Add MrGameEng.Net: WebSocket transport + delta component replication

Browsers can't speak UDP, so WebSocket is the engine's one transport.
The server side is a dependency-free RFC 6455 implementation over
TcpListener (handshake, frame codec with masking and fragmentation,
ping/pong — unit-tested against the RFC example vectors); the client
wraps ClientWebSocket, which works on desktop and maps to the browser
WebSocket in Blazor WASM. Both sit behind the poll-based INetConnection
so simulation systems drain messages from their own thread; client
sends are chained fire-and-forget (no blocking — wasm-safe).

Replication is server-authoritative: games register unmanaged component
types in a ReplicationSchema (same order both sides, up to 32 types),
ReplicationServer snapshots entities carrying NetId once per send and
ships each connection only the components that changed since its last
snapshot — a reliable ordered transport needs no acks for deltas. New
connections receive the full state through the same path; despawns are
tracked by set difference. ReplicationClient applies snapshots to a
local EntityStore and raises EntitySpawned so the game can decorate
replicated entities with presentation components.

Covered by 16 tests including a real loopback exchange between
WebSocketClient and WebSocketServer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -64,6 +64,13 @@ Engine libraries (each feature is a namespaced subfolder of its host):
   **Collisions** (`MrGameEng.Collisions`: `Collider` component, spatial hash rebuilt per
   tick, pairs/queries/raycast, `scene.UseCollisions()` after movement systems).
   → `Core`, `Graphics` (Collisions needs `Transform2D`, `RectF`).
+- **`Net`** — multiplayer building blocks, browser-compatible by design: a dependency-free
+  RFC 6455 WebSocket server over `TcpListener` (browsers can't speak UDP, so WebSocket is
+  the engine's one transport), a `ClientWebSocket`-based client (works in Blazor WASM),
+  both behind the poll-based `INetConnection`; server-authoritative component replication
+  (`ReplicationSchema` of unmanaged components, `ReplicationServer` sending per-connection
+  deltas — no acks needed over a reliable ordered transport, `ReplicationClient` applying
+  snapshots to a local store, `NetId`). → `Core`.
 - **`UI`** — Myra integration (`scene.UseUI()` after `UseRenderer2D()`),
   **DevConsole** (`MrGameEng.DevConsole`: in-game console capturing `Core.Log`,
   `scene.UseDevConsole()` last in OnLoad) and **Inspector** (`MrGameEng.Inspector`: a

MrGameEng.sln

@@ -43,6 +43,10 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Host", "src\MrGam
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Host.Tests", "tests\MrGameEng.Host.Tests\MrGameEng.Host.Tests.csproj", "{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Net", "src\MrGameEng.Net\MrGameEng.Net.csproj", "{A4E754C7-C5FD-43A2-B345-34152D3A22D1}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MrGameEng.Net.Tests", "tests\MrGameEng.Net.Tests\MrGameEng.Net.Tests.csproj", "{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -257,6 +261,30 @@ Global
 		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x64.Build.0 = Release|Any CPU
 		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x86.ActiveCfg = Release|Any CPU
 		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2}.Release|x86.Build.0 = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|x64.Build.0 = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Debug|x86.Build.0 = Debug|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|Any CPU.Build.0 = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|x64.ActiveCfg = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|x64.Build.0 = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|x86.ActiveCfg = Release|Any CPU
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1}.Release|x86.Build.0 = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|x64.Build.0 = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Debug|x86.Build.0 = Debug|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|Any CPU.Build.0 = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|x64.ActiveCfg = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|x64.Build.0 = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|x86.ActiveCfg = Release|Any CPU
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -279,5 +307,7 @@ Global
 		{4407F6E6-0B65-41A3-ADFA-B78684A9B918} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
 		{59818072-0D2B-4007-A50F-1343FA189EC6} = {827E0CD3-B72D-47B6-A68D-7590B98EB39B}
 		{B3EB5318-5EC0-4F23-8ECA-0EC5A0C4DFD2} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
+		{A4E754C7-C5FD-43A2-B345-34152D3A22D1} = {827E0CD3-B72D-47B6-A68D-7590B98EB39B}
+		{434C24AC-08C6-483E-A2DE-6DBC8A4DE791} = {0AB3BF05-4346-4AA6-1389-037BE0695223}
 	EndGlobalSection
 EndGlobal

docs/architecture.md

@@ -39,6 +39,7 @@
 | `MrGameEng.Audio`            | Звуковые эффекты и музыка (NVorbis); `AudioManager` с `SoundVolume`/`MasterVolume` (одна ручка на эффекты и музыку) |
 | `MrGameEng.Content`          | Пайплайн контента. **Assets** (`MrGameEng.Assets`): runtime-загрузка без Content Pipeline, кэш, `AssetRef<T>`. **Atlases** (`MrGameEng.Atlases`): текстурные атласы — сборка из дерева картинок (`AtlasBuilder`) и рантайм-загрузка (`TextureAtlas`), CLI `tools/MrGameEng.AtlasTool`. **Mods** (`MrGameEng.Mods`): система модов — порядок загрузки, JSON-дефы, локализация, слияние деревьев контента |
 | `MrGameEng.Simulation`       | Детерминированные геймплей-примитивы без данных мира. **Pathfinding** (`MrGameEng.Pathfinding`): A*, Dijkstra, BFS, flow fields по гриду. **AI** (`MrGameEng.AI`): utility-ИИ — кривые отклика, соображения, действия, выбор (`UtilityAi<TContext>`), `Blackboard`. **Collisions** (`MrGameEng.Collisions`): компонент `Collider`, spatial hash, пары/запросы/raycast |
+| `MrGameEng.Net`              | Мультиплеер, совместимый с браузером по построению: WebSocket-сервер (RFC 6455 поверх `TcpListener`, без зависимостей — браузер не умеет UDP, поэтому транспорт движка один — WebSocket), клиент на `ClientWebSocket` (работает в Blazor WASM), оба за poll-интерфейсом `INetConnection`; server-authoritative репликация компонентов: `ReplicationSchema` (unmanaged-компоненты, до 32 типов), `ReplicationServer` (пер-соединенческие дельты против последнего отправленного — ack не нужны поверх надёжного упорядоченного транспорта), `ReplicationClient` (применение снапшотов в локальный `EntityStore`), компонент `NetId` |
 | `MrGameEng.UI`               | Игровой UI на [Myra](https://github.com/rds1983/Myra): `Desktop` на сцену, виджеты, скининг. **DevConsole** (`MrGameEng.DevConsole`): ингейм-консоль — логи `Log`, команды, история, автодополнение. **Inspector** (`MrGameEng.Inspector`): ECS-дебагер в духе Chrome DevTools — дерево сущностей по архетипам, компоненты/поля с правкой простых полей, выбор кликом по миру с подсветкой, вкладка перфа рендера (`scene.UseInspector(renderer)`, F1) |
 | `MrGameEng.Assets.Generator` | Roslyn incremental source generator: классы с типизированными хендлами ресурсов (отдельный анализатор netstandard2.0) |
 
@@ -72,6 +73,7 @@
 ```
 MrGameEng.Host        ─┐
 MrGameEng.Audio       ─┤
+MrGameEng.Net         ─┤
 MrGameEng.Graphics    ─┼──► MrGameEng.Core ──► Friflo.Engine.ECS
                        │
 MrGameEng.Content     ─┤

src/MrGameEng.Net/INetConnection.cs

@@ -0,0 +1,25 @@
+namespace MrGameEng.Net;
+
+/// <summary>
+/// A bidirectional, reliable, ordered binary message channel (a WebSocket under the hood).
+/// Receiving is poll-based to fit the simulation loop: incoming messages queue up on a
+/// background reader and are drained with <see cref="TryReceive"/> from the tick. Sending
+/// never blocks the caller. Implementations are safe to use from one simulation thread.
+/// </summary>
+public interface INetConnection
+{
+    /// <summary>Connection id, unique within its owner (server-assigned; 0 for a client's own connection).</summary>
+    int Id { get; }
+
+    /// <summary>False once the peer disconnected or the connection failed; sends become no-ops.</summary>
+    bool IsOpen { get; }
+
+    /// <summary>Queues one binary message for delivery. No-op when the connection is closed.</summary>
+    void Send(ReadOnlySpan<byte> message);
+
+    /// <summary>Dequeues the next received binary message, if any.</summary>
+    bool TryReceive(out byte[] message);
+
+    /// <summary>Closes the connection.</summary>
+    void Close();
+}

src/MrGameEng.Net/MrGameEng.Net.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Friflo.Engine.ECS" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="MrGameEng.Net.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MrGameEng.Core\MrGameEng.Core.csproj" />
+  </ItemGroup>
+</Project>

src/MrGameEng.Net/Replication/NetId.cs

@@ -0,0 +1,14 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Marks an entity as replicated and identifies it across the network. The server assigns
+/// values (see <see cref="ReplicationServer.NextNetId"/>); the client creates a local
+/// entity with the same <see cref="Value"/> when the first snapshot arrives.
+/// </summary>
+public struct NetId : IComponent
+{
+    /// <summary>Network-wide entity id, unique per server world.</summary>
+    public int Value;
+}

src/MrGameEng.Net/Replication/ReplicationClient.cs

@@ -0,0 +1,92 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Client side of replication: applies snapshot messages from a
+/// <see cref="ReplicationServer"/> to a local <see cref="EntityStore"/>. Unknown net ids
+/// spawn local entities (carrying <see cref="NetId"/>), known ones get their changed
+/// components overwritten, despawns delete. The game decorates replicated entities with
+/// presentation components (sprites etc.) on top — replication never touches types outside
+/// its <see cref="ReplicationSchema"/>.
+/// </summary>
+public sealed class ReplicationClient
+{
+    /// <summary>Number of replicated entities currently alive locally.</summary>
+    public int EntityCount => _entities.Count;
+
+    /// <summary>Raised after an entity is created from a snapshot. Hook presentation setup here.</summary>
+    public event Action<Entity>? EntitySpawned;
+
+    private readonly ReplicationSchema _schema;
+    private readonly EntityStore _store;
+    private readonly Dictionary<int, Entity> _entities = [];
+
+    /// <summary>Creates a replication client writing into <paramref name="store"/>.</summary>
+    public ReplicationClient(ReplicationSchema schema, EntityStore store)
+    {
+        _schema = schema;
+        _store = store;
+    }
+
+    /// <summary>Applies every message queued on <paramref name="connection"/>.</summary>
+    public void Pump(INetConnection connection)
+    {
+        while (connection.TryReceive(out var message))
+        {
+            Apply(message);
+        }
+    }
+
+    /// <summary>Applies one snapshot message to the local store.</summary>
+    public void Apply(byte[] message)
+    {
+        using var reader = new BinaryReader(new MemoryStream(message));
+        if (reader.ReadByte() != ReplicationMessage.Snapshot)
+        {
+            return; // незнакомый тип сообщения — пропускаем, это не снапшот
+        }
+
+        var slots = _schema.Slots;
+        var count = reader.ReadInt32();
+        for (var record = 0; record < count; record++)
+        {
+            var netId = reader.ReadInt32();
+            var op = reader.ReadByte();
+            if (op == ReplicationMessage.OpDespawn)
+            {
+                if (_entities.Remove(netId, out var dead))
+                {
+                    dead.DeleteEntity();
+                }
+
+                continue;
+            }
+
+            var mask = reader.ReadUInt32();
+            var spawned = false;
+            if (!_entities.TryGetValue(netId, out var entity))
+            {
+                entity = _store.CreateEntity(new NetId { Value = netId });
+                _entities[netId] = entity;
+                spawned = true;
+            }
+
+            foreach (var slot in slots)
+            {
+                if ((mask & (1u << slot.Bit)) == 0)
+                {
+                    continue;
+                }
+
+                var data = reader.ReadBytes(slot.Size);
+                slot.Apply(entity, data, 0);
+            }
+
+            if (spawned)
+            {
+                EntitySpawned?.Invoke(entity);
+            }
+        }
+    }
+}

src/MrGameEng.Net/Replication/ReplicationSchema.cs

@@ -0,0 +1,64 @@
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// The set of component types a game replicates, registered in the same order on the
+/// server and every client (the order defines the wire ids). Components must be
+/// unmanaged structs — they are blitted to the wire as raw bytes, so server and client
+/// must run on the same engine version. Up to 32 types.
+/// </summary>
+public sealed class ReplicationSchema
+{
+    internal sealed class ComponentSlot
+    {
+        public required int Bit;
+        public required int Size;
+        public required Func<Entity, byte[], bool> TryWrite;
+        public required Action<Entity, byte[], int> Apply;
+    }
+
+    internal readonly List<ComponentSlot> Slots = [];
+
+    /// <summary>
+    /// Registers component type <typeparamref name="T"/> for replication. Returns this
+    /// schema for fluent chaining.
+    /// </summary>
+    public ReplicationSchema Register<T>()
+        where T : unmanaged, IComponent
+    {
+        if (Slots.Count == 32)
+        {
+            throw new InvalidOperationException(
+                "ReplicationSchema supports at most 32 component types."
+            );
+        }
+
+        var size = Unsafe.SizeOf<T>();
+        Slots.Add(
+            new ComponentSlot
+            {
+                Bit = Slots.Count,
+                Size = size,
+                TryWrite = (entity, buffer) =>
+                {
+                    if (!entity.HasComponent<T>())
+                    {
+                        return false;
+                    }
+
+                    MemoryMarshal.Write(buffer, in entity.GetComponent<T>());
+                    return true;
+                },
+                Apply = (entity, data, offset) =>
+                {
+                    var value = MemoryMarshal.Read<T>(data.AsSpan(offset, size));
+                    entity.AddComponent(value);
+                },
+            }
+        );
+        return this;
+    }
+}

src/MrGameEng.Net/Replication/ReplicationServer.cs

@@ -0,0 +1,174 @@
+using Friflo.Engine.ECS;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Server-authoritative component replication. Each call to <see cref="Send"/> snapshots
+/// every entity carrying <see cref="NetId"/> and sends each connection only what changed
+/// since that connection's previous snapshot (per-component deltas; a new connection gets
+/// the full state the same way). Deltas need no acknowledgements because the transport is
+/// reliable and ordered. Call at the desired send rate (e.g. every Nth simulation tick),
+/// from the simulation thread.
+/// </summary>
+public sealed class ReplicationServer
+{
+    private sealed class ConnectionState
+    {
+        // По netId: последний отправленный блоб каждого зарегистрированного компонента.
+        public readonly Dictionary<int, byte[]?[]> LastSent = [];
+    }
+
+    private readonly ReplicationSchema _schema;
+    private readonly ArchetypeQuery<NetId> _query;
+    private readonly Dictionary<INetConnection, ConnectionState> _states = [];
+
+    // Снапшот текущего тика, переиспользуется между соединениями.
+    private readonly List<(int NetId, byte[]?[] Components)> _current = [];
+    private readonly HashSet<int> _currentIds = [];
+    private int _nextNetId;
+
+    /// <summary>Creates a replication server over <paramref name="store"/>.</summary>
+    public ReplicationServer(ReplicationSchema schema, EntityStore store)
+    {
+        _schema = schema;
+        _query = store.Query<NetId>();
+    }
+
+    /// <summary>Allocates the next free network id for a newly spawned replicated entity.</summary>
+    public int NextNetId() => ++_nextNetId;
+
+    /// <summary>
+    /// Snapshots the world once and sends per-connection deltas. Closed connections are
+    /// forgotten; brand-new ones receive the full state.
+    /// </summary>
+    public void Send(IReadOnlyList<INetConnection> connections)
+    {
+        CaptureCurrentState();
+
+        foreach (var connection in connections)
+        {
+            if (!connection.IsOpen)
+            {
+                continue;
+            }
+
+            if (!_states.TryGetValue(connection, out var state))
+            {
+                state = new ConnectionState();
+                _states[connection] = state;
+            }
+
+            var message = BuildDelta(state);
+            if (message is not null)
+            {
+                connection.Send(message);
+            }
+        }
+
+        // Забываем состояние умерших соединений, чтобы не копить мусор.
+        foreach (var dead in _states.Keys.Where(c => !c.IsOpen).ToList())
+        {
+            _states.Remove(dead);
+        }
+    }
+
+    private void CaptureCurrentState()
+    {
+        _current.Clear();
+        _currentIds.Clear();
+        var slots = _schema.Slots;
+        _query.ForEachEntity(
+            (ref NetId netId, Entity entity) =>
+            {
+                var components = new byte[]?[slots.Count];
+                foreach (var slot in slots)
+                {
+                    var buffer = new byte[slot.Size];
+                    components[slot.Bit] = slot.TryWrite(entity, buffer) ? buffer : null;
+                }
+
+                _current.Add((netId.Value, components));
+                _currentIds.Add(netId.Value);
+            }
+        );
+    }
+
+    private byte[]? BuildDelta(ConnectionState state)
+    {
+        using var stream = new MemoryStream();
+        using var writer = new BinaryWriter(stream);
+        writer.Write(ReplicationMessage.Snapshot);
+        var countPosition = stream.Position;
+        writer.Write(0); // количество записей, допишем в конце
+        var records = 0;
+
+        foreach (var (netId, components) in _current)
+        {
+            if (!state.LastSent.TryGetValue(netId, out var lastSent))
+            {
+                lastSent = new byte[]?[components.Length];
+                state.LastSent[netId] = lastSent;
+            }
+
+            uint mask = 0;
+            for (var bit = 0; bit < components.Length; bit++)
+            {
+                var current = components[bit];
+                if (current is null)
+                {
+                    continue;
+                }
+
+                if (lastSent[bit] is null || !current.AsSpan().SequenceEqual(lastSent[bit]))
+                {
+                    mask |= 1u << bit;
+                    lastSent[bit] = current;
+                }
+            }
+
+            if (mask == 0)
+            {
+                continue;
+            }
+
+            writer.Write(netId);
+            writer.Write(ReplicationMessage.OpUpsert);
+            writer.Write(mask);
+            for (var bit = 0; bit < components.Length; bit++)
+            {
+                if ((mask & (1u << bit)) != 0)
+                {
+                    writer.Write(components[bit]!);
+                }
+            }
+
+            records++;
+        }
+
+        // Сущности, которые соединение знает, а в мире их больше нет.
+        foreach (var known in state.LastSent.Keys.Where(id => !_currentIds.Contains(id)).ToList())
+        {
+            state.LastSent.Remove(known);
+            writer.Write(known);
+            writer.Write(ReplicationMessage.OpDespawn);
+            records++;
+        }
+
+        if (records == 0)
+        {
+            return null;
+        }
+
+        stream.Position = countPosition;
+        writer.Write(records);
+        return stream.ToArray();
+    }
+}
+
+/// <summary>Wire constants shared by <see cref="ReplicationServer"/> and <see cref="ReplicationClient"/>.</summary>
+internal static class ReplicationMessage
+{
+    internal const byte Snapshot = 1;
+    internal const byte OpUpsert = 0;
+    internal const byte OpDespawn = 1;
+}

src/MrGameEng.Net/WebSocketClient.cs

@@ -0,0 +1,141 @@
+using System.Collections.Concurrent;
+using System.Net.WebSockets;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// Client side of <see cref="INetConnection"/>: a thin wrapper over the BCL
+/// <see cref="ClientWebSocket"/>, which works on desktop and inside Blazor WebAssembly
+/// (where it maps to the browser's WebSocket). Receiving runs on a background task into a
+/// queue; sends are chained fire-and-forget so the caller — and the browser's single
+/// thread — never blocks.
+/// </summary>
+public sealed class WebSocketClient : INetConnection, IDisposable
+{
+    /// <inheritdoc />
+    public int Id => 0;
+
+    /// <inheritdoc />
+    public bool IsOpen => !_closed && _socket.State == WebSocketState.Open;
+
+    private readonly ClientWebSocket _socket;
+    private readonly ConcurrentQueue<byte[]> _inbox = new();
+    private readonly CancellationTokenSource _shutdown = new();
+    private Task _sendTail = Task.CompletedTask;
+    private volatile bool _closed;
+
+    private WebSocketClient(ClientWebSocket socket) => _socket = socket;
+
+    /// <summary>
+    /// Connects to <paramref name="uri"/> (ws:// or wss://) and starts the receive loop.
+    /// </summary>
+    public static async Task<WebSocketClient> ConnectAsync(
+        Uri uri,
+        CancellationToken cancellationToken = default
+    )
+    {
+        var socket = new ClientWebSocket();
+        await socket.ConnectAsync(uri, cancellationToken).ConfigureAwait(false);
+        var client = new WebSocketClient(socket);
+        _ = Task.Run(client.ReceiveLoop);
+        return client;
+    }
+
+    /// <inheritdoc />
+    public void Send(ReadOnlySpan<byte> message)
+    {
+        if (!IsOpen)
+        {
+            return;
+        }
+
+        var copy = message.ToArray();
+        // Отправки сцеплены в хвост: ClientWebSocket не терпит параллельных SendAsync,
+        // а блокировать поток нельзя (в wasm это смерть).
+        lock (_shutdown)
+        {
+            _sendTail = _sendTail.ContinueWith(
+                _ => SendCore(copy),
+                CancellationToken.None,
+                TaskContinuationOptions.None,
+                TaskScheduler.Default
+            );
+        }
+    }
+
+    private async Task SendCore(byte[] message)
+    {
+        try
+        {
+            await _socket
+                .SendAsync(
+                    message,
+                    WebSocketMessageType.Binary,
+                    endOfMessage: true,
+                    _shutdown.Token
+                )
+                .ConfigureAwait(false);
+        }
+        catch (Exception)
+        {
+            Close();
+        }
+    }
+
+    /// <inheritdoc />
+    public bool TryReceive(out byte[] message) => _inbox.TryDequeue(out message!);
+
+    /// <inheritdoc />
+    public void Close()
+    {
+        if (_closed)
+        {
+            return;
+        }
+
+        _closed = true;
+        _shutdown.Cancel();
+        _socket.Dispose();
+    }
+
+    /// <summary>Closes the connection.</summary>
+    public void Dispose() => Close();
+
+    private async Task ReceiveLoop()
+    {
+        var buffer = new byte[64 * 1024];
+        var message = new MemoryStream();
+        try
+        {
+            while (!_closed)
+            {
+                var result = await _socket
+                    .ReceiveAsync(buffer, _shutdown.Token)
+                    .ConfigureAwait(false);
+                if (result.MessageType == WebSocketMessageType.Close)
+                {
+                    break;
+                }
+
+                message.Write(buffer, 0, result.Count);
+                if (result.EndOfMessage)
+                {
+                    if (result.MessageType == WebSocketMessageType.Binary)
+                    {
+                        _inbox.Enqueue(message.ToArray());
+                    }
+
+                    message.SetLength(0);
+                }
+            }
+        }
+        catch (Exception)
+        {
+            // обрыв или закрытие — штатное завершение цикла
+        }
+        finally
+        {
+            Close();
+        }
+    }
+}

src/MrGameEng.Net/WebSocketProtocol.cs

@@ -0,0 +1,248 @@
+using System.Buffers.Binary;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace MrGameEng.Net;
+
+/// <summary>WebSocket frame opcodes used by the server.</summary>
+internal enum WebSocketOpcode : byte
+{
+    Continuation = 0x0,
+    Text = 0x1,
+    Binary = 0x2,
+    Close = 0x8,
+    Ping = 0x9,
+    Pong = 0xA,
+}
+
+/// <summary>
+/// Minimal RFC 6455 building blocks for the server side: the upgrade handshake and frame
+/// encode/decode over a <see cref="Stream"/>. Kept free of sockets so the protocol logic is
+/// unit-testable; <see cref="WebSocketServer"/> wires it to TCP.
+/// </summary>
+internal static class WebSocketProtocol
+{
+    private const string HandshakeGuid = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
+
+    /// <summary>Computes the Sec-WebSocket-Accept value for a client's Sec-WebSocket-Key.</summary>
+    internal static string AcceptKey(string secWebSocketKey)
+    {
+        var bytes = Encoding.ASCII.GetBytes(secWebSocketKey + HandshakeGuid);
+        return Convert.ToBase64String(SHA1.HashData(bytes));
+    }
+
+    /// <summary>
+    /// Reads the HTTP upgrade request from <paramref name="stream"/> (up to the blank line)
+    /// and extracts the Sec-WebSocket-Key header. Returns false on a malformed request.
+    /// </summary>
+    internal static bool TryReadHandshakeKey(Stream stream, out string key)
+    {
+        key = "";
+        var buffer = new byte[8 * 1024];
+        var length = 0;
+        // Читаем до конца заголовков (\r\n\r\n); запрос маленький, побайтовое чтение не больно.
+        while (length < buffer.Length)
+        {
+            var read = stream.Read(buffer, length, 1);
+            if (read == 0)
+            {
+                return false;
+            }
+
+            length++;
+            if (
+                length >= 4
+                && buffer[length - 4] == (byte)'\r'
+                && buffer[length - 3] == (byte)'\n'
+                && buffer[length - 2] == (byte)'\r'
+                && buffer[length - 1] == (byte)'\n'
+            )
+            {
+                break;
+            }
+        }
+
+        var request = Encoding.ASCII.GetString(buffer, 0, length);
+        foreach (var line in request.Split("\r\n"))
+        {
+            var separator = line.IndexOf(':');
+            if (separator < 0)
+            {
+                continue;
+            }
+
+            if (
+                line[..separator]
+                    .Trim()
+                    .Equals("Sec-WebSocket-Key", StringComparison.OrdinalIgnoreCase)
+            )
+            {
+                key = line[(separator + 1)..].Trim();
+                return key.Length > 0;
+            }
+        }
+
+        return false;
+    }
+
+    /// <summary>Writes the 101 Switching Protocols response completing the handshake.</summary>
+    internal static void WriteHandshakeResponse(Stream stream, string secWebSocketKey)
+    {
+        var response =
+            "HTTP/1.1 101 Switching Protocols\r\n"
+            + "Upgrade: websocket\r\n"
+            + "Connection: Upgrade\r\n"
+            + $"Sec-WebSocket-Accept: {AcceptKey(secWebSocketKey)}\r\n"
+            + "\r\n";
+        var bytes = Encoding.ASCII.GetBytes(response);
+        stream.Write(bytes, 0, bytes.Length);
+    }
+
+    /// <summary>
+    /// Encodes one complete (FIN) frame. Server frames are unmasked per RFC 6455;
+    /// <paramref name="maskKey"/> is for tests that emulate a client.
+    /// </summary>
+    internal static byte[] EncodeFrame(
+        ReadOnlySpan<byte> payload,
+        WebSocketOpcode opcode,
+        byte[]? maskKey = null
+    )
+    {
+        var masked = maskKey is not null;
+        var headerLength =
+            2
+            + payload.Length switch
+            {
+                <= 125 => 0,
+                <= ushort.MaxValue => 2,
+                _ => 8,
+            };
+        var frame = new byte[headerLength + (masked ? 4 : 0) + payload.Length];
+        frame[0] = (byte)(0x80 | (byte)opcode);
+        switch (payload.Length)
+        {
+            case <= 125:
+                frame[1] = (byte)payload.Length;
+                break;
+            case <= ushort.MaxValue:
+                frame[1] = 126;
+                BinaryPrimitives.WriteUInt16BigEndian(frame.AsSpan(2), (ushort)payload.Length);
+                break;
+            default:
+                frame[1] = 127;
+                BinaryPrimitives.WriteUInt64BigEndian(frame.AsSpan(2), (ulong)payload.Length);
+                break;
+        }
+
+        var offset = headerLength;
+        if (masked)
+        {
+            frame[1] |= 0x80;
+            maskKey!.CopyTo(frame, offset);
+            offset += 4;
+            for (var i = 0; i < payload.Length; i++)
+            {
+                frame[offset + i] = (byte)(payload[i] ^ maskKey[i % 4]);
+            }
+        }
+        else
+        {
+            payload.CopyTo(frame.AsSpan(offset));
+        }
+
+        return frame;
+    }
+
+    /// <summary>
+    /// Reads one frame. Returns false on a clean end of stream. Masked payloads are unmasked.
+    /// </summary>
+    internal static bool TryReadFrame(
+        Stream stream,
+        out WebSocketOpcode opcode,
+        out bool fin,
+        out byte[] payload
+    )
+    {
+        opcode = WebSocketOpcode.Close;
+        fin = true;
+        payload = [];
+
+        var header = new byte[2];
+        if (!TryReadExactly(stream, header))
+        {
+            return false;
+        }
+
+        fin = (header[0] & 0x80) != 0;
+        opcode = (WebSocketOpcode)(header[0] & 0x0F);
+        var masked = (header[1] & 0x80) != 0;
+        long length = header[1] & 0x7F;
+        if (length == 126)
+        {
+            var extended = new byte[2];
+            if (!TryReadExactly(stream, extended))
+            {
+                return false;
+            }
+
+            length = BinaryPrimitives.ReadUInt16BigEndian(extended);
+        }
+        else if (length == 127)
+        {
+            var extended = new byte[8];
+            if (!TryReadExactly(stream, extended))
+            {
+                return false;
+            }
+
+            length = (long)BinaryPrimitives.ReadUInt64BigEndian(extended);
+        }
+
+        if (length > MaxPayloadBytes)
+        {
+            return false; // защита от злонамеренной длины — соединение закроется
+        }
+
+        var maskKey = new byte[4];
+        if (masked && !TryReadExactly(stream, maskKey))
+        {
+            return false;
+        }
+
+        payload = new byte[length];
+        if (!TryReadExactly(stream, payload))
+        {
+            return false;
+        }
+
+        if (masked)
+        {
+            for (var i = 0; i < payload.Length; i++)
+            {
+                payload[i] ^= maskKey[i % 4];
+            }
+        }
+
+        return true;
+    }
+
+    /// <summary>Upper bound for a single frame payload accepted by the server.</summary>
+    internal const int MaxPayloadBytes = 16 * 1024 * 1024;
+
+    private static bool TryReadExactly(Stream stream, byte[] buffer)
+    {
+        var offset = 0;
+        while (offset < buffer.Length)
+        {
+            var read = stream.Read(buffer, offset, buffer.Length - offset);
+            if (read == 0)
+            {
+                return false;
+            }
+
+            offset += read;
+        }
+
+        return true;
+    }
+}

src/MrGameEng.Net/WebSocketServer.cs

@@ -0,0 +1,279 @@
+using System.Collections.Concurrent;
+using System.Net;
+using System.Net.Sockets;
+using MrGameEng.Core;
+
+namespace MrGameEng.Net;
+
+/// <summary>
+/// A dependency-free WebSocket server (RFC 6455 over <see cref="TcpListener"/>) for
+/// dedicated servers. Accepting and reading happen on background tasks; the simulation
+/// drains new connections with <see cref="TryAcceptConnection"/> and reads messages by
+/// polling each connection — nothing here touches the ECS world from another thread.
+/// Binary messages only; pings are answered automatically.
+/// </summary>
+public sealed class WebSocketServer : IDisposable
+{
+    /// <summary>The port the server listens on.</summary>
+    public int Port { get; }
+
+    /// <summary>Snapshot of currently open connections.</summary>
+    public IReadOnlyList<INetConnection> Connections
+    {
+        get
+        {
+            lock (_connections)
+            {
+                return _connections.Where(c => c.IsOpen).Cast<INetConnection>().ToArray();
+            }
+        }
+    }
+
+    private readonly TcpListener _listener;
+    private readonly List<ServerConnection> _connections = [];
+    private readonly ConcurrentQueue<ServerConnection> _accepted = new();
+    private readonly CancellationTokenSource _shutdown = new();
+    private int _nextConnectionId;
+    private bool _started;
+
+    /// <summary>Creates a server for <paramref name="port"/> on all interfaces. Call <see cref="Start"/> to listen.</summary>
+    public WebSocketServer(int port)
+    {
+        Port = port;
+        _listener = new TcpListener(IPAddress.Any, port);
+    }
+
+    /// <summary>Starts listening and accepting connections in the background.</summary>
+    public void Start()
+    {
+        if (_started)
+        {
+            return;
+        }
+
+        _started = true;
+        _listener.Start();
+        Task.Run(AcceptLoop);
+        Log.Info($"WebSocketServer listening on port {Port}");
+    }
+
+    /// <summary>Dequeues a connection that completed its handshake since the last call.</summary>
+    public bool TryAcceptConnection(out INetConnection connection)
+    {
+        if (_accepted.TryDequeue(out var accepted))
+        {
+            connection = accepted;
+            return true;
+        }
+
+        connection = null!;
+        return false;
+    }
+
+    /// <summary>Stops listening and closes every connection.</summary>
+    public void Dispose()
+    {
+        _shutdown.Cancel();
+        _listener.Stop();
+        lock (_connections)
+        {
+            foreach (var connection in _connections)
+            {
+                connection.Close();
+            }
+
+            _connections.Clear();
+        }
+    }
+
+    private async Task AcceptLoop()
+    {
+        while (!_shutdown.IsCancellationRequested)
+        {
+            TcpClient client;
+            try
+            {
+                client = await _listener.AcceptTcpClientAsync(_shutdown.Token);
+            }
+            catch (Exception) when (_shutdown.IsCancellationRequested)
+            {
+                return;
+            }
+            catch (Exception exception)
+            {
+                Log.Warning($"WebSocketServer accept failed: {exception.Message}");
+                continue;
+            }
+
+            _ = Task.Run(() => Handshake(client));
+        }
+    }
+
+    private void Handshake(TcpClient client)
+    {
+        try
+        {
+            client.NoDelay = true;
+            var stream = client.GetStream();
+            if (!WebSocketProtocol.TryReadHandshakeKey(stream, out var key))
+            {
+                client.Dispose();
+                return;
+            }
+
+            WebSocketProtocol.WriteHandshakeResponse(stream, key);
+            var connection = new ServerConnection(
+                Interlocked.Increment(ref _nextConnectionId),
+                client
+            );
+            lock (_connections)
+            {
+                _connections.RemoveAll(c => !c.IsOpen);
+                _connections.Add(connection);
+            }
+
+            _accepted.Enqueue(connection);
+            connection.StartReceiveLoop();
+            Log.Info($"WebSocketServer: connection #{connection.Id} accepted");
+        }
+        catch (Exception exception)
+        {
+            Log.Warning($"WebSocketServer handshake failed: {exception.Message}");
+            client.Dispose();
+        }
+    }
+
+    private sealed class ServerConnection : INetConnection
+    {
+        public int Id { get; }
+        public bool IsOpen => !_closed;
+
+        private readonly TcpClient _client;
+        private readonly NetworkStream _stream;
+        private readonly ConcurrentQueue<byte[]> _inbox = new();
+        private readonly object _sendLock = new();
+        private volatile bool _closed;
+
+        internal ServerConnection(int id, TcpClient client)
+        {
+            Id = id;
+            _client = client;
+            _stream = client.GetStream();
+        }
+
+        internal void StartReceiveLoop() => Task.Run(ReceiveLoop);
+
+        public void Send(ReadOnlySpan<byte> message)
+        {
+            if (_closed)
+            {
+                return;
+            }
+
+            var frame = WebSocketProtocol.EncodeFrame(message, WebSocketOpcode.Binary);
+            try
+            {
+                lock (_sendLock)
+                {
+                    _stream.Write(frame, 0, frame.Length);
+                }
+            }
+            catch (Exception)
+            {
+                Close();
+            }
+        }
+
+        public bool TryReceive(out byte[] message) => _inbox.TryDequeue(out message!);
+
+        public void Close()
+        {
+            if (_closed)
+            {
+                return;
+            }
+
+            _closed = true;
+            try
+            {
+                _client.Dispose();
+            }
+            catch (Exception)
+            {
+                // соединение уже мертво — закрытие не должно бросать
+            }
+        }
+
+        private void ReceiveLoop()
+        {
+            var pending = new List<byte>();
+            var pendingOpcode = WebSocketOpcode.Binary;
+            try
+            {
+                while (!_closed)
+                {
+                    if (
+                        !WebSocketProtocol.TryReadFrame(
+                            _stream,
+                            out var opcode,
+                            out var fin,
+                            out var payload
+                        )
+                    )
+                    {
+                        break;
+                    }
+
+                    switch (opcode)
+                    {
+                        case WebSocketOpcode.Ping:
+                            lock (_sendLock)
+                            {
+                                var pong = WebSocketProtocol.EncodeFrame(
+                                    payload,
+                                    WebSocketOpcode.Pong
+                                );
+                                _stream.Write(pong, 0, pong.Length);
+                            }
+
+                            continue;
+                        case WebSocketOpcode.Pong:
+                            continue;
+                        case WebSocketOpcode.Close:
+                            lock (_sendLock)
+                            {
+                                var close = WebSocketProtocol.EncodeFrame(
+                                    [],
+                                    WebSocketOpcode.Close
+                                );
+                                _stream.Write(close, 0, close.Length);
+                            }
+
+                            return;
+                    }
+
+                    if (opcode != WebSocketOpcode.Continuation)
+                    {
+                        pendingOpcode = opcode;
+                        pending.Clear();
+                    }
+
+                    pending.AddRange(payload);
+                    if (fin && pendingOpcode == WebSocketOpcode.Binary)
+                    {
+                        _inbox.Enqueue(pending.ToArray());
+                        pending.Clear();
+                    }
+                }
+            }
+            catch (Exception)
+            {
+                // обрыв соединения — штатный путь завершения цикла
+            }
+            finally
+            {
+                Close();
+            }
+        }
+    }
+}

tests/MrGameEng.Net.Tests/MrGameEng.Net.Tests.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit.v3" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\MrGameEng.Net\MrGameEng.Net.csproj" />
+  </ItemGroup>
+</Project>

tests/MrGameEng.Net.Tests/ReplicationTests.cs

@@ -0,0 +1,200 @@
+using System.Collections.Concurrent;
+using Friflo.Engine.ECS;
+using MrGameEng.Net;
+using Xunit;
+
+namespace MrGameEng.Net.Tests;
+
+public class ReplicationTests
+{
+    private struct TestPosition : IComponent
+    {
+        public float X;
+        public float Y;
+    }
+
+    private struct TestHealth : IComponent
+    {
+        public int Value;
+    }
+
+    private sealed class FakeConnection : INetConnection
+    {
+        public int Id { get; init; }
+        public bool IsOpen { get; set; } = true;
+        public readonly ConcurrentQueue<byte[]> Sent = new();
+
+        public void Send(ReadOnlySpan<byte> message) => Sent.Enqueue(message.ToArray());
+
+        public bool TryReceive(out byte[] message) => Sent.TryDequeue(out message!);
+
+        public void Close() => IsOpen = false;
+    }
+
+    private static ReplicationSchema MakeSchema() =>
+        new ReplicationSchema().Register<TestPosition>().Register<TestHealth>();
+
+    [Fact]
+    public void FirstSnapshot_SpawnsEntities_OnTheClient()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 10f, Y = 20f },
+            new TestHealth { Value = 7 }
+        );
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = -3f, Y = 4f }
+        );
+
+        server.Send([connection]);
+        client.Pump(connection);
+
+        Assert.Equal(2, client.EntityCount);
+        var first = FindByNetId(clientStore, 1);
+        Assert.Equal(10f, first.GetComponent<TestPosition>().X);
+        Assert.Equal(7, first.GetComponent<TestHealth>().Value);
+        var second = FindByNetId(clientStore, 2);
+        Assert.False(second.HasComponent<TestHealth>());
+    }
+
+    [Fact]
+    public void UnchangedWorld_SendsNothing()
+    {
+        var serverStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var connection = new FakeConnection();
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 1f }
+        );
+
+        server.Send([connection]);
+        Assert.Single(connection.Sent);
+
+        server.Send([connection]);
+        Assert.Single(connection.Sent); // дельта пустая — второго сообщения нет
+    }
+
+    [Fact]
+    public void ChangedComponent_IsTheOnlyThingResent()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+
+        var entity = serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 1f },
+            new TestHealth { Value = 100 }
+        );
+        server.Send([connection]);
+        client.Pump(connection);
+
+        entity.AddComponent(new TestPosition { X = 5f, Y = 6f }); // здоровье не трогаем
+        server.Send([connection]);
+
+        Assert.True(connection.Sent.TryDequeue(out var delta));
+        // Запись: type(1) + count(4) + netId(4) + op(1) + mask(4) + TestPosition(8) — без TestHealth.
+        Assert.Equal(22, delta!.Length);
+
+        client.Apply(delta);
+        var replicated = FindByNetId(clientStore, 1);
+        Assert.Equal(5f, replicated.GetComponent<TestPosition>().X);
+        Assert.Equal(100, replicated.GetComponent<TestHealth>().Value);
+    }
+
+    [Fact]
+    public void DeletedEntity_DespawnsOnTheClient()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+
+        var entity = serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 1f }
+        );
+        server.Send([connection]);
+        client.Pump(connection);
+        Assert.Equal(1, client.EntityCount);
+
+        entity.DeleteEntity();
+        server.Send([connection]);
+        client.Pump(connection);
+
+        Assert.Equal(0, client.EntityCount);
+    }
+
+    [Fact]
+    public void LateJoiner_GetsFullState()
+    {
+        var serverStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var early = new FakeConnection { Id = 1 };
+        var late = new FakeConnection { Id = 2 };
+
+        serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 9f, Y = 9f }
+        );
+        server.Send([early]);
+        server.Send([early, late]); // мир не менялся: early — тишина, late — полный стейт
+
+        Assert.Single(early.Sent);
+        Assert.Single(late.Sent);
+
+        var clientStore = new EntityStore();
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        client.Pump(late);
+        Assert.Equal(1, client.EntityCount);
+        Assert.Equal(9f, FindByNetId(clientStore, 1).GetComponent<TestPosition>().X);
+    }
+
+    [Fact]
+    public void EntitySpawned_FiresOncePerEntity()
+    {
+        var serverStore = new EntityStore();
+        var clientStore = new EntityStore();
+        var server = new ReplicationServer(MakeSchema(), serverStore);
+        var client = new ReplicationClient(MakeSchema(), clientStore);
+        var connection = new FakeConnection();
+        var spawns = 0;
+        client.EntitySpawned += _ => spawns++;
+
+        var entity = serverStore.CreateEntity(
+            new NetId { Value = server.NextNetId() },
+            new TestPosition { X = 1f, Y = 1f }
+        );
+        server.Send([connection]);
+        client.Pump(connection);
+        entity.AddComponent(new TestPosition { X = 2f, Y = 2f });
+        server.Send([connection]);
+        client.Pump(connection);
+
+        Assert.Equal(1, spawns);
+    }
+
+    private static Entity FindByNetId(EntityStore store, int netId)
+    {
+        foreach (var entity in store.Entities)
+        {
+            if (entity.HasComponent<NetId>() && entity.GetComponent<NetId>().Value == netId)
+            {
+                return entity;
+            }
+        }
+
+        throw new InvalidOperationException($"Entity with NetId {netId} not found.");
+    }
+}

tests/MrGameEng.Net.Tests/WebSocketLoopbackTests.cs

@@ -0,0 +1,93 @@
+using System.Net;
+using System.Net.Sockets;
+using MrGameEng.Net;
+using Xunit;
+
+namespace MrGameEng.Net.Tests;
+
+public class WebSocketLoopbackTests
+{
+    [Fact]
+    public async Task ClientAndServer_ExchangeBinaryMessages_OverLoopback()
+    {
+        var port = FreePort();
+        using var server = new WebSocketServer(port);
+        server.Start();
+
+        using var client = await WebSocketClient.ConnectAsync(
+            new Uri($"ws://localhost:{port}/"),
+            new CancellationTokenSource(TimeSpan.FromSeconds(10)).Token
+        );
+
+        var connection = await WaitFor(
+            () => server.TryAcceptConnection(out var c) ? c : null,
+            "server accept"
+        );
+
+        // Сервер → клиент.
+        connection.Send([1, 2, 3, 250]);
+        var received = await WaitFor(
+            () => client.TryReceive(out var m) ? m : null,
+            "client receive"
+        );
+        Assert.Equal(new byte[] { 1, 2, 3, 250 }, received);
+
+        // Клиент → сервер (ClientWebSocket маскирует кадры — сервер обязан размаскировать).
+        client.Send([9, 8, 7]);
+        var echoed = await WaitFor(
+            () => connection.TryReceive(out var m) ? m : null,
+            "server receive"
+        );
+        Assert.Equal(new byte[] { 9, 8, 7 }, echoed);
+
+        Assert.True(connection.IsOpen);
+        Assert.True(client.IsOpen);
+    }
+
+    [Fact]
+    public async Task ClosingTheClient_ClosesTheServerConnection()
+    {
+        var port = FreePort();
+        using var server = new WebSocketServer(port);
+        server.Start();
+
+        var client = await WebSocketClient.ConnectAsync(
+            new Uri($"ws://localhost:{port}/"),
+            new CancellationTokenSource(TimeSpan.FromSeconds(10)).Token
+        );
+        var connection = await WaitFor(
+            () => server.TryAcceptConnection(out var c) ? c : null,
+            "server accept"
+        );
+
+        client.Close();
+
+        await WaitFor(() => connection.IsOpen ? null : "closed", "server-side close");
+        Assert.False(connection.IsOpen);
+    }
+
+    private static async Task<T> WaitFor<T>(Func<T?> poll, string what)
+        where T : class
+    {
+        for (var i = 0; i < 200; i++)
+        {
+            if (poll() is { } result)
+            {
+                return result;
+            }
+
+            await Task.Delay(25);
+        }
+
+        throw new TimeoutException($"Timed out waiting for {what}.");
+    }
+
+    private static int FreePort()
+    {
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+}

tests/MrGameEng.Net.Tests/WebSocketProtocolTests.cs

@@ -0,0 +1,88 @@
+using MrGameEng.Net;
+using Xunit;
+
+namespace MrGameEng.Net.Tests;
+
+public class WebSocketProtocolTests
+{
+    [Fact]
+    public void AcceptKey_MatchesRfc6455Example()
+    {
+        // Пример рукопожатия прямо из RFC 6455, раздел 1.3.
+        Assert.Equal(
+            "s3pPLMBiTxaQ9kYGzzhZRbK+xOo=",
+            WebSocketProtocol.AcceptKey("dGhlIHNhbXBsZSBub25jZQ==")
+        );
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(125)]
+    [InlineData(126)]
+    [InlineData(70_000)]
+    public void Frame_Roundtrips_Unmasked(int payloadLength)
+    {
+        var payload = MakePayload(payloadLength);
+        var frame = WebSocketProtocol.EncodeFrame(payload, WebSocketOpcode.Binary);
+
+        using var stream = new MemoryStream(frame);
+        Assert.True(
+            WebSocketProtocol.TryReadFrame(stream, out var opcode, out var fin, out var decoded)
+        );
+        Assert.Equal(WebSocketOpcode.Binary, opcode);
+        Assert.True(fin);
+        Assert.Equal(payload, decoded);
+    }
+
+    [Fact]
+    public void Frame_Roundtrips_Masked()
+    {
+        var payload = MakePayload(1000);
+        var frame = WebSocketProtocol.EncodeFrame(
+            payload,
+            WebSocketOpcode.Binary,
+            maskKey: [0x12, 0x34, 0x56, 0x78]
+        );
+
+        using var stream = new MemoryStream(frame);
+        Assert.True(WebSocketProtocol.TryReadFrame(stream, out _, out _, out var decoded));
+        Assert.Equal(payload, decoded);
+    }
+
+    [Fact]
+    public void TryReadHandshakeKey_ExtractsHeader()
+    {
+        var request =
+            "GET /chat HTTP/1.1\r\n"
+            + "Host: localhost\r\n"
+            + "Upgrade: websocket\r\n"
+            + "Connection: Upgrade\r\n"
+            + "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n"
+            + "Sec-WebSocket-Version: 13\r\n"
+            + "\r\n";
+        using var stream = new MemoryStream(System.Text.Encoding.ASCII.GetBytes(request));
+
+        Assert.True(WebSocketProtocol.TryReadHandshakeKey(stream, out var key));
+        Assert.Equal("dGhlIHNhbXBsZSBub25jZQ==", key);
+    }
+
+    [Fact]
+    public void TryReadFrame_TruncatedStream_ReturnsFalse()
+    {
+        var frame = WebSocketProtocol.EncodeFrame(MakePayload(100), WebSocketOpcode.Binary);
+        using var stream = new MemoryStream(frame, 0, frame.Length - 10);
+
+        Assert.False(WebSocketProtocol.TryReadFrame(stream, out _, out _, out _));
+    }
+
+    private static byte[] MakePayload(int length)
+    {
+        var payload = new byte[length];
+        for (var i = 0; i < length; i++)
+        {
+            payload[i] = (byte)(i * 31);
+        }
+
+        return payload;
+    }
+}