Add support for MQTT 3.1.1 Bridge flag.

Source/MQTTnet/Client/Options/MqttClientOptions.cs

@@ -98,6 +98,16 @@ namespace MQTTnet.Client
         /// </summary>
         public ushort TopicAliasMaximum { get; set; }
 
+        /// <summary>
+        ///     If set to true, the bridge will attempt to indicate to the remote broker that it is a bridge not an ordinary
+        ///     client.
+        ///     If successful, this means that loop detection will be more effective and that retained messages will be propagated
+        ///     correctly.
+        ///     Not all brokers support this feature so it may be necessary to set it to false if your bridge does not connect
+        ///     properly.
+        /// </summary>
+        public bool TryPrivate { get; set; } = true;
+
         /// <summary>
         ///     Gets or sets the user properties.
         ///     In MQTT 5, user properties are basic UTF-8 string key-value pairs that you can append to almost every type of MQTT

Source/MQTTnet/Client/Options/MqttClientOptionsBuilder.cs

@@ -96,16 +96,6 @@ namespace MQTTnet.Client
             return this;
         }
 
-        /// <summary>
-        ///     Sets the timeout which will be applied at socket level and internal operations.
-        ///     The default value is the same as for sockets in .NET in general.
-        /// </summary>
-        public MqttClientOptionsBuilder WithTimeout(TimeSpan value)
-        {
-            _options.Timeout = value;
-            return this;
-        }
-
         public MqttClientOptionsBuilder WithConnectionUri(Uri uri)
         {
             if (uri == null)
@@ -289,6 +279,16 @@ namespace MQTTnet.Client
             return this;
         }
 
+        /// <summary>
+        ///     Sets the timeout which will be applied at socket level and internal operations.
+        ///     The default value is the same as for sockets in .NET in general.
+        /// </summary>
+        public MqttClientOptionsBuilder WithTimeout(TimeSpan value)
+        {
+            _options.Timeout = value;
+            return this;
+        }
+
         public MqttClientOptionsBuilder WithTls(MqttClientOptionsBuilderTlsParameters parameters)
         {
             _tlsParameters = parameters;
@@ -322,6 +322,20 @@ namespace MQTTnet.Client
             return this;
         }
 
+        /// <summary>
+        ///     If set to true, the bridge will attempt to indicate to the remote broker that it is a bridge not an ordinary
+        ///     client.
+        ///     If successful, this means that loop detection will be more effective and that retained messages will be propagated
+        ///     correctly.
+        ///     Not all brokers support this feature so it may be necessary to set it to false if your bridge does not connect
+        ///     properly.
+        /// </summary>
+        public MqttClientOptionsBuilder WithTryPrivate(bool tryPrivate = true)
+        {
+            _options.TryPrivate = true;
+            return this;
+        }
+
         public MqttClientOptionsBuilder WithUserProperty(string name, string value)
         {
             if (name is null)

Source/MQTTnet/Formatter/MqttConnectPacketFactory.cs

@@ -33,7 +33,8 @@ namespace MQTTnet.Formatter
                 RequestResponseInformation = clientOptions.RequestResponseInformation,
                 SessionExpiryInterval = clientOptions.SessionExpiryInterval,
                 TopicAliasMaximum = clientOptions.TopicAliasMaximum,
-                UserProperties = clientOptions.UserProperties
+                UserProperties = clientOptions.UserProperties,
+                TryPrivate = clientOptions.TryPrivate
             };
 
             if (!string.IsNullOrEmpty(clientOptions.WillTopic))

Source/MQTTnet/Formatter/MqttPacketFormatterAdapter.cs

@@ -95,6 +95,10 @@ namespace MQTTnet.Formatter
             var protocolName = _bufferReader.ReadString();
             var protocolLevel = _bufferReader.ReadByte();
 
+            // Remove the mosquitto try_private flag (MQTT 3.1.1 Bridge).
+            // This flag is accepted but not yet used.
+            protocolLevel &= 0x7F;
+            
             if (protocolName == "MQTT")
             {
                 if (protocolLevel == 5)

Source/MQTTnet/Formatter/V3/MqttV3PacketFormatter.cs

@@ -175,12 +175,18 @@ namespace MQTTnet.Formatter.V3
                 throw new MqttProtocolViolationException("MQTT protocol name do not match MQTT v3.");
             }
 
+            var tryPrivate = (protocolVersion & 0x80) > 0;
+            protocolVersion &= 0x7F;
+            
             if (protocolVersion != 3 && protocolVersion != 4)
             {
                 throw new MqttProtocolViolationException("MQTT protocol version do not match MQTT v3.");
             }
 
-            var packet = new MqttConnectPacket();
+            var packet = new MqttConnectPacket
+            {
+                TryPrivate = tryPrivate
+            };
 
             var connectFlags = _bufferReader.ReadByte();
             if ((connectFlags & 0x1) > 0)
@@ -409,7 +415,14 @@ namespace MQTTnet.Formatter.V3
             ValidateConnectPacket(packet);
 
             bufferWriter.WriteString("MQIsdp");
-            bufferWriter.WriteByte(3); // Protocol Level 3
+
+            var protocolVersion = 3;
+            if (packet.TryPrivate)
+            {
+                protocolVersion |= 0x80;
+            }
+            
+            bufferWriter.WriteByte((byte)protocolVersion);
 
             byte connectFlags = 0x0;
             if (packet.CleanSession)

Source/MQTTnet/Packets/MqttConnectPacket.cs

@@ -64,6 +64,8 @@ namespace MQTTnet.Packets
 
         public List<MqttUserProperty> WillUserProperties { get; set; }
 
+        public bool TryPrivate { get; set; }
+        
         public override string ToString()
         {
             var passwordText = string.Empty;