feat: implement authenticated NAT mediator (#11)

Closes #11
2026-07-16 07:37:02 +02:00
parent 1baa1055dc
commit 6d076c281a
19 changed files with 1660 additions and 174 deletions
@@ -42,6 +42,21 @@ if (!registered.IsSuccess || registered.Value is null)
 Load `publisherCredential` from the game's deployment secret boundary; never
 embed it in a client build or source control. A successful registration returns a
 `PublishedSession` containing the lease and host-presence capabilities.
+Send a periodic presence request from the host's gameplay `NetManager` using the
+server-controlled refresh interval and the fixed-size native token:
+
+```csharp
+string presenceToken = NatPunchRequestTokenCodec.Encode(
+    NatPunchPeerRole.HostPresence,
+    session.HostPresenceHandle,
+    session.HostPresenceCapability);
+gameplayNetManager.NatPunchModule.SendNatIntroduceRequest(mediator, presenceToken);
+```
+
+The same codec creates `Host` tokens for host-polled invitations and `Client`
+tokens for a created join attempt. Always send them from the same LiteNetLib
+socket that will carry the direct game connection; the mediator ignores any
+caller-supplied public endpoint.

 Lease renewal is explicit and caller-controlled:

@@ -23,6 +23,8 @@ public static class ContractLimits
    public const int OpaqueHttpCredentialMaxCharacters = 1_024;
    public const int UdpCapabilityMaxCharacters = 192;
    public const int ConnectionTicketMaxCharacters = 192;
+    public const int DerivedCredentialCharacters = 43;
+    public const int NatPunchRequestTokenCharacters = 192;
    public const int LiteNetLibNatTokenMaxCharacters = 256;
    public const int SessionCapacityMaxPlayers = 10_000;
 }
@@ -0,0 +1,131 @@
+namespace FinalFactory.Rendezvous.Contracts;
+
+public enum NatPunchPeerRole
+{
+    HostPresence = 1,
+    Host = 2,
+    Client = 3,
+}
+
+public sealed class NatPunchRequestToken
+{
+    public NatPunchPeerRole Role { get; set; }
+    public MediationHandle MediationHandle { get; set; }
+    public string Capability { get; set; } = string.Empty;
+
+    public override string ToString() => "[NatPunchRequestToken: capability redacted]";
+}
+
+public static class NatPunchRequestTokenCodec
+{
+    public const int EncodedLength = ContractLimits.NatPunchRequestTokenCharacters;
+
+    private const string VersionPrefix = "rv1:";
+    private const int HandleLength = 32;
+    private const int CapabilityLength = ContractLimits.DerivedCredentialCharacters;
+    private const char Separator = ':';
+    private const char Padding = '.';
+
+    public static string Encode(
+        NatPunchPeerRole role,
+        MediationHandle mediationHandle,
+        string capability)
+    {
+        if (!TryGetRoleCode(role, out char roleCode)
+            || mediationHandle.Value == Guid.Empty
+            || capability is null
+            || capability.Length != CapabilityLength
+            || !ContractValidation.IsCapabilityValid(capability))
+        {
+            throw new ArgumentException("The NAT punch request token fields are invalid.");
+        }
+
+        string payload = string.Concat(
+            VersionPrefix,
+            roleCode,
+            Separator,
+            mediationHandle.Value.ToString("N"),
+            Separator,
+            capability);
+        return payload.PadRight(EncodedLength, Padding);
+    }
+
+    public static bool TryDecode(string? encoded, out NatPunchRequestToken? token)
+    {
+        token = null;
+        if (encoded is null
+            || encoded.Length != EncodedLength
+            || !encoded.StartsWith(VersionPrefix, StringComparison.Ordinal)
+            || !TryParseRole(encoded[VersionPrefix.Length], out NatPunchPeerRole role))
+        {
+            return false;
+        }
+
+        int roleSeparator = VersionPrefix.Length + 1;
+        int handleOffset = roleSeparator + 1;
+        int capabilitySeparator = handleOffset + HandleLength;
+        int capabilityOffset = capabilitySeparator + 1;
+        int paddingOffset = capabilityOffset + CapabilityLength;
+        string handleText = encoded.Substring(handleOffset, HandleLength);
+        if (encoded[roleSeparator] != Separator
+            || encoded[capabilitySeparator] != Separator
+            || !Guid.TryParseExact(handleText, "N", out Guid handle)
+            || handle == Guid.Empty
+            || !string.Equals(handleText, handle.ToString("N"), StringComparison.Ordinal)
+            || !ContainsOnlyPadding(encoded, paddingOffset))
+        {
+            return false;
+        }
+
+        string capability = encoded.Substring(capabilityOffset, CapabilityLength);
+        if (!ContractValidation.IsCapabilityValid(capability))
+        {
+            return false;
+        }
+
+        token = new NatPunchRequestToken
+        {
+            Role = role,
+            MediationHandle = new MediationHandle(handle),
+            Capability = capability,
+        };
+        return true;
+    }
+
+    private static bool TryGetRoleCode(NatPunchPeerRole role, out char code)
+    {
+        code = role switch
+        {
+            NatPunchPeerRole.HostPresence => 'p',
+            NatPunchPeerRole.Host => 'h',
+            NatPunchPeerRole.Client => 'c',
+            _ => default,
+        };
+        return code != default;
+    }
+
+    private static bool TryParseRole(char code, out NatPunchPeerRole role)
+    {
+        role = code switch
+        {
+            'p' => NatPunchPeerRole.HostPresence,
+            'h' => NatPunchPeerRole.Host,
+            'c' => NatPunchPeerRole.Client,
+            _ => default,
+        };
+        return role != default;
+    }
+
+    private static bool ContainsOnlyPadding(string value, int offset)
+    {
+        for (int index = offset; index < value.Length; index++)
+        {
+            if (value[index] != Padding)
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
@@ -316,6 +316,9 @@ internal sealed class JoinAttemptService(
        ContractValidation.IsCapabilityValid(hostCapability)
        && ContractValidation.IsCapabilityValid(clientCapability)
        && ContractValidation.IsConnectionTicketValid(ticket)
+        && hostCapability.Length == ContractLimits.DerivedCredentialCharacters
+        && clientCapability.Length == ContractLimits.DerivedCredentialCharacters
+        && ticket.Length == ContractLimits.DerivedCredentialCharacters
        && hostCapability.Length <= ContractLimits.LiteNetLibNatTokenMaxCharacters
        && clientCapability.Length <= ContractLimits.LiteNetLibNatTokenMaxCharacters;

@@ -133,12 +133,19 @@ builder.Services
    .BindConfiguration(UdpMediatorOptions.SectionName)
    .ValidateDataAnnotations()
    .Validate(
-        options => IPAddress.TryParse(options.ListenAddress, out _),
-        $"{UdpMediatorOptions.SectionName}:ListenAddress must be an IP address.")
+        options => IPAddress.TryParse(options.ListenAddress, out IPAddress? address)
+                   && address.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork,
+        $"{UdpMediatorOptions.SectionName}:ListenAddress must be an IPv4 address.")
+    .Validate(
+        options => string.IsNullOrWhiteSpace(options.Ipv6ListenAddress)
+                   || (IPAddress.TryParse(options.Ipv6ListenAddress, out IPAddress? address)
+                       && address.AddressFamily == System.Net.Sockets.AddressFamily.InterNetworkV6),
+        $"{UdpMediatorOptions.SectionName}:Ipv6ListenAddress must be an IPv6 address when configured.")
    .ValidateOnStart();
 builder.Services.AddSingleton<UdpMediatorService>();
 if (!isOpenApiGeneration)
 {
+    builder.Services.AddSingleton<NatMediationProcessor>();
    builder.Services.AddHostedService(static services =>
        services.GetRequiredService<UdpMediatorService>());
 }
@@ -4,6 +4,7 @@ namespace FinalFactory.Rendezvous.Server.State;

 internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousStore
 {
+    private static readonly TimeSpan UdpMaintenanceInterval = TimeSpan.FromSeconds(1);
    private readonly object _gate = new();
    private readonly EphemeralStoreOptions _options;
    private readonly IMonotonicClock _monotonicClock;
@@ -19,6 +20,8 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
    private readonly Dictionary<string, TimeSpan> _replay = new(StringComparer.Ordinal);
    private readonly Dictionary<string, TimeSpan> _revocations = new(StringComparer.Ordinal);
    private TimeSpan? _drainDeadline;
+    private TimeSpan _nextUdpMaintenance;
+    private long _maintenanceSweepCount;
    private bool _available = true;

    public InMemoryEphemeralRendezvousStore(
@@ -38,6 +41,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
    }

    public Guid InstanceId { get; }
+    internal long MaintenanceSweepCount => Interlocked.Read(ref _maintenanceSweepCount);

    public bool IsAvailable
    {
@@ -270,6 +274,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto

        if (!_presenceHandles.TryGetValue(command.Handle, out SessionListingId listingId)
            || !_listings.TryGetValue(listingId, out ListingEntry? entry)
+            || entry.LeaseDeadline <= now
            || entry.Definition.HostPresenceFingerprint != command.CapabilityFingerprint)
        {
            return new(StoreResultCode.NotFound);
@@ -285,7 +290,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
            command.LocalEndpoint,
            now + _options.PresenceLifetime);
        return new(StoreResultCode.Success, Snapshot(entry));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);

    public StoreResult<IReadOnlyList<StoredListing>> BrowseVisibleListings(
        VisibleListingQuery query,
@@ -446,13 +451,18 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
            return new(StoreResultCode.NotFound);
        }

+        if (attempt.IntroductionConsumed)
+        {
+            return new(StoreResultCode.Conflict);
+        }
+
        RemoveAttempt(command.AttemptId);
        return new(StoreResultCode.Success, true);
    }, cancellationToken);

    public StoreResult<StoredJoinAttempt> BindAttemptEndpoint(
        BindAttemptEndpointCommand command,
-        CancellationToken cancellationToken = default) => Atomic<StoredJoinAttempt>(_ =>
+        CancellationToken cancellationToken = default) => Atomic<StoredJoinAttempt>(now =>
    {
        ArgumentNullException.ThrowIfNull(command);
        if (command.Handle.Value == Guid.Empty
@@ -469,7 +479,13 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
        }

        if (!_attemptHandles.TryGetValue(command.Handle, out JoinAttemptId attemptId)
-            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt))
+            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt)
+            || attempt.Deadline <= now)
+        {
+            return new(StoreResultCode.NotFound);
+        }
+
+        if (!HasFreshHostPresence(attempt, now))
        {
            return new(StoreResultCode.NotFound);
        }
@@ -503,7 +519,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
        }

        return new(StoreResultCode.Success, Snapshot(attempt));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);

    public StoreResult<IntroductionEndpoints> ConsumeIntroduction(
        MediationHandle handle,
@@ -515,7 +531,13 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
        }

        if (!_attemptHandles.TryGetValue(handle, out JoinAttemptId attemptId)
-            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt))
+            || !_attempts.TryGetValue(attemptId, out AttemptEntry? attempt)
+            || attempt.Deadline <= now)
+        {
+            return new(StoreResultCode.NotFound);
+        }
+
+        if (!HasFreshHostPresence(attempt, now))
        {
            return new(StoreResultCode.NotFound);
        }
@@ -540,7 +562,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
            Snapshot(attempt),
            attempt.HostEndpoint,
            attempt.ClientEndpoint));
-    }, cancellationToken);
+    }, cancellationToken, eagerCleanup: false);

    public StoreResult<bool> ConsumeConnectionTicket(
        ConsumeConnectionTicketCommand command,
@@ -687,18 +709,38 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
        }
    }

-    private StoreResult<T> Atomic<T>(Func<TimeSpan, StoreResult<T>> operation, CancellationToken cancellationToken)
+    private StoreResult<T> Atomic<T>(
+        Func<TimeSpan, StoreResult<T>> operation,
+        CancellationToken cancellationToken,
+        bool eagerCleanup = true)
    {
        cancellationToken.ThrowIfCancellationRequested();
        lock (_gate)
        {
            cancellationToken.ThrowIfCancellationRequested();
            TimeSpan now = _monotonicClock.Elapsed;
-            Cleanup(now);
+            // Authenticated UDP duplicates need O(1) store work. Their operations
+            // check exact resource deadlines and amortize physical expiry removal.
+            bool drainExpired = _drainDeadline is TimeSpan drainDeadline
+                && now >= drainDeadline;
+            if (drainExpired || eagerCleanup || now >= _nextUdpMaintenance)
+            {
+                Cleanup(now);
+                _nextUdpMaintenance = now + UdpMaintenanceInterval;
+            }
+
            return operation(now);
        }
    }

+    private bool HasFreshHostPresence(AttemptEntry attempt, TimeSpan now) =>
+        _listings.TryGetValue(attempt.Command.ListingId, out ListingEntry? listing)
+        && listing.LeaseDeadline > now
+        && _presence.TryGetValue(
+            listing.Definition.HostPresenceHandle,
+            out PresenceEntry? presence)
+        && presence.Deadline > now;
+
    private StoreResult<T>? CheckNewWorkAdmission<T>(string subject)
    {
        if (!_available)
@@ -718,6 +760,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto

    private void Cleanup(TimeSpan now)
    {
+        _maintenanceSweepCount++;
        if (_drainDeadline is TimeSpan drainDeadline && now >= drainDeadline)
        {
            ClearActiveState();
@@ -0,0 +1,73 @@
+using System.Buffers.Binary;
+using System.Net;
+using System.Text;
+using FinalFactory.Rendezvous.Contracts;
+
+namespace FinalFactory.Rendezvous.Server.Transport;
+
+internal static class LiteNetNatRequestCodec
+{
+    private const byte NatMessageProperty = 17;
+    private const int TypeIdentifierLength = 8;
+    private const int TokenLengthPrefix = NatPunchRequestTokenCodec.EncodedLength + 1;
+    // LiteNetLib 2.1.4's private NatIntroduceRequest type ID. The native socket
+    // integration test deliberately fails if a package upgrade changes this wire value.
+    private static ReadOnlySpan<byte> RequestTypeIdentifier =>
+        [0x88, 0xbe, 0x10, 0x26, 0xbf, 0xb1, 0x66, 0x9c];
+
+    public static bool TryDecode(
+        ReadOnlySpan<byte> datagram,
+        out IPEndPoint? claimedLocalEndpoint,
+        out string? token)
+    {
+        claimedLocalEndpoint = null;
+        token = null;
+        if (datagram.Length < 1 + TypeIdentifierLength + 1 + 4 + 2 + 2
+            + NatPunchRequestTokenCodec.EncodedLength
+            || datagram[0] != NatMessageProperty
+            || !datagram.Slice(1, TypeIdentifierLength).SequenceEqual(RequestTypeIdentifier))
+        {
+            return false;
+        }
+
+        int offset = 1 + TypeIdentifierLength;
+        int addressLength = datagram[offset++] switch
+        {
+            0 => 4,
+            1 => 16,
+            _ => 0,
+        };
+        int expectedLength = offset + addressLength + 2 + 2
+            + NatPunchRequestTokenCodec.EncodedLength;
+        if (addressLength == 0 || datagram.Length != expectedLength)
+        {
+            return false;
+        }
+
+        IPAddress localAddress = new(datagram.Slice(offset, addressLength));
+        offset += addressLength;
+        int localPort = BinaryPrimitives.ReadUInt16LittleEndian(datagram.Slice(offset, 2));
+        offset += 2;
+        int encodedTokenLength = BinaryPrimitives.ReadUInt16LittleEndian(datagram.Slice(offset, 2));
+        offset += 2;
+        if (localPort == 0 || encodedTokenLength != TokenLengthPrefix)
+        {
+            return false;
+        }
+
+        ReadOnlySpan<byte> tokenBytes = datagram.Slice(
+            offset,
+            NatPunchRequestTokenCodec.EncodedLength);
+        for (int index = 0; index < tokenBytes.Length; index++)
+        {
+            if (tokenBytes[index] > 0x7f)
+            {
+                return false;
+            }
+        }
+
+        claimedLocalEndpoint = new(localAddress, localPort);
+        token = Encoding.ASCII.GetString(tokenBytes);
+        return true;
+    }
+}
@@ -0,0 +1,323 @@
+using System.Net;
+using System.Net.Sockets;
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.JoinAttempts;
+using FinalFactory.Rendezvous.Server.Sessions;
+using FinalFactory.Rendezvous.Server.State;
+
+namespace FinalFactory.Rendezvous.Server.Transport;
+
+internal interface INatIntroductionSink
+{
+    void Introduce(NatIntroductionPlan plan);
+}
+
+internal sealed record NatIntroductionPlan(
+    IPEndPoint HostLocal,
+    IPEndPoint HostPublic,
+    IPEndPoint ClientLocal,
+    IPEndPoint ClientPublic,
+    string ConnectionTicket)
+{
+    public override string ToString() => "[NatIntroductionPlan: endpoints and ticket redacted]";
+}
+
+internal enum NatMediationResult
+{
+    Dropped = 0,
+    HostPresenceAccepted = 1,
+    HostPresenceRejected = 2,
+    WaitingForPeer = 3,
+    Introduced = 4,
+    Duplicate = 5,
+    Rejected = 6,
+}
+
+internal sealed class NatMediationProcessor(
+    IEphemeralRendezvousStore store,
+    ISessionCapabilityService capabilities,
+    JoinAttemptService joinAttempts)
+{
+    public NatMediationResult ProcessDatagram(
+        ReadOnlySpan<byte> encoded,
+        IPEndPoint observedPublicEndpoint,
+        INatIntroductionSink introductionSink,
+        CancellationToken cancellationToken = default)
+    {
+        if (!RendezvousUdpCodec.TryDecode(encoded, out PresenceDatagram? datagram, out _)
+            || datagram is null
+            || datagram.Capability.Length != ContractLimits.DerivedCredentialCharacters
+            || !IPAddress.TryParse(datagram.LocalAddress, out IPAddress? localAddress))
+        {
+            return NatMediationResult.Dropped;
+        }
+
+        IPEndPoint claimedLocalEndpoint = new(localAddress, datagram.LocalPort);
+        NatPunchPeerRole role = datagram.MessageType == UdpPresenceMessageType.ClientPresence
+            ? NatPunchPeerRole.Client
+            : NatPunchPeerRole.HostPresence;
+        bool observedIpv6 = observedPublicEndpoint.AddressFamily == AddressFamily.InterNetworkV6
+            && !observedPublicEndpoint.Address.IsIPv4MappedToIPv6;
+        if (role == NatPunchPeerRole.Client && observedIpv6)
+        {
+            return NatMediationResult.Dropped;
+        }
+
+        NatMediationResult result = ProcessRequest(
+            claimedLocalEndpoint,
+            observedPublicEndpoint,
+            NatPunchRequestTokenCodec.Encode(role, datagram.MediationHandle, datagram.Capability),
+            introductionSink,
+            cancellationToken);
+        if (role != NatPunchPeerRole.HostPresence
+            || result != NatMediationResult.HostPresenceRejected
+            || observedIpv6)
+        {
+            return result;
+        }
+
+        return ProcessRequest(
+            claimedLocalEndpoint,
+            observedPublicEndpoint,
+            NatPunchRequestTokenCodec.Encode(
+                NatPunchPeerRole.Host,
+                datagram.MediationHandle,
+                datagram.Capability),
+            introductionSink,
+            cancellationToken);
+    }
+
+    public NatMediationResult ProcessRequest(
+        IPEndPoint claimedLocalEndpoint,
+        IPEndPoint observedPublicEndpoint,
+        string token,
+        INatIntroductionSink introductionSink,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(claimedLocalEndpoint);
+        ArgumentNullException.ThrowIfNull(observedPublicEndpoint);
+        ArgumentNullException.ThrowIfNull(introductionSink);
+
+        if (!NatPunchRequestTokenCodec.TryDecode(token, out NatPunchRequestToken? request)
+            || request is null
+            || !TryCreateObservedEndpoint(observedPublicEndpoint, out ObservedEndpoint publicEndpoint)
+            || !capabilities.TryFingerprint(request.Capability, out SecretFingerprint fingerprint))
+        {
+            return NatMediationResult.Dropped;
+        }
+
+        ObservedEndpoint? localEndpoint = TryCreatePrivateCandidate(
+            claimedLocalEndpoint,
+            publicEndpoint.AddressFamily,
+            out ObservedEndpoint candidate)
+            ? candidate
+            : null;
+
+        if (request.Role == NatPunchPeerRole.HostPresence)
+        {
+            StoreResult<StoredListing> presence = store.BindHostPresence(new(
+                request.MediationHandle,
+                fingerprint,
+                publicEndpoint,
+                localEndpoint), cancellationToken);
+            return presence.Succeeded
+                ? NatMediationResult.HostPresenceAccepted
+                : NatMediationResult.HostPresenceRejected;
+        }
+
+        AttemptPeerRole role = request.Role switch
+        {
+            NatPunchPeerRole.Host => AttemptPeerRole.Host,
+            NatPunchPeerRole.Client => AttemptPeerRole.Client,
+            _ => default,
+        };
+        if (role == default)
+        {
+            return NatMediationResult.Dropped;
+        }
+
+        StoreResult<StoredJoinAttempt> bound = store.BindAttemptEndpoint(new(
+            request.MediationHandle,
+            role,
+            fingerprint,
+            publicEndpoint,
+            localEndpoint), cancellationToken);
+        if (!bound.Succeeded || bound.Value is null)
+        {
+            return bound.Code == StoreResultCode.ReplayRejected
+                ? NatMediationResult.Rejected
+                : NatMediationResult.Dropped;
+        }
+
+        StoredJoinAttempt attempt = bound.Value;
+        if (attempt.IntroductionConsumed)
+        {
+            return NatMediationResult.Duplicate;
+        }
+
+        if (attempt.HostEndpoint is null || attempt.ClientEndpoint is null)
+        {
+            return NatMediationResult.WaitingForPeer;
+        }
+
+        if (attempt.HostEndpoint.PublicEndpoint.AddressFamily
+            != attempt.ClientEndpoint.PublicEndpoint.AddressFamily)
+        {
+            return NatMediationResult.Rejected;
+        }
+
+        StoreResult<IntroductionEndpoints> consumed = store.ConsumeIntroduction(
+            request.MediationHandle,
+            cancellationToken);
+        if (!consumed.Succeeded || consumed.Value is null)
+        {
+            return consumed.Code == StoreResultCode.ReplayRejected
+                ? NatMediationResult.Duplicate
+                : NatMediationResult.Rejected;
+        }
+
+        JoinAttemptServiceResult<ConnectionTicketGrant> ticket = joinAttempts.IssueConnectionTicket(
+            consumed.Value.Attempt);
+        if (!ticket.Succeeded || ticket.Value is null)
+        {
+            return NatMediationResult.Rejected;
+        }
+
+        try
+        {
+            introductionSink.Introduce(CreatePlan(consumed.Value, ticket.Value.Ticket));
+            return NatMediationResult.Introduced;
+        }
+        catch (Exception exception) when (exception is SocketException
+                                          or InvalidOperationException
+                                          or ArgumentException)
+        {
+            return NatMediationResult.Rejected;
+        }
+    }
+
+    private static NatIntroductionPlan CreatePlan(
+        IntroductionEndpoints endpoints,
+        string connectionTicket)
+    {
+        IPEndPoint hostPublic = ToIpEndpoint(endpoints.Host.PublicEndpoint);
+        IPEndPoint clientPublic = ToIpEndpoint(endpoints.Client.PublicEndpoint);
+        bool sameNat = hostPublic.Address.Equals(clientPublic.Address);
+        IPEndPoint hostLocal = sameNat && endpoints.Host.LocalEndpoint is { } hostCandidate
+            ? ToIpEndpoint(hostCandidate)
+            : hostPublic;
+        IPEndPoint clientLocal = sameNat && endpoints.Client.LocalEndpoint is { } clientCandidate
+            ? ToIpEndpoint(clientCandidate)
+            : clientPublic;
+        return new(hostLocal, hostPublic, clientLocal, clientPublic, connectionTicket);
+    }
+
+    private static bool TryCreateObservedEndpoint(
+        IPEndPoint source,
+        out ObservedEndpoint endpoint)
+    {
+        endpoint = default;
+        if (source.Port is < 1 or > 65_535)
+        {
+            return false;
+        }
+
+        IPAddress address = source.Address.IsIPv4MappedToIPv6
+            ? source.Address.MapToIPv4()
+            : source.Address;
+        if (address.Equals(IPAddress.Any)
+            || address.Equals(IPAddress.IPv6Any)
+            || address.IsIPv6Multicast
+            || IsIpv4MulticastOrBroadcast(address)
+            || (address.AddressFamily == AddressFamily.InterNetworkV6
+                && !IsGlobalIpv6(address)))
+        {
+            return false;
+        }
+
+        AddressFamilyKind family = address.AddressFamily switch
+        {
+            AddressFamily.InterNetwork => AddressFamilyKind.Ipv4,
+            AddressFamily.InterNetworkV6 => AddressFamilyKind.Ipv6,
+            _ => default,
+        };
+        if (family == default)
+        {
+            return false;
+        }
+
+        endpoint = new(family, address.ToString(), source.Port);
+        return true;
+    }
+
+    private static bool TryCreatePrivateCandidate(
+        IPEndPoint source,
+        AddressFamilyKind publicFamily,
+        out ObservedEndpoint endpoint)
+    {
+        endpoint = default;
+        if (source.Port is < 1 or > 65_535)
+        {
+            return false;
+        }
+
+        IPAddress address = source.Address.IsIPv4MappedToIPv6
+            ? source.Address.MapToIPv4()
+            : source.Address;
+        AddressFamilyKind family = address.AddressFamily switch
+        {
+            AddressFamily.InterNetwork => AddressFamilyKind.Ipv4,
+            AddressFamily.InterNetworkV6 => AddressFamilyKind.Ipv6,
+            _ => default,
+        };
+        if (family != publicFamily || !IsPrivateUnicast(address))
+        {
+            return false;
+        }
+
+        endpoint = new(family, address.ToString(), source.Port);
+        return true;
+    }
+
+    private static bool IsPrivateUnicast(IPAddress address)
+    {
+        byte[] bytes = address.GetAddressBytes();
+        return address.AddressFamily switch
+        {
+            AddressFamily.InterNetwork => bytes[0] == 10
+                || (bytes[0] == 172 && bytes[1] is >= 16 and <= 31)
+                || (bytes[0] == 192 && bytes[1] == 168),
+            AddressFamily.InterNetworkV6 => (bytes[0] & 0xfe) == 0xfc,
+            _ => false,
+        };
+    }
+
+    private static bool IsGlobalIpv6(IPAddress address) =>
+        !address.Equals(IPAddress.IPv6Loopback)
+        && !address.Equals(IPAddress.IPv6Any)
+        && !address.IsIPv6LinkLocal
+        && !address.IsIPv6Multicast
+        && !address.IsIPv6SiteLocal
+        && !IsPrivateUnicast(address)
+        && !IsDocumentationIpv6(address);
+
+    private static bool IsIpv4MulticastOrBroadcast(IPAddress address)
+    {
+        if (address.AddressFamily != AddressFamily.InterNetwork)
+        {
+            return false;
+        }
+
+        byte[] bytes = address.GetAddressBytes();
+        return bytes[0] >= 224 || bytes.All(static value => value == byte.MaxValue);
+    }
+
+    private static bool IsDocumentationIpv6(IPAddress address)
+    {
+        byte[] bytes = address.GetAddressBytes();
+        return bytes[0] == 0x20 && bytes[1] == 0x01 && bytes[2] == 0x0d && bytes[3] == 0xb8;
+    }
+
+    private static IPEndPoint ToIpEndpoint(ObservedEndpoint endpoint) =>
+        new(IPAddress.Parse(endpoint.Address), endpoint.Port);
+}
@@ -18,9 +18,17 @@ public sealed class UdpMediatorOptions
    [Required]
    public string ListenAddress { get; set; } = "0.0.0.0";

+    public string? Ipv6ListenAddress { get; set; }
+
    /// <summary>
    /// Gets or sets the UDP port. Zero requests an ephemeral port for tests.
    /// </summary>
    [Range(0, 65_535)]
    public int Port { get; set; } = 9050;
+
+    [Range(1, 4_096)]
+    public int MaxDatagramsPerPoll { get; set; } = 256;
+
+    [Range(1, 100)]
+    public int PollIntervalMilliseconds { get; set; } = 2;
 }
@@ -1,161 +1,136 @@
+using System.Diagnostics;
 using System.Net;
 using System.Net.Sockets;
 using FinalFactory.Rendezvous.Contracts;
-using FinalFactory.Rendezvous.Server.Sessions;
-using FinalFactory.Rendezvous.Server.State;
+using LiteNetLib;
+using LiteNetLib.Layers;
 using Microsoft.Extensions.Options;

 namespace FinalFactory.Rendezvous.Server.Transport;

-/// <summary>
-/// Owns the cancellable UDP socket used by the future NAT mediator.
-/// </summary>
 internal sealed partial class UdpMediatorService : BackgroundService
 {
    private readonly ILogger<UdpMediatorService> _logger;
    private readonly UdpMediatorOptions _options;
-    private readonly IEphemeralRendezvousStore _store;
-    private readonly ISessionCapabilityService _capabilities;
-    private UdpClient? _udpClient;
+    private readonly NatMediationProcessor _processor;
+    private LiteNetManager? _manager;
+    private LiteNetIntroductionSink? _introductionSink;

-    /// <summary>
-    /// Initializes a new UDP mediator service.
-    /// </summary>
    public UdpMediatorService(
        IOptions<UdpMediatorOptions> options,
        ILogger<UdpMediatorService> logger,
-        IEphemeralRendezvousStore store,
-        ISessionCapabilityService capabilities)
+        NatMediationProcessor processor)
    {
        _options = options.Value;
        _logger = logger;
-        _store = store;
-        _capabilities = capabilities;
+        _processor = processor;
    }

-    /// <summary>
-    /// Gets the bound endpoint after startup completes.
-    /// </summary>
    public IPEndPoint? LocalEndpoint { get; private set; }
+    public IPEndPoint? LocalIpv6Endpoint { get; private set; }

-    /// <inheritdoc />
    public override Task StartAsync(CancellationToken cancellationToken)
    {
        cancellationToken.ThrowIfCancellationRequested();
-
-        if (_udpClient is not null)
+        if (_manager is not null)
        {
            throw new InvalidOperationException("The UDP mediator is already running.");
        }

        IPAddress listenAddress = IPAddress.Parse(_options.ListenAddress);
-        UdpClient udpClient = new(new IPEndPoint(listenAddress, _options.Port));
-        _udpClient = udpClient;
-        IPEndPoint localEndpoint =
-            (IPEndPoint?)udpClient.Client.LocalEndPoint
-            ?? throw new InvalidOperationException("The UDP socket did not expose its bound endpoint.");
-        LocalEndpoint = localEndpoint;
+        if (listenAddress.AddressFamily != AddressFamily.InterNetwork)
+        {
+            throw new InvalidOperationException("The required UDP listen address must be IPv4.");
+        }

-        LogMediatorListening(_logger, localEndpoint.Address, localEndpoint.Port);
+        IPAddress? ipv6ListenAddress = string.IsNullOrWhiteSpace(_options.Ipv6ListenAddress)
+            ? null
+            : IPAddress.Parse(_options.Ipv6ListenAddress);
+        if (ipv6ListenAddress is not null
+            && ipv6ListenAddress.AddressFamily != AddressFamily.InterNetworkV6)
+        {
+            throw new InvalidOperationException("The optional UDP IPv6 listen address must be IPv6.");
+        }

+        EventBasedLiteNetListener listener = new();
+        RendezvousPacketLayer packetLayer = new(_processor);
+        LiteNetManager manager = new(listener, packetLayer)
+        {
+            NatPunchEnabled = true,
+            IPv6Enabled = ipv6ListenAddress is not null,
+            UnsyncedEvents = true,
+            MaxPacketPerManualReceive = _options.MaxDatagramsPerPoll,
+        };
+        manager.NatPunchModule.UnsyncedEvents = true;
+        _introductionSink = new(manager.NatPunchModule);
+        packetLayer.Attach(_introductionSink);
+
+        if (!manager.StartInManualMode(
+                listenAddress,
+                ipv6ListenAddress ?? IPAddress.IPv6Any,
+                _options.Port))
+        {
+            _introductionSink = null;
+            manager.Stop();
+            throw new InvalidOperationException("The UDP mediator could not bind its LiteNetLib socket.");
+        }
+
+        _manager = manager;
+        LocalEndpoint = new(listenAddress, manager.LocalPort);
+        LocalIpv6Endpoint = ipv6ListenAddress is null
+            ? null
+            : new(ipv6ListenAddress, manager.LocalPort);
+        LogMediatorListening(_logger, listenAddress, manager.LocalPort);
        return base.StartAsync(cancellationToken);
    }

-    /// <inheritdoc />
    public override async Task StopAsync(CancellationToken cancellationToken)
    {
        await base.StopAsync(cancellationToken).ConfigureAwait(false);
-        _udpClient?.Dispose();
-        _udpClient = null;
-        LocalEndpoint = null;
+        StopManager();
        LogMediatorStopped(_logger);
    }

-    /// <inheritdoc />
    public override void Dispose()
    {
-        _udpClient?.Dispose();
-        _udpClient = null;
-        LocalEndpoint = null;
+        StopManager();
        base.Dispose();
    }

-    /// <inheritdoc />
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
-        UdpClient udpClient = _udpClient
+        LiteNetManager manager = _manager
            ?? throw new InvalidOperationException("The UDP mediator socket was not initialized.");
-
+        long previous = Stopwatch.GetTimestamp();
        try
        {
            while (!stoppingToken.IsCancellationRequested)
            {
-                UdpReceiveResult received = await udpClient
-                    .ReceiveAsync(stoppingToken)
-                    .ConfigureAwait(false);
-                ProcessDatagram(received.Buffer, received.RemoteEndPoint, stoppingToken);
-                // Bootstrap deliberately emits no UDP response. Protocol handling lands in #11.
+                manager.PollEvents();
+                manager.NatPunchModule.PollEvents();
+                long current = Stopwatch.GetTimestamp();
+                manager.ManualUpdate((float)Stopwatch.GetElapsedTime(previous, current).TotalMilliseconds);
+                previous = current;
+                await Task.Delay(_options.PollIntervalMilliseconds, stoppingToken).ConfigureAwait(false);
            }
        }
        catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
        {
-            // Expected during normal shutdown.
-        }
-        catch (ObjectDisposedException) when (stoppingToken.IsCancellationRequested)
-        {
-            // Disposing the socket is the fallback that releases a blocked receive.
        }
        finally
        {
            LocalEndpoint = null;
+            LocalIpv6Endpoint = null;
        }
    }

-    internal UdpPresenceProcessingResult ProcessDatagram(
-        ReadOnlySpan<byte> encoded,
-        IPEndPoint observedSource,
-        CancellationToken cancellationToken = default)
+    private void StopManager()
    {
-        ArgumentNullException.ThrowIfNull(observedSource);
-        if (!RendezvousUdpCodec.TryDecode(encoded, out PresenceDatagram? datagram, out _)
-            || datagram is null
-            || !_capabilities.TryFingerprint(datagram.Capability, out SecretFingerprint fingerprint))
-        {
-            return UdpPresenceProcessingResult.Dropped;
-        }
-
-        if (datagram.MessageType != UdpPresenceMessageType.HostPresence)
-        {
-            return UdpPresenceProcessingResult.ClientPresenceDeferred;
-        }
-
-        AddressFamilyKind publicFamily = observedSource.AddressFamily switch
-        {
-            AddressFamily.InterNetwork => AddressFamilyKind.Ipv4,
-            AddressFamily.InterNetworkV6 => AddressFamilyKind.Ipv6,
-            _ => 0,
-        };
-        if (publicFamily == 0)
-        {
-            return UdpPresenceProcessingResult.Dropped;
-        }
-
-        ObservedEndpoint publicEndpoint = new(
-            publicFamily,
-            observedSource.Address.ToString(),
-            observedSource.Port);
-        ObservedEndpoint localEndpoint = new(
-            datagram.AddressFamily,
-            datagram.LocalAddress,
-            datagram.LocalPort);
-        StoreResult<StoredListing> bound = _store.BindHostPresence(new(
-            datagram.MediationHandle,
-            fingerprint,
-            publicEndpoint,
-            localEndpoint), cancellationToken);
-        return bound.Succeeded
-            ? UdpPresenceProcessingResult.HostPresenceAccepted
-            : UdpPresenceProcessingResult.HostPresenceRejected;
+        LiteNetManager? manager = Interlocked.Exchange(ref _manager, null);
+        _introductionSink = null;
+        LocalEndpoint = null;
+        LocalIpv6Endpoint = null;
+        manager?.Stop();
    }

    [LoggerMessage(
@@ -172,12 +147,62 @@ internal sealed partial class UdpMediatorService : BackgroundService
        Level = LogLevel.Information,
        Message = "UDP mediator stopped")]
    private static partial void LogMediatorStopped(ILogger logger);
-}

-internal enum UdpPresenceProcessingResult
-{
-    Dropped = 0,
-    HostPresenceAccepted = 1,
-    HostPresenceRejected = 2,
-    ClientPresenceDeferred = 3,
+    private sealed class LiteNetIntroductionSink(NatPunchModule module) : INatIntroductionSink
+    {
+        public void Introduce(NatIntroductionPlan plan) => module.NatIntroduce(
+            plan.HostLocal,
+            plan.HostPublic,
+            plan.ClientLocal,
+            plan.ClientPublic,
+            plan.ConnectionTicket);
+    }
+
+    private sealed class RendezvousPacketLayer(NatMediationProcessor processor) : PacketLayerBase(0)
+    {
+        private INatIntroductionSink? _sink;
+
+        public void Attach(INatIntroductionSink sink) => _sink = sink;
+
+        public override void ProcessInboundPacket(
+            ref IPEndPoint endPoint,
+            ref byte[] data,
+            ref int length)
+        {
+            bool isFrozenEnvelope = length >= 2
+                && data[0] == RendezvousUdpCodec.MagicFirst
+                && data[1] == RendezvousUdpCodec.MagicSecond;
+            INatIntroductionSink? sink = _sink;
+            if (isFrozenEnvelope)
+            {
+                if (sink is not null)
+                {
+                    _ = processor.ProcessDatagram(data.AsSpan(0, length), endPoint, sink);
+                }
+            }
+            else if (sink is not null
+                     && LiteNetNatRequestCodec.TryDecode(
+                         data.AsSpan(0, length),
+                         out IPEndPoint? claimedLocalEndpoint,
+                         out string? token)
+                     && claimedLocalEndpoint is not null
+                     && token is not null)
+            {
+                _ = processor.ProcessRequest(claimedLocalEndpoint, endPoint, token, sink);
+            }
+
+            // Every inbound packet is consumed here. NatPunchModule is used only for outbound introductions.
+            Drop(ref length);
+        }
+
+        public override void ProcessOutBoundPacket(
+            ref IPEndPoint endPoint,
+            ref byte[] data,
+            ref int offset,
+            ref int length)
+        {
+        }
+
+        private static void Drop(ref int length) => length = 0;
+    }
 }
@@ -2,7 +2,9 @@
  "Rendezvous": {
    "Udp": {
      "ListenAddress": "0.0.0.0",
-      "Port": 9050
+      "Port": 9050,
+      "MaxDatagramsPerPoll": 256,
+      "PollIntervalMilliseconds": 2
    }
  },
  "Logging": {