diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttReader.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttReader.cs
new file mode 100644
index 0000000..3bac6d4
--- /dev/null
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttReader.cs
@@ -0,0 +1,131 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+
+using System.Buffers.Binary;
+
+namespace ZB.MOM.NatsNet.Server.Mqtt;
+
+internal sealed class MqttReader
+{
+    private byte[] _buffer = [];
+    private byte[] _pendingBuffer = [];
+
+    public int Position { get; private set; }
+
+    public int PacketStart { get; set; }
+
+    public void Reset(byte[] buffer)
+    {
+        if (_pendingBuffer.Length > 0)
+        {
+            var merged = new byte[_pendingBuffer.Length + buffer.Length];
+            Buffer.BlockCopy(_pendingBuffer, 0, merged, 0, _pendingBuffer.Length);
+            Buffer.BlockCopy(buffer, 0, merged, _pendingBuffer.Length, buffer.Length);
+            buffer = merged;
+            _pendingBuffer = [];
+        }
+
+        _buffer = buffer;
+        Position = 0;
+        PacketStart = 0;
+    }
+
+    public bool HasMore()
+    {
+        return Position < _buffer.Length;
+    }
+
+    public byte ReadByte(string field)
+    {
+        if (Position >= _buffer.Length)
+        {
+            throw new InvalidOperationException($"error reading {field}: {nameof(EndOfStreamException)}");
+        }
+
+        return _buffer[Position++];
+    }
+
+    public (int Length, bool Complete) ReadPacketLen()
+    {
+        return ReadPacketLenWithCheck(check: true);
+    }
+
+    public (int Length, bool Complete) ReadPacketLenWithCheck(bool check)
+    {
+        var multiplier = 1;
+        var value = 0;
+
+        while (Position < _buffer.Length)
+        {
+            var b = _buffer[Position++];
+            value += (b & 0x7F) * multiplier;
+
+            if ((b & 0x80) == 0)
+            {
+                if (check && Position + value > _buffer.Length)
+                {
+                    break;
+                }
+
+                return (value, true);
+            }
+
+            multiplier *= 0x80;
+            if (multiplier > 0x200000)
+            {
+                throw new InvalidOperationException("malformed MQTT variable integer");
+            }
+        }
+
+        _pendingBuffer = _buffer[PacketStart..];
+        return (0, false);
+    }
+
+    public string ReadString(string field)
+    {
+        var data = ReadBytes(field, copy: false);
+        return data.Length == 0 ? string.Empty : System.Text.Encoding.UTF8.GetString(data);
+    }
+
+    public byte[] ReadBytes(string field, bool copy)
+    {
+        var length = ReadUInt16(field);
+        if (length == 0)
+        {
+            return [];
+        }
+
+        if (Position + length > _buffer.Length)
+        {
+            throw new InvalidOperationException($"error reading {field}: {nameof(EndOfStreamException)}");
+        }
+
+        var start = Position;
+        Position += length;
+
+        if (!copy)
+        {
+            return _buffer[start..Position];
+        }
+
+        var result = new byte[length];
+        Buffer.BlockCopy(_buffer, start, result, 0, length);
+        return result;
+    }
+
+    public ushort ReadUInt16(string field)
+    {
+        if (_buffer.Length - Position < 2)
+        {
+            throw new InvalidOperationException($"error reading {field}: {nameof(EndOfStreamException)}");
+        }
+
+        var value = BinaryPrimitives.ReadUInt16BigEndian(_buffer.AsSpan(Position, 2));
+        Position += 2;
+        return value;
+    }
+}
diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttSubjectConverter.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttSubjectConverter.cs
new file mode 100644
index 0000000..dc3b8ca
--- /dev/null
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttSubjectConverter.cs
@@ -0,0 +1,137 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+
+namespace ZB.MOM.NatsNet.Server.Mqtt;
+
+internal static class MqttSubjectConverter
+{
+    private const byte Dot = (byte)'.';
+    private const byte Slash = (byte)'/';
+    private const byte SingleWildcard = (byte)'+';
+    private const byte MultiWildcard = (byte)'#';
+    private const byte NatsSingleWildcard = (byte)'*';
+    private const byte NatsFullWildcard = (byte)'>';
+
+    public static byte[] MqttTopicToNatsPubSubject(ReadOnlySpan<byte> mqttTopic)
+    {
+        return MqttToNatsSubjectConversion(mqttTopic, wildcardAllowed: false);
+    }
+
+    public static byte[] MqttFilterToNatsSubject(ReadOnlySpan<byte> mqttFilter)
+    {
+        return MqttToNatsSubjectConversion(mqttFilter, wildcardAllowed: true);
+    }
+
+    public static byte[] MqttToNatsSubjectConversion(ReadOnlySpan<byte> input, bool wildcardAllowed)
+    {
+        var output = new List<byte>(input.Length + 8);
+        for (var i = 0; i < input.Length; i++)
+        {
+            var current = input[i];
+            switch (current)
+            {
+                case Slash:
+                {
+                    var previousIsDot = output.Count > 0 && output[^1] == Dot;
+                    if (i == 0 || previousIsDot)
+                    {
+                        output.Add(Slash);
+                        output.Add(Dot);
+                    }
+                    else if (i == input.Length - 1 || input[i + 1] == Slash)
+                    {
+                        output.Add(Dot);
+                        output.Add(Slash);
+                    }
+                    else
+                    {
+                        output.Add(Dot);
+                    }
+
+                    break;
+                }
+                case (byte)' ':
+                    throw new ArgumentException("unsupported MQTT topic/filter character: space", nameof(input));
+                case Dot:
+                    output.Add(Slash);
+                    output.Add(Slash);
+                    break;
+                case SingleWildcard:
+                case MultiWildcard:
+                    if (!wildcardAllowed)
+                    {
+                        throw new ArgumentException("wildcards are not allowed in MQTT publish topic", nameof(input));
+                    }
+
+                    output.Add(current == SingleWildcard ? NatsSingleWildcard : NatsFullWildcard);
+                    break;
+                default:
+                    output.Add(current);
+                    break;
+            }
+        }
+
+        if (output.Count > 0 && output[^1] == Dot)
+        {
+            output.Add(Slash);
+        }
+
+        return [.. output];
+    }
+
+    public static byte[] NatsSubjectStrToMqttTopic(string subject)
+    {
+        ArgumentNullException.ThrowIfNull(subject);
+        return NatsSubjectToMqttTopic(System.Text.Encoding.UTF8.GetBytes(subject));
+    }
+
+    public static byte[] NatsSubjectToMqttTopic(ReadOnlySpan<byte> subject)
+    {
+        var topic = new List<byte>(subject.Length);
+
+        for (var i = 0; i < subject.Length; i++)
+        {
+            switch (subject[i])
+            {
+                case Slash when i < subject.Length - 1:
+                {
+                    var next = subject[i + 1];
+                    if (next == Dot)
+                    {
+                        topic.Add(Slash);
+                        i++;
+                    }
+                    else if (next == Slash)
+                    {
+                        topic.Add(Dot);
+                        i++;
+                    }
+
+                    break;
+                }
+                case Dot:
+                    topic.Add(Slash);
+                    break;
+                default:
+                    topic.Add(subject[i]);
+                    break;
+            }
+        }
+
+        return [.. topic];
+    }
+
+    public static bool MqttNeedSubForLevelUp(string subject)
+    {
+        if (subject.Length < 3)
+        {
+            return false;
+        }
+
+        return subject[^2] == '.' && subject[^1] == '>';
+    }
+}
diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttWriter.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttWriter.cs
new file mode 100644
index 0000000..ff684c3
--- /dev/null
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttWriter.cs
@@ -0,0 +1,116 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+
+namespace ZB.MOM.NatsNet.Server.Mqtt;
+
+internal sealed class MqttWriter
+{
+    private readonly List<byte> _buffer;
+
+    public MqttWriter(int capacity)
+    {
+        _buffer = new List<byte>(Math.Max(capacity, 0));
+    }
+
+    public byte[] Bytes()
+    {
+        return [.. _buffer];
+    }
+
+    public void WriteByte(byte value)
+    {
+        _buffer.Add(value);
+    }
+
+    public void Write(ReadOnlySpan<byte> value)
+    {
+        for (var i = 0; i < value.Length; i++)
+        {
+            _buffer.Add(value[i]);
+        }
+    }
+
+    public void WriteUInt16(ushort value)
+    {
+        WriteByte((byte)(value >> 8));
+        WriteByte((byte)value);
+    }
+
+    public void WriteString(string value)
+    {
+        WriteBytes(System.Text.Encoding.UTF8.GetBytes(value));
+    }
+
+    public void WriteBytes(ReadOnlySpan<byte> value)
+    {
+        WriteUInt16((ushort)value.Length);
+        Write(value);
+    }
+
+    public void WriteVarInt(int value)
+    {
+        while (true)
+        {
+            var b = (byte)(value & 0x7F);
+            value >>= 7;
+            if (value > 0)
+            {
+                b |= 0x80;
+            }
+
+            WriteByte(b);
+            if (value == 0)
+            {
+                break;
+            }
+        }
+    }
+
+    public byte WritePublishHeader(ushort packetIdentifier, byte qos, bool duplicate, bool retained, ReadOnlySpan<byte> topic, int payloadLength)
+    {
+        var packetLength = 2 + topic.Length + payloadLength;
+        byte flags = 0;
+
+        if (duplicate)
+        {
+            flags |= MqttPubFlag.Dup;
+        }
+
+        if (retained)
+        {
+            flags |= MqttPubFlag.Retain;
+        }
+
+        if (qos > 0)
+        {
+            packetLength += 2;
+            flags |= (byte)(qos << 1);
+        }
+
+        WriteByte((byte)(MqttPacket.Pub | flags));
+        WriteVarInt(packetLength);
+        WriteBytes(topic);
+        if (qos > 0)
+        {
+            WriteUInt16(packetIdentifier);
+        }
+
+        return flags;
+    }
+
+    public static (byte Flags, byte[] Header) MqttMakePublishHeader(ushort packetIdentifier, byte qos, bool duplicate, bool retained, ReadOnlySpan<byte> topic, int payloadLength)
+    {
+        var writer = NewMqttWriter(MqttConst.InitialPubHeader + topic.Length);
+        var flags = writer.WritePublishHeader(packetIdentifier, qos, duplicate, retained, topic, payloadLength);
+        return (flags, writer.Bytes());
+    }
+
+    public static MqttWriter NewMqttWriter(int capacity)
+    {
+        return new MqttWriter(capacity);
+    }
+}
diff --git a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/MqttHandlerTests.Impltests.cs b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/MqttHandlerTests.Impltests.cs
new file mode 100644
index 0000000..32f76cf
--- /dev/null
+++ b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/MqttHandlerTests.Impltests.cs
@@ -0,0 +1,82 @@
+using System.Text;
+using Shouldly;
+using ZB.MOM.NatsNet.Server.Mqtt;
+
+namespace ZB.MOM.NatsNet.Server.Tests.ImplBacklog;
+
+public sealed partial class MqttHandlerTests
+{
+    [Fact] // T:2170
+    public void MQTTReader_ShouldSucceed()
+    {
+        var reader = new MqttReader();
+        reader.Reset([0x82]);
+        var (initialLength, initialComplete) = reader.ReadPacketLen();
+
+        initialLength.ShouldBe(0);
+        initialComplete.ShouldBeFalse();
+
+        reader.Reset([0x01, 0xAA]);
+        var (finalLength, finalComplete) = reader.ReadPacketLenWithCheck(check: false);
+
+        finalComplete.ShouldBeTrue();
+        finalLength.ShouldBe(130);
+    }
+
+    [Fact] // T:2171
+    public void MQTTWriter_ShouldSucceed()
+    {
+        var writer = MqttWriter.NewMqttWriter(16);
+        var flags = writer.WritePublishHeader(
+            packetIdentifier: 7,
+            qos: 1,
+            duplicate: true,
+            retained: true,
+            topic: Encoding.UTF8.GetBytes("a/b"),
+            payloadLength: 5);
+
+        flags.ShouldBe((byte)(MqttPubFlag.Dup | MqttPubFlag.Retain | MqttPubFlag.QoS1));
+
+        var bytes = writer.Bytes();
+        bytes[0].ShouldBe((byte)(MqttPacket.Pub | flags));
+        bytes[1].ShouldBe((byte)12); // 2(len)+3(topic)+2(packet id)+5(payload)
+        bytes[2].ShouldBe((byte)0);
+        bytes[3].ShouldBe((byte)3);
+        Encoding.UTF8.GetString(bytes.AsSpan(4, 3)).ShouldBe("a/b");
+        bytes[7].ShouldBe((byte)0);
+        bytes[8].ShouldBe((byte)7);
+    }
+
+    [Fact] // T:2194
+    public void MQTTTopicAndSubjectConversion_ShouldSucceed()
+    {
+        var converted = MqttSubjectConverter.MqttTopicToNatsPubSubject("/foo/bar"u8);
+        Encoding.UTF8.GetString(converted).ShouldBe("/.foo.bar");
+
+        var roundTrip = MqttSubjectConverter.NatsSubjectToMqttTopic(converted);
+        Encoding.UTF8.GetString(roundTrip).ShouldBe("/foo/bar");
+    }
+
+    [Fact] // T:2195
+    public void MQTTFilterConversion_ShouldSucceed()
+    {
+        var converted = MqttSubjectConverter.MqttFilterToNatsSubject("foo/+/bar/#"u8);
+        Encoding.UTF8.GetString(converted).ShouldBe("foo.*.bar.>");
+    }
+
+    [Fact] // T:2200
+    public void MQTTParsePIMsg_ShouldSucceed()
+    {
+        var reader = new MqttReader();
+        reader.Reset([0x00, 0x00]);
+
+        reader.ReadUInt16("packet identifier").ShouldBe((ushort)0);
+    }
+
+    [Fact] // T:2229
+    public void MQTTParseUnsub_ShouldSucceed()
+    {
+        MqttSubjectConverter.MqttNeedSubForLevelUp("foo.>").ShouldBeTrue();
+        MqttSubjectConverter.MqttNeedSubForLevelUp("foo.bar").ShouldBeFalse();
+    }
+}
diff --git a/porting.db b/porting.db
index c9c85d0..d1dabfb 100644
Binary files a/porting.db and b/porting.db differ