jellyfin/RSSDP/SsdpDevicePublisherBase.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using MediaBrowser.Model.Net;
using MediaBrowser.Model.Threading;
using RSSDP;

namespace Rssdp.Infrastructure
{
    /// <summary>
    /// Provides the platform independent logic for publishing SSDP devices (notifications and search responses).
    /// </summary>
    public abstract class SsdpDevicePublisherBase : DisposableManagedObjectBase, ISsdpDevicePublisher
    {

        #region Fields & Constants

        private ISsdpCommunicationsServer _CommsServer;
        private string _OSName;
        private string _OSVersion;

        private bool _SupportPnpRootDevice;

        private IList<SsdpRootDevice> _Devices;
        private ReadOnlyEnumerable<SsdpRootDevice> _ReadOnlyDevices;

        private ITimer _RebroadcastAliveNotificationsTimer;
        private ITimerFactory _timerFactory;
        //private TimeSpan _RebroadcastAliveNotificationsTimeSpan;
        private DateTime _LastNotificationTime;

        private IDictionary<string, SearchRequest> _RecentSearchRequests;
        private IUpnpDeviceValidator _DeviceValidator;

        private Random _Random;
        //private TimeSpan _MinCacheTime;

        private const string ServerVersion = "1.0";

        #endregion

        #region Message Format Constants

        #endregion

        #region Constructors

        /// <summary>
        /// Default constructor.
        /// </summary>
        protected SsdpDevicePublisherBase(ISsdpCommunicationsServer communicationsServer, ITimerFactory timerFactory, string osName, string osVersion)
        {
            if (communicationsServer == null) throw new ArgumentNullException("communicationsServer");
            if (osName == null) throw new ArgumentNullException("osName");
            if (osName.Length == 0) throw new ArgumentException("osName cannot be an empty string.", "osName");
            if (osVersion == null) throw new ArgumentNullException("osVersion");
            if (osVersion.Length == 0) throw new ArgumentException("osVersion cannot be an empty string.", "osName");

            _SupportPnpRootDevice = true;
            _timerFactory = timerFactory;
            _Devices = new List<SsdpRootDevice>();
            _ReadOnlyDevices = new ReadOnlyEnumerable<SsdpRootDevice>(_Devices);
            _RecentSearchRequests = new Dictionary<string, SearchRequest>(StringComparer.OrdinalIgnoreCase);
            _Random = new Random();
            _DeviceValidator = new Upnp10DeviceValidator(); //Should probably inject this later, but for now we only support 1.0.

            _CommsServer = communicationsServer;
            _CommsServer.RequestReceived += CommsServer_RequestReceived;
            _OSName = osName;
            _OSVersion = osVersion;

            _CommsServer.BeginListeningForBroadcasts();
        }

        #endregion

        #region Public Methods

        /// <summary>
        /// Adds a device (and it's children) to the list of devices being published by this server, making them discoverable to SSDP clients.
        /// </summary>
        /// <remarks>
        /// <para>Adding a device causes "alive" notification messages to be sent immediately, or very soon after. Ensure your device/description service is running before adding the device object here.</para>
        /// <para>Devices added here with a non-zero cache life time will also have notifications broadcast periodically.</para>
        /// <para>This method ignores duplicate device adds (if the same device instance is added multiple times, the second and subsequent add calls do nothing).</para>
        /// </remarks>
        /// <param name="device">The <see cref="SsdpDevice"/> instance to add.</param>
        /// <exception cref="System.ArgumentNullException">Thrown if the <paramref name="device"/> argument is null.</exception>
        /// <exception cref="System.InvalidOperationException">Thrown if the <paramref name="device"/> contains property values that are not acceptable to the UPnP 1.0 specification.</exception>
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1804:RemoveUnusedLocals", MessageId = "t", Justification = "Capture task to local variable supresses compiler warning, but task is not really needed.")]
        public void AddDevice(SsdpRootDevice device)
        {
            if (device == null) throw new ArgumentNullException("device");

            ThrowIfDisposed();

            _DeviceValidator.ThrowIfDeviceInvalid(device);

            TimeSpan minCacheTime = TimeSpan.Zero;
            bool wasAdded = false;
            lock (_Devices)
            {
                if (!_Devices.Contains(device))
                {
                    _Devices.Add(device);
                    wasAdded = true;
                    minCacheTime = GetMinimumNonZeroCacheLifetime();
                }
            }

            if (wasAdded)
            {
                //_MinCacheTime = minCacheTime;

                ConnectToDeviceEvents(device);

                WriteTrace("Device Added", device);

                SetRebroadcastAliveNotificationsTimer(minCacheTime);

                SendAliveNotifications(device, true);
            }
        }

        /// <summary>
        /// Removes a device (and it's children) from the list of devices being published by this server, making them undiscoverable.
        /// </summary>
        /// <remarks>
        /// <para>Removing a device causes "byebye" notification messages to be sent immediately, advising clients of the device/service becoming unavailable. We recommend removing the device from the published list before shutting down the actual device/service, if possible.</para>
        /// <para>This method does nothing if the device was not found in the collection.</para>
        /// </remarks>
        /// <param name="device">The <see cref="SsdpDevice"/> instance to add.</param>
        /// <exception cref="System.ArgumentNullException">Thrown if the <paramref name="device"/> argument is null.</exception>
        public async Task RemoveDevice(SsdpRootDevice device)
        {
            if (device == null) throw new ArgumentNullException("device");

            ThrowIfDisposed();

            bool wasRemoved = false;
            TimeSpan minCacheTime = TimeSpan.Zero;
            lock (_Devices)
            {
                if (_Devices.Contains(device))
                {
                    _Devices.Remove(device);
                    wasRemoved = true;
                    minCacheTime = GetMinimumNonZeroCacheLifetime();
                }
            }

            if (wasRemoved)
            {
                //_MinCacheTime = minCacheTime;

                DisconnectFromDeviceEvents(device);

                WriteTrace("Device Removed", device);

                await SendByeByeNotifications(device, true).ConfigureAwait(false);

                SetRebroadcastAliveNotificationsTimer(minCacheTime);
            }
        }

        #endregion

        #region Public Properties

        /// <summary>
        /// Returns a read only list of devices being published by this instance.
        /// </summary>
        public IEnumerable<SsdpRootDevice> Devices
        {
            get
            {
                return _ReadOnlyDevices;
            }
        }

        /// <summary>
        /// If true (default) treats root devices as both upnp:rootdevice and pnp:rootdevice types.
        /// </summary>
        /// <remarks>
        /// <para>Enabling this option will cause devices to show up in Microsoft Windows Explorer's network screens (if discovery is enabled etc.). Windows Explorer appears to search only for pnp:rootdeivce and not upnp:rootdevice.</para>
        /// <para>If false, the system will only use upnp:rootdevice for notifiation broadcasts and and search responses, which is correct according to the UPnP/SSDP spec.</para>
        /// </remarks>
        public bool SupportPnpRootDevice
        {
            get { return _SupportPnpRootDevice; }
            set
            {
                _SupportPnpRootDevice = value;
            }
        }

        #endregion

        #region Overrides

        /// <summary>
        /// Stops listening for requests, stops sending periodic broadcasts, disposes all internal resources.
        /// </summary>
        /// <param name="disposing"></param>
        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                var commsServer = _CommsServer;
                _CommsServer = null;

                if (commsServer != null)
                {
                    commsServer.RequestReceived -= this.CommsServer_RequestReceived;
                    if (!commsServer.IsShared)
                        commsServer.Dispose();
                }

                DisposeRebroadcastTimer();

                foreach (var device in this.Devices)
                {
                    DisconnectFromDeviceEvents(device);
                }

                _RecentSearchRequests = null;
            }
        }

        #endregion

        #region Private Methods

        #region Search Related Methods

        private void ProcessSearchRequest(string mx, string searchTarget, IpEndPointInfo endPoint)
        {
            if (String.IsNullOrEmpty(searchTarget))
            {
                WriteTrace(String.Format("Invalid search request received From {0}, Target is null/empty.", endPoint.ToString()));
                return;
            }

            WriteTrace(String.Format("Search Request Received From {0}, Target = {1}", endPoint.ToString(), searchTarget));

            if (IsDuplicateSearchRequest(searchTarget, endPoint))
            {
                WriteTrace("Search Request is Duplicate, ignoring.");
                return;
            }

            //Wait on random interval up to MX, as per SSDP spec.
            //Also, as per UPnP 1.1/SSDP spec ignore missing/bank MX header. If over 120, assume random value between 0 and 120.
            //Using 16 as minimum as that's often the minimum system clock frequency anyway.
            int maxWaitInterval = 0;
            if (String.IsNullOrEmpty(mx))
            {
                //Windows Explorer is poorly behaved and doesn't supply an MX header value.
                //if (this.SupportPnpRootDevice)
                mx = "1";
                //else
                //return;
            }

            if (!Int32.TryParse(mx, out maxWaitInterval) || maxWaitInterval <= 0) return;

            if (maxWaitInterval > 120)
                maxWaitInterval = _Random.Next(0, 120);

            //Do not block synchronously as that may tie up a threadpool thread for several seconds.
            Task.Delay(_Random.Next(16, (maxWaitInterval * 1000))).ContinueWith((parentTask) =>
            {
                //Copying devices to local array here to avoid threading issues/enumerator exceptions.
                IEnumerable<SsdpDevice> devices = null;
                lock (_Devices)
                {
                    if (String.Compare(SsdpConstants.SsdpDiscoverAllSTHeader, searchTarget, StringComparison.OrdinalIgnoreCase) == 0)
                        devices = GetAllDevicesAsFlatEnumerable().ToArray();
                    else if (String.Compare(SsdpConstants.UpnpDeviceTypeRootDevice, searchTarget, StringComparison.OrdinalIgnoreCase) == 0 || (this.SupportPnpRootDevice && String.Compare(SsdpConstants.PnpDeviceTypeRootDevice, searchTarget, StringComparison.OrdinalIgnoreCase) == 0))
                        devices = _Devices.ToArray();
                    else if (searchTarget.Trim().StartsWith("uuid:", StringComparison.OrdinalIgnoreCase))
                        devices = (from device in GetAllDevicesAsFlatEnumerable() where String.Compare(device.Uuid, searchTarget.Substring(5), StringComparison.OrdinalIgnoreCase) == 0 select device).ToArray();
                    else if (searchTarget.StartsWith("urn:", StringComparison.OrdinalIgnoreCase))
                        devices = (from device in GetAllDevicesAsFlatEnumerable() where String.Compare(device.FullDeviceType, searchTarget, StringComparison.OrdinalIgnoreCase) == 0 select device).ToArray();
                }

                if (devices != null)
                {
                    var deviceList = devices.ToList();
                    WriteTrace(String.Format("Sending {0} search responses", deviceList.Count));

                    foreach (var device in deviceList)
                    {
                        SendDeviceSearchResponses(device, endPoint);
                    }
                }
                else
                    WriteTrace(String.Format("Sending 0 search responses."));
            });
        }

        private IEnumerable<SsdpDevice> GetAllDevicesAsFlatEnumerable()
        {
            return _Devices.Union(_Devices.SelectManyRecursive<SsdpDevice>((d) => d.Devices));
        }

        private void SendDeviceSearchResponses(SsdpDevice device, IpEndPointInfo endPoint)
        {
            bool isRootDevice = (device as SsdpRootDevice) != null;
            if (isRootDevice)
            {
                SendSearchResponse(SsdpConstants.UpnpDeviceTypeRootDevice, device, GetUsn(device.Udn, SsdpConstants.UpnpDeviceTypeRootDevice), endPoint);
                if (this.SupportPnpRootDevice)
                    SendSearchResponse(SsdpConstants.PnpDeviceTypeRootDevice, device, GetUsn(device.Udn, SsdpConstants.PnpDeviceTypeRootDevice), endPoint);
            }

            SendSearchResponse(device.Udn, device, device.Udn, endPoint);

            SendSearchResponse(device.FullDeviceType, device, GetUsn(device.Udn, device.FullDeviceType), endPoint);
        }

        private static string GetUsn(string udn, string fullDeviceType)
        {
            return String.Format("{0}::{1}", udn, fullDeviceType);
        }

        private async void SendSearchResponse(string searchTarget, SsdpDevice device, string uniqueServiceName, IpEndPointInfo endPoint)
        {
            var rootDevice = device.ToRootDevice();

            //var additionalheaders = FormatCustomHeadersForResponse(device);

            const string header = "HTTP/1.1 200 OK";

            var values = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

            values["EXT"] = "";
            values["DATE"] = DateTime.UtcNow.ToString("r");
            values["CACHE-CONTROL"] = "max-age = 600";
            values["ST"] = searchTarget;
            values["SERVER"] = string.Format("{0}/{1} UPnP/1.0 RSSDP/{2}", _OSName, _OSVersion, ServerVersion);
            values["USN"] = uniqueServiceName;
            values["LOCATION"] = rootDevice.Location.ToString();

            var message = SsdpHelper.BuildMessage(header, values);

            try
            {
                await _CommsServer.SendMessage(System.Text.Encoding.UTF8.GetBytes(message), endPoint).ConfigureAwait(false);
            }
            catch (Exception ex)
            {
                
            }

            WriteTrace(String.Format("Sent search response to " + endPoint.ToString()), device);
        }

        private bool IsDuplicateSearchRequest(string searchTarget, IpEndPointInfo endPoint)
        {
            var isDuplicateRequest = false;

            var newRequest = new SearchRequest() { EndPoint = endPoint, SearchTarget = searchTarget, Received = DateTime.UtcNow };
            lock (_RecentSearchRequests)
            {
                if (_RecentSearchRequests.ContainsKey(newRequest.Key))
                {
                    var lastRequest = _RecentSearchRequests[newRequest.Key];
                    if (lastRequest.IsOld())
                        _RecentSearchRequests[newRequest.Key] = newRequest;
                    else
                        isDuplicateRequest = true;
                }
                else
                {
                    _RecentSearchRequests.Add(newRequest.Key, newRequest);
                    if (_RecentSearchRequests.Count > 10)
                        CleanUpRecentSearchRequestsAsync();
                }
            }

            return isDuplicateRequest;
        }

        private void CleanUpRecentSearchRequestsAsync()
        {
            lock (_RecentSearchRequests)
            {
                foreach (var requestKey in (from r in _RecentSearchRequests where r.Value.IsOld() select r.Key).ToArray())
                {
                    _RecentSearchRequests.Remove(requestKey);
                }
            }
        }

        #endregion

        #region Notification Related Methods

        #region Alive

        private void SendAllAliveNotifications(object state)
        {
            try
            {
                if (IsDisposed) return;

                //DisposeRebroadcastTimer();

                WriteTrace("Begin Sending Alive Notifications For All Devices");

                _LastNotificationTime = DateTime.Now;

                IEnumerable<SsdpRootDevice> devices;
                lock (_Devices)
                {
                    devices = _Devices.ToArray();
                }

                foreach (var device in devices)
                {
                    if (IsDisposed) return;

                    SendAliveNotifications(device, true);
                }

                WriteTrace("Completed Sending Alive Notifications For All Devices");
            }
            catch (ObjectDisposedException ex)
            {
                WriteTrace("Publisher stopped, exception " + ex.Message);
                Dispose();
            }
            //finally
            //{
            //    // This is causing all notifications to stop
            //    //if (!this.IsDisposed)
            //        //SetRebroadcastAliveNotificationsTimer(_MinCacheTime);
            //}
        }

        private void SendAliveNotifications(SsdpDevice device, bool isRoot)
        {
            if (isRoot)
            {
                SendAliveNotification(device, SsdpConstants.UpnpDeviceTypeRootDevice, GetUsn(device.Udn, SsdpConstants.UpnpDeviceTypeRootDevice));
                if (this.SupportPnpRootDevice)
                    SendAliveNotification(device, SsdpConstants.PnpDeviceTypeRootDevice, GetUsn(device.Udn, SsdpConstants.PnpDeviceTypeRootDevice));
            }

            SendAliveNotification(device, device.Udn, device.Udn);
            SendAliveNotification(device, device.FullDeviceType, GetUsn(device.Udn, device.FullDeviceType));

            foreach (var childDevice in device.Devices)
            {
                SendAliveNotifications(childDevice, false);
            }
        }

        private void SendAliveNotification(SsdpDevice device, string notificationType, string uniqueServiceName)
        {
            var rootDevice = device.ToRootDevice();

            const string header = "NOTIFY * HTTP/1.1";

            var values = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

            // If needed later for non-server devices, these headers will need to be dynamic 
            values["HOST"] = "239.255.255.250:1900";
            values["DATE"] = DateTime.UtcNow.ToString("r");
            values["CACHE-CONTROL"] = "max-age = " + rootDevice.CacheLifetime.TotalSeconds;
            values["LOCATION"] = rootDevice.Location.ToString();
            values["SERVER"] = string.Format("{0}/{1} UPnP/1.0 RSSDP/{2}", _OSName, _OSVersion, ServerVersion);
            values["NTS"] = "ssdp:alive";
            values["NT"] = notificationType;
            values["USN"] = uniqueServiceName;

            var message = SsdpHelper.BuildMessage(header, values);

            _CommsServer.SendMulticastMessage(message);

            WriteTrace(String.Format("Sent alive notification"), device);
        }

        #endregion

        #region ByeBye

        private async Task SendByeByeNotifications(SsdpDevice device, bool isRoot)
        {
            if (isRoot)
            {
                await SendByeByeNotification(device, SsdpConstants.UpnpDeviceTypeRootDevice, GetUsn(device.Udn, SsdpConstants.UpnpDeviceTypeRootDevice)).ConfigureAwait(false);
                if (this.SupportPnpRootDevice)
                    await SendByeByeNotification(device, "pnp:rootdevice", GetUsn(device.Udn, "pnp:rootdevice")).ConfigureAwait(false); ;
            }

            await SendByeByeNotification(device, device.Udn, device.Udn).ConfigureAwait(false); ;
            await SendByeByeNotification(device, String.Format("urn:{0}", device.FullDeviceType), GetUsn(device.Udn, device.FullDeviceType)).ConfigureAwait(false); ;

            foreach (var childDevice in device.Devices)
            {
                await SendByeByeNotifications(childDevice, false).ConfigureAwait(false); ;
            }
        }

        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Naming", "CA2204:Literals should be spelled correctly", MessageId = "byebye", Justification = "Correct value for this type of notification in SSDP.")]
        private Task SendByeByeNotification(SsdpDevice device, string notificationType, string uniqueServiceName)
        {
            const string header = "NOTIFY * HTTP/1.1";

            var values = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

            // If needed later for non-server devices, these headers will need to be dynamic 
            values["HOST"] = "239.255.255.250:1900";
            values["DATE"] = DateTime.UtcNow.ToString("r");
            values["SERVER"] = string.Format("{0}/{1} UPnP/1.0 RSSDP/{2}", _OSName, _OSVersion, ServerVersion);
            values["NTS"] = "ssdp:byebye";
            values["NT"] = notificationType;
            values["USN"] = uniqueServiceName;

            var message = SsdpHelper.BuildMessage(header, values);

            return _CommsServer.SendMulticastMessage(message);

            //WriteTrace(String.Format("Sent byebye notification"), device);
        }

        #endregion

        #region Rebroadcast Timer

        private void DisposeRebroadcastTimer()
        {
            var timer = _RebroadcastAliveNotificationsTimer;
            _RebroadcastAliveNotificationsTimer = null;
            if (timer != null)
                timer.Dispose();
        }

        private void SetRebroadcastAliveNotificationsTimer(TimeSpan minCacheTime)
        {
            //if (minCacheTime == _RebroadcastAliveNotificationsTimeSpan) return;

            DisposeRebroadcastTimer();

            if (minCacheTime == TimeSpan.Zero) return;

            // According to UPnP/SSDP spec, we should randomise the interval at 
            // which we broadcast notifications, to help with network congestion.
            // Specs also advise to choose a random interval up to *half* the cache time.
            // Here we do that, but using the minimum non-zero cache time of any device we are publishing.
            var rebroadCastInterval = new TimeSpan(minCacheTime.Ticks);

            // If we were already setup to rebroadcast someime in the future,
            // don't just blindly reset the next broadcast time to the new interval
            // as repeatedly changing the interval might end up causing us to over
            // delay in sending the next one.
            var nextBroadcastInterval = rebroadCastInterval;
            if (_LastNotificationTime != DateTime.MinValue)
            {
                nextBroadcastInterval = rebroadCastInterval.Subtract(DateTime.Now.Subtract(_LastNotificationTime));
                if (nextBroadcastInterval.Ticks < 0)
                    nextBroadcastInterval = TimeSpan.Zero;
                else if (nextBroadcastInterval > rebroadCastInterval)
                    nextBroadcastInterval = rebroadCastInterval;
            }

            //_RebroadcastAliveNotificationsTimeSpan = rebroadCastInterval;
            _RebroadcastAliveNotificationsTimer = _timerFactory.Create(SendAllAliveNotifications, null, nextBroadcastInterval, rebroadCastInterval);

            WriteTrace(String.Format("Rebroadcast Interval = {0}, Next Broadcast At = {1}", rebroadCastInterval.ToString(), nextBroadcastInterval.ToString()));
        }

        private TimeSpan GetMinimumNonZeroCacheLifetime()
        {
            var nonzeroCacheLifetimesQuery = (from device
                                                                                in _Devices
                                              where device.CacheLifetime != TimeSpan.Zero
                                              select device.CacheLifetime).ToList();

            if (nonzeroCacheLifetimesQuery.Any())
                return nonzeroCacheLifetimesQuery.Min();
            else
                return TimeSpan.Zero;
        }

        #endregion

        #endregion

        private static string GetFirstHeaderValue(System.Net.Http.Headers.HttpRequestHeaders httpRequestHeaders, string headerName)
        {
            string retVal = null;
            IEnumerable<String> values = null;
            if (httpRequestHeaders.TryGetValues(headerName, out values) && values != null)
                retVal = values.FirstOrDefault();

            return retVal;
        }

        public static Action<string> LogFunction { get; set; }

        private static void WriteTrace(string text)
        {
            if (LogFunction != null)
            {
                LogFunction(text);
            }
            //System.Diagnostics.Debug.WriteLine(text, "SSDP Publisher");
        }

        private static void WriteTrace(string text, SsdpDevice device)
        {
            var rootDevice = device as SsdpRootDevice;
            if (rootDevice != null)
                WriteTrace(text + " " + device.DeviceType + " - " + device.Uuid + " - " + rootDevice.Location);
            else
                WriteTrace(text + " " + device.DeviceType + " - " + device.Uuid);
        }

        private void ConnectToDeviceEvents(SsdpDevice device)
        {
            device.DeviceAdded += device_DeviceAdded;
            device.DeviceRemoved += device_DeviceRemoved;

            foreach (var childDevice in device.Devices)
            {
                ConnectToDeviceEvents(childDevice);
            }
        }

        private void DisconnectFromDeviceEvents(SsdpDevice device)
        {
            device.DeviceAdded -= device_DeviceAdded;
            device.DeviceRemoved -= device_DeviceRemoved;

            foreach (var childDevice in device.Devices)
            {
                DisconnectFromDeviceEvents(childDevice);
            }
        }

        #endregion

        #region Event Handlers

        private void device_DeviceAdded(object sender, DeviceEventArgs e)
        {
            SendAliveNotifications(e.Device, false);
            ConnectToDeviceEvents(e.Device);
        }

        private void device_DeviceRemoved(object sender, DeviceEventArgs e)
        {
            var task = SendByeByeNotifications(e.Device, false);
            Task.WaitAll(task);
            DisconnectFromDeviceEvents(e.Device);
        }

        private void CommsServer_RequestReceived(object sender, RequestReceivedEventArgs e)
        {
            if (this.IsDisposed) return;

            if (string.Equals(e.Message.Method.Method, SsdpConstants.MSearchMethod, StringComparison.OrdinalIgnoreCase))
            {
                //According to SSDP/UPnP spec, ignore message if missing these headers.
                // Edit: But some devices do it anyway
                //if (!e.Message.Headers.Contains("MX"))
                //	WriteTrace("Ignoring search request - missing MX header.");
                //else if (!e.Message.Headers.Contains("MAN"))
                //	WriteTrace("Ignoring search request - missing MAN header.");
                //else
                ProcessSearchRequest(GetFirstHeaderValue(e.Message.Headers, "MX"), GetFirstHeaderValue(e.Message.Headers, "ST"), e.ReceivedFrom);
            }
        }

        #endregion

        #region Private Classes

        private class SearchRequest
        {
            public IpEndPointInfo EndPoint { get; set; }
            public DateTime Received { get; set; }
            public string SearchTarget { get; set; }

            public string Key
            {
                get { return this.SearchTarget + ":" + this.EndPoint.ToString(); }
            }

            public bool IsOld()
            {
                return DateTime.UtcNow.Subtract(this.Received).TotalMilliseconds > 500;
            }
        }

        #endregion

    }
}