request was not received within the timeout window.");
            }
        }

        private async Task SendHeadersAsync(ClientMetadata client, WatsonMessage msg, CancellationToken token)
        { 
            byte[] headerBytes = _MessageBuilder.GetHeaderBytes(msg);
            await client.DataStream.WriteAsync(headerBytes, 0, headerBytes.Length, token).ConfigureAwait(false);
            await client.DataStream.FlushAsync(token).ConfigureAwait(false);
        }
        
        private async Task SendDataStreamAsync(ClientMetadata client, long contentLength, Stream stream, CancellationToken token)
        {
            if (contentLength <= 0) return;

            long bytesRemaining = contentLength;
            int bytesRead = 0;

            while (bytesRemaining > 0)
            {
                if (bytesRemaining >= _Settings.StreamBufferSize)
                {
                    client.SendBuffer = new byte[_Settings.StreamBufferSize];
                }
                else
                {
                    client.SendBuffer = new byte[bytesRemaining];
                }

                bytesRead = await stream.ReadAsync(client.SendBuffer, 0, client.SendBuffer.Length, token).ConfigureAwait(false);
                if (bytesRead > 0)
                {
                    await client.DataStream.WriteAsync(client.SendBuffer, 0, bytesRead, token).ConfigureAwait(false);
                    bytesRemaining -= bytesRead;
                }
            } 

            await client.DataStream.FlushAsync(token).ConfigureAwait(false);
        }

        #endregion

        #region Tasks

        private async Task MonitorForIdleClients(CancellationToken token)
        {
            try
            {
                Dictionary<Guid, DateTime> lastSeen = null;

                while (true)
                {
                    token.ThrowIfCancellationRequested();

                    await Task.Delay(5000, _Token).ConfigureAwait(false);

                    if (_Settings.IdleClientTimeoutSeconds > 0)
                    {
                        lastSeen = _ClientManager.AllClientsLastSeen();

                        if (lastSeen != null && lastSeen.Count > 0)
                        {
                            DateTime idleTimestamp = DateTime.UtcNow.AddSeconds(-1 * _Settings.IdleClientTimeoutSeconds);

                            foreach (KeyValuePair<Guid, DateTime> curr in lastSeen)
                            {
                                if (curr.Value < idleTimestamp)
                                {
                                    _ClientManager.AddClientTimedout(curr.Key);
                                    _Settings.Logger?.Invoke(Severity.Debug, _Header + "disconnecting client " + curr.Key + " due to idle timeout");
                                    await DisconnectClientAsync(curr.Key, MessageStatus.Timeout, true);
                                }
                            }
                        }
                    } 
                }
            }
            catch (TaskCanceledException)
            {

            }
            catch (OperationCanceledException)
            {

            }
        }
                
        #endregion

        #endregion
    }
}