snap

app/build.gradle

@@ -49,11 +49,10 @@ android {
         buildConfigField 'String', 'BUILD_TIME_DISPLAY', "\"${buildTimeDisplay.replace('\\\\', '\\\\\\\\').replace('\"', '\\\\\"')}\""
         externalNativeBuild {
             cmake {
-                def cmakeArgs = ["-DANDROID_STL=c++_shared"]
+                arguments "-DANDROID_STL=c++_shared"
                 if (resolveCcachePath) {
-                    cmakeArgs += "-DANDROID_CCACHE=${resolveCcachePath}"
+                    arguments "-DANDROID_CCACHE=${resolveCcachePath}"
                 }
-                arguments cmakeArgs
                 cppFlags ""
             }
         }

app/src/main/AndroidManifest.xml

@@ -79,6 +79,11 @@
             android:stopWithTask="false"
             android:foregroundServiceType="mediaProjection|microphone" />
 
+        <service
+            android:name=".sender.SenderPreviewService"
+            android:exported="false"
+            android:foregroundServiceType="mediaProjection" />
+
         <service
             android:name=".receiver.ReceiverCastService"
             android:exported="false"

app/src/main/java/com/foxx/androidcast/CastSettingsBinder.java

@@ -158,9 +158,14 @@ public final class CastSettingsBinder {
         };
         bindStringSpinner(spinner, labels, values, settings.getTransport(), v -> {
             settings.setTransport(v);
-            updateStreamProtectionEnabled(activity, settings);
+            refreshStreamProtectionUi(activity, settings);
         });
-        updateStreamProtectionEnabled(activity, settings);
+        refreshStreamProtectionUi(activity, settings);
+    }
+
+    private static void refreshStreamProtectionUi(AppCompatActivity activity, CastSettings settings) {
+        updateStreamProtectionSpinnerEnabled(activity, settings, R.id.spinner_sender_stream_protection,
+                R.id.text_stream_protection_hint, R.id.spinner_sender_transport);
     }
 
     private static void readTransport(AppCompatActivity activity, CastSettings settings, int spinnerId) {
@@ -404,7 +409,16 @@ public final class CastSettingsBinder {
     }
 
     private static void bindStreamProtection(AppCompatActivity activity, CastSettings settings) {
-        Spinner spinner = activity.findViewById(R.id.spinner_sender_stream_protection);
+        bindStreamProtectionSpinner(activity, settings, R.id.spinner_sender_stream_protection,
+                R.id.text_stream_protection_hint, R.id.spinner_sender_transport);
+    }
+
+    private static void bindStreamProtectionSpinner(AppCompatActivity activity, CastSettings settings,
+            int spinnerId, int hintId, int transportSpinnerId) {
+        Spinner spinner = activity.findViewById(spinnerId);
+        if (spinner == null) {
+            return;
+        }
         CastSettings.StreamProtection[] modes = CastSettings.StreamProtection.values();
         String[] labels = {
                 activity.getString(R.string.option_auto),
@@ -422,7 +436,23 @@ public final class CastSettingsBinder {
         }
         SettingsEnumSpinner.bind(spinner, modes, labels, selectable, current,
                 settings::setStreamProtection);
-        updateStreamProtectionEnabled(activity, settings);
+        Spinner transportSpinner = activity.findViewById(transportSpinnerId);
+        if (transportSpinner != null) {
+            transportSpinner.setOnItemSelectedListener(new android.widget.AdapterView.OnItemSelectedListener() {
+                @Override
+                public void onItemSelected(android.widget.AdapterView<?> parent, android.view.View view,
+                        int position, long id) {
+                    readTransport(activity, settings, transportSpinnerId);
+                    updateStreamProtectionSpinnerEnabled(activity, settings, spinnerId, hintId,
+                            transportSpinnerId);
+                }
+
+                @Override
+                public void onNothingSelected(android.widget.AdapterView<?> parent) {
+                }
+            });
+        }
+        updateStreamProtectionSpinnerEnabled(activity, settings, spinnerId, hintId, transportSpinnerId);
     }
 
     private static void readStreamProtection(AppCompatActivity activity, CastSettings settings) {
@@ -430,14 +460,16 @@ public final class CastSettingsBinder {
                 settings::setStreamProtection);
     }
 
-    private static void updateStreamProtectionEnabled(AppCompatActivity activity, CastSettings settings) {
-        Spinner protection = activity.findViewById(R.id.spinner_sender_stream_protection);
+    private static void updateStreamProtectionSpinnerEnabled(AppCompatActivity activity,
+            CastSettings settings, int spinnerId, int hintId, int transportSpinnerId) {
+        Spinner protection = activity.findViewById(spinnerId);
         if (protection == null) {
             return;
         }
+        readTransport(activity, settings, transportSpinnerId);
         boolean udp = CastConfig.TRANSPORT_UDP.equals(settings.getTransport());
         protection.setEnabled(udp);
-        TextView hint = activity.findViewById(R.id.text_stream_protection_hint);
+        TextView hint = activity.findViewById(hintId);
         if (hint != null) {
             hint.setVisibility(udp ? View.VISIBLE : View.GONE);
         }

app/src/main/java/com/foxx/androidcast/PictureInPictureHelper.java

@@ -45,12 +45,13 @@ public final class PictureInPictureHelper {
         return true;
     }
 
-    public static PictureInPictureParams buildParams(int videoWidth, int videoHeight) {
+    public static PictureInPictureParams buildParams(Context context, int videoWidth, int videoHeight) {
         PictureInPictureParams.Builder builder = new PictureInPictureParams.Builder()
                 .setAspectRatio(aspectRatio(videoWidth, videoHeight));
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
-            builder.setAutoEnterEnabled(true);
-            builder.setSeamlessResizeEnabled(true);
+            boolean allowed = isAllowedForApp(context);
+            builder.setAutoEnterEnabled(allowed);
+            builder.setSeamlessResizeEnabled(allowed);
         }
         return builder.build();
     }
@@ -59,7 +60,7 @@ public final class PictureInPictureHelper {
         if (!isSupported()) {
             return;
         }
-        activity.setPictureInPictureParams(buildParams(videoWidth, videoHeight));
+        activity.setPictureInPictureParams(buildParams(activity, videoWidth, videoHeight));
     }
 
     public static boolean enter(@NonNull Activity activity, int videoWidth, int videoHeight) {
@@ -67,7 +68,7 @@ public final class PictureInPictureHelper {
             return false;
         }
         try {
-            return activity.enterPictureInPictureMode(buildParams(videoWidth, videoHeight));
+            return activity.enterPictureInPictureMode(buildParams(activity, videoWidth, videoHeight));
         } catch (IllegalStateException e) {
             return false;
         }

app/src/main/java/com/foxx/androidcast/diagnostics/CastDiagnosticsFormatter.java

@@ -22,7 +22,8 @@ public final class CastDiagnosticsFormatter {
     public static String formatHtml(StreamMetrics metrics, String transport, String videoCodec,
             int streamW, int streamH, int renderW, int renderH, boolean idle,
             CastSettings localSettings, CastSettings remoteSettings,
-            NetworkStatsSnapshot peer, NetworkStatsSnapshot lastLocal, int viewZoomPercent) {
+            NetworkStatsSnapshot peer, NetworkStatsSnapshot lastLocal, int viewZoomPercent,
+            CastMtuSampler.Info mtuInfo) {
         float inFps = metrics.getInFps();
         float renderFps = metrics.getRenderFps();
         int bwInKbps = metrics.getBwInKbps();
@@ -103,6 +104,7 @@ public final class CastDiagnosticsFormatter {
         html.append("<font color='").append(COLOR_DIM).append("'>Transport: ")
                 .append(escape(transport != null ? transport.toUpperCase() : "?"))
                 .append("</font><br/>");
+        appendMtuSection(html, mtuInfo);
 
         if (videoCodec != null && !videoCodec.isEmpty()) {
             html.append("Codec: <font color='").append(COLOR_OK).append("'>")
@@ -251,6 +253,12 @@ public final class CastDiagnosticsFormatter {
             html.append("<br/>");
         }
 
+        CpuLoadSampler.Sample cpu = CpuLoadSampler.sample();
+        html.append("CPU: ").append(vuBarHtml(cpu.loadPercent)).append(' ')
+                .append(colored(cpu.loadPercent + "%", cpu.loadPercent > 85))
+                .append(' ').append(CpuLoadSampler.formatCoreSuffix(cpu));
+        html.append("<br/>");
+
         for (String hint : hints) {
             html.append("<font color='").append(COLOR_WARN).append("'>")
                     .append(escape(hint)).append("</font><br/>");
@@ -270,6 +278,29 @@ public final class CastDiagnosticsFormatter {
         return html.toString();
     }
 
+    private static void appendMtuSection(StringBuilder html, CastMtuSampler.Info mtu) {
+        if (mtu == null) {
+            return;
+        }
+        html.append("<font color='").append(COLOR_DIM).append("'>MTU:</font> ");
+        if (mtu.interfaceName.length() > 0) {
+            html.append(escape(mtu.interfaceName)).append(' ');
+        }
+        if (mtu.linkMtu > 0) {
+            html.append("link ").append(mtu.linkMtu);
+        } else {
+            html.append("link ?");
+        }
+        html.append(" · dg ").append(mtu.castDatagramBytes);
+        html.append(" frag ").append(mtu.castFragPayloadBytes);
+        html.append(" · IPv4 ").append(mtu.ipv4WireBytes);
+        if (mtu.headroomBytes >= 0) {
+            html.append(" · room ").append(colored(String.valueOf(mtu.headroomBytes),
+                    mtu.headroomBytes < 100));
+        }
+        html.append("<br/>");
+    }
+
     private static void appendUdpLossSection(StringBuilder html, NetworkStatsSnapshot peer,
             NetworkStatsSnapshot local) {
         long rx = pick(peer, local, true, false);
@@ -314,7 +345,7 @@ public final class CastDiagnosticsFormatter {
                 html.append(' ').append(colored("aud-dec " + aDecErr, true));
             }
             if (aDrop > 0) {
-                html.append(' ').append(colored("aud-q " + aDrop, true));
+                html.append(' ').append(colored("aud-drop " + aDrop, true));
             }
             if (dropAudio > 0) {
                 html.append(' ').append(colored("snd-aud " + dropAudio, true));
@@ -364,9 +395,15 @@ public final class CastDiagnosticsFormatter {
             html.append(" · ").append(escape(summary));
         }
         long fecPkts = val(peer, local, p -> p.fecStubPackets, l -> l.fecStubPackets);
+        long fecOk = val(peer, local, p -> p.fecPacketsDecoded, l -> l.fecPacketsDecoded);
+        long fecFail = val(peer, local, p -> p.fecDecodeFailures, l -> l.fecDecodeFailures);
         long nackReq = val(peer, local, p -> p.nackStubRequests, l -> l.nackStubRequests);
-        if (fecPkts > 0) {
-            html.append("<br/>FEC packets=").append(fecPkts);
+        if (fecPkts > 0 || fecOk > 0 || fecFail > 0) {
+            html.append("<br/>FEC enc=").append(fecPkts).append(" dec=").append(fecOk);
+            if (fecFail > 0) {
+                html.append(" <font color='").append(COLOR_WARN).append("'>fail=").append(fecFail)
+                        .append("</font>");
+            }
         }
         if (nackReq > 0) {
             html.append("<br/>NACK events=").append(nackReq);

app/src/main/java/com/foxx/androidcast/diagnostics/CastMtuSampler.java

@@ -0,0 +1,67 @@
+package com.foxx.androidcast.diagnostics;
+
+import android.content.Context;
+import android.net.ConnectivityManager;
+import android.net.LinkProperties;
+import android.net.Network;
+import android.os.Build;
+
+import com.foxx.androidcast.CastConfig;
+
+/**
+ * Link MTU from the OS plus derived cast UDP sizing (see {@link com.foxx.androidcast.network.UdpCastTransport}).
+ */
+public final class CastMtuSampler {
+    /** ACUD header: magic(4) + type(1) + msgId(4) + fragIdx(2) + fragCnt(2) + totalLen(4). */
+    public static final int ACUD_HEADER_BYTES = 17;
+    private static final int IPV4_UDP_OVERHEAD = 20 + 8;
+
+    public static final class Info {
+        public final int linkMtu;
+        public final String interfaceName;
+        public final int castDatagramBytes;
+        public final int castFragPayloadBytes;
+        public final int ipv4WireBytes;
+        /** {@code linkMtu - ipv4WireBytes}, or {@code -1} when link MTU is unknown. */
+        public final int headroomBytes;
+
+        Info(int linkMtu, String interfaceName, int castDatagramBytes, int castFragPayloadBytes,
+                int ipv4WireBytes, int headroomBytes) {
+            this.linkMtu = linkMtu;
+            this.interfaceName = interfaceName != null ? interfaceName : "";
+            this.castDatagramBytes = castDatagramBytes;
+            this.castFragPayloadBytes = castFragPayloadBytes;
+            this.ipv4WireBytes = ipv4WireBytes;
+            this.headroomBytes = headroomBytes;
+        }
+    }
+
+    private CastMtuSampler() {}
+
+    public static Info sample(Context context) {
+        int linkMtu = 0;
+        String iface = "";
+        if (context != null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            ConnectivityManager cm = context.getSystemService(ConnectivityManager.class);
+            if (cm != null) {
+                Network network = cm.getActiveNetwork();
+                LinkProperties lp = network != null ? cm.getLinkProperties(network) : null;
+                if (lp != null) {
+                    linkMtu = lp.getMtu();
+                    if (lp.getInterfaceName() != null) {
+                        iface = lp.getInterfaceName();
+                    }
+                }
+            }
+        }
+        return calculate(linkMtu, iface);
+    }
+
+    public static Info calculate(int linkMtu, String interfaceName) {
+        int datagram = CastConfig.UDP_CHUNK_SIZE;
+        int fragPayload = Math.max(0, datagram - ACUD_HEADER_BYTES);
+        int wire = datagram + IPV4_UDP_OVERHEAD;
+        int headroom = linkMtu > 0 ? linkMtu - wire : -1;
+        return new Info(linkMtu, interfaceName, datagram, fragPayload, wire, headroom);
+    }
+}

app/src/main/java/com/foxx/androidcast/diagnostics/CpuLoadSampler.java

@@ -0,0 +1,234 @@
+package com.foxx.androidcast.diagnostics;
+
+import java.io.BufferedReader;
+import java.io.FileReader;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Locale;
+
+/** Samples device or process CPU load (0–100% smoothed). */
+public final class CpuLoadSampler {
+    private static final long MIN_INTERVAL_MS = 200;
+    private static final int CORE_BUSY_THRESHOLD_PERCENT = 12;
+    private static final int USER_HZ = 100;
+
+    private static final Object LOCK = new Object();
+    private static long[] prevTotals;
+    private static long[][] prevPerCpu;
+    private static long prevSelfJiffies;
+    private static long lastSampleMs;
+    private static boolean procStatReadable = true;
+    private static Sample cached = new Sample(0, Runtime.getRuntime().availableProcessors(), -1);
+
+    public static final class Sample {
+        public final int loadPercent;
+        public final int totalCores;
+        /** Cores above {@link #CORE_BUSY_THRESHOLD_PERCENT} usage; {@code -1} if unavailable. */
+        public final int activeCores;
+
+        Sample(int loadPercent, int totalCores, int activeCores) {
+            this.loadPercent = Math.max(0, Math.min(100, loadPercent));
+            this.totalCores = Math.max(1, totalCores);
+            this.activeCores = activeCores;
+        }
+    }
+
+    private CpuLoadSampler() {}
+
+    /** Prime baseline so the first displayed sample is not stuck at 0%. */
+    public static void warmUp() {
+        synchronized (LOCK) {
+            lastSampleMs = 0;
+            prevTotals = null;
+            prevPerCpu = null;
+            prevSelfJiffies = 0;
+        }
+        sample();
+        try {
+            Thread.sleep(MIN_INTERVAL_MS + 50);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+        sample();
+    }
+
+    public static Sample getCached() {
+        synchronized (LOCK) {
+            return cached;
+        }
+    }
+
+    public static Sample sample() {
+        synchronized (LOCK) {
+            long now = System.currentTimeMillis();
+            if (now - lastSampleMs < MIN_INTERVAL_MS && prevTotals != null) {
+                return cached;
+            }
+            int totalCores = Math.max(1, cached.totalCores);
+            int deviceLoad = -1;
+            int active = -1;
+
+            if (procStatReadable) {
+                ProcStat current = readProcStat();
+                if (current != null) {
+                    totalCores = Math.max(1, current.perCpu.size());
+                    if (prevTotals != null) {
+                        deviceLoad = percentDelta(prevTotals, current.totals);
+                        active = countActiveCores(prevPerCpu, current.perCpu, totalCores);
+                    }
+                    prevTotals = current.totals;
+                    prevPerCpu = copyPerCpu(current.perCpu);
+                } else {
+                    procStatReadable = false;
+                }
+            }
+
+            int processLoad = sampleProcessLoad(now);
+            int load = deviceLoad >= 0 ? deviceLoad : processLoad;
+            if (deviceLoad >= 0 && processLoad > load) {
+                load = processLoad;
+            }
+
+            if (prevTotals != null || prevSelfJiffies > 0) {
+                cached = new Sample(load, totalCores, active);
+            } else if (processLoad > 0) {
+                cached = new Sample(processLoad, totalCores, -1);
+            }
+            lastSampleMs = now;
+            return cached;
+        }
+    }
+
+    private static int sampleProcessLoad(long now) {
+        long jiffies = readSelfCpuJiffies();
+        if (jiffies < 0) {
+            return 0;
+        }
+        int load = 0;
+        if (prevSelfJiffies > 0 && lastSampleMs > 0) {
+            long wallMs = now - lastSampleMs;
+            if (wallMs > 0) {
+                long delta = jiffies - prevSelfJiffies;
+                float pct = delta * 1000f * 100f / (USER_HZ * wallMs * cached.totalCores);
+                load = Math.round(Math.max(0f, Math.min(100f, pct)));
+            }
+        }
+        prevSelfJiffies = jiffies;
+        return load;
+    }
+
+    private static long readSelfCpuJiffies() {
+        try (BufferedReader reader = new BufferedReader(new FileReader("/proc/self/stat"))) {
+            String line = reader.readLine();
+            if (line == null) {
+                return -1;
+            }
+            int close = line.lastIndexOf(')');
+            if (close < 0 || close + 2 >= line.length()) {
+                return -1;
+            }
+            String[] parts = line.substring(close + 2).split("\\s+");
+            if (parts.length < 2) {
+                return -1;
+            }
+            return Long.parseLong(parts[0]) + Long.parseLong(parts[1]);
+        } catch (IOException | NumberFormatException ignored) {
+            return -1;
+        }
+    }
+
+    private static int countActiveCores(long[][] prev, List<long[]> curr, int totalCores) {
+        if (prev == null || curr.isEmpty() || prev.length != curr.size()) {
+            return -1;
+        }
+        int active = 0;
+        for (int i = 0; i < curr.size(); i++) {
+            if (percentDelta(prev[i], curr.get(i)) >= CORE_BUSY_THRESHOLD_PERCENT) {
+                active++;
+            }
+        }
+        return Math.min(active, totalCores);
+    }
+
+    private static int percentDelta(long[] prev, long[] curr) {
+        long prevIdle = prev[3] + prev[4];
+        long currIdle = curr[3] + curr[4];
+        long prevTotal = sumTimes(prev);
+        long currTotal = sumTimes(curr);
+        long totalDelta = currTotal - prevTotal;
+        if (totalDelta <= 0) {
+            return 0;
+        }
+        long idleDelta = currIdle - prevIdle;
+        float busy = (totalDelta - idleDelta) * 100f / totalDelta;
+        return Math.round(Math.max(0f, Math.min(100f, busy)));
+    }
+
+    private static long sumTimes(long[] times) {
+        long sum = 0;
+        for (long t : times) {
+            sum += t;
+        }
+        return sum;
+    }
+
+    private static long[][] copyPerCpu(List<long[]> list) {
+        long[][] out = new long[list.size()][];
+        for (int i = 0; i < list.size(); i++) {
+            out[i] = list.get(i).clone();
+        }
+        return out;
+    }
+
+    private static ProcStat readProcStat() {
+        try (BufferedReader reader = new BufferedReader(new FileReader("/proc/stat"))) {
+            ProcStat stat = new ProcStat();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                if (line.startsWith("cpu ")) {
+                    stat.totals = parseCpuLine(line);
+                } else if (line.startsWith("cpu") && line.length() > 3 && Character.isDigit(line.charAt(3))) {
+                    long[] cpu = parseCpuLine(line);
+                    if (cpu != null) {
+                        stat.perCpu.add(cpu);
+                    }
+                } else if (!line.startsWith("cpu")) {
+                    break;
+                }
+            }
+            return stat.totals != null ? stat : null;
+        } catch (IOException ignored) {
+            return null;
+        }
+    }
+
+    private static long[] parseCpuLine(String line) {
+        String[] parts = line.trim().split("\\s+");
+        if (parts.length < 5) {
+            return null;
+        }
+        long[] times = new long[Math.min(10, parts.length - 1)];
+        for (int i = 0; i < times.length; i++) {
+            try {
+                times[i] = Long.parseLong(parts[i + 1]);
+            } catch (NumberFormatException e) {
+                return null;
+            }
+        }
+        return times;
+    }
+
+    private static final class ProcStat {
+        long[] totals;
+        final List<long[]> perCpu = new ArrayList<>();
+    }
+
+    /** HTML suffix: {@code (2/8)} or {@code (8 cores)}. */
+    public static String formatCoreSuffix(Sample sample) {
+        if (sample.activeCores >= 0 && sample.activeCores <= sample.totalCores) {
+            return String.format(Locale.US, "(%d/%d)", sample.activeCores, sample.totalCores);
+        }
+        return String.format(Locale.US, "(%d cores)", sample.totalCores);
+    }
+}

app/src/main/java/com/foxx/androidcast/network/CastProtocol.java

@@ -33,6 +33,8 @@ public final class CastProtocol {
     public static final byte MSG_CODEC_SELECTED = 15;
     /** UDP NACK: payload is big-endian {@code messageId} (4 bytes). */
     public static final byte MSG_NACK = 16;
+    /** Receiver capability/settings (same payload as {@link #MSG_CAST_SETTINGS}). */
+    public static final byte MSG_RECEIVER_CAST_SETTINGS = 17;
     /** UDP outer wrapper: payload is {@link CastPacketFramer} wire bytes. */
     public static final byte MSG_WIRE_PACKET = 127;
 
@@ -268,7 +270,7 @@ public final class CastProtocol {
         return local.getTransport().equals(remote.getTransport());
     }
 
-    private static final byte NETWORK_STATS_VERSION = 4;
+    private static final byte NETWORK_STATS_VERSION = 5;
 
     public static byte[] networkStatsPayload(NetworkStatsSnapshot s) throws IOException {
         java.io.ByteArrayOutputStream bos = new java.io.ByteArrayOutputStream();
@@ -318,6 +320,10 @@ public final class CastProtocol {
             dos.writeLong(s.fecStubPackets);
             dos.writeLong(s.nackStubRequests);
         }
+        if (NETWORK_STATS_VERSION >= 5) {
+            dos.writeLong(s.fecPacketsDecoded);
+            dos.writeLong(s.fecDecodeFailures);
+        }
         dos.flush();
         return bos.toByteArray();
     }
@@ -372,6 +378,10 @@ public final class CastProtocol {
                     s.fecStubPackets = dis.readLong();
                     s.nackStubRequests = dis.readLong();
                 }
+                if (ver >= 5 && dis.available() > 0) {
+                    s.fecPacketsDecoded = dis.readLong();
+                    s.fecDecodeFailures = dis.readLong();
+                }
             }
         }
         return s;

app/src/main/java/com/foxx/androidcast/network/CastSession.java

@@ -13,12 +13,17 @@ import java.util.List;
 public class CastSession {
     public static final class HandshakeResult {
         public final String senderName;
+        /** Sender cast settings from handshake. */
         public final CastSettings settings;
+        /** Receiver advertisement ({@code AUTO} = auto-detected FEC capability). */
+        public final CastSettings receiverAdvertisement;
         public final String negotiatedVideoMime;
 
-        public HandshakeResult(String senderName, CastSettings settings, String negotiatedVideoMime) {
+        public HandshakeResult(String senderName, CastSettings settings,
+                CastSettings receiverAdvertisement, String negotiatedVideoMime) {
             this.senderName = senderName;
             this.settings = settings;
+            this.receiverAdvertisement = receiverAdvertisement;
             this.negotiatedVideoMime = negotiatedVideoMime;
         }
     }
@@ -60,13 +65,16 @@ public class CastSession {
         transport.send(CastProtocol.MSG_CODEC_CAPS, CastProtocol.codecCapsPayload(CodecCatalog.localEncoderMimes()));
 
         String negotiated = null;
+        CastSettings receiverAdvertisement = null;
         long deadline = System.currentTimeMillis() + 15_000;
         while (System.currentTimeMillis() < deadline) {
             CastProtocol.Message msg = transport.receive(2_000);
             if (msg == null) {
                 continue;
             }
-            if (msg.type == CastProtocol.MSG_CODEC_SELECTED) {
+            if (msg.type == CastProtocol.MSG_RECEIVER_CAST_SETTINGS) {
+                receiverAdvertisement = CastProtocol.parseCastSettings(msg.payload);
+            } else if (msg.type == CastProtocol.MSG_CODEC_SELECTED) {
                 negotiated = CastProtocol.parseCodecSelected(msg.payload);
                 break;
             }
@@ -76,7 +84,7 @@ public class CastSession {
         }
         settings.setNegotiatedVideoMime(negotiated);
         CodecPriorityCatalog.logReceivedCodec("sender", negotiated, settings);
-        return new HandshakeResult(deviceName, settings, negotiated);
+        return new HandshakeResult(deviceName, settings, receiverAdvertisement, negotiated);
     }
 
     public HandshakeResult serverHandshake(String expectedPin, CastSettings localSettings) throws IOException {
@@ -117,13 +125,20 @@ public class CastSession {
                     break;
             }
             if (authed && remote != null && senderCaps != null) {
+                CastSettings receiverAdvert = new CastSettings();
+                receiverAdvert.copyFrom(localSettings);
+                receiverAdvert.setStreamProtection(
+                        com.foxx.androidcast.network.transport.StreamProtectionCapability
+                                .handshakeAdvertisement());
+                transport.send(CastProtocol.MSG_RECEIVER_CAST_SETTINGS,
+                        CastProtocol.castSettingsPayload(receiverAdvert));
                 List<String> receiverCaps = CodecCatalog.localDecoderMimes();
                 transport.send(CastProtocol.MSG_CODEC_CAPS, CastProtocol.codecCapsPayload(receiverCaps));
                 String mime = CodecNegotiator.negotiate(senderCaps, receiverCaps,
                         PassthroughCodecPolicy.forNegotiation(remote.getVideoCodec()), remote, "receiver");
                 transport.send(CastProtocol.MSG_CODEC_SELECTED, CastProtocol.codecSelectedPayload(mime));
                 remote.setNegotiatedVideoMime(mime);
-                return new HandshakeResult(senderName, remote, mime);
+                return new HandshakeResult(senderName, remote, receiverAdvert, mime);
             }
         }
         throw new IOException("Handshake timeout");

app/src/main/java/com/foxx/androidcast/network/CastTransportFactory.java

@@ -57,17 +57,27 @@ public final class CastTransportFactory {
         }
     }
 
-    /** Apply protection after handshake using negotiated local/remote preferences. */
-    public static void applyNegotiatedUdpProtection(UdpCastTransport udp, CastSettings local,
-            CastSettings remote) {
+    /**
+     * Apply protection after handshake using negotiated local/remote preferences.
+     *
+     * @param localIsReceiver true when {@code local} is the receiver's capability from settings.
+     * @return negotiation details (prompts, agreed mode) for UI/logging.
+     */
+    public static com.foxx.androidcast.network.transport.StreamProtectionNegotiation applyNegotiatedUdpProtection(
+            UdpCastTransport udp, CastSettings local, CastSettings remote, boolean localIsReceiver) {
         if (udp == null || local == null) {
-            return;
+            return com.foxx.androidcast.network.transport.StreamProtectionNegotiation.none();
         }
-        CastSettings.StreamProtection mode = com.foxx.androidcast.network.transport.StreamProtectionNegotiator
-                .resolve(local.getStreamProtection(), remote != null ? remote.getStreamProtection() : null);
+        com.foxx.androidcast.network.transport.StreamProtectionNegotiation negotiated =
+                com.foxx.androidcast.network.transport.StreamProtectionNegotiator.negotiate(
+                        local.getStreamProtection(),
+                        remote != null ? remote.getStreamProtection() : null,
+                        localIsReceiver);
         CastSettings effective = new CastSettings();
         effective.copyFrom(local);
-        effective.setStreamProtection(mode);
-        applyUdpProtection(udp, effective);
+        effective.setStreamProtection(negotiated.agreed);
+        udp.setProtectionEngine(com.foxx.androidcast.network.transport.StreamProtectionFactory.create(
+                negotiated.agreed, udp.getLossStats(), negotiated.nackEnabled));
+        return negotiated;
     }
 }

app/src/main/java/com/foxx/androidcast/network/UdpCastTransport.java

@@ -1,5 +1,7 @@
 package com.foxx.androidcast.network;
 
+import android.util.Log;
+
 import com.foxx.androidcast.CastConfig;
 import com.foxx.androidcast.network.transport.CombinedProtectionEngine;
 import com.foxx.androidcast.network.transport.FecProtectionEngine;
@@ -27,6 +29,7 @@ import java.util.concurrent.ConcurrentHashMap;
 
 /** Best-effort message transport over UDP with fragmentation. */
 public class UdpCastTransport implements CastTransport, TransportStatsProvider {
+    private static final String TAG = "UdpCastTransport";
     private static final byte[] MAGIC = {'A', 'C', 'U', 'D'};
     /** magic(4) + type(1) + msgId(4) + fragIdx(2) + fragCnt(2) + totalLen(4) */
     private static final int HEADER = 17;
@@ -45,7 +48,12 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
     private final RetransmitCache retransmitCache = new RetransmitCache();
     private CastFramingContext framingContext;
     private CastSessionGate inboundSessionGate;
-    private static final long REASSEMBLY_TIMEOUT_MS = 3_000;
+    private static final long REASSEMBLY_TIMEOUT_MIN_MS = 3_000;
+    private static final long REASSEMBLY_TIMEOUT_MAX_MS = 20_000;
+    /** Large VP9 keyframes need many UDP fragments per FEC shard; allow extra time to collect shards. */
+    private static final long FEC_GROUP_TIMEOUT_MS = 12_000;
+    private static final int REASSEMBLY_LOG_EVERY = 16;
+    private int reassemblyExpireLogCount;
 
     public boolean isListening() {
         return listening && socket != null && !socket.isClosed();
@@ -81,8 +89,8 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
         }
         socket = new DatagramSocket(null);
         socket.setReuseAddress(true);
-        socket.setReceiveBufferSize(512 * 1024);
-        socket.setSendBufferSize(512 * 1024);
+        socket.setReceiveBufferSize(2 * 1024 * 1024);
+        socket.setSendBufferSize(2 * 1024 * 1024);
     }
 
     @Override
@@ -142,9 +150,9 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
         sendOnce(type, body, false);
     }
 
-    void retransmitCached(byte wireType, byte[] body) {
+    void retransmitCached(byte wireType, byte[] body, int messageId) {
         try {
-            sendOnce(wireType, body, false);
+            sendOnceWithId(wireType, body, false, messageId);
         } catch (IOException ignored) {
         }
     }
@@ -156,12 +164,15 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
             synchronized (lossStats) {
                 lossStats.nackCacheMisses++;
             }
+            if ((++reassemblyExpireLogCount % REASSEMBLY_LOG_EVERY) == 1) {
+                Log.w(TAG, "NACK cache miss id=" + messageId);
+            }
             return;
         }
         synchronized (lossStats) {
             lossStats.nackRetransmits++;
         }
-        retransmitCached(entry.wireType, entry.body);
+        retransmitCached(entry.wireType, entry.body, messageId);
     }
 
     @Override
@@ -199,6 +210,17 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
         for (int i = 0; i < group.totalShards(); i++) {
             byte[] shardWire = fecEngine.encodeShardWire(group, baseId, i);
             sendOnceWithId(wireType, shardWire, cache, baseId + i);
+            if (i + 1 < group.totalShards() && shardWire.length > 8_000) {
+                paceBetweenLargeShards();
+            }
+        }
+    }
+
+    private static void paceBetweenLargeShards() {
+        try {
+            Thread.sleep(2);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
         }
     }
 
@@ -213,6 +235,7 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
         }
         int chunkSize = CastConfig.UDP_CHUNK_SIZE - HEADER;
         int fragments = Math.max(1, (body.length + chunkSize - 1) / chunkSize);
+        boolean paceFragments = fragments > 24;
         for (int i = 0; i < fragments; i++) {
             int offset = i * chunkSize;
             int len = Math.min(chunkSize, body.length - offset);
@@ -222,6 +245,9 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
             synchronized (lossStats) {
                 lossStats.datagramsSent++;
             }
+            if (paceFragments && i + 1 < fragments && (i & 7) == 7) {
+                paceBetweenLargeShards();
+            }
         }
     }
 
@@ -259,30 +285,55 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
     private void expireStaleFecGroups() {
         if (fecEngine == null || nackEngine == null) {
             if (fecEngine != null) {
-                fecEngine.expireStaleGroups(REASSEMBLY_TIMEOUT_MS, null);
+                fecEngine.expireStaleGroups(FEC_GROUP_TIMEOUT_MS, null);
             }
             return;
         }
-        fecEngine.expireStaleGroups(REASSEMBLY_TIMEOUT_MS, nackEngine::requestNack);
+        fecEngine.expireStaleGroups(FEC_GROUP_TIMEOUT_MS, nackEngine::requestNack);
     }
 
     private void expireStaleReassembly() {
         long now = System.currentTimeMillis();
         for (Map.Entry<Integer, Reassembly> e : pending.entrySet()) {
             Reassembly r = e.getValue();
-            if (r != null && now - r.startedMs > REASSEMBLY_TIMEOUT_MS) {
+            if (r != null && now > r.deadlineMs) {
                 pending.remove(e.getKey());
                 synchronized (lossStats) {
                     lossStats.reassemblyIncomplete++;
                 }
+                maybeLogReassemblyExpired(e.getKey(), r);
                 int messageId = e.getKey();
                 if (nackEngine != null) {
                     nackEngine.requestNack(messageId);
+                } else if ((++reassemblyExpireLogCount % REASSEMBLY_LOG_EVERY) == 2) {
+                    Log.w(TAG, "Reassembly expired without NACK id=" + messageId);
                 }
             }
         }
     }
 
+    private void maybeLogReassemblyExpired(int messageId, Reassembly r) {
+        if ((++reassemblyExpireLogCount % REASSEMBLY_LOG_EVERY) != 1) {
+            return;
+        }
+        Log.w(TAG, "UDP reassembly timeout id=" + messageId + " frags=" + r.received + "/"
+                + r.fragCount + " bytes=" + r.totalLen);
+    }
+
+    private static long reassemblyDeadlineMs(int fragCount, int totalLen) {
+        if (fragCount <= 1) {
+            return System.currentTimeMillis() + REASSEMBLY_TIMEOUT_MIN_MS;
+        }
+        int chunkSize = Math.max(1, CastConfig.UDP_CHUNK_SIZE - HEADER);
+        int expectedFrags = Math.max(fragCount, (totalLen + chunkSize - 1) / chunkSize);
+        long budget = REASSEMBLY_TIMEOUT_MIN_MS + expectedFrags * 200L + (totalLen / 2048);
+        if (fragCount > 1) {
+            budget = Math.max(8_000L, budget);
+        }
+        budget = Math.min(REASSEMBLY_TIMEOUT_MAX_MS, budget);
+        return System.currentTimeMillis() + budget;
+    }
+
     private CastProtocol.Message parsePacket(byte[] data, int length) throws IOException {
         if (length < HEADER) {
             synchronized (lossStats) {
@@ -495,6 +546,7 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
         final int fragCount;
         final int totalLen;
         final long startedMs;
+        final long deadlineMs;
         final byte[][] parts;
         final int[] sizes;
         int received;
@@ -503,6 +555,7 @@ public class UdpCastTransport implements CastTransport, TransportStatsProvider {
             this.fragCount = fragCount;
             this.totalLen = totalLen;
             this.startedMs = System.currentTimeMillis();
+            this.deadlineMs = reassemblyDeadlineMs(fragCount, totalLen);
             this.parts = new byte[fragCount][];
             this.sizes = new int[fragCount];
         }

app/src/main/java/com/foxx/androidcast/network/control/NetworkStatsSnapshot.java

@@ -47,6 +47,8 @@ public class NetworkStatsSnapshot {
     public long recvDamagedVideoFrames;
     public String protectionMode = "NONE";
     public long fecStubPackets;
+    public long fecPacketsDecoded;
+    public long fecDecodeFailures;
     public long nackStubRequests;
 
     public NetworkStatsSnapshot copy() {
@@ -90,6 +92,8 @@ public class NetworkStatsSnapshot {
         c.recvDamagedVideoFrames = recvDamagedVideoFrames;
         c.protectionMode = protectionMode;
         c.fecStubPackets = fecStubPackets;
+        c.fecPacketsDecoded = fecPacketsDecoded;
+        c.fecDecodeFailures = fecDecodeFailures;
         c.nackStubRequests = nackStubRequests;
         return c;
     }

app/src/main/java/com/foxx/androidcast/network/transport/CombinedProtectionEngine.java

@@ -20,7 +20,9 @@ public final class CombinedProtectionEngine implements StreamProtectionEngine {
 
     @Override
     public StreamProtectionMode mode() {
-        return StreamProtectionMode.FEC_NACK;
+        return fec.mode() == StreamProtectionMode.FEC_REED_SOLOMON
+                ? StreamProtectionMode.FEC_REED_SOLOMON
+                : StreamProtectionMode.FEC_3_4;
     }
 
     @Override

app/src/main/java/com/foxx/androidcast/network/transport/FecProtectionEngine.java

@@ -8,6 +8,8 @@ import com.foxx.androidcast.network.transport.fec.FecShardWire;
 import com.foxx.androidcast.network.transport.fec.ProtectionEnvelope;
 import com.foxx.androidcast.network.transport.fec.ReedSolomonFec;
 
+import android.util.Log;
+
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
@@ -16,9 +18,13 @@ import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 
-/** Reed-Solomon FEC: v2 sends one shard per UDP message; v1 monolithic decode still accepted inbound. */
+/** Reed-Solomon FEC: one shard per UDP message; v1 monolithic decode still accepted inbound. */
 public final class FecProtectionEngine implements StreamProtectionEngine {
+    private static final String TAG = "FecProtection";
     private static final AtomicInteger NEXT_GROUP_ID = new AtomicInteger(1);
+    private static final int LOG_EVERY_N = 32;
+    private int inboundShardRejectLogCount;
+    private int inboundDecodeFailLogCount;
 
     private final byte mode;
     private final int dataShards;
@@ -53,11 +59,25 @@ public final class FecProtectionEngine implements StreamProtectionEngine {
         return protectionMode;
     }
 
+    /** Pick shard counts so each ACUD logical message stays small on lossy Wi‑Fi. */
+    static int[] shardPlanForPayload(int payloadLength, int defaultData, int defaultParity) {
+        if (payloadLength > 64_000) {
+            return new int[] {16, 8};
+        }
+        if (payloadLength > 16_000) {
+            return new int[] {8, 4};
+        }
+        return new int[] {defaultData, defaultParity};
+    }
+
     public FecEncodedGroup encodeGroup(byte[] payload) throws IOException {
         if (payload == null || payload.length == 0) {
             throw new IOException("empty payload");
         }
-        ReedSolomonFec fec = new ReedSolomonFec(dataShards, parityShards);
+        int[] plan = shardPlanForPayload(payload.length, dataShards, parityShards);
+        int k = plan[0];
+        int m = plan[1];
+        ReedSolomonFec fec = new ReedSolomonFec(k, m);
         byte[][] shards = fec.encode(payload);
         int groupId = NEXT_GROUP_ID.getAndIncrement();
         if (stats != null) {
@@ -65,7 +85,7 @@ public final class FecProtectionEngine implements StreamProtectionEngine {
                 stats.fecPacketsEncoded += shards.length;
             }
         }
-        return new FecEncodedGroup(mode, groupId, payload.length, dataShards, parityShards, shards);
+        return new FecEncodedGroup(mode, groupId, payload.length, k, m, shards);
     }
 
     public byte[] encodeShardWire(FecEncodedGroup group, int baseMessageId, int shardIndex)
@@ -116,6 +136,17 @@ public final class FecProtectionEngine implements StreamProtectionEngine {
             assembly = prev != null ? prev : created;
         }
         if (!assembly.addShard(shard)) {
+            maybeLogShardReject(shard, assembly);
+            byte[] retry = assembly.tryDecode();
+            if (retry != null) {
+                inboundGroups.remove(shard.groupId);
+                if (stats != null) {
+                    synchronized (stats) {
+                        stats.fecPacketsDecoded++;
+                    }
+                }
+                return new ShardDecodeResult(ShardDecodeResult.Status.OK, retry);
+            }
             return new ShardDecodeResult(ShardDecodeResult.Status.PENDING, null);
         }
         byte[] decoded = assembly.tryDecode();
@@ -126,6 +157,7 @@ public final class FecProtectionEngine implements StreamProtectionEngine {
                         stats.fecDecodeFailures++;
                     }
                 }
+                maybeLogDecodeFail(shard.groupId, assembly.receivedCount(), assembly.getTotalShards());
                 inboundGroups.remove(shard.groupId);
                 return new ShardDecodeResult(ShardDecodeResult.Status.FAILED, null);
             }
@@ -212,6 +244,22 @@ public final class FecProtectionEngine implements StreamProtectionEngine {
         return decoded.payload;
     }
 
+    private void maybeLogShardReject(FecShardWire.Parsed shard, FecShardReassembly assembly) {
+        if ((++inboundShardRejectLogCount % LOG_EVERY_N) != 1) {
+            return;
+        }
+        Log.w(TAG, "FEC shard reject gid=" + shard.groupId + " idx=" + shard.shardIndex
+                + " shardLen=" + shard.shardData.length + " recv=" + assembly.receivedCount()
+                + "/" + assembly.getTotalShards());
+    }
+
+    private void maybeLogDecodeFail(int groupId, int received, int total) {
+        if ((++inboundDecodeFailLogCount % LOG_EVERY_N) != 1) {
+            return;
+        }
+        Log.w(TAG, "FEC RS decode failed gid=" + groupId + " shards=" + received + "/" + total);
+    }
+
     private static boolean looksLikeWireHeader(byte[] payload) {
         if (payload == null || payload.length < 4) {
             return false;

app/src/main/java/com/foxx/androidcast/network/transport/ProtectionNegotiationPrompts.java

@@ -0,0 +1,56 @@
+package com.foxx.androidcast.network.transport;
+
+import android.content.Context;
+
+import com.foxx.androidcast.CastSettings;
+import com.foxx.androidcast.R;
+
+/** Formats {@link StreamProtectionNegotiation.Prompt} for logs and on-screen status. */
+public final class ProtectionNegotiationPrompts {
+    private ProtectionNegotiationPrompts() {}
+
+    public static String format(Context context, StreamProtectionNegotiation.Prompt prompt,
+            boolean localIsReceiver) {
+        if (context == null || prompt == null) {
+            return "";
+        }
+        String local = label(context, prompt.localPreference);
+        String remote = label(context, prompt.remotePreference);
+        String role = context.getString(prompt.target == StreamProtectionNegotiation.Prompt.Target.SENDER
+                ? R.string.protection_negotiation_role_sender
+                : R.string.protection_negotiation_role_receiver);
+        return context.getString(prompt.messageResId, local, remote, role);
+    }
+
+    public static String summarize(Context context, StreamProtectionNegotiation negotiation) {
+        if (context == null || negotiation == null) {
+            return "";
+        }
+        String mode = label(context, negotiation.agreed);
+        String nack = negotiation.nackEnabled
+                ? context.getString(R.string.protection_negotiation_nack_on)
+                : context.getString(R.string.protection_negotiation_nack_off);
+        return context.getString(R.string.protection_negotiation_agreed, mode, nack);
+    }
+
+    public static String label(Context context, CastSettings.StreamProtection mode) {
+        if (context == null || mode == null) {
+            return "?";
+        }
+        switch (mode) {
+            case FEC_3_4:
+                return context.getString(R.string.protection_fec_34);
+            case FEC_REED_SOLOMON:
+                return context.getString(R.string.protection_fec_rs);
+            case NACK:
+                return context.getString(R.string.protection_nack);
+            case FEC_NACK:
+                return context.getString(R.string.protection_fec_nack);
+            case AUTO:
+                return context.getString(R.string.option_auto);
+            case NONE:
+            default:
+                return context.getString(R.string.protection_none);
+        }
+    }
+}

app/src/main/java/com/foxx/androidcast/network/transport/RetransmitCache.java

@@ -5,7 +5,8 @@ import java.util.Map;
 
 /** Ring buffer of recent UDP logical messages for NACK retransmission. */
 public final class RetransmitCache {
-    private static final int DEFAULT_CAPACITY = 64;
+    /** VP9+FEC uses 7 message ids per frame; must cover several seconds of in-flight shards. */
+    private static final int DEFAULT_CAPACITY = 4096;
 
     private final int capacity;
     private final LinkedHashMap<Integer, CachedMessage> map;

app/src/main/java/com/foxx/androidcast/network/transport/StreamProtectionCapability.java

@@ -0,0 +1,46 @@
+package com.foxx.androidcast.network.transport;
+
+import com.foxx.androidcast.CastSettings;
+
+import java.util.EnumSet;
+
+/** Runtime FEC/NACK implementations on this device (not a user setting). */
+public final class StreamProtectionCapability {
+    private StreamProtectionCapability() {}
+
+    /** Wire value: receiver supports negotiation from {@link #receiverImplementations()}. */
+    public static CastSettings.StreamProtection handshakeAdvertisement() {
+        return CastSettings.StreamProtection.AUTO;
+    }
+
+    public static boolean isAutoAdvertisement(CastSettings.StreamProtection mode) {
+        return mode == null || mode == CastSettings.StreamProtection.AUTO;
+    }
+
+    /** Modes this build can encode/decode over UDP. */
+    public static EnumSet<CastSettings.StreamProtection> receiverImplementations() {
+        return EnumSet.of(
+                CastSettings.StreamProtection.FEC_3_4,
+                CastSettings.StreamProtection.FEC_REED_SOLOMON,
+                CastSettings.StreamProtection.NACK,
+                CastSettings.StreamProtection.FEC_NACK,
+                CastSettings.StreamProtection.NONE);
+    }
+
+    public static boolean supports(CastSettings.StreamProtection mode) {
+        CastSettings.StreamProtection n = StreamProtectionNegotiator.normalize(mode);
+        return receiverImplementations().contains(n);
+    }
+
+    public static EnumSet<CastSettings.StreamProtection> resolveRemoteCapability(
+            CastSettings.StreamProtection remote) {
+        if (isAutoAdvertisement(remote)) {
+            return receiverImplementations();
+        }
+        CastSettings.StreamProtection n = StreamProtectionNegotiator.normalize(remote);
+        if (supports(n)) {
+            return EnumSet.of(n);
+        }
+        return EnumSet.of(CastSettings.StreamProtection.NONE);
+    }
+}

app/src/main/java/com/foxx/androidcast/network/transport/StreamProtectionFactory.java

@@ -1,7 +1,6 @@
 package com.foxx.androidcast.network.transport;
 
 import com.foxx.androidcast.CastSettings;
-import com.foxx.androidcast.network.CastProtocol;
 import com.foxx.androidcast.network.UdpCastTransport;
 import com.foxx.androidcast.network.transport.fec.ProtectionEnvelope;
 
@@ -16,17 +15,22 @@ public final class StreamProtectionFactory {
 
     public static StreamProtectionEngine create(CastSettings.StreamProtection setting,
             TransportLossStats stats) {
+        return create(setting, stats, wantsBundledNack(setting));
+    }
+
+    public static StreamProtectionEngine create(CastSettings.StreamProtection setting,
+            TransportLossStats stats, boolean nackEnabled) {
         CastSettings.StreamProtection mode = StreamProtectionNegotiator.normalize(setting);
         RetransmitCache cache = new RetransmitCache();
         switch (mode) {
             case NONE:
                 return StreamProtectionEngine.noop();
             case FEC_3_4:
-                return new FecProtectionEngine(ProtectionEnvelope.MODE_FEC_34,
-                        FEC34_DATA, FEC34_PARITY, stats);
+                return wrapFec(new FecProtectionEngine(ProtectionEnvelope.MODE_FEC_34,
+                        FEC34_DATA, FEC34_PARITY, stats), stats, cache, nackEnabled);
             case FEC_REED_SOLOMON:
-                return new FecProtectionEngine(ProtectionEnvelope.MODE_REED_SOLOMON,
-                        RS_DATA, RS_PARITY, stats);
+                return wrapFec(new FecProtectionEngine(ProtectionEnvelope.MODE_REED_SOLOMON,
+                        RS_DATA, RS_PARITY, stats), stats, cache, nackEnabled);
             case NACK:
                 return new NackProtectionEngine(stats, cache);
             case FEC_NACK:
@@ -38,6 +42,20 @@ public final class StreamProtectionFactory {
         }
     }
 
+    private static StreamProtectionEngine wrapFec(FecProtectionEngine fec,
+            TransportLossStats stats, RetransmitCache cache, boolean nackEnabled) {
+        if (!nackEnabled) {
+            return fec;
+        }
+        return new CombinedProtectionEngine(fec, new NackProtectionEngine(stats, cache));
+    }
+
+    private static boolean wantsBundledNack(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.FEC_3_4
+                || mode == CastSettings.StreamProtection.FEC_NACK
+                || mode == CastSettings.StreamProtection.FEC_REED_SOLOMON;
+    }
+
     public static void wireUdp(UdpCastTransport udp, StreamProtectionEngine engine) {
         if (udp == null || engine == null) {
             return;

app/src/main/java/com/foxx/androidcast/network/transport/StreamProtectionNegotiation.java

@@ -0,0 +1,45 @@
+package com.foxx.androidcast.network.transport;
+
+import com.foxx.androidcast.CastSettings;
+
+/** Outcome of matching local/remote {@link CastSettings.StreamProtection} preferences. */
+public final class StreamProtectionNegotiation {
+    public final CastSettings.StreamProtection agreed;
+    public final boolean nackEnabled;
+    public final Prompt prompt;
+
+    public StreamProtectionNegotiation(CastSettings.StreamProtection agreed, boolean nackEnabled,
+            Prompt prompt) {
+        this.agreed = agreed != null ? agreed : CastSettings.StreamProtection.NONE;
+        this.nackEnabled = nackEnabled;
+        this.prompt = prompt;
+    }
+
+    public static StreamProtectionNegotiation none() {
+        return new StreamProtectionNegotiation(CastSettings.StreamProtection.NONE, false, null);
+    }
+
+    public static final class Prompt {
+        public enum Target {
+            /** Ask the casting phone to change stream protection in settings. */
+            SENDER,
+            /** Ask the receiver device to change stream protection in settings. */
+            RECEIVER
+        }
+
+        public final Target target;
+        /** String resource id for a short user-visible hint. */
+        public final int messageResId;
+        public final CastSettings.StreamProtection localPreference;
+        public final CastSettings.StreamProtection remotePreference;
+
+        public Prompt(Target target, int messageResId,
+                CastSettings.StreamProtection localPreference,
+                CastSettings.StreamProtection remotePreference) {
+            this.target = target;
+            this.messageResId = messageResId;
+            this.localPreference = localPreference;
+            this.remotePreference = remotePreference;
+        }
+    }
+}

app/src/main/java/com/foxx/androidcast/network/transport/StreamProtectionNegotiator.java

@@ -1,32 +1,242 @@
 package com.foxx.androidcast.network.transport;
 
 import com.foxx.androidcast.CastSettings;
+import com.foxx.androidcast.R;
 
-/** Picks an agreed stream protection mode for both peers. */
+import java.util.EnumSet;
+
+/** Picks agreed stream protection from receiver capability vs sender preference. */
 public final class StreamProtectionNegotiator {
     private StreamProtectionNegotiator() {}
 
-    /**
-     * Agreed mode for this session. Any mismatch, missing remote preference, or {@code NONE}
-     * on either side yields {@link CastSettings.StreamProtection#NONE} (plain UDP).
-     */
+    /** @deprecated Use {@link #negotiate(CastSettings.StreamProtection, CastSettings.StreamProtection, boolean)}. */
+    @Deprecated
     public static CastSettings.StreamProtection resolve(CastSettings.StreamProtection local,
             CastSettings.StreamProtection remote) {
-        CastSettings.StreamProtection a = normalize(local);
-        CastSettings.StreamProtection b = normalize(remote);
-        if (a == CastSettings.StreamProtection.NONE || b == CastSettings.StreamProtection.NONE) {
-            return CastSettings.StreamProtection.NONE;
+        return negotiate(local, remote, false).agreed;
     }
-        if (a == b) {
-            return a;
+
+    /**
+     * @param local sender preference when {@code localIsReceiver} is false; ignored on receiver
+     *              (capability is auto-detected)
+     * @param remote sender preference when {@code localIsReceiver}; receiver wire adv. when sender
+     */
+    public static StreamProtectionNegotiation negotiate(CastSettings.StreamProtection local,
+            CastSettings.StreamProtection remote, boolean localIsReceiver) {
+        EnumSet<CastSettings.StreamProtection> receiverCaps;
+        CastSettings.StreamProtection senderPref;
+        if (localIsReceiver) {
+            receiverCaps = StreamProtectionCapability.receiverImplementations();
+            senderPref = remote;
+        } else {
+            senderPref = local;
+            receiverCaps = StreamProtectionCapability.resolveRemoteCapability(remote);
+        }
+        return negotiateWithReceiverCaps(receiverCaps, senderPref, local, remote, localIsReceiver);
+    }
+
+    private static StreamProtectionNegotiation negotiateWithReceiverCaps(
+            EnumSet<CastSettings.StreamProtection> receiverCaps,
+            CastSettings.StreamProtection senderPref,
+            CastSettings.StreamProtection localRaw,
+            CastSettings.StreamProtection remoteRaw,
+            boolean localIsReceiver) {
+        CastSettings.StreamProtection sender = normalize(senderPref);
+        CastSettings.StreamProtection receiverLabel = localIsReceiver
+                ? StreamProtectionCapability.handshakeAdvertisement()
+                : pickRepresentativeCap(receiverCaps);
+
+        if (sender == CastSettings.StreamProtection.NONE) {
+            if (receiverCaps.stream().anyMatch(StreamProtectionNegotiator::isFec)) {
+                StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                        ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                        : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+                return new StreamProtectionNegotiation(
+                        CastSettings.StreamProtection.NONE,
+                        false,
+                        new StreamProtectionNegotiation.Prompt(
+                                target,
+                                R.string.protection_negotiation_use_none,
+                                receiverLabel,
+                                senderPref));
+            }
+            return StreamProtectionNegotiation.none();
+        }
+
+        if (receiverCaps.contains(sender)) {
+            return agreeExact(sender, receiverLabel, senderPref);
+        }
+
+        if (isFec34(sender) && normalize(remoteRaw) == CastSettings.StreamProtection.FEC_NACK) {
+            return fec34WithoutNack(receiverLabel, senderPref, localIsReceiver);
+        }
+
+        if (isNackOnly(sender) && receiverCaps.stream().anyMatch(StreamProtectionNegotiator::isFec)) {
+            CastSettings.StreamProtection fec = receiverCaps.contains(CastSettings.StreamProtection.FEC_NACK)
+                    ? CastSettings.StreamProtection.FEC_NACK
+                    : receiverCaps.contains(CastSettings.StreamProtection.FEC_3_4)
+                    ? CastSettings.StreamProtection.FEC_3_4
+                    : CastSettings.StreamProtection.FEC_REED_SOLOMON;
+            return agreeFecWithNack(fec, receiverLabel, senderPref, localIsReceiver);
+        }
+
+        if (isFec(sender)) {
+            if (isFec34(sender) && receiverCaps.contains(CastSettings.StreamProtection.FEC_REED_SOLOMON)) {
+                return negotiateFecFamilyMismatch(
+                        CastSettings.StreamProtection.FEC_REED_SOLOMON,
+                        CastSettings.StreamProtection.FEC_3_4,
+                        receiverLabel,
+                        senderPref,
+                        localIsReceiver);
+            }
+            if (sender == CastSettings.StreamProtection.FEC_REED_SOLOMON
+                    && receiverCaps.contains(CastSettings.StreamProtection.FEC_3_4)) {
+                return negotiateFecFamilyMismatch(
+                        CastSettings.StreamProtection.FEC_REED_SOLOMON,
+                        CastSettings.StreamProtection.FEC_3_4,
+                        receiverLabel,
+                        senderPref,
+                        localIsReceiver);
+            }
+            StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                    ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                    : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+            return new StreamProtectionNegotiation(
+                    CastSettings.StreamProtection.NONE,
+                    false,
+                    new StreamProtectionNegotiation.Prompt(
+                            target,
+                            R.string.protection_negotiation_use_none,
+                            receiverLabel,
+                            senderPref));
+        }
+
+        StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+        return new StreamProtectionNegotiation(
+                CastSettings.StreamProtection.NONE,
+                false,
+                new StreamProtectionNegotiation.Prompt(
+                        target,
+                        R.string.protection_negotiation_mismatch,
+                        receiverLabel,
+                        senderPref));
+    }
+
+    private static CastSettings.StreamProtection pickRepresentativeCap(
+            EnumSet<CastSettings.StreamProtection> caps) {
+        if (caps.contains(CastSettings.StreamProtection.FEC_REED_SOLOMON)) {
+            return CastSettings.StreamProtection.FEC_REED_SOLOMON;
+        }
+        if (caps.contains(CastSettings.StreamProtection.FEC_3_4)) {
+            return CastSettings.StreamProtection.FEC_3_4;
+        }
+        if (caps.contains(CastSettings.StreamProtection.FEC_NACK)) {
+            return CastSettings.StreamProtection.FEC_NACK;
+        }
+        if (caps.contains(CastSettings.StreamProtection.NACK)) {
+            return CastSettings.StreamProtection.NACK;
         }
         return CastSettings.StreamProtection.NONE;
     }
 
+    private static StreamProtectionNegotiation negotiateFecFamilyMismatch(
+            CastSettings.StreamProtection agreed,
+            CastSettings.StreamProtection other,
+            CastSettings.StreamProtection receiverLabel,
+            CastSettings.StreamProtection senderPref,
+            boolean localIsReceiver) {
+        StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+        boolean nack = wantsNack(other);
+        if (isFecNack(other)) {
+            nack = false;
+        }
+        return new StreamProtectionNegotiation(
+                agreed,
+                nack,
+                new StreamProtectionNegotiation.Prompt(
+                        target,
+                        R.string.protection_negotiation_fec_family_rs,
+                        receiverLabel,
+                        senderPref));
+    }
+
+    private static StreamProtectionNegotiation fec34WithoutNack(
+            CastSettings.StreamProtection receiverLabel,
+            CastSettings.StreamProtection senderPref,
+            boolean localIsReceiver) {
+        StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+        return new StreamProtectionNegotiation(
+                CastSettings.StreamProtection.FEC_3_4,
+                false,
+                new StreamProtectionNegotiation.Prompt(
+                        target,
+                        R.string.protection_negotiation_fec34_no_nack,
+                        receiverLabel,
+                        senderPref));
+    }
+
+    private static StreamProtectionNegotiation agreeFecWithNack(
+            CastSettings.StreamProtection fecMode,
+            CastSettings.StreamProtection receiverLabel,
+            CastSettings.StreamProtection senderPref,
+            boolean localIsReceiver) {
+        StreamProtectionNegotiation.Prompt.Target target = localIsReceiver
+                ? StreamProtectionNegotiation.Prompt.Target.SENDER
+                : StreamProtectionNegotiation.Prompt.Target.RECEIVER;
+        return new StreamProtectionNegotiation(
+                fecMode,
+                true,
+                new StreamProtectionNegotiation.Prompt(
+                        target,
+                        R.string.protection_negotiation_nack_to_fec,
+                        receiverLabel,
+                        senderPref));
+    }
+
+    private static StreamProtectionNegotiation agreeExact(
+            CastSettings.StreamProtection mode,
+            CastSettings.StreamProtection receiverLabel,
+            CastSettings.StreamProtection senderPref) {
+        boolean nack = wantsNack(mode);
+        if (mode == CastSettings.StreamProtection.FEC_3_4) {
+            nack = true;
+        }
+        return new StreamProtectionNegotiation(mode, nack, null);
+    }
+
     public static CastSettings.StreamProtection normalize(CastSettings.StreamProtection mode) {
         if (mode == null || mode == CastSettings.StreamProtection.AUTO) {
             return CastSettings.StreamProtection.NONE;
         }
         return mode;
     }
+
+    private static boolean wantsNack(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.NACK
+                || mode == CastSettings.StreamProtection.FEC_NACK;
+    }
+
+    private static boolean isNackOnly(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.NACK;
+    }
+
+    private static boolean isFec(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.FEC_3_4
+                || mode == CastSettings.StreamProtection.FEC_REED_SOLOMON
+                || mode == CastSettings.StreamProtection.FEC_NACK;
+    }
+
+    private static boolean isFec34(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.FEC_3_4;
+    }
+
+    private static boolean isFecNack(CastSettings.StreamProtection mode) {
+        return mode == CastSettings.StreamProtection.FEC_NACK;
+    }
 }

app/src/main/java/com/foxx/androidcast/network/transport/TransportLossStatsBridge.java

@@ -33,6 +33,8 @@ public final class TransportLossStatsBridge {
             s.recvDamagedVideoFrames = loss.recvDamagedVideoFrames;
             s.protectionMode = loss.protectionMode.name();
             s.fecStubPackets = loss.fecPacketsEncoded;
+            s.fecPacketsDecoded = loss.fecPacketsDecoded;
+            s.fecDecodeFailures = loss.fecDecodeFailures;
             s.nackStubRequests = loss.nackRequestsSent + loss.nackRequestsReceived;
             long lossEvents = loss.estimatedUdpLossEvents();
             if (loss.datagramsReceived > 0 && lossEvents > 0) {

app/src/main/java/com/foxx/androidcast/network/transport/fec/FecShardWire.java

@@ -7,14 +7,19 @@ import java.io.DataOutputStream;
 import java.io.IOException;
 
 /**
- * Per-datagram FEC shard (v2). Each shard is its own ACUD logical message so loss of one
+ * Per-datagram FEC shard. Each shard is its own ACUD logical message so loss of one
  * fragment does not invalidate the whole Reed–Solomon block.
+ *
+ * <p>v2 used a single-byte shard length (max 255), which breaks VP9 keyframes. v3 uses uint16.
  */
 public final class FecShardWire {
     private static final byte[] MAGIC = {'A', 'C', 'P', 'F'};
-    public static final byte VERSION_SHARD = 2;
-    /** Bytes after {@link #MAGIC} before shard payload. */
-    private static final int HEADER_AFTER_MAGIC = 19;
+    /** Legacy on-wire format; shard length field is 1 byte (max 255). */
+    public static final byte VERSION_SHARD_V2 = 2;
+    /** Current format; shard length field is uint16. */
+    public static final byte VERSION_SHARD = 3;
+    private static final int HEADER_AFTER_MAGIC_V2 = 19;
+    private static final int HEADER_AFTER_MAGIC_V3 = 20;
 
     private FecShardWire() {}
 
@@ -31,6 +36,9 @@ public final class FecShardWire {
         if (shardData == null) {
             throw new IOException("shardData required");
         }
+        if (shardData.length > 0xFFFF) {
+            throw new IOException("shard too large: " + shardData.length);
+        }
         ByteArrayOutputStream bos = new ByteArrayOutputStream(32 + shardData.length);
         DataOutputStream dos = new DataOutputStream(bos);
         dos.write(MAGIC);
@@ -43,18 +51,17 @@ public final class FecShardWire {
         dos.writeInt(originalLen);
         dos.writeByte(dataShards);
         dos.writeByte(parityShards);
-        dos.writeByte(shardData.length);
+        dos.writeShort(shardData.length);
         dos.write(shardData);
         return bos.toByteArray();
     }
 
     public static boolean isShardPacket(byte[] wire) {
-        Parsed p = tryParse(wire);
-        return p != null;
+        return tryParse(wire) != null;
     }
 
     public static Parsed tryParse(byte[] wire) {
-        if (wire == null || wire.length < MAGIC.length + HEADER_AFTER_MAGIC) {
+        if (wire == null || wire.length < MAGIC.length + HEADER_AFTER_MAGIC_V2) {
             return null;
         }
         for (int i = 0; i < MAGIC.length; i++) {
@@ -66,7 +73,7 @@ public final class FecShardWire {
             DataInputStream dis = new DataInputStream(new ByteArrayInputStream(wire, MAGIC.length,
                     wire.length - MAGIC.length));
             byte ver = dis.readByte();
-            if (ver != VERSION_SHARD) {
+            if (ver != VERSION_SHARD_V2 && ver != VERSION_SHARD) {
                 return null;
             }
             byte mode = dis.readByte();
@@ -77,14 +84,15 @@ public final class FecShardWire {
             int originalLen = dis.readInt();
             int dataShards = dis.readUnsignedByte();
             int parityShards = dis.readUnsignedByte();
-            int shardSize = dis.readUnsignedByte();
+            int shardSize = ver == VERSION_SHARD ? dis.readUnsignedShort() : dis.readUnsignedByte();
             if (dataShards < 1 || parityShards < 1 || totalShards != dataShards + parityShards) {
                 return null;
             }
-            if (shardIndex >= totalShards || originalLen < 1) {
+            if (shardIndex >= totalShards || originalLen < 1 || shardSize < 1) {
                 return null;
             }
-            if (wire.length < MAGIC.length + HEADER_AFTER_MAGIC + shardSize) {
+            int headerAfterMagic = ver == VERSION_SHARD ? HEADER_AFTER_MAGIC_V3 : HEADER_AFTER_MAGIC_V2;
+            if (wire.length < MAGIC.length + headerAfterMagic + shardSize) {
                 return null;
             }
             byte[] shardData = new byte[shardSize];

app/src/main/java/com/foxx/androidcast/receiver/AudioDecoder.java

@@ -166,23 +166,33 @@ public class AudioDecoder {
         Log.i(TAG, "AudioTrack buf=" + bufSize + " playState=" + track.getPlayState());
     }
 
+    /** Drops queued AAC input without tearing down the decoder (e.g. after video keyframe wait). */
+    public void clearInputQueue() {
+        inputQueue.clear();
+        retryFrame = null;
+    }
+
     public void queueFrame(long ptsUs, byte[] data) {
         if (data == null || data.length == 0) {
             return;
         }
         stats.framesQueued++;
-        if (!inputQueue.offer(new PendingFrame(ptsUs, data.clone()))) {
-            PendingFrame dropped = inputQueue.poll();
-            if (dropped != null) {
-                stats.queueDrops++;
+        PendingFrame frame = new PendingFrame(ptsUs, data.clone());
+        if (inputQueue.offer(frame)) {
+            return;
+        }
+        int dropped = inputQueue.size() + (retryFrame != null ? 1 : 0);
+        inputQueue.clear();
+        retryFrame = null;
+        if (dropped > 0) {
+            stats.queueDrops += dropped;
             logDropBurst();
         }
-            if (!inputQueue.offer(new PendingFrame(ptsUs, data.clone()))) {
+        if (!inputQueue.offer(frame)) {
             stats.queueDrops++;
             logDropBurst();
         }
     }
-    }
 
     private void logDropBurst() {
         long now = System.currentTimeMillis();

app/src/main/java/com/foxx/androidcast/receiver/LibvpxVideoDecoder.java

@@ -11,8 +11,10 @@ import java.io.IOException;
 /** Software VP8/VP9 decode via libvpx, rendering to a {@link Surface}. */
 public final class LibvpxVideoDecoder implements VideoDecoderSink {
     private static final String TAG = "LibvpxVideoDecoder";
+    private static final int DECODE_FAIL_LOG_EVERY = 16;
 
     private final Object lock = new Object();
+    private int decodeFailLogCount;
     private long nativeHandle;
     private Surface outputSurface;
     private Listener listener;
@@ -47,6 +49,8 @@ public final class LibvpxVideoDecoder implements VideoDecoderSink {
             }
             if (NativeCodecBridge.vpxDecodeFrame(nativeHandle, data, ptsUs, outputSurface)) {
                 notifyRenderedLocked();
+            } else if ((++decodeFailLogCount % DECODE_FAIL_LOG_EVERY) == 1) {
+                Log.w(TAG, "vpx decode failed len=" + data.length + " key=" + keyFrame);
             }
         }
     }

app/src/main/java/com/foxx/androidcast/receiver/ReceiverCastService.java

@@ -25,6 +25,8 @@ import com.foxx.androidcast.IntentExtras;
 import com.foxx.androidcast.R;
 import com.foxx.androidcast.CastConfig;
 import com.foxx.androidcast.diagnostics.CastDiagnosticsFormatter;
+import com.foxx.androidcast.diagnostics.CastMtuSampler;
+import com.foxx.androidcast.diagnostics.CpuLoadSampler;
 import com.foxx.androidcast.media.codec.CodecSessionRegistry;
 import com.foxx.androidcast.media.codec.PassthroughCodecPolicy;
 import com.foxx.androidcast.media.codec.VideoCodecBackend;
@@ -169,6 +171,7 @@ public class ReceiverCastService extends Service {
         videoDecodeThread = new HandlerThread("VideoDecode");
         videoDecodeThread.start();
         videoDecodeHandler = new Handler(videoDecodeThread.getLooper());
+        CpuLoadSampler.warmUp();
         mainHandler.post(idleWatchdog);
     }
 
@@ -337,7 +340,7 @@ public class ReceiverCastService extends Service {
 
             @Override
             public void onAudioFrame(long ptsUs, byte[] data) {
-                if (castEnded) {
+                if (castEnded || awaitingKeyframe || streamIdle) {
                     return;
                 }
                 final int frameBytes = data != null ? data.length : 0;
@@ -476,7 +479,7 @@ public class ReceiverCastService extends Service {
             return;
         }
         streamIdle = false;
-        awaitingKeyframe = true;
+        enterAwaitingKeyframe();
         openPlaybackAwaiting(R.string.playback_awaiting_stream);
         scheduleDecoderConfigureRetries();
     }
@@ -485,7 +488,7 @@ public class ReceiverCastService extends Service {
     private void enterAwaitingMode(int messageResId, boolean fullReset) {
         streamIdle = true;
         hasPendingConfig = false;
-        awaitingKeyframe = true;
+        enterAwaitingKeyframe();
         resetDecoderState();
         if (videoDecoder != null) {
             videoDecoder.release();
@@ -500,7 +503,7 @@ public class ReceiverCastService extends Service {
 
     private void resetStreamState() {
         hasPendingConfig = false;
-        awaitingKeyframe = true;
+        enterAwaitingKeyframe();
         pendingWidth = 0;
         pendingHeight = 0;
         displayWidth = 0;
@@ -516,6 +519,19 @@ public class ReceiverCastService extends Service {
         decoderHeight = 0;
     }
 
+    private void enterAwaitingKeyframe() {
+        awaitingKeyframe = true;
+        flushAudioInput();
+    }
+
+    private void flushAudioInput() {
+        synchronized (audioDecoderLock) {
+            if (audioDecoder != null) {
+                audioDecoder.clearInputQueue();
+            }
+        }
+    }
+
     private void onAudioConfigReceived(int sampleRate, int channels, byte[] csd0, boolean accepted) {
         pendingAudioRate = sampleRate;
         pendingAudioChannels = channels;
@@ -559,7 +575,7 @@ public class ReceiverCastService extends Service {
             streamMetrics.reset();
         }
         reconfiguring = true;
-        awaitingKeyframe = true;
+        enterAwaitingKeyframe();
         openPlaybackAwaiting(R.string.playback_awaiting_stream);
         pendingWidth = size.width;
         pendingHeight = size.height;
@@ -796,7 +812,8 @@ public class ReceiverCastService extends Service {
                 remoteCastSettings,
                 peer,
                 lastLocalStats,
-                PlaybackViewState.getZoomPercent());
+                PlaybackViewState.getZoomPercent(),
+                CastMtuSampler.sample(this));
     }
 
     private TransportLossStats buildReceiverLossSnapshot() {

app/src/main/java/com/foxx/androidcast/receiver/ReceiverPlaybackActivity.java

@@ -73,6 +73,7 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
     private boolean bound;
     private boolean autoStartListening;
     private boolean listeningCancelled;
+    private boolean autoPipAttempted;
 
     private final ICastStatusCallback.Stub statusCallback = new ICastStatusCallback.Stub() {
         @Override
@@ -182,11 +183,14 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
         awaitingMessage = findViewById(R.id.text_awaiting_message);
         pipButton = findViewById(R.id.button_pip);
         if (pipButton != null) {
-            pipButton.setOnClickListener(v -> tryEnterPictureInPicture());
+            pipButton.setOnClickListener(v -> tryEnterPictureInPicture(true));
         }
         textureView.setSurfaceTextureListener(surfaceListener);
         applyAspectRatio();
         updatePipUi();
+        if (AppPreferences.isReceiverPipEnabled(this) && PictureInPictureHelper.isSupported()) {
+            PictureInPictureHelper.applyParams(this, 16, 9);
+        }
 
         autoStartListening = getIntent().getBooleanExtra(EXTRA_AUTO_START_LISTENING, false);
         if (autoStartListening) {
@@ -245,14 +249,14 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
         mainHandler.post(diagnosticsRefresh);
         updateDiagnosticsVisibility();
         updatePipUi();
+        maybeAutoEnterPip();
     }
 
     @Override
     public void onUserLeaveHint() {
         super.onUserLeaveHint();
-        if (castSessionActive && AppPreferences.isReceiverPipEnabled(this)
-                && PermissionHelper.hasReceiverPlaybackReady(this)) {
-            tryEnterPictureInPicture();
+        if (shouldAutoEnterPipOnLeave()) {
+            tryEnterPictureInPicture(false);
         }
     }
 
@@ -268,6 +272,7 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
         super.onRequestPermissionsResult(requestCode, permissions, grantResults);
         if (requestCode == PermissionHelper.REQUEST_RECEIVER_PLAYBACK) {
             updatePipUi();
+            maybeAutoEnterPip();
         }
     }
 
@@ -546,7 +551,9 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
         if (pipButton == null) {
             return;
         }
-        boolean show = castSessionActive && AppPreferences.isReceiverPipEnabled(this)
+        boolean sessionVisible = castSessionActive || streamRendering
+                || (awaitingOverlay != null && awaitingOverlay.getVisibility() == View.VISIBLE);
+        boolean show = sessionVisible && AppPreferences.isReceiverPipEnabled(this)
                 && PictureInPictureHelper.isSupported()
                 && PermissionHelper.hasReceiverPlaybackReady(this);
         pipButton.setVisibility(show ? View.VISIBLE : View.GONE);
@@ -555,23 +562,64 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
         }
     }
 
-    private void tryEnterPictureInPicture() {
-        if (!castSessionActive || !AppPreferences.isReceiverPipEnabled(this)) {
+    private boolean shouldAutoEnterPipOnLeave() {
+        if (!AppPreferences.isReceiverPipEnabled(this)
+                || !PermissionHelper.hasReceiverPlaybackReady(this)) {
+            return false;
+        }
+        if (castSessionActive || streamRendering) {
+            return true;
+        }
+        return bound && awaitingOverlay != null
+                && awaitingOverlay.getVisibility() == View.VISIBLE;
+    }
+
+    private void maybeAutoEnterPip() {
+        if (!autoStartListening || autoPipAttempted || inPictureInPicture) {
+            return;
+        }
+        if (!AppPreferences.isReceiverPipEnabled(this)
+                || !PictureInPictureHelper.isSupported()) {
             return;
         }
         if (!PermissionHelper.hasReceiverPlaybackReady(this)) {
             PermissionHelper.remindForReceiverPlayback(this);
             return;
         }
-        if (!PictureInPictureHelper.isSupported()) {
+        autoPipAttempted = true;
+        mainHandler.post(() -> tryEnterPictureInPicture(false));
+    }
+
+    private void tryEnterPictureInPicture(boolean userInitiated) {
+        if (!AppPreferences.isReceiverPipEnabled(this)) {
+            return;
+        }
+        if (!castSessionActive && !streamRendering
+                && (awaitingOverlay == null || awaitingOverlay.getVisibility() != View.VISIBLE)) {
+            if (userInitiated) {
                 Toast.makeText(this, R.string.pip_unavailable, Toast.LENGTH_SHORT).show();
+            }
+            return;
+        }
+        if (!PermissionHelper.hasReceiverPlaybackReady(this)) {
+            if (userInitiated) {
+                PermissionHelper.remindForReceiverPlayback(this);
+            }
+            return;
+        }
+        if (!PictureInPictureHelper.isSupported()) {
+            if (userInitiated) {
+                Toast.makeText(this, R.string.pip_unavailable, Toast.LENGTH_SHORT).show();
+            }
             return;
         }
         if (!PictureInPictureHelper.isAllowedForApp(this)) {
+            if (userInitiated) {
                 PictureInPictureHelper.showEnableInSettingsDialog(this);
+            }
             return;
         }
-        if (!PictureInPictureHelper.enter(this, videoWidth, videoHeight)) {
+        if (!PictureInPictureHelper.enter(this, videoWidth, videoHeight) && userInitiated) {
             PictureInPictureHelper.showEnableInSettingsDialog(this);
         }
     }
@@ -582,7 +630,12 @@ public class ReceiverPlaybackActivity extends DrawerHostActivity {
             handle.setVisibility(inPictureInPicture ? View.GONE : View.VISIBLE);
         }
         if (pipButton != null) {
-            pipButton.setVisibility(inPictureInPicture || !castSessionActive ? View.GONE : View.VISIBLE);
+            boolean showPip = (castSessionActive || streamRendering
+                    || (awaitingOverlay != null && awaitingOverlay.getVisibility() == View.VISIBLE))
+                    && AppPreferences.isReceiverPipEnabled(this)
+                    && PictureInPictureHelper.isSupported()
+                    && PermissionHelper.hasReceiverPlaybackReady(this);
+            pipButton.setVisibility(inPictureInPicture || !showPip ? View.GONE : View.VISIBLE);
         }
         if (inPictureInPicture) {
             if (diagnosticsText != null) {

app/src/main/java/com/foxx/androidcast/receiver/ReceiverSession.java

@@ -14,6 +14,8 @@ import com.foxx.androidcast.network.CastSession;
 import com.foxx.androidcast.network.CastSessionGate;
 import com.foxx.androidcast.network.CastTransport;
 import com.foxx.androidcast.network.CastTransportFactory;
+import com.foxx.androidcast.network.transport.ProtectionNegotiationPrompts;
+import com.foxx.androidcast.network.transport.StreamProtectionNegotiation;
 import com.foxx.androidcast.network.UdpCastTransport;
 import com.foxx.androidcast.network.quic.QuicCronetCastTransport;
 import com.foxx.androidcast.network.transport.TransportLossStats;
@@ -74,6 +76,7 @@ public class ReceiverSession {
     private CastSession session;
     private CastTransport listenTransport;
     private final CastSessionGate inboundSessionGate = new CastSessionGate();
+    private boolean loggedFirstVideoFrame;
     private NetworkFeedbackManager networkFeedback;
     private PassThroughNetworkControlPlane.StatsEnricher statsEnricher;
 
@@ -195,6 +198,7 @@ public class ReceiverSession {
                     listenTransport = null;
                 }
                 inboundSessionGate.reset();
+                loggedFirstVideoFrame = false;
                 networkFeedback = null;
                 listener.onDisconnected();
             }
@@ -237,6 +241,12 @@ public class ReceiverSession {
                 case CastProtocol.MSG_VIDEO_FRAME:
                     CastProtocol.VideoFrame vf = CastProtocol.parseVideoFrame(msg.payload);
                     recordNetworkBytes(vf.data != null ? vf.data.length : 0);
+                    if (!loggedFirstVideoFrame) {
+                        loggedFirstVideoFrame = true;
+                        Log.i(TAG, "First video over network "
+                                + (vf.keyFrame ? "KEY" : "P") + " len="
+                                + (vf.data != null ? vf.data.length : 0));
+                    }
                     listener.onVideoFrame(vf.ptsUs, vf.keyFrame, vf.data);
                     break;
                 case CastProtocol.MSG_AUDIO_CONFIG:
@@ -286,7 +296,16 @@ public class ReceiverSession {
             udp = ((QuicCronetCastTransport) listenTransport).getUdpTransport();
         }
         if (udp != null) {
-            CastTransportFactory.applyNegotiatedUdpProtection(udp, localSettings, remoteSettings);
+            StreamProtectionNegotiation negotiated = CastTransportFactory.applyNegotiatedUdpProtection(
+                    udp, localSettings, remoteSettings, true);
+            String summary = ProtectionNegotiationPrompts.summarize(appContext, negotiated);
+            Log.i(TAG, summary);
+            postStatus(summary);
+            if (negotiated.prompt != null) {
+                String hint = ProtectionNegotiationPrompts.format(appContext, negotiated.prompt, true);
+                Log.i(TAG, hint);
+                postStatus(hint);
+            }
         }
     }
 

app/src/main/java/com/foxx/androidcast/sender/AudioEncoder.java

@@ -88,18 +88,18 @@ public class AudioEncoder implements AudioEncoderSink {
 
     public void stop() {
         running = false;
-        MediaCodec c;
-        synchronized (inputLock) {
-            c = codec;
-            codec = null;
-        }
-        if (drainThread != null) {
+        Thread drain = drainThread;
+        drainThread = null;
+        if (drain != null) {
             try {
-                drainThread.join(1000);
+                drain.join(1000);
             } catch (InterruptedException ignored) {
                 Thread.currentThread().interrupt();
             }
         }
+        synchronized (inputLock) {
+            MediaCodec c = codec;
+            codec = null;
             if (c != null) {
                 try {
                     c.stop();
@@ -111,17 +111,33 @@ public class AudioEncoder implements AudioEncoderSink {
                 }
             }
         }
+    }
 
     private void drainLoop() {
         MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
         boolean configSent = false;
-        while (running) {
-            int index = codec.dequeueOutputBuffer(info, 10_000);
+        while (true) {
+            MediaCodec c;
+            synchronized (inputLock) {
+                if (!running) {
+                    break;
+                }
+                c = codec;
+                if (c == null) {
+                    break;
+                }
+            }
+            int index;
+            try {
+                index = c.dequeueOutputBuffer(info, 10_000);
+            } catch (IllegalStateException e) {
+                break;
+            }
             if (index == MediaCodec.INFO_TRY_AGAIN_LATER) {
                 continue;
             }
             if (index == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED) {
-                MediaFormat fmt = codec.getOutputFormat();
+                MediaFormat fmt = c.getOutputFormat();
                 if (!configSent && callback != null) {
                     ByteBuffer csd = fmt.getByteBuffer("csd-0");
                     callback.onConfig(
@@ -135,7 +151,7 @@ public class AudioEncoder implements AudioEncoderSink {
             if (index < 0) {
                 continue;
             }
-            ByteBuffer buffer = codec.getOutputBuffer(index);
+            ByteBuffer buffer = c.getOutputBuffer(index);
             if (buffer != null && info.size > 0 && (info.flags & MediaCodec.BUFFER_FLAG_CODEC_CONFIG) == 0) {
                 byte[] data = new byte[info.size];
                 buffer.position(info.offset);
@@ -145,7 +161,11 @@ public class AudioEncoder implements AudioEncoderSink {
                     callback.onEncodedFrame(info.presentationTimeUs, data);
                 }
             }
-            codec.releaseOutputBuffer(index, false);
+            try {
+                c.releaseOutputBuffer(index, false);
+            } catch (IllegalStateException ignored) {
+                break;
+            }
         }
     }
 

app/src/main/java/com/foxx/androidcast/sender/MultiCastCoordinator.java

@@ -11,6 +11,8 @@ import com.foxx.androidcast.network.CastProtocol;
 import com.foxx.androidcast.network.CastSession;
 import com.foxx.androidcast.network.CastTransport;
 import com.foxx.androidcast.network.CastTransportFactory;
+import com.foxx.androidcast.network.transport.ProtectionNegotiationPrompts;
+import com.foxx.androidcast.network.transport.StreamProtectionNegotiation;
 import com.foxx.androidcast.network.UdpCastTransport;
 import com.foxx.androidcast.network.quic.QuicCronetCastTransport;
 
@@ -99,7 +101,7 @@ public final class MultiCastCoordinator {
         session = createSession(peerSettings);
         connectWithRetry(session, t.host, t.port);
         CastSession.HandshakeResult hs = session.clientHandshake(deviceName, pin, peerSettings);
-        applyNegotiatedProtection(session, peerSettings, hs.settings);
+        applyNegotiatedProtection(session, peerSettings, hs.receiverAdvertisement);
         peers.add(new ActivePeer(t, session, hs.settings, hs.negotiatedVideoMime));
         Log.i(TAG, "Peer ready " + t.name + " " + hs.negotiatedVideoMime
                 + " score="
@@ -128,7 +130,7 @@ public final class MultiCastCoordinator {
     CastSession session = createSession(peerSettings);
     connectWithRetry(session, target.host, target.port);
     CastSession.HandshakeResult hs = session.clientHandshake(deviceName, pin, peerSettings);
-    applyNegotiatedProtection(session, peerSettings, hs.settings);
+    applyNegotiatedProtection(session, peerSettings, hs.receiverAdvertisement);
     peers.add(new ActivePeer(target, session, hs.settings, hs.negotiatedVideoMime));
     if (keyframeRequester != null) {
       keyframeRequester.requestKeyframe();
@@ -202,8 +204,8 @@ public final class MultiCastCoordinator {
     return session;
   }
 
-  private static void applyNegotiatedProtection(CastSession session, CastSettings local,
-      CastSettings remote) {
+  private void applyNegotiatedProtection(CastSession session, CastSettings local,
+      CastSettings receiverAdvertisement) {
     if (session == null) {
       return;
     }
@@ -215,7 +217,13 @@ public final class MultiCastCoordinator {
       udp = ((QuicCronetCastTransport) transport).getUdpTransport();
     }
     if (udp != null) {
-      CastTransportFactory.applyNegotiatedUdpProtection(udp, local, remote);
+      CastSettings remote = receiverAdvertisement;
+      StreamProtectionNegotiation negotiated = CastTransportFactory.applyNegotiatedUdpProtection(
+              udp, local, remote, false);
+      Log.i(TAG, ProtectionNegotiationPrompts.summarize(appContext, negotiated));
+      if (negotiated.prompt != null) {
+        Log.i(TAG, ProtectionNegotiationPrompts.format(appContext, negotiated.prompt, false));
+      }
     }
   }
 

app/src/main/java/com/foxx/androidcast/sender/ScreenCastService.java

@@ -158,6 +158,7 @@ public class ScreenCastService extends Service implements
         }
         CastNotifications.createChannels(this);
         com.foxx.androidcast.network.CastTransportFactory.init(this);
+        com.foxx.androidcast.diagnostics.CpuLoadSampler.warmUp();
         projectionThread = new HandlerThread("CastProjection");
         projectionThread.start();
         projectionHandler = new Handler(projectionThread.getLooper());
@@ -411,6 +412,7 @@ public class ScreenCastService extends Service implements
     }
 
     private void startCaptureInternal(int resultCode, Intent data, CastSettings settings) throws IOException {
+        SenderPreviewService.stop(this);
         MediaProjectionManager mgr =
                 (MediaProjectionManager) getSystemService(MEDIA_PROJECTION_SERVICE);
         projection = mgr.getMediaProjection(resultCode, data);

app/src/main/java/com/foxx/androidcast/sender/SenderActivity.java

@@ -464,10 +464,14 @@ public class SenderActivity extends DrawerHostActivity {
             if (resultCode == Activity.RESULT_OK && data != null) {
                 screenPreview.setProjectionResult(resultCode, data);
                 previewConsentDeclined = false;
+                findViewById(R.id.panel_preview).post(() -> {
+                    refreshPreview();
+                    findViewById(R.id.panel_preview).postDelayed(this::refreshPreview, 200);
+                });
             } else {
                 previewConsentDeclined = true;
-            }
                 refreshPreview();
+            }
             return;
         }
         if (requestCode != REQUEST_MEDIA_PROJECTION) {

app/src/main/java/com/foxx/androidcast/sender/SenderCapturePreview.java

@@ -19,6 +19,7 @@ import android.widget.TextView;
 
 import androidx.core.content.ContextCompat;
 
+import com.foxx.androidcast.CastActiveState;
 import com.foxx.androidcast.CastSettings;
 import com.foxx.androidcast.R;
 import com.foxx.androidcast.sender.calibration.TvCalibrationGenerator;
@@ -62,6 +63,10 @@ public final class SenderCapturePreview {
     public void updateForMode(CastSettings.CaptureMode mode) {
         activeMode = mode;
         stopCamera();
+        if (CastActiveState.isSenderCasting()) {
+            showCastingPlaceholder();
+            return;
+        }
         if (mode == CastSettings.CaptureMode.CAMERA) {
             useFront = false;
             startCameraPreview();
@@ -71,7 +76,7 @@ public final class SenderCapturePreview {
         } else if (mode == CastSettings.CaptureMode.CALIBRATION_TEST) {
             showCalibrationPreview();
         } else if (screenPreview != null && screenPreview.needsProjection(mode)) {
-            if (screenPreview.hasProjection()) {
+            if (screenPreview.hasConsent()) {
                 labelView.setVisibility(android.view.View.GONE);
                 textureView.setVisibility(android.view.View.VISIBLE);
                 screenPreview.attach(textureView, mode);
@@ -92,6 +97,16 @@ public final class SenderCapturePreview {
         labelView.setText(labelView.getContext().getString(textRes));
     }
 
+    public void showCastingPlaceholder() {
+        if (screenPreview != null) {
+            screenPreview.pausePreview();
+        }
+        stopCamera();
+        textureView.setVisibility(android.view.View.GONE);
+        labelView.setVisibility(android.view.View.VISIBLE);
+        labelView.setText(labelView.getContext().getString(R.string.preview_casting_active));
+    }
+
     private void showCalibrationPreview() {
         if (screenPreview != null) {
             screenPreview.stop();

app/src/main/java/com/foxx/androidcast/sender/SenderPreviewService.java

@@ -0,0 +1,64 @@
+package com.foxx.androidcast.sender;
+
+import android.app.Service;
+import android.content.Context;
+import android.content.Intent;
+import android.os.Build;
+import android.os.IBinder;
+
+import androidx.annotation.Nullable;
+
+import com.foxx.androidcast.CastNotifications;
+
+/**
+ * Foreground service so {@link SenderScreenPreview} can use MediaProjection on Android 10+.
+ * Stopped when screen cast {@link ScreenCastService} starts (that service holds projection FGS).
+ */
+public final class SenderPreviewService extends Service {
+    private static volatile boolean running;
+
+    public static boolean isRunning() {
+        return running;
+    }
+
+    public static void start(Context context) {
+        Intent intent = new Intent(context, SenderPreviewService.class);
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+            context.startForegroundService(intent);
+        } else {
+            context.startService(intent);
+        }
+    }
+
+    public static void stop(Context context) {
+        context.stopService(new Intent(context, SenderPreviewService.class));
+    }
+
+    @Override
+    public void onCreate() {
+        super.onCreate();
+        CastNotifications.createChannels(this);
+        running = true;
+    }
+
+    @Override
+    public int onStartCommand(Intent intent, int flags, int startId) {
+        startForeground(CastNotifications.ID_SENDER + 100,
+                CastNotifications.sender(this, getString(com.foxx.androidcast.R.string.preview_screen_active),
+                        SenderPreviewService.class));
+        return START_STICKY;
+    }
+
+    @Override
+    public void onDestroy() {
+        running = false;
+        stopForeground(STOP_FOREGROUND_REMOVE);
+        super.onDestroy();
+    }
+
+    @Nullable
+    @Override
+    public IBinder onBind(Intent intent) {
+        return null;
+    }
+}

app/src/main/java/com/foxx/androidcast/sender/SenderScreenPreview.java

@@ -16,6 +16,7 @@ import android.view.Surface;
 import android.view.TextureView;
 import android.view.WindowManager;
 
+import com.foxx.androidcast.CastActiveState;
 import com.foxx.androidcast.CastSettings;
 
 /**
@@ -105,6 +106,9 @@ public final class SenderScreenPreview {
         if (projection != null || resultData == null) {
             return;
         }
+        if (!CastActiveState.isSenderCasting() && !SenderPreviewService.isRunning()) {
+            SenderPreviewService.start(appContext);
+        }
         MediaProjectionManager mgr =
                 appContext.getSystemService(MediaProjectionManager.class);
         projection = mgr.getMediaProjection(resultCode, resultData);
@@ -133,8 +137,14 @@ public final class SenderScreenPreview {
             DisplayMetrics metrics = new DisplayMetrics();
             WindowManager wm = (WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE);
             wm.getDefaultDisplay().getRealMetrics(metrics);
-            int w = Math.max(320, textureView.getWidth() > 0 ? textureView.getWidth() : metrics.widthPixels / 2);
-            int h = Math.max(240, textureView.getHeight() > 0 ? textureView.getHeight() : metrics.heightPixels / 2);
+            int viewW = textureView.getWidth();
+            int viewH = textureView.getHeight();
+            if (viewW <= 0 || viewH <= 0) {
+                textureView.post(() -> ensureVirtualDisplay(textureView, mode));
+                return;
+            }
+            int w = Math.max(320, viewW);
+            int h = Math.max(240, viewH);
             SurfaceTexture st = textureView.getSurfaceTexture();
             if (st == null) {
                 return;
@@ -181,6 +191,9 @@ public final class SenderScreenPreview {
             }
             projection = null;
         }
+        if (!CastActiveState.isSenderCasting()) {
+            SenderPreviewService.stop(appContext);
+        }
     }
 
     /** Pause preview surface only; keep MediaProjection consent for cast START. */

app/src/main/res/values-ru/strings.xml

@@ -149,6 +149,8 @@
     <string name="capture_camera_front">Камера (фронт)</string>
     <string name="label_mute">Тихо</string>
     <string name="preview_placeholder">Предпросмотр\n(трансляция начнется после нажатия кнопки)</string>
+    <string name="preview_screen_active">Предпросмотр экрана</string>
+    <string name="preview_casting_active">Трансляция — предпросмотр приостановлен</string>
     <string name="preview_camera">Предпросмотр с камеры</string>
     <string name="preview_calibration">Калибрационная таблица</string>
     <string name="receiver_use_settings_hint">Stream options are in Android Cast settings.</string>

app/src/main/res/values/strings.xml

@@ -133,7 +133,17 @@
     <string name="receiver_display_auto_fit">AUTO (fits screen)</string>
     <string name="receiver_display_adaptive">Adaptive (experimental)</string>
     <string name="label_stream_protection">Stream protection (UDP)</string>
-    <string name="stream_protection_hint">Hint only — negotiated with receiver. UDP transports only.</string>
+    <string name="stream_protection_hint">Sender preference — receiver FEC/NACK is auto-detected at connect. UDP only.</string>
+    <string name="protection_negotiation_role_sender">sender</string>
+    <string name="protection_negotiation_role_receiver">receiver</string>
+    <string name="protection_negotiation_nack_on">NACK on</string>
+    <string name="protection_negotiation_nack_off">NACK off</string>
+    <string name="protection_negotiation_agreed">Stream protection: %1$s (%2$s)</string>
+    <string name="protection_negotiation_use_none">No protection this session (receiver %1$s, sender %2$s). Set %3$s stream protection to None or match the peer.</string>
+    <string name="protection_negotiation_mismatch">Protection mismatch (receiver %1$s, sender %2$s). Casting without protection; align %3$s settings.</string>
+    <string name="protection_negotiation_fec_family_rs">Using Reed–Solomon FEC (receiver %1$s, sender %2$s). For a direct match, set %3$s to FEC (Reed–Solomon).</string>
+    <string name="protection_negotiation_fec34_no_nack">Using FEC 3/4 without NACK (receiver %1$s, sender %2$s). NACK disabled for this session.</string>
+    <string name="protection_negotiation_nack_to_fec">Using FEC with NACK (receiver %1$s, sender %2$s).</string>
     <string name="protection_fec_34">FEC 3/4</string>
     <string name="protection_fec_rs">FEC (Reed-Solomon)</string>
     <string name="protection_nack">NACK</string>
@@ -149,6 +159,8 @@
     <string name="capture_camera_front">Camera (front)</string>
     <string name="label_mute">Mute</string>
     <string name="preview_placeholder">Source preview\n(screen capture starts after START)</string>
+    <string name="preview_screen_active">Screen preview</string>
+    <string name="preview_casting_active">Casting — preview paused</string>
     <string name="preview_camera">Camera preview</string>
     <string name="preview_calibration">Calibration test pattern</string>
     <string name="receiver_use_settings_hint">Stream options are in Android Cast settings.</string>

app/src/test/java/com/foxx/androidcast/diagnostics/CastMtuSamplerTest.java

@@ -0,0 +1,27 @@
+package com.foxx.androidcast.diagnostics;
+
+import static org.junit.Assert.assertEquals;
+
+import com.foxx.androidcast.CastConfig;
+
+import org.junit.Test;
+
+public class CastMtuSamplerTest {
+    @Test
+    public void calculate_wifiMtu1500() {
+        CastMtuSampler.Info info = CastMtuSampler.calculate(1500, "wlan0");
+        assertEquals(1500, info.linkMtu);
+        assertEquals(CastConfig.UDP_CHUNK_SIZE, info.castDatagramBytes);
+        assertEquals(CastConfig.UDP_CHUNK_SIZE - CastMtuSampler.ACUD_HEADER_BYTES,
+                info.castFragPayloadBytes);
+        assertEquals(CastConfig.UDP_CHUNK_SIZE + 28, info.ipv4WireBytes);
+        assertEquals(1500 - info.ipv4WireBytes, info.headroomBytes);
+    }
+
+    @Test
+    public void calculate_unknownLink_noHeadroom() {
+        CastMtuSampler.Info info = CastMtuSampler.calculate(0, "");
+        assertEquals(-1, info.headroomBytes);
+        assertEquals(1228, info.ipv4WireBytes);
+    }
+}

app/src/test/java/com/foxx/androidcast/network/UdpCastTransportFecTest.java

@@ -7,6 +7,8 @@ import com.foxx.androidcast.CastConfig;
 import com.foxx.androidcast.CastSettings;
 import com.foxx.androidcast.network.transport.StreamProtectionFactory;
 
+import com.foxx.androidcast.network.CastFramingContext;
+
 import org.junit.Test;
 
 import java.net.DatagramPacket;
@@ -24,8 +26,8 @@ public class UdpCastTransportFecTest {
             receiver.prepareListen(portB);
             sender.connect("127.0.0.1", portB);
             CastSettings fec = settingsWith(CastSettings.StreamProtection.FEC_3_4);
-            CastTransportFactory.applyNegotiatedUdpProtection(receiver, fec, fec);
-            CastTransportFactory.applyNegotiatedUdpProtection(sender, fec, fec);
+            CastTransportFactory.applyNegotiatedUdpProtection(receiver, fec, fec, true);
+            CastTransportFactory.applyNegotiatedUdpProtection(sender, fec, fec, false);
 
             byte[] payload = new byte[900];
             for (int i = 0; i < payload.length; i++) {
@@ -58,6 +60,54 @@ public class UdpCastTransportFecTest {
         }
     }
 
+    @Test
+    public void fecShardGroup_wireFramedLargeKeyframe_delivers() throws Exception {
+        int portB = 38000 + (int) (System.nanoTime() % 2000);
+        UdpCastTransport sender = new UdpCastTransport();
+        UdpCastTransport receiver = new UdpCastTransport();
+        CastFramingContext framing = new CastFramingContext(42, 1, 0, 0);
+        try {
+            receiver.prepareListen(portB);
+            sender.connect("127.0.0.1", portB);
+            sender.setFramingContext(framing);
+            receiver.setFramingContext(framing);
+            CastSettings fec = settingsWith(CastSettings.StreamProtection.FEC_3_4);
+            CastTransportFactory.applyNegotiatedUdpProtection(receiver, fec, fec, true);
+            CastTransportFactory.applyNegotiatedUdpProtection(sender, fec, fec, false);
+
+            byte[] video = new byte[171_077];
+            for (int i = 0; i < video.length; i++) {
+                video[i] = (byte) (i & 0xFF);
+            }
+            byte[] videoMsg = CastProtocol.videoFramePayload(0, true, video);
+
+            Thread t = new Thread(() -> {
+                try {
+                    sender.send(CastProtocol.MSG_VIDEO_FRAME, videoMsg);
+                } catch (Exception e) {
+                    throw new RuntimeException(e);
+                }
+            });
+            t.start();
+
+            AtomicReference<byte[]> received = new AtomicReference<>();
+            long deadline = System.currentTimeMillis() + 15_000;
+            while (System.currentTimeMillis() < deadline && received.get() == null) {
+                CastProtocol.Message msg = receiver.receive(500);
+                if (msg != null && msg.type == CastProtocol.MSG_VIDEO_FRAME) {
+                    CastProtocol.VideoFrame vf = CastProtocol.parseVideoFrame(msg.payload);
+                    received.set(vf.data);
+                }
+            }
+            t.join(3000);
+            assertNotNull(received.get());
+            assertArrayEquals(video, received.get());
+        } finally {
+            sender.close();
+            receiver.close();
+        }
+    }
+
     private static CastSettings settingsWith(CastSettings.StreamProtection protection) {
         CastSettings s = new CastSettings();
         s.setTransport(CastConfig.TRANSPORT_UDP);

app/src/test/java/com/foxx/androidcast/network/transport/FecProtectionEngineShardTest.java

@@ -4,6 +4,7 @@ import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
 
 import com.foxx.androidcast.network.CastProtocol;
 import com.foxx.androidcast.network.transport.fec.FecEncodedGroup;
@@ -12,6 +13,20 @@ import com.foxx.androidcast.network.transport.fec.ProtectionEnvelope;
 import org.junit.Test;
 
 public class FecProtectionEngineShardTest {
+    @Test
+    public void largePayload_usesMoreSmallerShards() throws Exception {
+        TransportLossStats stats = new TransportLossStats();
+        FecProtectionEngine fec = new FecProtectionEngine(
+                ProtectionEnvelope.MODE_FEC_34, 4, 3, stats);
+        byte[] payload = new byte[180_000];
+        FecEncodedGroup group = fec.encodeGroup(payload);
+        assertEquals(16, group.dataShards);
+        assertEquals(8, group.parityShards);
+        assertEquals(24, group.totalShards());
+        int shardBytes = group.shards[0].length;
+        assertTrue(shardBytes < 16_000);
+    }
+
     @Test
     public void encodeAndDecodeShards_viaEngine() throws Exception {
         TransportLossStats stats = new TransportLossStats();

app/src/test/java/com/foxx/androidcast/network/transport/StreamProtectionFactoryTest.java

@@ -27,6 +27,23 @@ public class StreamProtectionFactoryTest {
         StreamProtectionEngine engine = StreamProtectionFactory.create(
                 CastSettings.StreamProtection.FEC_3_4, new TransportLossStats());
         assertEquals(StreamProtectionMode.FEC_3_4, engine.mode());
+        assertTrue(engine instanceof CombinedProtectionEngine);
+    }
+
+    @Test
+    public void fec34_withoutNack_isFecOnly() {
+        StreamProtectionEngine engine = StreamProtectionFactory.create(
+                CastSettings.StreamProtection.FEC_3_4, new TransportLossStats(), false);
+        assertEquals(StreamProtectionMode.FEC_3_4, engine.mode());
+        assertTrue(engine instanceof FecProtectionEngine);
+    }
+
+    @Test
+    public void fecRs_withNack_isCombined() {
+        StreamProtectionEngine engine = StreamProtectionFactory.create(
+                CastSettings.StreamProtection.FEC_REED_SOLOMON, new TransportLossStats(), true);
+        assertEquals(StreamProtectionMode.FEC_REED_SOLOMON, engine.mode());
+        assertTrue(engine instanceof CombinedProtectionEngine);
     }
 
     @Test

app/src/test/java/com/foxx/androidcast/network/transport/StreamProtectionNegotiatorTest.java

@@ -1,6 +1,10 @@
 package com.foxx.androidcast.network.transport;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
 
 import com.foxx.androidcast.CastSettings;
 
@@ -14,18 +18,93 @@ public class StreamProtectionNegotiatorTest {
     }
 
     @Test
-    public void mismatchFallsBackToNone() {
-        assertEquals(CastSettings.StreamProtection.NONE,
-                StreamProtectionNegotiator.resolve(
-                        CastSettings.StreamProtection.FEC_NACK,
-                        CastSettings.StreamProtection.NACK));
+    public void bothFec34_agreesWithNack() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_3_4,
+                CastSettings.StreamProtection.FEC_3_4,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_3_4, n.agreed);
+        assertTrue(n.nackEnabled);
+        assertNull(n.prompt);
     }
 
     @Test
-    public void matchingModesPreserved() {
-        assertEquals(CastSettings.StreamProtection.FEC_3_4,
-                StreamProtectionNegotiator.resolve(
+    public void senderFec34_receiverAuto_usesFec34() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
                 CastSettings.StreamProtection.FEC_3_4,
-                        CastSettings.StreamProtection.FEC_3_4));
+                CastSettings.StreamProtection.AUTO,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_3_4, n.agreed);
+        assertTrue(n.nackEnabled);
+    }
+
+    @Test
+    public void receiverSide_senderFec34_usesFec34() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.AUTO,
+                CastSettings.StreamProtection.FEC_3_4,
+                true);
+        assertEquals(CastSettings.StreamProtection.FEC_3_4, n.agreed);
+        assertTrue(n.nackEnabled);
+    }
+
+    @Test
+    public void senderNone_receiverAuto_noProtectionWithPrompt() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.NONE,
+                CastSettings.StreamProtection.AUTO,
+                false);
+        assertEquals(CastSettings.StreamProtection.NONE, n.agreed);
+        assertNotNull(n.prompt);
+    }
+
+    @Test
+    public void senderFec34_receiverRsAdvert_usesRs() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_3_4,
+                CastSettings.StreamProtection.FEC_REED_SOLOMON,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_REED_SOLOMON, n.agreed);
+        assertNotNull(n.prompt);
+    }
+
+    @Test
+    public void senderFec34_receiverAuto_usesFec34WithNack() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_3_4,
+                CastSettings.StreamProtection.AUTO,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_3_4, n.agreed);
+        assertTrue(n.nackEnabled);
+    }
+
+    @Test
+    public void senderFec34_receiverAdvertFecNack_usesFec34WithoutNack() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_3_4,
+                CastSettings.StreamProtection.FEC_NACK,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_3_4, n.agreed);
+        assertFalse(n.nackEnabled);
+    }
+
+    @Test
+    public void senderFecNack_receiverAuto_keepsFecNack() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_NACK,
+                CastSettings.StreamProtection.AUTO,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_NACK, n.agreed);
+        assertTrue(n.nackEnabled);
+    }
+
+    @Test
+    public void fecNackWithNackOnly_usesFecNack() {
+        StreamProtectionNegotiation n = StreamProtectionNegotiator.negotiate(
+                CastSettings.StreamProtection.FEC_NACK,
+                CastSettings.StreamProtection.AUTO,
+                false);
+        assertEquals(CastSettings.StreamProtection.FEC_NACK, n.agreed);
+        assertTrue(n.nackEnabled);
     }
 }

app/src/test/java/com/foxx/androidcast/network/transport/fec/FecShardReassemblyTest.java

@@ -39,6 +39,31 @@ public class FecShardReassemblyTest {
         assertArrayEquals(payload, recovered);
     }
 
+    @Test
+    public void vp9SizedPayload_recoversAllShards() throws Exception {
+        byte[] payload = new byte[54_000];
+        for (int i = 0; i < payload.length; i++) {
+            payload[i] = (byte) (i & 0xFF);
+        }
+        ReedSolomonFec fec = new ReedSolomonFec(4, 3);
+        byte[][] shards = fec.encode(payload);
+        FecShardReassembly asm = null;
+        for (int i = 0; i < shards.length; i++) {
+            byte[] wire = FecShardWire.encodeShard(
+                    ProtectionEnvelope.MODE_FEC_34, 42, 900, i, shards.length,
+                    payload.length, 4, 3, shards[i]);
+            FecShardWire.Parsed p = FecShardWire.tryParse(wire);
+            assertNotNull(p);
+            if (asm == null) {
+                asm = new FecShardReassembly(p);
+            }
+            assertTrue(asm.addShard(p));
+        }
+        byte[] recovered = asm.tryDecode();
+        assertNotNull(recovered);
+        assertArrayEquals(payload, recovered);
+    }
+
     @Test
     public void insufficientShards_returnsNull() throws Exception {
         byte[] payload = new byte[] {1, 2, 3, 4, 5};

app/src/test/java/com/foxx/androidcast/network/transport/fec/FecShardWireTest.java

@@ -5,9 +5,52 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 
+import java.io.ByteArrayOutputStream;
+import java.io.DataOutputStream;
+
 import org.junit.Test;
 
 public class FecShardWireTest {
+    @Test
+    public void largeShard_roundTrip() throws Exception {
+        byte[] payload = new byte[54_000];
+        for (int i = 0; i < payload.length; i++) {
+            payload[i] = (byte) (i & 0xFF);
+        }
+        ReedSolomonFec fec = new ReedSolomonFec(4, 3);
+        byte[][] shards = fec.encode(payload);
+        byte[] shard = shards[0];
+        assertTrue(shard.length > 255);
+        byte[] wire = FecShardWire.encodeShard(
+                ProtectionEnvelope.MODE_FEC_34, 99, 1000, 0, shards.length,
+                payload.length, 4, 3, shard);
+        FecShardWire.Parsed p = FecShardWire.tryParse(wire);
+        assertNotNull(p);
+        assertArrayEquals(shard, p.shardData);
+    }
+
+    @Test
+    public void v2SmallShard_stillParses() throws Exception {
+        byte[] shard = new byte[200];
+        ByteArrayOutputStream bos = new ByteArrayOutputStream();
+        DataOutputStream dos = new DataOutputStream(bos);
+        dos.write(new byte[] {'A', 'C', 'P', 'F'});
+        dos.writeByte(FecShardWire.VERSION_SHARD_V2);
+        dos.writeByte(ProtectionEnvelope.MODE_FEC_34);
+        dos.writeInt(1);
+        dos.writeInt(10);
+        dos.writeByte(0);
+        dos.writeByte(7);
+        dos.writeInt(800);
+        dos.writeByte(4);
+        dos.writeByte(3);
+        dos.writeByte(shard.length);
+        dos.write(shard);
+        FecShardWire.Parsed p = FecShardWire.tryParse(bos.toByteArray());
+        assertNotNull(p);
+        assertArrayEquals(shard, p.shardData);
+    }
+
     @Test
     public void encodeParse_roundTrip() throws Exception {
         byte[] shard = new byte[64];

ndk/README.md

@@ -8,7 +8,7 @@ The app loads `libandroidcast_codecs.so` for availability probes and future soft
 |-----------|------|--------|-------|
 | FEC 3/4 + RS | Yes | — | v2 **per-shard** on wire (`FecShardWire`); v1 monolithic decode still accepted |
 | NACK + retransmit cache | Yes | — | UDP `MSG_NACK`, combined with FEC when enabled |
-| libvpx VP8/VP9 | Native | Encode + decode | Screen cast via ImageReader; receiver libvpx or MediaCodec fallback |
+| libvpx VP8/VP9 | Native | Encode + decode | Screen cast via ImageReader; receiver libvpx (decoder uses `VPX_CODEC_USE_ERROR_CONCEALMENT` when supported) or MediaCodec fallback |
 | Opus / Speex | Stub | Probe only | Submodules under `third-party/` |
 
 ## Enable libvpx (developer)

ndk/jni/libvpx_bridge.c

@@ -342,9 +342,15 @@ Java_com_foxx_androidcast_media_codec_jni_NativeCodecBridge_nativeVpxDecoderCrea
         return 0L;
     }
     ctx->magic = VPX_DEC_MAGIC;
+    vpx_codec_iface_t *iface = pick_decoder_iface(mime_utf);
     vpx_codec_dec_cfg_t cfg = {0};
     cfg.threads = 1;
-    if (vpx_codec_dec_init(&ctx->codec, pick_decoder_iface(mime_utf), &cfg, 0) != VPX_CODEC_OK) {
+    /* vpxdec.c: dec_flags |= VPX_CODEC_USE_ERROR_CONCEALMENT when --error-concealment */
+    int dec_flags = 0;
+    if (vpx_codec_get_caps(iface) & VPX_CODEC_CAP_ERROR_CONCEALMENT) {
+        dec_flags |= VPX_CODEC_USE_ERROR_CONCEALMENT;
+    }
+    if (vpx_codec_dec_init(&ctx->codec, iface, &cfg, dec_flags) != VPX_CODEC_OK) {
         free(ctx);
         (*env)->ReleaseStringUTFChars(env, mime, mime_utf);
         return 0L;
@@ -381,9 +387,8 @@ Java_com_foxx_androidcast_media_codec_jni_NativeCodecBridge_nativeVpxDecodeFrame
     vpx_codec_err_t err = vpx_codec_decode(&ctx->codec, (const uint8_t *) bytes, (unsigned int) len,
                                             NULL, 0);
     (*env)->ReleaseByteArrayElements(env, data, bytes, JNI_ABORT);
-    if (err != VPX_CODEC_OK) {
-        return JNI_FALSE;
-    }
+    /* With error concealment, vpx_codec_decode may warn yet still output a frame (see vpxdec.c). */
+    (void) err;
 
     ANativeWindow *window = ANativeWindow_fromSurface(env, surface);
     if (window == NULL) {

scripts/build-native-codecs.sh

@@ -225,7 +225,7 @@ cp -f "$LIBVPX_A" "$OUT/libvpx.a"
 cp -f "$OUT/libvpx.a" "$PROJECT_OUT/libvpx.a"
 
 echo ""
-echo "Built: $PROJECT_OUT/libvpx.a"
+echo "Built: $PROJECT_OUT/libvpx.a (size $(ls -lah "${PROJECT_OUT}/libvpx.a" | awk '{print $5}'))"
 if [[ -n "$STAGING" ]]; then
   echo "(staged under $STAGING because of spaces in repo path)"
 fi

scripts/capture-vp9-fec-logs.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Capture VP9+FEC cast logs from sender (.106) and receiver (.39).
+# Usage: ./scripts/capture-vp9-fec-logs.sh [label]
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+CAP="$ROOT/.log_capture"
+TS="$(date +%Y%m%d_%H%M%S)"
+LABEL="${1:-fec}"
+mkdir -p "$CAP"
+
+SENDER="192.168.33.106:5555"
+RECEIVER="192.168.33.39:5555"
+
+adb connect "$SENDER" >/dev/null 2>&1 || true
+adb connect "$RECEIVER" >/dev/null 2>&1 || true
+
+FILTER='FEC|Fec|fec|UdpCast|StreamProtection|Protection|reasm|ReedSolomon|shard|ScreenCast|Libvpx|ReceiverCast|ReceiverSession|VideoDecoder|NativeCodec|CodecPriority|CastFanout|CastSendPump|NetworkFeedback|transportLoss|decode|keyframe|awaiting|AudioDecoder|drop'
+
+echo "==> Clearing logcat buffers"
+adb -s "$SENDER" logcat -c 2>/dev/null || true
+adb -s "$RECEIVER" logcat -c 2>/dev/null || true
+
+SPID=$(adb -s "$SENDER" shell pidof com.foxx.androidcast 2>/dev/null | tr -d '\r' || true)
+RPID=$(adb -s "$RECEIVER" shell pidof com.foxx.androidcast 2>/dev/null | tr -d '\r' || true)
+echo "==> PIDs sender=$SPID receiver=$RPID"
+
+echo "==> Recording 90s (start cast now)..."
+timeout 90 adb -s "$SENDER" logcat -v threadtime 2>&1 | grep -iE "$FILTER" > "$CAP/${LABEL}_sender_106_${TS}.log" &
+PID_S=$!
+timeout 90 adb -s "$RECEIVER" logcat -v threadtime 2>&1 | grep -iE "$FILTER" > "$CAP/${LABEL}_receiver_39_${TS}.log" &
+PID_R=$!
+wait "$PID_S" "$PID_R" 2>/dev/null || true
+
+# Snapshot dumps after live capture
+adb -s "$SENDER" logcat -d --pid="$SPID" -t 8000 > "$CAP/${LABEL}_sender_pid_${TS}.log" 2>&1 || true
+adb -s "$RECEIVER" logcat -d --pid="$RPID" -t 8000 > "$CAP/${LABEL}_receiver_pid_${TS}.log" 2>&1 || true
+adb -s "$SENDER" logcat -d -b crash -t 50 2>&1 > "$CAP/${LABEL}_sender_crash_${TS}.log" || true
+adb -s "$RECEIVER" logcat -d -b crash -t 50 2>&1 > "$CAP/${LABEL}_receiver_crash_${TS}.log" || true
+
+echo "==> Saved under $CAP/${LABEL}_*_${TS}.log"
+wc -c "$CAP"/${LABEL}_*_${TS}.log 2>/dev/null | tail -6